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1 Energy in buidings

2 About this free course This free course is an adapted extract from the Open University courses T212 Energy and sustainabiity and T313 Renewabe energy This version of the content may incude video, images and interactive content that may not be optimised for your device. You can experience this free course as it was originay designed on OpenLearn, the home of free earning from The Open University There you aso be abe to track your progress via your activity record, which you can use to demonstrate your earning. Copyright 2016 The Open University Inteectua property Uness otherwise stated, this resource is reeased under the terms of the Creative Commons Licence v4.0 Within that The Open University interprets this icence in the foowing way: Copyright and rights faing outside the terms of the Creative Commons Licence are retained or controed by The Open University. Pease read the fu text before using any of the content. We beieve the primary barrier to accessing high-quaity educationa experiences is cost, which is why we aim to pubish as much free content as possibe under an open icence. If it proves difficut to reease content under our preferred Creative Commons icence (e.g. because we can t afford or gain the cearances or find suitabe aternatives), we wi sti reease the materias for free under a persona enduser icence. This is because the earning experience wi aways be the same high quaity offering and that shoud aways be seen as positive even if at times the icensing is different to Creative Commons. When using the content you must attribute us (The Open University) (the OU) and any identified author in accordance with the terms of the Creative Commons Licence. The Acknowedgements section is used to ist, amongst other things, third party (Proprietary), icensed content which is not subject to Creative Commons icensing. Proprietary content must be used (retained) intact and in context to the content at a times. The Acknowedgements section is aso used to bring to your attention any other Specia Restrictions which may appy to the content. For exampe there may be times when the Creative Commons Non- Commercia Shareaike icence does not appy to any of the content even if owned by us (The Open University). In these instances, uness stated otherwise, the content may be used for persona and noncommercia use. We have aso identified as Proprietary other materia incuded in the content which is not subject to Creative Commons Licence. These are OU ogos, trading names and may extend to certain photographic and video images and sound recordings and any other materia as may be brought to your attention. Unauthorised use of any of the content may constitute a breach of the terms and conditions and/or inteectua property aws. We reserve the right to ater, amend or bring to an end any terms and conditions provided here without notice. A rights faing outside the terms of the Creative Commons icence are retained or controed by The Open University. Head of Inteectua Property, The Open University 2 of 87 Monday 18 June 2018

3 Contents Introduction 4 Learning Outcomes 5 1 The importance of energy use in buidings 6 2 Reducing space heating demand Heating a house Cutting buiding fabric heat osses Cutting ventiation osses How much insuation does a buiding need? 37 3 Improving heating system efficiency Gas boiers Oi-fired boiers Eectric heating Combined Heat and Power Generation Heating systems and CO 2 emissions 61 4 Saving eectricity Eectrica appiances Energy-efficient ighting 70 5 Home energy assessment 75 Concusion 78 End-of-course quiz 79 Keep on earning 82 Gossary 83 References 85 Acknowedgements 87 3 of 87 Monday 18 June 2018

4 Introduction Introduction This free course, Energy in buidings, ooks at the importance of energy in buidings in the UK, particuary housing. Topics covered incude reducing the heating demand of houses, improving their heating systems and reducing the eectricity used by appiances and ighting. These are a methods of reducing their overa CO 2 emissions. This OpenLearn course is an adapted extract from the Open University courses T213 Energy and sustainabiity and T313 Renewabe energy. It shoud take about 10 hours study time. You wi need a cacuator handy (perhaps within your computer) to answer some of the course questions. 4 of 87 Monday 18 June 2018

5 Learning Outcomes After studying this course, you shoud be abe to: understand the main ways in which a house oses heat energy carry out basic U-vaue cacuations for windows and insuation materias understand the factors infuencing heating system efficiency carry out basic cacuations concerning the efficiencies and CO 2 emissions of different heating systems carry out basic cacuations concerning ighting.

6 1 The importance of energy use in buidings 1 The importance of energy use in buidings Appreciating the importance of energy use in buidings requires a ook at UK nationa energy statistics. These use two categories of energy: Primary energy this is essentiay energy in its raw form. Exampes incude crude oi before it is refined and the fues used to generate eectricity: coa, natura gas and nucear heat. Deivered (or fina) energy this is the energy that the consumer actuay receives (and pays for): refined petro and diese, mains eectricity, piped natura gas. The statistics aso spit energy use into different sectors: the domestic sector peope s homes; the services sector shops, offices, schoos, etc.; transport and, finay, industry. Box 1 Energy units Perhaps the most famiiar energy unit is the kiowatt-hour (kwh). Househod gas and eectricity bis are normay expressed in these. In eectrica terms this is the amount of energy used by a 1 kiowatt (kw) appiance, such as a sma eectric fire, in one hour. The prefix kio means 1000 and is shortened to k. 1 kw = 1000 watts. Most of the energy cacuations in this course are in watts, kiowatts and kiowatt-hours. Energy statistics may use a scientific unit of the joue (J). This is the (tiny) amount of energy used by a 1 watt device in 1 second. 1 kiowatt-hour = 3.6 miion joues. Larger energy units use other prefixes. Those used in this course are: mega shortened to M. 1 MJ = 1 miion joues and 1 kwh = 3.6 MJ giga shortened to G. 1 GJ = 1000 MJ tera shortened to T. 1 TJ = 1000 GJ peta shortened to P. 1 PJ = 1000 TJ Figure 1 shows the breakdown of UK primary and deivered energy for 2009 in different ways. The top bar shows the actuay primary fues used. The three bars beow show the deivered energy use expressed in three ways: by fue, by energy sector and by end use. 6 of 87 Monday 18 June 2018

7 1 The importance of energy use in buidings Figure 1 UK primary and deivered energy use for 2009 (Sources DECC, 2010a, DECC, 2010b) Note that athough UK primary energy consumption in 2009 was neary 9000 PJ, the deivered energy use was ony 6000 PJ. As shown in the second bar of Figure 1, there were osses of amost 3000 PJ in conversion and deivery. Most of this is waste heat rejected from power stations in cooing towers or into the sea. Typicay generating 1 kwh of eectricity in a conventiona therma power station requires between 2 and 3 kwh of primary energy input. It can be seen in the third bar that the deivered (or fina) energy use in the domestic and services sector is about 2600 PJ. This is amost entirey consumed within buidings. Adding the fraction of industria energy use that is aso used in buidings means that energy in buidings makes up amost haf of the UK s deivered energy use. The domestic and services sectors aso account for two thirds of the UK s eectricity use. When this and its attendant generation energy osses are taken into account, buidings are responsibe for neary haf of UK primary energy use and about 35% of nationa greenhouse gas emissions (CCC, 2012). Energy use is, of course, ony the means to provide various energy services. The rea task is to provide these at a ower energy and environmenta cost. The energy services in buidings incude: the provision of comfortabe homes and working environments hot water for washing cooking food safe chied food storage adequate ighting for homes and offices the abiity to use eectronic equipment for communication and entertainment (and producing and studying materia such as this course). 7 of 87 Monday 18 June 2018

8 1 The importance of energy use in buidings The fourth bar of Figure 1 shows that over 2000 PJ of deivered energy were used for space heating (i.e. warming the interna spaces of buidings) and water heating. These appications ony require ow temperature heat (ess than 60 C). Space heating is a prime target for energy efficiency programmes. There is a considerabe potentia for using some of the ow temperature waste heat from power stations for heating buidings using combined heat and power generation (CHP), as is widey done in countries such as Denmark. Of the energy use in buidings about two-thirds was used in the domestic sector and about a further 20% in the services sector. Within the domestic sector there is a famiiar range of energy uses (see Figure 2). Figure 2 UK domestic sector energy consumption by end use, 2009 tota 1800 PJ (Source: DECC, 2014a) In 2009 about 60% of the deivered energy was used for space heating and 20% for hot water. Whie these particuar figures have ony changed sowy over the past 40 years, the amount of eectricity for other appiances (e.g. radios, TVs, computers and other eectronic devices) has increased by a factor of more than eight. Given that the domestic sector accounts for 35% of UK eectricity demand this is a matter of concern and is discussed in Section 4.1. The services sector contains a wide range of different buidings such as offices, schoos, shops and hospitas. Athough just over haf of the deivered energy was used for space heating, this sector uses arge amounts of eectricity, about 30% of UK demand. A arge amount of this is for ighting (see Figure 3) in retai premises over 30% of the energy use in 2009 was for ighting. 8 of 87 Monday 18 June 2018

9 2 Reducing space heating demand Figure 3 UK services sector energy consumption by end use, 2009 tota 700 PJ (Source DECC, 2014a) Activity 1 In 2009 which sector used more eectricity for ighting: the domestic sector or service sector? Answer In the domestic sector ighting made up 3% of 1800 PJ tota energy use, i.e. 54 PJ. In the services sector it made up 21% of 700 PJ, i.e. 147 PJ or neary three times as much as in the domestic sector. 2 Reducing space heating demand Heat energy is used in houses and offices to provide the energy service of comfort. This means providing space heating in winter with some kind of heating system and possiby cooing in summer. The heating system wi usuay aso provide hot water, discussed in Section 3. 9 of 87 Monday 18 June 2018

10 2 Reducing space heating demand A itte history Life indoors in the UK was very cod in winter in the past. Houses in the nineteenth and eary twentieth centuries had a centra fire (usuay fueed by coa), often aso used for cooking and water heating, and were it with either oi or gas amps. They had to be we ventiated both to suppy the combustion air for the fire and to get rid of the fumes from the amps. The basic principe of keeping warm was to wear ots of cothes, sit as cose as possibe to the fire during the day and retreat under a thick pie of bankets in bed at night. In the nineteenth century, offices did introduce the reative uxury of centra heating, fed from arge coa-fired boiers. Most pre-1918 buidings in the UK have soid brick was, usuay two bricks thick, though they may be three or more bricks thick in taer buidings. Buidings of this age sti make up about 20% of the housing stock. UK buiding standards improved sowy throughout the twentieth century. In the 1920s cavity was (an air gap between two separate skins of brick) were introduced, argey as a method of preventing damp penetration. The coa fire remained the norma mode of heating in UK homes we into the 1960s. These homes weren t very warm a survey in showed average whoe-house temperatures ranging from 12.4 C to 14.2 C (Danter, 1951). With the introduction of North Sea gas in the 1970s there aso came gas-fired centra heating. The proportion of the housing stock with centra heating rose from 31% in 1970 to over 96% in 2009 (DECC, 2014a). It became expected that houses shoud be fuy heated to an acceptabe comfort temperature. Concerns about death rates, particuary of the very young and the edery, have given rise to the concept of fue poverty. A househod is currenty (2015) considered to be fue poor if in order to achieve a satisfactory heating regime (21 C for the main iving area, and 18 C for other occupied rooms): they have required fue costs that are above average eve and were they to spend that amount, they woud be eft with a residua income beow the officia poverty ine (DECC, 2014b). It is estimated that in 2012 approximatey 10% of a Engish househods were in fue poverty. Not surprisingy, fue poverty is most common in the homes with the worst energy efficiency standards. Loft insuation was ony introduced into the buiding reguations for new UK houses in 1974 and then ony to a depth of 25 mm. Since then standards for new buidings have steadiy improved and government campaigns have encouraged househoders to insta insuation. However, there is sti a considerabe proportion of the existing housing stock that is reativey poory insuated. The picture for the services sector is not much better. The 1960s saw a fashion for curtain wa office construction where a stee or concrete frame was used to provide the structure and the was were argey made of thin concrete panes and arge sheets of singegazing. These offices were hard to heat in winter and often overheated in summer. Fortunatey office buidings tend to be reguary refurbished as new occupants come and go, but even so, making major improvements to the therma performance can be difficut. The insuation standards of new and refurbished buidings are covered by buiding reguations. The responsibiity for these in the UK is devoved to the regions, i.e. Scotand has sighty different reguations from those of Engand and Waes and Northern Ireand. 10 of 87 Monday 18 June 2018

11 2 Reducing space heating demand The reguations for the Repubic of Ireand tend to foow a simiar pattern to those in the UK. Where references are made to the buiding reguations in this course they refer to those for Engand and Waes. Since 2006, the reguations covering the therma performance of buidings have been mainy worded in terms of target CO 2 emissions rather than specific insuation eves. There may, however, be specified minimum standards for windows and suggested insuation eves for other parts of the buiding fabric and guidance for heating and ventiating systems. 2.1 Heating a house Athough it is common to think of a house being heated soey by some form of heating system, in practice it is ikey to be warmed by energy from three sources: the heating system free heat gains from occupants, ights, appiances and from hot water use passive soar gains from soar energy penetrating the windows. In a reay ow-energy house design, free heat and soar gains may provide more usefu heating than the heating system itsef. Obviousy in order to achieve a ow overa space heating demand it is necessary to reduce the heat osses. Figure 4 shows a sma house and iustrates the ways in which heat fows into and out of a house. The osses are particuary important. There are: fabric heat osses those through the buiding fabric itsef, i.e. the was, roof, foor and windows ventiation osses due to air moving through the buiding fue heat osses since the heating system is not 100% efficient. These basic osses aso appy to arger buidings. 11 of 87 Monday 18 June 2018

12 2 Reducing space heating demand Figure 4 Heat fows through a house There are three ways of reducing the space heating energy use discussed in this course: 1 cutting the fabric heat osses by the use of insuation (Section 2.2) 2 cutting the ventiation oss by making the buiding more airtight and possiby using mechanica ventiation with heat recovery (Section 2.3) 3 instaing a more efficient heating system (Section 3). 2.2 Cutting buiding fabric heat osses Heat oss mechanisms Heat energy wi fow through any object when the temperature on the two sides is different. The rate of this energy fow (i.e. the number of watts) depends on: the temperature difference between the two sides the tota area avaiabe for the fow the insuating quaities of the materia. 12 of 87 Monday 18 June 2018

13 2 Reducing space heating demand It is obvious that more heat is ost through a arge area of wa or window than a sma one, and on a cod day than a warm one. In order to understand how this heat oss occurs, and how it can be minimised, it is necessary to ook at the three mechanisms invoved in the transmission of heat: conduction, convection and radiation. Whie conduction is the main mechanism in insuated was and roofs, a three pay an important roe in windows. This section discusses ways of reducing heat oss through windows and was, and introduces some of the parameters used to describe how we (or bady) objects conduct heat Insuated windows Figure 5 shows a cutaway diagram of a doube-gazed window. This may take the form of a singe-gazed window pus a separate extra pane of secondary gazing or, more commony, a seaed doube-gazing unit, usuay with a space between the panes of about 16 mm. However, the basic principes are the same. Figure 5 How heat escapes from a seaed doube-gazed window in winter. 13 of 87 Monday 18 June 2018

14 2 Reducing space heating demand Conduction In any materia, heat energy wi fow by this mechanism from hotter to coder regions. The rate of fow wi depend on the area and temperature difference, and on the therma conductivity of the materia, a topic that wi be discussed in more detai ater. Generay, metas have very high therma conductivities and can transmit arge amounts of heat for sma temperature differences. In a window, heat conduction takes pace through the frame, which may be made of wood, pastic or meta. In high-quaity windows, frames often incude an insuated therma break to minimise heat oss. Further heat conduction takes pace through sti air within the window but this is imited because sti air is a good therma insuator. Insuators require a arge temperature differentia to conduct a reativey sma amount of heat. Most practica forms of insuation rey on very sma pockets of trapped air, for exampe between the panes of gazing, as bubbes in a pastic medium, or between the fibres of minera woo. Convection Whie sti air is a good insuator, moving air can carry heat from a warm surface to a cooer one. A warmed fuid, such as air, wi expand as it warms, becoming ess dense and rising as a resut, creating a fuid fow known as convection. This is one of the principa modes of heat transfer through windows. It occurs between the air and the gass on the inside and outside surfaces, and, in doube-gazing, in the gas trapped between the panes. The convection effects can be reduced by fiing doube-gazing with heavier, ess mobie gas moecues. Argon is most commony used; the heavier gases krypton and xenon give better performance but are more expensive. These gases are a by-products of the industria iquefaction of air to produce oxygen. Convection can aso be reduced by imiting the space avaiabe for gas movement. This is the principe used in most insuation materias. Various forms of transparent insuation have been deveoped that use a transparent pastic medium containing bubbes of trapped insuating gas. These materias coud eventuay revoutionise the construction of windows and was, but at present the materias are expensive, not robust and need protection from the rigours of weather and utravioet ight. Aternativey the gazing can be evacuated. Convection currents cannot fow in a vacuum. However, a very high vacuum is required and it needs to ast for the whoe ife of the window, which may be 50 years or more. Aso the window wi need interna structura spacers to stop it coapsing inwards under the air pressure on the outside. These spacers conduct heat across the gap, sighty reducing the overa performance. The vacuum gap does not need to be very thick and is ony 0.2 mm wide in commercia vacuum doubegazing units. Note that these convection currents are ony concerned with what happens inside a doube-gazing unit. There are aso ikey to be movements of air through the seas around the edges of windows, but that is a matter of ventiation and infitration (see Section 2.3). A simper way to reduce the convection effects is to insert extra panes of gass or transparent pastic fim between the panes of doube-gazing turning it into tripe- or quadrupe-gazing. 14 of 87 Monday 18 June 2018

15 2 Reducing space heating demand Radiation Heat energy can be radiated, in the same manner as it is radiated from the sun to the earth. The quantity of radiation is highy dependent on the temperature difference between the radiating body and its surroundings. The roof of a buiding, for exampe, wi radiate heat away at night to the cod atmosphere. In a doube-gazed window heat wi be directy radiated from the inner pane across the gap to the outer one. The amount of radiation aso depends on the surface s emissivity. Most materias used in buidings have high emissivities of approximatey 0.9, that is, they radiate 90% of the theoretica maximum for a given temperature. Other surfaces can be produced that have ow emissivities. This means that athough they may be hot, they wi radiate itte heat outwards. Low-e coatings are now normay used inside doube-gazing to cut radiated heat osses from the inner pane to the outer one across the air gap. There are two basic types. Hard coat uses a thin ayer of tin oxide, giving an emissivity of Soft coat uses very thin ayers of opticay transparent siver sandwiched between ayers of meta oxide and gives an emissivity of 0.05 or better U-Vaues Conduction, convection and radiation a contribute to the compex process of heat oss through a wa, window, roof, etc. In practice, the actua therma performance of any particuar buiding eement is usuay specified by a U-vaue, defined so that: heat fow through one square metre = U-vaue temperature difference. The U-vaue is thus the heat fow per square metre divided by the temperature difference. The heat fow has units of watts (W) and the temperature difference has units of kevins (K). The U-vaue thus has the units of watts per square metre kevin - W / m 2 K. In this course we have used the convention that units in a divisor are given a negative power, so this becomes W m -2 K -1. The ower the U-vaue, the better the insuation performance. Box 2 Degrees Cesius or kevins? Temperatures can be measured in degrees Cesius ( C) or kevins (K). Kevins are more ikey to be used in officia documents and scientific papers. The size of a degree is the same on both scaes, so temperature differences are identica in C and K. The Kevin scae is used to measure the absoute temperature, that is the temperature above absoute zero. Absoute zero has been measured as about 273 C, so 0 C is 273 K, and the temperature in kevins can be obtained simpy by adding 273 to the Cesius temperature. Note that a temperature expressed in kevins doesn't need a degree sign. Current UK buiding reguations use the kevin in specifying U-vaues and other therma quantities, and it is used in this section. In practice, U-vaues are widey quoted as W m -2 C 1 in architectura iterature and the trade press, simpy because the degree Cesius is more famiiar. Technica iterature can use a range of different presentations of the units of U- vaues: W m -2 K 1, W/m 2 K, W m -2 C 1 and W/m 2 C. These are a identica. 15 of 87 Monday 18 June 2018

16 2 Reducing space heating demand Window U-vaues Tabe 2 gives some indicative U-vaues for different gazing options. Note that these are ony indicative and better U-vaues for doube and tripe gazing are commerciay avaiabe. By way of comparison, a soid brick wa has a U-vaue of W m 2 K -1, and 10 cm of opaque fibregass insuation one of about 0.4 W m 2 K 1. Tabe 2 Indicative U-vaues for windows with wood or PVC-U frames Gazing type Singe-gazing 4.8 Doube-gazing (norma gass, air-fied) 2.7 Doube-gazing (hard coat ow-e, emissivity = 0.15, air-fied) 2.0 Doube-gazing (hard coat ow-e, emissivity = 0.2, argon-fied) 2.0 Doube-gazing (soft coat ow-e, emissivity = 0.05, argon-fied) 1.7 Tripe gazing (soft coat ow-e, emissivity = 0.05, argon-fied) 1.3 U-vaue/ W m 2 K 1 (Source: BRE, 2012) UK buidings have traditionay used singe-gazing. It has been estimated that in 1974 ess than 10% of British housing had any form of doube-gazing (BRE, 2008). It was ony made mandatory for new houses in Engand and Waes in 2002 and this was extended to repacement windows for a existing houses in This requirement aso insisted that windows with wood or PVC-U frames shoud have U-vaue better than 2.0 W m 2 K 1 or better than 2.2 W m 2 K 1 for auminium or stee frames. These vaues can ony be achieved by the use of ow-e coatings. They represent a considerabe improvement on the U-vaues of 4.8 W m 2 K 1 and 5.7 W m 2 K 1 respectivey for singe-gazing. There has been continuing deveopment in improving window U-vaues through the use of soft-coat ow-e gass, insuated spacers at the edge of the panes and better insuated frames. Currenty commerciay avaiabe doube gazed windows may have U-vaues of 1.5 W m -2 K -1 or better. It is aso important that windows have a good transparency to et in ight and soar gains and are airtight when cosed. Currenty windows in the UK are sod with a Window Energy Rating on a scae A G which takes into account the U-vaue, airtightness and transparency. A reasonaby airtight doube gazed window with a U-vaue of 1.6 W m -2 K -1 and a soar transparency of 50% woud get a C rating. Under the 2013 UK buiding reguations this rating is now the minimum acceptabe for new and repacement windows and even this standard may be tightened in the next few years. Essentiay to get a good rating a doube-gazed window must have a good ow-e coating and use insuating spacers around the edges of the gass and have a good transparency. Argon-fied doube-gazed units require a reativey thick gap of mm between the panes. This makes their use difficut in retrofitting oder timber sash windows, for exampe in historic isted buidings. For these, simmer doube-gazing units can be used to meet the required UK window reguation U-vaues, using a krypton/xenon fiing in an 8 mm gap or aternativey vacuum gazing with ony a 0.2 mm gap. 16 of 87 Monday 18 June 2018

17 2 Reducing space heating demand By 2012 over 79% of the UK housing stock had fu whoe-house doube-gazing (DECC, 2014a). Activity 2 Tabe 2 gives typica U-vaues of various types of window gazing system. What is the rate of heat oss in watts through a arge singe-gazed window with an area of 2 m 2, on a day when the outdoor and indoor temperatures are 5 C and 20 C respectivey? Answer Tabe 2 shows that the U-vaue for this window is 4.8 W m 2 K 1 The heat oss heat fow through one square metre = U-vaue temperature difference. The tota oss rate is thus (20 5) = 144 watts Activity 3 If the temperature difference remained the same throughout 24 hours, what woud be the tota heat oss in kiowatt-hours over the day? What woud it have been using the best of the gazing types shown in Tabe 2? Answer For a singe gazed window the heat oss over 24 hours wi be 144 watts 24 hours = 3456 watt-hours or just over 3.4 kiowatt-hours. For a tripe-gazed window with a U-vaue of 1.3 W m 2 K 1 the heat oss rate woud be (20 5) = 39 W and the tota heat oss over 24 hours woud be = 936 Wh, or under 1 kwh Insuation materias and their properties The heat fow through was, roofs and foors can be reduced by incorporating one or more of a range of insuating materias. In these conduction is the main mechanism of heat fow. In order to understand the insuation thickness required to achieve a given therma performance for a buiding, it is necessary to ook at heat fow in more numerica detai. As described above, heat energy wi fow through any substance where the temperature on the two sides is different, and the rate of this energy fow depends on: the temperature difference, T in T out, between the two sides (often written as ΔT) the tota area avaiabe for the fow the insuating quaities of the materia its thickness and its therma conductivity. The therma conductivity is denoted by the symbo λ (Greek ambda), athough wi aso find the symbo k used. In this course we have used λ. It is usuay expressed in terms of the rate of heat fow in watts that woud fow across a one metre cube of the materia with a temperature difference of one degree (kevin or Cesius) across it (see Figure 6): λ = heat fow per square metre of area divided by temperature difference per metre of thickness. 17 of 87 Monday 18 June 2018

18 2 Reducing space heating demand The heat fow per square metre has units of W / m 2. Temperature difference per metre of thickness has units of K / m. Heat fow per square metre divided by temperature difference per metre has units of: (W / m 2 ) / (K / m) or (W / m 2 ) (m / K) or W m / m 2 K or W / m K Therma conductivity, λ, thus has units of watts per metre kevin, W / m K or W m -1 K -1. The ower the conductivity the better the eve of insuation. Figure 6 Heat fow and therma conductivity Tabe 3 gives therma conductivities for some common buiding materias, together with their densities; generay the higher the density, the higher the therma conductivity. Tabe 3 Therma conductivities of common buiding materias Materia Density/ kg m 3 Auminium (window frames) Stee wa ties Reinforced concrete (2% stee) Window gass Brickwork (outer eaf) Paster (dense) Therma conductivity/ W m 1 K 1 18 of 87 Monday 18 June 2018

19 2 Reducing space heating demand Lightweight aggregate concrete Aerated concrete Aerated concrete (ower density) Timber (softwood) (Source: Buiding Reguations Part L2, ODPM, 2001 and manufacturer s iterature) Metas have very high therma conductivities and can transmit arge amounts of heat for sma temperature differences. Meta window frames, intes over windows and fixings used for insuation can transmit considerabe amounts of heat even though they ony have a sma tota area. These are often referred to as therma bridges or cod bridges. Window gass has a high conductivity, so using thicker gass wi have amost no effect on their overa U-vaue. Structura buiding materias such as brick and concrete have ower conductivities but the potentia heat osses are sti considerabe due to the arge surface areas of was and roofs. Insuation materias make use of the fact that sti air, or other gases with a reasonaby arge moecuar weight, are good therma insuators. Most practica forms of insuation rey on using very sma pockets of these gases. There are four kinds of commercia insuation materia: various grades of aerated concrete containing sma bubbes of air foamed gass containing sma bubbes of air various forms of woo made up of fibres with air hed trapped between them pastic foams containing sma bubbes of gas. Vacuum insuated panes (VIPs) with consideraby better performance are becoming increasingy avaiabe, but are very expensive. Their appication for refrigerators is described ater in Section Aerated concrete, whose therma properties are isted in Tabe 3, is not as physicay strong as its dense counterpart. There is a trade-off of compressive strength against therma insuation performance. In practica construction this materia can be used to form the inner eaf of a cavity wa suppementing the main insuation, which is ikey to be some form of minera woo or pastic foam within the cavity. Figure 7 shows sampes of these commony used insuation materias. Figure 7 Sampe insuation materias(eft to right: fibregass, ceuose fibre, dense rock woo, expanded poystyrene, extruded poystyrene and poyisocyanurate foam) 19 of 87 Monday 18 June 2018

20 2 Reducing space heating demand Woo and pastic foam insuation materias are very ight; their densities are typicay ony kg m 3. Tabe 4 beow gives some sampe conductivity vaues for them, taken from manufacturers iterature. Tabe 4 Sampe therma conductivity vaues for insuation materias Insuation materia Therma conductivity/ W m 1 K 1 Foamed gass Sheep s woo, ceuose fibre, minera woo, gass fibre woo Expanded and extruded poystyrene foam Poyurethane foam Poyisocyanurate (PIR) foam Phenoic foam Sheep s woo has, of course, been used in cothes as an insuating materia for humans for thousands of years. Ony now is it considered cheap enough to be used for oft insuation for buidings. Ceuose fibre insuation is made from shredded recyced newspaper treated with a minera fire retardant. It can be bown into wa cavities or oft spaces with a specia machine. The most commony avaiabe forms of insuation materia are minera woo (often caed rockwoo or earth woo ) and gass fibre woo. Modern manufactured rock woo is the resut of discoveries made in Hawaii of the effects of superheated steam on moten rock during vocanic eruptions. In the manufacturing process a suitabe rock is meted at over 1500 C. It is then spun out through sma hoes on the perimeter of a centrifuge to produce ong, thin fibres. Gass fibre manufacture is simiar. The fibres are then coated in pastic resin and formed into insuation batts (square fat pieces of insuation, as opposed to ros). Pastic foam insuation materias are made by bowing a gas into moten pastic. Expanded poystyrene is a very famiiar exampe widey used for packaging; other pastics used incude urea formadehyde, poyurethane, poyisocyanurate and phenoic resin. Their foams have different properties. Poystyrene foam, for exampe, can be made extremey strong and rigid. It is water-resistant and sufficienty strong to carry the weight of vehices and so it can be used under factory foors. It can aso be produced in bocks that can be quicky cipped together to buid insuating shuttering into which concrete can be poured (see Figure 8). 20 of 87 Monday 18 June 2018

21 2 Reducing space heating demand Figure 8 A poystyrene house.despite its soid appearance, this house buit in Miton Keynes in 1986 used a basic framework of insuating poystyrene bockwork into which concrete was poured. A brick skin was then added on the outside and the inside was pastered. The best therma insuation performance is achieved by poyurethane, poyisocyanurate and phenoic foams, which can have two-thirds of the conductivity of woo materias. Any practica insuation choice must take into account factors such as cost, ease of handing, compression resistance and water resistance. There is aso considerabe debate on the reative environmenta friendiness of different materias. The naturay occurring rock fibre, asbestos, is now argey banned because of associated heath probems (the fibres are fine and britte and tend to break down into a fine dust that can be breathed in). Modern manufactured minera woo and fibregass woo are safer than asbestos, though it is advisabe to wear a face mask when using them in confined spaces. They need high temperatures and a ot of energy in their manufacture. Pastic foams use oi-based chemicas and, being pastic, are infammabe and can produce toxic smoke when burning. Unti the 1990s chorofuorocarbons (CFCs) were used for bowing these foams. As a resut of concerns about environmenta effects these have been repaced with other gases with a ower ozone depetion potentia, such as hydrochorofuorocarbons (HCFCs), carbon dioxide, and more recenty pentane. The use of pentane, however, increases the potentia fammabiity and suitabe precautions have to be taken with its use. The environmenta impact of insuation materias can be reduced through recycing. Recyced materias can be used in gass fibre and some pastic foams, and recyced newspaper is the main ingredient of ceuose fibre insuation. Typicay, in a roof where 21 of 87 Monday 18 June 2018

22 2 Reducing space heating demand there was no insuation to start with, the energy used in insuation manufacture wi be saved within a year Conductivity and U-vaues the basics U-vaues are used, as with windows, to describe the overa therma performance of a buiding eement such as a wa or roof. However these are ikey to be made up of mutipe ayers of different materias (paster, brick, insuation, etc.) each of which wi have different therma properties. Therma conductivities are usefu for comparing the therma properties of different materias (Tabes 3 and 4), but for practica purposes we need to know the precise therma resistance of a particuar thickness of insuation materia. This therma resistance is anaogous to eectrica resistance using heat fow as current and temperature difference as votage. The higher the resistance the greater the reduction in heat fow for a given temperature difference. Consider a sab of a particuar materia t metres thick, with temperatures T in and T out on the two sides and a heat fow Q watts through each square metre (see Figure 9). Figure 9 Heat fow through a thin sab of materia The temperature difference per metre of thickness for this sab is (T in T out )/t, so it foows from the definition of therma conductivity (λ) that the heat fow per square metre is: Q = (T in T out ) λ/t watts The therma resistance (R) of the sab is defined as the temperature difference divided by the heat fow per square metre: R = (T in T out )/Q m 2 K W 1 = t/λ m 2 K W 1 22 of 87 Monday 18 June 2018

23 2 Reducing space heating demand The thicker the insuation, the greater the vaue of R. Thus for a ayer of insuation 100 mm (0.100 m) thick, with a therma conductivity of W m 1 K 1 the therma resistance of the 100 mm ayer wi be: R = t / λ = 0.1 / 0.04 = 2.5 m 2 K W -1 Finay, since the U-vaue of a sab is the heat fow divided by the temperature difference, R = 1/U (and U = 1/R). Thus the U-vaue of this sab of insuation on its own is 1/2.5 = 0.4 W m -2 K 1. An aternative way of ooking at this probem is to say that a 1 metre-thick sab of this insuation wi have a U-vaue on its own of 0.04 W m -2 K 1. Reducing the thickness by a factor of 10 to 100 mm wi increase the heat fow and give a U-vaue of 0.4 W m -2 K 1. Remember: U-vaues and therma resistances are usuay properties of whoe buiding eements, whie λ-vaues are properties of particuar materias A good insuation materia wi have a ow λ vaue A we insuated piece of buiding fabric wi have a ow U-vaue and a high therma resistance. Activity 4 Just considering the materias aone, which has a higher therma resistance: a piece of soid brick 250 mm thick with a conductivity of 0.77 W m 1 K 1 or 25 mm of insuation with a conductivity of W m 1 K 1? Answer A 250 mm thick piece of soid brick has a therma resistance of t/λ = 0.25 / 0.77 = m 2 K W -1. A 25 mm thickness of insuation has a therma resistance of t/λ = / 0.04 = m 2 K W -1. The insuation wins! Using insuation Loft insuation The most famiiar use of woo-type insuation is in oft insuation. Since its first introduction into the UK Buiding Reguations for housing, the recommended thickness has increased from 25 mm in 1974 to 270 mm at present (2015) for both new buid and for existing houses. Figure 10 shows the U-vaues resuting from different thicknesses. 23 of 87 Monday 18 June 2018

24 2 Reducing space heating demand Figure 10 Loft insuation thicknesses and U-vaues (Source: EST, 2002) Loft insuation is usuay sod in ros. A base ayer of 100 thickness is roed out between the wooden ceiing joists (Figure 11) and then a further ayer 170 mm thick is added over the top, bocking the upward heat oss through the ceiing joists. This is ikey to achieve a U-vaue of about 0.15 W m -2 K 1, a tweve-fod improvement compared to an uninsuated oft. Figure 11 Roing out a oft insuation base ayer between the ceiing joists It is important to make sure that any insuation does not bock gaps at the eaves, to aow air movement within the oft space (see Section 2.3). Insuation shoud be omitted under 24 of 87 Monday 18 June 2018

25 2 Reducing space heating demand any water tanks in the oft space to aow heat from the house to reach them. They, and any water pipes in the oft space, shoud be propery insuated to make sure they don t freeze in winter. Cavity wa insuation in existing buidings The was of buidings can be insuated in many ways. In existing buidings with cavity was insuation can be inserted into the cavity as ong as the buiding isn t exposed to high winds with driving rain, since the main function of the cavity is to stop damp penetration through the wa. A brick was may at first sight ook the same, but on coser inspection the outer skin of a cavity wa, as shown in Figure 12, wi be seen to be made up of bricks a aid side-on (stretchers). A soid brick wa wi aso incude bricks aid end-on at right anges (headers). Figure 12 Soid brick and cavity was Hoes can be dried in the outer brick skin and foam insuation can be injected into the gap, or rock woo or poystyrene beads can be bown into it. Typicay this wi improve the U-vaue from about 1.5 W m 2 K 1 to about 0.6 W m 2 K 1. Interna insuation of existing soid was A typica soid brick wa two bricks (about 230 mm or 9 inches) thick has a reativey poor U-vaue of W m -2 K -1. Such soid was can be dry ined. This invoves putting a ayer of insuation on the inside faced with pasterboard. This is fairy simpe to do (as ong as the occupants don t mind the disturbance to the interior of the house). Sheets of foambacked pasterboard can be gued to the wa (Figure 13(a)), or aternativey insuated battens can be screwed to the wa with a ayer of insuation between them and covered with a surface ayer of pasterboard screwed to the battens (Figure 13(b)). 25 of 87 Monday 18 June 2018

26 2 Reducing space heating demand Figure 13 (a) Internay insuating a wa with foam-backed pasterboard gued to the wa (b) Interna insuation using minera woo fitted between insuating battens and covered with pasterboard The U-vaues that can be achieved are mainy dependent on how much reduction in the interior room sizes can be toerated. For exampe the 2013 UK buiding reguations suggest a target vaue of 0.3 W m -2 K -1. This woud require the use of 75 mm of poyisocyanurate foam. Externa insuation of soid was Aternativey soid was can be externay insuated, usuay with a thick ayer of poystyrene foam which is then either covered with a ayer of cement render or a specia cadding ayer (see Figures 14(a) and 14(b)). Figure 14 (a) detais of externa insuation (b) externa insuation advertised at a trade fair. The outer surface can be rendered as in (a) or even cad in a thin ayer of imitation bricks Externa insuation is commony used in the refurbishment of tower bocks of fats. Athough it is reativey expensive, it is possibe to achieve good U-vaues of better than 0.3 W m 2 K 1 with 100 mm of foam insuation. 26 of 87 Monday 18 June 2018

27 2 Reducing space heating demand New buidings In new construction, brick was can be buit with cavities and insuation batts buit in. If the buiding is in an area of driving rain then an air gap may aso have to be incuded. Insuating aerated concrete bockwork can aso be used to buid the inner eaf (see Figure 15). Figure 15 Insuation inserted into the cavity of a new wa (Source: EST, 2005) A wa U-vaue of 0.18 W m -2 K -1 is currenty (2015) suggested by the UK Buiding Reguations. This can be bettered by making the cavity as wide as necessary (200 mm to 300 mm) to incorporate more insuation and, if necessary, to retain an air gap to prevent damp penetrating the wa. Timber frame construction can aso use considerabe thicknesses of insuation; 200 mm or more of wa insuation is commony used in Scandinavia and Germany. Foors The foors of buidings can aso be we insuated. Modern UK construction often uses thick sheets of poystyrene or poyurethane foam. In oder buidings with suspended timber foors, sheets of insuation materia can be inserted under the foorboards between the joists. The 2013 UK Buiding Reguations suggest a foor U-vaue of 0.22 W m -2 K -1 for new buidings and 0.25 W m -2 K -1 for refurbishment projects. This is ikey to require the use of poystyrene insuation more than 100 mm thick. 27 of 87 Monday 18 June 2018

28 2 Reducing space heating demand Cacuating U-vaues of mutipe ayers of materias Any thorough anaysis of the thickness of insuation required to meet a specified U-vaue wi require some detaied cacuations. The earier discussion of the basics of U-vaues ony considered the therma resistance of a singe sab of a buiding materia. In any practica buiding eement there wi be extra therma resistances, particuary those of the thin ayers of air adhering to the outermost and innermost ayers of the materia, and the air in any substantia gap between the ayers. Tabe 5 gives standard therma vaues used for these. Note that the outside surface resistance is much ower than the vaue used for the inside surface. This is because the air is ess ikey to be sti on the outside and wi thus provide a reativey poorer insuation performance. Tabe 5 Therma resistances for surfaces and air gaps Layer Resistance / Inside surface (R si ) 0.13 Air gap 0.18 Outside surface (R so ) 0.04 m 2 K W 1 The therma resistances of the components of a buiding eement can be added in series as in Figure 16, to give a tota therma resistance (rather ike adding eectrica resistances in series). The tota therma resistance of a practica buiding eement wi thus consist of the sum of those of a its ayers pus the inside and outside surface resistances. Figure 16 Summing therma resistances Taking, for exampe, a wa construction with four ayers, the tota therma resistance, R T, wi be: R T = R so + R 1 + R 2 + R 3 + R 4 + R si m 2 K W 1 The U-vaue of this wa is its inverse = 1/R T W m 2 K 1 For exampe the wa shown in Figure 15 consists of the foowing ayers: 115 mm common brick, a 115 mm cavity fied with minera woo (conductivity W m 1 K 1 ), 115 mm of aerated concrete bockwork (density 460 kg m 3 ) and a 13 mm ayer of paster on the inside. Using the conductivity vaues in Tabe 4 we can cacuate its U-vaue by summing the various therma resistances as shown in Tabe of 87 Monday 18 June 2018

29 2 Reducing space heating demand Tabe 6 Cacuation of therma resistances Layer Thickness /m Conductivity/ W m 1 K 1 Outside therma resistance 0.04 Resistance/ m 2 K W 1 Brick 115 mm /0.77 = 0.15 Minera woo 115 mm /0.035 = 3.29 Aerated concrete bock 115 mm /0.11 = 1.05 Dense paster 13 mm /0.57 = 0.02 Inside therma resistance 0.13 Tota therma resistance 4.67 The overa U-vaue is then: U = 1/R = 1/4.67 = 0.21 W m 2 K 1 In practice, buiding eements do not simpy consist of fat ayers. The wa construction above is ikey to use thin meta wa ties securing the outer brickwork to the inner eaf of bockwork. This wi create a therma bridge bypassing the insuation and reducing its performance. Depending on the detais a more reaistic U-vaue for this sort of construction might be about 0.25 W m 2 K 1. Simiary, in Figure 10, the base ayer of oft insuation ony bocks the fow of heat over a certain area. There is a parae heat-fow path through the wood of the joists supporting the ceiing. This fow is bocked by the top insuation ayer. A certain aowance aways has to be made for therma bridges, but the mathematics is not simpe. Activity 5 Ignoring the therma resistance of the panes of gass, use the data in Tabe 5 to estimate the U-vaue of a doube-gazed window. Answer The tota therma resistance of the window is the sum of the resistances of the inside ayer, the air gap between the panes and the outside surface ayer. Tota resistance = = 0.35 m 2 K W 1 U = 1/R = 1/0.35 = 2.86 W m 2 K 1 This answer is very cose to the vaue of 2.7 W m 2 K 1 given in Tabe 2 for air-fied doube-gazing, though this aso takes the heat oss through the window frame into account. Activity 6 What is the therma resistance of a 4 mm thick sheet of window gass? (You wi need to ook back to Tabe 3 in Section ) Is doubing the thickness of the gass ikey to improve significanty the overa U-vaue of a doube-gazed window? 29 of 87 Monday 18 June 2018

30 2 Reducing space heating demand Answer Tabe 3 gives the conductivity of gass as 1.05 W m 1 K 1. The therma resistance of a 4 mm thickness wi thus be ony 0.004/1.05 = m 2 K W 1. This is ony about 1% of the cacuated tota therma resistance of the window in Activity 5. Doubing the thickness of the gass wi doube its therma resistance but won t make much difference to the overa window U-vaue. Activity 7 (a) Exporing the improvement in U-vaue resuting from the introduction of cavity wa construction Tabe 6 above shows a cacuation of the U-vaue of a modern muti-ayer wa. A norma pre-1919 UK house is ikey to have soid was two bricks thick, with each brick being 115 mm thick (see Figure 12(a)). Later construction used cavity was with an air gap between the two skins of brick as iustrated in Figure 12(b). Tabe 7 is interactive and aows you to change the wa construction in the third ayer giving three options: a soid brick wa two bricks thick a cavity wa a soid brick wa three bricks thick. The overa cacuated U-vaue appears at the bottom. Which of the foowing gives the ower U-vaue? (i) (ii) adding a cavity to a two-brick soid wa or increasing the thickness of the soid wa to three bricks thickness? Tabe 7 Interactive content is not avaiabe in this format. (b) Exporing the benefits of cavity wa insuation and the thickness of insuation needed to meet future UK U-vaue standards The interactive Tabe 8 aows you to cacuate the U-vaue of a cavity wa fied with insuation (as shown in Figure 15). It aso aows you to change the inner eaf between brick and aerated concrete. (Note that you wi need to cick on the cacuate button to produce the answer at the bottom.) Tabe 8 Interactive content is not avaiabe in this format. (i) Start by cacuating the U-vaue of a cavity wa with a brick outer skin in ayer 2, a brick inner skin in ayer 4 and insuation in a 50 mm cavity. A typica conductivity vaue to use for bown minera woo cavity insuation might be W m 1 K 1. The properties of other types of insuation have been given in Tabe 4. This shoud give a U-vaue of 0.52 W m -2 K 1. How does this compare to the U-vaue of the uninsuated cavity wa in part (a) of this activity? 30 of 87 Monday 18 June 2018

31 2 Reducing space heating demand (ii) (iii) (iv) Next expore the improvement in U-vaue by changing the inner eaf of the wa to insuating aerated concrete in ayer 4. Remember to cick on cacuate to produce the fina U-vaue. Increase the thickness of the insuated cavity to 100 mm or 150 mm. What is the U-vaue now? Future UK houses may need was with a U-vaue of 0.15 W m -2 K 1 or better. What is the minimum insuation thickness they wi need using minera woo? What is the answer if they used poyisocyanurate foam with a conductivity of W m 1 K 1? Answer (a) (i) Adding an air gap to produce a cavity wa decreases the U-vaue from 2.03 W m -2 K 1 to 1.49 W m -2 K 1. (ii) Increasing the thickness of the soid wa to three bricks thickness reduces the U- vaue to 1.56 W m -2 K 1. The cavity wa gives the greater decrease in U-vaue. (b) (i) Fiing the cavity with minera woo insuation reduces the U-vaue from 1.49 to 0.52 W m -2 K 1, amost a three-fod improvement. (ii) Changing the inner eaf from brick to aerated concrete improves it further to 0.36 W m -2 K 1. (iii) Increasing the insuation thickness to 100 mm improves the U-vaue to 0.24 W m - 2 K 1 and 150 mm gives 0.18 W m -2 K 1. (iv) The minimum cavity thickness to achieve a U-vaue of 0.15 W m -2 K 1 with minera woo is 180 mm. This figure comes down to ony 120 mm if poyisocyanurate foam is used. 2.3 Cutting ventiation osses Buidings aso ose heat by ventiation, i.e., the passage of air through them. In houses this normay means the controabe air movement through openabe windows, extractor fans, or, in the case of arger buidings, a mechanica ventiation system. However there is aso an uncontroed component caed infitration. This is the air fow through gaps in the fabric of the buiding cracks around windows, doors and eectrica or pumbing outets, or between skirting boards and foors. In common use, the term infitration is used as a component of ventiation rather than something competey different. Some form of ventiation in a buiding is essentia. For exampe in a house it is needed in iving spaces: to provide combustion air in winter for boiers, fires and gas cookers, athough it is not necessary for heating systems with baanced fues (see Section 3.1) or for eectric fires 31 of 87 Monday 18 June 2018

32 2 Reducing space heating demand to remove moisture from kitchens, toiets and bathrooms, which shoud be equipped with controabe ventiation openings and/or their own extractor fans to provide fresh air for occupants and to keep them coo in summer. Ventiation is aso needed in other areas of the house, to remove moisture in the roof space or oft above the insuation, or under suspended ground foors (which are usuay of wood, but in more recent construction can be made of concrete). Figure 17 iustrates the ventiation and infitration air paths through a norma house and aso where it is important to maintain essentia ventiation. Note that an air fow must be maintained through the oft space and not be bocked by insuation pushed into the eaves of the roof. Figure 17 Air eakage paths through a norma house The main driving forces for this air movement are the buoyancy (or stack) effect of warm air and the wind pressure on a buiding. Warm air inside a buiding in winter is ess dense than cod air outside and, ike a hot air baoon, wi tend to rise. This has the effect of sucking in cod air from outside into the rooms on the ground foor. Wind pressure wi attempt to force air through gaps in the was on the windward side of the buiding and out again on the eeward side. Wind speeds increase with height above the ground, so winddriven infitration in high-rise buidings can be a major probem. Houses are normay naturay ventiated, i.e. they are dependent mainy on the stack effect to provide adequate air movement. In arger buidings mechanica ventiation is often used. This is often aso the means of space heating, with air being centray preheated (or cooed in summer) before being distributed throughout the buiding and extracted again through more ductwork. The term air conditioning normay impies the use of mechanica ventiation with centra air cooing. 32 of 87 Monday 18 June 2018

33 2 Reducing space heating demand The key factor in determining the ventiation heat oss in a buiding is the ventiation rate, i.e. the average rate at which air fows through it. Any warm air that escapes through the windows, doors and various gaps in the outer fabric is immediatey repaced by a new suppy of fresh cod air from outside. We may be unaware of how substantia this invisibe air reay is an average house contains about a quarter of a tonne of it! The ventiation rate is normay specified as the number of compete air changes that take pace per hour (ACH). Actuay measuring this scientificay is a fairy compex process. Typicay, in a new, we-buit, naturay ventiated house where windows are cosed, and with few gaps in the buiding fabric, it might take two hours for the air to be competey repaced by new, incoming air. We woud say that the ventiation rate of this house was 0.5 ACH. If the voume of a house is V m 3, and the air change rate is n ACH, then the tota amount of air passing through it per hour wi be n V m 3. This air needs to be heated up through the temperature difference ΔT between the externa temperature and the interna temperature. The energy required to raise one cubic metre of air through one kevin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m 3 K 1. Thus the tota ventiation heat oss, Q v, wi be: Q v = 0.33 n V ΔT watts For any given buiding, the actua ventiation rate wi depend on its age and ocation. Many buidings buit before 1918 had an open coa fire and chimney for amost every room. They are aso ikey to have been designed for gas ighting, with high ceiings and air bricks in the was to remove the combustion fumes. Draughty wooden ground foors are aso common. Since the pressure of the wind on a house has a great infuence, buidings in shetered ocations are ikey to have a ower air change rate than those in exposed positions. For exampe, a house buit before 1918 might have an average ventiation rate of over 2 ACH in an exposed ocation. After 1920, houses and offices were designed for eectric ighting and had ower ceiings. It was ony in the 1970s, with the advent of cheaper eectricity and gas centra heating, that houses began to be buit without open firepaces. They coud then (theoreticay at east) be designed to be reasonaby airtight. Section ooks at how to reduce heat oss by improving the airtightness of buidings. Heat oss can aso be reduced by recovering some of the heat from ventiation air before it is reeased. This is the topic of Section Airtightness Proper airtightness is the key to minimising air infitration. In existing housing this means using draught-proof strips, repacing eaky windows and bocking unused chimneys. The atter may be difficut since it is often necessary to maintain a sma air-fow through them to remove any moisture penetrating into them. It means paying carefu attention to bocking a the unwanted air eakage paths shown in Figure 17, whie maintaining the essentia ones. In new construction attention to detai is important. It is easy to eave air gaps around windows and where pipes penetrate was. Sheet pastic vapour barriers are often buit into was, especiay in timber-framed construction. For reay good airtightness these vapour barriers must be taped together where they join, so that they cover the whoe buiding enveope. This is a highy skied job. 33 of 87 Monday 18 June 2018

34 2 Reducing space heating demand The overa airtightness of a buiding can be assessed with a pressure test. Usuay one of the externa doors is repaced with a frame carrying a caibrated eectric fan (see Figure 18). This bows a arge amount of air into the buiding at a known rate, in order to set up a standard pressure difference between the inside and outside of 50 Pascas. This is roughy equivaent to the effects of a gae-force wind. The overa air eakage rate of the buiding at this pressure difference can be worked out and is usuay expressed in cubic metres per hour per square metre of buiding enveope area (i.e. area of was, roof, etc.). The ower the figure, the more airtight the construction. Probem areas can be identified and investigated using a sma smoke generator aowing the air eakage to be ceary seen. Figure 18 A bower door being used to pressurise a house to test its airtightness (courtesy Leeds Metropoitan University) 34 of 87 Monday 18 June 2018

35 2 Reducing space heating demand Ventiation heat recovery Many office buidings use mechanica ventiation driven by eectric fans. One way of reducing ventiation heat oss is to use mechanica ventiation with heat recovery (MVHR), which invoves aowing warm outgoing air to preheat cod incoming air. This can be done by passing both air streams through a heat exchanger (see Figure 19), consisting of mutipe ayers of thin, fat, meta or pastic pates with incoming and outgoing air passing through aternate ayers. This gives a arge area through which heat can fow. Obviousy such a system can be used ony if the inet and outet ducts are adjacent to each other. Figure 19 Fat pate heat exchanger MVHR is a mixed bessing. On the one hand it gives controabe ventiation adjustabe to amost every room. On the other it requires compex ductwork and air pumping, which can consume considerabe amounts of eectricity. MVHR systems are avaiabe for domestic appications (see Figure 20) but it is essentia that they are instaed in buidings that are airtight to start with, otherwise any attempt to pump air around the system may just increase the fow of air through unwanted air infitration paths. However, once the fabric heat osses of a buiding have been tacked with thick insuation and high-performance windows, this may be the ony satisfactory way to dea with the remaining major heat oss, that from ventiation. 35 of 87 Monday 18 June 2018

36 2 Reducing space heating demand Figure 20 Whoe-house mechanica ventiation with heat recovery (Source: Nichos, 2002) Activity 8 What is the difference between ventiation and infitration? Answer Ventiation is air movement that is considered essentia and may be deiberatey encouraged by opening windows or using mechanica ventiation. Infitration is uncontroed air movement through various cracks in the buiding fabric or through air paths such as chimneys. 36 of 87 Monday 18 June 2018

37 2 Reducing space heating demand Activity 9 A sma 19th century terraced house has a voume of 240 m 3, an air change rate of 2 ACH and an inside outside temperature difference, ΔT of 20 K. What is its ventiation heat oss rate in kiowatts? Answer Using the equation given above: Q v = 0.33 n V ΔT watts and taking n = 2 ACH, V = 240 m 3 and ΔT = 20 K Q v = watts = 3.17 kw(i.e. rather a ot). 2.4 How much insuation does a buiding need? A further reated question is how arge a heating system wi it need? Obviousy the severity of the winters at the buiding ocation is a key factor. There are two measures for this: the winter design temperature and the number of degree days. The winter design temperature is that of the codest weather ikey to occur on the worst winter days at a particuar ocation. It is used to size the heating system. For exampe the design temperature for London is 2 C, whie for Berin it is 11 C. The number of degree days is a measure of the average externa temperature over the winter months and can be used to estimate the heating fue bis Cacuating the tota heat oss of a house If we know the U-vaues of a the eements of the externa fabric of a buiding, its voume and its average ventiation rate, then we can cacuate its overa heat oss coefficient (or heat transfer coefficient). We can define this as the tota space heating energy fow rate in watts divided by the temperature difference between the inside and outside air. Let us take a reasonaby modern end-of-terrace house insuated to standards suggested in the 2002 Buiding Reguations for Engand and Waes. Its dimensions are shown in Figure 21 and its tota foor area (upstairs and downstairs) is 96 m of 87 Monday 18 June 2018

38 2 Reducing space heating demand Figure 21 An exampe of an end of terrace house The tota fabric heat oss fow rate, Q f, wi be the sum of a the U-vaues of the individua eements of the externa fabric, was, roof, foor, windows and doors mutipied by their respective areas mutipied by the inside outside temperature difference, ΔT. Q f = (ΣU x A x ) ΔT watts - (note: the Σ symbo means sum of ) The tota fabric contribution to the overa heat oss coefficient is then: Q f /ΔT = ΣU x A x W K 1 This is cacuated in Tabe 7. Tabe 7 House fabric eements and heat oss Eement Area / m 2 U-vaue / W m 2 K 1 Contribution to heat oss coefficient /W K 1 Foor = 12 Roof = 7.7 Was = 28 Windows and doors = 40 Tota 87.7 Note that we assume that the adjacent house wi be at the same interna temperature, so that there wi be no heat oss through the (uninsuated) party wa between them. Aso, in practice there wi be extra conduction heat osses caed cod bridges through items such as pipework running through was and the meta intes over windows, but we wi ignore these here. We must aso incude the ventiation heat oss, which is: 38 of 87 Monday 18 June 2018

39 2 Reducing space heating demand Q v = 0.33 n V ΔT watts where n is the number of air changes per hour (ACH) and V is the voume of the house (m 3 ). The ventiation contribution to the overa heat oss coefficient is then: Q v /ΔT = 0.33 n V W K 1 Assuming an air change rate of 0.5 ACH (which requires reasonaby airtight construction) and taking the voume of the house as 240 m 3 : Q v /ΔT = = 39.6 W K 1 Summing the fabric and ventiation contributions gives a tota whoe-house heat oss coefficient of: (Q f + Q v )/ΔT = = W K 1 Figure 22 gives the percentage breakdown of these osses, which shows their reative importance and gives a cue as to where to ook for further improvements. Figure 22 Percentage breakdown of heat oss coefficient We can use this whoe house heat oss coefficient to estimate a suitabe size for the heating system. If we assume an interna temperature of 20 C and site the house in London, for exampe, which has a winter design externa temperature of 2 C, then the heating system must be abe to maintain a temperature difference of 22 K. An estimate of the necessary heating system size, Q h, woud thus be: Q h = = 2800 W In practice there is aways an extra aowance to cope with warming up a house if it has been eft empty for a few days. 39 of 87 Monday 18 June 2018

40 2 Reducing space heating demand If the house were situated in Berin, which has much coder weather and a design temperature of 11 C, then the heating system woud have to maintain a worst-case temperature difference of 31 C and woud have to be rated at = 3950 W. Obviousy the more insuation and the better the airtightness, the smaer (and hopefuy cheaper) the heating system can be Degree days We can aso use this heat oss coefficient together with the number of degree days to understand how much space heating energy a buiding might use in different ocations. Over a ong period, such as a day or so, the heat oss from a buiding wi be proportiona to the average temperature difference between the interior and the outside air. If on a given day the average interna temperature was 20 C and the average externa temperature was 10 C, then the difference woud be 10 C. We woud describe that particuar day as having 10 degree days. If on another day the average interna temperature was the same and the externa temperature was zero, 0 C, i.e. an average difference of 20 C, we woud describe that day as having 20 degree days and expect the buiding to ose twice as much heat as on the first day. However if the average externa temperature was higher than the interior, then there woud not be any heating requirement, and the number of degree days woud be zero (rather than a negative number). The tota heating requirement over a month wi be proportiona to the sum of a the degree days of the individua days. Tabe 8 gives some ong-term averages for sampe UK ocations. Given their ong history of use, it is not surprising that they are normay produced in the UK with a standard indoor base temperature of 60 F, equivaent to 15.5 C. Tabe 8 20-year averages of degree days ( ) to base 15.5 C for sampe UK areas South Western London (Thames Vaey) Midands Northern Ireand Borders North-East Scotand January February March Apri May June Juy August September October November December Tota (Source: EST, 2005) 40 of 87 Monday 18 June 2018

41 2 Reducing space heating demand Tabe 8 gives an annua tota of 1889 degree days for the London area. A first estimate of an annua heating energy consumption of our house in watt-hours woud be the heat oss coefficient, W K 1, mutipied by the number of degree days mutipied by 24 (to convert from days to hours). Dividing by 1000 then gives the resut in kiowatthours (kwh). Annua consumption = /1000 = 5771 kwh If the house had been ocated in Berin, instead, which has 2600 degree days, then the heating oad woud have been much higher: Annua consumption = /1000 = 7944 kwh Put another way, it woud have to be better insuated to achieve the same heating demand. We can go further and say that if we managed to trim 1 W K 1 off the heat oss coefficient by better insuation or airtightness, then the margina saving in space heating demand woud be = 45.3 kwh in London or = 62.4 kwh in Berin. This coud then be used to anayse the reative cost effectiveness of further energy-saving investments. Activity 10 Based on the degree day data, is our sampe house ikey to have a higher heating demand in Berin or north-east Scotand? Answer The more degree-days, the higher the ikey heating demand. North-east Scotand has sighty more degree days, 2638, than Berin with 2600, so our sampe house is ikey to have the highest heating demand in Scotand Baance point temperature In practice, the degree day concept shoud be treated with caution when deaing with weinsuated buidings. It may seem strange that degree days are to a base of 15.5 C and not the actua interna temperature of the buiding. The degree day base temperature is assumed to be that beow which the heating system is ikey to be needed. This is in fact ikey to be a function of the amount of insuation. This probem is expored in Figure 23 which shows the estimated daiy heating requirements for a 1970s house, with and without extra insuation. 41 of 87 Monday 18 June 2018

42 2 Reducing space heating demand Figure 23 Degree days and baance point On any given day the gross space heating wi depend on the externa temperature; the coder it is, the more heating energy wi be required. However, there wi aways be a certain amount of interna free heat and soar gains. The space heating system itsef wi ony need to suppy heat if these are not sufficient to keep the house warm enough. We can describe this as the net space heating demand. This wi ony be required if the externa temperature fas beow a certain baance point temperature. Beow this the free heat gains cease to be sufficient to keep the house at the chosen interna temperature, assumed to be 20 C in Figure 23. In this particuar house in its uninsuated state the baance point appears to be just under 15 C; it is certainy consideraby ower than 20 C. We coud say that the heating demand of this house requires degree days to the base 14.5 C rather than 15.5 C. In fact the degree day concept is very od and predates any notion of insuation in the UK buiding stock. An assumption of a baance point temperature of 15.5 C was quite appropriate for estimating the space heating consumption of the buidings of the 1950s and 1960s. However, it is sti usefu in basic energy monitoring and targeting in buidings today Improving insuation standards By the standards of the whoe British housing stock the heat oss coefficient for the sampe house of 127 W K 1 cacuated above is quite good. In 2006 the average British dweing 42 of 87 Monday 18 June 2018

43 2 Reducing space heating demand had an estimated heat oss of about 250 W K 1 and this is a considerabe improvement on the figure of 375 W K 1 for 1970 (BRE, 2008). How do these improving insuation standards affect the tota amount of heat that a space heating system must suppy? Figure 24 shows the estimated gross heating demand of a poory insuated 1970s UK house (simiar houses wi be found right across Northern and Centra Europe). This peaks at neary 100 kwh per day in midwinter. Figure 24 Contributions to the gross space heating demand in a typica poory insuated UK house of the 1970s This heating demand wi be higher in the cod midwinter months than in the warmer spring and autumn ones. In summer, when the outside air temperature is high, this gross heating requirement fas to under 20 kwh per day. Free heat gains are ikey to contribute throughout the year, peaking sighty in midwinter when there is most artificia ighting use. Soar gains are ikey to give most contribution in autumn and spring. Not a of the soar gains wi be usefu in heating the house; some are ikey to cause overheating and the occupants may open the windows to dissipate any surpus gains. For this particuar house, over a whoe year, out of a tota gross heating demand of kwh, 5000 kwh have come from free heat gains and 3000 kwh from soar gains. Put another way, in this perfecty ordinary house 14% of the gross heating demand is suppied by soar gains. The net space heating demand, to be suppied by the norma heating system, is the remaining heat requirement, kwh. This wi have to be suppied over a heating season from mid-september to the end of May. Figure 25 shows the gross and net space heating demands over the year for a more modern house with a better standard of insuation, incuding doube-gazing and insuated was, foor and roof. The gross heating demand now peaks at just over 50 kwh per day in 43 of 87 Monday 18 June 2018

44 2 Reducing space heating demand midwinter. Over the whoe year it has been roughy haved to about kwh, but now the free heat and soar gains are sufficient to keep the house at a comfortabe temperature for more of the year. The heating season is now shorter, ony from October to Apri and the net space heating demand has faen to ony 4000 kwh, a reduction of 70%. Figure 25 Contribution to the gross space heating demand in a moderatey we-insuated house Passivhaus design If a house were sufficienty we-insuated super-insuated the free heat and soar gains aone shoud be sufficient to keep the interna spaces warm in a except the very codest weather. A space heating system as such might be very sma, or even unnecessary. It might be thought that as energy efficiency standards improve the eve of free heat gains in houses might be ikey to fa. However, over the past 40 years the deivered energy per UK househod for cooking, ights and appiances has actuay risen by about 30%. This trend is discussed in Section 4. Super-insuated buiding design has been heaviy promoted in Germany, both for new construction and the refurbishment of existing buidings. In the ate 1990s an estate of apartment bocks in Ludwigshafen in south-west Germany, originay constructed in the 1950s, was given a thorough therma modernisation (see Figure 26(a)) incuding: at east 200 mm of foam insuation on the roof and in the was (see Figure 26(b)) tripe-gazed windows with ow-e gass with a U-vaue of 0.8 W m 2 K 1 mechanica ventiation with heat recovery a combined heat and power (CHP) unit (effectivey a sma power station) using a fue ce. This technoogy is described ater in section of 87 Monday 18 June 2018

45 2 Reducing space heating demand Figure 26 (a) The Brunck Estate in Ludwigshafen, Germany (b) Laying bocks of foam insuation on a roof Monitoring showed that the net space heating energy use fe by a factor of seven from 210 kwh per square metre of foor area per year to ony 30 kwh m 2 yr 1. This is equivaent to 3 itres of heating oi hence the project name, the 3-Liter-Haus (Luwoge, 2006). Subsequent projects have used even thicker insuation. The German Passivhaus programme has haved this energy target to 15 kwh m 2 yr -1 using fabric U-vaues of approximatey 0.1 W m 2 K 1. The German cimate is coder than that of most of the UK. The house is considered passive in the sense that it does not need substantia heating. Such a target woud mean a net space heating demand of under 1500 kwh yr 1 for our sampe house with a 96 m 2 foor area. In the UK Passivhaus design is being promoted by the Buiding Research Estabishment and their introductory primer incudes various housing exampes. Activity 11 In 2013 there were 27 miion homes in the UK and their average space heating demand was about kwh yr -1. In a home insuated to a Passivhaus standard this coud be reduced to ony 1500 kwh yr -1. (a) (b) Cacuate the tota 2013 UK domestic space heating demand in petajoues (you wi need to ook back to Box 1 in Section 1 for the conversion factors). What woud the nationa domestic space heating demand become, in petajoues, if the whoe UK housing stock were insuated to a Passivhaus standard. Woud you consider the potentia energy saving significant given the overa nationa energy use shown in Figure 1 in Section 1? Answer (a) From Box 1, 1 kwh = 3.6 MJFor each home, annua space heating demand = kwh = MJ = MJ = 36 GJ Nationa domestic space heating demand = GJ = GJ = 972 PJ 45 of 87 Monday 18 June 2018

46 3 Improving heating system efficiency (b) For a Passivhaus home, annua space heating demand= 1500 kwh = = 5400 MJ = 5.4 GJ Nationa domestic space heating demand = GJ = GJ = PJ Saving = PJ = PJ Figure 1 shows a tota deivered energy consumption of about 6000 PJ of which about 2200 PJ are devoted to space and water heating. So a saving of over 800 PJ woud indeed be significant. 3 Improving heating system efficiency Over the past 50 years the UK has been transformed from a country where the majority of homes were heated with individua coa fires to one where centra heating is amost universa. In 2009, an estimated 97% of British homes were centray heated. An open coa fire may seem very cosy and traditiona, but in practice most of the heat disappears up the chimney. The therma efficiency of a heating system = usefu heat output / fue energy input. That of a coa fire is particuary ow. Tabe 9 iustrates the enormous variation in therma efficiencies of different space heating systems. Tabe 9 Therma efficiencies of different space heating systems Heating system Seasona average efficiency Open coa fire approx. 32% New coa boier approx. 70% Eectric fire 100% Oder gas fire approx. 50% Typica 1980s wa-hung gas boier approx. 65% Gas condensing boier >85% In the UK, gas is the dominant centra heating fue (see Figure 27) and it aso suppies haf of non-centray heated homes. Renewabe energy and energy from waste meet about 1% of domestic heating energy demand. 46 of 87 Monday 18 June 2018

47 3 Improving heating system efficiency Figure 27 GB domestic centra heating (CH) fues, 2009 (DECC, 2014a) This growth of centra heating use has been a mixed bessing for overa energy demand. The average UK dweing today is better insuated than its 1970 counterpart, and space heating system efficiencies have increased, from an average of about 50% in 1970 to neary 75% in 2006 (BRE, 2008). It might therefore be thought that the space heating demand of the average UK home woud have faen dramaticay. However, much of the potentia energy saving has been taken up as increased interna temperatures. Homes are now better heated and estimated average interna temperatures have risen from about 12 C to 18 C over the same period, increasing heat oss. It is even possibe that this temperature trend wi continue in the future to eves of 22 C or more found in housing in Germany and Scandinavia. Figure 28 shows the ayout of a typica domestic wet centra heating system. Water is circuated through a boier, which may be fueed by gas, oi or soid fue (or even an eectric heat pump). The heated water fows through the radiators and through a heat exchanger in a hot water storage cyinder, which hods the domestic hot water used for washing, etc. The storage cyinder (if present) shoud be we-insuated most are now suppied with a sprayed ayer of foam insuation. 47 of 87 Monday 18 June 2018

48 3 Improving heating system efficiency Figure 28 A diagram of a radiator, or wet, centra heating system There are many variants of this basic system. The most common is the combination or combi boier. This suppies heat to radiators but dispenses with the hot water cyinder, acting as an instantaneous heater for domestic hot water. Such a system shoud obviousy be propery controed for maximum efficiency. A good system is ikey to be controed by: preset time switches, or a timing programmer a room thermostat that senses the air temperature, usuay somewhere in the midde of the house a hot water cyinder thermostat that senses the requirement for domestic hot water. In addition individua radiators shoud be equipped with thermostatic radiator vaves (TRVs) that provide extra oca contro in individua rooms. Systems for arger buidings are ikey to foow a simiar pattern, but may have mutipe zones that can be separatey controed. Large buidings such as hospitas are ikey to use warm air heating. Air is centray heated (or cooed) and then bown through ducts to individua rooms. 3.1 Gas boiers Figure 29 shows a simpified diagram of a typica 1980s domestic boier. Combustion air enters at the bottom from the interior of the house to feed a arge gas burner heating a set of finned heat exchangers. Water is pumped through these and circuated out to the heating system. The burnt gases exit at the top into a fue to the outside air. Such a boier might have an output of 15 kw and an overa therma efficiency of 65%. 48 of 87 Monday 18 June 2018

49 3 Improving heating system efficiency Figure 29 Schematic of a typica 1980s gas boier More recent gas boier designs have incorporated a number of improvements: 1 Baanced fues Here the burner and heat exchanger are totay seaed off from the interior air of the buiding and the combustion air is bown through the boier using an eectric fan. Air is drawn in from the outside and the burnt gases are bown out again using a pair of concentric pipes via a singe fue termina on the outside of the buiding (Figure 30). Athough this design requires some consumption of eectricity, it aows the buiding to be made more airtight. 49 of 87 Monday 18 June 2018

50 3 Improving heating system efficiency Figure 30 A boier with a baanced fue 2 Eectronic spark ignition This has repaced the permanenty burning piot ight used in many oder boier designs which coud consume 10% of the tota gas used. 3 Condensing gas boiers (see Figure 31) The main component of natura gas is methane, CH 4. When this burns it produces a arge amount of water vapour: CH O 2 CO H 2 Omethane + oxygen carbon dioxide + water vapour 50 of 87 Monday 18 June 2018

51 3 Improving heating system efficiency Figure 31 Condensing gas boier In a non-condensing boier this water vapour is ost with the fue gases. In a condensing boier the heat exchanger is made sufficienty arge so that the return water from the heating system, which may be at about 50 C or ower, can coo the fue gas sufficienty to enabe the water vapour to be condensed out recovering its atent heat of vaporization. The energy content of a fue can be quoted as higher or ower vaues, depending on whether or not this atent heat of vaporization is recovered. The potentia energy savings in using a condensing boier depend on the difference between the higher and ower heating vaues (HHV and LHV). Tabe 10 gives sampe figures. Tabe 10 Higher and Lower Heating Vaues of Fues Higher heating vaue/ MJ kg 1 Lower heating vaue/ MJ kg 1 Ratio Natura gas Liquefied petroeum gas (LPG) Light heating oi (Sources: DECC, 2010a) Where natura gas is the fue, using a condensing boier can increase the amount of heat extracted from the gas by up to 10% and the overa boier efficiency to 90% or more. 51 of 87 Monday 18 June 2018

52 3 Improving heating system efficiency 3.2 Oi-fired boiers Oi-fired boiers are usuay simiar in design to gas boiers but need to incorporate a device to vaporise the heating oi before combustion. Condensing boiers are avaiabe for use with ight heating oi, but the potentia efficiency improvements are sighty ower, about 6%, because of the ower proportion of hydrogen, burning to water vapour, in the fue. Depending on its age and design, a gas or oi-fired boier can have an efficiency from 55% to over 90%. This can obviousy have a profound effect on the energy consumption of a buiding. In order to simpify things for purchasers, boiers have been given an Energy Rating Labe. Those with a seasona efficiency of more than 90% are A rated, whie those with an efficiency between 86% and 90% are B rated. Those with an efficiency of beow 70% woud get a G rating. From 2005 the UK Buiding Reguations ony aowed A and B rated boiers to be sod. Athough average heating efficiencies have improved over the years to neary 75% in 2006 for space heating there is sti a arge potentia for improvement. Activity 12 How does a baanced fue boier save heating energy compared to an oder type of boier? Answer A baanced fue boier does not need to draw its combustion air from inside the buiding. As a resut the buiding can be made more airtight. Activity 13 Why is a condensing boier more efficient than a non-condensing one, and what is the margin of improvement that might be expected with natura gas as fue? Answer A condensing gas boier contains an extra heat exchanger that aows the fue gases to be cooed enough to condense the water vapour in them. This aows the atent heat of vaporization of the water to be recovered. For natura gas this can represent an increase in efficiency of up to 10%. 3.3 Eectric heating On-peak eectric resistance heating In the UK, eectricity has aways been consideraby more expensive than gas or soid fue. Its use for heating has thus been rather restricted. It wasn t unti the 1960s that eectricity prices had faen sufficienty against gas and coa that the idea of a-eectric homes coud be promoted. 52 of 87 Monday 18 June 2018

53 3 Improving heating system efficiency Resistance heating using a radiant eectric fire or an immersion heater for domestic hot water has been widey promoted as a cean form of heating. Indeed it is very efficient and cean at the point of use, and especiay so compared to the dirt and air poution of coa fires. However, the inefficiency and poution is effectivey transferred to a remote power station. This is refected in the higher CO 2 emission figures compared to gas heating, as discussed in Section 3.5. Whie water heating is needed year-round, space heating is something required ony for a part of the year (and possiby ony a few winter months in a reay we-insuated buiding). Thus, athough an on-peak eectric fire is cheap for the consumer to buy, it may be expensive for the industry to suppy, requiring a power pant that sits ide from spring to autumn. Off-peak storage heating The use of off-peak night-time eectricity goes some way to reducing the probems of onpeak eectric heating use, aowing a daytime oad to be shifted to the night. It aowed the demand on the Nationa Grid to be smoothed out and was heaviy promoted in the 1960s and 1970s. Night-time eectricity coud be sod at a specia ow rate that coud compete with gas and coa. Domestic hot water coud be heated with an immersion heater in a hot water cyinder, but space heating required storage heaters. Figure 32 shows a typica design. Figure 32 Cut-away drawing of an off-peak storage heater It is basicay a set of high-density bocks (simiar to bricks) threaded with resistance heating eements inside an insuated casing. The eectricity suppy is remotey turned on and off by a time switch (sometimes radio controed by the Nationa Grid operator). The storage heater itsef usuay has two manua contros (often poory understood): 53 of 87 Monday 18 June 2018

54 3 Improving heating system efficiency an input contro that sets how much energy needs to be stored overnight according to the weather an output contro that aows air to circuate through the bocks and into the room. The reative ack of contro compared to the simpe thermostat of a centra heating system has ong been a source of dissatisfaction with this type of heating. Competition from cheap North Sea gas from the 1970s onwards effectivey stopped the spread of eectric space heating. Today it is mainy confined to rura areas beyond the gas grid and to urban tower bocks where the use of gas is prohibited for fear of exposions Heat pumps Instead of generating heat by burning a fue, a heating system can use heat drawn from the outside environment, the air or the ground, using a heat pump. Sma heat pumps are actuay famiiar objects. Every domestic refrigerator uses one. Figure 33 shows the key eements of the most basic type of heat pump. Figure 33 Schematic diagram of a heat pump The heat pumping process is made possibe by the use of a specia refrigerant iquid that bois at a ow temperature at atmospheric pressure (typicay about 15 ºC). At point 1 in Figure 33 the refrigerant is a coo iquid. In order to convert a iquid to a vapour, it must be given energy the atent heat of vaporization. The refrigerant absorbs heat in a heat exchanger (the evaporator) and vaporises. At point 2 the vapour enters an eectricay driven compressor that increases its pressure and temperature. At point 3 the hot vapour enters another heat exchanger (the condenser) where it condenses to a warm iquid and gives up its atent heat of vaporisation. 54 of 87 Monday 18 June 2018

55 3 Improving heating system efficiency At point 4 it is forced through a fine expansion vave or throtte where it oses pressure and drops in temperature. It then repeats the cyce. Overa, heat is pumped from a ow temperature in the evaporator to a higher one in the condenser. In a domestic refrigerator, the heat is absorbed in an evaporator inside the refrigerated compartment, thus owering its temperature, to a condenser on the back of the refrigerator, where the heat is reeased, warming one s kitchen in the process. In buidings a heat pump may be used for heating or cooing (more commony known as air conditioning). When used for heating the evaporator is ocated somewhere in the externa environment. An air source heat pump is ikey to have a fan coi unit such as that shown in Figure 34(a). A ground source heat pump uses pipes buried in the soi. These may be aid in a shaow trench, as in Figure 34(b) or in a deep vertica borehoe that may be 10 metres or more deep. Figure 34 (a) A fan-coi evaporator unit for an air source heat pump (b) evaporator heat exchanger cois about to be buried for a ground-source heat pump Heat is then pumped from the outside to a condenser inside the buiding, normay connected to a norma centra heating system. The temperature of the heat is sufficient to be usefu for heating purposes. Energy (usuay eectrica) is, of course, required to operate the compressor. A key performance parameter is the coefficient of performance (COP). COP = heat output from condenser / eectrica work input Typicay 1 kwh of eectricity can be used to produce 3 or more kwh of heat. This might seem to defy the aw of conservation of energy, but the maximum COP is imited by the aws of thermodynamics. It is possibe to have a high COP if the temperature difference through which the heat is raised is sma, but the vaue may drop significanty if the 55 of 87 Monday 18 June 2018

56 3 Improving heating system efficiency temperature difference is arge. This may be particuary true where domestic hot water has to be heated to 60 ºC in midwinter. In order to function in midwinter, the evaporator has to be abe to absorb heat from the externa environment even though the externa temperature may be very ow (down to beow 5 ºC in the UK). This requirement potentiay imits the performance of air-source heat pumps. Burying the evaporator coi in the ground (or in a ake or river) provides a more stabe temperature environment in extreme winter conditions and can resut in higher COPs. In an air source heat pump, the heat that is drawn from the externa environment is taken immediatey from the outside air, cooing it in the process. In a ground source heat pump, the same process takes pace but by cooing the ground (by ony a degree or two) so that heat fows down into it from the air over a arge area and a ong time period. A heating system using a heat pump is ikey to incude fans and pumps and may require top-up direct eectric heating in mid-winter. A more accurate measure of the overa performance is thus the system performance factor (SPF). This is the ratio of the annua heat produced by the heating system to the eectricity consumed. Measured SPF vaues in the UK have ranged from 2.2 to 4.0, with ground source heat pumps performing better than air source ones (EST, 2013). Put simpy: Usefu heating energy produced = eectricity consumed SPF Athough heat pumps driven by gas engines or absorption heat cyces have been produced, this is essentiay an eectric heating technoogy. It can be thought of as recaiming some of the primary energy osses at the power station. A modern gas fueed combined cyce gas turbine (CCGT) power station can have an efficiency of 50%. It thus takes 2 kwh of gas to produce 1 kwh of eectricity. However, the heat pump can turn this back into 2.5 kwh of heat. overa therma efficiency = 2.5 kwh / 2 kwh = 125% This makes it an attractive aternative to gas heating, athough the tota capita costs of heat pump and power station are considerabe. 3.4 Combined Heat and Power Generation An aternative to using individua boiers and eectric heaters in buidings is combined heat and power generation (CHP). This uses the waste heat from power stations, of many possibe sizes, which has great efficiency benefits. The technica potentia for this is enormous. Figure 1 in Section 1 shows amost 3000 PJ of energy ost in conversion and deivery. The buk of this, about 2000 PJ, was ost as ow-grade waste heat from power stations. This figure shoud be compared with the 2400 PJ, shown in the bottom bar of Figure 1, of ow temperature heat used for space and water heating. Denmark in particuar has adopted a nationa poicy of widespread use of CHP with district heating (DH), aso caed community heating, which invoves distributing heat through arge insuated pipes under the streets. By 2007 over 60% of Danish homes were fed by DH and 80% of the DH pants were fed by CHP (DEA, 2011). By comparison, DH suppied ony about 2% of the nationa heat demand in the UK in 2009 (Pöyry, 2009). 56 of 87 Monday 18 June 2018

57 3 Improving heating system efficiency District heating does not necessariy need to be inked to power stations. It can be suppied from heat-ony boiers. It does aow the use of a wide range of fues, such as energy from waste, biomass, or even soar energy. In the UK in 2013 there was about 6.0 gigawatts of CHP eectricity generation capacity (usuay written 6.0 GWe). The buk of this was arge gas or steam turbine pant in industry. In buidings in 2013 there was about 445 MWe of CHP pant spread over 1400 separate instaations (DECC, 2014a). Most of these were spark-ignition gas engines, ike the exampe shown in Figure 35. Figure 35 A sma- scae CHP unit before instaation; the engine is at the front and the generator at the rear (courtesy of Cogenco) Such units are essentiay heavy-duty orry engines fueed by gas and driving a generator. Natura gas is a very cean fue, so units can be designed to run for 5000 hours a year or more with minima maintenance, equivaent to a quarter of a miion mies for a orry. Such units normay run in conjunction with an extra gas boier which can provide more heat at times of peak demand. Typicay units woud range in size from about 50 kwe up to 1 MWe, with eectrica efficiencies of 25% up to 40%, arge units being more efficient. Prime users are buidings with arge heat and eectricity oads such as hospitas and hotes. Leisure centres with swimming poos are particuary appropriate because of the very arge ow-temperature heat requirements of the poos, which aow CHP units to be run in a condensing mode with an amost 90% overa fue efficiency. Other schemes have been set up specificay for arge bocks of fats, such as the Pimico scheme in centra London which suppies over 3000 homes. This is a technoogy that has been heaviy promoted in the UK in recent years. 57 of 87 Monday 18 June 2018

58 3 Improving heating system efficiency Stiring engine domestic micro-chp Over the past 15 years, sma CHP units of approximatey 1 kw eectrica output have been deveoped, intended as repacement domestic gas boiers. These use a sma Stiring engine to drive a generator. The Stiring engine is an externa combustion engine widey used in the nineteenth century, but it was widey repaced in the twentieth century by the sma petro engine. However, unike the petro engine, it has the advantage of being virtuay sient, making a good candidate for domestic appications. The engine and generator are packaged as a condensing gas boier that aso generates eectricity. Figure 36 shows the basic ayout of a singe cyinder design. The design is basicay simiar to the boier shown in Figure 36, but extra high temperature heat is suppied to the hot end of the Stiring engine, and ow temperature waste heat is recovered from the cod end. A simiar design using a four-cyinder engine has been marketed. Figure 36 Schematic diagram of a singe-cyinder domestic micro-chp unit The eectrica generation efficiency for these commercia units is ower than for the arger sma-scae CHP units described above. However, prototype machines have been buit with comparabe efficiencies. These units are primariy designed to meet the heat oad of a house but not to generate continuousy the fu eectricity demand as we. If the eectricity demand for the house is arger than the output of the generator, a the eectricity wi be used. If the output exceeds the demand, then it wi be exported to the grid via a suitabe two-way meter Fue ce CHP Fue ces can produce eectricity and heat without combustion. They can be thought of as arge batteries which ony work when suppied with a stream of hydrogen gas as fue, which ends up being oxidised to water. CHP units using fue ces are now becoming commerciay avaiabe and are widey used in the USA and Japan. In 2001 a 200 kwe phosphoric acid fue ce (PAFC) was instaed at a eisure centre in Woking as part of a scheme incorporating a further 1 MWe of gas engine CHP and 9 kw 58 of 87 Monday 18 June 2018

59 3 Improving heating system efficiency of photovotaic panes. The hydrogen to run the fue ce was produced by reacting natura gas with steam, aso producing carbon dioxide. The individua fue ce eements are stacked together to produce approximatey 400 vots DC, which is connected to the grid via an AC inverter. Hot water from the fue ce is used to provide heating and to run an absorption chier unit for cooing the eisure centre. Performance monitoring showed that the fue ce operated with approximatey the same eectrica generation efficiency, 37%, as a comparabe gas engine CHP unit (DTI, 2005). At present, capita costs and maintenance costs are higher than for gas engines, but these coud fa. Fue ce CHP has two main advantages over engine-driven CHP: the units are amost sient, with no moving parts, and they have high eectrica efficiencies, which in the onger term coud reach 50%, i.e. competitive with arge combined cyce gas turbine (CCGT) power stations (IEA, 2005) Using waste heat from arge power stations This option is itte-used in the UK, since the trend has been to buid arge coa-fired power stations we away from major buit-up areas. However, many gas-fired power stations are cose to major urban centres. For exampe the 1 GWe Barking CCGT station is ess than 20 km from centra London and pumps hundreds of megawatts of waste heat into the Thames. There is obviousy enormous potentia for using waste heat from such arge power stations, but it comes at a cost of reduced eectricity generation efficiency. Typicay there wi be a oss of one kwh of eectricity generated for every six kwh of usefu heat produced. However in a purpose buit combined cyce gas turbine (CCGT) power station specificay designed for combined heat and power operation, this coud be improved to a oss of one kwh of eectricity for every 10 kwh of heat produced (Pöyry, 2009) Which form of CHP is best? There are many possibiities for CHP. Which is best is a matter of ocation and trade-offs of cost and performance. Generay, the arger the CHP unit, the higher the eectricity generation efficiency, the ower the capita costs per kw of capacity and the ower the maintenance costs per kwh of eectricity produced. The distribution of heat from arger CHP units and arge power stations requires extensive networks of district heating pipes (Figure 37). 59 of 87 Monday 18 June 2018

60 3 Improving heating system efficiency Figure 37 District heating pipes being aid. The fow and return pipes are surrounded with insuation and encased in a singe pipe outer casing The technica potentia for CHP with district heating in the UK is arge, but at present it has to compete with riva cheap natura gas for individua home heating systems. However it is possibe that by 2050 the twin probems of reducing nationa CO 2 emissions and the decine in North Sea gas production wi mean that natura gas boiers wi have been phased out, argey removing the need for a natura gas distribution network (DECC, 2010c). Aternative methods of heating buidings wi have to be found. 60 of 87 Monday 18 June 2018

61 3 Improving heating system efficiency A 2005 Internationa Energy Agency report (IEA, 2005) modeed the heating needs of a European city of about inhabitants (based on Leicester). It compared different forms of CHP with a base case of using individua condensing gas boiers and eectricity from new, high-efficiency CCGT pant. It concuded that the east-cost soution, and the one with the argest CO 2 emission savings, was city-wide CHP using heat and eectricity from a CCGT power station. However, this eaves many buidings in suburban areas for which individua sma CHP units (or possiby eectric heat pumps) may we be the best soution. However, if sma fue ce CHP units can be produced with high efficiencies and ow maintenance costs then these coud become serious chaengers to city-wide CHP. The costs of district heating networks are high, but then so are those of riva technoogies such as heat pumps. If suitabe ow interest finance were avaiabe it coud be economic to extend district heating to between 3.3 and 7.9 miion UK househods (mainy in dense urban areas), incuding miion m 2 of commercia foor space. This woud represent between 6% and 14% of the UK heat demand (Pöyry, 2009) 3.5 Heating systems and CO 2 emissions In assessing any particuar buiding we are ikey to be interested in its tota fue consumption and its CO 2 emissions. The previous sections have given some guidance on estimating the ikey net space heating demand. Adding an aowance for water heating, the tota annua fue use for heating can thus be cacuated as: annua fue use = annua heat demand / heating system therma efficiency Not a heating fues produce the same amount of CO 2 for a given amount of heat produced. Tabe 11 gives the emission factors for the main UK heating fues. Note that natura gas is consideraby better than eectricity and coa. Eectricity is particuary bad because of the arge heat osses that take pace at conventiona power stations. Tabe 11 CO 2 emission factors for heating fues Natura gas 0.22 Liquefied petroeum gas (LPG) 0.24 Heating oi (gas oi) 0.30 House coa 0.39 Eectricity 0.52 Emissions CO 2 / kg kwh 1 (Source: BRE, 2012) Using these figures and ikey heating system efficiencies we can cacuate the tota annua CO 2 production. As described at the beginning of Section 3 the therma efficiency of a heating system = usefu heat output / fue input. So annua fue use = usefu heat demand / therma efficiency annua CO 2 production = annua fue use CO 2 emission factor 61 of 87 Monday 18 June 2018

62 4 Saving eectricity Consider, for exampe, a house with a tota estimated net space heating demand of kwh yr 1 pus a water heating demand of 5000 kwh yr 1. What are its ikey annua CO 2 emissions if it is heated (a) by a condensing natura gas boier with a therma efficiency of 90% or (b) using off-peak storage heating with an assumed efficiency of 100%? (a) (b) Using a condensing gas boier: tota annua fue use = ( )/0.90 = kwh yr 1 CO 2 emissions = = 3667 kg yr 1 Using off-peak eectric storage heaters: tota annua fue use = ( ) / 1.0 = kwh yr 1 annua CO 2 emissions = = 7800 kg yr 1 Thus the eectric heating option produces we over twice the CO 2 emissions of the gas option. Activity 14 Supposing the house described above is in a rura ocation and doesn t have access to mains natura gas. What are the ikey annua CO 2 emissions if it is heated: (a) by a condensing iquefied petroeum gas (LPG) boier with a therma efficiency of 93%? (b) by a ground source heat pump with a system performance factor of 2.80? Which has the ower annua CO 2 emissions? Answer (a) Using a LPG condensing gas boier:tota annua fue use = ( ) / 0.93 = kwh yr -1 From Tabe 11, the emission factor for LPG is 0.24 kg CO 2 per kwh So annua CO 2 emissions = = 3871 kg yr -1 (b) Using a heat pump:the usefu heat produced by the heat pump = SPF eectricity used. So eectricity used = usefu heat / SPF = ( ) / 2.80 = 5357 kwh yr -1 Annua CO 2 emissions = = 2786 kg yr -1 The heat pump wins! 4 Saving eectricity Cutting eectrica demand in buidings requires tacking a whoe range of uses incuding the ifestye changes that increased eectricity use has brought to our society. Mains eectricity started as a ighting technoogy, but then rapidy diversified into motive power and then a host of eectrica appiances. Many abour-saving devices ike vacuum 62 of 87 Monday 18 June 2018

63 4 Saving eectricity ceaners and washing machines were introduced in the 1920s and 1930s to cope with a servant shortage after Word War I. Refrigerators and freezers have aowed shopping to be done once a week rather than daiy and aowed the storage of pre-packaged convenience foods. In parae there has been a growth of eectronic entertainment, for exampe radio, teevision, hi-fi, and the computers and internet for fun rather than work (see Energy Saving Trust, 2006, Rise of the Machines). It raises questions of what exacty is the modern good ife? Is it just one of owning ots of appiances to make our ife easier so that we can sump in a chair and be entertained by the atest eectronic gadgets (Figure 38)? Figure 38 A 1990s vision of the good ife Figure 39 shows the growth in UK ownership of some of these appiances since Given that the number of UK househods grew from 19 miion in 1970 to neary 27 miion in 2010, the average 2010 househod woud appear to own 1 home computer and 2.2 teevisions. 63 of 87 Monday 18 June 2018

64 4 Saving eectricity Figure 39 Estimated ownership of appiances in the UK (Source: DECC, 2011) Looking to the future, the UK popuation is ikey to increase from 62 miion in 2010 to around 67 miion by The average househod size is aso faing. In 2010 it was about 2.3 persons per househod, but this is ikey to fa to 2.1 persons by The tota number of househods coud thus rise to neary 32 miion by 2050 (Boardman et a., 2005). Into every new househod goes another set of appiances. In 2010 eectricity use for cooking, domestic appiances and ighting amounted to about a quarter of nationa eectricity demand. Figure 40 shows the trends in consumption since 1970 for different categories. Athough programmes of promoting energy efficient ighting and fridges appear to have had an effect since 2000 (and dramaticay so since 2007), the gains have been counterbaanced by the growth in consumption by a host of other eectronic devices: TVs, mobie phones, home computers, etc. Figure 40 UK estimated domestic eectricity use by cooking, ights and appiances Note: 1 TWh = 1000 miion kwh (DECC, 2014a) 64 of 87 Monday 18 June 2018

65 4 Saving eectricity 4.1 Eectrica appiances Standby power Perhaps the most disturbing waste of eectricity is that from miions of sma devices that are not fuy switched off but are eft on standby. The situation was summarised by a 2005 House of Lords Seect Committee report: The effect of standby consumption, at a nationa eve, is breathtaking: in the United Kingdom teevision sets aone consume some 90 miion kwh per month in standby mode. This is approximatey equivaent to the continuous output of a sma 120 MW power station. It transates into greenhouse gas emissions approaching tc/year [haf a miion tonnes of CO 2 ]. Moreover, these figures appy ony to teevisions, and fai to take account of a the other forms of equipment audio equipment, video or DVD payers, computers, photocopiers which revert to standby mode when not in use. The Government estimate that overa no ess than 760 miion kwh per month of eectricity are consumed by appiances not actuay in use the equivaent of 1 GW continuous output, or some 2.25 percent of tota United Kingdom eectricity consumption, producing of the order of 1.2 MtC per annum [4 miion tonnes CO 2 ]. (House of Lords Science & Technoogy Committee, 2005) The probem with devices such as teevisions is that they have been designed for convenience, with itte regard to energy use. To aow the TV to be turned on by a remote contro from an armchair, the power suppy is eft running permanenty, just to power a sma infrared sensor and a reay. The situation has not improved with the deveopment of digita teevision and set top boxes to interface with sateite dishes or optic fibre cabes. A major probem arises with mains adapters and battery chargers. Eectronic devices are sod wordwide. In order to simpify compiance with safety reguations about high votages in appiances, they are often designed to be run from ow-votage suppies. A separate adaptor is then suppied to provide the required votage (6V or 12V) from the oca mains suppy, 120 vots AC or 230 vots AC. Thus athough the actua eectronic device may be switched on and off, the mains adaptor or charger usuay remains pugged in and permanenty switched on. The resut is a continua dissipation of heat. The soution is better design and enforced internationa standards, such as the 2005 European Energy-using Products (EuPs) Directive. The reguation for standby power says quite simpy As a genera principe, the energy consumption of EuPs in stand-by or off-mode shoud be reduced to the minimum necessary for their proper functioning (CEC, 2005). A ong-term target for a maximum standby power is 1 watt. This Directive became aw in the UK in Teevisions and consumer eectronics In the 1980s a arge cathode ray tube (CRT) coour teevision using vaves woud have consumed about 500 watts. The transistor equivaent of the 1990s (with the same screen size) consumed ony about 100 watts. The more recent deveopment of fat-screen TVs has aowed arger screen sizes and pushed the power consumption back up. A arge pasma screen TV might consume 300 watts. More recent designs with LED backighting 65 of 87 Monday 18 June 2018

66 4 Saving eectricity are more energy efficient, but the savings coud easiy be negated if screen sizes continue to increase (Figure 41). Figure 41 A arge fat-screen TV may use more eectricity than its cathode ray tube predecessor by virtue of its sheer screen size These, and other eectronic devices, represent an increasing use of eectricity in the home. In the office, the increased use of eectronic equipment is a source of excess heat that may promote the instaation of air conditioning. Yet the eectronics industry is capabe of great feats of energy efficiency if pressed. The modern aptop computer is carefuy designed to eke out the maximum period of operation for the minimum battery capacity Refrigerators In domestic use a refrigerator usuay has a main cabinet with a temperature of around +5 C and may have an icebox (freezer compartment) maintained at 5 C. A freezer has a ower interna temperature of around 18 C. Combinations, known as fridge-freezers, have become increasingy common in recent years. 66 of 87 Monday 18 June 2018

67 4 Saving eectricity In 2013 domestic refrigeration in the UK used an estimated 13 TWh of eectricity, about 4% of tota nationa demand (DECC, 2014a). This figure is down from a peak of about 17 TWh around 1990 due to the improved energy efficiency of fridges on sae. However these figures have to be compared with the 3 TWh used for refrigeration by the food processing industry. This presumaby handes exacty the same food and stores it for onger periods, but it does so in physicay arger and more energy-efficient cod stores. Unti the 1980s there was itte interest in the UK in producing energy efficient fridges. It paid designers to maximise the interna storage space even if that meant cutting down on insuation thickness. Eectricity bis were a secondary consideration. Yet the potentia for saving was enormous. The average consumption for a European 200-itre arder fridge (i.e. one without a freezer compartment) in 1973 was 550 kwh yr 1. A Danish unit produced in 1988 showed that eectricity consumption coud be cut to 90 kwh y 1. Better insuation reduced the heat oss and an improved COP (coefficient of performance) was achieved by fitting a arger evaporator and condenser (Nørgård, 1989). In 1994 a European energy abeing scheme was set up in part to promote ow-energy fridges. The ratings of this scheme were not very ambitious and the best fridges ony used haf of the eectricity of the worst. It has proved easy to better the origina A rating so today (2015) many manufacturers offer A+ and even A++ fridges and freezers. Current A rated fridge designs use about 25 mm of insuation thickness, whie freezers use about 50 mm. Improved efficiency can aways be achieved with thicker insuation. However, the externa dimensions are fixed in practice by the standard units of width and height of fitted kitchens, so improved efficiency wi usuay mean ess interna storage space. The way forward may be through the use of innovative insuation, such as vacuum insuation panes. Vacuum insuated panes (VIP) Sir James Dewar, a scientist at Oxford University, invented a vacuum fask in 1892, as a container for ow-temperature iquefied gases. This consisted of two gass fasks one inside the other with a vacuum in between ensuring that no heat coud fow by conduction across the gap. The gass surfaces were aso given a ow-emissivity coating of siver to prevent heat radiating across it. This was turned into a commercia product, the Thermos fask, in 1904 by two German gass-bowers. It has been the standard receptace for carrying hot drinks to picnics ever since. Large fasks of up to 125 itres capacity were deveoped for cod food storage in the ate 1920s, but were overtaken by the rise of the modern refrigerator. The deveopment of fat vacuum pane insuation, ike that of vacuum doube-gazing, is fraught with many difficuties. The most basic is that of resisting the pressure of the air. A pane one square metre in size woud have to resist a force equivaent to about 10 tonnes. One approach is to take a pastic foam or an aeroge made from siica, wrap it in a thick airtight pastic cover and pump any air or gases out. As ong as the foam has sufficient strength not to coapse under the atmospheric pressure, therma conductivities of under W m 1 K 1 can be achieved at pressures of around 1 thousandth of an atmosphere. This represents a four-fod improvement over the best non-evacuated pastic foam insuation. There are sti many probems to be soved to maintain this performance over a ong period of time since any pinhoes in the outer ayer coud destroy the vacuum. Athough VIPs are now commerciay avaiabe, they are expensive and there are ikey to be trade-offs of production cost against ife expectancy. 67 of 87 Monday 18 June 2018

68 4 Saving eectricity If this can be deveoped and the manufacturers reuctance to trade storage voume for insuation thickness can be overcome, then the fridges of 2050 coud each use under 100 kwh yr 1. UK domestic tota eectricity use by cod appiances coud have faen by a factor of five from its peak 1990 eve to ony 3.5 TWh yr 1 (Boardman et a., 2005). Activity 15 Given that insuation is cheap, why might refrigerator manufacturers be reuctant to use thicker insuation in their designs? Answer Because the externa dimensions of fridges are imited by the standard heights and widths of fitted kitchens. Thicker insuation wi thus mean ess storage space inside Washing machines and wet appiances Washing machines and heated dryers, together with dishwashers, accounted for about 13 TWh of eectricity in 2013 (DECC 2014a). Few dramatic improvements seem to be expected. The energy use is mainy dependent on the amount of hot water used and need for tumbe drying. Most washers aready have an economy cyce, which uses ower temperatures and ess hot water but more agitation over a onger period. The use of high-speed spin dryers reduces the need for heat energy in tumbe drying. Like refrigerators these have an energy rating scae and the energy abes (Figure 42) aso specify the water consumption. 68 of 87 Monday 18 June 2018

69 4 Saving eectricity Figure 42 Energy abe for a washing machine Simiary, energy-efficiency improvement in dishwashers has concentrated on using ess water, since heating the water is where most of the energy is used. Athough ownership is expected to increase, UK domestic energy use in wet appiances may reduce by 20% by 2050 (Boardman et a., 2005) Cooking Energy use in domestic cooking has been steadiy decining since the 1970s, in part due to the increased use of microwave ovens. These are more efficient than conventiona ovens since the microwaves heat the water moecues within the food rather than merey raising the temperature of the outside. With them has come an increased use of preprepared meas. This raises the question of the tota energy used in producing these. A 2005 report estimated that the tota energy input for a chicken ready mea to the dinner pate was 9.7 kwh kg 1, incuding ony about 1 kwh for the fina reheating. The aternative of buying and cooking a whoe chicken woud have a higher overa energy input, of 87 Monday 18 June 2018

70 4 Saving eectricity kwh kg 1, of which over 6 kwh woud be the fina cooking in the home (AEA Technoogy, 2005). As with wet appiances, few breakthroughs in cooking technoogy seem ikey in the foreseeabe future. 4.2 Energy-efficient ighting In 2009 ighting in buidings accounted for about 17% of UK eectricity use. The domestic sector used approximatey 16 TWh and the services sector a further 39 TWh, of which over a third was used in retai premises. As shown earier in Figure 40, the promotion of ow energy ighting has produced a dramatic reductions in eectricity use. It is estimated that domestic ighting eectricity use fe by about a quarter between 2002 and 2013 (DECC, 2014a). This is equivaent to cosing a arge 500 megawatt power station. There is sti penty of potentia for further savings Lamp types Athough traditiona incandescent eectric ight bubs (aso known as Genera Lighting Service or GLS amps) have been sod by wattage as 60 watt or 100 watt amps, what actuay counts is the amount of ight they produce. This is measured in units of umens (m). For exampe a 100 watt GLS amp produces about 1200 umens. Modern amp packaging now ceary states the ight output aowing a cear comparison between the wide range of amp technoogies now on sae. Tabe 12 gives a brief summary of the types of amps currenty avaiabe. Tabe 12 Descriptions of amp types Lamp type Genera ighting service (GLS) Tungsten haogen Detais The common incandescent ight bub Miniature incandescent amps, often run on a 12 V suppy from a transformer Aso avaiabe for mains use and sod as repacements for norma incandescent ight bubs Fuorescent high-pressure mercury discharge amp (MBF) Meta haide high-pressure mercury discharge amp (MBIF) Compact fuorescent amp (CFL) Tri-phosphor tubuar fuorescent High-pressure sodium discharge amp (SON) Low-pressure sodium discharge amp (SOX) and higher-efficacy version (SOX-E) Light-emitting diode (LED) Buish white ight, often used for shop ighting Buish white ight, used for street ighting Repacement for norma incandescent ight bubs Standard type for office and shop ighting Orange-white ight, used for street ighting Pure orange ight, used for street ighting Avaiabe in white and a range of coours Repacement for norma incandescent ight bubs and beginning to be used for street ighting 70 of 87 Monday 18 June 2018

71 4 Saving eectricity An important factor in choosing a amp is its uminous efficacy, the amount of ight emitted in umens per watt of eectricity used (umens W -1 ). This figure is aso ikey to be found on modern amp packaging. Another important factor is the coour rendering index (CRI), which measures how we the ight from a amp imitates the broad spectrum of white ight from the sun. GLS and tungsten haogen amps have very good coour rendering whie that from orange sodium street amps is very poor. Figure 43 shows the efficacies of the different amp types isted in Tabe 12 and how, for a given type, the efficacy increases with power. Figure 43 The efficacies of various amps (adapted from Beggs, 2002) and manufacturers' iterature Very high efficacies are avaiabe for street ighting, but most domestic appications require amps of under 1000 umens output. It is obvious that there is a arge gap at the eft 71 of 87 Monday 18 June 2018

72 4 Saving eectricity side of Figure 43. Unti the appearance of CFLs there were no high-efficacy ow-powered amps, yet this is where most domestic ighting appications ie. For domestic use the standard ight bub (for over 70 years) has been the 800 umen 60 watt GLS incandescent amp. Its attractions are its famiiarity and its good coour rendering, yet it may ony have an efficacy of 13 umens W 1. Its rivas, the compact fuorescent amp (CFL) and new white LED amps are consideraby more efficient and have a onger ife expectancy but do not have the same quaity of coour rendering. LED amps give fu brightness when switched on unike CFLs which may take severa minutes to warm up. Athough the sae of conventiona incandescent amps is being phased out in Europe and many other countries as an energy efficiency measure, sma tungsten haogen incandescent amps remain on sae. These are ony 30% more efficient than GLS amps but do have a good coour rendering index LED deveopment The deveopment of LED ighting is progressing very rapidy. They were originay introduced as sma ow power indicator amps originay in red but progressing through orange and green to bue. The basic white amp is in fact a bue LED coated with a phosphor based on cerium and yttrium that converts some of the bue ight to a broad yeow spectrum. Since 2005 commercia amps have progressed from a buish white to a warm white and the efficacy has improved from 25 umens W 1 to 70 umens W 1 making them competitive with compact fuorescent amps. At the time of writing (ate 2015) LED amps with a good coour rendering index approaching that of incandescent amps are just becoming commerciay avaiabe. Their ife expectancy ( hours) is 20 times that of a GLS amp and 3 times that of a CFL. This has made them of interest for street ighting projects where the cost of physica repacement is high. Athough they are currenty mass-produced as sma ights of usuay of under 1 watt output, these can be buit up into arger modues. These have ony been avaiabe in ow powers but at the time of writing (2015) LED repacements for 100 watt GLS amps are becoming avaiabe. In the onger term these deveopments have the potentia to produce dramatic cuts in domestic ighting consumption. It has been suggested that by 2050 this coud have faen by a factor of four from current eves to around 4 TWh yr 1 (Boardman et a., 2005) Lighting and iuminance Efficient ighting isn t just about using high-efficacy amps. It is aso about propery iuminating the things that we want to see, such as a book. A typica incandescent ight bub emits ight in a directions. We tak of this fow of ight as the uminous fux emitted by the amp. When a amp is mounted in a ight fitting or uminaire, the fow of uminous fux wi be concentrated in one particuar direction (e.g. downwards in the case of a norma ceiing uminaire). A certain number of umens are therefore concentrated in a particuar direction. When we come to the practica aspects of seeing what we are doing we need to know about the amount of ight faing on a given surface. This is known as the iuminance and has the SI unit, the ux. One ux is defined as a uminous fux of 1 umen faing on an area of 1 m of 87 Monday 18 June 2018

73 4 Saving eectricity For exampe, the diners (circa 1910) in Figure 44(a) may be enjoying their eectric ight. They are aso being energy-efficient. Hanging the bub cose over the tabe may have been necessary, not just because of the ow efficacy of incandescent amps, but aso because they were probaby paying, in rea terms, about four times more than today s price for eectricity. Figure 44 (a) An eary advertisement for (rather expensive) eectric ighting (b) the inverse square aw of iumination Are they ikey to be getting an adequate iuminance on their tabe? The energy radiated by the amp spreads out over a arger area as the distance from the amp increases (Figure 44(b)). Suppose that the tabe, with an area A square metres, receives the fu beam when the amp is at a distance d from it. If the amp was at a distance 2d, the beam woud spread over four times the origina area. At this distance the tabe woud ony receive a quarter of the fu beam. In other words, doubing the distance has reduced the iuminance by a factor of four. This inverse square aw appies to any simpe ight source (except where the beam is focused by enses or mirrors). If we assume in Figure 44(b) that the amp is rated at 40 watts and has an efficacy of 5 m W 1 (a typica figure for 1910), then it wi be producing 200 umens. The uminaire (the ampshade) may be ony 75% efficient, so a tota of ony 150 umens wi be projected downwards. If we assume that haf of this, 75 umens, actuay shines on the tabe, and that this has an area of 1 m 2, then the iuminance is 75 m m 2 or ux. Artificia ighting schemes are usuay specified as being capabe of suppying a specified number of ux on a horizonta surface the working pane. Generay the more demanding the work, the higher the required eve of iuminance. Tabe 13 gives some recommended vaues. Tabe 13 Recommended iuminance eves Task Circuation areas (stairs, corridors) 150 Cassroom desk 300 Recommended iuminance /ux 73 of 87 Monday 18 June 2018

74 4 Saving eectricity Office desk, aboratory bench 500 Eectronics assemby 1000 Hospita operating theatre 2000 The danger of a ist ike this is that it can be taken as a specification for a whoe buiding and resut in permanenty overit spaces in shops and offices. This is one reason why the services sector uses so much eectricity for ighting. A more energy efficient approach is to empoy task ighting individua controabe ighting where it is actuay needed. LEDs have a significant advantage over other ight sources in that they are directiona. The ight ony emerges from one side of the semiconductor chips. This is one reason that they have been so successfu when used in torches and why they are now being used in street ighting. Athough white LEDs produce fewer umens per watt than a sodium street amp, they can direct that ight more efficienty to where it is actuay needed. Aso, the best use shoud be made of dayight. Sunight is extremey bright and its iuminance can reach ux on the ground. However as a ight source it suffers from rapid fuctuations caused by passing couds. Under overcast sky conditions norma dayight has an iuminance of at east 5000 ux and is more constant. Dayight penetrating windows into a buiding can provide iuminance eves of around ux in typica naturay it offices for most of the day. The difficuty is in persuading occupants to turn off the artificia ights. Activity 16 Cacuate the new iuminance eve (in ux) on the dinner tabe of Figure 44(a) in each of the foowing cases: (a) the amp power remains the same but its efficacy is doubed to 10 umens W 1. (b) the amp is unchanged but its distance from the tabe is haved. Answer (a) (b) If the efficacy of the amp is doubed, then it wi produce 400 umens instead of ony 200 and the iuminance on the tabe wi aso doube from 75 ux to 150 ux. If the distance between the amp and the tabe is haved, then the iuminance on the tabe wi increase by a factor of four, from 75 ux to 300 ux (i.e. adequate for a cassroom desk in Tabe 13. Activity 17 Why did some nineteenth century hospitas site their operating theatres on the top foor of the buiding? Answer Tabe 13 recommends that an iuminance of 2000 ux shoud be used in operating theatres. Before the advent of modern high-intensity eectric ight, the ony way to get this was to perform operations in bright dayight. 74 of 87 Monday 18 June 2018

75 5 Home energy assessment 5 Home energy assessment So far this course has described a wide range of different technoogies. But how do you assess what makes a reay energy efficient home without actuay reading the fue bis? Which retrofit measures are ikey to be most cost-effective or produce the argest reduction in CO 2 emissions? Fortunatey home energy rating computer modes can hep. The need for an energy mode to assess housing energy use was identified in the UK in the 1980s. The Buiding Research Estabishment deveoped a domestic energy mode (BREDEM) in the 1990s. This forms the core of the Standard Assessment Procedure (SAP). Since 2006 this has been the key cacuation too to show compiance with the Buiding Reguations for houses and fats. It has been through severa revisions and the current (2015) version is SAP 2012 (BRE, 2012). Athough the actua cacuation procedure ony covers about six pages, the need to dea with a possibe types of houses and fats and their heating systems requires a arge amount of expanatory tabes. Its use in setting the UK Buiding Reguations has been a itte controversia. From a point of view of setting enforceabe standards, it woud be simpest to say that the eements of buidings shoud use a given thickness of insuation or achieve a given U-vaue. This is known as the eementa approach. If the requirement is to set the overa fue cost or CO 2 emissions, then a more compicated cacuation is required to take into account the different sizes of buidings, the heating system efficiency, etc. Finay, there is the probem of what to do about the different cimates across the country. A house on a hitop in Cumbria is ikey to have a coder cimate than one on the coast of Cornwa. Shoud it be required to have better insuation standards? Athough the 2009 version of the SAP methodoogy did not distinguish between different cimate zones, the revised 2012 version has now taken this into consideration. Then there is the question of how much of house energy use shoud be reguated. The choice of a gas or eectric cooker obviousy infuences energy use and carbon dioxide emissions, but is a matter of choice for the occupant, not the buider. As a resut this is eft out of the SAP cacuations. The fu version of SAP is used in assessing new buidings, but for assessing existing homes a reduced version, know as RdSAP is used. This is used in home energy audits, particuary to produce Energy Performance Certificates (EPCs). These have been required for homes for sae in the UK since 2007 and newy rented properties since They can ony be issued by quaified energy auditors. In its origina form the SAP assessment was ony concerned with the cost of energy for space and water heating, ventiation and ighting. It is adjusted for foor area. It is expressed on a scae of 1 to 100, the higher the number the ower the running costs. For exampe a pre-1919 house with soid was, no oft insuation and no centra heating woud have a SAP rating of just over 20. This was about the average rating for the UK housing stock in 1973 (DECC, 2014a). A newer house meeting the 2006 Buiding Reguations for Engand and Waes woud have a SAP rating of about 85. Its fue costs per square metre of foor area woud be about a fifth of those for the oder uninsuated house. Since the rating is cost based, eectricay heated houses tend to have worse SAP ratings than gas-heated ones. The rating aso aows for houses to have their own generation technoogies such as soar water heaters, PV panes or micro-chp units. 75 of 87 Monday 18 June 2018

76 5 Home energy assessment However, the need to reduce overa CO 2 emissions has now become the main driving force of the Buiding Reguations. An Environmenta Impact Rating (EIR), based on CO 2 emissions per square metre of foor area, was added. This, ike SAP, is expressed on a scae of The higher the number the better the standard. In order to make things easier to understand for the consumer, the scaes have been simpified into A-G bands (as shown in Tabe 14). Tabe 14 Environmenta Impact Rating scae SAP or EIR rating Labe band 92 A B C D E F 1 20 G The sampe energy performance certificate shown beow in Figure 45 gives an Energy Efficiency (SAP) rating of 55 (a D rating) with a potentia to increase it to 85 (B rating). The house was aso given an Environmenta Impact Rating (EIR) of ony an F (not shown in the figure). This was mainy due to its high CO 2 emissions because it was totay eectricay heated. 76 of 87 Monday 18 June 2018

77 5 Home energy assessment Figure 45 A sampe Energy Performance Certificate It is estimated that the average SAP rating of the UK housing stock has improved from about 18 in 1970 to 57 in A thorough assessment of the potentia savings in a home is ikey to require a carefu energy audit. This video shows how one set of homeowners in Oxford made changes to the energy efficiency of their home. It aso shows that home energy efficiency is ony one factor in a wider appreciation of a ow carbon ifestye. Video content is not avaiabe in this format. 1 The eco-house video 77 of 87 Monday 18 June 2018

78 Concusion Concusion This free course has described a weath of opportunities for saving energy in domestic buidings. The buk of this potentia ies in heat energy savings through the arge-scae appication of very ordinary technoogies: thicker insuation, doube- or tripe- gazing and condensing gas boiers. To achieve arge CO 2 emissions cuts from the UK buiding stock we wi need further steps. These might incude digging up the streets to distribute waste heat from power stations to most city-centre buidings, or the depoyment of miions of heat pumps or domestic micro-chp units. The technica potentia for cutting eectricity use is enormous. The promotion of owenergy refrigerators and ighting has been reducing domestic energy demand since the 1990s. Yet it does seem an uphi strugge against a tide of new eectronic devices that are designed to be attractive and convenient rather than energy efficient. Most importanty, we ive in a cuture where the provision of energy services, i.e. buidings that are warm in winter and adequatey it, has been seen argey as a matter of suppying cheap gas and eectricity. The needs of facing up to cimate change and diminishing North Sea gas suppies wi require changing attitudes to energy saving. Further information Further information on ow energy housing and saving energy in the home can be found at the foowing web sites (a accessed 17 March 2015): Association of Environment Conscious Buiders (AECB) Buiding Research Estabishment (BRE) 78 of 87 Monday 18 June 2018

79 End-of-course quiz Centre for Aternative Technoogy Energy Saving Trust Nationa Energy Foundation Superhomes End-of-course quiz Finay check your understanding of the course materia with this quiz of tweve short questions: Question 1 What are the three main modes of heat oss from a house? Provide your answer... Answer As described in Section 2.1 they are: fabric heat osses those through the buiding fabric itsef, i.e. the was, roof, foor and windows ventiation osses due to air moving through the buiding fue heat osses since the heating system is not 100% efficient Question 2 What can be done to cut each of these three heat osses? Provide your answer... Answer As described in Section 2.1: Fabric osses can be cut by the use of insuation Ventiation osses can be cut by making the buiding more airtight (and possiby using mechanica ventiation with heat recovery) Fue osses can be cut by instaing a more efficient heating system Question 3 What are the three important mechanisms invoved in the transmission of heat, particuary in windows? Provide your answer of 87 Monday 18 June 2018

80 End-of-course quiz Answer As described in Section 2.2 they are: Conduction Convection Radiation Question 4 Why are ow-e coatings used in doube and tripe gazed windows? Provide your answer... Answer As described in Section they are used to reduce the heat radiated from an inner pane to an outer one. Question 5 Which buiding eement is ikey to have the best insuation performance one with a high U-vaue or one with a ow U-vaue? Provide your answer... Answer As described in Section 2.2.2, the ower the U-vaue, the better the insuation performance. Question 6 Which type of insuation is strong enough to be used for shuttering for making concrete buidings? Provide your answer... Answer As described in Section and shown in Figure 8, poystyrene insuation can be used for making concrete buidings. Question 7 Briefy describe two ways of internay insuating or dry ining a soid brick wa. Provide your answer of 87 Monday 18 June 2018

81 End-of-course quiz Answer As described in Section and shown in Figure 13: 1 Sheets of foam-backed pasterboard can be gued to the wa 2 Insuated battens can be screwed to the wa with a ayer of insuation between them and covered with a surface ayer of pasterboard. Question 8 Give three reasons why ventiation is needed in a buiding. Provide your answer... Answer As described in Section 2.3 ventiation is needed to provide combustion air in winter for boiers, fires and gas cookers (athough it is not necessary for heating systems with baanced fues or for eectric fires) to remove moisture from kitchens, toiets and bathrooms to provide fresh air for occupants and to keep them coo in summer. Question 9 Why is a condensing gas boier more efficient than a non-condensing one? Provide your answer... Answer As described in Section 3.2 a condensing gas boier recovers the atent heat of vaporization of the water vapour produced when natura gas burns. Question 10 Expain why the use of eectricity as a heating fue invoves far higher CO 2 emissions than using natura gas in a efficient boier. Provide your answer... Answer As expained in Section 3.5, and shown in Tabe 11, the emission factor for eectricity is particuary high because of the arge heat osses that take pace at conventiona power stations. 81 of 87 Monday 18 June 2018

82 Keep on earning Question 11 Light emitting diode (LED) amps are increasingy on sae in UK shops for domestic use. What other ow energy ighting technoogy are they competing with? Provide your answer... Answer As described in Section compact fuorescent amps (CFLs) are aso sod for domestic use. Tungsten haogen amps are aso avaiabe but these are ony sighty more efficient than conventiona incandescent amps. Question 12 What two ratings appear on a home Energy Performance Certificate? Provide your answer... Answer As described in Section 5: 1 An Energy Efficiency or SAP rating concerned with energy costs 2 An Environment Impact Rating (EIR) concerned with CO 2 emissions Keep on earning 82 of 87 Monday 18 June 2018