NEED FOR A CORRECT GLASS DESCRIPTIVE CODE: ENERGY-CODING

Transcription

1 NEED FOR A CORRECT GLASS DESCRIPTIVE CODE: ENERGY-CODING Carl Axel Lorentzen BSc (Eng) Technical Advisory Service, Pilkington Danmark A/S, Oslo Plads 14, 2100 Copenhagen, Denmark Phone , Fax , cal@pilkington.dk Abstract - Glazing for daylight in windows are today not only two pieces of glass, but gives many different functions depending of the application. The performance figures of an double-glazing unit, depend on the properties of the two glasses, the spacer width and the filling of the enclosed space. In the past it was nearly only a matter of defining a U-value, to take care of heat-loss. Today, the description of a glass unit with different glasses are just as important and should also give the performance data for light transmittance: τv and total solar energy transmittance (solar factor): g. Glass Performance should always be given by: U value (W/m²K) / τv % (light transmittance) / g % (solar factor) 1. INTRODUCTION In the current debate on energy-saving concepts, glass facade designs have become a prominent theme. The optimisation of facade design is one of the reasons to investigate new energy-saving ideas. Central to this planning is the overall optimisation of a buildings system and their interactions. 2. ENVIRONMENTAL CONTROL GLASS 2.1 Re-establishing the concept of the window. Float glass was invented by Pilkington in the 1950s and now over 90% of the worlds window glass is made by this process. It is available in thickness ranging from 0,5 to 25mm. In the first two post-war decades, the inappropriate use of large areas of clear single glazing, particularly in highrise commercial buildings, led to buildings which suffered from overheating, excessive glare and high fuel consumption. A reaction to this in the 1970s and 1980s was the deep-plan, fully air-conditioned and artificially-lit office building. However, such intensively serviced buildings generally had even higher energy consumption, and the artificial and uniform nature of their interiors was not always appreciated by occupants, even sometimes being linked with the Sick Building Syndrome. In the 1990s, with a new generation of environmental control glasses that transmit light in varying degrees and an increased understanding of window management techniques as: *External and internal light shelves *Brise-soleils *Canopies *Photo-electric controls the window can be restored to its traditional role of permitting daylight and view, without letting occupations suffer from excessive solar gain and glare. Furthermore, buildings with windows designed to admit, yet control, natural light and ventilation can often eliminate the need for air-conditioning. Energy-economy and energyefficiency, a variety of design options, and a more pleasing internal environment are the true real-life benefits of a naturally-lit building. Light shelves are being used more and more in both external and internal applications to help increase the amount of light penetrating buildings and thus reduce lighting energy costs. 2.2 Glass and how to use it. Glass is both functional and appealing, it keeps the weather out and the warmth in, it saves energy, reduces heating bills, provides comfort and safety and gives light and space. It also does a whole lot more... Clear Glass Clear floatglass Extra clear Glass Optiwhite Thermal Insulation Hardcoated LowE Glass Softcoated LowE Glass Solar Control Class -Body tinted glass -Reflective coatings -Reflective coatings on a body tinted single glass -High performance coatings (incl LowE) Insulating Units -Incorporation of blinds and louvres internall, externally or between the panes Tabel 1: Glass types Glass introduce an innovative new method to classify, or group, glass products by: PRIMARY FUNCTION: *Thermal Insulation * Solar Control *Noise Control * Fire Resistance * Safety and Security * Privacy and Decoration * Mechanical Strength

2 2.3 Thermal Insulation Heat naturally takes the path of least resistance which, in the case of the fabric of a building, is typically the window glass or, correctly speaking, poor or inadequate glazing - such as the single pane glazing still commonly found in many buildings. Insulating Units, Low Emissivity (LowE) and Coated Glasses is a range of products which make it more difficult for heat to escape from buildings, offering designers a choice of insulation levels and aesthetic options. 2.4 Solar Control Buildings need to provide a comfortable environment for the occupants, keeping them cool in summer, warm in winter, whilst at the same time making the building an 'efficient performer' in the use of energy for heating, cooling and lighting. Today glass offers a wide range of performance options which enable the designer to balance these often conflicting needs, whilst the wide range of colours and glass types - body tintedreflective, neutral and (with High Perfomance glass: again) clear - ensure that the aesthetics of building need not be compromised, especially when used with contrasting or harmonising spandrel panels. 3. THERMAL INSULATION With increasing awareness of ecological problems and the condition of the environment, the reduction of energy consumption in all aspects of modern life is an important consideration. Glass has now been developed with coatings and products which reduce the heat lost through windows. Products which mean design and aesthetics do not have to be compromised when energy efficient buildings are demanded. 3.1 Mechanisms of heat loss through glazing. There are three stages of heat loss through glass products: *Heat loss to the internal glass surface from the room surfaces *Heat loss through the glass product *Heat loss from the outermost glass surface Heat loss to the internal surface, from the room, whenever the glass surface is at a lower temperature than the internal air temperature and the room surface temperature. Heat is lost in two ways: *by exchange of long wavelength radiation between the glass surface and room surfaces *by conduction and convection from room air moving over the glass surface Heat loss through the glass product. The provision of an airspace provides a number of opportunities for increasing the thermal resistance of the glazing: *Increase the width of the airspace *Incorporate low emissivity coatings *Use gas with lower thermal conductivity *Inhibit convection in the airspace *Evacuate the airspace Incorporation of low emissivity coating on the glass surface provides the possibility of reducing the long wave radiation exchange between panes. The hard coated low emissivity products, enables their incorporation into secondary frames applied to existing windows. The softer low emissivity coated products are restricted to use in insulating units. Gases with lower thermal conductivity, such as argon and krypton, provide further improvements in U value Heat loss from the outermost glass surface, as the inner surface. The thermal transmittance of the complete window is made up of three components: *Glass product (discussed previously) *The Spacer in the insulating units *The frame The spacer effect can add as much as 10% to the U- values of the window The thermal insulation of the frame is important, since the overall percentage area of the framing can be over 50% of the window opening. 3.2 Thermal transmittance: U-value. Heat loss is quantified by the thermal transmittance or U value. For the purpose of calculating U values, standardised conditions are used. These conditions may vary, depending on which standard is referred to. The U value, usually expressed in S.I units of W/m²K. Glazing Unit Energy-Coding Glass and airspace (mm) U value (W/m²K) / tv % (light transmittance) / g % (solar factor) Clear Floatglass air air Optiwhite + LowE-glass (type 1) 4-15-*4 (1) argon 4-15-*4 (3) argon Clear Float + *LowE-glass (Type 1,2 or 3) 4-sasch-*4 (1) air sasch-*4(1) air 4-15-*4 (2)argon 4-15-*4 (3)argon *4 (3)argon 4* *4 (3)krypton 2,9/80/76 1,9/74/68 1,5/77/78 1,1/76/64 1,8/75/71 1,3/69/63 1,3/77/66 1,1/75/59 0,9/68/52 0,4/62/40 Tabel 2: LowE-glass with emissivity below 0,2: (1) type: hard-coated(e=0,16). (2) type: soft-coated (e=0,1) (3) type: soft-coated (e=0,04) Calculated after DS/EN673 and DS/EN 410

3 3.3 Effective U value. Glass, which is transparent to solar radiation, also allows solar heat energy to enter a building. The U-value conventionally used to calculate heat loss ignores this fact and consequently can result in an inaccurate or misleading measure of overall performance, particularly where windows are concerned. However, the philosophy of combining heat losses with solar gains through the modification of the conventional U-value is defined by the 'Effective U value' (or alternatively, 'Energy Balance Value'). The proportion of solar heat transmitted by building materials which can contribute to heating needs - useful solar heat gain - is subtracted from the conventional U-value. 4. SOLAR CONTROL GLASS Various techniques are available to control the amount of solar heat gain coming through windows, including the use of external and internal shading (either fixed or adjustable), and solar control glasses. 4.1 Basic Principles Glass transmits solar radiation from the sun by three mechanisms: reflection, transmission and absorption, which for solar control purpose are defined in terms of the following parameters: Reflectance, Absorptance, Direct Transmittance and Total Transmittance. All the solar radiant heat properties are angle dependent. Solar control glass can be achieved by use of: *Body tinted glass *Reflective coated *Combinations of body tinted and reflective coatings in a single glass *Special high performance Insulating Units - HP-Glass *Incorporation of blinds and louvres internall, externally or between the panes 4.2 Daylight in Solar Control glass Reduction in total solar transmittance will usually decrease the transmission of the visible part of the solar spectrum. However, some body tints and coatings are able to preferentially attenuate the non-visible solar radiation leaving the transmission of the greater proportion of the visible radiation largely unchanged. This performances are preferred for passive solar applications. 4.3 High performance The ultimate to date in environmental glass, is that it has both solar control and low-emissivity properties. This glass offers high light and low heat transmissions, and is also capable of the highest levels of thermal insulation according to need and how it is used. Roof glazing for shopping centres need a high performance glazing system: to get solar light in, to avoid excessive solar heat gain, but also to minimise heat loss in winter. The total benefit is: a comfortable environment with a minimum of heat energy consumption. Glazing Unit Enegy-Coding Glass and airspace (mm) U value (W/m²K) / tv % (light transmittance) / g % (solar factor) Clear Floatglass air air 2,8/80/76 1,9/72/67 Solar Control Glass Body tinted + *LowE-glass (3) 6 grey-15-*6 argon 1,1/35/33 6 green-15-*6 argon 1,1/61/35 Solar Control Glass Reflective coated +*LowE-glass (3) 6 silver-15-*6 argon 1,1/17/15 6 blue-15-*6 argon 1,1/25/20 6 grey-15-*6 argon 1,1/26/24 Solar Control Glass HP (coated incl. LowE) + Clear Float (or *LowE-glass (3)) 4 clear-15-4 argon 1,1/65/44 6 neutral-15-6 argon 1,3/54/44 6 silver-15-6 argon 1,1/41/27 6brillant-15-6 argon 1,1/66/34 6 brillant-15-*6 argon 1,0/60/29 Tabel 3: LowE-glass (3) type: soft-coated (e=0,04) Calculated after DS/EN673 and DS/EN Glazing with Blinds The decision to install blinds and the type selected may be influenced by considerations other than solar control (e.g. to provide privacy, or prevent glare). The use of blinds in windows will affect the shading coefficient of the window. It will depend upon: - The solar properties of the glass and the blind material - Heat transfer coefficients at the window surfaces - Geometry of the blind - Angular position of the sun The important solar optical properties of the blind material are the reflectance, absorptance and transmittance of the material to solar radiation. Different coloured blind materials have varying solar performances. White blinds tend to give high performance, and dark colours low. In practice, louvres are often adjusted to avoid direct sun penetration. A typical arrangement, especially with horizontal louvres, is to set them at 45º to the plane of the window. 4.5 Thermal Safety One effect of high performance blinds is to reflect a large proportion of the transmitted energy for a second pass

4 through the glazing so that the energy absorbed in the glass is increased and the temperature of the glass rises. This increased temperature can increase the risk of thermal fracture in the glass. The risk of fracture due to thermal stress can be overcome by toughening the glass. 4.6 Colour The use of colour is another key component in the language of architecture. The various glasses exhibit different colour appearances from outside the building. High reflectance glasses which are usually coated produce stronger effects than body-tinted glasses. However, the amount of light transmitted from the building interior, through the glass, affects perception: high relative lighting levels internally will diminish the reflected colour and increase transparency. The strong external appearance of solar control glasses can be matched, or complemented by a range of glass spandrel panels. 5. DESIGN The design and specification of windows and glazing demand that several (often conflicting) requirements need to be met, and it is impossible to consider any one in isolation. However, when considering the use of solar control glass, it is convenient to consider two areas of application, since hot climates and temperate climates have different requirements for glass performance. 5.1 Hot Climates The main requirements for the use of glass in hot climates is to provide high levels of solar control to minimise solar heat gains and air conditioning load, and to avoid overheating. Related to this are the needs to insulate the building from heat conducted from the outside and to provide some control of glare arising from reflection from the ground, surrounding buildings and bright areas of sky (excluding direct sunlight), as well as considering the traditional functions of natural lighting and view out. To meet some or all of these needs, the solar transmission of glasses used in hot climates will be wide ranging with, in some cases, light and solar heat transmissions less than 10%. For improved solar control a low emissivity glass, glazed as inner pane of an insulating unit, acts as a second line of defence. Heat absorbed by the solar control glass is reflected back by the low emissivity coating, to provide better solar control. In hot climates the improved U-value produced by the use of a hard coated low emissivity glass also reduces the conduction gains across the glazing - an important factor in extensively glazed air conditioned buildings. In hot humid climates condensation can form on the outside of windows in air conditioned buildings. The higher insulation of insulating units incorporating hard coated low-e glass, means that in air conditioned buildings, the outer glass is warmer and the incidence of condensation is reduced. 5.2 Temperate Climates Glass performance in temperate climates has to balance the need to provide solar control and reduce summertime overheating against the need to provide high levels of natural illumination and the benefits of passive solar heating. The required total solar transmission and light transmission will not be as low as those demanded in hot climates. To allow for passive solar design, the performance range could be: Total transmission 20% to 70% Light transmission 35% to 80% U value 1,0 to 2,0 W/m²K These performance parameters for glass need relating to the specific application, since there is no one ideal glazing solution for all applications. However, as a general principle, high thermal insulation with solar control is a requirement for temperate climates, and since some solar control coatings exhibit low emissivity, it is possible to combine these functions in the same glazing solution. 6. SOLAR GAIN AND COMFORT In addition to the general 'building performance' needs outlined above, it is necessary to consider how a solar control glass interacts with the design of air conditioning to provide comfortable working conditions. 6.1 Solar Gains Solar radiation through windows causes the air temperature in a room to rise, and it is the task of the designer to ensure that this temperature does not cause discomfort by specifying comfortable design conditions, and by providing appropriate plant and equipment to meet them. To this end, the designer will have to undertake calculations to assess the effect of various glazing options on the solar heat gain that his ventilation or air conditioning equipment need to cope with, and choose the best solution for the specific application in question. Solar radiation is not the only source of heat which contributes to the 'total heat' within a building. Other sources include: - Conduction gains and losses through glazing - Ventilation by incoming warm air - Internal sources of heat (by lighting, occupants and electrical equipment). Most calculations of this nature are now commonly carried out by computer, using approved industry standard methods of calculation.

5 6.2 Direct Radiation and Comfort Whilst air conditioning can provide comfortable conditions for the building and occupants as a whole, the effect of solar radiation falling directly on people situated close to the glazing needs to be treated separately. An occupant receiving direct solar radiation can feel uncomfortably hot even when room temperatures are being maintained at a comfortable level by means of air conditioning or mechanical ventilation. As a general guide, highly reflective glasses with relatively low direct solar transmittance will be most effective at combating the localised overheating of occupants situated near windows. For example, a person sitting near a window glazed in clear glass on a summer's day would experience a dry resultant temperature in excess of 30ºC when the window was receiving the maximum solar radiation of about 750 W/m², even though the internal air temperature is being maintained at 21ºC. To maintain comfort, the dry resultant temperature should remain below 26ºC. The use of a medium performance body tinted solar control glass would reduce this to about 28ºC, which would still be too warm. A high performance glass highly reflective coated glasses however, would provide a dry resultant temperature of the order of 24-25ºC, which would be deemed comfortable. Solar gains into a building can be determined from a knowledge of the following: The position of the sun in relation to each elevation of the building Levels of solar radiation are dependent upon whether or not the sun is relatively high in the sky (altitude) and to the North, South, East or West (azimuth) The intensity of the solar radiation incident upon the faces of the building Other sources: ventilation by incoming warm air and internal sources of heat The surface areas exposed to the sun A large glazed area will potentially allow more solar gains to enter a building than smaller areas of glazing The date and time of day This is related to the relative movements of the sun and earth Shading effects Presence of blinds, overhangs, nearby buildings etc., may prevent solar radiation entering a building Type of glass Different glasses will transmit, reflect and absorb different proportions of the sun's energy Structure of the building A building constructed of heavyweight materials will heat up - and cool down - more slowly than one made with lightweight materials 7. CONCLUSION Need for a Descriptive Code. Glazing for daylight in windows are today not only two pieces of glass, but gives many different functions depending of the application. The performance figures of an double-glazing unit, depend on the properties of the two glasses, the spacer width and the filling of the enclosed space. In the past it was nearly only a matter of defining a U- value, to take care of heat-loss. Today, the description of a glass unit with different glasses are just as important and should also give the performance data for solar light transmittance and solar radiant heat: Total Solar Transmittance. Glass Performance should be given by: U value (W/m²K) tv % (light transmittance) g % (solar factor) Total Solar Transmittance - and Fire Resistance Sound Insulation (Rw in db) Safety (Toughened or Laminated) Privacy and Decoration Colour (appearance) REFERENCES Ruders og vinduers energimæssige egenskaber. Kompendier 1-5. IBE/DTU, Copenhagen 2000 (See more: Energimærkning. Tekniske Bestemmelser for ruder. April Teknologisk Institut, Århus BPS publikation 131. Fem glasfacader Carl Axel Lorentzen. SBI Direction 192: Glass in Building. SBI, denmark Button, D. and R. Dunning. July FENESTRATION An investigation into long term requirements for fenestration. Pilkington Glass Limited and the Department of Energy (UK).