The first line of defence: Passive design at an urban scale

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1 Proceedings of Conference: Air Conditioning and the Low Carbon Cooling Challenge, Cumberland Lodge, Windsor, UK, July London: Network for Comfort and Energy Use in Buildings, The first line of defence: Passive design at an urban scale Becci Taylor MEng MA(Cantab) CEng MIMechE & Philip Guthrie MEnvSci Arup Abstract Low carbon building design starts with appropriate passive design features of buildings such as orientation, form and envelope. For the optimum benefit from such features, they must be considered from the inception of the project and influence the architectural response. However, in an urban setting, at this stage many opportunities have already been missed. The true, and often overlooked first line of defence for reduction of building cooling loads is at the masterplanning stage of developments. Poorly designed cities can exacerbate the urban heat island effect which results in higher temperatures, higher heat gain to buildings and increased cooling loads. The performance of heat rejection equipment is reduced by higher external temperatures that will tend to increase the carbon intensity of the cooling process. These passive design measures also serve to make the external urban spaces more habitable, usable and appealing. This paper presents practical case studies of masterplanning projects which demonstrate these concepts. It discusses the methodology for the development of design principles for urban masterplans such as environmental design relating to air movement and radiant fields. Such design measures at an urban scale can act to reduce urban heat island effects in order to reduce heat gain to buildings and to minimise energy required for cooling. Decisions which can be made at masterplanning stage such as centralised heat rejection and transportation strategies have an impact on the urban heat island and sustainability of cities. The paper explores feedback loops within the urban environment relating to such effects. This paper is a high level review of the current research and technical understanding of urban design issues which impact the energy consumption of cities, focusing on those most relevant to hot climates. It is intended to stimulate discussion and encourage the pursuit of further research. Keywords: Masterplanning, Urban Heat Island, Microclimate, Passive Design. 1. Introduction The hierarchy of low carbon building design is frequently summarised as: 1. Passive design of building envelope and form 2. Efficient system design 3. Renewable power This paper would like to propose as an additional tier, the passive design of the city, or of a group of buildings, as the first line of defence in this hierarchy. Additionally, the use of district systems for heat rejection can bring benefit.

2 Figure 1: Revised hierarchy of low carbon building design. In many modern developments climatic issues are of far lower priority than was necessarily the case before air conditioning. Indeed, often western (temperate) planning attitudes or building styles have been applied to developments in much hotter climates. This impacts both the energy consumption of buildings and the external spaces around them. In addition, measures to reduce the urban heat island may often be missed, augmenting the effects of hot climates. Recently urban and building design teams have been increasingly considering microclimatic issues in developments. This work includes projects from the very large masterplan to individual external spaces such as courtyards. Such work can be generally grouped into two main areas: those in hot climates where it is desired to reduce the impact of the extreme climate; and those in temperate climates where the amenity of outdoor public realm can be improved by maximising the use of sunny and wind sheltered environments. Section 3 of this paper is a high level review of the current research and technical understanding of urban design issues which impact the energy consumption of cities, focusing on those most relevant to hot climates. It is written from a practical perspective with design case studies, and is intended to stimulate discussion and feedback regarding the various issues, many of which would benefit from additional analytical study and empirical testing. The reduction of the urban heat island, urban scale considerations of building design and site wide strategies for transport and systems, can lead to advantages including reduced cooling loads, improved daylighting and greater potential for natural ventilation. Section 4 reviews the additional advantages due to improvements in external comfort conditions, leading to increased usability of outdoor spaces. 2. Background 2.1. Urban Heat Island The urban space bounded by buildings is called the urban canopy layer. The air in the canopy layer is usually warmer than that of the surrounding countryside, this is called the urban heat island effect. Santamouris (2001) states that the urban canopy layer includes an unlimited number of microclimates generated by different configurations of urban spaces or forms.

3 Within these configurations are varied micro conditions (such as vegetation, albedo or surface materials), which drive and create the microclimates found within a city. The urban heat island effect is a well documented phenomenon, first noted by Howard (1833), but there are different theories as to the primary forces driving this effect. There have been a number of research programmes to measure the urban heat island, comparing city temperatures to rural equivalents. A GLA (2006) study measured a temperature difference of 8-9K in London compared to a rural reference on a number of occasions in the very warm summer of A mean heat island intensity of magnitude 5K was noted in Gotenberg, Sweden, ranging from 3.5K in the winter to 6K in the summer (Eliasson 1996). Greater Urban-Rural temperature differences were observed in some Indian cities, Pune and Bombay recorded up to 10K (Padmanabhamurty 1990/91). Santamouris et al (2001) noted an urban heat island intensity exceeding 10ºC in Athens. The main factors contributing to the urban heat island include the following: Albedo Albedo is the ratio of electromagnetic radiation (EM radiation) reflected from a surface to the amount incident upon it. At a given solar input, albedo will regulate the short-wave absorption of a surface, this reflectance property is governed by several factors, of which colour or spectral reflectance is one. Other factors determining albedo are roughness and purity of the surface (Berdahl and Bretz 1997). Taha et al. (1992) demonstrate differences in surface temperatures between different materials and coloured cities, making hypothetical approximations of a white coloured city comparing it to an average city (Figure 2). Figure 2: Albedo and surface temperature for different surface materials and colours. (Taha et al. 1992)

4 Figure 2 demonstrates a general trend for lower the albedo to result in lower surface temperatures. Vegetated surfaces are the exception, absorbing solar radiation without an increase in surface temperature Long-wave Radiation Incoming solar heat flux is balanced by an outgoing long-wave sensible heat flux and a latent heat flux. The sensible heat flux is calculated from the Stefan-Boltzmann law. The latent heat flux is larger the more moisture in the surface, often parameterised by the Bowen ratio (Oke 1987). Both the reflection of short-wave radiation and the emission of long-wave radiation have the potential to interact with and be absorbed by another surface, increasing its sensible heat (Figure 3). An increase in the sensible heat of a surface impacts its own radiation emission. Figure 3: Radiation reflected and emitted from: a) a surface in an open environment b) surfaces within a canyon. The increase in both the number of emitting surfaces and the amount of radiation emitted by each within a confined space alters the net radiation of the whole urban environment tending to make the radiation field higher than that of a rural environment Airflow The surface roughness of the city has implications on airflows across the city and within the microclimates. When an air mass encounters a building it is forced around it producing a series of eddy flows, due to unsteady separation of the flow from the buildings.this forcing and deflection of airflows can result in a restriction of air movement at low level. Spaces between building rows are often classified in terms of height to width ratio (H/W), Oke (1981) demonstrates a ratio of less than 0.3 creates flows similar to that of a single building. With ratios of the flows of the individual building begin to interfere with one another (Oke 1988), models by Hunter et. al. (1990/91) demonstrate how air flows move from isolated roughness to wake interference flow (Figure 9), canyons with a greater H/W

5 ratio display skimming flow and the formation of a stable lee vortex within the canyon itself, this was first noted by Albrecht (1933) and has been confirmed numerous times since. Georgii (1970) found that at wind speeds of less than 2 m/s, fresh air entering the space between buildings from above did not reach the street level, a vortex forms above 2 m/s, but only at speeds of 5 m/s or above does complete ventilation occur. This can lead to the entrainment of pollutants within the urban environment which has additional impacts on the urban heat island (see section 1.2.4). The higher building density of urban environments creates different air flow characteristics to rural areas. Wide streets: H/W < approx 0.35 Wake interference flow: 0.70 < H/W < approx 0.35 Narrow streets skimming flow: H/W > 0.70 Figure 4: Interference of buildings and canyons on urban air flow (adapted from Santamouris 2001) Air Pollution Pollution has an impact on solar radiation. There are two mechanisms by which it might cause solar attenuation in the atmosphere. Some gases, such as H 2 O, CO 2, CH 4, N 2 O and O 3, exhibit strong infra-red radiation absorption properties, (Tsangrassoulis 2001) heating the atmosphere this is often termed the greenhouse effect (Figure 5). Increased release of such pollutants within the urban environment can create a local scale greenhouse by trapping radiation at low level.

6 Long-wave radiation Figure 5: Radiation attenuation by pollutants through absorption The other mechanism is the scattering of radiation. Pollutant aerosol particles are the most effective at diffracting short-wave radiation (Ten Brink et. al. 1997). This process deflects light away from parallel to incident radiation and no light is lost (Figure 6). This process (in most cases) has no direct influence on the atmospheric temperature, however it has an impact on the surface energy balance. An exception to this is dark aerosols which absorb radiation and therefore affect local air temperatures. Diffuse scatter Figure 6: Light attenuation by pollutants through reflection and scattering Evaporation Surfaces of the urban environment are usually designed to quickly remove standing water and to prevent water infiltrating into them. As a result of this surface alteration, when rain ceases, only a thin film of water remains to be evaporated (Landsberg 1970). The evaporation of water enables heat, which ordinarily would become sensible heat of the surface, to be removed and used in the latent heat transfer of the water. Soil and vegetated areas have a large water storage capacity, which can gradually evaporate and therefore remove substantial amounts of heat. In addition the ability of plants to transpire, allowing heat loss through water

7 evaporation from vegetation surfaces, means that unlike tarmaced surfaces, vegetated surfaces can be at a lower temperature than the surrounding air. Cities tend to have less soil and vegetation than rural areas and therefore less evaporation. The benefit of evaporative cooling to lower surface temperatures are therefore reduced and as a result forms a contribution to the urban heat island Anthropogenic heat Fossil fuel combustion and other processes from transport, buildings, or industrial processes emit not only pollution but heat into the surroundings as a waste product. This additional heat can have an impact on the local surrounding microclimates. These processes are more prevalent and more densely situated in urban environments Thermal Comfort Overview In the majority of cases, energy consuming systems are used within buildings to provide comfortable conditions for working and living. In certain specialised applications, such as museums or data centres, closely controlled climate conditioning is used to provide conditions necessary for technology or material conservation. This is an important distinction as improvements in thermal comfort are often achievable using lower energy solutions and through different means than tight climate control. The expectation of internal conditions in hot climates is often cooler than that needed for thermal comfort, leading to excessively energy intensive systems. It is not generally necessary to condition offices and residential accommodation to 21 C when it is 45 C outside. There is an opportunity to reduce energy consumption by challenging this norm. Increased temperature set points lead to lower fabric gains, lower infiltration gains and lower fresh air cooling loads. Improvements in external thermal comfort are important to encourage changes in behaviour and reduce energy consumption, particularly relating to transportation Definition Comfort is a complicated and subjective parameter to quantify. Authors such as Fanger (1970) have produced extensive and comprehensive studies in indoor climates but issues are more complex in transitional, naturally ventilated (free running) and outdoor spaces. Comfort within these environments involves different conditions and issues not encountered in studies performed on indoor comfort (Givoni et al. 2003). The perceptive and physical elements of comfort can be defined separately. Thermal sensation is an objective response to an environment as a function of environmental variables; thermal comfort is the subjective response that includes psychological factors in addition to the thermal sensation. Thermal comfort is harder to predict as it varies from person to person. Predictions of thermal sensation are usually based around the following environmental variables (shown diagrammatically in figure 7):

8 - Short wave radiant field (solar) - Long wave radiant field - Air velocity - Dry bulb temperature - Humidity And the following person related variables: - Clothing factors - Metabolism (activity) Figure 7: The physical factors affecting thermal sensation For steady and uniform conditions, thermal sensation is linearly correlated with thermal comfort (Zhang & Zhao, 2008). Dynamic effects and non-uniform conditions have been found to cause the two to differ such phenomena are likely to occur in external settings. In addition, the following perceptive variables may affect thermal comfort: - Freedom of choice - Regional expectation - Seasonal expectation - Recent history - Activity - Quality / type of environment This breakdown between physical and psychological elements is shown in figure 8.

9 Figure 8: Additive factors of thermal comfort Physiological equivalent temperature Thermal sensation variables form inputs to the heat balance model of the human body, this is the basis of the physiological equivalent temperature (PET) (Hoppe, 1999). PET is defined as the air temperature at which, in a typical indoor setting (without wind and solar radiation), the heat budget of the human body is balanced with the same core and skin temperature as under the complex outdoor conditions to be assessed. This is calculated using the Munich Energybalance Model for Individuals (MEMI) (Hoppe, 1994). PET uses degrees centigrade to provide a comprehendible dimension to the scale, but it is still difficult to interpret the meaning this has for comfort of people in external environments. Referencing indoor environments and their specific comfort models excludes the psychological difference due to being outdoors. Wider ranging conditions tend to be tolerated outdoors, and indeed, field surveys show that PET may overestimate external thermal discomfort (Ali-Toudert & Mayer 2006). 3. Urban scale design to reduce building energy consumption 3.1. Overview In hot climates, the overall reduction of the urban heat island serves to reduce building energy consumption by mitigating a temperature build up in the urban landscape, leading to lower fabric gains to buildings, lower fresh air cooling loads and improved efficiency of heat rejection. Urban design strategies can improve the viability of natural ventilation, reducing cooling and ventilation system energy consumption. Solar gains can be reduced while providing increased opportunity for daylighting. The importance of the urban scale factors which impact energy consumption is increased by their positive reinforcement of each other. For example, improving external thermal comfort can lead to reduced car use, which reduces anthropogenic heat production, which in turn improves external comfort. Centralising heat rejection can reduce the ambient temperature, which will result in reduced cooling loads and heat rejection.

10 Figure 9: Positive feedback mechanisms in urban design for reduced energy consumption 3.2. Materials Material selection has a significant effect on the urban environment, influencing surface energy balances and visual fields. The mass of the materials provide a dynamic element to the urban environment, creating a heat store and leading to elevated night time temperatures. In very hot climates this can have a detrimental impact on evening comfort (and may be of benefit in cooler climates). This thermal storage effect is a contributor to the urban heat island. Lower surface temperatures reduce the temperatures in the radiant field, significantly improving comfort. They also can contribute to a reduction in the temperature of the ambient air as the convection from a given surface is lower. Associated temperature reductions can have significant impacts on the conduction gains to building fabric and ventilation loads. High albedo materials reduce the amount of solar radiation absorbed through building envelopes and into surfaces such as paving and keep surfaces cooler. These materials include, white paint, white plaster, light coloured stone, gravel, and shiny aluminium. The best thermally and visually performing materials must be selected - reflection of light can lead to visual glare problems. Non visual radiation reflections may also be problematic ideally surfaces should be diffusive, avoiding any specular or glossy finishes. The emissivity of a material is the ratio of energy radiated to energy radiated by a black body at the same temperature. Materials with high emissivities are good emitters of long-wave energy and readily release the energy that has been absorbed as short-wave radiation. Any materials in the sun should ideally be high albedo and low mass to prevent the build up of heat.

11 3.3. Vegetation Planted surfaces tend to have significantly lower surface temperatures than hard surfaces. This reduces the radiant field and can cool air blowing over the surface. Figure 10: Effects of vegetation in the local environment Research carried out in London in 2005 (Guthrie 2006) compared the near surface temperatures of vegetated surfaces against paved ones (Figure 11). This research showed cooler near surface temperatures for vegetated as opposed to paved surfaces. Figure 11: The relationship between surface temperature and corresponding air temperature according to surface type. Research into the effects of vegetation has found that it can create a cooling effect, not only within its own environment, but also upon its surrounding area (Taha 1997; Elisson 1996; Jauregui 1990/91; Ca et al. 1998; Lewis et al. 1971). Some of these studies find even small

12 vegetated areas provide temperature relief that stretch to surrounding areas (Shashua-Bar & Hoffman 2000; Honjo & Takakura 1990/91; Saito et. al. 1990/91). Yu & Hien (2006) found average temperature reductions due to a city park in Singapore through field measurements and numerical study using Envi-met. This can lead to a direct reduction of cooling load. Similar studies in different climates would be of use. The cooling effect of vegetated surfaces is partly due to evapotranspiration from leaves. This effect can be particularly noticeable below trees in the evening when the upper part of a tree s leaf canopy loses heat to the sky by transpiration. The leaves cool the air around them, which becomes heavier and sinks. Solar radiation is mostly absorbed in the leaves of plants, so that the reflected radiation is very small, i.e. they have a low albedo. Therefore, the leaves of trees can be used to intercept solar radiation before it strikes buildings without producing undesirable reflection. The use of ground planting adjacent to shaded areas or windows can reduce diffuse or reflected radiation. Numerical studies by Alexandri and Jones (2008) into the effect of green roofs and walls show reduced convective and radiative heat fluxes compared to concrete, along with a heat sink due to the evaporative transfer. The resulting lower surface and air temperatures were found to be very effective in reducing air temperatures within canyons, particularly in hot, dry climates. Significant reductions in building cooling loads were found where walls and roofs were greened. Many of the benefits of vegetation relate closely to the evaporative functions of leaves. Such benefits must be quantified to assess against any resource usage, particularly in water scarce locations, where water often has an associated energy cost Water bodies Water has a very low reflectivity, and therefore reflects little solar radiation towards occupied zones. This means that unshaded water absorbs a lot of solar radiation, but this does not necessarily produce a significant increase of water temperature due to the large thermal capacity of water and evaporation at its surface. Water will generally be cooler than surrounding hard surfaces and will therefore tend to reduce the radiant temperature and improve comfort. Rivers also create continuous ventilation corridors, which tends to improve comfort. The cool surface of water can cause a reduction in air temperature. This will be most effective where air travels over large surfaces of water. Research comparing air adjacent to the Thames in London to a street canyon 200m away (Graves et. al. 2001) demonstrated a cooling potential of 0.6ºC. It is suggested that the extent to which the urban heat island is affected by bodies of water may be an area for further research Geometrical arrangements to optimise shade and daylighting Site layouts can be designed to minimise solar radiation impinging on buildings and reduce cooling loads. In hot climates, the vernacular architecture tends to use narrow street canyons to provide shade both streets and buildings. Traditional height/width ratios however may not be possible in trafficked streets or where access is required for emergency vehicles etc. Around tropical latitudes, streets oriented along a north south axis will provide reduced radiant field compared to east west orientated streets. A visual solar study is shown in figure

13 12 at 25 N. Ali-Toudert & Mayer s 2006 numerical study using ENVI-met shows the translation to lower radiant and air temperatures, which will lead to lower energy consumption. Figure 12: September solar analysis of N/S and E/W streets at latitude 25 N Canyon comfort studies by Johansson (2006) find that for both Fez, Morocco at a latitude of 34 N and Colombo, Sri Lanka (6.9 N), H/W ratios of north south streets should be at least 2, and east west streets at least 4. These ratios were found to be at odds with the planning regulations in Fez. There is however a design challenge to minimise solar gains while providing sufficient daylighting. Natural daylight coupled with daylighting sensing controls can significantly reduce the use of artificial lighting, provides a more pleasant environment and can improve productivity. Sites can be laid out according to their latitude and climate in order to allow shading of direct solar radiation to be achieved while daylight levels are realised. This provides potential to reduce energy consumption due to reduced lighting and heat gains. For example, for latitudes in the tropics, better daylighting, reduced glare and simpler shading strategies will be achieved by preferentially situating glazing on the north façade, followed by the south. The shading of east and west facing windows must be designed carefully, to allow daylight levels to be achieved but without increasing solar gains. The use of surrounding buildings and other obstructions to provide shading of this low angle sun can significantly reduce the additional shading required at the glazing unit.

14 Figure 14a: Façade exposed to low angle sun in a wide (low H/W ratio) canyon Figure 13: Daylight study around a building geometry. Figure 14b: Façade shaded from low angle sun due to higher H/W ratio of canyon 3.6. Geometrical arrangements to improve air movement Streets can be oriented along the prevailing wind directions to provide a constant low speed aeration path. Our work on master plans shows that whilst aligning the streets with prevailing wind improves ventilation for the wind aligned streets, in an orthogonal city grid layout, the streets perpendicular to the wind have lower rates of ventilation. To enhance the ventilation in the streets perpendicular to the prevailing wind directions, wider streets (lower H/W) or higher buildings on the downwind side of the street to encourage downdraughts can be used (Figure 11). Alternatively, wind catching structures can draw wind into these streets. A design approach may be taken that minimises those streets which require ventilation in this direction, forming well shaded pedestrian streets instead. This approach was taken for a masterplan in Doha, Qatar. Streets were frequently aligned with the prevailing wind direction from the sea. The building heights increased in the direction of the wind, producing height differentials to aid ventilation of streets perpendicular to the wind.

15 Figure 15: The use of differential building heights to improve ventilation in street canyons. Alternatively, a higher overall average ventilation rate may be achieved in all the streets if the streets are aligned at 45 º to the prevailing wind direction. These principles were investigated in a masterplan in Jeddah, Saudi Arabia. A generic residential block was repeated in an orthogonal grid pattern and tested with parallel winds and wind at 45º using advanced computational fluid dynamics (CFD) to model of airflows. Figure 16: Wind flow around a standard courtyard arrangement, flowing perpendicular to streets (top) and at 45º (bottom), (the 3D geometry of the modelled residential block is shown on the right). Wide streets will tend to increase air movement, which may well be at odds with the need for shade. Contradictions such as this challenge the designer and should be considered in the

16 context of the masterplan. A planning solution ought to regard trafficked streets with increased ventilation requirements differently to those designed for pedestrians. The principles of airflow are well understood, and can be analysed analytically using CFD. However the interaction between air and the heat transfer processes at the surfaces of the urban environment is more difficult to model numerically. The development of design guidance with quantitative rules of thumb would be useful for designers at the early design stages Natural Ventilation The potential for natural ventilation is improved by any reduction of external air temperatures. A lower radiant field resulting from effective urban design will reduce the heat gain and further improve the viability of natural ventilation of buildings to provide comfortable working and living environments. External pollutant levels and noise may prevent natural ventilation, particularly for noise sensitive buildings such as schools. Removing cars and noisy plant equipment from the city will greatly enhance this potential and improve the air quality. The potential for natural ventilation can be improved by the use of building geometry to provide high pressure differentials for cross flow ventilation. Differential building heights or wind catching structures can improve the wind environment available to ventilate buildings Roof tops The construction of roof top areas impacts the overall urban heat island of a city. The outer surface albedo reflects solar radiation from the urban environment. This is recognised in Leadership in Energy & Environmental Design (LEED) accreditation scheme, credit Sustainable Sites 7.2 for New Construction (V2.2): Heat Island Effect: Roof. To gain the credit, buildings must use roofing materials with a low Solar Reflectance Index (SRI) or install vegetated roofs. Non roof effects are also included in credit 7.1 which encourages shading and appropriate surface material usage Courtyards Courtyards are common urban devices which provide good shading and natural ventilation opportunities. The design of courtyards with regard to height to width ratios, various openings (to external) and surrounding building heights, improves the environments within these exterior spaces by altering the potential airflow and ventilation exchange. The courtyards can therefore lead to lower fabric, solar and infiltration gains. The opportunity for cross flow ventilation from narrow plan building massing can be improved by cooler external temperatures produced in the courtyard. These principles were investigated and analysed using CFD, in a masterplan in Jeddah where the properties of a generic residential block were studied from the standard 2:1 H/W ratio (A) to a 1:4 H/W ratio (B), then taking the standard again block again and opening the ground floor. The improved ventilation rates within the courtyards can be seen in case B. Greater benefits could be achieved with the addition of vegetation or water features into the courtyard. This is suggested as a possible area for further numerical and experimental study.

17 A - block with central courtyard H/W ratio = 2:1 B - block with central courtyard H/W ratio = 1:4 Figure 17: CFD images of air flow around courtyard building forms Transition spaces, colonnades etc The inclusion of transition spaces can protect entrances. More enclosed spaces yield the most benefit from shade and air temperature reduction, but must allow for adequate pedestrian circulation. Numerical studies of colonnades (galleries) (Ali-Toudert & Mayer 2008) have been carried out, but site measurements are required to test analysis, particularly due to the limits of the software employed Reduction of car use A reduction in car use reduces anthropogenic heat input to the city. This may be achieved by persuasive or legislative planning. The provision of more comfortable external environments for pedestrians can encourage walking and the use of public transport (this is covered in section 4). Reduced car use also reduces pollution which can reduce the urban heat island Centralised systems Lower external temperatures can improve the efficiency of heat rejection. It is suggested that heat rejection is centralised where possible and carefully placed to reduce further anthropogenic heat input. Such planning of systems at an urban scale provides the possibility to place heat rejection devices in better ventilated environments and improve their efficiency, further reducing energy consumption. Centralisation provides improved scope for the use of new technologies such as solar powered district systems.

18 Figure 18: Unplanned organisation of cooling heat rejection equipment Renewables The potential for renewable energy production associated with buildings is limited by the availability of resource. In sunny climates, the unshaded surface area available for photovoltaic cells and solar water heaters should be maximised. Consideration of solar powered renewables at an urban scale is crucial to ensure that the maximum potential is achieved, and no significant shading is caused by adjacent buildings. It would be of interest to consider the impact of renewables, for instance changes in surface temperatures, on the urban heat island, as there is a potential conflict between urban energy generation and climatic issues. 4. External Comfort and Microclimate Design 4.1. General overview In addition to the benefits for buildings described in the previous section, in hot climates, mitigation of the urban heat island will tend to improve the comfort of unconditioned spaces and external spaces. The principles can be used at smaller scales to further enhance comfort this type of work is referred to as Microclimate Design. The energy consumption of cities can be reduced by improving external comfort to walking, cycling and the use of public transport. In addition, the area of conditioned buildings may be reduced by providing retail circulation outside of conditioned malls in passively improved areas. Benefits extend to improved quality of life and competitive advantage to businesses located in enhanced areas.

19 The aim of microclimate design in climates with very hot seasons such as the Middle East is to extend the usability of the external environment further from the cold season towards the hot season Shading Shade is the single most important factor in achieving external comfort in hot climates as the radiative flux from direct sunlight has a strong influence on the heat balance of the body. Diffuse radiation in climates where the solar radiation levels are very high can be a significant radiative flux both to people and surfaces, so must be considered in design Materials Appropriate surface properties, along with the use of mass if surfaces are well shaded can lead to significant reduction of the long wave radiant field and air temperature of external areas, as described in section 3. This can lead to reduction in thermal sensation and greatly improve external comfort. These principles formed part of the design guidelines for a masterplan in Doha, Qatar. The properties of shade canopies must be considered to prevent high surface temperatures radiating to shaded areas. It may be useful to use low emissivity materials for the underside of shading structures as they will emit less radiation and therefore appear cooler to a person below. However, this will have a complex effect on the radiant field and is recommended for further study. The long wave radiant field in an urban environment is difficult to predict at concept design stage, particularly where dynamic effects are important. Often there is little detail available and simplification is required in order to make analyses computationally sensible. The importance of long wave radiation in the prediction of comfort means this could benefit from further research. Envi-met offers a simple approximation and would benefit from further experimental validation Vegetation Trees and planted pergolas or canopies are very useful to provide shaded areas both along walking routes and external dwelling areas while providing cooling due to evapotranspiration from leaves, particularly in the evenings. Air moving through planted structures may be cooled, further improving comfort Water The use of water in external environments provides cooler surfaces, reducing the mean radiant field, and evaporation can provide cooler air temperatures. This was applied to the concept design of a coastal development in Singapore; air was encouraged onto site over the large areas of water, the sea wall was designed to bring water into the park, and the local cooling effect was increased by placing boardwalks over the water s edge so that the park users could walk over the cool surface. In a project in Zaragoza, Spain, water sprays were used to improve the comfort along a covered bridge. The water sprays cooled the air through evaporative cooling.

20 Figure 19: Evaporative cooling analysis along the Zaragoza bridge. The presence of a small amount of water provides a significant psychological comfort benefit, particularly through the sound of moving water and the production of a pleasant view. Although these effects are difficult to measure or evaluate they should be considered in design as will contribute to the overall perception of comfort beyond the physical sensation. The use of water is not a new principle, it has been incorporated into design in many cultures throughout history. For example water fountains and pools have been used to create external microclimates and benefit indoor spaces in the Alhambra in Granada, Spain. (Figure 20). Figure 20: Water features at Alhambra The extent of the improvement to exterior spaces is not adequately quantified, therefore it is suggested that this is an area in which more research is required. This is of particular importance in locations where water is scarce as resource use must be assessed against benefit. 4.4 Building massing The appropriate building massing can improve external comfort due to the provision of air movement and shading, in the manner described in section 3. North to south canyons provide better shaded environments in tropical latitudes - a masterplan in Doha utilised this principle

21 to provide more comfortable narrow pedestrian alleys running north to south. Additional design measures were required on east to west routes such as overhangs and street shading. In addition, the provision of comfortable areas to walk or dwell may change with the time of day and sun angle. This may be used to plan the usage of courtyard areas. Shading may be augmented with canopies, such as those shown in Figure 21. Ali-Toudert and Mayer (2006, 2007) have conducted numerous numerical studies investigating canyon design and external comfort, which support these principles. Figure 21: Street and courtyard shading in Granada, Spain 4.5 Colonnades and enclosure Colonnades are a device primarily designed to produce shaded circulation. They can also be configured to take advantage of the cool temperatures of neighbouring air-conditioned spaces and the thermal mass of shaded construction materials which can reduce the radiant temperature field to below the air temperature. Comfort conditions are improved by increasing enclosure to: - increase shade, - reduce air exchange - increase the area of cool surfaces. Operative temperature can be used as a measure of comfort relating to radiant field and air temperature. The graph below shows the results of dynamic thermal analysis of a number of colonnade designs for a masterplan in Doha, Qatar. The best colonnade reduces the annual exceedance of an operative temperature of 35 C from 40% to 9%. This colonnade is well enclosed to minimise diffuse as well as direct solar gain, and makes maximum use of shaded thermal mass.

22 Figure 22: Percentage of year for which test colonnades exceed operative temperatures. This work was carried out to encourage the client not to condition all retail circulation space. It would be of value to follow this up with empirical study. If cool air sources can be utilised without additional energy input, this air can be trapped by enclosure at low level, making use of the greater density of cool air. In a development in Dubai, an existing design was modified to protect external eating areas while exhausting waste conditioned air locally to the spaces. This allowed waste coolth to improve the comfort of the areas. 5. Conclusions and areas for further research For the maximum impact on designs and therefore the largest impact on energy use, it is necessary to consider the urban design principles discussed here at the earliest stage of a project. This is most effectively done through design workshops and outline studies. To facilitate this process more effectively, it is useful to have rules of thumb for the environmental designer to explain principles and benefits to the design team. Evidence of improvements to energy use and external comfort are important to ensure the microclimate design strategies are supported through the design process. To date, there are a number of numerical studies which have been carried out to look into various urban planning issues. It is proposed that further studies be carried out considering the various issues in a range of climates. This would facilitate better understanding of the various design options in a range of climatic conditions. As designs progress, analysis is carried out in order to predict conditions and support the implementation of energy saving site wide strategies. The industry would benefit from experimental feedback from strategies, providing evidence based research to test analytical predications. This type of research is of particular relevance in climates such as the Middle East, where many masterplans are currently being designed and built and offer opportunity for academic research. It is proposed that linkages with designers may be made to carry out a

23 type of post-occupancy evaluation of the external environment compared to the design intent. Geometrical issues are well understood numerically, but would benefit from further evidence based measurement. The interaction of various principles in strategies such as high mass colonnades would be of particular interest. Other design strategies such as material selection, the use of water and vegetation would benefit from a greater understanding and evidence based research, improving numerical representation. An area of particular interest is the experimentally validated numerical prediction of long wave radiant fields. It is suggested that the impact of renewables on the urban heat island requires consideration. The communication of benefits to clients is made difficult by the need to explain the complex principles of thermal comfort. PET is currently the most universal scale available for measuring thermal sensation, but is not a direct measure of comfort, with its various psychological factors. It is proposed that a subjective layer needs to be superimposed onto PET results, devised for the specific application of interest. Ideally, communication of comfort should be in the language of the audience, related in terms of tangible experience. This might be done using comparative scales, presenting the comfort of a space relative to a familiar baseline (i.e. typical English summer day) or as a yearly exceedence. The complex consideration of comfort under broader environmental and social conditions needs to be considered in order to aid and achieve lower energy solutions. 6. References Albrecht, F. (1933) Untersuchungen der verticalen Luftzirkulation in der Grossstadt. Meteorologische Zeitschrift 50, Alexandri E, Jones P. Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Building and Environment 43 (2008) Ali-Toudert, F. A., & H. Mayer (2006) Numerical study on the effects of aspect ratio and orientation of an urban street canyon on outdoor thermal comfort in hot and dry climate. Building and Environment 41: Ali-Toudert, F and Mayer, H. Effects of asymmetry, galleries, overhanging facades and vegetation on thermal comfort in urban street canyons. Solar Energy 81 (2007) Berdahl P. and Bretz S. E. (1997) Preliminary survey of the solar reflectance of cool roofing materials. Energy and Buildings 25, Ca V. T., Asaeda T. and Abu E. M. (1998) Reductions in air conditioning energy caused by a nearby park. Energy and Buildings 29, Eliasson I. (1996) Urban nocturnal temperatures, street geometry and land use. Atmospheric Environment 30, Fanger P. O. (1970) Thermal Comfort: Analysis and applications in environmental engineering. Danish Technical Press, Copenhagen. Chapter 2, Georgii H. W. (1970) The effects of air pollution on urban climates. In WMO technical note no. 108, Givoni B., Noguchi M., Saaroni H., Pochter O., Yaacov Y., Feller N., Becker S. (2003) Outdoor comfort research issues. Energy and Buildings 35, Graves H., Watkins R., Westbury P. and Littlefair P. (2001) Cooling buildings in London: Overcoming the heat island. BRE Report. CRC, Garston.

24 Guthrie, P. (2006) A Study of the Development of the Heat Island Effect in Local Microclimates and its Relationship with Human Comfort. Masters Research Project Southampton University, Environmental Science. Honjo, T. and Takakura T. (1990/91) Simulation of thermal effects of urban green areas on their surrounding areas. Energy and Buildings 15, Höppe P (1994) Die Wärmebilanzmodelle MEMI und IMEM zur Bewertung der thermischen Beanspruchung am Arbeitsplatz. Verh Dtsch Ges Arbeitsmed Umweltmed 34: Hoppe, P. (1999) The physiological equivalent temperature a universal index for the biometeorological assessment of the thermal environment. International Journal of Biometeorlogy 43: Howard L. (1833) Climate of London deduced from meteorological observations, 3 rd ed. in 3 volumes. Harvey and Darton, London. Hunter L. J., Watson I. D. and Johnson G. T. (1990/91) Modeling air flow regimes in urban canyons. Energy and Buildings 15, Jauregui, E. (1990/91) Influence of a large urban park on temperature and corrective precipitation in a tropical city. Energy and Buildings 15, Johansson, E. Urban Design and Outdoor Thermal Comfort in Warm Climates, PhD Thesis 2006 Landsberg H. E. (1970) Micrometeorological temperature differentiation through urbanisation. In WMO technical note no. 108, LEED for New Construction & Major Renovations Version 2.2 (2005) U.S. Green Building Council Lewis J., Nicholas F., Seales S. and Woollum C. (1971) Some effects of urban morphology on street level temperature at Washington D.C. Journal of Washington Academic Science 61, Lyall, I.T. (1977) The London Heat-Island in June-July Weather, Vol.32, No. 8, pp Oke T. R. (1981) Canyon geometry and the nocturnal urban heat island: Comparison of scale model and field observations. Journal of Climatology 1, Oke T. R. (1987) Boundary Layer Climates, 2 nd ed. Routledge, London. 9, 265, Oke T. R. (1988) Street design and urban canopy layer climate. Energy and Buildings 11, Padmanabhamurty B. (1990/91) Microclimates in tropical urban complexes. Energy and Buildings 15, Saito I., Ishihara O., and Katayama T. (1990/91) Study of the effect of green areas on the thermal environment in an urban area. Energy and Buildings 15, Santamouris, M. (2001) Energy and Climate in the Urban Built Environment. James and James Ltd., London Santamouris, M., N. Papanikolaou, I. Livada, I. Koronakis, C. Georgakis, A. Argiriou and D. N. Assimakopoulos. (2001) On the Impact of Urban Climate on the Energy Consumption of Buildings. Solar Energy 70: Shashua-bar L. and Hoffman M. E. (2000) Vegetation as a climatic component in the design of an urban street: An empirical model for predicting the cooling effect of urban green areas with trees. Energy and Buildings 31, Taha H. (1997) Urban climates and heat islands: albedo, evapotranspiration, and anthropogenic heat. Energy and Buildings 25, Taha H., Sailor D., and Akbari H. (1992) High-Albedo Materials for Reducing Building Cooling Energy Use. Lawrence Berkeley National Laboratory Report LBL-31721, Berkeley, California.

25 Ten Brink H., Kruisz C., Kos G. and Berner A. (1997) Composition/size of the lightscattering aerosol in the Netherlands. Atmospheric Environment 31, Tsangrassoulis A. (2001) Short-wave radiation. In: Energy and Climate in the Urban Built Environment. Santamouris M. (2001) (ed) James and James Ltd., London Yu, C and Hien, W.N. Thermal benefits of city parks. Energy and Buildings 38 (2006) Zhang Y, Zhao R. Relationship between thermal sensation and comfort in non-uniform and dynamic environments. Building and Environment (2008), doi: /j.buildenv