- Email: [email protected]
Energy and the city: density, buildings and transport
Energy and Buildings 35 (2003) 3–14
Energy and the city: density, buildings and transport Koen Steemers* Department of Architecture, The Martin Centre for Architectural and Urban Studies, University of Cambridge, 6 Chaucer Road, Cambridge CB2 2EB, UK
Abstract Cities by definition are a focal point of energy consumption. Their forms have a significant bearing on the balance of building and transport energy use, which are the two sectors that are directly affected by urban planning (the third being industrial). This paper establishes the relative magnitudes of building energy use in comparison to transport, and points out the interrelationships between the two in the context of the cities and of a temperate climate. The main part of the paper assesses the building energy trends and implications of urban form, with a particular reference to the effect of varying density, and presents strategic findings. It calls for continued research and development, particularly in the field of modelling the urban microclimate as a function of design, as well as comfort research with an emphasis on outdoor comfort. Urban microclimate and comfort are the themes of this journal, and this paper aims to set the scene. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Urban planning; Urban form; Density; Microclimate; Transport
1. Urban density—transport versus building energy The relative low cost of energy during the last century has allowed for the increasing dispersal of activities and decreasing densities of many of our cities. These lower densities have resulted in greater physical separation and diffuse dispersal of activities, making mass transport increasingly difficult. In combination with increasing wealth, this has resulted in increased private vehicular traffic (currently accounting for about 70% of Europe’s passenger transport). As the private vehicle typically consumes more than twice the energy per passenger per kilometer than a train, and almost four times that of a bus, the energy (and pollution) implications of an urban layout that does not maximise public transport are likely to be very significant [1]. This implies that it is essential to plan cities for efficient transport use. Although this is undoubtedly true, it is important to consider building energy use, which in UK accounts for over half of the total energy consumed (in the European Union this figure is 41%, and in US 36%) and an equivalent proportion of pollution generated. This compares with less than a quarter each for transport and industry. Although these figure are notional figures and not specific to urban areas, the 2:1 ratio of building:transport energy use (for UK) is likely to be a conservative estimate for cities. * Tel: þ44-1223-331712. E-mail address: [email protected] (K. Steemers).
This is partly because of the availability of urban public transport systems, the shorter distances for walking, the increased traffic congestion and limited parking, which suggest that there is likely to be proportionally less transport energy use. For example, in central London only 10% of commuters arrive by car compared to a national average of 40%. On the other hand, there is a concentration of office, retail and other commercial building types in cities, which have a greater level of energy consumption per square meter than housing. An air-conditioned office building to current building regulations consumes approximately six times the energy per square meter of a house. So, it would be fair to say that the energy and environmental implications of buildings are at least twice as significant as those for transport. For example, for London the ratio of energy used in buildings to transport is approximately 2.2:1 (including goods, taxis and air transport!) (Fig. 1) [2]. Despite the fact that building energy use is much greater than transport energy, there are a number of factors that have given the transport debate more urgency. (i) First, the very local pollution—both atmospheric and noise—are more immediately perceptible than those associated with buildings (power stations being generally located on the edge of or outside urban areas). (ii) Second, the rate of replacement of old vehicles with new ones (increasingly more efficient) is rapid compared to buildings (typically buildings may have a design life about 10 times longer than that of cars).
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Fig. 1. Energy use breakdown for London [9].
This means that transport policies will have a much greater chance of having a short-term benefit compared to building-related proposals, as the replacement rate of cars is well within the time scale of targets such as those set by the Kyoto agreement. (iii) Third and finally, cars are not only associated with environmental issues, but also with accidents, fatalities and social erosion [3], bringing transport still higher up the political agenda. It has been shown that cities of a high density, such as, for example Hong Kong, have a far lower transport energy demand per capita than low density cities such as Houston, by a factor of 18. On average, when comparing 10 major cities in the US with 12 European cities, European cities are
five times as dense but the US cities consume 3.6 times as much transport energy per capita [4]. The conclusion often drawn from such data is that dense cities are low energy cities. However, it is not clear to what extent density is the cause, and increased energy use is the effect. For instance, it can be argued that where private car use is less—due to, for example economic conditions—cities are denser. Interestingly, historic European cities, such as Paris, lie at a national ‘optimum’—achieving moderate energy use for modest densities—whilst sustaining a rich urban life (Fig. 2). It is not evident that moving towards increasing urban density will lead to reduced car traffic—in fact, in the short-term the opposite is likely to be the case. In the absence of extra capacity in the form of effective integrated public transport, increasing the density will inevitably increase traffic and
Fig. 2. Transport energy vs. urban density for 32 cities [11].
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pollution. The challenge is, thus, to anticipate and implement public transport systems before increasing densities to ensure that more existing and future inhabitants use public transport instead of private cars and, thus, to shift the balance away from the cars. However, the long-term perspective of global environmental concerns (climate change, ozone depletion, acid rain, reducing fossil fuels, etc.) clearly means that building energy use needs to be addressed, particularly in relation to urban areas. This is combined with increasing urbanisation resulting in a predominantly urban global population. It is, thus, essential to minimise the deleterious global and local effects of urbanisation to ensure that the life nurturing qualities of the city can be restored and maintained. Building energy use is a predominant factor in this context.
2. Density and building energy The concentration of activities and people in cities is often perceived to be the main source of environmental problems. However, such concentration can have environmental advantages achieved, for example through the sharing of resources. Most obviously, more intense use of land and sharing of infrastructure—energy and water supply, drainage, roads, buildings and public transport—reduces the energy per capita associated with its construction (and possibly maintenance) and benefits from an economy of scale by comparison to a more dispersed urban configuration. An example of this is the use of combined heat and power (CHP) and district heating (DH) energy provision. MicroCHP has the potential to deliver thermal (55%) and electrical (30%) energy locally to a neighbourhood at a high efficiency—typically 85% or more overall—and reduces the transmission and distribution losses of more centralised power stations. This compares with an efficiency of approximately 30% for an average UK power plant to deliver electricity. However, for the balance of heat and power to be used optimally the energy demand should not only be localised but also mixed, combining housing with other commercial activities. Mixed use, high density neighbourhoods have the further potential advantage of local employment, commerce, retail, leisure, etc. which in turn may reduce the distances that people need to travel to access
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such facilities. The notion of ‘‘decentralised concentration and high density mixed land-use’’, thus, seems to be reinforced by energy considerations [5]. However, when one considers the implications of high density on the demand side of building energy use and on building integrated renewable energy production (such as photo-voltaics), does the balance begins to tip in favour of lower densities? This balance is likely to depend on building type, and for the purposes of this paper, we will consider domestic and office use as the predominant types, in the context of UK. 2.1. Domestic buildings Taking UK as an example to demonstrate the point, the energy demand in housing is dominated by space heating, which on average accounts for 60% of the total energy (Fig. 3) [6]. It is the space heating that will be most affected by design, the remaining consumption being largely determined by occupant needs and not strongly dependent on climate. In dispersed developments with the possibility of greater solar access, passive solar design will have greater potential to reduce space heating demands and, thus, overall energy use in housing. In order to explore the effect of increasing density on energy, we need to consider what parameters can be altered to change the density of housing. There are a number of ways in which the urban form of a city can change to increase density: (i) by increasing building depth; (ii) by increasing building height or reducing spacing (i.e. changing height:width ratios between buildings); (iii) by increasing ‘compactness’ (e.g. apartments instead of detached housing, where the building depth and height characteristics may theoretically remain unaffected). The first—increasing building depth—is perhaps particularly common in terraced housing in UK cities, where extensions to the rear in the form of conservatories, living rooms, kitchens and bathrooms are commonplace. As a result, the building envelope:floor areas ratio is usually (though not necessarily) increased, and the availability of daylight and sunlight to the interior is reduced. The energy implications of conventional extensions (as opposed to a passive solar extension) are an increase in both the heating
Fig. 3. Energy use breakdown for UK housing [13].
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Fig. 4. Effect of obstruction on space heating for housing [14].
and lighting loads. Overall, these increases are very small— in the order of 5–10% for a two storey addition to a three storey terraced house. Housing densities can also be increased, by increasing the average building heights (or reducing the spacing between them), which can be defined by the obstruction angles to the facades. An obstruction of 308 to the south fac¸ade of a passive solar house1 can increase heating energy by 22% compared to an unobstructed fac¸ade (Fig. 4). However, conventional, non-solar housing will also require between 6 and 15% more energy for heating compared to a passive solar house [7]. In an urban context, the solar potential for housing is reduced, largely due to the presence of obstructions but also because of planning constraints on orientation (Fig. 5). For the average UK dwelling, the heating contribution of solar energy is small, 10–15%, compared to a passive solar house—in the order of 40%—thus, the energy consequences of obstructions for a non-solar urban dwelling is small. Thus, there is in the above example, about a one-third increase in heating energy for an urban house as compared to a green field passive solar house. However, it should be noted that there will be a significantly wider range of energy use in an urban environment as some dwellings will be more 1 A ‘passive solar’ house here is defined as a house with 75% of its glazing to the south and 25% to the north, compared to a ‘non-solar’ house which here is taken to be a house with west and east facing facades each with 50% of the glazing. In both the cases, the assumption is that the houses are designed according to, in 1990, Building Regulations (see [7] for further details).
obstructed and facing north whilst others may benefit from unobstructed solar gains. The average one-third increase compares to a heat saving of 40% while comparing detached housing with apartments (Fig. 6) [7]. Thus, the way to increase density and energy efficiency simultaneously is to increase ‘compactness’ of the urban fabric whilst maintaining a limited building depth (in the order of 10–12 m), and where an appropriate solar orientation is ensured to access light, sun and air. The conclusion would seem to be that for energy use in UK housing the arguments for and against densification are finely balanced and will probably depend on the infrastructure issues mentioned earlier. However, as the average obstruction angles increase above about 308, the balance will begin to swing against densification. An average obstruction angle of 308 equates to a theoretical plot ratio of up to 2.5 (assuming the terrace or courtyard form with a plan depth of 10 m). Such a figure results in densities in the order of 200 dwellings per hectare (DPH) (assuming 125 m2 per dwelling)—well above the 50 and 75 DPH discussed by the Urban Task Force [8]. This compares with the current density of new housing in UK of an average of 25 DPH [9], Ebenezer Howard’s 45 DPH for the Garden City [10], the new PPG3 guidance of up to 50 DPH [9], and Kowloon’s 1000 DPH in Hong Kong [10] (Fig. 7). The conclusion is that relatively high housing densities can be achieved before a negative impact on the energy demand becomes significant. 2.2. Non-domestic buildings The non-domestic sector is very diverse, so for the purposes of this paper, the focus is on UK office buildings—the predominant non-domestic building type. Unlike housing, space heating for offices in UK is not the predominant issue in terms of primary energy use and energy cost. Typically, the artificial lighting demand, and in the case of an air-conditioned office, the fan power and refrigeration loads, are often equally significant (dependent on the type of office building). If one excludes office equipment, as a non-design related energy demand, the energy breakdown for a typical air conditioned office in UK is: 44% for air conditioning, 34% lighting and 22% heating (Fig. 8) [11]. It is, therefore, clear that avoiding, or at least reducing air conditioning, is a primary objective to reduce energy demand. The secondary approach is to reduce reliance on artificial lighting by increasing and exploiting daylight availability. Finally, reducing heat losses is likely to be the least significant energy efficiency strategy and, therefore, should not be used as a determinant for building form, except where it compliments the previous two aspects (i.e. excessively compact and deep plan forms may reduce heat loss but will dramatically increase both mechanical ventilation and artificial lighting). The first two aspects—avoiding air conditioning and increasing daylight availability—are broadly complimentary in terms of building form implications. They both point
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Fig. 5. Effect of orientation on space heating for housing [14].
towards shallow plan building forms in order to enable natural ventilation and daylight penetration. Typical plan depths for naturally ventilated and day-lit office buildings are 12–15 m. The implication of limiting the plan depths of buildings might suggest that densities would decrease for low energy offices, compared to more conventional deep plan buildings. However, this is not necessarily the case. The avoidance of air conditioning may save up to 1 m depth of services zone per floor (i.e. 25% of the building volume assuming an initial 4 m floor-to-floor height) and a further 3– 5% of the volume for reduced plant size [12]. Therefore,
avoiding air conditioning can save a maximum of approximately 30% of the building volume. This may more than compensate for the reduced plan depths resulting in, for example, courtyard or finger-plan forms. The benefits of avoiding air conditioning through adopting shallow plan depths are typically that naturally ventilated offices use <40% of the primary energy. This is an enormous potential advantage. However, there are limitations in adopting natural ventilation in the urban context. Apart from internal gains levels (which are independent of context), the most often quoted constraints in cities are noise
Fig. 6. Relationship between building form and heat loss [14].
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Fig. 7. Dwellings and people per hectare for housing [3,15,16].
and air pollution. Both these factors frequently result in the adoption of air conditioning. The main cause of noise and pollution is traffic and, thus, as mentioned before, this issue needs to be addressed in advance of expecting a significant reduction in energy consumption for office buildings. In the meantime, there are building design strategies that can be implemented which limit the need for air conditioning. The most obvious is that it may only be necessary to mechanically ventilate the street facing zone (and/or to locate less noise sensitive accommodation there), allowing the rest of the building to be naturally ventilated from a more quiet and
cleaner courtyard, garden or atrium. Such a ‘mixed-mode’ strategy to the servicing of offices will go some way towards limiting energy use whilst maintaining the potential for future adaptation to full natural ventilation. Having established that it is in principle possible to maintain a given density of development and significantly reduced energy use by limiting air conditioning, it would seem appropriate to investigate the energy impact of increasing the density on office buildings. The primary energy consumption of an average naturally ventilated office building in UK is dominated by artificial lighting and heating
Fig. 8. Average energy use breakdown for air-conditioned offices [17].
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Fig. 9. Average energy use breakdown for naturally ventilated offices [17].
Fig. 10. Effect of increasing the building depth on energy use for a mixed mode office.
loads (Fig. 9). As the level of obstruction to buildings will affect daylight and sunlight availability, both the lighting and thermal loads will be affected by increasing urban densities. If again we consider two possible strategies for increasing density as being increasing building depths and heights, some initial analysis can be carried out. By increasing the building depth of offices the availability of natural ventilation and daylight reduces, resulting in an anticipated increase in mechanical ventilation and artificial lighting. However, heat losses are likely to diminish because the surface:volume ratio reduces with increasing plan depths. Using the LT method [11] to determine the energy use implications, it can be seen that for a low energy naturally
ventilated office (with mechanical fresh air supply for deep plan areas) the energy use doubles when the plan depth is doubled from 12 to 24 m. However, for an efficient airconditioned building the energy use increase is only 20%, although the total energy is almost three times larger than the naturally ventilated option at a depth of 12 m (Fig. 10).2 Thus, increasing density by increasing building depths will inevitably result in increased energy use for office buildings, although the relative increase is less for airconditioned buildings. Nevertheless, on average deep plan, 2 Assumptions regarding the office buildings modelled include low energy, optimised control systems, a 50% fac¸ade glazing ratio, with a lighting datum of 300 lx and 30 W/m2 of internal gains, located in southern UK (see the LT method for more details [11]).
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Fig. 11. Effect of obstruction angle on energy use for offices [17].
air-conditioned buildings still consume typically twice as much energy as equivalent mixed-mode buildings (naturally ventilated and mechanical fresh air supply), suggesting that the avoidance of air conditioning by improving the urban microclimate is a key factor. The alternative strategy for increasing density is to increase the average building height (or reduce the spacing, which will have the same consequence for sunlight and daylight availability). Using the same assumptions as in the example given previous, it is possible to predict the energy consequences of increasing obstructions. From LT energy analysis, it can be seen that the total energy use would be expected to increase by 23% for air conditioned and 45% for non-air conditioned offices for an obstruction of 308 compared to an unobstructed situation (Fig. 11).
3. City texture Despite the clarity and insights gained from looking at urban density in terms of simple parameters, such as obstruction angles or plan depth, it is important to move from the theoretical description to study the formal complexity of a real bit of city. The study of urban form has been an ongoing pre-occupation at the Martin Centre for Architectural and Urban Studies for over 30 years—starting with the seminal work of Leslie Martin and Lionel March [13,14]. The current research builds on this work and focuses on the relationship between urban ‘texture’ and environmental characteristics. It has already been pointed out that the
energy performance of buildings is linked to the quality of the urban environment, and although not the specific focus of this paper, research into the solar, wind, pollution and acoustic effects of urban texture is going on. A number of recent research projects have developed analytical techniques, which categorise the physical urban geometry and assess the consequence of this on environmental performance [15–17]. One of the techniques developed by the Martin Centre’s urban environmental research team assesses energy demand in terms of urban form. This technique, referred to as ‘LT urban’, combines the LT energy analysis tool mentioned previously with computer-based image processing [18] to extract data related to building form for large urban areas (typically 400 m 400 m). To demonstrate the technique and its application to establishing the relationship between urban density and building energy use, a 400 m 400 m part of London is used (Fig. 12). As we are primarily interested in built form, numerous assumptions have to be made about the detailed characteristics of individual buildings, such as glazing ratio, U-values, systems, etc. These have been standardised, based on a detailed survey of the area and making informed estimates where necessary. The base-case urban form is altered by adjusting the building heights so as to produce a range of urban densities from half to double the existing. The energy consequences of increasing the average obstruction angles are significant—for example, 108 increase in obstruction results in approximately 10% increase in energy (Fig. 13). For a density range of plot ratios from 1.25:1 to 5:1 the results show that doubling the
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Fig. 12. A digital elevation model (DEM) is used to determine and map energy use for the London area under investigation (left, aerial view of London site; middle, DEM of the site; right, energy demand (kW h/{ m}2 per year)).
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Fig. 13. Effect of obstruction angle on energy use for naturally ventilated offices on the London site.
density typically increases energy consumption by in the order of 25% for this whole section of the city (Fig. 14). It is interesting to note that optimising the glazing ratios in response to the level of obstruction reduces the effect, so that doubling the density now results in a 21% increase in energy.
This highlights that other parameters, at the level of individual buildings, will change the relationship of energy to urban density, and that they should respond to the specific urban context. Glazing ratios in particular—as they represent the main interaction between building and climate—
Fig. 14. Effect of density on energy use for naturally ventilated offices on the London site.
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Fig. 15. Average optimum glazing ratios at different floor heights for the London site.
affect the energy performance and can be ‘optimised’ to minimise energy demand. This is demonstrated by determining the optimum glazing ratio for the London site, which for all orientations show that glazing should reduce as you go up a building’s fac¸ade (Fig. 15). Optimum glazing ratios vary less in response to orientation compared to green field sites, as in an urban context the obstructions reduce the differences in terms of daylight and particularly sunlight availability.
4. Conclusions The order of magnitude of energy implications in relation to urban density has been demonstrated for domestic and office buildings in UK’s temperate climate. For dwellings, the energy implications of compact densification are balanced between the benefits from reduced heat losses and the non-benefits of reduced solar and daylight availability. For office buildings, increasing urban density increases energy use because of the reduced availability of daylight in particular. However, this increase is significantly less than the energy increase of changing from a naturally ventilated office to an air-conditioned office. This change is only possible to prevent if the urban environment is less polluted and noisy. This is where the link between transport and building energy becomes evident, as cars are the major cause of urban pollution. Although overall urban transport consumes less than half the energy of urban buildings, the implications of reducing private car use in
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favour of clean and efficient public transport can be significant for building energy use. Thus, the move should initially be towards improving the urban environment, so that the energy benefits will outweigh the non-benefits. Increasing the urban population is not likely in the first instance to improve the urban environment, and the worst will be the increase in noise and pollution in the short-term before the necessary level of investment in transport infrastructure is made. The result may thus be increased nondomestic building energy use due to the adoption of airconditioning. Combined with climate change, the same effect may also occur in the domestic sector. It is clear that the political will is to a large extent and needs to be in place to encourage sustainable development. However, there is a need for an analytical approach to provide information on which the decisions will be based. The potential role for research is evident. ‘‘Towns and cities should be well designed, be more compact and connected, support a range of diverse uses within a sustainable environment which is well integrated with public transport and adaptable to change’’ [8]. It is not difficult to agree with this statement and its vision. What is important is to develop the necessary techniques to inform the balance, sequence and implementation of decisions to achieve the desired results. This issue of Energy and Buildings begins to provide the required knowledge, with a particular reference to urban environmental and related comfort issues. It is by making our environments more comfortable that we can increase the use of outdoor space for energy efficient movement (i.e. walking and cycling) and exploit these beneficial urban microclimates to improve the energy performance of buildings.
Acknowledgements Koen Steemers is joint Director of the Martin Centre where he heads the urban environmental research team—he particularly wishes to thank Nick Baker, Sam Lawton, Carlo Ratti and Dana Raydan for their support. Dr. Steemers is coordinator of EU research projects such as ‘‘Assessing the Potential for Renewable Energy in Cities (PRECis)’’ and ‘‘Project Towards Zero Emission (ZED) Urban Development’’, as well as ‘‘Sustainable Building Form’’ funded by the Tyndall Centre for Climate Change Research, on these aspects this paper is based.
References [1] J. Whitelegg, Transport for a Sustainable Future: the Case for Europe, Wiley, Chichester, 1993. [2] Ecoenergia: rational and renewable energies for the cities of the future, in: Proceedings of the Report of Congress and Workshop, EA.UE, European Academy of the Urban Environment, Berlin, 1994. [3] D. Appleyard, Liveable Streets, University of California Press, Berkeley, 1981.
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[4] P. Newman, J. Kenworthy, Cities and automobile dependence, Gower Technical, Aldershot, 1989. [5] T. Elkin, D. McLaren, Reviving the city: towards sustainable urban development, Friends of The Earth, London, 1991. [6] BRE, Domestic Energy Fact File, Building Research Establishment, Garston, Watford, 1992. [7] S. Yannas, Solar Energy and Housing Design, Vol. 1, Architectural Association, London, 1994. [8] Towards an Urban Renaissance, The Urban Task Force, E&FN Spon, London, June 1999. [9] J. Lewis, Radical PPG3 unveiled, Building Design, 10 March 2000. [10] E. Scoffham, B. Vale, How compact is sustainable—how sustainable is compact? in: M. Jenks, E. Burton, K. Williams (Eds.), The Compact City: A Sustainable Form? E&FN Spon, London, 1996. [11] N. Baker, K. Steemers, Energy and Environment in Architecture: A Technical Design Guide, E&FN Spon, London, 2000. [12] J. Berry, A. Todd, Space for services: four strategic planning, Architects Journal, February 1986.
[13] L. Martin, L. March, Land use and built forms, Cambridge Research, April 1966. [14] L. Martin, L. March, (Eds.), Urban Space and Structures, Cambridge University Press, Cambridge, 1972. [15] N. Baker, K. Steemers, D. Crowther, M. Nikolopoulou, P. Richens, The urban porosity model: simplified parameters as indicators of environmental performance, in: H.S. Stephens (Ed.), Solar Energy in Architecture and Urban Planning, Bedford, 1996, pp. 300–305. [16] K. Steemers, N. Baker, D. Crowther, J. Dubiel, M. Nikolopoulou, C. Ratti, City Texture and Microclimate, Urban Design Studies, Vol. 3, University of Greenwich, Kent, 1997, pp. 25–50. [17] K. Steemers, C. Ratti, Informing bioclimatic urban design, in: M. Voyatzaki (Ed.), Architecture and Engineering: The Teaching of Architecture for Multidisciplinary Practice, European Association for Architectural Education, 1999. [18] C. Ratti, P. Richens, Urban texture analysis with image processing techniques, in: Proceedings of the CAADFutures99 Conference, Atlanta, June 1999.
Energy and the city: density, buildings and transport Koen Steemers* Department of Architecture, The Martin Centre for Architectural and Urban Studies, University of Cambridge, 6 Chaucer Road, Cambridge CB2 2EB, UK
Abstract Cities by definition are a focal point of energy consumption. Their forms have a significant bearing on the balance of building and transport energy use, which are the two sectors that are directly affected by urban planning (the third being industrial). This paper establishes the relative magnitudes of building energy use in comparison to transport, and points out the interrelationships between the two in the context of the cities and of a temperate climate. The main part of the paper assesses the building energy trends and implications of urban form, with a particular reference to the effect of varying density, and presents strategic findings. It calls for continued research and development, particularly in the field of modelling the urban microclimate as a function of design, as well as comfort research with an emphasis on outdoor comfort. Urban microclimate and comfort are the themes of this journal, and this paper aims to set the scene. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Urban planning; Urban form; Density; Microclimate; Transport
1. Urban density—transport versus building energy The relative low cost of energy during the last century has allowed for the increasing dispersal of activities and decreasing densities of many of our cities. These lower densities have resulted in greater physical separation and diffuse dispersal of activities, making mass transport increasingly difficult. In combination with increasing wealth, this has resulted in increased private vehicular traffic (currently accounting for about 70% of Europe’s passenger transport). As the private vehicle typically consumes more than twice the energy per passenger per kilometer than a train, and almost four times that of a bus, the energy (and pollution) implications of an urban layout that does not maximise public transport are likely to be very significant [1]. This implies that it is essential to plan cities for efficient transport use. Although this is undoubtedly true, it is important to consider building energy use, which in UK accounts for over half of the total energy consumed (in the European Union this figure is 41%, and in US 36%) and an equivalent proportion of pollution generated. This compares with less than a quarter each for transport and industry. Although these figure are notional figures and not specific to urban areas, the 2:1 ratio of building:transport energy use (for UK) is likely to be a conservative estimate for cities. * Tel: þ44-1223-331712. E-mail address: [email protected] (K. Steemers).
This is partly because of the availability of urban public transport systems, the shorter distances for walking, the increased traffic congestion and limited parking, which suggest that there is likely to be proportionally less transport energy use. For example, in central London only 10% of commuters arrive by car compared to a national average of 40%. On the other hand, there is a concentration of office, retail and other commercial building types in cities, which have a greater level of energy consumption per square meter than housing. An air-conditioned office building to current building regulations consumes approximately six times the energy per square meter of a house. So, it would be fair to say that the energy and environmental implications of buildings are at least twice as significant as those for transport. For example, for London the ratio of energy used in buildings to transport is approximately 2.2:1 (including goods, taxis and air transport!) (Fig. 1) [2]. Despite the fact that building energy use is much greater than transport energy, there are a number of factors that have given the transport debate more urgency. (i) First, the very local pollution—both atmospheric and noise—are more immediately perceptible than those associated with buildings (power stations being generally located on the edge of or outside urban areas). (ii) Second, the rate of replacement of old vehicles with new ones (increasingly more efficient) is rapid compared to buildings (typically buildings may have a design life about 10 times longer than that of cars).
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Fig. 1. Energy use breakdown for London [9].
This means that transport policies will have a much greater chance of having a short-term benefit compared to building-related proposals, as the replacement rate of cars is well within the time scale of targets such as those set by the Kyoto agreement. (iii) Third and finally, cars are not only associated with environmental issues, but also with accidents, fatalities and social erosion [3], bringing transport still higher up the political agenda. It has been shown that cities of a high density, such as, for example Hong Kong, have a far lower transport energy demand per capita than low density cities such as Houston, by a factor of 18. On average, when comparing 10 major cities in the US with 12 European cities, European cities are
five times as dense but the US cities consume 3.6 times as much transport energy per capita [4]. The conclusion often drawn from such data is that dense cities are low energy cities. However, it is not clear to what extent density is the cause, and increased energy use is the effect. For instance, it can be argued that where private car use is less—due to, for example economic conditions—cities are denser. Interestingly, historic European cities, such as Paris, lie at a national ‘optimum’—achieving moderate energy use for modest densities—whilst sustaining a rich urban life (Fig. 2). It is not evident that moving towards increasing urban density will lead to reduced car traffic—in fact, in the short-term the opposite is likely to be the case. In the absence of extra capacity in the form of effective integrated public transport, increasing the density will inevitably increase traffic and
Fig. 2. Transport energy vs. urban density for 32 cities [11].
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pollution. The challenge is, thus, to anticipate and implement public transport systems before increasing densities to ensure that more existing and future inhabitants use public transport instead of private cars and, thus, to shift the balance away from the cars. However, the long-term perspective of global environmental concerns (climate change, ozone depletion, acid rain, reducing fossil fuels, etc.) clearly means that building energy use needs to be addressed, particularly in relation to urban areas. This is combined with increasing urbanisation resulting in a predominantly urban global population. It is, thus, essential to minimise the deleterious global and local effects of urbanisation to ensure that the life nurturing qualities of the city can be restored and maintained. Building energy use is a predominant factor in this context.
2. Density and building energy The concentration of activities and people in cities is often perceived to be the main source of environmental problems. However, such concentration can have environmental advantages achieved, for example through the sharing of resources. Most obviously, more intense use of land and sharing of infrastructure—energy and water supply, drainage, roads, buildings and public transport—reduces the energy per capita associated with its construction (and possibly maintenance) and benefits from an economy of scale by comparison to a more dispersed urban configuration. An example of this is the use of combined heat and power (CHP) and district heating (DH) energy provision. MicroCHP has the potential to deliver thermal (55%) and electrical (30%) energy locally to a neighbourhood at a high efficiency—typically 85% or more overall—and reduces the transmission and distribution losses of more centralised power stations. This compares with an efficiency of approximately 30% for an average UK power plant to deliver electricity. However, for the balance of heat and power to be used optimally the energy demand should not only be localised but also mixed, combining housing with other commercial activities. Mixed use, high density neighbourhoods have the further potential advantage of local employment, commerce, retail, leisure, etc. which in turn may reduce the distances that people need to travel to access
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such facilities. The notion of ‘‘decentralised concentration and high density mixed land-use’’, thus, seems to be reinforced by energy considerations [5]. However, when one considers the implications of high density on the demand side of building energy use and on building integrated renewable energy production (such as photo-voltaics), does the balance begins to tip in favour of lower densities? This balance is likely to depend on building type, and for the purposes of this paper, we will consider domestic and office use as the predominant types, in the context of UK. 2.1. Domestic buildings Taking UK as an example to demonstrate the point, the energy demand in housing is dominated by space heating, which on average accounts for 60% of the total energy (Fig. 3) [6]. It is the space heating that will be most affected by design, the remaining consumption being largely determined by occupant needs and not strongly dependent on climate. In dispersed developments with the possibility of greater solar access, passive solar design will have greater potential to reduce space heating demands and, thus, overall energy use in housing. In order to explore the effect of increasing density on energy, we need to consider what parameters can be altered to change the density of housing. There are a number of ways in which the urban form of a city can change to increase density: (i) by increasing building depth; (ii) by increasing building height or reducing spacing (i.e. changing height:width ratios between buildings); (iii) by increasing ‘compactness’ (e.g. apartments instead of detached housing, where the building depth and height characteristics may theoretically remain unaffected). The first—increasing building depth—is perhaps particularly common in terraced housing in UK cities, where extensions to the rear in the form of conservatories, living rooms, kitchens and bathrooms are commonplace. As a result, the building envelope:floor areas ratio is usually (though not necessarily) increased, and the availability of daylight and sunlight to the interior is reduced. The energy implications of conventional extensions (as opposed to a passive solar extension) are an increase in both the heating
Fig. 3. Energy use breakdown for UK housing [13].
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Fig. 4. Effect of obstruction on space heating for housing [14].
and lighting loads. Overall, these increases are very small— in the order of 5–10% for a two storey addition to a three storey terraced house. Housing densities can also be increased, by increasing the average building heights (or reducing the spacing between them), which can be defined by the obstruction angles to the facades. An obstruction of 308 to the south fac¸ade of a passive solar house1 can increase heating energy by 22% compared to an unobstructed fac¸ade (Fig. 4). However, conventional, non-solar housing will also require between 6 and 15% more energy for heating compared to a passive solar house [7]. In an urban context, the solar potential for housing is reduced, largely due to the presence of obstructions but also because of planning constraints on orientation (Fig. 5). For the average UK dwelling, the heating contribution of solar energy is small, 10–15%, compared to a passive solar house—in the order of 40%—thus, the energy consequences of obstructions for a non-solar urban dwelling is small. Thus, there is in the above example, about a one-third increase in heating energy for an urban house as compared to a green field passive solar house. However, it should be noted that there will be a significantly wider range of energy use in an urban environment as some dwellings will be more 1 A ‘passive solar’ house here is defined as a house with 75% of its glazing to the south and 25% to the north, compared to a ‘non-solar’ house which here is taken to be a house with west and east facing facades each with 50% of the glazing. In both the cases, the assumption is that the houses are designed according to, in 1990, Building Regulations (see [7] for further details).
obstructed and facing north whilst others may benefit from unobstructed solar gains. The average one-third increase compares to a heat saving of 40% while comparing detached housing with apartments (Fig. 6) [7]. Thus, the way to increase density and energy efficiency simultaneously is to increase ‘compactness’ of the urban fabric whilst maintaining a limited building depth (in the order of 10–12 m), and where an appropriate solar orientation is ensured to access light, sun and air. The conclusion would seem to be that for energy use in UK housing the arguments for and against densification are finely balanced and will probably depend on the infrastructure issues mentioned earlier. However, as the average obstruction angles increase above about 308, the balance will begin to swing against densification. An average obstruction angle of 308 equates to a theoretical plot ratio of up to 2.5 (assuming the terrace or courtyard form with a plan depth of 10 m). Such a figure results in densities in the order of 200 dwellings per hectare (DPH) (assuming 125 m2 per dwelling)—well above the 50 and 75 DPH discussed by the Urban Task Force [8]. This compares with the current density of new housing in UK of an average of 25 DPH [9], Ebenezer Howard’s 45 DPH for the Garden City [10], the new PPG3 guidance of up to 50 DPH [9], and Kowloon’s 1000 DPH in Hong Kong [10] (Fig. 7). The conclusion is that relatively high housing densities can be achieved before a negative impact on the energy demand becomes significant. 2.2. Non-domestic buildings The non-domestic sector is very diverse, so for the purposes of this paper, the focus is on UK office buildings—the predominant non-domestic building type. Unlike housing, space heating for offices in UK is not the predominant issue in terms of primary energy use and energy cost. Typically, the artificial lighting demand, and in the case of an air-conditioned office, the fan power and refrigeration loads, are often equally significant (dependent on the type of office building). If one excludes office equipment, as a non-design related energy demand, the energy breakdown for a typical air conditioned office in UK is: 44% for air conditioning, 34% lighting and 22% heating (Fig. 8) [11]. It is, therefore, clear that avoiding, or at least reducing air conditioning, is a primary objective to reduce energy demand. The secondary approach is to reduce reliance on artificial lighting by increasing and exploiting daylight availability. Finally, reducing heat losses is likely to be the least significant energy efficiency strategy and, therefore, should not be used as a determinant for building form, except where it compliments the previous two aspects (i.e. excessively compact and deep plan forms may reduce heat loss but will dramatically increase both mechanical ventilation and artificial lighting). The first two aspects—avoiding air conditioning and increasing daylight availability—are broadly complimentary in terms of building form implications. They both point
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Fig. 5. Effect of orientation on space heating for housing [14].
towards shallow plan building forms in order to enable natural ventilation and daylight penetration. Typical plan depths for naturally ventilated and day-lit office buildings are 12–15 m. The implication of limiting the plan depths of buildings might suggest that densities would decrease for low energy offices, compared to more conventional deep plan buildings. However, this is not necessarily the case. The avoidance of air conditioning may save up to 1 m depth of services zone per floor (i.e. 25% of the building volume assuming an initial 4 m floor-to-floor height) and a further 3– 5% of the volume for reduced plant size [12]. Therefore,
avoiding air conditioning can save a maximum of approximately 30% of the building volume. This may more than compensate for the reduced plan depths resulting in, for example, courtyard or finger-plan forms. The benefits of avoiding air conditioning through adopting shallow plan depths are typically that naturally ventilated offices use <40% of the primary energy. This is an enormous potential advantage. However, there are limitations in adopting natural ventilation in the urban context. Apart from internal gains levels (which are independent of context), the most often quoted constraints in cities are noise
Fig. 6. Relationship between building form and heat loss [14].
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Fig. 7. Dwellings and people per hectare for housing [3,15,16].
and air pollution. Both these factors frequently result in the adoption of air conditioning. The main cause of noise and pollution is traffic and, thus, as mentioned before, this issue needs to be addressed in advance of expecting a significant reduction in energy consumption for office buildings. In the meantime, there are building design strategies that can be implemented which limit the need for air conditioning. The most obvious is that it may only be necessary to mechanically ventilate the street facing zone (and/or to locate less noise sensitive accommodation there), allowing the rest of the building to be naturally ventilated from a more quiet and
cleaner courtyard, garden or atrium. Such a ‘mixed-mode’ strategy to the servicing of offices will go some way towards limiting energy use whilst maintaining the potential for future adaptation to full natural ventilation. Having established that it is in principle possible to maintain a given density of development and significantly reduced energy use by limiting air conditioning, it would seem appropriate to investigate the energy impact of increasing the density on office buildings. The primary energy consumption of an average naturally ventilated office building in UK is dominated by artificial lighting and heating
Fig. 8. Average energy use breakdown for air-conditioned offices [17].
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Fig. 9. Average energy use breakdown for naturally ventilated offices [17].
Fig. 10. Effect of increasing the building depth on energy use for a mixed mode office.
loads (Fig. 9). As the level of obstruction to buildings will affect daylight and sunlight availability, both the lighting and thermal loads will be affected by increasing urban densities. If again we consider two possible strategies for increasing density as being increasing building depths and heights, some initial analysis can be carried out. By increasing the building depth of offices the availability of natural ventilation and daylight reduces, resulting in an anticipated increase in mechanical ventilation and artificial lighting. However, heat losses are likely to diminish because the surface:volume ratio reduces with increasing plan depths. Using the LT method [11] to determine the energy use implications, it can be seen that for a low energy naturally
ventilated office (with mechanical fresh air supply for deep plan areas) the energy use doubles when the plan depth is doubled from 12 to 24 m. However, for an efficient airconditioned building the energy use increase is only 20%, although the total energy is almost three times larger than the naturally ventilated option at a depth of 12 m (Fig. 10).2 Thus, increasing density by increasing building depths will inevitably result in increased energy use for office buildings, although the relative increase is less for airconditioned buildings. Nevertheless, on average deep plan, 2 Assumptions regarding the office buildings modelled include low energy, optimised control systems, a 50% fac¸ade glazing ratio, with a lighting datum of 300 lx and 30 W/m2 of internal gains, located in southern UK (see the LT method for more details [11]).
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Fig. 11. Effect of obstruction angle on energy use for offices [17].
air-conditioned buildings still consume typically twice as much energy as equivalent mixed-mode buildings (naturally ventilated and mechanical fresh air supply), suggesting that the avoidance of air conditioning by improving the urban microclimate is a key factor. The alternative strategy for increasing density is to increase the average building height (or reduce the spacing, which will have the same consequence for sunlight and daylight availability). Using the same assumptions as in the example given previous, it is possible to predict the energy consequences of increasing obstructions. From LT energy analysis, it can be seen that the total energy use would be expected to increase by 23% for air conditioned and 45% for non-air conditioned offices for an obstruction of 308 compared to an unobstructed situation (Fig. 11).
3. City texture Despite the clarity and insights gained from looking at urban density in terms of simple parameters, such as obstruction angles or plan depth, it is important to move from the theoretical description to study the formal complexity of a real bit of city. The study of urban form has been an ongoing pre-occupation at the Martin Centre for Architectural and Urban Studies for over 30 years—starting with the seminal work of Leslie Martin and Lionel March [13,14]. The current research builds on this work and focuses on the relationship between urban ‘texture’ and environmental characteristics. It has already been pointed out that the
energy performance of buildings is linked to the quality of the urban environment, and although not the specific focus of this paper, research into the solar, wind, pollution and acoustic effects of urban texture is going on. A number of recent research projects have developed analytical techniques, which categorise the physical urban geometry and assess the consequence of this on environmental performance [15–17]. One of the techniques developed by the Martin Centre’s urban environmental research team assesses energy demand in terms of urban form. This technique, referred to as ‘LT urban’, combines the LT energy analysis tool mentioned previously with computer-based image processing [18] to extract data related to building form for large urban areas (typically 400 m 400 m). To demonstrate the technique and its application to establishing the relationship between urban density and building energy use, a 400 m 400 m part of London is used (Fig. 12). As we are primarily interested in built form, numerous assumptions have to be made about the detailed characteristics of individual buildings, such as glazing ratio, U-values, systems, etc. These have been standardised, based on a detailed survey of the area and making informed estimates where necessary. The base-case urban form is altered by adjusting the building heights so as to produce a range of urban densities from half to double the existing. The energy consequences of increasing the average obstruction angles are significant—for example, 108 increase in obstruction results in approximately 10% increase in energy (Fig. 13). For a density range of plot ratios from 1.25:1 to 5:1 the results show that doubling the
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Fig. 12. A digital elevation model (DEM) is used to determine and map energy use for the London area under investigation (left, aerial view of London site; middle, DEM of the site; right, energy demand (kW h/{ m}2 per year)).
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Fig. 13. Effect of obstruction angle on energy use for naturally ventilated offices on the London site.
density typically increases energy consumption by in the order of 25% for this whole section of the city (Fig. 14). It is interesting to note that optimising the glazing ratios in response to the level of obstruction reduces the effect, so that doubling the density now results in a 21% increase in energy.
This highlights that other parameters, at the level of individual buildings, will change the relationship of energy to urban density, and that they should respond to the specific urban context. Glazing ratios in particular—as they represent the main interaction between building and climate—
Fig. 14. Effect of density on energy use for naturally ventilated offices on the London site.
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Fig. 15. Average optimum glazing ratios at different floor heights for the London site.
affect the energy performance and can be ‘optimised’ to minimise energy demand. This is demonstrated by determining the optimum glazing ratio for the London site, which for all orientations show that glazing should reduce as you go up a building’s fac¸ade (Fig. 15). Optimum glazing ratios vary less in response to orientation compared to green field sites, as in an urban context the obstructions reduce the differences in terms of daylight and particularly sunlight availability.
4. Conclusions The order of magnitude of energy implications in relation to urban density has been demonstrated for domestic and office buildings in UK’s temperate climate. For dwellings, the energy implications of compact densification are balanced between the benefits from reduced heat losses and the non-benefits of reduced solar and daylight availability. For office buildings, increasing urban density increases energy use because of the reduced availability of daylight in particular. However, this increase is significantly less than the energy increase of changing from a naturally ventilated office to an air-conditioned office. This change is only possible to prevent if the urban environment is less polluted and noisy. This is where the link between transport and building energy becomes evident, as cars are the major cause of urban pollution. Although overall urban transport consumes less than half the energy of urban buildings, the implications of reducing private car use in
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favour of clean and efficient public transport can be significant for building energy use. Thus, the move should initially be towards improving the urban environment, so that the energy benefits will outweigh the non-benefits. Increasing the urban population is not likely in the first instance to improve the urban environment, and the worst will be the increase in noise and pollution in the short-term before the necessary level of investment in transport infrastructure is made. The result may thus be increased nondomestic building energy use due to the adoption of airconditioning. Combined with climate change, the same effect may also occur in the domestic sector. It is clear that the political will is to a large extent and needs to be in place to encourage sustainable development. However, there is a need for an analytical approach to provide information on which the decisions will be based. The potential role for research is evident. ‘‘Towns and cities should be well designed, be more compact and connected, support a range of diverse uses within a sustainable environment which is well integrated with public transport and adaptable to change’’ [8]. It is not difficult to agree with this statement and its vision. What is important is to develop the necessary techniques to inform the balance, sequence and implementation of decisions to achieve the desired results. This issue of Energy and Buildings begins to provide the required knowledge, with a particular reference to urban environmental and related comfort issues. It is by making our environments more comfortable that we can increase the use of outdoor space for energy efficient movement (i.e. walking and cycling) and exploit these beneficial urban microclimates to improve the energy performance of buildings.
Acknowledgements Koen Steemers is joint Director of the Martin Centre where he heads the urban environmental research team—he particularly wishes to thank Nick Baker, Sam Lawton, Carlo Ratti and Dana Raydan for their support. Dr. Steemers is coordinator of EU research projects such as ‘‘Assessing the Potential for Renewable Energy in Cities (PRECis)’’ and ‘‘Project Towards Zero Emission (ZED) Urban Development’’, as well as ‘‘Sustainable Building Form’’ funded by the Tyndall Centre for Climate Change Research, on these aspects this paper is based.
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[4] P. Newman, J. Kenworthy, Cities and automobile dependence, Gower Technical, Aldershot, 1989. [5] T. Elkin, D. McLaren, Reviving the city: towards sustainable urban development, Friends of The Earth, London, 1991. [6] BRE, Domestic Energy Fact File, Building Research Establishment, Garston, Watford, 1992. [7] S. Yannas, Solar Energy and Housing Design, Vol. 1, Architectural Association, London, 1994. [8] Towards an Urban Renaissance, The Urban Task Force, E&FN Spon, London, June 1999. [9] J. Lewis, Radical PPG3 unveiled, Building Design, 10 March 2000. [10] E. Scoffham, B. Vale, How compact is sustainable—how sustainable is compact? in: M. Jenks, E. Burton, K. Williams (Eds.), The Compact City: A Sustainable Form? E&FN Spon, London, 1996. [11] N. Baker, K. Steemers, Energy and Environment in Architecture: A Technical Design Guide, E&FN Spon, London, 2000. [12] J. Berry, A. Todd, Space for services: four strategic planning, Architects Journal, February 1986.
[13] L. Martin, L. March, Land use and built forms, Cambridge Research, April 1966. [14] L. Martin, L. March, (Eds.), Urban Space and Structures, Cambridge University Press, Cambridge, 1972. [15] N. Baker, K. Steemers, D. Crowther, M. Nikolopoulou, P. Richens, The urban porosity model: simplified parameters as indicators of environmental performance, in: H.S. Stephens (Ed.), Solar Energy in Architecture and Urban Planning, Bedford, 1996, pp. 300–305. [16] K. Steemers, N. Baker, D. Crowther, J. Dubiel, M. Nikolopoulou, C. Ratti, City Texture and Microclimate, Urban Design Studies, Vol. 3, University of Greenwich, Kent, 1997, pp. 25–50. [17] K. Steemers, C. Ratti, Informing bioclimatic urban design, in: M. Voyatzaki (Ed.), Architecture and Engineering: The Teaching of Architecture for Multidisciplinary Practice, European Association for Architectural Education, 1999. [18] C. Ratti, P. Richens, Urban texture analysis with image processing techniques, in: Proceedings of the CAADFutures99 Conference, Atlanta, June 1999.