Environmentally Friendly Cities

Proceedings of Plea 1998, Passive and Low Energy Architecture, 1998, Lisbon, Portugal, June 1998

Eduardo Maldonado, Simos Yannas, Simos Yannas

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781134256297
• Edition: 1
• Topic: Architecture
• Subtopic: Architecture General

The 15th Passive and Low Energy Architecture (PLEA) conference considered the issues of sustainability and environmental friendliness at the city scale. Some 150 papers address the many and varied questions faced by architects and planners in reducing the impact on the environment of cities and their buildings.

Urban Planning

Living with the City

urban design and environmental sustainability

SIMOS YANNAS

Environment & Energy Studies Programme
Architectural Association Graduate School
34–36 Bedford Square, London WC1B 3ES, UK
[email protected]

Abstract

The paper looks at the parameters affecting the urban microclimate and their implications for building design and for thermal comfort outdoors. A review of key principles is followed by illustrations from case studies in different urban environments.

INTRODUCTION

With the fossil fuel consumption which accompanies them, and the resulting air pollution, noise, and heat island effect, our daily urban activities have caused changes in climate and air quality which threaten the environmental sustainability of cities. Researchers have been reporting on heat island observations at different urban centres for some thirty years. A discussion of some of the underlying phenomena is given in [1]. This also quotes values of mean minimum urban temperatures as being 5–6K higher than those of the surrounding countryside, with differences rising to 6–8 K in large cities on calm and clear nights. The maximum difference between urban and rural temperatures has been found to correlate with population size [2]. For North America, such correlation yields an estimated urban-rural difference of 2.5 K for a town of one thousand inhabitants, rising to 12 K for a city with a population of one million. For European cities the correlation was reported to yield smaller differences suggesting heat island effects of lesser intensity; this has been explained as being due to lower buildings and wider street canyons compared to North America. Surely, however, the lower European energy consumption per capita must also be a factor, since energy use is the main anthropogenic contributor to the heat island, in this instance from heat exhausted from buildings and cars.

In these proceedings, a review of estimates of urban heat island is given by Santamouris et al [3] who also report on recent measurements in Athens. The latter show mean differences of 7–8 K between urban and reference suburban temperatures, with maximum differences as high as 17 K measured during daytime in summer.

Given that in Southern Europe summer temperatures exceed 30°C even in the countryside, such high increments can only be seen as detrimental to outdoor comfort in cities, as well as fostering the continuing growth in the installation and operation of air conditioning appliances. In the heating season on the other hand, differences of this magnitude could eliminate mechanical heating for much of the time. Both [3] and Gomez et al [4] report on parks as being considerably cooler than street canyons thus confirming the importance of vegetation for summer cooling in urban centres.

With quantitative indicators such as these directing attention to the issue, a number of questions arise regarding what might be meaningful actions by designers and policy makers.

More specifically:

• What are the processes and constituents of urban warming ? Can we predict, anticipate and control these processes ?

• What might be the energy and/or comfort penalties if building design continued to ignore the issue ? Have buildings which were designed as “bioclimatic” or low-energy, but with no account of urban warming, shown signs of being overdesigned for winter and underdesigned for summer ?

• If the only weather data designers have are from suburban airports, how can the urban warming effect be accounted for in building and urban design ? what analytic tools can be used to predict or simulate this effect ?

• What are the implications on outdoor thermal comfort and air quality in cities?

• Is the surrounding countryside also forming a heat island owing to processes of global warming ? In these proceedings, for example, Roaf et al [5] quote a predicted increase in summertime temperature by 2–3 K in Southern Europe over the next thirty years; would such increases apply on top of current urban temperatures or are they part of what we already observe today ?

It would seem that at a moment in time when many might have thought little was left to know about bioclimatic design, complexity and uncertainty have increased rather than decrease, and our ever more sophisticated analytic tools are still not good enough to inform design in an integrated manner. The next section of the paper considers some of the main processes affecting urban microclimates and the parameters designers need to consider and may be able to influence. Members of our group are currently involved in projects dealing with such issues in London, Athens, Lisbon, Rio de Janeiro, Dhaka and Chandigarh. Some of these are illustrated in the following sections of the paper and in other papers in these proceedings.

THE URBAN MICROCLIMATE

Beneath its “pollution dome” and canopy layer, the urban environment has become not unlike the interior of a building. Such analogy seems quite appropriate when examining the processes and interacting factors which characterise the urban microclimate and its deviation from the climate of the surrounding countryside. According to [1] the main contributing factors to higher urban temperatures are the following:

a) air pollution, which reduces incoming solar radiation

b) built form and surface properties which increase absorption of shortwave radiation but reduce the dissipation of longwave radiation

c) the production of heat by buildings and traffic

d) reduced airflow and humidity.

Effect of pollution dome

Air pollution reduces the transmissivity of the urban atmosphere. Part of the solar radiation directed toward the city is retained by the pollution dome over it. Part of the direct radiation which is let through becomes diffused in the process. As a result the amount of solar radiation reaching an urban surface is less than that falling on an equivalent surface outside the city; it also has a different composition. Taking London as an example, in the thirty years prior to the Clean Air Act of 1956 midwinter values of bright sunshine had dropped to less than half those of the adjacent countryside. Reductions of such magnitude can seriously affect thermal comfort outdoors, as well as the heating and cooling loads of buildings. Clearly, if one were to calculate a building’s solar gains with solar data from outside the city, it is likely that the contribution from solar gains would be overestimated and the building’s net heating demand underestimated. Summer predictions could be similarly affected. Moreover, any changes in the composition of the radiation (e.g. less direct and higher diffused component) could lead to further discrepancy between assumed and actual climatic conditions. Thus a rather paradoxical question arises for designers. What climate should we be designing for ? that which should have been, but seems to be no longer; or that which seems to be emerging, but should not have been ? In London the Clean Air Act succeeded in clearing the smog, and within a decade midwinter sunshine values had nearly doubled and were approaching those of surrounding rural areas. With air pollution increasing again, this time due to traffic rather than domestic and industrial energy use, the knowledge that the effects can be reversed is important. Clearly, however, if the urban climate can change drastically in relatively short periods of time we need to consider ways for making buildings climatically more adaptable.

Urban form and surface properties

Compared to open country, built urban sites have a larger area of exposed surfaces per unit area of ground covered. Owing to such larger area, potentially more solar radiation could be collected on a built urban site than on a flat open terrain, especially in winter. However, in addition to reductions in incoming radiation due to pollution, view of the sun is also a variable on urban sites. In the city, a surface’s view of the sun at any given time is largely determined by the built form and street widths and orientations. In midwinter overshadowing is often considerable on high density sites. Thus built density and built form become critical considerations.

The albedo of surfaces is the next important factor for consideration. This determines the absorption of solar radiation and thus the resulting rise in surface temperatures, which in turn influence the temperature of adjacent air layers. In central urban areas most ground finishes and other surfaces are of low albedo leading to high absorption of solar radiation. More careful choice of texture and colour of surface materials to suit the particular climatic conditions can make a considerable difference. An assessment of the effect of built form and surface albedo on absorbed solar radiation and air temperatures at four urban sites in Lisbon is given in [6].

The thermal capacity of surface materials is another important factor in its action as daytime heat sink. The effect is to reduce temperature fluctuation during the day, releasing stored heat after sunset. However, many researchers have observed that in clear weather the heat island effect is actually more pronounced after sunset. This is because in urban areas, the restricted sky view from street canyons reduces the cooling achieved by longwave radiation, which is also inhibited by the pollution dome. In the open country, on the other hand, outward longwave radiation is at its maximum with a clear sky. Thus rural areas cool faster and the urban-rural temperature difference becomes more pronounced at such times. In his study of urban canyons in desert areas [7], Pearlmutter observes this phenomenon as a warming-up of the urban canyon after sunset; he reports that during daytime compact canyons acted as a “cool island” because of the shading and heat storage provided by the surrounding surfaces.

Sky view is thus a very important consideration for cooling as well as for solar access. The combined importance of built form, canyon geometry, sky view, albedos, and thermal capacity has been highlighted repeatedly in field measurements. In a number of field studies carried out by members of our group in Central London this year, differences in air temperature of 1.5–2 K, as well as significant differences in humidity, air movement and illuminance, were recorded in adjacent spots only a few metres apart owing to these factors; these are documented and discussed in references [8] to [13]. In the Copacabana area of Rio de Janeiro significant temperature differences were also found between spots on the coastal avenue and the adjacent streets and are documented in [14]. The implications of such local variations are very important. Firstly, they show that the urban tissue is not homogeneous, either along a horizontal section from centre to outskirts, or vertically from ground to the urban canopy, and that generalised models would be of limited value. Secondly, the existence of such differences suggests that pockets of improved microclimate can be created by appropriate interventions, thus starting a reversal of the heat island effect.

Heat from buildings and cars

It is generally assumed that in winter heat production due to space heating and other energy uses in the city, including traffic, produces as m...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• Invited Lectures
• Urban Planning
• Bioclimatic Planning of Outdoor Spaces
• Solar Architecture
• Retrofitting
• Strategies, Systems and Components
• Daylighting
• Passive Cooling and Natural Ventilation
• Energy and Environmental Impact of Building Materials
• Design Tools and Analysis Techniques
• Regulations, Solar Access and Certification
• Education
• Design Support Schemes
• THERMIE Demonstration Projects
• Author Index