Assessing the Urban Microclimate:

Introducing innovative modelling techniques

KOEN STEEMERS AND MARIALENA NIKOLOPOULOU

The Martin Centre for Architectural and Urban Studies, University of Cambridge Department of Architecture,6 Chaucer Road, Cambridge CB2 2EB, UK. Tel: ++44 1223 331700 Fax: ++44 1223 331701, e-mail: kas11@cam.ac.uk

ABSTRACT

This paper introduces a new EU-funded research project ("PRECis - assessing the Potential for RenewableEnergy in Cities") and summarises the findings of a recently completed project ("Project ZED - towardsZero Emission urban Development"). Project ZED demonstrated the potential of defining key urban formcharacteristics, using image processing software, and linking these to environmental performance of asection of a city, assessed using computer and physical simulation. Project PRECis goes on to refine andvalidate this work by relating the theoretical work to actual urban projects as well as microclimatic andenergy use data. The result of the research is the development of a simplified urban bioclimatic design toolwhich responds to intermediate or neighbourhood scale issues.

INTRODUCTION

Cities present us with a dilemma. On the one hand, theconcentration of activities provides a necessary andstimulating social, cultural and economic milieu. On theother hand, the high density of buildings and transport useconsumes energy and generates atmospheric and noisepollution. This degrades the urban microclimate, whichaffects both the well-being of people and, crucially, byincreasing the demand for sealed indoor environments andair-conditioning, the success of low-energy building designstrategies. Air-conditioned and artificially lit buildings cantypically double building energy use (and buildings accountfor half of our total energy consumption). In addition,uncomfortable [1] and unhealthy urban conditionsdiscourage people from walking, cycling or using publictransport, thereby encouraging private vehicle use, which inturn leads to more energy consumption and atmosphericpollution. Poor urban microclimates thus have significantimplications for energy use as well as health.

However, it is clear that urban microclimates varyconsiderably, both from city to city and within cities, someareas, for example, being more effectively ventilated bywind than others. This variation can be seen to result froma complex interaction between urban form, climate andhuman activity. Urban form interacts with climate in otherways too, particularly in relation to solar access, withimplications for daylight availability and passive solardesign solutions, as well as active solar systems (collectors,photovoltaic cells, etc.). In other words, if we are to extendrenewable energy use in cities (which is now where mostEuropeans live and where most building stock is located),there is an urgent need for developing tools to facilitatebioclimatic urban design.

Unfortunately, no simple methods currently exist that cancharacterise the microclimate of urban areas at aneighbourhood scale, that is to say at the intermediate scalebetween the city block and the city region. Individualtechniques, such as CFD, allow environmental effectsaround buildings to be investigated, but they tend to becumbersome and expensive to use, and to require specialisttraining. Moreover, there is at present no way ofsummarising such effects for an urban area as a whole andcomparing them to established norms; the urbanequivalents of minimum Daylight Factor or air changes perhour have not yet been defined.

However, recent research at the Martin Centre has shownthat through the application of image processing techniquesto three-dimensional urban textures, connections can bemade between urban form, microclimatic characteristicsand the potential for renewable energies. This opens up thepossibility of a significant advance in predicting urbanmicroclimates and in being able to evaluate, without theneed for elaborate models, the environmental impact ofalternative proposals for change. These could relate eitherto a major new development and its immediatesurroundings or to incremental measures for graduallyimproving the environmental performance of an existingarea as a whole.

The central purpose of the research described in this paperis to characterise the microclimate within cities in relationto urban form. The specific aims are:

• To describe a simplified urban porosity model [2] thatdefines the microclimatic characteristics of an urbantexture, in overall terms, so that as the physical form ofthe urban texture is changed, the likely effect onenvironmental conditions, energy use and the potentialfor renewable energy can be determined.

• To demonstrate the application of the model and theenvironmental assessment techniques involved to aselection of representative existing European urbantextures.

• To show the application of the model and associatedtechniques to case study urban sites where majorinterventions are planned so that alternative proposalscan be assessed, and in this way demonstrating the roleof the model as a practical bioclimatic urban design aid.

PROJECT ZED

The full title of this project was: "Project ZED: TowardsZero Emission Urban Development – The interrelationshipbetween energy, buildings, people and microclimate" [3, 4].It was coordinated by the Martin Centre, funded under theEuropean Commission's APAS Programme (DGXII) –contract number RENA-CT94-0016 – and was completedin 1997. The contractors were:

• The Martin Centre for Architectural and Urban Studies,Department of Architecture, University of Cambridge

• Future Systems (architects, London)

• RP+K Sozietät (services engineers, Berlin)

• BDSP (environmental engineers, London)

• GRECO (School of Architecture at Toulouse)

• TÜV Rheinland (Köln).

The objectives of Project ZED were: to analyse andunderstand the pattern of various urban typologies fromplanning, microclimatic and energy use perspectives; to usethis knowledge to develop new analysis techniques [4]; touse the findings and existing expertise to inform designinterventions that demonstrate innovative strategies whichresult in zero emission urban development [5]; and topropose a framework for dissemination and education [6](Figure 1).

Of particular relevance here is the theoretical research tounderstand significant environmental aspects of the urbancontext. In order to determine the energy potential ofbuildings in the urban context it is necessary to be able todetermine the nature of the urban microclimate. Littleunderstanding exists in terms of the relationship betweencomplex urban form (and therefore questions of planning)and the microclimate (closely related to energy, comfortand health issues). This work has developed innovativeand simplified techniques and shows how they can be usedto predict the relative environmental performance of anypart of a city, either existing or proposed.

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Figure 1: Organisational diagram for Project ZED.

The theoretical research carried out for this projectdemonstrates the significant relationships between urbanform and environmental characteristics. In particular, theprocesses of radiation exchange and ventilation of urbaninterstices show a good correlation with urban formparameters. The increase in solar energy absorption withinurban texture correlates well with the volume to surfaceratio. And street ventilation as a function of wind directionrelates to the directionality of 'permeability' described bythe 'permeability rose', a new urban form descriptor definedin the project. Such new urban form parameters have beendeveloped in this project and can be extracted usingrelatively simple image processing techniques to analyseany urban texture (Figure 2).

Project ZED has demonstrated the validity of a simplifiedurban microclimatic model, describing the keyenvironmental processes that influence building energyuse, renewable energy and environmental quality. Theurban parameters enable the intermediate scale to bemodelled with more detail than existing regional climatemodels, yet not requiring the precise description ofindividual buildings. The model provides synoptic resultsrather than single time frame snapshots.

It is thus possible to analyse urban form in relation toenvironmental performance (i.e. the urban microclimate).This can then inform the design of urban projects whichminimise energy demand and provide on-site renewableenergy to meet this demand, with the target of zeroemission. Finally, the knowledge and information gainedcan be made accessible to a wide range of decision makersto inform the way forward Towards Zero Emission UrbanDevelopment.

PRECIS

The current research project is entitled: "PRECis: assessingthe Potential for Renewable Energy in Cities – Thecharacterisation of urban microclimates, urban form and theenvironmental factors affecting renewable energy use." Itis coordinated by the Martin Centre, funded by theEuropean Commission's JOULE Programme (DGXII) -contract number JOR3-CT97-0192 – and commenced inFebruary 1998. The contractors are:

• The Martin Centre for Architectural and Urban Studies,Department of Architecture, University of Cambridge

• CFD norway (CFD experts, N)

• Ecole d'Ingeneurs de Fribourg (Radiance experts, CH)

• Centre for Renewable Energy Sources (CRES, GR)

• Politecno di Torino (POLITO, I)

• Municipality of Grugliasco (GRGL, I)

The main objectives and technical approach aresummarised below:

1. To establish quantified generalised relationshipsbetween microclimatic characteristics at aneighbourhood scale and the geometric morphologicalproperties of urban form.

2. To develop a simplified urban model that describes themicroclimatic characteristics of urban texture so that asthe physical form of urban texture is changed, the likelyeffect on environmental conditions, energy use andpotential for renewable energy can be determined.

3. As a first step in validation, to apply the model andenvironmental assessment techniques to a selection of12 representative existing European urban textures.

4. To apply the model and associated techniques to threedetailed case study urban sites in Italy, UK and Norway,where major interventions are planned so thatalternatives can be assessed in collaboration with themunicipalities.

5. To assist municipalities, and for use more widely, indrawing up of bioclimatic urban design guidelines andpotential implications for bylaws, legislation, etc.

The principal beneficiaries of the project will be planners,architects and related urban professionals, for whom themodel will be a major advance as a bioclimatic urbandesign aid, applicable throughout Europe. It will providean objective means of assessing the environmental impactor potential of incremental or major urban change. Theproject will thus have wide applicability and contribute toenhancing the quality of life through improved urbanenvironments.

Project Coordin.

Microclimate data collation

3-D urban representation

Urban form parameters

Microclimatic modelling

Developing the model

Energy & RE module

Case studies

Urban design guidelines

Dissemination

Figure 3: Organisation of Project PRECis

REFERENCES

1 M. Nikolopoulou, N. Baker and K. Steemers. Thermalcomfort in outdoor urban spaces, PLEA 98 (these proceedings).

2 N. Baker, D. Crowther, M. Nikolopoulou, P. Richens and K.Steemers. The Urban Porosity Model: Simplified Parameters asIndicators of Environmental Performance, Solar Energy inArchitecture and Urban Planning, H. S. Stephens, Bedford, 1996.

3 K. Steemers, N. Baker, D. Crowther, M. Nikolopoulou and R.Clocquet. Project ZED: Modelling Environmental Characteristicsof Urban Forms, Solar Energy in Architecture and UrbanPlanning, H. S. Stephens, Bedford, 1996.

4 K. Steemers, N. Baker, D. Crowther and M. Nikolopoulou.Urban Design: Microclimate, Buildings and Energy, PLEA 96(Passive and Low Energy Architecture Conference), Louvain-la-Neuve, Belgium, 1996.

5 S. Stankovic, K. Steemers and J. Kaplicky. Building as aWind Concentrator: Assessment of Wind Energy GeneratingPotential in a Moderately Windy Urban Environment, SolarEnergy in Architecture and Urban Planning, H. S. Stephens,Bedford, 1996.

6 L. Adolphe. Towards a geocoded information system forenvironmental conscious design of urban projects, Solar Energy inArchitecture and Urban Planning, H. S. Stephens, Bedford, 1996.