PRECis - Title
PRECis: Assessing the Potential for Renewable Energy in Cities.
The characterisation of urban microclimates, urban form and the environmental factors affecting energy use.
Coordinator: The Martin Centre for Architectural and Urban Studies.Department of Architecture.University of Cambridge.
Koen Steemers.Joint Director of the Martin Centre.
PRECis - Partners
University of CambridgeDr Koen Steemers (United Kingdom)
CFD norway as. Prof. Helge Norstrud (Norway)
Ecoles d’Ingénieurs de Fribourg Dr. Raphael Compagnon (Switzerland)
Centre for Renewable Energy SourcesDr Argiro Dimoudi (Greece)
Politecnico di TorinoProf. Mario Grosso (Italy)
Municipality of Grugliasco- associated to POLITO (Italy)
PRECis - Objectives
To demonstrate and quantify the relationships between the urban form parameters of a neighbourhood and the energy and environmental performance
To assess a series of urban planning strategies and case studies, ranging from minimal intervention to complete new urban plans
To develop techniques and guidelines which can assess and characterise any urban texture in terms of the potential for renewable energy
PRECis - Background
City scale (macroscale)Urban geographyCity shapeTransportInfrastructure
Intermediate scale (mesoscale)Urban block / neighbourhoodPublic spaceUrban designOutside / inside relationships
Building scaleBuilding formEnvelopeFabricMaterials
PRECis - Background
‘Cities of the industrial era have consciously excluded natural
processes, substituting mechanical devices made possible by
intensive use of fossil fuels… Thus, we might see our overwhelming
problems of depletion and pollution as largely outgrowths of our
ways of shaping the urban environment’.
Lyle, J.T. 'Regenerative Design for Sustainable Development', John Wiley and Sons, Inc., 1993
PRECis - Background
Background
• The urban context by definition has the highest density of
energy consumption and associated local environmental
problems.
• Improving the urban microclimate can bring environmental
and energy benefits.
• A better outdoor microclimate (cleaner, less noisy, cooler,
etc.) can reduce reliance on mechanical systems in urban
buildings.
PRECis - Potential
Potential energy savings
• reduced reliance on air conditioning can halve
energy use
• optimising form can reduce energy further by
approximately 25%
• optimising glazing ratios can give an average 10%
reduction in energy use
PRECis - Potential
Potential environmental benefits
• Reduced pollution
• More effective pollution dispersal
• Less noise propogation
• Increased daylight / sunlight availability or shade
• Healthier and more pleasant outdoor environment
• Increased walking and reduced reliance on transport
PRECis - State of the Art: Heat island
The urban heat island (Turban-rural) Montreal on 7 March 1968 at 7am, with light wind and cloudless sky represented as isotherms or ‘island’ of higher temperature above urban areas. Source: Oke, 1986
Air temperature (red) measured in urban canyon H/W 5.5 in vernacular Moroccan city of Fez (hot-arid climate), in summer (top) and winter (bottom) compared with meteorological airport air temperature (black)- low during daytime and high during night-time for summer clear nights. Source: Rosenlund et al, 2000
PRECis - State of the Art: Temp. v. Geometry
The relationship between maximum heat island intensity observed in a settlement and the canyon sky view factor (city texture) in its central area. Source: Oke, 1981
PRECis - State of the Art: Climate v. design
Recommended urban morphological responses to major climatic regions. Source: Keplinger, 1978
PRECis - State of the Art: Density
Would an energy-efficient environment mean great sacrifices in terms of amenity, equity or aesthetics?
LEFT: Density vs. urban form- same density, different ground coverage.Source: Urban Task Force, 1999
PRECis - The approach
PRECis - Environmental and form parameters
PRECis - Scales of investigation
Building and street specifics-Obstructions-Building depth-H/W canyon ratio-Street configuration
-Building use-Envelope U-value-Façade reflectances-Façade roughness
Immediate neighbourhood(effect of planted park)
Directional porosityCity frontal areaCity textureEdge effectPADCdCp
PRECis - Technique
Trondheim Cambridge Il Borgo Pathsia Pérolles Plaka
PRECisbase sites
Existing Proposals Generic urban forms Other real sites
Cambridge (UK) 1 3 Martin & March 6 Pescara (IT) 3Trondheim (N) 1 1 Street Junctions 8 London (UK) 1Fribourg (CH) Toulouse (FR) 1
Pérolles 1 5† Berlin (G) 1Grugliasco (IT) Cambridge
(UK)3
Il Borgo 1 3‡
Paradiso 1Athens (GR)
Patission 1Plaka 1
SUBTOTAL 7 12 14 9
TOTAL 42 (28 real sites + 14 generic sites)
Digital Elevation Model (DEM): image where each pixel has a grey-level proportional to its height. A DEM is equivalent to a full 3-dimensional description of the urban surface.
PRECis - Wind and pollution
Computed pressure coefficient and particle streak lines for wind direction of 270 and (left) =0.14, and (right) =0.28, for a generic urban area; =0.14 represents the turbulent flow over a smooth surface, =0.28 represents the turbulent flow over medium sized vegetation and small buildings
Pressure distribution on buildings superimposed with pollutant particle streak lines on the proposed scheme in the Trondheim site.
PRECis - Wind: simplified model
Correlation between ventilation rate (red) and Cd (Coefficient of Drag) (black) for the Trondheim site (bottom right); calculations done with CFD
PRECis - Wind and energy use
CPCalc+
Cd (Coefficient of Drag)
ESP-r
Investigating effect of urban drag coefficient Cd on airflow rates and energy loads (winter energy loss and summer cooling) and potential RE through natural ventilation
Three-step processing of an existing urban layout with Mapping Technique to reach the Reference Model Array (RMA, right), preserving geometric properties such as Plan Area Density (PAD)
PRECis - Wind and planning
Downwind wake core of buildings are plotted for the prevalent wind direction, representing an area of calm where wind velocity is reduced to under 50% of the upstream velocity.
PRECis - Vegetation and temperature
Rule of thumb:
As a rule of thumb, a 0.8K reduction in ambient air temperature is to be expected for a 10% increase to the ratio of green to ground surface built area
PRECis - Form and light
PRECis - Form parameters
London site view 400 x 400m
London Digital Elevation Model (DEM)
London axonometric obtained from a DEM
Aerial photo of LondonTotenham Court Road area
PRECis - Environmental parameters
Shadow casting performed on London DEM
Directional porosity rose for London DEM, varying with orientation
PRECis - Form and light
BOTTOM: comparative average sky view factors for 3 European cities - London shows higher potential for urban heat island occurrence.FAR BOTTOM: comparative average sky view factor values along the height of the street canyon, indicating potential daylighting inside buildings
PRECis - Form and energyLT (lighting and thermal) method: manual
spreadsheet method that allows the prediction of heating, lighting, ventilating and cooling energy consumption in non-domestic buildings and to help designers to determine how energy consumption of a building relates to architectural parameters at the concept design stage.
Passive zone Façade orientation Urban Horizon Angle at 6m high
Energy use (MWh/sqm/yr)
PRECis - Wind, light and noise
PRECis - Light and noise
A - 2.7 H/WD - 0.9 H/W
Sound pressure level extremesAverage sound pressure level in all streets
53.4 dB 67 dB
C - 0.45 H/W A - 2.7 H/W
View factor extremesAverage view factor from street to sky
0.81 0.37
PRECis - Case studies
FORM INDICATORS
Density (low - high)
Grain size (coarse - fine)
Compactness (compact - dispersed)
Street configuration (orthogonal - organic)
Skyline profile (rough - uniform)
Trondheim Cambridge Il Borgo Pathsia London Plaka
ENVIRONMENTALPARAMETERS
Plan area density + Surface to volume ratio
Surface to volume ratio
Porosity + surface to volume ratio
Directionality
Directional porosity + drag coefficient
Site coverage
Surface toVolume
ratio
35%
0.14
20%
0.26
30%
0.35
51%
0.25
55%
0.21
49%
0.31
…. Design guidelines
PRECis
‘Evolutionary approachrather than revolutionary’
‘Remedial interventionrather than radical
‘Urban microsurgery’
Minimal interventionwith optimal environmental benefit
...
THE END