PRECis Title

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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.
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PRECis - Partners
University of Cambridge Dr Koen Steemers (United
Kingdom) CFD norway as. Prof. Helge
Norstrud (Norway) Ecoles dIngénieurs de
Fribourg Dr. Raphael Compagnon (Switzerland) Cen
tre for Renewable Energy Sources Dr Argiro
Dimoudi (Greece) Politecnico di Torino Prof.
Mario Grosso (Italy) Municipality of
Grugliasco - associated to POLITO (Italy)
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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
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PRECis - Background
City scale (macroscale) Urban geography City
shape Transport Infrastructure
Building scale Building form Envelope Fabric M
aterials
Intermediate scale (mesoscale) Urban block /
neighbourhood Public space Urban
design Outside / inside relationships
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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
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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.

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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

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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

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PRECis - State of the Art Heat island
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
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
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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
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PRECis - State of the Art Climate v. design
Recommended urban morphological responses to
major climatic regions. Source Keplinger, 1978
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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
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PRECis - The approach
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PRECis - Environmental and form parameters
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PRECis - Scales of investigation
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PRECis - Technique
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.
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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.
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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
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PRECis - Wind and energy use
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
CPCalc
Cd (Coefficient of Drag)
ESP-r
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)
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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.
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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
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PRECis - Form and light
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PRECis - Form parameters
London site view 400 x 400m
Aerial photo of LondonTotenham Court Road area
London axonometric obtained from a DEM
London Digital Elevation Model (DEM)
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PRECis - Environmental parameters
Directional porosity rose for London DEM, varying
with orientation
Shadow casting performed on London DEM
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PRECis - Form and light
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LT (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.
PRECis - Form and energy
Passive zone
Façade orientation
Urban Horizon Angle at 6m high
Energy use (MWh/sqm/yr)
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PRECis - Wind, light and noise
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PRECis - Light and noise
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PRECis - Case studies
Site coverage Surface toVolume ratio
35 0.14
20 0.26
30 0.35
51 0.25
55 0.21
49 0.31
ENVIRONMENTALPARAMETERS Plan area density
Surface to volume ratio Surface to volume
ratio Porosity surface to volume
ratio Directionality Directional porosity drag
coefficient
FORM INDICATORS Density (low - high) Grain size
(coarse - fine) Compactness (compact -
dispersed) Street configuration (orthogonal -
organic) Skyline profile (rough - uniform)
. Design guidelines
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PRECis
THE END
Evolutionary approachrather than
revolutionary Remedial interventionrather
than radical Urban microsurgery Minimal
interventionwith optimal environmental
benefit...