Review and Critical Analysis of International UHI Studies

At a City scale one study focuses on the Greater Toronto Area (GTA) in Canada [033]. This study
simulates the potential of the heat island mitigation strategies (i.e. cool roofs/pavements, shade
trees, wind‐shielding and urban vegetation) to reduce cooling energy use in buildings in the GTA
region. It accounts for both the direct and indirect effects. The study focuses on single family
residential and offices and retail stores. They model defined the variety of building types
available (according to their characteristics); simulate annual cooling/heating energy use and
peak demand using the DOE‐2 building simulation model thus determining the direct and
indirect energy and demand savings for each heat island reduction strategy, finally they
estimates the total roof area of air‐conditioned buildings in the GTA, using existing data sources
and thus were able to determine the metropolitan‐wide effects of heat island reduction strategies.
The gas and electricity prices were used to provide a financial estimate as to the extent of savings.
Potential annual savings for GTA were found to be $11m for the heat island strategies deployed
(with residential sector accounting for 59% of the savings). It is important to note that the DOE‐2
model has been addressed in both this paper and others to underestimate the cooling‐energy
savings and potential of reflective roofs (by as much as a factor of two); the method used to
estimate the actual roof area for commercial buildings was based on population and residential
roof area – thus further research in this area is required to generate more accurate results and
thus economic assessments. The findings indicate that the indirect savings potential appeared to
be smaller than the total potential energy savings. Therefore the greatest economic benefits arise
through mass implementation of cool roofs and shade trees that make both direct and indirect
savings over other indirect saving methods (e.g. cool pavements – only indirect). Moreover, the
study also does not address the mass use of UHI mitigating technologies in other building types
within the region (e.g. hospitals, schools, restaurants, etc) thus additional financial savings aren’t
accounted for and may be underestimated [033].
The State and National scale studies use a similar approach to arrive at the same conclusions,
therefore the principle study that is used to provide the figure for the State and National scales is
examined in closer detail [025, 027, 028, 112, 176]. This study example the energy and thus
monetary savings resulting from the mass implementation of cool‐colored roofs on residential
and commercial buildings in the main US metropolitan areas. The analysis is based on
simulations of building energy use ‐ using the DOE‐2 building energy simulation program. They
specify the prototypical buildings, simulate each prototypes using two heating systems
(gas/electricity); then they enter this data with the weather data in to the model and run the
simulation for buildings with light and dark colored roofs in a range of climates monitoring the
simulation outputs (for energy use of the air‐conditioning and the heating systems). Economic
estimates are then evaluated from the savings generated by changing from dark to cool‐colored
roofs. In order to obtain the National savings the Metropolitan Statistical Area results are then
extrapolated to the entire US. Annual savings for the state of California were found to be $100m
per year in 2005 [025, 112]. The studies highlight that the largest savings are in individual
buildings in the hottest and sunniest climates. It is important to note that the DOE‐2 simulation
results have been found to provide conservative estimates in comparison to field measurements.
Furthermore, there is no reference in this or any other study with adapting the internal a/c system
to the new cool‐roofing design – e.g. buildings cooling load is shown to decrease with the use of a
cool roof thus smaller units will suffice.
Review and Critical Analysis of International UHI Studies
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