212
7.4 Feasibility Study: UHI Mitigation Strategies
by Simulations
Many mitigation measures can be adopted and have been proposed by various
researchers, which could be classifi ed as measures that could only be implemented
during the design and planning stage (eg sky view factor and building material etc.)
and those that could also be implemented after the design and planning stages (eg
green areas and roof spray cooling) (Rizwan et al. 2008 ). RayMan model was used
in order to quantify possible increasing in thermal comfort as a consequence of
some possible mitigating measures of both types. Main inputs of the model relate to
the outdoor environment conditions: dry-bulb air temperature and RH, wind velocity, Bowen ratio (ratio of sensible over latent heat fl ux in evapotranspiration, fi xed
at 1.5) and cloud cover (fi xed at 1 okta). Other inputs are the albedo and emissivity
of surfaces, fi xed respectively at 0.30 and 0.95, typical values of urban
environment.
The following limitations in RayMan analysis must be highlighted:
– emissivity is considered the same for all the different kinds of surfaces;
– consequences of higher albedos cannot be correctly evaluated: the lower surfaces
temperature would not be estimated as it is given by the air temperature (input of
the software).
For these reasons next simulations concern topology modifi cations only (height
and distance of buildings, presence of green); obviously these are mitigation strategies that can be implemented during the design and planning stage only.
As described by Noro et al. ( 2015 ) and Busato et al. ( 2014 ), a slightly warm
PMV was obtained for via Rinaldi (Padua old town) and in via Pindemonte, the pilot
area (high density population residential zone): for the latter the modifi cation in
thermal comfort with some characteristics of the site was evaluated. Table 7.7
reports the results considering different layout of buildings:
– considering the actual situation;
– increasing the street width from 15 to 25 m;
– limiting the maximum height of buildings to 12 and 6 m;
– having a garden in front of the point of measurements instead of an apartment
building.
Every simulation was repeated using the same values of environmental variables
(air temperature and RH, wind velocity) as measured during the experimental sessions. Results in Table 7.7 show that an increase in Sky View Factor (SVF) thus
allowing a more effective nightly cooling of surfaces and a decrease of T mr . In particular, limiting the maximum height of buildings to 6 m would be the action with
the most relevant effect. The mean radiant temperature was shown to decrease by
2.6 °C and PMV by 0.2. Anyway, the night effects of an increased SVF were probably underestimated by RayMan, because the mean radiant temperature and so
PMV and PET were calculated by knowledge of air temperature (input) that is
M. Noro et al.
7.4 Feasibility Study: UHI Mitigation Strategies
by Simulations
Many mitigation measures can be adopted and have been proposed by various
researchers, which could be classifi ed as measures that could only be implemented
during the design and planning stage (eg sky view factor and building material etc.)
and those that could also be implemented after the design and planning stages (eg
green areas and roof spray cooling) (Rizwan et al. 2008 ). RayMan model was used
in order to quantify possible increasing in thermal comfort as a consequence of
some possible mitigating measures of both types. Main inputs of the model relate to
the outdoor environment conditions: dry-bulb air temperature and RH, wind velocity, Bowen ratio (ratio of sensible over latent heat fl ux in evapotranspiration, fi xed
at 1.5) and cloud cover (fi xed at 1 okta). Other inputs are the albedo and emissivity
of surfaces, fi xed respectively at 0.30 and 0.95, typical values of urban
environment.
The following limitations in RayMan analysis must be highlighted:
– emissivity is considered the same for all the different kinds of surfaces;
– consequences of higher albedos cannot be correctly evaluated: the lower surfaces
temperature would not be estimated as it is given by the air temperature (input of
the software).
For these reasons next simulations concern topology modifi cations only (height
and distance of buildings, presence of green); obviously these are mitigation strategies that can be implemented during the design and planning stage only.
As described by Noro et al. ( 2015 ) and Busato et al. ( 2014 ), a slightly warm
PMV was obtained for via Rinaldi (Padua old town) and in via Pindemonte, the pilot
area (high density population residential zone): for the latter the modifi cation in
thermal comfort with some characteristics of the site was evaluated. Table 7.7
reports the results considering different layout of buildings:
– considering the actual situation;
– increasing the street width from 15 to 25 m;
– limiting the maximum height of buildings to 12 and 6 m;
– having a garden in front of the point of measurements instead of an apartment
building.
Every simulation was repeated using the same values of environmental variables
(air temperature and RH, wind velocity) as measured during the experimental sessions. Results in Table 7.7 show that an increase in Sky View Factor (SVF) thus
allowing a more effective nightly cooling of surfaces and a decrease of T mr . In particular, limiting the maximum height of buildings to 6 m would be the action with
the most relevant effect. The mean radiant temperature was shown to decrease by
2.6 °C and PMV by 0.2. Anyway, the night effects of an increased SVF were probably underestimated by RayMan, because the mean radiant temperature and so
PMV and PET were calculated by knowledge of air temperature (input) that is
M. Noro et al.