Review and Critical Analysis of International UHI Studies

4.5.2 Review for Cool Pavements:
4.5.2.1 Cool Pavement Principles
Similar to the principle of cool roofs, a low albedo of the paving material, implies that more of the
sun’s energy is absorbed thus directly increasing the pavement temperature and indirectly
heating the surrounding air and helping the formation of urban smog. When solar radiation is
reflected from a pavement surface some of it escapes the urban canopy and enters into space;
however, depending upon the sky view factor some may be intercepted and partially absorbed
by exterior building walls. The larger the thermal emittance coefficient (more significant for
pavements than roofs) the less heat is stored within the paving surface and thus there is less heat
stored to release after sunset hours. These two fundamental properties maximize the amount of
heat that is radiated away from the paving surface. This results in a cooler pavement than ones
built from materials that act to retain the sun’s solar energy as heat within their structure. In
principle, the cooler pavements will tend keep the surrounding air cooler and thus reduce the
demands on cooling loads. With similar logic, the cool pavement tends to increase the demand on
the heating load during the heating seasons in non‐equatorial climates. This is ode to less of the
solar energy being converted into heat in the pavement structure and thus less heat is conducted
into the surrounding air which would act to warm buildings and therefore reduce the heating
load. Nonetheless, as there is less solar energy in the winter than the summer the quantity of
solar energy that is converted into heat in the pavements is greater during the summer months
than in the winter months. Raising the albedo, thermal emittance and porosity acts to displace
more summer heat gain than winter heat gain.
Pavements can reflect a significant portion of the solar radiation away from the urban surface and
mostly direct it back towards the sky vault. The increase in albedo (along with the other key
properties that help define a cool pavement) have the potential to significantly affect the urban
air temperatures and thus reduce the A/C loads, thereby reducing the energy use and CO2
emissions. In theory, there are two mechanisms by which the cool pavements can mitigate global
warming:
a) by indirectly lowering the atmospheric temperature they can lower the demand on space
conditioning loads – associated to power‐related emissions;
b) by directly reducing the amount of solar radiation converted into tropospheric heat.
Cool pavements will indirectly lower the total annual space conditioning loads if they cause a
greater reduction in the cooling energy than they cause an increase in heating energy over the
year. This depends on:
� The degree to which there is a change in the pavement material temperature from
implementing cool pavement technologies;
� The climates in which the pavements are designed to operate – i.e. permeable pavements
often maintain their “cool” status better in wet conditions through evaporative cooling;
Review and Critical Analysis of International UHI Studies
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