Environmental Engineering Reference
In-Depth Information
Materials with high albedo values are usually light-coloured, natural or artifi-
cial materials, widely used in vernacular architecture. However, in recent decades,
“cool'' artificial materials have been developed. The main types of cool materials are
membranes, paints, anticorrosive and waterproofing coatings, cement products, pre-
painted steel and mortar. The characteristic of these products is that their albedo is
much higher than a conventional material of the same colour.
Cool materials are usually associated with “cool roofs''. Products for cool
roofing are made of highly reflective and emissive materials that can remain approx-
imately 28-33
◦
C cooler than conventional materials during peak summer weather
(EPA4). The large-scale use of cool materials in urban areas might lead to indi-
rect energy savings, due to the reduced air temperature brought about by increased
solar reflectance. The indirect benefits that arise from this ambient cooling of a
city or neighbourhood will in turn decrease the need for air-conditioning. Com-
puter simulation results, calibrated with actual measurements, have shown that
an increase in roof albedo from 0.2 to 0.78 reduced the cooling energy consump-
tion of a house in Sacramento (USA) by 78% (Bretz et al., 1989). Multi-year
observations show a temperature reduction of 0.3 K/decade because of the massive
construction of high-albedo greenhouses in the Almeria area of Spain (Campra et al.,
2008).
Cool roofs typically yield measured summertime daily air-conditioning savings and
peak demand reductions of 10-30% (Kolokotsa, 2008).
Although highly reflective materials have been extensively tested in cool roof appli-
cations, existing data on their potential to mitigate heat islands when used in pavements
and other urban structures are very limited. Conventional paving materials, such as
asphalt and concrete tiles, have solar reflectances of 5-40%. That means that they
absorb a great amount of the incident energy (EPA5). Ground-covering materials of
high solar reflectance, emittance and permeability, when applied on a part of a pave-
ment that is not shaded, can help reduce surface and ambient temperatures. In Flisvos
Park, in the municipality of Paleo Faliro, Greece, 4500 m
2
of cool paving materials
were applied. It was found that, under specific climatic conditions, the used materials
may reduce the peak daily ambient temperature of a typical summer day by up to
1.9 K, while surface temperatures were reduced by up to 12
◦
C (Santamouris et al.,
2012).
Cool roofs and cool pavement technologies were considered in combina-
tion in research by Millstein and Menon (2011). It was assumed that roofs
and pavements represent 25% and 35% of the urban area respectively,
while
the considered albedo increase for roofs and pavements was
0.15
respectively. It was calculated that cool roofs and cool pavements contribute to
decreasing afternoon summertime temperatures by 0.11-0.53 K. For some of the
urban locations studied,
+
0.25 and
+
no statistically significant temperature reductions were
found.
Rosenfeld et al. (1995) assumed an increase in the average albedo of 0.13, and
in particular from 0.13 to 0.26 for an area of 100,000 km
2
in Los Angeles. It was
calculated that the peak impact of the albedo change occurs in the early afternoon,
the potential cooling exceeding 3 K at 3 p.m. Simulations carried out under differ-
ent boundary conditions indicated that the expected peak summertime temperature
reductions are between 2 and 4 K.
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