Environmental Engineering Reference
In-Depth Information
Air temperatures in densely built urban areas are higher than the temperatures
of the surrounding rural countryside. The phenomenon known as “heat island'' arises
from many factors, the more important of which are summarized by Oke et al. (1991).
Urban heat island studies refer usually to the “urban heat island intensity'', which is
the maximum temperature difference between the city and the surrounding area. Data
compiled by various sources show that heat island intensity can be as high as 15 C.
Extensive studies on the heat island intensity in Athens, involving more than 30 urban
stations, show that urban stations present higher temperatures, of between 5 C and
15 C, compared to reference suburban stations.
Heat island, the most documented phenomenon of climate change, has a very
important impact on the energy consumption of buildings (Akbari et al., 1992).
Increased urban temperatures exacerbate the cooling load of buildings, increase
peak electricity demand for cooling and decrease the efficiency of air conditioners
(Santamouris et al., 2001). In parallel, high urban temperatures considerably decrease
the cooling potential of natural and night ventilation techniques and increase pollution
levels. Recent research has provided data on the amplitude and the characteristics of
heat island phenomenon in many European and US cities.
The impact of heat island on the cooling energy consumption of buildings is quite
important. Studies have shown that cooling energy consumption may be doubled due to
increased ambient temperatures in the affected areas (Santamouris et al., 2001; Hassid
et al., 2000). At the same time, the environmental quality in the overheated zones is
worsening as pollution is increasing (Stathopoulou et al., 2008), and the ecological
footprint of the city is growing seriously (Santamouris et al., 2007a). As reported, for
US cities with populations larger than 100,000, the peak electricity load will increase
1.5% to 2% for every 1 F increase in temperature. Taking into account that urban
temperatures during summer afternoons in the US have increased by 2-4 F over the
last 40 years, it can be assumed that 3-8% of the current urban electricity demand is
used to compensate for the heat island effect alone.
The heat island phenomenon may occur during the day or the night. Its intensity
is mainly determined by the thermal balance of the urban region and can result in up
to 10 C of temperature difference.
To counterbalance the impact of the heat island phenomenon, efficient mitigation
techniques have been proposed and applied (Santamouris et al., 2007b). This involves:
the use of advanced cooling materials for the urban environment that are able to
reflect solar radiation and amortize and dissipate heat (Doulos et al., 2004; Zinzi,
2010); strategic landscaping of cities, including appropriate selection and placing of
green areas, use of vegetative roofs (Niachou et al., 2001; Santamouris et al., 2007c;
Sfakianaki et al., 2009); solar control systems; and the use of heat sinks such as the
ground, water and ambient air, to dissipate excess heat.
Urban regeneration may be a very powerful tool to meet the objectives of sus-
tainable development through the rehabilitation of existing cities and building stock,
the recycling of previously developed land and the retention of greenfield sites. In
particular, refurbishment of existing urban environments should be seen as an excel-
lent opportunity to implement sustainability notions and as a start to adopting these
principles as a guide within which other considerations may trade off.
Although mitigation techniques have been extensively tested in various small-scale
applications, existing data on their potential to mitigate heat islands when used in
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