The contribution of bioclimatic architecture in the improvement of outdoor urban spaces - Solar Energy Sciences and Engineering Applications - page 625

pavements and other urban structures are very limited. Some applications of cooling
materials combined with other mitigation techniques to improve the environmental
quality of open spaces are already reported for Tirana (Albania) and Athens (Greece)
(Fintikakis et al., 2011; Gaitani et al., 2011).
This chapter aims to present the existing knowledge on heat island mitigation
techniques and to provide information on existing applications. The main mitigation
techniques are presented in a simple and comprehensive way, while some of the well-
known and evaluated applications of different mitigation technologies are described.
19.2 MITIGATION STRATEGIES
The gathering of a great proportion of the world's population in cities, the need for
cheap and readily available housing, the extended infrastructure and the alienation
from traditional ways of living and working have caused the expansion of the manmade
landscape. The consequences of urbanization in the natural landscape are important:
the coverage of soil and permeable surfaces with non-permeable materials that absorb
heat, and the decrease of forests and planted areas dramatically increase surface and
air temperatures. These two parameters cause thermal discomfort to the inhabitants of
cities, simultaneously leading to increased electricity consumption of urban buildings.
Heat island is one of the most documented climatic phenomena of the contem-
porary urban environment, mainly caused by urbanization, while global warming is
another aggravating factor and an inevitable reality. Heat island effect is present in
urban and suburban areas, causing city centres to have considerably higher tempera-
tures than the suburbs. The phenomenon is present both during day and night, during
all seasons of the year and concerns cities in every part of the world. The intensity
of the phenomenon, meaning the temperature difference between the centre and the
periphery of cities, depends on various factors, including local weather, geomorphol-
ogy, urban geometry, anthropogenic heat, quality of the materials used for groundcover
and buildings, the quantity and type of green spaces, etc.
Planners and scientists can do little to harness weather, geomorphology and even
urban geometry. However, there are advances in new technologies that can contribute
to the improvement of the thermal properties of the built environment and to reduce
the intensity and the impacts of heat islands. The aim of these mitigation techniques
is to reduce the energy consumption of urban buildings and the concentration of pol-
lutants, so as to reduce temperature levels in the city centres and to establish comfort
conditions for city dwellers, users, etc. These goals are usually achieved by ameliorating
the thermal and optical properties of urban materials, in particular solar reflectance,
thermal emissivity and heat capacity, by blocking the incident solar radiation and by
increasing the cooling load of open spaces. These parameters largely determine the
urban air and surface temperatures.
The main mitigation techniques for reducing urban heat islands include:
an increase in planted areas;
use of materials of high albedo values, and cooling materials;
shading of the surfaces covered with high heat capacity - low thermal emissivity
materials;
use of thermal sinks.