The contribution of bioclimatic architecture in the improvement of outdoor urban spaces - Solar Energy Sciences and Engineering Applications - page 626

19.2.1 Planted areas
Planted urban surfaces, except from being aesthetically appealing, help to decrease
urban temperatures and reduce the energy consumption of buildings (Georgi et al.,
2010). Green spaces that help cool the environment can be applied in two forms: (i)
planted public areas, such as parks, gardens, and trees on the sides of roads or on
squares; and (ii) planted roofs (green roofs).
19.2.1.1 Green public spaces
Trees and vegetation contribute to the reduction of air and surface temperatures
through evapotranspiration and shading of adjacent urban surfaces. Evapotran-
spiration is “the combined loss of water to the atmosphere by evaporation and
transpiration'' (Santamouris et al., 2001, Shashua-Bar et al., 2011). Evaporation is the
process through which a liquid is converted into a gas and transpiration is the process
through which the water inside the body of a plant is released as water vapour in the
atmosphere. External temperatures can be reduced by up to 6 ◦ C by using groundcover
or lawn instead of paving, thus improving both comfort and energy efficiency of build-
ings, so that heating and cooling requirements are reduced (Sustainability Victoria,
Landscape Design).
Shade provided by trees contributes to temperature reduction, as it blocks the
solar radiation reaching paved surfaces and building envelopes. Shaded surfaces may
be 5 ◦ -20 ◦ C cooler than unshaded ones (EPA1, 2007). Cooler surfaces also lessen heat
island effect by reducing heat transfer to the surrounding air.
Cooler walls decrease the quantity of heat transmitted to buildings, thus lowering
cooling costs. The savings associated with shading from trees can be up to US$200/tree,
depending on the climate of the specific area (Akbari, 2002).
Joint studies by the Lawrence Berkeley National Laboratory (LBNL) and the Sacra-
mento Municipal Utility District (SMUD) placed varying numbers of trees around
houses to shade windows and then measured the buildings' energy use. The cooling
energy savings ranged between 7% and 47% and were greatest when trees were planted
to the west and southwest of buildings (EPA2).
Apart from cooling buildings during the warm months, deciduous trees allow solar
radiation to reach building façades through their branches during winter. At the same
time, when properly situated, trees can create wind barriers, protecting buildings and
reducing heat losses during the cold period.
The impact of trees largely depends on their number and positioning. A small
number of trees has a limited effect, compared to urban parks and gardens, which
extend their positive effects to the surrounding built environment. A study of two
urban parks in Singapore showed that the average temperatures obtained in both
parks were lower and the range of the standard deviation obtained from the parks was
smaller compared to those of the built environment. Also, a maximum 1.3 K average
temperature difference was observed around parks. It was also shown that energy
consumption for cooling can be reduced by almost 10% for buildings situated close
to urban parks (Chen and Wong, 2006).
However, planting trees comes with problems that need to be taken into account,
so as to select appropriate species and position trees so that they can have maximum
effect. Such problems include interfering with aboveground and underground utilities,