Thesis document - Jana Milosovicova - Urban Design English

transformed into latent heat and thus contributes to evaporative cooling,the non-respirative built surfaces in urban environments absorbheat, store it and then slowly release the trapped energy in form ofheat. This energy absorbed by various materials (construction materials,pavement, soil, etc.), known as ‘storage heat’, is expressed bythe specific heat capacity 13 (Sass, online). The values for commonlyused materials are shown in Table 1.“(…) water and vegetation play a decisiverole in the transformation of solarenergy falling on the earth’s surface(…)” (Kravčík et al. 2007, p. 8)Table 1. Specific heat capacity of various materials (Source: EngineerToolBox, online)MaterialSpecific Heat Capacity- cp - kcal/kg o C(4186.8 J/kg K = 1 kcal/kg o C)MaterialSpecific Heat Capacity- cp - kcal/kg o C(4186.8 J/kg K = 1 kcal/kg o C)Asphalt 0.22 Sand 0.19Brick, common 0.22 Sandstone 0.22Clay 0.22 Soil, dry 0.19Concrete, stone 0.18 Soil, wet 0.35Concrete, light 0.23 Stone 0.2Glass 0.2 Water 4.19Granite 0.19 Wood, balsa 0.7Limestone 0.2 Wood, oak 0.48Plaster, light 0.24 Wood, white pine 0.6Some authors suggest not to attribute much weighting to the storageheat parameter. Oke, for example, “found no significant correlation betweentypes of man-made surfaces and heat island intensity. The onlysignificant difference observed was between man-made and naturalsurfaces. Thus, concrete surfaces and asphalt paving, brick walls andcinder blocks, all contribute more or less equally to the problem ofurban heat build-up, while tree-covered areas show remarkable reductionin heat build-up” (Oke 1981 in Emmanuel 2005, p. 24).This implies that in the city – where most of surfaces are impermeableto water and the precipitation runs off rapidly into the sewer system(or is collected into rainwater tanks) rather than being availablefor evapotranspiration and its cooling effect on-site (Sass, online), thelarge-scale removal of vegetation and draining is connected with theabsorption of solar radiation, formation of “hot plates” on land and withthe release of a colossal amount of the stored heat. “Sensible heat releasedfrom just 10 km 2 of drained land (a small town) for a sunny dayis comparable with the installation power of all the power plants in theSlovak Republic (6,000 MW)” (Kravčík et al. 2007, p. 28).The distribution of solar energy on Earth is also determined by the amountof reflected solar radiation. Albedo, or surface reflectance, expressesthe ratio of reflected radiation out of total radiation. For example, vegetationreflects about 5-30% of shortwave solar radiation and hence hasan albedo of 0.05-0.3; a colour-painted wall reflects up to 35% (albedo0.35), while a white-painted wall reflects up to 90% of solar radiationand has an albedo of 0.9 (fig. 13) (Kravčík et al. 2007, p. 25).13 Energy needed to raise the temperature of 1 kg of a substance by 1 degreeFig. 13 Albedo of various materials in urbanenvironment2 Literature review: The impact of built structures on climate, energy flow and the water cycle17