Green Roofs versus Ponds and High Albedo Materials as Passive Cooling Techniques of Urban Spaces. more

1 Green Roofs versus Ponds and High Albedo Materials as Passive Cooling Techniques of Urban Spaces Eleftheria Alexandri1 and Phil Jones2 Welsh School of Architecture, Bute Building, King Edward VII Avenue, Cardiff CF10 3NB, Wales, United Kingdom 1 AlexandriE@cf.ac.uk 2 JonesP@cf.ac.uk Abstract : In this paper the effect of vegetation, water and high albedo of materials is examined in the built environment. The high capacity and low albedo urban building materials are mostly responsible for the formation of raised urban temperatures. If these materials are covered with materials of low heat capacity and high albedo, this phenomenon of excessive heat gains in the built environment could be moderated. The necessity of covering existing or new urban structures with high albedo materials, with plants or water is investigated. With the use of dynamic, heat and mass transfer algorithms these techniques are investigated for three cities in hot climates; Athens (hot and dry), Mumbai (hot and humid) and Riyadh (hot and arid). A comparison is done between the techniques and the different climates and conclusions are drawn, regarding which technique could prove more beneficial for mitigating raised urban temperatures for each climate. Key words: urban temperature, microclimate of buildings 1. Introduction Most cities, especially the ones in hot zones, suffer from raised urban temperatures and other climatic alterations, generally known as the heat island effect. Although raised temperatures might be beneficial during cold periods, for warm and hot periods they may be responsible for the loss of lives [1]. Apart from that, they are responsible for greater needs of energy consumption for cooling city centres. In the instance of Athens, the heat island intensity reaches up to 12oC in summer, which is responsible for 50% larger cooling loads in the city centre than in the suburban areas [2]. It is impossible to argue about designing with passive cooling strategies in buildings in the centres of cities, unless these raised urban temperatures are mitigated. Very old techniques of mitigating raised temperatures around buildings are placing vegetation both on the building envelope or very close to it, water features near the building, as well as covering the building elements with high albedo materials (e.g. white paint). This paper investigates these three techniques as a modern approach to the moderation of the raised temperatures which contemporary cities face. 2 2. Methodology A one-dimensional, prognostic (dynamic) micro scale model has been developed and programmed in C++, describing heat and mass transfer in a typical urban canyon [3]. The model is capable of describing thermal phenomena in building materials, vegetation, soil, water and the air outside the buildings and has been validated with an experiment [4]. It is based on heat and mass transfer algorithms, taking into consideration the effect of humidity gradiets on temperature. In this way, the effect of plants on climates with similar thermal characteristics, but different humidity concentrations can be examined in detail. Surface and air temperatures above four types of roofs are examined for three types of hot climates. A plain concrete roof is examined, and a concrete roof covered with high albedo material (Picture 1a). Temperatures of the plain concrete roof are compared with temperatures on and above a high albedocoated concrete roof, green roof (Picture 1b) and a roof covered with 20cm of water (Picture 1c). Water is at 20oC, at midnight and is not renewed for the whole day under investigation. The thermal properties of the concrete and the green roof are given in [3]. The three climates for which temperatures on and above these roofs are examined are the Mediterranean climate (hot and dry summer), with climatic data for the city of Athens, the rain forest climate (hot and humid), with Mumbai's climatic data and the desert climate (hot and dry), with the climatic data of Riyadh. The hottest period of all three cities is examined, which is July for Athens and Riyadh and May for Mumbai. All three cities are examined for a typical day of their hottest period. The term “typical” implies a day whose hourly values are the averaged hourly values of the month under investigation. The climatic data of all three cities have derived from METEONORM [5] and are input in the boundary node "urb", in Picture 1. urb 4.00 air' 0.40 urb urb 0.30 air 4.00 4.00 air' a1 0.40 air' 0.40 0.30 air 0.10 0.10 0.10 0.10 0.10 0.10 0.10 air 0.30 gr-out gr-in s,out s c,s c c,in in 0.30 a1 0.30 0.10 0.10 0.10 0.10 0.10 a1 0.30 wa,s wa c,wa c c,in in 0.10 0.10 0.10 c,s c c,in in a b c Pond roof roof model Green roof model Concrete roof model roof model, (b) Green roof model and (c) pond model Picture 1. (a) Concrete 3 3. Results Comparison of Green and Concrete Roofs For a typical day in Athens and Riyadh in July and a typical day in Mumbai in May, a comparison is made between calculated temperatures for a concrete and a green roof. The results are shown in Picture 2-Picture 10, for the three cities. Surface and air temperatures at the green roof are symbolised as [gr r], and at the concrete roof as [con]. In the instance of Athens, concrete roofs (node c,s, Picture 1a) reach peak surface temperatures of the magnitude of 54.6oC and a day-time average temperature of 43.1oC (Picture 2). At the same time, the surface temperature of the green roof (node gr-out, Picture 1b) reaches a peak of only 34.9oC and a day-time average of 26.1oC (Picture 2). The two roofs reach an average day-time surface temperature difference of the magnitude of 17.0oC. During peak time this difference reaches 26.2oC. Regarding the boundary air temperature (0.30m above the roof, a1 node in Picture 1), its day-time average difference reaches 10.2oC (Picture 3). During peak time this difference reaches 22.4oC. For the air node 1m above the roof (air’ node, as seen in Picture 1), these differences become smaller, but still sensible, with a 3.1oC day-time average and a 7.3oC maximum (Picture 4). For Mumbai, with its higher amounts of solar radiation and higher air temperature, the differences between the two roofs are even larger; surface temperatures reach a day-time average difference of 17.4oC, with 27.6oC maximum (Picture 5). For the boundary air layer, these values become 11.4oC and 23.8oC, respectively (Picture 6), while for the air node 1m above the roof they lower to 3.6oC and 8.0oC, respectively (Picture 7). The results for arid and hot Riyadh display the largest differences between concrete and green roofs. Surface temperature differences reach a 21.0oC daytime average and 29.5oC maximum (Picture 8). For the boundary air layer, these differences become 16.5oC and 27.1oC, respectively (Picture 9), while the air layer 1m above the roof reaches 5.2oC day-time average and 8.8oC maximum decrease (Picture 10). In all cases examined, it has been shown that in all three dry, humid and arid climates, green roofs are capable of lowering not only surface temperatures, but also air temperatures above them (Table 1). The largest temperature decreases are noted for the arid climate of Riyadh (21.0oC day-time average and 29.5oC maximum, for the surface temperature) and the smallest ones for Athens (17.0oC day-time average and 26.2oC maximum, for the surface temperature). Table 1. Average (ave) and maximum (max) differences between temperatures on and above a concrete and a green roof City ∆Ts ( C) max Athens Mumbai Riyadh 26.2 27.6 29.5 ave 17.0 17.4 21.0 o ∆ta1 ( C) Max 22.4 23.8 27.1 ave 10.2 11.4 16.5 o ∆Tair' ( C) Max 7.3 8.0 8.8 ave 3.1 3.6 5.2 o Comparison of Pond and Concrete Roofs When comparing air and surface temperature differences between concrete and pond roofs for all three climates, the differences are of a similar magnitude as the ones discussed in the previous paragraph (comparison of Table 1 and Table 2). In Picture 2-Picture 10 surface and air temperatures at the pond roof are 4 symbolised as [water]. For Athens, the day-time average surface temperature decrease, when the pond roof is compared with the concrete roof, is 15.3oC, with a 28.1oC maximum (Picture 2). For the boundary air layer (a1), these differences become 8.9oC and 16.8oC, respectively (Picture 3). For the air node 1m above the roof, the differences reach 5.5oC maximum and 3.1oC day-time average (Picture 4). For humid Mumbai, the differences between the pond and the concrete roof are smaller than those between the green and the concrete roof. Although surface temperature decreases are quite similar for the maximum and the average (27.8oC maximum and 13.9oC day-time average decrease), air temperature decreases are much smaller, reaching 2.9oC day-time average and 5.5oC maximum 1m above the roof (Picture 7). Regarding Riyadh, the null heat capacity of the covering plants of green roofs has a greater effect than pond roofs for lowering the raised temperatures of a concrete roof. Surface temperature reaches a maximum decrease of 22.9oC and a day-time average of 10.8oC (Picture 8). For the air above the roof these decreases become even smaller, reaching 13.9oC maximum and 6.5oC day-time average at node a1 (Picture 9) and 4.6oC and 2.3oC, respectively, at node air' (Picture 10). As can be observed in Table 2, the climate that benefits the most from pond roofs is that of dry Athens, with the air 1m above the roof reaching a day-time average temperature decrease of the magnitude of 3.1oC. The smallest decreases are noted in arid Riyadh, with a respective value of 2.3oC. Table 2. Average and maximum differences between temperatures on and above a concrete and a pond roof City Athens Mumbai Riyadh ∆Ts ( C) max ave 28.1 15.3 27.8 13.9 22.9 10.8 o ∆Ta1 ( C) max ave 16.8 8.9 16.6 8.1 13.9 6.5 o ∆Tair' ( C) max ave 5.5 3.1 5.5 2.9 4.6 2.3 o Comparison of High Albedo Coated Roofs and Concrete Roofs High albedo roofs (such as white ones) have been proven to cool down urban temperatures [6], [7], [8]. By decreasing surface temperature, with a coating which does not absorb a large part of the incoming radiation, the air temperature is also lowered. Nonetheless, such coatings need frequent maintenance, as, in most cases they loose 25% or more of their albedo within three years [9]. In this paragraph, the white concrete roof which compared with a concrete roof without any coating (with a 0.23 albedo) in two stages. As a concrete roof with white coating (with a 0.70 albedo) and a concrete roof with white coating, 3 years after it has been applied (with a 0.53 albedo). In Picture 2-Picture 10, surface and air temperatures at the white concrete are symbolised as [wh-con], and as [old-wh-con] for the three-year-old, white-painted concrete roof. When comparing a newly coated white roof with a plain concrete roof (Table 3), the temperature distributions due to the albedo changes are quite noticeable, but smaller than in the previous cases. In the instance of Athens, surface temperature decreases by a 12.4oC day-time average and 20.9oC maximum. Air temperature decreases are smaller, with 7.0oC day-time average and 12.4oC maximum for the boundary air layer and 4.1oC and 2.2oC, respectively, for the air layer 1m above the roof. Temperature differences between the white 5 concrete and the concrete roof are similar for Mumbai, with the surface temperature reaching decreases of the magnitude of 11.7oC for the day-time average and 20.8oC for the maximum, 6.7oC and 12.4oC, respectively, for a1 node and 2.1oC and 4.1oC, respectively, for air' node. For the higher temperatures of Riyadh these changes become smaller, with the maximum surface temperature decrease reaching 17.9oC and a day-time average of 10.4oC. The boundary air node reaches respective values of 10.8oC and 6.1oC and the air layer 1m above the roof reaches decreases of 3.5oC maximum and 1.9oC day-time average. Three years after the white coating has been applied, temperature decreases lower by almost half (Table 4). In the instance of Athens, surface temperature decreases reach only 11.9oC maximum and 7.1oC day-time average, while the boundary air layer reaches 7.0oC maximum temperature decrease and 4.0oC day-time average. For the air layer 1m above the roof these decreases are even smaller, reaching 2.3oC maximum and 1.3oC day-time average. The decreases are of the same magnitude for Mumbai as well, reaching from 11.8oC maximum and 6.6oC day-time average for the surface temperature to 2.0oC and 1.1oC, respectively, for the air layer 1m above the roof. For Riyadh, the temperature differences between the concrete and the three-year-old white coated roof are even smaller, reaching from 10.1oC maximum and 5.9oC day-time average for the surface to 2.0oC and 1.1oC, respectively, for the air layer 1m above the roof. In general, white coated building materials can lower their surface temperature and the air temperature above them quite significantly, but not as much as green and pond roofs, as will be argued in the following paragraphs. Due to the heat capacity of the building material and the lack of evaporation, this decrease is smaller than for pond and green roofs. As the white coating gets older, the cooling of urban spaces from high albedo materials lowers significantly, as the albedo decreases with time. Table 3. Average and maximum differences between temperatures on and above a concrete and a white-coated roof City Athens Mumbai Riyadh ∆Ts ( C) max 20.9 20.8 17.9 ave 12.4 11.7 10.4 o ∆Ta1 ( C) Max 12.4 12.4 10.8 ave 7.0 6.7 6.1 o ∆Tair' ( C) max 4.1 4.1 3.5 ave 2.2 2.1 1.9 o Table 4. Average and maximum differences between temperatures on and above a concrete and a three-year-old white-coated roof City Athens Mumbai Riyadh ∆Ts ( C) max 11.9 11.8 10.1 ave 7.1 6.6 5.9 o ∆Ta1 ( C) Max 7.0 7.0 6.1 ave 4.0 3.8 3.5 o ∆Tair' ( C) max 2.3 2.3 2.0 ave 1.3 1.2 1.1 o Green Roofs versus White Concrete Roofs In the case of Athens, as can be observed in Picture 2-Picture 4, both the surface temperature of the green roof and the air temperature above it are higher than the white concrete’s temperatures in the morning, for 7 hours, with a 3.5oC average difference. For the boundary air layer this difference becomes 4.1oC and for the air layer 1m above the roof, it reaches only 1.3oC. Later on during the day, as heat is stored up in the white concrete roof, its surface and air temperatures become higher than those of the green roof’s, reaching a much larger average difference of 8.8oC for the surface, 8.1oC for the boundary layer and 2.7oC for the air layer 1m above the roof. Three years after the coating has been applied ([old-wh-con] case), surface temperature of plants exceed that of the roof’s only by a negligible 0.2oC (Picture 2). The air above it slightly exceeds it by 1.6oC in the boundary layer (Picture 3) and by 0.6oC at 1m height above the roof, in the morning (Picture 4). The rest of the day, the green roof shows much lower temperatures, by an average difference of 10.1oC for the surface, 11.2oC for the boundary air layer and 3.2oC for the air layer 1m above the roof. For Mumbai, with its slightly higher irradiation, the morning hours when the green roof has higher surface temperatures than the newly coated white concrete one, are 6, less than that for Athens. During this period, the average difference between the surface temperatures reaches 3.1oC (Picture 5). The boundary air layer reaches 3.5oC (Picture 6) and the air layer 1m above the roof 1.1oC (Picture 7), all three differences being slightly smaller than those of Athens. The rest of the day the differences between the lowered temperatures above the green roof and the white concrete ones are even larger, reaching an average difference of 10.5oC for the surface, 10.2oC for the boundary air layer and 3.4oC for the air layer 1m above the roof. Three years after the coating has been applied, green roofs in Mumbai have a far cooler behaviour than the old white-coated concrete surface. Green surface temperature is still lower than that of the old white concrete surface, while the air temperature above the green roof exceeds that above the old white concrete roof by an average of 1.6oC at the boundary layer and 0.5oC at 1m height, for only 4 hours. For the rest of the day, temperature at the boundary air layer above the green roof is lower than that above the old white roof by 11.4oC average and at 1m height this difference becomes 3.8oC. Regarding surface temperature, plants have a lower surface temperature than the old white roof throughout the whole day, by a 13.2oC average. The optimum conditions for the green roof to cool its surroundings are those of the arid climate of Riyadh, with its high temperatures and insolation in July. Its surface temperature does not exceed the surface temperature of the white concrete roof (Picture 8), nor do air temperatures above it (Picture 9-Picture 10). For the freshly white coated roof, the day-time average difference between its surface temperature and the surface temperature of plants on the concrete roof, reaches 8.8oC. The day-time average temperature difference for the boundary air layer is 9.2oC and for the air layer 1m above the roof 2.8oC. After three years, when the white paint’s albedo has been reduced by 25%, these differences become even larger, with the green roof having a smaller surface temperature by a day-time average of 14.1oC. The difference between the two boundary air layers reaches 11.9oC and at 1m above the roof the daytime average difference is of the magnitude of 3.9oC. The day-time average differences between a newly coated white concrete roof and a green roof are summarised in Table 5. The differences between the three-year-old white concrete roof and the green roof are given in Table 6. A distinction is made between the periods that the green roof has higher temperatures ([gr r]>[wh-con]) and the periods when its temperatures are lower ([gr r]<[wh-con]). In general, Riyadh seems to benefit more by green roofs than 6 7 by white painted concrete ones. The city which benefits the most from white painted roofs is Athens. Yet again, the periods when green roofs offer cooler effects than the white painted roofs are longer and of larger intensity, for all three climates. In all cases, the white coating loses its high albedo relatively quickly, and the amplitude of its difference with green roofs becomes even larger for all three cities examined. Table 5. Average temperature differences (in C) between a white coated concrete roof and a green roof City ∆Ts ([grr]>[whcon]) 3.5 3.1 ∆Ts ([grr]<[whcon]) 8.8 10.5 8.8 ∆Ta1 ([grr]>[whcon]) 4.1 3.5 o o Athens Mumbai Riyadh ∆Ta1 ([grr]<[whcon]) 8.1 10.2 9.2 ∆Tair' ([grr]>[whcon]) 1.3 1.1 - ∆Tair' ([grr]<[whcon]) 2.7 3.4 2.8 Table 6. Average temperature differences (in C) between a three-year-old white coated concrete roof and a green roof City ∆Ts ([grr]>[oldwh-con]) 0.2 ∆Ts ([grr]<[oldwh-con]) 10.1 13.2 14.1 ∆Ta1 ([grr]>[oldwh-con]) 1.6 1.6 ∆Ta1 ([grr]<[oldwh-con]) 11.2 11.4 11.9 ∆Tair' ([grr]>[oldwh-con]) 0.6 0.5 ∆Tair' ([grr]<[oldwh-con]) 3.2 3.8 3.9 Athens Mumbai Riyadh Green Roofs versus Pond Roofs As can be observed in Picture 2-Picture 10, in all climates examined, in the morning and early afternoon hours, green roofs have higher temperatures than pond roofs, whose starting water temperature is 20oC at midnight. However, in the late afternoon and evening hours, due to the high heat capacity of water, pond roofs have much larger temperatures than green roofs in all three climates examined. As water does not have the capacity of regulating its temperature as plants do, it may raise its temperature to high levels when it is not regularly cooled with water of lower temperature. The differences between the pond roof and the green roof are given in Table 7. A distinction is made between the periods that the green roof has higher temperatures ([gr r]>[water]) and the periods when its temperatures are lower ([gr r]<[water]). It can be observed that for all three climates, the latter are much greater than the first ones. In the instance of Athens, the surface temperature differences between the green and the pond roof reach an average of 6.0oC when water surface temperature is lower than that of plants, and an average of 10.1oC when plants surface temperature is smaller than water surface temperature. The same analogy stands true for air temperature, as well. At the boundary air layer these average differences become 5.5oC and 10.4oC, respectively, while for the air layer 1m above the roof they are 1.7oC and 3.1oC, respectively. For Mumbai, these differences are of the same magnitude, reaching 5.2oC when the surface temperature of the green roof is greater than that of the pond roof and 10.9oC when the green roof has a smaller surface temperature than the pond roof. For the boundary air layer these values become 4.6oC and 11.2oC, respectively, while for the air layer 1m above the roof, these values become 1.5oC and 3.8oC. For the higher solar radiation of Riyadh the difference between green and pond roofs is larger. For the period when the pond roof has smaller temperatures than the green roof, the differences are negligible (0.8oC for the surface, 0.2oC for the boundary air node and null for the air node 1m above the roof). In contrast, the differences when the green roof has smaller temperatures than the pond roof are much larger, reach 13.6oC for the surface, 14.0oC for the boundary air layer and 4.3oC for the air layer 1m above the roof. Due to the heat stored in water, its temperatures may reach undesirable peaks, if the water is not renewed with new quantities of cooler water. On the contrast, plants do not need such treatment; as their thermal mass is unimportant, they do not store significant amounts of heat, thus their temperature drops quickly when solar radiation lowers. It is therefore more appropriate to use green roofs in climates with high solar radiation, such as Riyadh. Table 7. Average temperature differences (in C) between a pond roof and a green roof City ∆Ts([grr]> [water]) 6.0 5.2 0.8 ∆Ts([grr]< [water]) 10.1 10.9 13.6 ∆Ta1([grr]> [water]) 5.5 4.6 0.2 ∆Ta1([grr]< [water]) 10.4 11.2 14.0 ∆Tair'([grr]> [water]) 1.7 1.5 ∆Tair'([grr]< [water]) 3.1 3.8 4.3 o 8 Athens Mumbai Riyadh Pond Roofs versus White-Coated Concrete Roofs The temperature differences between the pond and the white coated concrete roofs are not as large as those between the green and the white coated roofs for all three climates examined. Still, they are quite significant, as can be observed in Table 8. It can be observed in Picture 2-Picture 10 that pond roofs tend to have larger temperatures than white-coated roofs from the evening hours onwards, due to the heat stored in water's larger heat capacity. In the morning and afternoon hours, for all climates examined, temperatures on and above pond roofs are smaller than those of the white-coated roofs, and this difference is larger than the evening one. In the instance of Athens, the average temperature difference when the water surface is smaller than the white-coated one (morning and afternoon) reaches 4.4oC, while the evening average difference (when the surface water temperature is greater than the white-coated concrete surface) is only 2.4oC. The differences are similar for the air nodes, reaching average differences of 0.9oC for the first case and 0.3oC for the second case. The differences are similar for the climate of Mumbai. The surface temperature differences when the pond roof shows smaller temperatures than the white roof reach an average of 4.3oC and 3.0oC for the period when the pond roof has greater surface temperatures than the white-coated roof. These differences become 0.8oC and 0.4oC, respectively, for the air layer 1m above the roof. For Riyadh the differences between the two types of roofs are not similar to those for the climates of Athens and Mumbai. As the great irradiation amounts of Riyadh and its high ambient temperatures are responsible for smaller temperature decreases from pond roofs, the difference for the evening period is greater than that for the afternoon period, when comparing surface temperatures. The surface temperature difference when the pond roof shows smaller temperatures than the white roof reach an average of 3.5oC, while it reaches 4.0oC, for the period that the surface pond temperature is greater than the white-coated surface temperature. Regarding air temperatures, due to the low humidity concentrations of Riyadh which allow for greater evaporation rates from the pond, are inverse, the average temperature difference is 2.0oC for the first case and 1.7oC for the second one at the boundary air layer, while they are both of the magnitude of 0.6oC for the air layer 1m above the roof. Three years after the coating has been applied, the pond roof has much smaller temperatures than the white-coated roof (Table 9). The differences during nighttime, when the pond roofs shows larger temperatures than the old white-coated roof are quite small. In the example of Athens, only 1.5oC average difference is reached between the surface temperatures of the pond and the old white concrete roof, at night-time, while a 9.3oC difference is reached during day-time, when pond roof shows smaller temperatures than the three-year-old white painted roof. Differences of a similar magnitude are observed at the boundary layer (0.6oC and 5.4oC, respectively), while 1m above the roof the pond roof shows only smaller temperatures by 1.9oC. The differences are of a similar magnitude for Mumbai (from 1.6oC and 9.8oC, respectively for the surface temperatures to 0.2oC and 1.7oC, respectively, for the air layer 1m above the roof) and slightly larger for the first case and slightly smaller for the second case for Riyadh (2.9oC and 7.4oC, respectively, for the surface and 0.4oC and 1.5oC, respectively for the air' layer). Table 8. Average temperature differences (in C) between a white coated concrete roof and a pond roof City ∆Ts([water]> ∆Ts([water]< ∆Ta1([water] ∆Ta1([water] ∆Tair'([water ∆Tair'([water [wh-con]) [wh-con]) >[wh-con]) <[wh-con]) ]>[wh-con]) ]<[wh-con]) 2.4 3.0 4.0 4.4 4.3 3.5 1.0 1.3 1.7 o o 9 Athens Mumbai Riyadh 2.7 2.7 2.0 0.3 0.4 0.6 0.9 0.8 0.6 Table 9. Average temperature differences (in C) between a three-year-old white coated concrete roof and a pond roof City ∆Ts([water]> ∆Ts([water]< ∆Ta1([water] ∆Ta1([water] ∆Tair'([water] ∆Tair'([water] [old-wh-con]) [old-wh-con]) >[old-wh<[old-wh>[old-wh<[old-whcon]) con]) con]) con]) 1.5 1.6 2.9 9.3 9.8 7.4 0.6 0.7 1.3 5.4 5.6 4.7 0.2 0.4 1.9 1.7 1.5 Athens Mumbai Riyadh 10 60 Ts,c[con] Ts,c[wh-con] 50 Ts,c[old-wh-con] Ts,wa[water] Temperature ( C) 40 Tlgr-out[gr r] o 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Picture 2. Surface temperature of the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Athens 50 45 40 35 Temperature ( C) 30 25 20 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Ta1[con] Ta1[wh-con] Ta1[old-wh-con] Ta1[gr r] Ta1[water] o Picture 3. Air temperature 0.3m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Athens 40 35 30 Temperature ( C) 25 20 15 Tair'[con] 10 5 0 1 Tair'[wh-con] Tair'[old-wh-con] Tair'[gr r] Tair'[water] 3 5 7 9 11 13 15 17 19 21 23 o Time (Hours) Picture 4. Air temperature 1m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Athens 11 70 Ts,c[con] 60 50 Temperature ( C) o Ts,c[wh-con] Ts,c[old-wh-con] Tlgr-out[gr r] Ts,wa[water] 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Picture 5. Surface temperature of the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in May in Mumbai 50 45 40 35 Temperature ( C) 30 25 20 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Ta1[con] Ta1[wh-con] Ta1[old-wh-con] Ta1[gr r] Ta1[water] o o Picture 6. Air temperature 0.3m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in May in Mumbai 40 35 30 Temperature ( C) 25 20 15 Tair'[con] 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Tair'[wh-con] Tair'[old-wh-con] Tair'[gr r] Tair'[water] Picture 7. Air temperature 1m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in May in Mumbai 12 70 Ts,c[con] 60 50 Temperature ( C) o Ts,c[wh-con] Ts,c[old-wh-con] Tlgr-out[gr r] Ts,wa[water] 40 30 20 10 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Picture 8. Surface temperature of the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Riyadh 60 50 Temperature ( C) 40 o 30 20 Ta1[con] Ta1[wh-con] Ta1[old-wh-con] Ta1[gr r] Ta1[water] 10 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Picture 9. Air temperature 0.3m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Riyadh 50 45 40 35 Temperature ( C) 30 25 20 Tair'[con] 15 10 5 0 1 3 5 7 9 11 13 15 17 19 21 23 Time (Hours) Tair'[wh-con] Tair'[old-wh-con] Tair'[gr r] Tair'[water] o Picture 10. Air temperature 1m above the concrete [con], white concrete [wh-con], old white concrete [old-wh-con], green [gr r] and pond roof [water], for a typical day in July in Riyadh 13 4. Conclusions From this theoretical study, the thermal effect in the vicinity of buildings of concrete, green, pond and high albedo roofs has been examined for three different climatic types (hot and dry Athens, hot and humid Mumbai and hot and arid Riyadh). It has been proved that the more solar radiation a roof receives and the drier the air is, the more beneficial it is to cover the roof with vegetation. For Riyadh, the roof’s surface temperature decreases by an average of 21.0oC and the air temperature 1m above it by 5.2oC, while for Mumbai these numbers become 17.4oC and 3.6oC, respectively. Pond roofs may prove beneficial for climates with mild ambient temperatures and less extreme solar radiation peaks than Riyadh. In the instance of Athens, the maximum surface temperature difference between the green roof and the concrete roof reaches 26.2oC, while the respective difference between the pond and the concrete roof is slightly larger, reaching 28.1oC. In general, pond roofs have lower temperatures than green roofs during morning and afternoon hours. In the instance of Mumbai they reach decreases from 5.2oC for the surface to 1.5oC at the air layer 1m above the roof. Nonetheless, due to the high heat capacity of water, evening temperatures are much larger on and above pond roofs than green roofs. In the late afternoon and evening hours, they have more than double larger temperatures than green roofs, reaching differences from 10.9oC, to 3.8oC, respectively, in the example of Mumbai. White-coated roofs lower their temperatures significantly, from a maximum surface temperature decrease of 20.9oC for Athens to 17.9oC for Riyadh. In the instance of Riyadh, where ambient air temperatures are quite raised, white paints are not as effective as the evaporating surfaces of water and the evapotranspiring characteristics of plants. Nonetheless, if white coating is not renewed, it becomes less effective with time, as its albedo lowers. For a threeyear-old white-coated roof, the surface temperature decrease reaches a maximum of only 11.9oC for Athens and 10.1oC for Riyadh. Regarding the comparison between green roofs and white roofs, it has been pointed out that although white roofs might result in lower temperatures during the morning, nonetheless, green roofs result in lower temperatures throughout the afternoon and evening and their effects are of a much larger intensity. In the case of arid Riyadh, green roofs have lower temperatures than white ones, even in the morning (Table 5). As the albedo of white roofs lowers quite quickly, a comparison has also been carried out with roofs of lowered reflectivity (Table 6), where differences are even larger and green roofs have lower temperatures than the old white ones, practically the whole day. From this comparison, it can be argued that in order to decrease urban temperatures, it is more effective, from a thermal point of view, to cover roofs with vegetation or water than with white paint. The drier a climate is, the greater the effect of transpiration or evapotranspiration; it is more beneficial for arid climates to cover urban surfaces with vegetation or water than with white coating. If water is preferred, it should be kept in mind that water should be renewed or proper night radiative cooling techniques should be applied, so that the excessive heat stored during the day can be removed from the water. White paints should be renewed quite often, as their albedo tends to lower quite soon, leading to raised surface and air temperatures. Apart from the climatic characteristics, the choice of the cooling technique depends on the time of the day lower temperatures are desired. Green roofs 14 may have raised temperatures than white-coated and pond roofs in the morning and early afternoon hours, but they lower their temperatures quite significantly in the evening, due to their negligible heat capacity. In contrast, both pond and white-coated roofs have lower temperatures than green roofs in the morning and afternoon, but their temperatures are more than double greater in the evening, when compared with green roofs. In general, due to the fact that green roofs do not store heat and do not raise their temperature beyond a limit, when compared to all types of roof covering, they show the comparatively lowest temperatures, of a larger intensity and for a longer periods. 5. Refrences : [1] Santamouris, M., (Ed), Energy and Climate in the Urban Built Environment, James & James, London, 2001. [2] White, K.S., (et al.), Technical Summary, Climate Change 2001: Impacts, Adaptation and Vulnerability, In: McCarthy, J.J., (et al.), (Eds), Climate Change 2001: Impacts, Adaptation and Vulnerability, United Nations’ Intergovernmental Panel on Climate Change (IPCC) Cambridge University Press, Cambridge, pp. 19-73, 2001. [3] Alexandri, E. & Jones, P., The Thermal Effects of Green Roofs and Green Façades on an Urban Canyon, PLEA 2004, Eindhoven, 2004. [4] Alexandri, E. & Jones, P., Heat Transfer Modelling versus Heat and Mass Transfer Modelling in the Building Envelope; Comparison with Experimental Results, PLEA 2005, Beirut, 2005. [5] Remund, J., (et al.), Meteonorm, Version 3.0, Meteotest, Bern, 1997. [6] Akbari, H., (et al.), Peak Power and Cooling Energy Savings of High-Albedo Roofs, Energy and Buildings, 25, pp. 117-126, 1997. [7] Konopacki, S., (et al.), Demonstration of Energy Savings of Cool Roofs, A Report Prepared for the U.S. Environmental Protection Agency, Heat Island Project, University of California, Berkeley, 1998. [8] Taha, H., Urban Climates and Heat Islands: Albedo, Evapotranspiration, and Anthropogenic Heat, Energy and Buildings, 25, pp. 99-103, 1997. [9] Unknown, Cool Roof Rating Council Product Listing, As of May 5, 2003. Available from http://www.coolroofs.org [Accessed 13-5-2003]