The effect of urban environment on the cooling degree hours and its effect on the C.O.P. of air-conditioning unit

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1 International Workshop on Energy Performance and Environmental The effect of urban environment on the cooling degree hours and its effect on the C.O.P. of air-conditioning unit N.M. Papanikolaou, M. Santamouris and I. Livada Section Applied Physics, Physics Department, University of Athens ABSTRACT In the framework of the European program of»polis", an extensive network of 27 selfregistering stations, which were recording air temperatures inside the urban environment of the city of Athens for a time period of two years, from August 996 until August 998, was installed. The present study concerns the degree hours during the summer period, for seven of the above stations, and their differentiation, concerning the particular urban characteristics of each station, such as the orientation, plantation, and the adjacency to the sea. Finally, the effect of the urban environment on the C.O.P. of an air-conditioning unit was also examined.. INTRODUCTION The phenomenon of urbanization is intense worldwide. As it results from relative studies it is expected that by the year 2, 8% of the world population will be living in urban environment. According to relative studies (Balaras et al., 993a, b; Livada-Tselepidaki et al., 994, 995a, b; Tselepidaki(Livada) et al., 99, 992, 993a, b; Tsinonis et al., 993), which were conducted right after the heat wave of 987, which caused an important number of deaths in the city of Athens (Katsouyianni et al., 993; Livada-Tselepidaki et al., 994, 995), it became perceptible that the data from the three meteorological stations of the city did not suffice for the study of the urban heat island of Athens. In the framework of the European program of»polis»27 self-registering stations were placed in the basin of Athens. Those stations were used to record air temperatures, for the analytic study of the phenomenon of heat island. The detailed treatment and study of the data that was recorded, for the time period of August 996 to August of 998, showed that the prevailing meteorological conditions influence uniformly the basin of Athens. As a result, the observed differentiations were due to the local phenomena that assist the maintenance of high temperatures, in certain areas (Livada et al., 998, 22). 2. THE DATA USED From a total of 27 stations, we will examine seven of them, which were selected in order to examine the influence on the air temperatures, of a typical urban environment as well as the effect of sea and the plantation in the reduction of the urban heat island. The particular characteristics of each station, which are under review, are presented in Table. Furthermore, the measurements of air temperatures that were received by the station (St. A) of National Observatory of Athens (latitude = 37 o 58 N, longitude = 2 ó 43 E and altitude = 7m) for the period 95-24, will be taken into consideration, so as to compare the examined summer period (June - September of 996,997 and 998), in relation to a long-lasting time series of measurements. 3. PERSISTENCE OF HOURS & DEGREE HOURS OF AIR-TEMPERATURE EQUAL OR HIGHER THAN 3 O C DURING A DAY For the reviewed 7 stations as well as for the St. A, the following were calculated for each

2 2 International Workshop on Energy Performance and Environmental Table : General description of the station sites. Station Observations-description The region is an open area, on a hill, with important presence of Station Α: plantation. It is one of the three Observatory meteorological station of the city of Athens Station 4: Koridallos Station 6: Ag. Kosmas Station 7: Stournari Station 2: University Club Station3: Solonos & Mavromihali Station9: National Garden Station 22: Pedio tou Areos The station was placed 5m from the ground, inside an urban canyon with hight (H) m which was the same as the width(w). The orientation of the station was North-western. The area is on the west of the center of Athens The station was placed on the roof of two storyed building in the region of Ag. Kosmas by the sea. The region was open with significnat presence of plantation. The station was 5m from the ground, inside urban canyon with H / W and with 5 m width(w). The orientation of the station was South. The station was placed 5m from the ground in the fringes of a square. The orientation of the station was South-eastern. The station was placed m from the ground, inside an urban canyon with H/W 2,5 and with m width (W). The orientation of station was North-western. The station was placed in the National Garden. The region has dense vegetation. The station was placed in the thickets of Pedio tou Areos" and was found under continuous shading by the trees. The region has dense vegetation and is irrigated each morning. month: (a) the number of hours with temperature equal to or higher than the 3 o C, per month (Table 2), (b) the corresponding degree hours (DH) (Table 3), (c) the daily persistence of the hourly air temperatures which were equal or higher than 3 o C (Table 4). The stations, based on the statistical control of mean values of hours that present temperatures equal or higher than 3 o C, can be separated in two groups. The first group includes the stations that were placed in thickets (St. A., St9 and St 22) or by the sea (St 6) and the second, those which were placed in a typical urban environment (inside urban canyons) in the city centre (St.7, St.2 and St.3) and its western parts (St.4). As shown in Tables 2 and 3, between the two groups an important differentiation is observed, in the monthly number of hours with temperature equal to or higher than 3 o C, as well as the corresponding number of DH. The lowest values for both sizes were observed in the first group of stations (StA, St6, St9 and St 22) and the highest in the second group (St4, St7, St 2 and St 3). It is marked that station 2, which was placed in the centre of the city, with southeastern orientation, where there was no shading by other buildings, particularly from the sunrise until midday, the observed values of DH are often double, compared to the number of DH of its neighboring station (St.3), which is Northern-western oriented. Beyond the orientation these higher values of station 2 are also due to the significant traffic of the area, as well as to the existence of a basic terminal station of urban buses. The immediate higher values are presented in St 4, which was placed in the western urban region of the basin of Athens. Moreover, this station presents the higher frequency of hours with high temperatures after 4. (6-97%). This particular behavior is owed to the orientation of the station, where in combination Table2: Monthly values with hours equal or greater than 3 o C. St.Α St St St St St St St St.Α St St St St St St St

3 International Workshop on Energy Performance and Environmental 3 Table 3: Monthly values of degree hours (DH) on the base of 3 o C. St.Α St St St St St St St St.Α St St St St St St St with the small aspect ratio of street (H/W=), it results in the acceptance of the direct solar radiation, from the early afternoon hours until the sunset. The analysis showed that, even though statistically important persistence is generally observed, the highest values were presented once again in the centre of the city and in the St 4 i.e. in the stations, which they were placed inside urban canyons. 4. PERSISTENCE DURING A MONTH In order to examine the phenomenon of persistence of synoptic conditions, which encourage the appearance of at least one hour per day, with air temperature equal to or higher than 3 o C, the coefficients of autocorrelation (Ri), with lag, 2, 3, 4, 5, 6, and 7 days, were calculated for Besson's coefficient of persistence National Obs.of Athens (St.A) June July August September Years Figure : Besson s coefficient of persistence for June, July, August and September Table 4: Monthly values of the Besson s coefficient of persistence (R B +). St.Α St St St St St St St St.Α St St St St St St St each month and each station. As it appears from Figures 2a, b, June, which is characterized by large intervals with constant synoptic conditions the persistence reached the 5 up to 6 successive days. Exception constitute, stations 22 and 6 where due to their particular local characteristics (irrigation and adjacency to the sea respectively) the persistence is statistically significant for the duration of 2 to 3 days. In July the statistically significant persistence fluctuated in much lower values ( to 2 successive days), for all stations (Figure 3a, b). This observation should be attributed to the appearance of predominant North-Northeastern winds of Etesian, which winds during this month encourages cooling the city. The above explanation is also strengthened by the results of August, where the statistically significant values of autocorrelation coefficients are from two to four successive days for all the stations (Figure 4a, b). The statistically significant persistence in September is limited in one day in the first group of stations, while in the second group it approaches three days (Figs. 5a, b). 5. THE C.O.P. ISSUE From the comparison of the above elements for the two groups of stations we draw the conclusion that the energy needs for air conditioning are higher in a typical urban environment. This

4 4 International Workshop on Energy Performance and Environmental June (a) St.A St.6 St.9 St June ( b) St.4 St.7 St.2 St.3 Figure 2: (Ri) during June for non-urban (a) and urban (b) stations July (a) St.A St.6 St.9 St July ( b) St.4 St.7 St.2 St.3 Figure 3: (Ri) during July for non urban (a) and urban (b) stations. is comprehensible because, as it has already been reported, the hours where air temperatures are presented to be equal or higher than 3 o C, in typical urban stations (group 2), for the summer period that we examine they are in average more than 75% concerning the reference station StA. The corresponding comparison with the degree hours leads to 37%. The examination of the mean monthly performance of a split air conditioning unit (C.O.P), using the relative tables of the constructor, under the measured air temperatures of the exterior environment (Table 5) results in the fact that the mean monthly C.O.P. of the aircondition unit for the urban stations (group 2) is smaller by 4,3%, concerning the stations of the first group were placed in thickets or by the sea. In addition, the mean minimum monthly C.O.P (Table 6) is smaller by 7%, concerning the sta August (a) St.A St.6 St.9 St August (b) St.4 St.7 St.2 St.3 Figure 4: (Ri) during August for non urban (a) and urban (b) stations September (a) St.A St.6 St September (b) St.4 St.7 St.2 St.3 Figure 5: (Ri) during September for non urban(a) and urban(b) stations.

5 International Workshop on Energy Performance and Environmental 5 tions of the first group. These have as a result, the needed cooling load and the peak-cooling load to be higher in typical urban environment (urban canyon) in order to cover the more unfavorable conditions of air conditioning. 6. CONCLUSIONS The conclusions of this study are as follows: A. The effect of vegetation and the sea in the microclimate of a region is obvious. Stations in thickets in the centre of the city of Athens or by the sea, are present during the examined period in the present study, much smaller number of hours with air temperatures equal to or higher than 3 o C, compared Table 5: Monthly average values of C.O.P. St.6 4,33 4,468 4,336 4,35 4,358 St.9 4,33 4,46 4,27 4,258 4,36 St.22 4,345 4,429 4,472 St.4 4,348 4,427 4,242 4,26 4,266 St.7 4,49 4,59 4,24 4,4 4,22 St.2 3,92 4,59 4, 3,968 4,6 St.3 4,326 4,458 4,248 4,87 4,36 St.6 4,4 St.9 4,432 4,384 4,75 4,234 St.22 4,47 4,344 4,47 St.4 4,3 4,98 4, 4,3 St.7 4,238 4,2 4,65 4,4 St.2 4,48 4,8 3,875 3,892 St.3 4,435 Table 6: Monthly minimum values of C.O.P. St.6 3,695 4,86 3,786 3,649 3,832 St.9 3,74 4,32 3,5 3,455 4,6 St.22 3,272 4,4 4,289 St.4 3,775 4,74 3,23 2,998 3,443 St.7 3,432 3,477 3,523 3,237 3,432 St.2 2,655 3,26 2,98 2,598 2,85 St.3 3,775 4,3 3,535 3,443 3,82 St.6 4,4 St.9 4,4 3,786 2,529 3,5 St.22 4,3 3,9 3,992 St.4 3,729 3,55 2,735 2,678 St.7 2,46 3,283 2,62 2,826 St.2 3,69 2,655 2,483 2,438 St.3 4,83 to those that are found in a typical urban environment, with dense layout and intense circulatory problem. B. The statistically significant persistence of appearance in a day, of air temperatures equal to or higher than 3 o C, as well as the corresponding degree hours are much higher in the stations inside the typical urban environment in comparison with those that are found in thickets or by the sea, for all the months examined. C. The western orientation in combination with the H/W= of St 4 has as result, after the sunset, the maintenance of high temperatures, because of the stored heat in the walls. D. The use of the dense network of stations helped considerably in the study of the phenomenon of urban heat island of Athens. Also, it helped in the definition of parameters that increases (circulatory, dense layout, orientation) or decreases (green, sea) the intensity of the phenomenon. Ε. The mean C.O.P. of an air-condition unit is at 4,3% lower in an urban canyon than those which were placed in thickets or by the sea. In addition, the mean minimum monthly value of C.O.P. is at 7% lower. This resulted in the need of higher cooling and peak-cooling load. REFERENCES Balaras, C., I. Livada-Tselepidaki, M. Santamouris and D. Asimakopoulos, 993a. Calculation and statistical analysis of the Environmental Cooling Power Index for Athens, Greece. J. Energy Conversion and Management. 34(2): Balaras, C., I. Livada-Tselepidaki, M. Santamouris and D. Asimakopoulos, 993b. Analysis of thermal confort conditions in Athens, Greece. Energy Conversion and Management. 34(4): Katsouyianni, K., A. Pantazopoulou, G. Touloumi, K. Moustris, I. Livada-Tselepidaki, D. Asimakopoulos, G. Poulopoulou and D. Trichopoulos, 993. Evidence for Interaction between Air Pollution and High Temperature in the Causation of Excess Mortality. Archives of Environmental Health. 48(4): Livada-Tselepidaki, I., M. Santamouris, D. Asimakopoulos and S. Kontoyiannidis, 994. On the variability of cooling degree days in an urban environmental. Application to Athens, Greece. J.Energy and Buildings. 2: Livada-Tselepidaki, I., D. Asimakopoulos, K. Katsouyianni, K. Moustris, G. Touloumi and A. Pantazopoulou, The use of complex thermohygrometric

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