Thermal Comfort Conditions for a Tropical City, Salvador Brazil

Transcription

1 September 2004 Page 1 of 1 Thermal Comfort Conditions for a Tropical City, Salvador Brazil Telma Andrade 1,2, Jussana Nery 1, Tereza Freire 1, Lutz Katzschner 3 and Danilo Fortuna 1 1 Federal University of Bahia, Salvador, Brazil 2 Faculty of Technology and Sciences, Salvador, Brazil 3 University of Kassel, Kassel, Germany ABSTRACT: Taking the example of Salvador (Brazil), a city located at 13º south of the equator, thermal comfort conditions were analysed in order to give architects and urban planners background information for buildings and open space design. The analysis was conducted by applying the Test Reference Year (TRY) database, obtained from the airport of Salvador meteorological station, to calculate hourly thermal comfort index, PET (Physiological Equivalent Temperature) in ºC. Air velocity, air temperature and humidity, and global radiation were the available data gathered. To verify the possible differences between the TRY and the meteorological station data, a first evaluation was made by comparing the monthly average air temperature calculated from both databases. The results obtained were the hourly PET data selected as representative days of each month of TRY airport data. To compare this with the urban situations, case studies with field measurements were carried out to see the differences between airport data and the densely populated areas. The paper concludes by pointing out the differences found and advises that the thermal index PET database should be used to describe the thermal comfort situation in a tropical city. However if should be taken cautiously, because it may underestimate heat storage. Conference Theme: Comfort and well-being in urban spaces Keywords: Thermal comfort; Urban climate; Physiological Equivalent Temperature; Tropical climatological measurements 1. INTRODUCTION Access to hourly climatic data in some tropical regions is usually extremely difficult to obtain, either because the historical series is incomplete or because the meteorological stations are only now beginning to use automatic registration systems. Airports can be used as a reliable source for these data, although due to its function they are located at the outskirts of the cities. On the other hand, meteorological stations, generally situated closer to city centres, publish average monthly data over a long period of time. Hourly behaviour of climatic parameters representing typical days is useful for human thermal comfort approach. This paper attempts to assess the urban thermal comfort by adopting a thermal index derived from the human energy balance the Physiological Equivalent Temperature PET (ºC) [1], using the Test Reference Year TRY calculated by Lamberts (2002) with available hourly meteorological data from the airport of Salvador, a city situated in the northeastern Brazilian coast at 13 south of latitude and 38 of longitude west. Goulart [2] defines TRY after ASHRAE [1993] cap. 24. It consists in selecting a climatic year from hourly historical series by successive discarding years with highest and lowest average monthly temperatures. TRY for Salvador is dated 1961, selected from a 10- year period from and the database was made available by Lamberts [3] for the development of the present research. The thermal comfort index used, PET, is the result of the integral effect of climatic variables interfering with heat exchange between the body and its environment and also takes into account the clothing insulation and activity level. TRY database complies of air temperature (T a ºC), wet bulb temperature (W bt in ºC), air velocity (v in m/s) and global radiation (R in W/m 2 ) derived from sky condition and the solar constant. To evaluate the deviation from Ondina Meteorological Station data [4], which is located within the coastal urban area, mean monthly temperature values were plotted against mean temperature values calculated for the Test Reference Year (TRY). Also field measurements of air and globe temperatures, relative humidity and air velocity were carried out in selected points within the urban fabric and a quick comparison made (Fig. 1).

2 September 2004 Page 2 of 2 to those under the conditions being assessed. Comfort limits range from 22-24ºC [8]. The PET index expresses the perception of thermal condition integrating the climatic parameters: air temperature (T a in o C); (T mrt in o C); vapour pressure (e in hpa) and air velocity (v in m s -1 ). The indoor reference climate assumes that mean radiant temperature equals air temperature, the air velocity is established as 0.1 m s -1 ; water vapour pressure is set to 12 hpa, which represents a relative humidity of 50% at air temperature of 20ºC [1]. Rayman 1.2 software has been developed by Rutz [9] to calculate PET index. Figure 1 Map of Salvador-Bahia-Brazil (13º south), Airport, Ondina Met station and measuring points. 2. URBAN CLIMATE OF SALVADOR Analysing the climate of Salvador, as a first approach, Nery [5] stated the climatic condition of the city promotes positive thermal stress during the whole year, attenuated during winter season (June, July and August). This was corroborated by the values of PET index resulting from measured climatic data that showed heat stress in summer and even winter afternoons within urban fabric. These outcomes emphasize the need to provide shading through vegetation and to preserve ventilation paths for thermal comfort in tropical cities, as expected [6]. However, Goulart et al [2] plotted TRY climatic data of Salvador on Givoni bioclimatic chart [7] and their findings indicated that in three per cent of the total number of hours of the year, climatic sensation is below lower thermal comfort limit. The climatological data from the period of [4] show that the average monthly maximum air temperatures are quite homogeneous, with the highest value of 30 C (February and March) and the lowest of 26,2 C (July), whereas the minimum is about 24ºC in summer and 21 ºC in winter. Salvador has a warm humid maritime type of climate and its relative humidity does not vary much annually. Indeed, its average annual range is about 4% from 79% (February) to 83,1% (May). Typically, the wind pattern of Salvador is characterized by southeasterly Trade Winds. The mean wind speed is 3.2 m s -1 for winter and 2.8 m s -1 in summer. June has the lowest insolation with hours and January the highest with hours. 3. THE PET INDEX (ºC) The PET index was introduced by Höppe [1] and is defined as the physiological equivalent temperature at any given place (outdoors or indoors) and is equivalent to the air temperature at which, in a typical indoor setting, the heat balance of the human body (work metabolism 80 W of light activity, added to basic metabolism; heat resistance of clothing 0.9 clo is maintained with core and skin temperatures equal 4. METHODOLOGY To verify the possible differences between the TRY and the meteorological station data, a first evaluation was made by comparing the monthly average air temperature calculated from both databases. The hourly climatic data for the TRY was used to calculate the thermal comfort PET index (ºC) by running the Rayman software. Available hourly TRY data were: air temperature (ºC), air humidity (ºC), air velocity (m s -1 ) and global radiation (W/m 2 ). When inserting global radiation data directly into the software, the resulting PET values obtained were out of the expected range, presenting negative values if global radiation was equal or below about 300 W/m 2. To solve this problem, mean radiant temperature values were calculated from Stefan law (1) using the given global radiation data, as globe temperature (T g o C) data were not available. This procedure was not valid in the absence of solar radiation. Tmrt = 4 εσ R (K) (1) Where, R is taken as global radiation (W/m 2 ), σ equals 5.67X10-8 W/m 2 K 4 is Stefan-Boltzman constant, ε is the emittance, T mrt, mean radiant temperature in ºC. In the absence of solar radiation T mrt equals T a. From morning to about 16:00 hours ε was considered equal to 1, as solar radiation is predominant. After 16:00 hours, ε was taken as 0,9 [10], due to building materials estimated emittance for long wave radiation. To select the representative days for each month of TRY, highest and lowest average daily PET values were chosen and compared to the PET comfort limits. Besides, two points were selected within the built area and the climatic parameters measured (air temperature, relative humidity, air velocity and globe temperature) using Smetek Meteorological Station System, from 7:00 to 23:00 hours on January 19 th. To work out T mrt from measured T g, equation 2 was used (after Belding s formula) [11]: T mrt= [T g + 0,24(v) 1/2 (T g-t a)] (ºC) (2)

3 September 2004 Page 3 of 3 5. RESULTS Figure 2 shows a comparison between the average monthly air temperature calculated from both TRY and Meteorological Station climatic databases. Figure 2: Average air temperature from TRY and Salvador climatic data. Hourly PET results are shown graphically in a temporal distribution. They were calculated from TRY values and represent the warmer and cooler days of each month (Figure 3). The highest and lowest PET values are also indicated as well as the PET comfort limits.

4 September 2004 Page 4 of 4 Figure 3: Hourly PET (ºC) results for each month, calculated from TRY airport data. Figure 4 shows PET results from the measuring locations within the urban area carried out on January 19 th over PET curves from TRY for January. Figure 4: PET (ºC) values calculated from TRY and from measured points in the urban area in January. 6. DISCUSSION The results in Figure 2 present TRY air temperature values ( C) slightly lower than the monthly climatic data for winter. TRY database were gathered from Salvador airport, white monazitic sand, open space, no buildings and practically no vegetation. Ondina Meteorological Station is located on the Atlantic coast within the urban fabric, partially built and with shading vegetation. The two places

5 September 2004 Page 5 of 5 differ, especially by the white sand whose reflexivity is high and heat storage low. This possibly accounts for the slightly lower air temperature values, especially in wintertime. Figure 3 shows the hourly PET results for each month of the year. For the warmer day curve, regardless of the season PET values above upper comfort limit (24ºC) are observed throughout the year around midday from about 10:00 to 16:00 hours. In relation to the lower comfort limit (22ºC) PET values were slightly below or within the comfort zone for the evening and night period. The lowest values occur, in general, between 4:00 hours and dawn. However, there are two distinct periods during the year concerning the calculated values for the warmer day. From October to March peak PET values are around 40 ºC, reaching 46 ºC in January. From May to August the curves tend to flatten during the whole day with maximum peak values of about 27ºC. April and September curves are intermediate and present similar behaviour to each other with peak values of 35 ºC and 34 ºC, respectively (Figure 3). For the cooler day curve, from May to August all PET values are below the lower comfort limit, reaching 14ºC in June at 4:00 hours. The curves represented in Figure 4 show that PET values for the built environment were on average 15ºC higher than the upper comfort limit (24 ºC). They probably represent the effect of heat storage and the influence of the built fabric. Wind velocity can be reduced and shading provided for some periods of the day by buildings seem to create a more uniform, albeit warmer, urban climate conditions. 7. CONCLUSION The results show TRY air temperature values ( C) slightly lower than the monthly climatic data for winter. In addition, PET values found for the airport of Salvador indicate some periods of negative thermal stress evaluated by its lower comfort limit of 22 ºC. To obtain a database, which expresses the thermal conditions for Salvador, a tropical city, the Test Reference Year (TRY) was translated into PET - Physiological Equivalent Temperature index values. The comparison between PET results originated from TRY and those from inner city measuring points indicates deviation from airport open area values, possibly meaning that city structure accumulates heat. Thus, the curves are more uniformly higher, except for the peak value at the airport at 13:00 hours. Airport data tend to underestimate heat storage, as hypothesized, and should be used with some caution when designing building and urban spaces. They may tend to produce positive thermal stress environment, which in turn could require postoccupancy corrections or adaptations to achieve thermal comfort. biometeorological assessment of the thermal environment. International Journal of Biometeorology : [2] GOULART, S.; LAMBERTS, R.; FIRMINO, S. Dados Climáticos para Projeto e Avaliação Energética de Edificações para 14 Cidades Brasileiras. Florianópolis, PROCEL, 1998 [3] LAMBERTS, R. Dados cedidos pelo Prof. Dr. Roberto Lamberts da UFSC. Florianópolis, [4] INMET. Normais Climatológicas de cidades brasileiras Brasília, Ministério da Agricultura, [5] NERY, J. et. al. Primeira aproximação para estudo de clima urbano em Salvador. In Anais (do) IV Encontro Nacional de Conforto no Ambiente Construído. Salvador, FAUFBA / LACAM; ANTAC, [6] Andrade, T. C. Q.; Nery, J. M. F.G.; Freire, T. M.M.; Katzschner, Lutz. (2002) Thermal Comfort and Urban Climate in a Tropical City Salvador/Brazil. Revista Baiana de Tecnologia TECBAHIA. V.17, N.3, Set/Dez. 2002, pp [7] GIVONI, B Comfort, climate analysis and buiding design guidelines. Energy and Buidings 18, [8] MATZARAKIS, A.; MAYER, H.; IZIOMON, M. Applications of a universal thermal index: physiological equivalent temperature. International Journal of Biometeorology Issue 2: [9] RUTZ et al. ( 2000). [10] SANTAMOURIS, M., ASIMAKOPOULOS, D. (1996) Design Source on Passive Solar Architecture. Energy Conservation in Buildings, CIENE, Athens. [11] GIVONI, B Man, Climate and Architecture. London. 2 nd Ed. Applied Science Publishers, p-24. REFERENCES [1] HÖPPE, P. The physiological equivalent temperature a universal index for the