01-014 The 2005 World Sustainable Building Conference, DEVELOPMENT OF A PASSIVE COOLING STRATEGY USING DOUBLE-ROOFING SYSYTEM WITH RAINWATER SPRAYING AND ITS FIELD TESTING IN TERMS OF THE INDOOR THERMAL ENVIRONMENT Itaru TAKAHASHI Ph.D 1 Akihiko KUROIWA 2 1 Department of Architecture, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan 2 Architect A, 3-18-53-301 Iguchi, Mitaka, Tokyo 181-0011, Japan, arck@nifty.com Keywords: double-roofing, passive cooling, rainwater, evaporation, exergy Summary This paper describes passive cooling effect of double roofing-system with rainwater spraying. The upper roof is thermally-well insulated so that it functions as shading. The lower roof, namely the ceiling is composed of glass-fiber cloth of 1mm and zinc board of 0.5mm so that it functions as a radiant cooling panel. The evaporation of rainwater taking place on the external surface of the ceiling provides the occupants with the coolness into indoor space. The authors made a couple of field measurement of the indoor thermal environment of a wooden house in Tokyo the roof of which is equipped with double-roofing system. The following are findings. In a room with the double-roofing system, the residents did not feel discomfort during the period of the measurement, in which the MRT ranged 27~32 C, the indoor air velocity 0.5~1.0m/s, the room air temperature 28~32 C, while outside air temperature 26.5~37 C. The residents felt comfortably cool due to the combination of solar control of the upper roof and also bamboo made sudare screen, natural ventilation and radiant cooling by the ceiling. The internal surface of the ceiling absorbed heat of 20~70W/m 2 of the sum of radiation and convection from indoor space. The internal surface of the ceiling of the double-roof emitted radiant cool exergy of 0.065W/m 2 and convective cool exergy of 0.19W/m 2 and thereby the residents might feel comfortably cool. 1. Introduction In tropical and temperate regions including Japan, the excessive use of air conditioners in summer causes increase of electricity use, bad influence on the temperature regulation of human body and urban heat island phenomenon, so that it is vitally important to develop passive cooling strategies that can provide us with both of thermal comfort and low energy use, namely low exergy consumption (IEA-ECBCS-Annex37, 2004). From this point of view, various passive strategies such as shading, natural ventilation, evaporative cooling and so on have studied, developed and tested. Roof spraying is relatively often used among the strategies of evaporative cooling. However, the amount of heat that the internal surface of the ceiling absorbs from indoor space becomes small, because the roof sprayed is necessarily thermally-well insulated for both of winter and the period without spraying in summer. Therefore, the authors developed a passive cooling strategy using double-roofing system with rainwater spraying. We designed and completed a wooden house with this cooling system in the suburb of Tokyo in1999 (Takahashi, 2001). The upper roof is thermally-well insulated so that it functions as shading, and the lower roof, namely the ceiling is composed of glass-fiber cloth of 1mm and zinc board of 0.5mm so that it functions as a radiant cooling panel. The evaporation of rainwater taking place on the external surface of the ceiling provides the occupants with the coolness into indoor space. The device for thermal insulation and the device for evaporative cooling are separately equipped with this double-roofing system, so that the temperature of the internal surface of the ceiling can go down at most the wet bulb temperature and the combination with natural ventilation and shading can provide the occupants comfortable coolness. 2. Double-Roofing Cooling System Figure 1 shows a sectional view of the living room and the kitchen in which this cooling system is installed. Photo 1 shows the house in which double-roofing cooling system is installed. The upper roof is composed of the surface cover of 2mm of galvanized steel board, a closed cavity of 90mm and a foamed polystyrene board of 75mm at the bottom and functions as shading and thermal insulation. The lower roof, namely the ceiling is composed of glass-fiber cloth of 1mm and zinc board of 0.5mm the internal surface of which is painted white so that it functions as a radiant cooling panel. - 91 -
Figure 1 Sectional view of the living room and the kitchen Photo 1 Façade of a house using double using double-roofing system with rainwater spraying. -roofing system with rainwater spraying. The surface temperature of the ceiling can immediately decrease, if the evaporation occurs. A thin glassfiber cloth is placed on the external surface of the thin zinc ceiling board in order to keep the external surface of the glass-cloth and the zinc board wet and evaporating. The four covers consisting of surface cover galvanized steel board of 2mm and foamed polystyrene board of 60mm enclose the air gap between the upper roof and the ceiling for thermal insulation in winter. The screens made of bamboo and called sudare are installed at the south and the east windows as shading. The windows are placed on the south wall and the west, and also the north so that the air breezes through from the south. The walls are thermally-well insulated by a glass-wool cloth of 50mm thickness. The combination of shading, natural ventilation and radiant cooling by the ceiling can effectively provide the residents with acceptable and comfortable enough range of coolness. The rainwater is collected by the rainwater collector installed on the rooftop, when it rains. The rainwater to be used for evaporation is supplied into the glass-fiber cloth from the rainwater tank by gravity; so that there is no use of electricity. A solenoid valve connected with the rainwater pipe alone uses electricity. Therefore this cooling system uses very little electricity. The solenoid valve is controlled by a remote control device equipped with the bath room which is located at the second floor. The rainwater is supplied four times a day: 4:50~4:54, 10:50~10:54, 16:50~16:54, 22:50~22:54. The bed room is located at the west of the second floor. This room has a single roof which is the same as the upper roof of the living room. The residents open the windows alone during the daytime in summer for natural ventilation because of security. 3. Measurement of Indoor Thermal Environment The authors made a couple of field measurement of the indoor thermal environment of the wooden house in Tokyo which is equipped with the double-roofing cooling system from the 19th to the 22nd of August in 2000 and from the 6th to the 12th of August in 2002. We measured horizontal solar radiation, outside air temperature and humidity, air temperature and humidity in the air gap between the upper roof and the ceiling, room air temperature and humidity, temperature of the internal surfaces of the walls, temperature of the internal and external surface of the upper roof, temperatures of the internal surface of the ceiling, room air velocity and so on every one minute during the period of the measurement. We gave five placards which are written as discomfort, hot, neutral, slightly chilly and comfortably cool expressed in Japanese. The residents are husband and wife over 70 years old. They wore a short sleeve shirt and a short pant or a summer skirt. The value of clothing insulation was 0.3~0.4clo. Whenever the resident feels some sensation which is similar to the sensation written in the placards, he (she) is asked to stand the similar placards in a bottle on the dining table in the living room. When the resident feels a sensation excluding five sensations, we also asked them to stand no placards. The votes and behaviors of the residents were recorded by a digital camera every 10 minutes during the period of the measurement (Takahashi, 2000). We asked the residents to open the windows in the nighttime during the period of the measurement in 2002.
4. Passive Cooling Effect Measured Figure 2 shows outdoor climate, indoor thermal environment of the double-roofing room (living room and kitchen) and thermal sensation vote in the double-roofing room measured in 2002. The highest of the horizontal solar radiation is about 1000W/m 2 from the 8th to the 11th of August in 2002. The outside air temperature measured under the rainwater collector ranges 26.5~37 C. The outdoor environment was very severe during the period of the measurement. The room air temperature varies 28~32 C and becomes close to the outside air temperature in day time due to natural ventilation. The temperature of the internal surface of the ceiling varies 24.5~28 C excluding the period from 16:00 to 17:00 of the 8th and the 10th. In the period from 16:00 to 17:00 of the 8th and the 10th, the external surface of the ceiling became partially dry because it was just before spraying, so that the temperature of the internal surface of the ceiling suddenly went up and down. The mean radiant temperature (MRT) varies 27~32 C and goes between the room air temperature and the internal surface of the ceiling. In contrast with convective air conditioning, the MRT becomes lower than the room air temperature and indoor radiant environment is improved by the combination between evaporative cooling on the external surface of the ceiling and solar control of the bamboo screen of sudare. Most of the room air velocity becomes 0.5~1.0m/s, so that the air sufficiently breezed. Relative humidity of the living room varies 45~75%. The residents voted neutral, slightly chilly or comfortably cool excluding hot voted during the period from 16:00 to 18:00 of the 10th. They voted no discomfort. Figure 2 Outdoor climate (upper), indoor thermal environment of the double-roofing room (middle) and thermal sensation vote (lower) measured in 2002. Wall Ceiling Wall 30 C 32 C 29 C Human body Single-roofing room (Bed room) Ceiling Wall 27 C Human body Double- roofing room (Living room and kitchen) Ceiling 28 C (Outside air temperature 31 C, at 14:00 on the 20th of August in 2000) Photo 2 Thermograph of the single-roofing room (left) and the double-roofing room (right) measured in 2000.
Photo 2 shows the thermograph of the single-roofing room (left) and the double-roofing room (right) measured at 14:00 on the 20th of August in 2000. In the single-roofing room, the surface temperature of the ceiling and walls are 1~2 C lower than the outside air temperature. On the other hand, in the double-roofing room, the surface temperature of the ceiling and walls are 3~4 C lower than the outside air temperature. Table 1 shows the consciousness and lifestyle of the residents in passive and active cooling that was grasped by having interviews with the residents. The residents originally do not use and like air conditioners. In the double-roofing room, the residents feel comfortably cool around their heads. On the other hand, in the single-roofing room, the residents feel hot and discomfort around their heads. The residents have felt acceptable cool in summer except a few days in the double-roofing room without use of air conditioner since 2000. The residents feel hot and use a small fan, when air does not breeze around noon, or internal heat increases by cooking. Table 1 Consciousness and lifestyle of the residents in passive and active cooling The residents originally do not use and like air conditioners. In the double-roofing room, the residents feel comfortably cool around their heads. In the single-roofing room, the residents feel hot and discomfort around their heads. The residents have felt acceptable cool in summer except a few days in the double-roofing room since 2000. The residents feel hot and use a small fan, when the air does not breeze around noon, or internal heat increases by cooking. Figure 3 Vertical temperature distribution of the living Figure 4 Heat absorption of the internal and the room measured on the 9th of August in 2002. external surface of the ceiling of the double roof. Figure 3 shows the vertical temperature distribution of the living room measured on the 9th of August in 2002. The surface temperature of the floor and room air temperatures at the height of 0.1m, 0.6m, 1.1m, 1.6 and 2.1m are about 28 C and the surface temperature of the ceiling at the height of 2.6m is 24.5 C at 6:00. While the outside air temperature is 27.3 C. The surface temperature of the ceiling is 24.5 C and is alone lower than the other temperatures also at 15:00. In contrast with convective air conditioning, the surface temperature of the ceiling is alone lower than the other temperatures due to evaporative cooling at the external surface of the ceiling, so that it makes residents feel comfortably cool around their heads. Figure 4 shows heat absorption of the internal and the external surface of the ceiling. Red circles show the amount of the heat that the internal surface of the ceiling absorbs from the indoor space by the sum of radiation and convection. Blue dots show the amount of the heat that the external surface of the ceiling absorbs from the air gap and the lower surface of the upper roof by the sum of radiation and convection. Horizontal axis shows the amount of the heat that the external surface of the ceiling emits into the outdoor space by evaporation. The sum of the heat absorption from the internal surface of the ceiling and the heat
absorption from the external surface of the ceiling is equivalent to the heat emission from the external surface of the ceiling by evaporation. The heat absorption of the internal surface of the ceiling varies 20~70W/m 2 and is equivalent to about 40% of the heat emission of the external surface. On the other hand, the heat absorption of the external surface of the ceiling varies 20~110W/m 2 and is equivalent to about 60% of the heat emission of the external surface. It is necessary to make the amount of the heat absorption of the internal surface of the ceiling more than the heat absorption of the external surface for cooling of indoor space. 5. Exergy Flows through Roofs There are various energy flows and matter flows such as solar radiation, convective heat, effective radiation, rainwater, water vapor and so on in buildings with passive cooling strategies. Exergy is the concept, in which quantifies the potential of energy and matter to disperse in the course of their diffusion into their environment, so that we can discuss the potential for cooling contained by thermal energy, the potential for heating contained by thermal energy and the potential for diffusing contained by matter such as water in a unique index as values of exergy (Shukuya, 2002, 2004). The exergy balance of the upper surface of the roof is given by the following equation. 2 2 h T T h T T C T T T T ITRF To sdir sdif qer Toser sgouto T T T T ro sou o co sou o o sou sol sou o sou o sou sou (1) where is solar absorptivity of the roof [-] (=0.7), I TRF is solar radiation flowing into the surface of the roof [W/m 2 ], T o is the environmental temperature [K] (=outside air temperature), s dir is the entropy of the direct solar radiation [W/m 2 K], s dif is the entropy of the diffuse radiation [W/m 2 K], is mean emissivity of the roof [-] (=0.9), q er is the energy of effective radiation [W/m 2 ], s er is the entropy of effective radiation [W/m 2 K], s gou is the entropy generation rate caused by the heat transfer on the upper surface of the roof [ W/m 2 K], h ro is the radiant heat coefficient of the upper surface of the roof [W/m 2 K] (=6W/m 2 K), h co is the convective heat coefficient of the upper surface of the roof [W/m 2 K] (=16W/m 2 K), C o is the thermal conduction of the of the upper surface of the roof [W/m 2 K] (=0.476W/m 2 K), T sou is the temperature of the upper surface of the roof [K], T sol is the temperature of the lower surface of the roof [K] (Shukuya, 2004). On the left-hand side of equation (1), the first term means solar exergy absorbed, the second the exergy of effective radiation absorbed and the third exergy consumption rate. On the right-hand side, the first term means thermal exergy of radiation flowing into outside air, the second thermal exergy of convection flowing into outside air and the third the exergy of thermal conduction flowing from the upper surface to the lower surface. Equation (1) mathematically expresses that the solar exergy and the exergy of the effective radiation flow into the upper surface of the roof, a large portion of them is consumed by heat transfer and the rest is discharged out of the upper surface. We also derived exergy balance equation of the external surface of the ceiling of the double roof, exergy balance equation of the internal surfaces of the ceiling of the single-roof and exergy balance equation of the internal surfaces of the ceiling of the double roof. The exergy emitted by body whose temperature is higher than its environment is an ability of thermal energy emitted by the body to disperse into the environment. On the other hand, the exergy emitted by body whose temperature is lower than its environment is an ability of thermal energy absorbed by the body to let the thermal energy in the environment flow into it and to cool space or body. We call the former warm exergy and the latter cool exergy. The exergy contained by liquid water is an ability of water to evaporate and to diffuse into its environment such as outside air. We call it wet exergy (Shukuya, 2004). Figure 5 shows exergy balances through the single roof (left) and the double roof (right) which are the results of the exergy analysis using the data measured at 14:00 on the 20th of August in 2000. Orange arrows show warm exergy, dark blue arrows cool exergy and light blue arrows wet exergy. In the case of the single-roofing room, solar exergy of 472 W/m 2 and the exergy of effective radiation of 0.33 W/m 2 are absorbed by the upper surface of the roof and 91% of the sum of them is consumed by the heat transfer from solar radiation and effective radiation to thermal conduction, convection and radiation at the upper surface of the roof. As a result, radiant warm exergy of 0.13 W/m 2 and convective warm exergy of 0.1 W/m 2 are emitted from the internal surface of the ceiling. All of the radiant exergy flows which flow from the walls, the windows and the floor into the internal surface of the ceiling is warm exergy. Warm exergy flows from the ceiling might make the residents feel hot and discomfort around their heads as shown in Table 1. In the case of the double-roofing room, the values of solar exergy and the exergy of effective radiation absorbed by the upper surface of the upper roof are the same as the case of the single-roofing room.
effective 8.4 Effective radiation 0.33 34 472 430 warm Warm 0. 1 0.0001 0.13 0.009 8.4 effective radiation Effective radiation 0.33 34 472 Wet wet 6.3 436 Cool cool 0.0005 0.19 0.065 0.00085 0.006 0.005 0.002 0.0003 0.0004 0.00002 0.002 0.0066 0.00004 ( east (East window) (West (west wall) wall) (E (east ast window) window) (west wall) 0.007 (east wall) (east wall) 0.015 (East wall) (West wall) (East wall) Single-roofing room Double-roofing room Figure 5 Exergy balances through the single roof (left) and the double roof (right). Orange arrows show warm exergy and dark blue arrows cool exergy. Wavy arrows show radiant exergy and the arrows curved thermal exergy of convection. Light blue arrows show wet exergy of water and values in squares show exergy consumption. Wet exergy of liquid water sprayed of 6.3 W/m 2 is thrown into the external surface of the ceiling and a large portion of its exergy is consumed by evaporation. As a result, the surface temperature of the ceiling becomes lower than outside air temperature. Radiant cool exergy of 0.065 W/m 2 and convective cool exergy of 0.19 W/m 2 are emitted from the internal surface of the ceiling. Radiant cool exergy of 0.015 W/m 2 alone from the floor flows into the ceiling, because its angle factor between the floor and the ceiling is the largest among the angle factors between the other parts and the ceiling. Cool exergy flows from the ceiling might make the residents feel comfortably cool around their heads as shown in Table 1. 6. Conclusion Followings are findings. In the double-roofing room, the residents did not feel discomfort during the period of the measurement, in which the MRT ranged 27~32 C, the indoor air velocity 0.5~1.0m/s, the room air temperature 28~32 C, while outside air temperature 26.5~37 C. The residents felt comfortably cool due to the combination of solar control of the upper roof and also bamboo made sudare screen, natural ventilation and radiant cooling by the ceiling. The internal surface of the ceiling absorbed heat of 20~70W/m 2 of the sum of radiation and convection from the indoor space. The internal surface of the ceiling of the doubleroofing room emitted radiant cool exergy of 0.065W/m 2 and convective cool exergy of 0.19W/m 2 and thereby the residents might feel comfortably cool. Acknowledgement We would like to thank Mr. and Mrs. Sugiyama who are the residents of the house with double-roofing system for their cooperating on our field measurement. We would also like to thank Dr. Masanori Shukuya who is a professor of the graduate school of Musashi Institute of Technology and Mr. Kazuaki Mori who is a researcher of Hazama Corporation for their cooperating on our planning and field measurement. References IEA-ECBCS-Annex37. 2004, Guidebook for Low-Exergy Heating and Cooling of Buildings. Takahashi, I. and Kuroiwa, A. 2001, Natural Cooling System Utilizing Evaporation of Rainwater, Regenwassernutzung und bewirtschaftung im internationalen Kontext fbr (International Regenwassertage 2001 in Mannheim), pp. 217-110. Takahashi, I., Saito, M. and Shukuya, M. et al. 2000, Difference in Thermal Sensation and Behavioral Pattern of the Occupants between Passive and Active Cooling Strategies, Proceeding of the 17th International PLEA Conference, pp. 593-598. Shukuya, M. and Hammasche, A. 2002, Introduction to the Concept of Exergy for a Better Understanding of Low-Temperature-Heating and High-Temperature- Cooling Systems, VTT RESEARCH-2158, pp. 1-41. Shukuya, M., Nishikawa, R., Takahashi, I., Saito, M., Asada, H. and Isawa, K. 2004, Theory on Exergy and Environment, HOKUTO Publisher (in Japanese).