Thermal Performance of a Passive Cooling Louver System to Form Cool Microclimate in Urban Residential Outdoor Spaces

Transcription

1 Thermal Performance of a Passive Cooling Louver System to Form Cool Microclimate in Urban Residential Outdoor Spaces Yukari Hirayama, MAg Isamu Ohta, PhD [Misawa Homes Institute of Research and Development Co., Ltd.] Yukari_Kezuka@home.misawa.co.jp Akira Hoyano, PhD [The Open University of Japan] Takashi Asawa, PhD [Tokyo Institute of Technology] ABSTRACT A passive cooling louver system (PC louver), which is an aluminum louver partition coated with hydrophilic and water absorbing film, was developed in order to form a cool microclimate in urban residential outdoor spaces in hot and humid regions. The combination of hydrophilicity and water absorption of the film enhances water diffusion of the surface of PC louver. Hence, when a PC louver is watered from the top, the entire surface becomes wet, which enhances its evaporative cooling effect. The PC louver was designed to shade direct, provide radiation cooling and ventiration cooling with cooled airflow. In this research, the thermal performance of the developed PC louver in outdoor environment was evaluated. As a result, the PC louver s surface was fully wetted and its surface temperature was approximately the ambient wet bulb temperature throughout the day. Temperature of the air passing through the PC louver decreased 2 3 C, which achieved the expected cooling performance as a passive cooling system. INTRODUCTION In urban cities in hot and humid regions, the outdoor thermal environment has become a serious issue as the danger of heat stroke has increased. In these regions with a large amount of rainfall during the summer, the application of a passive cooling design using solar shading and evaporative cooling is focused (Hoyano et al., 1995). In residential area, it is expected to form cool microclimate in semioutdoor spaces, so as to improve thermal comfort for both indoor and outdoor spaces. Therefore, we developed a Passive Cooling Louver System (PC louver) as a residential exterior item (Figure 1). Louvers in general are able to shade while penetrating air flow. In addition, by wetting the louver s surface, the surface temperature of the louver is expected to decrease by the latent heat of the evaporating water. Thus by adding the function that enables to wet the louver s entire surface, the following passive cooling effects can be expected : 1) solar shading, 2) radiation cooling, 3) ventilation cooling with cool airflow. In this paper we first summarize required performances of PC louver and approaches to satisy these requirements. Subsequently, we verified the thermal performance of the PC louver through experiments in an outdoor environment. Besides, since it is necessary to predict the cooling effects at the architectural design stage, the thermal performance of the PC louver is evaluated in means of revealing the heat balance of the PC louver, in order to build a heat transfer model. However, this paper describes the achievements of thremal performances of the PC louver compared to the required performances, and the construction of modeling is performed in the next step. 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 1

2 DEVELOPMENT OF A PASSIVE COOLING LOUVER SYSTEM Cooling Potential and Required Performance of the PC louver Among hot and humid regions, there are cities that the relative humidity during the daytime decreases significantly. For example Tokyo, Japan is a seasonally hot and humid climate, but while the air temperature is above 3 C during the day, relative humidity tends to decrease to 4 5 %, and wet bulb temperature at about 2 25 C, which indicates enough potential as a cooling source (AIJ, 25). In the previous research (Hoyano et al., 1995), they developed a moist and void brick wall as a passive cooling wall (PCW), and verified that the wet bricks inner surface temperature lowered to almost the ambient wet bulb temperature in an outdoor environment. Moreover, in the previous measurement and simulation of a semi-enclosed space using PCWs, the mean radiant temperature near PCWs were 2 4 C less than the ambient air temperature and the air temperature passing through the PCW was lowered to 3 C at the maximum. (Shirai et al., 1997; He et al., 29). The high performance of a PCW is well known through the previous research, but since a PCW is a wall of bricks, its form and utilization is more suitable in public spaces rather than in residences. Besides, moist bricks are effective to provide cooling effect continuously, but being moisted constantly is not a good condition for durability. Therefore, as shown in Table 1, we summarized the requirements for the development of the PC louver to satisfy performances as a passive cooling system together satisfying requirements as an exterior item in residences. Surface Specifications of the PC louver For the basic material of the PC louver, aluminium was chosen for its strength and durability. For the surface layer, it is important to wet the entire surface of the louver in order to minimize the surface temperature distribution. Thus, a hydrophilic resin with porous particles (Figure 2(a)) and photocatalyst Louver s function in general Shading. Penetrating airflow. Additional function as a PC louver Radiation cooling. Generating cool air. Required performance of a PC louver Radiation cooling by lowering the surface temperature of the louver to the ambient wet bulb temperature. Generating cool air with air temperature 2-3 C lower than the ambient air temperature. Indoor Solar radiation from the upper side is shaded by the eaves Cool Air Flow Wet Surface Shading from Solar Radiation Cool Radiance Figure 1 Image of passive cooling effects using a PC louver in semi-outdoor space of a residance. Table 1. Required performances and methods for the development of a PC louver. Required performances Methods 1) Shade direct to the subject space. Adjust pitch of the louver s slats. Wind Outdoor 2) Wet the entire surface of the louver and create uniform surface temperature distribution. 3) Prevent algae, mold or smudge by letting the louver s surface dry easily when it is not in use. 4) Enhance heat transfer between cooled louver s surface and air passing through the louver. 5) Lower surface temperature of the louver immediately after watering. 6) Enable to maximize the shape factor to the subject space. Enhance the hydrophilicity of the louver s surface. Separate surface layer and base layer, and wet only the surface layer. Enhance the surface ratio of the louver s slat to the louver s vertical plane. Lower heat capacity of the louver by using the hollow aluminium slats. Manufacture the louver with compact depth and flexible width and height. 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 2

3 Principal component Particle size distribution Volumetric specific gravity Water absorption rate SiO % 3 1 μm % porous particles 1 [μm] SEM image of porous particles Surface Layer Base Layer photocatalyst Porous particles δ =.5 mm Hydrophilic resin δ =.5 mm Aluminum matrix δ = 1.2 mm (a) Characteristics of porous particles. (b) Cross section of the slat s surface. Figure 2 The characteristics of the slat s surface of the PC louver. (Terrace side) water drop from upper layer (Outside) (Terrace side) (Outside) water diffusion by hydrophilicity, capillarity and gravity 3 (a) Image of water flow. (b) Image of the transmittance of direct. Figure 3 Description of the PC louver s cross section. (TiO 2 ) was coated to the aluminum base (Figure 2(b)). Hydrophilicity of the resin and the porous particles were enhanced by the photocatalyst, and the capillary force of the porous particles aided the diffusibility of the surface water. Form and Sectional Composition of the PC Louver The form and the sectional composition of the PC louver were determined mainly with the consideration of water flow and the transmittance. The louver s slats are titled 3 down toward the terrace side in order to drain water mainly to the terrace side. In addition, by cutting the edge of slats toward the center, water is led to drop near the center of the next slat (Figure 3(a)). Space between slats affects the quantity of transmittance, evaporation, air permeability, and heat transfer between louver s surface and air passing through the louver. Among these factors, we mainly focused on the effect of solar shading because the amount of solar energy is larger compared to other factors. By narrowing a space between the slats to 1 mm, the louver system allows direct to transmit only at solar altitude lower than 13, which corresponds to approximately an hour before and after the sunset and sunrise (Figure 3(b)). THERMAL CHARACTERISTICS OF THE PASSIVE COOLING LOUVER SYSTEM Aim of the Experiment water drop to lower layer water diffusion by hydrophilicity and capillarity force 1 mm 2 mm [mm] Transmittance of direct solar radiation <13 An outdoor experiment was conducted in order to verify thermal performance of the PC louver in means of clarifying the heat balance of the PC louver s surface. The heat transfer model is expected to be inserted in a microclimate simulation tool (Asawa et al., 28), which the authours have developed. In order to analyze the spatial distribution of the microclimate in the simulation, the heat transfer model of PC louver is required to be simplified to reduce calculation load. Therefore, a distribution of the surface temperature of the PC louver is verified at the experiment in order to discuss the possibility to treat PC louver s surface as a thermally equivalent semi-permeable vertical plane. A description of the PC louver s heat transfer model is shown in Figure 4. 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 3

4 Solar Radiation absorption ar s T w in Heat flux by water flow α w ( T w -T s ) T s : average of the circumference surface temperature of slats Long wave Radiation ε ( R L σt s 4 ) Convection Heat Flux by Airflow α c ( T aw - T s ) Latent Heat Flux βk (X s X a ) T w Measurement Methods Figure 4 A description of heat balance at the PC louver s surface. Modeled as an equivalent semi-permeable vertical plane A terrace with the PC louver was constructed in semi-outdoor space of a detached house, facing the southwest direction (22 ). The site was open to the predominant wind direction, thus wind flowed into the terrace from the front to diagonal direction of the louver. Two PC louver planes on the front side of the house were attached to the pergola, which the east side louver was wetted during the experiment (the wet PC louver) and the west side louver was kept dried (the dry PC louver) for comparison purposes. The scheme of the PC louver and the measurments are shown in Figure 5 and Table 2. The measurement was conducted on Oct.1 st 212 and Aug. 4 th 213 to Nov. 29 th 213. Air temperature at at the vicinity of the PC louver was measured using aφ =.1 mm T-type thermocouple set inside a forced draft Φ = 13 mm polyvinyl cylinder (air velocity of approximately 1.5 m/s), in order to reduce the influence of. Ambient dry bulb and wet bulb temperature were measured inside a forced draft Φ = 15 mm aluminum cylinder (air velocity of approximately 3 m/s). A water tube with Φ = 6 mm was inserted through the strut to the beam of the pergola. The amount of water supplied was measured right below the water drip tube and the amount of water drainage was measured at bottom of PC louver using a tipping-bucket rain gauge. During the experiment, tap water was supplied at approximately.4 kg/min per PC louver s vertical plane. Although, for hot and humid regions with a large amount of rainfall during summer, where the amount of precipitation is enough to cover the evaporation amount, we are working to construct a system to use rain and supply it to the PC louver. Measurement Results Equivalent Semi-permeable Vertical Plane Wet and Dry PC Louver s Surface Temperature The distribution of the wet state of the louver was difficult to measure, thus surface temperature distribution was measured using an infrared camera. Figure 6 (a) shows the surface temperature distribution of the terrace side and the outside of the PC louver at 13: on Aug. 7 th 213, as a representative day of a clear sunny day. Overall, there was about a 3 C range in surface temperature distribution of the wet PC louver. As shown in Figure 6 (b), the outside surface of the louver had a large distribution of due to its ragged form, but the difference in surface temperature was small. This indicates that the quantity of does not determin the surface temperature of the wet louver, but the latent heat, as the quantity of evaporation shown in Figure 9, is the main factor. The small distribution in the wet PC louver s surface temperature also indicates that the convective heat flux of water flow is small compared to other heat fluxes at the experimented amount of watter supply. This is also confirmed by the calculation of He et al., (28) that only a few centimeters of the wet surface s temperature are affected by the water s temperature. From these result, the small range of surface temperature distribution was confirmed when the entire surface is wet. The range was small enough to use the average temperature as a representative temperature for an equivalent vertical plane, when discussing on the microclimate in residential space. T w out 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 4

5 Dry PC louver Wet PC louver Awning on ceiling (solar transmittance 8%) Pergola storing water drip tube PC louver South-west side Terrace side Outside 1.5 N Legend : Ambient air temperature Air temperature Surface temperature North-west side Wind direction and velocity Total horizontal or vertical Terrace floor with permeable resin pavement Figure 5 Section of PC louver and measurement points. N Unit : [m] Table 2. Descriptions of the Measurement Sensors. Measurement Measuring point Sensor type Resolution Interval ±.3 Ambient air temperature Weather Transmitter GL+9m ±3% (<9%RH) Ambient relative humidity (WXT52, VAISALA) ±5% (>9%RH) 1min. Ambient wind direction Ambient wind velocity Dry bulb temperature Wet bulb temperature Total horizontal Total vertical Wind direction Wind velocity GL+2.3m GL+1.5m 1m outside from PC louver GL+2.6m (top of pergola) GL+1.5m 1m outside from PC louver GL+1.5m.15m inside terrace from PC louver GL+1.5m,.1m in and outside of PC louver Wind vane anemometer Φ.1mmT-type thermocouple (internal forced draft cylinder) Pyranometer (sensitive waveband: μm) 3-D Ultrasonic wind sensor ±5 ±.3m/s.1 ±5% ±2 ±.1m/s Air temperature Φ.1mmT-type thermocouple (internal forced draft cylinder).1 Surface temperature GL+1.5m Φ.1mm T-type thermocouple.1 Surface temperature GL+1.5m Infrared camera (sensitive waveband:8-14 μm) ±2 Water supply Below water drip tube Weight scale.1g 1sec. / 1sec. / 1min. arbitrary Water drainage Bottom of PC louver Tipping-bucket rain gauge (φ.2 m) ±3% 1min. Terrace side Wet Louver Dry Louver Outside Dry Louver Wet Louver Outside of the Wet Louver Surface Temp.[ ] Aug. 7 th 13: Ta:35.3 WB:24.3 Surface Temp.[ ] (a) Terrace side and outside of the PC louver. (b) Enlarged diagram of outside. Figure 6 Surface temperature distribution of the PC louver. Aug. 7 th 13: Ta:35.3 WB:24.3 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 5

6 Air Temp. [ ] Solar Radiation [W/m 2 ] DB, WB, Surface temp. [ ] : (b) Total 6:horizontal 12: 18: : 6: and wind 12: velocity 18: 1 : 6: 12: 18: : 6: 12: 18: : : (c) Dry 6: and wet 12: bulb 18: temp. : and surface 6: temp. 12:of 18: PC louver : 6: 12: 18: : 6: 12: 18: : 42 Watering : (a) Ambient air temp. and humidity Relative Humidity Air Temperature Solar radiation Figure 7 Diurnal surface temperature change of the wet PC louver and the dry PC louver. [W/m 2 ] Vertical Solar Radiation Wind velocity Dry PC louver outer side bottom side terrace side upper side Dry Bulb Absolute humidity Total vertical Reflected Absorbed 2 Transmitted 1: 11: 12: 13: 14: 15: 16: Time (Nov. 29 th 213) Water tem (inside water drip tube) 24 outer side 22 bottom side Wet PC Wet Bulb 2 louver terrace side upper side 18 : 12: : 12: : 12: : 12: : Aug. 4 th Aug. 5 th Aug. 6 th Aug. 7 th Figure 8 Quantity of Solar reflectance and transmittance. Diurnal surface temperature change of the PC louver is shown in Figure 7. From Aug. 4 th 213 to Aug. 7 th 213, water was supplied continuously during the daytime as shown in the white bands in Figure 7. Surface temperature of the dry PC louver exceeded 4 C, while the surface temperature of the upper side and terrace side of the wet PC louver was only 1.5 C higher than the ambient wet bulb temperature, which indicates the effect of evaporative cooling at the louver s surface. Transmittance and Absorption of Solar Radiation The amount of transmitted was approximately 5 W/m 2 during daytime in a sunny day, which is about 5 % of the incident (Figure 6). Direct was calculated to transmit at an hour before and after the sunset and sunrise, but solar transmittance did not increase, since the quantity of was already small at these times. Solar radiation absorption was calculated by the deduction of transmitted and reflected from incident, and the absorption was approximately 47% during the daytime RH [%] Absolute humidity [g/kg ] Wind velocity [m/s] 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 6

7 Solar radiation [W/m 2 ] Wind velocity [m/s] Temperature [ ] Total horizontal Latent heat [MJ/(m 2 h)] Amount of evaporation [kg/(m 2 h)] Sep. 19 th 213 Figure 9 Evaporation rate and daily amount. Figure 1 Air temperature at the vicinity of the PC louver. (Data interval: 1sec.) Figure 11 Cooling efficiency of air passing through the PC louver. Evaporation Rate Evaporation rate per vertical surface plane of a representative sunny day is shown in Figure 9. Water was supplied continuously for 24 hours during this period. From the experimental results, 14 kg/m 2 of water evaporated in a day, which is equivalent to 34 MJ/m 2 of latent heat flux. This is about two to three times larger than that of standard water retentive pavements. Air temperature in front and behind the PC louver Wind direction in Figure 1 is shown by considering the normal direction from outside to PC louver as. When wind speed was larger than.5 m/s, air temperature at the vicinity of the PC louver varied depending on wind s direction. When wind direction is to ±45 at 13:56 13:58, air temperature decreased approximately 2 C in the terrace side compared to the outside air temperature. When wind direction is ±45 to ±9 at 14:2 14:3, air temperature difference was not significant. Air temperature in terrace side also decreased with a breeze (wind velocity less than.5 m/s) at 13:59 and 14:4. Here, the cooling effeciency of wind penetrating : 6: 12: 18: 24: 13:55: (b) Velocity 13:56: and direction 13:57: of wind 13:58: inflow 13:59: 14:: 14:1: 14:2: 14:3: 14:4: 14:5: Velocity Direction (a) Total horizontal :55: (c) Air temperature 13:56: 13:57: and surface 13:58: temperature 13:59: 14:: 14:1: 14:2: 14:3: 14:4: 14:5: 36 Dry Bulb Outside air temp Terrace side air temp. 24 Wet Upper side Bulb surface temp :55 13:56 13:57 13:58 13:59 14: 14:1 14:2 14:3 14:4 14:5 Aug. 7 th 213 Cooling efficiency [-] Oct. 1 st Wind velocity [m/s] Daily amount of evaporation [kg/(m 2 d)] Bottom side surface temp Wind from normal Direction of PC louver [ ] 3th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 7

8 the PC louver is evaluated by the following index η, based on the ambient wet bulb temperature: η = (T a T l )/(T a - T wb ) (1) Figure 11 shows the calculated data when wind continuously passed through the louver from the normal direction for more than 3 sec. ƞ tended to stabilize at.2 when wind velocity is larger than 2 m/s, and distributed between.4 at breeze. This is a similar feature to the former PCW that the maximum value of η is recognized at breeze, but stabilizes at smaller value as wind velocity increases. CONCLUSION This study investigated the potential use of an evaporative cooling system during daytime in urban cities in hot and humid regions. A Passive Cooling Louver System, coated with hydrophilic resin, porous particles, and a photocatalyst was developed as an exterior material for residences. From the outdoor experiment the following thermal performances were revealed: 1) The surface temperature of the wet PC louver was approximately the ambient wet bulb temperature. 2) The distribution of the wet PC louver s surface temperature was small enough that it can be modeled as an averaged value of an equivalent vertical plane. 3) Transmittance of direct solar radiance is few. 4) The maximum amount of daily evaporation of the PC louver is approximately 14 kg/m 2 ( 34 MJ/m 2 of latent heat) per vertical plane. 5) Air temperature at the vicinity of PC louver decreased by 3 C at most. From these results, the required cooling performance as a development of the PC louver was confirmed. For the next step, we will construce a thermal transfer model of the PC louver and incorporate it to a microclimate simulation tool, in order to aid spatial design to form cool microclimate. ACKNOWLEDGMENTS The development of the surface layer of the PC louver was jointly carried out AICA Kogyo Co., Ltd. The authors are grateful for the assistance from AICA Kogyo Co., Ltd. NOMENCLATURE Φ = Diameter [mm] ε = Emissivity [-] R L = Long wave radiation [W/m 2 ] σ = Stefan-Boltzmann constant [-] T s = Surface temperature [ C ] T aw = Air temperature at windward of PC louver [ C] T wb = Wet bulb temperature [ C ] T al = Air temperature at leeward of PC louver[ C] T w = Water temperature [ C] η = Cooling efficiency [-] k = mass transfer coefficient [-] β = Evaporation efficiency [-] α c = Convection coefficient between air and the surface of PC louver[w/(m 2 K)] α w = Heat transfer coefficient between water and the surface of PC louver[w/(m 2 K)] REFERENCES Hoyano, A. et.al Design and development of an evaporative-cooling prototype wall made of waterpermeable ventilable bricks. The International Solar Energy Society 1995 Solar World Congress, AIJ. 25. Automated meteorological Data Acquisition System, AMeDAS Shirai, K. et al Investigation of thermal comfort in an outdoor space partly enclosed by a passive cooling wall made of water-permeable ventilable bricks. Proceedings of ISES 1997 Solar World Congress He, J., and Hoyano, A., 29. A 3D-CAD-based simulation tool for prediction and evaluation of the thermal improvement effect of passive cooling walls in the developed urban locations. Solar Energy, 83: He, J., and Hoyano, A., 28. A numerical simulation method for analyzing the thermal improvement effect of super-hydrophilic photocatalyst-coated building surfaces with water film on the urban/built environment. Energy and Buildings, 4: Asawa T. et al., 28. Thermal design tool for outdoor spaces based on heat balance simulation using a 3D- CAD system. Building and Environment, Vol.43, 12: th INTERNATIONAL PLEA CONFERENCE December 214, CEPT University, Ahmedabad 8