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Performance analysis on a building-integrated solar heating and cooling panel
Renewable Energy 74 (2015) 627e632
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis on a building-integrated solar heating and cooling panel Cui Yong a, Wang Yiping b, c, Zhu Li c, * a
Tianjin University Research Institute of Architectural Design, Tianjin 300072, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China c School of Architecture, Tianjin University, Tianjin 300072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 May 2014 Accepted 27 August 2014 Available online
The solar heating and nocturnal radiant cooling techniques are combined aiming at a novel solar heating and cooling panel (termed as SHCP) to be easily assembled as construction components for building roofing or envelope and also compatible with surroundings for its versatile coating colors, which can remove the double-skin mode from conventional solar equipment. SHCP has two functions for heating and cooling collecting. In this paper, the heating and cooling performances were analyzed in detail based on a small scale experimental system and effects of air gap and coatings were investigated. The results show that in sunny day of extreme cold January in Tianjin, China, the daily average heat-collecting efficiency is 39% with the maximum of 65%, while in sunny night during hot seasons the average cooling capacity can reach 87 W/m2. When two different coatings were sprayed on SHCP without air gap, its heating and cooling performances were all analyzed, the daily average heat-collecting efficiency was 39% and 27% with the maximum points of 65% and 49%, respectively, and the cooling capacity was almost the same of 30 W/m2 in January. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Solar panel Radiant cooling Building integration Monitoring
1. Introduction Currently, energy consumption in buildings mainly consists of domestic hot water, heating, cooling, ventilating, lighting and electrical appliance, which accounts for approximately 27.5% of the total energy use in China, and energy consumption for heating and cooling is 63% of the whole energy consumption in buildings. It is believed that such a proportion will be greatly increased with the rapid economic development of China. Consequently, it is of great importance in the building field to exploit renewable energy, which can minimize the energy expenditure and improve thermal comfort. And it is necessary to realize the integration of solar energy equipments with the building, which can ensure the integrality and aesthetics of buildings. Solar thermal utilization is a mature technology, but nocturnal radiation cooling is a developing technology and has the advantages of energy saving and easy integration, which transmits thermal radiation on the earth through the two atmospheric windows (8 mm ~ 13 mm and 13.5 mm ~ 16 mm) to the sky, since the sky temperature is much lower than the temperature of most of objects
* Corresponding author. Tel./fax: þ86 022 27404771. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.renene.2014.08.076 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
on the earth. Considerable researches have been carried out to study the nocturnal radiant cooling technology under different climate conditions. Evyatar Erell and Yair Etzion have set up a radiant cooling system whose cooling output was 80 W/m2 in the arid area [1]. Maal and Kodah have built another radiant cooling system whose cooling output was 13 MJ/(m2 night) in Jordan [2]. A model building utilizing radiant cooling technology for preserving vegetable has been built in Japan, and the cooling capacity is 40e60 W/m2 [3]. Shuo Zhang and Jianlei Niu have studied a nocturnal radiative cooling system combined with microencapsulated phase change material (MPCM) slurry storage system [4]. The cooling energy consumption and the effect of energy-free nocturnal radiation application were simulated based on hour-byhour calculations in five typical cities across China. The results showed the energy saving potential in Lanzhou and Urumqi can reach 77% and 62% for low-rise buildings. The present hybrid system was recommended to be used in northern and central China cities where the weather is dry and the ambient temperature is low at night. Giuseppe Oliveti studied the radiation exchange between the building walls and the surrounding environment (sky, ground) in the infrared band and determined the radiation heat transfer efficiency between the vertical wall and the sky, between the vertical wall and the ground, also the level roof and the sky [5]. Ursula Eicker and Antoine Dalibard developed a new type of PVT system to
628
C. Yong et al. / Renewable Energy 74 (2015) 627e632
generate electricity and cooling [6]. In Madrid, the average cooling capacity measured was 60e65 W/m2. Moien Farmahini Farahani studied a multi-step refrigeration system of the nocturnal radiative cooling technology combined with two stage evaporation refrigeration and verified the feasibility in four different climatic conditions [7]. The results showed that the multi-step refrigeration system could be the choice for refrigeration in summer. Moien Farmahini Farahani studied a set of two step cooling system including night radiative cooling unit, cooling coil unit and indirect evaporative cooling unit [8]. In summer night, the water in the cooling coil was cooled through radiative cooling way and stored in the tank. During the day, the outdoor hot air passed through the cooling coil and indirect evaporative cooling unit to be cooled. As for the dual-functional system that solar heating is coupled with sky radiant cooling, the solar wall [9,10] and solar roofing system [11] are both the passive heating and cooling technique, using the facades and roof of buildings as solar collector or radiator and air as the working fluid, to supply the requirement energy for buildings. Evyatar. Erell and Yair. Etzion studied the heating performance of the radiative cooling system mentioned in Ref. [1], the average heat-collecting output of the system was 370 W/m2 under the sunny but cool conditions of typical Sde-Boker winter [1,12]. M. Matsuta et al. studied a solar heating and radiative cooling system using a solar collector-sky radiator, the maximum collective and radiative capacities obtained were 610 W/m2 and 51 W/m2, respectively for a clear day and night [13]. Patent ZL03154771.1 presents a sort of solar collector wall that can be used as construction component of building envelope, which realizes the perfect combination of solar energy equipments with the building [14]. In this paper, the nocturnal radiant cooling technology was coupled with the solar collector wall mentioned above to produce a building-integrated solar heating and cooling panel, termed as SHCP hereinafter. Experimental apparatus were set up for detailed study on its performance. 2. System descriptions 2.1. Structure of SHCP The cross-section profiles of different SHCP is shown in Fig. 1. Fig 1a gives the sectional profile of the SHCP without air gap, which mainly concerns about heating function. The structure of SHCP is
from outside functional and protective coating, metal plate, heatconductive filling adhesive, metal tubes, tube-fixing metal fittings with grooves and finally to the inside insulation layer which also functions as the building envelope insulation layer. When local night ambient temperature is low enough to be used for cooling, an air gap was added between the plate and insulation layer but with adjustable inlet and outlet, which is demonstrated in Fig. 1b. The panel investigated here has specified structure size and materials. Acrylic resin coating of about ten years' life length was sprayed manually on an aluminum plate with length of 2000 mm and width of 1000 mm and thickness of 1 mm; copper tubes of 8 mm diameter were mechanically fixed at a space 150 mm to the plate by 10 mm wide aluminum stripes with grooves and conductively fixed using self-made conductive glue with the conductivity of 3.12 W/(m K); high heat-proof performance polystyrene insulation board with 90 mm thickness was used and if needed a 50 mm air gap was added. 2.2. Experimental system An experimental system was set up to test the performance of the SHCP designed above with necessary auxiliary parts shown in Fig. 2, where two 30 L tanks were used for heating and cooling storage. All the exposed surfaces of the system besides SHCP were insulated by rubber-plastic with 30 mm thickness. The SHCP was set vertically facing south aimed to act as building envelope. The red line represents the heating cycle while the blue line (in web version) gives the cooling cycle, and the arrows show the working fluid flow path and orientation. Working fluid is water in this work, while 29% ethanol solution as working fluid in winter to prevent freezing. SHCP was used as solar heat collector at daytime while acts as radiator at night. When it is the heating cycle, the pump1, rotameter1 and valve1 are turned on with pump2, rotameter2 and valve2 closed. The working fluid in the bottom of heat-storage tank is pumped out to be heated by incident solar irradiation on SHCP, and then flows back to the top of heat-storage tank. When outlet temperature of working fluid in SHCP is lower than inlet temperature, it comes to the cooling cycle, valves and pumps are set in reverse manually. Working fluid in the top of cool-storage tank is pumped into the tubes of the panels and cooled by means of radiation and sometimes coupled by natural convection when the
Fig. 1. Cross-section profiles of different SHCP.
C. Yong et al. / Renewable Energy 74 (2015) 627e632
629
CP SH valvel1 coolstorage tank valvel2
rotameter2 heatstorage tank pump2
rotameter1 pump1
Fig. 2. Flow chart of heating and cooling panel testing system.
ambient temperature is low enough, and afterward flows back to the bottom of tank. 2.3. Measurement Local solar irradiation on horizontal plane was measured by solar cells calibrated by an Eppley pyranometer with the accuracy of ±5%, and then calculated to the solar irradiation on vertical plane through Erbs model [15]. Wind speed was measured by anemometer. All temperatures were measured using PT-100 thermo resistances with measurement accuracy of 0.1 C, and the PT-100 thermo resistances were calibrated individually under different temperatures to obtain the calibration curves to be used for data processing. The PT-100 thermo resistance was set in a thermometer shelter about 1.5 m above the ground to get the ambient temperature. The inlet and outlet temperatures of the working fluid in SHCP were measured by PT-100 thermo resistances in the copper casings. The tank fluid temperatures of both tanks were measured by PT-100 thermo resistances in copper casings of the tanks' center, respectively. And also the temperature distribution on the SHCP surfaces was measured by PT-100 thermo resistances with distances of 25 mm and 600 mm in width and length directions. For air gap type SHCP, the temperature distribution along the air gap was also measured. A rotameter was used to manage the flow rate of the working fluid. The signals were all linked to a SWP automatic real-time datalogging system and then to a computer. 3. Performance evaluation The heating and cooling capacity are calculated based on the measured inlet and outlet temperature.
Q ¼ mCP ðTout Tin Þ
(1)
where, Q is the heating or cooling capacity (W), m is the mass flux of the fluid (kg/s), CP is the specific heat of the fluid (kJ/kg K), Tin and Tout are the inlet and outlet temperature of the working fluid ( C), respectively. The calculated heat-collecting capacity above divided by solar irradiation would obtain the heat-collecting efficiency of the system:
h¼
Q AI
(2)
where, h is the heat-collecting efficiency of the system, A is the area of SHCP (m2), I is solar energy absorbed per SHCP area (W/m2). 4. Results and discussion The heating performance of SHCP without air gap in winter and cooling performance with air gap in summer were analyzed in detail and the effects of air gap and coatings on the heating and cooling performances of SHCP were investigated. 4.1. Heating performances Fig. 3 shows the heat-collecting performance analyses of SHCP without air gap based on the measured climate conditions and temperature data in a sunny day of January. The outside coating has properties of 0.7 of absorptivity, 0.96 of emissivity and 0.3 of reflectivity, and the mass flow rate was set at 0.005 kg/s. Solar radiation measured on the horizontal surface was conversed to that on the vertical surface, both of the data were shown in Fig. 3a together with ambient air temperature. It can be seen that the solar radiation on vertical surface was about 40% smaller than that on horizontal plane, so the maximum solar radiation was only about 200 W/m2 at 12:00. Because the weather turned grey, the solar radiation had a dip at 15:00. Fig. 3b shows the inlet and outlet temperatures of working fluid in SHCP. The highest outlet temperature was 14 C with temperature difference about 8 C, but it was too low to be used directly in winter, so can be stored in the heat-storage tank as heat source of heat pump or heat pipe. The reason maybe came from low solar radiation to some extent; maybe relied on the non-perfect design of SHCP with bigger space between copper tubes and less number of copper tubes. We will optimize the SHCP structure and operational parameters by mathematical model later. Unglazed SHCP, lower air temperature and solar radiation made lower panel surface temperature, so that small heat was transferred to working fluid which results in small effective heat conductive surface area and little heat-collecting time during 8:30 to 11:20am in Fig. 3c. Due to the low final temperature, the efficiencies were pretty high with an average value of 39% and a peak point of 65%
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C. Yong et al. / Renewable Energy 74 (2015) 627e632
collector with single function of only collecting heat, which has two functions of heating and cooling.
4.2. Cooling performances Fig. 4 shows the cooling performance of the same SHCP with air gap in a sunny day of hot season, and the mass flow rate was set at 0.007 kg/s. The radiant cooling experiment was conducted from 18:00pm to 6:00am next day. Ambient air temperature was from 29 C to 16 C, and wind speed changed from 9.0 m/s to 1.0 m/s, both of the data were shown in Fig. 4a. Fig. 4b shows the inlet and outlet working fluid temperature of SHCP, and the panel surface 30
(a) o
ambient temperature wind speed
o
tenperature( C)
26
9.0
24
8.0
22
5.0
20
4.0
18
2.0
16
1.0
14 18:59:00
20:59:00
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0:59:00
2:59:00
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time
35
(b)
outlet temperature inlet temperature panel surface temperature
temperature(oC)
30
25
20
15 18:12:00
20:42:00
23:12:00
1:42:00
4:12:00
6:42:00
1:42:00
4:12:00
6:42:00
time 140
(c)
Fig. 3. Heating performance analysis of SHCP without air gap.
and the total heat-collecting capacity was 74 MJ among 2.5 h. The sudden increases were caused by short-time clouds shading and large thermal capacity of working fluid medium, which was corresponding to the solar radiation curve. To compare with the flat-plate solar collector on the market, whose average daily heat-collecting efficiency is generally 48%e 58%, the average heat-collecting efficiency of SHCP is slightly smaller. The reasons are that the SHCP is unglazed and is set vertically in this research. But SHCP is different from flat-plate solar
cooling capacity (W/m2)
120 100 80 60 40 20 0 18:12:00
20:42:00
23:12:00
time Fig. 4. Cooling performance analysis of SHCP with air gap.
wind speed(m/s)
28
(a)
631
solar irradiation on vertical SHCP ambient air temperature
6
2
solar irradiation(W/m )
o
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ambient air temperature( C)
C. Yong et al. / Renewable Energy 74 (2015) 627e632
4
150
2
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0 50 -2 0 -4 12:17:00 16
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(b)
temperature( C)
14 o
12 10 8 6 4 2 0
inlet temperature with coating A outlet temperature with coating A inlet temperature with coating B outlet temperature with coating B
-2 -4 12:17:00
100
16:17:00
20:17:00
(c)
0:17:00
4:17:00
8:17:00
capacity with coating material A capacity with coating material B
2
capacity(W/m )
80 60
cooling capacity
40 20 0
heat-collecting capacity
-20 -40 -60 -80 12:17:00
heat-collecting efficiency
0.7
16:17:00
20:17:00
0:17:00
(d)
4:17:00
8:17:00
with coating A with coating B
0.6 0.5 0.4 0.3 0.2 0.1 0.0 8:57:00
9:37:00
time
10:17:00
10:57:00
Fig. 5. Performance comparison with different outside coatings.
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temperature. The outlet temperature was always less than inlet temperature at night due to radiation and sometimes coupled by natural convection. A decrease in the outlet temperature during this particular night between 21:30 and 22:00 was due to a decrease in ambient air temperature of 2 C in this interval. This change was probably accompanied by the results of an increase in wind speed from 1 m/s to 2 m/s. At the beginning of cooling cycle, the panel surface temperature was higher than the outlet temperature of working fluid, because of the high ambient temperature and diffused sunlight, so that the received energy was larger than the energy loss. With time, the panel was cooled by radiation with the low temperature sky and convection with air, but due to the thermal resistance between the panel and the working fluid, the panel surface temperature decreased to below the inlet and outlet working fluid temperature. Fig. 4c shows that the cooling capacity was small in the early and last hours during the cooling cycle, which can be explained by the effect of the energy stored in the system during day time due to high ambient temperature and diffuse solar irradiation. Up to 22:00 a large part of the nocturnal cooling was used to cool down the working fluid effectively. The average cooling capacity was about 87 W/m2 at night. To compare with the conventional radiant cooling system with cooling output of 40 W/m2e80 W/m2, the average cooling capacity of SHCP is about 87 W/m2. The unglazed SHCP would transfer heat with surrounding and cold sky by radiation method, with ambient air and air in the air gap behind SHCP through convection way for reducing the temperature of working fluid to achieve the purpose of cooling. The SHCP is set vertically, but the cooling performance is be better than that of the horizontal radiant cooling system. And the SHCP is different from the conventional radiantor with single function of only collecting cooling, which has two functions of heating and cooling.
night, and the absence of air gap and low temperature difference between surface temperature of SHCP and air temperature reduced convective cooling, so the final cooling capacity at night was the equilibrium between radiant and convective cooling. The average cooling capacity of the two systems were almost the same of 30 W/ m2 due to the almost the same emissivity of two coatings. Fig. 5d shows the comparison curves of heat-collecting efficiency with two coatings from 8:30 to 11:20am. The daily average heat-collecting efficiency was 39% with the maximum efficiency of 65% with coating A, and the daily average heat-collecting efficiency was 27% with the maximum point of 49% with coating B.
4.3. Influence of coating on performance of the system
References
Another panel was prepared for parallel experiments to investigate the influence coming from a different acrylic acid resin coating, which has the absorptivity, reflectivity, and emissivity of 0.4, 0.6 and 0.92 [16]. Fig. 5 shows the comparison process on the two systems under the same climate conditions and temperature data. The mass flow rate of the two systems was both set at 0.005 kg/s. The maximum solar irradiation on vertical SHCP was about 200 W/m2, average ambient temperature was about 1 C, and wind speed was changed from 4.7 m/s to 1.8 m/s in January, the data were shown in Fig. 5a. Fig. 5b shows the inlet and outlet temperature of the two systems. The vertical line was the turning point between heating cycle and cooling cycle. The inlet and outlet of the cycle, the inlet temperatures of the two systems were almost the same, but due to higher absorptivity and lower reflectivity of coating A, the outlet temperature was higher about 4 C than that with coating B. While it is the cooling cycle, the temperature difference between inlet and outlet of the two systems was almost the same about 4 C, but the temperatures with coating A were lower than those with coating B due to the emissivity of the two coatings. Fig. 5c compares the heat-collecting capacity of the two systems from 8:30 to 12:45 and cooling capacity from 12:45 to 8:15 next day. The heat-collecting capacity firstly increased and then decreased, which was the equilibrium between absorbed and dispersed energy from SHCP. The higher mass flow rate reduced the temperature difference between inlet and outlet, resulting in an increase in the mean surface temperature of SHCP, thus in turn results in increased radiant cooling, but the presence of cloud prevents long-wave radiation from SHCP surface under a foggy
[1] Erell Evyatar, Etzion Yair. Heating experiments with a radiative cooling system. Build Environ 1996;31:509e17. [2] Al-Nlmr MA, Kodah Z, Nassa B. A theoretical and experimental investigation of a radiative cooling system. Sol Energy 1998;63:367e73. [3] Effects of the passive use of nocturnal radiative cooling in fresh vegetable cooling. 2001. [4] Zhang Shuo, Niu Jianlei. Cooling performance of nocturnal radiative cooling combined with microencapsulated phase change material (MPCM) slurry storage. Energy Build 2012;54:122e30. [5] Oliveti Giuseppe, Arcuri Natale, De Simone Marilena, Bruno Roberto. Experimental evaluations of the building shell radiant exchange in clear sky conditions. Sol Energy 2012;86:1785e95. [6] Eicker Ursula, Dalibard Antoine. Photovoltaic-thermal collectors for night radiative cooling of buildings. Sol Energy 2011;85:1322e35. [7] Farmahini-Farahani Moien, Heidarinejad Ghassem. Increasing effectiveness of evaporative cooling by pre-cooling using nocturnally stored water. Appl Therm Eng 2012;38:117e23. [8] Farmahini Farahani Moien, Heidarinejada Ghassem, Delfani Shahram. Energy Build 2010;42:2131e8. [9] Zalewski L, Lassue S, Duthoit B. Study of solar walls-validating a simulation model. Build Environ 2002;37:109e21. [10] Hirunlabh J, Kongduang W, Namprakai P. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renew Energy 1999;18: 109e19. [11] http://www.omsolar.net. [12] Erell Evyatar, Etzion Yair. A combined hybrid radiative cooling and heating system for arid zones. In: Sayigh AM, editor. Energy and the environment into the 1990s, proceedings of the 1st world renewable energy congress, reading, U K. Pergamon; 1990. p. 2723e7. Oxford. [13] Matsuta M, Terada S, Ito H. Solar heating and radiative cooling using a solar collector-sky radiator with a spectrally selective surface. Sol Energy 1987;39: 183e6. [14] Wang Yiping, Yuan Bing, Zhang Jinli. The solar collector wall. China Patent 2003, ZL03154771.1. [15] Erbs DG, Klein SA, Duffie JA. Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation. Sol Energy 1982;28. [16] Shaoyan Ge, Hongyue Na. Thermal radiation property and measurement. China: Science and Technique Press; 1989.
5. Conclusions The solar heating and nocturnal radiant cooling techniques are combined to produce the novel solar heating and cooling panel to realize the perfect integration between buildings and solar systems and provide thermal comfort in buildings. The experiment results show that in extreme cold January in Tianjin, China, the daily average heat-collecting efficiency was 39% with the maximum of 65%, while during hot seasons the average cooling capacity can reach 87 W/m2. When two different acrylic acid resin coatings were sprayed on SHCP without air gap, the daily average heat-collecting efficiency was 39% and 27% with the maximum points of 65% and 49%, respectively, and the cooling capacity was almost the same of 30 W/m2 in January. Due to non-perfect design of SHCP, the cooling capacity is not enough. A mathematical model will be presented to optimize the design parameters of SHCP to allow for maximum utilization of solar irradiation and nocturnal radiant cooling. The mathematical model of SHCP will be built to optimize the structure of SHCP and operational condition. The economy of the system including SHCP, heat pump or heat pipe and indoor terminal will also be investigated in the later work.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Performance analysis on a building-integrated solar heating and cooling panel Cui Yong a, Wang Yiping b, c, Zhu Li c, * a
Tianjin University Research Institute of Architectural Design, Tianjin 300072, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China c School of Architecture, Tianjin University, Tianjin 300072, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 May 2014 Accepted 27 August 2014 Available online
The solar heating and nocturnal radiant cooling techniques are combined aiming at a novel solar heating and cooling panel (termed as SHCP) to be easily assembled as construction components for building roofing or envelope and also compatible with surroundings for its versatile coating colors, which can remove the double-skin mode from conventional solar equipment. SHCP has two functions for heating and cooling collecting. In this paper, the heating and cooling performances were analyzed in detail based on a small scale experimental system and effects of air gap and coatings were investigated. The results show that in sunny day of extreme cold January in Tianjin, China, the daily average heat-collecting efficiency is 39% with the maximum of 65%, while in sunny night during hot seasons the average cooling capacity can reach 87 W/m2. When two different coatings were sprayed on SHCP without air gap, its heating and cooling performances were all analyzed, the daily average heat-collecting efficiency was 39% and 27% with the maximum points of 65% and 49%, respectively, and the cooling capacity was almost the same of 30 W/m2 in January. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Solar panel Radiant cooling Building integration Monitoring
1. Introduction Currently, energy consumption in buildings mainly consists of domestic hot water, heating, cooling, ventilating, lighting and electrical appliance, which accounts for approximately 27.5% of the total energy use in China, and energy consumption for heating and cooling is 63% of the whole energy consumption in buildings. It is believed that such a proportion will be greatly increased with the rapid economic development of China. Consequently, it is of great importance in the building field to exploit renewable energy, which can minimize the energy expenditure and improve thermal comfort. And it is necessary to realize the integration of solar energy equipments with the building, which can ensure the integrality and aesthetics of buildings. Solar thermal utilization is a mature technology, but nocturnal radiation cooling is a developing technology and has the advantages of energy saving and easy integration, which transmits thermal radiation on the earth through the two atmospheric windows (8 mm ~ 13 mm and 13.5 mm ~ 16 mm) to the sky, since the sky temperature is much lower than the temperature of most of objects
* Corresponding author. Tel./fax: þ86 022 27404771. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.renene.2014.08.076 0960-1481/© 2014 Elsevier Ltd. All rights reserved.
on the earth. Considerable researches have been carried out to study the nocturnal radiant cooling technology under different climate conditions. Evyatar Erell and Yair Etzion have set up a radiant cooling system whose cooling output was 80 W/m2 in the arid area [1]. Maal and Kodah have built another radiant cooling system whose cooling output was 13 MJ/(m2 night) in Jordan [2]. A model building utilizing radiant cooling technology for preserving vegetable has been built in Japan, and the cooling capacity is 40e60 W/m2 [3]. Shuo Zhang and Jianlei Niu have studied a nocturnal radiative cooling system combined with microencapsulated phase change material (MPCM) slurry storage system [4]. The cooling energy consumption and the effect of energy-free nocturnal radiation application were simulated based on hour-byhour calculations in five typical cities across China. The results showed the energy saving potential in Lanzhou and Urumqi can reach 77% and 62% for low-rise buildings. The present hybrid system was recommended to be used in northern and central China cities where the weather is dry and the ambient temperature is low at night. Giuseppe Oliveti studied the radiation exchange between the building walls and the surrounding environment (sky, ground) in the infrared band and determined the radiation heat transfer efficiency between the vertical wall and the sky, between the vertical wall and the ground, also the level roof and the sky [5]. Ursula Eicker and Antoine Dalibard developed a new type of PVT system to
628
C. Yong et al. / Renewable Energy 74 (2015) 627e632
generate electricity and cooling [6]. In Madrid, the average cooling capacity measured was 60e65 W/m2. Moien Farmahini Farahani studied a multi-step refrigeration system of the nocturnal radiative cooling technology combined with two stage evaporation refrigeration and verified the feasibility in four different climatic conditions [7]. The results showed that the multi-step refrigeration system could be the choice for refrigeration in summer. Moien Farmahini Farahani studied a set of two step cooling system including night radiative cooling unit, cooling coil unit and indirect evaporative cooling unit [8]. In summer night, the water in the cooling coil was cooled through radiative cooling way and stored in the tank. During the day, the outdoor hot air passed through the cooling coil and indirect evaporative cooling unit to be cooled. As for the dual-functional system that solar heating is coupled with sky radiant cooling, the solar wall [9,10] and solar roofing system [11] are both the passive heating and cooling technique, using the facades and roof of buildings as solar collector or radiator and air as the working fluid, to supply the requirement energy for buildings. Evyatar. Erell and Yair. Etzion studied the heating performance of the radiative cooling system mentioned in Ref. [1], the average heat-collecting output of the system was 370 W/m2 under the sunny but cool conditions of typical Sde-Boker winter [1,12]. M. Matsuta et al. studied a solar heating and radiative cooling system using a solar collector-sky radiator, the maximum collective and radiative capacities obtained were 610 W/m2 and 51 W/m2, respectively for a clear day and night [13]. Patent ZL03154771.1 presents a sort of solar collector wall that can be used as construction component of building envelope, which realizes the perfect combination of solar energy equipments with the building [14]. In this paper, the nocturnal radiant cooling technology was coupled with the solar collector wall mentioned above to produce a building-integrated solar heating and cooling panel, termed as SHCP hereinafter. Experimental apparatus were set up for detailed study on its performance. 2. System descriptions 2.1. Structure of SHCP The cross-section profiles of different SHCP is shown in Fig. 1. Fig 1a gives the sectional profile of the SHCP without air gap, which mainly concerns about heating function. The structure of SHCP is
from outside functional and protective coating, metal plate, heatconductive filling adhesive, metal tubes, tube-fixing metal fittings with grooves and finally to the inside insulation layer which also functions as the building envelope insulation layer. When local night ambient temperature is low enough to be used for cooling, an air gap was added between the plate and insulation layer but with adjustable inlet and outlet, which is demonstrated in Fig. 1b. The panel investigated here has specified structure size and materials. Acrylic resin coating of about ten years' life length was sprayed manually on an aluminum plate with length of 2000 mm and width of 1000 mm and thickness of 1 mm; copper tubes of 8 mm diameter were mechanically fixed at a space 150 mm to the plate by 10 mm wide aluminum stripes with grooves and conductively fixed using self-made conductive glue with the conductivity of 3.12 W/(m K); high heat-proof performance polystyrene insulation board with 90 mm thickness was used and if needed a 50 mm air gap was added. 2.2. Experimental system An experimental system was set up to test the performance of the SHCP designed above with necessary auxiliary parts shown in Fig. 2, where two 30 L tanks were used for heating and cooling storage. All the exposed surfaces of the system besides SHCP were insulated by rubber-plastic with 30 mm thickness. The SHCP was set vertically facing south aimed to act as building envelope. The red line represents the heating cycle while the blue line (in web version) gives the cooling cycle, and the arrows show the working fluid flow path and orientation. Working fluid is water in this work, while 29% ethanol solution as working fluid in winter to prevent freezing. SHCP was used as solar heat collector at daytime while acts as radiator at night. When it is the heating cycle, the pump1, rotameter1 and valve1 are turned on with pump2, rotameter2 and valve2 closed. The working fluid in the bottom of heat-storage tank is pumped out to be heated by incident solar irradiation on SHCP, and then flows back to the top of heat-storage tank. When outlet temperature of working fluid in SHCP is lower than inlet temperature, it comes to the cooling cycle, valves and pumps are set in reverse manually. Working fluid in the top of cool-storage tank is pumped into the tubes of the panels and cooled by means of radiation and sometimes coupled by natural convection when the
Fig. 1. Cross-section profiles of different SHCP.
C. Yong et al. / Renewable Energy 74 (2015) 627e632
629
CP SH valvel1 coolstorage tank valvel2
rotameter2 heatstorage tank pump2
rotameter1 pump1
Fig. 2. Flow chart of heating and cooling panel testing system.
ambient temperature is low enough, and afterward flows back to the bottom of tank. 2.3. Measurement Local solar irradiation on horizontal plane was measured by solar cells calibrated by an Eppley pyranometer with the accuracy of ±5%, and then calculated to the solar irradiation on vertical plane through Erbs model [15]. Wind speed was measured by anemometer. All temperatures were measured using PT-100 thermo resistances with measurement accuracy of 0.1 C, and the PT-100 thermo resistances were calibrated individually under different temperatures to obtain the calibration curves to be used for data processing. The PT-100 thermo resistance was set in a thermometer shelter about 1.5 m above the ground to get the ambient temperature. The inlet and outlet temperatures of the working fluid in SHCP were measured by PT-100 thermo resistances in the copper casings. The tank fluid temperatures of both tanks were measured by PT-100 thermo resistances in copper casings of the tanks' center, respectively. And also the temperature distribution on the SHCP surfaces was measured by PT-100 thermo resistances with distances of 25 mm and 600 mm in width and length directions. For air gap type SHCP, the temperature distribution along the air gap was also measured. A rotameter was used to manage the flow rate of the working fluid. The signals were all linked to a SWP automatic real-time datalogging system and then to a computer. 3. Performance evaluation The heating and cooling capacity are calculated based on the measured inlet and outlet temperature.
Q ¼ mCP ðTout Tin Þ
(1)
where, Q is the heating or cooling capacity (W), m is the mass flux of the fluid (kg/s), CP is the specific heat of the fluid (kJ/kg K), Tin and Tout are the inlet and outlet temperature of the working fluid ( C), respectively. The calculated heat-collecting capacity above divided by solar irradiation would obtain the heat-collecting efficiency of the system:
h¼
Q AI
(2)
where, h is the heat-collecting efficiency of the system, A is the area of SHCP (m2), I is solar energy absorbed per SHCP area (W/m2). 4. Results and discussion The heating performance of SHCP without air gap in winter and cooling performance with air gap in summer were analyzed in detail and the effects of air gap and coatings on the heating and cooling performances of SHCP were investigated. 4.1. Heating performances Fig. 3 shows the heat-collecting performance analyses of SHCP without air gap based on the measured climate conditions and temperature data in a sunny day of January. The outside coating has properties of 0.7 of absorptivity, 0.96 of emissivity and 0.3 of reflectivity, and the mass flow rate was set at 0.005 kg/s. Solar radiation measured on the horizontal surface was conversed to that on the vertical surface, both of the data were shown in Fig. 3a together with ambient air temperature. It can be seen that the solar radiation on vertical surface was about 40% smaller than that on horizontal plane, so the maximum solar radiation was only about 200 W/m2 at 12:00. Because the weather turned grey, the solar radiation had a dip at 15:00. Fig. 3b shows the inlet and outlet temperatures of working fluid in SHCP. The highest outlet temperature was 14 C with temperature difference about 8 C, but it was too low to be used directly in winter, so can be stored in the heat-storage tank as heat source of heat pump or heat pipe. The reason maybe came from low solar radiation to some extent; maybe relied on the non-perfect design of SHCP with bigger space between copper tubes and less number of copper tubes. We will optimize the SHCP structure and operational parameters by mathematical model later. Unglazed SHCP, lower air temperature and solar radiation made lower panel surface temperature, so that small heat was transferred to working fluid which results in small effective heat conductive surface area and little heat-collecting time during 8:30 to 11:20am in Fig. 3c. Due to the low final temperature, the efficiencies were pretty high with an average value of 39% and a peak point of 65%
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collector with single function of only collecting heat, which has two functions of heating and cooling.
4.2. Cooling performances Fig. 4 shows the cooling performance of the same SHCP with air gap in a sunny day of hot season, and the mass flow rate was set at 0.007 kg/s. The radiant cooling experiment was conducted from 18:00pm to 6:00am next day. Ambient air temperature was from 29 C to 16 C, and wind speed changed from 9.0 m/s to 1.0 m/s, both of the data were shown in Fig. 4a. Fig. 4b shows the inlet and outlet working fluid temperature of SHCP, and the panel surface 30
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Fig. 3. Heating performance analysis of SHCP without air gap.
and the total heat-collecting capacity was 74 MJ among 2.5 h. The sudden increases were caused by short-time clouds shading and large thermal capacity of working fluid medium, which was corresponding to the solar radiation curve. To compare with the flat-plate solar collector on the market, whose average daily heat-collecting efficiency is generally 48%e 58%, the average heat-collecting efficiency of SHCP is slightly smaller. The reasons are that the SHCP is unglazed and is set vertically in this research. But SHCP is different from flat-plate solar
cooling capacity (W/m2)
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Fig. 5. Performance comparison with different outside coatings.
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temperature. The outlet temperature was always less than inlet temperature at night due to radiation and sometimes coupled by natural convection. A decrease in the outlet temperature during this particular night between 21:30 and 22:00 was due to a decrease in ambient air temperature of 2 C in this interval. This change was probably accompanied by the results of an increase in wind speed from 1 m/s to 2 m/s. At the beginning of cooling cycle, the panel surface temperature was higher than the outlet temperature of working fluid, because of the high ambient temperature and diffused sunlight, so that the received energy was larger than the energy loss. With time, the panel was cooled by radiation with the low temperature sky and convection with air, but due to the thermal resistance between the panel and the working fluid, the panel surface temperature decreased to below the inlet and outlet working fluid temperature. Fig. 4c shows that the cooling capacity was small in the early and last hours during the cooling cycle, which can be explained by the effect of the energy stored in the system during day time due to high ambient temperature and diffuse solar irradiation. Up to 22:00 a large part of the nocturnal cooling was used to cool down the working fluid effectively. The average cooling capacity was about 87 W/m2 at night. To compare with the conventional radiant cooling system with cooling output of 40 W/m2e80 W/m2, the average cooling capacity of SHCP is about 87 W/m2. The unglazed SHCP would transfer heat with surrounding and cold sky by radiation method, with ambient air and air in the air gap behind SHCP through convection way for reducing the temperature of working fluid to achieve the purpose of cooling. The SHCP is set vertically, but the cooling performance is be better than that of the horizontal radiant cooling system. And the SHCP is different from the conventional radiantor with single function of only collecting cooling, which has two functions of heating and cooling.
night, and the absence of air gap and low temperature difference between surface temperature of SHCP and air temperature reduced convective cooling, so the final cooling capacity at night was the equilibrium between radiant and convective cooling. The average cooling capacity of the two systems were almost the same of 30 W/ m2 due to the almost the same emissivity of two coatings. Fig. 5d shows the comparison curves of heat-collecting efficiency with two coatings from 8:30 to 11:20am. The daily average heat-collecting efficiency was 39% with the maximum efficiency of 65% with coating A, and the daily average heat-collecting efficiency was 27% with the maximum point of 49% with coating B.
4.3. Influence of coating on performance of the system
References
Another panel was prepared for parallel experiments to investigate the influence coming from a different acrylic acid resin coating, which has the absorptivity, reflectivity, and emissivity of 0.4, 0.6 and 0.92 [16]. Fig. 5 shows the comparison process on the two systems under the same climate conditions and temperature data. The mass flow rate of the two systems was both set at 0.005 kg/s. The maximum solar irradiation on vertical SHCP was about 200 W/m2, average ambient temperature was about 1 C, and wind speed was changed from 4.7 m/s to 1.8 m/s in January, the data were shown in Fig. 5a. Fig. 5b shows the inlet and outlet temperature of the two systems. The vertical line was the turning point between heating cycle and cooling cycle. The inlet and outlet of the cycle, the inlet temperatures of the two systems were almost the same, but due to higher absorptivity and lower reflectivity of coating A, the outlet temperature was higher about 4 C than that with coating B. While it is the cooling cycle, the temperature difference between inlet and outlet of the two systems was almost the same about 4 C, but the temperatures with coating A were lower than those with coating B due to the emissivity of the two coatings. Fig. 5c compares the heat-collecting capacity of the two systems from 8:30 to 12:45 and cooling capacity from 12:45 to 8:15 next day. The heat-collecting capacity firstly increased and then decreased, which was the equilibrium between absorbed and dispersed energy from SHCP. The higher mass flow rate reduced the temperature difference between inlet and outlet, resulting in an increase in the mean surface temperature of SHCP, thus in turn results in increased radiant cooling, but the presence of cloud prevents long-wave radiation from SHCP surface under a foggy
[1] Erell Evyatar, Etzion Yair. Heating experiments with a radiative cooling system. Build Environ 1996;31:509e17. [2] Al-Nlmr MA, Kodah Z, Nassa B. A theoretical and experimental investigation of a radiative cooling system. Sol Energy 1998;63:367e73. [3] Effects of the passive use of nocturnal radiative cooling in fresh vegetable cooling. 2001. [4] Zhang Shuo, Niu Jianlei. Cooling performance of nocturnal radiative cooling combined with microencapsulated phase change material (MPCM) slurry storage. Energy Build 2012;54:122e30. [5] Oliveti Giuseppe, Arcuri Natale, De Simone Marilena, Bruno Roberto. Experimental evaluations of the building shell radiant exchange in clear sky conditions. Sol Energy 2012;86:1785e95. [6] Eicker Ursula, Dalibard Antoine. Photovoltaic-thermal collectors for night radiative cooling of buildings. Sol Energy 2011;85:1322e35. [7] Farmahini-Farahani Moien, Heidarinejad Ghassem. Increasing effectiveness of evaporative cooling by pre-cooling using nocturnally stored water. Appl Therm Eng 2012;38:117e23. [8] Farmahini Farahani Moien, Heidarinejada Ghassem, Delfani Shahram. Energy Build 2010;42:2131e8. [9] Zalewski L, Lassue S, Duthoit B. Study of solar walls-validating a simulation model. Build Environ 2002;37:109e21. [10] Hirunlabh J, Kongduang W, Namprakai P. Study of natural ventilation of houses by a metallic solar wall under tropical climate. Renew Energy 1999;18: 109e19. [11] http://www.omsolar.net. [12] Erell Evyatar, Etzion Yair. A combined hybrid radiative cooling and heating system for arid zones. In: Sayigh AM, editor. Energy and the environment into the 1990s, proceedings of the 1st world renewable energy congress, reading, U K. Pergamon; 1990. p. 2723e7. Oxford. [13] Matsuta M, Terada S, Ito H. Solar heating and radiative cooling using a solar collector-sky radiator with a spectrally selective surface. Sol Energy 1987;39: 183e6. [14] Wang Yiping, Yuan Bing, Zhang Jinli. The solar collector wall. China Patent 2003, ZL03154771.1. [15] Erbs DG, Klein SA, Duffie JA. Estimation of the diffuse radiation fraction for hourly, daily and monthly-average global radiation. Sol Energy 1982;28. [16] Shaoyan Ge, Hongyue Na. Thermal radiation property and measurement. China: Science and Technique Press; 1989.
5. Conclusions The solar heating and nocturnal radiant cooling techniques are combined to produce the novel solar heating and cooling panel to realize the perfect integration between buildings and solar systems and provide thermal comfort in buildings. The experiment results show that in extreme cold January in Tianjin, China, the daily average heat-collecting efficiency was 39% with the maximum of 65%, while during hot seasons the average cooling capacity can reach 87 W/m2. When two different acrylic acid resin coatings were sprayed on SHCP without air gap, the daily average heat-collecting efficiency was 39% and 27% with the maximum points of 65% and 49%, respectively, and the cooling capacity was almost the same of 30 W/m2 in January. Due to non-perfect design of SHCP, the cooling capacity is not enough. A mathematical model will be presented to optimize the design parameters of SHCP to allow for maximum utilization of solar irradiation and nocturnal radiant cooling. The mathematical model of SHCP will be built to optimize the structure of SHCP and operational condition. The economy of the system including SHCP, heat pump or heat pipe and indoor terminal will also be investigated in the later work.