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Experience on integration of solar thermal technologies with green buildings
ARTICLE IN PRESS
Renewable Energy 33 (2008) 1904–1910 www.elsevier.com/locate/renene
Experience on integration of solar thermal technologies with green buildings X.Q. Zhai, R.Z. Wang, Y.J. Dai, J.Y. Wu, Q. Ma Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China Received 26 August 2006; accepted 29 September 2007 Available online 3 January 2008
Abstract The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings, which are for the demonstration of flat and villa, respectively. Here, a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply was designed and constructed for the office building. However, only solar hot-water systems were installed for the flat and villa. All the three solar thermal systems have continuously run for 2 years. Two different integrated approaches have been put into practice in the two green residential buildings. It is shown that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. The solar-powered integrated energy system has the advantage of high utilization ratio with different functions according to different seasons. It is testified to be capable of taking on about 70% of the yearly building load regarding the involved space under the weather condition of Shanghai. r 2007 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Green building; Integrated approach; Solar thermal technology
1. Introduction 1.1. Concept of green building The concept of green building has stirred extensive interest among the building and energy researches all over the world. Green buildings are examples of applied ecology, where designers understand the constitution, organization, and structure of ecosystems, and the impacts of architecture are considered from an environmental perspective. By utilizing the concepts, methods, and language of ecology, designers can create architecture that intentionally engages the natural system of a site [1]. As for energy consumption of green buildings, it is highly suggested to reduce fossil fuel by making use of renewable energy, such as solar energy, wind energy and geothermic energy. Being abundant and clean, solar energy is receiving much attention in green building energy system. Corresponding author. Tel./fax: (86-21) 34206296.
E-mail address: [email protected] (X.Q. Zhai). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.09.027
Generally, the newer green buildings combine several of solar technologies. They may be both energy efficient, solar heated and cooled, and PV powered, i.e. they are simply ‘‘solar buildings’’ [2]. 1.2. Solar thermal technologies based on solar collectors Solar collectors can be integrated into building facades due to the fact that integrating solar systems in the building envelope often is a necessity if the systems are to be economically feasible. Presently, solar collectors have been used in a variety of applications including solar hot-water supplying, solar space heating and cooling. Solar water collectors have undergone a rapid development; they are installed with the main purpose of preheating domestic hot water and/or to cover a fraction of the space heating demand. With regard to air-conditioning system, considering the problem of peak load of electricity consumption in summer due to electric chillers, the idea of solar cooling is intriguing from demand side considerations: the chilling demand at least to a significant extent runs parallel to the
ARTICLE IN PRESS X.Q. Zhai et al. / Renewable Energy 33 (2008) 1904–1910
availability of solar radiation. Therefore, the interest in solar cooling by sorption systems has been prevalent for several decades [3]. In most of the solar cooling systems, hot water driven single-stage lithium bromide absorption chillers were commonly used. Evacuated tubes or other high-grade solar collectors were adopted to provide a hot water temperature of 88–90 1C as a heat source to drive the chiller. Experimental results on the performance of such systems were reported by several researchers [4,5]. Compared with the existing absorption systems, adsorption systems can be built in small scale and can be operated with no moving parts, which means that the rectifier or solution pump is not needed. Also, there exists no corrosion problem in adsorption systems. Theocharis Tsoutsos et al. [6] reported that the combination of an adsorption chiller with solar collectors offers a technically simple and energy saving solution. Wang [7] suggested that for the minitype solar airconditioning system, solar adsorption cooling system will be a better chance. Because of the intermittent nature of solar energy, intermittent adsorption refrigeration cycles have long been considered as logical approaches to solar cooling systems [8]. Therefore, up to now, the solarpowered adsorption systems have mostly been intermittent and used only for ice making application. For applications such as air conditioning, two or more adsorption beds can be used to produce a cooling effect continuously. Numerical simulations have been done to investigate the performance of a solar-powered adsorption air conditioning system driven by simple flat plate solar collectors [9]. As for working pairs, a silica gel/water adsorption refrigerator uses waste heat at below 100 1C, which would be suitable for a wider range of solar collector types [10]. 1.3. Main work of this paper The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings which are for the demonstration of flat and villa, respectively. As demonstration projects, they contain multiple green energy technologies, such as solar thermal technology, solar photovoltaic, natural ventilation, natural lighting, indoor virescence, and the like. Here, we designed a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply for the office building. However, only solar hot-water systems were designed for the flat and villa. All the three systems have continuously run for 2 years. In this paper, the integration of solar thermal systems with green buildings was introduced and main performances of the systems were summarized. 2. Present state of solar thermal utilization in buildings of China In China, solar collectors have undergone a rapid development with an annual average growth of 30% since
1905
1980. By the end of 2005, a total of over 60,000,000 m2 solar collectors have been put into use nationwide. They are installed with the main purpose of hot water supply in residential buildings. Currently, solar water heaters have accounted for about 10% market of the water heating devices. There is still a great market potential for solar water heaters in China. Solar collectors have become an important symbol of green buildings. In the 2008 Olympic projects of Beijing, about 90% domestic hot water will be provided by solar collectors, which contributes greatly to the concept of green Olympics. However, this application mainly for obtaining hot water through solar energy is not very consistent with the order of nature. In winter, it is convenient to combine hotwater system with floor heating system just through increasing the collector area. A typical instance of solarpowered floor heating system is the newly built Lasa Railway Station in the famous Qingzang Railway project. Whereas, for summer with high solar radiant intensity and high ambient air temperature, the demand for airconditioning is in preference to hot water, this phenomenon is obvious especially in the south of China. Solarpowered air-conditioning system would be a perfect scheme because it not only makes the best use of solar energy, but also converts low-grade energy (solar energy) into highgrade energy for comfort. The research of solar cooling systems in China were mainly centered on the solar absorption air-conditioning systems. A large-scale solar absorption air-conditioning system driven by evacuated tubular solar collectors was built in Rushan, Shandong Province. The cooling capacity of this system is about 100 kW, with the average COP of 0.57 in 6 h effective operation [5]. Another influential solar absorption airconditioning system with the same cooling capacity driven by flat-plate solar collectors was constructed in Jiangmen, Guangdong Province. The experimental results showed that average COP is 0.4 [11]. As for mini type solarpowered air conditioning system, Shanghai Jiao Tong University broke through key technological difficulties in 2004, and invented a silica gel-water adsorption chiller, which has been put into practice in the green office building of Shanghai Research Institute of Building Science. The main problems resisting the further development of solar thermal utilization in buildings include (a) neglect of integration of solar collectors with buildings in the design processes; (b) lack of highly efficient solar-powered integrated energy system. Therefore, our experience in the green buildings aims at investigating feasible approaches for the integration of solar thermal systems with buildings in China. 3. Experience on integration of solar thermal utilization with green residential buildings Solar heating could be an important contributor in the residential buildings for hot-water supply. Currently, the familiar approach for integration of solar collectors into
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X.Q. Zhai et al. / Renewable Energy 33 (2008) 1904–1910
residential buildings in China is to install collectors on the south tilted roof. However, integration of solar collectors into building roof has the disadvantage of serious thermal loss led by long pipelines. Furthermore, the roof area is always not enough to install solar collectors especially for multi-story buildings and high-rise buildings, which are widespread in large cities like Shanghai. Then, solar collectors can be considered to integrate with the south envelops, and form consecutive facades with aesthetic compatibility. The feasible methods include integrations of solar collectors with balconies, walls and awnings. 3.1. Integration of solar hot-water system with flat A three-storied green building was built for the demonstration of flat, where the first floor is for ordinary single-storied flat and the upper two floors are for duplex flat. The solar collectors were installed on the sideboards of balconies. According to the dimension of balconies, we customized 0.75 m-high U-type evacuated tubular solar collectors with CPC, and placed 5.6 m2 solar collectors for the first floor, 2.7 and 4.2 m2 for the second floor and third floor, respectively. Fig. 1 shows the effect of integration of solar collectors and the flat. Here, solar collectors act as not only the heat source of hot-water system, but also the decoration of balconies. This demonstration project serves as a good example of both building integration and of a sensible combination of functions. Moreover, it provides a feasible design method for multi-story buildings and highrise buildings especially for residential buildings. Besides solar collector arrays, the solar hot-water system of the single-storied flat is mainly composed of a solar collecting pump, a constant pressure tank and a heat storage water tank of 0.3 m3 in volume. They are connected through copper pipes and valves to form a closed circulating system with a setting pressure of 0.4 MPa. The domestic hot water is heated by the heat exchanger inside the heat storage water tank. Similar solar hot-water system was constructed for the
Fig. 1. Effect of integration of solar collectors and flat.
duplex flat by the parallel connection of solar collector arrays on the second floor and the third floor. In order to evaluate thermal performance of this kind of installation, the solar collector array of the first floor was chosen as a representative to test on the efficiency characteristic. The instantaneous experimental collector efficiency data were expressed as the function of (TinTa)/ I. According to the experimental results of this summer, the instantaneous solar collecting efficiency can be denoted as Z ¼ 0:38 0:98ðT in T a Þ=I,
(1)
where Z is instantaneous solar collecting efficiency, Tin and Ta are respectively inlet temperature of solar collector array and air temperature (1C), I is the solar intensity (W/m2). 3.2. Integration of solar hot-water system with villa In the villa, because the whole roof is occupied by technologies of solar photovoltaic and virescence, we customized 1 m-high U-type evacuated tubular solar collectors with CPC in terms of the dimension of awning, on which we installed 4.6 m2 solar collectors at 101 tilted angle as shown in Fig. 2. Such design provides another example of how a solar element could be used in the original design in a logical manner, especially for those without enough roof area. The solar hot-water system in the villa is similar with those of flat. In terms of experimental results, the instantaneous solar collecting efficiency can be expressed as Z ¼ 0:41 1:67ðT in T a Þ=I.
(2)
4. Experience on integration of solar thermal utilization with green office building 4.1. Integration of solar collectors and green office building As the power to drive adsorption chillers and the heat source for the floor heating and natural ventilation, the
Fig. 2. Effect of integration of solar collectors and villa.
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solar collectors are the most important parts. We installed 150 m2 solar collectors on the roof of the green building, wherein U-type evacuated tubular solar collectors with CPC of area 90 m2 were placed on the west side (SCW), and the other 60 m2 heat pipe evacuated tubular solar collectors on the east side (SCE). For the purpose of efficient utilization of solar energy, the architects designed a steel structure roof, facing due south and tilted at an angle of 401 to the ground surface, on which the solar collectors were mounted and integrated with the building perfectly. Fig. 3 shows the appearance of the green office building integrated with solar collectors. All solar collectors of both sides were divided into three parallel rows, as shown in Fig. 4. The collector units in each row were connected in a series arrangement for the purpose of obtaining hot water with relatively high temperature, which plays an important role in improving performance of the solar energy system. Such an arrangement of solar collectors not only guarantees high system performance but also enhances the architectural expression of the building. Besides, it provides a feasible idea for integration of solar collectors and civil buildings especially for public buildings. 4.2. Design of solar-powered integrated energy system An integrated energy system based on solar thermal technologies was designed and set up for building area of 460 m2. As an office building, the hot water demand is not as significant as that in residential buildings. So, the solarpowered integrated system design of the green building is mainly focused on floor heating in winter and airconditioning in summer. Another novel design is natural ventilation enhanced by solar hot water, which is effective and necessary to solve the problem of surplus hot water in transitional seasons. Moreover, it provides a new method for the design of solar-enhanced natural ventilation. Except for solar collectors, the solar-powered integrated energy system mainly includes two adsorption chillers,
Fig. 3. The external appearance of the green office building integrated with solar collectors.
Fig. 4. Arrangement of solar collector arrays. (a) Heat pipe evacuated tubular solar collector arrays. (b) U-type pipe evacuated tubular solar collector arrays.
floor heating pipes, finned tube heat exchangers, circulating pumps, and a cooling tower. Besides, a hot water storage tank of 2.5 m3 in volume is employed to collect solar heat, thereby providing hot water for the integrated solar energy system. All components are connected by tubes and valves to form the whole circulating system. The flow diagram of the integrated solar energy system is shown in Fig. 5, where AD1 and AD2 are two adsorption chillers, CT is a cooling tower, WT is a hot water storage tank, P1 and P2 are two solar collecting pumps, P3 and P4 are hot water pump and cooling water pump, respectively. Through valves located on the pipes, the solar-powered integrated energy system can be switched to different operating modes according to different seasons: (1) In summer, the cooling load of air-conditioning area under design condition is 60 kW, thereinto, 15 kW is sensible heat cooling load met by solar-adsorption airconditioning system, which is discussed in this paper. The other 45 kW is latent heat cooling load taken on by
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Fig. 5. The flow diagram of the integrated solar energy system, showing different operating modes: floor radiation heating (FH), natural ventilation enhanced by solar hot water (NV), air-conditioning (AC).
a liquid-desiccant system, which is constructed by Shanghai Research Institute of Building Science. Thereby, the hybrid air-conditioning system deals with cooling and humidity loads independently, and the fan coils inside air-conditioning rooms realize dry operating mode. (2) In winter, solar-powered floor heating system is used to satisfy heating load of the green building. (3) In transitional seasons, solar hot water is pumped into finned tube heat exchangers to induce stack pressure, which is capable of improving natural ventilation. (4) The system can be used to supply hot water as long as a heat exchanger is installed in parallel with what mentioned above. In order to complete the solar-powered air-conditioning system, we chose the environment friendly silica-gel/water as the working pair and invented an adsorption chiller, which is capable of working from 55 to 95 1C. Fig. 6 shows the photo of the silica gel-water adsorption chiller. Owing to its practicability in low temperature, the chiller is testified to be suitable for solar-powered air-conditioning system. The performance test shows that the chiller attains rated refrigerating capacity of 8.5 kW when the hot water temperature is 85 1C, and the corresponding COP is 0.4 [12]. As for heating system, we selected cuprotherm floor heating system produced by Wieland Ltd. of Shanghai. The floor heating coil pipes are made of high-quality pure copper with the dimension of F12 0.7 mm, as shown in Fig. 7. They were fixed on the 30 mm thick polystyrene insulation layer with spacing interval 200 mm. And then crushed stone concrete was poured with the thickness of 70 mm. There is an air channel under the roof of the green building, which is designed by architects for indoor air exhaust through natural ventilation. In order to enhance
Fig. 6. The photo of the silica gel-water adsorption chiller.
natural ventilation by stack pressure, we installed seven groups of heat exchange elements inside the air channel. Each group consists of three parallel finned tube heat exchangers as shown in Fig. 8. The finned tube heat exchanger is made of a 3-m-long copper tube with 540 square fins. The diameter of the tube is 20 mm and the sectional dimension of the square fins is 102 mm 102 mm. 4.3. Summary of all-year operation of the solar-powered integrated energy system From September 2004 to August 2006, the solarpowered integrated energy system has been continuously in operation under different modes according to different seasons. Based on all-the-year-round experimental data, it is concluded that under the weather condition of Shanghai, 150 m2 vacuum tube solar collector arrays can be used to
ARTICLE IN PRESS X.Q. Zhai et al. / Renewable Energy 33 (2008) 1904–1910
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whole power consumption was 1.87 kW, and the average electric COP was 8.19 during an 8 h operation, and the maximum exceeded 10. In addition, under typical working condition in transition seasons (daily solar insolation of 17 MJ/m2 and average ambient temperature of 10 1C), solar-enhanced natural ventilation air flow rate induced by stack pressure is doubled compared with conventional natural ventilation. Consequently, the system is capable of inducing air change rate of 3 ACH and supplying hot water for the office building. Also can be obtained is that the solar fraction for the system in winter is 56%, correspondingly, 75% in summer, and 68% in transition seasons. Then the mean annual solar fraction of the system nearly reaches 70% through weighted average of those in different seasons and the corresponding days. Fig. 7. The photo of floor heating coil pipe.
5. Conclusion Different integrated approaches were carried out in the green buildings of Shanghai Research Institute of Building Science. A solar-powered integrated energy system involving heating, air-conditioning, natural ventilation and hot water supplying was constructed for the green office building, which realizes high integration of solar thermal technologies. However, only solar hot-water systems were designed for the two green residential buildings. As a brief summary, it is wished to emphasize the significant points of this work in the following.
Fig. 8. Finned tube heat exchanger inside air channel.
satisfy heating and air-conditioning for covered area of 460 m2. Under typical working condition in winter (daily solar insolation of 18 MJ/m2 and average ambient temperature of 1.98 1C), the average floor temperature and indoor air temperature is about 23.71 and 17.10 1C, respectively, which satisfies indoor thermal environment. Under typical working condition in summer (daily solar insolation of 20.36 MJ/m2 and average ambient temperature of 31.66 1C), the average refrigerating output of solar powered air-conditioning system is 15.31 kW during efficient operation of 8 h; moreover, the maximum attains 20 kW. With regard to heat consumption of two adsorption chillers, the average system COP is 0.35, and average solar COP is 0.15 concerning daily solar insolation. In the solarpowered air-conditioning system, it is important to reduce electric power consumption; consequently, electric COP is another important index to evaluate performance of the system. Thus, considering the solar collecting pump (P1), hot water pump (P2) and cooling water pump (P3), the
(1) Our experience in green residential buildings shows that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. Solar collectors therefore could be used in the original design in a logical manner for the purpose of supplying hot water as well as improving building facade. The integration of solar collectors with balconies provides a feasible design method for multi-story buildings and high-rise buildings especially for residential buildings, which are widespread in large cities like Shanghai. (2) Our experience in the green office building shows that silica gel/water adsorption chiller invented by our research group has been verified as a successful solar air conditioning unit. Under the weather condition of Shanghai, 150 m2 vacuum tube solar collector arrays can be used to satisfy heating and air-conditioning for covered area of 460 m2. In addition, they are capable of inducing natural ventilation by stack pressure and supplying hot water for the office building. The solarpowered integrated energy system can take on about 70% of the yearly building load regarding the involved space. Such designs have the advantage of high utilization ratio with different functions according to different seasons, which makes the solar-powered integrated energy systems more economical.
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In China, with the implement of ‘‘Renewable Energy Law’’ since January the 1st of 2006, enough emphasis has been put by the governments of all provinces. Taking Shanghai for example, the government has issued ‘‘Program of Exploiting and Using Solar Energy’’, which calls for notable effect of solar thermal utilization in 3–5 years. Accordingly, by the year 2007, there will be 100,000 m2 (solar collecting area) solar hot water systems integrated with buildings in Shanghai. It is reasonable to expect that the solar thermal utilization will play greater role in building energy systems in the coming years. Acknowledgments This work is supported by the Shanghai Commission of Science and Technology under the contract No. 03DZ12012. Reference [1] Olgyay V, Herdt J. The application of ecosystems services criteria for green building assessment. Sol Energy 2004;77(4):389–98. [2] Hestnes AG. Building integration of solar energy systems. Sol Energy 1999;67(4–6):181–7.
[3] Ziegler F. Recent developments and future prospects of sorption heat pump systems. Int J Therm Sci 1999;38(3):191–208. [4] Li ZF, Sumathy K. Experimental studies on a solar powered air conditioning system with partitioned hot water storage tank. Sol Energy 2001;71(5):285–97. [5] Zinian H, Ning Z, et al. Design and performance of a solar absorption air-conditioning and heat-supply system (in Chinese). Acta Energiae Solaris Sinica 2001;22(1):6–11. [6] Tsoutsos T, Anagnostou J, Pritchard C, Karagiorgas M, Agoris D. Solar cooling technologies in Greece. An economic viability analysis. Appl Therm Eng 2003;23(11):1427–39. [7] Wang RZ. Adsorption refrigeration in Shanghai Jiao Tong University. Renew Sust Energ Rev 2001;5(1):1–37. [8] Sumathy K, Yeung KH, Yong L. Technology development in the solar adsorption refrigeration systems. Prog Energ Combust 2003; 29(4):301–27. [9] Yong L, Sumathy K. Modeling and simulation of a solar powered two bed adsorption air conditioning system. Energ Convers Manage 2004;45(17):2761–75. [10] Dieng AO, Wang RZ. Literature review on solar adsorption technologies for ice-making and air-conditioning purposes and recent developments in solar technology. Renew Sust Energ Rev 2001; 5(4):313–42. [11] Jianhong L, Weibing M, et al. 100 kW solar-powered air-conditioning system (in Chinese). Acta Energiae Solaris Sinica 1999;20(3):239–43. [12] Wang DC, Xia ZZ, Wu JY, et al. Study of a novel silica gel-water adsorption chiller. Part I. Design and performance prediction. Int J Refrig 2005;28(7):1073–83.
Renewable Energy 33 (2008) 1904–1910 www.elsevier.com/locate/renene
Experience on integration of solar thermal technologies with green buildings X.Q. Zhai, R.Z. Wang, Y.J. Dai, J.Y. Wu, Q. Ma Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China Received 26 August 2006; accepted 29 September 2007 Available online 3 January 2008
Abstract The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings, which are for the demonstration of flat and villa, respectively. Here, a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply was designed and constructed for the office building. However, only solar hot-water systems were installed for the flat and villa. All the three solar thermal systems have continuously run for 2 years. Two different integrated approaches have been put into practice in the two green residential buildings. It is shown that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. The solar-powered integrated energy system has the advantage of high utilization ratio with different functions according to different seasons. It is testified to be capable of taking on about 70% of the yearly building load regarding the involved space under the weather condition of Shanghai. r 2007 Elsevier Ltd. All rights reserved. Keywords: Solar energy; Green building; Integrated approach; Solar thermal technology
1. Introduction 1.1. Concept of green building The concept of green building has stirred extensive interest among the building and energy researches all over the world. Green buildings are examples of applied ecology, where designers understand the constitution, organization, and structure of ecosystems, and the impacts of architecture are considered from an environmental perspective. By utilizing the concepts, methods, and language of ecology, designers can create architecture that intentionally engages the natural system of a site [1]. As for energy consumption of green buildings, it is highly suggested to reduce fossil fuel by making use of renewable energy, such as solar energy, wind energy and geothermic energy. Being abundant and clean, solar energy is receiving much attention in green building energy system. Corresponding author. Tel./fax: (86-21) 34206296.
E-mail address: [email protected] (X.Q. Zhai). 0960-1481/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2007.09.027
Generally, the newer green buildings combine several of solar technologies. They may be both energy efficient, solar heated and cooled, and PV powered, i.e. they are simply ‘‘solar buildings’’ [2]. 1.2. Solar thermal technologies based on solar collectors Solar collectors can be integrated into building facades due to the fact that integrating solar systems in the building envelope often is a necessity if the systems are to be economically feasible. Presently, solar collectors have been used in a variety of applications including solar hot-water supplying, solar space heating and cooling. Solar water collectors have undergone a rapid development; they are installed with the main purpose of preheating domestic hot water and/or to cover a fraction of the space heating demand. With regard to air-conditioning system, considering the problem of peak load of electricity consumption in summer due to electric chillers, the idea of solar cooling is intriguing from demand side considerations: the chilling demand at least to a significant extent runs parallel to the
ARTICLE IN PRESS X.Q. Zhai et al. / Renewable Energy 33 (2008) 1904–1910
availability of solar radiation. Therefore, the interest in solar cooling by sorption systems has been prevalent for several decades [3]. In most of the solar cooling systems, hot water driven single-stage lithium bromide absorption chillers were commonly used. Evacuated tubes or other high-grade solar collectors were adopted to provide a hot water temperature of 88–90 1C as a heat source to drive the chiller. Experimental results on the performance of such systems were reported by several researchers [4,5]. Compared with the existing absorption systems, adsorption systems can be built in small scale and can be operated with no moving parts, which means that the rectifier or solution pump is not needed. Also, there exists no corrosion problem in adsorption systems. Theocharis Tsoutsos et al. [6] reported that the combination of an adsorption chiller with solar collectors offers a technically simple and energy saving solution. Wang [7] suggested that for the minitype solar airconditioning system, solar adsorption cooling system will be a better chance. Because of the intermittent nature of solar energy, intermittent adsorption refrigeration cycles have long been considered as logical approaches to solar cooling systems [8]. Therefore, up to now, the solarpowered adsorption systems have mostly been intermittent and used only for ice making application. For applications such as air conditioning, two or more adsorption beds can be used to produce a cooling effect continuously. Numerical simulations have been done to investigate the performance of a solar-powered adsorption air conditioning system driven by simple flat plate solar collectors [9]. As for working pairs, a silica gel/water adsorption refrigerator uses waste heat at below 100 1C, which would be suitable for a wider range of solar collector types [10]. 1.3. Main work of this paper The green buildings of Shanghai Research Institute of Building Science include an office building for the demonstration of public building and two residential buildings which are for the demonstration of flat and villa, respectively. As demonstration projects, they contain multiple green energy technologies, such as solar thermal technology, solar photovoltaic, natural ventilation, natural lighting, indoor virescence, and the like. Here, we designed a solar-powered integrated energy system including heating, air-conditioning, natural ventilation and hot water supply for the office building. However, only solar hot-water systems were designed for the flat and villa. All the three systems have continuously run for 2 years. In this paper, the integration of solar thermal systems with green buildings was introduced and main performances of the systems were summarized. 2. Present state of solar thermal utilization in buildings of China In China, solar collectors have undergone a rapid development with an annual average growth of 30% since
1905
1980. By the end of 2005, a total of over 60,000,000 m2 solar collectors have been put into use nationwide. They are installed with the main purpose of hot water supply in residential buildings. Currently, solar water heaters have accounted for about 10% market of the water heating devices. There is still a great market potential for solar water heaters in China. Solar collectors have become an important symbol of green buildings. In the 2008 Olympic projects of Beijing, about 90% domestic hot water will be provided by solar collectors, which contributes greatly to the concept of green Olympics. However, this application mainly for obtaining hot water through solar energy is not very consistent with the order of nature. In winter, it is convenient to combine hotwater system with floor heating system just through increasing the collector area. A typical instance of solarpowered floor heating system is the newly built Lasa Railway Station in the famous Qingzang Railway project. Whereas, for summer with high solar radiant intensity and high ambient air temperature, the demand for airconditioning is in preference to hot water, this phenomenon is obvious especially in the south of China. Solarpowered air-conditioning system would be a perfect scheme because it not only makes the best use of solar energy, but also converts low-grade energy (solar energy) into highgrade energy for comfort. The research of solar cooling systems in China were mainly centered on the solar absorption air-conditioning systems. A large-scale solar absorption air-conditioning system driven by evacuated tubular solar collectors was built in Rushan, Shandong Province. The cooling capacity of this system is about 100 kW, with the average COP of 0.57 in 6 h effective operation [5]. Another influential solar absorption airconditioning system with the same cooling capacity driven by flat-plate solar collectors was constructed in Jiangmen, Guangdong Province. The experimental results showed that average COP is 0.4 [11]. As for mini type solarpowered air conditioning system, Shanghai Jiao Tong University broke through key technological difficulties in 2004, and invented a silica gel-water adsorption chiller, which has been put into practice in the green office building of Shanghai Research Institute of Building Science. The main problems resisting the further development of solar thermal utilization in buildings include (a) neglect of integration of solar collectors with buildings in the design processes; (b) lack of highly efficient solar-powered integrated energy system. Therefore, our experience in the green buildings aims at investigating feasible approaches for the integration of solar thermal systems with buildings in China. 3. Experience on integration of solar thermal utilization with green residential buildings Solar heating could be an important contributor in the residential buildings for hot-water supply. Currently, the familiar approach for integration of solar collectors into
ARTICLE IN PRESS 1906
X.Q. Zhai et al. / Renewable Energy 33 (2008) 1904–1910
residential buildings in China is to install collectors on the south tilted roof. However, integration of solar collectors into building roof has the disadvantage of serious thermal loss led by long pipelines. Furthermore, the roof area is always not enough to install solar collectors especially for multi-story buildings and high-rise buildings, which are widespread in large cities like Shanghai. Then, solar collectors can be considered to integrate with the south envelops, and form consecutive facades with aesthetic compatibility. The feasible methods include integrations of solar collectors with balconies, walls and awnings. 3.1. Integration of solar hot-water system with flat A three-storied green building was built for the demonstration of flat, where the first floor is for ordinary single-storied flat and the upper two floors are for duplex flat. The solar collectors were installed on the sideboards of balconies. According to the dimension of balconies, we customized 0.75 m-high U-type evacuated tubular solar collectors with CPC, and placed 5.6 m2 solar collectors for the first floor, 2.7 and 4.2 m2 for the second floor and third floor, respectively. Fig. 1 shows the effect of integration of solar collectors and the flat. Here, solar collectors act as not only the heat source of hot-water system, but also the decoration of balconies. This demonstration project serves as a good example of both building integration and of a sensible combination of functions. Moreover, it provides a feasible design method for multi-story buildings and highrise buildings especially for residential buildings. Besides solar collector arrays, the solar hot-water system of the single-storied flat is mainly composed of a solar collecting pump, a constant pressure tank and a heat storage water tank of 0.3 m3 in volume. They are connected through copper pipes and valves to form a closed circulating system with a setting pressure of 0.4 MPa. The domestic hot water is heated by the heat exchanger inside the heat storage water tank. Similar solar hot-water system was constructed for the
Fig. 1. Effect of integration of solar collectors and flat.
duplex flat by the parallel connection of solar collector arrays on the second floor and the third floor. In order to evaluate thermal performance of this kind of installation, the solar collector array of the first floor was chosen as a representative to test on the efficiency characteristic. The instantaneous experimental collector efficiency data were expressed as the function of (TinTa)/ I. According to the experimental results of this summer, the instantaneous solar collecting efficiency can be denoted as Z ¼ 0:38 0:98ðT in T a Þ=I,
(1)
where Z is instantaneous solar collecting efficiency, Tin and Ta are respectively inlet temperature of solar collector array and air temperature (1C), I is the solar intensity (W/m2). 3.2. Integration of solar hot-water system with villa In the villa, because the whole roof is occupied by technologies of solar photovoltaic and virescence, we customized 1 m-high U-type evacuated tubular solar collectors with CPC in terms of the dimension of awning, on which we installed 4.6 m2 solar collectors at 101 tilted angle as shown in Fig. 2. Such design provides another example of how a solar element could be used in the original design in a logical manner, especially for those without enough roof area. The solar hot-water system in the villa is similar with those of flat. In terms of experimental results, the instantaneous solar collecting efficiency can be expressed as Z ¼ 0:41 1:67ðT in T a Þ=I.
(2)
4. Experience on integration of solar thermal utilization with green office building 4.1. Integration of solar collectors and green office building As the power to drive adsorption chillers and the heat source for the floor heating and natural ventilation, the
Fig. 2. Effect of integration of solar collectors and villa.
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solar collectors are the most important parts. We installed 150 m2 solar collectors on the roof of the green building, wherein U-type evacuated tubular solar collectors with CPC of area 90 m2 were placed on the west side (SCW), and the other 60 m2 heat pipe evacuated tubular solar collectors on the east side (SCE). For the purpose of efficient utilization of solar energy, the architects designed a steel structure roof, facing due south and tilted at an angle of 401 to the ground surface, on which the solar collectors were mounted and integrated with the building perfectly. Fig. 3 shows the appearance of the green office building integrated with solar collectors. All solar collectors of both sides were divided into three parallel rows, as shown in Fig. 4. The collector units in each row were connected in a series arrangement for the purpose of obtaining hot water with relatively high temperature, which plays an important role in improving performance of the solar energy system. Such an arrangement of solar collectors not only guarantees high system performance but also enhances the architectural expression of the building. Besides, it provides a feasible idea for integration of solar collectors and civil buildings especially for public buildings. 4.2. Design of solar-powered integrated energy system An integrated energy system based on solar thermal technologies was designed and set up for building area of 460 m2. As an office building, the hot water demand is not as significant as that in residential buildings. So, the solarpowered integrated system design of the green building is mainly focused on floor heating in winter and airconditioning in summer. Another novel design is natural ventilation enhanced by solar hot water, which is effective and necessary to solve the problem of surplus hot water in transitional seasons. Moreover, it provides a new method for the design of solar-enhanced natural ventilation. Except for solar collectors, the solar-powered integrated energy system mainly includes two adsorption chillers,
Fig. 3. The external appearance of the green office building integrated with solar collectors.
Fig. 4. Arrangement of solar collector arrays. (a) Heat pipe evacuated tubular solar collector arrays. (b) U-type pipe evacuated tubular solar collector arrays.
floor heating pipes, finned tube heat exchangers, circulating pumps, and a cooling tower. Besides, a hot water storage tank of 2.5 m3 in volume is employed to collect solar heat, thereby providing hot water for the integrated solar energy system. All components are connected by tubes and valves to form the whole circulating system. The flow diagram of the integrated solar energy system is shown in Fig. 5, where AD1 and AD2 are two adsorption chillers, CT is a cooling tower, WT is a hot water storage tank, P1 and P2 are two solar collecting pumps, P3 and P4 are hot water pump and cooling water pump, respectively. Through valves located on the pipes, the solar-powered integrated energy system can be switched to different operating modes according to different seasons: (1) In summer, the cooling load of air-conditioning area under design condition is 60 kW, thereinto, 15 kW is sensible heat cooling load met by solar-adsorption airconditioning system, which is discussed in this paper. The other 45 kW is latent heat cooling load taken on by
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Fig. 5. The flow diagram of the integrated solar energy system, showing different operating modes: floor radiation heating (FH), natural ventilation enhanced by solar hot water (NV), air-conditioning (AC).
a liquid-desiccant system, which is constructed by Shanghai Research Institute of Building Science. Thereby, the hybrid air-conditioning system deals with cooling and humidity loads independently, and the fan coils inside air-conditioning rooms realize dry operating mode. (2) In winter, solar-powered floor heating system is used to satisfy heating load of the green building. (3) In transitional seasons, solar hot water is pumped into finned tube heat exchangers to induce stack pressure, which is capable of improving natural ventilation. (4) The system can be used to supply hot water as long as a heat exchanger is installed in parallel with what mentioned above. In order to complete the solar-powered air-conditioning system, we chose the environment friendly silica-gel/water as the working pair and invented an adsorption chiller, which is capable of working from 55 to 95 1C. Fig. 6 shows the photo of the silica gel-water adsorption chiller. Owing to its practicability in low temperature, the chiller is testified to be suitable for solar-powered air-conditioning system. The performance test shows that the chiller attains rated refrigerating capacity of 8.5 kW when the hot water temperature is 85 1C, and the corresponding COP is 0.4 [12]. As for heating system, we selected cuprotherm floor heating system produced by Wieland Ltd. of Shanghai. The floor heating coil pipes are made of high-quality pure copper with the dimension of F12 0.7 mm, as shown in Fig. 7. They were fixed on the 30 mm thick polystyrene insulation layer with spacing interval 200 mm. And then crushed stone concrete was poured with the thickness of 70 mm. There is an air channel under the roof of the green building, which is designed by architects for indoor air exhaust through natural ventilation. In order to enhance
Fig. 6. The photo of the silica gel-water adsorption chiller.
natural ventilation by stack pressure, we installed seven groups of heat exchange elements inside the air channel. Each group consists of three parallel finned tube heat exchangers as shown in Fig. 8. The finned tube heat exchanger is made of a 3-m-long copper tube with 540 square fins. The diameter of the tube is 20 mm and the sectional dimension of the square fins is 102 mm 102 mm. 4.3. Summary of all-year operation of the solar-powered integrated energy system From September 2004 to August 2006, the solarpowered integrated energy system has been continuously in operation under different modes according to different seasons. Based on all-the-year-round experimental data, it is concluded that under the weather condition of Shanghai, 150 m2 vacuum tube solar collector arrays can be used to
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whole power consumption was 1.87 kW, and the average electric COP was 8.19 during an 8 h operation, and the maximum exceeded 10. In addition, under typical working condition in transition seasons (daily solar insolation of 17 MJ/m2 and average ambient temperature of 10 1C), solar-enhanced natural ventilation air flow rate induced by stack pressure is doubled compared with conventional natural ventilation. Consequently, the system is capable of inducing air change rate of 3 ACH and supplying hot water for the office building. Also can be obtained is that the solar fraction for the system in winter is 56%, correspondingly, 75% in summer, and 68% in transition seasons. Then the mean annual solar fraction of the system nearly reaches 70% through weighted average of those in different seasons and the corresponding days. Fig. 7. The photo of floor heating coil pipe.
5. Conclusion Different integrated approaches were carried out in the green buildings of Shanghai Research Institute of Building Science. A solar-powered integrated energy system involving heating, air-conditioning, natural ventilation and hot water supplying was constructed for the green office building, which realizes high integration of solar thermal technologies. However, only solar hot-water systems were designed for the two green residential buildings. As a brief summary, it is wished to emphasize the significant points of this work in the following.
Fig. 8. Finned tube heat exchanger inside air channel.
satisfy heating and air-conditioning for covered area of 460 m2. Under typical working condition in winter (daily solar insolation of 18 MJ/m2 and average ambient temperature of 1.98 1C), the average floor temperature and indoor air temperature is about 23.71 and 17.10 1C, respectively, which satisfies indoor thermal environment. Under typical working condition in summer (daily solar insolation of 20.36 MJ/m2 and average ambient temperature of 31.66 1C), the average refrigerating output of solar powered air-conditioning system is 15.31 kW during efficient operation of 8 h; moreover, the maximum attains 20 kW. With regard to heat consumption of two adsorption chillers, the average system COP is 0.35, and average solar COP is 0.15 concerning daily solar insolation. In the solarpowered air-conditioning system, it is important to reduce electric power consumption; consequently, electric COP is another important index to evaluate performance of the system. Thus, considering the solar collecting pump (P1), hot water pump (P2) and cooling water pump (P3), the
(1) Our experience in green residential buildings shows that, for new buildings, solar collectors can be mounted on balconies and awnings besides roofs, on condition that solar systems become part of the general building design. Solar collectors therefore could be used in the original design in a logical manner for the purpose of supplying hot water as well as improving building facade. The integration of solar collectors with balconies provides a feasible design method for multi-story buildings and high-rise buildings especially for residential buildings, which are widespread in large cities like Shanghai. (2) Our experience in the green office building shows that silica gel/water adsorption chiller invented by our research group has been verified as a successful solar air conditioning unit. Under the weather condition of Shanghai, 150 m2 vacuum tube solar collector arrays can be used to satisfy heating and air-conditioning for covered area of 460 m2. In addition, they are capable of inducing natural ventilation by stack pressure and supplying hot water for the office building. The solarpowered integrated energy system can take on about 70% of the yearly building load regarding the involved space. Such designs have the advantage of high utilization ratio with different functions according to different seasons, which makes the solar-powered integrated energy systems more economical.
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In China, with the implement of ‘‘Renewable Energy Law’’ since January the 1st of 2006, enough emphasis has been put by the governments of all provinces. Taking Shanghai for example, the government has issued ‘‘Program of Exploiting and Using Solar Energy’’, which calls for notable effect of solar thermal utilization in 3–5 years. Accordingly, by the year 2007, there will be 100,000 m2 (solar collecting area) solar hot water systems integrated with buildings in Shanghai. It is reasonable to expect that the solar thermal utilization will play greater role in building energy systems in the coming years. Acknowledgments This work is supported by the Shanghai Commission of Science and Technology under the contract No. 03DZ12012. Reference [1] Olgyay V, Herdt J. The application of ecosystems services criteria for green building assessment. Sol Energy 2004;77(4):389–98. [2] Hestnes AG. Building integration of solar energy systems. Sol Energy 1999;67(4–6):181–7.
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