- Email: [email protected]
water adsorption chiller
Applied Thermal Engineering 29 (2009) 2100–2105
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller W.-S. Chang *, C.-C. Wang, C.-C. Shieh Energy & Environment Research Laboratories, Industrial Technology Research Institute, Bldg. 64, 195, Sec. 4, Chung Hsing Rd., Chutung, Hsinchu 310, Taiwan
a r t i c l e
i n f o
Article history: Received 2 May 2008 Accepted 31 October 2008 Available online 24 November 2008 Keywords: Solar Adsorption cooling Silica gel
a b s t r a c t In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Adsorption refrigeration technology is attracting more and more attention in recent years because it can save energy and is environmentally friendly. Adsorption cycles can be driven by low-grade waste heat or solar energy under 80 °C. They do not have to use ozone-depleting chlorofluorocarbons (CFCs) and do not need electricity or fossil fuels as driving sources. Silica gel–water adsorption chiller can be used in combination with solar energy because of the possibility of using the lowgrade solar energy under 80 °C, which can be easily obtained with flat-plate collectors or vacuum tube collectors. Although the adsorption chillers are thought to be very promising in the future for the application of solar cooling and waste heat recovery, the wide spread of this technology is not yet possible. The reason is mostly attributed to the poor COP value (COP, coefficient of performance, defined as the ratio of the output cooling power and the input heating power) and higher product cost of adsorption chillers. In summer of a subtropical land like Taiwan, about one third of the electricity may be consumed for air-conditioning in buildings. The need of air-conditioning is consistent with the solar radiation of the day and the season. If one can use the excessive solar energy,
* Corresponding author. Tel.: +886 3 6570882; fax: +886 3 6570883. E-mail address: [email protected] (W.-S. Chang). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.10.021
the total system efficiency of energy utilization would be significantly improved. In an authors’ previous study [1] an integrated, one-bed, closedtype silica gel–water adsorption chiller was developed and experimentally studied. Flat-tube heat exchangers with corrugated fins were adopted as an adsorber and evaporator/condenser. To further realize commercialization of this kind of adsorption chiller, a twobed silica gel–water adsorption chiller that can provide chilled water continuously was also developed and studied [2]. Many researchers have devoted themselves to adsorption refrigeration technology and many studies have been conducted. Among these works the silica gel–water adsorption systems have been analytically [3–7] and experimentally [8–11] investigated. Recently, a solar-powered hybrid energy system with adsorption chiller in an ecological building has been conducted. The adsorption cooling COP of 0.28 has been attained and the average efficient (Collector efficiency is defined as the ratio of the absorbed energy and the input irradiation.) of collectors (U-type and heat pipe evacuated collectors) is about 40% [12]. Additionally, some field tests of the solar adsorption cooling system driven with allglass evacuated tube collectors were introduced. Daily solar cooling COPs ranging from 0.1 to 0.13 were reported [13–14]. In this paper, a two-bed silica gel–water adsorption chiller with plate fin and tube heat exchangers was newly developed. In order to conduct a field test, a solar-powered compound system for heating and cooling was designed and constructed in a golf course located in Hsinchu, Taiwan.
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W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
Nomenclature
2. System description A schematic diagram of the entire system is shown in Fig. 1. The system described in this paper is mainly composed of three subsystems, i.e., the solar heating circuit, the hot water supply circuit and the adsorption cooling circuit. The solar heating circuit possesses 108.5 m2 flat-plate solar collectors, whose efficiency curve is described as gc ¼ 0:76 4:7 ðT i TGamb Þ, mounted on the roof ground surface of the building. Collector efficiency gc is defined as the ratio of the absorbed energy and the input irradiation. It is experimentally decided and described by three parameters, i.e., inlet water temperature of collector Ti, ambient air temperature Tamb, and global irradiance G. Because of lower thermal loss factor of 4.7, the collector can obtain higher water temperature than 65 °C without significantly loosing the collector efficiency. The collectors are installed tilled at the angle of 20° to the ground and at an azimuth of 30° south by east in order to be in line with the orientation of the building. Furthermore, because most of the hot water will be used around noon for the air-conditioning, it is also appropriate to collect more solar heat in the morning. A solar hot water storage tank of 1300 L in volume is used to store solar heat with the design temperature of 80 °C. On the test run a temperature of 90 °C could be reached. This storage tank would not only provide hot water higher than 65 °C as heat source to drive the adsorption chiller, but also provide 50 °C hot water to the dormitory. The hot water supply circuit is connected to the solar heating circuit via a plate heat exchanger. The cold makeup water will be heated to 50 °C by the solar hot water and stored in two buffer tanks (each 1000 L). The hot water will be supplied to the dormitory for bathing use of 50 persons for a day. Eventually, the backup gas-fired boiler will be automatically turned on to heat the water to the required temperature.
inlet water temperature of collector [°C] ambient air temperature [°C] global irradiance [W/m2] collector efficiency [-]
Ti Tamb G
gc
The core component of the cooling circuit is an adsorption chiller with about 10 kW cooling power, which was newly developed in this study. This adsorption chiller is driven by the solar hot water with the temperature of 65–95 °C and produce chilled water. The chilled water will then be circulated to the employee restaurant for 3 h around noon (from 11:00 to 14:00) in the summer time (from May to September). The installation location Shinchu is located in the northern Taiwan. The geographical position of Shinchu is at latitude of 24.8° north and a longitude of 121° east. It belongs to the subtropical climate zone. Fig. 2 shows the monthly-average daily irradiation on a horizontal surface and the ambient air temperature. The irradiation in summer can be almost three times higher than in winter.
35
16 Horizontal irradiance in Shinchu Average ambient temperature
14 12
Data for years 1998 to 2003
30 25
10 20 8 15 6 10
4
Temperature [ºC]
SCP
coefficient of performance, ratio of the output cooling power and input heating power specific cooling power, produced cooling power per kg adsorbent [W/kg]
Irradiation [MJ/(m2.day)]
COP
5
2 0
0 1
2
3
4
5
6
7
8
9
10
11
12
Month of the year Fig. 2. Monthly average of horizontal irradiation and ambient air temperature in Shinchu.
Fig. 1. Schematic diagram of the system for heating and cooling.
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Because the solar hot water is usually consumed for the purpose of bathing in Taiwan, the solar heat gain will always not be consistent with the heat demand by season. If the installed collector area is determined by the winter hot water demand, it will lead to excessive solar heat gain in summer. Fig. 3 shows the predicted daily solar heat gain of the collector field. The hot water demand is indicated as a solid curve. It is assumed that 50 L water of 60 °C is needed for each person. Fifty persons are considered. The daily hot water demand varies from 464 MJ/day in January to 324 MJ/day in July because of different makeup water temperature, which is assumed to be equal to the ambient air temperature. We can see that the solar heat is insufficient from December to February. The backup gas-fired boiler would be necessarily brought into operation. The white bars from May to September indicate the solar heat used as driving heat source of the adsorption chiller. Even though part of the solar energy is used for air-conditioning, the rest heat is still enough for the dormitory hot water demand. Fig. 5. Photograph of the adsorption chiller.
3. Adsorption chiller Figs. 4 and 5 show a schematic view and a photograph of the adsorption chiller developed in this study, respectively. It consists of two identical units, the so-called integrated, one-bed, closedtype silica gel–water adsorption chiller. Each unit mainly consists of two heat exchangers which are located in a vacuum chamber.
Daily heat quantity [MJ/day] Daily hot water demand [MJ/day]
900 Solar heat for air-conditioning
800
Solar heat for dormitory
700 600
Daily hot water demand for dormitory
500 400 300 200 100 0 1
2
3
4
5
6
7
8
9
10
11
12
Month of the year Fig. 3. Daily hot water demand and predicted solar heat gain used for dormitory and for air-conditioning.
Hot water outlet Cooling water outlet
Hot water inlet Cooling water inlet
Adsorber (1), packed with silica/gel
Adsorber (2), packed with silica/gel Vacuum chamber
Evap./Cond. (1)
Evap./Cond. (2)
Chilled water inlet Chilled water outlet
4-way valve
Cooling water inlet
Cooling water outlet
Fig. 4. Schematic of the adsorption chiller.
The upper one, i.e., the adsorber is composed of copper plate fin and tube heat exchangers (920.8*550*60 mm, fin pitch 2.54 mm). Fig. 6 shows detailed geometry of this component. Silica gel with corn diameter of 0.5–1.5 mm is packed between the fins together with the PVAc (polyvinyl acetate) binder. There are four vertically arranged adsorption heat exchangers in one adsorption bed and 62.64 kg silica gel is employed. The lower one, i.e., the evaporator/condenser is composed of five fin-tube heat exchangers (serve as evaporator and condenser), which are horizontally placed on the bottom of the vacuum chamber. This evaporator/condenser heat exchanger (900*190.5*60 mm, fin pitch 1.8 mm) has similar construction like the adsorber heat exchanger and is used as an evaporator during adsorption cycles and as a condenser during desorption cycles. The function exchange between evaporator and condenser is controlled by three 4-way valves. With this kind of design, the chiller architecture is simplified and the chamber volume is reduced. These two chiller units described above were connected with totally six 4-way valves. While the chiller 1 is in the desorption/ condensation modes, the chiller 2 is in the adsorption/evaporation modes. By appropriate switch of the valves, it is then possible to produce chilled water continuously. Heat recovery processes for desorber/adsorber and cold recovery processes for condenser/ evaporator were conducted to improve the COP. For every chiller unit, there exist two main operating modes, i.e., the desorption/condensation mode and adsorption/evaporation mode. During desorption cycles, the silica gel packed between the fins in the adsorber is heated by hot water flowing through the heat transfer tubes. Then the water vapor is desorbed from the silica gel and condenses on the surface of the condenser, through which cooling water circulates. After the desorption cycle, the operating mode is switched into the adsorption mode. The adsorber is cooled by cooling water, and the evaporator is supplied with the water to be chilled. Water evaporates from the evaporator and is adsorbed by the dry silica gel. As the evaporation proceeds, the chilled water outlet temperature drops. Between the two main operating modes, the heat recovery processes for desorber/adsorber were conducted by introducing the hotter water in the desorber into the colder adsorber of the other chiller unit. Similarly the cold recovery processes for condenser/ evaporator were done by introducing the chilled water in the evaporator into the condenser of the other chiller unit. The adsorption chiller has been tested in the laboratory. Fig. 7 shows the result of the test runs. The cooling power and the COP increase with decreasing cooling water temperatures as expected, because lower adsorption temperatures lead to higher adsorptive
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Fig. 6. A detailed figure of the adsorber of plate fin and tube exchangers.
12 0.42
8
0.40 0.38
6 4 2 0 24
Test conditions Hot water inlet temp.=80 ,flow rate=1.43kg/s Cooling water flow rate=2.02kg/s (adsorption) flow rate=2.06kg/s (desorption) Chilled water inlet temp.=14 , flow rate=1.25kg/s Cycle time: 744sec, Heat recovery time: 15sec.
0.36 COP
0.34 0.32 0.30
25
26
27
28
29
30
31
32
Cooling water temperature[ºC]
Heat Transfer Rate [kW]
Cooling power [kW]
10
COP
0.44
Cooling Power
180 150 120 90 60 30 0 -30 -60 -90 -120 -150 -180 -210 -240 759
Desorber Evaporator
Adsorber
Hot water inlet
85
Temperature [ºC]
Hot water outlet
55 45
Cooling water outlet (adsorber)
Cooling water outlet (condenser)
35 25
Cooling water inlet
Chilled water inlet
Chilled water outlet
15 5 759
1518
2277
3036
3795
Fig. 9. Measured heat transfer rates during the operation (80/25/14 °C).
In the beginning of a cycle, there is a peak of the chilled water outlet temperature. This resulted from the fact that the rest water with higher temperature during the desorption/condensation mode flowed into the chilled water outlet circuit. This is a negative characteristic of this kind of integrated adsorption chiller and should be avoided as far as possible. One can conduct heat recovery of the evaporator and condenser with a few seconds time delay (relative to heat recovery of desorber and adsorber) to depress this temperature peak. Alternatively, one can add a buffer tank to the chilled water outlet circuit downstream to diminish the temperature fluctuation. 4. Field test results
75 65
1518
Operating Time [sec]
Fig. 7. Experimental results by variation of cooling water inlet temperature.
capacity and then higher cooling power. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP of 0.37 can be achieved. The corresponding SCP (specific cooling power, defined as the produced cooling power per kg adsorbent) is about 72 W/(kg adsorbent). Figs. 8 and 9 show the measured temperatures and heat transfer rates under the test conditions of 80 °C hot water, 25 °C cooling water, and 14 °C chilled water inlet temperatures. After the heat recovery processes the remaining water temperature in the desorber fell to about 57 °C while the water temperature in the adsorber rose to about 53 °C.
Condenser
2277
3036
3795
Operating time [sec] Fig. 8. Measured temperatures during the operation (80/25/14 °C).
Fig. 10 shows a typical system operation result in the cooling period on a normal sunny day (September/2/2006). The chiller was operated from 10:40 to 13:40 for 3 h. To clearly show the chiller efficiency, the COP of the adsorption chiller and correspondingly the collector efficiency were indicated cycle by cycle. In this period for air-conditioning, the average solar radiation is about 721.8 W/m2. The temperature of the hot water tank ranges from 71 °C to 80 °C, with an average value of 75.2 °C. The average solar energy collecting efficiency is about 28.4%. The highest COP of the adsorption chiller achieves 0.6. The average cooling power and COP is 8.39 kW and 0.452, respectively. The corresponding efficiency of solar cooling system is about 12.8% (product of the efficiency of the solar heating).
2
10
Local time
Dec_05
Apr_01
0 Nov_21
0
20
5
Nov_07
10
30
Oct_24
20
40 10
Oct_10
30
Sep_26
40
50
Sep_12
50
60 15
Aug_29
60
70
20
Jun_16
70
90 80
May_13
80
100
Collector efficiency Cooling power
25
Apr_29
Collector efficiency Temperature_solar hot water tank
Daily irradiation COPchiller
Apr_15
Irradiance COP_adsorption cooling
Irradiance [W/m2]
90
1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Cooling power[kW]; Irradiation[MJ/m ]
30
100
COPchiller[%]; Collector efficiency[%]
W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
10 :5 4: 0 11 0 :0 8: 00 11 :2 1: 0 11 0 :3 5: 00 11 :4 8: 0 12 0 :0 2: 0 12 0 :1 5: 00 12 :2 9: 0 12 0 :4 2: 0 12 0 :5 6: 00 01 :0 9: 0 01 0 :2 3: 00 13 :3 6: 00
COP [%]; Efficiency [%]; Temperature [ºC]
2104
Fig. 10. A typical system operation result during the use of air-conditioning in one day (2 September 2006).
Fig. 12. Daily monitored data of collector and chiller efficiency from April to December.
Fig. 11 shows the detailed data on the same day (September/2/ 2006). Water in the storage tank was heated from 53 °C to 80 °C before 11:30, while, during the cooling demand time, the hot water inlet temperature remained above 68 °C. The fluctuation of chilled water outlet temperature is apparent to see in the figure. This would be partly eliminated by introducing a small buffer tank installed after the chilled water outlet of the adsorption chiller. As shown in the chilled water inlet temperature (equal to the return temperature from the blower in the restaurant), the temperature was clearly reduced. Fig. 12 shows the monitored results in the period from April to December of the year 2006. The adsorption chiller was operated from July to October. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW.
powered compound system for heating and cooling was designed and constructed in a golf course to conduct the field test. Based on the experimental results in the laboratory and on the test field, the following results were obtained. (1) Test runs of this adsorption chiller were conducted in the laboratory. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP of 0.37 can be achieved. It has provided a SCP of about 72 W/(kg adsorbent). (2) Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW (3) A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively. (4) By introducing a small buffer tank installed after the chilled water outlet of the adsorption chiller, the fluctuation of chilled water outlet temperature could be apparently reduced.
5. Conclusion In this paper, a two-bed silica gel–water adsorption chiller with plate fin and tube heat exchangers was newly developed. A solar-
80
Hot water inlet 70
Temperature [ºC]
60 50
Hot water outlet
Cooling water inlet
40 30
Chilled water inlet 20
Chilled water outlet 10 06:00
08:00
10:00
12:00
14:00
16:00
18:00
Local time Fig. 11. Hot water and chilled water temperature change in a typical system operation in one day (2 September 2006).
W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
Acknowledgements The authors wish to thank the Energy Bureau, Ministry of Economic Affairs, Taiwan, ROC, for the financial support on this study. References [1] W.S. Chang, C.C. Wang, C.C. Shieh, Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed, Applied Thermal Engineering 27 (13) (2007) 2195–2199. [2] W.S. Chang, C.C. Wang, C.C. Shieh, Design and experiment of a solar-powered compound system for heating and cooling, HPC’06, Heat Powered Cycles, Newcastle, England, 11–14 September, 2006. [3] B.B. Saha, A. Akisawa, T. Kashiwagi, Computer simulation of a silica gel–water adsorption refrigeration cycle – the influence of operating conditions on cooling output and COP, ASHRAE Transactions: Research 101 (2) (1995) 348– 357. [4] E.C. Boelman, B.B. Saha, T. Kashiwagi, Parameter study of a silica-gel water adsorption refrigeration cycle – the influence of thermal capacitance and heat exchanger UA-values on cooling capacity, power density and COP, ASHRAE Transactions: Research 103 (1) (1997) 139–148. [5] H.T. Chua, K.C. Ng, A. Malek, T. Kashiwagi, A. Akisawa, B.B. Saha, Modeling the performance of two-bed gel–water adsorption chiller, International Journal of Refrigeration 22 (1999) 194–204.
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[6] H.T. Chua, K.C. Ng, W. Wang, C. Yap, X.L. Wang, Transient modeling of a two-bed silica gel–water adsorption chiller, International Journal of Heat and Mass Transfer 47 (2004) 659–669. [7] K.C.A. Alam, B.B. Saha, Y.T. Kang, A. Akisawa, T. Kashiwagi, Heat exchanger design effect on the system performance of silica gel adsorption refrigeration systems, International Journal of Heat and Mass Transfer 43 (24) (2000) 4419– 4431. [8] A. Sakoda, M. Suzuki, Simultaneous transport of heat and adsorbate in closed type adsorption cooling system utilizing solar heat, Transactions of the ASME Journal of Solar Energy Engineering 108 (1986) 239–245. [9] E.C. Boelman, B.B. Saha, T. Kashiwagi, Experimental investigation of a silica gel–water adsorption refrigeration cycle – the influence of operating conditions on cooling output and COP, ASHRAE Transactions: Research 101 (2) (1995) 358–366. [10] B.B. Saha, T. Kashiwagi, Experimental investigation of an advanced adsorption refrigeration, ASHRAE Transactions: Research (1997) 50–58. [11] K. Oertel, M. Fischer, Adsorption cooling system for cold storage using methanol/silica gel, Applied Thermal Engineering 18 (1998) 773–786. [12] X.Q. Zhai, R.Z. Wang, Y.J. Dai, J.Y. Wu, Y.X. Xu, Design and experimental analysis of solar-powered hybrid energy system in the ecological building, The 2005 World Sustainable Building Conference, Tokyo, 27–29 September 2005, pp. 1079–1084. [13] H.L. Luo, R.Z. Wang, Y.J. Dai, J.Y. Wu, J.M. Shen, B.B. Zhang, An efficient solarpowered adsorption chiller and its application in low-temperature grain, Solar Energy 81 (2007) 607–613. [14] H.L. Luo, Y.J. Dai, R.Z. Wang, J.Y. Wu, Y.X. Xu, J.M. Shen, Experimental investigation of a solar adsorption chiller used for grain depot cooling, Applied Thermal Engineering 26 (2006) 1218–1225.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Design and performance of a solar-powered heating and cooling system using silica gel/water adsorption chiller W.-S. Chang *, C.-C. Wang, C.-C. Shieh Energy & Environment Research Laboratories, Industrial Technology Research Institute, Bldg. 64, 195, Sec. 4, Chung Hsing Rd., Chutung, Hsinchu 310, Taiwan
a r t i c l e
i n f o
Article history: Received 2 May 2008 Accepted 31 October 2008 Available online 24 November 2008 Keywords: Solar Adsorption cooling Silica gel
a b s t r a c t In this paper, a solar-powered compound system for heating and cooling was designed and constructed in a golf course in Taiwan. An integrated, two-bed, closed-type adsorption chiller was developed in the Industrial Technology Research Institute in Taiwan. Plate fin and tube heat exchangers were adopted as an adsorber and evaporator/condenser. Some test runs have been conducted in the laboratory. Under the test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP (coefficient of performance for cooling) of 0.37 can be achieved. It has provided a SCP (specific cooling power) of about 72 W/(kg adsorbent). Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW. A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Adsorption refrigeration technology is attracting more and more attention in recent years because it can save energy and is environmentally friendly. Adsorption cycles can be driven by low-grade waste heat or solar energy under 80 °C. They do not have to use ozone-depleting chlorofluorocarbons (CFCs) and do not need electricity or fossil fuels as driving sources. Silica gel–water adsorption chiller can be used in combination with solar energy because of the possibility of using the lowgrade solar energy under 80 °C, which can be easily obtained with flat-plate collectors or vacuum tube collectors. Although the adsorption chillers are thought to be very promising in the future for the application of solar cooling and waste heat recovery, the wide spread of this technology is not yet possible. The reason is mostly attributed to the poor COP value (COP, coefficient of performance, defined as the ratio of the output cooling power and the input heating power) and higher product cost of adsorption chillers. In summer of a subtropical land like Taiwan, about one third of the electricity may be consumed for air-conditioning in buildings. The need of air-conditioning is consistent with the solar radiation of the day and the season. If one can use the excessive solar energy,
* Corresponding author. Tel.: +886 3 6570882; fax: +886 3 6570883. E-mail address: [email protected] (W.-S. Chang). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.10.021
the total system efficiency of energy utilization would be significantly improved. In an authors’ previous study [1] an integrated, one-bed, closedtype silica gel–water adsorption chiller was developed and experimentally studied. Flat-tube heat exchangers with corrugated fins were adopted as an adsorber and evaporator/condenser. To further realize commercialization of this kind of adsorption chiller, a twobed silica gel–water adsorption chiller that can provide chilled water continuously was also developed and studied [2]. Many researchers have devoted themselves to adsorption refrigeration technology and many studies have been conducted. Among these works the silica gel–water adsorption systems have been analytically [3–7] and experimentally [8–11] investigated. Recently, a solar-powered hybrid energy system with adsorption chiller in an ecological building has been conducted. The adsorption cooling COP of 0.28 has been attained and the average efficient (Collector efficiency is defined as the ratio of the absorbed energy and the input irradiation.) of collectors (U-type and heat pipe evacuated collectors) is about 40% [12]. Additionally, some field tests of the solar adsorption cooling system driven with allglass evacuated tube collectors were introduced. Daily solar cooling COPs ranging from 0.1 to 0.13 were reported [13–14]. In this paper, a two-bed silica gel–water adsorption chiller with plate fin and tube heat exchangers was newly developed. In order to conduct a field test, a solar-powered compound system for heating and cooling was designed and constructed in a golf course located in Hsinchu, Taiwan.
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W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
Nomenclature
2. System description A schematic diagram of the entire system is shown in Fig. 1. The system described in this paper is mainly composed of three subsystems, i.e., the solar heating circuit, the hot water supply circuit and the adsorption cooling circuit. The solar heating circuit possesses 108.5 m2 flat-plate solar collectors, whose efficiency curve is described as gc ¼ 0:76 4:7 ðT i TGamb Þ, mounted on the roof ground surface of the building. Collector efficiency gc is defined as the ratio of the absorbed energy and the input irradiation. It is experimentally decided and described by three parameters, i.e., inlet water temperature of collector Ti, ambient air temperature Tamb, and global irradiance G. Because of lower thermal loss factor of 4.7, the collector can obtain higher water temperature than 65 °C without significantly loosing the collector efficiency. The collectors are installed tilled at the angle of 20° to the ground and at an azimuth of 30° south by east in order to be in line with the orientation of the building. Furthermore, because most of the hot water will be used around noon for the air-conditioning, it is also appropriate to collect more solar heat in the morning. A solar hot water storage tank of 1300 L in volume is used to store solar heat with the design temperature of 80 °C. On the test run a temperature of 90 °C could be reached. This storage tank would not only provide hot water higher than 65 °C as heat source to drive the adsorption chiller, but also provide 50 °C hot water to the dormitory. The hot water supply circuit is connected to the solar heating circuit via a plate heat exchanger. The cold makeup water will be heated to 50 °C by the solar hot water and stored in two buffer tanks (each 1000 L). The hot water will be supplied to the dormitory for bathing use of 50 persons for a day. Eventually, the backup gas-fired boiler will be automatically turned on to heat the water to the required temperature.
inlet water temperature of collector [°C] ambient air temperature [°C] global irradiance [W/m2] collector efficiency [-]
Ti Tamb G
gc
The core component of the cooling circuit is an adsorption chiller with about 10 kW cooling power, which was newly developed in this study. This adsorption chiller is driven by the solar hot water with the temperature of 65–95 °C and produce chilled water. The chilled water will then be circulated to the employee restaurant for 3 h around noon (from 11:00 to 14:00) in the summer time (from May to September). The installation location Shinchu is located in the northern Taiwan. The geographical position of Shinchu is at latitude of 24.8° north and a longitude of 121° east. It belongs to the subtropical climate zone. Fig. 2 shows the monthly-average daily irradiation on a horizontal surface and the ambient air temperature. The irradiation in summer can be almost three times higher than in winter.
35
16 Horizontal irradiance in Shinchu Average ambient temperature
14 12
Data for years 1998 to 2003
30 25
10 20 8 15 6 10
4
Temperature [ºC]
SCP
coefficient of performance, ratio of the output cooling power and input heating power specific cooling power, produced cooling power per kg adsorbent [W/kg]
Irradiation [MJ/(m2.day)]
COP
5
2 0
0 1
2
3
4
5
6
7
8
9
10
11
12
Month of the year Fig. 2. Monthly average of horizontal irradiation and ambient air temperature in Shinchu.
Fig. 1. Schematic diagram of the system for heating and cooling.
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Because the solar hot water is usually consumed for the purpose of bathing in Taiwan, the solar heat gain will always not be consistent with the heat demand by season. If the installed collector area is determined by the winter hot water demand, it will lead to excessive solar heat gain in summer. Fig. 3 shows the predicted daily solar heat gain of the collector field. The hot water demand is indicated as a solid curve. It is assumed that 50 L water of 60 °C is needed for each person. Fifty persons are considered. The daily hot water demand varies from 464 MJ/day in January to 324 MJ/day in July because of different makeup water temperature, which is assumed to be equal to the ambient air temperature. We can see that the solar heat is insufficient from December to February. The backup gas-fired boiler would be necessarily brought into operation. The white bars from May to September indicate the solar heat used as driving heat source of the adsorption chiller. Even though part of the solar energy is used for air-conditioning, the rest heat is still enough for the dormitory hot water demand. Fig. 5. Photograph of the adsorption chiller.
3. Adsorption chiller Figs. 4 and 5 show a schematic view and a photograph of the adsorption chiller developed in this study, respectively. It consists of two identical units, the so-called integrated, one-bed, closedtype silica gel–water adsorption chiller. Each unit mainly consists of two heat exchangers which are located in a vacuum chamber.
Daily heat quantity [MJ/day] Daily hot water demand [MJ/day]
900 Solar heat for air-conditioning
800
Solar heat for dormitory
700 600
Daily hot water demand for dormitory
500 400 300 200 100 0 1
2
3
4
5
6
7
8
9
10
11
12
Month of the year Fig. 3. Daily hot water demand and predicted solar heat gain used for dormitory and for air-conditioning.
Hot water outlet Cooling water outlet
Hot water inlet Cooling water inlet
Adsorber (1), packed with silica/gel
Adsorber (2), packed with silica/gel Vacuum chamber
Evap./Cond. (1)
Evap./Cond. (2)
Chilled water inlet Chilled water outlet
4-way valve
Cooling water inlet
Cooling water outlet
Fig. 4. Schematic of the adsorption chiller.
The upper one, i.e., the adsorber is composed of copper plate fin and tube heat exchangers (920.8*550*60 mm, fin pitch 2.54 mm). Fig. 6 shows detailed geometry of this component. Silica gel with corn diameter of 0.5–1.5 mm is packed between the fins together with the PVAc (polyvinyl acetate) binder. There are four vertically arranged adsorption heat exchangers in one adsorption bed and 62.64 kg silica gel is employed. The lower one, i.e., the evaporator/condenser is composed of five fin-tube heat exchangers (serve as evaporator and condenser), which are horizontally placed on the bottom of the vacuum chamber. This evaporator/condenser heat exchanger (900*190.5*60 mm, fin pitch 1.8 mm) has similar construction like the adsorber heat exchanger and is used as an evaporator during adsorption cycles and as a condenser during desorption cycles. The function exchange between evaporator and condenser is controlled by three 4-way valves. With this kind of design, the chiller architecture is simplified and the chamber volume is reduced. These two chiller units described above were connected with totally six 4-way valves. While the chiller 1 is in the desorption/ condensation modes, the chiller 2 is in the adsorption/evaporation modes. By appropriate switch of the valves, it is then possible to produce chilled water continuously. Heat recovery processes for desorber/adsorber and cold recovery processes for condenser/ evaporator were conducted to improve the COP. For every chiller unit, there exist two main operating modes, i.e., the desorption/condensation mode and adsorption/evaporation mode. During desorption cycles, the silica gel packed between the fins in the adsorber is heated by hot water flowing through the heat transfer tubes. Then the water vapor is desorbed from the silica gel and condenses on the surface of the condenser, through which cooling water circulates. After the desorption cycle, the operating mode is switched into the adsorption mode. The adsorber is cooled by cooling water, and the evaporator is supplied with the water to be chilled. Water evaporates from the evaporator and is adsorbed by the dry silica gel. As the evaporation proceeds, the chilled water outlet temperature drops. Between the two main operating modes, the heat recovery processes for desorber/adsorber were conducted by introducing the hotter water in the desorber into the colder adsorber of the other chiller unit. Similarly the cold recovery processes for condenser/ evaporator were done by introducing the chilled water in the evaporator into the condenser of the other chiller unit. The adsorption chiller has been tested in the laboratory. Fig. 7 shows the result of the test runs. The cooling power and the COP increase with decreasing cooling water temperatures as expected, because lower adsorption temperatures lead to higher adsorptive
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Fig. 6. A detailed figure of the adsorber of plate fin and tube exchangers.
12 0.42
8
0.40 0.38
6 4 2 0 24
Test conditions Hot water inlet temp.=80 ,flow rate=1.43kg/s Cooling water flow rate=2.02kg/s (adsorption) flow rate=2.06kg/s (desorption) Chilled water inlet temp.=14 , flow rate=1.25kg/s Cycle time: 744sec, Heat recovery time: 15sec.
0.36 COP
0.34 0.32 0.30
25
26
27
28
29
30
31
32
Cooling water temperature[ºC]
Heat Transfer Rate [kW]
Cooling power [kW]
10
COP
0.44
Cooling Power
180 150 120 90 60 30 0 -30 -60 -90 -120 -150 -180 -210 -240 759
Desorber Evaporator
Adsorber
Hot water inlet
85
Temperature [ºC]
Hot water outlet
55 45
Cooling water outlet (adsorber)
Cooling water outlet (condenser)
35 25
Cooling water inlet
Chilled water inlet
Chilled water outlet
15 5 759
1518
2277
3036
3795
Fig. 9. Measured heat transfer rates during the operation (80/25/14 °C).
In the beginning of a cycle, there is a peak of the chilled water outlet temperature. This resulted from the fact that the rest water with higher temperature during the desorption/condensation mode flowed into the chilled water outlet circuit. This is a negative characteristic of this kind of integrated adsorption chiller and should be avoided as far as possible. One can conduct heat recovery of the evaporator and condenser with a few seconds time delay (relative to heat recovery of desorber and adsorber) to depress this temperature peak. Alternatively, one can add a buffer tank to the chilled water outlet circuit downstream to diminish the temperature fluctuation. 4. Field test results
75 65
1518
Operating Time [sec]
Fig. 7. Experimental results by variation of cooling water inlet temperature.
capacity and then higher cooling power. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP of 0.37 can be achieved. The corresponding SCP (specific cooling power, defined as the produced cooling power per kg adsorbent) is about 72 W/(kg adsorbent). Figs. 8 and 9 show the measured temperatures and heat transfer rates under the test conditions of 80 °C hot water, 25 °C cooling water, and 14 °C chilled water inlet temperatures. After the heat recovery processes the remaining water temperature in the desorber fell to about 57 °C while the water temperature in the adsorber rose to about 53 °C.
Condenser
2277
3036
3795
Operating time [sec] Fig. 8. Measured temperatures during the operation (80/25/14 °C).
Fig. 10 shows a typical system operation result in the cooling period on a normal sunny day (September/2/2006). The chiller was operated from 10:40 to 13:40 for 3 h. To clearly show the chiller efficiency, the COP of the adsorption chiller and correspondingly the collector efficiency were indicated cycle by cycle. In this period for air-conditioning, the average solar radiation is about 721.8 W/m2. The temperature of the hot water tank ranges from 71 °C to 80 °C, with an average value of 75.2 °C. The average solar energy collecting efficiency is about 28.4%. The highest COP of the adsorption chiller achieves 0.6. The average cooling power and COP is 8.39 kW and 0.452, respectively. The corresponding efficiency of solar cooling system is about 12.8% (product of the efficiency of the solar heating).
2
10
Local time
Dec_05
Apr_01
0 Nov_21
0
20
5
Nov_07
10
30
Oct_24
20
40 10
Oct_10
30
Sep_26
40
50
Sep_12
50
60 15
Aug_29
60
70
20
Jun_16
70
90 80
May_13
80
100
Collector efficiency Cooling power
25
Apr_29
Collector efficiency Temperature_solar hot water tank
Daily irradiation COPchiller
Apr_15
Irradiance COP_adsorption cooling
Irradiance [W/m2]
90
1200 1100 1000 900 800 700 600 500 400 300 200 100 0
Cooling power[kW]; Irradiation[MJ/m ]
30
100
COPchiller[%]; Collector efficiency[%]
W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
10 :5 4: 0 11 0 :0 8: 00 11 :2 1: 0 11 0 :3 5: 00 11 :4 8: 0 12 0 :0 2: 0 12 0 :1 5: 00 12 :2 9: 0 12 0 :4 2: 0 12 0 :5 6: 00 01 :0 9: 0 01 0 :2 3: 00 13 :3 6: 00
COP [%]; Efficiency [%]; Temperature [ºC]
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Fig. 10. A typical system operation result during the use of air-conditioning in one day (2 September 2006).
Fig. 12. Daily monitored data of collector and chiller efficiency from April to December.
Fig. 11 shows the detailed data on the same day (September/2/ 2006). Water in the storage tank was heated from 53 °C to 80 °C before 11:30, while, during the cooling demand time, the hot water inlet temperature remained above 68 °C. The fluctuation of chilled water outlet temperature is apparent to see in the figure. This would be partly eliminated by introducing a small buffer tank installed after the chilled water outlet of the adsorption chiller. As shown in the chilled water inlet temperature (equal to the return temperature from the blower in the restaurant), the temperature was clearly reduced. Fig. 12 shows the monitored results in the period from April to December of the year 2006. The adsorption chiller was operated from July to October. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily average COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW.
powered compound system for heating and cooling was designed and constructed in a golf course to conduct the field test. Based on the experimental results in the laboratory and on the test field, the following results were obtained. (1) Test runs of this adsorption chiller were conducted in the laboratory. Under the standard test conditions of 80 °C hot water, 30 °C cooling water, and 14 °C chilled water inlet temperatures, a cooling power of 9 kW and a COP of 0.37 can be achieved. It has provided a SCP of about 72 W/(kg adsorbent). (2) Some field tests have been performed from July to October 2006 for providing air-conditioning and hot water. The efficiency of the collector field lies in 18.5–32.4%, with an average value of 27.3%. The daily COP of the adsorption chiller lies in 33.8–49.7%, with an average COP of 40.3% and an average cooling power of 7.79 kW (3) A typical daily operation shows that the efficiency of the solar heating system, the adsorption cooling and the entirely solar cooling system is 28.4%, 45.2%, and 12.8%, respectively. (4) By introducing a small buffer tank installed after the chilled water outlet of the adsorption chiller, the fluctuation of chilled water outlet temperature could be apparently reduced.
5. Conclusion In this paper, a two-bed silica gel–water adsorption chiller with plate fin and tube heat exchangers was newly developed. A solar-
80
Hot water inlet 70
Temperature [ºC]
60 50
Hot water outlet
Cooling water inlet
40 30
Chilled water inlet 20
Chilled water outlet 10 06:00
08:00
10:00
12:00
14:00
16:00
18:00
Local time Fig. 11. Hot water and chilled water temperature change in a typical system operation in one day (2 September 2006).
W.-S. Chang et al. / Applied Thermal Engineering 29 (2009) 2100–2105
Acknowledgements The authors wish to thank the Energy Bureau, Ministry of Economic Affairs, Taiwan, ROC, for the financial support on this study. References [1] W.S. Chang, C.C. Wang, C.C. Shieh, Experimental study of a solid adsorption cooling system using flat-tube heat exchangers as adsorption bed, Applied Thermal Engineering 27 (13) (2007) 2195–2199. [2] W.S. Chang, C.C. Wang, C.C. Shieh, Design and experiment of a solar-powered compound system for heating and cooling, HPC’06, Heat Powered Cycles, Newcastle, England, 11–14 September, 2006. [3] B.B. Saha, A. Akisawa, T. Kashiwagi, Computer simulation of a silica gel–water adsorption refrigeration cycle – the influence of operating conditions on cooling output and COP, ASHRAE Transactions: Research 101 (2) (1995) 348– 357. [4] E.C. Boelman, B.B. Saha, T. Kashiwagi, Parameter study of a silica-gel water adsorption refrigeration cycle – the influence of thermal capacitance and heat exchanger UA-values on cooling capacity, power density and COP, ASHRAE Transactions: Research 103 (1) (1997) 139–148. [5] H.T. Chua, K.C. Ng, A. Malek, T. Kashiwagi, A. Akisawa, B.B. Saha, Modeling the performance of two-bed gel–water adsorption chiller, International Journal of Refrigeration 22 (1999) 194–204.
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