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Experimental investigation of a solar adsorption chiller used for grain depot cooling
Applied Thermal Engineering 26 (2006) 1218–1225 www.elsevier.com/locate/apthermeng
Experimental investigation of a solar adsorption chiller used for grain depot cooling H.L. Luo a, Y.J. Dai a, R.Z. Wang a b
a,*
, J.Y. Wu a, Y.X. Xu a, J.M. Shen
b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Branch of Jiangsu Province, Chinese State Grain Administration Company, Nanjing 210012, China Received 13 January 2005; accepted 14 October 2005 Available online 20 December 2005
Abstract The solar cooling technology is attractive since cooling load of building is roughly in phase with solar energy availability. In this study, a solar adsorption chiller was built and tested with aim of developing an alternative refrigeration system used for grain cooling storage. This solar adsorption chiller consists of four subsystems, namely, a solar water heating unit with 49.4 m2 solar collecting area, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. In order to achieve continuous refrigeration, two adsorption units are operated out-of-phase with mass recovery cycle in the adsorption chiller. Field test results show that, under the climatic conditions of daily solar radiation being about 16–21 MJ/m2, this solar adsorption chiller can furnish 14–22 °C chilled air with an average cooling output ranging from about 3.2–4.4 kW, its daily solar cooling COP (coefficient of performance) is about 0.1–0.13. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Solar; Adsorption chiller; Grain depot cooling
1. Introduction In grain storage, the spoilage of stored grain is mainly due to the attack of insects and moulds. Low temperature storage can prevent development of insect and mould, inhibit respiration of stored grain and extend its storage time. Hence, the storage temperature is one of the most important factors that determine the quality of stored grain. Mechanical ventilators and grain chillers are commonly used devices for controlling storage temperature in China [1]. For mechanical ventilators, the initial and operation costs are low. Nevertheless, sole using mechanical ventilators can not make the temperature of stored grain drop to a suitable point in sweltering summer and autumn. The grain chiller, characterized by *
Corresponding author. Tel.: +86 21 6293 3838; fax: +86 21 6293 3250. E-mail address: [email protected] (R.Z. Wang). 1359-4311/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.036
great cooling power and large electric power consumption, is essentially an electrically powered vapor compression refrigeration system. Although it is an effective device to decrease grain temperature in depot, the grain chiller is economically adverse for operation of a grain depot because of its high operation costs. Hence, it is necessary to develop alternative refrigeration device for grain cooling storage. Use of solar cooling technology in grain cooling storage system is very attractive since cooling load of grain depot is roughly in phase with solar energy availability. Different solar regenerated desiccant cooling devices have been proposed for grain storage in previous studies [2–4]. Such devices focus on reducing the humidity of ambient air before it is forced through grain. A challenge still exits to make the temperature of stored grain drop to a suitable point without, or with minimal electric power consumption in sweltering summer and autumn in hot areas. Dai et al. [5] theoretically investigated a solar
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Nomenclature Ac COP Cpw I(t) Ih m_ Qc T t
solar collecting area (m2) coefficient of performance isobaric specific heat of water (J/kg °C) solar radiation intensity on the top surface of collector (W/m2) hourly solar radiation (MJ/m2) mass flow rate (kg/s) cooling power (W) temperature (°C) time (s)
Greek symbol g collector efficiency
powered solid adsorption–desiccant cooling hybrid system used for grain cooling. The analysis results indicate that such system can provide cold air below 20 °C for grain cooling storage. Many researches on solar adsorption refrigeration have also been done during the past decades. Pons et al. [6] experimentally investigated a solar adsorption ice maker with 6 m2 collector areas, which could produce 30–35 kg ice per day under the solar radiation of about 22 MJ/m2 day. Critoph [7] comprehensively studied the performance limitations of adsorption cycles for solar cooling. Boubakri et al. [8] studied the limits of ice production by means of adsorptive collector–condenser technology. Wang et al. [9] introduced a hybrid system of solar powered water heater and adsorption ice maker. Recently, Yong et al. [10] simulated the performance of a solar powered adsorption air conditioning system. All these studies reveal that, in general, the solar adsorption refrigeration device is feasible for areas with abundant solar resources.
Subscripts ad adsorber am ambient bottom bottom part c solar collector cw chilled water ev evaporator hw hot water i instantaneous in inlet out outlet up upper part wt water tank
In this study, a solar adsorption chiller used for grain depot cooling was designed, constructed and field tested. The effects of operating parameters, such as solar hot water temperature, heating/cooling time, and mass recovery process on system performance were also examined.
2. Description of the solar adsorption chiller The solar adsorption chiller used for grain depot cooling mainly consists of four subsystems, namely, a solar water heating unit, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. A schematic diagram of this solar adsorption chiller is shown in Fig. 1. The solar water heating unit is utilized to produce hot water to drive the adsorption chiller. The cooling tower offers the cooling water to cool the condensers and adsorbers. The cooling production is Legend
Solar collector array Water pump Pyranometer
Water valve Flow meter
Vacuum valve
Wind valve H Hygrometer
T
Platinum resistance Cooling tower
T
Hot water inlet
T
T
V11
V12 V13 T
Condenser 1 Baffle plate
T
V7
V6
V5
V4
V3
V2
V8
V1
V0
Evaporator 1
V9
T
Adsorber 1
Partitioned hot water tank V10 Fan coil unit Grain depot
Cooling water inlet Adsorber 2 Evaporator 2
T
T H H
Condenser 2
T
T
Chilled water tank
Second stage evaporator
Chilled water inlet
Fig. 1. Schematic diagram of the solar adsorption chiller used for grain depot cooling.
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transferred to a grain depot through a fan coil unit. A wind valve is employed to control the outlet air temperature of the fan coil unit. The characteristics and main parameters of the solar adsorption chiller are shown in Table 1. The solar water heating unit (see Fig. 1) includes an all-glass evacuated tube collector array, a water pump and a partitioned hot water tank, etc. The partitioned hot water tank is divided into two parts by a baffle plate; the upper part has two fifths of the volume of the tank. The water pump, controlled by a differential temperature controller, independently works without considering whether the adsorption chiller is in the operation. In the early morning of sunny days, valve 12 is kept open, and valve 13 is closed. The upper part water of the tank is quickly heated by solar collectors and provides energy for the adsorption chiller. When water temperature in the upper part of the tank exceeds about 70 °C, valve 13 is opened also. The whole tank will be used as the heat reservoir to the adsorption chiller. Such a design is aiming at starting the adsorption chiller as early as possible in the morning. The solar adsorption chiller is designed for inhibiting temperature rise of stored grain in hot seasons. No auxiliary heat source is provided in order to reduce operation cost of grain storage. The adsorption chiller stops running when water temperature in the upper part of the tank is below about 65 °C in the afternoon. The available hot water temperature of evacuated tube solar water heating unit is about 60–90 °C. Hence, silica gel–water, which is suited for use of low heat Table 1 Characteristics and parameters of the solar adsorption chiller Subsystem
Characteristics and parameters
Solar water heating unit Collector type All-glass evacuated tube collector Collecting area 49.4 m2 Pump Rated power: 290 W; rated head: 25 m; rated flow rate: 0.00042 m3/s Hot water tank Capacity: 0.6 m3; Height: 1.2 m Adsorption chiller Adsorbent Hot water pump Chilled water pump Chilled water tank Cooling tower Fan Pump Fan coil unit Fan Heat exchanger
Micro-pore silica gel; mass: 50 kg/adsorber Rated power: 250 W; rated head: 10m; rated flow rate: 0.0013 m3/s Rated power: 120 W; Rated head: 8 m; rated flow rate: 0.00054 m3/s Capacity: 0.03 m3 Rated power: 250 W; rated flow rate: 0.306 m3/s Rated power: 370 W; Rated head: 12 m; rated flow rate: 0.0017 m3/s
source such as hot water [11,12], is chosen as adsorption working pair. The adsorption chiller (see Fig. 1) includes two identical adsorption units and a second stage evaporator with methanol working fluid. Each adsorption unit consists of one adsorber, one condenser and one evaporator (the first stage evaporator), which are housed in a vacuum chamber. When the adsorber is heated in one adsorption unit, water vapor desorbs from the silica gel enclosed in the adsorber with the increase of silica gel temperature. The desorbed water vapor is condensed via condenser and enters into evaporator in liquid state. When the adsorber is cooled, water vapor from evaporator is adsorbed by the silica gel, and cooling effect is, thus, produced in the evaporator during this stage. Continuous refrigeration can be achieved with two adsorption units to shift between adsorption and desorption phases. During the operation of the adsorption chiller, liquid methanol in the second stage evaporator is continuously heated by circulating chilled water. The evaporated methanol vapor rises and is condensed on the surface of the first stage evaporator in the adsorption unit being in adsorption phase, then drops back to the bottom of the second stage evaporator. In this way, cooling output of the first stage evaporator can be transferred to the methanol in the second stage evaporator and then to circulating chilled water. Such a design can suppress the heat transfer between circulating chilled water and the first stage evaporator of the adsorption unit being in desorption phase effectively. The advantages of mass recovery have been demonstrated in previous literatures [13,14]. With the help of a vacuum valve (V11) and 11 water valves (V0–V10), the adsorption chiller can be operated with a two-adsorber continuous refrigeration cycle with mass recovery. The operation scheme of this refrigeration cycle is listed in Table 2. Different from conventional two-adsorber cycle with mass recovery, there are two switching stages in this adsorption refrigeration cycle. During switching stage, the resident hot water in the adsorber being in desorption phase is pushed into the other adsorber being in adsorption phase by cooling water (see Fig. 1 and Table 2). So, the adsorber being in adsorption phase is heated preliminarily, and the sensible heat of resident hot water in the adsorber being in desorption phase is recovered partially. When the resident hot water in the adsorber being in desorption phase is almost pushed out, the switching stage terminates. Hence, the time of switching stage, namely, the preliminary heating time is determined by cooling water flowrate.
3. Performance index Rated power: 750 W; rated flow rate: 0.417 m3/s Type: tube-fin; Heat exchange area: 58.7 m2
In general, the performance of solar collector is represented in terms of instantaneous efficiency, gi, and
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Table 2 Operation scheme of the two-adsorber continuous refrigeration cycle with mass recovery Cycle stage
Adsorber 1
Adsorber 2
Opened valves
Closed valves
Heating/cooling Mass recovery (adsorber 1 ! adsorber 2) Switching Heating/cooling Mass recovery (adsorber 2 ! adsorber 1) Switching
Heating Heating Cooling Cooling Cooling Preliminary heating
Cooling Cooling Preliminary heating Heating Heating Cooling
V0, V0, V0, V1, V1, V2,
V1, V1, V1, V0, V0, V0,
V3, V3, V6, V2, V2, V4,
V4, V4, V9, V5, V5, V8,
V7 V7, V11 V10 V6 V6, V11 V10
V2, V2, V2, V3, V3, V1,
V5, V5, V3, V4, V4, V3,
V6 , V6 , V4 , V7 , V7 , V5 ,
V8, V8, V5, V8, V8, V6,
V9, V9, V7, V9, V9, V7,
V10, V11 V10 V8, V11 V10, V11 V10 V9, V11
daily efficiency, gday. They can be calculated by the following equations, respectively [15]: gi ¼ gday
m_ c C pw ðT c-out T c-in Þ Ac I ðtÞ R m_ c C pw ðT c-out T c-in Þ dt R ¼ Ac I ðtÞ dt
ð1Þ ð2Þ
The cycle COP of the adsorption chiller, COPcycle, is defined as ratio between the useful cooling output of the second stage evaporator and the energy supplied by the solar water heating unit: R m_ cw C pw ðT ev-in T ev-out Þ dt COPcycle ¼ R ð3Þ m_ hw C pw ðT ad-in T ad-out Þ dt The solar cooling COP of the system, COPsolar, is defined as ratio between the useful cooling output of the second stage evaporator and the total incident solar energy on the surface of solar collectors: R m_ cw C pw ðT ev-in T ev-out Þ dt R COPsolar ¼ ð4Þ Ac I ðtÞ dt The cooling power of the system, Qc, can be calculated by Qc ¼ m_ cw C pw ðT ev-in T ev-out Þ
ð5Þ
Fig. 2. Photographs of the solar adsorption chiller installed in a grain depot. (a) The adsorption chiller and (b) the solar collector array.
A RS232 bus is used as a communication passage between the Keithley 2700 multimeter/data acquisition system and a computer. The solar adsorption chiller was installed in a grain depot in Jiangsu Province, China. Photographs of the experimental setup are shown in Fig. 2. From July to September of 2004, a series of field tests on the performance of the solar adsorption chiller were carried out.
4. Experimentation 5. Results and discussion In order to acquire necessary data to evaluate the performance of the solar adsorption chiller, a series of sensors and instruments are employed. A pyranometer (TBQ-2 Type) with an accuracy of ±2% is used to measure the solar radiation on the top surface of the collectors. The temperature measurements are performed with a series of platinum resistance temperature sensors (pt100) properly placed (see Fig. 1). All these temperature sensors are calibrated, and the accuracy of the measured temperatures is estimated to be within ±0.2 °C. Four turbine flow meters are used to measure the water flow rate of cycle pumps. Furthermore, two hygrometers are used to monitor the outlet and inlet air relative humidity of fan coil unit. The pyranometer, hygrometers, flow meters and temperature sensors are connected to a Keithley 2700 multimeter/data acquisition system.
5.1. Solar water heating unit Fig. 3 shows the variations of solar radiation, I(t), upper and bottom part water temperature inside the partitioned hot water tank, Twt-up and Twt-bottom, outlet water temperature of the solar collector array,Tc-out, and ambient temperature, Tam, with time in a test day. In the early morning, the water temperature in the upper part of the tank increases quickly. The adsorption chiller begins to run at about 9:40 h. The water temperature inside the partitioned hot water tank fluctuates periodically because of the running of the adsorption chiller. The temperature fluctuation in the upper part of the tank generally is less than 5 °C during one cycle. This indicates that the tank volume and collector area match
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100
1000 Twt-up Twt-bottom Tc-out Tam I(t)
70
800 2
80
Solar radiation (W/m )
Temperature ( oC)
90
600
60
400
50 200 40 0
30
05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 Local time (h) Fig. 3. Variations of solar radiation, ambient temperature and water temperature with time (August 6, 2004).
the heat load of the adsorption chiller on the whole. The solar radiation reaches its maximum at about 12:30 h. However, water temperature in the upper part of the tank reaches its maximum at about 13:50 h. This phenomenon indicates that the energy gain from the solar water heating unit is greater than the energy consumption of the adsorption chiller before 13:50 h. Fig. 4 shows the measured instantaneous efficiency of the allglass evacuated tube collector in the same test day. Although its outlet temperature exceeds about 75 °C after 10:30 h, the all-glass evacuated tube collector still has good thermal performance; its instantaneous efficiency varies within about 0.25–0.55. 5.2. Performance of the adsorption chiller Before the adsorption chiller is put into practical operation for grain depot cooling, its performance under different test conditions is measured with a test facility.
The test facility consists of a hot water system, a cooling water system and a chilled water system. In the hot water system, water in hot water tank is heated by a steam boiler via a steam-water heat exchanger; a steam flow regulation valve is employed to control hot water temperature. In the chilled water system, chilled water temperature is controlled by regulating the city water flowrate entered into the chilled water tank. The redundant water in chilled water tank is drained into cooling water tank through an overflow valve. In the cooling water system, cooling water temperature is also controlled by regulating the city water flowrate entered into the cooling water tank. In this test facility, the fluctuations of hot water, chilled water and cooling water temperature can be controlled within ±0.8 °C during test operation. The performance of the adsorption chiller under two different cycle modes is shown in Fig. 5. Here, Qc and COPcycle represent average cooling power and cycle COP during one cycle, respectively. It is seen that mass recovery process is helpful for improvement of the performance of the adsorption chiller. Compared with the cycle mode without mass recovery, cycle COP and cooling power can be increased by about 14.6% and 11.2%, respectively with 1-min mass recovery process when heating/cooling time is fixed at 900 s. Also found is that, with 1-min mass recovery process, a peak cooling power of 5.03 kW is achieved when heating/cooling time is fixed at 720 s, the corresponding cycle COP is about 0.317. With 1-min mass recovery process, a peak cycle COP of 0.324 is achieved when heating/cooling time is fixed at 900 s, the corresponding cooling power is 4.96 kW. These indicate that the proper heating/cooling time for this adsorption chiller is about 720–900 s at the hot water inlet temperature of 85 °C. For the adsorption chiller, the suitable mass recovery time is about 60 s [16].
70
6
60
COPcycle
ηi (%)
5
50
0.25
Preliminary heating time:40 s Hot water inlet temperature: 85 oC Chilled water inlet temperature: 18oC Cooling water inlet temperature: 32oC
0.20
40
30
0.15 300
4
Cooling power (kW)
0.30
3 400
500
600
700
800
900
Heating/cooling time (s)
20 08
09
10
11
12
13 14 15 Local time (h)
16
17
18
19
Fig. 4. Instantaneous efficiency of the all-glass evacuated tube collector (August 6, 2004).
COPcycle with mass recovery (60 s) Qc with mass recovery (60 s)
COPcycle without mass recovery Qc without mass recovery
Fig. 5. Variations of cycle COP and cooling power with heating/ cooling time.
H.L. Luo et al. / Applied Thermal Engineering 26 (2006) 1218–1225
4
0.20 0.16 0.12 55
60
Mass recovery time: 60 s Preliminary heating time: 40 s 3 Heating/cooling time: 900 s Chilled water inlet temperature: 18 oC Cooling water inlet temperature: 32 oC 2 70 75 80 85 90 95
65 Hot water inlet temperature ( oC)
Fig. 6. Variations of cycle COP and cooling power with hot water inlet temperature.
Variations of average cooling power and cycle COP during one cycle with hot water inlet temperature are plotted in Fig. 6. It is found that both cooling power and cycle COP increase with the increase of hot water inlet temperature. When hot water inlet temperature is above about 75 °C, the increment rate becomes slow. At the hot water inlet temperature of 65 °C, the cooling power and cycle COP still reach 2.97 kW and 0.232, respectively. These indicate that the adsorption chiller can be powered by solar hot water effectively. The average cooling power and cycle COP during one cycle of the adsorption chiller under different chilled water inlet temperature conditions are presented in Fig. 7. The chilled water is cycled between the fan coil unit and the second stage evaporator in order to remove the cooling production of the adsorption chiller. Both cooling power and cycle COP increase quickly with the increase of chilled water inlet temperature. At the chilled water inlet water temperature of 20 °C, the cooling power reaches 5.29 kW, and the corresponding cycle
Fig. 8 shows the temperature variations of hot water, chilled water and outlet air of the fan coil unit during one cycle with mass recovery. The corresponding instantaneous cycle COP and cooling power of the solar adsorption chiller are presented in Fig. 9. During the mass recovery process, the pressure in the adsorption unit being in desorption phase decreases rapidly; it in return improves desorption effect, and results in increase of the demand for desorption heat, at last leads to decrease of the hot water outlet temperature, Thw-out. In contrast, water evaporation in the adsorption unit
25 80 70 20
60 50
15
40 Hot water flow rate: 0.001 m 3/s Chilled water flow rate: 0.0005 m3/s Cooling water inlet temperature: 31.6 - 33.9 o C
10 0
500
Tair-out
1000 1500 Time (s)
Tev-in
Tev-out
0.30 0.25
Thw-out
0.20
Mass recovery time: 60 s Preliminary heating time: 40 s Heating/cooling time: 900 s Hot water inlet temperature: 85 oC Cooling water inlet temperature: 32 oC
0.15
3
Qc
6
2
COPcycle
0.9 0.8 0.7 0.6
4
0.5 0.4 0.3
2
0.2
0.10
0.1
1 12
Thw-in
Fig. 8. Variations of Thw-in, Thw-out, Tev-in, Tev-out and Tair-out with time.
5 4
20
1.0
Cooling power (kW)
COPcycle Qc
30
2000
6
0.35
COPcycle
5.3. Transient performance of the solar adsorption chiller
Cooling power (kW)
COPcycle
0.24
Cooling power (kW)
5 0.28
COPcycle
0.32
COP is 0.331. To improve cooling power and cycle COP, it is reasonable to increase the chilled water inlet water temperature of the adsorption chiller appropriately.
Hot water temperature o C
6 COPcycle Qc
Chilled water or air temperature o C
0.36
1223
14 16 18 20 Chilled water inlet temperature (oC)
Fig. 7. Variations of cycle COP and cooling power with chilled water inlet temperature.
0.0
0 0
500
1000 Time (s)
1500
2000
Fig. 9. Instantaneous performance of the solar adsorption chiller.
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5
32
4
28 3 24 20
2
16 1
Qc T am
12
COPcycle 16:50 - 17:50
15:50 - 16:50
14:50 - 15:50
8
13:50 - 14:50
12:50 - 13:50
11:50 - 12:50
0
10:50 - 11:50
Ih T ev-out
COPcycle (%); Temperature ( o C)
36
09:50 - 10:50
Cooling power (kW); Hourly solar radiation (MJ/m2)
being in adsorption phase is suppressed greatly due to pressure increment occurred in mass recovery process. Consequently, the chilled water outlet temperature of the adsorption chiller, Tev-out, increases gradually. The air outlet temperature of the fan coil unit, Tair-out, fluctuates a little with the change of Tev-out. For grain storage, the effects of such little temperature fluctuation are negligible. It is seen in Fig. 9 that the instantaneous cycle COP changes in a wide range with the variations of the inlet and the outlet temperatures of hot water. However, the cooling power of the adsorption chiller only changes a little during one cycle except the mass recovery and switching period. Fig. 10 shows the weather conditions (hourly solar radiation, Ih, and mean ambient temperature), hourly mean chilled water outlet temperature and performance of the solar adsorption chiller in a typical test day. The operating conditions are listed in Table 3. During the period of 12:00–15:00 h, the solar radiation is intensive and the ambient temperature is relatively high. Consequently, the cooling load of grain depot is also relatively high. As expected, the cooling output of the solar
Local time Fig. 10. Weather conditions and hourly performance of the solar adsorption chiller in a test day (August 6, 2004).
Table 3 Operating conditions during the test operation of the solar adsorption chiller Parameters
Value
Heating/cooling time (s) Mass recovery time (s) Preliminary heating time (s) Hot water flow rate (m3/s) Cooling water flow rate (m3/s) Chilled water flow rate (m3/s) Hot water inlet temperature (°C) Outlet air temperature of the fan coil unit (°C)
900 60 40 0.001 0.0014 0.0005 P65 14–22
adsorption chiller is roughly in phase with cooling load of grain cooling storage. 5.4. Daily performance of the solar adsorption chiller This solar adsorption chiller is used to cool the upper air space of a testing grain depot. The operating conditions are the same as those listed in Table 3. The representative test results are presented in Table 4. Here, Qc represents the average cooling power over the working time in one day. It is seen that, the solar water heating unit can power the adsorption chiller for about 6.5– 8.5 h during one day, average cooling power of the solar adsorption chiller is about 3.2–4.4 kW. In areas with abundant solar resources, it is reasonable to expect such solar adsorption chiller to run effectively.
6. Conclusions A solar adsorption chiller was developed for grain depot cooling. Based on the test results, the following conclusions can be drawn. (1) The solar water heating unit, the silica gel–water adsorption chiller, the cooling tower and the fan coil unit match well in this study. This adsorption chiller can run effectively when the hot water temperature is roughly above 65 °C. (2) This adsorption chiller can be operated with a twoadsorber continuous refrigeration cycle with mass recovery. One-minute mass recovery process improves the cycle COP and cooling power of the adsorption chiller effectively.
Table 4 Daily performance of the solar adsorption chiller Test date
Solar radiation (MJ/m2)
Ambient temperature (°C)
Working time (min)
gday
COPsolar
Qc (kW)
2004/07/31 2004/08/06 2004/08/09 2004/08/15 2004/08/26 2004/09/19
19.6 20.3 17.4 19.5 18.7 16.2
28.5–37.6 29.4–37.5 26.8–36.7 27.3–36.4 26.9–36.3 25.2–33.6
474 508 423 474 457 382
0.45 0.46 0.42 0.45 0.44 0.40
0.123 0.125 0.096 0.131 0.124 0.109
4.19 4.14 3.25 4.43 4.21 3.87
H.L. Luo et al. / Applied Thermal Engineering 26 (2006) 1218–1225
(3) With a daily solar radiation of 16–21 MJ/m2 this solar adsorption chiller can provide 6.5–8.5 h of cooling with an average cooling power of 3.2– 4.4 kW and with a solar cooling COP of about 0.1–0.13. (4) The cooling output of this solar adsorption chiller is roughly in phase with cooling load of grain depot. In areas with abundant solar resources, such solar adsorption chiller may provide an alternative way for grain cooling storage.
Acknowledgements This work was supported by National Key Technology R&D program under contract no. 2004BA523B02, the National Key Fundamental Research Program under contract no. G2000026309. The authors thank a lot to Mr. Z.H. Wang, B.B. Zhang, Y.R. Tong, et al. for their assistance to this work.
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Experimental investigation of a solar adsorption chiller used for grain depot cooling H.L. Luo a, Y.J. Dai a, R.Z. Wang a b
a,*
, J.Y. Wu a, Y.X. Xu a, J.M. Shen
b
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China Branch of Jiangsu Province, Chinese State Grain Administration Company, Nanjing 210012, China Received 13 January 2005; accepted 14 October 2005 Available online 20 December 2005
Abstract The solar cooling technology is attractive since cooling load of building is roughly in phase with solar energy availability. In this study, a solar adsorption chiller was built and tested with aim of developing an alternative refrigeration system used for grain cooling storage. This solar adsorption chiller consists of four subsystems, namely, a solar water heating unit with 49.4 m2 solar collecting area, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. In order to achieve continuous refrigeration, two adsorption units are operated out-of-phase with mass recovery cycle in the adsorption chiller. Field test results show that, under the climatic conditions of daily solar radiation being about 16–21 MJ/m2, this solar adsorption chiller can furnish 14–22 °C chilled air with an average cooling output ranging from about 3.2–4.4 kW, its daily solar cooling COP (coefficient of performance) is about 0.1–0.13. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Solar; Adsorption chiller; Grain depot cooling
1. Introduction In grain storage, the spoilage of stored grain is mainly due to the attack of insects and moulds. Low temperature storage can prevent development of insect and mould, inhibit respiration of stored grain and extend its storage time. Hence, the storage temperature is one of the most important factors that determine the quality of stored grain. Mechanical ventilators and grain chillers are commonly used devices for controlling storage temperature in China [1]. For mechanical ventilators, the initial and operation costs are low. Nevertheless, sole using mechanical ventilators can not make the temperature of stored grain drop to a suitable point in sweltering summer and autumn. The grain chiller, characterized by *
Corresponding author. Tel.: +86 21 6293 3838; fax: +86 21 6293 3250. E-mail address: [email protected] (R.Z. Wang). 1359-4311/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.036
great cooling power and large electric power consumption, is essentially an electrically powered vapor compression refrigeration system. Although it is an effective device to decrease grain temperature in depot, the grain chiller is economically adverse for operation of a grain depot because of its high operation costs. Hence, it is necessary to develop alternative refrigeration device for grain cooling storage. Use of solar cooling technology in grain cooling storage system is very attractive since cooling load of grain depot is roughly in phase with solar energy availability. Different solar regenerated desiccant cooling devices have been proposed for grain storage in previous studies [2–4]. Such devices focus on reducing the humidity of ambient air before it is forced through grain. A challenge still exits to make the temperature of stored grain drop to a suitable point without, or with minimal electric power consumption in sweltering summer and autumn in hot areas. Dai et al. [5] theoretically investigated a solar
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Nomenclature Ac COP Cpw I(t) Ih m_ Qc T t
solar collecting area (m2) coefficient of performance isobaric specific heat of water (J/kg °C) solar radiation intensity on the top surface of collector (W/m2) hourly solar radiation (MJ/m2) mass flow rate (kg/s) cooling power (W) temperature (°C) time (s)
Greek symbol g collector efficiency
powered solid adsorption–desiccant cooling hybrid system used for grain cooling. The analysis results indicate that such system can provide cold air below 20 °C for grain cooling storage. Many researches on solar adsorption refrigeration have also been done during the past decades. Pons et al. [6] experimentally investigated a solar adsorption ice maker with 6 m2 collector areas, which could produce 30–35 kg ice per day under the solar radiation of about 22 MJ/m2 day. Critoph [7] comprehensively studied the performance limitations of adsorption cycles for solar cooling. Boubakri et al. [8] studied the limits of ice production by means of adsorptive collector–condenser technology. Wang et al. [9] introduced a hybrid system of solar powered water heater and adsorption ice maker. Recently, Yong et al. [10] simulated the performance of a solar powered adsorption air conditioning system. All these studies reveal that, in general, the solar adsorption refrigeration device is feasible for areas with abundant solar resources.
Subscripts ad adsorber am ambient bottom bottom part c solar collector cw chilled water ev evaporator hw hot water i instantaneous in inlet out outlet up upper part wt water tank
In this study, a solar adsorption chiller used for grain depot cooling was designed, constructed and field tested. The effects of operating parameters, such as solar hot water temperature, heating/cooling time, and mass recovery process on system performance were also examined.
2. Description of the solar adsorption chiller The solar adsorption chiller used for grain depot cooling mainly consists of four subsystems, namely, a solar water heating unit, a silica gel–water adsorption chiller, a cooling tower and a fan coil unit. A schematic diagram of this solar adsorption chiller is shown in Fig. 1. The solar water heating unit is utilized to produce hot water to drive the adsorption chiller. The cooling tower offers the cooling water to cool the condensers and adsorbers. The cooling production is Legend
Solar collector array Water pump Pyranometer
Water valve Flow meter
Vacuum valve
Wind valve H Hygrometer
T
Platinum resistance Cooling tower
T
Hot water inlet
T
T
V11
V12 V13 T
Condenser 1 Baffle plate
T
V7
V6
V5
V4
V3
V2
V8
V1
V0
Evaporator 1
V9
T
Adsorber 1
Partitioned hot water tank V10 Fan coil unit Grain depot
Cooling water inlet Adsorber 2 Evaporator 2
T
T H H
Condenser 2
T
T
Chilled water tank
Second stage evaporator
Chilled water inlet
Fig. 1. Schematic diagram of the solar adsorption chiller used for grain depot cooling.
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transferred to a grain depot through a fan coil unit. A wind valve is employed to control the outlet air temperature of the fan coil unit. The characteristics and main parameters of the solar adsorption chiller are shown in Table 1. The solar water heating unit (see Fig. 1) includes an all-glass evacuated tube collector array, a water pump and a partitioned hot water tank, etc. The partitioned hot water tank is divided into two parts by a baffle plate; the upper part has two fifths of the volume of the tank. The water pump, controlled by a differential temperature controller, independently works without considering whether the adsorption chiller is in the operation. In the early morning of sunny days, valve 12 is kept open, and valve 13 is closed. The upper part water of the tank is quickly heated by solar collectors and provides energy for the adsorption chiller. When water temperature in the upper part of the tank exceeds about 70 °C, valve 13 is opened also. The whole tank will be used as the heat reservoir to the adsorption chiller. Such a design is aiming at starting the adsorption chiller as early as possible in the morning. The solar adsorption chiller is designed for inhibiting temperature rise of stored grain in hot seasons. No auxiliary heat source is provided in order to reduce operation cost of grain storage. The adsorption chiller stops running when water temperature in the upper part of the tank is below about 65 °C in the afternoon. The available hot water temperature of evacuated tube solar water heating unit is about 60–90 °C. Hence, silica gel–water, which is suited for use of low heat Table 1 Characteristics and parameters of the solar adsorption chiller Subsystem
Characteristics and parameters
Solar water heating unit Collector type All-glass evacuated tube collector Collecting area 49.4 m2 Pump Rated power: 290 W; rated head: 25 m; rated flow rate: 0.00042 m3/s Hot water tank Capacity: 0.6 m3; Height: 1.2 m Adsorption chiller Adsorbent Hot water pump Chilled water pump Chilled water tank Cooling tower Fan Pump Fan coil unit Fan Heat exchanger
Micro-pore silica gel; mass: 50 kg/adsorber Rated power: 250 W; rated head: 10m; rated flow rate: 0.0013 m3/s Rated power: 120 W; Rated head: 8 m; rated flow rate: 0.00054 m3/s Capacity: 0.03 m3 Rated power: 250 W; rated flow rate: 0.306 m3/s Rated power: 370 W; Rated head: 12 m; rated flow rate: 0.0017 m3/s
source such as hot water [11,12], is chosen as adsorption working pair. The adsorption chiller (see Fig. 1) includes two identical adsorption units and a second stage evaporator with methanol working fluid. Each adsorption unit consists of one adsorber, one condenser and one evaporator (the first stage evaporator), which are housed in a vacuum chamber. When the adsorber is heated in one adsorption unit, water vapor desorbs from the silica gel enclosed in the adsorber with the increase of silica gel temperature. The desorbed water vapor is condensed via condenser and enters into evaporator in liquid state. When the adsorber is cooled, water vapor from evaporator is adsorbed by the silica gel, and cooling effect is, thus, produced in the evaporator during this stage. Continuous refrigeration can be achieved with two adsorption units to shift between adsorption and desorption phases. During the operation of the adsorption chiller, liquid methanol in the second stage evaporator is continuously heated by circulating chilled water. The evaporated methanol vapor rises and is condensed on the surface of the first stage evaporator in the adsorption unit being in adsorption phase, then drops back to the bottom of the second stage evaporator. In this way, cooling output of the first stage evaporator can be transferred to the methanol in the second stage evaporator and then to circulating chilled water. Such a design can suppress the heat transfer between circulating chilled water and the first stage evaporator of the adsorption unit being in desorption phase effectively. The advantages of mass recovery have been demonstrated in previous literatures [13,14]. With the help of a vacuum valve (V11) and 11 water valves (V0–V10), the adsorption chiller can be operated with a two-adsorber continuous refrigeration cycle with mass recovery. The operation scheme of this refrigeration cycle is listed in Table 2. Different from conventional two-adsorber cycle with mass recovery, there are two switching stages in this adsorption refrigeration cycle. During switching stage, the resident hot water in the adsorber being in desorption phase is pushed into the other adsorber being in adsorption phase by cooling water (see Fig. 1 and Table 2). So, the adsorber being in adsorption phase is heated preliminarily, and the sensible heat of resident hot water in the adsorber being in desorption phase is recovered partially. When the resident hot water in the adsorber being in desorption phase is almost pushed out, the switching stage terminates. Hence, the time of switching stage, namely, the preliminary heating time is determined by cooling water flowrate.
3. Performance index Rated power: 750 W; rated flow rate: 0.417 m3/s Type: tube-fin; Heat exchange area: 58.7 m2
In general, the performance of solar collector is represented in terms of instantaneous efficiency, gi, and
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Table 2 Operation scheme of the two-adsorber continuous refrigeration cycle with mass recovery Cycle stage
Adsorber 1
Adsorber 2
Opened valves
Closed valves
Heating/cooling Mass recovery (adsorber 1 ! adsorber 2) Switching Heating/cooling Mass recovery (adsorber 2 ! adsorber 1) Switching
Heating Heating Cooling Cooling Cooling Preliminary heating
Cooling Cooling Preliminary heating Heating Heating Cooling
V0, V0, V0, V1, V1, V2,
V1, V1, V1, V0, V0, V0,
V3, V3, V6, V2, V2, V4,
V4, V4, V9, V5, V5, V8,
V7 V7, V11 V10 V6 V6, V11 V10
V2, V2, V2, V3, V3, V1,
V5, V5, V3, V4, V4, V3,
V6 , V6 , V4 , V7 , V7 , V5 ,
V8, V8, V5, V8, V8, V6,
V9, V9, V7, V9, V9, V7,
V10, V11 V10 V8, V11 V10, V11 V10 V9, V11
daily efficiency, gday. They can be calculated by the following equations, respectively [15]: gi ¼ gday
m_ c C pw ðT c-out T c-in Þ Ac I ðtÞ R m_ c C pw ðT c-out T c-in Þ dt R ¼ Ac I ðtÞ dt
ð1Þ ð2Þ
The cycle COP of the adsorption chiller, COPcycle, is defined as ratio between the useful cooling output of the second stage evaporator and the energy supplied by the solar water heating unit: R m_ cw C pw ðT ev-in T ev-out Þ dt COPcycle ¼ R ð3Þ m_ hw C pw ðT ad-in T ad-out Þ dt The solar cooling COP of the system, COPsolar, is defined as ratio between the useful cooling output of the second stage evaporator and the total incident solar energy on the surface of solar collectors: R m_ cw C pw ðT ev-in T ev-out Þ dt R COPsolar ¼ ð4Þ Ac I ðtÞ dt The cooling power of the system, Qc, can be calculated by Qc ¼ m_ cw C pw ðT ev-in T ev-out Þ
ð5Þ
Fig. 2. Photographs of the solar adsorption chiller installed in a grain depot. (a) The adsorption chiller and (b) the solar collector array.
A RS232 bus is used as a communication passage between the Keithley 2700 multimeter/data acquisition system and a computer. The solar adsorption chiller was installed in a grain depot in Jiangsu Province, China. Photographs of the experimental setup are shown in Fig. 2. From July to September of 2004, a series of field tests on the performance of the solar adsorption chiller were carried out.
4. Experimentation 5. Results and discussion In order to acquire necessary data to evaluate the performance of the solar adsorption chiller, a series of sensors and instruments are employed. A pyranometer (TBQ-2 Type) with an accuracy of ±2% is used to measure the solar radiation on the top surface of the collectors. The temperature measurements are performed with a series of platinum resistance temperature sensors (pt100) properly placed (see Fig. 1). All these temperature sensors are calibrated, and the accuracy of the measured temperatures is estimated to be within ±0.2 °C. Four turbine flow meters are used to measure the water flow rate of cycle pumps. Furthermore, two hygrometers are used to monitor the outlet and inlet air relative humidity of fan coil unit. The pyranometer, hygrometers, flow meters and temperature sensors are connected to a Keithley 2700 multimeter/data acquisition system.
5.1. Solar water heating unit Fig. 3 shows the variations of solar radiation, I(t), upper and bottom part water temperature inside the partitioned hot water tank, Twt-up and Twt-bottom, outlet water temperature of the solar collector array,Tc-out, and ambient temperature, Tam, with time in a test day. In the early morning, the water temperature in the upper part of the tank increases quickly. The adsorption chiller begins to run at about 9:40 h. The water temperature inside the partitioned hot water tank fluctuates periodically because of the running of the adsorption chiller. The temperature fluctuation in the upper part of the tank generally is less than 5 °C during one cycle. This indicates that the tank volume and collector area match
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100
1000 Twt-up Twt-bottom Tc-out Tam I(t)
70
800 2
80
Solar radiation (W/m )
Temperature ( oC)
90
600
60
400
50 200 40 0
30
05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 Local time (h) Fig. 3. Variations of solar radiation, ambient temperature and water temperature with time (August 6, 2004).
the heat load of the adsorption chiller on the whole. The solar radiation reaches its maximum at about 12:30 h. However, water temperature in the upper part of the tank reaches its maximum at about 13:50 h. This phenomenon indicates that the energy gain from the solar water heating unit is greater than the energy consumption of the adsorption chiller before 13:50 h. Fig. 4 shows the measured instantaneous efficiency of the allglass evacuated tube collector in the same test day. Although its outlet temperature exceeds about 75 °C after 10:30 h, the all-glass evacuated tube collector still has good thermal performance; its instantaneous efficiency varies within about 0.25–0.55. 5.2. Performance of the adsorption chiller Before the adsorption chiller is put into practical operation for grain depot cooling, its performance under different test conditions is measured with a test facility.
The test facility consists of a hot water system, a cooling water system and a chilled water system. In the hot water system, water in hot water tank is heated by a steam boiler via a steam-water heat exchanger; a steam flow regulation valve is employed to control hot water temperature. In the chilled water system, chilled water temperature is controlled by regulating the city water flowrate entered into the chilled water tank. The redundant water in chilled water tank is drained into cooling water tank through an overflow valve. In the cooling water system, cooling water temperature is also controlled by regulating the city water flowrate entered into the cooling water tank. In this test facility, the fluctuations of hot water, chilled water and cooling water temperature can be controlled within ±0.8 °C during test operation. The performance of the adsorption chiller under two different cycle modes is shown in Fig. 5. Here, Qc and COPcycle represent average cooling power and cycle COP during one cycle, respectively. It is seen that mass recovery process is helpful for improvement of the performance of the adsorption chiller. Compared with the cycle mode without mass recovery, cycle COP and cooling power can be increased by about 14.6% and 11.2%, respectively with 1-min mass recovery process when heating/cooling time is fixed at 900 s. Also found is that, with 1-min mass recovery process, a peak cooling power of 5.03 kW is achieved when heating/cooling time is fixed at 720 s, the corresponding cycle COP is about 0.317. With 1-min mass recovery process, a peak cycle COP of 0.324 is achieved when heating/cooling time is fixed at 900 s, the corresponding cooling power is 4.96 kW. These indicate that the proper heating/cooling time for this adsorption chiller is about 720–900 s at the hot water inlet temperature of 85 °C. For the adsorption chiller, the suitable mass recovery time is about 60 s [16].
70
6
60
COPcycle
ηi (%)
5
50
0.25
Preliminary heating time:40 s Hot water inlet temperature: 85 oC Chilled water inlet temperature: 18oC Cooling water inlet temperature: 32oC
0.20
40
30
0.15 300
4
Cooling power (kW)
0.30
3 400
500
600
700
800
900
Heating/cooling time (s)
20 08
09
10
11
12
13 14 15 Local time (h)
16
17
18
19
Fig. 4. Instantaneous efficiency of the all-glass evacuated tube collector (August 6, 2004).
COPcycle with mass recovery (60 s) Qc with mass recovery (60 s)
COPcycle without mass recovery Qc without mass recovery
Fig. 5. Variations of cycle COP and cooling power with heating/ cooling time.
H.L. Luo et al. / Applied Thermal Engineering 26 (2006) 1218–1225
4
0.20 0.16 0.12 55
60
Mass recovery time: 60 s Preliminary heating time: 40 s 3 Heating/cooling time: 900 s Chilled water inlet temperature: 18 oC Cooling water inlet temperature: 32 oC 2 70 75 80 85 90 95
65 Hot water inlet temperature ( oC)
Fig. 6. Variations of cycle COP and cooling power with hot water inlet temperature.
Variations of average cooling power and cycle COP during one cycle with hot water inlet temperature are plotted in Fig. 6. It is found that both cooling power and cycle COP increase with the increase of hot water inlet temperature. When hot water inlet temperature is above about 75 °C, the increment rate becomes slow. At the hot water inlet temperature of 65 °C, the cooling power and cycle COP still reach 2.97 kW and 0.232, respectively. These indicate that the adsorption chiller can be powered by solar hot water effectively. The average cooling power and cycle COP during one cycle of the adsorption chiller under different chilled water inlet temperature conditions are presented in Fig. 7. The chilled water is cycled between the fan coil unit and the second stage evaporator in order to remove the cooling production of the adsorption chiller. Both cooling power and cycle COP increase quickly with the increase of chilled water inlet temperature. At the chilled water inlet water temperature of 20 °C, the cooling power reaches 5.29 kW, and the corresponding cycle
Fig. 8 shows the temperature variations of hot water, chilled water and outlet air of the fan coil unit during one cycle with mass recovery. The corresponding instantaneous cycle COP and cooling power of the solar adsorption chiller are presented in Fig. 9. During the mass recovery process, the pressure in the adsorption unit being in desorption phase decreases rapidly; it in return improves desorption effect, and results in increase of the demand for desorption heat, at last leads to decrease of the hot water outlet temperature, Thw-out. In contrast, water evaporation in the adsorption unit
25 80 70 20
60 50
15
40 Hot water flow rate: 0.001 m 3/s Chilled water flow rate: 0.0005 m3/s Cooling water inlet temperature: 31.6 - 33.9 o C
10 0
500
Tair-out
1000 1500 Time (s)
Tev-in
Tev-out
0.30 0.25
Thw-out
0.20
Mass recovery time: 60 s Preliminary heating time: 40 s Heating/cooling time: 900 s Hot water inlet temperature: 85 oC Cooling water inlet temperature: 32 oC
0.15
3
Qc
6
2
COPcycle
0.9 0.8 0.7 0.6
4
0.5 0.4 0.3
2
0.2
0.10
0.1
1 12
Thw-in
Fig. 8. Variations of Thw-in, Thw-out, Tev-in, Tev-out and Tair-out with time.
5 4
20
1.0
Cooling power (kW)
COPcycle Qc
30
2000
6
0.35
COPcycle
5.3. Transient performance of the solar adsorption chiller
Cooling power (kW)
COPcycle
0.24
Cooling power (kW)
5 0.28
COPcycle
0.32
COP is 0.331. To improve cooling power and cycle COP, it is reasonable to increase the chilled water inlet water temperature of the adsorption chiller appropriately.
Hot water temperature o C
6 COPcycle Qc
Chilled water or air temperature o C
0.36
1223
14 16 18 20 Chilled water inlet temperature (oC)
Fig. 7. Variations of cycle COP and cooling power with chilled water inlet temperature.
0.0
0 0
500
1000 Time (s)
1500
2000
Fig. 9. Instantaneous performance of the solar adsorption chiller.
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5
32
4
28 3 24 20
2
16 1
Qc T am
12
COPcycle 16:50 - 17:50
15:50 - 16:50
14:50 - 15:50
8
13:50 - 14:50
12:50 - 13:50
11:50 - 12:50
0
10:50 - 11:50
Ih T ev-out
COPcycle (%); Temperature ( o C)
36
09:50 - 10:50
Cooling power (kW); Hourly solar radiation (MJ/m2)
being in adsorption phase is suppressed greatly due to pressure increment occurred in mass recovery process. Consequently, the chilled water outlet temperature of the adsorption chiller, Tev-out, increases gradually. The air outlet temperature of the fan coil unit, Tair-out, fluctuates a little with the change of Tev-out. For grain storage, the effects of such little temperature fluctuation are negligible. It is seen in Fig. 9 that the instantaneous cycle COP changes in a wide range with the variations of the inlet and the outlet temperatures of hot water. However, the cooling power of the adsorption chiller only changes a little during one cycle except the mass recovery and switching period. Fig. 10 shows the weather conditions (hourly solar radiation, Ih, and mean ambient temperature), hourly mean chilled water outlet temperature and performance of the solar adsorption chiller in a typical test day. The operating conditions are listed in Table 3. During the period of 12:00–15:00 h, the solar radiation is intensive and the ambient temperature is relatively high. Consequently, the cooling load of grain depot is also relatively high. As expected, the cooling output of the solar
Local time Fig. 10. Weather conditions and hourly performance of the solar adsorption chiller in a test day (August 6, 2004).
Table 3 Operating conditions during the test operation of the solar adsorption chiller Parameters
Value
Heating/cooling time (s) Mass recovery time (s) Preliminary heating time (s) Hot water flow rate (m3/s) Cooling water flow rate (m3/s) Chilled water flow rate (m3/s) Hot water inlet temperature (°C) Outlet air temperature of the fan coil unit (°C)
900 60 40 0.001 0.0014 0.0005 P65 14–22
adsorption chiller is roughly in phase with cooling load of grain cooling storage. 5.4. Daily performance of the solar adsorption chiller This solar adsorption chiller is used to cool the upper air space of a testing grain depot. The operating conditions are the same as those listed in Table 3. The representative test results are presented in Table 4. Here, Qc represents the average cooling power over the working time in one day. It is seen that, the solar water heating unit can power the adsorption chiller for about 6.5– 8.5 h during one day, average cooling power of the solar adsorption chiller is about 3.2–4.4 kW. In areas with abundant solar resources, it is reasonable to expect such solar adsorption chiller to run effectively.
6. Conclusions A solar adsorption chiller was developed for grain depot cooling. Based on the test results, the following conclusions can be drawn. (1) The solar water heating unit, the silica gel–water adsorption chiller, the cooling tower and the fan coil unit match well in this study. This adsorption chiller can run effectively when the hot water temperature is roughly above 65 °C. (2) This adsorption chiller can be operated with a twoadsorber continuous refrigeration cycle with mass recovery. One-minute mass recovery process improves the cycle COP and cooling power of the adsorption chiller effectively.
Table 4 Daily performance of the solar adsorption chiller Test date
Solar radiation (MJ/m2)
Ambient temperature (°C)
Working time (min)
gday
COPsolar
Qc (kW)
2004/07/31 2004/08/06 2004/08/09 2004/08/15 2004/08/26 2004/09/19
19.6 20.3 17.4 19.5 18.7 16.2
28.5–37.6 29.4–37.5 26.8–36.7 27.3–36.4 26.9–36.3 25.2–33.6
474 508 423 474 457 382
0.45 0.46 0.42 0.45 0.44 0.40
0.123 0.125 0.096 0.131 0.124 0.109
4.19 4.14 3.25 4.43 4.21 3.87
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(3) With a daily solar radiation of 16–21 MJ/m2 this solar adsorption chiller can provide 6.5–8.5 h of cooling with an average cooling power of 3.2– 4.4 kW and with a solar cooling COP of about 0.1–0.13. (4) The cooling output of this solar adsorption chiller is roughly in phase with cooling load of grain depot. In areas with abundant solar resources, such solar adsorption chiller may provide an alternative way for grain cooling storage.
Acknowledgements This work was supported by National Key Technology R&D program under contract no. 2004BA523B02, the National Key Fundamental Research Program under contract no. G2000026309. The authors thank a lot to Mr. Z.H. Wang, B.B. Zhang, Y.R. Tong, et al. for their assistance to this work.
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