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heating system
Energy Conversion and Management 51 (2010) 1643–1650
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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Development of hybrid solar-assisted cooling/heating system B.J. Huang *, J.H. Wu, H.Y. Hsu, J.H. Wang Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
a r t i c l e
i n f o
Article history: Available online 20 March 2010 Keywords: Ejector cooling system Ejector Heat pump Solar cooling Solar ejector cooling Solar energy
a b s t r a c t A solar-assisted ejector cooling/heating system (SACH) was developed in this study. The SACH combines a pump-less ejector cooling system (ECS) with an inverter-type heat pump (R22) and is able to provide a stable capacity for space cooling. The ECS is driven by solar heat and is used to cool the condenser of the R22 heat pump to increase its COP and reduce the energy consumption of the compressor by regulating the rotational speed of the compressor through a control system. In a complete SACH system test run at outdoor temperature 35 °C, indoor temperature 25 °C and compressor speed 20–80 Hz, and the ECS operating at generator temperature 90 °C and condensing temperature 37 °C, the corresponding condensing temperature of the heat pump in the SACH is 24.5–42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11–2.50. This indicates that the use of ECS in SACH can effectively reduce the condensing temperature of the heat pump by 12.6–7.3 °C and reduce the power consumption by 81.2–34.5%. The SACH can also supply heat from the heat pump. At ambient temperature from 5 °C to 35 °C, the heating COPh is in the range 2.0–3.3. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The ejector cooling system (ECS) using low boiling point refrigerant is suitable for solar cooling application due to its simple design and low cost. Huang et al. [1,2] has shown that the COP of an ECS using R141b, with a proper design of ejector and system structure, can reach 0.54 at generator temperature 84 °C, condenser temperature 28 °C, and evaporator temperature 8 °C. This makes the ECS become competitive to the sorption (absorption or adsorption) system that is much more complicated in design and more expensive [3–12]. In the ECS, the condenser temperature must be lower than the critical condensing temperature (critical point) to obtain a high performance. Fig. 1 shows the double-choking phenomenon of ECS [1,2]. Therefore, the ejector should be operated at critical mode in order to obtain a better performance. If the ECS was driven by solar energy, it always requires a back-up system to make up the heat required to keep a constant cooling capacity for space cooling during cloudy or rainy periods (Fig. 2). Heat supplied by fossil fuel or electricity was generally adopted. This however causes a problem of additional investment of heaters and low efficiency in heat supply. The present study intends to develop a hybrid solar-assisted ejector cooling/heating system (SACH) in which a conventional inverter-type air-conditioner (heat pump) made of variablespeed compressor is connected in series with a solar ejector * Corresponding author. Tel.: +886 2 2363 4790; fax: +886 2 2364 0549. E-mail address: [email protected] (B.J. Huang). 0196-8904/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.07.026
cooling system (see Fig. 3). The solar ejector cooling system is used to cool the condenser of the air conditioner to reduce the condensing temperature and increase the COP to reduce the power consumption of the compressor when solar irradiation is available. The cooling load is directly supplied by the inter-type heat pump. During cloudy or rainy periods or at night, the SACH will provide the entire cooling load from the heat pump as usual. The SACH can also produce hot water by the heat pump year round to supply heat, in addition to the direct heat supply from the solar collector. This will make the solar cooling/heating system more economical. The present paper reports the experimental results of this study.
2. Experimental setup 2.1. System configuration of SACH The SACH consists of three major parts: a pump-less ejector cooling system, a solar collector system, and an inverter-type heat pump with variable-speed compressor. Fig. 4 is the schematic diagram of a practical SACH. A small SACH prototype using an inverter-type air-conditioner (heat pump) with rated cooling capacity 2 kW was built and tested in the present study. The cooling capacity of the ejector cooling system (ECS) is 700 W rated at condenser temperature 32 °C, generator temperature 90 °C, and evaporator temperature 7 °C. Table 1 shows the specification of the air conditioner used in the SACH. The overall system design specification is shown in Table 2.
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Nomenclature COP COPc COPej COPh Pc Pco qc,e
Critical model (double-choking)
Entrainment ratio ω
Tc,c Wc
coefficient of performance coefficient of cooling performance coefficient of ejector cooling performance coefficient of heating performance critical condenser pressure, MPa condenser pressure of limiting condition, MPa cooling capacity, kW
ω
Greek symbol entrainment ratio
x
(single-choking) Sub-critical model Back-flow model (malfunction)
constant
ω <0
critical point
Pc∗
Pco
condensing temperature of heat pump, °C power consumption of inverter-type air conditioner kW
Pc
A test performed for a commercial inverter-type air-conditioner shows that the cooling COP increases at a rate about 0.12 per °C condensing temperature drop. This means that 20 °C reduction in condensing temperature will increase COP by 2.4, about twice the original COP or save about 50% compressor power consumption. To work as a heat pump for heating, the condenser of the R22 heat pump is connected to a 100 L hot water tank inside which a condenser coil made of 30 m copper tube (OD 10 mm) is immersed. A schematic of the experimental system is shown in Fig. 5. The system was installed with 36 T-type thermocouples with an uncertainty of ±0.7 °C, and five pressure transducers with a ±1% uncertainty (see Fig. 5). The power consumption of the compressor was measured by a power meter within a ±1.5% uncertainty. 2.2. Pump-less ejector cooling system
Fig. 1. Double-choking phenomenon of ECS.
The pump-less ejector cooling system was developed by Huang et al. [5,13] by combining the concept of heat-driven pump (HDP)
Fig. 2. Conventional solar cooling system.
Fig. 3. Hybrid solar-assisted cooling/heating system (SACH).
B.J. Huang et al. / Energy Conversion and Management 51 (2010) 1643–1650
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Fig. 4. The schematic diagram of a practical SACH.
Table 1 Specification of the inverter-type air-conditioner used in SACH. Refrigerant Input voltage Compressor displacement Compressor frequency Compressor input power Rated cooling capacity at 54.4 °C condenser/7 °C evaporator Rated COP
R22 AC 220 V 11.3 c.c./rev 20–80 Hz 0.3–1.1 kW 0.8–2.9 kW 2.6
Table 2 Overall system design specification of SACH. 1. Heat pump (inverter-type) – cooling mode Toshiba HD160X1-S12F Refrigerant Compressor input power, kW Condenser capacity, kW Condensing temperature, °C Evaporator temperature, °C Cooling capacity, kW COPc
R22 0.3 1.7 30 7 1.4 4.66
2. Ejector cooling system Refrigerant Generator temperature, °C Generator heat input, kW Condenser capacity, kW Condensing temperature, °C Evaporator temperature, °C Evaporator capacity, kW COPej
R365mfc 90 3.4 5.1 37 20 1.7 0.5
with the design of ECS. The ECS utilized a multi-function generator (MFG) to eliminate the mechanical pump. The MFG acts as a generator for vapor generation and as a feed pump for returning liquid to the generator. The schematic of the ejector cooling system with multi-function generator (ECS/MFG) is shown in Fig. 6. There are two MFGs (generators) in the ECS/MFG. Each MFG consists of a vapor generator (boiler) and an evacuation chamber. The vapor generator is a heat exchanger like a conventional boiler for heating the liquid in order to generate vapor at high pressure. The evacuation chamber is composed of a cooling jacket and a liquid tank. The cooling jacket provides a cooling effect to depressurize the whole generator in order to intake the liquid from the condenser. The two MFGs (A and B) operate interchangeably through the control of switching valves. The operation of each MFG is divided into four phases: pressurizing, vapor discharge, depressurizing, and liquid intake. While MFG A is operating at
depressurizing, liquid intake, and pressurizing phases, MFG B is operating at the vapor discharge phase. The cooling effect of the ECS/MFG is generated only at the vapor discharge phase. The design of a MFG thus requires the total time duration for depressurizing, liquid intake and pressurizing phases shorter than the period of discharge phase. The pressure variation of two MFGs is shown in Fig. 7. For R365mfc, the ejector design for SACH is based on experimental results [13] and the system design conditions (Table 2) with 2.8 mm for the nozzle throat diameter, 5.1 mm for the nozzle exit diameter, 9 mm for constant-area chamber diameter, 65° for inlet converging angle. The ejector area ratio of constant-area section to nozzle throat is 10.33. This R365mfc ejector has been shown to have better performance than R141b ejector at evaporator temperature higher than 15 °C [13]. The condenser of the ECS was cooled by a compact water cooling tower utilizing cellulose pad as the filling material [14]. For experimental purpose, the ECS was driven by hot steam generated from a steam generator to simulate solar heating. The heating power can be adjusted by regulating the electric power input to a 6 kW electric heater using a SCR and a PID controller in order to obtain a designated generator temperature of ECS. 2.3. Indoor and outdoor chamber for test In order to test the performance of the SACH at different ambient conditions, an outdoor environmental chamber was built to simulate ambient condition. The internal size of the ambient environmental chamber is 0.85 m 0.70 m 0.60 m which can install the condenser of the air conditioner. A 1.2 kW electric heater with air blower was installed inside the chamber. The air temperature is then controlled by a PID controller through a SCR. The air temperature inside the outdoor chamber can be controlled in the range 5– 40 °C. Another indoor environmental chamber for simulate the conditions of a cooling room or a house was also built. The internal size is L 1.60 m D 0.65 m H 1.50 m which can install the evaporator of the air conditioner. One kilowatt electric heater with hot air blower was installed inside the chamber. The air temperature is controlled by a PID controller with a SCR. The air temperature inside the indoor chamber can be controlled in the range 20–45 °C. 2.4. Central control system design of SACH The central control system of the SACH consists of three subsystems: pump-less ejector cooling system control, room temperature
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Fig. 5. Schematic description of experimental setup.
Fig. 6. ECS with multi-function generator (ECS/MFG).
control through heat pump (compressor speed), and hot water heating control. The whole central control system was built in a microprocessor PIC16F877. The control of ECS for full-cycle operation follows the same method in [13]. The rotation speed of the compressor is automatically varied according to the room temperature. The compressor speed will be increased to provide higher cooling capacity when the room temperature is higher than 25 °C (normal setting value). The relation between the compressor rotation speed and the room temperature is shown in Fig. 8, which is taken from the supplier of the air conditioner. For heating mode, the compressor runs in full speed (80 Hz) and the temperature sensor installed in the water tank is
used to switch ON/OFF the heat pump according to the setting temperature. For system testing, the central control system performs time scheduling control for room cooling mode starting at 9 AM till 4 PM and water heating mode starting from 6 PM till 10 PM. To shift the electricity load of the SACH to midnight to reduce the peak load problem, the present SACH also installed an ice storage device in parallel with the evaporator of the air conditioner. The ice storage capacity is 2 RT-H using a 100 L ice tank. The ice making is scheduled to start at 10:30 PM till 7:30 AM. An additional indoor air heat exchanger to deliver cooling capacity to room from the ice storage tank is designed with a cold water circulation system. The
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Fig. 7. Pressure variation of two MFGs.
5
2.5
COPc Cooling capacity (qc,e) Power consumption (Wc )
4
2
1.5
2
1
1
0.5
kW
COPc
3
Fig. 8. Relation between the compressor rotation speed and the room temperature.
cooling capacity provided by the ice storage is around 800 W. This makes the maximum total cooling capacity of the SACH to be 2.2 kW including ice melting.
0 0
20
40
60
80
0 100
Compressor frequency (Hz) 3. Test results of SACH
Fig. 9. Cooling performance test at various compressor speed.
3.1. Cooling performance test at steady state for air conditioner alone The COPc of the air conditioner is defined as the ratio of the cooling capacity at the evaporator, qc,e, to the power input of the air conditioner, Wc.
COP c ¼
qc;e Wc
temperature varies from 37.1 °C to 49.3 °C, and the cooling capacity from 0.79 to 2.08 kW, input power varies from 0.28 to 1.12 kW, COPc from 2.82 to 1.86. 3.2. Startup test at fixed cooling load
ð1Þ
For air conditioner alone, the cooling performance test at fixed compressor speed (80 Hz) was run first at ambient temperature 35 °C. The input power is 1.12 kW and the cooling capacity is 2.08 kW at condensing temperature 49.3 °C and evaporating temperature 3.6 °C. The cooling COPc is 1.86. The vapor exhaust temperature of the compressor is 62 °C. The vapor at the evaporator exit is 7 °C superheated. Similar tests were run with different compressor speed. The results of Fig. 9 show that the cooling capacity as well as the input power increases with increasing compressor speed. Since the rate of change with compressor speed for the cooling capacity is larger, the cooling COP decreases with increasing compressor speed. At 20–80 Hz, the corresponding condensing
To test the performance of compressor speed variation during startup, the air conditioner alone is run at 1 kW cooling load and at 35 °C ambient temperature and 25 °C indoor temperature. Fig. 10 shows that the compressor run at 77 Hz at beginning then continues to vary the speed for about 4 min with maximum current 4 A. The compressor speed as well as input power decreases when the indoor temperature approaching 25 °C (setting value). The measured overall COPc during the start period is 2.11. 3.3. Heating performance test The heating performance of the SACH for water heating is run at ambient temperature 25 °C and full compressor speed (80 Hz). The
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Temperature ( oC)
Cold dowm
Steady state
60 40
Indoor environmental temperature
20 25 oC
0
Current (A)
6 4 2 0 0
600
1200
1800 Time (s)
2400
3000
3600
Fig. 10. Startup test at fixed cooling load. Fig. 12. Variation of COPh with ambient temperature.
120
100
Temperature (ºC)
ature is 16.6 °C, the generator temperature is 90 °C, and the condensing temperature is 33 °C. Table 3 summarizes the cooling performance of the air conditioner alone and the SACH at compressor speed 80 Hz. It is seen that the cooling COPc increases from 1.86 to 2.5, about 34%. This is mainly due to the decrease in condensing temperature of the heat pump. Fig. 13 shows that the SACH can provide a continuous cooling performance of ECS at compressor speed 80 Hz. Fig. 14 is the comparison of the thermodynamic cycle of the two operations.
Compressor outlet Compressor inlet Condenser Evaporator Water tank (top) Water tank (middle) Water tank (bottom)
80
60
40
3.5. Overall cooling performance test of SACH and energy saving 20
0
0
1200
2400
3600
Time (s) Fig. 11. Heating performance test at 25 °C ambient temperature.
initial water temperature in tank is 27 °C. Fig. 11 shows that it takes about 60 min to reach 57 °C and the heating COPh is 2.94, energy consumption is 0.0115 kW h per liter hot water. The temperature rise speed is 0.47 °C/min and the highest temperature at compressor exhaust is 70 °C. Fig. 12 shows the variation of COPh with ambient temperature from 5 °C to 35 °C and COPh is in the range 2.0–3.3. The heating performance is satisfactory [15]. 3.4. Cooling performance test of SACH at fixed compressor speed For the performance test of SACH, the pump-less ejector cooling system is first tested with the heat pump working at fixed compressor speed (80 Hz) and at ambient temperature 35 °C. At steady state, the input power is 0.98 kW and the cooling capacity is 2.44 kW at condensing temperature 42 °C and evaporating temperature 2.5 °C. The cooling COPc of SACH is 2.50. The vapor temperature at the compressor exhaust is 49 °C. The vapor at the evaporator exit is 5 °C superheated. The vapor temperature at the compressor exhaust is 49 °C. For the ECS, the condensing temper-
It needs to understand the amount of reduction in input power of the heat pump in the operation of SACH due to the heat-driven ECS. We run the SACH at 35 °C ambient temperature and at different compressor speeds. The results are shown in Fig. 15. In Fig. 15, the cooling performance for air conditioner alone is compared with SACH. It is seen that energy saving is achieved for all compressor speed, especially at lower frequency. Table 4 summarizes all the test results and comparison. It is seen that, at compressor speed 20–80 Hz, for the ECS operated at 90 °C generator temperature and 37 °C condensing temperature, the corresponding condensing temperature of the SACH is 24.5– 42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11–2.50. This indicates that the use of ECS in SACH can reduce the condensing temperature of the heat pump
Table 3 Cooling performance of the air conditioner alone and SACH at compressor speed 80 Hz.
Ambient temperature, °C Condensing temperature, °C Evaporating temperature, °C Vapor temperature at compressor exit, °C Degree of superheating at evaporator exit inlet, °C Input power, kW Cooling capacity, kW COPc
Air conditioner alone
SACH
35 49.2 3.6 62 7 1.12 2.08 1.86
35 42 2.5 49 5 0.98 2.44 2.5
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120
3
80
q c,e (kW)
Temperature (ºC)
SACH air conditioner alone
MFG_B temperature
MFG_A temperature
Generator temperature
2 1 0 1.5
40
W c (kW)
Condenser temperature Evaporator temperature
0 0
600
1200
1800
2400
3000
3600
SACH air conditioner alone
1 0.5
Time(s) 0 6
Fig. 13. Continuous cooling performance of ECS at compressor speed 80 Hz.
COP c
SACH air conditioner alone
4000
3
2000 1800 1600 1400 1200 1000
2
4 2 0 80
31 21
T c,c (oC)
Pressure (kPa)
Combined ejector No ejector
800 4
600
1
41
SACH air conditioner alone
60 40 20
11
0
20
400
40
60
80
100
Compressor frequency (Hz) Fig. 15. Performance of SACH at different compressor speeds.
200 150
200
250
300
350
400
450
Enthalpy (kJ/kg) Fig. 14. Comparison of the thermodynamic cycle of the two operations.
by 12.6–7.3 °C and reduce the compressor power consumption by 81.2–34.5%.
3.6. Complete system test of SACH The SACH built in the present study includes a central control system to perform a full-day test at various modes, including water
heating, space cooling, and ice storage, etc. The simulated ambient temperature is 35 °C during daytime and 30 °C at night. The cooling load at daytime is fixed at 1.5 kW. The continuous test run has been carried out to simulate a practical application of SACH. Fig. 16 shows the test results. It is seen from the compressor current variation that the central control system runs very well. During the period 9:00–11:30 AM, the cooling capacity is mainly supplied by ice melting and the rather high electric current (3.5 A) is due to the additional power consumption of the circulation pump (0.76 kW) installed in the ice storage system. The SACH takes over the cooling load supply after 11:30 AM and the reduction in compressor input power (2 A current) is clearly observed.
Table 4 Performance comparison of air conditioner alone and SACH. Outdoor temperature 35 °C; room temperature 25 °C Compressor speed (Hz)
Condensing temperature of heat pump, Tc,c (°C) Air conditioner alone, Tc,c1
20 40 60 80
20 40 60 80
37.1 42.1 45.8 49.3 Power consumption
SACH, Tc,c2
Temperature drop of SACH Tc,c1 Tc,c2 (°C)
24.5 12.6 34.0 8.1 38.2 7.6 42.0 7.3 of compressor, Wc (kW)
Cooling capacity, qc,e (kW) Air conditioner alone, qc,e1
SACH, qc,e2
Increase = (qc,e1 qc,e2)/ qc,e1, (%)
0.79 1.45 1.81 2.08 COPe
1.02 1.70 2.12 2.44
29.4 17.4 17.2 17.1
Air conditioner alone, Wc1
SACH, Wc2
Reduction = (Wc1 Wc2)/Wc1, (%)
Air conditioner, COPc1
SACH, COPc2
Increase = (COPc1 COPc2)/ COPc1, (%)
0.28 0.52 0.78 1.12
0.20 0.45 0.68 0.98
28.6 13.5 13.5 12.9
2.82 2.79 2.32 1.86
5.11 3.78 3.14 2.50
81.2 35.6 35.4 34.5
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B.J. Huang et al. / Energy Conversion and Management 51 (2010) 1643–1650
In a complete SACH system test run at outdoor temperature 35 °C, indoor temperature 25 °C and compressor speed 20–80 Hz, and for the ECS operating at 90 °C generator temperature and 37 °C condensing temperature, the corresponding condensing temperature of heat pump in the SACH is 24.5–42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11– 2.50. This indicates that the use of ECS in SACH can reduce the condensing temperature of the heat pump by 12.6–7.3 °C and reduce the power consumption by 81.2–34.5%. The present study also developed a central control system for the SACH to operate at different modes, including hot water heating, space cooling, and ice storage at night. It is shown that the full-day system performance is satisfactory. The performance of the SACH developed in the present study can be further improved by better system matching design. The rated cooling capacity of the ECS used in the present SACH is rather small compared to the size of heat pump which is the minimum size commercially available. This results in a less energy saving at higher compressor speed during high solar radiation periods. The heat source used in the present SACH is supplied from an electric heater. In the future, we will build a solar heating system using vacuum-tube collector and test with the SACH to demonstrate a real solar cooling/heating application.
Melting ice and Air condition
Freeze
Heating
Temperature (ºC)
80 60 Compressor outlet
40 20 Compressor Inlet
0
Temperature (ºC)
-20 60
Water tank (average)
40
20 Ice storage tank
Current (A)
0 10 8 6
Acknowledgment 4 2 0 18:00
22:00
2:00
6:00
10:00
14:00
18:00
This publication is based on the work jointly supported by Award No. KUK-C1-014-12, made by King Abdullah University of Science and Technology (KAUST) and the Project No. 97-D0137-1 made by Energy Bureau, Ministry of Economic Affairs, Taiwan.
Time Fig. 16. Complete system test of SACH.
The water heating mode runs from 18:00 PM at water temperature 27 °C and stops at 19:20 PM with hot water temperature reaching 57 °C. The measured heating COPh is 2.90 and the energy consumption is 0.0137 kW h per liter of hot water. The water temperature rise speed is 0.432 °C/min. 4. Discussion and conclusions A solar-assisted ejector cooling/heating system (SACH) was developed in this study. The SACH combines a pump-less ejector cooling system (ECS) with an inverter-type heat pump (R22) and is able to produce a stable capacity for space cooling. The cooling effect of ECS is generated by solar heat and is used to cool the condenser of the R22 heat pump to increase its COP and reduce the energy consumption of the compressor by regulating the rotational speed of the compressor using a control system. The ECS uses a multi-function generator (MFG) as a vapor generator and a thermal pumping device to replace the mechanical circulation pump for refrigerant circulation. The experiment of ECS operating at full-cycle using R365mfc has shown that the COPe of ECS can reach 0.49 with cooling capacity of 1.57 kW at generator temperature 90 °C, condenser temperature 35.4 °C, and evaporator temperature 19.4 °C (to cool the condenser of the heat pump).
References [1] Huang BJ, Chang JM. Empirical correlation for ejector design. Int J Refrig 1999;22:379–88. [2] Huang BJ, Chang JM, Wang CP, Petrenko VA. A 1D analysis of ejector performance. Int J Refrig 1999;22:354–64. [3] Nguyen VM, Riffat SB, Doherty PS. Development of a solar-powered passive ejector cooling system. Appl Therm Eng 2001;21:157–68. [4] Srisastra P, Aphornratana S, Sriveerakul T. Development of a circulating system for a jet refrigeration cycle. Int J Refrig 2008;31:921–9. [5] Huang BJ, Hu SS, Lee SH. Development of an ejector cooling system with thermal pumping effect. Int J Refrig 2006;29:476–84. [6] Sokolov M, Hershgal D. Enhanced ejector refrigeration cycles powered by low grade heat. Part 1. Systems characterization. Int J Refrig 1990;12:351–6. [7] Sokolov M, Hershgal D. Enhanced ejector refrigeration cycles powered by low grade heat. Part 2. Design procedures. Int J Refrig 1990;12:357–63. [8] Sokolov M, Hershgal D. Enhanced ejector refrigeration cycles powered by low grade heat. Part 3. Experimental results. Int J Refrig 1991;14:24–31. [9] Arbel A, Sokolov M. Revisiting solar-powered ejector air conditioner-the greener the better. Sol Energy 2004;77:57–66. [10] Sun DW. Solar powered combined ejector-vapour compression cycle for air conditioning and refrigeration. Energy Convers Manage 1997;38:479–91. [11] Sun DW. Evaluation of a combined ejector-vapour compression refrigeration. Int J Energy 1998;22:333–42. [12] Huang BJ, Petrenko VA, Chang JM, Lin CP, Hu SS. A combined-cycle refrigeration system using ejector-cooling cycles as the bottom cycle. Int J Refrig 2001;24:391–9. [13] Wang Jin Hua, Wu JH, Hu SS, Huang BJ. Performance of ejector cooling system with thermal pumping effect using R141b and R365mfc. Appl Therm Eng 2009;29:1904–12. [14] Hu SS, Huang BJ. Study of a high efficiency residential split water-cooled air conditioner. Appl Therm Eng 2005;25:1599–613. [15] Huang BJ, Lee CP. Long-term performance of solar-assisted heat pump water heater. J Renew Energy 2003;29:633–9.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Development of hybrid solar-assisted cooling/heating system B.J. Huang *, J.H. Wu, H.Y. Hsu, J.H. Wang Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
a r t i c l e
i n f o
Article history: Available online 20 March 2010 Keywords: Ejector cooling system Ejector Heat pump Solar cooling Solar ejector cooling Solar energy
a b s t r a c t A solar-assisted ejector cooling/heating system (SACH) was developed in this study. The SACH combines a pump-less ejector cooling system (ECS) with an inverter-type heat pump (R22) and is able to provide a stable capacity for space cooling. The ECS is driven by solar heat and is used to cool the condenser of the R22 heat pump to increase its COP and reduce the energy consumption of the compressor by regulating the rotational speed of the compressor through a control system. In a complete SACH system test run at outdoor temperature 35 °C, indoor temperature 25 °C and compressor speed 20–80 Hz, and the ECS operating at generator temperature 90 °C and condensing temperature 37 °C, the corresponding condensing temperature of the heat pump in the SACH is 24.5–42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11–2.50. This indicates that the use of ECS in SACH can effectively reduce the condensing temperature of the heat pump by 12.6–7.3 °C and reduce the power consumption by 81.2–34.5%. The SACH can also supply heat from the heat pump. At ambient temperature from 5 °C to 35 °C, the heating COPh is in the range 2.0–3.3. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The ejector cooling system (ECS) using low boiling point refrigerant is suitable for solar cooling application due to its simple design and low cost. Huang et al. [1,2] has shown that the COP of an ECS using R141b, with a proper design of ejector and system structure, can reach 0.54 at generator temperature 84 °C, condenser temperature 28 °C, and evaporator temperature 8 °C. This makes the ECS become competitive to the sorption (absorption or adsorption) system that is much more complicated in design and more expensive [3–12]. In the ECS, the condenser temperature must be lower than the critical condensing temperature (critical point) to obtain a high performance. Fig. 1 shows the double-choking phenomenon of ECS [1,2]. Therefore, the ejector should be operated at critical mode in order to obtain a better performance. If the ECS was driven by solar energy, it always requires a back-up system to make up the heat required to keep a constant cooling capacity for space cooling during cloudy or rainy periods (Fig. 2). Heat supplied by fossil fuel or electricity was generally adopted. This however causes a problem of additional investment of heaters and low efficiency in heat supply. The present study intends to develop a hybrid solar-assisted ejector cooling/heating system (SACH) in which a conventional inverter-type air-conditioner (heat pump) made of variablespeed compressor is connected in series with a solar ejector * Corresponding author. Tel.: +886 2 2363 4790; fax: +886 2 2364 0549. E-mail address: [email protected] (B.J. Huang). 0196-8904/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.07.026
cooling system (see Fig. 3). The solar ejector cooling system is used to cool the condenser of the air conditioner to reduce the condensing temperature and increase the COP to reduce the power consumption of the compressor when solar irradiation is available. The cooling load is directly supplied by the inter-type heat pump. During cloudy or rainy periods or at night, the SACH will provide the entire cooling load from the heat pump as usual. The SACH can also produce hot water by the heat pump year round to supply heat, in addition to the direct heat supply from the solar collector. This will make the solar cooling/heating system more economical. The present paper reports the experimental results of this study.
2. Experimental setup 2.1. System configuration of SACH The SACH consists of three major parts: a pump-less ejector cooling system, a solar collector system, and an inverter-type heat pump with variable-speed compressor. Fig. 4 is the schematic diagram of a practical SACH. A small SACH prototype using an inverter-type air-conditioner (heat pump) with rated cooling capacity 2 kW was built and tested in the present study. The cooling capacity of the ejector cooling system (ECS) is 700 W rated at condenser temperature 32 °C, generator temperature 90 °C, and evaporator temperature 7 °C. Table 1 shows the specification of the air conditioner used in the SACH. The overall system design specification is shown in Table 2.
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Nomenclature COP COPc COPej COPh Pc Pco qc,e
Critical model (double-choking)
Entrainment ratio ω
Tc,c Wc
coefficient of performance coefficient of cooling performance coefficient of ejector cooling performance coefficient of heating performance critical condenser pressure, MPa condenser pressure of limiting condition, MPa cooling capacity, kW
ω
Greek symbol entrainment ratio
x
(single-choking) Sub-critical model Back-flow model (malfunction)
constant
ω <0
critical point
Pc∗
Pco
condensing temperature of heat pump, °C power consumption of inverter-type air conditioner kW
Pc
A test performed for a commercial inverter-type air-conditioner shows that the cooling COP increases at a rate about 0.12 per °C condensing temperature drop. This means that 20 °C reduction in condensing temperature will increase COP by 2.4, about twice the original COP or save about 50% compressor power consumption. To work as a heat pump for heating, the condenser of the R22 heat pump is connected to a 100 L hot water tank inside which a condenser coil made of 30 m copper tube (OD 10 mm) is immersed. A schematic of the experimental system is shown in Fig. 5. The system was installed with 36 T-type thermocouples with an uncertainty of ±0.7 °C, and five pressure transducers with a ±1% uncertainty (see Fig. 5). The power consumption of the compressor was measured by a power meter within a ±1.5% uncertainty. 2.2. Pump-less ejector cooling system
Fig. 1. Double-choking phenomenon of ECS.
The pump-less ejector cooling system was developed by Huang et al. [5,13] by combining the concept of heat-driven pump (HDP)
Fig. 2. Conventional solar cooling system.
Fig. 3. Hybrid solar-assisted cooling/heating system (SACH).
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Fig. 4. The schematic diagram of a practical SACH.
Table 1 Specification of the inverter-type air-conditioner used in SACH. Refrigerant Input voltage Compressor displacement Compressor frequency Compressor input power Rated cooling capacity at 54.4 °C condenser/7 °C evaporator Rated COP
R22 AC 220 V 11.3 c.c./rev 20–80 Hz 0.3–1.1 kW 0.8–2.9 kW 2.6
Table 2 Overall system design specification of SACH. 1. Heat pump (inverter-type) – cooling mode Toshiba HD160X1-S12F Refrigerant Compressor input power, kW Condenser capacity, kW Condensing temperature, °C Evaporator temperature, °C Cooling capacity, kW COPc
R22 0.3 1.7 30 7 1.4 4.66
2. Ejector cooling system Refrigerant Generator temperature, °C Generator heat input, kW Condenser capacity, kW Condensing temperature, °C Evaporator temperature, °C Evaporator capacity, kW COPej
R365mfc 90 3.4 5.1 37 20 1.7 0.5
with the design of ECS. The ECS utilized a multi-function generator (MFG) to eliminate the mechanical pump. The MFG acts as a generator for vapor generation and as a feed pump for returning liquid to the generator. The schematic of the ejector cooling system with multi-function generator (ECS/MFG) is shown in Fig. 6. There are two MFGs (generators) in the ECS/MFG. Each MFG consists of a vapor generator (boiler) and an evacuation chamber. The vapor generator is a heat exchanger like a conventional boiler for heating the liquid in order to generate vapor at high pressure. The evacuation chamber is composed of a cooling jacket and a liquid tank. The cooling jacket provides a cooling effect to depressurize the whole generator in order to intake the liquid from the condenser. The two MFGs (A and B) operate interchangeably through the control of switching valves. The operation of each MFG is divided into four phases: pressurizing, vapor discharge, depressurizing, and liquid intake. While MFG A is operating at
depressurizing, liquid intake, and pressurizing phases, MFG B is operating at the vapor discharge phase. The cooling effect of the ECS/MFG is generated only at the vapor discharge phase. The design of a MFG thus requires the total time duration for depressurizing, liquid intake and pressurizing phases shorter than the period of discharge phase. The pressure variation of two MFGs is shown in Fig. 7. For R365mfc, the ejector design for SACH is based on experimental results [13] and the system design conditions (Table 2) with 2.8 mm for the nozzle throat diameter, 5.1 mm for the nozzle exit diameter, 9 mm for constant-area chamber diameter, 65° for inlet converging angle. The ejector area ratio of constant-area section to nozzle throat is 10.33. This R365mfc ejector has been shown to have better performance than R141b ejector at evaporator temperature higher than 15 °C [13]. The condenser of the ECS was cooled by a compact water cooling tower utilizing cellulose pad as the filling material [14]. For experimental purpose, the ECS was driven by hot steam generated from a steam generator to simulate solar heating. The heating power can be adjusted by regulating the electric power input to a 6 kW electric heater using a SCR and a PID controller in order to obtain a designated generator temperature of ECS. 2.3. Indoor and outdoor chamber for test In order to test the performance of the SACH at different ambient conditions, an outdoor environmental chamber was built to simulate ambient condition. The internal size of the ambient environmental chamber is 0.85 m 0.70 m 0.60 m which can install the condenser of the air conditioner. A 1.2 kW electric heater with air blower was installed inside the chamber. The air temperature is then controlled by a PID controller through a SCR. The air temperature inside the outdoor chamber can be controlled in the range 5– 40 °C. Another indoor environmental chamber for simulate the conditions of a cooling room or a house was also built. The internal size is L 1.60 m D 0.65 m H 1.50 m which can install the evaporator of the air conditioner. One kilowatt electric heater with hot air blower was installed inside the chamber. The air temperature is controlled by a PID controller with a SCR. The air temperature inside the indoor chamber can be controlled in the range 20–45 °C. 2.4. Central control system design of SACH The central control system of the SACH consists of three subsystems: pump-less ejector cooling system control, room temperature
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Fig. 5. Schematic description of experimental setup.
Fig. 6. ECS with multi-function generator (ECS/MFG).
control through heat pump (compressor speed), and hot water heating control. The whole central control system was built in a microprocessor PIC16F877. The control of ECS for full-cycle operation follows the same method in [13]. The rotation speed of the compressor is automatically varied according to the room temperature. The compressor speed will be increased to provide higher cooling capacity when the room temperature is higher than 25 °C (normal setting value). The relation between the compressor rotation speed and the room temperature is shown in Fig. 8, which is taken from the supplier of the air conditioner. For heating mode, the compressor runs in full speed (80 Hz) and the temperature sensor installed in the water tank is
used to switch ON/OFF the heat pump according to the setting temperature. For system testing, the central control system performs time scheduling control for room cooling mode starting at 9 AM till 4 PM and water heating mode starting from 6 PM till 10 PM. To shift the electricity load of the SACH to midnight to reduce the peak load problem, the present SACH also installed an ice storage device in parallel with the evaporator of the air conditioner. The ice storage capacity is 2 RT-H using a 100 L ice tank. The ice making is scheduled to start at 10:30 PM till 7:30 AM. An additional indoor air heat exchanger to deliver cooling capacity to room from the ice storage tank is designed with a cold water circulation system. The
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Fig. 7. Pressure variation of two MFGs.
5
2.5
COPc Cooling capacity (qc,e) Power consumption (Wc )
4
2
1.5
2
1
1
0.5
kW
COPc
3
Fig. 8. Relation between the compressor rotation speed and the room temperature.
cooling capacity provided by the ice storage is around 800 W. This makes the maximum total cooling capacity of the SACH to be 2.2 kW including ice melting.
0 0
20
40
60
80
0 100
Compressor frequency (Hz) 3. Test results of SACH
Fig. 9. Cooling performance test at various compressor speed.
3.1. Cooling performance test at steady state for air conditioner alone The COPc of the air conditioner is defined as the ratio of the cooling capacity at the evaporator, qc,e, to the power input of the air conditioner, Wc.
COP c ¼
qc;e Wc
temperature varies from 37.1 °C to 49.3 °C, and the cooling capacity from 0.79 to 2.08 kW, input power varies from 0.28 to 1.12 kW, COPc from 2.82 to 1.86. 3.2. Startup test at fixed cooling load
ð1Þ
For air conditioner alone, the cooling performance test at fixed compressor speed (80 Hz) was run first at ambient temperature 35 °C. The input power is 1.12 kW and the cooling capacity is 2.08 kW at condensing temperature 49.3 °C and evaporating temperature 3.6 °C. The cooling COPc is 1.86. The vapor exhaust temperature of the compressor is 62 °C. The vapor at the evaporator exit is 7 °C superheated. Similar tests were run with different compressor speed. The results of Fig. 9 show that the cooling capacity as well as the input power increases with increasing compressor speed. Since the rate of change with compressor speed for the cooling capacity is larger, the cooling COP decreases with increasing compressor speed. At 20–80 Hz, the corresponding condensing
To test the performance of compressor speed variation during startup, the air conditioner alone is run at 1 kW cooling load and at 35 °C ambient temperature and 25 °C indoor temperature. Fig. 10 shows that the compressor run at 77 Hz at beginning then continues to vary the speed for about 4 min with maximum current 4 A. The compressor speed as well as input power decreases when the indoor temperature approaching 25 °C (setting value). The measured overall COPc during the start period is 2.11. 3.3. Heating performance test The heating performance of the SACH for water heating is run at ambient temperature 25 °C and full compressor speed (80 Hz). The
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Temperature ( oC)
Cold dowm
Steady state
60 40
Indoor environmental temperature
20 25 oC
0
Current (A)
6 4 2 0 0
600
1200
1800 Time (s)
2400
3000
3600
Fig. 10. Startup test at fixed cooling load. Fig. 12. Variation of COPh with ambient temperature.
120
100
Temperature (ºC)
ature is 16.6 °C, the generator temperature is 90 °C, and the condensing temperature is 33 °C. Table 3 summarizes the cooling performance of the air conditioner alone and the SACH at compressor speed 80 Hz. It is seen that the cooling COPc increases from 1.86 to 2.5, about 34%. This is mainly due to the decrease in condensing temperature of the heat pump. Fig. 13 shows that the SACH can provide a continuous cooling performance of ECS at compressor speed 80 Hz. Fig. 14 is the comparison of the thermodynamic cycle of the two operations.
Compressor outlet Compressor inlet Condenser Evaporator Water tank (top) Water tank (middle) Water tank (bottom)
80
60
40
3.5. Overall cooling performance test of SACH and energy saving 20
0
0
1200
2400
3600
Time (s) Fig. 11. Heating performance test at 25 °C ambient temperature.
initial water temperature in tank is 27 °C. Fig. 11 shows that it takes about 60 min to reach 57 °C and the heating COPh is 2.94, energy consumption is 0.0115 kW h per liter hot water. The temperature rise speed is 0.47 °C/min and the highest temperature at compressor exhaust is 70 °C. Fig. 12 shows the variation of COPh with ambient temperature from 5 °C to 35 °C and COPh is in the range 2.0–3.3. The heating performance is satisfactory [15]. 3.4. Cooling performance test of SACH at fixed compressor speed For the performance test of SACH, the pump-less ejector cooling system is first tested with the heat pump working at fixed compressor speed (80 Hz) and at ambient temperature 35 °C. At steady state, the input power is 0.98 kW and the cooling capacity is 2.44 kW at condensing temperature 42 °C and evaporating temperature 2.5 °C. The cooling COPc of SACH is 2.50. The vapor temperature at the compressor exhaust is 49 °C. The vapor at the evaporator exit is 5 °C superheated. The vapor temperature at the compressor exhaust is 49 °C. For the ECS, the condensing temper-
It needs to understand the amount of reduction in input power of the heat pump in the operation of SACH due to the heat-driven ECS. We run the SACH at 35 °C ambient temperature and at different compressor speeds. The results are shown in Fig. 15. In Fig. 15, the cooling performance for air conditioner alone is compared with SACH. It is seen that energy saving is achieved for all compressor speed, especially at lower frequency. Table 4 summarizes all the test results and comparison. It is seen that, at compressor speed 20–80 Hz, for the ECS operated at 90 °C generator temperature and 37 °C condensing temperature, the corresponding condensing temperature of the SACH is 24.5– 42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11–2.50. This indicates that the use of ECS in SACH can reduce the condensing temperature of the heat pump
Table 3 Cooling performance of the air conditioner alone and SACH at compressor speed 80 Hz.
Ambient temperature, °C Condensing temperature, °C Evaporating temperature, °C Vapor temperature at compressor exit, °C Degree of superheating at evaporator exit inlet, °C Input power, kW Cooling capacity, kW COPc
Air conditioner alone
SACH
35 49.2 3.6 62 7 1.12 2.08 1.86
35 42 2.5 49 5 0.98 2.44 2.5
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120
3
80
q c,e (kW)
Temperature (ºC)
SACH air conditioner alone
MFG_B temperature
MFG_A temperature
Generator temperature
2 1 0 1.5
40
W c (kW)
Condenser temperature Evaporator temperature
0 0
600
1200
1800
2400
3000
3600
SACH air conditioner alone
1 0.5
Time(s) 0 6
Fig. 13. Continuous cooling performance of ECS at compressor speed 80 Hz.
COP c
SACH air conditioner alone
4000
3
2000 1800 1600 1400 1200 1000
2
4 2 0 80
31 21
T c,c (oC)
Pressure (kPa)
Combined ejector No ejector
800 4
600
1
41
SACH air conditioner alone
60 40 20
11
0
20
400
40
60
80
100
Compressor frequency (Hz) Fig. 15. Performance of SACH at different compressor speeds.
200 150
200
250
300
350
400
450
Enthalpy (kJ/kg) Fig. 14. Comparison of the thermodynamic cycle of the two operations.
by 12.6–7.3 °C and reduce the compressor power consumption by 81.2–34.5%.
3.6. Complete system test of SACH The SACH built in the present study includes a central control system to perform a full-day test at various modes, including water
heating, space cooling, and ice storage, etc. The simulated ambient temperature is 35 °C during daytime and 30 °C at night. The cooling load at daytime is fixed at 1.5 kW. The continuous test run has been carried out to simulate a practical application of SACH. Fig. 16 shows the test results. It is seen from the compressor current variation that the central control system runs very well. During the period 9:00–11:30 AM, the cooling capacity is mainly supplied by ice melting and the rather high electric current (3.5 A) is due to the additional power consumption of the circulation pump (0.76 kW) installed in the ice storage system. The SACH takes over the cooling load supply after 11:30 AM and the reduction in compressor input power (2 A current) is clearly observed.
Table 4 Performance comparison of air conditioner alone and SACH. Outdoor temperature 35 °C; room temperature 25 °C Compressor speed (Hz)
Condensing temperature of heat pump, Tc,c (°C) Air conditioner alone, Tc,c1
20 40 60 80
20 40 60 80
37.1 42.1 45.8 49.3 Power consumption
SACH, Tc,c2
Temperature drop of SACH Tc,c1 Tc,c2 (°C)
24.5 12.6 34.0 8.1 38.2 7.6 42.0 7.3 of compressor, Wc (kW)
Cooling capacity, qc,e (kW) Air conditioner alone, qc,e1
SACH, qc,e2
Increase = (qc,e1 qc,e2)/ qc,e1, (%)
0.79 1.45 1.81 2.08 COPe
1.02 1.70 2.12 2.44
29.4 17.4 17.2 17.1
Air conditioner alone, Wc1
SACH, Wc2
Reduction = (Wc1 Wc2)/Wc1, (%)
Air conditioner, COPc1
SACH, COPc2
Increase = (COPc1 COPc2)/ COPc1, (%)
0.28 0.52 0.78 1.12
0.20 0.45 0.68 0.98
28.6 13.5 13.5 12.9
2.82 2.79 2.32 1.86
5.11 3.78 3.14 2.50
81.2 35.6 35.4 34.5
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In a complete SACH system test run at outdoor temperature 35 °C, indoor temperature 25 °C and compressor speed 20–80 Hz, and for the ECS operating at 90 °C generator temperature and 37 °C condensing temperature, the corresponding condensing temperature of heat pump in the SACH is 24.5–42 °C, cooling capacity 1.02–2.44 kW, input power 0.20–0.98 kW, and cooling COPc 5.11– 2.50. This indicates that the use of ECS in SACH can reduce the condensing temperature of the heat pump by 12.6–7.3 °C and reduce the power consumption by 81.2–34.5%. The present study also developed a central control system for the SACH to operate at different modes, including hot water heating, space cooling, and ice storage at night. It is shown that the full-day system performance is satisfactory. The performance of the SACH developed in the present study can be further improved by better system matching design. The rated cooling capacity of the ECS used in the present SACH is rather small compared to the size of heat pump which is the minimum size commercially available. This results in a less energy saving at higher compressor speed during high solar radiation periods. The heat source used in the present SACH is supplied from an electric heater. In the future, we will build a solar heating system using vacuum-tube collector and test with the SACH to demonstrate a real solar cooling/heating application.
Melting ice and Air condition
Freeze
Heating
Temperature (ºC)
80 60 Compressor outlet
40 20 Compressor Inlet
0
Temperature (ºC)
-20 60
Water tank (average)
40
20 Ice storage tank
Current (A)
0 10 8 6
Acknowledgment 4 2 0 18:00
22:00
2:00
6:00
10:00
14:00
18:00
This publication is based on the work jointly supported by Award No. KUK-C1-014-12, made by King Abdullah University of Science and Technology (KAUST) and the Project No. 97-D0137-1 made by Energy Bureau, Ministry of Economic Affairs, Taiwan.
Time Fig. 16. Complete system test of SACH.
The water heating mode runs from 18:00 PM at water temperature 27 °C and stops at 19:20 PM with hot water temperature reaching 57 °C. The measured heating COPh is 2.90 and the energy consumption is 0.0137 kW h per liter of hot water. The water temperature rise speed is 0.432 °C/min. 4. Discussion and conclusions A solar-assisted ejector cooling/heating system (SACH) was developed in this study. The SACH combines a pump-less ejector cooling system (ECS) with an inverter-type heat pump (R22) and is able to produce a stable capacity for space cooling. The cooling effect of ECS is generated by solar heat and is used to cool the condenser of the R22 heat pump to increase its COP and reduce the energy consumption of the compressor by regulating the rotational speed of the compressor using a control system. The ECS uses a multi-function generator (MFG) as a vapor generator and a thermal pumping device to replace the mechanical circulation pump for refrigerant circulation. The experiment of ECS operating at full-cycle using R365mfc has shown that the COPe of ECS can reach 0.49 with cooling capacity of 1.57 kW at generator temperature 90 °C, condenser temperature 35.4 °C, and evaporator temperature 19.4 °C (to cool the condenser of the heat pump).
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