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Performance test of solar-assisted ejector cooling system
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Performance test of solar-assisted ejector cooling system Bin-Juine Huang*, Wei-Zhe Ton, Chen-Chun Wu, Hua-Wei Ko, Hsien-Shun Chang, Hang-Yuen Hsu, Jen-Hao Liu, Jia-Hung Wu, Rue-Her Yen Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
article info
abstract
Article history:
A solar-assisted ejector cooling/heating system (SACH-2k) is built and test result is
Received 10 February 2010
reported. The solar-driven ejector cooling system (ECS) is connected in series with an
Received in revised form
inverter-type air conditioner (IAC). Several advanced technologies are developed in SACH-
25 March 2013
k2, including generator liquid level control in ECS, the ECS evaporator temperature control,
Accepted 22 June 2013
and optimal control of fan power in cooling tower of ECS.
Available online 12 December 2013
From the field test results, the generator liquid level control performs quite well and keeps stable performance of ejector. The ECS evaporator temperature control also
Keywords:
performs satisfactorily to keep ejector performance normally under low or fluctuating solar
Solar refrigeration
radiation. The fan power control system cooling tower performs stably and reduces the
Ejector
power consumption dramatically without affecting the ECS performance. The test results
Solar energy
show that the overall system COPo including power consumptions of peripheral increases
Energy saving
from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43%. The highest COPo is 4.5. ª 2013 Elsevier Ltd and IIR. All rights reserved.
Test de performance d’un syste`me de refroidissement a` e´jecteur fonctionnant partiellement a` l’e´nergie solaire Mots cle´s : Froid solaire ; Ejecteur ; Economie d’e´nergie ; Energie solaire
1.
Introduction
The solar heating system built for heating purpose in winter may produce too much heat in summer and could even
damage the system if it was not properly protected. Solar cooling technology is thus promising in converting excess solar heat into cooling for space cooling in summer. Ejector cooling system (ECS) using low boiling point refrigerant is suitable for solar cooling application due to its simple design
* Corresponding author. E-mail addresses: [email protected], [email protected] (B.-J. Huang). 0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.06.009
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Nomenclature COPo
overall coefficient of performance (COP) of solar-assisted cooling system, including power consumption of peripheral, dimensionless solar radiation intensity incident upon the IT collector slope, W m2 ITf filtered solar radiation intensity incident upon the collector slope, W m2 QA heat transfer rate of auxiliary heater, W condenser heat rejection rate, W Qc cooling load or evaporator heat transfer rate, W Qe cooling capacity of ejector cooling system (ECS), QECS W generator heat input in ejector cooling system, Qg W cooling capacity of inverter-type air QIAC conditioner, W cooling load of cooling room, W QL ambient temperature, C Ta condenser temperature of ECS, C Tc critical condensing temperature of ECS, C Tc evaporator temperature of ECS, C Te setting value of ECS evaporator temperature, C Te, set generator temperature of ECS, C Tg collector inlet temperature, C Ti room temperature, C Troom power consumption of IAC, W WIAC Wsolar pump pumping power of solar collector pump, W power consumption of cooling tower including WCT fan and water pump, W Wfreon pump power consumption of circulation pump of ECS, W h solar collector efficiency
and low cost. Many researches on ejector cooling technology driven by low-grade energy have been carried out extensively during the past (Huang and Chang, 1999; Huang et al., 1999; Nguyen et al., 2001; Huang et al., 2006; Sokolov and Hershgal, 1990a,b, 1991; Arbel and Sokolov, 2004; Sun, 1997, 1998; Huang et al., 2001). The technology is becoming mature for commercialization. The COP (coefficient of performance) of ECS can reach 0.5 or higher (Huang and Chang, 1999; Huang et al., 1999) which is competitive to the absorption or adsorption system that is much more complicated in design and more expensive. If the ECS was driven by solar thermal energy, it requires a back-up heater to make up the heat in order to keep a constant
Fig. 1 e Conventional solar cooling system.
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cooling capacity for space cooling during cloudy or rainy periods (Fig. 1). 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 New Energy Center at National Taiwan University has been devoted to the development of solar-assisted ejector cooling/heating system (SACH). Two types of SACH were developed. The SACH consists of a conventional inverter-type air conditioner (IAC) made of variable-speed compressor which is connected in series or parallel with a solar-heated ejector cooling system. The first system SACH-1 is in series configuration as shown in Fig. 2 (Huang et al., 2010a). SACH-1 combines a pumpeless ejector cooling system with an inverter-type air conditioner to provide a stable space cooling. The ECS is driven by solar heat and is used to sub-cool the condenser of the IAC to increase its COP and reduce the power input of the compressor by automatically regulating the rotational speed of the compressor. The cooling load is supplied by the IAC only. A SACH-1 was built and run at outdoor. It shows that the use of ECS in SACH-1 can effectively reduce the condensing temperature of the heat pump by 7.3e12.6 C and reduce the input power of compressor by 34.5e81.2% (Huang et al., 2010a). The other system SACH-2 is in parallel configuration as shown in Fig. 3 (Huang et al., 2010b). In SACH-2, the ECS is driven by solar heat and connected in parallel with an IAC. The cooling load is supplied by ECS when solar energy is available and the input power of IAC can be reduced. In variable weather, ECS will probably operate at off-design condition of ejector and the cooling capability of ECS can be lost completely. In order to make the ejector operate normally at critical or non-critical double-choking condition to obtain a better performance, Huang et al. (2010b) further added a control system in SACH-2 to control the evaporating temperature of ECS to avoid ejector failure during unsteady or low solar radiation. This makes the ECS always produce cooling effect even at low solar radiation periods while the ejector performs at off-design conditions. The energy saving of IAC is experimentally shown 50e70%, not including power consumptions of solar heating system, refrigerant pump of ECS, and fan and pump in water condenser of ECS. The long-term performance test results show that the daily energy saving is around 30e70% as compared to the energy consumption of IAC without solar-driven ECS. The total energy saving of IAC is 52% tested from September 7, 2009 to March 20, 2010. Huang et al. (2012) further developed a maximum-power point tracking control technology (MPPT) for solar heating system to minimize the pumping power consumption with optimal heat collection. The proportional-integral (PI) feedback tracking control system was developed with a tracking filter to determine the instantaneous optimal tracking target online. The test results show that the mass flowrate for different days is between 18.1 and 22.9 kg min1 with pumping power between 77 and 140 W, which is greatly reduced from the standard design value (450 W) for the total collector area 25.9 m2. SACH-2 is designed in parallel configuration whose ECS may operate at a very low COP at high condensing temperature when ambient temperature is high, even using water cooling. This will cause very low system efficiency of SACH-2
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Fig. 2 e The schematic diagram of SACH-1 (Huang et al., 2010a).
and not economical when it is used in hot climate. Besides, the power consumptions of peripheral in SACH-2 need to be reduced in order to make this technology economical. The present study further modifies the design of SACH-2 using series configuration similar to SACH-1 (Huang et al., 2010a) but using a refrigerant pump in ECS and several advanced control technologies to keep stable performance of ECS and minimize power consumption of peripherals. This type of SACH is called “SACH-k2”. In series configuration of SACH, the ECS is used to cool the condenser of the IAC. The ECS does not directly provide cooling effect to the cooling room. Hence, the design of evaporator temperature of ECS can be raised and the COP of ECS can be increased. In SACH-k2, the cooling load is steadily supplied by IAC. The solar-driven ECS acts as a device for energy saving when solar energy is available. Several advanced technologies have been developed in SACH-k2: (1) the refrigerant pump used in ECS requires a generator liquid level control in ECS to eliminate large level
variation during variable solar radiation and keep stable performance of ejector; (2) the fan power consumption in cooling tower of ECS needs to be reduced. The present study focuses on the development of these technologies and running a field test to measure the overall performance efficiency of SACH-k2, including energy consumption of all the peripheral equipment.
2.
Experimental setup
SACH-k2 was developed using several advanced technologies. The design of SACH-k2 includes: (1) system configuration of SACH-k2; (2) solar heating system; (3) ejector cooling system (ECS); (4) IAC retrofitting; (4) cooling space.
2.1.
System configuration of SACH-k2
SACH-k2 consists of 3 major parts: a solar heating system, an ejector cooling system (ECS) with water-cooled condenser,
Fig. 3 e Solar-assisted cooling/heating system in parallel configuration (SACH-2) (Huang et al., 2010b).
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Fig. 4 e Schematic diagram of SACH-k2.
and an inverter-type air conditioner (IAC). The schematic diagram is shown in Fig. 4. ECS is coupled with IAC at the intercooler, which is the evaporator of ECS or the sub-cooler of IAC. The cooling load of the cooling room is designed at 3.5 kW (1RT) which matches the rated cooling capacity of the IAC used. The cooling capacity of the ECS is 5.6 kW which is able to cool the condenser of the IAC at a lower temperature to increase the COP of IAC. The system specification is shown in Table 1. R245fa is used as the working fluid of ECS.
2.2.
Solar heating system
The solar heating system used in SACH-k2 is the same as that used in the study of MPPT (maximum-power-point tracking) control of pump (Huang et al., 2012). The solar heating system consists of 24 flow-through vacuum-tube collectors (Model EZL100-6) with 26 m2 total absorber area. Eight collectors are connected in series and three in parallel. Glycol solution is
pumped from the buffer tank through the solar collector and absorbs solar energy to heat the generator of ECS. The flow then returns to the buffer tank as shown in Fig. 4. An inverter for rotational speed control of the circulation pump was installed and a PC-based control system was developed for the MPPT control of the circulation pump. This solar heating system can supply hot water at temperature in the range 70e130 C to drive the ejector cooling system. A buffer tank (200 L) is used as a storage for stable pumping. The test of solar collector shows that the thermal efficiency of the solar heating system is 0.6 at water inlet temperature 100 C (Huang et al., 2010a,b).
2.3.
Ejector cooling system (ECS)
The flow diagram of ECS is as shown in Fig. 4. ECS and IAC are linked at the intercooler which is the evaporator of ECS and the condenser of IAC. The cooling capacity of ECS is 5.6 kW rated at condenser temperature 40 C, generator
Table 1 e System design specification of SACH-k2. Design specification of IAC IAC model: Refrigerant: Condenser temperature, C Evaporator temperature, C Condenser heat rejection, kW Cooling capacity, kW Compressor input, kW COPIAC Design specification of ECS Refrigerant Generator temperature, C Condensing temperature, C Evaporator temperature, C Generator heat input, kW Condenser capacity, kW Evaporator capacity, kW COP
MA732BVY8 R22 54 8 8 1.2e4.5 0.4e1.125 3.25 R245fa 100 40 20 8.9 14.6 5.6 0.63
Table 2 e Design specification of ECS. Refrigerant Cooling capacity (kW) Operating temperature Generator temperature ( C) Evaporator temperature ( C) Condenser temperature ( C) Flowrate Primary flowrate (kg s1) Entrained flowrate (kg s1) Compression ratio Entrainment ratio Heat transfer rate Generator (kW) Evaporator (kW) Condenser (kW) COP
R245fa 5.6 100 20 40 0.030 0.022 2.04 0.73 8.9 5.6 14.5 0.63
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Table 3 e Ejector design. Nozzle
Constant-area section
Area ratio
Throat dia. (Dt)
Exit dia. (D1)
Dia. (D3)
Entrance angle
A3/At
2.72 mm
5.52 mm
8.71 mm
65
10.33
temperature 100 C, and evaporator temperature 20 C. R245fa is chosen as the working fluid and the estimated COP of ECS is 0.63, not including pumping power. The ECS design specification is shown in Table 2. The ejector design using 1D ejector model (Huang et al., 1999) for ECS of SACH-k2 is shown in Table 3.
2.3.1.
Control of generator liquid level
The generator of ECS is designed in shell-tube type as shown schematically in Fig. 5. Liquid refrigerant (R245fa) is in the shell side and the heating fluid (glycol water from solar collector) is in the tube side. Refrigerant vapor is generated in the shell side to drive the ejector. Instantaneous variation of solar radiation intensity may result in large variation of liquid level in the generator of ECS. Conventional high-low level control (ON/OFF) will cause large variation in generator temperature/ pressure and unstable performance of ECS. A level control system is thus developed in the present study. A refrigerant gear pump with variable-speed motor (300e2000 rpm, maximum power input 750 W, maximum pumping head 140 kg cm2) was used for liquid level control in generator. The feedback control system uses a float-type magnetic liquid level gage to measure the level and to vary the flowrate by regulating motor speed of pump to control the level. The liquid level should be controlled at certain level of accuracy in order to provide stable pressure to the ejector. The liquid level should be higher than the heating tubes to ensure better heat exchanging. The level control system will accurately control liquid level at 12 1 cm from the bottom of the generator. Fig. 6 is the feedback control system to adjust the inlet liquid flowrate to the generator. The proportionalintegral control algorithm (PI) was implemented in the controller.
Fig. 5 e Schematic of generator design.
Fig. 6 e Feedback generator level control system.
2.3.2.
Cooling tower design
SACH-k2 uses water cooling for the condenser of ECS. A highperformance water cooling tower was used in the present study. The design follows the research results of Hu and Huang (2005). Cellulous pads (0.45 m 0.45 m 0.6 m) were used to construct the cooling tower matrix. The air fan consumes 380 W at full speed with flowrate 1.08e1.25 m3 s1. The water circulation pump consumes 45 W at flowrate 32e38 L min1 and 3.8e5.4 mH2O head. The schematic of water cooling tower is shown in Fig. 7. The real hardware is shown in Fig. 8.
2.3.3. tower
Power consumption control system of ECS cooling
SACH-k2 uses water cooling for condenser of ECS as described above. Fan and pump will consume electrical energy during operation (Fig. 7). ECS must be operated at a condenser temperature Tc below the critical temperature Tc to keep the ejector working under double-choking condition (Huang et al., 1999). Usually, the operating point of Tc is always below the critical temperature Tc . This means that Tc can be raised to Tc without decreasing cooling effect of ECS. In the other words, the fan or pump speed can be reduced to increase the cooling water temperature as well as the condenser temperature to save energy. The condensing pressure will increase with decreasing fan speed as well as input power without changing the cooling effect of ECS (entrainment ratio of ejector) until the condensing pressure larger than the critical condensing pressure. An feedback control system using a micro-processor is designed which will reduce the fan speed but still keeping the ejector working at double-choking condition. An inverter is connected to the fan motor so that it can be triggered by the
Fig. 7 e Schematic of water cooling tower.
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Fig. 10 e Decomposition of filter F in the feedback tracking control system.
Fig. 8 e Water cooling tower.
controller. The step-down algorithm was used to adjust the fan speed gradually from the maximum value until the evaporator temperature of ECS starting to rise.
2.3.4.
Evaporator temperature control of ECS
In ECS, the condensing temperature must be lower than the critical condensing temperature (critical point) such that the ejector can operate at double-choking condition (Huang et al., 1985). For an ejector with fixed geometry, the critical condensing temperature depends on the evaporator temperature and the generator temperature (Huang et al., 1985) which varies with solar radiation intensity. The ejector of ECS will probably operate at off-design conditions due to the variation of solar radiation intensity. Huang et al. (2010b) have developed an optical control technique to make the ejector operate at critical or noncritical double-choking condition to obtain a better performance under variable solar radiation. The same technology is used in SACH-k2 but with some adjustment of parameters. An electronic expansion valve was installed in the suction line of the ejector (at the evaporator suction inlet) to regulate the opening of the expansion valve to control the evaporator temperature. A feedback tracking control system was then designed to adjust the evaporator temperature which may vary with instantaneous solar radiation intensity IT(t), as shown in Fig. 9. A filter F is needed to convert the signal IT(t) into the setting value of evaporator temperature Te, set(t) for tracking control as shown in Fig. 10.
Fig. 9 e Feedback control of evaporator temperature.
Since the setting of evaporator temperature Te, set directly depends on the generator temperature Tg. A converter (Te signal converter) can be defined as a functional relation of generator temperature, Fe. The generator temperature will further depend on the solar radiation intensity. Another converter (Tg signal converter) can be defined as a functional relation of solar radiation intensity incident upon solar collector ITf, Fg. The two signal converters Fe and Fg can be determined from the field test of the SACH-2k. The low-pass filter Fs in Fig. 10 is used to filter the fast variation of solar radiation signal, i.e. low pass, since only lowfrequency content of the solar radiation variation will affect the response of the generator temperature. Hence, the filter Fs is designed as a low-pass filter using moving average, MA filter, with fixed time interval 8 min. To determine the two signal converters Fg and Fe, SACH-2k was run outdoor continuously and the electronic expansion valve was adjusted manually to regulate the evaporator temperature according to the generator temperature such that the ECS still produce cooling effect at off-design condition. From the energy balance between the solar heating system and the generator, the determined generator temperature depends on the solar radiation intensity. Fig. 11 shows that Tg varies linearly with solar radiation intensity ITf. Each data points are taken from steady-state performance at about 20e30 min time interval. A linear relation is derived for the Tgsignal converter Fg as Tg ¼ 0:0292 Itf þ 74:327
(1)
Fig. 12 shows that Te decreases linearly with increasing Tg. A linear relation is derived for the Te-signal converter Fe as Te ¼ 0:5549Tg þ 70:527 dTc ; dTc ¼ 36:7 Tc
(2)
The controller C(s) of the tracking feedback control system uses the proportional control algorithm with proportional gain Kp ¼ 0.33.
Fig. 11 e Variation of Tg with irradiation.
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30
Table 5 e Design of intercooler.
Te (oC)
25
Tube side (water)
20
Temperature ( C) Pressure (MPa) Flowrate (kg s1) Enthalpy (kJ kg1) Max heat rate (kW)
15 10 5
Shell side (R245fa)
Inlet
Outlet
Inlet
Outlet
25 0.1013 0.26 104.83
35 0.1013 0.26 146.63
78 0.143 0.052 462.43
40 0.121 0.052 252.57
10.9
0 80
85
90
95
100
Tg (oC)
Fig. 12 e Variation of Te with Tg.
2.4.
Inverter-type air conditioner (IAC)
The inverter-type air conditioner (IAC) used in SACH-k2 is purchased from market then retrofitted in the lab. MA732BVY8 made by TECO Co was selected. The IAC can supply cooling load 1.2e4.5 kW at different frequency. The specification of IAC is shown in Table 4. The intercooler, which is the evaporator of ECS or condenser of IAC, is installed between the condenser and the evaporator of IAC (Fig. 4) to sub-cool the IAC refrigerant. Refrigerant R22 is then recharged after retrofitting. The design of intercooler is shell-tube type and the design specification is shown in Table 5. The indoor air cooler is installed in the cooling room. The outdoor unit of IAC is installed inside the package of ECS.
2.5.
Cooling room
3.
Test results of SACH-k2
SACH-k2 is tested outdoor with solar heating. The performance of generator liquid level control, power consumption of ECS cooling tower, evaporator temperature control of ECS, and overall system performance were measured.
3.1.
The cooling room is a small concrete room with inside dimension L2.1 m H3.2 m W2.35 m. A calibration was run to measure the variation of heat load of the room with indoor/ outdoor temperature difference. The heat load was determined by directly measuring the cooling capacity of the indoor evaporator of IAC and the temperature difference between indoor and outdoor (Fig. 13). It is seen that the heat load of the room is not high enough to match the cooling capacity of IAC (3.5 kW). Hence, a 2.6 kW electric heater was put inside the room to supply additional heat to the room during experiment.
2.6.
installed inside the cooling room with refrigerant line connected to the package. Solar hot water is circulated through the package to heat the ECS. A PC-based control and monitoring system was developed to control the operation of cooling room, solar heating system, ECS, IAC, fans and pumps etc. Test data were recorded every 10 s. The PC-based control system is also used to carry out the optimal power consumption control of ECS cooling tower (fan and pump) and generator liquid level of ECS, and pumping power of solar heating system.
Generator liquid level control of ECS
The liquid level control test in ECS was performed with solar heating. In a variable weather with high fluctuation of solar irradiation, the liquid level was controlled between 11 cm and 12.5 cm for the level setting at 12 cm, about 8 to 4% error, as shown in Fig. 15. Only slight level variation occurs during the abrupt change of solar irradiation. In partly-cloudy weather with moderate fluctuation of solar irradiation, the liquid level was controlled between 11.6 and 12.4 cm for the level setting at 12 cm, about 3% error, as shown in Fig. 16. This indicates that the liquid level control system is robust with respect to the abrupt change of solar radiation.
System integration of SACH-k2
SACH-k2 is designed in package in which IAC and ECS are packed together as shown in Fig. 14. Indoor air fan unit is
Table 4 e Specification of IAC. IAC model Refrigerant Condensing temperature, C Evaporating temperature, C Condenser heat rejection, kW Cooling capacity, kW Compressor input, kW COPIAC
MA732BVY8 R22 54 8 5.6 1.2e4.5 0.4e1.125 3.25
Fig. 13 e Heat load of cooling room.
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3.2.
Fig. 14 e Package of SACH-k2.
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Cooling tower power consumption of ECS
The power consumption of fan in water cooling tower is 630 W at full speed. The fan control system will reduce the fan power consumption without affecting the cooling performance of ECS. Fig. 17 shows that the fan power consumption drops from 630 W to 150 W without affecting the performance (condenser and evaporator temperature stay approximately steady). At the end of test near 13:20, while the solar radiation drops, the fan power is reduced to 110 W at condenser temperature around 39 C. The average power consumption during the test period 11:00e13:15 is 155 W which means 75% energy reduction. Fig. 18 shows that the fan power consumption drops to 90 W (minimum) in a clear weather. The average power consumption during the test period 11:30e12:50 is 124 W, which
Fig. 15 e Test result of level control in variable weather.
Fig. 16 e Test result of level control in partly-cloudy day.
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Fig. 17 e Test result of cooling tower fan control.
means 80% energy reduction. But ESC can still operate under critical mode.
3.3.
Evaporator temperature control of ECS
In order to make ejector operate under critical or non-critical double-choking condition, an electronic expansion valve was installed at the evaporator inlet to regulate the opening of the expansion valve to control the evaporator temperature according to the variation of solar radiation. The feedback tracking control system is as shown in Fig. 9.
The test result shown in Fig. 19 was performed at average ambient temperature 34 C and cooling room temperature 26.9 C with additional electric heating load 2.6 kW. It is seen that the tracking of Tg and Te was satisfactory. A larger control error on Tg was observed as solar radiation drops and varies. The control error of Te is less than 1.5 C at stable solar radiation and about 3 C at abrupt solar radiation change. Fig. 20 shows that at solar irradiation about 790W m2, the power consumption of IAC decreases from 992 W to 512 W, a reduction of 48%, when the solar-driven ECS is activated and operating at Tg ¼ 96 C and Te ¼ 20 C, and IAC condenser
Fig. 18 e Test result of cooling tower fan control.
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Fig. 19 e Performance test of SACH-k2.
temperature at 38 C. The ECS is shown still running at low and variable solar radiation. The result shown in Fig. 21 was from the test performed at average ambient temperature 28 C and cooling room temperature 26.1 C with additional electric heating load 2.6 kW. It is seen that the tracking of Te was satisfactory. The ECS is running well at the periods of unstable solar radiation.
3.4.
Overall system performance of SACH-k2
The system performance test of SACH-k2 was carried out with all the implemented control systems. Cooling fans, evaporator temperature and MPPT of water pump of solar heating system
were optimized. Fig. 22 shows that the power consumption of IAC alone was 1087 W. Additional electric heating load 2.6 kW in the cooling room was applied during the test. The power consumption of IAC dropped to 600 W in average after ECS was turned on. Power consumption of IAC saved 44.8%. Fig. 23 shows that the power consumption of IAC alone was 1030 W with an electric heating load 2.6 kW in the cooling room applied during the test. The power consumption of IAC dropped to 561 W in average during 11:00e13:15 after ECS was turned on. Power consumption of IAC saved 45.6%. Fig. 24 shows the performance of SACH-k2 in cloudy day. In the beginning, the power consumption of IAC was 1030 W. Additional electric heating load 2.6 kW in the cooling room
Fig. 20 e Performance test of SACH-k2.
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Fig. 21 e Performance test of SACH-k2.
was applied during the test. The power consumption of IAC dropped to 630 W in average during 14:00e14:50 after ECS was turned on. Power consumption of IAC saved 37.4%. The test result at clear weather is shown in Fig. 25. The average power consumption of IAC alone was 926 W. Additional electric heating load 2.6 kW in the cooling room was applied during the test. The power consumption of IAC then dropped to 485 W in average during 11:40e12:50 after ECS was turned on. Power consumption of IAC saved 47.6%.
In summary, all the test results show that the IAC condenser was sub-cooled about 10e20 C and the average power consumption reduced by 37.4e47.6%. Tables 6e8 summarize the performance of IAC alone and SACH-k2. For the test runs, the generator liquid level control in ECS performs quite well and keeps stable performance of ejector under variable solar radiation. The ECS evaporator temperature control also performs satisfactorily to keep ejector performance normally under low solar radiation. The fan power
Ambient temp: 34oC Cooling room temp: 26.9oC Generator temp Tg
Solar irradiation IAC power consumption
Condenser temp of IAC
ECS evaporator temp Te
Fig. 22 e Test result of SACH-k2.
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heater: 2.6kW Generator temp
Solar irradiation
100
Temperature(oC)
1,200
1,000
80
800
60
600
IAC power consumption
40
400
Condenser temp of IAC 20
200 Ambient temp: 28oC
ECS evaporator temp
Solar radiation(W/m2) / IAC power consumption(W)
SCH-k2 (May 26, 2011)
120
Cooling room temp: 26.1oC 0 10:45:48
0 11:15:51
11:45:32
12:15:12
12:44:54
13:15:03
13:45:09
Fig. 23 e Test result of SACH-k2.
control system in cooling tower of ECS performs stably and reduces the power consumption dramatically. The overall coefficient of performance (COP) of SACH-k2, COPo, including the power consumption of all the peripheral devices is defined as: COPo ¼ QL = WIAC þ Wsolar pump þ WCT þ Wfreon pump
(3)
where WIAC is the power consumption of IAC, Wsolar pump is the pumping power of solar collector pump, WCT is the power
consumption of cooling tower including fan and water pump, Wfreon pump is the circulation pump of ECS. It is seen from Table 8 that COPo increases from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43% increase. The IAC power input decreases about 45%. The solar-driven ECS sub-cools the condenser of IAC about 15 C and improves the performance of IAC. In addition, the power consumption of peripheral devices in SACH-k2 is reduced by using advanced control technology, including the MPPT control of solar heating system (Huang et al., 2012). The highest COPo of SACH-k2
Fig. 24 e Test result of SACH-k2.
184
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May 29, 2011
120
1200 Cooling room temp 23.2oC
Generator temp Tg
800
IAC power consumption
60
600
400
40
Solar irradiation(W/m2) /
80
Temperature(oC)
1000 IAC power consumption(W)
Solar irradiation
100
Condenser temp of IAC 200
20 ECS evaporator temp 0 11:07:04
0 11:38:13
12:09:04
12:40:00
13:10:58
Fig. 25 e Test result of SACH-k2.
Table 6 e Power consumption of peripheral in SACH-k2. Date
Power consumption (W) Solar heating system
Fan of ECS cooling tower
Pump of ECS cooling tower
Refrigerant pump of ECS
Total power of peripheral equipment
115.9 85 111.1 93.4 140.3
139.3 127 155.3 107.8 123.8
28 28 28 28 28
37 45 47 42 52
320.2 285 341.4 271.2 344.1
2011/05/09 2011/05/18 2011/05/26a 2011/05/26b 2011/05/29
is 4.5 which reach the target of solar cooling technology suggested by Wiemken et al. (2010).
4.
Discussion and conclusions
A solar-assisted ejector cooling/heating system SACH-k2 which is in series configuration and uses a circulation pump in ECS is developed in the present study. In SACH-k2, the ECS
is driven by solar heat and produces cooling effect to cool the condenser of IAC. The solar-driven ECS acts as a device for energy saving when solar energy is available. SACH-k2 also requires more advanced technologies for operating control to keep normal performance of ECS under variable solar radiation. Besides, the power consumption of peripheral in SACHk2 needs to be reduced. Several advanced technologies are developed in SACH-k2 including generator liquid level control in ECS, the ECS
Table 7 e Overall system performance of SACH-k2. System
IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2
Date
2011/05/09 2011/05/18 2011/05/26a 2011/05/26b 2011/05/29
IAC intercooler temperature ( C) Inlet
Outlet
38.5 38.6 36.0 36.3 36.6 36.3 34.7 35 32.7 32.5
37.6 23.6 33.0 21.4 35.2 21.5 34.3 22.8 31.7 17.3
Power consumption of IAC (W)
Total power consumption (W)
Total cooling capacity (W)
COPo
1087 600 961 515 1031 561 1006 630 926 485
1087 920 961 800 1031 903 1006 901 926 829
3196 3731 3200 3600 3073 3804 3180 3537 3000 3582
2.94 4.06 3.30 4.50 2.96 4.22 3.16 3.93 3.24 4.32
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Table 8 e Performance of IAC alone and SACH-k2. Date Test condition Ambient temperature, C IAC subcooling, C IAC power, kW Total peripheral power, kW Cooling capacity, kW COPo
05/09 IAC 34 e 1.09 e 3.2 2.94
SACH-k2 15 0.6 0.32 3.7 4.06
05/18 IAC 29.4 e 0.96 e 3.2 3.3
evaporator temperature control, and the optimal control of fan power in cooling tower of ECS. From the results of test runs, the generator liquid level control in ECS performs quite well and keeps stable performance of ejector under variable solar radiation. The ECS evaporator temperature control also performs satisfactorily to keep ejector performance normally under low and variable solar radiation. The fan power control system in cooling tower of ECS performs stably and reduces the power consumption dramatically. The COPo increases from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43%. The IAC power input decreases about 45%. The solar-driven ECS sub-cools the condenser of IAC by 10e20 C and improves the performance of IAC. The highest COPo of SACH-k2 is 4.5 which reach the target of solar cooling technology suggested by Wiemken et al. (2010). The present test results of SACH-k2 indicate that the solar thermally-driven ejector air conditioning technology which integrates different advanced technologies, including MPPT of solar heating system, ECS generator liquid level control, ECS condenser temperature control, fan power control of ECS cooling tower, becomes more mature toward commercialization. Further performance improvement of SACH-k2 is still possible. We found that there is excess cooling capacity generated by ECS during high solar radiation periods, due to fixed heat exchanger design of intercooler. By increasing the size of intercooler, COPo may be increased further. This means that it is possible to obtain a COPo higher than 6.0.
Acknowledgments This publication is based on the work supported by Award No.KUK-C1-014-12, made by King Abdullah University of Science and Technology (KAUST).
references
Arbel, A., Sokolov, M., 2004. Revisiting solar-powered ejector air conditioner-the greener the better. Sol. Energy 77, 57e66.
SACH-k2 15.2 0.51 0.29 3.6 4.50
05/26 IAC 28 e 1.03 e 3.1 2.96
SACH-k2 14.8 0.63 0.27 3.8 4.22
05/29 IAC 25 e 0.93 e 3.0 3.24
SACH-k2 15.2 0.48 0.34 3.6 4.32
Huang, B.J., Jiang, C.B., Hu, F.L., 1985. Ejector performance characteristics and design analysis of jet refrigeration systems. Trans. ASME, J. Eng. Gas Turb. Power 107, 792e802. Huang, B.J., Chang, J.M., 1999. Empirical correlation for ejector design. Int. J. Refrigeration 22, 379e388. Huang, B.J., Chang, J.M., Wang, C.P., Petrenko, V.A., 1999. A 1-D analysis of ejector performance. Int. J. Refrigeration 22, 354e364. Huang, B.J., Petrenko, V.A., Chang, J.M., Lin, C.P., Hu, S.S., 2001. A combined-cycle refrigeration system using ejector-cooling cycles as the bottom cycle. Int. J. Refrigeration 24, 391e399. Huang, B.J., Hu, S.S., Lee, S.H., 2006. Development of an ejector cooling system with thermal pumping effect. Int. J. Refrigeration 29, 476e484. Huang, B.J., Wu, J.H., Hsu, H.Y., Wang, J.H., 2010a. Development of hybrid solar-assisted cooling/heating system. Energy Convers. Manag. 51, 1643e1650. Huang, B.J., Yen, C.W., Wu, J.H., Liu, J.H., Hsu, H.Y., Petrenko, V.O., Chang, J.M., Lu, C.W., 2010b. Optimal control and performance test of solar-assisted cooling system. Appl. Therm. Eng. 30, 2243e2252. Huang, Bin-Juine, Ton, Wei-Zhe, Wu, Chen-Chun, Ko, Hua-Wei, Chang, Hsien-Shun, Yen, Rue-Her, Wang, Jiunn-Cherng, 2012. Maximum-power-point tracking control of solar heating system. Sol. Energy 86, 3278e3287. Hu, S.S., Huang, B.J., 2005. Study of a high efficiency residential split water-cooled air conditioner”. Appl. Therm. Eng. 25, 1599e1613. Nguyen, V.M., Riffat, S.B., Doherty, P.S., 2001. Development of a solar-powered passive ejector cooling system. Appl. Therm. Eng. 21, 157e168. Sokolov, M., Hershgal, D., 1990a. Enhanced ejector refrigeration cycles powered by low grade heat. Part 1. Systems characterization. Int. J. Refrigeration 12, 351e356. Sokolov, M., Hershgal, D., 1990b. Enhanced ejector refrigeration cycles powered by low grade heat. Part 2. Design procedures. Int. J. Refrigeration 12, 357e363. Sokolov, M., Hershgal, D., 1991. Enhanced ejector refrigeration cycles powered by low grade heat. Part 3. Experimental results. Int. J. Refrigeration 14, 24e31. Sun, D.W., 1997. Solar powered combined ejector-vapour compression cycle for air conditioning and refrigeration. Energy Convers. Manag. 38, 479e491. Sun, D.W., 1998. Evaluation of a combined ejector-vapour compression refrigeration. Int. J. Energy 22, 333e342. Wiemken, E., Wewio´r, J.W., Elias, A.P., Nienborg, B., Koch, L., Sept 28eOct 1, 2010. Performance and perspectives of solar cooling, proceedings of EuroSun 2010. In: International Conference on Solar Heating, Cooling and Buildings (Graz, Austria).
Available online at www.sciencedirect.com
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journal homepage: www.elsevier.com/locate/ijrefrig
Performance test of solar-assisted ejector cooling system Bin-Juine Huang*, Wei-Zhe Ton, Chen-Chun Wu, Hua-Wei Ko, Hsien-Shun Chang, Hang-Yuen Hsu, Jen-Hao Liu, Jia-Hung Wu, Rue-Her Yen Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
article info
abstract
Article history:
A solar-assisted ejector cooling/heating system (SACH-2k) is built and test result is
Received 10 February 2010
reported. The solar-driven ejector cooling system (ECS) is connected in series with an
Received in revised form
inverter-type air conditioner (IAC). Several advanced technologies are developed in SACH-
25 March 2013
k2, including generator liquid level control in ECS, the ECS evaporator temperature control,
Accepted 22 June 2013
and optimal control of fan power in cooling tower of ECS.
Available online 12 December 2013
From the field test results, the generator liquid level control performs quite well and keeps stable performance of ejector. The ECS evaporator temperature control also
Keywords:
performs satisfactorily to keep ejector performance normally under low or fluctuating solar
Solar refrigeration
radiation. The fan power control system cooling tower performs stably and reduces the
Ejector
power consumption dramatically without affecting the ECS performance. The test results
Solar energy
show that the overall system COPo including power consumptions of peripheral increases
Energy saving
from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43%. The highest COPo is 4.5. ª 2013 Elsevier Ltd and IIR. All rights reserved.
Test de performance d’un syste`me de refroidissement a` e´jecteur fonctionnant partiellement a` l’e´nergie solaire Mots cle´s : Froid solaire ; Ejecteur ; Economie d’e´nergie ; Energie solaire
1.
Introduction
The solar heating system built for heating purpose in winter may produce too much heat in summer and could even
damage the system if it was not properly protected. Solar cooling technology is thus promising in converting excess solar heat into cooling for space cooling in summer. Ejector cooling system (ECS) using low boiling point refrigerant is suitable for solar cooling application due to its simple design
* Corresponding author. E-mail addresses: [email protected], [email protected] (B.-J. Huang). 0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.06.009
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Nomenclature COPo
overall coefficient of performance (COP) of solar-assisted cooling system, including power consumption of peripheral, dimensionless solar radiation intensity incident upon the IT collector slope, W m2 ITf filtered solar radiation intensity incident upon the collector slope, W m2 QA heat transfer rate of auxiliary heater, W condenser heat rejection rate, W Qc cooling load or evaporator heat transfer rate, W Qe cooling capacity of ejector cooling system (ECS), QECS W generator heat input in ejector cooling system, Qg W cooling capacity of inverter-type air QIAC conditioner, W cooling load of cooling room, W QL ambient temperature, C Ta condenser temperature of ECS, C Tc critical condensing temperature of ECS, C Tc evaporator temperature of ECS, C Te setting value of ECS evaporator temperature, C Te, set generator temperature of ECS, C Tg collector inlet temperature, C Ti room temperature, C Troom power consumption of IAC, W WIAC Wsolar pump pumping power of solar collector pump, W power consumption of cooling tower including WCT fan and water pump, W Wfreon pump power consumption of circulation pump of ECS, W h solar collector efficiency
and low cost. Many researches on ejector cooling technology driven by low-grade energy have been carried out extensively during the past (Huang and Chang, 1999; Huang et al., 1999; Nguyen et al., 2001; Huang et al., 2006; Sokolov and Hershgal, 1990a,b, 1991; Arbel and Sokolov, 2004; Sun, 1997, 1998; Huang et al., 2001). The technology is becoming mature for commercialization. The COP (coefficient of performance) of ECS can reach 0.5 or higher (Huang and Chang, 1999; Huang et al., 1999) which is competitive to the absorption or adsorption system that is much more complicated in design and more expensive. If the ECS was driven by solar thermal energy, it requires a back-up heater to make up the heat in order to keep a constant
Fig. 1 e Conventional solar cooling system.
173
cooling capacity for space cooling during cloudy or rainy periods (Fig. 1). 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 New Energy Center at National Taiwan University has been devoted to the development of solar-assisted ejector cooling/heating system (SACH). Two types of SACH were developed. The SACH consists of a conventional inverter-type air conditioner (IAC) made of variable-speed compressor which is connected in series or parallel with a solar-heated ejector cooling system. The first system SACH-1 is in series configuration as shown in Fig. 2 (Huang et al., 2010a). SACH-1 combines a pumpeless ejector cooling system with an inverter-type air conditioner to provide a stable space cooling. The ECS is driven by solar heat and is used to sub-cool the condenser of the IAC to increase its COP and reduce the power input of the compressor by automatically regulating the rotational speed of the compressor. The cooling load is supplied by the IAC only. A SACH-1 was built and run at outdoor. It shows that the use of ECS in SACH-1 can effectively reduce the condensing temperature of the heat pump by 7.3e12.6 C and reduce the input power of compressor by 34.5e81.2% (Huang et al., 2010a). The other system SACH-2 is in parallel configuration as shown in Fig. 3 (Huang et al., 2010b). In SACH-2, the ECS is driven by solar heat and connected in parallel with an IAC. The cooling load is supplied by ECS when solar energy is available and the input power of IAC can be reduced. In variable weather, ECS will probably operate at off-design condition of ejector and the cooling capability of ECS can be lost completely. In order to make the ejector operate normally at critical or non-critical double-choking condition to obtain a better performance, Huang et al. (2010b) further added a control system in SACH-2 to control the evaporating temperature of ECS to avoid ejector failure during unsteady or low solar radiation. This makes the ECS always produce cooling effect even at low solar radiation periods while the ejector performs at off-design conditions. The energy saving of IAC is experimentally shown 50e70%, not including power consumptions of solar heating system, refrigerant pump of ECS, and fan and pump in water condenser of ECS. The long-term performance test results show that the daily energy saving is around 30e70% as compared to the energy consumption of IAC without solar-driven ECS. The total energy saving of IAC is 52% tested from September 7, 2009 to March 20, 2010. Huang et al. (2012) further developed a maximum-power point tracking control technology (MPPT) for solar heating system to minimize the pumping power consumption with optimal heat collection. The proportional-integral (PI) feedback tracking control system was developed with a tracking filter to determine the instantaneous optimal tracking target online. The test results show that the mass flowrate for different days is between 18.1 and 22.9 kg min1 with pumping power between 77 and 140 W, which is greatly reduced from the standard design value (450 W) for the total collector area 25.9 m2. SACH-2 is designed in parallel configuration whose ECS may operate at a very low COP at high condensing temperature when ambient temperature is high, even using water cooling. This will cause very low system efficiency of SACH-2
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Fig. 2 e The schematic diagram of SACH-1 (Huang et al., 2010a).
and not economical when it is used in hot climate. Besides, the power consumptions of peripheral in SACH-2 need to be reduced in order to make this technology economical. The present study further modifies the design of SACH-2 using series configuration similar to SACH-1 (Huang et al., 2010a) but using a refrigerant pump in ECS and several advanced control technologies to keep stable performance of ECS and minimize power consumption of peripherals. This type of SACH is called “SACH-k2”. In series configuration of SACH, the ECS is used to cool the condenser of the IAC. The ECS does not directly provide cooling effect to the cooling room. Hence, the design of evaporator temperature of ECS can be raised and the COP of ECS can be increased. In SACH-k2, the cooling load is steadily supplied by IAC. The solar-driven ECS acts as a device for energy saving when solar energy is available. Several advanced technologies have been developed in SACH-k2: (1) the refrigerant pump used in ECS requires a generator liquid level control in ECS to eliminate large level
variation during variable solar radiation and keep stable performance of ejector; (2) the fan power consumption in cooling tower of ECS needs to be reduced. The present study focuses on the development of these technologies and running a field test to measure the overall performance efficiency of SACH-k2, including energy consumption of all the peripheral equipment.
2.
Experimental setup
SACH-k2 was developed using several advanced technologies. The design of SACH-k2 includes: (1) system configuration of SACH-k2; (2) solar heating system; (3) ejector cooling system (ECS); (4) IAC retrofitting; (4) cooling space.
2.1.
System configuration of SACH-k2
SACH-k2 consists of 3 major parts: a solar heating system, an ejector cooling system (ECS) with water-cooled condenser,
Fig. 3 e Solar-assisted cooling/heating system in parallel configuration (SACH-2) (Huang et al., 2010b).
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Fig. 4 e Schematic diagram of SACH-k2.
and an inverter-type air conditioner (IAC). The schematic diagram is shown in Fig. 4. ECS is coupled with IAC at the intercooler, which is the evaporator of ECS or the sub-cooler of IAC. The cooling load of the cooling room is designed at 3.5 kW (1RT) which matches the rated cooling capacity of the IAC used. The cooling capacity of the ECS is 5.6 kW which is able to cool the condenser of the IAC at a lower temperature to increase the COP of IAC. The system specification is shown in Table 1. R245fa is used as the working fluid of ECS.
2.2.
Solar heating system
The solar heating system used in SACH-k2 is the same as that used in the study of MPPT (maximum-power-point tracking) control of pump (Huang et al., 2012). The solar heating system consists of 24 flow-through vacuum-tube collectors (Model EZL100-6) with 26 m2 total absorber area. Eight collectors are connected in series and three in parallel. Glycol solution is
pumped from the buffer tank through the solar collector and absorbs solar energy to heat the generator of ECS. The flow then returns to the buffer tank as shown in Fig. 4. An inverter for rotational speed control of the circulation pump was installed and a PC-based control system was developed for the MPPT control of the circulation pump. This solar heating system can supply hot water at temperature in the range 70e130 C to drive the ejector cooling system. A buffer tank (200 L) is used as a storage for stable pumping. The test of solar collector shows that the thermal efficiency of the solar heating system is 0.6 at water inlet temperature 100 C (Huang et al., 2010a,b).
2.3.
Ejector cooling system (ECS)
The flow diagram of ECS is as shown in Fig. 4. ECS and IAC are linked at the intercooler which is the evaporator of ECS and the condenser of IAC. The cooling capacity of ECS is 5.6 kW rated at condenser temperature 40 C, generator
Table 1 e System design specification of SACH-k2. Design specification of IAC IAC model: Refrigerant: Condenser temperature, C Evaporator temperature, C Condenser heat rejection, kW Cooling capacity, kW Compressor input, kW COPIAC Design specification of ECS Refrigerant Generator temperature, C Condensing temperature, C Evaporator temperature, C Generator heat input, kW Condenser capacity, kW Evaporator capacity, kW COP
MA732BVY8 R22 54 8 8 1.2e4.5 0.4e1.125 3.25 R245fa 100 40 20 8.9 14.6 5.6 0.63
Table 2 e Design specification of ECS. Refrigerant Cooling capacity (kW) Operating temperature Generator temperature ( C) Evaporator temperature ( C) Condenser temperature ( C) Flowrate Primary flowrate (kg s1) Entrained flowrate (kg s1) Compression ratio Entrainment ratio Heat transfer rate Generator (kW) Evaporator (kW) Condenser (kW) COP
R245fa 5.6 100 20 40 0.030 0.022 2.04 0.73 8.9 5.6 14.5 0.63
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Table 3 e Ejector design. Nozzle
Constant-area section
Area ratio
Throat dia. (Dt)
Exit dia. (D1)
Dia. (D3)
Entrance angle
A3/At
2.72 mm
5.52 mm
8.71 mm
65
10.33
temperature 100 C, and evaporator temperature 20 C. R245fa is chosen as the working fluid and the estimated COP of ECS is 0.63, not including pumping power. The ECS design specification is shown in Table 2. The ejector design using 1D ejector model (Huang et al., 1999) for ECS of SACH-k2 is shown in Table 3.
2.3.1.
Control of generator liquid level
The generator of ECS is designed in shell-tube type as shown schematically in Fig. 5. Liquid refrigerant (R245fa) is in the shell side and the heating fluid (glycol water from solar collector) is in the tube side. Refrigerant vapor is generated in the shell side to drive the ejector. Instantaneous variation of solar radiation intensity may result in large variation of liquid level in the generator of ECS. Conventional high-low level control (ON/OFF) will cause large variation in generator temperature/ pressure and unstable performance of ECS. A level control system is thus developed in the present study. A refrigerant gear pump with variable-speed motor (300e2000 rpm, maximum power input 750 W, maximum pumping head 140 kg cm2) was used for liquid level control in generator. The feedback control system uses a float-type magnetic liquid level gage to measure the level and to vary the flowrate by regulating motor speed of pump to control the level. The liquid level should be controlled at certain level of accuracy in order to provide stable pressure to the ejector. The liquid level should be higher than the heating tubes to ensure better heat exchanging. The level control system will accurately control liquid level at 12 1 cm from the bottom of the generator. Fig. 6 is the feedback control system to adjust the inlet liquid flowrate to the generator. The proportionalintegral control algorithm (PI) was implemented in the controller.
Fig. 5 e Schematic of generator design.
Fig. 6 e Feedback generator level control system.
2.3.2.
Cooling tower design
SACH-k2 uses water cooling for the condenser of ECS. A highperformance water cooling tower was used in the present study. The design follows the research results of Hu and Huang (2005). Cellulous pads (0.45 m 0.45 m 0.6 m) were used to construct the cooling tower matrix. The air fan consumes 380 W at full speed with flowrate 1.08e1.25 m3 s1. The water circulation pump consumes 45 W at flowrate 32e38 L min1 and 3.8e5.4 mH2O head. The schematic of water cooling tower is shown in Fig. 7. The real hardware is shown in Fig. 8.
2.3.3. tower
Power consumption control system of ECS cooling
SACH-k2 uses water cooling for condenser of ECS as described above. Fan and pump will consume electrical energy during operation (Fig. 7). ECS must be operated at a condenser temperature Tc below the critical temperature Tc to keep the ejector working under double-choking condition (Huang et al., 1999). Usually, the operating point of Tc is always below the critical temperature Tc . This means that Tc can be raised to Tc without decreasing cooling effect of ECS. In the other words, the fan or pump speed can be reduced to increase the cooling water temperature as well as the condenser temperature to save energy. The condensing pressure will increase with decreasing fan speed as well as input power without changing the cooling effect of ECS (entrainment ratio of ejector) until the condensing pressure larger than the critical condensing pressure. An feedback control system using a micro-processor is designed which will reduce the fan speed but still keeping the ejector working at double-choking condition. An inverter is connected to the fan motor so that it can be triggered by the
Fig. 7 e Schematic of water cooling tower.
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177
Fig. 10 e Decomposition of filter F in the feedback tracking control system.
Fig. 8 e Water cooling tower.
controller. The step-down algorithm was used to adjust the fan speed gradually from the maximum value until the evaporator temperature of ECS starting to rise.
2.3.4.
Evaporator temperature control of ECS
In ECS, the condensing temperature must be lower than the critical condensing temperature (critical point) such that the ejector can operate at double-choking condition (Huang et al., 1985). For an ejector with fixed geometry, the critical condensing temperature depends on the evaporator temperature and the generator temperature (Huang et al., 1985) which varies with solar radiation intensity. The ejector of ECS will probably operate at off-design conditions due to the variation of solar radiation intensity. Huang et al. (2010b) have developed an optical control technique to make the ejector operate at critical or noncritical double-choking condition to obtain a better performance under variable solar radiation. The same technology is used in SACH-k2 but with some adjustment of parameters. An electronic expansion valve was installed in the suction line of the ejector (at the evaporator suction inlet) to regulate the opening of the expansion valve to control the evaporator temperature. A feedback tracking control system was then designed to adjust the evaporator temperature which may vary with instantaneous solar radiation intensity IT(t), as shown in Fig. 9. A filter F is needed to convert the signal IT(t) into the setting value of evaporator temperature Te, set(t) for tracking control as shown in Fig. 10.
Fig. 9 e Feedback control of evaporator temperature.
Since the setting of evaporator temperature Te, set directly depends on the generator temperature Tg. A converter (Te signal converter) can be defined as a functional relation of generator temperature, Fe. The generator temperature will further depend on the solar radiation intensity. Another converter (Tg signal converter) can be defined as a functional relation of solar radiation intensity incident upon solar collector ITf, Fg. The two signal converters Fe and Fg can be determined from the field test of the SACH-2k. The low-pass filter Fs in Fig. 10 is used to filter the fast variation of solar radiation signal, i.e. low pass, since only lowfrequency content of the solar radiation variation will affect the response of the generator temperature. Hence, the filter Fs is designed as a low-pass filter using moving average, MA filter, with fixed time interval 8 min. To determine the two signal converters Fg and Fe, SACH-2k was run outdoor continuously and the electronic expansion valve was adjusted manually to regulate the evaporator temperature according to the generator temperature such that the ECS still produce cooling effect at off-design condition. From the energy balance between the solar heating system and the generator, the determined generator temperature depends on the solar radiation intensity. Fig. 11 shows that Tg varies linearly with solar radiation intensity ITf. Each data points are taken from steady-state performance at about 20e30 min time interval. A linear relation is derived for the Tgsignal converter Fg as Tg ¼ 0:0292 Itf þ 74:327
(1)
Fig. 12 shows that Te decreases linearly with increasing Tg. A linear relation is derived for the Te-signal converter Fe as Te ¼ 0:5549Tg þ 70:527 dTc ; dTc ¼ 36:7 Tc
(2)
The controller C(s) of the tracking feedback control system uses the proportional control algorithm with proportional gain Kp ¼ 0.33.
Fig. 11 e Variation of Tg with irradiation.
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30
Table 5 e Design of intercooler.
Te (oC)
25
Tube side (water)
20
Temperature ( C) Pressure (MPa) Flowrate (kg s1) Enthalpy (kJ kg1) Max heat rate (kW)
15 10 5
Shell side (R245fa)
Inlet
Outlet
Inlet
Outlet
25 0.1013 0.26 104.83
35 0.1013 0.26 146.63
78 0.143 0.052 462.43
40 0.121 0.052 252.57
10.9
0 80
85
90
95
100
Tg (oC)
Fig. 12 e Variation of Te with Tg.
2.4.
Inverter-type air conditioner (IAC)
The inverter-type air conditioner (IAC) used in SACH-k2 is purchased from market then retrofitted in the lab. MA732BVY8 made by TECO Co was selected. The IAC can supply cooling load 1.2e4.5 kW at different frequency. The specification of IAC is shown in Table 4. The intercooler, which is the evaporator of ECS or condenser of IAC, is installed between the condenser and the evaporator of IAC (Fig. 4) to sub-cool the IAC refrigerant. Refrigerant R22 is then recharged after retrofitting. The design of intercooler is shell-tube type and the design specification is shown in Table 5. The indoor air cooler is installed in the cooling room. The outdoor unit of IAC is installed inside the package of ECS.
2.5.
Cooling room
3.
Test results of SACH-k2
SACH-k2 is tested outdoor with solar heating. The performance of generator liquid level control, power consumption of ECS cooling tower, evaporator temperature control of ECS, and overall system performance were measured.
3.1.
The cooling room is a small concrete room with inside dimension L2.1 m H3.2 m W2.35 m. A calibration was run to measure the variation of heat load of the room with indoor/ outdoor temperature difference. The heat load was determined by directly measuring the cooling capacity of the indoor evaporator of IAC and the temperature difference between indoor and outdoor (Fig. 13). It is seen that the heat load of the room is not high enough to match the cooling capacity of IAC (3.5 kW). Hence, a 2.6 kW electric heater was put inside the room to supply additional heat to the room during experiment.
2.6.
installed inside the cooling room with refrigerant line connected to the package. Solar hot water is circulated through the package to heat the ECS. A PC-based control and monitoring system was developed to control the operation of cooling room, solar heating system, ECS, IAC, fans and pumps etc. Test data were recorded every 10 s. The PC-based control system is also used to carry out the optimal power consumption control of ECS cooling tower (fan and pump) and generator liquid level of ECS, and pumping power of solar heating system.
Generator liquid level control of ECS
The liquid level control test in ECS was performed with solar heating. In a variable weather with high fluctuation of solar irradiation, the liquid level was controlled between 11 cm and 12.5 cm for the level setting at 12 cm, about 8 to 4% error, as shown in Fig. 15. Only slight level variation occurs during the abrupt change of solar irradiation. In partly-cloudy weather with moderate fluctuation of solar irradiation, the liquid level was controlled between 11.6 and 12.4 cm for the level setting at 12 cm, about 3% error, as shown in Fig. 16. This indicates that the liquid level control system is robust with respect to the abrupt change of solar radiation.
System integration of SACH-k2
SACH-k2 is designed in package in which IAC and ECS are packed together as shown in Fig. 14. Indoor air fan unit is
Table 4 e Specification of IAC. IAC model Refrigerant Condensing temperature, C Evaporating temperature, C Condenser heat rejection, kW Cooling capacity, kW Compressor input, kW COPIAC
MA732BVY8 R22 54 8 5.6 1.2e4.5 0.4e1.125 3.25
Fig. 13 e Heat load of cooling room.
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3.2.
Fig. 14 e Package of SACH-k2.
179
Cooling tower power consumption of ECS
The power consumption of fan in water cooling tower is 630 W at full speed. The fan control system will reduce the fan power consumption without affecting the cooling performance of ECS. Fig. 17 shows that the fan power consumption drops from 630 W to 150 W without affecting the performance (condenser and evaporator temperature stay approximately steady). At the end of test near 13:20, while the solar radiation drops, the fan power is reduced to 110 W at condenser temperature around 39 C. The average power consumption during the test period 11:00e13:15 is 155 W which means 75% energy reduction. Fig. 18 shows that the fan power consumption drops to 90 W (minimum) in a clear weather. The average power consumption during the test period 11:30e12:50 is 124 W, which
Fig. 15 e Test result of level control in variable weather.
Fig. 16 e Test result of level control in partly-cloudy day.
180
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Fig. 17 e Test result of cooling tower fan control.
means 80% energy reduction. But ESC can still operate under critical mode.
3.3.
Evaporator temperature control of ECS
In order to make ejector operate under critical or non-critical double-choking condition, an electronic expansion valve was installed at the evaporator inlet to regulate the opening of the expansion valve to control the evaporator temperature according to the variation of solar radiation. The feedback tracking control system is as shown in Fig. 9.
The test result shown in Fig. 19 was performed at average ambient temperature 34 C and cooling room temperature 26.9 C with additional electric heating load 2.6 kW. It is seen that the tracking of Tg and Te was satisfactory. A larger control error on Tg was observed as solar radiation drops and varies. The control error of Te is less than 1.5 C at stable solar radiation and about 3 C at abrupt solar radiation change. Fig. 20 shows that at solar irradiation about 790W m2, the power consumption of IAC decreases from 992 W to 512 W, a reduction of 48%, when the solar-driven ECS is activated and operating at Tg ¼ 96 C and Te ¼ 20 C, and IAC condenser
Fig. 18 e Test result of cooling tower fan control.
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181
Fig. 19 e Performance test of SACH-k2.
temperature at 38 C. The ECS is shown still running at low and variable solar radiation. The result shown in Fig. 21 was from the test performed at average ambient temperature 28 C and cooling room temperature 26.1 C with additional electric heating load 2.6 kW. It is seen that the tracking of Te was satisfactory. The ECS is running well at the periods of unstable solar radiation.
3.4.
Overall system performance of SACH-k2
The system performance test of SACH-k2 was carried out with all the implemented control systems. Cooling fans, evaporator temperature and MPPT of water pump of solar heating system
were optimized. Fig. 22 shows that the power consumption of IAC alone was 1087 W. Additional electric heating load 2.6 kW in the cooling room was applied during the test. The power consumption of IAC dropped to 600 W in average after ECS was turned on. Power consumption of IAC saved 44.8%. Fig. 23 shows that the power consumption of IAC alone was 1030 W with an electric heating load 2.6 kW in the cooling room applied during the test. The power consumption of IAC dropped to 561 W in average during 11:00e13:15 after ECS was turned on. Power consumption of IAC saved 45.6%. Fig. 24 shows the performance of SACH-k2 in cloudy day. In the beginning, the power consumption of IAC was 1030 W. Additional electric heating load 2.6 kW in the cooling room
Fig. 20 e Performance test of SACH-k2.
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Fig. 21 e Performance test of SACH-k2.
was applied during the test. The power consumption of IAC dropped to 630 W in average during 14:00e14:50 after ECS was turned on. Power consumption of IAC saved 37.4%. The test result at clear weather is shown in Fig. 25. The average power consumption of IAC alone was 926 W. Additional electric heating load 2.6 kW in the cooling room was applied during the test. The power consumption of IAC then dropped to 485 W in average during 11:40e12:50 after ECS was turned on. Power consumption of IAC saved 47.6%.
In summary, all the test results show that the IAC condenser was sub-cooled about 10e20 C and the average power consumption reduced by 37.4e47.6%. Tables 6e8 summarize the performance of IAC alone and SACH-k2. For the test runs, the generator liquid level control in ECS performs quite well and keeps stable performance of ejector under variable solar radiation. The ECS evaporator temperature control also performs satisfactorily to keep ejector performance normally under low solar radiation. The fan power
Ambient temp: 34oC Cooling room temp: 26.9oC Generator temp Tg
Solar irradiation IAC power consumption
Condenser temp of IAC
ECS evaporator temp Te
Fig. 22 e Test result of SACH-k2.
183
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heater: 2.6kW Generator temp
Solar irradiation
100
Temperature(oC)
1,200
1,000
80
800
60
600
IAC power consumption
40
400
Condenser temp of IAC 20
200 Ambient temp: 28oC
ECS evaporator temp
Solar radiation(W/m2) / IAC power consumption(W)
SCH-k2 (May 26, 2011)
120
Cooling room temp: 26.1oC 0 10:45:48
0 11:15:51
11:45:32
12:15:12
12:44:54
13:15:03
13:45:09
Fig. 23 e Test result of SACH-k2.
control system in cooling tower of ECS performs stably and reduces the power consumption dramatically. The overall coefficient of performance (COP) of SACH-k2, COPo, including the power consumption of all the peripheral devices is defined as: COPo ¼ QL = WIAC þ Wsolar pump þ WCT þ Wfreon pump
(3)
where WIAC is the power consumption of IAC, Wsolar pump is the pumping power of solar collector pump, WCT is the power
consumption of cooling tower including fan and water pump, Wfreon pump is the circulation pump of ECS. It is seen from Table 8 that COPo increases from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43% increase. The IAC power input decreases about 45%. The solar-driven ECS sub-cools the condenser of IAC about 15 C and improves the performance of IAC. In addition, the power consumption of peripheral devices in SACH-k2 is reduced by using advanced control technology, including the MPPT control of solar heating system (Huang et al., 2012). The highest COPo of SACH-k2
Fig. 24 e Test result of SACH-k2.
184
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May 29, 2011
120
1200 Cooling room temp 23.2oC
Generator temp Tg
800
IAC power consumption
60
600
400
40
Solar irradiation(W/m2) /
80
Temperature(oC)
1000 IAC power consumption(W)
Solar irradiation
100
Condenser temp of IAC 200
20 ECS evaporator temp 0 11:07:04
0 11:38:13
12:09:04
12:40:00
13:10:58
Fig. 25 e Test result of SACH-k2.
Table 6 e Power consumption of peripheral in SACH-k2. Date
Power consumption (W) Solar heating system
Fan of ECS cooling tower
Pump of ECS cooling tower
Refrigerant pump of ECS
Total power of peripheral equipment
115.9 85 111.1 93.4 140.3
139.3 127 155.3 107.8 123.8
28 28 28 28 28
37 45 47 42 52
320.2 285 341.4 271.2 344.1
2011/05/09 2011/05/18 2011/05/26a 2011/05/26b 2011/05/29
is 4.5 which reach the target of solar cooling technology suggested by Wiemken et al. (2010).
4.
Discussion and conclusions
A solar-assisted ejector cooling/heating system SACH-k2 which is in series configuration and uses a circulation pump in ECS is developed in the present study. In SACH-k2, the ECS
is driven by solar heat and produces cooling effect to cool the condenser of IAC. The solar-driven ECS acts as a device for energy saving when solar energy is available. SACH-k2 also requires more advanced technologies for operating control to keep normal performance of ECS under variable solar radiation. Besides, the power consumption of peripheral in SACHk2 needs to be reduced. Several advanced technologies are developed in SACH-k2 including generator liquid level control in ECS, the ECS
Table 7 e Overall system performance of SACH-k2. System
IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2 IAC alone SACH-k2
Date
2011/05/09 2011/05/18 2011/05/26a 2011/05/26b 2011/05/29
IAC intercooler temperature ( C) Inlet
Outlet
38.5 38.6 36.0 36.3 36.6 36.3 34.7 35 32.7 32.5
37.6 23.6 33.0 21.4 35.2 21.5 34.3 22.8 31.7 17.3
Power consumption of IAC (W)
Total power consumption (W)
Total cooling capacity (W)
COPo
1087 600 961 515 1031 561 1006 630 926 485
1087 920 961 800 1031 903 1006 901 926 829
3196 3731 3200 3600 3073 3804 3180 3537 3000 3582
2.94 4.06 3.30 4.50 2.96 4.22 3.16 3.93 3.24 4.32
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Table 8 e Performance of IAC alone and SACH-k2. Date Test condition Ambient temperature, C IAC subcooling, C IAC power, kW Total peripheral power, kW Cooling capacity, kW COPo
05/09 IAC 34 e 1.09 e 3.2 2.94
SACH-k2 15 0.6 0.32 3.7 4.06
05/18 IAC 29.4 e 0.96 e 3.2 3.3
evaporator temperature control, and the optimal control of fan power in cooling tower of ECS. From the results of test runs, the generator liquid level control in ECS performs quite well and keeps stable performance of ejector under variable solar radiation. The ECS evaporator temperature control also performs satisfactorily to keep ejector performance normally under low and variable solar radiation. The fan power control system in cooling tower of ECS performs stably and reduces the power consumption dramatically. The COPo increases from 2.94e3.3 (IAC alone) to 4.06e4.5 (SACH-k2), about 33e43%. The IAC power input decreases about 45%. The solar-driven ECS sub-cools the condenser of IAC by 10e20 C and improves the performance of IAC. The highest COPo of SACH-k2 is 4.5 which reach the target of solar cooling technology suggested by Wiemken et al. (2010). The present test results of SACH-k2 indicate that the solar thermally-driven ejector air conditioning technology which integrates different advanced technologies, including MPPT of solar heating system, ECS generator liquid level control, ECS condenser temperature control, fan power control of ECS cooling tower, becomes more mature toward commercialization. Further performance improvement of SACH-k2 is still possible. We found that there is excess cooling capacity generated by ECS during high solar radiation periods, due to fixed heat exchanger design of intercooler. By increasing the size of intercooler, COPo may be increased further. This means that it is possible to obtain a COPo higher than 6.0.
Acknowledgments This publication is based on the work supported by Award No.KUK-C1-014-12, made by King Abdullah University of Science and Technology (KAUST).
references
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SACH-k2 15.2 0.51 0.29 3.6 4.50
05/26 IAC 28 e 1.03 e 3.1 2.96
SACH-k2 14.8 0.63 0.27 3.8 4.22
05/29 IAC 25 e 0.93 e 3.0 3.24
SACH-k2 15.2 0.48 0.34 3.6 4.32
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