Optimal control and performance test of solar-assisted cooling system

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Optimal control and performance test of solar-assisted cooling system

B.J. Huang

a,*

_{, C.W. Yen}

a

_{, J.H. Wu}

a

_{, J.H. Liu}

a

_{, H.Y. Hsu}

a

_{, V.O. Petrenko}

a

_{, J.M. Chang}

b

_{, C.W. Lu}

b

a_{Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan}

b_{Department of Refrigeration, Air Conditioning and Energy Engineering, National Chin-Yi University of Technology, Taichung, Taiwan}

a r t i c l e i n f o

Article history: Received 15 April 2010 Accepted 2 June 2010 Available online 19 June 2010 Keywords: Solar energy Ejector system Ejector cooling Solar cooling

a b s t r a c t

The solar-assisted cooling system (SACH) was developed in the present study. The ejector cooling system (ECS) is driven by solar heat and connected in parallel with an inverter-type air conditioner (A/C). The cooling load can be supplied by the ECS when solar energy is available and the input power of the A/C can be reduced. In variable weather, the ECS will probably operate at off-design condition of ejector and the cooling capability of the ECS can be lost completely. In order to make the ejector operate at critical or non-critical double-choking condition to obtain a better performance, an electronic expansion valve was installed in the suction line of the ejector to regulate the opening of the expansion valve to control the evaporator temperature. This will make the SACH always produce cooling effect even at lower solar radiation periods while the ejector performs at off-design conditions. The energy saving of A/C is experimentally shown 50e70% due to the cooling performance of ECS. The long-term performance test results show that the daily energy saving is around 30e70% as compared to the energy consumption of A/C alone (without solar-driven ECS). The total energy saving of A/C is 52% over the entire test period. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Ejector cooling system (ECS) using low boiling point refrigerant is suitable for solar cooling application due to its simple design and low cost[1e13]. If the ECS was driven by solar energy, it requires a back-up heater to make up the heat in order to keep a constant cooling capacity for space cooling during cloudy or rainy periods. 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). The SACH consists of a conventional inverter-type air conditioner (A/C) made of variable-speed compressor connected in series or parallel with a solar ejector cooling system. SACH-1 is in series conﬁguration and linked with a pump-less ejector cooling system with an inverter-type air conditioner to provide a stable space cooling [13]. SACH-2 is in parallel conﬁguration as shown inFig. 1. The ejector cooling system (ECS) driven by solar heat is connected in parallel with an inverter-type air conditioner. The energy consumption of the air conditioner can be reduced by regulating the rotational speed of the

compressor when the ECS is operating. During cloudy or rainy periods or at night, SACH-2 will provide the entire cooling load from the inverter-type air conditioner (heat pump) as usual.

In the ECS, the condenser temperature must be lower than the critical condensing temperature (critical point) such that the ejector can operate at double-choking condition to obtain a better performance [2]. Otherwise, the cooling capacity will drop dramatically and the ECS may even loss the cooling performance completely and induce a reverse operation (heat-ing). For a ﬁxed geometry ejector which is designed for a particular double-choking critical condition (design point), the ECS will operate at off-design condition if the generator, evap-orator, and condenser temperatures are not at the design point due to the environmental variations. For an ejector withﬁxed geometry, the critical condensing temperature depends on the generator temperature[2]which will vary with solar radiation intensity in solar cooling application. Therefore, the cooling performance of ECS may cease and a heating performance may be induced during the periods of lower solar radiation. The SACH will have a serious problem under variable weather condition, if this is not solved.

To solve this problem, the expansion valve installed in the suction line of the ejector (at the evaporator inlet) can be replaced by an electronic expansion valve to regulate the opening to control the suctionflowrate to the ejector. The valve is completely closed when a reverse flow will occur. The preliminary field test result

* Corresponding author. Tel.: þ886 2 23634790; fax: þ886 2 23640549. E-mail address:[email protected](B.J. Huang).

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using a simple on/off control of the valve has shown that the performance of SACH-2 was improved[14]. However, it still needs a good feedback control system to control the valve opening according to the variations of environment.

In the present study, an optimal control technology is further developed to cope with thisﬁeld operation problem of SACH-2. The opening of the electronic expansion is regulated automatically to control the evaporator temperature according to the variation of solar radiation intensity. This will make the SACH-2 always produce cooling effect even at lower solar radiation periods while the ejector performs at off-design conditions.

2. Experimental setup 2.1. System design of SACH-2

SACH-2 consists of 3 subsystems: an ejector cooling system, a solar collector system, and an inverter-type air conditioner with variable-speed compressor. Fig. 2 is the schematic diagram of a practical SACH-2. The SACH-2 uses an inverter-type air condi-tioner with rated cooling capacity 3.6 kW (1RT). The cooling capacity of the ECS is designed at 1.8 kW rated at critical condenser temperature 38C, generator temperature 100C, and evaporator

Fig. 1. Solar-assisted cooling/heating system in parallel conﬁguration (SACH-2).

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temperature 8C. The overall system design speciﬁcation is shown inTable 1. The ECS refrigerant is R365mfc and the nozzle design is listed inTable 2with 5.3 mm in the nozzle throat diameter, 10 mm in the nozzle exit diameter, 15.5 mm in constant-area chamber diameter, and 65in inlet converging angle. The ejector area ratio of constant-area section to nozzle throat is 8.55 which is designed for the operating condition of critical condenser temperature 38C, generator temperature 100C, and evaporator temperature 8C. The evaporator cooling capacity of the ECS is designed at 1.8 kW which will reduce the power consumption of inverter-type air conditioner by 50e70% at clear weather.

The ejector cooling system and the inverter-type air conditioner operate independently. There is a microprocessor-based central control system to effectively regulate the two systems according to the solar irradiation and solar collector system performance to reduce the input power of the inverter-type air conditioner. The condenser of the ejector cooling system is cooled by a conventional water cooling tower. An electronic expansion valve was installed in the suction line of the ejector (at the evaporator inlet) to regulate the opening to control the ﬂowrate through the evaporator according to the variation of solar radiation intensity.

2.2. Solar heating system

The solar heating system consists of 48 sets vacuum-tube solar collectors with 51.8 m2total collector area (Table 2), a circulation pump, and a buffer tank used to stabilize the temperature to heat the generator of the ejector cooling system. The instantaneous thermal ef_{ficiency of the solar collector is 0.615 at water inlet} temperature 120C. The measured solar energy collection ef fi-ciency is shown inFig. 3. The test result shows that the solar energy collection efficiency is around 0.6 when the buffer tank tempera-ture reaches 100C.

A PC-based control system was designed in the present study to control the ON/OFF of the circulation pump according to solar radiation variation. Temperature-difference controller based on the temperature difference between the collector outlet and the buffer tank was employed.

2.3. ECS control system design

In the ECS, the condensing temperature must be lower than the critical condensing temperature (critical point) such that the ejector can operate at double-choking condition. For an ejector withﬁxed geometry, the critical condensing temperature depends on the evaporator temperature and the generator temperature[2]

which varies with solar radiation intensity. The ejector of the ECS will probably operate at off-design conditions due to the variation of solar radiation intensity.

The performance of an ECS operating at off-design condition can be analyzed using the performance map of the ejector [15], as shown inFig. 4. At aﬁxed condensing temperature which is lower

Table 1

Overall system design speciﬁcation of SACH-2.

1. Inverter-type air-conditioner Hitachi RAS-32JQ

Refrigerant R410a

Input voltage, V AC 220

Compressor frequency, Hz 20e80

Compressor input power, kW 0.26e1.09

Cooling capacity at 54.4C condenser/7C evaporator, kW 1.0e4.2

Rated COP 3.85

2. Ejector cooling system

Refrigerant R365mfc

Generator temperature,C 100

Generator heat input, kW 11

Condenser capacity, kW 12.8

Condensing temperature,_C ₃₈

Evaporator temperature,_C ₈

Evaporator cooling capacity, kW 1.8

COPej 0.16

Table 2

Speciﬁcation of the vacuum-tube solar collector.

Total absorber area 1.08 m2

No. of vacuum tubes 6

Collector tube dimensions F100 2000 mm

Collection efﬁciency 0.615 @120C

Fig. 3. Measured instantaneous solar collector efﬁciency of a single collector.

Fig. 4. Performance map of aﬁxed-geometry ejector.

Fig. 5. Feedback control structure for ECS.

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than the critical condensing temperature (i.e., non-critical double-choking condition), the evaporator temperature and cooling capacity will decrease to TeLif the generator temperature increases

to TgH. This follows the operating line ZeA inFig. 4. Furthermore, if

the generator temperature decreases to TgLand the ejector wants to

operate at a critical double-choking condition, the evaporator temperature needs to be increased to TeH in order to keep the

ejector working at double-choking condition. This follows the operating line ZeB inFig. 4. The operating condition of constant condensing temperature happens very often since the water temperature of the condenser cooling tower of the ECS varies very little if the ambient temperature varies slowly.

In order to make the ejector operate at critical or non-critical double-choking condition to obtain a better performance, an electronic expansion valve was installed in the suction line of the ejector (at the evaporator inlet) to regulate the opening of the expansion valve to control the evaporator temperature. A feedback tracking control system was then designed according to the struc-ture shown in Fig. 5. The tracking control is for adjusting the evaporator temperature Te(t) which may vary with instantaneous

solar radiation intensity IT(t). Hence, aﬁlter F is needed to convert

the signal IT(t) into the setting value of evaporator temperature Te, set(t) for tracking control as shown inFig. 6.

Since the setting of evaporator temperature Te,setwill directly

depend on the generator temperature Tg. A converter (Tesignal

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 Feand Fgcan be determined from theﬁeld test of the

SACH-2.

To determine the two signal converters Feand Fg, the SACH-2

was run 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. Table 3 are the measured Tewith respect to Tgat different ambient temperature.

Each data points are taken from steady-state performance at about 20e30 min time interval.Fig. 7 shows that Tedecreases linearly

with increasing Tg. A linear relation can be derived for the Te-signal

converter Feas

Te ¼ 0:7446Tgþ 86:1; (1)

From the energy balance of the solar heating system and the generator, the generator temperature will depend on the solar radiation intensity and the cooling load.Table 4shows the variation of Tgwith solar radiation intensity ITfand ambient temperature Ta.

Each data points are taken from steady-state performance at about 20e30 min time interval.Fig. 8 shows that Tgincreases linearly

with ITfat ambient temperature in the range 27.8e35.5C. A linear

relation can be derived for the Tg-signal converter Fgas

Tg ¼ 0:0216ITfþ 85:84; (2)

The low-passﬁlter FsinFig. 6is used toﬁlter the fast variation of

solar radiation signal, i.e. low pass, since only low-frequency content of the solar radiation variation will affect the response of the generator temperature. Hence, theﬁlter Fsis designed as a

low-passfilter using moving average, i.e. MA filter, with fixed time interval 8 min.

The controller C(s) of the tracking feedback control system uses the proportional control algorithm with proportional gain 0.33. A PC-based control system was developed to control the solar heating system, the ECS and the whole SACH-2 operation. The monitoring system collects the data every 10 s.Fig. 9shows the outlook of the whole SACH-2.

Fig. 6. Decomposition ofﬁlter F in the feedback tracking control system.

Table 3

Experimental results of Teregulation.

Ta(C) 35.5 32.7 Tg(C) Te(C) 85 24.8 20.7 90 21.0 17.2 95 17.1 13.6 100 13.2 10.0 105 9.4 6.4 110 5.5 2.8

Fig. 7. Experimental results of Teregulation.

Table 4

Experimental results of Tgvariation with solar radiation intensity.

Date 2009/10/2 2009/10/14 2009/10/26 2009/11/24 Ta(C) 35.5 32.7 30.7 27.8 ITf(W/m2) Tg(C) 500 98.8 96.1 95 94.5 600 100.2 98.4 97.7 97.9 700 101.6 100.8 100.4 101.2 800 103 103.2 103.1

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Fig. 9. Outlook of SACH-2 (left: ECS, right: solar heating system).

Fig. 10. Performance of SACH-2 (2010/1/20)e fair sunny day.

Fig. 11. Performance of SACH-2 (2010/1/20)e fair sunny day.

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3. Field test of SACH-2

Theﬁeld test of SACH-2 was run continuously to monitor the system performance, including the regulation of evaporator temperature and the performance of ECS, the power input to the air conditioner (A/C), and the power input reduction (energy saving) of the A/C at various weather conditions.

3.1. Performance of SACH at various weather

Fig. 10shows the test results on a fair sunny day without cloud but having weaker solar radiation. The tested indooreoutdoor temperature difference is 5.9C which corresponds to a medium cooling load. It is seen that the evaporator temperature setting value varies with the generator temperature. The control system tends to track the evaporator temperature setting values. The

maximum tracking error is about 5C which happens during low solar radiation periods around 15:30PM. During 10:40e15:00, the generator temperature is mostly higher than 95C near the design point (100C) and the evaporator temperature around 6C (near the design point 8C). The expansion valve is fully opened.

It is seen fromFig. 11that the ECS still works at off-design point of ejector with double-choking and provides cooling power after 15:30PM when solar radiation intensity drops while the generator temperature is only around 70C. The evaporator temperature is changed to about 20C. After 16:00PM, the ECS ceases to work and the power input of the A/C is 0.68 kW which represents the power input to the cooling load of the room without the ECS. During 15:30PM to 16:00PM, the input power to A/C is around 0.31 kW while the ECS is working. This means that a reduction of input power of A/C is around 54% due to the cooling performance of ECS driven by solar heat.

Fig. 12. Performance of SACH-2 (2010/1/31)e partly cloudy day.

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There is time lag around 10:00AM due to thermal response of the generator temperature. During the period 11:00AM to 15:00PM, the ECS performs very well at high solar radiation intensity. The generator temperature is mostly higher than 95C near the design point (100C) and the evaporator temperature around 7C (near the design point 8C). The input power of the A/C is almost zero, i.e. 100% energy saving, since the ECS supplies almost all the cooling load. The test results have shown that the tracking control system works and the ECS can produce cooling effect at off-design conditions at low solar radiation periods around 15:30PM.

Fig. 12 shows the test results on a partly cloudy day with indooreoutdoor temperature difference 4.4_{C (low cooling load). It}

is seen that the evaporator temperature setting value varies with the generator temperature. And the tracking control system tends to track the evaporator temperature setting values. The maximum tracking error is about 8C which happens during a sudden drop

and rise of solar radiation and generator temperature around 11AM. During 12:00e14:30, the generator temperature is mostly higher than 95C near the design point (100C) and the evaporator temperature around 6C (near the design point 8C).

Fig. 13shows that the cooling power from ECS varies with the solar radiation intensity. The input power of A/C is around 0.66 kW at 16PM when the ECS ceases to work. During the period 14:30PM to 15:30PM at very low solar radiation intensity, the ECS still works at off-design conditions and provide some cooling power. The input power of A/C is around 0.31 kW. This means that the reduction of input power of A/C is around 53% due to the performance of ECS.

Fig. 14 shows the test results on a cloudy day with indooreoutdoor temperature difference 6.7_{C (medium cooling}

load). It is seen that the evaporator temperature setting value varies with the generator temperature. Before 9:40AM, the solar radiation is low and varies. The SACH-2 is at start-up period and the

Fig. 14. Performance of SACH-2 (2009/12/24)e cloudy day.

Fig. 15. Performance of SACH-2 (2009/12/24)e cloudy day.

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expansion valve is closed. The tracking control is not activated. After that, the tracking control system starts to track the evaporator temperature setting values. The maximum tracking error is about 5C which happens when there is a sudden drop of solar radiation and generator temperature around 10:50AM, 12:10PM, 12:43PM and after 13:50PM. After 13:00PM, the solar radiation intensity drops quickly and the generator temperature decreases to below 80C. After 14:00PM at low solar radiation (<300 W/m2_{), the}

generator temperatureﬂuctuates between 60_{C and 80}_C. Fig. 15shows that the cooling power from ECS varies with the solar radiation intensity. The input power of A/C is around 0.55 kW before 9:40AM when the ECS has not started to work yet. During the period 10:00AM to 13:30PM at moderate solar radiation intensity, the ECS works and provide some cooling power. The input power of A/C is around 0.2 kW. This means that the reduction of input power of A/C is around 63% due to the cooling performance of ECS. After

Fig. 16. Long-term performance test results of SACH-2.

Fig. 17. Input power of A/C alone (without ECS).

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14:00PM, the ECS ceases at low solar radiation (< 300 W/m2₎

and the space cooling was completely provided by A/C. The power input of A/C increases to 0.55 kW.

SACH-2 has been tested continuously for about 7 months since Sept 7, 2009.Fig. 16shows the variation of daily-total cooling power of ECS and the solar irradiation. It is seen that the daily-total cooling power provided by the ECS is more related to the ambient temperature.

In order to determine the input power reduction of A/C in SACH-2, the input power of A/C with respect to the room temperature difference Ta Troomhas to be determined experimentallyﬁrst as

the baseline. The data taken from running the A/C alone are used to determine the correlation. Fig. 17is the correlation between the input power of A/C alone and the room temperature difference Ta Troom. This result is used to calculate the reduction of A/C input

power in the long-termﬁeld test of SACH-2.

Fig. 18shows that the daily energy saving of SACH-2 is around 30e80% as compared to the energy consumption of A/C alone (without solar-driven ECS). Table 5 summarizes the long-term performance test results of SACH-2 from 2009/09/07 to 2010/03/20. It is seen that the total energy saving of A/C due to the cooling performance of ECS is 52%.

4. Discussions and conclusion

The solar-assisted cooling system (SACH-2) was developed in the present study. The solar ejector cooling system is connected in parallel with an inverter-type air conditioner. The cooling load is supplied by the ECS when solar energy is available and the input power of the inverter-type A/C can be reduced by regulating the rotational speed of the compressor. During cloudy or rainy periods

Table 5

Long-term performance test results of SACH-2 (2009/09/07e2010/03/20). Data Solar irradiation,

HTMJ/m2day Ta (_C) Troom (_C) Room temp difference Ta Troom(C) Power consumption of A/C, EAC(kWh) Cooling power from ECS, LECS(kWh) Power consumption of A/C alone, EAC 0(kWh) ESAVING (kWh) Daily-total energy saving (%) 2009/9/7 13.7 36.6 27.6 9.1 3.12 12.55 5.20 2.07 39.9 2009/9/22 11.2 33.7 26.8 6.9 2.23 9.29 4.20 1.96 46.8 2009/10/2 13.6 36.0 27.0 9.0 1.99 11.81 5.12 3.13 61.1 2009/10/8 12.6 32.7 24.6 8.1 1.93 9.60 4.75 2.82 59.4 2009/10/9 13.5 33.6 25.2 8.5 1.61 10.77 4.92 3.32 67.4 2009/10/14 13.3 33.0 26.5 6.5 2.53 12.89 4.01 1.49 37.1 2009/10/15 12.6 33.0 27.0 6.0 2.07 14.32 3.80 1.73 45.5 2009/10/22 12.7 30.7 24.9 5.8 2.25 10.05 3.71 1.46 39.4 2009/10/23 12.9 30.5 24.8 5.6 1.56 9.41 3.61 2.05 56.7 2009/10/26 13.1 31.0 24.4 6.6 1.77 7.45 4.06 2.29 56.4 2009/10/30 12.3 31.0 25.1 6.0 1.84 13.92 3.77 1.93 51.2 2009/11/4 11.5 28.2 22.8 5.4 1.61 14.69 3.49 1.89 54.0 2009/11/5 11.8 30.2 24.5 5.7 2.45 12.25 3.64 1.24 34.0 2009/11/6 11.6 31.2 24.3 6.9 2.45 11.71 4.18 1.73 41.4 2009/11/16 11.5 28.7 21.9 6.8 1.24 10.48 4.17 2.93 70.2 2009/11/19 11.9 25.4 21.6 3.9 1.12 11.17 2.82 1.69 60.1 2009/11/25 12.0 29.3 22.6 6.8 1.52 10.85 4.14 2.63 63.4 2009/11/26 11.2 28.3 22.3 6.0 1.59 10.07 3.80 2.21 58.2 2009/11/27 11.9 29.0 24.1 4.9 1.47 10.81 3.29 1.83 55.5 2009/12/1 10.7 24.0 21.9 2.1 1.20 9.69 1.98 0.77 39.2 2009/12/2 10.4 25.0 20.4 4.5 1.89 9.26 3.12 1.45 46.4 2009/12/4 9.9 23.2 20.2 3.0 1.64 10.80 2.40 0.99 41.1 2009/12/8 9.5 27.9 21.0 6.9 2.05 6.75 4.20 2.14 51.1 2009/12/9 10.6 28.7 22.6 6.1 3.02 5.46 3.82 1.05 27.4 2009/12/15 11.7 28.4 22.4 6.0 2.08 8.96 3.81 1.75 46.0 2009/12/16 9.9 23.9 21.7 2.2 1.80 7.62 2.03 0.23 38.4 2009/12/21 10.5 20.5 17.6 2.9 1.36 6.54 2.38 1.27 53.4 2009/12/22 10.6 25.4 18.3 7.0 1.42 6.95 4.27 2.85 66.7 2009/12/23 7.6 25.9 19.5 6.4 2.12 4.33 3.96 2.21 56.0 2009/12/24 8.9 26.2 19.7 6.5 1.39 5.85 4.04 2.65 65.5 2010/1/4 10.9 22.9 18.8 4.08 1.77 7.90 2.90 1.14 39.2 2010/1/5 9.6 21.7 19.2 2.45 1.16 7.82 2.15 0.99 46.2 2010/1/11 9.2 22.9 20.5 2.45 0.78 8.06 2.15 1.37 63.8 2010/1/16 11.2 23.0 18.1 4.92 1.20 6.96 3.29 2.09 63.5 2010/1/17 11.9 24.7 18.9 5.81 1.27 8.27 3.70 2.43 65.6 2010/1/19 12.2 26.8 20.2 6.61 1.42 7.67 4.07 2.65 65.0 2010/1/21 10.6 27.8 22.7 5.10 2.03 5.54 3.37 1.34 39.7 2010/1/27 10.5 25.2 19.6 5.61 1.21 5.55 3.61 2.39 66.3 2010/1/28 10.1 28.1 21.1 7.03 2.17 5.38 4.26 2.09 49.0 2010/1/29 12.1 27.9 21.4 6.54 2.02 8.71 4.04 2.01 49.9 2010/2/1 11.9 29.0 22.6 6.44 1.96 8.87 3.99 2.03 50.9 2010/2/3 11.7 24.0 22.0 2.01 0.35 8.24 1.95 1.59 82.0 2010/2/4 11.9 23.2 21.9 1.38 1.21 8.04 1.66 0.45 27.0 2010/2/24 13.7 28.0 20.4 7.58 1.39 8.25 4.52 3.12 69.1 2010/2/26 12.1 30.3 23.1 7.22 3.03 5.44 4.35 1.32 30.4 2010/3/4 12.0 30.4 24.2 6.20 1.92 7.93 3.88 1.95 50.4 2010/3/15 12.4 30.6 24.3 6.31 2.84 3.71 3.93 1.10 27.9 2010/3/18 11.7 26.7 22.5 4.15 1.26 5.90 2.93 1.67 56.9 2010/3/19 13.2 29.8 22.8 6.95 0.95 7.01 4.22 3.28 77.6 2010/3/20 10.3 32.1 25.0 7.06 2.47 5.36 4.28 1.81 42.3 Total 88.76 436.93 181.94 94.57 52.0%

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able. The performance of the ECS will be at off-design conditions and the cooling capability of the ECS can be lost completely in cloudy or variable weather. In order to make the ejector operate at critical or non-critical double-choking condition to obtain a better performance, an electronic expansion valve was installed in the suction line of the ejector to regulate the opening of the expansion valve to control the evaporator temperature. A feedback tracking control system was designed for adjusting and tracking the evap-orator temperature which may vary with solar radiation intensity. This will make the SACH-2 always produce cooling effect even at lower solar radiation periods while the ejector performs at off-design conditions.

Field test results show that the regulation of the expansion valve is satisfactory and the ECS works properly and supplies cooling load at variable weather. The A/C input power reduction can be achieved at low solar radiation periods while the ejector is working at off-design condition. The reduction of input power of A/C is 50e70% due to the cooling performance of ECS. The long-term performance test results of SACH-2 from 2009/09/07 to 2010/03/20 show that the daily energy saving of SACH-2 is around 30_{e70% as compared to} the energy consumption of A/C alone (without solar-driven ECS). It is seen that the total energy saving of A/C due to the cooling performance of ECS is 52% over the entire test period.

Thefield test of SACH-2 at different weathers in the present study has verified the feasibility of optimal performance control of SACH-2 which keeps the ECS working at off-design double-choking condition by regulating the evaporator temperature. The feedback tracking control system is satisfactory although some defects still exist. The tracking error needs to be improved. This can be done by modifying thefilter correlation F.

Theﬁlter F is used to convert the instantaneous solar radiation intensity IT(t) into the setting value of evaporator temperature Te, set(t) for tracking control. The two signal converters Feand Fg(Fig. 6)

were determined experimentally fromﬁeld test. The SACH-2 was run 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 point of the ejector. The Te-signal converter Fewhich

converts Tginto Tewas derived (Fig. 7) from the test data collected

at the near-steady periods, about 20_{e30 min time interval, with} variable solar radiation intensity. The experiment was performed at 6 different generator temperatures over the range 85e110_{C. It}

takes two working days for thisﬁeld experiment. It will take longer for larger temperature range. However, this seems not necessary.

The determination of Tg-signal converter Fgis rather

compli-cated since the generator temperature will depend on the solar radiation intensity and the space cooling load (related to ambient temperature). It takes four working days in this experiment. More studies are required to obtain a better converter. The ﬁeld test results of SACH-2 presented previously show that the expansion valve operates at saturation state (full-open) very often, even at ejector off-design point (100C). The size of the chosen expansion valve may be too small and needs re-examined and replaced.

In the future work, improvement of theﬁlter Fecan also be made

using a precise ejector performance map if available. The feedback

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).

Nomenclature

HT daily-total solar radiation intensity incident upon the

collector slope, MJ m2day1

IT solar radiation intensity incident upon the collector slope,

W m2

ITf low-passedﬁltered signal of IT, W m2

Ta ambient temperature,C

Tg generator temperature of ECS,C

Te evaporator temperature of ECS,C

Te,set evaporator temperature setting of ECS,C

Ti collector inlet temperature,C

Troom room temperature,C

Qc condenser heat rejection rate in the heat-driven cooler, W

Qe evaporator heat transfer rate in split-type

air conditioner, W

Qg generator heat transfer rate in ejector cooling system, W

h

solar collector efﬁciency References

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