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Experimental assessment of solar absorption-subcooled compression hybrid cooling system
Solar Energy 185 (2019) 245–254
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Experimental assessment of solar absorption-subcooled compression hybrid cooling system
T
⁎
Jianting Yua,b,c, Zeyu Lia,b,c, , Erjian Chena,b,c, Yongrui Xua,b,c, Hongkai Chena,b,c, Le Wangd a
School of Electric Power, South China University of Technology, Guangzhou 510640, China Guangdong Province Key Laboratory of High Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China c Guangdong Province Engineering Research Center of High Efficient and Low Pollution Energy Conversion, Guangzhou 510640, China d State Key Laboratory of Compressor Technology, Hefei, Anhui 230031, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar cooling Experiment Absorption Subcooled compression Hybrid system
The solar absorption-subcooled compression hybrid cooling system (SASCHCS) is to be the economically feasible solution for the high-rise building. But the system has been seldom studied experimentally in the existing open literatures. Therefore, the prototype of SASCHCS is developed to measure the operational performance. The absorption subsystem is driven by the 27 m2 of stationary compound parabolic collector exclusively. And the test is based on the sunny day without and with fluctuation as well as cloudy day of subtropical Guangzhou. It is found that the outlet temperature of chilled water in the absorption chiller exceeds 22 °C since the cooling output of absorption subsystem serves as the subcooling power of compression subsystem. Consequently, the low grade solar energy of which temperature is higher than 60 °C can be used. The peak instantaneous cooling power and coefficient of performance (COP) of absorption subsystem is 4 kW and 0.69, respectively. Besides, the daily mean solar COP (SCOP) of absorption subsystem/daily average rise of COP in the compression subsystem on the sunny day, sunny day with fluctuation and cloudy day is 0.21/22.2%, 0.2/19.8% and 0.13/13.3%, respectively. The paper is helpful to adequately realize the real operation performance of SASCHCS and promote its further improvement.
1. Introduction Energy scarcity and environmental damage are two major issues restricting the social and economic development. It was predicted that the global energy demand rises by two times in 2050 (Guo and Zhou, 2017). The air conditioning plays an important role in the total energy demand in respect that the facility consumption accounts for nearly 50% of building energy demand (Pérez-Lombard et al., 2008). Besides, the excessive use of air conditioning deteriorates the global warming, in turn the increasing surrounding temperature simulates the growth of refrigeration system. Therefore, decreasing the consumption of air conditioning becomes increasingly popular in recent years (Aliane et al., 2016). Considering that solar energy is the most abundant renewable energy and the solar irradiance matches the cooling load of office building, the solar thermal cooling is to be a promising alternative to the conventional vapour compression chiller (Otanicar et al., 2012). The number of solar cooling facility rises rapidly in recent years. There are about 1000 solar cooling plants installed worldwide (Eicker
⁎
et al., 2015). Additionally, up to 1200 solar cooling systems are ready to build all over the world (Montagnino, 2017). The solar LiBr/H2O absorption chiller is the most economical in all the solar thermal cooling due to its relatively low investment cost and high performance (Kim and Infante Ferreira, 2008). It was reported that the solar absorption chiller lowers 11–48% of primary energy consumption and 16.7% of CO2 emission compared with the vapour compression chiller (Asdrubali and Grignaffini, 2005). Furthermore, it was concluded that the solar absorption chiller is suitable for the large-size building (Zhai et al., 2011) and the location in which the electric price is high (Mammoli et al., 2010). Although the performance of solar absorption chiller is acceptable for the residential building (Chen et al., 2017), its feasibility is poor for the high-rise building. The primary obstacle is that the area of collector is limited by the roof area so that the excessive auxiliary thermal is required to cover the huge cooling load of building (Fong et al., 2010). Consequently, the operational cost of corresponding plant goes up remarkably and is even higher than that of vapour compression chiller (Noro and Lazzarin, 2014). Similarly, the total cost of solar absorption chiller exceeds the one of traditional air conditioning (Eicker
Corresponding author at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. E-mail address: [email protected] (Z. Li).
https://doi.org/10.1016/j.solener.2019.04.055 Received 18 February 2019; Received in revised form 31 March 2019; Accepted 17 April 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 185 (2019) 245–254
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Nomenclature
A COP COP∗ Cp I m Q SCOP t T W
Subscripts
area of solar collector [m2] coefficient of performance increasing of coefficient of performance specific heat [kJ/kg °C] solar radiation [W/m2] mass flow rate [kg/s] energy gain/consumption [kW] solar coefficient of performance time [s] temperature [°C] work [kW]
a c col e g i k o ref s
absorption compression/Conde-nser collector evaporator generator inlet heat exchange outlet reference system solution
Greek symbols η
efficiency
solution of such hybrid cycle (Jain et al., 2015). Shirazi et al. (2016) performed the 3E assessment of this solution and found that the payback period is 36.2 yrs, 61.3% less than that of traditional solar absorption chiller with backup thermal. Another solution called solar absorption-subcooled compression hybrid cooling system (SASCHCS) was presented by Li et al. (2016), aiming to further shorten the payback period of solar cooling for the high-rise building. The cooling capacity of absorption subsystem serves as the subcooling power of compression subsystem in the SASCHCS so that the evaporator temperature of absorption subsystem grows and the exergy destruction of compression subsystem falls. Thus, it was found that the payback period of SASCHCS is close to the one of solar photovoltaic cooling (the most economical solution in recent) for the high-rise building (Li and Liu, 2019).
and Pietruschka, 2009) so that the government subsidy is still necessary for the solar cooling (Hang et al., 2011). Accordingly, to propose an economically feasible solution of solar refrigeration for the high-rise building is extremely important. One of the feasible solutions of solar cooling for the high-rise building is based on the absorption-compression hybrid refrigeration cycle in which the absorption subsystem is exclusively driven by solar energy and the compression subsystem serves as the backup refrigeration. The advantage of this solution lies in that the consumption of auxiliary energy goes down notably owing to the high performance of compression subsystem. The absorption-compression hybrid cooling facility with the parallel configuration, in which the absorption and compression subsystems are coupled by evaporation, is the simplest
Fig. 1. Schematic flow diagram of SACSHCS. 246
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the similar configuration is also found in the refrigeration system with the natural working fluid. It was reported that the performance of transcritical CO2 chiller goes up dramatically by the dedicated subcooling (Gullo et al., 2016). Llopis et al. (2016) found that the COP regarding the transcritical CO2 refrigeration system with the dedicated subcooling is 30.3% better than that of traditional facility without the subcooling by experiment. Furthermore, it was demonstrated that the performance of such machine is independent upon the refrigerant type of dedicated subcooling cycle (Llopis et al., 2015). It is gotten that there are few experiment researches of SASCHCS in the existing open literatures. Therefore, we developed the prototype of SASCHCS and performed the experimental study in the previous foundation. The test is based on three typical days including the sunny day without and with fluctuation as well as cloudy day. The novelty of paper is the demonstration of actual performance regarding the hybrid system in various typical days. The paper is helpful to adequately realize the real operation performance of SASCHCS and promote its further improvement.
The existing studies of SASCHCS mainly concern on the thermodynamic analysis as well as system design. The cycle characteristic of hybrid system and the coupling of two subsystems were investigated by Xu et al. (2016). Subsequently, Xu et al. (2017) carried out the 4E analysis of SASCHCS to derive the suitable working condition of facility. Li et al. (2018) built an easily solved off-design model of SASCHCS by the combination of characteristic equation of absorption subsystem and lumped parameter model of compression subsystem to simulate the system performance for different conditions. It was shown that the cooling output and coefficient of performance (COP) of absorption subsystem strongly relies on the compressor speed. For the facility design, it was obtained that the condenser and evaporator size of compression subsystem as well as cooling capacity of absorption subsystem is critical from the exergoeconomic viewpoint (Li et al., 2017a,b). The exergoeconomic design criterion of absorption subsystem size was studied in detail and presented by Jing et al. (2018). Liu et al. (2018) performed the 3E design with respect to the cooling power of absorption subsystem based on the annual data and found that the most suitable design should be in terms of the May meteorological data. Considering the cost-effective design of SASCHCS can be only guaranteed in the design case instead of the entire working range in the existing exergoeconomic design, the design guideline of hybrid system based on the variable condition was proposed by Jing et al. (2019) to keep the low product cost flow rate and relative cost difference according to the varied operation condition. In addition to the SASCHCS,
2. Prototype The construction and working principle of SASCHCS prototype is illustrated in Fig. 1. The picture of prototype is demonstrated in Fig. 2. The facility is placed in South China University of Technology (Guangzhou, 23.18 N, 113.27 E). It contains three subsystems: solar
Fig. 2. Picture of prototype. 247
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The experiment is carried out by the following steps:
thermal driving subsystem (collector and storage tank), absorption subsystem and compression subsystem. There is a closed water circulation between the evaporator of absorption subsystem and the subcooler of compression subsystem to deliver the cooling power of absorption chiller to the subcooler. Moreover, the heat dissipation of absorption subsystem and compression one is by the same cooling tower in parallel. The collectors are the stationary compound parabolic collector (CPC) installed towards the south with 20° tilted angle. Its aperture area is 27 m2. The working fluid of collector is water. The volume of storage tank is 200 L. The collector hot water pump P1 transfers the solar thermal from the collectors to the storage tank. And the generator hot water pump P2 circulates the hot water between the storage tank and the generator. The absorption subsystem is the water cooled single effect LiBr/H2O absorption chiller. The rated cooling capacity is 4.5 kW in 95 °C hot water. The size of absorption subsystem is designed according to the existing solar field (the specific size of absorption subsystem is about 0.167 kW/m2). All the heat exchangers except the solution heat exchanger (SHE) are the horizontal falling film heat exchanger. The SHE is the plate heat exchanger. The area of generator, condenser, evaporator, absorber and SHE is 0.79 m2, 0.46 m2, 0.73 m2, 0.79 m2 and 0.38 m2, respectively. The cooling water of condenser and absorber is driven by the water pump P3 and P4, respectively. Moreover, the chilled water is driven by pump P5 to offer the subcooling power of compression subsystem. The compression subsystem is the water cooled vapour compression chiller. The nominal cooling power is 34 kW. The size of compression subsystem is designed by the appropriate size ratio of absorption subsystem and compression subsystem (Li et al., 2016). Its refrigerant is R410A. A variable frequency scroll compressor is employed. All the heat exchangers are the plate heat exchanger. The area of condenser, evaporator and subcooler is 4.1 m2, 3.0 m2 and 0.36 m2, respectively. The electronic expansion valve (EEV) is set at the fixed superheating mode. The corresponding superheating is 8 °C. The gas liquid separator located between the evaporator and compressor guarantees that only the vapour refrigerant flows into the compressor. In particularly, the receiver installed between the condenser and subcooler is to access the remarkable subcooling (Koeln and Alleyne, 2014). The volume of separator and receiver is 8 L and 6 L, respectively. In addition, the cooling water and chilled water is driven by the water pump P6 and P7, respectively. The flow rate of each water pump is listed at Table 1. It is mentioned that all the flow rates are constant during the experiment. The higher flow rate of chilled water in the compression subsystem is to avoid the frosting of copper tube located between the EEV outlet and evaporator inlet by low evaporator temperature (< 5 °C). Besides, the compressor speed is reduced so that the cooling output of compression subsystem is controlled at about 15 kW during the experiment, also preventing the above-mentioned frosting led by high cooling power. Especially, a rotary vane vacuum pump is used to keep the vacuum of absorption subsystem (works 3 times in a month).
Step1: The collector hot water pump P1 is turned on to heat the water inside the storage tank when the tank is filled with water. Subsequently, the cooling water pump P6, chilled water pump P7 and compressor are started. Step2: The generator hot water pump P2 is turned on when the top layer temperature of storage tank is high enough to drive the absorption subsystem. Meanwhile, the water pump P3, P4 and P5 are started to activate the absorption subsystem. In addition, the system is turned off and the experiment finishes when the inlet temperature of generator hot water is less than 60 °C. For the k-th heat exchanger, the corresponding heat load is derived:
Qk = mk Cp |Tok − Tik |
(1)
The instantaneous COP of absorption subsystem and compression subsystem is calculated as follows:
COPa =
Qea Qg
(2)
COPc =
Qec W
(3)
The daily mean efficiency of collectors, daily mean COP of absorption subsystem and compression subsystem as well as daily mean solar COP (SCOP) is:
∫ Qcol dt ∫ IA col dt
(4)
COPa . m =
∫ Qea dt ∫ Qg dt
(5)
COPc . m =
∫ Qec dt ∫ Wdt
(6)
SCOPm =
∫ Qea dt ∫ IA col dt
ηcol . m =
(7)
COP ∗
The relative increase of COP ( ) is defined to assess the performance growth of SASCHCS compared to the reference system (the compression subsystem without the subcooling is used as the reference system):
COP ∗ =
COPc, m − COPref COPref
(8)
It is noteworthy that the reference system operates in the identical condition with the SASCHCS, i.e., inlet temperature and flow rate of cooling water and chilled water as well as compressor speed of both systems is same. The relative mean error (RME) is calculated by the following expression (Pei et al., 2012) N
3. Experimental procedure
RME =
The meteorological data is measured by the weather station of which mode is DAVIS Vantage Pro2 (deviation of solar irradiance and surrounding temperature is 0.5% of the reading and ± 0.5 °C, respectively). The temperature is tested by the T-type thermocouples (deviation is 0.1% of the reading) located at the outside surface of copper pipe with insulation. The water flow rate is measured by the turbine volumetric flow meters (deviation is less than 1% of the reading). The power meter (deviation is 0.5%) is employed to test the compressor work directly. The experimental data is recorded and transferred to computer by Agilent 34970A and it is updated by the data acquisition program (compiled with LABVIEW) at intervals of 10 s.
RE =
∑1 |RE| (9)
N
dy ∂y dx1 ∂y dx2 ∂y dx n = + + ...+ y ∂x1 y ∂x2 y ∂x n y
(10)
The corresponding RME of experimental data is shown in Table 2. Table 1 Flow rate of water pump.
3
Flow rate of water pump (m /h)
248
P1
P2
P3
P4
P5
P6
P7
0.8
1.0
0.8
1.0
1.0
2.9
3.9
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subsystem is quadratic as well. The peak is 4 kW during the test, which is 11.1% less than the nominal. It is thought that the difference is primarily caused by the pressure drop and heat loss. Besides, it is seen that the cooling output of absorption subsystem changes less from 12:00 to 16:00. The higher the cooling power of absorption chiller is, the lower the temperature of its chilled water becomes. Hence, the variation of chilled water temperature is inversely coincident to that of cooling capacity in the absorption subsystem. It is observed that the least inlet/ outlet temperature of chilled water in the absorption subsystem is 26 °C/22.5 °C. As expected, the relatively higher temperature of chilled water is attributed to that the cooling capacity of absorption subsystem serves as the subcooling power of compression subsystem. Nevertheless, the actual chilled water temperature of absorption subsystem is about 10 °C less than the simulation in our previous study (Li et al., 2016). Such difference is caused by the additional temperature drop of heat transfer led by the separate assembling of evaporator in the absorption chiller and subcooler in the prototype. It should be mentioned that the above-mentioned assembling is extremely convenient for the development of hybrid system though it leads to the extra exergy destruction. The working characteristic of compression subsystem on 25 August is displayed in Fig. 8. Similarly, the corresponding sudden variation is led by the periodic oil return of compressor as well. It is found that the condenser temperature of compression subsystem enhances from 37 °C to 40 °C and the evaporator temperature of compression subsystem goes down from 8.5 °C to 7 °C owing to the working of absorption subsystem. The above-mentioned phenomenon is primarily attributed to the improvement of heat transfer rate in the heat exchanger. But the compressor work nearly maintains at 3.05 kW during the entire experiment, which can be explained by the reduction of suction density as well as enhancement of pressure ratio (Li et al., 2018). Additionally, the variation of cooling power and subcooling in the compression subsystem is similar to the one of subcooling power in respect that they are influenced by the cooling capacity of absorption subsystem. It is found that the daily mean cooling output of compression subsystem grows from 15.5 kW to 18.95 kW and the subcooling nearly maintains at 15 °C due to the subcooling power offered by the absorption subsystem. The COP of absorption subsystem and compression one on 25 August is shown in Fig. 9. The behavior of COP in the absorption subsystem is quadratic, which is similar to that of inlet temperature in the generator hot water. The peak COP of absorption subsystem reaches to 0.69 near 14:00. It is noteworthy that the significant decrease of heat load in the generator as well as the evaporator inertia leads to the unreasonable rise of COP in the absorption subsystem at the final part of experiment (15:00–17:33), which was also reported by González-Gil et al. (2011). In addition, the COP of compression subsystem increases
Table 2 RME of experimental data. Varibles
Qcol
Qg
Qea
Qec
ηcol
SCOP
COPa
COPc
RME
1.97%
3.29%
2.35%
1.79%
5.9%
0.64%
1.19%
0.69%
4. Results and discussion Although the daily meteorological data is varied, it can be primarily divided into three types in terms of the available solar irradiance: sunny day (the solar irradiance is relatively stable and strong), sunny day with fluctuation (the solar irradiance is moderate and changes frequently, i.e., sometimes is sunny and sometimes is cloudy) and cloudy day (the solar irradiance is weak). It can be inferred that the behavior of solar cooling in the same typical day is similar. Hence, the experiment in three typical days is implemented to adequately examine the prototype performance. The experiment result associated with the sunny day without fluctuation, sunny day with fluctuation and cloudy day is carried out on 25 August, 05 September and 10 September, respectively. The inlet temperature of cooling water of 25 August, 05 September and 10 September is 32 °C, 32 °C and 31 °C, respectively. The solar irradiance and surrounding temperature of three days are displayed in Figs. 3 and 4, respectively. It is seen that the ambient temperature and the maximal solar irradiance of 05 September approaches to that of 25 August. The fluctuation of solar irradiance on 05 September is mainly from 9:00 to 15:00. And the highest surrounding temperature of sunny day and cloudy day is 36 °C and 30 °C, respectively. 4.1. Results of sunny day without fluctuation (25 August 2018) The experimental data of collector on 25 August is demonstrated in Fig. 5. The hot water pump P1 is turned on at 10:05. The collector temperature rises quickly and reaches to 95 °C at 10:28. Subsequently, the sudden drop of collector temperature is caused by the start of absorption subsystem. It is observed that the outlet temperature of collector goes up to the 94 °C and gradually reduces to 60 °C after the operation of absorption subsystem. Meanwhile, the inlet temperature of collector nearly maintains at 71 °C before 15:30 and goes down to 56 °C slowly in the end of test. In addition, the change of instantaneous useful heat in the collector is coincident to that of collector outlet temperature. It goes up to 18 kW and then gradually drops to 13 kW after the operation of absorption subsystem. The daily mean useful heat of collector is 11.30 kW. On the other hand, the average efficiency of collector is 0.69 during the most period of experiment, benefiting from the low collector temperature as well as strong solar irradiance (Nie et al., 2017). The measurement data of generator at 25 August is exhibited in Fig. 6. The hot water pump P2 is turned on to drive the absorption subsystem when the top layer temperature of storage tank equals to 87 °C (at 10:28). Accordingly, the inlet temperature of generator hot water comes down to 79 °C sharply due to the consumption of generator. Subsequently, it grows to 93 °C and gradually falls to 60 °C at 17:33. Furthermore, it is seen that the temperature drop of hot water in the generator is about 6 °C in most time of experiment. The heat load of generator is similar to the inlet temperature of generator hot water, as expected. It rises to 9 kW at first and goes down to 2.8 kW at end of experiment. With the heat delivering from the hot water, the solution temperature enhances by 15 °C when LiBr/H2O flows through the generator. The temperature of chilled water and cooling power of absorption subsystem at 25 August is shown in Fig. 7. It is noteworthy that the sudden growth of temperature as well as cooling capacity is caused by the periodic oil return of compressor. Similar to the inlet temperature of generator hot water, the trend of cooling capacity in the absorption
Solar radiation (W/m2)
800
600
400
200
0 00:00
04:00
08:00
25-Aug-2018
12:00
16:00
Time (hr) 05-Sep-2018
Fig. 3. Daily solar irradiance. 249
20:00
24:00
10-Sep-2018
10 30 8
34
Temperature ( C)
Surrounding temperature (°C)
36
32 30
28
25 6 20 4
15
2
26
04:00
08:00
12:00
16:00
20:00
Time (hr) 05-Sep-2018
25-Aug-2018
10 10:00
24:00
11:00
12:00
13:00
14:00
15:00
16:00
Time (hr) Toea Tiea
10-Sep-2018
17:00
0 18:00
Qea
Fig. 7. Experimental data of absorption subsystem evaporator on 25-Aug-2018.
Fig. 4. Daily surrounding temperature.
24
60
30 90
70 18
60 50
12
40 6
30
Toc
20 10:00
11:00
12:00
Tic 13:00
Temperature ( C)
Temperature (°C)
24
Useful heat of collector (kW)
50
80
18
40 12 30 6 20 0
10
Qcol 14:00
15:00
16:00
17:00
0 10:00
0 18:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
Cooling output of compression subsystem and compressor work (kW)
24 00:00
Cooling capacity of absorption subsystem (kW)
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-6 18:00
Time (hr)
Time(hr)
Tc
Fig. 5. Measurement data of collector on 25-Aug-2018.
Te
Ts
Qec
W
Fig. 8. Measurement data of compression subsystem on 25-Aug-2018. 10
150
1.0
9
6
90 4
75 60
2
6
3 0.4 0 0.2
45 30 10:00
0.6
COPc
Temperature (°C)
105
0.8
COPa
8 120
Heat load of generator (kW)
135
COPa 11:00
12:00
Tig
13:00
14:00
15:00
Time (hr) Tog Tisg
16:00
17:00
Tosg
0 18:00
0.0 10:00
Qg
11:00
12:00
13:00
14:00
COPc 15:00
16:00
17:00
-3 18:00
Time (hr)
Fig. 6. Test data of generator on 25-Aug-2018.
Fig. 9. COP of 25-Aug-2018.
from 5.10 to 6.21 after the working of the absorption subsystem.
Fig. 10. The P1 is turned on at 9:30 to heat the water of storage tank by solar energy. It is clear that the temperature and useful heat of collector fluctuates dramatically because of the frequent change of solar irradiance. Moreover, the notable fluctuation of solar irradiance also deteriorates the collector performance compared to the data of 25 August,
4.2. Results of sunny day with fluctuation (05 September 2018) The experimental data of collector on 05 September is exhibited in 250
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4.3. Results of cloudy day (10 September 2018)
80
The measurement of collector on 10 September is demonstrated in Fig. 15. The water pump P1 is turned on at 10:00. Since the solar irradiance is weak on the cloudy day, the heating of hot water before the start of absorption subsystem is longer. It takes 95 mins to heat the top layer water of the storage tank to the setting temperature. Subsequently, the outlet temperature of collector reduces from 74 °C to 60 °C after the work of absorption subsystem. Additionally, the weak solar irradiance seriously decreases the collector performance, i.e., the daily average useful heat of collector on 10 September is 45.2% poorer than that on 25 August. And the daily mean efficiency of collector on 10 September is 0.61 and goes down by 11.6% compared to the data on 25 August. The experimental data of generator on 10 September is displayed in Fig. 16. The absorption subsystem is activated to work when the top layer temperature of storage tank is heated to 75 °C (at 11:35). It is seen that the temperature of generator hot water gradually comes down in respect that the consumption of generator exceeds the solar thermal gain. Compared to the data of 25 August, the daily mean heat load of generator on 10 September falls by 15.8%. And the temperature improvement of solution in the generator is about 10.5 °C. The temperature of chilled water and cooling capacity in the absorption subsystem on 10 September is exhibited in Fig. 17. It is obvious that the cooling power of absorption subsystem is poor on the cloudy day, i.e., the daily average one drops by 23.4% compared with the data of 25 August. Besides, the minimum inlet and outlet temperature of chilled water in the absorption subsystem is 27 °C and 25 °C, respectively. The thermodynamic characteristic of compression subsystem on 10 September is shown in Fig. 18. It is obtained that the condenser temperature, evaporator one and cooling capacity of compression subsystem vary slightly due to the weak subcooling power. The condenser temperature grows from 36 °C to 38.5 °C and the evaporator temperature falls from 10.2 °C to 8.6 °C after the operation of absorption subsystem. Additionally, the daily mean cooling output of compression subsystem enhances from 15.3 kW to 17.9 kW by the subcooling. The corresponding subcooling of compression subsystem is about 10 °C. The COP of absorption subsystem and compression one on 10 September is demonstrated in Fig. 19. It is easily inferred that the performance of hybrid system reduces dramatically due to the weak solar irradiance. The daily average COP of absorption subsystem on 10 September is 9% lower than that on 25 August. On the other hand, the COP of compression subsystem goes up from 5.20 to 5.91 by the subcooling, which means the corresponding rise of COP in the compression
Temperature (°C)
60
18
50 12
40 30
6 20
Toc
10 09:00
Tic
Useful heat of collector (kW)
24 70
Qcol 0
10:00
11:00
12:00
13:00
Time (hr)
14:00
15:00
16:00
Fig. 10. Measurement data of collector on 05-Sep-2018.
i.e., the highest outlet temperature of collector (after the working of absorption chiller) is 85 °C. Furthermore, the daily mean useful heat of collector on 05 September is 9.64 kW and 14.7% less than the one on 25 August. Similarly, the daily average efficiency of collector on 05 September is 0.64 and 7.2% lower than that on 25 August. The measurement of generator on 05 September is displayed in Fig. 11. The absorption subsystem is activated to work as the top layer temperature reaches to 80 °C (at 9:50). It is clear that the temperature of hot water and solution in the generator changes frequently as a result of fluctuation in the collector outlet temperature. In addition, the temperature of generator hot water, consumption of generator and temperature rise of solution on 05 September are all lower than that on 25 August due to the drop of useful heat in the collector. It is obtained that the temperature growth of solution and the peak inlet temperature of generator hot water are 13 °C, 83 °C, respectively. And the daily average heat load of generator on 05 September is 10.2% weaker than that on 25 August. The chilled water temperature and cooling output of absorption subsystem on 05 September is shown in Fig. 12. The cooling power of absorption subsystem becomes instable and goes down owing to the similar behavior of inlet temperature in the generator hot water, as expected. It is found that the daily mean cooling capacity of absorption subsystem on 05 September is 12% poorer than the one on 25 August. And the lowest inlet/outlet temperature of chilled water in the absorption subsystem is 28 °C/25 °C. The measurement data of thermodynamic characteristic in the compression subsystem on 05 September can be seen in Fig. 13. There is not significant fluctuation of condenser temperature, evaporator temperature and compressor work in the compression subsystem though the solar irradiance changes frequently. It is observed that the condenser temperature rises from 39 °C to 41 °C and the evaporator temperature drops from 11 °C to 10 °C after the working of absorption subsystem. The compressor work is nearly fixed at 3.05 kW regardless of subcooling. However, the cooling output and subcooling of compression subsystem after the operation of absorption subsystem emerges the notable fluctuation, as expected. The daily average cooling capacity of compression subsystem goes up from 16.4 kW to 19.3 kW by the subcooling. And the corresponding subcooling is 14 °C. The COP of absorption and compression subsystems on 05 September is shown in Fig. 14. It is obvious that the fluctuation of solar irradiance is adverse to the performance of SASCHCS. The daily average COP of absorption subsystem on 05 September is 2% less than that on 25 August. Furthermore, the COP of compression subsystem grows from 5.30 to 6.36 by the subcooling. It is derived that the above-mentioned enhancement of COP in the compression subsystem at 05 September is 7% weaker than the one at 25 August.
10
150 135
8
Temperature (°C)
120 105
6
90 4
75 60
2 45 30
0 10:00
11:00
Tig
12:00
13:00
Time (hr) Tog Tisg
14:00
15:00
16:00
Tosg
Fig. 11. Test data of generator on 05-Sep-2018. 251
Qg
Heat load of generator (kW)
90
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25
6
20
4
15
2
0
10 12:00
13:00
14:00
Time (hr) Toea Tiea
15:00
16:00
Toc
Qea
40 12 30
8
20
4 0
10
-4
Tc
Time (hr) Te Ts
14:00
15:00
Qec
11:00
Qcol 0 13:00
12:00
Time (hr)
10
135 120
Temperature (°C)
Temperature (°C)
16
13:00
Tic
150
Cooling output of compression subsystem and compressor work (kW)
20
50
12:00
6
40 10:00
24
11:00
12
Fig. 15. Measurement data of collector on 10-Sep-2018.
60
10:00
60
50
Fig. 12. Experimental data of absorption subsystem evaporator on 05-Sep2018.
0 09:00
18
8
105 6 90 4
75 60
2
45 0
30 11:30
12:00
12:30
Time (hr) Tog Tisg
Tig
16:00
13:00
Tosg
Qg
Fig. 16. Test data of generator on 10-Sep-2018.
W
Fig. 13. Measurement data of compression subsystem on 05-Sep-2018.
10
30
8
8
1.2
6
0.8
4
0.6
2
0.4
0
Temperature (°C)
1.0
COPc
COPa
25 6 20 4
15
0.2
0.0 09:00
COPa
-2
COPc
10 11:30
-4 10:00
11:00
12:00
13:00
14:00
15:00
Heat load of generator (kW)
11:00
70
2
12:00
Time (hr)
Tochw
16:00
Time (hr)
0 13:00
12:30
Tichw
Cooling capacity of absorption subsystem (kW)
10:00
24
Useful heat of collector (kW)
8
80
Temperature (°C)
Temperature (°C)
30
30
Cooling capacity of absorption subsystem (kW)
10
35
Qea
Fig. 17. Experimental data of absorption subsystem evaporator on 10-Sep2018.
Fig. 14. COP of 05-Sep-2018.
subsystem is 0.71.
specifically in Fig. 20. In general, the stronger solar irradiance is favorable for the operation of facility. The daily mean efficiency of collector on 25 August, 05 September and 10 September is 0.692, 0.640 and 0.614, respectively. It is derived that the daily average COP of absorption subsystem on 25 August, 05 September and 10 September is
4.4. Summary of experiment The performance indicator of three typical days is shown 252
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Temperature (°C)
50 12
40
30 6 20 0
10
0 10:00
10:30
11:00
Tc
11:30
Time (hr) Te Ts
12:00
-6 13:00
12:30
Qce
and 38.1% compared with the one of 25 August. The detailed results of experiment days are listed in Table 3. It is noteworthy that the daily calculation is based on experimental period. The daily mean solar irradiance of 25 August, 05 September and 10 September is 604.7 W/m2, 557.6 W/m2 and 373.3 W/m2, respectively. It is well known that the stronger the solar irradiance is, the higher the energy gain/consumption becomes. As a consequence, the daily average useful heat of collector on 25 August, 05 September and 10 September is 11.30 kW, 9.64 kW and 6.19 kW, respectively. And it is gotten that the daily mean consumption of generator on 25 August, 05 September and 10 September is 6.99 kW, 6.28 kW and 5.88 kW, respectively. Since the higher heat load of generator and temperature of hot water results in better cooling output, the daily average cooling capacity of absorption subsystem on 25 August, 05 September and 10 September is 3.62 kW, 3.18 kW and 2.77 kW, respectively. Similarly, the daily mean enhancement of cooling power for the compression subsystem on 25 August, 05 September and 10 September is 3.44 kW, 3.20 kW and 2.04 kW, respectively. In general, it is displayed that the daily average SCOP and daily mean COP of absorption subsystem is 0.13–0.2 and 0.43–0.54, respectively from the measurement of 15 days, depending on the solar irradiance. The 15-day experiment of prototype is carried out and the results of three typical days are discussed in detail. Subsequently, the long term measurement of SASCHCS for different seasons is to be performed. And the consumption of cooling tower in the absorption subsystem will be measured to analyze the energy saving more exactly.
Cooling output of compression subsystem and compressor work (kW)
18
W
2.0
8
1.5
6
1.0
4
0.5
2
COPa
0.0 10:00
10:30
COPc
11:00
11:30
12:00
Time (hr)
12:30
COPc
COPa
Fig. 18. Measurement data of compression subsystem on 10-Sep-2018.
5. Conclusion The prototype of SASCHCS is developed and measured on the sunny day without and with fluctuation as well as cloudy day of subtropical Guangzhou. The experiment results are summarized as follows: (1). The absorption subsystem produces the high temperature cooling since it serves as the heat sink of subcooler in the compression subsystem, i.e., the outlet temperature of chilled water in the absorption subsystem exceeds 22 °C. Thereby, the low grade solar thermal of which temperature is greater 60 °C can be employed. The highest instantaneous cooling power and COP of absorption chiller is 4 kW and 0.69, respectively. (2). For the sunny day (25 August) of which daily mean solar irradiance is 604.7 W/m2, the daily average cooling capacity, COP and SCOP of absorption subsystem are 3.62 kW, 0.52 and 0.21, respectively. And it is obtained that the daily mean COP of compression subsystem goes up by 22.2% by the subcooling power offered by the absorption chiller. (3). It is observed that the temperature of hot water, subcooling and cooling output of two subsystems change frequently on the sunny day with fluctuation (05 September). The corresponding daily average solar irradiance is 557.6 W/m2. It is exhibited that the daily mean cooling capacity, COP and SCOP of absorption subsystem are 3.18 kW, 0.51 and 0.20, respectively. Additionally, the daily average COP of compression subsystem rises by 19.8% by the cooperation of absorption chiller. (4). The facility performance goes down remarkably on the cloudy day (10 September). The daily mean solar irradiance is 373.3 W/m2. It is derived that the daily average cooling capacity, COP and SCOP of absorption subsystem are 2.77 kW, 0.47 and 0.13, respectively. On the other hand, the improvement of daily mean COP in the compression subsystem is just 13.3% due to the weak output of absorption subsystem.
0 13:00
Fig. 19. COP of 10-Sep-2018. 0.5
0.4
0.6 0.5
0.3
COP*
Energy conversion efficiency
0.7
0.4 0.3
0.2
0.2 0.1 0.1 0.0
25-Aug-2018 col
05-Sep-2018 COPa
SCOP
10-Sep-2018
0.0
COP*
Fig. 20. Performance indicator of three days.
0.517, 0.507 and 0.470, respectively. Similarly, the daily mean SCOP of absorption subsystem on 25 August, 05 September and 10 September is 0.21, 0.20 and 0.13, respectively. Besides, the relative increase of COP in the hybrid system on 25 August, 05 September and 10 September is 22.2%, 19.8% and 13.3%, respectively. It is exhibited that the daily mean SCOP of 05 September and 10 September comes down by 4.7%
Acknowledgement This work is supported by: (1) Natural Science Foundation of Guangdong Province under the contract No. 2018A030313310, (2) Key 253
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Table 3 Summary of experiment results. Day
Duration (min) Entire
Data of three typical days 25Aug 448 05 Sep 403 10 Sep 177 Data of other days 01 Aug 432 07 Aug 343 08 Aug 506 09 Aug 511 13 Aug 134 14 Aug 242 22 Aug 314 24 Aug 421 04 Sep 224 06 Sep 438 11 Sep 492 24 Sep 391
Daily energy gain/consumption (kWh)
Performance indicator
SASCHCS
I
Qc
Qg
Qea
Qec
ηc
SCOP
COPa
425 383 82
121.96 101.17 29.73
84.43 64.78 18.26
49.51 40.07 8.04
25.61 20.3 3.78
134.17 123.84 49.01
0.69 0.64 0.61
0.21 0.20 0.13
0.52 0.51 0.47
342 298 453 456 87 198 252 368 120 363 441 333
93.88 72.72 136.41 131.53 25.18 52.47 73.61 99.18 48.23 99.9 126.23 90
62.48 49.19 94.59 88.07 15.45 34.52 48. 64.53 32.18 64.09 86.01 57.81
34.04 31.99 52.86 50.02 8.84 19.77 27.02 36.68 12.41 36.15 45.66 35.3
17.31 14.75 25.17 24.73 4.03 9.69 11.71 19.04 6.28 19.02 24.44 17.07
119.82 94.29 144.08 142.08 37.21 67.42 84.12 117.36 60.49 125 137.36 112.75
0.67 0.68 0.69 0.67 0.61 0.66 0.65 0.65 0.67 0.64 0.68 0.64
0.18 0.20 0.18 0.19 0.16 0.18 0.16 0.19 0.13 0.19 0.19 0.19
0.51 0.46 0.48 0.49 0.46 0.49 0.43 0.52 0.51 0.53 0.54 0.48
Laboratory of Compressor Technology under the contract No. SKLYSJ201806, (3) Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes under the contract No. KLB10004.
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Experimental assessment of solar absorption-subcooled compression hybrid cooling system
T
⁎
Jianting Yua,b,c, Zeyu Lia,b,c, , Erjian Chena,b,c, Yongrui Xua,b,c, Hongkai Chena,b,c, Le Wangd a
School of Electric Power, South China University of Technology, Guangzhou 510640, China Guangdong Province Key Laboratory of High Efficient and Clean Energy Utilization, South China University of Technology, Guangzhou 510640, China c Guangdong Province Engineering Research Center of High Efficient and Low Pollution Energy Conversion, Guangzhou 510640, China d State Key Laboratory of Compressor Technology, Hefei, Anhui 230031, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar cooling Experiment Absorption Subcooled compression Hybrid system
The solar absorption-subcooled compression hybrid cooling system (SASCHCS) is to be the economically feasible solution for the high-rise building. But the system has been seldom studied experimentally in the existing open literatures. Therefore, the prototype of SASCHCS is developed to measure the operational performance. The absorption subsystem is driven by the 27 m2 of stationary compound parabolic collector exclusively. And the test is based on the sunny day without and with fluctuation as well as cloudy day of subtropical Guangzhou. It is found that the outlet temperature of chilled water in the absorption chiller exceeds 22 °C since the cooling output of absorption subsystem serves as the subcooling power of compression subsystem. Consequently, the low grade solar energy of which temperature is higher than 60 °C can be used. The peak instantaneous cooling power and coefficient of performance (COP) of absorption subsystem is 4 kW and 0.69, respectively. Besides, the daily mean solar COP (SCOP) of absorption subsystem/daily average rise of COP in the compression subsystem on the sunny day, sunny day with fluctuation and cloudy day is 0.21/22.2%, 0.2/19.8% and 0.13/13.3%, respectively. The paper is helpful to adequately realize the real operation performance of SASCHCS and promote its further improvement.
1. Introduction Energy scarcity and environmental damage are two major issues restricting the social and economic development. It was predicted that the global energy demand rises by two times in 2050 (Guo and Zhou, 2017). The air conditioning plays an important role in the total energy demand in respect that the facility consumption accounts for nearly 50% of building energy demand (Pérez-Lombard et al., 2008). Besides, the excessive use of air conditioning deteriorates the global warming, in turn the increasing surrounding temperature simulates the growth of refrigeration system. Therefore, decreasing the consumption of air conditioning becomes increasingly popular in recent years (Aliane et al., 2016). Considering that solar energy is the most abundant renewable energy and the solar irradiance matches the cooling load of office building, the solar thermal cooling is to be a promising alternative to the conventional vapour compression chiller (Otanicar et al., 2012). The number of solar cooling facility rises rapidly in recent years. There are about 1000 solar cooling plants installed worldwide (Eicker
⁎
et al., 2015). Additionally, up to 1200 solar cooling systems are ready to build all over the world (Montagnino, 2017). The solar LiBr/H2O absorption chiller is the most economical in all the solar thermal cooling due to its relatively low investment cost and high performance (Kim and Infante Ferreira, 2008). It was reported that the solar absorption chiller lowers 11–48% of primary energy consumption and 16.7% of CO2 emission compared with the vapour compression chiller (Asdrubali and Grignaffini, 2005). Furthermore, it was concluded that the solar absorption chiller is suitable for the large-size building (Zhai et al., 2011) and the location in which the electric price is high (Mammoli et al., 2010). Although the performance of solar absorption chiller is acceptable for the residential building (Chen et al., 2017), its feasibility is poor for the high-rise building. The primary obstacle is that the area of collector is limited by the roof area so that the excessive auxiliary thermal is required to cover the huge cooling load of building (Fong et al., 2010). Consequently, the operational cost of corresponding plant goes up remarkably and is even higher than that of vapour compression chiller (Noro and Lazzarin, 2014). Similarly, the total cost of solar absorption chiller exceeds the one of traditional air conditioning (Eicker
Corresponding author at: School of Electric Power, South China University of Technology, Guangzhou 510640, China. E-mail address: [email protected] (Z. Li).
https://doi.org/10.1016/j.solener.2019.04.055 Received 18 February 2019; Received in revised form 31 March 2019; Accepted 17 April 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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Nomenclature
A COP COP∗ Cp I m Q SCOP t T W
Subscripts
area of solar collector [m2] coefficient of performance increasing of coefficient of performance specific heat [kJ/kg °C] solar radiation [W/m2] mass flow rate [kg/s] energy gain/consumption [kW] solar coefficient of performance time [s] temperature [°C] work [kW]
a c col e g i k o ref s
absorption compression/Conde-nser collector evaporator generator inlet heat exchange outlet reference system solution
Greek symbols η
efficiency
solution of such hybrid cycle (Jain et al., 2015). Shirazi et al. (2016) performed the 3E assessment of this solution and found that the payback period is 36.2 yrs, 61.3% less than that of traditional solar absorption chiller with backup thermal. Another solution called solar absorption-subcooled compression hybrid cooling system (SASCHCS) was presented by Li et al. (2016), aiming to further shorten the payback period of solar cooling for the high-rise building. The cooling capacity of absorption subsystem serves as the subcooling power of compression subsystem in the SASCHCS so that the evaporator temperature of absorption subsystem grows and the exergy destruction of compression subsystem falls. Thus, it was found that the payback period of SASCHCS is close to the one of solar photovoltaic cooling (the most economical solution in recent) for the high-rise building (Li and Liu, 2019).
and Pietruschka, 2009) so that the government subsidy is still necessary for the solar cooling (Hang et al., 2011). Accordingly, to propose an economically feasible solution of solar refrigeration for the high-rise building is extremely important. One of the feasible solutions of solar cooling for the high-rise building is based on the absorption-compression hybrid refrigeration cycle in which the absorption subsystem is exclusively driven by solar energy and the compression subsystem serves as the backup refrigeration. The advantage of this solution lies in that the consumption of auxiliary energy goes down notably owing to the high performance of compression subsystem. The absorption-compression hybrid cooling facility with the parallel configuration, in which the absorption and compression subsystems are coupled by evaporation, is the simplest
Fig. 1. Schematic flow diagram of SACSHCS. 246
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the similar configuration is also found in the refrigeration system with the natural working fluid. It was reported that the performance of transcritical CO2 chiller goes up dramatically by the dedicated subcooling (Gullo et al., 2016). Llopis et al. (2016) found that the COP regarding the transcritical CO2 refrigeration system with the dedicated subcooling is 30.3% better than that of traditional facility without the subcooling by experiment. Furthermore, it was demonstrated that the performance of such machine is independent upon the refrigerant type of dedicated subcooling cycle (Llopis et al., 2015). It is gotten that there are few experiment researches of SASCHCS in the existing open literatures. Therefore, we developed the prototype of SASCHCS and performed the experimental study in the previous foundation. The test is based on three typical days including the sunny day without and with fluctuation as well as cloudy day. The novelty of paper is the demonstration of actual performance regarding the hybrid system in various typical days. The paper is helpful to adequately realize the real operation performance of SASCHCS and promote its further improvement.
The existing studies of SASCHCS mainly concern on the thermodynamic analysis as well as system design. The cycle characteristic of hybrid system and the coupling of two subsystems were investigated by Xu et al. (2016). Subsequently, Xu et al. (2017) carried out the 4E analysis of SASCHCS to derive the suitable working condition of facility. Li et al. (2018) built an easily solved off-design model of SASCHCS by the combination of characteristic equation of absorption subsystem and lumped parameter model of compression subsystem to simulate the system performance for different conditions. It was shown that the cooling output and coefficient of performance (COP) of absorption subsystem strongly relies on the compressor speed. For the facility design, it was obtained that the condenser and evaporator size of compression subsystem as well as cooling capacity of absorption subsystem is critical from the exergoeconomic viewpoint (Li et al., 2017a,b). The exergoeconomic design criterion of absorption subsystem size was studied in detail and presented by Jing et al. (2018). Liu et al. (2018) performed the 3E design with respect to the cooling power of absorption subsystem based on the annual data and found that the most suitable design should be in terms of the May meteorological data. Considering the cost-effective design of SASCHCS can be only guaranteed in the design case instead of the entire working range in the existing exergoeconomic design, the design guideline of hybrid system based on the variable condition was proposed by Jing et al. (2019) to keep the low product cost flow rate and relative cost difference according to the varied operation condition. In addition to the SASCHCS,
2. Prototype The construction and working principle of SASCHCS prototype is illustrated in Fig. 1. The picture of prototype is demonstrated in Fig. 2. The facility is placed in South China University of Technology (Guangzhou, 23.18 N, 113.27 E). It contains three subsystems: solar
Fig. 2. Picture of prototype. 247
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The experiment is carried out by the following steps:
thermal driving subsystem (collector and storage tank), absorption subsystem and compression subsystem. There is a closed water circulation between the evaporator of absorption subsystem and the subcooler of compression subsystem to deliver the cooling power of absorption chiller to the subcooler. Moreover, the heat dissipation of absorption subsystem and compression one is by the same cooling tower in parallel. The collectors are the stationary compound parabolic collector (CPC) installed towards the south with 20° tilted angle. Its aperture area is 27 m2. The working fluid of collector is water. The volume of storage tank is 200 L. The collector hot water pump P1 transfers the solar thermal from the collectors to the storage tank. And the generator hot water pump P2 circulates the hot water between the storage tank and the generator. The absorption subsystem is the water cooled single effect LiBr/H2O absorption chiller. The rated cooling capacity is 4.5 kW in 95 °C hot water. The size of absorption subsystem is designed according to the existing solar field (the specific size of absorption subsystem is about 0.167 kW/m2). All the heat exchangers except the solution heat exchanger (SHE) are the horizontal falling film heat exchanger. The SHE is the plate heat exchanger. The area of generator, condenser, evaporator, absorber and SHE is 0.79 m2, 0.46 m2, 0.73 m2, 0.79 m2 and 0.38 m2, respectively. The cooling water of condenser and absorber is driven by the water pump P3 and P4, respectively. Moreover, the chilled water is driven by pump P5 to offer the subcooling power of compression subsystem. The compression subsystem is the water cooled vapour compression chiller. The nominal cooling power is 34 kW. The size of compression subsystem is designed by the appropriate size ratio of absorption subsystem and compression subsystem (Li et al., 2016). Its refrigerant is R410A. A variable frequency scroll compressor is employed. All the heat exchangers are the plate heat exchanger. The area of condenser, evaporator and subcooler is 4.1 m2, 3.0 m2 and 0.36 m2, respectively. The electronic expansion valve (EEV) is set at the fixed superheating mode. The corresponding superheating is 8 °C. The gas liquid separator located between the evaporator and compressor guarantees that only the vapour refrigerant flows into the compressor. In particularly, the receiver installed between the condenser and subcooler is to access the remarkable subcooling (Koeln and Alleyne, 2014). The volume of separator and receiver is 8 L and 6 L, respectively. In addition, the cooling water and chilled water is driven by the water pump P6 and P7, respectively. The flow rate of each water pump is listed at Table 1. It is mentioned that all the flow rates are constant during the experiment. The higher flow rate of chilled water in the compression subsystem is to avoid the frosting of copper tube located between the EEV outlet and evaporator inlet by low evaporator temperature (< 5 °C). Besides, the compressor speed is reduced so that the cooling output of compression subsystem is controlled at about 15 kW during the experiment, also preventing the above-mentioned frosting led by high cooling power. Especially, a rotary vane vacuum pump is used to keep the vacuum of absorption subsystem (works 3 times in a month).
Step1: The collector hot water pump P1 is turned on to heat the water inside the storage tank when the tank is filled with water. Subsequently, the cooling water pump P6, chilled water pump P7 and compressor are started. Step2: The generator hot water pump P2 is turned on when the top layer temperature of storage tank is high enough to drive the absorption subsystem. Meanwhile, the water pump P3, P4 and P5 are started to activate the absorption subsystem. In addition, the system is turned off and the experiment finishes when the inlet temperature of generator hot water is less than 60 °C. For the k-th heat exchanger, the corresponding heat load is derived:
Qk = mk Cp |Tok − Tik |
(1)
The instantaneous COP of absorption subsystem and compression subsystem is calculated as follows:
COPa =
Qea Qg
(2)
COPc =
Qec W
(3)
The daily mean efficiency of collectors, daily mean COP of absorption subsystem and compression subsystem as well as daily mean solar COP (SCOP) is:
∫ Qcol dt ∫ IA col dt
(4)
COPa . m =
∫ Qea dt ∫ Qg dt
(5)
COPc . m =
∫ Qec dt ∫ Wdt
(6)
SCOPm =
∫ Qea dt ∫ IA col dt
ηcol . m =
(7)
COP ∗
The relative increase of COP ( ) is defined to assess the performance growth of SASCHCS compared to the reference system (the compression subsystem without the subcooling is used as the reference system):
COP ∗ =
COPc, m − COPref COPref
(8)
It is noteworthy that the reference system operates in the identical condition with the SASCHCS, i.e., inlet temperature and flow rate of cooling water and chilled water as well as compressor speed of both systems is same. The relative mean error (RME) is calculated by the following expression (Pei et al., 2012) N
3. Experimental procedure
RME =
The meteorological data is measured by the weather station of which mode is DAVIS Vantage Pro2 (deviation of solar irradiance and surrounding temperature is 0.5% of the reading and ± 0.5 °C, respectively). The temperature is tested by the T-type thermocouples (deviation is 0.1% of the reading) located at the outside surface of copper pipe with insulation. The water flow rate is measured by the turbine volumetric flow meters (deviation is less than 1% of the reading). The power meter (deviation is 0.5%) is employed to test the compressor work directly. The experimental data is recorded and transferred to computer by Agilent 34970A and it is updated by the data acquisition program (compiled with LABVIEW) at intervals of 10 s.
RE =
∑1 |RE| (9)
N
dy ∂y dx1 ∂y dx2 ∂y dx n = + + ...+ y ∂x1 y ∂x2 y ∂x n y
(10)
The corresponding RME of experimental data is shown in Table 2. Table 1 Flow rate of water pump.
3
Flow rate of water pump (m /h)
248
P1
P2
P3
P4
P5
P6
P7
0.8
1.0
0.8
1.0
1.0
2.9
3.9
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subsystem is quadratic as well. The peak is 4 kW during the test, which is 11.1% less than the nominal. It is thought that the difference is primarily caused by the pressure drop and heat loss. Besides, it is seen that the cooling output of absorption subsystem changes less from 12:00 to 16:00. The higher the cooling power of absorption chiller is, the lower the temperature of its chilled water becomes. Hence, the variation of chilled water temperature is inversely coincident to that of cooling capacity in the absorption subsystem. It is observed that the least inlet/ outlet temperature of chilled water in the absorption subsystem is 26 °C/22.5 °C. As expected, the relatively higher temperature of chilled water is attributed to that the cooling capacity of absorption subsystem serves as the subcooling power of compression subsystem. Nevertheless, the actual chilled water temperature of absorption subsystem is about 10 °C less than the simulation in our previous study (Li et al., 2016). Such difference is caused by the additional temperature drop of heat transfer led by the separate assembling of evaporator in the absorption chiller and subcooler in the prototype. It should be mentioned that the above-mentioned assembling is extremely convenient for the development of hybrid system though it leads to the extra exergy destruction. The working characteristic of compression subsystem on 25 August is displayed in Fig. 8. Similarly, the corresponding sudden variation is led by the periodic oil return of compressor as well. It is found that the condenser temperature of compression subsystem enhances from 37 °C to 40 °C and the evaporator temperature of compression subsystem goes down from 8.5 °C to 7 °C owing to the working of absorption subsystem. The above-mentioned phenomenon is primarily attributed to the improvement of heat transfer rate in the heat exchanger. But the compressor work nearly maintains at 3.05 kW during the entire experiment, which can be explained by the reduction of suction density as well as enhancement of pressure ratio (Li et al., 2018). Additionally, the variation of cooling power and subcooling in the compression subsystem is similar to the one of subcooling power in respect that they are influenced by the cooling capacity of absorption subsystem. It is found that the daily mean cooling output of compression subsystem grows from 15.5 kW to 18.95 kW and the subcooling nearly maintains at 15 °C due to the subcooling power offered by the absorption subsystem. The COP of absorption subsystem and compression one on 25 August is shown in Fig. 9. The behavior of COP in the absorption subsystem is quadratic, which is similar to that of inlet temperature in the generator hot water. The peak COP of absorption subsystem reaches to 0.69 near 14:00. It is noteworthy that the significant decrease of heat load in the generator as well as the evaporator inertia leads to the unreasonable rise of COP in the absorption subsystem at the final part of experiment (15:00–17:33), which was also reported by González-Gil et al. (2011). In addition, the COP of compression subsystem increases
Table 2 RME of experimental data. Varibles
Qcol
Qg
Qea
Qec
ηcol
SCOP
COPa
COPc
RME
1.97%
3.29%
2.35%
1.79%
5.9%
0.64%
1.19%
0.69%
4. Results and discussion Although the daily meteorological data is varied, it can be primarily divided into three types in terms of the available solar irradiance: sunny day (the solar irradiance is relatively stable and strong), sunny day with fluctuation (the solar irradiance is moderate and changes frequently, i.e., sometimes is sunny and sometimes is cloudy) and cloudy day (the solar irradiance is weak). It can be inferred that the behavior of solar cooling in the same typical day is similar. Hence, the experiment in three typical days is implemented to adequately examine the prototype performance. The experiment result associated with the sunny day without fluctuation, sunny day with fluctuation and cloudy day is carried out on 25 August, 05 September and 10 September, respectively. The inlet temperature of cooling water of 25 August, 05 September and 10 September is 32 °C, 32 °C and 31 °C, respectively. The solar irradiance and surrounding temperature of three days are displayed in Figs. 3 and 4, respectively. It is seen that the ambient temperature and the maximal solar irradiance of 05 September approaches to that of 25 August. The fluctuation of solar irradiance on 05 September is mainly from 9:00 to 15:00. And the highest surrounding temperature of sunny day and cloudy day is 36 °C and 30 °C, respectively. 4.1. Results of sunny day without fluctuation (25 August 2018) The experimental data of collector on 25 August is demonstrated in Fig. 5. The hot water pump P1 is turned on at 10:05. The collector temperature rises quickly and reaches to 95 °C at 10:28. Subsequently, the sudden drop of collector temperature is caused by the start of absorption subsystem. It is observed that the outlet temperature of collector goes up to the 94 °C and gradually reduces to 60 °C after the operation of absorption subsystem. Meanwhile, the inlet temperature of collector nearly maintains at 71 °C before 15:30 and goes down to 56 °C slowly in the end of test. In addition, the change of instantaneous useful heat in the collector is coincident to that of collector outlet temperature. It goes up to 18 kW and then gradually drops to 13 kW after the operation of absorption subsystem. The daily mean useful heat of collector is 11.30 kW. On the other hand, the average efficiency of collector is 0.69 during the most period of experiment, benefiting from the low collector temperature as well as strong solar irradiance (Nie et al., 2017). The measurement data of generator at 25 August is exhibited in Fig. 6. The hot water pump P2 is turned on to drive the absorption subsystem when the top layer temperature of storage tank equals to 87 °C (at 10:28). Accordingly, the inlet temperature of generator hot water comes down to 79 °C sharply due to the consumption of generator. Subsequently, it grows to 93 °C and gradually falls to 60 °C at 17:33. Furthermore, it is seen that the temperature drop of hot water in the generator is about 6 °C in most time of experiment. The heat load of generator is similar to the inlet temperature of generator hot water, as expected. It rises to 9 kW at first and goes down to 2.8 kW at end of experiment. With the heat delivering from the hot water, the solution temperature enhances by 15 °C when LiBr/H2O flows through the generator. The temperature of chilled water and cooling power of absorption subsystem at 25 August is shown in Fig. 7. It is noteworthy that the sudden growth of temperature as well as cooling capacity is caused by the periodic oil return of compressor. Similar to the inlet temperature of generator hot water, the trend of cooling capacity in the absorption
Solar radiation (W/m2)
800
600
400
200
0 00:00
04:00
08:00
25-Aug-2018
12:00
16:00
Time (hr) 05-Sep-2018
Fig. 3. Daily solar irradiance. 249
20:00
24:00
10-Sep-2018
10 30 8
34
Temperature ( C)
Surrounding temperature (°C)
36
32 30
28
25 6 20 4
15
2
26
04:00
08:00
12:00
16:00
20:00
Time (hr) 05-Sep-2018
25-Aug-2018
10 10:00
24:00
11:00
12:00
13:00
14:00
15:00
16:00
Time (hr) Toea Tiea
10-Sep-2018
17:00
0 18:00
Qea
Fig. 7. Experimental data of absorption subsystem evaporator on 25-Aug-2018.
Fig. 4. Daily surrounding temperature.
24
60
30 90
70 18
60 50
12
40 6
30
Toc
20 10:00
11:00
12:00
Tic 13:00
Temperature ( C)
Temperature (°C)
24
Useful heat of collector (kW)
50
80
18
40 12 30 6 20 0
10
Qcol 14:00
15:00
16:00
17:00
0 10:00
0 18:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
Cooling output of compression subsystem and compressor work (kW)
24 00:00
Cooling capacity of absorption subsystem (kW)
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-6 18:00
Time (hr)
Time(hr)
Tc
Fig. 5. Measurement data of collector on 25-Aug-2018.
Te
Ts
Qec
W
Fig. 8. Measurement data of compression subsystem on 25-Aug-2018. 10
150
1.0
9
6
90 4
75 60
2
6
3 0.4 0 0.2
45 30 10:00
0.6
COPc
Temperature (°C)
105
0.8
COPa
8 120
Heat load of generator (kW)
135
COPa 11:00
12:00
Tig
13:00
14:00
15:00
Time (hr) Tog Tisg
16:00
17:00
Tosg
0 18:00
0.0 10:00
Qg
11:00
12:00
13:00
14:00
COPc 15:00
16:00
17:00
-3 18:00
Time (hr)
Fig. 6. Test data of generator on 25-Aug-2018.
Fig. 9. COP of 25-Aug-2018.
from 5.10 to 6.21 after the working of the absorption subsystem.
Fig. 10. The P1 is turned on at 9:30 to heat the water of storage tank by solar energy. It is clear that the temperature and useful heat of collector fluctuates dramatically because of the frequent change of solar irradiance. Moreover, the notable fluctuation of solar irradiance also deteriorates the collector performance compared to the data of 25 August,
4.2. Results of sunny day with fluctuation (05 September 2018) The experimental data of collector on 05 September is exhibited in 250
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4.3. Results of cloudy day (10 September 2018)
80
The measurement of collector on 10 September is demonstrated in Fig. 15. The water pump P1 is turned on at 10:00. Since the solar irradiance is weak on the cloudy day, the heating of hot water before the start of absorption subsystem is longer. It takes 95 mins to heat the top layer water of the storage tank to the setting temperature. Subsequently, the outlet temperature of collector reduces from 74 °C to 60 °C after the work of absorption subsystem. Additionally, the weak solar irradiance seriously decreases the collector performance, i.e., the daily average useful heat of collector on 10 September is 45.2% poorer than that on 25 August. And the daily mean efficiency of collector on 10 September is 0.61 and goes down by 11.6% compared to the data on 25 August. The experimental data of generator on 10 September is displayed in Fig. 16. The absorption subsystem is activated to work when the top layer temperature of storage tank is heated to 75 °C (at 11:35). It is seen that the temperature of generator hot water gradually comes down in respect that the consumption of generator exceeds the solar thermal gain. Compared to the data of 25 August, the daily mean heat load of generator on 10 September falls by 15.8%. And the temperature improvement of solution in the generator is about 10.5 °C. The temperature of chilled water and cooling capacity in the absorption subsystem on 10 September is exhibited in Fig. 17. It is obvious that the cooling power of absorption subsystem is poor on the cloudy day, i.e., the daily average one drops by 23.4% compared with the data of 25 August. Besides, the minimum inlet and outlet temperature of chilled water in the absorption subsystem is 27 °C and 25 °C, respectively. The thermodynamic characteristic of compression subsystem on 10 September is shown in Fig. 18. It is obtained that the condenser temperature, evaporator one and cooling capacity of compression subsystem vary slightly due to the weak subcooling power. The condenser temperature grows from 36 °C to 38.5 °C and the evaporator temperature falls from 10.2 °C to 8.6 °C after the operation of absorption subsystem. Additionally, the daily mean cooling output of compression subsystem enhances from 15.3 kW to 17.9 kW by the subcooling. The corresponding subcooling of compression subsystem is about 10 °C. The COP of absorption subsystem and compression one on 10 September is demonstrated in Fig. 19. It is easily inferred that the performance of hybrid system reduces dramatically due to the weak solar irradiance. The daily average COP of absorption subsystem on 10 September is 9% lower than that on 25 August. On the other hand, the COP of compression subsystem goes up from 5.20 to 5.91 by the subcooling, which means the corresponding rise of COP in the compression
Temperature (°C)
60
18
50 12
40 30
6 20
Toc
10 09:00
Tic
Useful heat of collector (kW)
24 70
Qcol 0
10:00
11:00
12:00
13:00
Time (hr)
14:00
15:00
16:00
Fig. 10. Measurement data of collector on 05-Sep-2018.
i.e., the highest outlet temperature of collector (after the working of absorption chiller) is 85 °C. Furthermore, the daily mean useful heat of collector on 05 September is 9.64 kW and 14.7% less than the one on 25 August. Similarly, the daily average efficiency of collector on 05 September is 0.64 and 7.2% lower than that on 25 August. The measurement of generator on 05 September is displayed in Fig. 11. The absorption subsystem is activated to work as the top layer temperature reaches to 80 °C (at 9:50). It is clear that the temperature of hot water and solution in the generator changes frequently as a result of fluctuation in the collector outlet temperature. In addition, the temperature of generator hot water, consumption of generator and temperature rise of solution on 05 September are all lower than that on 25 August due to the drop of useful heat in the collector. It is obtained that the temperature growth of solution and the peak inlet temperature of generator hot water are 13 °C, 83 °C, respectively. And the daily average heat load of generator on 05 September is 10.2% weaker than that on 25 August. The chilled water temperature and cooling output of absorption subsystem on 05 September is shown in Fig. 12. The cooling power of absorption subsystem becomes instable and goes down owing to the similar behavior of inlet temperature in the generator hot water, as expected. It is found that the daily mean cooling capacity of absorption subsystem on 05 September is 12% poorer than the one on 25 August. And the lowest inlet/outlet temperature of chilled water in the absorption subsystem is 28 °C/25 °C. The measurement data of thermodynamic characteristic in the compression subsystem on 05 September can be seen in Fig. 13. There is not significant fluctuation of condenser temperature, evaporator temperature and compressor work in the compression subsystem though the solar irradiance changes frequently. It is observed that the condenser temperature rises from 39 °C to 41 °C and the evaporator temperature drops from 11 °C to 10 °C after the working of absorption subsystem. The compressor work is nearly fixed at 3.05 kW regardless of subcooling. However, the cooling output and subcooling of compression subsystem after the operation of absorption subsystem emerges the notable fluctuation, as expected. The daily average cooling capacity of compression subsystem goes up from 16.4 kW to 19.3 kW by the subcooling. And the corresponding subcooling is 14 °C. The COP of absorption and compression subsystems on 05 September is shown in Fig. 14. It is obvious that the fluctuation of solar irradiance is adverse to the performance of SASCHCS. The daily average COP of absorption subsystem on 05 September is 2% less than that on 25 August. Furthermore, the COP of compression subsystem grows from 5.30 to 6.36 by the subcooling. It is derived that the above-mentioned enhancement of COP in the compression subsystem at 05 September is 7% weaker than the one at 25 August.
10
150 135
8
Temperature (°C)
120 105
6
90 4
75 60
2 45 30
0 10:00
11:00
Tig
12:00
13:00
Time (hr) Tog Tisg
14:00
15:00
16:00
Tosg
Fig. 11. Test data of generator on 05-Sep-2018. 251
Qg
Heat load of generator (kW)
90
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25
6
20
4
15
2
0
10 12:00
13:00
14:00
Time (hr) Toea Tiea
15:00
16:00
Toc
Qea
40 12 30
8
20
4 0
10
-4
Tc
Time (hr) Te Ts
14:00
15:00
Qec
11:00
Qcol 0 13:00
12:00
Time (hr)
10
135 120
Temperature (°C)
Temperature (°C)
16
13:00
Tic
150
Cooling output of compression subsystem and compressor work (kW)
20
50
12:00
6
40 10:00
24
11:00
12
Fig. 15. Measurement data of collector on 10-Sep-2018.
60
10:00
60
50
Fig. 12. Experimental data of absorption subsystem evaporator on 05-Sep2018.
0 09:00
18
8
105 6 90 4
75 60
2
45 0
30 11:30
12:00
12:30
Time (hr) Tog Tisg
Tig
16:00
13:00
Tosg
Qg
Fig. 16. Test data of generator on 10-Sep-2018.
W
Fig. 13. Measurement data of compression subsystem on 05-Sep-2018.
10
30
8
8
1.2
6
0.8
4
0.6
2
0.4
0
Temperature (°C)
1.0
COPc
COPa
25 6 20 4
15
0.2
0.0 09:00
COPa
-2
COPc
10 11:30
-4 10:00
11:00
12:00
13:00
14:00
15:00
Heat load of generator (kW)
11:00
70
2
12:00
Time (hr)
Tochw
16:00
Time (hr)
0 13:00
12:30
Tichw
Cooling capacity of absorption subsystem (kW)
10:00
24
Useful heat of collector (kW)
8
80
Temperature (°C)
Temperature (°C)
30
30
Cooling capacity of absorption subsystem (kW)
10
35
Qea
Fig. 17. Experimental data of absorption subsystem evaporator on 10-Sep2018.
Fig. 14. COP of 05-Sep-2018.
subsystem is 0.71.
specifically in Fig. 20. In general, the stronger solar irradiance is favorable for the operation of facility. The daily mean efficiency of collector on 25 August, 05 September and 10 September is 0.692, 0.640 and 0.614, respectively. It is derived that the daily average COP of absorption subsystem on 25 August, 05 September and 10 September is
4.4. Summary of experiment The performance indicator of three typical days is shown 252
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Temperature (°C)
50 12
40
30 6 20 0
10
0 10:00
10:30
11:00
Tc
11:30
Time (hr) Te Ts
12:00
-6 13:00
12:30
Qce
and 38.1% compared with the one of 25 August. The detailed results of experiment days are listed in Table 3. It is noteworthy that the daily calculation is based on experimental period. The daily mean solar irradiance of 25 August, 05 September and 10 September is 604.7 W/m2, 557.6 W/m2 and 373.3 W/m2, respectively. It is well known that the stronger the solar irradiance is, the higher the energy gain/consumption becomes. As a consequence, the daily average useful heat of collector on 25 August, 05 September and 10 September is 11.30 kW, 9.64 kW and 6.19 kW, respectively. And it is gotten that the daily mean consumption of generator on 25 August, 05 September and 10 September is 6.99 kW, 6.28 kW and 5.88 kW, respectively. Since the higher heat load of generator and temperature of hot water results in better cooling output, the daily average cooling capacity of absorption subsystem on 25 August, 05 September and 10 September is 3.62 kW, 3.18 kW and 2.77 kW, respectively. Similarly, the daily mean enhancement of cooling power for the compression subsystem on 25 August, 05 September and 10 September is 3.44 kW, 3.20 kW and 2.04 kW, respectively. In general, it is displayed that the daily average SCOP and daily mean COP of absorption subsystem is 0.13–0.2 and 0.43–0.54, respectively from the measurement of 15 days, depending on the solar irradiance. The 15-day experiment of prototype is carried out and the results of three typical days are discussed in detail. Subsequently, the long term measurement of SASCHCS for different seasons is to be performed. And the consumption of cooling tower in the absorption subsystem will be measured to analyze the energy saving more exactly.
Cooling output of compression subsystem and compressor work (kW)
18
W
2.0
8
1.5
6
1.0
4
0.5
2
COPa
0.0 10:00
10:30
COPc
11:00
11:30
12:00
Time (hr)
12:30
COPc
COPa
Fig. 18. Measurement data of compression subsystem on 10-Sep-2018.
5. Conclusion The prototype of SASCHCS is developed and measured on the sunny day without and with fluctuation as well as cloudy day of subtropical Guangzhou. The experiment results are summarized as follows: (1). The absorption subsystem produces the high temperature cooling since it serves as the heat sink of subcooler in the compression subsystem, i.e., the outlet temperature of chilled water in the absorption subsystem exceeds 22 °C. Thereby, the low grade solar thermal of which temperature is greater 60 °C can be employed. The highest instantaneous cooling power and COP of absorption chiller is 4 kW and 0.69, respectively. (2). For the sunny day (25 August) of which daily mean solar irradiance is 604.7 W/m2, the daily average cooling capacity, COP and SCOP of absorption subsystem are 3.62 kW, 0.52 and 0.21, respectively. And it is obtained that the daily mean COP of compression subsystem goes up by 22.2% by the subcooling power offered by the absorption chiller. (3). It is observed that the temperature of hot water, subcooling and cooling output of two subsystems change frequently on the sunny day with fluctuation (05 September). The corresponding daily average solar irradiance is 557.6 W/m2. It is exhibited that the daily mean cooling capacity, COP and SCOP of absorption subsystem are 3.18 kW, 0.51 and 0.20, respectively. Additionally, the daily average COP of compression subsystem rises by 19.8% by the cooperation of absorption chiller. (4). The facility performance goes down remarkably on the cloudy day (10 September). The daily mean solar irradiance is 373.3 W/m2. It is derived that the daily average cooling capacity, COP and SCOP of absorption subsystem are 2.77 kW, 0.47 and 0.13, respectively. On the other hand, the improvement of daily mean COP in the compression subsystem is just 13.3% due to the weak output of absorption subsystem.
0 13:00
Fig. 19. COP of 10-Sep-2018. 0.5
0.4
0.6 0.5
0.3
COP*
Energy conversion efficiency
0.7
0.4 0.3
0.2
0.2 0.1 0.1 0.0
25-Aug-2018 col
05-Sep-2018 COPa
SCOP
10-Sep-2018
0.0
COP*
Fig. 20. Performance indicator of three days.
0.517, 0.507 and 0.470, respectively. Similarly, the daily mean SCOP of absorption subsystem on 25 August, 05 September and 10 September is 0.21, 0.20 and 0.13, respectively. Besides, the relative increase of COP in the hybrid system on 25 August, 05 September and 10 September is 22.2%, 19.8% and 13.3%, respectively. It is exhibited that the daily mean SCOP of 05 September and 10 September comes down by 4.7%
Acknowledgement This work is supported by: (1) Natural Science Foundation of Guangdong Province under the contract No. 2018A030313310, (2) Key 253
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Table 3 Summary of experiment results. Day
Duration (min) Entire
Data of three typical days 25Aug 448 05 Sep 403 10 Sep 177 Data of other days 01 Aug 432 07 Aug 343 08 Aug 506 09 Aug 511 13 Aug 134 14 Aug 242 22 Aug 314 24 Aug 421 04 Sep 224 06 Sep 438 11 Sep 492 24 Sep 391
Daily energy gain/consumption (kWh)
Performance indicator
SASCHCS
I
Qc
Qg
Qea
Qec
ηc
SCOP
COPa
425 383 82
121.96 101.17 29.73
84.43 64.78 18.26
49.51 40.07 8.04
25.61 20.3 3.78
134.17 123.84 49.01
0.69 0.64 0.61
0.21 0.20 0.13
0.52 0.51 0.47
342 298 453 456 87 198 252 368 120 363 441 333
93.88 72.72 136.41 131.53 25.18 52.47 73.61 99.18 48.23 99.9 126.23 90
62.48 49.19 94.59 88.07 15.45 34.52 48. 64.53 32.18 64.09 86.01 57.81
34.04 31.99 52.86 50.02 8.84 19.77 27.02 36.68 12.41 36.15 45.66 35.3
17.31 14.75 25.17 24.73 4.03 9.69 11.71 19.04 6.28 19.02 24.44 17.07
119.82 94.29 144.08 142.08 37.21 67.42 84.12 117.36 60.49 125 137.36 112.75
0.67 0.68 0.69 0.67 0.61 0.66 0.65 0.65 0.67 0.64 0.68 0.64
0.18 0.20 0.18 0.19 0.16 0.18 0.16 0.19 0.13 0.19 0.19 0.19
0.51 0.46 0.48 0.49 0.46 0.49 0.43 0.52 0.51 0.53 0.54 0.48
Laboratory of Compressor Technology under the contract No. SKLYSJ201806, (3) Key Laboratory of Efficient and Clean Energy Utilization of Guangdong Higher Education Institutes under the contract No. KLB10004.
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