# Experimental Study of a Novel Direct-Expansion Variable Frequency Finned Solar/Air-Assisted Heat Pump Water Heater

Authors: Jing Qin; Jie Ji; Wenzhu Huang; Hong Qin; Mawufemo Modjinou; Guiqiang Li
Journal: International Journal of Photoenergy
 (2018)

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## Abstract

A novel direct expansion variable frequency finned solar/air-assisted heat pump water heater was fabricated and tested in the enthalpy difference lab with a solar simulator. A solar/air source evaporator-collector with an automatic lifting glass cover plate was installed on the system. The system could be operated in three modes, namely, air, solar, and dual modes. The effects of the ambient temperature, solar irradiation, compressor frequency, and operating mode on the performance of this system were studied in this paper. The experimental results show that the ambient temperature, solar irradiation, and operating mode almost have no effect on the energy consumption of the compressor. When the ambient temperature and the solar irradiation were increased, the COP was found to increase with decreasing heating time. Also, when the compressor frequency was increased, an increase in the energy consumption of the compressor and the heat gain of the evaporator were noted with a decrease in the heating time.


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## Body

## 1. Introduction

Water heating consumes nearly 20% of the total energy consumption for an average family [1]. Solar energy is a clean, inexhaustible, and abundant energy resource [2]. The development of an affordable and effective clean energy technologies such as a combination of solar energy and heat pump can have huge long-term benefits, which is the concept of solar-assisted heat pump (SAHP) [3].The concept of direct expansion solar-assisted heat pump (DX-SAHP) was first presented by Sporn and Ambrose [4]. While the “indirect-expansion SAHP” system has an intermediate heat exchanger between the solar water circuit and a water circuit, a DX-SAHP uses a two-phase solar collector to function directly as an evaporator. The evaporator-collector configuration favourably reduces the number of components in use, which can lower the cost of the system and avoid the nighttime freeze-up problem of a traditional water collector [5].For the SAHP system, the match between different components is critical. Liu et al. [6] found that the mass flow rate of refrigerant can match well with the thermal load of the evaporator by means of compressor frequency modulation. In this case,the compressor frequency should be adjusted according to the ambient conditions. Chaturvedi et al. [7] showed that the coefficient of performance (COP) of the system can be enhanced extensively by lowering the compressor frequency when the ambient temperatures are higher. Moreno-Rodríguez et al. [8] showed that when the condenser water flow rate is lower than 0.114 kg/s, a higher COP can be obtained by reducing the compressor frequency. However, when the water flow rate is over 0.174 kg/s, lowing the compressor frequency will drastically lead to a lower COP.SAHP is an effective way for utilizing solar energy, but solar energy is an intermittent energy which changes greatly depending on time and weather [9]. For this reason, supplementary heat source is required to ensure the continuous and reliable operation of the SAHP. Air source heat pump (ASHP) can operate more reliably by absorbing heat from the ambient air [10]. However, the application of single ASHP is also dependent on the weather conditions. Especially in winter when the evaporating temperature decreases, there is a significant drop in the heating capacity and energy efficiency of the system [11] in good shape. Since SAHP and ASHP have their respective advantages and disadvantages, many researchers combined SAHP and ASHP to improve the reliability of the system. The current dual source heat pump system is made up of different heat exchangers to utilize different heat sources, which would involve more complicated equipment and cause higher cost and other related issues [12–14]. A dual source evaporator-collector, which is made up of finned copper tubes with selective absorption coating is fabricated in this study to improve the performance of the dual source heat pump system. The dual source evaporator-collector configuration could effectively reduce the number of components in use and absorb more heat from solar and air than flat plate evaporator through the fins.Glazed and unglazed flat plate solar collectors are two major collector types mostly used in SAHP systems [15]. When the temperature of the collector system is higher than the ambient temperature, the use of glazed solar collector will reduce heat loss [16], while, when the temperature of the collector is lower than the ambient temperature, the use of unglazed solar collector produces high heat efficiency [17]. Therefore, an automatic lifting glass cover plate, which could automatically cover and uncover the evaporator-collector, is expected to enhance the performance of SAHP system.Therefore, there are three approaches to enhance the performance of a solar-assisted heat pump water heater (SAHPWH) system: (1) variable frequency compressor; (2) a dual source evaporator-collector made up of finned copper tubes with selective absorption coating; and (3) an automatic lifting glass cover plate. However, there is no system in published experimental studies that applied all three components above to improve the performance of SAHPWH system. In this paper, a novel DX-SAHPWH system that applied all three components above was fabricated and tested experimentally.Most studies of SAHP were implemented in outdoor environment where constant solar radiation and ambient temperature are hard to maintain [18]. In this study, the experiment was carried out under constant and controlled solar irradiation and ambient temperature in the enthalpy difference lab with a solar simulator to produce better results.In this paper, a novel DX-SAHPWH system was experimentally investigated and tested in the enthalpy difference lab with a solar simulator. The effects of the ambient temperature, solar irradiation, compressor frequency, and the operating modes on the performance of this system were studied.


## 2. System Description and Test Apparatus

### 2.1. System Description

Figure1 shows the schematic diagram of the DX-SAHPWH system. This system mainly consists of a solar/air source horizontal finned evaporator-collector, a R410a hermetic rotary DC inverter compressor, a plate-type heat exchanger condenser, a 150 L pressure bearing structure water-storage tank, and an electronic expansion valve.Figure 1
Configuration diagram of the DX-SAHPWH system. (a) Schematic. (b) Photo. 1: solar/air source horizontal finned evaporator-collector; 2: automatic lifting glass cover plate; 3: fan; 4: electronic expansion valve; 5: accumulator; 6: condenser; 7: water tank; 8: variable frequency compressor; 9: strainer.
(a)
(b)In this system, the refrigerant absorbs energy from the air and solar; and gets vaporized in the evaporator-collector. Then, it is compressed in the compressor and becomes superheated vapor of high temperature and pressure. The vapor enters the condenser and gets condensed into liquid. The heat rejected in this procedure is absorbed by water-storage tank. Therefore, the water is heated. After this, the refrigerant passes through the electronic expansion valve where it expands irreversibly and adiabatically. Finally, the refrigerant flows back to the evaporator-collector and gets vaporized again, the cycle to continues.As shown in Figure2, the evaporator-collector is made up of finned copper tubes with selective absorption coating to increase the area for absorbing solar and air energy, its main parameters are shown in Table 1. A fan is also installed at the back of the evaporator-collector. Besides, an automatic lifting glass cover plate, which can be used to cover and uncover the evaporator-collector automatically, was fabricated on the top of the evaporator-collector, as shown in Figure 3.Figure 2
Diagram of the evaporator-collector. (a) Front view. (b) Lateral view.
(a)
(b)Table 1
Parameters of evaporator-collector.
Length
1475 mm
Width
800 mm
Solar absorptivity of selective absorption coating
0.95
Transmittance of glass plate
0.85
External diameter of copper tube
9.52 mm
Thickness of copper tube
0.35 mm
Number of tube rows
16
Thickness of fin
0.35 mm
Punching diameter of fin
10 mm
Height of fin
15 mm
Vertical pitch of fin
3 mm
Number of fins
208
Angle of fin and copper tube
28°Figure 3
Automatic lifting glass cover plate.Based on the operating conditions of the glass cover plate and fan, this system could be operated in three modes, namely, air, solar, and dual modes, although the system could absorb energy from both air and solar in three working modes, as shown in Table2.Table 2
Three operating modes of the DX-SAHPWH system.
Component
Air mode
Solar mode
Dual mode
Glass cover plate
Uncover
Cover
Uncover
Fan
Fast
Off
Slow


### 2.2. Test Apparatus

To study the performance of the DX-SAHPWH system, the system has been tested in the enthalpy difference lab with a solar simulator. The enthalpy difference lab can maintain constant ambient temperature and humidity by operating the air handling units (AHU), as shown in Figure4. The AHU consists of coolers, heaters, humidifier, and fan, which offer cooling capacity, heating capacity, controllable humidity, and air cycle in the lab, respectively. The solar simulator was then configured in the lab to simulate solar irradiation on the surface of the evaporator-collector. As the spectrum distribution satisfies the national class B level standard, and the heterogeneity and instability of the solar simulator are under 5%, the solar simulator effectively simulated the solar irradiation for the study. Its luminous area is 2 × 2 m2, and the adjustable range of the irradiance is from 0 to 1200 W/m2. The solar simulator was set parallel to the collectors. Hence, the influence of solar irradiation on this system can be studied.Figure 4
Configuration diagram of the enthalpy difference lab. (a) Schematic. (b) Photo.
(a)
(b)Ambient dry bulb and wet bulb temperature of the enthalpy difference lab was measured using platinum resistance thermometer (with grade A accuracy). The temperatures of water in the storage tank, at the inlet and outlet of the evaporator-collector, the compressor, the condenser, and the electronic expansion valve were measured using T-type copper-constantan thermocouples. Also, the refrigerant pressure in evaporator-collector and condenser was measured using Huba pressure sensor. The solar irradiance and the energy consumption of the system were also measured and recorded during the experiments. A digital power meter (YOKOGAWA WT230) was used to measure the energy consumption of the system. The experimental data was recorded automatically for every 6 seconds by a data logger (Agilent 34970A). The details of the sensors and measurement instruments are shown in Table3.Table 3
Characteristics of sensors and measurement instruments.
Parameter
Range
Accuracy
Measuring apparatus
Temperature (°C)
−100 to 100
±0.2
T-type thermocouple
Pressure (MPa)
0–3.04
0.3%
Huba pressure sensor
Irradiance (W/m2)
0–2000
2%
Pyranometer
Power (kW)
0–12
1.2%
YOKOGAWA power sensor WT230


## 2.1. System Description

Figure1 shows the schematic diagram of the DX-SAHPWH system. This system mainly consists of a solar/air source horizontal finned evaporator-collector, a R410a hermetic rotary DC inverter compressor, a plate-type heat exchanger condenser, a 150 L pressure bearing structure water-storage tank, and an electronic expansion valve.Figure 1
Configuration diagram of the DX-SAHPWH system. (a) Schematic. (b) Photo. 1: solar/air source horizontal finned evaporator-collector; 2: automatic lifting glass cover plate; 3: fan; 4: electronic expansion valve; 5: accumulator; 6: condenser; 7: water tank; 8: variable frequency compressor; 9: strainer.
(a)
(b)In this system, the refrigerant absorbs energy from the air and solar; and gets vaporized in the evaporator-collector. Then, it is compressed in the compressor and becomes superheated vapor of high temperature and pressure. The vapor enters the condenser and gets condensed into liquid. The heat rejected in this procedure is absorbed by water-storage tank. Therefore, the water is heated. After this, the refrigerant passes through the electronic expansion valve where it expands irreversibly and adiabatically. Finally, the refrigerant flows back to the evaporator-collector and gets vaporized again, the cycle to continues.As shown in Figure2, the evaporator-collector is made up of finned copper tubes with selective absorption coating to increase the area for absorbing solar and air energy, its main parameters are shown in Table 1. A fan is also installed at the back of the evaporator-collector. Besides, an automatic lifting glass cover plate, which can be used to cover and uncover the evaporator-collector automatically, was fabricated on the top of the evaporator-collector, as shown in Figure 3.Figure 2
Diagram of the evaporator-collector. (a) Front view. (b) Lateral view.
(a)
(b)Table 1
Parameters of evaporator-collector.
Length
1475 mm
Width
800 mm
Solar absorptivity of selective absorption coating
0.95
Transmittance of glass plate
0.85
External diameter of copper tube
9.52 mm
Thickness of copper tube
0.35 mm
Number of tube rows
16
Thickness of fin
0.35 mm
Punching diameter of fin
10 mm
Height of fin
15 mm
Vertical pitch of fin
3 mm
Number of fins
208
Angle of fin and copper tube
28°Figure 3
Automatic lifting glass cover plate.Based on the operating conditions of the glass cover plate and fan, this system could be operated in three modes, namely, air, solar, and dual modes, although the system could absorb energy from both air and solar in three working modes, as shown in Table2.Table 2
Three operating modes of the DX-SAHPWH system.
Component
Air mode
Solar mode
Dual mode
Glass cover plate
Uncover
Cover
Uncover
Fan
Fast
Off
Slow


## 2.2. Test Apparatus

To study the performance of the DX-SAHPWH system, the system has been tested in the enthalpy difference lab with a solar simulator. The enthalpy difference lab can maintain constant ambient temperature and humidity by operating the air handling units (AHU), as shown in Figure4. The AHU consists of coolers, heaters, humidifier, and fan, which offer cooling capacity, heating capacity, controllable humidity, and air cycle in the lab, respectively. The solar simulator was then configured in the lab to simulate solar irradiation on the surface of the evaporator-collector. As the spectrum distribution satisfies the national class B level standard, and the heterogeneity and instability of the solar simulator are under 5%, the solar simulator effectively simulated the solar irradiation for the study. Its luminous area is 2 × 2 m2, and the adjustable range of the irradiance is from 0 to 1200 W/m2. The solar simulator was set parallel to the collectors. Hence, the influence of solar irradiation on this system can be studied.Figure 4
Configuration diagram of the enthalpy difference lab. (a) Schematic. (b) Photo.
(a)
(b)Ambient dry bulb and wet bulb temperature of the enthalpy difference lab was measured using platinum resistance thermometer (with grade A accuracy). The temperatures of water in the storage tank, at the inlet and outlet of the evaporator-collector, the compressor, the condenser, and the electronic expansion valve were measured using T-type copper-constantan thermocouples. Also, the refrigerant pressure in evaporator-collector and condenser was measured using Huba pressure sensor. The solar irradiance and the energy consumption of the system were also measured and recorded during the experiments. A digital power meter (YOKOGAWA WT230) was used to measure the energy consumption of the system. The experimental data was recorded automatically for every 6 seconds by a data logger (Agilent 34970A). The details of the sensors and measurement instruments are shown in Table3.Table 3
Characteristics of sensors and measurement instruments.
Parameter
Range
Accuracy
Measuring apparatus
Temperature (°C)
−100 to 100
±0.2
T-type thermocouple
Pressure (MPa)
0–3.04
0.3%
Huba pressure sensor
Irradiance (W/m2)
0–2000
2%
Pyranometer
Power (kW)
0–12
1.2%
YOKOGAWA power sensor WT230


## 3. Thermodynamic Analysis Methods

For this system, the total energy consumptionWsys is the sum of the energy consumption of the compressor, water pump, fan, and electric controller.
(1)Wsys=Wcom+Wpump+Wfan+Wele,where Wcom,Wpump,Wfan, and Wele represent the energy consumption of the compressor, water pump, fan, and electric controller, respectively. In this system, Wpump is 60 W and Wele is 10 W. Wfan is 0 W in solar mode and 100 W in dual and air modes.For the compressor, the refrigerant gas temperatures (Td,Ts) and pressures (Pd,Ps) at the discharge and suction ends of the compressor, respectively, are related by the compression index γ as follows:
(2)TdTs=PdPsγ−1/γ.The rotating speedn of the compressor is influenced by the inverter frequency.
(3)n=120×f×1−sp,where p is motor magnetic series and s is motor transfer ratio.The mass flow ratemr of the refrigerant is given by
(4)mr=ηvnVd60vs,where ηv is the volumetric efficiency, Vd is the displacement volume, and vs is the specific volume of refrigerant gas at compressor suction.According to the first law of thermodynamics, the heat absorbed by the evaporator-collectorQe should be equal to heat gain of the refrigerant.
(5)Qe=mrhr,out−hr,in,where hr,out is refrigerant enthalpy leaving the evaporator-collector and hr,in is refrigerant enthalpy entering the evaporator-collector.The theoretical power consumptionWth is given by
(6)Wth=ηvVdPsn60γγ−1PdPsγ−1/γ−1.In combination with the above two formulas,Wth could also be
(7)Wth=ηvVdPsn60γγ−1TdTs−1.The input power consumptionWcom of the compressor is given by
(8)Wcom=Wthηiηmηmo,where ηi,ηm, and ηmo are the indicated efficiency, mechanical efficiency, and motor efficiency, respectively.The instantaneous heat exchange rate of this system is calculated by(9)Qsys=cwmwdTwdt,where cw represents the specific heat capacity of water, mw is the gross mass of water in the tank, and Tw stands for the water temperature.The fundamental and predominant performance evaluation approach for the DX-SAHPWH system is coefficient of the performance (COP). It is defined as the ratio of heat exchange rate of the systemQsys to the energy consumption of the system Wsys. This description is mathematically stated as
(10)COP=QsysWsys.Based on (1), (9), and (10), instantaneous COP can be expressed by
(11)COPi=cwmwdTw/dtWcom+Wpump+Wfan+Wele.The average COP with the water temperature rising from the initial valueTini to the final value Tfin is
(12)COPm=∫Qsys∫Wsys=cwmwTfin−Tini∫Wcom+Wpump+Wfan+Wele.Since the heat absorbed by the evaporator-collector includes convectional and radiant heat exchanging,Qe can also be expressed by
(13)Qe=AKeiΔTm+αeR,where A is the heat exchanging area of the evaporator-collector. i=1,2,3, and Ke1,Ke2,Ke3 stands for convectional heat exchanging coefficient of the evaporator-collector in solar, dual, and air mode, respectively. According to the difference of wind speed in each mode, Ke1,Ke2,Ke3 is different, and Ke1≤Ke2<Ke3.ΔTm is the mean temperature difference of convectional heat exchanging; αe is the absorptivity of the evaporator-collector; R is the intensity of solar irradiation.


## 4. Experiment Results and Discussion

### 4.1. Effect of the Solar Irradiation on Heat Performance of the System

According to GB/T 23137-2008, the nominal working conditions of heat pump water heater were chosen as the test conditions, which was set to an ambient temperature of 20°C, an ambient relative humidity of 59%, and the water temperature rising from 15°C to 55°C. To investigate the effect of the solar irradiation on heat performance of this system, the experiments were taken under above conditions and the solar irradiation set at 500 W/m2 and 900 W/m2. The results are shown in Figures 5–9.Figure 5
Effect of the solar irradiation onWcom.Figure 6
Effect of the solar irradiation onQe.Figure 7
Effect of the solar irradiation onCOPi.Figure 8
Effect of the solar irradiation onCOPm.Figure 9
Effect of the solar irradiation on heating time.As illustrated in Figure5, Wcom increases significantly with the rising water temperature. The reason is that when Tw increases, Td,Ts,Pd, and Ps increase as well with a decrease in ηm and the increment of Td is significantly larger than the increment of Ts, so when Ps and Td/Ts both is increased, according to (7) and (8), Wcom will increase significantly. Besides when the solar irradiation increases from 500 W/m2 to 900 W/m2, Ps is found to be almost the same, so was Wcom. And when the operating mode is set different, Ps shows no significant difference, so Wcom is almost the same.As shown in Figure6, when solar irradiation increases from 500 W/m2 to 900 W/m2, the heat absorbed from solar irradiation by the evaporator-collector increases, so Qe increases. Qe decreases with the rising of the water temperature. The reason is that with the rising of the water temperature, the evaporator temperature increases, which means the mean temperature difference of convectional heat exchanging ΔTm decreases, thus the heat absorbed from air by the evaporator decreases.As shown in Figure7, when the solar irradiation is increased from 500 W/m2 to 900 W/m2, Wcom is almost the same and Qe is higher, so COPi is higher. And with the rising of the water temperature, Qe decreases and Wcom increased, so COPi decrease. And there are three reasons for high COPi when Tw is low. First, when Tw is low, Ps and Td/Ts are small, according to (7) and (8), Wcom is small. Second, the refrigerant temperature in the evaporator is very low when Tw is low, so the refrigerant could absorb more energy from air. Finally, before the system is turned on, the solar simulator needs to be adjusted for 50 min or so, and the evaporator-collector could absorb a lot of energy from the solar simulator during this period. After the system is turned on, the energy absorbed by the evaporator-collector will be transfered to the water. Therefore, when Tw are low, Wcom is low and Qe is very high, and so COPi is found to be very high.As described in Figures8 and 9, COPm is higher and heating time for the water temperature rising from 15°C to 55°C is less when the solar irradiation is higher, because the evaporator-collector can absorb more heat from solar irradiation under higher solar irradiation, which will speed up the heating process and increases the average COP. When the solar irradiation increases from 500 W/m2 to 900 W/m2, COPm increases by 0.20, 0.49, and 0.38 in air, dual, and solar modes, respectively. And the heating time decreases by 7.5 min, 23.5 min, and 39.5 min in air, dual, and solar modes, respectively.


### 4.2. Effect of the Ambient Temperature on Heat Performance of the System

To investigate the effect of the ambient temperature on heat performance of the system, experiments were done under the following conditions of water temperature rising from 15°C to 55°C, ambient relative humidity of 59%, the solar irradiation of 900 W/m2,the compressor frequency of 30 Hz, and the ambient temperature of 20°C and 30°C. Results are shown in Figures 10–12 and Table 4.Figure 10
Effect of the ambient temperature onWcom.Figure 11
Effect of the ambient temperature onQe.Figure 12
Effect of the ambient temperature onCOPi.Table 4
Effect of the ambient temperature on average COP and heating time.
Ambient temperature (°C)
Average COP
Heating time (min)
Air
Solar
Dual
Air
Solar
Dual
20
2.23
3.35
2.65
382.2
336.6
328.8
30
3.40
4.15
3.48
260
276.3
262.2As described in Figure10, when the ambient temperature rises from 20°C to 30°C, the increment of Ps can be neglected, so Wcom is almost the same.As presented in Figure11, when the ambient temperature is higher, the mean temperature difference of convectional heat exchanging ΔTm increases, so the energy absorbed from air by the evaporator-collector increases, which leads to a higher Qe.As shown in Figure12, when the ambient temperature rises from 20°C to 30°C, Wcom is almost the same, Qe is higher, so COPi is higher. It is noteworthy that after the water temperature rises to 52°C, COPi decreases below 1.00 in air mode, which means that the performance of this system is not better than electric water heater. The reason is that the heat exchange rate of the evaporator-collector is extremely low and the energy consumption of the compressor is too high after the water temperature rises to 52°C.As shown in Table4, when the ambient temperature is higher, COPm is higher and heating time is less. Because when the ambient temperature is higher, the mean temperature difference of convectional heat exchanging ΔTm increases; therefore, the evaporator-collector could absorb more energy from air, which will accelerate the heating process and raise average COP.


### 4.3. Effect of the Compressor Frequency on Heat Performance of the System

To investigate the effect of the compressor frequency on heat performance of the system, experiments were done under the following conditions of an ambient temperature of 20°C, the ambient relative humidity of 59%, the solar irradiation of 900 W/m2, and the compressor frequency of 30 Hz, 45 Hz, and 60 Hz. The results are shown in Figures 13–17.Figure 13
Effect of the compressor frequency onWcom.Figure 14
Effect of the compressor frequency onQe.Figure 15
Effect of the compressor frequency onCOPi.Figure 16
Effect of the compressor frequency onCOPm.Figure 17
Effect of the compressor frequency on heating time.As illustrated in Figure13, according to (3) and (7), when the compressor frequency increases, the rotating speed of compressor n increases, so Wcom increases. For the system working in different operating modes, Ps is almost the same, so Wcom is almost same.As presented in Figure14, Qe decreases with the increment of Tw and increases with the rising of the compressor frequency. According to (3), (4), and (5), when the compressor frequency f increases, the rotating speed n of the compressor increases, then the mass flow rate of the refrigerant mr increases, so Qe increases.As shown in Figure15, since the compressor frequency could both affect Wcom and Qe, the changing of COPi is more complicated under different compressor frequencies. In air mode, COPi is higher when the compressor frequency is higher. In solar mode, COPi is higher when the compressor frequency is lower. And the difference of COPi gradually decreased to 0 after the water temperature increases to 45°C. In dual mode, before the water temperature rises to 23°C, COPi is higher when the compressor frequency is lower and, afterwards, COPi is higher when the compressor frequency is higher.As shown in Figures16 and 17, heating time for the water temperature rising from 15°C to 55°C is less when the compressor frequency is higher. Because when the compressor frequency is higher, Qe is higher, which will speed up the heating process. In air and dual modes, COPm is higher when the compressor frequency is higher. So it is better to set the compressor frequency at 60 Hz for a higher COPm and less heating time under above conditions. In solar mode, COPm is lower when the compressor frequency is higher, so it is preferable to set the compressor frequency at 30 Hz for a higher COPm at the cost of longer heating time.


### 4.4. Comparison of Three Modes

Several experiments were tested in order to provide an operating strategy about the operating mode and the compressor frequency under certain ambient conditions, results are shown in Table5.Table 5
Average COP and heating time under different ambient and operation conditions.
Ambient conditions
Compressor frequency
Average COP
Heating time (min)
Ambient temperature
Solar irradiation
Air
Dual
Solar
Air
Dual
Solar
20°C
900 W/m2
30 Hz
2.23
2.65
3.35
382.2
328.8
336.3
45 Hz
3.37
2.54
3.29
191.9
186.7
237.3
60 Hz
3.85
3.66
2.92
129.7
139.3
201.4
30°C
900 W/m2
30 Hz
3.40
3.48
4.15
260.0
262.2
276.3
45 Hz
4.12
3.96
3.88
147.9
167.2
200.9
60 Hz
4.07
3.97
3.61
122.8
128.6
161.7It is generally accepted that when the average COP is higher and heating time is less, the performance of heat pump water heater system is better. As shown in Table5, under conditions that the solar irradiation is 900 W/m2 and the ambient temperature is 20°C and 30°C, in air and dual modes, it is better for this system to be operated at compressor frequency of 60 Hz for a relatively higher average COP and less heating time. And in solar mode, it is preferable to set the compressor frequency at 30 Hz for a higher average COP at the cost of longer heating time.Under the conditions that the solar irradiation is 900 W/m2 and the ambient temperature is 30°C, the system reaches the largest average COP of 4.15 in solar mode with the compressor frequency at 30 Hz. While under above conditions, the system shows better performance in air mode with the compressor frequency at 60 Hz, because the heating time is much less and the average COP is only a little lower than 4.15.As shown in Figure15, when the ambient temperature, the solar irradiation, and the water temperature are different, the corresponding operating mode and the compressor frequency for a largest instantaneous COP are different. Thus, the operating mode and the compressor frequency should be chosen according to the ambient temperature, the solar irradiation, and the water temperature.


## 4.1. Effect of the Solar Irradiation on Heat Performance of the System

According to GB/T 23137-2008, the nominal working conditions of heat pump water heater were chosen as the test conditions, which was set to an ambient temperature of 20°C, an ambient relative humidity of 59%, and the water temperature rising from 15°C to 55°C. To investigate the effect of the solar irradiation on heat performance of this system, the experiments were taken under above conditions and the solar irradiation set at 500 W/m2 and 900 W/m2. The results are shown in Figures 5–9.Figure 5
Effect of the solar irradiation onWcom.Figure 6
Effect of the solar irradiation onQe.Figure 7
Effect of the solar irradiation onCOPi.Figure 8
Effect of the solar irradiation onCOPm.Figure 9
Effect of the solar irradiation on heating time.As illustrated in Figure5, Wcom increases significantly with the rising water temperature. The reason is that when Tw increases, Td,Ts,Pd, and Ps increase as well with a decrease in ηm and the increment of Td is significantly larger than the increment of Ts, so when Ps and Td/Ts both is increased, according to (7) and (8), Wcom will increase significantly. Besides when the solar irradiation increases from 500 W/m2 to 900 W/m2, Ps is found to be almost the same, so was Wcom. And when the operating mode is set different, Ps shows no significant difference, so Wcom is almost the same.As shown in Figure6, when solar irradiation increases from 500 W/m2 to 900 W/m2, the heat absorbed from solar irradiation by the evaporator-collector increases, so Qe increases. Qe decreases with the rising of the water temperature. The reason is that with the rising of the water temperature, the evaporator temperature increases, which means the mean temperature difference of convectional heat exchanging ΔTm decreases, thus the heat absorbed from air by the evaporator decreases.As shown in Figure7, when the solar irradiation is increased from 500 W/m2 to 900 W/m2, Wcom is almost the same and Qe is higher, so COPi is higher. And with the rising of the water temperature, Qe decreases and Wcom increased, so COPi decrease. And there are three reasons for high COPi when Tw is low. First, when Tw is low, Ps and Td/Ts are small, according to (7) and (8), Wcom is small. Second, the refrigerant temperature in the evaporator is very low when Tw is low, so the refrigerant could absorb more energy from air. Finally, before the system is turned on, the solar simulator needs to be adjusted for 50 min or so, and the evaporator-collector could absorb a lot of energy from the solar simulator during this period. After the system is turned on, the energy absorbed by the evaporator-collector will be transfered to the water. Therefore, when Tw are low, Wcom is low and Qe is very high, and so COPi is found to be very high.As described in Figures8 and 9, COPm is higher and heating time for the water temperature rising from 15°C to 55°C is less when the solar irradiation is higher, because the evaporator-collector can absorb more heat from solar irradiation under higher solar irradiation, which will speed up the heating process and increases the average COP. When the solar irradiation increases from 500 W/m2 to 900 W/m2, COPm increases by 0.20, 0.49, and 0.38 in air, dual, and solar modes, respectively. And the heating time decreases by 7.5 min, 23.5 min, and 39.5 min in air, dual, and solar modes, respectively.


## 4.2. Effect of the Ambient Temperature on Heat Performance of the System

To investigate the effect of the ambient temperature on heat performance of the system, experiments were done under the following conditions of water temperature rising from 15°C to 55°C, ambient relative humidity of 59%, the solar irradiation of 900 W/m2,the compressor frequency of 30 Hz, and the ambient temperature of 20°C and 30°C. Results are shown in Figures 10–12 and Table 4.Figure 10
Effect of the ambient temperature onWcom.Figure 11
Effect of the ambient temperature onQe.Figure 12
Effect of the ambient temperature onCOPi.Table 4
Effect of the ambient temperature on average COP and heating time.
Ambient temperature (°C)
Average COP
Heating time (min)
Air
Solar
Dual
Air
Solar
Dual
20
2.23
3.35
2.65
382.2
336.6
328.8
30
3.40
4.15
3.48
260
276.3
262.2As described in Figure10, when the ambient temperature rises from 20°C to 30°C, the increment of Ps can be neglected, so Wcom is almost the same.As presented in Figure11, when the ambient temperature is higher, the mean temperature difference of convectional heat exchanging ΔTm increases, so the energy absorbed from air by the evaporator-collector increases, which leads to a higher Qe.As shown in Figure12, when the ambient temperature rises from 20°C to 30°C, Wcom is almost the same, Qe is higher, so COPi is higher. It is noteworthy that after the water temperature rises to 52°C, COPi decreases below 1.00 in air mode, which means that the performance of this system is not better than electric water heater. The reason is that the heat exchange rate of the evaporator-collector is extremely low and the energy consumption of the compressor is too high after the water temperature rises to 52°C.As shown in Table4, when the ambient temperature is higher, COPm is higher and heating time is less. Because when the ambient temperature is higher, the mean temperature difference of convectional heat exchanging ΔTm increases; therefore, the evaporator-collector could absorb more energy from air, which will accelerate the heating process and raise average COP.


## 4.3. Effect of the Compressor Frequency on Heat Performance of the System

To investigate the effect of the compressor frequency on heat performance of the system, experiments were done under the following conditions of an ambient temperature of 20°C, the ambient relative humidity of 59%, the solar irradiation of 900 W/m2, and the compressor frequency of 30 Hz, 45 Hz, and 60 Hz. The results are shown in Figures 13–17.Figure 13
Effect of the compressor frequency onWcom.Figure 14
Effect of the compressor frequency onQe.Figure 15
Effect of the compressor frequency onCOPi.Figure 16
Effect of the compressor frequency onCOPm.Figure 17
Effect of the compressor frequency on heating time.As illustrated in Figure13, according to (3) and (7), when the compressor frequency increases, the rotating speed of compressor n increases, so Wcom increases. For the system working in different operating modes, Ps is almost the same, so Wcom is almost same.As presented in Figure14, Qe decreases with the increment of Tw and increases with the rising of the compressor frequency. According to (3), (4), and (5), when the compressor frequency f increases, the rotating speed n of the compressor increases, then the mass flow rate of the refrigerant mr increases, so Qe increases.As shown in Figure15, since the compressor frequency could both affect Wcom and Qe, the changing of COPi is more complicated under different compressor frequencies. In air mode, COPi is higher when the compressor frequency is higher. In solar mode, COPi is higher when the compressor frequency is lower. And the difference of COPi gradually decreased to 0 after the water temperature increases to 45°C. In dual mode, before the water temperature rises to 23°C, COPi is higher when the compressor frequency is lower and, afterwards, COPi is higher when the compressor frequency is higher.As shown in Figures16 and 17, heating time for the water temperature rising from 15°C to 55°C is less when the compressor frequency is higher. Because when the compressor frequency is higher, Qe is higher, which will speed up the heating process. In air and dual modes, COPm is higher when the compressor frequency is higher. So it is better to set the compressor frequency at 60 Hz for a higher COPm and less heating time under above conditions. In solar mode, COPm is lower when the compressor frequency is higher, so it is preferable to set the compressor frequency at 30 Hz for a higher COPm at the cost of longer heating time.


## 4.4. Comparison of Three Modes

Several experiments were tested in order to provide an operating strategy about the operating mode and the compressor frequency under certain ambient conditions, results are shown in Table5.Table 5
Average COP and heating time under different ambient and operation conditions.
Ambient conditions
Compressor frequency
Average COP
Heating time (min)
Ambient temperature
Solar irradiation
Air
Dual
Solar
Air
Dual
Solar
20°C
900 W/m2
30 Hz
2.23
2.65
3.35
382.2
328.8
336.3
45 Hz
3.37
2.54
3.29
191.9
186.7
237.3
60 Hz
3.85
3.66
2.92
129.7
139.3
201.4
30°C
900 W/m2
30 Hz
3.40
3.48
4.15
260.0
262.2
276.3
45 Hz
4.12
3.96
3.88
147.9
167.2
200.9
60 Hz
4.07
3.97
3.61
122.8
128.6
161.7It is generally accepted that when the average COP is higher and heating time is less, the performance of heat pump water heater system is better. As shown in Table5, under conditions that the solar irradiation is 900 W/m2 and the ambient temperature is 20°C and 30°C, in air and dual modes, it is better for this system to be operated at compressor frequency of 60 Hz for a relatively higher average COP and less heating time. And in solar mode, it is preferable to set the compressor frequency at 30 Hz for a higher average COP at the cost of longer heating time.Under the conditions that the solar irradiation is 900 W/m2 and the ambient temperature is 30°C, the system reaches the largest average COP of 4.15 in solar mode with the compressor frequency at 30 Hz. While under above conditions, the system shows better performance in air mode with the compressor frequency at 60 Hz, because the heating time is much less and the average COP is only a little lower than 4.15.As shown in Figure15, when the ambient temperature, the solar irradiation, and the water temperature are different, the corresponding operating mode and the compressor frequency for a largest instantaneous COP are different. Thus, the operating mode and the compressor frequency should be chosen according to the ambient temperature, the solar irradiation, and the water temperature.


## 5. Conclusion

A novel DX-SAHPWH system was fabricated and tested experimentally in the enthalpy difference lab with a solar simulator. The effects of ambient temperature and solar irradiation on the performance of this system were analyzed. And the performance of the system in three operating modes with different compressor frequencies was compared by the paper.(1)
The ambient temperature, solar irradiation, and operating mode have almost no effect on the energy consumption of the compressor. When the ambient temperature and the solar irradiation are higher,Qe and COPi are higher as well.
(2)
With the rising of the water temperature,Wsys increases while Qe and COPi decrease. When the compressor frequency increases, Wsys and Qe both increase.
(3)
It is therefore recommended to operate this system in air and solar modes with the compressor frequency at 60 Hz under an ambient temperature condition of 20°C to 30°C and an average solar irradiation of 900 W/m2.
(4)
In order to enhance the performance of this system, the operating mode and the compressor frequency should be chosen according to the water and ambient temperature, and solar irradiation.



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*Source: 5986463-2018-04-22.xml*