A novel testing and theoretical approach for air-source heat-pump water heater with flash tank vapor-injection

Air-source heat-pump (ASHP) water heater provides an important low-carbon solution for domestic water heating, which regains interest in the context of pursuing global carbon neutrality. In this study, a novel testing and theoretical approach is proposed for ASHP water heater with flash tank vapor-injection by retrofitting the testing methods of state points. Five flash tank vapor-injections with different temperature levels are tested and verified the testing method for ASHP water heater. On basis of the tested compressor suction temperature, exhaust temperature, electronic expansion valve pre-throttle temperature, electronic expansion valve throttling temperature and vapor-injection temperature, a thermodynamic analysis is conducted to determine the system performances. The relationship between the vapor-injection pressure ratio, system heating capacity and COP is identified by adjusting the expansion valve opening degree of the system. The results show that the COP of the system firstly decreases and subsequently increases with the increase in vapor-injection pressure ratio, and the minimum COP appears when the vapor-injection pressure ratio is 0.3. This study provides new insights into the advanced ASHP water heater.


Introduction
An air-source heat-pump water-heater is an energy-saving device for producing hot water that absorbs lowgrade heating from air. Conventional air-source heat pumps will freeze when operating in low-temperature and high-humidity areas, and system heating efficiency will decrease significantly in low-temperature environments [1][2][3][4]. Therefore, numerous experimental and thermodynamic studies have been performed to improve heat pump efficiency in the winter [5][6][7][8].
Arai [9] proposes a heat pump with flash tank vapor-injection for a scroll compressor and discovers that the heating performance could be increased by ∼15% compared with the conventional scroll compressor enhanced vapor-injection system under low temperature conditions. Heo et al [10] build an enhanced vapor-injection airsource heat-pump test bench with a flash tank and discover that the system heating is related to the compressor frequency and ambient temperature. Xu et al [11] discover that the electronic expansion valve and proportional integral derivative controller can effectively regulate system operation during the testing of transient and steadystate operations of an enhanced vapor-injection system. Bake et al [12] conduct a test on carbon dioxide as a working-fluid-enhanced vapor-injection system and discovered that the system heating and COP were 10.7% and 6.7% higher than those of single-stage compressed enhanced vapor-injection systems, respectively. Wang et al [13] perform an experiment to compare the performances of the R410A enhanced vapor-injection air-source heat pump and ordinary air-source heat pump; they discover that at −17.8°C ambient temperature, using the R410A enhanced vapor-injection heat pump increased heat by 30% compared with the ordinary heat pump, and the COP increased by 20%. Sun et al [14] conduct an experimental study on the quasi-secondary compressed heat pump system of a rolling rotor-type air compressor and discovered that the heating capacity of the system increased gradually but the growth rate decrease gradually; additionally, the system COP increases and then decreases with the increase in vapor-injection pressure. Roh et al [15] test the effect of vapor-injection pressure on heat production in an enhanced vapor-injection system. Roh and Min [16] propose a vapor-injection heat pump system that directly injected vapor into an accumulator; compare with the conventional vapor-injection compressor air source heat pump, their proposed system could reduce the exhaust temperature of the compressor.Hua et al [17] intend to improve the energy efficiency of the solid desiccant heat pump (SDHP) system of the conventional air conditioning and experiments are conducted to evaluate the system in terms of the supply air condition, the electricity consumption and the coefficient of performance (COP). Wang et al [18] develops a novel frost-free ASHP system and the results show that the system thermodynamic characteristics are highly sensitive to the variation in the EEV( electronic expansion valve) openings of both the1st stage throttle and 2nd stage throttle.
All previous experimental studies focus on the system COP; it is uncommon to obtain the state point of the P-h chart indirectly and effectively to study the COP of the system operation. In this study, the state point of the P-h chart is determined by testing the temperature of the heat pump system during operation. Subsequently, a thermodynamic analysis is performed to study how the vapor-injection pressure ratio of the quasi-secondary compressed enhanced vapor-injection air source heat pump water heater (ASHP water heater with flash tank vapor-injection) system can be adjusted.
Guo et al [19] present a methodology for optimizing operations, which considers thermostatic and timing control patterns. Ma and Chai [20] establish a thermodynamic model for the compressed process of a scroll compressor with a vapor-injection port, and the calculation results show that the operating range of this system is larger than ordinary. Ibrahim et al [21] present a dynamic simulation model to predict the performance of an air-source heat-pump water heater and an optimal management model for an air-source heat-pump water heater, which is developed and applied for a typical winter day in Beirut. Xiao et al [22] establish a simulation model through a mathematical model and evaluate the performance of the enhanced vapor-injection system with R22, R290, and R23 as working fluids. Sirwan R et al [23] develop a mathematical model to calculate entropy increase and evaluate exergy loss based on the second law of thermodynamics in an ejector-absorption cooling combined system; they discover that adding a flash tank to the system could improve the system operation efficiency. Jaehyeok et al [24] propose an optimum cycle control method for a refrigerant injection heat pump with a double expansion sub-cooler based on an intermediate pressure and injection ratio. Qiao et al [25,26] discover that the electronic expansion valve opening significantly affected the system performance. Li et al [27] adjust the openness of an electronic expansion valve to regulate refrigerant volume and obtain the optimal openness of the R22 and R417A systems in the winter and under low-temperature conditions. Zheng et al [28] propose a novel vapor-injection cycle using a cascade condenser to liquefy separated vapor streams for evaporation and the amount and state of the injected stream is controlled via a branch circuit.
So, most related studies have focused on establishing a simulation model to predict the performance of an air-source heat-pump water heater, while few have focused on the relationship between vapor-injection pressure ratio, system COP, and capacity. This paper presents a thermodynamics analysis based on adjusting the expansion valve opening degree of the ASHP water heater with flash tank vapor-injection to test the compressor suction temperature, exhaust temperature, electronic expansion valve pre-throttle temperature, electronic expansion valve throttling temperature, and vapor-injection temperature.The calculating and thermal analysis process of ASHP water heater with flash tank vapor-injection are shown as figure 1.

Calculating principle and thermodynamic analysis of ASHP water heater with flash tank vapor-injection
For the enhanced vapor-injection mode, a scroll compressor is generally used and two intake pipes are adopted for the intake port of the compressor. One port is provided on the scroll to connect the vapor injection, while another port is connected to the suction side of the compressor. The five temperatures (compressor suction, exhaust , electronic expansion valve pre-throttle, electronic expansion valve throttling, and vapor-injection temperatures) are arranged in the main inlet and outlet of the compressor, front and rear of the electronic expansion valve, and the upper part of the flash tank. After testing the five temperatures, the vapor-injection pressure ratio of the system is studied by adjusting the expansion valve opening degree of the ASHP water heater with flash tank vapor-injection, and the relationship between the vapor-injection pressure ratio, system COP, and heat capacity is analyzed.

The ASHP water heater with flash tank vapor-injection system
In this study, the heat pump water heater system comprises a compressor, gas-liquid separator, water side heat exchanger , accumulator, dry filter, four-way reversing valve, electronic expansion valve, flash tank, and evaporator, as shown in figure 2.
As shown in figure 2, the high-temperature and high-pressure working fluids (R22) flow from the outlet of the enhanced vapor-injection compressor that passes through the condenser. After the first throttling, the working fluid is divided into two parts in the flash tank: one part of the working fluid passes through the flash tank and subsequently depressurizes the second expansion valve, and another part of the flash steam enters the suction end of the compressor. The ASHP water heater with a flash tank vapor-injection system can decrease the heat absorption capacity of the heat pump in the air by increasing the sub-cooling degree of the evaporator while increasing the temperature of the compressor working fluid by the vapor-injection system and increasing the efficiency of the system. This mode is used to upgrade the efficiency of the air-source heat pump in the winter and from the observed effects, the operation is relatively stable and the system efficiency and anti-frost effects are good.

The testing and thermodynamic analysis principle of the ASHP water heater with flash tank vaporinjection
For the ASHP water heater with a flash tank vapor-injection system, the testing and thermodynamic analysis are assumed as follows: (1) The refrigerant flow pressure loss is overlooked; (2) Due to the extremely short time of vapor injection, the process of compressing while vapor injection is simplified to a constant volume mixing and adiabatic depressurization process [29] ; (3) The pressure of the overheating section of the evaporator outlet is constant; (4) The pressure of the condenser is constant; As shown in figure 2(b), the calculation principle of the ASHP water heater with flash tank vapor-injection, compressor suction temperature, exhaust temperature, electronic expansion valve pre-throttle temperature, electronic expansion valve throttling temperature, and vapor-injection temperature are t , 1 t , 3 t , 4 ¢ t , 5 and t , respectively. According to the experimental temperature parameters t , 1 t , 3 t , 4 ¢ t , 5 and t , 6 the pressure parameter of each point can be directly obtained from the P-h chart: P , 1 P , 2 ¢ P , 2 ¢ P , 4 P , 5 and ¢ P , 5 and the corresponding state points 1, 5, and 6. The state points 2, 2′, 3, 4, 4′, and 5′ corresponding to the P-h chart must be calculated.

Calculating principle of the ASHP water heater with flash tank vapor-injection
The system is adiabatic compression process, and the entropy state point 2 can be calculated as follows where S 1 and S 2 are the entropy of state points 1 and 2, respectively (kJ kg −1 K −1 ), e is the isentropic efficiency, in which 0.8 was used in this study. Process 2′-3 is isentropic efficiency compressed, and if = ¢ S S 2 2 is assumed, then the entropy of state point 2′ is where S 2′ is the entropies of state point 2′, respectively (kJ kg −1 K −1 ).
where m is the main circuit flow; i is the vapor-injection circuit flow; and h 2 and h 6 are the enthalpies of state points 2 and 6, respectively (kJ kg −1 ).

The entropy of state point 3 is
The entropy of state point 4 is = ( ) P P , 5 4 3 where P 3 and P 4 are the state point pressures of state points 3 and 4, respectively (kPa).
The enthalpy of state point 4′ is where h 4 and h 4 ′ are the enthalpies of state of points 4 and 4′, respectively (kJ kg −1 ). The enthalpy of state of point 5′ is: where h 5 and h 5′ are the enthalpies of state of points 5 and 5′, respectively (kJ kg −1 ). By substituting these state points from the cycle into the heat production process, the heat capacity of the cycle can be obtained: where Q is the heating capacity (kW). The compressed power is where W 0 is the compressed power (kW). Therefore, the COP of a quasi-secondary compressed with flash tank enhanced vapor-injection heat-pump water heater is In the formula, h R is the heat pump efficiency, in which 0.8 was used in this study.

Thermodynamic analysis of the ASHP water heater with flash tank vapor-injection
For the ASHP water heater with flash tank vapor-injection, the vapor-injection pressure significantly affects the heating performance of the system. This paper introduces a vapor-injection pressure ratio β 1 , indicating the relationship between the vapor-injection pressure, condensing pressure, and evaporation pressure. Subsequently, the vapor-injection pressure ratio β 1 can be calculated as where P 2 is the vapor-injection pressure, P 1 the evaporation pressure, and P 3 the condensation pressure (kPa).

The compressed power
(1) Precompressed process Assuming that the system is isisentropic efficiency compressed, the compressed power is where k is the isentropic index of the refrigerant (1.19 for R22); R is the gas constant of refrigerant (96.16 × 10 -3 (kJ kg −1 K −1 ) for R22); W 1-2 is the compressed power of state points 1 to 2 (kW); P 1 and P 2 represent the evaporation and intermediate pressure , respectively (kPa); T 1 represents the compressor suction temperature (K).
(2) Compressed power of the enhanced vapor-injection compressed process The heat balance equation in the flash tank is [30].
where i is the mass flow of the vapor-injection circuit (kg s −1 ); m is the primary circuit mass flow (kg s −1 ); a is the relative vapor-injection flow; h 4 , h 5 , and h 6 (kJ kg −1 ) are the enthalpies of state of points 4, 5, and 6 in the P-h chart.
Find the enthalpy corresponding to the pressure of R22 through the P-H diagram, and fit the relationship between the pressure and enthalpy of R22. The relationship between saturated gas enthalpy (R22) and pressure satisfaction is The relationship between saturated liquid enthalpy (R22) and pressure satisfaction is The relationship between the two-phase zone enthalpy (R22) and pressure satisfaction is According to the conservation of mass, the conservation of energy and the gas differential equation of an unsteady thermal system, W 2-2 (kW) can be calculated as (3) Compressed power after vapor injection The compressed power during a compressed process after vapor injection can be written as represents the compressed power of state points 2′ to 3. P 2′ and P 3 are the refrigerant pressures of points 2′ and 3, respectively; T 2 and T 6 are the temperatures of state points 2 and 6, respectively; v 2 and v 2 ′ are the refrigerant specific volumes of points 2 and 2′, respectively (m 3 kg −1 ).
(4) Total compressed power of the system As shown from the P-h chart (figure 2), the system compressed power comprises pre-compressed power, compressed power of the enhanced vapor injection, and compressed power after vapor injection: The pressure value does not change significantly during the mixing process of the working fluid; subsequently, W 2-2′ can be ignored and equations (12), (17), and (18) are substituted into equation (19): Equations (11) and (16) are substituted into equation (20): In equation (21), x is the dryness of the working fluid (equal to 0.1), and the vapor-injection pressure ratio β 1 is variable.

Calculation of heat capacity
In the P-h chart (figure 2), the heat capacity of the ASHP water heater with flash tank vapor-injection is Process 4-4′ is adiabatic expansion: 4 4 According to the energy balance equation of the hypothesis and the vapor-injection process, the enthalpy of point 2′ is The pressure value does not change significantly during the mixing process of the working fluid; subsequently, W 2-2′ can be ignored and equation (26) can be written as Equations (14), (15), and (16) are substituted into equation (26):  (20) and (27). This method is extremely complicated to be calculated directly; therefore, the calculation diagram is used in this study.

Experimental test and verification
According to the experimental test procedure shown in figure 3, the ASHP water heater with flash tank vaporinjection (LNDKFX-17II) system was tested. The basic parameters of the quasi-secondary compressed with flash tank enhanced vapor-injection heat-pump water heater are shown in the following table 1.

Temperature sensor arrangement of the experiment
The experimental points of the five temperatures (discharge temperature of compressor, temperature before electronic expansion valve, temperature after electronic expansion valve, the temperature of flash vapor, and vapor-injection temperature) were arranged according to figure 3. The corresponding temperature on the system diagram are t , 1 t , 3 t , 4 ¢ t , 5 and t , 6 respectively, and the experiment layout is as shown in figure 3. According to the sensor arrangement requirement shown in figure 3. shows the heat preservation treatment of the water supply pipe joint and the temperature sensor. Because testing the pressure of each state point in figure 2 is difficult, the system state point was tested and calculated by testing the five temperatures and the COP was obtained using the state points. According to the test point arrangement of figure 3. shows the heat insulation treatment of the water supply pipe joint for the temperature sensor. The temperature sensor was a bare wire that was aligned with the testing point temperature. The sensors were placed close to the outer wall of the copper pipe of the hot water machine, fixed with aluminum foil paper and insulating tape, and with heat insulated using a foaming material.

Ambient condition of experiment
Because the experiment was performed in an air enthalpy laboratory, the air temperature of the evaporator could be adjusted. To understand the operation of the heat-pump water heater, the ambient temperature was controlled in the five experimental conditions, as shown in table 2.
The purpose of this experiment is to obtain the state point of the air-source heat pump in low-temperature environments. In conditions 1 and 2, the ambient dry temperature (15°C) and wet-bulb temperature (20°C) remained constant, while the inlet and outlet water temperatures were changed and controlled (from 15°C/20°C to 50°C/55°C). Experimental condition 3 involves extreme conditions for investigating the performance of the air-source heat-pump water heater in low-temperature environments. The ambient dry-bulb temperature of experimental conditions 4 and 5 are 2°C, and the wet-bulb temperature was 1°C (experimental conditions 4 and 5), which rendered it easier to frost the air-source heat pump.

Experimental results
According to the five experimental conditions in table 2, the tested temperature parameters t , 1 t , 3 t , 4 ¢ t , 5 and t 6 were analyzed to study the changes in various temperature parameters under different experimental conditions.
According to the five experimental conditions in table 2, the tested temperature parameters t , 1 t , 3 t , 4 ¢ t , 5 and t 6 were analyzed to study the changes in various temperature parameters under different conditions. Owing to the large fluctuation in ambient temperatures, the compressor suction temperature (t 1 ) was used as the basic   temperature (t 1 ) and other temperatures were compared with this temperature. During the experiment, frosting occurred in the air-cooled fins of the water heater operating in experimental condition 4, and the frost layer was thin. The front fins were slightly frosted, and the frosting parts appeared in the upper half of the fins. After a period of frosting, the heat pump entered the defrosting condition, and the defrosting time was short, i.e., ∼5 min. After the defrosting, the heat pump operated stably. The defrosting was performed twice in the experiment, and the heat pump operated well throughout the process. As shown in figure 4. the suction temperature (t 1 ) of the compressor decreased and the exhaust gas temperature(t 3 ) increased when the ambient temperature decreased. Furthermore, the suction temperature (t 1 ) of the compressor increased and the electronic expansion valve pre-throttle temperature (t 4 ) increased. As shown in figure 6, the temperature before the throttle valve (t 4 ) increased with the compressor suction temperature (t 1 ) first and subsequently decreased with the continual increase in the compressor suction temperature (t 1 ); t 6 is the temperature after throttling, which is consistent with the trend of t 5′ .
Based on the experimental data above and according to figure 5, each state point can be obtained and the COP of the heat pump can be calculated. The tested COP obtained from testing the five temperatures of the heat pump is compared with the COP obtained by directly testing the heat capacity and electric power, as shown in figure 7. When the ambient temperature was 20°C and 2°C, the corresponding system COP was approximately 6 and 2.0, respectively, indicating that the COP of the air-source heat pump was primarily affected by the ambient temperature. Although the value h R was small, the calculation results were reliable; based on the comparison between the test and actual COP, the experimental method proposed herein was simple and practical.

Thermodynamic analysis of ASHP water heater with flash tank vapor-injection
After the experimental data were verified, a thermodynamic analysis was performed based on the data, and the relationships of heat generation and COP of the ASHP water heater with flash tank vapor-injection system with β 1 are as shown in figures 6 and 7, respectively.
As shown in figure 6. when the ambient temperature remains the same, the heat generation of the ASHP water heater with a flash tank vapor-injection system increases gradually with the increase in the vapor-injection pressure ratio β 1 when β 1 is less than 0.3, but when the vapor-injection ratio is greater than 0.3, the system's heating energy growth rate becomes lower and lower or even decreases. This is because with the increase of the vapor-injection pressure ratio, the quality of vapor is increased, and the system's compression power and heating capacity are increased. However, the increase in the heating capacity is lower than the increase in the compressor power, so the COP of the system decreases continuously with the increase of the vapor injection pressure ratio. When the vapor injection pressure ratio exceeds 0.3, a certain amount of refrigerant liquid enters the intermediate mixing chamber of the compressor, so that the enthalpy difference between the inlet and outlet of the condenser is relatively reduced, which further affects the heating capacity and leads to the overall compression power decrease of the system.    As shown in figure 7. COP decreases first and then increases with the increase of the vapor-injection pressure ratio, and reaches a minimum value when β 1 is 0.3. The main reason for this is that as the vapor-injection pressure ratio increases, the compression power of the system increases first and then decreases. However, the heating capacity has been increasing, and although it has finally decreased, its reduction rate is less than the compression power rate.
It can be seen from the above analysis that when under low-temperature conditions. When the air pressure ratio β 1 is 0.3, the system COP is the minimum value, but the system heating capacity is the maximum value. To meet the heating capacity in a low-temperature environment, the best system COP must be abandoned to adjust the expansion valve opening degree of the ASHP water heater with flash tank vapor-injection and then to control the vapor-injection pressure ratio of the system, so that the operation of the system can meet the heating capacity of users.

Conclusion
A novel testing method of the ASHP water heater with flash tank vapor-injection verified by the actual COP was presented herein; additionally, a thermodynamic analysis was performed based on a test. Based on testing and theoretical studies, the conclusions obtained are as follows: 1. A simple and effective method to test the system COP was by testing the compressor suction temperature, exhaust temperature, electronic expansion valve pre-throttle temperature, electronic expansion valve throttling temperature, and vapor-injection temperature to obtain the state points in the P-h chart.
2. As the vapor-injection pressure increased, the heat capacity and COP of the ASHP water heater with flash tank vapor-injection system decreased first and subsequently decreased with the increase in vapor-injection pressure; when the vapor-injection pressure ratio was 0.3, the COP of the system exhibited a lower value.