The current problems in the application of heat pump water heaters: (1) The air source heat pump will cause frosting on the evaporator under certain meteorological conditions in winter. The frost layer not only increases the heat transfer resistance, but also blocks the fan passage caused by the fin passage. Decreased, causing the performance of the heat pump to drop, and even not working properly in severe cases; (2) The air source heat pump should work under different environmental conditions throughout the year, and the working conditions change greatly, especially in the winter when the ambient temperature is lower or the water temperature of the water heater is higher. High-pressure compressor pressure is relatively large, even beyond its allowable working range.

Ji Jie et al. proposed an air conditioner/water heater integrated machine that can effectively use condensation heat as a water heater, and conducted theoretical and experimental research on it. The results show that the air conditioner/water heater integrated machine can greatly improve energy utilization efficiency. Kim M et al. performed a numerical simulation of the dynamic performance of the heat pump water heater system and optimized the tank volume. These studies did not address the problems in heat pump applications.

In order to solve the problems of frosting of heat pump heat exchangers and safe operation of compressors, some companies in the United States, Australia and other countries have developed indoor heat pump water heaters, and placed compressors and evaporators indoors.

Morrison GL et al., Yu Leyuan et al. conducted an experimental study on this model. The results show that the heat pump heat exchanger does not have frosting problems and the system works stably. However, the heat pump system absorbs indoor air heat, which reduces the indoor temperature. Especially in winter, the impact of indoor heat pump water heaters on indoor ambient temperature may cause inconvenience to users. Sakellari D et al. numerically simulated the feasibility of using the concentrated exhaust heat of the building to heat the floor through the heat pump cycle. The results show that this method is feasible, but no corresponding experimental verification.

2 exhaust air heat recovery type heat pump water heater basic principle and characteristics in order to solve the problem of the outdoor heat exchanger frosting and the compressor pressure ratio caused by the use of heat pump water heaters due to geographical restrictions and the impact of indoor heat pump water heaters on the bathroom environment This paper proposes a heat recovery type heat pump water heater that utilizes indoor exhaust heat, and its system is mainly composed of water tank, compressor, evaporator, capillary and drying filter, gas-liquid separator and other accessories. Designed in the form of components, the evaporator is installed in the air exhaust of the bathroom, and the fan is used as the exhaust fan and the evaporator fan. The evaporator absorbs the heat of the exhaust of the bathroom, and the hot water in the water tank is heated by the heat pump to cool the bathroom exhaust after cooling. When it is discharged outside, the evaporator fan can be used as an exhaust fan alone when the heat pump water heater is not working.

The heat pump water heater utilizing exhaust heat has the following characteristics: (1) avoiding frosting on the surface of the heat exchanger outdoor heat exchanger because the heat pump evaporator is placed indoors, and generally the bathroom temperature is above 18 ° C, so the evaporator fins The surface does not cause frosting problems, overcomes the performance degradation and unstable work of the refrigeration system caused by frosting/defrosting, and the heat pump water heater can work efficiently and stably under any external weather conditions. Therefore, the waste heat recovery to the water heater can be free from geographical restrictions, and is widely used in various climatic conditions in the south and north, and expands the use range of the heat pump water heater, which is of great significance for promoting the use of the heat pump water heater.

(2) Avoid the problem that the compressor pressure ratio is too large when the heat pump works in winter conditions or when the outlet water temperature is high. The water temperature of the water heater required by the user is generally 45 to 50 ° C. For the heat pump system using R22 working fluid, the condensation is performed. The pressure is generally about 2. 4MPa. When the heat pump water heater is used in winter, the outdoor ambient temperature is lower, and the corresponding system evaporation pressure is also lower, which causes the compressor pressure to increase relatively, and on the other hand increases the compressor power consumption. The COP of the system is lowered, and the safety work of the compressor is extremely disadvantageous, and the heat pump water heater using the indoor exhaust heat is avoided, and the indoor exhaust air is stable in the range of 18-28 ° C for many seasons, and The heat pump system has a higher evaporation temperature, which makes the heat pump system compressor work within a relatively suitable pressure ratio range, so that the operational reliability is improved.

3 Experimental study on dynamic performance of heat pump water heaters 3. Experimental prototype In order to verify the dynamic performance of heat pump water heaters using indoor exhaust heat, the experimental prototypes were processed and tested. The experimental heat pump water heater prototype uses a two-way slotted finned tube heat exchanger with geometric parameters as shown; the exhaust fan is a forward multi-wing centrifugal fan with a wind volume of 650 m 3 /h. The compressor is rotor type, its standard 5毫米的管管。 The rated cooling capacity is 2. 2 kW; the throttling element uses a capillary with an inner diameter of 0. 5 mm. The water tank volume is 0. 1m 3 , the actual water injection amount in the experiment is 85.3 kg; the condensing coil in the water tank is coiled by a copper tube, and the length is about 36 m.

3. 2 Experimental device experiment is carried out in the performance test room of the enthalpy difference air conditioner. The heat pump system and the arrangement of the measuring points are shown as shown. The laboratory is divided into indoor and outdoor side environmental chambers. Each air chamber is equipped with an air treatment unit. The temperature and humidity of the environmental chamber are stabilized at the set value by the computer through PID control to meet the temperature and humidity conditions required for the experiment. Heat pump system evaporator fan (exhaust fan) flow is controlled by the indoor side environmental chamber variable frequency fan, the air volume is measured by standard nozzle, the system cooling capacity can be determined by measuring the difference between the inlet and outlet of the evaporator; the heat pump system consumes power and the refrigerant at each measuring point Parameters such as pressure, undercooling and superheat are collected by the test bench control system.

The bath water inlet and outlet and the water temperature in the water tank are measured by thermocouple. Due to the natural convection, there is water temperature stratification in the water tank. Therefore, a pair of thermocouples are arranged on the upper, middle and lower layers of the tank, respectively. The lower three layers of the midpoint. The thermocouples are calibrated in a constant temperature bath. All parameters are collected and processed using computer data. The accuracy of the measuring instruments and sensors is as shown.

In the experiment, the indoor and outdoor environment is first adjusted to the required working conditions. The indoor side environment is controlled at a dry bulb temperature θa of 18 ° C, a relative humidity of 60%, an exhaust fan air volume of 650 m 3 /h, and an outdoor side environment control. The ball temperature was 18 °C. After the indoor and outdoor environment is stable, the prototype of the tested heat pump is turned on, and the data of each measuring point is collected every 6s. The experimental result is shown as ~8.

4 Experimental results and analysis show the temperature rise curve of the water in the water tank. The upper water temperature reaches 50 °C after the sample machine is turned on for about 45 minutes, which can meet the requirements of bathing water. In addition, it can be seen from the figure that there is a serious stratification of the water temperature in the water tank, and the water temperature at the bottom of the water tank rises very slowly with time, and only rises by about 3 ° C during the entire heating process. The reason for this phenomenon is that the natural convection heat transfer of the water outside the condensing coil causes the hot water to collect in the upper layer. At the same time, the condensing coil of the heat pump unit is the working medium up and down type, and the R22 working fluid in the coil at the bottom of the water tank. In the state of being too cold, the heat exchange between the working medium and the water outside the coil is small, resulting in a small increase in the water temperature at the bottom of the tank.

The refrigerant temperature at the outlet of the condensing coil and the corresponding degree of subcooling are given. It can be seen from the figure that the condenser outlet refrigerant has a large degree of subcooling due to the lower water temperature outside the condensing coil; The increase of water temperature causes the condensation temperature to rise, while the water temperature at the bottom of the tank does not change much, causing the temperature of the refrigerant at the outlet of the condensing coil to increase slowly with the increase of the upper water temperature, while the subcooling of the refrigerant increases rapidly, and the heating process ends. The coldness reaches about 28 ° C, which is beneficial for increasing the heat capacity of the unit and increasing its coefficient of performance.

The condensing pressure, evaporation pressure and high and low pressure difference of the heat pump system are shown as a function of time. It can be seen from the figure that the heat pump prototype quickly reaches equilibrium after being turned on; as the water temperature in the water tank increases, the evaporation pressure increases approximately linearly. The condensing temperature is about 1. 6MPa. The condensing temperature is about 56 ° C, the system high and low pressure difference is about 1. 6MPa.

The curve of suction and exhaust steam temperature with heating time is given. With the increase of water temperature and compressor pressure ratio, the compressor exhaust steam temperature gradually increases; when the water temperature reaches 50 °C, the compressor exhaust steam temperature It is about 107 °C. Therefore, for the prototype of the experimental heat pump water heater using R22 working fluid, the bath water outlet temperature of 50 °C is the limit of its safe work. Further increase of the outlet water temperature will make the compressor pressure ratio and exhaust steam temperature exceed the safe range, and the safe operation of the heat pump system. unfavorable. It can also be seen that the suction temperature of the compressor does not change much 30 minutes before the heating process, and the speed of the suction temperature of the compressor increases at the later stage of the heating process. The reason is that the heat transfer temperature of the R22 and the air in the evaporator is reduced. Gradually reduced.

Shown is the change of heat and COP of heat pump water heater with running time. It is found that the heating capacity increases with the increase of water temperature in the water tank, but its growth rate decreases with the increase of heating time; the coefficient of performance COP runs with The time rises first and then falls, and the COP reaches a maximum in the water outlet temperature of 33 to 40 ° C, and its value is about 3.4. The average COP during the whole heating process is about 3. 15. The reason for the maximum COP is The operating conditions such as the condensing temperature and evaporating temperature of the system in the water outlet temperature range make the components of the heat pump system reach the best match, so the system COP is high.

The increase of calorific value over time is due to two factors: (1) the residual heat obtained from the exhaust air (ie the cooling capacity of the heat pump system) increases with the increase of the outlet temperature of the water tank, and its growth rate gradually increases. Slow down the trend. It can be seen that the increase of the cooling capacity of the heat pump system is due to the increase of the system evaporation pressure and the degree of supercooling; (2) As the water pump temperature increases, the high and low pressure difference of the heat pump system increases, resulting in a nearly linear increase in the power consumption of the compressor. As a result, the heat generated by the heat pump system increases.

5 Conclusion A heat pump water heater unit using indoor exhaust heat is proposed, which avoids the frost on the surface of the heat exchanger outdoor heat exchanger and the deterioration of the working conditions of the compressor under winter conditions, and does not affect the ambient temperature of the bathroom.

The experimental results show that the experimental heat pump water heater prototype can heat 85 kg water from 18 °C to 50 °C in 45 min at room temperature of 18 °C; the heat pump system performance coefficient reaches the maximum when the water tank outlet temperature is in the range of 33-40 °C. The maximum coefficient of performance is about 3.4. The average coefficient of performance during the entire heating process is 3. 15. The average power consumption is 800 W. When the outlet temperature of the water tank is 50 °C, the compressor discharge temperature is about 107 °C. The high and low pressure difference of the heat pump system is about 1.6. Further increase of the outlet water temperature will make the compressor pressure ratio and the exhaust steam temperature exceed the safe range, which is unfavorable for the safe operation of the heat pump system, that is, the prototype of the experimental heat pump water heater using the R22 working fluid, bathing The water outlet temperature of 50 ° C is the limit of its safe work.

At the same time, it was found that for the heat pump water heater system using the condensing coil type water tank, the refrigerant up-and-down arrangement can cause a serious stratification effect on the water temperature in the water tank, so that the refrigerant has a large degree of subcooling at the condenser outlet. And as the water temperature of the water tank rises, the degree of coldness increases linearly. For the end of the heating process of the experimental prototype, the degree of subcooling can reach 28 °C.

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