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Defrosting Method of Air Source Heat Pump Water Heater

The main body of the air source heat pump water heater is a heat pump system. It uses electric energy as the driving force, uses the refrigerant to absorb the low-grade heat energy in the environment, transfers the heat to the water through the heat exchanger in the heat pump cycle to obtain the hot water, and finally sends the hot water to the user through the water circulation system. There are two ways to heat water. One is to directly use the casing of the condenser as a water storage container. When in use, the condenser is filled with water. After the water temperature rises to meet the requirements for use, the hot water can be discharged from the upper outlet of the condenser, and the lower inlet automatically replenishes the cold water. This method is suitable for small domestic water heaters. Another way is to have a larger storage tank on one side of the condenser. The water in the storage tank is continuously sent by the water pump to the lower part of the condenser, and after heating, it is returned from the upper part to the storage tank. After such repeated cycles, the temperature of the water in the storage tank gradually increases.

Analysis of existing defrosting methods at home and abroad:
The air source heat pump water heater operates at a low temperature to heat. The outdoor operation environment is bad. When the surface temperature of the outdoor heat exchanger (acting from the evaporator when heating) is lower than the air dew point temperature and less than 0 °C, the surface of the outdoor heat exchanger will be frosted, which will affect the heating effect and operation. The formation and influencing factors of frost and ice are more complicated, and there are many studies on the heat and mass transfer performance of frosting under normal pressure at home and abroad. At present, the domestic defrosting methods of heat pump systems mainly have the following methods.

Natural defrosting: When the system needs defrosting, the air source heat pump water heater stops running for a certain period of time, using the heat in the surrounding environment to restore the evaporation temperature to above O °C, and the frost layer on the surface of the heat exchanger is melted. This is the easiest way. After defrost is finished, restart the refrigeration system to work. The biggest advantage of this defrost method is that the method is simple and does not consume additional energy. However, this defrosting method has certain defects. The defrosting time is longer, and heat loss is more. This causes the heat pump heating efficiency to decrease. The evaporators of air source heat pump water heaters are mostly placed in the open air. In areas with low temperatures in the north, most of the winter temperatures can reach below °C, and defrosting cannot be performed in this way.

Water flushing: Water flushing is to spray water on the cooling coil and rib surface to heat the pipeline. The frost can be removed in a short period of time even for large capacity coils. However, this type of defrost is a waste of water resources. When the temperature in the northern part is low and when the temperature of the water is too high, the hot steam generated during the watering will condense or freeze on the surface of the cold unit, increasing the heat transfer resistance of the surface of the cold unit. Moreover, if the residual water is not removed in time after the water is sprayed, the speed of frost formation will be accelerated in the next defrosting process.

Hot air defrosting: Hot air defrosting is to open the compressor exhaust into the evaporator, and use the heat of the exhaust to make the outer wall of the frost melt and fall off. In some cases, the heat of the hot part of the hot gas is used. More often, the hot gas is condensed in the evaporator, and its sensible heat and latent heat are used for defrosting. The advantage of hot gas defrosting is that the frost layer melts from the inside out.

Electric heating defrosting: electric heating defrosting is to provide defrosting heat by electric heating, and is mostly used on finned tube type chillers. The electric heating element is attached to the fins. When defrosting, the compressor and the fan of the chiller stop running, the solenoid valve is closed, and the electric heater starts to supply power to heat the defrosting. After the defrosting, the compressor starts running, the heating relay stops supplying power to the electric heater, the solenoid valve opens, and the refrigerant enters the evaporator. Electric heating defrosting has the advantages of simple system, complete defrosting and simple control, and is widely used in small devices. However, its disadvantage is that it consumes a lot of power and should not be used on large devices.

Defrost methods commonly used by domestic air source heat pump water heater manufacturers:

At present, most domestic air source heat pump water heater manufacturers mainly adopt hot gas defrosting methods, specifically reverse cycle defrosting and hot gas bypass defrosting.

Reverse Cycle Defrost: One is to install a temperature sensor on the outdoor evaporator coil to determine whether frost is formed by detecting the temperature of the outdoor coil. The other is to determine whether the outdoor evaporator is frosted by detecting the difference between the condenser coil temperature and the room temperature (or water temperature). That is, when the outdoor evaporator is frosted, the heat exchange efficiency is lowered, and the heat exchange amount of the condenser is lowered, and the coil temperature is lowered. When it is detected that the difference between the condenser coil temperature and the room temperature (or water temperature) is lower than a certain value, it can be judged that the outdoor heat exchanger is frosted more seriously.

When the defrosting starts, the reversing defrost program is started, and the four-way reversing valve acts to change the flow direction of the refrigerant, so that the unit is switched from the heating operation state to the cooling operation state. The high temperature gas discharged from the compressor is switched to the outdoor heat exchanger through a four-way valve for defrost. When the outdoor coil temperature rises to a certain temperature value, the defrosting is ended.

This kind of defrosting method will affect the water supply of the air source heat pump water heater. That is, during the defrosting, it is impossible to provide the user with hot water of an effective water temperature. At the same time, after defrosting, the original hot water temperature will decrease. From an energy perspective, the loss of this defrosting process is equivalent to a shutdown of twice the defrosting time. After calculation, the unit's heat supply will be reduced by about 10%. Moreover, the frequent reversal of the four-way valve will affect its reliability and life.

Hot gas bypass defrosting: it does not change the flow direction of the refrigerant, and the unit maintains the heating state during the defrosting process. The high temperature gas discharged from the compressor directly bypasses a part of it to the outdoor heat exchanger for defrost. Since the heat of the high-pressure side refrigerant is still from the heat absorbed in the evaporator, when the temperature is low and the defrost is not fast enough, there will be insufficient heat absorption, which will cause the host to enter a protective shutdown state. If a simple bypass path is adopted, the compressor liquid blow phenomenon is likely to occur. At the same time, during the defrosting process, as the displacement of the compressor is reduced, the effect of heating the hot water is affected, and the demand for the normal hot water amount cannot be satisfied.

Hot gas bypass defrosting: it does not change the flow direction of the refrigerant, and the unit maintains the heating state during the defrosting process. The high temperature gas discharged from the compressor directly bypasses a part of it to the outdoor heat exchanger for defrost. Since the heat of the high-pressure side refrigerant is still from the heat absorbed in the evaporator, when the temperature is low and the defrost is not fast enough, there will be insufficient heat absorption, which will cause the host to enter a protective shutdown state. If a simple bypass path is adopted, the compressor liquid blow phenomenon is likely to occur. At the same time, during the defrosting process, as the displacement of the compressor is reduced, the effect of heating the hot water is affected, and the demand for the normal hot water amount cannot be satisfied.

In order to improve the defrosting efficiency, some scholars have improved the defrost cycle and proposed some new defrost cycle methods.
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