The purpose of defrosting is to improve the performance of the unit and ensure the normal operation of the unit. At present, the research on air source heat pump defrosting technology mainly focuses on two aspects: accelerating the defrosting process and inhibiting the frosting technology. During the defrosting process, the operating state of the unit has not yet reached the defrosting requirement. At this time, the unit has entered the defrosting mode due to the automatic setting, or the thickness of the frost layer on the surface of the unit heat exchanger has reached the defrosting requirement. Without defrosting, this unbalanced supply and demand condition resulted in false defrosting of the unit, increased the burden on the unit, and greatly increased the power consumption of the system. Scholars have proposed various defrosting control methods for inverter air source heat pumps. Direct measurement is a relatively accurate control method, but it is restricted by the actual operating environment and engineering cost. Rarely used, indirect measurement is the most commonly used defrost control method at present, and there are the following three types:
(1) Time-temperature method
The most commonly used control method at present, the time-temperature defrosting control method is to set a temperature sensor on the surface of the heat exchanger. When the temperature of the surface of the outdoor evaporator heat exchange coil or fin surface is lower than a certain set value, the Record the running time of the compressor. When the time reaches the set value (usually 45min), the air source heat pump system enters the defrosting mode. The defrost mode program design logic is simple, it is a relatively simple, economical, and widely used defrost control method. However, in the time-temperature method, since the time setting must be a fixed value, in the actual operation process, this method cannot accurately grasp when the unit will defrost, and the phenomenon of “misdefrosting” of the unit occurs frequently, which seriously affects the unit’s performance. performance and efficiency.
(2) Fuzzy intelligent defrosting
Fuzzy intelligent defrosting is a modular intelligent defrosting control method, which includes data acquisition and A/D conversion module, input quantity fuzzification module, fuzzy reasoning module, defrosting control module, and monitoring and adjustment module. Logic operations are performed sequentially, and the specific operation process is shown in the figure. Although the automation module greatly improves the effectiveness of the defrosting process, it still cannot completely solve the problem of "false defrosting".
(3) Frost thickness method
The principle of the method is simple. The frost layer thickness sensor is used to detect the frost layer thickness on the surface of the evaporator coil. When the thickness reaches a predetermined defrosting thickness, the frost starts to be defrosted. This method needs to set the defrost thickness reasonably according to the actual pipe diameter and coil position. However, how to properly set the defrosting thickness is one of the key factors for the application of this method; meanwhile, high-gain signal amplifiers and sensors are required in terms of hardware, and the price is relatively expensive.
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