Intelligent control of degree of superheat for direct-expansion solar-assisted heat pump based on electronic expansion valve opening

Abstract: The idea of the combination of heat pump and solar energy has been proposed and developed by many researchers around the world. Solar energy can be used to heat the refrigerant in the evaporator of a heat pump, by employing a solar collector as the evaporator, which is called a direct-expansion solar-assisted heat pump (DX-SAHP) system. Solar energy is intermittent and unstable, and it has significant effect on the thermal performance of a DX-SAHP system. How to develop a control strategy to match environmental parameters with operating parameters under various operating conditions is critical for the system performance. Aiming at the operation control of the system, a DX-SAHP system was designed and built in Qingdao China, which could supply domestic hot water in a whole year. The system mainly consisted of a bare solar collector/evaporator with area of 1.56 m2, a rotary-type hermetic compressor with rated power of 400 W, an electronic expansion valve (EEV), and a micro-channel aluminum flat tube condenser with single surface area of 0.435 m2 surrounding a 195 L water tank. The system was charged with 800 g of R134a. The temperature was measured with the platinum resistance thermometers (PT100, with grade A accuracy). The pressure was measured by using pressure transducers with uncertainty of 0.1%. A pyranometer with sensitivity of 8.145 μV /(W/m2) was placed to measure the solar radiation intensity. The degree of superheat at the outlet of solar collector/evaporator was regulated by the EEV with full stroke of 500 steps. The control system was based on microcontroller. The opening of the EEV was regulated actively with the controller which communicated with a microcontroller. The output from the platinum resistance thermometers, the pressure transducers and the pyranometer was collected by a data acquisition logger at 5-second interval. The data acquisition logger transmitted the experimental data to the microcontroller via RS485. Based on the degree of superheat at the outlet of the solar collector/evaporator, a control strategy for the system was developed and tested over a wide range of operating conditions. The initial opening of the EEV was given with a mathematical correlation of solar radiation intensity and ambient temperature, which ensured that the degree of superheat could reach the setting range rapidly and smoothly. The control strategy of the DX-SAHP system was described completely as follows. Firstly, the average values of solar radiation intensity and ambient temperature in two minutes before the system starts were measured. Secondly, the compressor operated with the initial opening of the EEV calculated by the equation in 15 minutes. Then the degree of superheat was measured. The opening of the EEV was regulated in real time according to the presented control rules. In addition, the regulating interval was 90 seconds. It was also noted that when the discharge temperature of the compressor reached 105℃ or water temperature reached the set value, the compressor stopped at once. Experimental results showed that the control strategy could regulate the degree of superheat in the target range of 5-10℃ within 25 minutes after system start-up effectively. In the normal running stage of the system, the degree of superheat was controlled smoothly, and the maximum overshoot was below 4℃. The presented control strategy is expected to contribute to further studies and applications of DX-SAHP systems in the future. Compared with the traditional degree of superheat control method in the refrigeration system, the proposed method is convenient, stable and reliable.