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 m2surrounding 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.%针对直膨式太阳能热泵系统的运行控制问题,提出了一种基于电子膨胀阀开度的过热度控制策略,主要包括电子膨胀阀初始开度算法和过热度控制算法,通过对试验数据进行拟合,得到了电子膨胀阀初始开度与太阳辐射强度和环境温度的函数关系.设计并搭建了以制冷剂R134a为工质的直膨式太阳能热泵热水器试验平台,主要由裸板式太阳能集热/蒸发器、滚动转子式压缩机、微通道冷凝器、蓄热水箱和电子膨胀阀等组成.控制系统采用单片机作为主控制器,单片机与数据采集控制器之间采用RS485总线方式通讯.在此平台上,对提出的过热度控制策略进行了全工况测试.试验结果表明:在系统开机后的25 min内,过热度有效控制在目标范围5～10 ℃内;在系统正常运行阶段,过热度控制平稳,最大超调量小于4 ℃.所提出的全工况过热度控制策略有助于系统稳定高效运行.
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