Heat pumps with water as heat exchange medium applied in greenhouse heating have not been used for vegetable seedling cultivation. In this work, a multi-connected direct expansion capillary radiation heat pump (MDCRHP) was designed for vegetable seedling cultivation, and a closed local temperature control (CLTC) method was proposed to accurately control air and substrate temperatures in seedling beds, enhance the heating effect and reduce the equipment cost. The results show that the CLTC method can achieve good heating effect and quickly raise air/substrate temperature at a speed of 1 °C/min from 8 °C to about 20 °C within 12 min. The air and substrate temperature fields in the seedling beds were evenly distributed. The temperature differences at different horizontal positions and different heights were less than 1 °C, and the relative humidity was 100%, which is conducive to vegetable grafting seedling. It can be concluded from test results that the MDCRHP had strong adaptability for greenhouse heating and was obviously superior to the electric heating wire (EWH). The output capacity of the compressor can be controlled by adjusting the operation frequency to meet the temperature requirements at different stages of seedling cultivation. Under the conditions of similar external ambient temperature and heating effect in the seedling beds, the energy saving rate of the MDCRHP was 47–50.7% compared with the EWH. The cost of the MDCRHP is about 7.2% lower than that of the conventional heat pump. It takes 3–3.5 years to recover the equipment investment.

The performance improvement of heat pump depends on the appropriate injection style such as liquid injection, vapour injection and two-phase injection. In this work, experimental and theoretical investigation on the heat pump with R410A refrigerant reveals three kinds of injection styles can be selectively achieved in the same injection device by suitable control of the sub-cooling electronic expansion valve. Liquid injection and vapour injection improve the heating capacity (Qh), mainly due to the increase of heat exchange in the sub-cooler and compressor compression work, respectively. Two-phase injection lifts the Qh owing to these two effects. Subsequently, the effect of two-phase injection on improving the heat pump performance is superior to the other two injection styles. For instance, the maximum Qh were 14.4 kW at 60 rps with the liquid injection, 12.5 kW and 13.4 kW at 85 rps with vapour injection and two-phase injection, and 8.1%, 7.0% and 18.3% higher than those without injection, respectively. The corresponding coefficient of performance was improved by 10.4%, 4.5% and 10.6%, respectively. Furthermore, the model based on the pressure-enthalpy diagram has been developed to analyze the experimental results and expound the main factors affecting the heat pump performance under different ambient temperatures.