| An electrically driven air-source heat pump, which uses air as heat source and has low environmental pollution emission, is an ideal and clean heating alternative. Especially in spring and autumn season, and countryside or suburb of the city where centralized heating is incomplete, it is operated effciently and conveniently. However, a conventional air source heat pump may have low effciency and great reduction in heating capacity, and even, it cannot run steadily for long periods where the ambient temperatures is lower than -5℃. Of the many solutions, injecting refrigerant vapor in the the working chambers of compressor to compose a new heat pump system, (named Economized Vapor Injection Heat Pump System,abbreviated as EVI system) , has been recognized as an approach with good prospects both in technology and commerce. Nevertheless, there is some energy loss in the throttling element in the supplementary circuit in conventional EVI heat pump system, in order to recover the available energy, the EVI heat pump system coupled with ejector (named Economized Vapor Injection Heat Pump System Coupled with Ejector, abbreviated as EVIe system) was proposed. The main work of this paper is as follows:The attenuation characteristic of heating performance of conventional single-stage compression heat pump has been analysed. As the decrease mechanism of heat pump efficiency being studied, many factors, such as compression process, vapor suction superheating, discharge temperature and throttling process were taken into account, and then, the quasi-ideal heat pump cycle was presented. The quasi-ideal heat pump cycle has the highest heating performance and it is based on special working fluid. EVI heat pump can radically improve the compression process, and makes it approach to the quasi-ideal heat pump cycle much more. As ejector has advantages such as simple construction, low cost, no moving parts, safe for two-phase working fluid and so on, it can be used to replace throttling valve to recover the available energy in the supplementary circuit and the EVIe system was constructed. And the working of heat pump will be made steady and the heating capacity will be more improved.The thermodynamic model to analyze the EVIe system has been set up. Based on it, the working rule of heating performance improvement of EVIe has been analysed. Ejector replaces throttling valve to recover the available energy in the supplementary circuit, and the indicated efficiency was improved further. The EVIe system has lower discharge enthalpy and lower discharge temperature of the vapor from the compressor, and makes it run more steadily. the heating EER of the EVIe system can be about 4% higher than that of the system without ejector where the heating capacity keeps nearly constant and the power input was depressed slightly.Based on the working characteristics of ejector, how to decide the working state point was deeply investigated, and the method has been concluded. From the study of its working under the evaporating temperature of -20℃and -15℃, we find out that: the entrainment ratio has an appropriate value range, if it is too large, and the mixing pressure would be small and the vapor injection mass will minish; if it was too small, the mixing pressure would be big, under the situation, more vapor may be supply into the compressor, but the advantage of EVIe should decrease comparing with EVI system. The evaporating temperature should be higher than the design evaporating temperature. If the evaporation temperature becames lower than the designed one, the ejector will working in sub-critical condition, and the entrainment ratio decreases fiercely and even be malfunction. If the evaporating temperature higher than the designed one, the ejector owns steady entrainment ratio, but it doesn't own the best efficiency. The effect of evaporating temperature on vapor injection pressure and mass are different. The effect on vapor supplement pressure is faintness and increases as the vapor injection mass increases. The inspiration given us is: as designing the EVIe system, we must decide the evaporating temperature first and not the lower the better. When it is in the reasonable range, not only the ejector can work but the efficiency will be high.The design method of scroll compressor EVIe system was presented and the prototype ejector was established with the evaporation temperature of -20℃. The results show that the simulation can be adaptive depending on the practice working condition. The EVIe system was exergetically analyzed using experimental data and it is also compared with single-stage system and EVI system. EVI can improve the compression process and it owns higher exergy output. The ejector can effectively improve the performance of the heat pump, and the exergy efficiency could increase 3% to 5% by replacing the throttling valve in the supplementary circuit with an ejector, while the rate of exergy output would not decrease. The exergy loss in the compressor amounts to about 77% of the total of the heat pump.The effect of ejector structure on the system performance has been analysed. The favorable range of dimension of nozzle throat section area, the ratio of mixing chamber column segment section area to nozzle throat section area, and the ratio of inlet to outlet of mixing chamber taper segment are ascertained. When the hydraulic diameter of nozzle throat section area is 3.03×10-3～3.061×10-3, the ratio of mixing chamber column segment section area to nozzle throat section area is 2.67～2.89, the EVIe system will own best performance with the evaporating temperature from -15℃to -25℃and the discharge temperature is lower than 130℃. The appropriate value of the ratio of inlet to outlet of mixing chamber taper segmentβis between 1.5 and 2.
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