## 1 Introduction  
Due to air pollutant emissions such as particles, nitrogen oxides and sulfur oxides, coal and gas boiler heating systems were recognized as main causes of atmospheric pollution in North China which led to adverse impacts on human health [1; 2; 3]. Air source heat pump (ASHP) is becoming a promising device for space heating in North China due to its energy-savings, environmental friendliness, and application flexibility [4; 5]. ASHP has been carried out as a main method in the \"coal to electricity\" clean heating reform work and appointed to be a promoted substitution for existing small coal boilers in rural residential buildings of North China [5]. However, the utilization of ASHP for space heating still faces challenges such as heating capacity, frosting and low coefficient of performance (COP). The ozone depletion potential (ODP) and global warming potential (GWP) of refrigerants in ASHP constitute other challenges for environment. To evaluate ASHP for space heating, studies compared its initial investment, running cost and environmental impacts with conventional space heating systems [5; 6; 7; 8; 9; 10]. [PERSON] et al. [5] showed that low temperature ASHP heating mode has lower primary energy consumption and lower dioxide pollution emissions compared with coal boiler, gas boiler and direct electric heating mode. [PERSON] et al. [6] indicated that heat pump heating system would have lower cost and less environment impact than other systems if its COP is above 3. [PERSON] and Azapagic [7] showed that up to 36% of CO\({}_{2}\) emission can be saved with water source heat pump and 6% with ASHP in comparison with boiler in the UK. [PERSON] et al. [8] reported the application of ASHP for heating in Harbin which showed the temperature difference between indoor and outdoor air should be controlled within 41 \({}^{\circ}\)C to achieve an acceptable COP. [PERSON] et al. [9] evaluated solar thermal and ASHP combisystems which indicated that variations in electricity price affect the additional investment far more than other economic parameters and the potential for achieving cost benefit vary a lot depending on load and climate boundary conditions. [PERSON] and [PERSON] [10] indicated that heat pumps can realize emissions reductions when installed at high penetration levels combined with a grid decarbonization strategy. To promote ASHP for space heating, efforts have been taken on new technologies of ASHP including two-stage compression, quasi-two-stage compression, new throttling devices, thermal storage, and defrosting technologies. [PERSON] et al. [11] studied two-stage compression ASHP and found the seasonal COP with cylinder volume ratio of 2 is 10.3% and 17.6% higher than that of 3 when outdoor design temperature is \(-4\)\({}^{\circ}\)C and \(-8\)\({}^{\circ}\)C. [PERSON] and [PERSON] [12] analyzed a two-stage compression ASHP and found the \(COP_{h}\) of the heat pump can be maximized by optimally allocating thermal conductance inventory of the heat exchangers. [PERSON] and [PERSON] [13] did exergy analysis for quasi-two-stage compression heat pump coupled with ejector and indicated that the ejector could decrease exergy loss of compressor and improve the system exergy efficiency. [PERSON] et al. [14] compared the performance of ASHP water heater using different expansion devices and showed that short tube orifice is more suitable for heat pump water heater than capillary tube. [PERSON] et al. [15] found that sensible heat storage system can achieve improved indoor thermal comfort and lower energy consumption compared with normal VRV (Variable Refrigerant Volume) systems. [PERSON] et al. [16] proposed a novel defrosting method based on superheat degree control which demonstrated reasonable strategy that initiated defrosting before the performances of ASHP deteriorated rapidly. [PERSON] and [PERSON] [17] tested a low-temperature ASHP operating in a thermal buffer zone created by an enclosed balcony space which could improve the COP of ASHP in cold climates. [PERSON] et al.