**1. Introduction**

The development of efficient and ecofriendly engines which emit less waste heat is the focus of recent research to overcome the environmental issues of global warming and CO<sup>2</sup> emission [1]. Therefore, in the last decade, the research trend is shifting towards the development of efficient electric vehicles [2]. The challenge associated with full commercialization of electric vehicles is their lower driving range due to high power consumption by the thermal management system. The gasoline engines are using waste heat for cabin heating during cold climatic conditions but in the case of electric

vehicles, cabin heating energy is consumed from the battery, which reduces the driving range of the vehicles more in cold climatic conditions [3]. Currently, gasoline vehicles and electric vehicles are using positive temperature coefficient (PTC) heaters widely for heating purposes despite its higher cost above 2 kW and higher power consumption. The driving range of electric vehicles with PTC heaters has reduced up to 24% more than that of electric vehicles without PTC heaters [4]. Heat pumps are the best and efficient alternative for the PTC heater because the second law of thermodynamics states that the coefficient of performance of the heat pump is above 1.0. However, the heating performance of a heat pump system decreases drastically in cold climatic conditions for electric vehicles [5]. To overcome this limitation and develop an efficient heat pump system for cold regions, several studies of an improved model of heat pump system for electric vehicles are presented.

Zhang et al. have proposed an R134a-based economized vapor injection heat pump system for electric vehicles to eliminate the key issues of drainage associated with external heat exchangers and the decrease in heating performance during cold weather conditions. The proposed heat pump system showed a 57.7% improvement in heating capacity, 2097 W of maximum capacity and coefficient of performance of 1.25 under the ambient temperature of −20 ◦C [5]. Qin et al. have developed refrigerant injection air-source heat pump for electric vehicle to overcome the poor heating performance, high battery power consumption and operational safety of traditional air-source heat pump systems in cold climate conditions. The electric scroll compressor of the refrigerant injection air-source heat pump was provided with injection portholes and the effect of the size and shape of these injection portholes was analyzed with respect to the system performance. The larger injection portholes showed enhancement in heating capacity and the refrigerant injection air-source heat pump presented 28.6% higher heating performance compared with the traditional air-source heat pump [6,7]. To develop the efficient heat pump system with less battery power consumption and extended driving range for electric vehicles in cold weather conditions, Qin et al. have proposed air-source heat pumps with refrigerant injection. Under the ambient temperature of −20 ◦C, the heating performance of the heat pump was tested for various in-car inlet temperatures and different fresh-air ratios. In addition, the effects of refrigerant injection and dryness were also tested on the heating performance of the heat pump. The proposed heat pump showed 31% improved heating capacity compared with the traditional heat pump above a −10 ◦C in-car inlet temperature [8]. Qin et al. have proved that the air-source heat pump is a potential candidate to heat up the cabin of electric vehicle in cold weather conditions. The experimental study on the proposed heat pump showed coefficient of performance above 1.7 at the ambient temperature of −20 ◦C [9]. Zhou et al. have developed a heat pump system with defrost technology for the thermal management of electric vehicles under low temperature and high humidity ambient conditions. With the proposed heat pump system with defrost technology, the defrosting time of external heat exchanger could be controlled within 100 s at the ambient conditions of temperature of −20 ◦C and relative humidity of 80% [10]. Ahn et al. have investigated the heating performance of a dual source heat pump which used a combination of air and waste heat as the heat sources in electrical vehicles. At lower ambient temperatures, the proposed dual heat source heat pump showed better heating performance compared with a single heat source heat pump which used air or waste heat as heat source [11]. Jung et al. have developed a simulation model to analyze the coefficient of performance and isentropic efficiency of an R134a heat pump with vapor injection. At an ambient temperature of −10 ◦C, the heat pump with single injection port and dual injection port has shown the coefficient of performance higher by 7.5% and 9.8%, respectively, and the isentropic efficiency higher by 11.2% and 22.9%, respectively, compared with the conventional heat pump [12]. Zhang et al. have proposed a desiccant integrated heat pump system to reduce the heat demand inside the cabin of electric vehicles. At an ambient temperature of −20 ◦C, the proposed heat pump system has shown reductions of 42% and 38% in cabin heat load and compressor electric power, respectively, compared with the traditional heat pump system [13]. Choi et al. have designed heat pump system with vapor injection to evaluate the maximum heating capacity and coefficient of performance at lower ambient temperatures under the influence of various injection positions and different intermediate

pressures [14]. Lee et al. have experimentally analyzed the heating and cooling performances of hybrid heat pump which uses waste heat of electric devices for heating and air source for cooling of electric bus. The proposed hybrid heat pump shows a cooling capacity of 23 kW and heating coefficient of performance of 2.4 [15]. Kwon et al. have presented experimental and numerical studies on the heating performance of a heat pump system with vapor injection for electric vehicles in cold weather conditions. Compared with the Joule heating system for electric vehicles, vapor injection heat pump systems enabled the extended driving range with less battery power consumption and improved heating performance in cold weather conditions [16]. Liu et al. have proposed a propane-based heat pump system to improve the heating performance for electric vehicles in cold ambient conditions. The influences of compressor speed, outside ambient temperature, inside circulated air percentage, inside air volume flow rate and outside air velocity were experimentally investigated with respect to the performance of propane-based heat pump system. The propane-based heat pump system shows effective performance above the ambient temperature of −10 ◦C [17]. Li et al. have developed an R1234yf-based heat pump system and compared its performance with an R134a-based heat pump system for an electric vehicle under cold ambient conditions. The comparison of both heat pump systems was conducted for outside temperature, outside air velocity, inside temperature, inside air mass flow rate, compressor speed, charge, inner condenser width and economized vapor injection [18]. Ahn et al. have analyzed the heating performance and coefficient of performance of dehumidifying heat pump integrated with additional heat source for electric vehicles with less occupancy [19]. Bellocchi et al. have proposed a heat pump integrated with regenerative heat exchanger for electric vehicle HVAC which decreases the power consumption by 17–52% and driving range reduction up to 6% [20]. Lee et al. have experimentally investigated air-source heat pump system for electric vehicles to evaluate the steady state performances of heating capacity and coefficient of performance and transient temperature for cabin heating performance. The proposed air-source heat pump system presents heating capacity of 3.10 kW and coefficient of performance of 3.26 at the ambient temperature of −10 ◦C [21]. Li et al. have developed a hybrid model of an air-cycle heat pump system with turbocharger, blower and regenerated heat exchanger for electric vehicles. The performance of air-cycle heat pumps was numerically examined for three different positions of blower and results show that the blower installed before the compressor achieves higher coefficient of performance and heating capacity. Under the same operating conditions of electric vehicles, the air-cycle heat pump system with turbocharger power 23% compared with the positive temperature coefficient (PTC) system [22]. Cho et al. have analyzed the heating performance of an R-134a based heat pump system which uses waste heat of electrical devices for an electric bus. The behaviors of compressor work, heating capacity and coefficient of performance are investigated for the outdoor temperature and volume flow rate [23]. Lee et al. have developed R744 based electric air conditioning system for fuel cell electric vehicles which showed superior performance characteristics compared with the conventional R-134a-based air conditioning system. The developed system presented cooling capacity of 6.4 kW and coefficient of performance of 2.5 [24]. Lee et al. have proposed R744-based heat pump system with stack coolant heat source to reduce the power consumption and improve the driving range of fuel cell electric vehicles in cold climatic conditions. The proposed system showed heating capacity of 5 kW for coolant flow rate of 5 L/min and ambient temperature of −20 ◦C [25]. Shi et al. have presented the experimental study to analyze the performance of economized vapor injection heat pump system with R32 refrigerant. The effect of injection pressure on heat capacity and power consumption of system was analyzed. The economized vapor injection heat pump system with R32 refrigerant shows better performance than that with R410A refrigerant [26].

From the conducted literature review, the issues associated with the efficient performance of heat pump system of electric vehicle need to be addressed in future for full commercialization of electric vehicles. To improve the performance of heat pump systems of light-duty commercial vehicles, an additional high-pressure side chiller is installed at the discharge end of the electric compressor. The main objective of the present work is to experimentally investigate the cooling and heating

performance characteristics of the heat pump system with a high-pressure side chiller for light-duty commercial electric vehicle under the real road driving conditions.
