**1. Introduction**

In recent times, increasing demand of high energy efficiency and zero emission has resulted into shifting the major means of transportation towards the electric vehicle [1]. The low driving range and battery life are two major hurdles in the development of electric vehicles. The cabin is heated in winter using electric energy of the battery, which results into reduction in driving range of vehicle [2]. The charge-discharge performances of the batteries are degrading significantly as the temperature reduces. In cold weather conditions, to maintain the battery performance and battery life, effective battery thermal management in form of preheating of battery is essential [3]. Numerous studies have been conducted focusing on cabin heating and battery heating in cold weather conditions to improve the driving range and battery performance.

Guo et al. presented effective heating for battery cell and battery pack using echelon internal heating strategy [4]. Ruan et al. developed an optimal internal heating strategy

**Citation:** Lim, T.-K.; Garud, K.S.; Seo, J.-H.; Lee, M.-Y.; Lee, D.-Y. Experimental Study on Heating Performances of Integrated Battery and HVAC System with Serial and Parallel Circuits for Electric Vehicle. *Symmetry* **2021**, *13*, 93. https:// doi.org/10.3390/sym13010093

Received: 4 December 2020 Accepted: 4 January 2021 Published: 7 January 2021

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for rapid battery heating [5]. Lei et al. suggested an intermittent self-heating lithiumion battery method for heating of battery with temperature uniformity [6]. Shang et al. proposed buck-boost conversion based high frequency alternating current heater for battery heating in low temperature conditions. As the AC heating frequency increases, the heating speed and efficiency improve, due to lithium-ion transport and increase in heat generation of ohmic resistance [7]. Fan et al. concluded that the discharge rate has negligible effect on the heating performance of battery thermal management system compare with external heating source. In addition, a higher mass flow rate of the heating medium gives better heating performance [8]. Delos Reyes et al. investigated the behavior in driving ranges of Mitsubishi i-MiEV and Nissan Leaf for the ambient temperature variation in a range of 20 to −15 ◦C [9].

Positive temperature coefficient (PTC) heaters are widely used for cabin heating, however, they consume more energy. Therefore, heat pumps have been used as the replacement of PTC heater for cabin heating [10]. Zhang et al. showed coefficient of performance (COP) of 1.25 and improvement of 57.7% in heating capacity using economized vapor injection heat pump system [11]. Cho et al. proposed a coolant source heat pump which uses waste heat from electric devices for heating a passengers' compartment of electric bus [12]. Qin et al. presented that the air source heat pump with refrigerant injection shows enhancement in the heating capacity compared with the conventional air source heat pump for electric vehicles [13–15]. Ahn et al. investigated heating performances of air source heat pump, waste heat pump and dual source (air + waste heat) heat pump for electric vehicle, and concluded that the dual source heat pump shows superior heating performances compared with air source and waste heat pumps [16]. Lee et al. proposed an R744 based stack coolant heat pump, which attains a heating capacity of 5.0 kW at an ambient temperature of −20 ◦C [17]. Shi et al. suggested R32 based economized vapor injection heat pump system for temperature range of −2 to 15 ◦C, and showed higher coefficient of performance compare with conventional single stage heat pump [18]. Jung et al. showed that the single injection heat pump with optimum port angle of 440◦ and dual injection heat pump with optimum port angles of 535◦/355◦ present enhancement of 7.5% and 9.8%, respectively, in coefficient of performance, compared with non-injection heat pump at ambient temperature of −10 ◦C [19]. Patil et al. proposed a 2.0 kW burner that shows a maximum efficiency of 96.7% for the cabin heating of an electric vehicle [20]. Zhang et al. showed that the heat pump system with desiccant reduces cabin heat load and power consumption by 42% and 38%, respectively, at an ambient temperature of −20 ◦C, compared to traditional heat pump system [21]. Choi et al. investigated the heating performances of vapor injection heat pump system for cabin heating of electric vehicle in cold weather conditions [22,23]. Ahn et al. showed that the dual evaporator heat pump has 62% higher heating coefficient of performance for cabin in electric vehicle compare with conventional heat pump [24]. Lee et al. proposed a mobile heat pump which uses waste heat of electric devices for heating in an electric bus. The heating coefficient of performance of proposed heat pump is evaluated as 2.4 [25]. Liu et al. investigated heating performances of propane-based heat pump system for cabin heating in electric vehicles and found that the proposed system shows superior heating performance above the ambient temperature of −10 ◦C [26]. Li et al. compared heating performances of an R134a based heat pump and an R1234yf based heat pump, and showed that the R1234yf based heat pump is a potential candidate for the replacement of the R134a based heat pump for cabin heating in cold weather conditions [27]. Bellocchi et al. developed a heat pump with a regenerative heat exchanger, which reduces power consumption by 17–52% and reduces the decrease in driving range up to 6% for electric vehicles [28]. Lee et al. proposed air source heat pump system with a heating coefficient of 3.26 and a heating capacity of 3.10 kW at an ambient temperature of −10 ◦C for cabin heating in electric vehicle [29]. Lee et al. have experimentally investigated the performance characteristics of heat pump system integrated with a high pressure side chiller under cold and hot weather conditions for light duty commercial electric vehicles [30]. Jeffs et al. integrated five different heat

sources with heat pump system for efficient heating of battery and cabin. An energy saving of 14.8% was achieved with a heat pump system integrated with different heat sources [31]. Further, Jeff et al. proposed an optimal strategy for tradeoff heating between battery and cabin. The optimal strategy enables 6.2% of improvement in range for no battery heating and 5.5% of improvement in cabin comfort for full battery heating [32].

From the literature review, there are very few studies that discuss combined cabin and battery heating. Therefore, the objective of the present study is to develop an integrated system with serial and parallel circuits to enable the trade-off between battery heating and heating ventilation and air conditioning system (HVAC) (cabin) heating. The battery and HVAC heating performances, namely, battery out temperature, battery temperature rise rate, battery heating capacity, HVAC (cabin) heating capacity and total heating capacity, are compared experimentally for the integrated system with serial and parallel circuits under the influence of heater power and flow rate. In addition, an ANN model is developed in the present study, to accurately predict the battery and HVAC heating performances.
