*5.1. Heating Performances of Battery and HVAC for Integrated System with Parallel Circuit*

The experimental results for battery and HVAC heating performances of the integrated system with parallel circuit namely, battery out temperature, battery temperature rise rate, battery heating capacity and HVAC heating capacity at various heater powers and flow ratios are elaborated in this section.

#### 5.1.1. Battery out Temperature and Battery Temperature Rise Rate any flow ratio and heater power is low then the experiment is stopped. For all flow ratios,

The variation of battery out temperature with time for the integrated system with parallel circuit at various heater powers and flow ratios is presented in Figure 3a. The maximum battery heating temperature is cutoff at 40 ◦C because battery functions effectively below this temperature [8]. Therefore, the stopping criteria for the experiment at each flow rate and various heater powers is set as 40 ◦C, as shown in Figure 3a. The purpose of battery heating is to reach the maximum cut off temperature (40 ◦C) in minimum time using the proposed integrated heating system. If the experiment reaches the cut off temperature at any flow ratio and heater power, then it is stopped. However, if the experiments fail to reach cutoff temperature or the increasing gradient of temperature curve at any flow ratio and heater power is low then the experiment is stopped. For all flow ratios, as the heater power increases from 2 to 6 kW, the battery out temperature increases. This means higher heater power enables higher and faster heating of battery. A higher flow ratio indicates higher flow rate of working fluid for battery heating. Therefore, as the flow ratio increases from 2/8 to 8/2, the battery out temperature enhances towards the higher heating temperature. In the case of heater power of 2 kW, the battery heating temperature reaches to only 35 ◦C after 3000 s. The increasing gradient for each temperature curve increases slowly with time, hence, experiments are stopped on or before 3000 s, because, from the trends of temperature curve, it is not expected that they could reach to cutoff temperature. Whereas the battery heating temperature reaches to 40 ◦C within 2000 s for heater power of 4 kW and within 1000 s for heater power of 6 kW. as the heater power increases from 2 to 6 kW, the battery out temperature increases. This means higher heater power enables higher and faster heating of battery. A higher flow ratio indicates higher flow rate of working fluid for battery heating. Therefore, as the flow ratio increases from 2/8 to 8/2, the battery out temperature enhances towards the higher heating temperature. In the case of heater power of 2 kW, the battery heating temperature reaches to only 35 °C after 3000 s. The increasing gradient for each temperature curve increases slowly with time, hence, experiments are stopped on or before 3000 s, because, from the trends of temperature curve, it is not expected that they could reach to cutoff temperature. Whereas the battery heating temperature reaches to 40 °C within 2000 s for heater power of 4 kW and within 1000 s for heater power of 6 kW. The variation of the battery temperature rise rate with flow ratio for the integrated system with parallel circuit at various heater powers is shown in Figure 3b. Due to an increase in the battery out temperature with increase in the heater power, the battery temperature rise rate enhances with heater power. In addition, the higher flow rate of working fluid for battery heating increases the battery temperature rise rate as the flow ratio increases. As the flow ratio increases from 2/8 to 8/2, the battery temperature rise rate increases by 48.20% for heater power of 2 kW, 38.90% for heater power of 4 kW and 53.90% for heater power of 6 kW. The maximum battery temperature rise rate for heater powers of 2 kW, 4 kW and 6 kW are observed 0.39 °C/min, 0.66 °C/min and 1.17 °C/min, respectively, at the flow ratio of 8/2.

(**a**) Battery out temperature

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(**b**) Battery temperature rise rate

**Figure 3.** Variation of (**a**) battery out temperature and (**b**) battery temperature rise rate of parallel circuit for various heater power and flow ratios. **Figure 3.** Variation of (**a**) battery out temperature and (**b**) battery temperature rise rate of parallel circuit for various heater power and flow ratios.

> 5.1.2. Battery and HVAC Heating Capacities The behavior of battery heating capacity with flow ratio for the integrated system with a parallel circuit at various heater powers is shown in Figure 4a. The battery heating capacity increases with increase in flow ratio as well as heater power, due to an increase in the battery out temperature and the battery temperature rise rate with heater power and flow ratio. The battery heating capacity curves are less steep with the flow ratio for the lower heater powers of 2 kW and 4 kW, compared with the higher heater power of 6 kW. The battery heating capacities increase from 124.14 to 183.99 W, 224.62 to 311.65 W and 456.36 to 702.43 W for heater powers of 2 kW, 4 kW and 6 kW, respectively, when the flow ratio increases from 2/8 to 8/2. The maximum battery heating capacities for heater The variation of the battery temperature rise rate with flow ratio for the integrated system with parallel circuit at various heater powers is shown in Figure 3b. Due to an increase in the battery out temperature with increase in the heater power, the battery temperature rise rate enhances with heater power. In addition, the higher flow rate of working fluid for battery heating increases the battery temperature rise rate as the flow ratio increases. As the flow ratio increases from 2/8 to 8/2, the battery temperature rise rate increases by 48.20% for heater power of 2 kW, 38.90% for heater power of 4 kW and 53.90% for heater power of 6 kW. The maximum battery temperature rise rate for heater powers of 2 kW, 4 kW and 6 kW are observed 0.39 ◦C/min, 0.66 ◦C/min and 1.17 ◦C/min, respectively, at the flow ratio of 8/2.

#### powers of 4 kW and 6 kW are higher by 69.30% and 281.70%, respectively, than that for heater power of 2 kW. 5.1.2. Battery and HVAC Heating Capacities

The effect of flow ratio and heater power on HVAC heating capacity of the integrated system with parallel circuit is presented in Figure 4b. The lower flow ratio indicates higher flow rate of working fluid through HVAC and vice versa. Therefore, the HVAC heating capacity is higher at the lower flow ratio and lower at the higher flow ratio. This means HVAC heating capacity decreases as the flow ratio increases for all heater powers. The HVAC heating capacities decrease by 26.50%, 39.40% and 50.00% for heater powers of 2 kW, 4 kW and 6 kW, respectively, as the flow ratio increases from 2/8 to 8/2. The decreasing curve of HVAC heating capacity are steeper at higher heater power compared with lower heater power. The maximum HVAC heating capacities for heater powers of 2 kW, The behavior of battery heating capacity with flow ratio for the integrated system with a parallel circuit at various heater powers is shown in Figure 4a. The battery heating capacity increases with increase in flow ratio as well as heater power, due to an increase in the battery out temperature and the battery temperature rise rate with heater power and flow ratio. The battery heating capacity curves are less steep with the flow ratio for the lower heater powers of 2 kW and 4 kW, compared with the higher heater power of 6 kW. The battery heating capacities increase from 124.14 to 183.99 W, 224.62 to 311.65 W and 456.36 to 702.43 W for heater powers of 2 kW, 4 kW and 6 kW, respectively, when the flow ratio increases from 2/8 to 8/2. The maximum battery heating capacities for heater

4 kW and 6 kW are observed at flow ratio of 2/8 which are 1315.50 W, 1915.22 W and

powers of 4 kW and 6 kW are higher by 69.30% and 281.70%, respectively, than that for heater power of 2 kW.

The effect of flow ratio and heater power on HVAC heating capacity of the integrated system with parallel circuit is presented in Figure 4b. The lower flow ratio indicates higher flow rate of working fluid through HVAC and vice versa. Therefore, the HVAC heating capacity is higher at the lower flow ratio and lower at the higher flow ratio. This means HVAC heating capacity decreases as the flow ratio increases for all heater powers. The HVAC heating capacities decrease by 26.50%, 39.40% and 50.00% for heater powers of 2 kW, 4 kW and 6 kW, respectively, as the flow ratio increases from 2/8 to 8/2. The decreasing curve of HVAC heating capacity are steeper at higher heater power compared with lower heater power. The maximum HVAC heating capacities for heater powers of 2 kW, 4 kW and 6 kW are observed at flow ratio of 2/8 which are 1315.50 W, 1915.22 W and 3482.23 W, respectively. Min et al. have proposed fuzzy logic based electric vehicle thermal management system to maintain the desire temperatures of battery and cabin [44]. Seo et al. have investigated heat transfer characteristics of an integrated heating system for thermal management of cabin and battery of electric vehicle [45]. In the case of the integrated system with a parallel circuit, the sum of heating capacities for battery and HVAC are not equivalent to the supplied input for all heater powers of 2 kW, 4 kW and 6 kW. The heat losses occurred in experimental components, valves, pipes, pipe fittings/connections and ambient from the heated working fluid. *Symmetry* **2021**, *13*, x FOR PEER REVIEW 10 of 25 3482.23 W, respectively. Min et al. have proposed fuzzy logic based electric vehicle thermal management system to maintain the desire temperatures of battery and cabin [44]. Seo et al. have investigated heat transfer characteristics of an integrated heating system for thermal management of cabin and battery of electric vehicle [45]. In the case of the integrated system with a parallel circuit, the sum of heating capacities for battery and HVAC are not equivalent to the supplied input for all heater powers of 2 kW, 4 kW and 6 kW. The heat losses occurred in experimental components, valves, pipes, pipe fittings/connections and ambient from the heated working fluid.

(**a**) Battery heating capacity

**Figure 4.** *Cont*.

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(**b**) Heating ventilation and air conditioning system (HVAC) heating capacity

**Figure 4.** Variation in (**a**) battery heating capacity and (**b**) HVAC heating capacity of parallel circuit with flow ratios for various heater powers. **Figure 4.** Variation in (**a**) battery heating capacity and (**b**) HVAC heating capacity of parallel circuit with flow ratios for various heater powers.

### *5.2. Heating Performances of Battery and HVAC for Integrated System with Serial Circuit 5.2. Heating Performances of Battery and HVAC for Integrated System with Serial Circuit*

The experimental results for battery and HVAC heating performances of the integrated system with serial circuit, such as battery out temperature, battery temperature rise rate, battery heating capacity and HVAC heating capacity at various heater powers are discussed in this section. The experimental results for battery and HVAC heating performances of the integrated system with serial circuit, such as battery out temperature, battery temperature rise rate, battery heating capacity and HVAC heating capacity at various heater powers are discussed in this section.

#### 5.2.1. Battery out Temperature and Battery Temperature Rise Rate 5.2.1. Battery out Temperature and Battery Temperature Rise Rate

The variation of battery out temperature with time for the integrated system with serial circuit at various heater powers is presented in Figure 5a. As the heater power increases from 2 kW to 6 kW, the battery out temperature increases. Hence, higher heater power shows faster battery heating performance compare with lower heater power. In the case of an integrated system with a serial circuit, full flow rate of working fluid is used in battery heating, therefore, the time needed to reach 40 °C is shorter than the integrated system with parallel circuit for all heater powers. The battery out temperature reaches to 40 °C within 400s for 6 kW heater power, which is within 600 s for 4 kW heater power and that within 1200 s for 2 kW heater power. Ruan et al. have also shown the heating temperature behavior of single cell battery over the time. The battery heating from temperature of −30 °C to 2.1 °C is achieved within 103 s [5]. The variation of battery out temperature with time for the integrated system with serial circuit at various heater powers is presented in Figure 5a. As the heater power increases from 2 kW to 6 kW, the battery out temperature increases. Hence, higher heater power shows faster battery heating performance compare with lower heater power. In the case of an integrated system with a serial circuit, full flow rate of working fluid is used in battery heating, therefore, the time needed to reach 40 ◦C is shorter than the integrated system with parallel circuit for all heater powers. The battery out temperature reaches to 40 ◦C within 400 s for 6 kW heater power, which is within 600 s for 4 kW heater power and that within 1200 s for 2 kW heater power. Ruan et al. have also shown the heating temperature behavior of single cell battery over the time. The battery heating from temperature of −30 ◦C to 2.1 ◦C is achieved within 103 s [5].

The variation of battery temperature rise rate of the integrated system with serial circuit for various heater powers is shown in Figure 5b. As a result of the increase in the battery out temperature with increase in the heater power, the battery temperature rise rate is higher at higher heater power. The battery temperature rise rates are 102.90% and The variation of battery temperature rise rate of the integrated system with serial circuit for various heater powers is shown in Figure 5b. As a result of the increase in the battery out temperature with increase in the heater power, the battery temperature rise rate is higher at higher heater power. The battery temperature rise rates are 102.90% and

204.60% higher for heater powers of 4 kW and 6 kW, respectively, compared to the battery

204.60% higher for heater powers of 4 kW and 6 kW, respectively, compared to the battery temperature rise rate at heater power of 2 kW. The battery temperature rise rate is higher for the integrated system with serial circuit compare to the integrated system with parallel circuit for all heater powers because full flow rate of working fluid is used for battery heating in the integrated system with serial circuit which significantly enhances the battery out temperature for the integrated system with serial circuit compared with the integrated system with parallel circuit. The battery temperature rise rates in the case of an integrated system with serial circuit are higher by 83.20%, 119.20% and 94.60% for heater powers of 2 kW, 4 kW and 6 kW, respectively, compared to the integrated system with parallel circuit. Guo et al. have achieved the battery cell heating from temperature of −20.30 ◦C to 10.02 ◦C within 13.70 min at an average temperature rise of 2.21 ◦C/min and battery pack heating from a temperature of −20.84 ◦C to 10 ◦C is achieved within 12.40 min at an average temperature rise of 2.47 ◦C/min [4]. circuit for all heater powers because full flow rate of working fluid is used for battery heating in the integrated system with serial circuit which significantly enhances the battery out temperature for the integrated system with serial circuit compared with the integrated system with parallel circuit. The battery temperature rise rates in the case of an integrated system with serial circuit are higher by 83.20%, 119.20% and 94.60% for heater powers of 2 kW, 4 kW and 6 kW, respectively, compared to the integrated system with parallel circuit. Guo et al. have achieved the battery cell heating from temperature of −20.30 °C to 10.02 °C within 13.70 min at an average temperature rise of 2.21 °C/min and battery pack heating from a temperature of −20.84 °C to 10 °C is achieved within 12.40 min at an average temperature rise of 2.47 °C/min [4].

temperature rise rate at heater power of 2 kW. The battery temperature rise rate is higher for the integrated system with serial circuit compare to the integrated system with parallel

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**(a)** Battery out temperature

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(**b**) Battery temperature rise rate

**Figure 5.** Variation of (**a**) Battery out temperature and (**b**) Battery temperature rise rate of serial circuit for various heater power. **Figure 5.** Variation of (**a**) Battery out temperature and (**b**) Battery temperature rise rate of serial circuit for various heater power.

#### 5.2.2. Battery and HVAC Heating Capacities The effect of heater power on battery heating capacity of the integrated system with 5.2.2. Battery and HVAC Heating Capacities

serial circuit is shown in Figure 6a. Due to higher battery heating performance at higher heater power, the battery heating capacity increases with increase in the heater power. The battery heating capacities at heater powers of 2 kW, 4 kW and 6 kW are 336.37 W, 682.75 W and 1025.16 W, respectively. As a result of higher battery heating temperature of the integrated system with serial circuit compare to integrated system with parallel circuit for all heater powers, the battery heating capacities of integrated system with serial circuit for the heater powers of 2 kW, 4 kW and 6 kW are higher by 82.80%, 119.10% and 45.90%, respectively, than those of integrated system with parallel circuit. The effect of heater power on HVAC heating capacity of the integrated system with The effect of heater power on battery heating capacity of the integrated system with serial circuit is shown in Figure 6a. Due to higher battery heating performance at higher heater power, the battery heating capacity increases with increase in the heater power. The battery heating capacities at heater powers of 2 kW, 4 kW and 6 kW are 336.37 W, 682.75 W and 1025.16 W, respectively. As a result of higher battery heating temperature of the integrated system with serial circuit compare to integrated system with parallel circuit for all heater powers, the battery heating capacities of integrated system with serial circuit for the heater powers of 2 kW, 4 kW and 6 kW are higher by 82.80%, 119.10% and 45.90%, respectively, than those of integrated system with parallel circuit.

serial circuit is shown in Figure 6b. As whole flow rate of working fluid is used for HVAC heating in the case of an integrated system with serial circuit, the HVAC heating capacity of integrated system with serial circuit is higher than that of integrated system with parallel circuit for all heater powers. The HVAC heating capacities of integrated system with serial circuit for heater powers of 2 kW, 4 kW and 6 kW are higher by 46.50%, 93.60% and 64.40%, respectively, than those of integrated system with parallel circuit. The HVAC heating capacity of integrated system with serial circuit also increases as the heater power increases. The HVAC heating capacities at heater powers of 4 kW and 6 kW are enhanced by 92.30% and 196.90%, compared to that of 2 kW. Zhang et al. have shown maximum cabin heating capacity of 2097 W using an economized vapor injection heat pump system whereas, Patil et al. have presented maximum cabin heating capacity of 2171.5 W using a 2.0 kW burner [11,20]. The effect of heater power on HVAC heating capacity of the integrated system with serial circuit is shown in Figure 6b. As whole flow rate of working fluid is used for HVAC heating in the case of an integrated system with serial circuit, the HVAC heating capacity of integrated system with serial circuit is higher than that of integrated system with parallel circuit for all heater powers. The HVAC heating capacities of integrated system with serial circuit for heater powers of 2 kW, 4 kW and 6 kW are higher by 46.50%, 93.60% and 64.40%, respectively, than those of integrated system with parallel circuit. The HVAC heating capacity of integrated system with serial circuit also increases as the heater power increases. The HVAC heating capacities at heater powers of 4 kW and 6 kW are enhanced by 92.30% and 196.90%, compared to that of 2 kW. Zhang et al. have shown maximum cabin heating capacity of 2097 W using an economized vapor injection heat pump system whereas, Patil et al. have presented maximum cabin heating capacity of 2171.5 W using a 2.0 kW burner [11,20].

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**Figure 6.** Effect of heater power on (**a**) battery heating capacity and (**b**) HVAC heating capacity of serial circuit. **Figure 6.** Effect of heater power on (**a**) battery heating capacity and (**b**) HVAC heating capacity of serial circuit.
