# A Study on Performance Characteristics of a Heat Pump System with High-Pressure Side Chiller for Light-Duty Commercial Electric Vehicles

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## Abstract

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## 1. Introduction

_{2}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.

## 2. Experimental Method

^{3}/h, the compressor speed was varied from 4000 rpm to 6000 rpm, the coolant inlet temperature was varied from 35 °C to 55 °C and the coolant volume flow rate was varied from 10 L/min to 20 L/min to evaluate the performance of heat pump system with a high-pressure side chiller. In the heating mode, the ambient air temperature and air temperature were set to −6.7 °C, air volume was fixed to 300 m

^{3}/h, coolant flow rate was set to 10 L/min, air velocity was varied from 3 m/s to 5 m/s, compressor speed was varied from 2000 rpm to 6000 rpm and the coolant temperature was varied from −6.7 °C to 50 °C to evaluate the performance of the heat pump system with a high-pressure side chiller.

## 3. Uncertainty Analysis and Data Reduction

#### 3.1. Uncertainty Analysis

#### 3.2. Data Reduction

^{3}), ${C}_{p,coolant}$ is specific heat capacity of coolant (kJ/kg·K), ${T}_{coolant.in}$ is coolant inlet temperature (°C), ${T}_{coolant.out}$ is coolant outlet temperature (°C), and ${P}_{high}$ and ${P}_{low}$ are high pressure (MPa) and low pressure (MPa).

## 4. Results and Discussion

#### 4.1. Performance of Heat Pump System with a High-Pressure Side Chiller in Cooling Mode

#### 4.1.1. Cooling Capacity, Compressor Work and COP at Various Compressor Speeds

^{3}/h, air velocity of 3 m/s and coolant inlet conditions of 35 °C and volume flow rate of 10 L/min. As presented in Figure 2a, the cooling capacity increased from 3.8 kW to 4.7 kW with the rise in the compressor speed from 4000 rpm to 6000 rpm. The cooling capacity increased with compressor speed due to the increase in refrigerant flow rate [31]. The cooling capacity increased from 3.8 kW to 4.4 kW with an enhancement of 15.8% for the increase in the compressor speed from 4000 rpm to 5000 rpm. However, the cooling capacity increased from 4.4 kW to 4.7 kW with an enhancement of 6.82% for the increase in the compressor speed from 5000 rpm to 6000 rpm. Beyond the compressor speed of 5000 rpm, the increase in the cooling capacity reduced due to the rapid rise in the pressure ratio [31]. The rise in the pressure ratio affected the cycle capacity by increasing the degree of superheat associated with vapor refrigerant discharges from compressor and reducing the degree of subcooling of liquid refrigerant discharges from the condenser. In addition, the pressure ratio affected the compressor work. As presented in Figure 2a, the compressor work also increased with the increase in compressor speed. With the increase in the compressor speed from 4000 rpm to 5000 rpm, the compressor work increased from 1.07 kW to 1.37 kW with an enhancement of 28.0%. The compressor work increased from 1.37 kW to 1.71 kW with an enhancement of 24.8% for the increase in the compressor speed from 5000 rpm to 6000 rpm. Unlike cooling capacity, the percentage enhancement in compressor work was not reduced drastically with the rise in compressor speed. The variations of cooling capacity and compressor work with various compressor speeds affected the coefficient of performance. The coefficient of performance is the ratio of cooling capacity to compressor work. With the increase in the compressor speed, the percentage enhancement in the cooling capacity decreased drastically but the percentage enhancement in compressor work was not decreased drastically. Therefore, the coefficient of performance decreased with the increase in the compressor speed as shown in Figure 2a. With the increase in the compressor speed from 4000 rpm to 5000 rpm, the coefficient of performance decreased from 3.54 to 3.21 with a reduction of 9.32%, whereas, the coefficient of performance decreased from 3.21 to 2.75 with a reduction of 14.3% for the increase in the compressor speed from 5000 rpm to 6000 rpm. The percentage reduction in the coefficient of performance increased beyond a compressor speed of 5000 rpm due to characteristic variations of cooling capacity and compressor work with compressor speed, as presented in Figure 2a. The working of the heat pump system with a high-pressure side chiller for various compressor speeds is shown on a P-h diagram in Figure 2b. With respect to the P-h diagram for the refrigeration cycle, as compressor speed increased, low-side pressure decreased to under 2 bar. However, high-side pressure was stable due to similar coolant conditions to be supplied. The portion of heat transfer rate between a refrigerant and a coolant at the developed chiller showed about one-third of the condensing heat capacity along with compressor speed.

#### 4.1.2. Cooling Capacity, Compressor Work and COP at Various Coolant Inlet Temperatures

^{3}/h, air velocity of 3 m/s, coolant volume flow rate of 10 L/min and compressor speed of 5000 rpm. The coolant inlet temperature had very little effect on the cooling capacity. As shown in Figure 3a, with the increase in the coolant inlet temperature from 35 °C to 45 °C, the cooling capacity remained constant at 4.15 kW, whereas, with an increase in the coolant inlet temperature from 45 °C to 55 °C, the cooling capacity decreased from 4.15 kW to 4.06 kW, a reduction of 2.2%. With the increase in the coolant inlet temperature, the compressor work increased because of the increase in the high-pressure side of the compressor. The compressor work increased from 1.36 kW to 1.58 kW with an enhancement of 16.2% for the increase in the coolant inlet temperature from 35 °C to 45 °C. However, with the increase in the coolant inlet temperature from 45 °C to 55 °C, the compressor work increased from 1.58 kW to 1.67 kW, an enhancement of 5.70%. With an increase in the coolant inlet temperature, the percentage enhancement in the compressor decreased as shown in Figure 3a. The coefficient of performance varied with coolant inlet temperature due to variations in cooling capacity and compressor work with coolant inlet temperature. As shown in Figure 3a, the coefficient of performance decreased with the increase in the coolant inlet temperature because the cooling capacity was not affected much, and compressor work increased with coolant inlet temperature. Because of the characteristic variation of cooling capacity and compressor work presented in Figure 3a, the coefficient of performance decreased from 3.06 to 2.64 with a reduction of 13.7% for the increase in the coolant inlet temperature from 35 °C to 45 °C, while it decreased from 2.64 to 2.43, a reduction of 7.95%, for the increase in the coolant inlet temperature from 45 °C to 55 °C. The refrigeration cycle of the heat pump system with a high-pressure side chiller for various coolant inlet temperatures on the P-h diagram is shown in Figure 3b. High-side pressure increased with coolant inlet temperature in order to have certain temperature difference. The proportion of the heat transfer rate in the developed chiller with the increase in coolant temperature varied from one quarter to three-quarters of the total condensing heat capacity due to heat transfer potential, such as temperature difference among fluids.

#### 4.1.3. Cooling Capacity, Compressor Work and COP at Various Coolant Volume Flow Rates

^{3}/h, air velocity of 3 m/s, coolant inlet conditions of 35 °C and compressor speed of 5000 rpm. The coolant volume flow rate had no significant effect on the cooling capacity as shown in Figure 4a. With increase in the coolant volume flow rate from 10 L/min to 15 L/min, the cooling capacity reduced by 1.47% from 4.09 W to 4.03 W, whereas the cooling capacity remained constant at 4.03 W when the coolant volume flow rate increased from 15 L/min to 20 L/min. The compressor work decreased with the increase in the coolant volume flow rate due to the fall in the high-pressure side of the compressor. However, the decrease in the compressor work with coolant volume flow rate was not significant as shown in Figure 4a. The compressor work decreased from 1.33 kW to 1.3 kW with a reduction of 2.26% for the increase in coolant volume flow rate from 10 L/min to 15 L/min. With the increase in the coolant volume flow rate from 15 L/min to 20 L/min, the compressor work decreased by 1.54% from 1.3 kW to 1.28 kW. Due to variations in the cooling capacity and compressor work with coolant volume flow rate, the coefficient of performance also showed variation with coolant volume flow rate. As shown in Figure 4a, the effect of coolant volume flow rate on coefficient of performance was less because of less effect of coolant volume flow rate on cooling capacity and compressor work. Due to characteristic behaviors of cooling capacity and compressor work with coolant volume flow rate, as presented in Figure 4a, the coefficient of performance increased from 3.08 to 3.1 with an enhancement of 0.65% for the increase in the coolant volume flow rate from 10 L/min to 15 L/min, whereas, when coolant volume flow rate increased from 15 L/min to 20 L/min, the coefficient of performance increased by 1.29% from 3.1 to 3.14. Figure 4b presents the P-h diagram for the refrigeration cycle of the heat pump system with a high-pressure side chiller with various coolant volume flow rates. The working of this refrigeration cycle on the P-h diagram for various coolant volume flow rates was similar to that explained for the refrigeration cycle on the P-h diagram for various compressor speeds.

#### 4.1.4. Pressure Characteristics

^{3}/h, and air velocity of 3 m/s. At coolant inlet conditions of 35 °C and 10 L/min, the compressor speed varied from 3000 rpm to 5000 rpm and pressure ratio was analyzed. With the increase in the compressor speed from 3000 rpm to 5000 rpm, the low pressure decreased; however, the high pressure remained constant in the range of 1100 kPa because the coolant and ambient conditions remained constant. As an effect of this the pressure ratio which is defined as the ratio of high pressure to low pressure, increased with the increase in the compressor speed. Pressure ratio increased by 26.8% from 3.05 to 3.87 with the increase in the compressor speed from 3000 rpm to 5000 rpm, as shown in Figure 5a. This was the reason behind the increase in the compressor power consumption as depicted Figure 2a. In the case when the coolant temperature increased from 35 °C to 55 °C, the low pressure remained constant, whereas the high pressure increased up to 1500 kPa. Therefore, the pressure ratio increased with the increase in the coolant temperature. The pressure ratio increased by 35.0% from 3.6 to 4.87 with the increase in the coolant temperature from 35 °C to 55 °C, as shown in Figure 5b. The effect of coolant temperature on the pressure ratio caused an increase in the compressor power consumption with an increase in the coolant temperature as shown in Figure 3a. However, the pressure ratio affected little with the variation of coolant volume flow rate from 10 L/min to 20 L/min because of the same operating conditions of coolant temperature and compressor speed.

#### 4.2. Performance of Heat Pump System with Higher Pressure Side Chiller in Heating Mode

#### 4.2.1. Heat Dissipation at Various Air Velocities

#### 4.2.2. Chiller Heat Transfer Rate at Various Compressor Speeds and Coolant Temperatures

^{3}/h, coolant flow rate of 10 L/min, and maximum temperature and maximum pressure of 120 °C and 2500 kPa. The chiller heat transfer rate increased with the increase in the compressor speed from 2000 rpm to 6000 rpm for the coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C. However, the chiller heat transfer rate increased for the increase in the compressor speed from 2000 rpm to 4000 rpm and reduced with the increase in the compressor speed from 4000 rpm to 6000 rpm for the coolant temperature of 0 °C. The compressor speed range became narrow with the increase in the coolant temperature to present the variation of chiller heat transfer rate. The variation of the chiller heat transfer rate is presented over the compressor speed range of 2000 rpm to 6000 rpm for coolant temperatures of−6.7 °C and 0 °C, that of 2000 rpm to 5000 rpm for the coolant temperatures of 10 °C, 20 °C and 30 °C and that of 2000 rpm to 4000 rpm for the coolant temperatures of 40 °C and 50 °C. With the increase in the compressor speed, the percentage increase in the chiller heat transfer rate decreased for coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C. The chiller heat transfer rate increased by 11.4%, 55.6%, 40.7%, 70.6%, 54.0% and 55.0% for the coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively with the increase in the compressor speed from 2000 rpm to 3000 rpm. The chiller heat transfer rate increased by 11.7%, 20.6%, 16.2%, 17.8%, 21.9% and 22.6% for the coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively, with the increase in the compressor speed from 3000 rpm to 4000 rpm. The chiller heat transfer rate increased by 11.6%, 1.56%, 7.29% and 11.0% for the coolant temperatures of −6.7 °C, 10 °C, 20 °C and 30 °C, respectively, with the increase in the compressor speed from 4000 rpm to 5000 rpm. Finally, the chiller heat transfer rate increased by 1.83% for the coolant temperatures of −6.7 °C with an increase in the compressor speed from 4000 rpm to 5000 rpm. In the case of the coolant temperature of 0 °C, when the compressor speed increased from 2000 rpm to 3000 rpm and 3000 rpm to 4000 rpm correspondingly, the chiller heat transfer rate increased by 21.6% and 11.4%, whereas the chiller heat transfer rate decreased by 2.90% and 3.20% with the increase in the compressor speed from 4000 rpm to 5000 rpm and 5000 rpm to 6000 rpm, respectively. The maximum chiller heat transfer rates of 4.91 kW, 4.73 kW, 3.83 kW, 3.84 kW, 3.37 kW and 3.05 kW were experimentally evaluated for the coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively at the highest compressor speeds of respective ranges, whereas the maximum chiller heat transfer rate of 3.50 W was experimentally evaluated at the middle of compressor speed range for a coolant temperature of 0 °C.

#### 4.2.3. Compressor Power Consumption at Various Compressor Speeds and Coolant Temperatures

^{3}/h, coolant flow rate of 10 L/min, maximum temperature of 120 °C and maximum pressure of 2500 kPa. For each coolant temperature, compressor power consumption increased with the increase in the compressor speed. In addition, the percentage increase in the compressor power consumption decreased with the increase in the compressor speed for each coolant temperature except 40 °C. The variation range of compressor power consumption with compressor speed decreased as the coolant temperature increased from −6.7 °C to 50 °C. The variation of compressor power consumption is presented over the compressor speed range of 2000 rpm to 6000 rpm for coolant temperatures of −6.7 °C and 0 °C. For coolant temperatures of 10 °C, 20 °C and 30 °C, the variation of compressor power consumption is presented over the compressor speed range of 2000 rpm to 5000 rpm. The variation of compressor power consumption is presented over the compressor speed range of 2000 rpm to 4000 rpm for coolant temperatures of 40 °C and 50 °C. The compressor power consumption increased by 76.3%, 64.2%, 76.5%, 51.8%, 44.0%, 35.7% and 35.7% for coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively, when the compressor speed increased from 2000 rpm to 3000 rpm. The compressor power consumption increased by 33.2%, 29.7%, 35.5%, 36.5%, 26.7%, 43.3% and 22.8% for coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively, when the compressor speed increased from 3000 rpm to 4000 rpm. The compressor power consumption increased by 21.6%, 17.4%, 22.8%, 19.7% and 18.1% for coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C and 30 °C, respectively, when the compressor speed increased from 4000 rpm to 5000 rpm. The compressor power consumption increased by 11.1% and 5.30% for coolant temperatures of −6.7 °C and 0 °C, respectively when the compressor speed increased from 5000 rpm to 6000 rpm. The minimum compressor power consumption of 0.48 kW, 0.44 kW, 0.60 kW, 0.70 kW, 0.86 kW, 0.95 kW and 1.05 kW were experimentally evaluated at the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively, for a compressor speed of 2000 rpm. The maximum compressor power consumption of 1.51 kW and 1.16 kW were evaluated at the coolant temperatures of −6.7 °C and 0 °C, respectively, for compressor speed of 6000 rpm. For coolant temperatures of 10 °C, 20 °C and 30 °C, the maximum compressor power consumptions were evaluated as 1.77 kW, 1.75 kW and 1.85 kW, respectively, at the compressor speed of 5000 rpm. The maximum compressor power consumptions of 1.84 kW and 1.75 kW were evaluated at the coolant temperatures of 40 °C and 50 °C, respectively, for compressor speed of 4000 rpm.

#### 4.2.4. System Efficiency at Various Compressor Speeds and Coolant Temperatures

^{3}/h and coolant flow rate of 10 L/min. The compressor speed range became narrow for the variation of system efficiency as the coolant temperature increased. The variation of system efficiency is presented over the compressor speed range of 2000 rpm to 6000 rpm for the coolant temperatures of −6.7 °C and 0 °C, that of 2000 rpm to 5000 rpm for coolant temperatures of 10 °C, 20 °C and 30 °C and that of 2000 rpm to 4000 rpm for coolant temperatures of 40 °C and 50 °C. For coolant temperatures of −6.7 °C, 0 °C, 10 °C and 20 °C, the system efficiency decreased with the increase in the compressor speed, whereas, for the coolant temperatures of 30 °C, 40 °C and 50 °C, the system efficiency increased, reached maximum value at compressor speed of 3000 rpm and decreased with further increase in the compressor speed. The system efficiency decreased by 35.5%, 25.5%, 10.8% and 7.51% for the coolant temperatures of −6.7 °C, 0 °C, 10 °C and 20 °C, respectively, and increased by 16.0%, 14.9% and 15.3% for the coolant temperatures of 30 °C, 40 °C and 50 °C, respectively, with the increase in the compressor speed from 2000 rpm to 3000 rpm. The system efficiency decreased by 16.2%, 15.6%, 11.2%, 15.4%, 6.34%, 15.5% and 1.49% with the increase in compressor speed from 3000 rpm to 4000 rpm for the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively. The system efficiency decreased by 8.63%, 17.4%, 17.6%, 11.3% and 6.62% with the increase in compressor speed from 4000 rpm to 5000 rpm for the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C and 30 °C, respectively. The system efficiency decreased by 7.72% and 8.82% with the increase in the compressor speed from 5000 rpm to 6000 rpm for the coolant temperatures of −6.7 °C and 0 °C, respectively. The system efficiency decreased as the coolant temperature increased from −6.7 °C to 50 °C for the same compressor speed. The coolant temperature of −6.7 °C showed the maximum system efficiency for the same compressor speed. The maximum system efficiencies of 7.21%, 5.81%, 4.08% and 3.08% were experimentally evaluated at the compressor speed of 2000 rpm for the coolant temperatures of −6.7 °C, 0 °C, 10 °C and 20 °C, respectively, whereas the maximum system efficiencies of 2.37%, 2.17% and 1.76% were experimentally evaluated at the compressor speed of 3000 rpm for the coolant temperatures of 30 °C, 40 °C and 50 °C, respectively.

#### 4.2.5. Heater Core Performance at Various Compressor Speeds and Coolant Temperatures

^{3}/h, coolant flow rate of 10 L/min, maximum temperature of 120 °C and maximum pressure of 2500 kPa. As the coolant temperature increased from −6.7 °C to 50 °C, the compressor speed range of 2000 rpm to 6000 rpm decreased for the variation of heater core performance. The heater core performance was evaluated at the maximum compressor speed of 6000 rpm for coolant temperatures of −6.7 °C and 0 °C, that of 5000 rpm for coolant temperatures of 10 °C, 20 °C and 30 °C and that of 4000 rpm for coolant temperatures of 40 °C and 50 °C. With the increase in the compressor speed, the heater core performance increased for the coolant temperatures of −6.7 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, whereas, for the coolant temperature of 0 °C the heater core performance showed parabolic variation with compressor speed. With the increase in the compressor speed from 2000 rpm to 3000 rpm, the heater core performance increased by 230%, 15.4%, 12.0%, 10.6%, 6.03%, 2.38% and 2.43% for the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively. With the increase in the compressor speed from 3000 rpm to 4000 rpm, the heater core performance enhanced by 35.4%, 7.34%, 5.62%, 4.38%, 1.60%, 2.98% and 0.98% for the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C, respectively. The heater performance enhanced by 28.0%, 1.95%, 2.36%, 1.52% and 2.43% for the coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C and 30 °C, respectively, when compressor speed increased from 4000 rpm to 5000 rpm. As the compressor speed increased from 5000 rpm to 6000 rpm, the heater core performance increased by 5.25% for coolant temperature of −6.7 °C and decreased by 5.80% for the coolant temperature of 0 °C. For the same compressor speed, the heater core performance enhanced with the increase in the coolant temperature. The heater core performance showed maximum behavior at the coolant temperature of 50 °C for same compressor speed. The maximum heater core performance of 0.36 kW was evaluated at compressor speed of 6000 rpm for the coolant temperature of −6.7 °C and that of 0.77 W at compressor speed of 5000 rpm for the coolant temperature of 0 °C. For the coolant temperatures of 10 °C, 20 °C and 30 °C, the corresponding maximum heater core performances of 1.85 kW, 2.75 kW and 3.58 kW were evaluated at the compressor speed of 5000 rpm. The maximum heater core performances of 4.35 kW and 4.98 kW were evaluated for coolant temperatures of 40 °C and 50 °C, respectively, at the compressor speed of 4000 rpm.

#### 4.2.6. Performance of Heat Pump System with a High-Pressure Side Chiller in Transient State

^{3}/h, coolant temperature and flow rate of −6.7 °C and 10 L/min, and compressor speed range of 3000 rpm to 6000 rpm. The variation in coolant temperature with time for compressor speeds of 3000 rpm, 4000 rpm, 5000 rpm and 6000 rpm is presented in Figure 11. Starting with the coolant temperature of −6.7 °C, for all compressor speeds, coolant temperature increased with time and converged, as shown in Figure 11. The increase in coolant temperature followed a smooth curve for compressor speeds of 3000 rpm, 4000 rpm and 5000 rpm, but, for the compressor speed of 6000 rpm, the increasing coolant temperature curve fluctuated due to experimental or environmental error. However, the curve converged accurately at the same time with other smooth curves. The convergence temperature was higher for a compressor speed of 6000 rpm followed by convergence temperatures corresponding to compressor speeds of 5000 rpm, 4000 rpm and 3000 rpm. However, the convergence temperature was obtained almost at the same time for all compressor speeds. The highest coolant convergence temperature of 22 °C was experimentally evaluated at the compressor speed of 6000 rpm followed by 20 °C, 16 °C and 13 °C at the compressor speeds of 5000 rpm, 4000 rpm and 3000 rpm, respectively. Even when starting with the lower coolant temperature of −6.7 °C, it took a longer time of 1 h 30 min to attain the convergence coolant temperature for all compressor speeds. This meant the time invested in cabin heating was longer for all compressor speeds. With reference to the ambient temperature of −6.7 °C, the temperature differences of 28.7 °C, 26.7 °C, 22.7 °C and 19.7 °C were evaluated at convergence time for compressor speeds of 6000 rpm, 5000 rpm, 4000 rpm and 3000 rpm, respectively.

#### 4.2.7. Pressure Characteristics

^{3}/h, and coolant flow rate of 10 L/min. In all cases of coolant temperature variation, as the compressor speed increased, pressure ratio had the same trend with cooling mode. However, pressure ratio in heating mode was higher than cooling mode by two to three times. In heating mode, because the tested system was exposed to temperatures under −6.7 °C, low pressure got down to below 100 kPa, similar to vacuum pressure, and high pressure led to applied coolant temperature increases up to 1500 kPa at the coolant temperature of 50 °C. As a result of that, pressure ratio had a wide range from 2.67 to 12.4, which led to the increase of compressor power consumption depicted in Figure 8.

## 5. Conclusions

- (a)
- In the cooling mode of heat pump system with a high-pressure side chiller, the system efficiency decreases by 16.4% on an average and the cooling capacity and compressor work enhances by 8.0% and 27.0%, respectively, on average with variation in compressor speed.
- (b)
- In the heating mode, the coolant gets heated by the discharged refrigerant of the electric compressor in the high-pressure side chiller and higher temperature coolant transfers heat to the inlet air of heater core so that the cabin heats up. In the high-pressure side chiller, the heat transfer rate is higher at the lower coolant temperature due to the higher temperature difference between the coolant and the refrigerant and because the coolant could absorb more heat from refrigerant at lower temperatures. However, with respect to the heater core to heat up the cabin because the coolant temperature is relatively low despite heat gain from the refrigerant, heat transfer rate in heater core is quantitatively low. On the contrary, when the coolant temperature is higher than ambient conditions, −6.7 °C, system efficiency of the tested heat pump is decreased due to the increase in compressor power consumption with higher pressure ratio.
- (c)
- In the transient mode, the coolant temperature converges to 22 °C, 20 °C, 16 °C and 13 °C after 1 h 30 min for the compressor speeds of 6000 rpm, 5000 rpm, 4000 rpm and 3000 rpm, respectively. The attained temperature differences are 28.7 °C, 26.7 °C, 22.7 °C and 19.7 °C for 6000 rpm, 5000 rpm, 4000 rpm and 3000 rpm, respectively, at the convergent stage.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Effect of compressor speed on (

**a**) cooling capacity, compressor work and coefficient of performance (COP) and (

**b**) P-h diagram of heat pump system with chiller during the cooling mode.

**Figure 3.**Effect of coolant inlet temperature on (

**a**) cooling capacity, compressor work and coefficient of performance (COP) and (

**b**) P-h diagram of heat pump system with chiller during the cooling mode.

**Figure 4.**Effect of coolant volume flow rate on (

**a**) cooling capacity, compressor work and coefficient of performance (COP) and (

**b**) P-h diagram of heat pump system with chiller during the cooling mode.

**Figure 5.**Pressure ratio characteristics with (

**a**) compressor speed and (

**b**) coolant temperature in cooling mode.

**Figure 6.**Heat dissipation performance of the outdoor heat exchanger (condenser) of heat pump system with chiller for various air velocities.

**Figure 8.**Behavior of compressor power consumption for coolant temperatures of −6.7 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C and 50 °C and compressor speeds of 2000 rpm, 3000 rpm, 4000 rpm, 5000 rpm and 6000 rpm.

**Figure 9.**Variation of system efficiency for compressor speed range of 2000 rpm to 6000 rpm and coolant temperature range of −6.7 °C to 50 °C.

**Figure 10.**Variation of heater core performance for various compressor speeds and coolant temperatures.

**Figure 11.**Variation in coolant temperature with time for compressor speeds of 3000 rpm, 4000 rpm, 5000 rpm and 6000 rpm.

**Figure 12.**Pressure ratio characteristics with compressor speed and coolant temperature in heating mode.

Component | Specification |
---|---|

Chiller | Plate-type inner-fin brazed-aluminum heat exchanger 150.3 W × 82.7 H × 57.5 D |

Condenser (Size, mm) | PF(parallel flow)-type louvered-fin brazed-aluminum heat exchanger 568 W × 382 H × 20 D |

Evaporator (Size, mm) | Laminated-type louvered-fin brazed-aluminum heat exchanger 216 W × 200 H × 48 D |

Heater core (Size, mm) | PF(parallel-flow)-type louvered-fin brazed-aluminum heat exchanger 185 W × 165 H × 30 D |

Compressor (Displacement, cm ^{3}) | Scroll type (36.0) |

Expansion device | Electric expansion valve (EEV) |

Water pump (Coolant) | Electric-driven water [ump] (ethylene glycol:water = 50:50) |

Instrument | Range | Accuracy |
---|---|---|

Thermocouples (T-type) | −25~100 °C | ±0.1 °C |

Pressure gage (Sensors, PI3H) | Max. 2500 kPa | ±0.1% |

Different pressure transducer (Sensors, EJX110) | 0~150 mmAq | ±0.15% |

Mass flow rate (Coriolis type) | Max. 600 kg/h | ±0.2% |

Volume flow rate (Electromagnetic type) | 0.1~10 m/s | ±0.5% |

Humidity/Temperature Module (EE99-1) | 0~100% RH | ±2.0% |

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## Share and Cite

**MDPI and ACS Style**

Lee, M.-Y.; Garud, K.S.; Jeon, H.-B.; Lee, H.-S.
A Study on Performance Characteristics of a Heat Pump System with High-Pressure Side Chiller for Light-Duty Commercial Electric Vehicles. *Symmetry* **2020**, *12*, 1237.
https://doi.org/10.3390/sym12081237

**AMA Style**

Lee M-Y, Garud KS, Jeon H-B, Lee H-S.
A Study on Performance Characteristics of a Heat Pump System with High-Pressure Side Chiller for Light-Duty Commercial Electric Vehicles. *Symmetry*. 2020; 12(8):1237.
https://doi.org/10.3390/sym12081237

**Chicago/Turabian Style**

Lee, Moo-Yeon, Kunal Sandip Garud, Han-Byeol Jeon, and Ho-Seong Lee.
2020. "A Study on Performance Characteristics of a Heat Pump System with High-Pressure Side Chiller for Light-Duty Commercial Electric Vehicles" *Symmetry* 12, no. 8: 1237.
https://doi.org/10.3390/sym12081237