4.2.1. Heat Dissipation at Various Air Velocities

The heat dissipation performance of the outdoor heat exchanger (condenser) of the heat pump system with a high-pressure side chiller for various air velocities is presented in Figure 6. The performance of the heat exchanger with air velocity was experimentally evaluated at the inlet temperature of 80 ◦C, inlet pressure of 1600 kPa, inlet SH of 25 and outlet SC of 8. With the increase in the air velocity, the mass flow rate of air increased, hence the air carried more heat from the outdoor heat exchanger, which resulted in the increase in the heat dissipation of the outdoor heat exchanger [11]. The heat dissipation of the outdoor heat exchanger increased linearly with the increase in the air velocity, as shown in Figure 6. The heat dissipation of the outdoor heat exchanger increased by 19% from 9.3 kW to 11.1 kW with the increase in the air velocity from 3 m/s to 4 m/s, whereas the heat dissipation increased by 13.7% from 11.1 W to 12.6 kW when the air velocity increased from 4 m/s to 5 m/s.

5 m/s.

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

pressure side chiller under heating mode is presented in this section.

4.2.1. Heat Dissipation at Various Air Velocities

This section elaborates the performance characteristics of the heat pump system with a high‐ pressure side chiller, namely, heat dissipation, chiller heat transfer rate, compressor power consumption, system efficiency, heater core performance and pressure characteristics for the steady state conditions of various compressor speeds, coolant temperatures and air velocities in heating mode. In addition, a brief discussion about the transient behavior of heat pump system with a high‐

The heat dissipation performance of the outdoor heat exchanger (condenser) of the heat pump system with a high‐pressure side chiller for various air velocities is presented in Figure 6. The performance of the heat exchanger with air velocity was experimentally evaluated at the inlet temperature of 80 °C, inlet pressure of 1600 kPa, inlet SH of 25 and outlet SC of 8. With the increase in the air velocity, the mass flow rate of airincreased, hence the air carried more heat from the outdoor heat exchanger, which resulted in the increase in the heat dissipation of the outdoor heat exchanger [11]. The heat dissipation of the outdoor heat exchanger increased linearly with the increase in the air velocity, as shown in Figure 6. The heat dissipation of the outdoor heat exchanger increased by 19% from 9.3 kW to 11.1 kW with the increase in the air velocity from 3 m/s to 4 m/s, whereas the heat dissipation increased by 13.7% from 11.1 W to 12.6 kW when the air velocity increased from 4 m/s to

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

### system with chiller for various air velocities. 4.2.2. Chiller Heat Transfer Rate at Various Compressor Speeds and Coolant Temperatures

4.2.2. Chiller Heat Transfer Rate at Various Compressor Speeds and Coolant Temperatures The effect of compressor speed and coolant temperature on the heat transfer rate of chiller is presented in Figure 7. Chiller heat transfer rate for various compressor speeds and coolant temperatures was experimentally recorded for ambient and air temperatures of −6.7 °C, air volume flow rate of 300 m3/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 The effect of compressor speed and coolant temperature on the heat transfer rate of chiller is presented in Figure 7. Chiller heat transfer rate for various compressor speeds and coolant temperatures was experimentally recorded for ambient and air temperatures of −6.7 ◦C, air volume flow rate of 300 m<sup>3</sup> /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

range for a coolant temperature of 0 °C.

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.

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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

**Figure 7.** Effect of compressor speed and coolant temperature on heat transfer rate of chiller.

**Figure 7.** Effect of compressor speed and coolant temperature on heat transfer rate of chiller. 4.2.3. Compressor Power Consumption at Various Compressor Speeds and Coolant Temperatures

The 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 is presented in Figure 8. The behavior of compressor power consumption for various compressor speeds and coolant temperatures were experimentally evaluated for ambient and air temperatures of −6.7 ◦C, air volume flow rate of 300 m<sup>3</sup> /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. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 19 of 25

**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 **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.

#### and 6000 rpm. 4.2.4. System Efficiency at Various Compressor Speeds and Coolant Temperatures

4.2.4. System Efficiency at Various Compressor Speeds and Coolant Temperatures The 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 is shown in Figure 9. The variation of system efficiency with compressor speed and coolant temperature was experimentally evaluated at ambient and air temperatures of −6.7 °C, maximum temperature of 120 °C, maximum pressure of 2500 kPa, air volume flow rate of 300 m3/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 The 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 is shown in Figure 9. The variation of system efficiency with compressor speed and coolant temperature was experimentally evaluated at ambient and air temperatures of −6.7 ◦C, maximum temperature of 120 ◦C, maximum pressure of 2500 kPa, air volume flow rate of 300 m<sup>3</sup> /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

°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 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. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 20 of 25 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.

**Figure 9.** Variation of system efficiency for compressor speed range of 2000 rpm to 6000 rpm and **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.

#### coolant temperature range of −6.7 °C to 50 °C. 4.2.5. Heater Core Performance at Various Compressor Speeds and Coolant Temperatures

4.2.5. Heater Core Performance at Various Compressor Speeds and Coolant Temperatures The variation of heater core performance for various compressor speeds and coolant temperatures is shown in Figure 10. The variation is presented at the ambient and air temperatures of −6.7 °C, air volume flow rate of 300 m3/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 The variation of heater core performance for various compressor speeds and coolant temperatures is shown in Figure 10. The variation is presented at the ambient and air temperatures of −6.7 ◦C, air volume flow rate of 300 m<sup>3</sup> /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,

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 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. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 21 of 25 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.

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

**Figure 10.** Variation of heater core performance for various compressor speeds and coolant 4.2.6. Performance of Heat Pump System with a High-Pressure Side Chiller in Transient State

temperatures. 4.2.6. Performance of Heat pump system with a High‐pressure Side Chiller in Transient State The transient performance of heat pump system with a high‐pressure side chiller during the heating mode was experimentally evaluated for the ambient and air temperature of −6.7 °C, air volume flow rate of 300 m3/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 The transient performance of heat pump system with a high-pressure side chiller during the heating mode was experimentally evaluated for the ambient and air temperature of −6.7 ◦C, air volume flow rate of 300 m<sup>3</sup> /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

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 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. 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 compressorspeeds. 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.

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

#### 5000 rpm and 6000 rpm. 4.2.7. Pressure Characteristics

in Figure 8.

4.2.7. Pressure Characteristics Pressure ratio in heating mode was analyzed like cooling mode in the tested system, as shown in Figure 12. The variations of pressure ratio with coolant operating conditions and compressor speed were experimentally recorded for HVAC inlet air conditions of temperature of −6.7 °C, air flow rate of 300 m3/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 Pressure ratio in heating mode was analyzed like cooling mode in the tested system, as shown in Figure 12. The variations of pressure ratio with coolant operating conditions and compressor speed were experimentally recorded for HVAC inlet air conditions of temperature of −6.7 ◦C, air flow rate of 300 m<sup>3</sup> /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.

wide range from 2.67 to 12.4, which led to the increase of compressor power consumption depicted

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

#### mode. **5. Conclusions**

**5. Conclusions** The cooling and heating performance characteristics of the heat pump system with a high‐ pressure side chiller are experimentally investigated under real road driving conditions for light‐ duty commercial electric vehicles. The critical findings from the experimental investigations into the The cooling and heating performance characteristics of the heat pump system with a high-pressure side chiller are experimentally investigated under real road driving conditions for light-duty commercial electric vehicles. The critical findings from the experimental investigations into the heat pump system with a high-pressure side chiller in cooling and heating modes are summarized as below.


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:** Conceptualization, M.-Y.L.; K.S.G.; H.-B.J. and H.-S.L.; methodology, M.-Y.L.; K.S.G.; H.-B.J. and H.-S.L.; formal analysis, M.-Y.L.; K.S.G.; H.-B.J. and H.-S.L.; investigation, M.-Y.L.; H.-B.J. and H.-S.L.; resources, M.-Y.L.; K.S.G.; H.-B.J. and H.-S.L.; data curation, M.-Y.L. and H.-S.L.; writing—original draft preparation, M.-Y.L.; K.S.G. and H.-S.L.; writing—review and editing, M.-Y.L.; K.S.G. and H.-S.L.; visualization, M.-Y.L. and H.-S.L.; supervision, M.-Y.L. and H.-S.L.; project administration, H.-S.L.; funding acquisition, H.-S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Ministry of Trade, Industry & Energy(MOTIE), Korea Evaluation Institute of Industrial Technology (KEIT) through the Industrial Technology Innovation Program (20003988, Development of 4 kW heating and cooling modularization technology using waste heat recovery and alternative refrigerant on electric vehicle's exclusive platform).

**Acknowledgments:** This work was supported by the Ministry of Trade, Industry & Energy(MOTIE), Korea Institute for Advancement of Technology (KIAT) through the Automobile Parts Cluster Construction Program for the Environment-Friendly Vehicles (P0000760, Development of Integrated Smart Air-Conditioning System for Electric Commercial Vehicles), Korea Evaluation Institute of Industrial Technology(KEIT) through the Industrial Technology Innovation Program (20003988, Development of 4 kW heating and cooling modularization technology using waste heat recovery and alternative refrigerant on electric vehicle's exclusive platform) and the National Research Foundation of Korea (NRF) grant funded by the Korea government(MSIT) (No. 2020R1A2C1011555).

**Conflicts of Interest:** The authors declare no conflict of interest.
