Heat pump systems for HVAC in electric vehicles

In electric vehicles (EVs), the heating, ventilation and air-conditioning (HVAC) system typically uses the same electric motor used to propel the vehicle. However, the electric heaters and AC compressors have a high energy consumption. As a result, the driving range of the EV is greatly reduced, typically by up to 40%. [1] 

 

Experimental studies suggest that a refrigerant-based heat pump system offers sharply increased energy efficiency with the possibility to increase driving range and reduce CO2 emissions. It has also been reported that several manufacturers in the automotive industry are adopting heat pump systems for their EVs. Furthermore, various low-GWP refrigerants have been examined in the literature for the optimisation of heat pump systems in EVs. [2] 

 

CO₂ (R744) heat pump system with intermediate cooling compressor ^[3] 

 

Various experimental studies appear to support the development of CO₂ (R744)-based heat pump systems for EVs. However, research shows that the utilization of internal heat exchanger results in a high discharge temperature. High discharge temperature is particularly problematic in heat pump systems for EVs mainly, because of their wide operating temperature range. These systems have to operate in extremely hot and cold regions, where the discharge temperature can typically reach 130 °C and even up to 150 °C. Solutions are thus needed to address the issue of high discharge temperature and improve performance in extremely harsh conditions. 

 

A recent study proposed a CO₂ system for EVs that introduces the concept of intermediate cooling and uses a compressor with two additional ports reserved for intermediate cooling. According to the researchers, this technology contributes to a 19.8% increase in the maximum cooling capacity and a 12.5% improvement in the optimum COP versus the basic CO₂ system under extremely hot ambient conditions (45 °C). As for the heating mode, when comparing the proposed CO₂ heat pump with an intermediate cooling compressor (ICC) and a CO₂ heat pump without ICC, the authors found that the system with ICC offered better heating performance. They found 50–132% increase in heating capacity and an 18.9–61.9% improvement in COP as the ambient temperature decreases from 0°C to −20°C.  

 

They also found that the COP of the CO₂ heat pump system with ICC was approximately 1.70 at −20 °C/20 °C condition while the discharge temperature was controlled well within 100 °C. Furthermore, the optimum refrigerant charge seemed to increase with the ambient temperature in heating mode. In this study, the preferred refrigerant charge of the experimental system was about 0.75 kg to accommodate an ambient temperature of −20°C to 0°C. 

 

An innovative system for defogging in heating mode ^[1] 

 

Heat pump systems are generally not designed to reduce humidity within the cabin of an EV. Consequently, the inner surface of the windshield will fog up when the temperature of the glass drops below the dew point of the cabin air. This seriously impairs the driver’s visibility, which represents a major road safety risk. Therefore, effective solutions for simultaneously heating and dehumidifying the cabin air are required. 

 

A recent study in Taiwan proposed a novel HVAC system which provides an additional high-efficiency dehumidification function along with conventional heating and cooling capacities for EVs. The experimental apparatus consisted mainly in a rotary compressor, two indoor heat exchangers, an outdoor heat exchanger, two electric air-heaters, two expansion valves, two three-way valves, one four-way valve, and four single-way valves. 

 

The main findings of this study included the following: 

1. In cooling mode, a COP of 3.18 was achieved. The temperature inside the simulated cabin dropped below 25 °C in five minutes and dropped to around 15 °C under steady-state conditions.  

2. In heating mode, a COP of 3.3 was achieved. The cabin temperature rose to 30 °C in 10 min and increased to approximately 40 °C at steady state. 

3. In defogging mode, the dehumidification performance achieved was equal to 1.47 L/kWh, which meets the minimum requirement of the CNS 12492 standard in Taiwan. The air sucked from the test chamber at 27 °C was dehumidified and reduced to a temperature of around 17 °C by an evaporator and then passed through a condenser, where it was heated to a temperature of about 40 °C.  

 

Sources 

[1] Chang, T.-B.; Sheu, J.-J.; Huang, J.-W. High-Efficiency HVAC System with Defog/Dehumidification Function for Electric Vehicles. Energies 2021, 14, 46. https://doi.org/10.3390/en14010046 

[2] Wu, J., Zhou, G., & Wang, M. (2020). A comprehensive assessment of refrigerants for cabin heating and cooling on electric vehicles. Applied Thermal Engineering, 115258. https://doi.org/10.1016/j.applthermaleng.2020.115258 

[3] Chen, Y., Zou, H., Dong, J., Wu, J., Xu, H., & Tian, C. (2021). Experimental investigation on the heating performance of a CO₂ heat pump system with intermediate cooling for electric vehicles. Applied Thermal Engineering, 182, 116039. https://doi.org/10.1016/j.applthermaleng.2020.116039