Modeling of advanced heat pump cycles and aerodynamic design of a small-scale centrifugal compressor for electric vehicles

Abstract

Unlike conventional vehicles powered by internal combustion engines, electric vehicles do not have enough waste heat to provide sufficient heating to the cabin. Thus, an additional heating system, such as a heat pump, is needed. However, its performance decreases significantly when the ambient temperature is low. The new kangaroo heat pump cycle (KC) is proposed to increase the heating capacity in low-temperature climates. It is an enhanced flash tank-based vapor injection heat pump cycle (FT-VIC). A sub-cycle is added to the system to increase the refrigerant inlet quality entering the flash tank, which leads to a higher refrigerant mass flow rate and heating capacity. Because KC has a higher heating capacity, the heating needed from the electric heater can be reduced, thus reducing energy consumption and increasing the driving distance. In this study, thermodynamic models were developed for the basic heat pump cycle (BC), FT-VIC, and KC. And a new method evaluating the life cycle climate performance (LCCP) of electric vehicle heat pumps based on the SAE J2766 standard was proposed. Results show that KC is effective at low ambient temperatures. At -15°C, KC can save 13.8% energy compared to BC, and save 2.7% energy compared to FT-VIC. However, due to the additional weight, KC has a higher LCCP than other cycles. If the pressure ratio limit is removed and the compressor efficiencies are constant, KC can have a lower LCCP than other cycles in cold climates. Transient models were also developed to assess their performance in urban driving conditions. Results show that at the end of the simulation, the cabin room temperature of KC is 3.6°C and 7.0°C higher than that of FT-VIC and BC, respectively. However, due to the high pressure ratio and refrigerant mass flow rate, the accumulated power consumption of KC is 32.4% higher than FT-VIC and 64.4% higher than BC. Despite its high energy consumption, it is more efficient than adding heat from an electric heater. In addition, a small-scale centrifugal compressor was designed for an electric vehicle to reduce the compressor’s size and weight. Results show that its COP is 6.6% higher than that of the scroll compressor at the design point. However, its efficiency at the off-design point quickly drops. Future studies are needed to improve its off-design point performance.