The loss of winter driving range in electric vehicles equipped with conventional electric heating systems is well-documented—range being reduced by up to 60% and even more in severe cold. The advanced heat pump system in the Toyota Prius Prime plug-in (PHEV), developed in conjunction with Denso, is designed to deliver improvements compared with conventional electric heating and the heat pumps in the Nissan Leaf and Kia Soul EV. Their automotive heat pumps are efficient, but typically operate down only to 0° C/32° F.
The Prius Prime system uses 63% less energy than conventional heating and should extend driving range up to 21%, Toyota and Denso researchers reported in a presentation at the 2017 SAE World Congress (WCX17).
The Prime PHEV incorporates a concept from some static-mount commercial heat pumps: a refrigerant liquid-gas separator and refrigerant gas injection circuit to provide heat pump efficiency down to -10° C/14° F. The large static-mount heat pump separators would be impractical to package underhood. However, the Prime unit was described as similarly compact to an automotive thermostatic expansion valve.
Heat pump coming for EVs?
The Prime gasoline engine could be started and run to provide heat and dehumidification. But this would affect PHEV fuel economy, so the engine is kept off and the heat pump and a complex system of control valves and the liquid-gas separator is used in EV mode. Which indicates Toyota plans this system also for EVs.
A key reason heat pump performance is thermally-limited is because refrigerant mass-flow rate drops with ambient temperature. Like other automotive heat pumps, the Prime's has an under-dashboard internal condenser (the discharge source for all cabin heat). The compressor is a scroll-type Denso unit with a refrigerant gas injection port added. In heating mode the hot, high-temperature refrigerant gas is discharged from the compressor into the internal condenser, where it provides some cabin heat, condensing partly in the process. This contrasts with the front-mounted outer condenser, which in A/C mode condenses refrigerant by giving up heat to the ambient air.
The partly-condensed gas from the internal condenser then flows through an electric expansion valve for further decompression,and into the ultra-compact liquid-gas separator that is a key part of the refrigerant gas injection system. This gives this heat pump circuit extra performance in the heating mode.
From the separator, the gas goes to the compressor inlet. The liquid refrigerant condenses further as it goes through a throttle passage, and flows through to the outer condenser, where in heat mode it absorbs heat from the ambient air and flows to the internal condenser to contribute further to the passenger compartment warming.
Humidity control challenge
The cabin has a humidity/temperature sensor mounted to the windshield (measuring glass and adjacent interior air temperature, plus cabin humidity). However, actual control of dehumidification is another challenge for a heat pump system, because in EV mode there is no waste heat (from engine cooling system or exhaust system) to combine with heat from compressor operation.
However, the Toyota-Denso heat pump system also provides a way without having to start the engine. In fact, there are two circuits, a series and a parallel, each intended for a specific ambient temperature range (32-40° F/0-4° C for parallel circuit, 40-60° F/4-16° C for serial), the latter for the reduced amount of reheating necessary.
Both include a flow path through the evaporator, which vaporizes any liquid refrigerant and absorbs heat from the air going through that heat exchanger, based on signals from a temperature sensor between the fins. And both dehumidification circuits are able to operate in conjunction with cabin heating.
The humidity sensor also is part of the system configuration that provides operation in six different HVAC modes (cabin cooling, cabin heating, serial dehumidification for the cabin (with heating), parallel dehumidification for the cabin (with heating), defrosting the outer heat exchanger, and heat pump-generated cabin heating when ambient temperature is low.