Is free source energy the secret to tripling electric vehicle (EV) passenger heating system efficiency?

EVs have historically implemented positive temperature coefficient (PTC) heaters to raise the cabin temperature in heating mode. These devices generate heat by printing conductive ink onto polymer-based substrates. Its material properties allow the PTC heater to regulate the temperature without passively needing external controls. As a result of thermal self-regulation, PTC heaters deliver an efficiency of around 100%, comparable to that of an electric heater.

While this current state is a marked improvement over traditional fixed-resistance heaters that use coils and wires for heat generation, diverting electric energy to heat the cabin is parasitic to the cold-weather driving range, a critical hurdle for manufacturers. As a result, the industry searched for solutions to improve this limitation and landed on a solution initially developed for stationary heating: the heat pump.

Heat pumps and EVs

Heat pumps take heat from ambient air (outside the vehicle or waste heat from the battery or drivetrain) and transport it where needed. Internal combustion engine (ICE) vehicles do not need heat pumps due to the waste heat available from the ~70% of fuel energy lost in converting chemical-to-thermal-to-mechanical energy to drive the car.

There are two critical advantages the heat pump offers over a traditional system with a PTC heater. First, it is bi-directional, able to manage both heating and cooling cabin climate efficiently. Second, scavenging ambient heat enables up to three units of output energy for one unit of input energy, diverting more of the battery’s power toward vehicle propulsion vs. cabin climate, thereby extending the driving range.

Heat pump trends and innovations

In addition to the components required for basic vapor-compression refrigeration (compressor, condenser, expansion valve, evaporator), automotive heat pump systems have additional electronics that enable the technology. These are the PTC heater and electric expansion valve (EXV).

PTC heaters are still needed to kickstart heating at extremely cold temperatures in heat pump systems. They are also the current standard, as heat pump vehicles are still several years from mass adoption. One innovation from Mahle is a high-voltage PTC heater, offering improved safety, lower cost, smaller packaging envelope, and higher efficiency through intelligent electronics that constantly monitor and adjust the PTC element resistance to optimize temperature delivery. The improved efficiency of the Mahle unit can increase driving range by 20% when combined with a heat pump and operates down to -40oC.

Sanhua produces an EXV for fluorinated R-134a HFC and R-1234yf HFO refrigerant systems that embeds the control board and integrates the onboard diagnostics while operating between -40oC and 120oC. This combination delivers precision flow control, boosting COP by more than 10% while maximizing the heat exchangers’ effectiveness with precision superheat control. In addition, this improved accuracy can reduce excessive heat exchanger sizing to account for flow variability, reducing system cost while decreasing mass to extend the driving range.

Takeaways and future development

EVs are compelling the industry to look at thermal management holistically, using all available heat sources and demands to optimize the system for performance and range at all operating conditions. As a result, once they roll out systems with parallel heat pumps and battery/electronics cooling loops, OEMs will likely begin work to fully-integrated the two circuits. This approach will require innovation in (at least) sensors, control valves, heat exchangers, and sealing technology to manage the complexity. However, with the energy around EVs and the opportunity for market leadership at stake, it’s certainly a problem worth solving.