The electric vehicle market is very near the light-off point for which consumers and investors alike have been waiting. With a 2017 global market size near $120MM, the market is projected to exceed $560MM in 2025, a CAGR of over 22%. These numbers generate excitement, and pressure from China to take a leadership role in the global automotive market has compelled the US and Europe to ramp up their development efforts in kind.
EVs are generally less complicated than ICEs, with fewer parts and simple energy paths that enable market penetration by dozens of new OEMs. Despite those benefits, thermal management of both the battery and passenger comfort systems remains one of the biggest remaining hurdles to the global adoption of EVs. The combustion reaction in ICEs provides the engine with plenty of thermal energy, some of which heats the cabin and engine in cold weather. Conversely, EVs have to use battery capacity to heat the passenger cabin and regulate the battery’s temperature, both of which prefer to be between 20-25°C. This additional power-draw dramatically reducing the EV’s range. Even regenerative braking, a significant option to extend EV range, loses effectiveness in extremely cold climates.
Temperature extremes outside the desired range have varying effects on the battery and vehicle performance. Extreme heat accelerates the corrosion reaction, which shortens battery life. Extreme cold greatly increases the resistance to electron flow, but this effect only hurts performance as opposed to the battery itself. There are numerous workarounds that EV manufacturers suggest: preheating the cabin and battery while charging, discharging a stream of energy to decrease temperature differential between the battery and the ambient environment, and not letting the charge drop below 15-20%. These solutions incrementally help cold-weather range, but the solution to the problem is likely a [commercially-viable] thermodynamic one. Here are three innovative, enabling technologies for thermal management of EVs.
One solution is the heat pump, an external vapor-compression system with an independent compressor and expansion valve. The system transfers heat to where it is needed most, either to the cabin/battery in cold weather or away from those zones in warm weather. Nissan employed the first commercially-available heat pump on the 2013 Leaf. While they provide an attractive solution to the thermal management, a challenge OEMs have encountered with heat pumps is the battery capacity required to haul the additional system components counteracts the energy benefit they provide.
Heat pumps using traditional organic refrigerants typically only provide around 2-3 kW of heating energy, while peak load point for rapid charging or high-performance demands 15 kW or above at cold ambients. Using a more thermodynamically-efficient refrigerant, such as CO2, could provide the heating load, but comes at the cost of thick material grades dictated by its high operating pressures at typical run conditions. VW is rolling out a CO2 refrigerant EV in 2019.
Vehicle mass is indirectly related to range, and the more the EV weighs, the less sense the option makes. Another drawback of heat pumps is that that system does not operate below -10°C, which means another supplemental component, such as a resistive heater (and its requisite mass) would be needed to preheat the heat pump system.
Positive Temperature Coefficient (PTC) Heaters
If the EVs need a resistive heater, such as a positive temperature coefficient (PTC) device, to preheat the heating system, could the PTC heater alone handle the entire heating operation? Borg Warner rolled out a high-voltage PTC heater for Chinese EV manufacturer NIO. The heater rapidly warms the cabin and defrosts the windows, though the peak power output is only 7kW. Though the peak load is not yet high enough, this solution is a more robust one that can handle the extremely cold temperatures, where the EV needs heating the most.
Liquid Immersion Cooling
An emerging option for managing the battery temperature is dielectric liquid immersion, widely known as a leading cooling technology in the data center industry. Ricardo has explored liquid immersion as a battery chiller option, but to date, OEMs have not widely explored this approach. GRC and Engineered Fluids are two leading suppliers of liquid immersion technology. The objective of the approach is to trim the extreme temperatures the battery sees with a high-specific heat fluid that is safe around electronic components. Two-phase immersion cooling is a popular option for battery thermal management, but because it needs to operate at the saturation temperature, two-phase immersion has a limited operating window for a given heat transfer rate. Single-phase immersion had a broader range of applicability, and drastically extends the amount of time the battery takes to soak to extreme ambient temperatures. The high heat capacity also enables smaller components, helping the mass challenge, and dielectric fluids are available in environmentally-friendly chemistries.
Thermal management is one of the biggest hurdles EVs still face before they achieve cost parity with ICEs and the company or researcher that best solves that problem will be well-positioned for success when the volumes ramp up.