The role of coolants in e-mobility
While many of the world’s largest cities struggle with smog and air pollution, The 2015 Paris agreement advises to keep the average temperature rise below 2°C this century in order to slow down global warming and its consequences. Norms and regulations, becoming ever stringent, are subjected to the automotive sector to solve these urgent issues by reduction of exhaust gas pollution and greenhouse gas emissions.
To meet these regulations and avoid hefty fines, OEMs invest at unprecedented speed in emission-reducing technologies. Central in this recent revolution in mobility is electrification; be it through hybridization of today’s combustion engines, full battery electric vehicles (BEV), fuel-cell electric vehicles (FCEV) or other forms of eMobility.
In hybrid electric vehicles (HEVs) or plug-in electric vehicles (PHEVs) improvement in fuel economy and reduction of emissions typically comes at the cost of increase vehicle complexity. Next to an internal combustion engine (ICE), a small to medium size battery is fitted together with an electric motor to aid the engine at high loads (acceleration) and to recuperate energy (regenerative braking). In an even more drastic powertrain architecture, a downscaled ICE is put in series with a battery and an electric motor, where the ICE function is only to charge the battery while propulsion is executed by the electromotor (Range-extender BEV).
In all the above setups, thermal management of the ICE and electric powertrain is key for safety, lifetime and efficiency of the vehicle, avoiding battery thermal runaway, capacity loss, and sub-optimal working temperatures for battery, electric motors and power electronics. Where for HEVs it is clear that the ICE still requires the same cooling performance of today’s engine coolants, also in Battery Electric Vehicles water/glycol coolants play a critical role.
Early BEVs, especially in lower market segment, were, and still are, air cooled. It has been found, however, that battery degradation is more severe in such vehicles as air, with its mediocre heat transfer properties, is not able to maintain to battery at a balanced temperature throughout the pack. Moreover, fast charging is only allowed when a battery is heated to its optimal temperature region (20-35°C), also at sub-zero ambient temperatures.
Because of above requirements, water/glycol is the thermal management fluid of choice as the battery temperature can easily be levelled out due to the superior heat transfer. Battery heating is accomplished by efficiently taking away heat from the ICE, electric engine and/or power electronics. In BEVs, where the major source of heat, the ICE, is not present, coupling of the liquid cooling loop to the HVAC or heat pump allows for improved efficiency and climate comfort.
Therefore, in recent years, a clear shift towards liquid water/glycol cooling is witnessed for electric vehicles, especially at premium brands offering long-range (large battery packs) and fast charge capabilities. In the most preferred layout, today’s coolants flow through electrically insulated channels below the battery pack (bottom plate) and in the housing of eMotors to avoid leaks and short-circuiting.
The latter is also critical in fuel cell electric vehicles. Here, heat needs to be removed from the fuel cell stack where H2 is converted to electricity, which is subsequently stored in a battery. Due the large heat flux produced by the fuel cell stack, electric insulation is no longer viable as it creates a thermal barrier between stack and coolant. Low-conductive coolants, currently under development, or dielectric liquids are believed to be most apt in such situations.