Thermal Challenges of Electric Commercial Vehicles
Mahle thermal-management expert Laurent Art says solutions developed for passenger EVs, such as immersion cooling, can be scaled to truck applications.
Vehicle electrification poses new challenges for thermal management, driving development of new systems and components specific to battery and fuel-cell electric vehicles (EVs). Key components of the cooling circuit that typically were mechanically driven on an internal combustion engine (ICE) vehicle also need to be electrified, said Laurent Art, director pre-development thermal systems and components at Mahle. This drives the development of new components such as electrical fans, compressors and pumps.
The solutions developed for passenger EVs can to some extent be scaled to truck applications, Art told SAE Media. “This is, for example, the case for battery cooling where the same base technology is used in both cases.” Art’s scope of activity includes air-conditioning and powertrain-cooling systems for passenger vehicles and commercial trucks. He recently answered a few questions about the unique thermal requirements of BEVs and FCEVs.
What are the main thermal-management challenges with electric commercial vehicles?
One major challenge is to prevent the fuel cell stack from overheating. Unlike ICEs, most of the heat losses of a fuel cell powertrain need to be rejected via the coolant circuit, and at a lower temperature level than for an ICE. Therefore, a fuel cell vehicle requires much higher cooling performance than an ICE or even a BEV. To solve this challenge, Mahle has developed high-performance cooling systems for such applications.
Another important requirement is to keep the battery within tight temperature limits to avoid premature ageing and enable the battery to always deliver its full capacity. This means the battery needs to be actively cooled or heated depending on the operating conditions. Also, due to different temperature requirements for different subsystems (battery, e-motor, fuel cell stack), the cooling architecture of electrified trucks typically includes multiple coolant loops, as well as interaction between the refrigerant system and coolant circuit for the battery cooling.
For BEVs, as the temperature of the coolant and the amount of heat rejected by the powertrain are typically lower than for ICEs, the cabin heating must be supported by additional electrical heat.
How can EV driving range be improved?
Mahle is particularly active in the development of heat pump systems that can warm up the cabin by harvesting, via the refrigerant circuit, the waste heat from the e-powertrain and battery, as well as energy from the ambient air. This reduces the electrical power consumed for the cabin heating and therefore improves the driving range of the vehicle.
The benefits of heat pumps on vehicle driving range are dependent on the ratio of traction power to cabin heating power. Therefore, while heat pump systems are very attractive for light vehicles (battery range can be increased by up to 20% at an outside temperature of 0°C/32°F), we expect a lower penetration on commercial vehicles.
Do off-highway machines require bespoke solutions or present unique challenges?
Mahle also provides off-highway thermal management solutions and cooling/HVAC of agricultural and construction vehicles. There is a trend towards electrification of small construction machines, especially on inner-city construction sites. The same applies to work machines for municipal transport.
There is a need for thermal management for batteries as pure cooling modules with electric fan drives or as temperature-controlled HVAC systems. In addition, there are requirements for liquid cooling of traction motors and power electronics in separate cooling loops due to different temperature levels.
Any specific considerations to enable effective fast charging?
The amount of heat dissipated in the battery increases with the charging rate; therefore, adequate thermal management is a key enabler for fast charging. Coolant cooled batteries are currently mainstream on the market. In this case, the battery cells typically sit on a cooling plate circulated by cold coolant. It is critical to provide cold coolant to the cooling plate, to prevent battery-cell overheating during charging. For these applications, a high-capacity refrigerant circuit, based on a high-capacity compressor, high-performance condenser and chiller, supplies cold water to the battery.
Mahle also develops immersion cooling solutions, whereby the battery cells are directly cooled by a dielectric fluid. This further improves the cooling performance of the battery in comparison with coolant cooled batteries.
In the case of fast charging of trucks, the heat generated under [the Megawatt Charging System] will further increase. Therefore, we see an even bigger benefit with our immersion cooling solution to minimize the risk of derating during the charging operation. The cooling circuit must also be adapted to reject the heat out of the vehicle with upsizing or duplication of some components.
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