Thermal management challenges in EVs

  • 30-Sep-2009 01:12 EDT

Depending on the length of journey, the Rinspeed iChange concept car is either heated by a PTC high-voltage air heater or by a fuel-powered air heater.

Cars with a highly efficient combustion engine and all electric vehicles have one thing in common: They do not produce enough waste heat to warm up the interior. At the Frankfurt Motor Show, Eberspächer demonstrated new heating options for electric and hybrid vehicles. There is no silver bullet, though. The right choice of thermal management depends on the overall concept and drivetrain.

The seemingly logical choice to heat the interior of an electric vehicle (EV) is electricity. Or is it? While electricity is of course available in an EV, it is also precious. Heating a vehicle interior in winter and defrosting the windows requires about 5 kW, the supplier calculates. Looking at the Opel Ampera’s (Chevrolet Volt) battery capacity of 16 kW, for example—that is a large chunk of the charge.

“Also, when we discuss thermal management with vehicle manufacturers, it often turns out that heating the interior is not the only challenge. Instead, we find ourselves discussing the need to pre-heat other components such as the battery,” said Uwe Kohle, head of the supplier’s development of vehicle heaters.

“If additional components have to be heated, a coolant-based heating is usually the better choice,” Kohle continued, “while air heaters tend to be the fast and efficient solution to heat the cabin.”

However, that does not answer the question of where the heat is supposed to come from. In a hybrid vehicle or a range extender that has fuel on board, the heating can be based on fuel combustion. With an electric vehicle, the decision is more complex because the high energy demand of a heating system impacts range.

“We are currently talking to nearly all vehicle manufacturers planning to sell a vehicle with electric drive,” said Kohle. “Everybody in the industry is in the process of defining the right solution.”

The current thermal-management dilemma is exemplified by the dual approach in the Rinspeed iChange concept car. The supplier has integrated two heating systems in this EV: A newly developed PTC high-voltage air heater from Eberspächer catem is active during short trips and city traffic. It transforms 580 V onboard voltage into heat via a variable resistor with positive coefficient of temperature.

The big advantage is that the heater operates at 99% efficiency. As the PTC system’s heating capacity is 5.6 kW, this could drain the iChange’s LiFePO4 battery during longer trips and would impact the car’s approximate range of 90 km (56 mi).

Therefore, the concept car is equipped with a second heater for longer journeys. The fuel-powered heater, called Airtronic, consumes a maximum of 0.43 L (0.11 gal) per hour and works at 84% efficiency. Its downside is that the driver eventually has to refill the 3.5-L (0.9-gal) fuel tank, hidden behind the concept car’s license plate.

“Will drivers be willing to do that?” asks Kohle. “No one knows at the moment.”

Currently, the Airtronic is designed for E85 ethanol or biodiesel, but the supplier is already working on a water heater version that will operate with pure ethanol. Considering the need for low or zero emissions driving, the fuel heater is equipped with catalytic converter, NOx trap, and particulate filter.

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