• Wade Ortel

By Virtue of Inefficiency

Updated: Apr 19

The Greek letter eta is used to denote efficiency in most engineering calculations

Internal combustion engines, such as those found powering gasoline or diesel fueled vehicles, generate a lot of waste heat. Of the energy yielded by combustion, a larger portion is lost to the surroundings as heat than is available to propel the vehicle. While limited efficiency is certainly a drawback to thermal engines, in automotive applications, it does yield a favorable byproduct: cabin heating for cold weather.

Since modern internal combustion engines are operated at a constant and relatively high temperature for reasons of efficiency, there is a large, high temperature thermal reservoir available and well suited to providing heat for vehicle occupants. Specifically, heat is provided by diverting hot engine coolant to a small heat exchanger found within the vehicle’s interior. Heat is then transferred to the cabin by means of an electric blower. Other than the insignificant amount of electricity expended to power the blower, no additional energy input is required to heat the cabin of an internal combustion engine vehicle (ICEV).

Due to their high efficiency, limited waste heat is generated by the drive system of an electric vehicle (EV). Furthermore, what little heat is generated is necessarily available at a relatively low temperature. The lithium-ion batteries found in EVs perform best in a relatively narrow band around room temperature. Similarly, the performance of the power electronics found in the inverter of an EV also tend to degrade as their temperatures rise. Finally, there is no significant advantage and numerous disadvantages to running an electric motor at high temperatures.

As there is no appreciable source of waste heat at an appropriate temperature for cabin heating, electric vehicles must adopt a different strategy to maintain cabin habitability when ambient temperatures are low. It is important to stress that any technology employed to this end will consume energy in excess of that which is already required for vehicle locomotion. That is, in direct contrast to an ICEV, running the heater in an EV will increase its energy consumption and therefore decrease its range.

Conventionally, EVs have employed resistive elements for cabin heating, which work in much the same way as a household space heater. While this option provides the requisite functionality, using a heat pump system generally offers superior energy efficiency. Such a system can be added with a relatively simple and therefore inexpensive change; a minor alteration allows the air conditioning system to provide heating in addition to cooling. Since a heat pump transfers heat between thermal reservoirs rather than converting electricity directly into heat, the output of such a system can exceed the electrical input of the compressor. Thus, the energy required for cabin heating is significantly reduced, thereby increasing vehicle range. Many EVs utilize both a heat pump system and a conventional resistive heater. This allows the heating system to be performant at low temperatures and to offer rapid heating, all while offering high efficiency.

A further increase in range can be achieved by preheating the vehicle prior to driving. This way, the EV’s interior can be warmed using electricity drawn from the grid rather than its battery pack. While this does nothing to alter the total amount of energy consumed, it will reduce the amount of energy drawn from the battery pack, thereby increasing range. Another method to increase range in cold weather is to warm only the occupants, rather than the entire cabin. To this end, heated seats and steering wheels allow for relatively low cabin temperature without compromising occupant comfort.

While novel heating system designs and prudent planning can help to lessen the blow to EV range dealt by occupant heating, the fundamental challenge remains. On a more positive note, it is important to recognize that this obstacle only arises due to the high efficiency of EVs. In contrast, a gasoline or diesel fueled vehicle’s ability to freely heat its cabin is entirely due the inherent inefficiency of the technology that powers that vehicle.

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