A closer look at why heat pumps are dominating EV HVAC systems

Heating or cooling the interior cabin of an EV substantially affects the vehicle’s on-road efficiency, and the HVAC system often gets the proverbial “rented mule” treatment. In an ICE vehicle—particularly an underpowered one—there is a visceral reminder that running the AC, at least, costs something, since there is a palpable hit to acceleration whenever the compressor kicks in. In an EV, while using the HVAC won’t detract from the maximum output of the traction motor, it does still demand energy from the battery, so there will be a hit to range. Even more perverse is the fact that the abysmal energy conversion of an ICE—ranging from 30% for a modern gasoline design to as much as 50% for a comparable diesel—means there is plenty of waste heat available for heating the cabin, whereas the high efficiency of the EV drivetrain means a supplemental heat source is required, especially in colder climates (which is also where heat pumps perform poorly, but more on that later).

If you want to heat up a space that is cold, there are a number of options, from burning a fuel to directly converting electricity into heat (i.e. with a resistor) to actively transporting heat from a colder location to a warmer one with a device called, appropriately enough, a heat pump (note, however, that it takes work to oppose the Second Law of Thermodynamics, which states that heat naturally flows from hotter to colder locations). Ignoring the “burning fuel” option as anathema to the objectives of a magazine about EVs, the remaining two options—a resistive heating element and a heat pump—are both used in EVs. Resistors have the advantage of being very reliable—they are pretty much the simplest electronic component possible—and even when not the primary source of heat for an EV, are often used as a backup for a heat pump system (for when the outside air is too cold—the same is done with residential heat pumps). 

Heat pumps, however, have two huge advantages: They can operate in both directions (that is, provide heating or cooling of the cabin air) and they can transport more heat than the energy required to operate them. This ratio of heat delivered versus input power is known as the Coefficient of Performance, or COP, and as might be expected, a resistor will always have a COP of 1—it can’t deliver more heat than the energy supplied to it, of course—while all of the various types of heat pumps can achieve a COP of greater than 1, and if the cabin air is being heated, some of the waste heat from the heat pump goes into the cabin, boosting COP further still. However, most heat pumps (specifically, those that rely on a compressible refrigerant, which is the vast majority of them) can only work across a rather limited temperature range—if it gets too cold outside, they simply stop finding heat to pump into the cabin, hence the need for a backup heat source. Also, the COP goes down as the difference in the hot and cold side temperatures goes up—in other words, a heat pump trying to heat the interior of a car (or a house) when the ambient temperature is freezing cold might not be any more efficient than a resistive heater (this is another reason why a backup heat source is often needed). Furthermore, heat pumps using a compressible refrigerant are invariably more complicated than either a simple resistive, or even a fuel-burning, heater, which generally means they will be less reliable, and said refrigerant will either be inflammable (e.g. cyclopentane), a potent greenhouse gas (e.g. CFCs, or chlorofluorocarbons), or toxic (e.g. ammonia).

Regardless of whether the cabin air needs to be heated or cooled, it takes energy to perform such work, and the energy required is proportional to the volume of air and the temperature difference, to the first degree. Given that most EVs—even most buses—have relatively small cabin volumes compared to, say, a house, it wouldn’t seem like much heating/cooling capacity would be required. Unfortunately, most vehicles of any kind have little or no insulation to prevent the gain or loss of heat from the cabin and, worse, the copious amounts of glass and metal comprising the vehicle body make an efficient collector (or radiator) of heat, bearing more than a passing resemblance to a solar thermal panel for heating water. This means that the capacity of the heating/cooling system must be several times higher than would be expected on the basis of air volume or occupant load (after all, humans, cats, dogs, etc shed heat too). How much higher the capacity needs to be is a rather difficult question to answer, as an EV, unlike a house, might find itself in Anchorage, Alaska one week and Albuquerque, New Mexico the next, so it has to contend with radically different ambient temperatures and solar heat gain values (aka insolation, or the amount of power received from the sun over a given surface area). Consequently, there is more than a little hand-waving and guesstimating that goes into sizing the climate control system for a vehicle, and whereas with an ICE one can be profligate with the heating capacity, at least (after all, for every 100 kW of power output there’s likely to be 100-300 kW of waste heat available), the cooling system can’t be so grossly oversized without penalty, regardless of the traction system.  Furthermore, the heating system exacts no penalty on range for the ICE vehicle, but definitely increases energy consumption in an EV.

Before diving into practical examples, some explanation of the—often bewildering—terms, units and equations used in HVAC (Heating, Ventilation, Air-Conditioning) is in order, starting with the most obvious, which is that it takes energy to heat up a given mass of air, water, etc, and that the rate at which that heating occurs is defined as work, or power. In the US, the most common rating for refrigeration systems and air conditioners is the BTU, but note that this is actually a unit of energy, whereas a unit of work is intended (the correct unit would be BTU per hour, then). Fuel-burning heaters or heat pumps are also commonly specified in BTU (with the ”per hour” omitted, once again), but resistance heaters are almost always specified in watts (or kilowatts). To convert between the two, the ratio is 1 W = 3.412 BTU / hr. To bring all of this into a concrete example, I’ll use the specs from the window AC unit in my shop: it’s rated at 6,000 BTU on the box (though the data label does correctly refer to “BTU / hr”) and consumes 5.1 A at 115 VAC, or 586 W, which works out to a COP of 3.0 (that is, 6,000 / 3.412 = 1,758 W of heat moved vs 586 W used), which is fairly good, all things considered.

Moving on to an actual EV example, and starting at the simpler end of things, the electrical resistance heater used in the early models of the Nissan LEAF was rated at a maximum output of 5 kW, or approximately 17,000 BTU / hr, which for comparison’s sake is sufficient for a small house or large apartment here in central Florida, despite the 8x or so difference in air volumes! Given a traction battery capacity of 24 kWh, and a power demand of 15.5 kW to travel at 60 mph (100 kph), the max range will be 93 miles (150 km). Running the heater at the full 5 kW output the entire time, however, will increase the power draw to 20.5 kW, reducing the maximum range to 70 miles (113 km), or a reduction of approximately 25%!

A better solution—and one which has been all but universally adopted by EV OEMs—is to use a heat pump, and by far the most popular type is one in which a refrigerant gas is compressed to a hot gas, condensed back into a liquid, evaporated into a gas again (absorbing considerable heat in the process), then repeating the cycle, all in a closed loop. In ICE vehicles this system pumps heat in one direction, of course, but in EVs there is a compelling incentive to make the system bidirectional (using electrically-controlled valves), at which point the higher COP of a heat pump directly translates into a reduced impact on range (at least in heating mode). In fact, if the COP in cooling mode is n, then in heating mode it is theoretically n + 1, on the assumption that all the heat from the work going into the heat pump ends up getting transferred to the hot side (along with the heat picked up on the cold side). Reality is never quite as ideal as theory predicts, but it is nonetheless true that COP for a heat pump is always higher when viewed from the hot side, rather than the cold. Another trick that can be done with a heat pump (in cabin heating mode, anyway) is to capture the waste heat from the traction inverter, motor, etc, extending the temperature range over which it can operate without resorting to a backup source such as a resistance heater. This also lowers the temperature difference between the hot and cold sides, improving the COP as a consequence of improved Carnot, or thermodynamic, efficiency (heat pumps are the same as heat engines, just operating in reverse).

The compressible-gas heat pump gets all the action because it is the most efficient (i.e. it has the highest COP), but it does have some downsides—it requires a mechanical compressor, which will wear out over time, and a refrigerant gas that is toxic, inflammable or a terrible greenhouse gas. 

A purely electric type of heat pump is possible, however, and the most common example found these days relies on a phenomenon first noticed in 1834 by the French physicist Jean Peltier. Basically, when current passes through a junction of dissimilar metals (or semiconductors), one side gets colder than the ambient temperature, while the other side gets hotter. Reversing the current flips the assignment of hot and cold sides, while causing heat to flow across the junction will set up a current—i.e. the junction will act in similar fashion to a photovoltaic cell.

The problem with these so-called Peltier-effect thermoelectric heat pumps is that they don’t have a terribly impressive COP unless the temperature difference (dT) between the hot and cold sides is kept very low (i.e. a dT of 10° C or less), and the COP peaks at a current substantially less than the maximum rated value (i.e. it declines at both low and high currents). For example, at a 30° C dT and at 80% of maximum current, COP as viewed from the cold side will only be around 0.50 for the typical Peltier-effect device! Needless to say, that’s too high a penalty to pay, hence the continued dominance of compressible-refrigerant heat pumps in any application larger than coolers for 6-packs.

This article appeared in Charged Issue 56 – July/Aug 2021 – Subscribe now.