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I'm interested in electric cars and a specific model I'm interested is not equipped with a heat pump.... but does have AC. The discussion across the internet usually cites cost as a reason they did not include the heat pump. Given that an air conditioner IS a heat pump, and in their simplest forms the only difference is valving to make the refrigerant run in reverse, I assume that in practice there is something more complicated involved.

In practice, what are the differences between a "bi-directional" heat pump and a "uni-directional" AC/heat pump that would contribute to cost differences? I'm interesting in both specifics relating to vehicles, but also anything that might apply generally.

Sorry if this question is too engineery for this site?

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  • Glancing at the DIY EV forums it seems that people are doing this, but until a major EV manufacturer puts together a product the short answer is "making just one costs too much". Viz, economies of scale overwhelm everything else. Vehicle heat pumps need to cope with a lot more vibration than stationary ones (and there are a bunch of other design considerations) that make it impractical to use stationary heat pumps in cars. I suspect that until EVs standardise somewhat it won't be worth a third party manufacturer designing on on spec.
    – Móż
    Commented Oct 21, 2021 at 0:41
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    One place you do see reverse cycle aircons is in campervans and housetrucks, not just DIY ones but the more expensive commercially made ones too. But those are large and power hungry, so not suitable for cars (also, you probably don't want 10kW of heat in your car).
    – Móż
    Commented Oct 21, 2021 at 0:43

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This is slightly speculative, but I came across a sensible answer. In an electric car, the cooling system cools the cabin but also cools the battery. In some conditions, the passenger may request heat while the battery needs cooling. So to offer a bi-directional heat pump for the cabin, two systems become necessary, one for the cabin and one for the battery.

I saw the info about the shared cooling system in this walk through of the chevy bolt: https://youtu.be/5gDILvW5yMg?t=188

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    It's a good point, but a more complicated single system could pump heat from the battery/electronics into the cabin without the need for a second cooling loop and compressor
    – Chris H
    Commented Dec 21, 2021 at 14:05
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Whilst a heat pump is more efficient than just using direct electric to thermal energy e.g. using electrically resistant wire, the tiny air volume of a car does not need much energy to heat it so it might be that over the life time of operating the vehicle you could save X cost from a more efficient heat pump but it actually would cost you, the consumer and the planet, more in terms of emissions, natural resources, manufacturing costs etc to use a heat pump to do it.

Maybe. I'd like to see some back of the envelope calculations though before I was confident in this answer.

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