heat exchanger

[Jim GM4DHJ ...]

how does a heat exchanger heater work in an electric car?...how can it be more efficient than an resistive heater?...where does the energy come from ? ....

[Jim GM4DHJ ...]

does it take waste heat from the electric motor? ....can't think where else it would get the heat from.....

[John Rumm]

The better cars will have active cooling for the batteries. Many models also have air source heat pumps for the heating / air con.

[Jim GM4DHJ ...]

but where does the free heat come from? ....say in the nissan leaf where the batteries aren't cooled .....

[John Rumm]

Older models of Leaf just have resistive heaters - so no "free" heat. Newer ones an air source heat pump. So the heat is extracted from the outside air, which will end up colder than it was.

[Jim GM4DHJ ...]

how that work then ? ...

[Brian Reay]

My Outlander (a hybrid) has a coolant system for the motors. I assume, but I?ve never checked, it helps heat the interior in cold weather.

It seems to be similar to that used in a normal engine, you check in the same way etc. I?ve not checked how they organise the ?plumbing? but there are two header tanks so the systems aren?t common.

[John Rumm]

Exactly the same as a fridge... it has a hot side and a cold side. It pumps heat from the cold side to the hot. So with the fridge its from inside the box to outside. With air con its from inside the car to outside, and when operating the other way round in heat pump mode, from outside to inside.

In cars its typically doe by repeated phase changes of a high volatility low boiling point refrigerant. Vaporising and absorbing the latent heat of vaporisation, in the cool bit, then later condensing and giving up that latent heat.

On a smaller scale it can also be done solid state with Peltier effect junctions.

[Jim GM4DHJ ...]

thanks I understand now......

[Jim GM4DHJ ...]

I was thinking that is how it would work ....

[John Rumm]

A term you will often hear mentioned is "CoP" of Coefficient of Performance. So a heat pump that can move 3kW of heat for an electrical input of 1kW is said to have a CoP of three.

That means in cooling mode, you get rid of heat at 3kW and have to pay for power at 1kW to drive it. For heating you get heating at 4kW, for the same cost as 1kW (i.e. the heat pumped and the waste heat from the system ends up "inside")

[Roger Hayter]

Presumably a finite minimum amount of work has to be done to compress the gas for a given amount of latent heat. Do you know what the theoretical maximum CoP is for a typical gas, and do we get anywhere near it?

[newshound]

For a given gas, the COP depends on the working temperature (and pressure?) in the circuit and drops off when you are away from the "design" conditions. Presumably commercial systems are optimised to be working near the maximum. I would not be surprised if older systems with bad greenhouse gases performed slightly better than modern, greener ones.

[John Rumm]

Not something I have looked at in much detail, but this seems to cover it:

[tabbypurr]

modern fridges generally eat a good bit less electrickery than old 70s/80s R12 ones.

NT

[newshound]

It does indeed, great link!

[newshound]

Better thermal insulation because of the introduction of rating letters?

[Roger Hayter]

Thanks. Loosely speaking the unsurprising result is that the lower the temperature difference the more efficient is the heat transfer. Very interesting.

[John Rumm]

Some modern ones use inverters to drive the compressor, so they can modulate the output and load match.

[tabbypurr]

It's a combination of factors. I don't recall what the impact of the refrigerant is.

NT