My car is cold

By Marcus Wilson 16/10/2013

The last couple of days have seen our Engineering Design Show. This is where our 2nd/3rd/4th year Engineering students get to talk about and show off the various projects they've been working on in the last year. It's very interesting to see the range of activities going on, and there are some 'competitive events' – this year the 3rd year mechanical engineering students had to design a seed-planter to automatically put pine seeds into seed trays – accurately, quickly and cheaply. 

For me, the stand out talk was by one of the fourth years – Matt Dromgool. He talked about a solution to the problem of heating in electric cars.

This problem is something that had never occured to me, though it is extremely obvious when one thinks about it. In a petrol/diesel car, providing heating to the interior is easy. Just blow in some of the waste heat from the engine. A small radiator and fan does the job and often has the added bonus of saving your engine from overheating when the main fan fails. The one obvious disadvantage of this method is that you don't get instant heat when you start the engine from cold. When it's minus ten or lower outside, instant heat is often what you want. Some cars have wire-mesh heating elements built into the windscreen for quick defrosting on cold days. 

However, an electric engine doesn't generate much waste heat. There's not enough to warm the interior of the car (and, more importantly, the windscreen) adequately. So how do manufacturers of electric cars solve this problem? The simple solution is to stick in a resistive heater – just like a bar element on an electric fire. But where does the energy come from? From the battery, of course. The trouble is, to heat the car, you need a lot of heat. Matt put up some data from one electric car manufacturer that showed the range on a full charge dropping from about 160 km to about 50 km when the heater was turned on fully. That's a pretty severe drop in performance. 

Ironically, a lot of electric cars carry air conditioning systems. That's because many are little more than petrol cars with the petrol engine pulled out and an electric one slotted in – not much else gets changed – and the air conditioner stays in place. So an obvious solution is to allow  the air conditioner to run in reverse – to move heat from outside the car to inside, not the other way around. This is what a domestic heat pump does – the same system can either heat your house or cool it depending on the direction in which the fluid flows. So Matt's project looked at converting a conventional car air conditioner (which just does the cooling bit – it has no need to supply heat in a petrol car) and modifying it to allow it to run on a reverse cycle as well, to do the heating bit. A nice, simple, cost-effective solution. Sure, it still runs on electrical energy, but it uses much less energy than stuffing a resistive heating element in the car. It also carries the advantage that one can now get instant heating when required on really cold days. 

I wonder when we'll start seeing them in electric cars. 





0 Responses to “My car is cold”

  • The commercially-available Renault Zoe electric vehicle already has a reversible heat pump. I suspect there are others, but that’s one I recall from reading a review.