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Old 12-23-2011, 08:50 PM   #45 (permalink)
Ken Fry
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Originally Posted by Old Mechanic View Post
Why would the electric car manufacturers and home builders not utilise the heat created by the battery pack and electric motor to provide "free" heat in the passenger compartment, at least when it was available?

Batteries are like people: they like to be warm but not too warm. Like people, they get warm from working.

So when it's cold you may need to heat them, and when its warm you may need to cool them. If you pre-warm them in the winter (while charging) then they will stay warm as they discharge... but you can't extract any heat, because you want them warm. In the summer they are happy sitting around at 100 degrees, but once you start driving you need to cool them. However, taking that excess heat and putting it into the passenger compartment would make the passengers unhappy.

Often, the temperature of the batteries is not a concern, and just passive air cooling is all that is needed (or in the north, maybe a heating pad under the batteries). But routing cabin air exhaust out through the batteries is simple, and that serves to cool them when they are likely to be too hot, and warm them when they are likely to be too cold. In the winter, on a long uphill stretch, the batteries could get too warm potentially -- but "too warm" is 130-140 degrees, so then even the heated cabin air can be used for cooling the batteries. In a simple system, blowing past the batteries and into the cabin would be unwise (due to the safety issues from gases venting from batteries). In a more complicated system (with water cooled batteries, for instance) perhaps battery heat could be re-routed to do some good, but it is very rare on a cold day that the batteries are hot enough to produce useful heat.)

The motor will eventually become warm, and in the winter that warmth could be used for cabin heat. The amount of heat available is small (if the motor is efficient) so it could be hard to cost justify the ducting and safety devices (for example, to avoid having an overheated motor emitting toxic fumes into the cabin, etc.) If the motor is already water cooled, then using the warm water for cabin heat would be relatively easy and would make sense. It might not make economic sense, because electric heaters are so cheap -- you'd almost have to have a heater radiator already in the cabin -- and most EV's would not.

In some cases, manufacturers do some of these optimizations. The Prius has all sorts of clever ideas to do things like recycle heat. In other cases, these things cannot be cost justified, especially if the vehicle is already efficient: there is just not much waste heat to be had.

Good question though -- it makes sense, but is hard to cost justify.
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