Cars are frequently hotter than their surroundings when parked in the sun, and i imagine that in sunnier parts of the world the situation continues when the car is driven. Currently, the solution to this appears to be to use energy from the engine or possibly the battery to power an air con unit, thereby reducing the fuel economy.
I think there's another solution to this: use Stirling engines to reduce the temperature difference between the inside and the outside of the vehicle, then either store or use the energy generated to help the engine rather than hinder it. While it's parked in the sun, keep it cool with Stirling engines and store the energy generated, maybe in a flywheel, battery or electrolysing water to produce hydrogen fuel. When the car is started, add this energy to the engine.
When in motion, the interior of the car is still being heated by the sun but its exterior is being cooled by the movement of air over the body. This could also be used to power Stirling engines for the same purpose.
The result is a car which uses less energy for air conditioning, stays cool in the heat and is somewhat more efficient.
-- nineteenthly, Jun 05 2009

Stirling engine wikipedia page http://en.wikipedia...#Automotive_engines
[mitxela, Jun 06 2009]

The point is really that it may be fairly simple to cool a car without using energy from the engine. It wouldn't provide it with a huge boost, but it would be "free" in the sense that the greenhouse effect which heated the car in the first place provides the negative entropy. Right now, air conditioning unnecessarily wastes energy.
-- nineteenthly, Jun 06 2009


I think the general view on using Stirling engines in cars, for whatever purpose, is that it's a great idea, but until they make them reliable enough it isn't worth the hassle.
-- mitxela, Jun 06 2009


Ah, but this isn't so much about powering the car with a Stirling engine except just minimally. If the engine broke, the car would still go, it'd just be hot inside in certain weathers until the aircon's replaced or repaired. It won't grind to a halt, just have slightly higher fuel requirements.
Thanks for the link, and i quote "The main difficulties involved in using the Stirling engine in an automotive application are startup time, acceleration response, shutdown time, and weight". Dealing with those one by one:
* Startup time - the engine is triggered by thermostats and an AND gate type arrangement which turns it on when the temperature difference gets beyond a certain amount, so the work may have already been done by the time the driver gets in.
* Acceleration response - it doesn't need to accelerate.
* Shutdown time - again not a problem because the car is only going to reach the temperature of the outside, possibly including wind chill.
* Weight - I suspect the weight would be more of a problem for a main engine. This isn't a main engine, just an air conditioning system, and if car air conditioning involves pipes, pumps and in particular fluid, that's a substantial weight in itself.
-- nineteenthly, Jun 06 2009


Instead of using a Stirling engine, you could just use a thermopile. The temperature gradient set up between the hot inside and the cooler outside would induce a voltage potential in the thermopile and you could just power a small A/C device.
-- WhereYouAt, Jun 07 2009


//a small A/C device// AC usually takes a lot of power, turning it on will normally drop your fuel consumption by at least 10%.
-- mitxela, Jun 07 2009


This is the thing. If you use the heat itself as the source, you're maximising the efficiency. A thermocouple is not going to remove much heat in itself. It would remove a little and a larger size or higher number of thermocouples would remove more, but a Stirling engine would remove the heat in itself, rather like a heat sink. A heat sink, however, would be hard to streamline.
-- nineteenthly, Jun 07 2009



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