tomw wrote:
Sure the heat is going into the surrounding air, but a liquid-air heat exchanger permits much higher heat transfer than a finned heat sink
This isn't the case, Tom. It does not matter if the thing you are cooling has a solid or a liquid inside it. The heat transfer to air is set by its surface area and the amount of air moved over it.
A typical air-cooled heatsink is a block of cast or extruded aluminum. The fins are quite thick. They're easy to make, but have a rather low surface area compared to their weight.
A typical car radiator is much more efficient design for transferring heat to air. It *has* to be -- it needs to get rid of hundreds of times more heat than an EV controller! The radiator is far larger, has vastly more surface area due to having thousands of very thin fins, and get far more air blown across it. is lighter because it's using water instead of aluminum to move spread the heat around inside it.
You sometimes see this type of air-cooled heatsink as well. Instead of a cast or extruded block, it's a thin slab of aluminum or copper, with hundreds of thin sheet metal fins bonded to it. Coupled with a good fan, such a design is much more effective at transferring heat to the air -- just like a car radiator. But they cost more, so you don't see them as often.
That of course is why ice's have radiators rather than fans blowing directly on the ice.
That is a complex question. The reasons have more to do with the desire to make the engine as physically small as possible, and relocate its cooling somewhere else (to the radiator on the front of the vehicle). There are also factors like the need for hot water for a heater, and emission controls that require tight temperature regulation.
If you are using LiFePO4 cells in 115 F (46 C) ambient you can't afford a very large temperature difference between the cells and ambient due to decreased cell life at higher temperatures.
I agree. Pretty much all batteries have a much shorter life if exposed to high temperatures on a routine basis. It's the old Ahrrenius equation again -- life is reduced about 2:1 for every 10 deg C increase in temperature. 115 deg.F (45 deg.C is about 20 deg.C above ambient (25 deg.C), so have have a 4:1 reduction in life.
But there may be less complicated ways to reduce battery temperature than a liquid refrigeration system. Batteries have a tremendous thermal mass. If you put them in an insulated box, it takes days for their temperature to change. For example, you can cool them at night with ambient air that is 20 deg.F cooler, then seal them up during the day when it's hot.
-- For every complex problem, there is a solution that is simple, obvious, and wrong. -- H.L. Mencken -- Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
