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Would it be feasible to use a small Stirling (like the coffee cup or
MM-60) to recharge batteries? I'm talking AA, AAA, C type batteries.
I assume that it is possible, but does anybody know how long it would
take to do charge a battery?

Another thing that I was wondering about was the
possibility/feasibility of using the temperature difference between
the inside and outside of buildings in cold climates to produce
electricity. Any comments?

Thanks,
Duncan

I'm glad you brought this up Duncan as I've been thinking along the same lines.
To trickle charge the batteries you mention takes about 1/10C, or about a tenth of the total battery capacity. For example, if you have an AA battery with a rating of 1400mA/h, then you'll need to generate about 140mA to charge it in any reasonable time. The more current you can generate, the faster the battery will charge – to a point. Too much current will damage the battery. To estimate how long the cell will take to charge use: a charge rate of 0.1C will take about 10 hours, 0.5C about 2 hours, and 1C about an hour. I always tack on another 30% in time just to be sure (as heat is generated during charging).

I think the best, and safest, way to go about this project is to find a DC charger already built, and supply it with whatever voltage/current it needs. Let's guess that your charger has an input requirement of 12V and 3A maximum. You'd need a Stirling that can, after all losses, generate 36 watts of power. That's a lot of power for a little Stirling. It could be done, but you wouldn't want to use a low-temperature-differential type. Perhaps a Stirling powered by some type of combustion process would be better suited. Love to see it!

Second question:
Stirling's change a difference in temperature into mechanical work. There is a formula to figure out what the theoretical maximum performance (conversion ratio) is: measure the temperature difference, and divide it by the hottest temperature (do it in Kelvin). For example, say it is –20C outside, and 20C inside. Change to Kelvin (K = ° C + 273) And you get: 293K minus 253K divided by 293K = about 14% efficiency.
That is the theoretical maximum; in reality you can expect to get much, much less. Perhaps you can convert 20% of that theoretical 14% into mechanical work if you're lucky, or about 3% efficiency. Then if your generator is running at, say, 60% efficiency, you total now drops to under 2%.
If you were to dissipate the heat from within your building to the outside through a Stirling, you could only expect to get about 2% of the energy flowing outside returned to you as useful work. Pretty bad eh?
A better place to find useful energy in a cold climate is probably the furnace exhaust. Use that as the hot side, and the ambient air as the cold side, and you'll generate plenty of power. Or how about the exhaust stream from a gasoline generator, or even a parabolic solar reflector? It's just a matter of getting the temperature differential as big as you can.
I've been thinking, there may be a place where a small temperature differential could be put to practical use: Antarctica. In the middle of winter it may be possible to get a 60C difference using ocean water as the hot side, and the air as the cold side. Still, the conversion rate would be pathetic, but it would be neat to try. Solar panels love cold weather, and Antarctica has some of the worlds strongest winds that could be harnessed with a wind turbine…. So the Stirling would just be used to power the penguin rotisserie.


I'm somewhat of a plebe myself; so if any of my info is incorrect and you are a Stirling guru, please correct me

I think it would. I assume you mean nicads or nimh batteries? To use a 12Vdc charger to charge a couple of 1.25V batteries would be complete waste. You'd be better generating around 1.3V - 5V, and building your own charge controller or buying one that works on a low voltage.

If you wanted to charge a 1.25V 1400mah nicad at c/10 that would be 140ma. 0.14A * 1.25V is 0.175watts of electricity. You can charge nicads without any controller, just stop charging them after a set time. So I reckon a Stirling with about 9Watts worth of heat input could charge a nicad in 10 - 12hours at 2% efficiency.

Question here: A few posts above, the thermodynamic efficiency of heat transfer is
discussed. The question becomes, what is the "ideal energy". Theoretically, if one had two
volues of matter of essentially infinite size, but only 1 degree celcius different in
temperature, one could extract an unlimited amount of energy by transfer of energy from
"high to low", albeit the efficiency would be tiny.

In my proposed application, I'd be transferring heat from hot air to cold water, but the
temperature difference is between 10 and 20 degrees C. At 10 degrees C difference, the
maximum efficiency of transfer would be 3.3%, but 3.3% of what? (Yes, I know that there is
loss of eficiency further in the Stirling engine, and further loss in the electrical generator.)
In this application, the mass of hot air is large, and I can control the size of the mass of
cold water.

Thanks!

Adam