Running a Stirling Engine Using the Night Sky

I’m a failed inventor. I get strange ideas, try them out, and usually discover that I either got my physics wrong or built my prototype poorly, or that someone beat me to the punch thirty or three hundred years ago. I created what I thought was a great factoring algorithm for huge numbers and found out that Fermat developed it first. That’s the way it goes.

So here’s the latest invention. It’s not really an invention so much as an interesting application of an existing device.

Is it possible to make a heat engine that runs off the thermal differential between the night sky and the heat radiating from the ground? Stirling engines can run using fairly small temperature differences, such as the ambient air and the heat in the palm of your hand. You can get one of these hand-driven Stirling heat engines from ebay for under $50.  The real question isn’t so much whether you can make a device that runs off the heat sink of the night sky, but how much of a thermal delta you could provide to that engine.

How cold is the night sky? I’ve read that on a clear night, it can provide a radiative heat sink of -70˚C. Yeah, that’s negative 70 degrees centigrade. Pretty cold. Those with a background in heat transfer physics know that the other two forms of thermal transfer are conductive and convective, and with the right glass or plastic covered chamber, you can minimize those thermal paths so that your heat source/sink sees only the -70˚C of the night sky. This is why some telescopes have a problem with their optics freezing up. Really. It also explains why some windshields frost up even though it doesn’t reach freezing temperatures outside, and other related phenomena.

The other side of your Stirling heat engine could be getting its energy from the radiation from the ground, say, about 15˚C. Or if you heated up a tank of water during the day, maybe 40 or 50˚C. You could run your engine easily with a delta of 100 degrees, though as you thermal guys know, the engine would be a lot more efficient at higher temperatures during the day shift.

But, the hypothetical question is if you could run an engine off the night sky. My speculative answer is “yes”. What’s nifty about the ability to do this? Well, instead of dumping HEAT into the atmosphere to generate energy, which every power source on Earth currently does, you are actually dumping COLD (a.k.a, removing heat) into the atmosphere to run your engine. The net heat loss of your engine is NEGATIVE.

So yeah, we could cool down the Earth and generate energy at the same time. Crazy, huh? The biggest problem with the idea is that you could generate a lot more energy a lot more efficiently using a hot solar Stirling engine during the day at the same cost, while dumping more heat into the atmosphere.

And cost seems to drive everything except our self-preservation instinct.


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