Archive for the ‘Alternative Energy’ Category

Running a Stirling Engine Using the Night Sky

November 28, 2018

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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Photons Dancing on the Head of a Match

May 1, 2014

Who cares about angels on the end of a pin? Let’s get real; how many photons are there on the end of a match?

There’s a lot of data out there to help us calculate this; one article says that the human eye can detect a candle in the dark at 30 miles. Isn’t that something? The same article says it takes at least 54 photons just for the human eye to register an event. So that match (or candle) has to get 54 photons into that fraction of your eye that actually receives and focuses the photons. But the whole eye doesn’t actually receive the photons; it’s just the black opening in the middle. The largest it ever gets is about 7 millimeters diameter, which is 38.5 square millimeters in area, or .385 square centimeters.

How many events can the human eye see in one second? If we’re looking at the match from 30 miles away, and it looks continuous, then we are receiving over 50 frames a second (though the human eye has been recorded as being able to discern and identify an image in 1/255th of a second, we’ll be conservative). If the image were less than 50 times per second, we would detect a choppiness in the image; still, overlap in the match’s photon emissions could turn a choppy image into a smooth one. But lets assume we get a continuous 54 photons, all the time, at least 50 times a second; anything less would look like a flicker off.

Now we have everything we need to calculate how many photons are coming off the head of a match!

Just put an eyeball…or just the iris, the bit that receives the light, in every spot in a 30 mile radius, add up the total number of irises, multiply by the 50 times-per-second, times the 54 photons per eyeball, and we should have the number we need.

The sphere of irises is 4*pi*r-squared. Or 4*3.14*30*30 = 11,310 square miles, or 29,292 square kilometers. Or 29,292,000,000 square meters. Or 292,920,000,000,000 square centimeters. Since each dissected eyeball (just the iris, you see) only takes up .385 square centimeters, that’s about 760,000,000,000,000 irises stuffed carefully into the perimeter of the sphere to capture all the photons.  Just as a point of interest, that’s about 50,000 times the number of human eyeballs on Earth. Guess we’ll be dissecting all the other animals, too. May as well start with fisheyes; they’re sort of gross to begin with.

Each of those eyes is gathering 54 photons at least 50 times a second, so we get to multiply the 760 million million by another 250-ish, giving us a grand total of about 190,000 million million photons off the head of a match every second. Or, just because I like a lot of zeros, 190,000,000,000,000,000 photons. Every second. From a freakin’ MATCH HEAD. We are awash in a photon bath.

Now, leave the darkness of night, and realize that when you walk outside, you are no longer looking at one tiny spot radiating onto 190 quadrillion eyes, now you have a hemisphere of 30 cubic miles of daylight radiating onto your eyeball. Well, okay, you can’t look at the hemisphere all at the same time. Your turn to do the math! How many photons are hitting your eye every second? Hint; it’s an absolute crapload of photons.