Not an engine, but some friends of mine got a mylar sheet that's black on one side and reflective on the other. We tried it out in the desert tied onto trees/vehicles. You put the shiny side down, so the hot IR radiation of the earth is reflected away, and the black side sees the extremely cold (in IR) desert sky. If you put a little hole in the middle and put a bucket under it, you get a fair bit of water, because the mylar sheet gets about 20 degrees C below ambient and a lot of water condenses on it. (even in the desert)
Basically, have a highly reflective white coat on your roof, to reduce temperatures by about 3 Degrees Celsius.
Almost all homes in Urban India are made from concrete and bricks, which can hold a lot of heat.
I myself have been in houses that use this to cover only some rooms of the house (mainly the bedroom), and the temperature difference is definitely noticeable. It also makes the room livable in the extreme hot summers in India.
This is the opposite. It says, "Refelects [sic] 90% of solar infrared rays," because of its "High IR reflective Pigments [sic]," so its emissivity in the infrared is 0.1, but the IR-selective paints we're talking about here are optimized for high infrared emissivity, which means they absorb a lot of infrared.
Maybe there's some wiggle room here because solar infrared is mostly near IR and MWIR, and the place where we want high emissivity (absorptivity) is longwave IR, but to the extent that the advertisement makes any claims about infrared emissivity, it claims very low infrared emissivity, not high.
A paint with low emissivity across the spectrum will slow down the temperature rise when the sun is up, but also slow down the temperature drop when the sun is down. This can still make rooms livable, but it isn't the same as what you get with regular whitewash, where the temperature of the roof is actually lower than the temperature of the air around it.
IIRC, the papers they're working from mention that lime works very nearly as well as the baryta they're using. Guess what people have been painting their houses white with for several thousand years?
If anyone is interested in passive sub-ambient cooling (not for power generation, just for "free" cooling) I strongly recommend https://www.youtube.com/@Nighthawkinlight -- he has been doing a lot of experiments in this space and releasing recipes as he goes. Stuff you can do in your kitchen.
wiki article states "Up to 10,000 TWh/yr of power could be generated from OTEC without affecting the ocean's thermal structure". which converts to about 500GW which... isn't that much
10,000 TWh/y = 1e+7 GWh/y, divide it by 365.25 days/y to produce daily output of 27,379 GWh/day, then by 24 h/day to get pure power of 1,141 GW. It's still more than a terawatt, three orders of magnitude larger than the largest nuclear reactors.
> the generation of >400 milliwatts per square meter of mechanical power with a potential for >6 watts per square meter.
Keep in mind the power is fully mechanical so no electricity or control circuit is required. And based on the simplicity it seems like a good candidate to power something that you need to last 100 years with no maintenance for example.
I think the "last 100 years with no maintenance" is not likely feasible with this approach. The top plate has a coating that supports high infrared emissivity -- and I think it would need to be regularly cleaned to work well. And you can't really prevent it from getting dirty by enclosing it b/c that both substantially changes the performance and moves the maintenance burden to cleaning the enclosure.
Air bearings run dry until they get some moisture. Then they fail. Old joke about making radio enclosures: make it as watertight as possible, then drill a small hole on the bottom to let the water escape.
Until they are replaced with dust, pollution, hair, animals, leaf litter, aggressive plants, seismic events, pollen, skin particles, birdshit, fallen logs, slime mold, etc.
So what? It's research, not business. Surely you didn't expect they'd found a practical source of free energy that was ready to compete with solar but somehow nobody else bothered to try before?
https://www.i2cool.com/tideflow/uwJVdixI.html
https://baitykool.com/radiativeskycooling.html
Peak performance, I think. Considering that they got the black white sides flipped
It has pretty impressive performance.
Tech Ingredients did one or two vids as well - https://www.youtube.com/watch?v=dNs_kNilSjk
Was thinking of whipping up a batch for my rv.
https://www.amazon.in/EXCEL-CoolCoat%C2%AE-Reflective-Coatin...
Basically, have a highly reflective white coat on your roof, to reduce temperatures by about 3 Degrees Celsius.
Almost all homes in Urban India are made from concrete and bricks, which can hold a lot of heat.
I myself have been in houses that use this to cover only some rooms of the house (mainly the bedroom), and the temperature difference is definitely noticeable. It also makes the room livable in the extreme hot summers in India.
Maybe there's some wiggle room here because solar infrared is mostly near IR and MWIR, and the place where we want high emissivity (absorptivity) is longwave IR, but to the extent that the advertisement makes any claims about infrared emissivity, it claims very low infrared emissivity, not high.
A paint with low emissivity across the spectrum will slow down the temperature rise when the sun is up, but also slow down the temperature drop when the sun is down. This can still make rooms livable, but it isn't the same as what you get with regular whitewash, where the temperature of the roof is actually lower than the temperature of the air around it.
https://www.youtube.com/watch?v=5VSmBl8Rv_o
This one shows that it is not as unbelievable as it sounds :)
https://youtube.com/shorts/9KuTdPGqhVo
About two orders of magnitude weaker than solar panels, even over 24 hours.
E = (T2-T1) / T2
Not sure if you can get the MTBF on Stirling engines higher than on LFP batteries, though.
https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio...
[0]: https://en.wikipedia.org/wiki/The_Millennial_Project
There's a shore-based research OTEC in Hawaii, but the best is a floating, closed-loop OTEC in the ocean.
Source, page 39 of the full report:
https://www.iea.org/reports/global-energy-review-2025/electr...
10,000 TWh/y = 1e+7 GWh/y, divide it by 365.25 days/y to produce daily output of 27,379 GWh/day, then by 24 h/day to get pure power of 1,141 GW. It's still more than a terawatt, three orders of magnitude larger than the largest nuclear reactors.
https://youtu.be/duuk_r--lqU?t=99
Even though the video uses the sun to heat the oil, I would think it would be feasible to use geothermal heat instead.
Keep in mind the power is fully mechanical so no electricity or control circuit is required. And based on the simplicity it seems like a good candidate to power something that you need to last 100 years with no maintenance for example.
Video of how it works: https://www.youtube.com/watch?v=Q5QEBqjkNjo