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)
wombatpm 2 days ago [-]
Now you just need a couple of droids and you could go into the moisture farming business
sigmoid10 2 days ago [-]
What I really need is a droid that understands the binary language of moisture vaporators.
timdiggerm 2 days ago [-]
But can he speak Italian lawn games?
nmeagent 1 days ago [-]
I was going to look into this once, but instead I opted to go into Tosche station to pick up some power converters and unfortunately never quite got around to it.
1 days ago [-]
jeffrallen 2 days ago [-]
Nah, those are not the droids he's looking for.
viraj_shah 2 days ago [-]
Fascinating 20 degrees C is huge. What's a fair bit of water? At what time of day and how long did you collect water?
gsf_emergency_6 2 days ago [-]
These guys claim >40degC, and are deploying in Dubai..
Peak performance, I think. Considering that they got the black white sides flipped
cameldrv 1 days ago [-]
I didn't do the measurment and it's been a while so it's possible I misremembered the temperature delta, or maybe it was degrees F. It was about a 2-3 square meter sheet and it made about a liter of water overnight.
nickdothutton 1 days ago [-]
I think I read something similar in a "Boys survival book, desert chapter" in the early 80s.
kragen 2 days ago [-]
Wow, that's awesome, and a much bigger temperature difference than I would have guessed. Did you get frost?
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.
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.
kragen 2 days ago [-]
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.
schiffern 2 hours ago [-]
To be fair it does say "reflects solar infrared rays," which I would interpret as "the IR from the Sun" (aka NIR).
The product datasheet[0] claims a thermal emittance (aka LWIR) of 0.82. Having such a high value is typical for non-metallic surfaces.[1]
It kind of blew my mind when I first learned about this whole phenomenon (mostly from the YouTube series I posted). Not all white paints are equal and it’s kind of interesting to think that something that looks mostly identical to our eyes has very different (passive) properties in the infrared.
I think one of the things in the paints that Ben adds is a set of microspheres that reject incident incoming infrared beyond a certain angle but allow it to pass through when radiated. Something like that.
kragen 2 days ago [-]
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?
marcosdumay 1 days ago [-]
Not with the optimal mixture for maximum-packing of limestone nanospheres.
That effect is almost not perceptible in normal milled limestone.
gsf_emergency_6 2 days ago [-]
Vid of the engine in action, from the team that made the paper
There's a shore-based research OTEC in Hawaii, but the best is a floating, closed-loop OTEC in the ocean.
AstroNutt 2 days ago [-]
Interesting link.
I would think step 7 would come before step 6 though.
I thought about this for a few minutes and can't come with a reason otherwise.
adastra22 2 days ago [-]
The timelines are increasing powers of 2. It’ll take much longer to colonize all asteroids than to settle Mars.
andbberger 2 days ago [-]
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
pezezin 2 days ago [-]
10 000 TWh/yr is one third of the current total electric energy generation of the whole planet, is not a small amount.
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.
andbberger 2 days ago [-]
oops. yes. still not that much though. i mean it's a lot but it's "one more large industrialized country" a lot not "kardashev 2" a lot
IAmBroom 1 days ago [-]
Those goalposts of yours are on a FTL ship...
WJW 2 days ago [-]
Kardashev 2 has a Dyson sphere. Of course anything on a single planet can never have that much.
I've been stalking the citations for this paper for a while now. Surely people would be scrambling to replicate these results. It could truly be transformative for the world if it works and is scale-able.
The science looks good to me
Animats 2 days ago [-]
> 400 milliwatts per square meter
About two orders of magnitude weaker than solar panels, even over 24 hours.
E = (T2-T1) / T2
kragen 2 days ago [-]
Yes, but it works at night!
Not sure if you can get the MTBF on Stirling engines higher than on LFP batteries, though.
gsf_emergency_6 1 days ago [-]
And in the day time, if transparent and applied to solar panels, the "efficiency" gain (~10W psqm) itself will dwarf other considerations.( Remembering that bulk of radiative cooling shouldn't happen below ~1300nm) And then there's beating the efficacy of carbon capture at mitigating warming by orders of magnitudes
Nothing to sneeze at. Just be careful of midbrow high-effort dismissals from the old and wise:)
"Case Study 2: Solar Farm in Dubai
Problem: Solar panels lost 15–20% efficiency at 55°C+ temperatures.
Solution: Coated panels with i2Cool’s film.
Results:
Panel surface temperature: ▼25.7°C (from 58°C to 32.3°C)
Power output: ▲8% (equivalent to adding 2,400 new panels to a 30MW farm)"
scoopertrooper 2 days ago [-]
>400 milliwatts per square meter of mechanical power with a potential for >6 watts per square meter.
Animats 1 days ago [-]
What miracle gets a Stirling engine to be 15x more efficient? Stirling engines have been around for over a century.
burnt-resistor 1 days ago [-]
RF energy harvesting in urban areas results in about 0.5-5 mW/m^2. I would guess it would be about 1-2 orders of magnitude less in rural areas.
This is like shaving nickels to make money.
Certainly, there are better energy sources like the fusion reactor in the sky and building a fusion reactor (that's perpetually 30 years away).
TIL: Active nuclear reactors of all types around the world are mappable using antineutrino detectors. It would probably also expose the location of every stationary nuclear-powered ship and submarine too.
kogasa240p 2 days ago [-]
Since we're talking about stirling engines, I've always wondered how using geothermal heat for a larger stirling engine would work.
In one of the later Foundation series books, Isaac Asimov had a whole world run on this.
coder543 2 days ago [-]
Which book? Which world? I don't remember this, but it has been a few years.
monegator 2 days ago [-]
foundation and earth, if i recall correctly
blacksmith_tb 1 days ago [-]
Stirling engines are of course fun, but I wonder if the same approach but with the specially-coated radiator on a Peltier instead would net much - it'd avoid the moving-parts problem, at least.
AnimalMuppet 2 days ago [-]
400 milliwatts per square meter? That's interesting that they can do it at all, but that level is completely impractical for real use.
15155 2 days ago [-]
This is plenty of power to run a microcontroller and a radio (sporadically) with an energy-harvesting setup.
aetherspawn 2 days ago [-]
> 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.
abeppu 2 days ago [-]
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.
AnimalMuppet 2 days ago [-]
Mechanical things don't usually work for 100 years with no maintenance. Bearings run dry, if nothing else.
ufocia 2 days ago [-]
Air bearings always run dry without problems.
yetihehe 2 days ago [-]
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.
contingencies 2 days ago [-]
Until they are replaced with dust, pollution, hair, animals, leaf litter, aggressive plants, seismic events, pollen, skin particles, birdshit, fallen logs, slime mold, etc.
IAmBroom 1 days ago [-]
No moving parts in open water last without maintenance. Life, uh, finds a way.
foxglacier 2 days ago [-]
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?
nrhrjrjrjtntbt 2 days ago [-]
It is interesting to know if it has potential (pun intended) for some use. Even if that is some very niche thing.
bilsbie 1 days ago [-]
I wonder if you could harness different temperatures at different water depths.
Just like this scheme, it's not very economically efficient.
Carnot efficiency is proportional to the temperature ratio between the hot end and the cold end in degrees Kelvin. If both temperatures are in the 200's, then efficiency will be low.
OTEC does provide lots of potable water though, so that's one advantage.
burnt-resistor 1 days ago [-]
Davis is so frickin windy, wind power would be my first choice.
Secondarily, using a deep ground source heat pump to power a Stirling cycle engine would probably be much more powerful than harvesting a few mW from ambient temperature gradient between surface and air.
That's my 2 centidollars.
carabiner 2 days ago [-]
Cost to build, maintain this machine? $/watt?
rriley 2 days ago [-]
Great! Now I desperately need this Stirling engine for my morning coffee: https://a.co/d/6Ja2LeF
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.
The product datasheet[0] claims a thermal emittance (aka LWIR) of 0.82. Having such a high value is typical for non-metallic surfaces.[1]
[0] https://5.imimg.com/data5/CA/RO/MY-653008/excel-cool-coat.pd...
[1] https://ntrs.nasa.gov/api/citations/19840015630/downloads/19...
I think one of the things in the paints that Ben adds is a set of microspheres that reject incident incoming infrared beyond a certain angle but allow it to pass through when radiated. Something like that.
That effect is almost not perceptible in normal milled limestone.
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
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.
I've been stalking the citations for this paper for a while now. Surely people would be scrambling to replicate these results. It could truly be transformative for the world if it works and is scale-able.
The science looks good to me
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.
Nothing to sneeze at. Just be careful of midbrow high-effort dismissals from the old and wise:)
These guys are applying them to solar panels:
https://www.i2cool.com/tideflow/uwJVdixI.html
This is like shaving nickels to make money.
Certainly, there are better energy sources like the fusion reactor in the sky and building a fusion reactor (that's perpetually 30 years away).
TIL: Active nuclear reactors of all types around the world are mappable using antineutrino detectors. It would probably also expose the location of every stationary nuclear-powered ship and submarine too.
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.
https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversio...
Just like this scheme, it's not very economically efficient.
Carnot efficiency is proportional to the temperature ratio between the hot end and the cold end in degrees Kelvin. If both temperatures are in the 200's, then efficiency will be low.
OTEC does provide lots of potable water though, so that's one advantage.
Secondarily, using a deep ground source heat pump to power a Stirling cycle engine would probably be much more powerful than harvesting a few mW from ambient temperature gradient between surface and air.
That's my 2 centidollars.
Video of how it works: https://www.youtube.com/watch?v=Q5QEBqjkNjo