Here in Brazil we use a system with copper pipes in an insulated "box" to heat water during the day using this energy from the sun. A couple of years ago, in a cold winter day with a minimum of 5 degrees (celsius), just after the sunrise the water froze, broke the pipes and the glass above it. I couldn't understand what happened because the ambient temperature was above freezing point, maybe it was something like this effect?
Fascinating! That does seem like the most likely explanation.
This reminds me of the ancient ice ponds that made ice thousands of years ago in Persia. I read somewhere that they were able to make ice through a combination of radiative and evaporative cooling at night temperatures around the same as you experienced, about 5C.
Yes, I like this explanation, when that phenomenon happened in our house I thought one of our neighbours was dumb because he just put a blanket over the collector on the roof, as he said, "to keep it warm". But now I think he was right. Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating.
>Now, I don't know if this exists but it would be nice if there was a kind of glass that let radiation pass only to one side and not the other way, a kind of "valve", this could solve the problem of water frozing from radiation in our solar heating.
This doesn't exist and can't even in principle because it would be a fundamental violation of thermodynamics. Basically, it would be a Maxwell's demon for radiation that would allow you to arbitrarily reduce entropy.
But, it might be possible to have a material that has different characteristics at different temperatures, as long as it's symmetric. If it's nearly opaque to IR in its cold state, hopefully sunlight at dawn would warm it rapidly enough that it would automatically "shut off" on cold nights, and still "turn on" shortly after dawn even on cold days.
Yes I think that'd be ok. You'd basically just be taking advantage of the thermal gradient between the radiating body and the object being heated. It'd be analogous to adding more insulation as it heats up and removing insulation as it cools down.
You'd be slowing the flow of heat into your reservoir just as much as you'd be slowing down the loss of that heat later though.
You might still get some benefit in preventing freezing at the expense of needed a larger area to get the same amount of heat flow in to your water system though.
I don’t think maxwells demon is impossible, it just is a device for converting información to energy. In theory, many such devices may be possible for the conversion of information to both energy and matter. This seems like it might be support the simulation hypothesis.
The handwavey explanation in my head is that “ambient” for this emitter is not just the immediate physical environment, but also deep space, which is very very cold. This wouldn’t work if the temperature of space and the air were the same.
An emitter is also an absorber so if space were as hot as the emitter then it would not shed heat.
Yeah, on a clear night the pipes radiate heat to space but nothing much radiates the other way as space is cold and dark. It's also why the tops of cars get frosty on clear nights.
a thing to keep in mind with research papers is that in many cases they're concerned with finding how to get the largest effect, or demonstrate the effect in a way that's most clearly due to the reasons they claim and not some experimental error, or measure the effect most precisely, and only in rare cases are they concerned with how to get the effect most cheaply, a consideration which conflicts with the others
there are a lot of cheap materials that are transparent in the thermal infrared, like ldpe, potassium chloride, sodium chloride, silicon, and rock crystal. they're mostly a pain in one way or another (not that i have any experience with this)
Checking my understanding here: in a very simplified sense, is this saying that we’re taking energy, be it from ambient temperature during the evening or that plus solar radiation during the day, and essentially bottlenecking/‘forcing’ it to radiate in the spectrum that’s atmospherically transparent and can thus escape to space? Seems the use of insulating it in a vacuum would be to minimise heat loss until it’s cooled to the respective temperature that radiates in that specific spectrum.
You might like NightHawkInLight's videos on the topic[0] and Tech Ingredients[1] - they've been working on making paint using this technology with an at-home DIY process and experimenting with it.
You should take an infrared thermometer and point it to the clear sky. If the sky is very clear, you will get temperatures much lower than ambient. You just let heat radiate away, and this is enough to cool.
Why isn't this replacing aircon over the whole world? Unless that IR-transparent window is made of gold dust, it has to work out cheaper than the energy cost of running an air conditioner.
If my napkin math is right, solar panels give around 2x higher electricity production per square meter than these devices give cooling power. Then that goes into a COP of ~3 for your aircon, and the solar panels have have a factor of 6x better cooling performance per area covered.
Then factor in that electricity can be used for lots of other useful things than cooling, and that solar power variation during the day is perfectly matched with cooling demand, it's a no brainer.
Adding to everyone else, but local climate makes a big difference. This isn't very useful unless you have dry still air, and it works best at night, but could maybe work during the day if you have a good view of the sky but have blocked the sun.
Most places with dry still air don't need specific cooling at night, because "everything" already cools due to this effect, and it doesn't stay hot for long after the sun goes down. Areas that need a lot of cooling overnight tend to be humid, which disrupts radiative cooling. Also, it doesn't take much airflow for convection to transfer more heat than radiation, and most places have variable wind... so you can't really count on it.
> Most places with dry still air don't need specific cooling at night, because "everything" already cools due to this effect, and it doesn't stay hot for long after the sun goes down.
Exactly this. This is reflected as well in the article, even if not promintently: they have 30C Celsius at midday (which is hot) and 4-5C at night (which is cold). 25C between day and night is close to desert behavior as far as diurnal air temperature variation [1] is concerned.
my air conditioner is rejecting 3000 watts to the outdoors right now, even though my bedroom is the same temperature as the nighttime outdoors. doing that with radiative cooling would require 8 square meters of emitter area at 400 watts per square meter and a clear, low-humidity sky (not necessarily night, but in the daytime, you additionally have to keep the sun off the panels). my air conditioner's condenser is significantly less than 1 square meter as seen from the sky and also works when it's cloudy or humid outside
basically forced-air convection heat transfer is a motherfucking miracle, and vapor-compression refrigeration even more so
Theres videos on youtube by a dude that does Science experiments at home making radiant paint from common stuff, it's really good and easy to use and shows solid reductions below ambient.
Simple plastic film stretched over the opening would be a good start, perhaps two layers with some air between. You lose a little in radiation but gain heaps from stopping convection.
Consider this: quite powerful solar thermal generators [1] and furnaces [2] have been built, which use mirrors to concentrate sunlight. Do they create intense cold at night? No, because you can't focus coldness. The device in the article, and the one described above, depend on shielding a volume from terrestrial heat sources, and are effective only because the heat being transferred is minimized.
We've been pretty satisfied with Room and Board furniture, which is typically made in the US. While a bit more expensive it's not really that different from the $3K garbage price point and seems to last (so far).
Bluey is great. For younger ones, Tumble Leaf (sadly now discontinued) is also wonderful - clay-mation, gentle with lovely music and promoting a sense of discovery.
Most materials, aside from metals, of sufficient thickness have high high infrared emissivity. All white paints, for example, already did radiate heat upwards through the atmospheric window. These more reflective films are no more emissive than previous paints (they're all quite emissive to begin with!)
The only difference, optically speaking, really is that they are better solar reflectors.
The difference is that the wavelengths in which they reflect and emit are different. Where you emit, you absorb. White paint has a lower solar reflectance because it emits and absorbs across a broader band. The better PRDC coatings absorb/emit more tightly in the atmospheric IR window.
There’s been a ton of development in super reflective materials since the first demonstration of daytime radiative cooling at Stanford in 2014. Ultimately though, reflectivity does drop with dirt and pollution exposure over time, even for these kinds of materials. So most super reflective materials plateau at a lower number long term outdoors.
Also, just for reference, Spectralon, a sintered PTFE reflectance standard has had this level of solar reflectance for decades. So, in a sense, not that much new here. a sintered ceramic is gonna be expensive and will have a hard time competing with the simplicity of a paint based approach. Super white paints using various pigments have been well studied the last 5 years.
IMO simply being reflective is a dead end. Future materials will have to absorb wideband energy and actively emit it in an atmospheric bandgap to be effective enough to matter. I would think this would be an excellent application for quantum dots and have been sort of low-key waiting for an announcement or paper to drop for the last year or two.
We also need to reduce aircraft contrails. Contrails increase high-altitude clouds (cirrus and related), which blocks the heat that would have been lost by radiative cooling. It isn't hard to reduce contrails. We have the atmospheric data telling us the height of and where clouds are likely to form. Aircraft need to change altitude by 2000 ft from what was planned to avoid the cloud-forming regions.
That's not how clouds in the atmosphere work. Lower clouds reflect infrared back up and away from Earth. When large oceangoing vessels reduce their soot emissions, the formation of low altitude clouds declined letting more infrared in to the surface and increase the amount of heating in the lower atmosphere and surface of the earth. On the other hand, contrails hold heat in and prevent it from radiating out into space.
The "ideal" situation would be lots of low altitude clouds and no high-altitude clouds like Cirrus and contrails. This conjecture comes from the known property of low altitude clouds reflect the heat up and eliminated heat retention properties of high-altitude clouds.
We already pay to clean lots of buildings, and rain should help with that too, no? Seems like this would be a very solvable problem to me, but maybe I'm misunderstanding the problem fundamentally.
The less rain you get (i.e. during a drought) the more you tend to get dust in the air. We went through this last summer; my dog would kick up clouds of dirt dust running through our yard.
Worse than this, though, is the durability of the material. Lots of these super reflective paints don't hold up very well to rainwater (which itself is not especially clean or PH neutral) or seasonal extremes.
Typically, you wouldn't want to paint the sides of buildings, as that'll just reflect the light mostly down. You want it on rooftops, which most people don't pay to clean frequently or at all.
This makes sense; I guess it just becomes a new added expense with no major benefit aside from maybe saving on some cooling costs? Seems like a thing you'd be able to convince a company to buy into completely, but only once it's cheap and durable.
As a sidenote, this is from Gang Chen, former head of MIT's Mechanical Engineering department who was (rather shamefully in my opinion) wrongly targeted by Trump's China initiative. The government finally dropped the case acknowledging there was nothing there.
Some of this is universal, but much of her story is particular to how US med schools operate: their research faculty tend to be largely soft money in nature, so grant money is even more necessary than in other 'hard money' jobs in non-med school fields. Such a system is destined to fail when geniuses like Kariko pursue risky new territory for which large grants are hard to secure.
The really distasteful thing here is Penn as an institution. They have reaped the benefits of her work in terms of mRNA patent royalties (a very large number I believe), and of course reputationally. Yet, they treated her truly terribly and have never - and it seems like will never - acknowledge it. For example, Sean Grady, mentioned here as the one that essentially cleared out her lab in 2013 without telling her is the chair of neurosurgery at Penn Medicine. Will he apologize? I doubt it.
Some of this is universal, but much of her story is particular to how US med schools operate: their research faculty tend to be largely soft money in nature, so grant money is even more necessary than in other 'hard money' jobs in non-med school fields. Such a system is destined to fail when geniuses like Kariko pursue risky new territory for which large grants are hard to secure.
As a professor who served on a hiring committee this past year in a top-15 engineering department in the US (and not in CS), some of this did not really ring as true to me. Overall, I came away more pleasantly surprised by our process than how I might have assumed it to be as a PhD student (where I assumed it was entirely based on favoritism). Based on the intersection of publication record, research statement and topic areas we were recruiting for this cycle (which was clearly advertised in our position), there were really only about 20-25 applications which were plausibly competitive.
We ended up doing a standardized set of Zoom interviews for about 15 of them - here too, many candidates were simply unable to clearly explain their past work and make a clear pitch for their vision and work (even with slides). This is an essential skill for running a lab and one any prospective candidate should practice and refine! From that point on the whole department was involved and, at least for us, everyone had an equal say and felt safe speaking up. ~4-5 were invited for on-campus interviews and one candidate was liked the most by nearly everyone, leading to a consensus.
As a public university, there are a lot of things we do in a standardized way, and sets of personal questions etc. we do not ask. Is it fair? I'll leave others to judge, as selecting one person to hire out of ~200 applicants is not an easy task. But I don't think it was capricious either. At our department, we intend for everyone who comes as an assistant prof to be tenured, so it is very much a long-term hire and commitment we are making. We take that job seriously.
> "...many candidates were simply unable to clearly explain their past work and make a clear pitch for their vision and work (even with slides)..."
This is interesting, why do you think they fumbled? Is it a lack of practice/experience in presenting these ideas or is it an indicator of a grad student who has become a foot soldier for a PI?
I'm genuinely not sure! The virtual pre-screen interview has become the de facto norm in my field, so it's not like it's surprising or unexpected. And it's not like we are looking for TED speakers here - anyone should be capable with sufficient practice and preparation.
If anything, I've seen people that look great on paper, coming from the pedigreed lab/ and fancy PhD institution fail at this stage. So perhaps there's some overconfidence there. Indeed, no one in our department had any connection to, or prior knowledge of, the candidate who was ultimately given the offer. Once they got past the initial screening, through both virtual and in-person interviews, their technical depth and breadth, teaching skills and overall collegiality shone through.
All that being said, I don't doubt that all the questionable things mentioned in the article above occur, and perhaps widely. But, there is hope! And there's no better place to start than those of us who are younger faculty.
