A Rooftop Device That Can Make Solar Power and Cool Buildings | Hacker News Reader

A Rooftop Device That Can Make Solar Power and Cool Buildings

news.stanford.edu

63 pointsblue_devil25 comments

An interesting idea, but it's total wishcraft at this point. One significant thing the article misses is that if (big if!) they can make this work, it could significantly increase the power output of the solar cells, since PV cell voltage is inversely a function of junction temperature. (Current is a function of insolation. This is why PV panels make much more power on a cold, clear winter day than they do in the middle of a Texas summer - the extra voltage more than makes up for the smaller winter current. And why although they've never caught on for cost reasons, hybrid PV/thermal panels can make sense on two fronts.)

Afaik the radiative cooling element should not point to the sun for good performance. On the other hand you want to have your solar cell pointing to the sun. So at least one of them doesn't perform well.

Eg. on a building with tilted roof (prevalent in Middle Europe ) one would like to mount the former on the north side and the latter on the south side for good performance. Unfortunately the article does make no mention of it.

I think you can lens the lower radiation away from the sun. A fresnel-like arrangement that points light from under the panel in a direction other than that of the panel.

Solar panels can be mounted on a moving playform.

It's been a while since I brushed up my physics, but that's not how it used to work, IIRC. Every body (in the physics sense) radiates heat, and that's independent of the incident energy. When the incident energy is more than the radiated energy, there is net energy transfer into the body leading to temperature rise.

"...That Can Make Solar Power and Cool Buildings" but "What they weren’t able to test is whether the device also produced electricity. The upper layer in this experiment lacked the metal foil, normally found in solar cells, that would have blocked the infrared light from escaping."

They've invented an over-elaborate heat shield until they can demonstrate what the title promises.

What I'm more interested in is passive cooling for solar panels. We lose so much efficiency from higher temps. Placing them on our hot roofs, even with an air gap, is just ridiculous.

Combine that passive cooling with a heat pump to create hot water from PV panel waste heat and you'll both gain efficiency as well as a hot water source.

Here's a DIYer that did that but instead of hot water used a minisplit

https://renewablesystemstechnology.com/solar-heat-pump-syste...

Although a simpler solution to build due to the absence of liquid coolant I'd still choose a combination of PV-panels with liquid-based collectors attached to the back as this also makes it possible to circulate warm coolant through the collector to de-ice/snow the panels in wintertime.

How much efficiency do they lose? What else reduces efficiency, and is the trade off to moving them to a different location worse?

On the first one: let's look at a high end model like https://www.energysage.com/solar-panels/sunpower-corporation.... You will see the temperature coefficient of PMax and Voc in the specs.

The PMax in this case is -.38. You can predict a panel's loss of efficiency by multiplying the difference in degrees Celsius from standard testing conditions. Standard testing conditions for a solar panel is at 25 Celsius. A hot panel in summer will get up to 65 C. So 40 x -.38 in this case, or just a bit over a 15% loss.

That is not great, but 85% of max efficiency is not ridiculous. Panel orientation, angle, and shading can easily add up to more than that. The second question is a much harder one.

What's ridiculous is not having solar panels.

This is passive cooling for solar panels, basically. The title says its for cooling buildings but in reality they created a solar panel* with a cool bottom. It seems like they could thermally couple the two if they wanted.

*The solar panel was not able to produce electricity.

(video) https://www.ted.com/talks/aaswath_raman_how_we_can_turn_the_...

> "What if we could use the cold darkness of outer space to cool buildings on earth? In this mind-blowing talk, physicist Aaswath Raman details the technology he's developing to harness "night-sky cooling" -- a natural phenomenon where infrared light escapes earth and heads to space, carrying heat along with it -- which could dramatically reduce the energy used by our cooling systems (and the pollution they cause). Learn more about how this approach could lead us towards a future where we intelligently tap into the energy of the universe."

This makes no sense to me. The article states that there is one frequency of radiation that will pass through the atmosphere, so the device radiates at that wavelength. But why would it make any difference? The atmosphere might reflect most radiation back to earth, but not back to the point it came from. So it shouldn't matter what frequency the device is radiating - its radiation will be reflected elsewhere, and radiation from elsewhere will be reflected to it.

I don't understand your point maybe. The atmosphere can't reflect the radiation back -- it is unable to absorb energy in those ranges of wavelengths. The radiation passes through the atmosphere in to space unhindered.

I’ve always wondered that too.

The absorption and emission spectra of a material are equal, mostly. There are some exceptions and small errors, but for the most part a material that emits a specific set of wavelengths of light will absorb practically the same wavelengths.

The converse could be said that if you use the right material, it will absorb significantly less radiation from the atmosphere while emitting a similar amount.

However there are some bands with very high absorption and if you avoid emitting in that band you could possibly experience locally lower air temperatures or lower levels of a particular band. (I am only guessing that this would be a significant amount)

IANAPhysicist, but AFAIU, radiators work both ways. That is, they're also absorbers. So on the frequencies that don't pass through the atmosphere, "your heat" may end up elsewhere on the planet, but you'll also catch radiation emitted everywhere else. You'll eventually reach equilibrium with the average amount of heat that is "available" on this frequency. On the other hand, if your radiator is tuned to the frequency that passes through the atmosphere, you emit your heat, but don't absorb heat in return. The equilibrium point you'll be trying to reach is that of deep space.

> But why would it make any difference?

I think because at that frequency range the apparent temperature of the sky is low.

You basically design a material that is "black" (good absorber-emitter, conversely: bad reflector) in the infrared window[0] and "white" (good reflector/bad absorber-emitter) in the ranges where the bulk of the incoming radiation is.

That way you maximize the amount of energy it can shed and minimize the amount it absorbs. You're not cheating thermodynamics, you're just doing better than for example titanium dioxide paint or metal foil normally do because those also reflect as much sunlight as possible but at the cost of of not shedding heat through IR radiation.

[0] https://en.wikipedia.org/wiki/Infrared_window

Emitters in the infrared window have been in the news before[0], but has there been any realworld deployments?

[0] https://news.stanford.edu/news/2014/november/radiative-cooli...

A similar cooling effect, though less powerful, has been achieved in specially treated wood:

https://www.sciencemag.org/news/2019/05/engineered-wood-radi...

I wonder if they could make that material into shirts or hats?

Doesn't solar cell does that too ?

Make solar power?

Surely the sun makes the solar power and a solar panel turns that into electrical power?

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