One of the things limiting the efficiency of any air conditioner is the size of its heat exchanger. If I could run my hot freon lines somewhere over the house so that they could radiate to space at night, I would absolutely increase the efficiency of my A/C system. But piping and heat exchangers cost money.

Right now, A/C condensors are huge compared to 30 years ago. It used to be the mark of an A/C man to have scars on his forearms where he accidentally touched a hot Freon pipe. Now, you can't use those Freon-to-water heat exchangers for, "free hot water" because the Freon only gets about 20F above ambient temperature. But, yes, you could run pipes all over the place to radiate heat, thus reducing the necessary size of your condensor coils and fan horsepower. The problem is that there isn't much efficiency left to gain with that method (compared to what's available on the retail market) and you would have to custom design your system to get any gain at all. I messed about with this, and concluded that the retail products could kick my butt any time because they had computer aided design methods and years of expertise. A properly designed condensor uses the least amount of copper and aluminum to do the job, and it's a lot less pipe than any passive radiator ever thought about. You want to kick some efficiency numbers? Consider burying your pipe and conducting the heat into the earth. It's call a, "ground source" heat pump. Works like a champ in Chicago because the ground temp is about 52F. At that temperature, you don't even need Freon. Just cool a bunch of water and pump that!

 

It's a very clever way to get more value out of the evacuated tubes in a solar collector of that type. I saw a prototype installation years ago but haven't seen much since. I think the players went out of business.

They didn't, they've made gazillions - particularly in China:
from steps-centre.org/wp-content/uploads/WP-70-Solar.pdf

 

They didn't, they've made gazillions - particularly in China:
from steps-centre.org/wp-content/uploads/WP-70-Solar.pdf

View attachment 116888

View attachment 116889

I think wayneh was referring to the use of evacuated tubes for cooling purposes.

 

Well, to do what they did in the lab would be tough. But as noted, there has been a commercial product that comes very close, a black body radiator inside an evacuated clear tube.

No, these are NOT black body radiators. A BB radiator absorbs and radiates equally at all wavelengths, a BB radiator in an insulated evacuated tube would have reduced conduction and convection losses, but will still efficiently radiate. Under maximum solar irradiance, (about 1000W/m^2), a black body in an evacuated tube will get to about 91C (Stefan Boltzmann law: σT^4 = 1000). The tubes they were making at Sydney Uni in the 1970s, on an overcast day, would get to 250C (without focussing).

To increase the efficiency of the collector you want to high absorption of the solar radiation and low emission. At each wavelength the absorption and emission characteristics of a surface must be equal (basic thermodynamics), but they can be different at different wavelengths.

There are plenty of descriptions on the web, here's an idea (from http://www.almecogroup.com/uploads/1172-ALMECO_TinoxEnergy_ENG_S402_07_2014_mail.pdf)

There are lots of clever selective surface technologies being developed, wait for paints that keep your car cooler - these have the opposite filtering effect of a solar absorber.

 

The ones I saw resemble the tubes pictured here
http://solarproductmaker.com/1-5-evacuated-solar-heat-pipe-tube.html

 

A BB radiator absorbs and radiates equally at all wavelengths

Not even close. Wavelength is distributed over a Boltzmann distribution which depends on temperature.

 

Tesla23 said: ↑
A BB radiator absorbs and radiates equally at all wavelengths

Not even close. Wavelength is distributed over a Boltzmann distribution which depends on temperature.

Loose with words, I thought the meaning would be clear given we were talking about selective surfaces, a more precise statement



from p34 of
http://www.kostic.niu.edu/352/Heat_4e_Chap12-Radiation_Fundamentals_SUMMARY-PDF.pdf

The key here is that you can make a surface where the absorptivity/emissivity is a function of wavelength so that it gets hotter / colder than a black body. If you are trying to collect solar energy, a black body is not ideal as a black body at 100C radiates 1100W/m^2 - more than the solar irradiance at the equator. A selective surface can still absorb most of the solar radiation but will re-radiate much less.

 

A BB radiator absorbs and radiates equally at all wavelengths

Ah, I see your point;
"A BB radiator absorbs and radiates equally, at all wavelengths" versus
"A BB radiator absorbs and radiates, equally at all wavelengths"

I totally missed that subtle difference the first time through.

My previous use of the term was only to indicate that the same surface that was well suited to absorb sunlight is also pretty good at radiating also. It's not a black body by any stretch in either application, but tries to approach that. I believe it was blued or anodized metal in the system I saw.

 

Here's an interesting article, related to what's been discussed:

http://www.iflscience.com/technolog...ated-solar-captures-light-under-cloudy-skies/

Perhaps this tech could be used in cities like mine.

 

Cooling system could replace air con and drastically cut energy use

​

A heat-pump-like system that relies on a phenomenon called electrocaloric cooling could heat or chill a room almost twice as efficiently as standard air-conditioning units

 

I'm highly skeptical but would like to read the article. It appears to be behind a paywall. Any full copy out there?

 

..........but would like to read the article. It appears to be behind a paywall.........

Unfortunately, nowadays this seems to be the norm on most, if not all, linked articles.

 

I'm highly skeptical but would like to read the article. It appears to be behind a paywall. Any full copy out there?

It's not behind a paywall. It only requires you to register your basic info with New Scientist

 

Found it free via another path. It's vaporware, very similar to those "new battery technology" announcements designed to rustle up funding.

 

Getting back to the original idea, I have a better idea. Instead of wasting all that energy by beaming it into space, let’s beam it to the southern hemisphere where they need heating when we need air conditioning (and they can return the favor.)

 

Air-Conditioning Discovery Eliminates Harmful Gases

​

The technology combines a number of existing techniques and has “superlative performance,” says Neil Mathur, a materials scientist at the University of Cambridge, UK.

 

I don't think it is vaporware, here is a better explanation:

https://www.nature.com/articles/d41586-023-03595-1

like all these discoveries, whether it turns out to be useful comes down to engineering and price. It is interesting though..

 

Yeah, eliminate "Harmful-Gases", only use Propane and Butane.
Oh, but wait, then there's no way to Patent it and corner the Market,
I guess that idea's no good, damn.

I've been running a Propane/Butane mix in my Truck for ~4-Years,
it kicks butt, and it's dirt cheap, ( shipped in to the US from Canada ).
.
.
.

 

Hi,

The one about 'beaming' heat (or any energy) out into space or back from space to the earth for renewable energy makes me wonder what might get in the path of that 'beam'. Be it airplane, satellite, or flying farm animal it could be a problem.

Years ago I read about a technique where an air jet is shot over a venturi of some sort and the action caused real cooling. That might have been 20 years ago, no sign of anything like that yet.

30 years ago I used a wet sponge and small fan to keep cream for coffee from spoiling. The wet sponge is held up against one side of the small carton and the other sides insulated. It works for a day or two. It's just the effects of evaporation.
More recently I see devices sold on Amazon built like this for keeping things cool.

 

It's just the effects of evaporation.

As is conventional air-conditioning.