crashsite

First, let me apologize that this post is a bit long-winded. But, in an effort to head off extraneious and redundant input (good luck on that, huh?).....

Last summer a steam pipe under the streets of New York City burst. I was unaware of why there was live steam being piped around the city in the heat of summer. An internet search gave the answer. Several, actually. One was that the steam was used for facilities cooling.

It's always seemed counter-intuitive that the first step in removing heat is to add even more. For example, the flame in a gas refrigerator or the compressor in an air conditioner. But, I know the basic physics and it makes some sense. But, the secret is not the addition of heat, it's using the heat energy to create a thermal differential.

The modern home is essentially an insulated box. In the winter, the insulation keeps the heat that's generated by stoves, heaters, furnaces and mammals (people and pets) inside the living area. Pretty easy and logical. But, in the summer, the insulation is used to keep the heat out. Then, even more heat needs to be generated to power the air conditioner to cool it off.

Of course, the problem is that the sun beats indescriminately on the roof of the house and surrounding areas. There are some thermal gradients (shade under trees, ponds and pools, etc.) but, they are too generalized and too slight to be effectively utilized.

If a solar concentrator could be put...say...on the roof, it would serve two purposes. One is that it would shade part of the roof and the second is that it would concentrate the sunlight into a small area. We've all charred a piece of paper with a magnifying glass so this is readily understood. If a series of parabolic reflectors werre used, the area of the reflector would shade and the sun's light would concentrate onto a small target. If a series of fresnel lenses were used, again, it would effectively shade most of the roof while concentrating the light onto a small target.

That action does create a substantial thermal gradient. By passing a working fluid (coolant) throught the hot section, it would gain energy and then when brought to the cooler, shaded area, allowed to expand in a heat exchanger, you basically have a "gas refrigerator" without the gas flame. Then it needs only a low-power fan and ducting to distribute the cooled air into the home.

The amount of energy available is substantial. A rule-or-thumb is that strong, direct sunlight provides about 1kW per square meter. My house is about 1400 sq. feet and my garage is about another 600 sq. feet. Divide that by 27 (to stay in the English system) and cut that in half to give a conservative estimate (I'm somewhat more northern than the "hot belt" of the US) and, during the hot part of summer days, there's about 35 kW equivalency there.

My furnace is 20 kW and my wood stove....I can't even begin to convert. Bottom line is that even a small fraction of that 35 kW could be used to substantially cool my house (ie: covering only part of the roof with the apparatus and the inherent conversion inefficiency of it).

Complexity of the apparatus could be minimized by reducing or eliminating moving parts (sun tracking). It would only have to work during the hot part of the day. That would be late morning to mid afterneoon. The insulation in the home could hold the cool air in til the sun sets and things cool for the night. But, of course tracking could still be used, even in a simplified form (of which there are many possible options).

So, the true bottom line is...why isn't this commonly done?

Hero999

It can be done but I don't know if it would be better to use a solar panel to run a compressor based air conditioning unit.

Out of your 35kW of solar power you'll probably only be able to extract 20% at the most.

The amount of cooling power that will give you depends on the temperature differential between the inside of your house and the outside.

You could make the system more efficient, if you use the waste heat to heat water.

I imagine that this is not often done because it's fragile and high maintenance as you need to clean the reflectors quite frequently.

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Pommie

I am also amazed that these systems have not been commercialized.

Here is an article from 1996 about an ammonia absorption system.

You may also want to google Lithium Bromide absorption systems.

Mike.

Nigel Goodwin

Probably because they aren't viable?.

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justDIY

in the movie mosquito coast, a western inventor / scientist went crazy and forced his family from their suburban home to live in the jungles of south america. he brought air conditioning to the savages in the form of a wood fired ammonia absorption chiller. all was well till some pirates showed up, shot up the place, and then caused a big explosion when the ammonia tanks ruptured. I'm sure Hollywood exaggerated things with the help of gasoline, but thats the main reason ammonia isn't used much any more, it's a tad bit flammable

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Nigel Goodwin

Are you sure ammonia is flammable? - my school chemistry days are a LONG time ago, but I don't recall it was so? - could be wrong though?.

More likely it's just the American film makers usual imagination that ANYTHING you shoot blows up!

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crashsite

I don't know about that. I think they're pretty accurate. Like in the movie, "Them" when the ants make a ring around the outside of a car generator, with their bodies and shorted it out with a good deal of pyrotechnics. Please tell me at least THAT wasn't fake.........

justDIY

ammonia vapors are explosively flammable given a certain fuel-air mixture

http://www.occupationalhazards.com/N...f_Ammonia.aspx
http://www.labour.gov.on.ca/english/...4_ammonia.html

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Krumlink

I couldnt think of anything to say, but I did find a clip with an EXPLODING BABY

http://images.google.com/imgres?imgu...en-US%26sa%3DN

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Pommie

Ammonia isn't used anymore because anhydrous (or even just concentrated) Ammonia is a nasty substance to have around. That why I mentioned Lithium Bromide (and Chloride) absorption systems which are much safer.

Mike.
Here is a paper on solar powered LiCl aircon for anyone that's interested.

rjvh

the problem with the most alternative energy sources is that they are not 24/7 hours/days availible that needs than a form of storage because it seems that the consumption is always out of phase with the supply
the storage is now these days the big cost factor to make
add also the poor efficiency of transfering energy to an other form and it's clear that it is nice for a hobby but comercialy it still has to go a long way
it's a hobby for me but to sell it you can not compete

crashsite

But, this is the exception to the rule. It's most effective at the times it's needed most and is also at maximum power when maximum cooling is needed. It just seems like that much raw energy should be pretty easy to extract and put to use and I don't know why it's not routinely being done. But, maybe a better question is, "If I'm so darned smart about it, why haven't I done it?".

Nigel Goodwin

Try it and see? - I suspect you will find that it's NOT easy to extract

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crashsite

Oops, did I forget to mention the sinfully lazy part, too?

rjvh

the case of heating amonia in a pipe and condens it in the chloride solution that hapens in the day cycle to evaporate it again and that is the moment that you extract heat from (your room) the suroundings where you did store the liquid amonia/chloride solution that is thus after the day cycle and that is mostly the time of day that it will cool down already
for making an ice machine this works wel but cooling your house on the hottest moment of the day no
this is the working of an passive machine (no energie put into it from your side)
if you make a machine with a constant circulation in it it consumes already power for pumping the media through
than you have a the same as a fridge or an aircon
an active system is much more work and costs to maintain also
why do we use amonia chloride solution ?
this is only because it is capable of holding a good value of energie