Air-cycle air conditioner - Craig R Meyer

Aug. 8th, 2004

07:00 pm - Air-cycle air conditioner

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So this "air conditioning" thing is looking like a hit. Us in the rich northern hemisphere dig it plenty, which means that our sweat-shopping brethren in the south will be even hungrier for in once they can scratch up the dough/labor reform.

And at the same time, this ozone-eating refrigerant business just sucks. They say they're phasing out freon, but I say bullshit. It will take an all-out block-by-block invasion of China (and/or Monsanto) to close the books on that business.

So. Ridiculous growth industry, approaching environmental disaster. Well then!

So you know how refrigerators work, but let me remind you anyway:

1: A compressor, uh, compresses gaseous refrigerant (like freon), thus heating it up.

2: The hot refrigerant is cooled in a "heat dumper" heat exchanger (that heat going outside onto hapless sidewalk pedestrians), cooling it down to room temperature or so. In most modern systems the refrigerant condenses into a liquid here, but a phase change isn't required.

3: The medium-temperature refrigerant is expanded, thus making it way cold. In conventional condensed-liquid systems, this simply happens in a pressure-blowing "expansion valve". In all-gas systems, the refrigerant is expanded in some sort of engine.

4: The cold gaseous refrigerant goes through a "heat sucker" exchanger that sucks in heat from the building's inside air.

Keep in mind that the "heat-dumper" heat exchangers is a gas-to-gas type, unlike the liquid-to-gas type exemplified by a car's radiator. This is why a replacement car AC heat-dumping exchanger (aka "condenser") is more expensive than a new radiator, even though it's much smaller. Manufacturing a heat exchanger with all that exterior AND interior exchange area that doesn't also leak is a very tricky=pricey undertaking.

I have it on good faith from both theory and the literature that smaller temperature differences ("delta-T's") between the four steps above make for a more efficient, but necessarily bulkier and more expensive system. A lower delta-T across the condenser means that more exchange area (= $) is needed to reject the same amount of waste heat, etc. This is why Carrier requested a more stringent USA standard for AC efficiency (more money for them), and is also why Dick Cheney and His Idiotic Majesty said no (more money for them).

So. Suggestion:

An air-conditioning system that uses no refrigerant at all, but air itself. The key component is basically a car-style turbocharger that is driven by a high-speed direct-drive electric motor.

So, to transpose from the four steps above:

1: The compressor side of the turbo unit compresses some air, thus naturally also heating it up.

2: That compressed air passes through the same kind of "heat dumper" heat exchanger, cooling down a bit.

3: The compressed-but-cooled air then passes through the turbine side of the turbo unit, coming out at ambient pressure, but colder than it was going in.

4: Um, there is no fourth step. There is no "heat sucker" exchanger with which to transfer heat from the building's interior air to the "refrigerant" (air). The cold "refrigerant" goes right out into the room and that's it.

So. Since this is a low delta-T process (the compression ratio of a single-stage radial-flow turbocharger-style compressor is limited to 3 or 4), the overall efficiency is high.

What's better is that the step 4 heat exchanger isn't even there, thus reducing the cost and reducing that step's delta-T to zero zero zero.

The "heat-sucker" heat exchanger from step 2 IS there, alas, and it'll be a whopper, too, since the system delta-T is low. However, the good news is that it can leak, since leaking a little air out into the atmosphere is perfectly OK. The step 2 heat exchanger thus be correspondingly and significantly cheapened.

That electric-driven compressor/turbine unit will be tricky, of course. The main speed bump will be integrating the high-speed electric drive into turbocharger body, and the rotor into the turbine shaft. Super-high surface speeds preclude cheap iron-based magnetic materials as well. It's still doable however, and straightforward to those who really know that sort of thing.

(Look at the one-piece compressor/turbine/generator in the <>Capstone microturbine</a> for an example of that kind of direct-drive integration between a turbine electric machine.)

And then there's the reliability issue, since the thing must run for months at a time. Help is on the way, however! Check out how Capstone's turbine uses an air bearing. No friction! What a score. That's great, because it means that the friction/reliability issue has already been handled by someone, so we know it's not impossble.

And that's what I got. 'Could be neat.

Request for Help:
Turbine noise is more unnerving than compressor noise, so this probably wouldn't fly in a window-mounted unit. Can active sound cancellation help?

How small can the compresser/turbine/motor unit be before the efficiency goes to hell?

Below a certain size, is a free piston type of compressor/engine a better idea?

(2 comments/permalink | Leave a comment)


Date:July 8th, 2006 - 08:55 pm

air conditioner

Hi Craig - its a neat idea but i would have 2 big questions
1) is the electricity required to spool up a turbine to 60-70 thousand rpm against mounting back pressure going to be pretty expensive?
2) the cooling effect is mainly when the phaze change happens in refrigerent isnt it? ( A compressed ambient liquid as it takes in heat to become a gas gets really cold like freon out of the can) SO you might get some cooling but its only the amount based on cooling the compressed and heated air back to outside air temp and then allowing it to expand - which is some cooling but maybe not as much as with the phase change- otherwise why have refregerant at all?- I always wondered since water is 857 times more dense then air then why couldnt you have water spray (or soak) over the cooling lines to allow for much less power used in cooling.Cool thoughts
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Date:July 10th, 2006 - 10:52 pm

Re: air conditioner

Well yes, there's certainly an energy cost in building up all that rotational kinetic energy, but we can presume that it's either a magnetic or compressed-air bearing like that used in the Capstone microturbine generator. So very little energy is needed to maintain the high speed.

Well, in a regular air conditioner, that's right, most of the action is when the refrigerant goes from liquid to gas and back again. But it's perfectly possible to do it in an all-gas way, with the temperature changes coming along with pressure changes.

What makes this kind of all-gas scheme difficult, though, is the fact that you have to go through a huge volume of air. All of the air being cooled actually passes through the compression/expansion machine, which would necessitate a piston-based compressor/expander that's very large, loud and expensive. Also, the out-going air would smell oily and machine-y, which is certainly less than classy.

The high-speed turbo-compressor/expander I'm suggesting, though, if floating in a magnetic or compressed-air bearing, might make take care of these problems. Turbines go through lots more gas volume per size than piston machines do, there's no metal-metal sliding contact and the whole thing works without lubrication.

That leaves the big stinker, though, of using an air-air heat exchanger to release heat from the compressed (and not yet re-expanded) air. That will be a very big and surely expensive thing.

But that still leaves your fundamental question of why we have refrigerants at all.

Perhaps part of it is that by going back and forth between a liquid and high-pressure gas, that keeps the machinery and heat exchangers relatively small, as each cubic inch of gas/liquid packs a big punch.

Another good thing about a conventional refrigerant is that once you have this high-pressure liquid, you can pipe that to an expander far from the other more-imposing machinery. So in a car, for instance, the compressor and condensor are in the front of the car, while the expander is in the air vent near the interior. The thing I'm talking about, OTOH, dumps out its cold air right near where it got the hot air, like a window AC unit. And window units aren't classy in the slightest.
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