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Old 12-29-11, 10:49 AM   #1
Lex Parsimoniae
 
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Default CO2 in pump-less solar collectors??

Found this paper about using CO2 as a solar heat-transfer medium..

http://www.ewp.rpi.edu/hartford/~cha...c_tube_CO2.pdf

But, the conclusion of the solar-CO2 study, says it's too complicated (to paraphrase).

I started thinking about that heavy duty CO2 plumbing..
And where to find a good CO2 pump..?. That's going to be the hard part..


Solar space heating with hot air is so simple.. Can CO2 be done 'simple'?

I started wondering about a basic wall mounted space heater.


Cool air from inside the house flows into the bottom, is heated and flows out the top..
Seems like it would work, even when you don't have a fan running..?.

How could CO2 be made to work in a free-flow mode..?.
If your collector had high pressure copper pipes? CO2 replaces the water?

Or maybe use a CO2 Header line with evacuated tubes plugged into it? (but gas isolated).


Indoors, your HX would be arranged so the heated CO2 would flow into
the top and out the bottom, as it was cooled..

Spring loaded Check-valves on the input and output of the indoor HX would allow the gas coming
into the HX to condense (as it lost heat) dropping it's pressure.
This pressure drop would cause the input check valve to cycle open
and allow more heated CO2 to enter the indoor HX.
The cycle would be repeated continuously (a pumping action).
The indoor HX could be a water tank or a liquid-to-air coil (with fan).


When compared to using water, what (if anything) would be gained by using such a system?

Maybe due to the weight of H2O, it couldn't self-pump?
Would the heat transfer rate be a lot higher, using CO2?

I'm wondering if this scheme would even work at all.
If it is possible, wouldn't it have been done by now?

At least there would be no worries of frozen pipes on cold nights..

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Old 01-01-12, 11:44 AM   #2
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I'm not making sense of any of this. There is CO2 in the cavity in front of the air that you want in your building? That CO2 won't cause the air behind the plate to move.

In fact I think the plate is complicating things and shouldn't be there at all. I don't see any reason why you would need or want anything simpler than a large black surface at the back, a plate of glass on the front, some insulation covering the outside surface to minimize losses, a way to block airflow when there isn't enough sun to create heat, and a passageway that snakes the indoor air through so it flows past the entire hot surface to absorb its heat and then pass it to the inside by a fan or blower.

By adding the separator between them, the side facing the glass will be warmer than the side that need the heat transferred to it making the unit keep more heat on the glass side instead of just passing air through it. This is where the aluminum can heater guys are making their mistake, the sun hits the black painted cans and then it heats the area between the cans and the glass while cold indoor air passes through the cans and needs to wait for thermal transfer to actually pull the heat into the cans. Remove the separator and anyone with some wood, good water sealing paint, caulking, a fan, wood working skills, a piece of scrap glass, a drill, an insulated flap attached to some insulated tubing to bring the heat into the house and you've got a space heater that likely won't cost you even $100. I'm trying to find a piece of scrap glass or maybe a few that is as close as to size of a 4x8 sheet of plywood without going over, price is right style. I wonder if my neighbors and cars driving by will be upset by a solar heater mounted in the lawn visible from the road.

As far as I can tell, the cheapest method of solar heating is to do exactly this, build a DIY solar air heater. Even if it doesn't work as well as a professional heater it will easily pay for itself if done with scrap materials and maybe a purchased fan, short duct, and some plywood and a few short 2x4's cut into 2x2's.

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Old 01-01-12, 12:41 PM   #3
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I'm looking at the idea using using a refrigerant like CO2 (or even R410A or Propane)
in place of water (& glycol) as a solar heat transfer medium, because it seems
like it might be possible, to do without using a coolant pump..

If worked and could cheaply heat a hotwater tank in the basement,
it would be a very useful supplement to my DHW needs.

With hotwater in the collectors, you always need a pump running...
And, it runs all day, when it's sunny.

If a refrigerant could flow inside a loop, without using any electricity.?.
Why not?

The good thing about using water, is it can be heated up to 190F
pretty easily in a good panel..

Got to run..
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Old 01-01-12, 12:59 PM   #4
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This discussion is really most curious, especially if it relates at all to the paper cited in the beginning...

Quote:
Originally Posted by Xringer View Post
Found this paper about using CO2 as a solar heat-transfer medium..

http://www.ewp.rpi.edu/hartford/~cha...c_tube_CO2.pdf

But, the conclusion of the solar-CO2 study, says it's too complicated (to paraphrase).
It would really help to read and understand, at least a little bit of what the paper actually says.

It does NOT say that using CO2 in a solar heating application is too complicated. It DOES say that because of the transcritical nature (the CO2 is changing state between liquid and gas) the measurement is complicated because of the varying characteristics of the medium.

The most important thing it says is that the "annually-averaged collector efficiency is measured at 60.0%".

Compare this to this graph from BiS:


In the ranges shown in the graph, efficiency of a flat plate is averaging 50% while an evacuated tube is averaging 45%.

The paper says the average efficiency when using CO2 as the heat transport medium is 60%. Which is pretty significant, especially when you realize that CO2 is being used in exactly the same way as water, and is exploited for its phase change characteristics in evacuated tube collectors. So the proper comparison is 45% to 60%. This is a big deal.

It looks like what is missing from this discussion is an understanding of how evacuated tube collectors actually work, and the thermal aspects of phase change (which also happens to be the principle that is the basis for all vapor-compression machines).

You might want to start by understanding the principle behind heat pipes.


Otherwise, you have driven off the road and you are sitting in the ditch with your wheels spinning.

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Old 01-01-12, 01:05 PM   #5
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Sounds good, I wasn't reading what you said at first clearly about you using it for your hot water tank as long as the heated CO2 manages to get the heated water down to the hot water tank then that sounds good. Some pumps use a minuscule amount of power though, I think there was a thread here with someone discussing using a pump for off-grid and looking at the 10-20 watt range. If 10 watts could do it and the pump ran for 8 hours a day that is only 2.5 kwh. If you ran it cycling on only peak sun and disabled it when the temperature of the tank is getting higher than it should be you'd be in the sub 2kwh range. Probably about 25 cents of electricity or the same as a 1 degree setback your heat pumps for a night. I'm not saying its a bad idea or anything but in case CO2 doesn't work as you plan.
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Old 01-01-12, 03:00 PM   #6
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The drawing on the right seems like a neat setup.
It's moving solar heat into the copper heater.
I'm not concerned about the molecules inside the evaluated tubes.
We know they work really well and reach high temperatures.

But what would happen to CO2 or some other refrigerant inside the Copper Header.?.

If the heat expanded the refrigerant inside the header, could that expansion
/ increase in pressure, be used to 'pump' refrigerant around a loop??

The indoor part of the loop would be the HX, where the gas would be cooled.
(Transferring it's heat to water).

When cooling dropped the pressure inside the HX, more hot gas would
enter the input check valve.

Header
Collector_______HX
|--------->---------|
|---------<---------|

The pressure inside the Header would equalize to the whole loop.
As the HX cooled the gas, more gas would be drawn in from the header.
But as cool water dropped the pressure in the HX, some gas would
be drawn back up to the Header, (via the bottom check valve).
Maybe kinda like a Stirling engine action..?.

Would it flow? Or just equalize and the HX stays cold??
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Old 02-07-12, 09:44 PM   #7
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Your valves are spring-loaded, so they will stabilize, stretched a distance to match the pressure differential induced by the essentially constant loads imposed on both sides. If you could force the valves closed cyclically, to prevent pressures from stabilizing, it would pump. I'm sure it's possible to devise a passive mechanism to do this, but I'm not sure what the simplest would be.
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Old 02-09-12, 04:11 AM   #8
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Well, if you want to circulate water other than thermo-syphon and not use an electric pump, then there is a device called a pulse flow valve, see link PULSE FLOW VALVE FOR WET BACK | Trade Me
It fires bursts or pulses of heated water to your HCW or radiators.

I cannot see the issue with using a small 20w 12volt magnetic drive pump to circulate the water, just use a small 30W PV panel, these pumps have a life of approx 15,000 hrs. If sufficient volume of water is pumped to keep the in/out delta temperatures < 10 degrees then provided the pipe work is well insulated, losses are small.

To heat a house, a box on the exterior wall directly heating the room air would be the most efficient as if designed correctly with cooler air passing between the glazing and bits that get warmed by the sun, would prevent much of the re-radation back through the glazing.

You can always make a heat pipe to move the heat from the external solar panel, except diy ones only work in an upwards direction with the condensing area at the top of the pipe and heat source at the bottom. A 20mm dia copper evacuated tube containing R22 refrigerant will move 5 KW heat along its length over 10 meters. I still think water is simpler.

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Old 02-09-12, 07:58 AM   #9
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Quote:
Originally Posted by mrd View Post
Your valves are spring-loaded, so they will stabilize, stretched a distance to match the pressure differential induced by the essentially constant loads imposed on both sides. If you could force the valves closed cyclically, to prevent pressures from stabilizing, it would pump. I'm sure it's possible to devise a passive mechanism to do this, but I'm not sure what the simplest would be.
The pressures would stabilize, if the pressure was the same in both HXs.
But, when the sun comes out and heats up the outdoor HX,
the outdoor side pressure will build up and force hot gas indoors.
Which would equalize the pressure for a while.

But, perhaps a pumped system would be better..
Considering what would likely happen on a cold night..
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Old 02-09-12, 08:11 AM   #10
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Quote:
Originally Posted by Solar Mike View Post
A 20mm dia copper evacuated tube containing R22 refrigerant will move 5 KW heat along its length over 10 meters.
Mike, do you have more info on this? The amount of heat isn't such a big surprise, but the distance (10 meters or about 30 feet) is big news to me.

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