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gaiatechnician

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Posts: 36
Reply with quote  #1 
I now have a passable solar collector in the greenhouse, it transfers heat to water and it does warm the under soil quite a bit.  It used to be window screen, in the hope that it would be dual purpose.  (To cool the air when it gets too hot for the plants later in the summer). Anyways, changed it and it is now using an old oven pan as the collector. Water drips down and into the soil and takes solar heat with it.    Significantly better than last week.   Next phase is an outdoor collector. In this case, it would be somewhat similar to what other people have done.  I will use about 2 ft of windowscreen and the water will drip down it and air will pass through it.  Hopefully that way we get good transfer of heat from the air to the water. (thermal conduction via the aluminum wires).   My main question is "will a back pass collector work?" So, in this case there is a clear  plastic separation halfway in and the hottest air goes up the back, and the cooler air drops down the front of the collector closest to the face of the thing.  Can that even work?  I want it that way because in theory (my theory) there will be less heat loss that way.  So the water is dripping down the screen up top and the air will cool and you get a circulation of air that speeds up and is self sustaining. I will do some sort of diagram later.  Also dimensions? How wide would a back pass and front pass be to get roughly laminar air flow?  The idea is to put the water about 1 ft underground in a bed. Just one loop of 5/8 drip tube.  Maybe 10 ft of it. (the real cheap stuff).   Thanks Brian.  (I have a week off because Victoria froze up.) 

gaiatechnician

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Posts: 36
Reply with quote  #2 
I did a diagram that might help explain what I am doing.   So, I am only using window screen in the upper portion and it is just to transfer the heat from the air to the water.  I only did a couple of tests because I  have to cut the screen again to the right size.  It is too big at present.  Water throughput might  be quite slow, probably just a couple of liters per hour. At this time of year, on a sunny day, I am only expecting a 10 to 20 degree rise in temperature. I don't want more that that otherwise it might hurt the plants.  Hope that helps.  The unusual part would be the landscaping fabric on the back wall. I want it to act as absorber/transfer to the air and also as insulation.  Also, it would have circulating air.  but I thought with the air coming up the back like this, it might have a tiny bit less heat loss.  Perhaps that is bonkers.  Any thoughts?  Tomorrow is to be about -2 C with flurries so may or may not be sunny enough to put this into operation.  Brian collector.png.png 
mattie

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Reply with quote  #3 
Hello GT
Would it make sense for you to ensure that once the heat transfer occurs it is not allowed to reverse.What im saying is some sort of insulation between the screens to reduce this? Also because its an air flow and convective currents, the area where maximum heat transfer will occur should be at the top of the unit this may be useful to the design process.
Regards Mattie
stmbtwle

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Reply with quote  #4 
I think you're going the long way round relying on the backplate to heat the air and then then the air heat the water. Instead of screen use the landscape fabric you mentioned. It's black and will absorb the heat directly, and pass it directly to the water flowing over it.
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Garage_Hermit

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Reply with quote  #5 
Er, GT,

I am wondering if you could not use JUTE CLOTH like off a potato sack, say, to wick the water up from a container (long channel).

Capillary action should pull the water up the cloth screen, and then the latter should get heated up as it goes, and hopefully turn into to *water vapour*...

(it is a bit like the evaporative wall that Mattie posted some time ago, that I hardly understood, although it was a brilliant presentation !)

If you could then collect the water vapor, you ought to get warm water, I should of thought...

You would need a condenser of some sort, at the top of the JUTE screen...

Must admit, don't know how the condenser would work - probably a metal drum, with cold water feeding in and out of it... The water vapour is supposed to condense on the cold metal surface...

(Latter does not want to be TOO cold...)

The end result is (hopefully...) a tub full of warm water.

G_H

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JoeK

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Posts: 135
Reply with quote  #6 
I think you might find this "modified trickle down" collector interesting...seems basically what you've come up with already minus the airflow.

http://www.jc-solarhomes.com/MTD/MTD_solar_heating.htm

or this link talks more about the original trickle down roof design.

http://jc-solarhomes.com/solar%20thermal%20roof.htm

BUT I think you might find using the air itself an easier and perhaps overall better tactic for heating up growing beds. Just using air, which will naturally become moist and warm inside the cold frame. If you pump the moist warm air in a sunny cold frame underground into a perforated pipe, the cool conditions underground will  create a dew point that condenses that warm vapor back into a (minute) trickle of liquid. Literally would make a trickle if conditions are right. This phase change very efficiently transfers accumulated energy in the vapor into the soil, while the air leaving the pipe from the other end is dried and air-conditioned.  It's better cooling (for summer)than a swamp cooler, in which the air flow absorbs the water vapor (along with the heat it just absorbed changing into vapor).  In a well designed, active system return air is just a few degrees warmer than the soil temp, and dry and ready to absorb more warm vapor as it flows back around to the intake end of the system.  For best cooling in summer run the system at night with the vents/lid open and cool off the soil. Next day the "thermally empty" battery of the soil will keep conditions cooler by absorbing lots of the solar gain.  I would suggest leaving the fan off, or run as minimally as possible, during the day(in summer) and venting heat as much as possible out the vents/lid. some shade goes a long way too.

Check out this thread I started a while back. Still have to put up the west wall, then I'll  turn it on and update the thread.

http://www.simplysolar.supporttopics.com/post/subterranean-heating-and-cooling-6679334

System like this can be scaled all the way down to a single pipe with a tiny fan, not sure how you might go about setting up a thermosiphon to power the circulation but I'm sure it can be done and its worth some brainstorming for cold frame scale. Wouldn't need much flow, and running power to warm a cold frame seems a little silly to me.  I'm not too savvy in paint or anything so drawing up sketches on the computer is tough for me, I'd like to improve my drafting skills but perhaps I can just draw something up and take a photo of it...


more detailed info on the physics in this link

http://sunnyjohn.com/indexpages/shcs.htm
JoeK

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Posts: 135
Reply with quote  #7 
Ok I did a little thinking and drew a mockup that just might work. I think this might be all you need, but if you replaced the simple chimney pipe with a more efficient collector you could likely increase the "draw" quite a bit. The collector would just have to be designed so the intake draws cool dry air coming out of the ground and then as it's heated air should rise unobstructed until it exits out the top. It might be possible to take advantage of the rear (North) interior wall of the frame itself and modify it into a large flat collector. Could work very well actually and not really impinge on the growing space. This is sounding like a very good refinement if your willing to do a little more work.  It's already in my head, Back to the drawing board. This one piece,black pipe design is about the most simple, basic "collector" you could make.
Thermosiphon Coldframe.jpg 

I suspect that even a slight airflow would yield a significant advantage over a "plain" cold frame, but more is certainly better. For reference in active systems target flow is at least 5 times the volume of air enclosed, per hour. Up to 10 exchanges per hour has shown significant increase in heat transfer though. That means recycling the entire volume of air in the system every 6 minutes. And of course in an active system you would reverse the direction of airflow and use your fan to push hot/moist air down underground from the elevated pipe, allowing cool air to exit near the plant/soil level. Use the fan to push through the system, not pull, as positive pressure in the underground section will increase the transfer of moisture, heat, and air, into the soil.

gaiatechnician

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Posts: 36
Reply with quote  #8 
Seems like there is a  lots to like  in this air cycling idea, especially in a cold frame, because the air heats up and cools down so quickly in the tiny space above the soil.  It (the air movement) would totally even out the temperature and make for healthier plants.   BUT,  I don't have a way to do it right now. The only reason I could even do the drip wall thing was because I had a week off due to frozen ground (I am a stone mason).  5 air changes per hour seems like a lot though! My design limitation is that all my systems must run on 1 psi air. The Funny thing is you could probably easily do it if you had a bellows type set up with a bladder under the soil  or inside the cold frame. It could be like a lung, sucking down through the soil or filling and then  blowing down into and  up through the soil.  For a larger system than a cold frame,  a solar powered fan would probably be justified. Bit pricey to buy a solar panel and fan just for one cold frame.   Maybe I can figure out a way to do the bladder thing to move the air down  into the soil.    I could do it like a seesaw with a piston at one end. The 1 psi air can move water to lift and lower the see saw. (I made a see saw that worked just from the sun alternatively heating 2 fournliter milk jugs last year so it is definitely possible.    
Quote:
Originally Posted by JoeK
I think you might find this "modified trickle down" collector interesting...seems basically what you've come up with already minus the airflow.

http://www.jc-solarhomes.com/MTD/MTD_solar_heating.htm

or this link talks more about the original trickle down roof design.

http://jc-solarhomes.com/solar%20thermal%20roof.htm

BUT I think you might find using the air itself an easier and perhaps overall better tactic for heating up growing beds. Just using air, which will naturally become moist and warm inside the cold frame. If you pump the moist warm air in a sunny cold frame underground into a perforated pipe, the cool conditions underground will  create a dew point that condenses that warm vapor back into a (minute) trickle of liquid. Literally would make a trickle if conditions are right. This phase change very efficiently transfers accumulated energy in the vapor into the soil, while the air leaving the pipe from the other end is dried and air-conditioned.  It's better cooling (for summer)than a swamp cooler, in which the air flow absorbs the water vapor (along with the heat it just absorbed changing into vapor).  In a well designed, active system return air is just a few degrees warmer than the soil temp, and dry and ready to absorb more warm vapor as it flows back around to the intake end of the system.  For best cooling in summer run the system at night with the vents/lid open and cool off the soil. Next day the "thermally empty" battery of the soil will keep conditions cooler by absorbing lots of the solar gain.  I would suggest leaving the fan off, or run as minimally as possible, during the day(in summer) and venting heat as much as possible out the vents/lid. some shade goes a long way too.

Check out this thread I started a while back. Still have to put up the west wall, then I'll  turn it on and update the thread.

http://www.simplysolar.supporttopics.com/post/subterranean-heating-and-cooling-6679334

System like this can be scaled all the way down to a single pipe with a tiny fan, not sure how you might go about setting up a thermosiphon to power the circulation but I'm sure it can be done and its worth some brainstorming for cold frame scale. Wouldn't need much flow, and running power to warm a cold frame seems a little silly to me.  I'm not too savvy in paint or anything so drawing up sketches on the computer is tough for me, I'd like to improve my drafting skills but perhaps I can just draw something up and take a photo of it...


more detailed info on the physics in this link

http://sunnyjohn.com/indexpages/shcs.htm
JoeK

Registered:
Posts: 135
Reply with quote  #9 
Well I'll leave how to fit the idea into your cold frame up to you, since I don't understand where your 1psi air is coming from, nor quite how the bellows idea would work out. But if you got it working in another project then I bet your up to the task. Cold frames, to their advantage in this sense, are typically just big enough to contain the plants and don't have a lot of extra air space. That's why they can get overheated so quickly and easily... Ones I've seen have all been just a few cubic feet of volume, under 12 anyway, so changing them 5 times an hr really doesn't take much flow. 12 cubic ft 5 times per hour is exactly 1 cfm, hardly anything. 2cfm would give you 10 changes. sorry for American units btw but I've not gotten too good with Metrics yet. working on that. At rates like that I think a thermosiphon could be made to do well enough, especially if there was more collection area powering the convection than just the plain pipe on the drawing above.  Could perhaps even be easier (no moving parts to fail/maintain either) than the bellows rig,which seems like a complicated arrangement to me but maybe if I understood it as you do I wouldn't think so.
Rather difficult to install the underground section until things have thawed out, Though it should only be 1-2ft max underground. 2ft gives twice the effective thermal mass, but 1ft. transfers the heat back to the plants/roots more effectively. Any deeper and I think performance would drop off quickly as the plants just get too far to feel much benefit, especially without insulating to depth.
Interested to see what you come up with.
gaiatechnician

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Posts: 36
Reply with quote  #10 
Probably be too busy to do much for a while. We are having awful weather and I have to rush around for 4 or 5 hours work at the time in the lighter rain. If people can get a cold frame to thermo siphon, that would be great.  A big problem with cold frames is that the air in the frame heats up and cools down too quick. Every year, I read the sad stories from gardeners who come home to partially cooked seedlings because they forgot to vent their cold frame.  So if the heat siphons under the ground, it would save a lot of grief, and have a warmer (on average) cold frame into the bargain. Brian
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