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PSP greenhouse - black straw idea

paul wheaton

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posted 16 years ago

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So one of the big tools of making the PSP greenhouse work so well is the cold trench.

One of the things I wonder about when I look at it, is that in the winter when a bit of sun comes by, it seems that the cold air near the bottom isn't gonna warm up. As the air high up warms, it isn't gonna shoot some of that warm down low.

I wonder what might happen if a piece of copper tubing went from the lowest point to the highest point with a trip about an inch from the glass on the south side. And all the parts of the copper tubing that were exposed to sun were painted black. Then, as the copper tubing heated up, it would draw cold air from down low. As the really cold air is sucked up and converted to warm air, the cold air in the bottom would be replaced with warm-ish air. Which would warm the soil "down thar" and release that warmth later that night.

Silliness?

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Leah Sattler

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posted 16 years ago

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I think you are on to something. I think movement of air would work to some extent but I bet it would work even better with something akin to a hydronic radiant floor heating system using solar. copper is expensive and I think a black hose used in a closed system would be easier to work with. you could even create zones within the green house that could be re arranged in a snap. I'm not so sure you could use passive solar though. but I would think a small pump powered by solar could be obtained. I'm currently brainsstorming about this sort of thing due to the possibility of getting to build our own house so it is fresh in my mind.

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paul wheaton

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posted 16 years ago

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I'm thinking that it would be just one simple tube. No fan. No pump. Maybe something about eight feet long. About straw width.

Since it is black, when the sun hits it, it will warm. Since heat rises, the air in that part of the tube will rise - thus drawing air up the tube from the bottom.

Since cold air pools, then the pooled air will be sucked up, converted to warm air and be more at the top of the greenhouse.

I would think that if it were sunny for an hour, one little straw would probably suck up an awful lot of air from the bottom.

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Susan Monroe

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posted 16 years ago

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That is a viable idea, but not in much use.

In the book Solviva by Anna Edy, she got hold of an experiment by a man named Morse (1880s, I think) and tried her own adaption, and went own to design it into her home and greenhouse:

"I set about to create my own experimental version of Morse’s solar heater, to prove it to myself. I built it inexpensively on a wall of the barn I was temporarily living in, with black tar paper for collection surface and clear polyethylene for cover. I substituted Morse’s straight, vertical air-flow path with a winding path constructed with horizontal 4x4s painted black, thinking this might create even more heat because the air had to travel much further between the inlet ad the outlet. I completed the heater by sealing the plastic cover around the edges at about 11 a.m. on a sunny cold day in late March. I read the thermometer at the bottom vent where unheated, uninsulated air entered the solar heater: 32°F. By the time I got upstairs, some four minutes later, there was a stench of hot tar rushing into the room with the solar-heated air, and the thermometer at the top of the solar collector was already reading 110°F, a 78-degree temperature rise. Fearing another fire, I grabbed the fan I had waiting close by and stuck it into the top of the vent to pull the air faster through the solar heater, and soon the temperature of the incoming air was down to a safer 90°F, still a 58-degree rise.

"Although this was a very crude application of the Morse heater, it clearly proved the design to be at least as effective as Morse had described. In fact, it seemed that substituting his straight air path with a winding air path had increased the solar heat gain."

Sue

paul wheaton

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posted 16 years ago

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Excellent quote!

I just thought of a better way to express the problem I am trying to solve. (and, I admit, this problem is entirely in my head)

Without the straw, the temps at 3pm could look like this (each number representing a one foot high zone in the greenhouse):

100
95
90
80
70
60
50
45

And then at 6am:

45
44
43
42
41
40
39
38

With the black straw, I would like to think the 3pm picture might be more like

85
81
77
73
69
65
61
57

And since the low ground was warmed more during the day, the 6am temps would look more like

50
49
48
47
46
45
44
43

So i think the black tube isn't adding more heat. It's just getting it down into the low trench where it might buffer the heat better.

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Steve Nicolini

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posted 16 years ago

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Why only one tube? How about one in each corner of the greenhouse?

I like no pump no fan. I like simplicity. The straws might house a thin vine, too.

How wide are these straws in your head? Giant Paul straws? Or short guy straws?

paul wheaton

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posted 16 years ago

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I'm thinking that a soda fountain size straw about eight feet long.

No vines please - they would cover the black.

I would guess that one would do plenty just because in an hour it would have moved a whole lot of air.

I guess it could depend on how big the greenhouse is.

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Steve Nicolini

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posted 16 years ago

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I see. I was picturing a straw maybe 3/4 inch in diameter. Would it be possible to have too much flow?

paul wheaton

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posted 16 years ago

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I don't know about too much flow. I think that you want to not block the greenhouse glass too much. And I think that if you get the diameter too big, the hot air on the sun side might go up, and the cool air on the back side of the pipe might go down. But with a small diameter pipe, I think there isn't enough room for some air to go up and some to go down. So going up wins.

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Leah Sattler

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posted 16 years ago

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could it have the interior of the tube arranged to encourage the hot air to spiral it as it rises? I would have to really put my thinking cap on to get the interior of the pipe right to direct the hot air in a circular path but with a little quiet time I could figure it out

. it could be designed on a flat flexible surface the same interior dimensions of the pipe/straw and then rolled up and slipped in. that would eliminate the hot air up cold air down with in the tube possibiltiy and might even get more air flow over all.

If it was super light weight you could hang it on a fishing swival and the hot air might just spin it enought to prevent a cold side and a hot side. some blackened pvc might work. another option of getting the hot air rotating in the tube might be to drill holes in a spiral or just on one side on the top half then have it on a swivel. as hot air moves out the hole it might offer just enough propulsion to turn the straw.

or......instead of holes. cap the top and direct all the air to the side near the top. a small deflecter could direct the hot air "around" the tub at the top to really get it spinning on the swindle. I'm not sure that I am explaining that well, if you don't understand I will draw a picture.

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Leah Sattler

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posted 16 years ago

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ok did a drawing anyway. ( the top is capped not open )

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paul wheaton

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posted 16 years ago

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So you want it to spin so that the bottom part scoops the air?

What if the straw, all by itself, moves the air just as well without spinning?

So I kinda have a couple of missions here. One is to optimize the performance of the greenhouse. Another is to come up with a solution where cost is freaky small, and effort is freaky small. A copper straw painted black where the sun hits it seems to do the trick. I suppose most small diameter pipes would work about as well - so it could be a scrounged material.

Leah, are you thinking that the small pipe wouldn't work? Or that it would work poorly?

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Susan Monroe

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posted 16 years ago

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I'm thinking that a larger pipe would work better also.

You mentioned that you didn't want to block the greenhouse glass too much, but since the sun is moving all the time, the shadow created by a 1" pipe (for instance) wouldn't stay in one place long enough to cause any problem to the plants. And, many greenhouses have solid joints, so you could line the pipe alongside the joints. Or for that matter, the pipes could BE the supports, dual purpose.

Also, in the Solviva book, the author discovered that the sunnier the day, the faster the heat moved upward, even in her house. She had planned on growing vines over her south-facing windows to cut the heat, but never had to, because the thermal convection moved the heat fast enough that there was no problem of excess. And the fans she did use were cheap portable fans, $20 each.

She also had proven designs for composting toilets, and a rabbit- and chicken-heated greenhouse.

Sue

Leah Sattler

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posted 16 years ago

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The biggerit is the more air it will move to some extent and the better it will work. but I'm sure their is a threshhold where it is too big, especially if it isn't rotating to get sun on all of it. the thought of it "scooping the air" is a side note in the idea. it may not be feasable because of the air would be forcing it to spin the opposite dierction....I think. the rotation would insure that the pipe was exposed on all sides to the sun and wouldn't have a "cold side". I think the bigger the pipe, the more air it will move and the better it will work. if it can't spin then directing the air upward in a spiral and putting the outflow out the side rather than just an open top will keep the hot air from going up one side of the interior and the cold air down, although I don't think that is likely an issue, you pointed out that it is a possibility and I think it is.

an sliver of thought. the heat conductivity of copper may work for or against you. if as the cold air is sucked up a larger proportion of the heat will be lost due to the conductive nature of the copper. if it isn't rotating the "cold side" will lose heat while the hot side gains it. is that loss significant? will it be overshadowed by the heat gained through the relative conductivety of the copper that is exposed to the solar? is it a wash and a useless question?

if I were you I would do two. one pvc and one copper painted black. make sure they are placed in similiar areas to get the same sun. measure the temp of the air coming out of each to compare. the hotter the air the faster is will go up and the more cold air it will draw through.

idea #1,678,097. non rotating. cold side of tube PVC to prevent heat loss/cold gain, hot side sheetmetal/copper/something conductive. split a 4" (diameter)pvc lengthwise and replace it with a strip of salvaged black metal.

this is fun!

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Susan Monroe

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posted 16 years ago

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Okay, adding to your idea about heat loss from the non-sun side --- as long as it isn't rotating, why not use black-painted copper, with a half-circle of that foam pipe insulation on the back side? It could be painted, too, to reduce UV deterioration.

Also, what about being relatively low and horizontal, placed low in the greenhouse window? As long as somewhere else it went upward to take the heat... the heat expansion should more the air even if it was horizontal, wouldn't it?

Sue

Leah Sattler

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posted 16 years ago

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thats perfect sue. DUH! foam insulation. I guess i am thinking about what I could make one with scrounging what i already have. it taints my thinking.

the surface area could definitly be expanded on if the pipe was horizontal. i will add to that idea and suggest a l_l shape with holes drilled or a slot along the length of the bottom to draw cold air up.

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paul wheaton

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posted 16 years ago

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I think that the most important thing I'm drawing out of all of this is that the idea sounds worthy.

If Oehler can get tomatoes in december, perhaps this one little thing can get him tomatoes in january?

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Susan Monroe

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posted 16 years ago

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Leah, if the structure was wood-framed, what about routing out a groove, say on the 2" side of a 2x4, just deep enough to hold half of a 1" copper pipe. The wood behind the pipe would serve as the insulation. Paint the front of the pipe (and maybe the front of the wood, too). This way, it wouldn't interfere with sunlight any more than the frame would.

One step further...

The lower front part of a greenhouse is often solid (like the lowest two feet) because that's the coldest place. Suppose that section was a relatively narrow wooden box filled with vermiculite (etc) for insulation. Run black-painted copper pipe back and forth along the front of this box. The intake could be just lying on the floor, where it would take in the cold air. Then the air heats up in the horizontal pipe and thermal convection pulls it upward through uprights where it goes up near the ceiling. To adjust temps, it could be vented to the outside, or on cold days somehow it could be switched to go through a small fan that forces the warm air downward. So the air constantly circulates from the cold floor through the black pipes to the ceiling and downward again.

Think?

Sue

Leah Sattler

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posted 16 years ago

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that sounds like it has an excellent shot at being effective sue. I know paul was looking for something relatively simple and i don't know if that is too much for him or not.

I still think that having a relatively large diameter pipe would be best though. I would assume that because this is using almost passive movement of air that a small tube wouldn't create enough suction to move significant amounts of air over a relatively short period of time. also since a relatively small change in temp here is desired it pays to note the exchange of air from the floor to the ceiling would be relatively slow in terms of quantity (not air speed)then the overall circulation of air would be slowed. since a relatively small temp adjustment is desired, getting a large portion of the hotter air around is more important than getting a small amount of super heated air.

think of a big vent in the ceiling blowing warmish air vs a blowtorch. the big vent wins hands down for heating a room.

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Susan Monroe

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posted 16 years ago

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Oh, I'm all for larger pipe. But after reading Anna Edy's account of the great temperature rise (from 32F, yet), I wonder if there would be issues with black plastic pipe, say 3"? Fire hazard? Off-gassing?

Sue

Leah Sattler

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posted 16 years ago

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now I am getting artsy. in dream land it would be cool to build a solar chimney. made of brick or covered in ceramic tile. black stove pipe. I'm picturing something that looks reminiscent of a free standing fireplace. I'm thinking about whether something like this could be incorporated effectively into a passive solar home design.

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Susan Monroe

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Maybe I'm wrong, but I thought that solar chimneys were put in desert places to cool them.

Where's Mr. Google......

Sue

Leah Sattler

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posted 16 years ago

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I can see if the top of the chimeny was vented outside how that might cool something. but if the rising heated air is just released back into the place then it would help heat.

what about a flue that could change between outside or inside air intake. depending on weather conditions.

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Steve Nicolini

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posted 16 years ago

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Would you mind defining a "flue" for me?

Leah Sattler

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posted 16 years ago

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"an enclosed passage way for directing a current"

http://www.merriam-webster.com/dictionary/flue

could call it an air intake or vent too. one that can be switched from interior to exterior when desired. since the idea is that as the hot air draws up it will create a vaccum sucking cooler air from bottom up, which will in turn create a vaccum drawing air from other parts of the room down (theoretically). if you want it to cool the room, draw cool outside air. if you want it to heat draw already slightly warmed inside air. just a thought.

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Susan Monroe

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posted 16 years ago

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The only problem I see with venting warm air inside is that without a fan or something, it will stay near the ceiling, when you want it forced down near the floor. Like trying to catch a helium balloon that doesn't have a string tied to it.

Sue

paul wheaton

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posted 16 years ago

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I think that the idea is that without the straw, the cold air down low, will stay down low. And the air at the top will heat up plenty with the sun and stay there.

With the straw, you end up with the same overall heat in the greenhouse, but you have sucked up maybe two or three feet of cold air from the bottom. So the warm-ish air that is at about the three to four foot level gets moved down to the bottom.

But .... this is entirely speculative.

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Leah Sattler

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posted 16 years ago

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Maybe it could help cool and circulate fresh air in the summer and help circulate warm air down in the winter.

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paul wheaton

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This idea has popped into my head again and .... I still think it is an excellent idea. And if it isn't, I would really like to get a better understanding as to why it might not be such a great idea. Anybody?

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Joel Hollingsworth

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posted 16 years ago

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Two little things:

You don't want to mix the air at the bottom of the straw. The point is to draw in the coldest air available and heat it, so that no air remains so cold. Everything should be arranged to allow cold air to drain into this air intake with a laminar flow.

If you are using a thin copper tube, there's no reason for it to draw energy from inside the greenhouse. It can run outside the building, and loop around at any height you want, collecting energy that wouldn't be available to the plants, before returning to the building. A loose flapper valve, similar to the one in an old fireplace bellows but maybe made of mylar rather than leather, will keep this from flowing backward at night.

I've illustrated this crudely: pipe sections to remain insulated are shown in orange, with a solar collector as a black squiggle. The flapper valve is in grey.

siphon.PNG

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paul wheaton

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Does this mean that you think that a simple tube as described won't get the job done?

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Joel Hollingsworth

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I think it will work OK.

A lot of the current it would set up would be up the sides of it, which would draw air in a torus through about the area that the sun shines on. It wouldn't take in that much energy from your greenhouse, and most of that energy would be used circulating the upper layers of warm air. But some of it would go to the task you've laid out.

A design like mine would add energy from outside the greenhouse, and apply more of it to the job of pulling the coldest air away from the trench. But yours is much, much easier to build, so probably better overall.

It also occurred to me that one could put together a little heat exchanger loop with a negative-thermal-expansion medium and have the trench heating driven directly by negative convection, but it doesn't look like they sell that sort of thing yet.

Some of the zeolite minerals contract when heated, but you'd have to suspend them in a liquid at a rate that the liquid's expansion wouldn't overwhelm the zeolite's effect, which might not even be possible.

The more I think of it, the more sense it makes to just keep an air channel between the muntins and your glazing, and paint the inside of that channel black. This doesn't necessarily stop more light than the muntins otherwise would, and if there is any plumbing needed it would just be to feed the muntins with cold air and maybe to allow hot air out at the top. It would either change the design of a structural element (illustration, center), or add a few pieces (illustration, right), but it wouldn't mean building a new structural element just for this purpose.

muntins.PNG

Keralee Roberts

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posted 16 years ago

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Paul. I think you are onto a good idea here and it will likely work, the solar chimney is already a proven method for moving air effectively via convection. The only thing I question is using a small straw instead of large tubing. It would work but you will be greatly limiting the volume of flow, for no reason that I can see, unless you just happen to have a huge supply of tiny copper tubes (pricey) lying about? Black ABS pipe is very cheap and will likely do the trick quickly and easily. Or flexible drainage tubing, possibly even easier and cheaper, as no joints would be needed.

However, you have inspired me to do likewise but for a different purpose--I need to heat, or at least keep above freezing, a house. Since I have a good vertical southern exposure, I was thinking of box-type solar air heaters. But the tubes may be a nicer solution, I can cover each one with its own solar skin made of clear plastic soda bottles (tops and bottoms removed), run the air from the basement, outside the house vertically, to a collection plenum at a window opening, removable during summer. This would cost little to make and I think it will be easier to install/uninstall--I'm a one-woman show here, so maneuvering large collectors is not likely to happen.
What do you guys think?

Joel Hollingsworth

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posted 16 years ago

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I think Paul's objection is that a wider tube would tend to shade hte plants.

Black ABS piping has a solid inner and outer wall, but is foam through most of its thickness. I don't think the inside of the pipe would heat up much: most of the flow would be up the outside of it, and would hardly draw from the trench at all.

On the other hand, half an ABS drainpipe, with clear or translucent plastic on the side facing the sun, would pass more air than the same diameter of copper AFAIK.

Galvanized (cast iron) plumbing pipe is very conductive (partly made of graphite, one of the most heat-conductive substances) and cheaper than copper. I think it mostly comes painted black, too, although soot from a candle would be easy to apply.

paul wheaton

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posted 16 years ago

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I think using materials on hand would override anything else.

I think that something the size of a straw would get a good, one way draft going that would pull from the very bottom of the straw.

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Brian Adams

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posted 15 years ago

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paul wheaton wrote:
This idea has popped into my head again and .... I still think it is an excellent idea. And if it isn't, I would really like to get a better understanding as to why it might not be such a great idea. Anybody?

Part of me wants to say, "Don't fool with the design. It only works because there is no air movement and thus it traps the cold air in the pit."

But a slightly larger part of me wants to say, "There has to be a way to take advantage of the enormous temperature differences within the greenhouse."

What if you extended the tube and put it right back into the pit so it made a big 'U' shape. Wouldn't that force all the heated air from the black painted upper part back into the pit where it would be needed most?

Geoff Rich

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posted 15 years ago

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What if you flattened the top of the tube and created fan-like structure? It would increase surface area exposed to the sun and disperse the warm air over a broader area. Not a dramatic engineering change, easy to test, easy to back off from.

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