passive solar garden heater

This is an idea I have shared with others for about six years, but I don't think I have created a thread about it.  

The idea is to create something like a 40 foot deep well with steel casing.  But it's dry.  No water.  And then mount a 4 foot tall steel cap-ish thing that has glass on the south side.  (note, the glass must not block any light (as most exterior glass does))  And then there is a 3/4 inch black anodized tube that is exposed to the glass at the top and travels to near the bottom of the "well".  

When the sun shines, the tube pulls air from the bottom of the hole - thus the air temperature for the whole 40 feet is relatively homogenized.   This warms the area 40 feet down in the summer.   At night, the air in this pipe stratifies - so the warmest air is at the top.  

When the sun shines, the stuff at the bottom is heated.   All the rest of the time, the warmest air moves to the top.  

If this works, the heat from the summer will slowly be given off through the winter.    So maybe something like 50 to 60 degrees is slowly given off.

If this doesn't work, then then the temperature, around here, that deep will be something like 45 degree year round - so the heater will constantly give a temperature that is warmer than freezing.  

I know of some homesteaders that use a technique like this to keep animal waterers from freezing.  The idea is to upgrade this to try and store even more heat.


Here are my feeble images to try to "paint a picture" of what I am suggesting.

That's a sound idea because it'a based on a valid physical principles. I wonder if a Solatube might work as a cap for the well.



It's an efficient light gathering lens from all directions as the Sun moves. A section of reflective tube could extend down into the well and could have small perforations near the top to let the hot air escape.

So you would surround the top part with something like a horse trough and the water would not freeze?

What other applications can you see it being used for ?

I am wondering if it might help heat a small greenhouse?

I like this idea too.  I think Travis Johnson uses a culvert buried vertically below stock tanks to deice them.

I think the sun hitting the glass would need to heat the small tube more than it heats the casing.  Otherwise both will get hot and the air won't move up or down the small pipe.  If the top of the casing was lined with something reflective (aluminum trim coil, tinfoil), and the small pipe was black, then the light entering the casing would bounce around until it heats the small pipe, creating the airflow.

I bet the pipe would only need to go down 10' or so.  It would lose tons of heat to the surrounding soil.  Possibly it would lose all its heat.  In the winter you'd still get the stratification heating of the 45 degree deep earth (or 46 degree if the summer sun heat didn't all get dissipated).  

Arguably, what would happen if you just took a metal culvert, 2' in diameter, and sunk it vertically 10' deep with a cap on it at grade.  Possible bonus points if the bottom of the pipe found groundwater (more constant heat?).  Then at night the air at the top of the culvert would stay warmer (deep earth temp) and heat the soil right around the culvert.  Plant your growies one inch from the pipe in a circle and their roots may be kept warmer.  

Greg Mamishian wrote:That's a sound idea because it'a based on a valid physical principles. I wonder if a Solatube might work as a cap for the well.

Supposing that the anodized tube is still in the picture, I think it would be pretty good!  I think that exposing 4 feet of of southern exposure through glass would be even better.

Miles Flansburg wrote:So you would surround the top part with something like a horse trough and the water would not freeze?

That would be one possibility.   I am thinking that what I really want to do is grow a lemon tree, outdoors, in montana.

Mike Jay wrote:I like this idea too.  I think Travis Johnson uses a culvert buried vertically below stock tanks to deice them.

Do we have a thread here?

I think the sun hitting the glass would need to heat the small tube more than it heats the casing.  

agreed.

I bet the pipe would only need to go down 10' or so.  It would lose tons of heat to the surrounding soil.  Possibly it would lose all its heat.  In the winter you'd still get the stratification heating of the 45 degree deep earth (or 46 degree if the summer sun heat didn't all get dissipated).  

I agree that you can get most of the effect at just 10 feet.   But I am trying to shoot past harvesting the ambient ground temperature to making an attempt to SET the ambient ground temperature in this area.  

Maybe the first thing to do is a bit of an experiement.   With and without the annodized tube.   10 feet, 20 feet and 40 feet deep.  With a solar tube at the top, or something more elaborate.   And maybe something that is simply a metal cap.  

Arguably, what would happen if you just took a metal culvert, 2' in diameter, and sunk it vertically 10' deep with a cap on it at grade.  Possible bonus points if the bottom of the pipe found groundwater (more constant heat?).  Then at night the air at the top of the culvert would stay warmer (deep earth temp) and heat the soil right around the culvert.  Plant your growies one inch from the pipe in a circle and their roots may be kept warmer.  

I already have something kinda like that - a pump house.   All the plumbing does not break.   Now that I think about it - it has a metal lid, but I haven't thought to check it for snow in the winter.  I will try to be a bit more observant.

paul wheaton wrote:Do we have a thread here?

 Not a thread but a reply to a thread.  Here's a link to where Travis mentioned it in detail Who's using heated waterers?

I do like the idea of being able to set the ground temperature in the area.  I'm just not sure if that relatively small amount of sunlight will be able to overcome the thermal inertia and conduction of the dirt down deep.  I really don't know.  More experiments are needed!

Mike Jay wrote:I'm just not sure if that relatively small amount of sunlight will be able to overcome the thermal inertia and conduction of the dirt down deep.  I really don't know.  More experiments are needed!

Experiments, experiments!!!  Mike, I like you're questioning about the energy balance issue and the rate of that small area's energy collection vs. the rate of the earth's ability to move that heat away.  But then I start thinking about an array of these that are active all year such that during the summer you get a large hot zone of soil that goes down fairly deep....how long will that hold in there?  Seems like a very interesting way to add to the microclimate of a space.  It makes me think more about the tefa thread and perhaps modifying the radiator idea a bit like such:

That's a beautiful diagram Greg!  Is that the start of a new acronym?  GWPGRAYRPIESC?  What are the pebbley rectangles at the bottom of the pipes?  Wait... are they even pipes?

I like the array idea and using bricks as mass to hold the heat through the night (If I'm thinking the way you're thinking).  Would black bricks be better or glass on the South side?  I can see how having glass to suck in all the heat every day would be good.  Then again, having a black brick surface to radiate heat at night in the winter would also be good.

My hunch is that the energy balance of solar in vs earth heat moving would lead us to have shallower pipes in the ground.  If you can do twenty 10' pipes or five 40' pipes, I'm thinking the 10's would store the heat better in a more concentrated area.  Then as the earth sucks it away, it could be moving towards another pipe instead of into the depths.  Plus it would heat the roots of the lemons.  

I've realized that if you use a pipe (vs a stack of blocks), the digging part isn't that hard.  Assuming you have sandy soil, a post hole auger will make an 8" hole as deep as you want (straightness not guaranteed).

If you hit groundwater first, does that present an opportunity?  If the ground water is at a nice temp (say 45 degrees) and you sink the array of pipes down into it, would you have a permanent source of 45 degree air for the array?  And then the pipes wouldn't need the glass or the internal circulation riser pipe?  

I would think you are going to need a real grid of pipes,  say 5 pipes by 5 pipes or something to overcome the heat loss. I can't believe a single pipe surrounded by earth would be able to keep any heat for any length of time. I think it would lose all its heat almost immediately after the sun stops hitting it.  You may be able to plant a tree in the middle of a grid of pipes with enough room between them for the tree roots to grow through and still be able to access enough water. I think Mike's idea of shorter pipes being more efficient is accurate.  If you keep the pipes the same size,  they are going to collect a certain amount of heat from the sun,  and the longer they are,  the more ground to suck the heat away. I would be interested in trying it with even shorter pipes than mentioned,  say down to maybe 4 feet long.  It would be interesting to see where the sweet spot occurred,  but my guess is that would change constantly dependent on the amount of sun on any given day.

Oh, one more thought.  If the plants being protected drop their leaves in winter, the pipes could be really close together with the plant's branches intertwining around the pipes.  Then in winter the pipes would get sunlight and radiate it mere inches away from the branches.  In summer the heat isn't needed so the shaded pipes just hibernate.

This concept would mainly apply with dormant trees and arrays that don't/can't store summer energy for winter use, OR pipes that tap into ground water for their heat and don't need direct sunlight.

Mike Haasl wrote:That's a beautiful diagram Greg!  Is that the start of a new acronym?  GWPGRAYRPIESC?  What are the pebbley rectangles at the bottom of the pipes?  Wait... are they even pipes?

I like the array idea and using bricks as mass to hold the heat through the night (If I'm thinking the way you're thinking).  Would black bricks be better or glass on the South side?  I can see how having glass to suck in all the heat every day would be good.  Then again, having a black brick surface to radiate heat at night in the winter would also be good.

My hunch is that the energy balance of solar in vs earth heat moving would lead us to have shallower pipes in the ground.  If you can do twenty 10' pipes or five 40' pipes, I'm thinking the 10's would store the heat better in a more concentrated area.  Then as the earth sucks it away, it could be moving towards another pipe instead of into the depths.  Plus it would heat the roots of the lemons.  

I've realized that if you use a pipe (vs a stack of blocks), the digging part isn't that hard.  Assuming you have sandy soil, a post hole auger will make an 8" hole as deep as you want (straightness not guaranteed).

If you hit groundwater first, does that present an opportunity?  If the ground water is at a nice temp (say 45 degrees) and you sink the array of pipes down into it, would you have a permanent source of 45 degree air for the array?  And then the pipes wouldn't need the glass or the internal circulation riser pipe?  

That acronym is catchy, isn't it?  But I think we'll need a few more letters before we're done!  The pebbley rectangles at the bottom represent coarse gravel.  I figured I'd want the water table to not rise above the gavel and into the masonry tubes that way the air can still circulate from side to side while still allowing the cold air to drop down to the ground water and exchange with it.  I want the water to not freeze and I figure the gravel would help with that too. I do think the ground water would help a lot.

I'm thinking I'll put the collumns about 4' apart and they'd be the size of 2 cinder blocks side by side, either with or without brick cladding.   Here's what I was thinking about for a grid:

Then I figured I could add more off from it to produce a larger grid:

It's lots of work and some cash to get all the blocks, so I was thinking of trialing just 4 radiators surrounded by a wall of foil faced foam to see what happens over the coarse of a year.  Would love to see if citrus, figs, coffee or tea can make it.  What do you guys think....will this be an ok trial size?  Should be able to see some effect and measure it, no?  Anything else I should be considering?

I'm thinking figs might be the easiest.  They will drop their leaves and are relatively speaking hardy.  Hardy citrus and tea might be next easiest.  Not sure if coffee will make it, but would love it.  I was only planning on going down 6 feet with the pipes at my site by digging down somewhat and raising the soil level the rest of the way to make the 6 feet.  I was also thinking of digging out a walk way between the trees and filling it will coarse gravel that would also allow cold air to drop down in the paths to get it away from the trees...a sort of cold well design, perhaps extending that outside the cube to let the coldest air drain out of the cube....but not at all sure about that part.

I guess the bonus with the tall perimeter wall, besides blocking the wind and giving off a bit of radiated stored heat, would be that it would keep out the deer.  I don't mind a bit of deer browsing, but I wouldn't miss that.  They definitely like browsing my in ground fig trees.

Mike Haasl wrote:

paul wheaton wrote:Do we have a thread here?

 Not a thread but a reply to a thread.  Here's a link to where Travis mentioned it in detail Who's using heated waterers?

Thank you Mike!!!  Travis' post really gives me a lot of hope that the radiator array will have a good chance of working.

A form of this has been used with a house, I thought I saw a youtube video of it but it might have been a web page, but each end of a 100-200 foot long tube was black and inside a glass space, with a screen on each end to keep stuff out. One end was in a sort of chimney space and much higher up than the other end. The sun would heat up both ends, which would cause a natural draft, drawing the warm summer air through the tube and heating up the earth mass around the house. In the winter both ends were closed to prevent drawing cold winter air through which would cool the mass.

This is very similar to the PAHS system, except PAHS runs year round and there are 2 tubes, with one end outside and the other inside the house so that fresh air is cooled in the summer and warmed in the winter. It's something I would like to try including in my own home design, having these tubes going around the house several times under the waterproof, insulated "umbrella" that would extend out into the earth berm.

paul wheaton wrote: Maybe the first thing to do is a bit of an experiement.   With and without the annodized tube.   10 feet, 20 feet and 40 feet deep.  With a solar tube at the top, or something more elaborate.   And maybe something that is simply a metal cap.

Just curious Paul, for those depths would you use something like this to dig or do you have a better tool/method?

(Start at 6:20 in to skip past the dousing and get to the drilling)

That's what I'd use.  I've dug down to 14' with one and it was tolerable.  I'm guessing I could go to 20 without any help.  After that I'd probably rig up a tall tripod with a pulley.  Run a rope from the halfway point of the pipe to the pulley.  I can imagine that getting you to 30'.  I'm imagining 40' would require a pretty tall tripod or a different technique.

I read an article on storing heat by using rocks inside barrels of water. If the water is heated and the rocks in the container are heated it would provide heat at night. The containers are buried below ground. Also I used a Fresnel lens from an old flat screen TV to heat the black pipe in the glass like a magnifying glass focusing the lens focal point on the Black pipe increasing the radiant heat. It seems to work well. I'm going to try burring the barrels or lining the well wall with a plastic RCP and fil the bottom with gravel up to a certain point and hope that the rocks will retain heat to help facilitate the night radiation.

Mike Haasl wrote:That's what I'd use.  I've dug down to 14' with one and it was tolerable.  I'm guessing I could go to 20 without any help.  After that I'd probably rig up a tall tripod with a pulley.  Run a rope from the halfway point of the pipe to the pulley.  I can imagine that getting you to 30'.  I'm imagining 40' would require a pretty tall tripod or a different technique.

========
Am I missing something here? Doesn't heat work just like water and everything else in nature, ie. it seeks to balance out. To my thinking one can never appreciably increase the temperature of a hole put into this massive heat/cool/cold sink called Mother Earth. Any heat pulled from the Sun will dissipate to the Earth and the volume of heat one can gather is miniscule compared to the sucking power of that ENORMOUS mass of Earth which goes on pretty much forever, in terms of our tiny efforts so the temps really will never get much above any area of the country's annual average outdoor air temperature, aka ground temperature.

Of course, we can use ground heat and we do, in Geothemal uses but that means using some form of energy to "pump" that heat, to gather/condense that heat into usable heat. Also low temp uses like Travis's stock waterers, [I'm gonna go check Travis's idea out right after I post this] I'm guessing that this works less well the further north one goes where frost can be 9 plus feet deep. When it's minus 40 plus a huge wind chill at surface, one cannot, as far as I know, use ground heat to keep a stock waterer from freezing up.

I kinda feel like you're right Terry.  I'd love to see it work though so it's worth a shot.

Mike Haasl wrote:I kinda feel like you're right Terry.  I'd love to see it work though so it's worth a shot.

I love any idea too, Mike, 'cause it gets the creative juices flowing. I am in 9 foot frost country so our stock waterers, SWers, are often the heater type and they can make for huge power bills. There are a few other methods that work, a "new" SWer that is super insulated and it uses the power of a large herd, eg cattle, to constantly pull "heated" water aka ground water into the waterer thus keeping it from freezing - the river principle.

Another one that would work really well for folks in warmer winter climes is a large tank in a super insulated building, sealed well so so cold air movement either. This could be scaled down for the warmer winter temps areas but it works in our minus 40s and more climate. In really nutso temps the tank is partially drained to allow more warmer water to refill the tank.

Another is for folks to use their dugouts/ponds/... and use a constant flow of water pumped from deep in the water source to a TOTALLY UNINSULATED trough. Which is allowed to overflow from one end and the water is channeled back to the dugout or if that isn't possible it becomes runoff. It's much cheaper to pump water than heat it! These are being used at 54 degrees latitude.

Another idea is the SW that uses natural thermal flow, ie. cold water sinks and the "warm" water that is being warmed by the Earth rises. Two large tubes that hold a volume of water and do a natural heat exchange all based on the principles of Mom Nature. Have to find it. Will post again.

Mike Haasl wrote:I kinda feel like you're right Terry.  I'd love to see it work though so it's worth a shot.

Do you recall, ever know about the Montana - Passive Annual Heat Storage idea, by John Hait, Missoula?? Whitefish???.

I don't hear anything more about it. Did it "fail", fall flat because of the principle we are discussing. It sounded brilliant but maybe it had other health issues, moist ground heat moving/creating issues related to folks breathing them? ?? I dunno.

Mike Haasl wrote:I kinda feel like you're right Terry.  I'd love to see it work though so it's worth a shot.

Maybe a new thread. To my mind folks in the usa should have zero stock waterer problems using/adapting some of the principles that have been developed for SWers in super butt cold regions of Canada.

Terry Byrne wrote:

Mike Haasl wrote:I kinda feel like you're right Terry.  I'd love to see it work though so it's worth a shot.

Do you recall, ever know about the Montana - Passive Annual Heat Storage idea, by John Hait, Missoula?? Whitefish???.

I don't hear anything more about it. Did it "fail", fall flat because of the principle we are discussing. It sounded brilliant but maybe it had other health issues, moist ground heat moving/creating issues related to folks breathing them? ?? I dunno.

My understanding is that it worked, and it plays a big part in the wofati design (being the ati part). If people install air tubes but not a way to keep them clean, then mold would certainly be an issue. I plan to use the air pipes in my wofati design this year, and there will be fish tape inserted through each pipe and a spare tape kept on hand. Then once or twice a year, I will tie the spare tape and a towel that is soaked in something like vinegar that will discourage mold on to the original tape, pull it all through the pipe to wipe it clean, and will have the second tape remaining for the next cleaning. An air filter at the outer opening of the pipe should help reduce the cleaning load.

Digging down and transferring heat into the earth, say into a mass of bedrock, I think will be limited by the thermal conductivity of the mass. Solid rock will draw heat down and away, especially as you get deep enough to touch the water table. Once that happens, no amount of heat will help as the water will transfer heat away very fast. The PAHS system instead relies on dry dirt, which has a low insulative value when dry, as well as thermal mass. You keep that mass dry and insulated from temp swings and if using the air tubes to draw air through the mass, then over time the mass increases in temperature. As I live, cook, and probably heat with a RMH in the space, the walls and surrounding mass will got up in average temperature.

For a garden heater, you can use thermal mass as well. One of the more famous sites is the Central Rocky Mountain Permaculture Institute, featured in the book 'The Forest Garden Greenhouse' by Jerome Osentowski which details the methods they use to grow tropical plants at high/cold elevation. Air tubes that run under the beds and heat up the soil during the summer are then used to keep plants alive with warm roots and heated air. They use temp sensors and fans and I'm very tempted after I get my wofati built (which I plan to include a front porch earthship-ish greenhouse on the south side) to expand the greenhouse and include air tubes to charge up the soil. These greenhouses usually have insulation around the perimeter going down 5-10 feet deep, for a thermal break. Some also have insulation under the bottom, so you have a box of protected soil that you can then charge up to a higher average annual temperature. I think this is key a thermal battery that is contained below grade, otherwise the surrounding earth will always be pulling the temp back to the surrounding average.

There's also the rate of heat transfer which I think is a big limit here. Assuming the E-W-N walls were perfectly insulated, the south facing glazing (in the northern hemisphere) has a minimal R value and will radiate heat out when it's cold, and based on the surface area and temp diff indoor/outdoor (where thermal curtains can help), you can calculate a BTU/hour loss. So any heat well that draws heat from below grade will need to have a decent BTU/hour rate to keep the plants above freezing, and the rate of transfer is usually tied to delta temp. This is where having forced air using fans can really help, and where I believe a truly passive setup will struggle as the outdoor temps drop further below freezing, and the heat well transfer rates can't keep up to glazing BTU loss rates.

So the original post is titled growing citrus in the outdoors in the northern US. Which is not going to happen unless you find some method to hybridize citrus. First, the growing season isn't long enough.

Second, even if you keep the ground from freezing, the tree in the air will freeze. Citrus trees require massive amounts of water, and are accordingly structured. These are not hard woods. They are softer than pine.

Oranges, for example are harvested in november, December. In florida. And while a day or two of frost or even ice won't kill them, a week will. it can ruin the harvest if it happens at a bad time.

That said, you can do a combination of passive solar plus geothermal greenhouse that will grow citrus. One set up I've seen on you tube included an active compost heap as part of the path of the geothermal tubing. This seems to be a method that pops up in the uk. Personally, I'm a  little leery of it, since that is methane gas production, which is highly and easily combustible.

Also, the 40 ft hole? You're likely to have water infiltration, which will then probably freeze at the frost line. In the hole. Which if you've used pipe and not, say a steel girder, will break?

On the other hand, and you may be too far north for this to work, you can landscape in some ways that can effectively alter your agriculture zone? I can't remember the proper term. This is done by a partial terracing and using a pond as something of a solar battery. Think roman style amphitheater, with the pond as the stage. This creates a moisture and heat sink, with added solar exposure.

I do love the ideas you've all been tossing around, though.