Many folks talk about using the water-tank method of solar heating in their gh, and I'm sure there are those that swear by it. But the fact is, there simply is not a significant efficiency in the passive radiant effect of stored BTUs out of the tanks and into the gh proper. It's even questionable how efficiently the charging process occurs during the day, as the solar energy first must pass through the gh walls (and perhaps shade cloth), and then through a semi-insulating barrier holding the water in a limited length of time.

Much better results would be achieved with a simple active system, using pumps, a solar collector, night/day valves, and some sort of fan-driven exchanger...perhaps even a retrofitted auto radiator. When I was considering a ground-up design for a brand-new gh, I came up with an active solar system that didn't occupy gh space (heat storage below a paver floor) and otherwise had a negligible outside profile. I don't have my notes in front of me, but it would've added about $800 to my materials costs.

-Bruce

I'd love to see your active solar design with heat storage below the paver floor. I am in the early stages of planning a system like what you describe for a 16' X 8' lean-to greenhouse having a raised paver floor supported by mortar block walls. I want to use a solar-powered roof vent fan (solar collector outside, fan inside the greenhouse) and ducting to pull warm air from the top rear of the greenhouse and route it under the floor to exhaust vents at the front/short (south) wall of the greenhouse. I haven't done the calculations yet to see if this will (theoretically) do the job, but I had planned to use this in conjunction with black-painted, 55 gallon drums of water and have a thermostat-controlled electric space heater as a backup. Feel free to e-mail me at pclash@yahoo.com
Thanks!

Unfortunately, my drawings aren't detailed enough for someone else to make sense of them -- they were only for me to derive an accurate parts list. But I can describe it to you well enough for you to get the idea.

Damp earth holds nearly as many BTUs per cubic foot as water. When you have your site planned, excavate a pit below your floor to a depth of about 30". Line the pit with a good quality, thick tarp, mitreing corners as best you can. Cut pink poly panels to insulate the floor and sides, then lay another tarp over the pink poly (optional, but will keep the insulation dry). Fill in 6" or so, then start laying coils of 3/4" mainline tubing used for irrigation (parts available at DripWorks). Maintain a distance of approximately 8-10" between the coils, and give yourself enough extra out of the pit to work with later. Fill in about 8-10", then lay more coils. You'll need about 300 to 400 feet or so. It comes in 500-foot coils for about $125 including shipping.

The solar collector is a concrete slab, four feet by 12, and runs along a sunny wall on the ground. If the climate you live in is exceptionally cold, you'll need to insulate the slab with pink poly as well. Pour the concrete to a depth of 2" and let cure. Make a manifold by punching holes every inch in the 3/4" tubing for 1/4" connectors. There are two manifolds: one on each four-foot side of the collector. Run 1/4" irrigation tubing from one side of the collector to the other (you'll have 46 lengths if it's four feet wide), then pour another inch or two of concrete over the top. When the concrete is dry, cover with blacktop.

Build two hinged wooden covers (each four feet by six feet) over the solar collector, using pressure-treated 2x2s and plywood. Attach a chain on each side of the covers so that you can prop them open at an angle. On the bottom side of the covers, staple mylar film.

Leave enough extra tarp material to wrap up and over the edge of your foundation header, so that you won't get excessive water in the pit if it rains hard. When you have the floor leveled where you want it, sink a bucket in one corner which is the sump where your pump will move the water through your tubing. You could get fancy and caulk connectors through holes in the side of the bucket. An open-loop system will give you a lot less headaches than a closed-loop system, like the way hot water/baseboard heating systems use. You can have the pump operate off a timer, or a thermostat.

During the day the pump is driving the water through the collector and into the pit. At night you close a valve to shut off the collector, and open a valve to your heat exchanger. Like I mentioned, you could get a truck radiator...maybe even one with electric fans already mounted in place. They'll be DC, so you'll have to have an AC converter.

Another benefit to having the pit under the floor, is that the pavers will assist in transferring radiant heat upwards into the gh, adding more BTUs above and beyond what the exchanger is able to do.

You want to drive hot air into the floor. I worked out the details with my dad, who is a retired engineer, and he concluded that hot air is inefficient because of the pipe size diameter takes up too much space in the pit. You'd have to excavate a much more elaborate pit to achieve the same results as that I described above. If you still like the hot air idea, do it in tandem with the water circulation system. When you have your pit nearly filled, use those half-sized concrete blocks, laid side-to-side so that air can pass through the channels from one side of your gh to the other. Rig up a manifold on one side such that your fan can push the air into the blocks, heat them, then come up the other side. Lay your pavers over the top of the blocks. If the pavers "rock" from side to side because of the uneven blocks, then use a bit of sand on top of the blocks.

Unless you're in an extraordinarily cold area, I'd forget the 55-gallon drums. They take up too much space above ground. If you need more storage, dig the pit another six inches deeper. Here's an interesting exercise for you: calculate the volume of a 55-gallon drum, then calculate the volume derived from adding each inch to your pit. You'll be surprised!

That's about it, in a nutshell. Hope this helps.

-Bruce

I agree with others. In the usual configuration, it is not worth the space. If you can make the water do double duty, like rainwater storage, then maybe.

~tom made it work, but he really didn't have a GH per se, his structure looked more like a shed, very well insulated.

bcfromfl, thanks for the info. Sounds very similar to geothermal system. You've given me a lot to think about. I'll be calculating for a while... just picked up a passive solar house book which offers thermodynamic equations for a "solar slab". I will probably go the tandem approach with forced air through the slab with water circulation underneath. I'll let you know what the final configuration turns out to be... may be awhile. If I have my greenhouse filled with plants, tables and whatnot will I get enough direct illumination on the blacktop over the slab? I did the calculations you mentioned and would indeed have at least one barrel's worth of heat storage for each additional inch of pit depth (for damp earth). In your description above is the assumption that the soil will stay damp and if so how? Due to infiltrating moisture, occasional watering, moist greenhouse environment? I'm assuming the covers are to prevent losses at night.

Thanks for all the info folks. Seems like there's many different ways of approaching this. I've been thinking of building solar water heaters around the greenhouse and having pumps circulate the heated water in a mostly closed loop directly underneath the soil of the plants (using a 'grow box' type of design, and possibly a 'cozy coat' type surround). Maybe I'm getting a little carried away haha, but I'd like to try my hand at something elaborate.

Okay, I did the calculations (and there are a lot of them). I won't have enough thermal mass just using a mortar-block base with pavers on top to maintain temperature without a heating system for a 8X16 lean-to greenhouse. Adding 36" of soil with insulation below and on the sides provides just enough thermal mass to lose only 22 degrees overnight (given a 10 degree outside temp). If I add 12 55 gallon drums (stacked pyramid-style)I end up with only a 14 degree drop overnight(conservatively). Not too bad but I'd rather not have the space used up (and expense for the barrels). I have a couple of questions for bcfromfl:
1. Is it necessary to circulate the water during the day while the solar collector is warming? I'm assuming there would be static water in the lines. It seems like circulation would only be needed at night to transfer heat via the radiator to the air.
2. In your description above is the assumption that the soil will stay damp and if so how? Due to infiltrating moisture, occasional watering, moist greenhouse environment?
3. I'm assuming the covers are to prevent losses at night?

I was away for a couple of days.

#1. Absolutely! You're transferring BTUs from the collector into your pit during the day. It's essential for the operation of the system for the pit to be warmed. If you have direct sunlight, that collector will get hot very quickly.

Depending upon the angle of the sun, and hours of light, you should get about 30,000 to 40,000 BTUs per day into the pit. It can hold a lot more than that, but you're limited by the hours per day, and also the relative inefficiencies. If you had a warm spell during the winter where you didn't really need to run it at night, theoretically, you might be able to charge it with maybe a couple hundred thousand BTUs.

#2. You should wet the ground thoroughly once you've completed the structure. It should remain sufficiently damp after that point with the factors you mentioned. Make sure you wrap the tarp up and over the foundation such that outside rain won't be able to fill the pit -- otherwise, you'll have a swamp in there!

#3. The hinged covers have three purposes:

a) reflecting additional solar energy to the collector via the mylar film (make sure they're angled correctly when attaching the chains).

b) avoiding snow/ice/freezing at night during the winter. Depending upon your hardiness zone, it may also be necessary to circulate some water through the collector at night to avoid cracking the cement or damage to the water lines.

If severe freezing is a sure thing during the winter, then the collector should run day and night. Your "day/night" valve will just open the loop through the radiator, instead of shutting off flow to the collector too.

c) to protect the solar collector during the warmer months when the system won't be running.

Hope this helps.

-Bruce

BTW, I thought you might be interested in some of my observations. While I don't have the funds right now to build the "cadillac" gh of which the solar pit I described is a part of, I've been collecting data proving its value for over a year.

I have several thermometer/hygrometers at different spots in my present gh to monitor microclimates, as well as a temp probe about 8" in the ground under the floor. This probe shows me the benefit of a solar floor. During the summer months, my ground temperature will be 78-80F at that depth. As fall passes, this temperature will slowly drop into the 60s.

I also keep track of the outside/inside differential, and the change in the inside (gh) night temperatures as the ground cools. While the ground is still in the 60s, I can maintain a nighttime outside/inside differential that's 2F greater (under outside conditions that are comparable) than when the ground ultimately cools to about 57-58F by late December. This is only measuring the passive effect of the minimal ground temps rising up into the structure.

What this also shows is the benefit of a slight temperature difference of a solar mass. It's not necessary to have a large solar mass many degrees warmer than the gh you're trying to influence. Your only limiting factors are how quickly the pit can be charged, and the efficiency/speed with which the energy can be drawn back out via the radiator.

Hope this helps.

-Bruce

All those complicated doings! It's fine for a science project, but if you want a working, functioning GH in which you can spend the majority of your time actually growing plants as opposed to fiddling with your system and want to use geothermal energy...

It may not be geo energy so much as geo-properties, but if you simply excavate down to at least 2 -3 ft. below your frost line then erect the GH above it, you will have a near energy free GH.

With only a shadecloth, or if you glass glaze it; whitewash; it will stay comfortably cool in summer and with the use of mirrors (polished aluminum panels) to direct sunlight into it in winter, it will stay quite warm and never drop below the natural temp of the ground below the frost line; around 50 deg. There is your Solar energy!

At worst, in exceptionally deep cold spells, or when the sun is clouded over by sleet or snow, just a few oil filled radiators would keep it warm. Another natural factor works in favor of it: In really deep, bitter cold it tends to be too cold for either snow or sleet, and thereby, crisp, cold, but sunny, making the mirrors particularly effective.

Why do so many keep trying to reinvent the wheel? Pit GH's have been effectively growing plants in deep winter for a millnium and more, sans ANY technology as we know it.

The weakness in gh structures is that they must be covered in a material that will let sufficient light through for whatever plants the owner desires to cultivate...which typically means relatively poor insulative properties. I have an 8x14 structure that is reasonably airtight, yet even though my ground temp never sinks below ~57F (if I have several cloudy days in a row during the winter), it still requires 8500 BTUs per hour to maintain a +20F differential on cold nights.

Hanging a double-layer of plastic inside had no appreciable effect, as I have tested it both ways. I think what I might have gained in a smidgeon of extra insulation, was lost by fewer solar thermal units reaching the ground inside through the extra layer of plastic. It also was a royal pain to install and remove each year!

Depending upon what types of plants are selected/desired for cultivation, a pit gh may serve the owner just fine. But for plants requiring higher light (and therefore compromising the insulative properties of the covering), supplemental heating methods must be explored.

If I may suggest an observation, larger gh's tend to be more temperature stable and moderate than smaller ones. The thermal coefficients and heating/cooling techniques for a gh larger than, say, 25-30 feet in length would be vastly different (and simpler) than for one less than 15 feet. A higher roof means hot air can stratify well above plants, and a larger floor area means more potential for solar BTU storage. I'm not an engineer, but I think that ratios of cubic interior volume to square floor dimensions are probably valid.

Just my two cents...

-Bruce

Bruce, I don't disagree about larger spaces being easier to maintain at a consistant temp. However, properly sited and aimed, you really can use mirrors to get brilliant, natural light down into a pit while concurrently; increasing the temp.

Please keep in mind that once we drop into the teens and below, and last night, it was below 0, our ground temp is now on it's way to freezing all the way down to about 30 inches and once it does freeze down to the frost line, it stays there for months, even when we have short mid-winter warm spells. We might have days of mud and slush at the surface in February, but it's still frozen down below well into late March/early April.

I do not honestly believe there is a one size fits all for energy effecient and/or passive GH heating, cooling or lighting needs. Each region has it's own properties and for those of us in areas where the annual temps can swing more than 100 degrees, from high 90's in August to minus 15 in January, given the opportunity to use the natural thermal property of the earth below us, we are foolish to dismiss the notion out of hand simply because the "norm" has changed, but that "norm" of GH's all above ground even in our climate came about in the early years of the 20th Century, when gas and oil for heat and light was incredibly cheap.

Things are changing rapidly and we need to explore our past to see our future. If what worked before those days of cheap energy can be used again, we will- because all too soon, we will have no other choice.

I'm pleased to find this thread. In Montana our freeze line is about 5 feet so going quite that deep isn't practical. However, I do have a line on a cheap, used solar water panel and a huge pile of scrap soapstone so am thinking of a heat sink slab in the floor of my 8x16 greenhouse heated by hot water. Any thoughts on mountain the collector inside the greenhouse, near the ceiling? I figure that's the most efficient place, given my region.

Essentially I'm looking at a roughly six inch concrete and soapstone scrap slab with pex tubing running to the collector, with the top of the slab sloped to give me a basin that could hold large gravel and up to 150 gallons of water. The walking surface of gravel at ground level would be dry, with the floor drain's lip several inches below the level of gravel/floor. I'm hoping for a combination of concrete/soapstone and water as heat sink material plus some "standing water" under the gravel to help with humidity. Very dry here in Montana. All insulated around the edges and bottom with rigid insulation of course. would welcome any thoughts.

WOW!

I'm glad I stumbled on this thread :-D

I was having an issue with using so much space for water barrels, the underground system should be a life saver - at least a space saver LOL

Thank you all for the great insights.

Well, I try to use "distributed" thermal mass. While the 'permanent' GH has this built into the floor (concrete pavers on 17 tons of gravel), I use water-filled plastic jugs and 5 gal. carboys in almost every bit of floor space of the 'temporary' GH not used by the plants. The jugs that hold kitty litter are nicely-sized, convenient to carry, and best of all, *free.*

I have just built a green... hut... more or less based on the size. I have heard many ideas that I am going to try to put together and see what happens! Anyways, it is a 15 gallon white drum that I am not going to bother painting but I am going to use it with a solar heater. Youtube is the answer for this! I have also decided to put the solar heater inside the greenhouse in a sunny spot with a hose coming off the top end of the heater. (where the heat comes out) I then put about 6 foot of hose down into the drum with the end coming back out of the drum that is full of water with a small fan attached to the end of the hose. The hope is that as the air circulates through the hose that is down into the water and heat it up. Once warm, with the fan running all the time, hopefully when the sun goes down, the heat stored in the water will be obsorbed into the now cooler air passing through the heater and hose....? let ya know how it goes in the next week or so...!

Main issues:

The pipe in the barrel will become blocked as water condenses inside the pipe, especially in the daytime, ultimately no air will flow through the pipe ;) A small fan wont move enough air to make a difference. The barrel of water wont have sufficient mass/heat storage capacity to make a difference.