I'm a visitor from afar, new to these lands (forum). I'm uneducated and willing to hear all input, however caustic.

That said...

Is there any efficacy to installing a non-invasive GTS? By that, I mean a system that does not require irreversible modification to the property.

I understand the thermal coefficient for concrete is only 1/3 that of water, but is it possible to use a "contact reservoir" laid flush with a foundation concrete floor to induce a marginally efficient geothermal exchanger?


Here's my crazy idea. Build a subfloor atop the concrete, insulated to a degree (R3 maybe?). Below this, install channels of water, not pipes, that lay flush with the concrete. The width of the channel, juxtaposed to piping, will allow for a very low profile sheet of water to pass in close contact with the concrete concurrently optimizing the conduction surface. Radiation would be tampered with an insulated channel top cover, below the false floor.

It's just a crazy idea, but sometimes those are the best kind. Let me know what you know. I want to know more.

All it makes me imagine, is moisture problems and the likelihood of the water freezing as the heat is extracted out of it, then the slab freezing. I imagine that would destroy the foundation.

I have read that some folks are putting thermal loops deep in the ground under their foundations, but think even this might be problematical. Also there would probably not be enough feet of loop to supply sufficient heat.

You might be better off running PEX with water and glycol through the ground under the house, insulate the perimeter of the house down to 4 or even better 6 feet, and use a solar thermal heater to heat the ground all summer in the hope it would radiate the heat back into the house all winter.

-Rosalinda

Well, the environment is not a house, but a greenhouse, and the channel would be enclosed and insulated, so moisture buildup wouldn't occur unless there was a fault. We've also been thinking of building a a tromb wall along the north side, but we're trying to find ways of cutting overall climate control costs in the long term without modifying the property itself.  I think I might have confused some with the term "foundation." We're actually talking about a solid concrete slab.

The issue is that we can't dig so an in-ground system is unavailable to us. The unit has a concrete floor that spans the dimensions. Now if we cover this with a moisture barrier and solar reflector (white plastic sheet) we can prevent excess heat buildup through radiation absorbtion of the concrete (which would help in summer, but be a bugger in winter), but we though by adding an insulated, shaded subfloor, and using a thermal buffer such as water, we can potentially move excess heat from the environment into the subfloor and use the existing concrete floor as a less than optimal heat sink.

If we were able to dig, I'd be all over a 300' loop. But the property isn't ours. :(

Propylene Glycol was going to be added to lower the freezing temperate. The greenhouse itself is partially insulated, and of course, the environment will be kept near 76F as often as possible. The system would be used when temperature ranges are far outside nominal, to relieve stress on mechanical climate control.

Ahhh, that is a very different kettle of fish. An insulated false floor that you run the heated greenhouse air through, should heat the concrete and the ground underneath it, and mitigate the heat inside the greenhouse, but it wont carry over for a long period of time. I think economically, building the floor so it has hypocaust type air channels and using a couple of fans to move the air through the floor might work. Unless you were going to add hundreds of gallons of water, I do not think you will get a sufficient result - which brings me back to fish. Isn't that what some of those greenhouses that raise Tilapia do? Have tanks with the fish that they run the warm air through to grow the algae that feed the fish, then use the fish waste to feed the plants in the greenhouse?

-Rosalinda

We'll have plenty of water on hand, which is why the idea of a water channel occurred. The air could work, since, if the sub-floor was sealed and insulated, there would be a temperature difference between the upper and lower space, just from radiance reflection and the natural coolness of the concrete (you know what I mean...i don't know the word for it yet). but if I was circulating air from the greenhouse to the subfloor, i actually might worry about the mold.

My next question, does anyone know a good place to start researching the mathematics involved with calculating (roughly, we're gardeners not statisticians) the transfer of heat? I'm positive the specific math will be far too involved with too many variables, but I'm searching for the "cover all" equations, to guesstimate. As the system is passive and investment minimal, we're not too concerned about the overall efficiency. I'm just wondering if there's a way for me to have a more exacting game plan going into it.

Any ideas about a good material for the job?

The water will be distilled water & PG and will remain in a sealed tank system, so hopefully contamination and salts can be kept out of the equation. But I'm curious how a material like aluminum would work. It certainly has the most efficient exchange capacity for the dollar (250w/(m.k)) or maybe copper (400w/(m.k)and is widely available and easy to manipulate.

The concrete has a much lower thermal conductivity, but a much larger surface area than the channels, and is much thicker. I'm not too sure about the fine details of thermodynamics. Learnin'.

Creating the flush channel wouldn't be difficult with Aluminum. But what kind of sustainability would the system have?

assuming nothing but thermal stress over time, is the aluminum likely to corrode quickly? Would some kind of coated aluminum be more efficient?

Is there a thermally conductive plastic on the market at a reasonable price?

What are some common materials used for efficient transfer in a regular geothermal system?

Is copper used and if so (I'm sure this is silly but...) what keeps it from breaking down?

Man, sorry...I'm just so curious and naive to some of these concepts.

I don't think it possible to extract anywhere near the heat required from contact with the ground immediately under a slab.

The scope of work for modifying the floor of the greenhouse sounds extensive. If you take out the slab you can drill underneath, and replace. Or you could insulate the ground and do radiant and run with high efficency boiler would be more attractive than I assume gas fired hot air. I am going to install open loop geo in my wifes hobby greenhouse this summer. With "plenty of water on hand" have you looked at water to air open loop?
Eric

Modifications to the property must be easily reversible. Any drilling, slab splitting, or permanent installations are unfortunately out of the question. After some time, we may be able to purchase the property and convert to a "proper" GTS, but in the mean time, we must find a way to get the job done with the tools we are afforded. I understand the idea is not optimal or inline with industry standards.

But you've got to understand I'm coming from the horticulture community, where in a non-commercial (and sometimes commercial) sense, construction is often patchy and improvisational. I'm attempting to hybridize this modality with a more comprehensive forethought and expert opinion.

We will, of course, research and comply with all codes and hire professionals to install what we can not, but we are still in a position to try and maximize the energy potential of the system.


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I've found an in-line, duct, water-air heat exchanger we will be using to assist in temperature control. In fact, this will hopefully replace the need for an A/C all together. It's called the "Ice Box 8in." if you search (this forum won't let me paste things..."no rich content"...pshhh, I'm a proletariat, I don't have rich content...my content's on a budget).

The coolant will be required to run from the NE positioned cooling tank to the requisite S supply tanks. (These tanks will be insulated against radiant heat.) So, the passage of the coolant will run the width of the greenhouse. My thought was by creating a pre-reservoir heat sink on the return side of the system, I can help diffuse thermal buildup before the coolant is returned to the solar-electric heat exchanger. By adding an extra layer of thermal diffusion we were hoping to lower the consumption of electricity.

we were also looking at adding fins to the channel, or prefab heatsinks, to help exchange of the heat from the upper layer to the lower.

In absence of this method, a simply return pipe will be run, exposed to solar radiation, back to the cooling tank, essentially inverting the issue and causing excess use of electricity for cooling.

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What is the average temperature of non-exposed concrete slab? From my own observation, shaded concrete tends to stay close to the earth temperature.
What is the average temperature of the shallowest subterranean "zone", ignoring solar radiance?

Make sure you test any distilled water for pH - it tends to be acidic due to dissolved CO2 and will corrode any metal it comes in contact with (pumps, aluminum tanks etc).

What is the proper PH to prevent corrosion? 7?

Posted By GrapeStreet on 23 Mar 2011 04:36 PM 
After some time, we may be able to purchase the property and convert to a "proper" GTS, but in the mean time, we must find a way to get the job done with the tools we are afforded. I understand the idea is not optimal or inline with industry standards.

Nobody suggested your idea was not "optimal", we are saying it won't work.
The largest contributor to geo success is loop design. If you are unable to have a proper loop system installed, look at a different system.
Perhaps a wood boiler is possible?
joe

Nat gas and open loop geo are not options?

Yes you want a pH of 7 for pumps, metal containers etc.

Jon, it does not sound as if he can even drill a well for an open loop. Maybe one of those air to air heat pump systems?

If the greenhouse is large enough, large amounts of water storage might be the way to go, and then think up clever ways to utilize them for heat input and output. Not sure of your setup, but if you have benches, the water tanks can go under them if you have an active system to add heat to them.

-Rosalinda

Posted By GrapeStreet on 22 Mar 2011 02:20 PM

Is there any efficacy to installing a non-invasive GTS? By that, I mean a system that does not require irreversible modification to the property.

From what I know about Geothermal systems, the Geothermal unit is 1/3 the cost of the install, all the other parts that make up the geothermal system (piping, well, HVAC duct work, etc) 1/3 and the final 1/3 is labor. I recall someone asking about a house sized open hole he had, he wanted to know if he could save a lot on an install since he already had a hole in the ground. I told him in the grand scheme of things, a big hole in the ground really isn't saving him all that much on the total install. (Not to mention the hole wasn't large enough to accommodate the size of the system he wanted to install).

I'm not sure I exactly understand you goal here.  If the duct work is too small, it's going to have to be modified to make it larger to work with a geothermal system, and it will not be easily modified to undo the changes, however, it doesn't need to be unmodified, regardless of what heating method you switch back to, the larger duct work will accommodate it.

Weather you bury the ground loop under the basement floor or lay it on the floor with installation on top of it does not change the fact that the footprint is far too small heat the house on top of it. A rough estimate, a typical horizontal loop will be 400 to 600 feet long for each ton, so a 4 ton system will need anywhere from 1600 to 2400 feet of loop. And running it back and forth on a basement floor isn't going to help much, when installing in a yard, trenches should spaced four to five feet apart. So lets take a look at your basement, lets say it's 40ft x 60ft, that's about 10, 60 foot long runs laid down on your basement floor, 600 feet total, that at least 1000 feet short, and we will not even get into the thermal dynamics of heat transfer with cement here. Let just assume you dug up your yard up and laid 2400 feet of loop pipe and filled the trenches with cement instead of dirt. The thermal transfer wouldn't be any there near as good as dirt, you would have to add thousands of feet of extra pipe to make up for the reduced thermal transfer rate from the loop to the cement. 

In short would it work? Yes, for a week or two at the most, under a light heating load until the Geothermal heat pump extracted all of the heat out of the basement floor, then the system will be running strictly on electric resistant Aux heat the rest of the winter.  I'm not sure of the effects of freezing cement, but in a foundation, it can't be very good for it, not to mention it's going to cost you a fortunate to install. You might as well put a potbelly stove in the basement and use $1 bills to burn in it, it will probably be cheaper in the long run. It would be far cheaper to just heat the house with electric baseboard heat, without the additional $40,000 geothermal system install.

    

Posted By Rosalinda on 22 Mar 2011 04:37 PM
All it makes me imagine, is moisture problems and the likelihood of the water freezing as the heat is extracted out of it, then the slab freezing. I imagine that would destroy the foundation.

-Rosalinda

Actually I been thinking about this and I really don't think the a freezing slab would affect a house too much, after all I see sidewalks outside in the winter that suffer no ill effects. I believe it's the summer where you will see damage. Did you ever see a concrete highway on a hot summer day when there isn't enough space between the sections? The highway buckles. Now imagine a basement concrete slab, with no expansion joints, and much larger then a highway section, heated uniformly to 100+ degree. The slab would expand, pressing into the block walls. Since there is compacted dirt on the other side of the block, and they are hollow, they would give first, the weakest link. I guess it would depend on how much the slab expands, but I believe it would be enough to destroy the interior walls of the bottom blocks. With the loss of support of one of the walls of the blocks, and the weight of the house could cause the remaining bottom block to fail, causing the house to collapse 6 to 8 inches. This would be more than enough to sever any sewage or water piping. Also any supporting beaming in the middle of the house, usually made of hollow steel poles would get crushed as well.      

it would depend on how much the slab expands

Very very little (< .1 ").

I'm coming from the horticulture community, where in a non-commercial (and sometimes commercial) sense, construction is often patchy and improvisational. I'm attempting to hybridize this modality with a more comprehensive forethought and expert opinion.

You need to spend at least a couple of weeks reading some good thermodynamics text or on-line research. Your 'PATCHY' admission and type of questions do show a total lack of understanding of the basic thermodynamic principles, hence you need some more background to even ask an intelligent questions.

A good engineering analysis (other than on a free internet discussion board where all you get is often misinformed opinion) would run you a few grand at least.

e.g. Wylen and Sonntag, Wiley, "Fundementals of classical thermodynamics" or similar whould be in your local library.


My own worthless internet opinion (can provide the calculations for $700, heh, heh) is that your slab scheme is useless, akin to 2nd order prepetural motion machine.

One can, in theory, create a greenhouse (or any other enclosure) that will never freeze just due to ground heat passively coming from below it. This is what frost protected shallow foundations do. Practically, you can't afford the aerogels that it would require to insulate to the required level.

Jonr - I've read about aerogels...would they pass the sunlight needed in a greenhouse application?

What in Sam Hill does "hybridize this modality" mean? Should I be scornful or impressed?

Aerogels pass light well and evidently some are even fine for seeing through. But they are very expensive - hopefully this will change someday.