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Jeremy Franklin
Posts: 58
Location: Binghamton, NY
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This isn't technically Wofati, but since it's using a lot of the same physics, I thought you people might be able to help shed some light.

I'm wanting/wishing to run geothermal lines under my courtyard (1500 sq ft) as it is built.  I'm not looking to heat or cool the courtyard itself, but since it will be a high traffic area even in the dead of winter, I'd like to offset the temperature of the stone floor enough that you can comfortably walk barefoot on it in the summer, and have it be mostly free of snow and ice in the winter.  We're talking a very simple setup with one coiled loop of plastic pipe buried 10-12 feet under the courtyard, and another buried  maybe 6 inches beneath the surface, hooked together by a simple pump.

What I'm trying to figure out is, is the math even there?  In reading other geothermal forums, the general consensus seems to be that geothermal is insufficient for ice melt, due primarily to much higher energy level required to change from ice to water.  However, intuitively, it would seem like, if I took all the energy that was baked into a square foot of stone during a day in the middle of July, and relased it in a day in the middle of January, that seems like it would be enough to keep the stone free of ice and snow, at least most of the year.  However, I don't truly understand the maths involved.   Can I store all or even most of that energy for that long? How much earth battery would I need to store all that?  Most geothermal people start their calculations with how fast you lose your heat in your house.  Obviously, for an open air courtyard, that's going to be pretty high.  But their assumptions are also that A) you want to maintain a stable, comfortable temperature, which I don't, and B) that you're using a heat pump to compress the heat out of the water and distribute it to the air, which I'm not, and C) that ice melt needs to happen fairly immediately, whereas I'm perfectly happy if it needs to take 12 hours or so.

My scenario is this.  I'm in upstate, NY, so, for example, this last winter we got probably 6-8 decent snowfalls around 6 inches each, and one that was 30 inches of snow.  This is fairly typical.  Winter temperatures usually bottom out  around the 0-10F range with a 15-20 below zero event happening once every few years.  In the summer, it's typical to see 90-100F in July and August.  I have a beautiful, 27 acre property that is all gentle southern slope.  I think I have a pretty high water table, and there is a natural spring maybe 200 feet uphill from our build site.  Soil is pretty decent.  The entire property was young forest when we got it, and cattle pasture 40-50 years before that.  So we've got a middling clay/sand mix and a lot of organic material in the soil.  The courtyard will be surrounded by half-buried buildings, mostly heated, most in the hundred sq ft range. I can and will probably want to extend the umbrella out beyond them as well, possibly offering additional protection to the courtyard.  Where there aren't  buildings, there will be a 5-6 foot wall surrounding the courtyard, so hopefully wind scrubbing will be reduced.

It seems to me that my challenges are two-fold: 1) Can I hold the heat from summer into the winter? Especially if the water table wants to creep up into my battery? (For what it's worth, the ground is always most saturated in the Spring, and in the Fall the water table drops considerably, even sometimes drying out the natural spring for a month or two.) 2) is there any sense in trying to insulate the bottom coil to help retain the heat I'm depositing? I'm not sure I could ever afford to do that anyway, but if I could, would it be worth the effort and expense? 3) Can I effectively transfer the heat if the stones are just going to radiate it right to the atmosphere?  Is this a project that requires careful, thoughtful design? Or is it a lost cause that the math just doesn't support? 

Or is there another approach entirely that would be more appropriate?
 
Joseph Lofthouse
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Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
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I think that this is a brilliant idea!  We often have spring/fall snowstorms here, where the heat stored in pavement quickly melts any snow that falls onto it.

30"  of snow equals about 76 cm which contains about 3 to 8 cm of water/ice. Lets use 6 cm for our calculations... It takes 560 calories to melt that much ice.

If we assume that the density of the dirt in the storage battery is 1.5, and that it is at 16 C (61 F), then it is storing  24 calories per ml, -- 7200 calories in a column of dirt 10 feet deep, enough to melt  around 400 inches of snow. Then the math gets complicated.... The rate at which the snow can be melted, which depends on the design of the battery and specs of it's components.

How does that compare to using solar collectors? If we assume that solar radiation averages about 700 watts that equals 6 calories per cm^2 per hour, perhaps 50 calories per day per cm^2, so it would take 11 days to melt the ice if we used a solar collector with the same surface area as the courtyard.

If water flows through a thermal storage battery, it carries the stored heat away with it. This is often mitigated by placing an impermeable cap over the storage battery, and building the battery above the water table.

 
Travis Johnson
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It can be done. I think it was in Minnesota where they tested this very thing. They took a mile of black topped roadway and sank wells into it to help absorb the sun and then store the heat. They wanted to see if they could melt the ice from the road over the winter. Did they ever, they had so much heat they ended up heating a nearby apartment building they had so much excess heat.

My house is geothermally heated and cooled, and even unheated I have never seen it drop below 57 degrees, the ground temperature here in Maine. My chicken coop was heated geothermally too by just having a tile drain loop underground for 100 feet and back into the coop. A squirrel cage fan was enough to keep the coop 20 degrees warmer than outside. Most of the time that meant temps above freezing, even here in Maine.

The biggest challenge I have found personally with radiant heating and cooling is getting the proper amount of spacing in the pex tubing. I will admit I was off in my mudroom. I spaced the pex tubing a foot apart thinking in such a small room (13 x 9 ft) it would not need much to heat it. FAIL! I never thought that it also had a very small thermal mass in its concrete floor. I should have spaced them every six inches.

I would think in your case doing the heat transfer would be difficult, but doable. It is going to vary so greatly. Removing so much heat from bluerock paving in August at noon is going to take some closely woven pex to draw off the heat. Melting snow will be a cinch, there is plenty of radiant heat gadgets to accomplish that.
 
Jeremy Franklin
Posts: 58
Location: Binghamton, NY
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Thanks, guys.  So if I'm understanding it all correctly, the battery should have plenty of storage, unless we have a winter with more than 400 inches of snow, in which case, I've probably got bigger problems than if my geothermal courtyard is at ideal temperature.  And it sounds like we should have plenty of energy available, collected both during the summer, and even supplemented in the winter. In Joseph's example, he mentions that it would take 11 days worth of energy to melt 30 inches of snow (right? I'm reading that correctly?), Which would be our worst case scenario, and honestly, once a year or once every few years, if I have to go out and do a little shoveling, it won't kill me.  And if we only have three inches of snow, then it would only take one day from the battery, which would be plenty, as we only have maybe 15-20 total days of any appreciable snow accumulation (guessing).

Of course, I think that example assumes a perfect solar collector, which my stone courtyard certainly won't be, correct? Or is there such a thing as inefficiencies in a solar collector?

So then it comes down to transfer of heat, right?  Which is probably where I leave the expertise of the Wofati/earth berm clan, as we're then talking about ideal pex spacing and perhaps flow speed and volume. And then I'm asking questions about, do I just leave the pump running 24-7-365, or do I only turn it on during the day in the summer and leave it off at night and in the winter unless I'm trying to melt snow?
 
Joseph Lofthouse
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Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
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If I were making an active thermal storage battery, I'd put a temperature sensor in the surface slab, and in the midst of the storage mass. And only run the pump when it makes sense to run it.  During the summer it might be possible to store plain old ambient heat day and night... I greatly simplified things, but figured the collector was oriented towards the sun and only 70% efficient. and only collecting heat 8 hours per day. A horizontal surface would be less efficient in winter due to the low sun angle.
 
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