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Start with a few dozen very simply made heat pipes, each consisting of a 20 foot length of pipe, capped on both ends, containing low pressure water and steam, with no air or other gasses inside. Apply to the middle 1/3 of each heat pipe some thermal insulation.
Drill an appropriate number of evenly
spaced, 20.5 foot deep, holes into the ground, in the area you are going to build the driveway of your house.
Lower one heat pipe into each hole. Fill the bottom third with heat conductive bentonite, then the middle third with the cheapest structurally sound, nontoxic, fill material available, then fill the rest with more heat conductive bentonite.
Then build your driveway on top.
Not suitable for homes built on permafrost :)
Fascinating.
http://www.healingd.../bentonite-clay.htm
Off topic...but fascinating. [2 fries shy of a happy meal, Dec 10 2009]
Seasonal heat storage in sand
Seasonal_20Heat_20Storage_20in_20Sand
[afinehowdoyoudo, Dec 14 2009]
Jet powered snow blowers
http://www.darkroas...ks-for-fun-and.html
These machines are still used to remove snow from runways, and in some cases, for de-icing of planes in Russia. The engine is a MiG-15 or MiG-17 radial compressor engine with a lengthened jet exhaust. [wagster, Dec 17 2009]
[link]
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The physical components of the system are relatively inexpensive -- even the heat pipes. The real cost is that of the labor -- drilling a couple dozen holes 20 feet deep, and the work of manhandling the heat pipes into those holes, etc.. |
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Hmm... it occurs to me, that to avoid collapsing earlier holes as a consequence of drilling later ones, you'd probably need to do these a small number at a time -- drill a small number of holes, put in the heat pipes, put in the various fills, tamp them down, then do the next few holes, etc.. |
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This would be slower and costlier (but safer) than making *all* the holes, then putting in all the pipes, putting in all the fillers, then tamping it all down. |
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Good one. Depending on the ground, drilling those
holes could be fairly easy.. or pure hell.
Unfortunately my place tends towards the latter
condition. |
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I am presuming that these holes are so deep because you want to use the earth as a ... err, cold sink? This deep earth has a constant temperature because of the insulating overlying earth. Maybe you could avoid deep drilling by using the nearby house instead of overlying earth. Horizontal pipes could go under the driveway to run underneath the house. The house insulates the ground under it and so this ground should be comparable to that 20 feet deep. In houses without basements that sit on a concrete pad this would not really require much digging at all. |
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//How expensive is bentonite?//I pay about $8 for a 15kg box every month. |
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I bet Toaster has a cat. I am sorry. But I wonder why bentonite? Why not water full of antifreeze? |
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If there's a useable climatic variation(hotin the summer, coldin the winter) and the driveway is covered with a suitably solar-absorbent material then the earth below the drive way could be used as a heat store. |
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In the summer, the hot surface collects solar energy and that in turn replenishes the underground reservoir. |
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In the winter, a control system maintains the surface a +1C,just enough to stop snow from settling. |
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But you need to handle the runoff of water at just above freezing as it will form sheet ice as soon as it exits the warmed area. |
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The holes are deep so to ensure that their bottoms are several feet below the water table. |
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This deep water is used as a heat source, but instead of circulating the water directly (as in an open loop geothermal system), we suck out it's heat via heat pipes. |
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Regarding the cost... I googled for "bentonite clay cost", and the first page found (titled "Bentonite Treatments") indicates $340/tonne. Is a tonne any different from a ton? |
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As for why bentonite... well, it is a better heat conductor for most types of soil. And you can't fill the space between the pipe and the hole's sides with water and antifreeze, and expect the hole to stay filled, unless your "ground" is solid rock. |
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There's zero risk of creating permafrost below the driveway if the bottoms of the heat pipes are below the water table, because the slow horizontal movement of water through the ground will conduct heat to any frozen water. |
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If the water table is so low that the heat pipes are dry... the risk is relatively low, since summertime heat will be conducted downwards, hopefully providing enough thermal energy to replenish that which was used over the winter. |
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To avoid wasting the earths heat warming the ground unnecessarily, we could include a thermostat valve in each heat pipe, which would be closed when the temperature of the ground near the surface is above 33F. |
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8/7, good point about the runoff, I hadn't thought of that. |
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I can see two relatively easy options, though. |
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The cheap solution, just drain the water directly into your town's sewers... since sewers are kept warm due to household waste water, they rarely freeze. |
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The eco-friendly solution: build the driveway out of water permeable pavers. After all, since we've gone to the trouble of making sure that the ground below the driveway isn't frozen, then that ground should be able to accommodate the melt water from a typical snowfall. |
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If necessary, the above two solutions could be combined -- porous pavers to let most of the water into the ground, and a "proper" drain so that once the ground becomes saturated, the excess can go into the sewer instead of onto the street. |
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//How expensive is bentonite?// |
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I think that's a matter between ben and his clientele. |
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While I think "ton" and "tonne" are interchangeable, mostly the latter is used to refer to a "metric tonne" which is 1,000kg or 2,200'ish pounds. |
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No way there's enough heat down there to melt up to 20 inches of snow in 24 hours, unless you actually do hit water and then force the heat out with a heat pump. It's common for the water table to be hundreds of feet down. |
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To heat a 1000sq ft house with a heat pump running off the heat in the ground, you need 4000 feet of pipe laid 4 feet apart below the frost line. I don't know how that would compare to a driveway, but it's a ballpark, anyway. |
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With the driveway melter, you need a lot of heat suddenly. It's no good to take 3 days to melt the snow, it needs to be melted all the time. |
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I you used permeable concrete or asphalt, would not the cold snow melt just drop down and absorb any excess heat left in the ground? |
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Or you could just bury spent fuel rods from nuclear power stations under your drive. It'll never freeze again. |
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// 20 inches of snow in 24 hours // |
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Let's say the snow has a density of 10% and it's at zero Centigrade (to make the math easier). |
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For a snow depth of 500mm over a square metre, that's half a cubic metre - 500 litres - which is 10% ice. So, 50 litres of ice; 320 Joules per gram to melt it. That's 16 MJ per square metre, averaged over 24 hours that's 185 Watts. |
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And that's neglecting losses to the atmosphere, and you still have water at 0C ready to refreeze at the smallest drop in temperature. |
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Not "bad science", but impractical and also economically doomed unless your heat pipes can harvest 200W per square metre for prolonged periods. |
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oxen_crossing, where do you live, that the water table is so far down? |
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And permeable concrete/asphalt is only permeable to liquid water, not solid snow/ice. |
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Only the meltwater will pass through it. And since the meltwater has already been heated to above freezing, it's not going to absorb more heat, until it reaches soil that's deep enough to have a temperature above the temperature of that water. |
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Thanks for the math, 8/7. So the system I've described will have a horrible time dealing with snow that's been plowed from the street onto the driveway, and a mediocre time trying to melt snow at the same rate as it falls. |
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However, I still think that the system will be able to accomplish it's most important purpose -- preventing a layer of ice from forming on the driveway. After all, we can drive through snow (if it's not too deep), or shovel it away, but ice is slippery and dangerous, and difficult to remove, normally requiring either an ice chopper, or chemical ice melter. |
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Goldbb, it's not that the water table is 4000 feet down it's that you need a large surface area to harvest the geothermal heat. the pipe is often run as a webbing or spirals in a hole then grouted (bentonite or other). |
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Drilling makes up much of the cost for geothermal. A commonly used RC drill rig can easily rent for $4,000 /day or much more. for this idea, such a rig could bang out 20 foot holes in as little as 30 minutes each (including setup, teardown, and move time) depending on overburden conditions and / or rock types. |
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Water is not required for geothermal. |
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Whoa, hold on. Heat pipes will only pump heat
uphill. The condensate return does not work going
against gravity. So there would be no replenishment
of heat in the underground storage during summer.
Also, in the winter there would be the problem of
the water condensing and freezing at the top of the
pipe instead of running back down. Also, what [8th
of 7] said. So its no good. Nice try though.. had me
fooled for days. |
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Well now... if you drill those holes down to the moho, you won't have any trouble keeping it heated. |
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The heat pipe will not replenish the heat of the earth in the summer, that is true. However, the summer sun will warm the surface of the earth, and ordinary conduction (admittedly very slow) will eventually warm the deep earth. |
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Freezing can't happen at the top of the heat pipe until the temperature of the *entire* heat pipe has reached the temperature of water's triple point. That's just how heat pipes work. I could provide a longer explanation, if necessary. |
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I'm not saying freezing can't happen, but once the ground at the *bottom* of the pipe gets that cold... well, it's not a big deal if the water in the pipe freezes (as long as the pipe doesn't burst from it, of course). |
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Some people like to have an ambulance stationed at the bottom of a cliff, others prefer to just build a railing at the top of the cliff. :-) |
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Instead of trying to engineer a solution to the problem, why not just engineer away the problem in the first place? If the snow doesn't land on the driveway, it doesn't need to be removed from the driveway. |
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You could engineer the dwelling so the cars are stored inside, say, in a garage. For example, townhouses in urban areas often have garage doors sitting right at the edge of the sidewalk. |
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You could also cover the driveway, like with a carport, adding some half-walls to limit blowing snow. |
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Intelligent engineering can prevent most of the problem. Then, your smaller problem is more easily solved, probably with direct electric resistive heating elements, powered by the grid. |
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Then you can use whatever clean power generation works at your site to offset power purchased from the grid, if you like. |
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A side benefit of the garage/carport is that it also eliminates dew and frost on the vehicles. |
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Please clarify: you have no circulation in the pipes (pumps or etc.)? The heat is only transferred by convection up from the ground? I'm confused. |
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I saw a VERY cool method of de-icing your drive the other day. Granted it's neither free-energy nor eco-friendly, and in fact it's so insane that it's a wonder anyone tried it at all. But they did. (link) |
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To be honest, I think that this idea belongs in reality and the idea in the link should have been confined to the halfbakery. |
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// it's a wonder anyone tried it at all. // |
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They have a similar device for NBC decontamination - a jet engine mounted on a sealed vehicle, with a water injection system. Used for "pressure washing" tanks on exiting contaminated ares. |
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oxen_crossing, the water in the pipes is not circulating in the conventional sense. |
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Instead, each pipe is a simple straight vertical tube, capped on the top and the bottom. Inside of each pipe is some gaseous H2O (steam) and some liquid H2O (water), and very importantly, no other fluid. This arrangement is what's known as a "heat pipe." |
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The pressure of this H2O is quite low -- well below atmospheric pressure, and consequently the boiling/condensing point of the H2O inside is low. |
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If any portion of the the pipe has a temperature below the condensing point of the steam inside, that steam will condense, releasing heat into the material of the pipe, and then (as the steam has become liquid water) dripping down the pipe to the bottom. |
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The condensation of that steam lowers the pressure throughout the interior of the pipe. The reduced pressure lowers the condensing point of the steam, which may eventually cause condensation from steam to water to stop occurring. |
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The reduced pressure also lowers the boiling point of the water at the bottom, however, and if the boiling point drops below the temperature of the pipe material at the bottom of the pipe, the water there will boil and turn into steam, absorbing heat from the pipe, and raising the pressure within the pipe. |
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These two processes (condensation and boiling) will continue, until no portion of the pipe in contact with the steam in the pipe has a temperature below the condensation point of that steam, and simultaneously no portion of the pipe in contact with the water in the pipe has a temperature above that water's boiling point. |
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Consequently, heat can be transferred through a heat pipe upwards against gravity very efficiently, but not so easily downwards. |
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This idea has been brewing in my brain for quite a while, just now found this on here.
I was just researching what refrigerant to use, to be honest I've never thought of using plain old water.
So far I had been planning on propane, tuned to the right pressure would boil at 40º. It was also just a big empty tube, no insulation in the middle. I imagine 3/4" copper pipe just hammered down, 2 or 3 per peice of sidewalk.
drill thru, blast it in, pressurize, a little quickcrete cap and move on. No need to repour the whole thing. |
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I have no idea how many watts it would be able to move.. if you catch a few inches of snow in one night it might take a day or two, but hopefully it would be enough to prevent the packed-down ice shell that survives a 45º day. |
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