[furrybill]

Folks,

I did some reading through the forum and didn't come across this question, probably because its a bit in la-la land, but I'm asking anyway. [By the way, the search function isn't working, is that because I've got scripts turned off or a function of the site?]

I've got a plumber friend who is very bright and has looked at all sorts of alternative heating methods for up here in relatively cold southern Maine. His preference is a pellet stove. Solar doesn't work too well at this latitude, and normal geothermal doesn't produce enough heat on the coldest days.

I was thinking about his comments and taking my desire to be independent of all inputs to an extreme and it occurred to me: why not drill deeper? If a geothermal well only gives you 50-55 degree water, just drill more! Hence my question. Would it be possible to drill, or how far would you have to drill, to get temps that would allow you to produce plenty of heat energy. What's more, how deep do you need to go to get water hot enough to let you produce electricity?

Is this totally out of the realm of possibility for a residential house? If it is possible, are we talking ridiculous cost?

Cheers, Bill

[purdum]

Not sure why "normal" geothermal wouldn't work for you, assuming you are referring to a ground-loop system rather than air loop. It's 7 degrees here in Michigan as I type this, and our geothermal heat pump is keeping the house at a comfortable 68 degrees. It often gets to 10 or 20 below zero and we've not noticed a significant drop in room temperature. The ability of a geothermal heat pump to keep up with heat loss in the house will depend mostly on how big the compressor is and how well your house is insulated.

[frankthetank]

You shouldn't need to go deeper. For how i understand it, the unit "extracts" heat from that 50F water raising it to over 100F... and dumping the "used" water into another well. I have noticed several people who aren't happy with the loops (maybe not enough of them or deep enough) vs the people who use a pump and dump system (2 wells usually).

A woodstove and a few years worth of wood probably wouldn't set you back more then $1500

Purdum-

What does that cost you to run during the winter? My neighbor has one with the loops and he isn't too happy with the performance. He also was too early and never got any government breaks to install it! I'm pretty sure his house is kept warmer then mine, and i know its well insulated (its brand new) but his bills are still higher then mine in the winter (i'm on NG). I do like his zone heating. It would be nice to be able to keep different rooms, different temps. The AC in the summer is basically the cost of running the pumps.

[Gerben]

I was thinking about his comments and taking my desire to be independent of all inputs to an extreme and it occurred to me: why not drill deeper? If a geothermal well only gives you 50-55 degree water, just drill more! Hence my question. Would it be possible to drill, or how far would you have to drill, to get temps that would allow you to produce plenty of heat energy. What's more, how deep do you need to go to get water hot enough to let you produce electricity?

Is this totally out of the realm of possibility for a residential house? If it is possible, are we talking ridiculous cost?

I have no idea about the geology of Main, but for an approximation: how deep do you need to go to get to 50-55 degree water. What is your average soil temperature at the surface. If you want to double the temperature difference, you'll have to drill approximately twice as deep.
Geothermal is used to make electricity, but only where there is favorable geology, like near Yellowstone or in Iceland.
It's out of the realm for a residential house. We'd be talking millions of US$ if in Main.

[mos6507]

I'm really warming up (no pun intended) to the idea of passivhaus (aka super-super-insulation). If you all but eliminate heat loss, then once you bring the house up to temperature, it takes very little energy to keep it there.

[purdum]

FranktheTank - we've been consistently running about $850-950 for the entire heating season, which averages 6000-6500 degree days. We replaced an old oil furnace, which was costing us $2400+ per year for heating, so we save several thousand per year. The geothermal had a payback period of 8 yrs and this is the fifth season of using (so yes, we also missed out on tax credits). We went with ground loops (300' of piping, 5' deep) because it was cheaper than drilling wells and we had the space to do it. NG is not available to us, just propane. The system runs in reverse in the summer to give us air conditioning, which we didn't have before, and cooling costs for the entire year are under $150. We've been very happy with the system - no oil fumes, no soot, relatively maintenance free, and it is zoned differently for 1st and 2nd floors. PM me and I can send you a Word document that has a full cost-benefit analysis.

[jbrovont]

Totally depends on the geology of the area, but for an idea of drilling somewhere that doesn't have a hotspot close to the surface, this might give you a starting point TauTona (3.9km deep)

The mine is so deep that temperatures in the mine can rise to dangerous levels. Air conditioning equipment is used to cool the mine from 55 °C down to a more tolerable 28 °C. The rock face temperature currently reaches 60 °C.

[PrestonSturges]

Maybe if you had an old oil well you'd get more heat

[SilentRunning]

You shouldn't need to go deeper. For how i understand it, the unit "extracts" heat from that 50F water raising it to over 100F... and dumping the "used" water into another well. I have noticed several people who aren't happy with the loops (maybe not enough of them or deep enough) vs the people who use a pump and dump system (2 wells usually).

We installed a geothermal unit this summer - we use 1.5 wells ( a well plus a shallow dry well) Such systems are called "open loops". They tend to be more efficient than the closed loop systems - and way more efficient than a closed loop that is too small. The only caveat is that your water can't be "hard" with an open loop.

What does that cost you to run during the winter? My neighbor has one with the loops and he isn't too happy with the performance. He also was too early and never got any government breaks to install it! I'm pretty sure his house is kept warmer then mine, and i know its well insulated (its brand new) but his bills are still higher then mine in the winter (i'm on NG). I do like his zone heating. It would be nice to be able to keep different rooms, different temps. The AC in the summer is basically the cost of running the pumps.

The bane of closed loop systems is not enough loop length. Such systems work great in the fall/early winter phase - but then the ground around the coils freezes and the system works harder and harder to extract heat from cold frozen ground, just as you are in the time of year when you need maximum heat.

With an open loop like ours you have an essentially infinite heat source at 50F. Based on data gathered so far, we're going to use <$500 to heat the house this winter.

[Mudpuppy]

As a rough base rule for every kilometer (0.6 miles) down you go the temperature increases by 20 degree`s celcius. Of course this is just an average. So you would need to go down miles before things warm up enough.

[Dreamtwister]

Would it be possible to drill

How deep you can drill is a function of technology and money. Essentially, the deeper you drill, the more expensive it is to do so. As far as I am aware, the deepest manmade borehole is the Kola Super Deep Borehole (Near the Norway border). Kola SG-3 is 12,261 meters deep.

or how far would you have to drill, to get temps that would allow you to produce plenty of heat energy.

As many have said, that greatly depends on the local geology. I know of one neighborhoodin Toronto that built a community geothermal project. They ripped out the road and went with an horizontal installation. It's only about 10 meters down. If they had gone vertical, they could have extracted more heat, but it would have raised the cost significantly.

What's more, how deep do you need to go to get water hot enough to let you produce electricity?

Madpuppy's calculations are approximately correct (barring variations in local geology). Geothermal electricity operates by heating water to the boiling point and using the steam to drive a turbine. That means unless you are living over a hotspot (If there were any of those in Maine, there would already be geothermal power plants there) you are looking at *at least* a 5 km borehole.

Is this totally out of the realm of possibility for a residential house? If it is possible, are we talking ridiculous cost?

Geothermal heating? No, it's perfectly doable. The biggest disadvantage is the capital costs (drilling). But if you can come up with around $25,000, there's no reason not to do it.

Geothermal electricity? Yes, it's almost certainly out of the question.

[Tanada]

Upon reflection it all depends on what liquid you are using as your boiling fluid. For example, if you had enough money and a waver to use Ammonia as your working fluid you could use a moderately deep bore hole to heat the Ammonia as a pressurized liquid, then allow it to boil before running the gassified Ammonia through a steam turbine. You would then use the spent Ammonia gas in a heat exchanger to heat water for your hydronic home heating system and/or hot water supply. The last step would be to compress the cooled Ammonia gas back into a liquid state and pump it back down into the loop to be reheated by the geothermal energy.

[JRP3]

We installed a geothermal unit this summer - we use 1.5 wells ( a well plus a shallow dry well) Such systems are called "open loops". They tend to be more efficient than the closed loop systems - and way more efficient than a closed loop that is too small. The only caveat is that your water can't be "hard" with an open loop.


The bane of closed loop systems is not enough loop length. Such systems work great in the fall/early winter phase - but then the ground around the coils freezes and the system works harder and harder to extract heat from cold frozen ground, just as you are in the time of year when you need maximum heat.

With an open loop like ours you have an essentially infinite heat source at 50F. Based on data gathered so far, we're going to use <$500 to heat the house this winter.

What about a vertical closed loop system? That way you avoid the shallow frozen ground problem and hard water isn't an issue. Probably more expensive than open loop vertical wells, don't know about horizontal closed loop. What's the energy use difference between pumping water up from a well compared to pumping horizontally through a loop?

[Mudpuppy]

Mos, super-insulation requires some sort of air-exchange otherwise the water vapor from gas combustion, your body, showers, etc. builds up in the house and causes rot. The air/heat exchange is expensive itself and requires energy.

Ventilation is very important to prevent the humidity buildup. But it should be relatively easy to make a simple countercurrent heat exchanger for that. Bear in mind when I say `it should be` I mean that although I have studied such systems in biological systems I have never actually built one. So this is a purely theoretical idea on how you might go about doing it. But countercurrent heat exchangers are already common for industrial purposes.

In a simple system if you had a long pipe that is insulated on the outside in which outside air was fan driven inside and then inside that larger pipe you had a smaller pipe for the outgoing air then the outgoing air should help to warm the incoming air and minimise heat (energy) loss.
Something like this system is what I am thinking of, with the exception that the see through larger pipe shown here would have to be insulated on the outside of it.
http://en.wikipedia.org/wiki/File:Tubul ... hanger.png
It would be best if the inner pipe was a copper pipe and the larger outer pipe could be a PVC plastic pipe such as what is used for spouting down pipes, but with insulation added to the outside. The inside copper pipe would be connected by insulated pipe to an inside vent, and one side of the outer pipe would also be connected to a different inside vent. The longer the pipe the more efficiant the heat exchange should be.