Energy Matters Forums

[taggertycyclist]

Did I blink, or is there a new sub-forum?

I've recently been turned on to the geothermal concept. Now, my tiny mind thought originally that geothermal related only to using the heat from underground, such as the geysers in New Zealand, or tapping into semi-active volcanos. Silly me.

I started thinking about it a bit more when someone on a cycling forum posted that the earth is a thermal mass, and that it has a pretty stable temperature when you dig down a little.

Now I have researched it a bit more, I am beginning to appreciate what it means, and I like the idea. BUT, I'd like to mine, if you like, the experiences of others.

Who has put down a shaft or two, or created an out-and-back tunnel or whatever to get the air circulating from house to underground and back to the house.

The concept seems so simple and, on first glance, cheap. But then I started to think about the cost of boring shafts like putting down water bores (pretty high cost per metre and there are certain issues related to machinery operating in the urban landscape, plus meeting up with other services and stuff like that).

Then we are hearing more in my region about bushfire bunkers. They don't interest me per se, but we all know how cool a buried, large diameter concrete culvert can be, so is there potential there?

It seems to me that if 6ft under terra firma is a constant, say, 15 deg, it would be ideal for heating in winter (heck, we've just survived living at 15 deg inside all this past winter), and for cooling in summer.

Idea, thoughts, experiences anyone? Gordon?

[taggertycyclist]

Unusual for someone to reply to their own post, isn't it?

But my Canadian wife has just pointed out to me that most Canadian residences have basements that are six foot or more under the land surface. And having lived for six months with her parents in Red Deer Alberta, on the cusp of one of their really cold winters (try -40 deg C), I can attest to the fact that the basements, with insulated walls and carpeting, help keep the heating bills down quite a bit.

And in summer, she says, the basements are "blissfully cool" (yes, temps in the mid-30s are not unusual). Her mother even runs the furnace fan (without the furnace lit, obviously) to take the cool air from downstairs to upstairs.

From what I have seen in Western Canada, the Canadians seem to have energy efficient house-building down to a fine art. They use pine building-grade plywood a lot, including for roofing (with a mastic tiling stapled over the top for weather protection) and the windows are double-glazed. There is a code requirement for insulation in the ceiling and walls, too. It is very rare to see corrugated galvanised or Colorbond steel roofing, on houses, along with concrete or terracotta tiles. And indication of how effective the insulation qualities are is that my in-laws home is almost silent inside when there is traffic and playing children on the street (yes, sometimes even at the same time!).

The houses are built so they are in a virtually airtight cocoon, (although wall ventilators and vapour barriers take care of various moisture issues) when compared with the breezy almost see-through construction of Australian houses, especially the rooves.

[Gordon-Loomberah]

Now, my tiny mind thought originally that geothermal related only to using the heat from underground, such as the geysers in New Zealand, or tapping into semi-active volcanos. Silly me.

No silliness, no tiny mind, you are correct!

I think Geothermal should really only refer to proper geothermal, ie very deep under the Earth's surface due to hot rocks.
Ground source more accurately describes near-surface sources of heat (or lack thereof in Canada!). Calling that source geothermal is probably the bright idea of some marketing type, trying to sex up their product description Unfortunately it seems to be spreading, leading to a blurring of its proper definition.
Near the surface, the temperature, once you get down approximately 2 metres, is very close to the average annual temperature of that area, and is almost constant year round. At 1m depth there is still a significant annual variation, but it's a lot less than at the surface. Here its about 16-17C at 2m.
A good way to cool a house in summer/warm it a bit in winter is to run a network of 100mm+ diameter pipes 2m underground and blow air into your house to take advantage of this constant temperature. With a sufficiently long run of pipes, preferably several runs in parallel so you dont need too huge a pump to force the air though, you can make a huge difference to the indoor temperature, often enough to remove the need for air conditioning. A one way flow with outside sourced air or a circulation of indoor air will work.
A couple of years ago I tracked down and visited one house with a small version of this- 2 pairs of 100mm pipes that run about 35 or 40metres, ~4m under ground, and the air comes out at 25C after entering at ~35C on the day I visited and measured it. That is nowhere near long enough a run to get the air down to the average deep ground temp, but it does keep a quite large room at a pleasant temperature in hot weather. I suspect you would need to run a couple of hundred metres of pipe to ensure you got the air down to the deep ground temperature. You also need a long enough run to dissipate the heat you are adding - the pipes need to pass through a sufficient volume of Earth so it doesnt heat up to much in hot spells... although if you had a large farm dam, much less pipe would be needed due to the greater thermal capacity and conductivity of water cf dirt/sand/rock.
A proper sized system like this has the potential to even out your annual indoor temperatures a lot, for quite a small running cost- just the fan(s) electricity. The initial cost of digging a few hundred metres of 2m deep trench could be quite high though. Also, the cost of the pipe will be high, a few years ago I priced 100mm PVC (6m lengths) at about $25 each. The idea of breathing in air that's passed through that much PVC, and no doubt picked up some outgassed solvents along the way, doesn't really appeal to me though. I looked around at other materials, polyethylene and concrete etc, and the costs were even higher. You need to consider condensation too- if the dew point of the atmosphere is above your underground temperature, then water will condense in the pipes- so some sort of drainage arrangement needs to be made, otherwise legions of legionnella could breed in it! - not what you want to be breathing in.
Of course water can be circulated in the pipes instead of air, and a heat exchanger used in the house- something as simple as a car radiator with a fan blowing through it would work perfectly well, but you wont get the same temperature drop as with a direct air system due to <100% efficiency in the heat exchanger.

Cave people had this sorted out millenia ago- live in a cave and you have direct access to the constant temperature air

Gordon

[Tracker]

I know that this is an "Old Thread", but the thought had arisen to get a local "Bore" guy in to sink as deep a bore as one could afford, and construct a "Coaxial Pipe" to sink down into the bore and use that as a heat sink.

ie blow air or pump water down the center and up the outside. I suspect that it would not even need to be concentric.. Preferred but not imperative..

It was just the thought that Bore-Hole machines are standard and potentially usable in a suburban back-yard.

Multiple holes could be drilled for increased energy conversion, and no one would know you even had it operating.

Trenching a hundred meters around your home would be most disruptive..

[GeoffHammond]

One of the subject houses on the TV show 'Grand Designs' did exactly this. The house foundation was a number of concrete piles poured into holes bored into the ground. Within the reinforcing of each pile was a serpentine of pipe - looked about 1". This went to a heat pump or exchanger or something and...

Alternatively, you could look for a block of land on which to build your house that had an old mine and draw air out of it. My mine is too far up the hill to use effectively... *sigh*

[rfitz42]

I lived on a farm for a year that had a 300' water pipe geothermal system attached to a heat pump. The pipe was supposed to be at 6" deep, but was not properly laid, and some areas were less that 1" deep. I liked the idea, but knew about the in-ground forced air system and wanted to go with that. I bought an old house and had to put in an underground system.
I found a 3/4 hp motor necessary to push the air through my 10" ductwork, which had to be 700 feet for my house. I have a 1/4 hp motor on each of the in-house zone entries for the air to suck it into the house, but never need to use them. It was necessary to drill drain holes in the bottom of the pipe to allow the condensation to drip out.
I'm not good at conversions to metric, so I'll just stick with feet and inches. The pipe runs between 6 & 8' deep and runs back and forth accross my lot. This explains why I need the 3/4 hp motor to push the air. I am heating to 68 degrees fairenheit in the winter and cooling to the same tempurate in the summer. My home is 5400 square feet with 11 foot ceilings, plus another 2700 for the basement. I have two 5 ton heat pumps for backup, but have not needed to use them since I installed my system.
I went to geothermal (the name of the style of energy used by the US Department of Energy whether appropriate or not) to replace a natural gas furnace the was 660,000 BTUs. It cost over US $900/month in the first year that I owned the house and I decided to get rid of it. You could heat a hotel with that monster unit. My in-ground cost of installation for geothermal was $40,000, and my ROI is just over six years. That is much cheaper than a car, and renewable energy.
If you contact the US DOE Building Technology Sector in Renewable Energy Department, you can get a mountain of information on setting up your system. It's like a plug and play for me- once installed, I have not needed to do anything to it, and I no longer pay to air condition or heat my home.
TRY IT, YOU'LL LIKE IT!

[Red]

Your local service station has the ground temps on the bowser by law. It is the temp at which the fuel was stored.

[bpratt]

Your local service station has the ground temps on the bowser by law. It is the temp at which the fuel was stored.

Certainly doesn't appear to be law in Queensland, as nowhere on the pump can you see the temperature displayed or a switch/button to display it.

[Red]

It should be there, written in small print even..

[GVZ]

Check out Geiger, "Climate Near the Ground" an old book if you can find it (http://www.amazon.com/Climate-Near-Grou ... 0674135008). Also go to David Holmgren's site to find explanation of his ground tube vented into a pantry to minimise need and use of a refrigerator (http://www.holmgren.com.au/) - see discussion here: http://www.permies.com/permaculture-for ... net-fridge.