Thread: domestic geothermal cooling?

Hi, The Grande Designs building show on the ABC thursdays had a similar installation done for a Castle a few weeks back. But they were heating the Castle by laying a black polypipe type setup in the ground over a large area about two feet down at a guess. I only remember because they were laying it in winter and there was ice on the ground and trees yet they were intent on getting heat from the pipes. Good luck with it.



Kazrod.

I've often wondered why there isn't a piping loop included in every driveway slab, for exactly this purpose.

Cheers,
Andrew

Originally Posted by contrebasse

How deep would I need to go?
How do I optimise heat exchange with the earth's thermal mass? Encase in concrete?
How would I calculate the length of piping needed?
Would this be possible as a low cost DIY project?

Would this work at all?????????????????????????????

Matthew

We have a commercial unit, installed about 11 years ago.

Ours uses a ground loop consisting of about 10 (cannot remember exactly sorry) 100mm diameter holes sunk to a depth of about 50 metres. Through these holes runs a continuous thickwall poly pipe that circulates water from the heat exchanger. Once the pipe was laid, a slurry of sand and alum or something like that was poured down the holes to create a thermal bridge between the pipe and the rock.

Calculating the length is an engineering exercise. It depends on the load from the heat pump, the type of material the pipe runs through (ie. how much thermal energy can it deliver, and how fast does it recover) If it's any consolation, the company got it wrong and had to come back and bore more holes...

Can you do it yourself? I'm sure you can, but you'll need some good help.. Challenges would be the engineering and the procurement or fabrication of specialised items like the heat exchanger.

woodbe.

there is usually a lot of information on this subject in the Alternative building type books in local libraries... might be a good way to start to educate yourself on options (I'm guessing that you will look at other options) and begin to get an idea of pricing.
From my understanding there is a 'lag' between seasons, i.e. it takes some time to warm cold ground, and to cool hot ground.

I'll keep an eye on this thread, as I've been wondering if there is a cost effective way of adding cooling to my Melbourne house... I've a shaded side area which requires the paving to be removed and a concrete slab laid.

A great topic for a thread.

The bottom of this page shows a picture of what you trying to do.

http://www.greenhouse.gov.au/yourhom...nical/fs44.htm

Originally Posted by Clinton1

ding there is a 'lag' between seasons, i.e. it takes some time to warm cold ground, and to cool hot ground.

Hi Clinton,

If you laid your pipes close to the surface, that is the case. Once you go down a couple of metres, the temperature doesn't vary much at all.

In the US, where a lake is in close proximity, a lot of these systems use lake water either directly, or by a loop placed on the lake bed, instead of deep underground pipes.

woodbe.

It just seemed too simple to be true. Surely ground loops can be drilled or laid just about anywhere at very reasonable cost. Why don't we hear more bout them?

Well I guess its more complicated than it seems. Woodbe put in 500m of pipe to cool I suppose an average house. Even with my longish suburban garden I'd be hard pressed to lay 500m of pipe horizontally. So I guess its vertical bores to get that kind of exchange going.

I have found a few links I'll share with you soon. And I found out that the average human emits about 450 watts. Now THAT's useful info!

I would think you'd want to use PEX instead of copper.

sh.t yeah, especially if I have to put in 500metres of the stuff!

I was thinking that copper would be a more efficient thermal coupler than PEX though. Maybe need less pipe??

I asked the same question. The answer was that we could have copper, and the price would go up substantially, but the real problem is that repairs are almost impossible. The poly pipe they installed is very heavy walled, and comes with a lifetime guarantee on the pipe and the joins.

Our place is not average. It's a bit of a monster, cavity stone 2 storey joint. Built by a mad italian after the war without a permit by all accounts. Even with the efficient a/c it costs a bomb to keep cool (23-24 degrees on the thermostat) I'm always on the lookout for methods of improving the building's thermal efficiency as a result.

woodbe.

That's right, copper is next to impossible to repair, as everyone who had copper in their slab in the 70's found out in U.S.

Plus PEX has no joins. I shudder to think how many you'd need in the backyard layout.

I poured a slab in my old house with two zones of PEX@200' apiece, and it was tied to the reo. The guys made no special concession about stepping around it, and in fact were standing on it most of the time they were screeding. No leaks yet (fingers x'ed). I was worried, but it is really tough stuff.

I checked out that house manual site. Could maybe have a well digger drill a hole(s) in the front yard and sink the pipe there?

The Gravity Wave Discovery Centre at Gingin in WA is built on a sand plain and uses straight ground water. They pump ground water up and pass it through a heat exchanger on their roof. The water is then deposited direct on the ground where it drains back into the soil. It's very effective but you obviously can only do this in a free draining soil.

Originally Posted by contrebasse

It just seemed too simple to be true. Surely ground loops can be drilled or laid just about anywhere at very reasonable cost. Why don't we hear more bout them?

Well I guess its more complicated than it seems. Woodbe put in 500m of pipe to cool I suppose an average house. Even with my longish suburban garden I'd be hard pressed to lay 500m of pipe horizontally. So I guess its vertical bores to get that kind of exchange going.

I have found a few links I'll share with you soon. And I found out that the average human emits about 450 watts. Now THAT's useful info!

Nah - human energy output is more like 120W on average, but varies according to what you are doing with the energy inputs (food) and stored energy (glucose & fat). So a sleeping person emits about 81 watts, a person sitting or standing (such as in an office) will emit about 100-130W, a moving person about 150-190W, a briskly walking person about 350-450W and athletes can emit up to 1600W running etc.

Of course the higher levels are not sustainable except those at rest ie: sleeping or doing our daily stuff - so about 120W is the average. But capturing that heat is another matter - with current thermoelectric materials we can only convert with 3% efficiency so using that usefully for other purposes is limited. Designers of commercial A/C do take into account body heat from expected occupants when looking at capacity need for buildings.

Geothermal at residential scale will become cheaper and is very effective. Ground based systems need to go down at least 3m so that the temperature remains largely stable all seasons (around 12-14 degrees). The principal is to have piping through which a slow-moving liquid is pumped (ethylene glycol is commonly used) to either transfer heat from the house to the colder earth in summer or heat from the earth to the colder house in winter. This is done via exchange units (specially designed heat pumps).

Any good energy bank will work - so a large body of water such as a dam or lake - or even a sufficiently large underground or shaded water tank that maintains a relatively stable temperature (and can be used for driniking or other uses too so long as the level does not drop too much). Anything of a DYI nature that is close to the surface (ie; above 3m) will not work very well and the effectiveness of the heat exchangers used is critical so this really is for specialists. Mind you not letting heat in for summer or out in winter is the best and cheapest DYI activity - deciduous shade trees, curtains & pelmets, awnings and pergolas with correctly angled slats for winter sun and summer shade, plenty of insulation in ceilings, walls and under floors and good design and siting generally. Not especially 'sexy' but easy to do and very cost effective than high tech solutions.

Regardless of the energy bank type the principal is that use of supplementary energy (usually fossil fuel sourced) is minimised as in summer the temperature is dropped from 35-40 to say 23-25 degrees (using the 10 degree cooler energy bank source) and in winter it has only to rise to 18-20 from an incoming temp from the energy bank of 12-14 rather than the outside air temp of perhaps 0 degrees or lower.

There are a number of installation in this area (ACT using 60-90,000 litres underground tanks. The Australian Geophysical Survey Office (AGSO) building in Canberra uses ground geothermal to heat and cool one of its large office buildings and a water based purpose built damn to do the same for another. This has been operating now for many years and the payback was 3-4 years at late 1990s energy prices.

So look around - there are small player doing these as well as some of the energy companies (such as Origin). As fossil fuel prices go up and real costs start being paid for carbon emissions this technology will become increasingly competitive (and will save money even for those who can afford to invest early).