Re the title of the thread, we got the permission, with gratifyingly enthusiastic Planning Officer support for the 'sustainable' objectives. However, investigation into AGS-like interseasonal storage isn't ready yet, so the project's now entering working drawings etc on a more conventional basis - this forum will be first to know.

Tom,

I like the idea of your non-insulated inter-seasonal thermal store, but I am wondering:

1. how well this will work if there is some kind of water table involved? will the dumped summer heat just be carried away by movement of water in the ground?

2. Have you calculated the amount of energy used to dump the heat into the ground over the summer - in terms of pumping?

3. If you have a heating need of say 15 MwH, how many square meters of solar do you need?

4.constant expansion and contraction of the ground underneath a house/possibly combined with drying or saturating the ground - could it lead to structural problems with the foundations?

well if your loss numbers are right then in the lowest case you would need more than 31m2 of solar dedicated to this endeavour,based on an isolation value of 1 mwh, and assuming 100% efficiency.

Interesting how much better it works on a large scale, and I assume no water table involved.

I have a heating/HW load of about 200,000 kWh at my present level of insulation, so these ideas kind of suit me. The ground floor of my property is around 500m2 and I was wondering whether it made sense to bore a hole in on an angle underneath the property, but this issue of the water table bothers me immensly - common sense tells me that a water table kills this idea - I mean to say you need to know the stratification of the ground - that surely isnt easy to find out.

Whereas if this could be done on a mass/town level, underneath a park for instance it could work quite well.

Of course to actually heat this heat dump would take quite some heat input too.

Better to insulate, use heat pumps and hope the government solves the renewables problem?

Perhaps a lake or dam would make a better inter-seasonal store.

But this link points to something that might have a better chance of working than bore holes.
http://www.icax.co.uk/thermalbank.html

I wonder if, with the bore holes, that you make a thermal store that is too large relative the the surface area available for heat transfer into and out of the thermal store. In short, any heat you input is quick conducted to a large area of soil, much larger than you actually need, at which point the overall temperature of that heat is so low that it cannot be extracted in the way you might hope.

Posted By: hotelRefurber
Actually new build is not something I am particularly interested in as the problems of being eco-friendly are essentially solved with passive house/thermal mass etc, etc. Thermal stores make sense there but really the problem is far less complicated/large than retro fit. Refit/refurb of our building stock is the largest problem we as a society need to look at in buildings.

Totally agree with that. Just about everything needs to be in your favour to make it viable, and sadly that seems unlikely on existing buildings, which is where our big problem is - as you say. Grrr.

Tom

I did my dissertation on this very area. Let us see if we can calculate something. We will need to know the thermal properties of the bungalow, mainly the W/m^2/K, surface area, volume, the thermal response time and the desired temperature. Then we will need to know the properties of the store, mainly the SHC (Granite is 0.8J/g/K I seem to remember), the mass of the store, the losses (W/kg/K and W/m^2/K), the shape of the store and the response time. Now we also need to know the insolence month by month (not just the total kWh for a year). On top of this to make the place easy to live with we will need to know the number and type of occupants i.e. when are they there and how often do they leave the building.

I know it is possible to heat a place like this but as a 'retrofit' I am a bit dubious from just reading through the thread. I would need much more information before I could calculate if it is possible.

Good luck with the idea though.

Nick

Just to say the dynamic modelling I attempted has come to a dead end, with the results being unreliable either way. I doubt there is software out there which can accurately predict heat flows through the depth of soil necessary

<blockquote><cite>Posted By: tony</cite>ralphd, you have a 20C temperature gradient -- I am looking for only 10 C -- there is a glimmer of hope for me then. http://www.tonyshouse.info/</blockquote>

Looking at what your doing, seems to be a controlled insertion of heat via a bore hole and an uncontrolled extraction of heat as that heat rises under your house. Right?

With that extraction of heat being uncontrolled one potential problem could be that the heat you insert is going to radiate in all directions so only a minority portion of it will radiate directly under your house. Because of the length of the bore hole most of the heat would radiate sideways, I think, and I guess you are hoping that it will then radiate upwards during winter. The problem might be that the heat does not move fast enough or in the direction you want.

I wonder if you could gain control of the heat extraction without using a heatpump, probably not.


If it does not work for you perhaps one option might be to retrofit horizontal pipe work in your basement floor for inserting heat into the ground (say a depth of 1 meter), that way the heat will be released, still uncontrolled, but closer to your house so you should collect significantly more of it. You could cover the floor with thick insulation in summer to encourage heat flow into the earth and remove in winter to encourate heat flow back into the house.

Either way, nice house you have there.

Posted By: Mike GeorgeJust to say the dynamic modelling I attempted has come to a dead end, with the results being unreliable either way. I doubt there is software out there which can accurately predict heat flows through the depth of soil necessary

I think the software that was used to model the DLSC borehole field would work. No idea what software was used or how to get it.
http://www.dlsc.ca/borehole.htm

-Ralph

Posted By: ralphd My projection is now for it to take ~5yrs for the heat stored in the summer to help warm the house in the winter (vs the 2-3yrs claimed by AGS proponents).

-Ralph

It looks to me as though the time taken for a store to approach equilibrium is a function of its size. If it is big enough to be reasonably efficient, it will take several years. If it only takes a year or even less the losses will be very high in relation to the heat retrievable. Sort of make sense intuitively too, I think.

Geoff

No, it should be in the library at Plymouth though. Basically came down to needing thirty times the air mass to stabilise for 24 hours. But mine was not a passive system. Was purposely agitated to even out the temperature stratification.

I may get around to re writing it over the summer (when I have finished my present course) into a more readable, less academic form and publish that as it was mainly about the mathematical modeling. Next project is a statistical review of solar energy rather than a steady state model that is usually used.

Nick

This is a very interesting discussion. On researching it on he net, it says that such a store needs to be soil not rock. Why is this the case. We are just buying a house and (without examination) I expect it is built on decomposing granite/shale - could this not act as a heat store?

Posted By: aviatrixThis is a very interesting discussion. On researching it on he net, it says that such a store needs to be soil not rock. Why is this the case. We are just buying a house and (without examination) I expect it is built on decomposing granite/shale - could this not act as a heat store?

Maybe the cost of drilling through rock would kill it. Another big negative would be water moving through the rocky debris carrying off the heat. On a yearlong timescale, the rate of movement would not need to be great to carry off the lot.

Make sure to paint the concrete black! There's a reason the Dutch used asphalt :)

Just ressurecting this to see how various contributors have been getting on with their AGS related projects ...
As for mine:
It's a Bungalow extension, and refurb of the existing roof.
Walls:150mm timberframe with 150mm EPS Platinum EWI.
Roof: 300mm Hemp in "double rafter"/"hidden purlin" (Bit like doing cross-battening using 100 by 50mm timber)

Still building! (One pair of hands belonging to someone with CFS)
Got as far as installing 100m of pipework under 24m2 of slates for the wet loops in SE and SW facing roof slopes.
The slate is single lapped, being fixed using "Nu-Lok" metal rails and channels.
(Less slate meaning less weight and quicker heat transfer)
Ground store is about 70m3 (insulated around periphery to about 2m, and laminated with insulation to force a 6m path for heat delivered)
Going for antifreeze, and solenoid valves as opposed to drain-down.
That's the nutshell ... Where's everyone at?