As part of this project: http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=10810&page=1#Item_1

I'm interested in creating a thermal heat store. I'm guided by price, so I'm looking at IBC containers.

I'm thinking of digging a 1.5x 1.5m trench one meter from my south facing wall.

Putting 6 or so IBC's in it so that 30-40cm of the tank protrudes above the ground, & filling them up & lidding.

Put seedbeds on top of them for starting plants early.

Then getting hold of old double glazed patio doors, stripping the frames off and making a 'lean to' structure that covers the IBC's and leans against the top of the house.

I guess I then need flaps at the top and bottom of my insulated strawbale wall to allow air circulation when air temp in the lean to is greater than the house, and closing when air temp is lower?

I was also pondering whether you could circulate warm water from a woodburner through these tanks to bring their temp in midwinter?

Questions:

1, Is this a mad idea? Does it have some hope of being useful?

2, Should I moisture protect and insulate these tanks from the surrounding soil? Or given it should be mostly dry soil, just let the heat seep into the soil?

3,How do you make/buy air flaps that will work? Like the ones used here: http://en.wikipedia.org/wiki/Trombe_wall

Thanks

Sam

plastic water tank in a cage

Why not 2000 litre black orange juice barrels, under £100 from agric merchants? Got 2 - that 4 tonnes of rainwater - to leaky-pipe/timer water the polytunnel (doesn't last long - amazing!)

At that price, yes def.

So would I, earth is not that great an insulator, even when dry, and it has a SPC of about a quarter of water but a density about 2.7 times greater, so will soon sap the heat out.

Me too please,
I pay £ 40 around here, and can only fit two at a time in the back of the van

Posted By: SteamyTeaearth is not that great an insulator, even when dry

But it is an insulator, in sufficient depth, just like any other material, even if wet with *static* water (but well short of saturated).

So if you simply buried your tanks, without insulation around, you'd have an effectively infinite hemispherical 'thickness' of insulation all around and under the tanks. That thickness of earth wouldn't be there over the tanks however, so you'd have to insulate over the tanks, just below ground surface, in a wide 'wing' extending outward by whatever the thickness of the ring of soil-as-insulation you deem to be operative horizontally around the tanks - because that thickness will be warm (but cooler to cold further outward) and mustn't loose heat upward to the surface.

Yes, it would take a year or more to 'fill' that hemispherical volume of earth with heat, building up a temp gradient warmest at the tank wall and going cooler, eventually to 'natural' soil temp = about 10C steady, which prevails below 1-2m deep. During that time, your captured heat will indeed seem to be just leaking away into the ground - but once that temp gradient has significantly equilibriated, further heat loss should continue to decline, year by year, to a v low level, as the temp gradient becomes even longer and 'flatter'.

You will then be sitting on an enormous hemispherical heat store, with your tanks at the centre, really only acting as a heat exchanger with the surrounding earth, where the vast bulk of the heat is stored. If the whole thing was buried under your floor slab, which was uninsulated so in direct contact with the heat store (with the 'wing' of insulation perhaps 6m wide all around the house), you'd be close to a classic PAHS or AGS (google for them) interseasonal solar-store house.

Vital would be that there should be no water movement through the earth that you deem to be the operative volume. A bit of static water is OK, just increases the heat storage capacity of the soil without seriously degrading its insulation. Moving water though will whip all your stored heat away in no time. That might be horizontally moving underground water, or it might be vertical rise-and-fall of a water table, or it might be rainfall draining vertically down.

The first two you'd have to check out geologically; the later you'd tackle by converting the insulation 'wing' into also an underground 'umbrella', by putting a polythene membrane under the insulation of the 'wing' - slightly domed, to drain rainwater off to the edge. Doesn't have to be perfect - can be fitted around extg trees/shrubs etc.

For the 'wing' of insulation, a good thickness of Leca maybe, doesn't mind getting wet so the membrane can go under it, beds down solid so the topsoil above isn't squishy, as it would be over e.g. EPS. Possible problem is that the soil layer over the 'wing' wd suffer great extremes of temp, both freeze and bake, because not stabilised by subsoil beneath.

The physicists amongst us will now disprove the principle, because though the outward temp gradient does indeed flatten to infinity, so does its spherical frontal area increase, squared. However they are missing something, because many prototype PAHS and AGS houses are working successfully.

First out of the traps - ST. We should both get a life, not sit on the computer.

Posted By: SteamyTeaWould it not just take on the average input temperature, just like it does now

True of any heat store, unless cleverly stratified etc - a separate question.

Posted By: SteamyTeaThe higher temperature you have your store at, the greater your energy losses

Again, true of any heat store - so an infinite thickness of 'free' insulation is jolly good!

Posted By: SteamyTeaI also suspect that within a few feet of your uninsulated inter seasonal store it is just the normal ground temperature, losses will be huge.

Why suspect that? You're describing the transient situation two months into the first summer of serious solar input into the store. Yes loss will be huge at that time - a spherically expanding step-change temp front encountering virgin-cold (well, 10C) 'natural' ground temp. So heat passes to the next outward spherical 'shell' of ground, and the step-change temp front continues its way outward. What would make it stop expanding outward at any particular radius 'within a few feet'? Why wouldn't it steadily peter out into a lengthening, flattening temp gradient to infinity?

Posted By: SteamyTeaAsk people with in-ground inter seasonal stores for some data, all you need it input energy and recoverable energy and the corresponding temperatures. They won't be very impressive. But I would like to see some month on month figures.

You got me - I don't have that. Anyone? Tony? Viking House?

Posted By: SteamyTeadon't get heat and temperature mixed up

As if I would ...

Posted By: SteamyTea... creates a negative temperature gradient ... heat will flow from the hotter to the colder, this will reduce the temperature in the store.

Just like your in-ground tank surrounded by '2m of 50 m.K^-1.W^-1' (does that mean a good insulator?) - only difference is mine is surrounded by an infinite thickness of soil as insulant.

Posted By: SteamyTeaAs heat flows into the ground it will warm up the ground a bit, but you have in effect an infinite heat sink.

I want to edit that:
'As heat flows into the spherical shell of ground-as-insulator it will warm up the spherical shell of ground-as-insulator a bit, beyond which you have in effect an infinite heat sink of more ground'.

and compare it to your case:
'As heat flows into the 2m thick shell of insulation it will warm up the shell of insulation a bit, beyond which you have in effect an infinite heat sink of ground'.

So no difference, except mine absorbs very much more heat in the process of establishing its temp gradient - but it gets there just the same within a year or more.

And in mine the spherical shell of ground-as-insulator grows in thickness, ultimately to infinity, pushing away the frontier of ground-as-infinite-heat-sink. My infinite thickness of ground-as-insulator will always trump your fixed-thickness of insulation, however thick you care to name!