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I have a large pile of old ceramic roof tiles (the curved ones you see on old Greek houses - my house is in Greece) and a approx 2 cubic metre concreted space under the house where I can stack them and insulate all round using expanded polystyrene packing I have accumulated over the years (not having the heart to chuck it away!). I am thinking of running a black plastic pipe coiled through the stack and using the store to preheat to the hot water cylinder, the central heating circuit in the winter, or both (using a solar water heater as the 'power' source). Although the result mught be relatively inefficient compared to other options, in terms of money and carbon footprint it will be practically free.
Anybody have experience of something similar, and/or could advise me of do's and don'ts etc.
I've mentioned it here before, I think, but a friend did something similar (but on a bigger scale) around 25 to 30 years ago. He was converting an old chapel (which coincidentally had been one where my great grandfather had been the minister back in the late 1890's) and built a square, well-insulated, room in the centre, maybe ten or twelve feet to a side. He filled this with large granite boulders, scrounged from the beach (one at the time, in the back of a 2CV). In between the boulders be looped heating coils taken from old storage heaters. The heating coils were charged up from a home made wind generator all year around, and to heat the building in winter he just blew air through the heat store and out through floor level vents in each room. It worked well, as I recall, even though the chapel wasn't that well insulated.
I think the problem with a thermal store that's only 2 m³ is going to be the amount of heat you can store in there. I don't know what the heat capacity of tile is, probably less than granite or concrete, and almost certainly a lot less than water (granite is around 1/5 the heat capacity of water). At a guess you're not going to achieve very high temperatures, and that, coupled with the low heat capacity, is going to mean that you won't store a lot of heat in there. Might be enough though, as ST says, try an experiment first.
As always, don't forget airtightness to go with your insulation.
As JSH says the specific heat capacities of most materials are pretty pathetic compared to water. The higher density compensates somewhat but still 2 m³ of water will probably store about twice as much heat as the same volume of ceramic. Ceramic can be heated over 100 °C and doesn't need to be boxed in, though.
Ceramic tiles will be about 0.8 kJ.kg^-1.K^-1 (most earth and stone is)
The density will be about 3000 kg.m^-3 (most earth and stone is)
So you will have about a 1/5.25 (19%) the SHC of water (not counting air gaps but air is 1 kJ.kg^-1.K^-1) and the density will be be 3 times. So 1/5.25 * 3 = 57% by volume.
Does not leak, you can pass air or water over it, can go to over 1000°C, is cheap and does not move.
The main thing is to think just how hot they will get and how much energy you can transfer at that temperature.
Lets say your really lucky and can get the store up to 50°C and you want your house at 20°C You will probably not get an efficiency better than 50%, so your store temperature will, on average be at 35°C. That is 15°C above your house temperature. Imagine a radiator that swings between 20°C and 50°C heating your house, will have to be a big radiator.
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