Water has a high specific heat capacity and is therefore a good store of heat energy: ~90kWh/t (so ~90kWh/m^3) for a delta-T of 80K or ~60kWh/t for a delta-T of 50K, which significantly outdoes many electrical batteries for example.
In slightly less intensely techie terms, that is equivalent to 90 units i(ie kilowatt-hours) of electricity going into a 1 tonne or 1 cubic-metre or 1000 litre tank (ie a big tank!).
In the mild winter of 2011/2012 we used the lowest-yet amount of gas for space/central heating ~2MWh though a more normal winter might be closer to 2.6MWh (@1000HDD12). We would hope to reduce that figure with improved heating efficiency such as better insulation, but that's the circa-2012 expectation.
An interseasonal store to carry 2.6MWh heat from summer to winter, to avoid burning fuel at all in winter for space heating, would take somewhere around 40t of water or two buried milk-tankers. Given the small size of our plot, difficulty of access, and the nest of sewers underneath us, that would be difficult to achieve never mind raising eyebrows. (We have had cars end up in the front gardens of neighbouring houses, but I think that the tankers might be considered to be overdoing things.)
The Xsorb system uses heat stored in latent heat of vaporisation (etc) of water bound to silica, activated alumina and zeolites, and if working as claimed offers some of the following features:
- Heat capacity 280kWh/m^3 (@850kg/m^3)
- Density 700--850kg/m^3
- Cost ~£1500/m^3 (thus ~£5k/MWh)
So for us, potentially, somewhat under 10m^3 or 8.5t (which might conceivably fit in our loft space for example), could carry our winter's heat requirement for us, and could be recharged in summer with solar squeezed into remaining roof space, in conjunction with oversized solar/PVT as a dump load in effect.
An additional possible plus for us is that heat is released upon absorption of humdity from air passed through the store. As we had rather too much RH in the 2011/2012 winter in spite of our MHRV, converting some of it into heat in effect could be a very good thing.
For us then, if the costing above is full system not just raw materials, then £15k of capex would avoid a current ~£150/year cost of gas, so would represent a 100-year payback, but might still be good from footprint point of view if the materials do not contain too much embedded energy, etc.