Earth Notes: On Xsorb Interseasonal Heat Storage (2008)

Water has a surprisingly high specific heat capacity. It is therefore a good store of heat energy in terms of both volume and weight per unit of energy stored. ~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 (ie kilowatt hours) of electricity going into a 1 tonne or 1 cubic-metre or 1000 litre water tank (ie a big tank).

(Note that as of 2018 the Sunamp heat batteries based on sodium acetate trihydrate have triple the storage density of water by volume. Magnetite is good too.)

In the mild winter of 2011 to 2012 we used the lowest-yet amount of gas for space/central heating ~2MWh. Though a more normal winter might demand closer to 2.6MWh (@1000HDD12: with 1000 Heating Degree Days at a base temperature of 12°C). We would hope to reduce that figure with improved heating efficiency such as better insulation, but that's the circa-2012 expectation.

Interseasonal

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.)

Xsorb

The Xsorb system uses heat stored in latent heat of vaporisation (etc) of water bound to silica, activated alumina and zeolites. If working as claimed, it 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. It 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 humidity 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. That 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.