Underground Hot Water storage

[AE-NMidlands]

[Just to say, not something I think I'd do, as heating needs are approaching minimal now (with A2A units), but ideas I am fascinated by, especially for new builds. And over 10yrs ago, someone on the MSE forum had bought a house with a large 'proper' rectangular swimming pool, and was looking to fill it in. I always wondered if it would make for a decent and cheap GSHP source, be it water, sand or both. Funny what you remember.]

or if the gradients / elevations (or powered fans) will work for you, fill it with big rocks with clear air passages, put in a Trombe wall (triple glazed s-facing black-painted wall absorber) and route the hot air through it to heat them up through the summer, then duct the hot air out of the top of the store to heat the house when needed later in the year...

[nowty]

I guess i have something similar in having an underground river which constantly replenishes itself to extract heat from with a heat pump.

Keeps the COP at a nice level even if its winter and sub zero outside.

Info on project here,
https://camelot-forum.co.uk/phpBB3/view ... f=17&t=487

[Countrypaul]

Hi Desp, I was also wondering about water flow. Not a day to day problem for say GSHP, but would add up for longer term storage. Might even be a benefit for GSHP carrying away cooled water, during the heating season.

I wonder if placing layers of PIR overlapping (perhaps 5 x 200mm sheets), then filling with sand/gravel/stone dust, would be enough. Not trying to stop water flow completely, but just slow it down enough to make it a non-ish issue. Or just lagging the inside with dpm layers.

Just thinking out loud, perhaps you want the sand wet, to increase total mass, so long as there is little to no flow allowing heat to escape. Perhaps minimal damp proofing is all that is needed, v's the cost of making it water proof?

I live on quite a steep hill, and water leachs through the ground, so in my case, I assume I'd need the store to be pretty well sealed. Are there giant plastic tubs you can buy, something like a cheap version of a hard plastic pond liner, but without the high expense of say a giant rainwater collection tank?

Presumably someone has already thought through most of this?

[Just to say, not something I think I'd do, as heating needs are approaching minimal now (with A2A units), but ideas I am fascinated by, especially for new builds. And over 10yrs ago, someone on the MSE forum had bought a house with a large 'proper' rectangular swimming pool, and was looking to fill it in. I always wondered if it would make for a decent and cheap GSHP source, be it water, sand or both. Funny what you remember.]

When you say giant plastic tubs, the most obvious are IBCs which tend to be 1000L (although other sizes are around) which can be easilyput besideeach other and stacked (max 5 high when full iirc).

[jonc_uk]

Just thinking out loud, perhaps you want the sand wet, to increase total mass, so long as there is little to no flow allowing heat to escape.

There are two different types of heat store being confused here.

Bear in mind that water has a much higher specific heat capacity than rock / sand (Water - 4200J/kgK, sand around 950J/kgK), however as you want to keep it as liquid, you are somewhat limited as to its maximum temperature.

For a sand battery, you might want to heat to say 500C. I would suggest putting something like that below ground would be a really bad idea. Can you imagine what would happen to your 500C store if water started to get in to it? The water would expand 1,600 times and kaboom..

For below ground, you want water only. For a sand battery, above ground and not able to get wet. eg. don't put it in your swimming pool plant room!

The other interesting point is how you extract heat from your 500C store - Caldera are generating steam via cast-in pipes, other systems use air passing through ducts within the store with a heat exchanger to water.

[Mart]

Many thanks JonC, I'd simply assumed that sand, stone etc could store more heat (sub 100C) than water because it was denser. Rookie mistake.

So, maybe dig a hole, insulate well, make it mostly waterproof, without worrying too much, then fill with IBC's, or large water tanks, and enjoy?

Now wondering what they can store, and within their limits, since not designed for ~100C I assume. I notice that HW tanks seem to be somewhere around 7kWh per 100lt, not sure if that's right. So you'd need 20,000lt* to store a decent amount of heat, perhaps 1,400kWh?

Then again, perhaps the HW tank is based on cold mains, to acceptable hot level, so being able to heat the store a tad higher, and drain the heat down to almost zero, allows for more energy?

Apologies I'm being so loose with terms, and knowledge, purely a fun thought process for me.

*I'm actually picturing a hole that could hold 3x3x3 IBC's, so getting on for 27,000lt. Then again, if you need a metre of insulation for seasonal storage, that takes you to a 5m3 hole, which is getting pretty damn big.

Maybe it's not worth the effort on a single domestic basis? Or let people like Caldera lead. I'll shut up now!

[Krill]

On an interesting note, if you were to fill that 27,000 litre store from a home tap, at a rate of 20 litres per minute, is would take 22.5 hours to fill and at Seven Trent prices cost you just less than £50, which actually appears more reasonable than I expected it to.

[Stig]

An example of a build with inter-seasonal heat store:
http://tonyshouse.readinguk.org/

Tony contributes on the green building forum (where I occasionally lurk).

[AE-NMidlands]

On an interesting note, if you were to fill that 27,000 litre store from a home tap, at a rate of 20 litres per minute, is would take 22.5 hours to fill and at Seven Trent prices cost you just less than £50, which actually appears more reasonable than I expected it to.

... plus the sewage treatment costs for that volume? The really big win for rainwater harvesting is that your sewage charges are a multiple of your water import... so if you use rainwater for flushing you side-step a big charge 2 ways: less import and much lower sewerage charges... It's on my to-do list!

I would fill the store with rainwater, having put a filter on the inlet and maybe dosing with some sort of biocide. I don't know what that would be, metabisulphite must be corrosive as it liberates SO2, I haven't thought any further...

[Moxi]

A simple sand filter or sand filter with an activated charcoal polish on top would suffice, if slow filtration is problematic or you don’t want back wash hassles then a uv tube would be fine, lots of uv tube solutions at aquatic pond shops where they are used for koi ponds etc.

If the water store is just for flushing the loo and has a frequent turn over then keeping it free from light and temperature fluctuations will generally keep it sweet enough without any invasive systems.

Moxi

[Krill]

On an interesting note, if you were to fill that 27,000 litre store from a home tap, at a rate of 20 litres per minute, is would take 22.5 hours to fill and at Seven Trent prices cost you just less than £50, which actually appears more reasonable than I expected it to.

... plus the sewage treatment costs for that volume? The really big win for rainwater harvesting is that your sewage charges are a multiple of your water import... so if you use rainwater for flushing you side-step a big charge 2 ways: less import and much lower sewerage charges... It's on my to-do list!

I would fill the store with rainwater, having put a filter on the inlet and maybe dosing with some sort of biocide. I don't know what that would be, metabisulphite must be corrosive as it liberates SO2, I haven't thought any further...

If the water is simply being used an energy store with no other energy input ie light, or nutrients, is there any need to treat it at all? Just look at nowtys' river.

Filling it via rain water was something I considered but the issue is time: if it was implemented in September how long would it take to fill? I think it might take all winter, at which point what is the opportunity cost?

In terms of sewerage, another £50 or so is my reckoning.