RMS messing about with a sand battery

[Swwils]

People are forgetting that even if you have an absolutely massive thermal store if it's sized beyond what you will take out of it - all you are doing is just losing your "charge" to losses.

So most will not be efficient past 1-2 days of capacity just by physics.

[Oldgreybeard]

I knew a chap many years ago that was converting an old chapel (by coincidence one where my great grandfather had been the minister in the 1890s) into a home. He build a concrete block room in the centre of the chapel, two storeys high, and filled it with big granite rocks he'd collected from the beach, one by one in his 2CV. He also collected lots of old storage heaters, stripped their cases off and fitted them at the base of the pile of rocks.

He insulated this room very well, and all the rooms in the house had a wall that abutted the central heat store room. Each room had a vent to the heat store room, that could be opened or closed, and each vent has a computer type fan that could suck warm air from the rock filled central room. He heated the rocks up all year around, using a home made wind turbine, that used a re-wound lorry alternator, I think. The chapel was on the top of a hill, so the wind turbine worked pretty well.

[Moxi]

People are forgetting that even if you have an absolutely massive thermal store if it's sized beyond what you will take out of it - all you are doing is just losing your "charge" to losses.

So most will not be efficient past 1-2 days of capacity just by physics.

But isnt thermal loss also a function of the delta T so a larger thermal store at lower temperature losses less than a smaller thermal mass at a higher temperature?

Moxi

[Oldgreybeard]

People are forgetting that even if you have an absolutely massive thermal store if it's sized beyond what you will take out of it - all you are doing is just losing your "charge" to losses.

So most will not be efficient past 1-2 days of capacity just by physics.

But isnt thermal loss also a function of the delta T so a larger thermal store at lower temperature losses less than a smaller thermal mass at a higher temperature?

Moxi

Very much so. I think the massive thermal store in that chapel was intended to run at a very low temperature differential, probably only around 10°C to 20°C above room temperature, for that reason. The low temperature differential allowed the insulation to be effective without taking up too much volume. The chap that built it was a former aerospace engineer, who'd dropped out of the rat race because of stress and a view that what he was doing at work was morally indefensible.

[Swwils]

Yes but think about the practicality.

The whole focus is to decouple the time when energy is consumed from a central resource and when energy is used in the house:

1. Massive thermal store that can store all your heat for a season: circa 4000 kw, is a tall order. Especially given the temperatures (and losses) that would require.

2. Smaller thermal store that lasts 1-2 days, taking use of a daily cheap rate. Everyone with an AGA familiar with this.

Even magic vacuum insulation 200mm thick would see a daily loss around 5% at significant temps - getting the heat out of a low temp system is the real challenge. I think the UK has a real bias towards DHW tanks because we love wet heating so much, so makes "sense" we have wet stores.

We really should be pushing for de-strat pumps.

[Oldgreybeard]

The chapel conversion I mentioned was off-grid, by choice rather than necessity. I think the sensible heat capacity of the thermal store was pretty massive, probably of the order of 1MWh to 2MWh, maybe a fair bit more. Heating was by relatively low temperature warm air, I don't remember what he did for hot water. For that project it made sense, as the chapel was a large building, and taking up a chunk of space smack bang in the middle of it for thermal storage didn't have much impact on the living space needed for just the three of them.

It was clearly an impractical solution for most homes, where space is at a premium, but for this project, being built on a shoestring budget, in what had been a ruin for many years, it seemed to work well.

[Gareth J]

You can go round and round in circles with it;
Water; cheap, energy dense, easy to exchange but has a maximum.
Solid materials; Upper limit bound by insulation performance more than anything so despite a lower heat capacity than water, can store more energy in the same space. But at high temperatures, very lossy.

Unbounded by space concerns, sand, or dry earth is interesting as it's such a relatively poor conductor. I imagine heating a storage heater block core to, say 500C via direct heating in summer, which is surrounded by a BIG volume of sand. Like a mini version of the earth. Keep cooling the extremities as you draw down the stored heat and delta TS, and therefore insulation requirements are keeps low, slowly the centre heat migrates out.

Take a bit of modelling to get it right though. And servicing the innerds might be a challenge.

[Oldgreybeard]

And servicing the innerds might be a challenge.

That it was! The chap that built the big thermal store in the chapel built a concrete block tunnel into it, about 4ft high, with a large foam insulation block stuffed into it as a door. The stripped down storage heaters were at the end of this, more or less in the centre at the bottom of the rock filled chamber. He built the insulated outer walls up as he added more rocks, so crawling into that access tunnel all I remember thinking was the mass of granite rocks above my head . . .

[GarethC]

https://electrek.co/2022/11/21/rondo-bi ... /#comments

Thoughts on this all?

[Oldgreybeard]

https://electrek.co/2022/11/21/rondo-bi ... /#comments

Thoughts on this all?

Looks exactly like a large version of the GEC Nightstor from around 40 to 50 years ago!

There seems to be a rash of re-invention of the high temperature thermal stores that were seen as the way ahead in the 1970's recently. Perhaps for the same reason. Back then, nuclear energy was seen as being able to generate clean and cheap electricity, town gas was being phased out because of the cost and vast amounts of pollution the coking plants created, and oil and coal were expensive.

It all got knocked on the head when North Sea gas came in, as that was dirt cheap by comparison, and quickly became the fuel of choice for pretty much anything that needed heat. Now gas has got more costly and we're more focussed on the environmental impact, it looks like electric storage heating is having a bit of a resurgence.