Marvin

The design is to protect the relays from over amperage. Actually today the tank was heated and some went in a storage heater.

Yes I have trouble..

Hi @ProDaveYes very complicated. Your quite right, I forgot to show the heater control switches.

HI @pdf27 Thank you for your work on sand calcs. My results have been similar, however, I am not yet convinced that the idea is dead because of materials now available. I have hourly records of temperature and PV supply and energy use for our home over a year and I am going to calculate the whole thing to try and find out what happens using different sizes and shapes with different thermal resistant materials being used. I will post the full information once completed - probably in February. Regards M

Assuming the theoretical set up as listed in my third posting above, my calculation is: Thermal conductivity of sand 0.25 https://www.geothermal-energy.org/pdf/IGAstandard/WGC/2010/2952.pdf Thermal conductivity of Mineral wool 0.035 W/mK https://www.google.com/search?q=thermal+conductivity+of+mineral+wool&sxsrf=ALiCzsZ55VU1Hax1EltJUgwhzjwpX2LASw%3A1672745639975&ei=pxK0Y9iSO9OThbIPt-6M6AU&ved=0ahUKEwiYgq6Np6v8AhXTSUEAHTc3A10Q4dUDCA8&uact=5&oq=thermal+conductivity+of+mineral+wool&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAzIFCAAQgAQyBggAEBYQHjIGCAAQFhAeMgYIABAWEB4yBQgAEIYDMgUIABCGAzIFCAAQhgMyBQgAEIYDOgQIABBDOggIABAWEB4QD0oECEEYAEoECEYYAFAAWL8RYIUUaABwAXgAgAFpiAGlB5IBBDExLjGYAQCgAQHAAQE&sclient=gws-wiz-serp 500 degrees temp change, one meter thick Sand, 400mm thick Mineral wool. My calculation suggests the loss to be about 5 Watts per hour. Where have I gone wrong??

Yes. mineral wool will take up to about 600C without problem, steel pipe, steel container, sand will go a lot higher.

by blowing air along a tube that runs into the sand and heating the air using an electrical resistance element at the start of the tube and circulating the air in the tube around the element and sand(simplified version omitting controls)

Continuing on with the theory of a sand thermal store... Assuming a theoretical one square meter of surface for thermal loss from inside to outside: 510C temp inside one meter thick sand, (ignoring tank resistance), 400 mm mineral insulation, (ignoring external weather protection) outside temperature -10C. The question is In one hour how much heat would be lost.

Thank you everybody for your input. Very informative. I think we all basically agree that sand can hold a lot of energy. What I want to calculate is the losses incurred from a particular design. This is not simple and related only to our property. I have calculated, and recorded the fact, that even in November and December there are days when despite all the household equipment, heating, electric vehicle and battery storage, we still send energy to the grid. In fact every month of the year there are days when this happens. The point is: IF we had a thermal store this energy could be used to reduce the loss from the already energised (is that the right word?) thermal store. A sort of top up. I will now be trying to define what thermal losses would be expected ( and will probably need @SteamyTea's help) from example designs of a store.

Hi Roger. No it doesn't matter apart from being able to get paid for any excess. I think the system would generate about 30,000kWh a year.

Hi @Roger440 The other cost challenge would be upgrading your connection to the grid but I didn't mention because as you say the cost turns out to be silly.

Hi @Onoff Yes Thanks. I don't need 80000kWh of power..... and might need planning permission.

Yes @Radian I have seen systems like that. What's that? 20 cubic meters? The costs would be exstoronate. 😂

They should of done the maths and not waste the time.

Are you referring to any one in particular? SBF?

This December we produced 169kWh from 5.12kW system in the sunniest part of the country which means you would need about a 30kW system. However we produced no PV some days and today 12kWh which means your system would produce about 70KWh. The panels would cover some 27m2. And so on. Your right. An awful lot. M

Hi @pdf27 Another great idea, however there is no roof left that can be used for solar anything. We have PV and an ASHP. Yes we produce about 2000kWh excess in the summer. 4m3 was the guess of the size as I still have to calculate the heat loss. No we are not going to put the thermal storage in the building. It must remain outside the building thermal envelope. We are looking to use about 500kWh of heat during the 6 months of the year. The maths is complicated because even on some days in January like today we produced more energy than we could store or use (there's just less days like this in January). Any idea how much a Sunamp system is per 100kWh? M

I hope to find out. We require (but for us the figures may be different from others) about 500kWh for the winter

Right, thanks @onoff. Specific heat recovery! sorry. Hi @SteamyTea I have looked at air and its a lot of bottles to produce 400kW and also the increase of pressure requires more energy to produce

Yes, but not as a single property storage for heating over the winter being supplied by excess PV power. (as far as I can find).

Yes, one of the things that confuses me is the rate of heat loss difference between a tubular shape and a square shape with the same volume and same thickness of thermally resistant material. 🙂 I'm looking at a tubular shape...

I need to keep the heat for at least 2 months, which is possible, but is it practical.

A luton vaN 18tons?? I think the tyres would burst

store spare energy during the summer for use over the winter (for heating only).