Marvin

IMHO this is a bit of muddled logic. Our diverter only diverts excess energy from our PV. Excess PV is all the bits which are not used and would otherwise go to the grid. Our diverter can channel 200 Watts for hours to the immersion heating up the water. It can also channel 3kW for 30 seconds. Neither of these energy profiles would produce more heat in the hot water tank using our ASHP. Our ASHP will not run on less than about 700Watts, but the diverter would stop these bits of energy from going to the grid and use them heating the water. During the winter months we have often heated the hot water on little more than the odd cloud break or the spare energy in between the ASHP cycling. I would recommend checking out a solar diverter to anyone who has PV and a hot water tank with an immersion.

We don't turn the MVHR off because we would have moisture issues in the ensuite, however, this year we aim to run the MVHR solely from PV, all year.

Hi @dnb Our weather is often so variable during a day ( in relation to sun and wind) that we've given up trying to predict it. Using brush strokes has been our best strategy. Using the results from the PGIS on a monthly basis has given us a general picture of what to expect regarding PV energy production. Here's our results for last year: Year Settings 2022 ERA5 6% loss Running Act sub diff % kWh Total Jan 2022 216 216 204 204 -12 94.4% Feb 2022 316 532 294 498 -22 93.6% Mar 2022 552 1,084 601 1099 49 101.4% Apr 2022 774 1,858 744 1843 -30 99.2% May 2022 809 2,667 818 2661 9 99.8% Jun 2022 821 3,488 902 3562 81 102.1% Jul 2022 836 4,324 816 4379 -20 101.3% Aug 2022 724 5,048 770 5149 46 102.0% Sep 2022 582 5,630 536 5603 -46 99.5% Oct 2022 393 6,023 362 5965 -31 99.0% Nov 2022 252 6,275 296 6262 44 99.8% Dec 2022 190 6,465 171 6433 -19 99.5% So in the year we produced 99.5% of what was expected. Temperatures are reasonably as expected except for the wind chill factor! We think that the winter production is lower than estimated because we are in a shallow with higher ground to the east and west so during the winter a bigger percentage is not produced. Again, if we decide to cook a cake for instance, because of the good insulation this can overheat the bungalow. Solar gain? Vacuuming! Ironing! Jolly good argument! All heat the house. Too many variables for my liking. I am now concentrating on reducing the purchased overnight energy using battery storage.

Hi @Alan Ambrose Do you have a water tank in the loft or a header tank for the heating?

Yes we have. Works fine on old style night storage heater. Spoke to company and they heat the office like that when spare energy available after heating hot water tank. I think they have a oil heater. As long as ALL the energy is used for resistive purposes. I.e purely heating a electric bar, not controlling like a timer etc.

Hi @RichardL Yes. SWMBO needs to be able to use it when I'm away. There's two camps on the choices of controls. Some new wave, some old school. You'll have to guess which camp we're in!

The aim with the hot water tank is to over heat it enough with otherwise unused energy so it lasts a few days. At present the PV runs at a good day (for winter) then 3 poor days, or about average day after day, or a mix of the 2. Yes we have to adjust things depending on the temperature etc, to achieve the best performance....

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