How many PV panels to keep a thermal store topped up with no use?

Bendy_House · 10 April at 2:56PM

Just thinking aloud.

Exploring bio-mass for a house that is often left unoccupied for a couple of weeks - sometimes even a couple of months - at a time. Biomass is usually used via a thermal store, and this has the benefit of being able to have other sources of heat added to it - solar heat panels, PVs via an immersion, solid-fuel stoves, etc.

As I understand it, biomass has the disadvantage of being a sluggish heat source, so to turn up to a cold house would likely be an unpleasant experience for a few hours? (Unless they can be turned on remotely using an App, like other boilers can? This would obviously be an auto-feed boiler.)

Anyhoo, if designing a new heating system from scratch, how well would a couple of PV panels manage to keep a thermal store nicely hot and ready to go when the house is unoccupied? Assuming, of course, an average amount of sun. There would be no consumption of that stored heat over the unoccupied periods, and thermal stores are presumably very highly insulated? So, pretty much 'anything' from an immersion heater should be useful? On arrival, this stored heat can be used to get the house's rads running right away, whilst the biomass boiler kicks in and takes over. That's the idea...

Average PV panels have around 250W output? A couple, 500W, so surely a useful amount? Over a period of, say, 2 weeks or potentially much more, there is almost bound to be some decent output, but what is the chance of it getting - or just maintaining - the TS at its normal operating temp over that period? In essence, would the output of, say, 2 panels be able to match the heat loss from the TS? The TS would likely be left in a charged state when the house is left, so the PVs would usually only need to counter heat loss.

Does anyone have an idea of this?!

If it's potentially a goer, what's the cheapest way to get a couple of panels and a ~1kW inverter setup? The immersion element would be able to control itself via its 'stat, so just shut off the power to it when demand has been met.

Thanks for any thoughts on this :-)

QrizB · 10 April at 9:05PM

Just looking at the solar aspect, it's going to depend on the time of year.

I have solar panels and live in the central south of England, a pretty good spot for solar (not the best, but better than many in the UK). In summer I can expect to get 4kWh in a day from each kW of panel, but in winter it's less than 1kWh and there are often periods where it's worse; from 11/12/21 to 10/01/22 I only managed to average 0.4kWh per kW of panel.

Let's say you want it to work all year round and you need 1kWh/day to make up the loss from your thermal store (that's a guess, it could be more or less; the Sunamps we were discussing in this thread loses between 0.48-0-84kWh/day, for example). That would need a 2.5kWp system, which at current prices would cost £3-4k for a professional installation.

QrizB · 10 April at 9:45PM

Sorry, replying to myself - here's a kit of parts for such a system, £1565:

https://www.bimblesolar.com/offgrid/complete-packages?product_id=2636

(That doesn't seem to include roof rails for the panels.)

Bendy_House · 10 April at 10:00PM

Thanks QrizB.

That makes sense. A mere couple of panels - ~500W max - would often struggle to light even a conventional bulb in winter :-(

QrizB · 11 April at 7:42AM

Assuming your house is has mains electricity plus a reliable internet connection, if the biomass boiler can't be directly automated the simplest and cheapest option might be to fit a smart switch (perhaps the Timeguard one, like 1961Nick or Reed_Richards did) to a conventional immersion heater in the heat store.

If you switch it on 3-4 hours before you arrive, you'll have 9-12kWh of stored heat ready for use.

Even at current electricity prices, 12kWh will cost less than £4. If you do that ten times a year you're looking at £40/yr on your electricity bill.

Bendy_House · 11 April at 8:37AM

That is a very good idea, QrizB! Thanks

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How many PV panels to keep a thermal store topped up with no use?

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