solar pv to storage heater

[jaymoseley]

Id like to find out if its possible to buy a few solar pv panels, and take the feed from that direct to a storage heater or underfloor heating pad. Not plug into the grid, or even the house electrical circuit. And not have the heater or heating coil attached to the mains either to avoid any shocks etc. Straight from pv to dedicated electric heater. It doesnt matter if the heat only comes in the day. Thats when I need the heat anyways. Ive seen underfloor heating kits. And seperate pv solar panels. Can the pv panel plug straight into the coil of the kit? One reason I ask is that a few months ago I had seen a solar powered pv panel to underfloor heating kit for a few hundred pounds online. Ive lost the link, but I know its possible. What Id like to do Is buy something like that, wrap the coil in bricks and some sort of cover and get cheap heat. Any thoughts advice etc? For instance can the electric output from a pv panel go straight to the heating coil and trickle feed it. and slowly heat the coil Or does it need an intermediate pulse convertor?

[Andy Burns]

What time of year do you want the heating?

What time of year do solar panels give their lowest output for the shortest time?

[Martin Brown]

It is possible to do it, but it is utterly pointless. The only time it will harvest vaguely decent amounts of power is when you are trying to keep the interior of the house cool in summer. Even then you would be much better off just harvesting hot water directly to a thermal store. You can have lashings of hot water in midsummer with the right kit.

The hours of daylight in a UK winter make solar PV a complete non starter. We have loads of solar PV powered "please go round the dangerous bend" signs round here. They sometime manage a feeble glower at you after a good sunny winters day but they are stone dead every frosty morning when they might actually do some good. Batteries in them are destroyed completely every winter and have to be replaced.

Avoid like the plague and refer to trading standards.

Even adding dilithium crystals won't make it remotely useful.

[Dave Liquorice]

And this is Solar PV at about 30% effciency not Solar Thermal which will be more than double that. Even in mid summer with bright sunshine all day you aren't going to get a great many kWHrs, it's not worth the effort with Solar PV. Solar thermal would be a better bet and dump the heat into a smallish thermal store and circulate that for heating but you still aren't going to get a great deal of heat and in winter more or less forget it.

[meow2222]

Possible yes, but it doesn't make sense to do things that way. Solar thermal harvests more power for much less cost.

NT

[Brian Gaff]

This sounds very inefficient if it would work at all. Brian

[Andrew Gabriel]

Grid tie inverters run the panels in series anyway - the inverters normally take 600VDC input, depending how many panels you have.

(That's why fire brigades won't enter houses with solar panels to put fires out.)

[dennis]

Pretty stupid really. A 240V AC supply can have about 350V peak on it which is only a bit less than 10 solar panels in series would be operating at. Any more than 10 panels would probably be two strings and be less than that.

[Dave Liquorice]

Yeah but the 240 mains supply has a big switch that can isolate the house wiring. With Solar PV and those volts and appreciable current are "just there". The grid tie invertor should shut down when it loses the grid but that DC is still present in wiring which is located where? At least CU's or cutouts are normally in fairly common places and within feet of where the mains enters the property.

[Capitol]

Isn't this an urban myth?

[Tim Lamb]

Set fire to Harry's house and find out?

[CB]

"wont enter" might be an urban myth, but "are urged to be cautious" looks more reasonable

[jaymoseley]

Interesting feedback thanks. Obviously not a good idea is the general consensus.

Just as a matter if interest I thought Id run these figures by and see if I can get a clearer picture... My electric storage heater uses 12kwh a night on cheap rate night tariff. (Thats 85 p/night) That gets it nice and hot. Im trying to calculate how many panels would give me the equivelent of

12kwh mains a day...so...

How does one calculate average annual UK output for each 100W panel? Assuming more in summer less in winter etc. And how can that be converted into 12kwh/day? In other words in the UK, if one wanted 12kwh for *general use heating* during the day, how many 100w panels, on average, would be needed to get the the equivelent of a mains 12kwh/day input? Thanks ahead.

[Chris J Dixon]

Try putting your details in

Chris

[Martin Brown]

Average 12kWh/day over the year would be doable if you had very deep pockets but there is a 5:1 ratio of average summer output to winter.

Ballpark figures for annual isolation at UK latitudes are

1800 MJ/m^2 direct sunshine 1700 MJ/m^2 diffuse light

I think that was for Brighton (worse the further north you go).

A commercial solar cell is no more than 25% efficient.

Being generous you can harvest 900MJ/m^2 = 100kWh/m^2 per year

Area needed therefore = 365*12/100 = 45m^2

But 5x that size if you want 12kWh of daily heating available in winter. Minor corrections due to solar PV working better in cold conditions and possible errors on my part converting units here and there. There are calculators about online for any latitude & climate.

If you are serious you would do well to borrow a copy of the now ancient book Sun Power An Introduction to the Applications of Solar Energy by J C McVeigh, Brighton Poly, Pergammon Press 1977.

It dates from the first OPEC induced oil shock but the engineering content is still just as valid today as it was then and it will avoid you falling for the spiel from snake oil and green energy salesmen.

[Andy Burns]

Going on all the optimal assumptions (You live in Cornwall, have a south facing roof inclined at 37 degrees) then 10.75kW of panels should get you min 12kWh/day of electricity in December, and max 56kWh/day in June.

43 x 250W panels of 1.6m x 1m, about £125+VAT each, ignoring whatever for wiring and an inverter if you want to run 240V heaters from them.

The site given earlier is ok.

[A_lurker]

In message , [e-mail] writes

On a 4kW SSE-facing array in Sussex, with some shading from trees in December/January, I get about 3.5MWh/year, so on average, just under

10kWh/day. But that's mostly in May to September. Daily outputs are as high as 25kWh in summer, but at this time of year 3 kWh would be exceptionally good, and a dull day might yield only 500Wh. So to guarantee 12kWh on any day, I make that nine hundred and sixty 100w panels :-(

HTH...

[dennis]

In winter a 2kW array will give you less than 1 kWhr if its cloudy and about 1.5 kWhr if its sunny. In summer its about 2kWhr and 10kWhr for the same array.

The same area of thermal panels will give you about four/five times as much.

With solar thermal you get a lower grade heat from flat panels than from evacuated tubes. The same amount of energy if the store is cold enough to make use of the lower grade.

From a cot point of view you should have solar PV fitted as the fits will pay fo it and return a profit. Then fit an immersun or similar to heat during the day when power is available.

[meow2222]

Flat panel solarthermal yield much more than tubes per area and per pound when heating cold water (and only when heating cold water).

NT

[Dave Liquorice]

Isn't the whole idea that the water doesn't stay cold?

The bottom of a thermal store ought to be at 40 to 50 C, coolest. Once the solar has been running for a while it'll be pushing 60 C or more. This'll be the minimum temperature of the circulating water, it needs to be highre to actually transfer any heat... Ours (tubes) circulates at about 80 C.