Solar PV Powered Immersion Heater

Mark Elvin · 11 Nov 2010

We are having a 2.5kw solar system fitted under the rent you roof scheme through British gas as it seems we have nothing to loose & a small amount to gain (typically over a year the system will provide 50-65% of your electricity requirements).

Anyway, this got me thinking about the manner in which we heat our water. Currently we use gas, set twice a day, once in the early morning, once mid afternoon.

I was thinking that if we did the early morning water heating using the gas & the mid afternoon using the solar electricity, the problem being the immersion is about 2.6kw, using more than the cells will produce on a perfect day.

So my question is - can you get l"low power immersions (say 500-1000w) that would enable me to heat the water using the immersion whilst leaving some power in hand?

htgeng · 11 Nov 2010

Mark Elvin said:
So my question is - can you get l"low power immersions (say 500-1000w) that would enable me to heat the water using the immersion whilst leaving some power in hand?

Yes they do exist. If you go to the navitron forum you will find info there. Also someone designed/built a module to switch on a 'load' when there was excess solar power being generated that wasn't used in the house. I think the guy is pretty coy about sharing the design details though.

Mark Elvin · 11 Nov 2010

htgeng said:
Mark Elvin said:

So my question is - can you get l"low power immersions (say 500-1000w) that would enable me to heat the water using the immersion whilst leaving some power in hand?

Click to expand...

Yes they do exist. If you go to the navitron forum you will find info there. Also someone designed/built a module to switch on a 'load' when there was excess solar power being generated that wasn't used in the house. I think the guy is pretty coy about sharing the design details though.

That's cool, at least I'm not hatching some hairbrained scheme.

Mark Elvin · 12 Nov 2010

Well it looks like the house consumes about 400-500w in "standby" mode (e.g when everybody is at work).

So hopefully the cells will produce something like 1500w+ on a dull day, thus meaning there is sufficient power in hand to heat the water through a 1000w immersion controlled by 7day timer.

Paul Barker · 12 Nov 2010

in your case there is no point supplying the grid, so I would enlist an electronics expert to provide equipment which detects when you are exporting and divert that in steps to low powered towel rail elements inserted into a cylinder through taps in the side. they are available as low powered as 250 watt, so you could have 250 watt steps, your monitoring system would switch in however many of the elements for what power is available and cut them one by one also.

But when the insentive for solar thermal comes on board you would better change to that.

the only real benefit of pv is the FIT which you have given away.

Mark Elvin · 12 Nov 2010

Paul Barker said:
But when the insentive for solar thermal comes on board you would better change to that.

Is there likey to be one?
For us the benefit was cheaper electricity bills, despoite not getting the FIT.

big-all · 12 Nov 2010

i would have a look at the small print in the contract
what you are planning is to micro manage the output and use all the power
i am sure there will be a clause that prevents you using more than a certain amount otherwise they wont get a return on there investment

Paul Barker · 12 Nov 2010

Mark Elvin said:
Paul Barker said:

But when the insentive for solar thermal comes on board you would better change to that.

Click to expand...

Is there likey to be one?
For us the benefit was cheaper electricity bills, despoite not getting the FIT.

ref the above
Yes I think there will be a solar thermal grant which will be a lump sum based on the expected hot water usage of your dwelling.

back on topic
it is a nice thought that you will get loads of electricity but in reality if you are at work in the day most of the time pv is generating it will feed back up the grid.

One very simple thing you could do is have a low powered imersion as you say on during daylight hours when you are out and throw a switch on all other electric demand except fridge freezer burglar smoke alarm.

the actual benefit in electricity bill savings is overplayed, since when you use it isn't when the sun is out.

i wouldn't give away rights to my roof for that little amount. Rights which I have to pass on when I sell the house, tied to the deeds. No thanks.

htgeng · 12 Nov 2010

Paul Barker said:
i wouldn't give away rights to my roof for that little amount. Rights which I have to pass on when I sell the house, tied to the deeds. No thanks.

There is a good point to that. The company takes a leasehold on your house I believe, all done with the mortgage company agreement if there is a mortgage. When the house is sold the buyers solicitor may warn them off. Consider if the roof leaks and needs replacing. All those panels would have to be carefully taken off.

As someone said you will have to micromanage the loading otherwise you could actually end up paying more if for example you left loads on and it went cloudy..

Mark Elvin · 1 Mar 2011

I have done more research & the device I'm looking for is called an EMMA.

Going to carefully check the contract to see if it prohibits use of one.

mikely · 1 Mar 2011

big-all said:
i would have a look at the small print in the contract
what you are planning is to micro manage the output and use all the power
i am sure there will be a clause that prevents you using more than a certain amount otherwise they wont get a return on there investment

The major part of the feed-in tariff is the generation tariff which is paid whether the power is exported or not. FIT generation tariff for PV < 4KW starts at 41.3p per KWh with an additional export price of 3p per KWh. The electricity exported may not be metered at all. The FIT rules allow the installer to assume an export of 50%. So they may just assume you use 50%. As this only affects the income from the export price, it doesn’t make much difference. See: http://www.energysavingtrust.org.uk...ity/Consumer-guidance-on-free-solar-PV-offers

I doubt that it is worth spending a lot of money trying to maximise use of the PV power; in fact none at all if the export is estimated. The energy saving trust estimate a saving of about £50 per installed KW per year, assuming 50% use of generated power. Note also they make the point that the installer may only offer generated electricity at a discounted price rather than free.

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Solar PV Powered Immersion Heater

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