Immersion Heater - PV electricity

[echase]

Hey guys,

I've taken a look at how much money I could save using solar PV to power my immersion heater:

Very comprehensive but needs more work. I have not checked through it all carefully but it may be about right for stepped controllers. But there is something wrong with your maths for proportional ones as even if you only put ¼ of your total generation into water heating you will displace around 600KWh pa. At an electricity cost of 13p that is £80. ¼ is conservative as it could be up to ½. So the benefit is much higher than you say.

I think there is a serious error in your “Ultimately, with just 3kWh capacity in the hot water tank we can only capture 1.1MWh a year even if the sun shone every day. That's 110L oil which is 72GBP” as 1.1MWh of electricity costs £140,000! Your oil cost of 6.6p per kWh is too cheap as it will cost significantly more than that per kW actually put into the water due to boiler and pipe losses particularly in summer when the lost heat has no value in home heating.

EMMA claim £454 benefit pa based on 13p electricity. http://www.coolpowerproducts.com/documents/EMMA-ROI-calculator-120216-web.xls I think that is over optimistic (in particular I’d challenge the 3375 W, 30% and 65% figures in that table) but it is based on a comprehensive calculation and 10% energy price inflation. Interestingly they claim a 20% improvement as they claim that heating water slower with a low power (or dimmed 3kW) element reduced churn in the tank and so reduces losses. (Rather contradicts the idea of having a pump to churn it for you).

Fitting some of these devices is so simple that I don’t think it’s essential to employ an electrician unless you are altering meter tails. Under Part P regs I think you will find that an electrician is not mandatory although clearly desirable. As this is the Electricians Forum lots of you out there will know better than me; I am only an electronics engineer.

 

[David Bowen]

Very comprehensive

Thanks

but needs more work.

Yep, I'm happy for folks to enlighten me, it's my first attempt.

But there is something wrong with your maths for proportional ones as even if you only put ¼ of your total generation into water heating you will displace around 600KWh pa. At an electricity cost of 13p that is £80. ¼ is conservative as it could be up to ½. So the benefit is much higher than you say.

I think there is a serious error in your “Ultimately, with just 3kWh capacity in the hot water tank we can only capture 1.1MWh a year even if the sun shone every day. That's 110L oil which is 72GBP” as 1.1MWh of electricity costs £140,000! Your oil cost of 6.6p per kWh is too cheap as it will cost significantly more than that per kW actually put into the water due to boiler and pipe losses particularly in summer when the lost heat has no value in home heating.

I think my setup would have an absolute maximum of 3kWh x 365 days = 1,095kWh for the year. I'd abbreviated that to 1.1MWh - Wolfram|Alpha. I should generate 3,116kWh per year according to the SAP figures for my PV so it would be a maximum of 1,095/3,116 = 35% I could capture.

You're right about oil not being 100% efficient but I don't have a sensible multiplier to use here - I'm open to suggestions.

At an electricity cost of 13p that is £80

Yep, if I was displacing electricity for heating water it would certiainly make the economics more compelling.

EMMA claim £454 benefit pa based on 13p electricity...

I believe you nice chaps will come up with a much cheaper device before EMMA breaks even.

in particular I’d challenge the 3375 W, 30% and 65% figures in that table

For reference, for the 21 December to 21 March period my solar generation was 397kWh. If I had a perfect proportional controller it could have used 93kWh so that's 23% of the generated power. We used 270kWh of the solar to power devices, so even if my immersion store was unlimited, we'd only have had 127kWh spare power for heating = 32%. Clearly this could be higher in the Summer, but with my limited immersion capacity I'm still stuck with the 35% upper bound.

A back-of-the-smoke-packet calculation: For September 21->March 21 I can capture 2 * 93kWh and then an upper bound of 183 days * 3kWh in the sunny months = 735kWh per year = 24% of my generation.

If I've got this wrong please come back to me as I'd love for this to be more lucrative.

 

[echase]

an absolute maximum of 3kWh x 365 days = 1,095kWh for the year. I'd abbreviated that to 1.1MWh - Wolfram|Alpha. I should generate 3,116kWh per year according to the SAP figures for my PV so it would be a maximum of 1,095/3,116 = 35% I could capture.

Why only 3kW per day? Most people use more hot water than that, maybe 6kWh. Are you very abstemious? If hot water is effectively free with a controller it makes sense to convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun. That would increase hot water usage whilst reducing electricity import.

You're right about oil not being 100% efficient but I don't have a sensible multiplier to use here - I'm open to suggestions.

Based on gas I’d say 50 -80%. Particularly bad if you run a pilot light all summer just for water heating, like I would have to if I left boiler on.

For reference, for the 21 December to 21 March period my solar generation was 397kWh. If I had a perfect proportional controller it could have used 93kWh so that's 23% of the generated power.

Don’t understand where the 93kWh comes from. I get much more than 32% in summer.

It’s a bit dangerous extrapolating winter figs to all year around. In absence of a full year’s figs better to take the benchmark figs from SAP calculation and use that. My 18 months of stats show I exceed the SAP fig by 10 - 20% in Hampshire.


Sorry my “1.1MWh of electricity costs £140,000” was wrong. Take off 3 zeros.

 

[David Bowen]

Why only 3kW per day?

Our immersion thermostat kicks in after we've consumed about 3kWh. Maybe we have an 11inch element? If the sun ever shines again I'm going to use my heat-sensor to see how much of the tank it's heating.

convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun. That would increase hot water usage whilst reducing electricity import.

At the moment we're using the solar electricity directly to power them and they heat up their water themselves. You're right though, it would be nice to buffer the solar power into the hot water tank as you describe if they had hot water inlets.

Based on gas I’d say 50 -80%. Particularly bad if you run a pilot light all summer just for water heating, like I would have to if I left boiler on.

Yes, as I mentioned in my article, the vagaries of local setups make the economics quite a local thing. Luckily for me I don't have a pilot light.

Don’t understand where the 93kWh comes from.

That's the amount I would have used in the three months from December with the best possible proportional controller, a 3kW immersion element and a 3kWh hot water capacity.

It’s a bit dangerous extrapolating winter figs to all year around.

I'm not. As in my article, I'm saying that September->December is the reflection of December->March in terms of solar input so I took that 93kWh and doubled it. Then I said that I was going to use the optimal case for the Summer, that is 3kWh per day.

Sorry my “1.1MWh of electricity costs £140,000” was wrong. Take off 3 zeros.

You're right - if I was displacing electrical heating and I got full solar power every day of the year I could save 140GBP. Of course I'm not actually going to achieve that figure. Also, that isn't a very high bar if I have to pay a few hundred pounds for a device, especially something like EMMA.

Thanks for your suggestions, keep them coming.

 

[suntrap]

If hot water is effectively free with a controller it makes sense to convert dishwasher and washing machine to hot fill to limit the time their elements need to come on at times there is no/limited sun.

I'm not sure that's such a good idea. I think washing machines work better with the water starting cold and then being heated - mine, for instance, churns around for quite some time before the element is activated. And if you plonk hot water straight into a dishwasher I think it tends to bake certain foods on.

 

[inie meanie]

Just to add to the calculation, I have just fitted a small storage heater in the lounge, so that when my water is up to temp it switches over to this. Now I know when it, s very dark etc it won't be so usefull but at the moment we don't need to switch the heating on in the evening because the lounge is warm enough, thus adding extra savings as heating on in the evenings can soon clock up a pound or two! Oh yeah heater was off ebay for just a few pounds. :- ) ( I do have 8kw on the roof so I have the capacity )

 

[echase]

if you plonk hot water straight into a dishwasher I think it tends to bake certain foods on.

I once had a dishwasher that I put on hot fill only and it worked very well. Speeds up the cycle.

Am still getting lots of private messages asking for kits. Please can I repeat that I am no longer doing kits as my design will be available as a commercially built unit. Can’t give a date yet.

 

[David Bowen]

Just to add to the calculation, I have just fitted a small storage heater in the lounge, so that when my water is up to temp it switches over to this. Now I know when it, s very dark etc it won't be so usefull but at the moment we don't need to switch the heating on in the evening because the lounge is warm enough, thus adding extra savings as heating on in the evenings can soon clock up a pound or two! Oh yeah heater was off ebay for just a few pounds. :- ) ( I do have 8kw on the roof so I have the capacity )

Good stuff. I have a temperature record for the whole of last year, but my solar didn't go in 'til October so I can't run a simulation of using the power when the temp is below some threshold. I'll have a ponder.

I also got a device so I could avoid using the boiler. It's called a termowentylator which makes me smile every time I say it. It's Polish for fan heater

 

[daveUK]

Hey guys,

I've taken a look at how much money I could save using solar PV to power my immersion heater:

How much water can I heat with spare Solar Power?

Notably, I've also run simulations for these device types:

* Switch that triggers based on solar output
* Switch that triggers when there's sufficient spare power
* Controller which ramps up the immersion in proportion to the spare power

Very interesting article which I think a lot of people on here could usefully read.

A couple of minor points.

I agree with echase that you've ignored the efficiency of fossil-fuel fired hot water heating. Before implementing my system for using free PV energy to heat water, I measured the efficiency of my gas-fired system (modern boiler but with fairly long poorly?-lagged run to hot water tank) at only 55%, which surprised me somewhat. At the marginal cost of my gas in summer (£0.081/kWh), this means water heating in summer using gas costs me about £0.15/kWh (cf. the figure you've used of £0.066/kWh). (This means at the moment it's actually cheaper for me to use paid-for electricity for hot water heating in summer at my marginal rate of £0.11/kWh) Obviously these figures are only pertinent to me (as your £0.066/kWh is to you) but illustrate the other end of the range of hot water heating costs.

Many of your comments refer to your assumed maximum usage of 3kWh hot water per day, which echase seems to dispute. I suspect it's a fairly appropriate figure for our 2-person household (but I do only wash when necessary!)

I'm happy to see that your figures suggest a simple single-element switched immersion system is optimal (in winter/spring) for a 1kW heater since I've gone down the cheap & cheerful 3kW-immersion-powered-by-site-transformer route which creates a 700W heater.


Thanks again.

 

[echase]

I measured the efficiency of my gas-fired system (modern boiler but with fairly long poorly?-lagged run to hot water tank) at only 55%, which surprised me somewhat. At the marginal cost of my gas in summer (£0.081/kWh), this means water heating in summer using gas costs me about £0.15/kWh (cf. the figure you've used of £0.066/kWh). (This means at the moment it's actually cheaper for me to use paid-for electricity for hot water heating in summer at my marginal rate of £0.11/kWh) QUOTE]

I have not measured mine but suspect it’s even less than 55% as I have a pilot light and very long pipes. I was using full price electricity instead of the boiler all summer. How do you get £0.081/kWh? Is that because in summer you fall below the usage required to get into the cheaper tariff? Not sure how they decide this tariff, is it based on a quarter by quarter usage or averaged over a year?

 

[johncooperst]

echase, good to see you are still keen to get your product to market
any ideas on dates for this?
have you recieved your test sample and destroyed it yet?
we are sat on the fence with our R&D at the moment as your product sounds perfect but its all about timing, take care

 

[echase]

echase, good to see you are still keen to get your product to market
any ideas on dates for this?
have you recieved your test sample and destroyed it yet?
we are sat on the fence with our R&D at the moment as your product sounds perfect but its all about timing, take care

Tried to send you a PM but your inbox is full.

Samples are too strong to break. Lots of them have passed all sorts of testing.

 

[johncooperst]

i have just emptied it if you are able to try again or you can mail me at [email protected]
thanks

 

[Keith Walker]

I have been looking at a unit to just that but it is SO expensive. I have 3.8 kw pv and would like to spill the excess to the immersion heater. Any chance of some info on how you are doing it?
Thanks

Hi,

I had the same requirements after getting a 2.8kW rent-a-roof scheme installed.

I have designed a circuit that will automatically use the excess power generated by your solar panels. It monitors the load and the generated solar power and uses the difference to control a 3kW dimmer.

I'm sharing my circuit and hopefully this will produce improvements to the circuit for all to use.

Details can be found at www.talkingsolar.co.uk
The website is still being developed (very slowly) so bear with me.

Cheers, Keith Walker

 

[JohnBoyZ]

Hi,

I had the same requirements after getting a 2.8kW rent-a-roof scheme installed.

I have designed a circuit that will automatically use the excess power generated by your solar panels. It monitors the load and the generated solar power and uses the difference to control a 3kW dimmer.

I'm sharing my circuit and hopefully this will produce improvements to the circuit for all to use.

Details can be found at www.talkingsolar.co.uk
The website is still being developed (very slowly) so bear with me.

Cheers, Keith Walker

Hi Keith,

Well done getting your system up and running and going to the trouble of launching the associated web site.
I had a quick look at the web site but couldn't see any details of the 3KW power controller, have you designed your own circuit or are you using a proprietary solution? That question also applies to mains filtering.

Regards
John