Immersion Heater - PV electricity

[David Bowen]

Hey there,

Just thought I'd let you know that I updated my web page based on your useful feedback. You can now change the figures it uses to work out the value of the different arrangements and it will highlight the best performing "simple" element choice:

How much water can I heat with spare Solar Power?

Hope that helps.

 

[echase]

Hey there,

Just thought I'd let you know that I updated my web page based on your useful feedback. You can now change the figures it uses to work out the value of the different arrangements and it will highlight the best performing "simple" element choice:

It’s better but would be nice if you made clear that your headline Summary applies to stepped controllers and not proportional ones.

I have a new installation in London where we have put a web based monitor with 3 current sensors on it: PV generation, grid power and power into the immersion. From this I can see that in last week it’s putting 1-8kWh into the immersion per day. As it’s a very rainy week it’s been limited by the amount of sunshine rather than the thermostat cutting out. During the more sunny days, and even those have been very patchy, it's been 6-8kWh.

So I conclude that your 3kWh is too low and that double or treble that is more representative of a typical household as per a Google search of suitable values. But I don’t yet have the demographics of the household so don’t know their water usage compared with yours. I am assuming they are using gas to provide top up.

 

[David Bowen]

It’s better but would be nice if you made clear that your headline Summary applies to stepped controllers and not proportional ones.

Done.

So I conclude that your 3kWh is too low and that double or treble that is more representative of a typical household

Yep, I understand that, but at the moment the immersion thermostat is stopping us putting in more than 3kWh. When the weather gets warm and we turn off the central heating I'll investigate how long the immersion element is. Also, as the days get longer we can heat the water between each shower / bath in the morning / evening so we might be able to consume more that way.

Thanks for your thoughts.

 

[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

Hi John,

I haven't put the 3kW dimmer on the website yet. I've also got a filter on the circuit. The circuit overall design is down to me but with a pick and mix from the internet. I've never disned a circuit before. so it may not be the best in the world but it seems to work for me.

Cheers, Keith

 

[Keith Walker]

The point of it not being a sine wave but part of one is using rectifiers to generate a voltage proportional to current only works when the two elements you are comparing are of the same current waveform shape. In your case with a dimmer type control load you will export at low and high powers and import at mid powers. this can be corrected but a better result is obtained if you use a detection circuit which measures true power then it is independent of load current shape and power factor.

Hi,

I agree that once the dimmer is running my load current will be a chopped waveform. I've added some capitors to smooth this out, so I'm comparing two dc voltages (not true power). I have had the circuit running and ended up taking power from the grid.
I've now changed the circuit so that it doesn't start to switch the dimmer on until 500 watts are being generated above the house load. The dimmer is controlled usings a LED, but the is not very goof at using very low levels of power, less that 25% of the waveform.
So I'm wasting some power but I have left the circuit driving a 2kW heater all day and no power has been use dfrom the grid. I will try reducing the 500 watts differential, but this will be trail and error.
I know my circuit is not the best in the world, but it's been a great winter project and my first design project (or should I say pick-and-mix from the internet project).

Cheers, Keith

 

[Keith Walker]

Hi,

Could you let me know wher you got the web mointor from and how much it cost?

Thanks, Keith

 

[chris_cronin]

echase quote:

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?

E.ON calculates the higher tariff on a daily basis ie in my case 900kWh/year divided by 365 (approx 2.5kWh) charged at the higher rate for each day first plus the remaining power used that same day at the lower rate.

 

[chris_cronin]

Hi All

just spent last 2 days reading all 38 pages on this thread!!!

I have a 3.92kW PV array & an air source heat pump.
I been using the heat pump for HW heating for 3 years, which works fine, but would like to add an immersion heater solution as well, to boost HW heating with surplus PV power both when the heat pump is running & when it isn't - it's not an effective solution to run the heat pump on surplus PV power only because of the time it takes for the heat pump to stabilise after startup.
With my current setup my heat pump can be running & I can be exporting to the grid under good solar conditions but equally I can be running the heat pump & importing some or all of the power to run it depending on solar conditions, time of day etc.
Since my PV installation (Nov 2011) I've exported an average of 24% to the grid.
I run 5 CT's - 3 on a Current Cost EnviR & processing 2 minutes data points through Techtoniq Energy Station 2 software.
I process this data daily to give me daily consumption/export figures using OpenOffice Calc to calculate whether I'm importing or exporting (hence the need for 3 CT's)
The other 2 CT's are used with openenergymonitor's EmonTX, EmonGLCD & NanodeRF methods as mentioned by Paul previously.

I have just ordered a Kemo M028N power control & Kemo M150 DC & Pulse converter to experiment with as PWM control seems to be the way to go but would appreciate any feedback.
My plan is to us the Arduino Uno PWM feature to be able to control the power supplied to a 3kW immersion element but haven't seen a suitable sketch yet!!

ps I seem to recall a previous enquiry within this topic from somebody asking whether anyone was using a heat pump - if you're that person reading this, you just found one

I'm not an electrical/electronics engineer (mechanical engineering is my forte) so be patient with me

 

[flyman]

Hi All

just spent last 2 days reading all 38 pages on this thread!!!

I have a 3.92kW PV array & an air source heat pump.
I been using the heat pump for HW heating for 3 years, which works fine, but would like to add an immersion heater solution as well, to boost HW heating with surplus PV power both when the heat pump is running & when it isn't - it's not an effective solution to run the heat pump on surplus PV power only because of the time it takes for the heat pump to stabilise after startup.
With my current setup my heat pump can be running & I can be exporting to the grid under good solar conditions but equally I can be running the heat pump & importing some or all of the power to run it depending on solar conditions, time of day etc.
Since my PV installation (Nov 2011) I've exported an average of 24% to the grid.
I run 5 CT's - 3 on a Current Cost EnviR & processing 2 minutes data points through Techtoniq Energy Station 2 software.
I process this data daily to give me daily consumption/export figures using OpenOffice Calc to calculate whether I'm importing or exporting (hence the need for 3 CT's)
The other 2 CT's are used with openenergymonitor's EmonTX, EmonGLCD & NanodeRF methods as mentioned by Paul previously.

I have just ordered a Kemo M028N power control & Kemo M150 DC & Pulse converter to experiment with as PWM control seems to be the way to go but would appreciate any feedback.
My plan is to us the Arduino Uno PWM feature to be able to control the power supplied to a 3kW immersion element but haven't seen a suitable sketch yet!!

ps I seem to recall a previous enquiry within this topic from somebody asking whether anyone was using a heat pump - if you're that person reading this, you just found one

I'm not an electrical/electronics engineer (mechanical engineering is my forte) so be patient with me

Chris,

PWM with Arduino is fairly simple ... for an example sketch check examples and fading in the Arduino software. AnalogWrite is available for certain pins and you'll simply map the result of inputs to the output pin.

Regards

 

[TechyTim]

Hey there,

Just thought I'd let you know that I updated my web page based on your useful feedback. You can now change the figures it uses to work out the value of the different arrangements and it will highlight the best performing "simple" element choice:

How much water can I heat with spare Solar Power?

Hope that helps.

David,
If you have read all the posts on this thread you will be aware that I am marketing a simple SolarHeat controller which does not measure the differential but is cheap(ish) and can be configured to switch at 1.5kW or 2.5kW output. I recommend this type for summer use only.

I have read your web posting with interest - very detailed and scientific but I have some concerns about your basics.
1. As others have said 3kWh is very low for the heat absorption available - either you are starting with less than half a tank of hot water or a short immersion heater - as a typical 100 litres tank requires 9kWh to heat from 10 DegC;
2. Your boiler efficiency may be 70% but this will be degraded by pipework losses and boiler cycling to a more typical 60% in the summer;
3. The orientation of your panels maybe a significant factor in output available so far as low winter sun will not be able to give much generation;
4. Your base load of 800W seems very high, 300W seems more typical - as with all good energy management you should go round turning off things on standby, etc.

Interesting to see how your calculations develop over the next few months as I reckon that 1.5kW differential is break-even with gas and a 4kW system should give significant savings over the summer with a 3kW heater.

TT.

 

[echase]

echase quote:


E.ON calculates the higher tariff on a daily basis ie in my case 900kWh/year divided by 365 (approx 2.5kWh) charged at the higher rate for each day first plus the remaining power used that same day at the lower rate.

Unless you have a smart meter I assume they don’t know what your daily usage is. So every time they do “know” (quarterly when readings taken or estimated) they will make your calculation and average it over the quarter. So assuming when averaged your daily usage is always >>2.5kWh it means you in reality pay the lower rate for any extra gas you use for water heating. It will only be the higher tariff if you use very little gas in summer, e.g. don’t have gas cooking, so bringing your base load usage without water and house heating below 2.5kWh.

To remind you all the original question was about whether using one unit of gas for water heating was priced at the higher or lower tariff.

 

[chris_cronin]

Unless you have a smart meter I assume they don’t know what your daily usage is. So every time they do “know” (quarterly when readings taken or estimated) they will make your calculation and average it over the quarter. So assuming when averaged your daily usage is always >>2.5kWh it means you in reality pay the lower rate for any extra gas you use for water heating. It will only be the higher tariff if you use very little gas in summer, e.g. don’t have gas cooking, so bringing your base load usage without water and house heating below 2.5kWh.

To remind you all the original question was about whether using one unit of gas for water heating was priced at the higher or lower tariff.

Sorry, maybe my explanation wasn't clear enough.
E.ON's higher rate tariff is 22.27p/kWh for the first 900kWh used in a year; thereafter the rate is reduced to the lower tariff of 10.13p/kWh used in the year.
This doesn't mean you pay at the higher rate until you have used the first 900kWh in the year up front.
They divide 900 by 365 days so each day you pay the higher rate on the first 900/365 (approx 2.5kWh) & then at the lower rate on the remainder of power used on that day.
So every customer pays the same for each days higher rate usage (assuming they use at least 2.5kWh), which, I assume replaces the old daily standing charge that no supplier seems to use any more.
This higher rate daily charge comes out at about £0.55 for the first 2.5kWh thereafter the charge depends on the extra days units used but charged at the lower rate.

The same method is used for charging for gas but with different higher & lower rates to electricity in my case 6.859p/kWh for the first 2,680 kWh used in a year then 2.98p/kWh for the remainder used in the same period
Hope this is a bit clearer?

 

[David Bowen]

I have read your web posting with interest - very detailed and scientific

Thank you.

1. As others have said 3kWh is very low for the heat absorption available - either you are starting with less than half a tank of hot water or a short immersion heater - as a typical 100 litres tank requires 9kWh to heat from 10 DegC;

Yes, I think we probably have a short immersion. I'll be checking that out when we get a warm day and the central heating stays off! I'm planning to check with an external heat sensor.

2. Your boiler efficiency may be 70% but this will be degraded by pipework losses and boiler cycling to a more typical 60% in the summer;

Agreed - and some folks believe even less. As it happens my old Camray is rated at 70%, but hopefully a newer condensing boiler at something like 93% efficient should come down to around 70% or better when taking pipes and other losses into account.

3. The orientation of your panels maybe a significant factor in output available so far as low winter sun will not be able to give much generation;

Yes, as I mention in the article, the panels are WSW which isn't terribly good. Also, it tends to be cloudy in the Vale of York. On the bright side, in the last month the panels offset 200kWh we would have pulled from the grid, but we did export 130kWh for our neighbours to use - if only there were some way to use that locally

4. Your base load of 800W seems very high, 300W seems more typical - as with all good energy management you should go round turning off things on standby, etc.

Indeed, it is high. Overnight it's down to about 350W but sadly during the day we're in the house. To be honest we get good value from the power we're using and tend to leave large loads until the sun is shining. With two young children who like muddy puddles, plus two adults doing computer-based work at home I sometimes think we are lucky to come in at 800W!

Interesting to see how your calculations develop over the next few months as I reckon that 1.5kW differential is break-even with gas and a 4kW system should give significant savings over the summer with a 3kW heater.

Yes, you could be right. Although my web page says "oil", you could insert values for a gas based system into those text boxes and see what it says. I'm hoping to update it when I have more data - it could be enough to capture 'til the Summer solstice and then double everything up for a rough idea.

By the way, feel free to ask if there's some way of presenting the data that would help explain things you're interested in. Sadly the calculations included over 1million readings each for the house and solar over three months so even if I make my Javascript very fast, it'll stress your computer doing the detailed calculations.

If you have read all the posts on this thread you will be aware that I am marketing a simple SolarHeat controller which does not measure the differential but is cheap(ish) and can be configured to switch at 1.5kW or 2.5kW output. I recommend this type for summer use only.

Sounds like a good plan. Best of luck with your sales.

 

[lioncrunch]

Hi echase and all,, I have one of these £32.00 gizmos,and yes it does do what it says, and works well"ish,,
as echase say's you are constantly wondering if it's set right, I have it on a1800w water boiler which boils in no time, then I have to alter it to run aNight storage radiator, which I rewired so as to put on one element or allthree 2500w, constantly watching the power generated, to see if I can switch onthe other elements, it's not ideal, if I set it to run one element 850w, thengo out and the sun shines I am wasting all that energy.
If you want to run 1KW all the time then I guess its fine.
Waiting for something more intelligent to come along

 

[lioncrunch]

I guess the spell check made a mess of this!!!!

 

[mightywalter]

Hi - new to the forum - haven't (unlike some new joiners) spent the full 2 days reading all 39 pages, but I have browsed thro' quite a few, suitably impressed with what seems to have been happening here over the past 18 months. What started out as a simple question, and thoughts of switching in an extra 1kW heating element, seems to have evolved almost into an online multi-participant product development workshop, with some deep discussions and brilliant ideas emerging.

Like others we've just had solar PV installed and have now realised the desirability of using it to heat our water also - at present we have no gas, so water & space heating are all electric (storage heaters for the latter). From the forum discussions I'm persuaded that I need to look beyond the Parsons Switch plus 1kW element towards a fully responsive device that will track the generation and use all the surplus available using the existing 3kW immersion element.

If I understand correctly a couple of the regular posters - echase, pmcalli, and a couple of others - have fairly advanced product designs. Is there an FAQ listing these, or links to obtain more information ? I'd like to get something installed in the next month or so, so I can benefit from the increasing sunshine

Many thanks,


Walter

 

[echase]

Hi echase and all,, I have one of these £32.00 gizmos,and yes it does do what it says, and works well"ish,,
as echase say's you are constantly wondering if it's set right,

A lot of people posting here are asking me if they can buy one of my ready built units. The best offer I can make is to say that they should be available from back end of this year, unless you also want a PV system, in which case you can buy a PV system with my unit now.

If you don’t want to build your own unit based on the designs offered here my recommendation would be to consider one of these £32 units (altough I have never had one in my hands) and use a less than 3kW load on it. That may recover much of its cost in the next year if you don’t mind constantly fiddling with it and worrying about the electricity you are “incorrectly” using/diverting. Then sell the £32 one on eBay and get one of mine which will pay for itself in much less time than your PV system will and needs no fiddling. That all assumes you have a >3kW PV system and do the installation yourself, otherwise the paybacks change.

 

[maits]

Inspired by some of the folk on this thread (and elsewhere) I spent some winter evenings putting together a combination of Arduino, Crydom SSR, current and voltage sensor, LCD display etc. and now have the capability to monitor export v import with fully dynamic proportionate load control to match the immersion load to net export from solar PV and the rest of the house. So far so good, and I thought I'd finished the difficult bit! I have a 210L Tribune unvented direct cylinder and can hook this up to the bottom immersion unit, but that still means I'll need a grid top-up if I use lots of water in the evening. What I want to do to finish off my system is to install a second cylinder upstream of my existing cylinder as a pre-heat (which lots of others have mentioned). That way if the sun shines a lot - or shines a little over two days when I don't use much hot water - the pre-heat tank should get hotter than my existing one, so I shouldn't need any grid top up at all, and all surplus generation can be harvested until the pre-heat tank tops out at say 85 degrees. But Heatrae Sadia say installing a pre-heat tank between the reducing valve and their cold inlet is against Building Regs so can't be done. Most of the unvented pre-heat cylinders out there seem to be marketed for use only with combi boilers. Has anyone installed two unvented tanks in series in a way that works for Building Regs? The acceptable alternative is to install in parallel and rely on mixing/ diverter valves but that adds complexity/ cost/ more things to go wrong and lacks the simplicity of a pre-heat.

 

[Whip1971]

I would recommend reading all the posts. I've just finished making pmcalli's gadget, it does exactly what I want, excess P.V. Going to a 3kW immersion. I would PM him mate, he's extreamely helpfull. Good luck !!

 

[Whip1971]

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