Immersion Heater - PV electricity

[pmcalli]

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

Had a look at your circuit. using precision rectifiers gives a solution but its performance is far from optimal when combined with a dimmer or proportional controller as these do not give a sine wave current which the solar panel inverter does. If you send me a pm with your email address I will send you a circuit using two chips to do the whole job but its not for publication.

 

[JohnBoyZ]

Hi Keith,

Even though I haven't got round to building one of these yet, I have to say pmcallis circuits are elegant solutions and also answer one of your web site forum questions about how to cope with a system where there is no henley block and PV uses a spare MCB in the existing CU.

Edit: Didn't mean to just big-up pmcalli, all the other contributors pauldreed, inie meanie, echase etc have done sterling work on this forum, if you havent done so already its worth reading this thread from start to finish.

Regards
John

Note to self, "Must get off my a** and build one"

 

[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,

Given time the power dimmer will be on the site. Time seems to be the one thing I'm very short of.
The next part of the circuit is the comparison. This will then give drive an LED which changes the resistance of a LDR (light dependant resistor). This change of resitance controls the 3kW dimmer.

Cheers, Keith

 

[Keith Walker]

Had a look at your circuit. using precision rectifiers gives a solution but its performance is far from optimal when combined with a dimmer or proportional controller as these do not give a sine wave current which the solar panel inverter does. If you send me a pm with your email address I will send you a circuit using two chips to do the whole job but its not for publication.

Hi John,

PM sent.

Very true I don't get a sine wave when the dimmer is chopping the load. But a couple of capitors gives me a rippled (not sure of the term required) dc output. This is compared and used to drive the dimmer.

Before I started of the mains dimmer I had the control circuit driving three leds. Load, solar power and output. It was so sensitive that if the load conditions was just right and my sub-woofer turned up (and the right sort of music, Queen We Will Rock You) the base beat could be seen on the leds. I was amazed! Anyway a chucky capitor leveled things out.

But the circuit still responds to changes in the washing machines cycle.
Videos to follow when time allows.

Cheers, Keith

 

[pmcalli]

Hi John,

PM sent.

Very true I don't get a sine wave when the dimmer is chopping the load. But a couple of capitors gives me a rippled (not sure of the term required) dc output. This is compared and used to drive the dimmer.

Before I started of the mains dimmer I had the control circuit driving three leds. Load, solar power and output. It was so sensitive that if the load conditions was just right and my sub-woofer turned up (and the right sort of music, Queen We Will Rock You) the base beat could be seen on the leds. I was amazed! Anyway a chucky capitor leveled things out.

But the circuit still responds to changes in the washing machines cycle.
Videos to follow when time allows.

Cheers, Keith

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.

 

[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.