Immersion Heater - PV electricity

It is a solar water controller which uses a small PV panel to provide all the power to run the controller and pump so there is no mains needed. But it can be mains powered if you want. It’s mainly designed to switch the water pump on and off in response to the solar power and temperatures in the system. It also has 2 relay outputs to power the gas boiler/immersion heating top up function. These would do 1kW with an external relay needed for more than that.

It would not connect as is to a PV electric system, but with simple modifications:

1) You could add CT inputs. To mimic your unit paukdreed it would need a couple of input circuits to convert the CT output to a 0-2.5V aprox DC value.
2) It could log the power in the PV system too which is useful in system where the inverter is in the loft and so its power meter is not visible.


As you designed and soldered your own unit mine is not difficult. There are no surface mount components. But it’s outside the capability of a complete novice in electronic assembly.

June 09 covered the design and software with July for how to build, test and connect it.
 
a simple thought...... dont forget to introduce a cylinder stat to your circuit so it wont continue to heat the cylinder to a dangerous temperature! :-)
 
The input circuitry would add significantly to the overall circuit design. A simple passive diode bridge would not suffice, as I am sure you will be aware. This part of the circuit design was the steepest learning curve for me, and took the longest time to sort out.
You say that there are no surface mount components, but surely there must be components to provide the right environment for the PIC to operate? (sorry never seen a PIC or had anything to do with them).
Can you change variables in the PIC once it has been programmed (such as histeresys, switching thresholds ect) without actually re-programming it?

Edit - Yes I've got an immersion heater with an integral thermostat set about 5 degrees higher than my gas boiler water thermostat. So the central heating boiler does not heat the water (unless its been a dull day!)
 
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There are extra components on the analogue inputs but they are little more than resistor dividers and overvoltage protection resistors. But you would need to convert the CT ouput to DC with something like an op amp.


My standard device has >20 variables that can be altered by menu. If I changed the software to provide this function I’d add extra variables to alter the extent of input power averaging, switching hysteresis, etc.
 
echase said:
There are extra components on the analogue inputs but they are little more than resistor dividers and overvoltage protection resistors. But you would need to convert the CT ouput to DC with something like an op amp.


My standard device has >20 variables that can be altered by menu. If I changed the software to provide this function I’d add extra variables to alter the extent of input power averaging, switching hysteresis, etc.
Click to expand...
I use op amps to monitor the CT's which give me 3 volts at 5kw power flow (which can be changed via a shunt resistance).
If I can help, especially with the 'front end' circuit - let me know.
I'll PM you my email address..
 
Hi Millerman,
Just picked up this topic and very interested in your unit can we talk more on this.
Have been trying for some time to use my surplus PV into I/H which is a standard 3kw unit and not much joy as yet. Latest idea was to run the I/H thru. a site tranny (240/110) with a adequet continious rating of say 1500va,a trial gave me about 650w at the I/H. Auto control sounds a better way fwd.
 
rondo said:
Hi Millerman,
Just picked up this topic and very interested in your unit can we talk more on this.
Have been trying for some time to use my surplus PV into I/H which is a standard 3kw unit and not much joy as yet. Latest idea was to run the I/H thru. a site tranny (240/110) with a adequet continious rating of say 1500va,a trial gave me about 650w at the I/H. Auto control sounds a better way fwd.
Click to expand...
Wouldn't it be more straightforward to simply fit a 1kw heater element and dispense with the transformer?
 
There has been a lot of debate on the use of a 1kw immersion heater as a load dump. My first thought was to use a 2kw immersion heater then changed my mind to a 1Kw immersion. For my water tank the cost as about £75 (stainless steel) Then i had my solar system fitted that was constantly producing over 2.5kw, in theory wasting 1.5Kw. So i have purchased a unit that can be connected to a 3kw immersion heater (hence no cost "immersion heater") and can be run using 100 watts or 3000 watts. Spending pennies on a system to run the spare energy is fine. I have elected to splash out and use a plc and power controller, the cost is around £375 but over the period of the solar system life, it's peanuts. It works and is fully reliable.
 
Hi Paul,I looked at fitting a 1kw I/h at the bottom of the cyl. on an essex flange (popular years ago) but to-days thinking is that current copper cyl,s are too thin walled so it,s not recomended.A straight swop with the 3kw 27" top fitted is not good because they are only 11" and only heat the top of cyl.So my next trial was a site trany 230v/110v,I was given some duff info by an lecy friend who said I would get about 1.4kw at the I/h,but as you will probably know it,s more like 650w.The trany has apparently to be of suffient continious rating (va) I tried one rated at 2000va and seemed ok ,but you,ve got to switch it off/on manually as required and it takes some time to top up temp.Dont use much H/W so not worth investing much and I,m retired so have the time to monitor it.I would be pleased to hear any views on this.
 
Hi Paul,thanks very much,I thought the power and size were fixed,a 1 kw 26"from the top would be great.I was looking at the ideas on this thread regarding auto control and Andy appears to have a cracking bit of kit he has made but as I explained to him we use very little hot water, so as much as I would have liked to go that way I could,nt justify the costs, "Gotta use it to save"
Once again many thanks. Ron.
 
mac1 said:
Hi
I noticed this on a website. It might be possible to use it to lower the power of the immersion to match the output of the PV system.
http://www.graham-laming.com/bd/Immersion_Heater_controller.pdf
Click to expand...
This will not work as expected!
If you research the technology you will find that because it 'chops' the sine wave, it will use more than 1kw for a short period of time and then use no power at all for the remainder of the cycle (wasting generated power).
If you are not familiar with the technology principles - then please take my word - it won't work! (but nice try)
 
Hi Millerman,

Please can you forward me more information on your appliance switch you mentioned in your post on 06-01-2011.

Many Thanks
 
Hi Paul
I am interested in your comment regarding the power control. Having just built an opto isolated triac control unit, I tested controling a 3kw kettle via the triac circuit as a test for my immersion heater control. Using a power meter at the plug of the kettle and whilst watching the output of my inverter and the mains input meter I performed vairious tests ranging form 300 watts to 2 kw taking it close to the total spare capacity that I was generating. At no time did my standard electricity consumption meter register any load. Can you provide a link to a paper / information that may cover your thoughts regarding the wave form cutting situation? Using a triac will provide a much more efficient way of heating your water as you will be able to drive the heater from a much lower level. It seems as though lots of people are catching on to the possibilities of using this spare power :-)
 
Please see Light Dimming | Micro Basics which shows the principles of how a Triac works and which I refer to below.
You will see that the power is controlled by firing the Triac at a certain phase in it's cycle, thereby reducing the power 'average' through the full cycle.
Now, if we assume (for example) that you are trying to reduce the power of a 3kw heater down to 1.5kw, this will mean that the Triac will fire at 90 degrees at both the positive and negative cycle (as shown on the 3rd diagram down).
You will note that the current flowing as the Triac fires will be the full current of a 3kw load, of which only 50% will be supplied by the invertor, and the rest via the grid supply.

- The overall power reduction is only achieved by supplying full power for 50% of the time.

A further consideration is that in the 90 degrees before the Triac fires, the power flow is zero - which is not making efficient usage of the power provided by the invertor....

It is many years since I studied Triacs, so would be grateful for confirmation from other board members that this is correct.
 
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Hi Paul
Yes I can see the logic of that. I wonder if the short duration of the extra spike is not seen by the input meter, almost like a transient spike. I willl carry out some more real life tests to see if this is what happens. I will get back to you.
 
I'm not sure how the digital input meters work, or how they deal with reactive/resistive loads, phase shifts ect, so can't really help there, but look forward to your results.
Maybe if instead of using a kettle for testing which will only run for a few minutes and be more difficult to measure as the input meters usually record in 1kwh units, you might consider an appliance which can be left running for an hour or so, such as a heater, tumble drier etc.
 
Hi Paul, aquired 1kw immersion heater from source you reccomended and fitted ,works a treat on topping up between boiler firings.
Would you know if there is a remote switching device available (to switch the 1kw 230v heater) trying to save my legs up and down the stairs. I guess that would be 230v x 5amp capacity?

Many Thanks Ron.
 
rondo said:
Hi Paul, aquired 1kw immersion heater from source you reccomended and fitted ,works a treat on topping up between boiler firings.
Would you know if there is a remote switching device available (to switch the 1kw 230v heater) trying to save my legs up and down the stairs. I guess that would be 230v x 5amp capacity?

Many Thanks Ron.
Click to expand...

The only commercially available 'automatic' switching unit that I am aware of is called EMMA - Solar and Wind Applications Ltd but it very expensive and probably not suitable for your needs.
However, there are a number of people who are developing switching units, and it is only a matter of time before an affordable unit is made available.
 
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rondo said:
Hi Paul, aquired 1kw immersion heater from source you reccomended and fitted ,works a treat on topping up between boiler firings.
Would you know if there is a remote switching device available (to switch the 1kw 230v heater) trying to save my legs up and down the stairs. I guess that would be 230v x 5amp capacity?

Many Thanks Ron.
Click to expand...
.....until a competitively priced 'automatic' switcher is developed, you could try something like this 3 Remote Control Mains Plug Sockets & Transmitter which although does not control the heater automatically, it would enable you to switch it on and off from downstairs, saving your legs!!
There are many similar ones for sale - many even cheaper, usually in big supermarkets/diy stores, and this particular one for example will switch 2900 watts, and because the remote control works by wireless (and not infrared), it will work through walls and ceilings, and up to 20 mtrs away.

...just a thought...
 
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Hi again Paul.

Well I have just run some tests using an accurate industrial power meter that shows direction of power flow and power factor as it is referenced to the voltage. I balanced my 3 kw kettle using my circuit running it at various loads depending on power that was available. My consumption meter is divided in 100ths of a kwhr and each digit takes 8 flashes from an led before it moves. I ran the export rate right down to 10 watts with some fluctuations but I kept it pretty level. At no time did the import meter show any input. I have discussed the principal with another electrical engineer who also believes that the power meter will average the consumption over the duration of the cycle. If this was not the case the rather expensive Emma unit would not work either. I am really happy with my tests and will now continue with the rest of the measurement side development. I do understand your principal of why it should bring power in during the cycle, but in real life tests it doesn't seem to work like that.
 
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inie meanie said:
Hi again Paul.

Well I have just run some tests using an accurate industrial power meter that shows direction of power flow and power factor as it is referenced to the voltage. I balanced my 3 kw kettle using my circuit running it at various loads depending on power that was available. My consumption meter is divided in 100ths of a kwhr and each digit takes 8 flashes from an led before it moves. I ran the export rate right down to 10 watts with some fluctuations but I kept it pretty level. At no time did the import meter show any input. I have discussed the principal with another electrical engineer who also believes that the power meter will average the consumption over the duration of the cycle. If this was not the case the rather expensive Emma unit would not work either. I am really happy with my tests and will now continue with the rest of the measurement side development. I do understand your principal of why it should bring power in during the cycle, but in real life tests it doesn't seem to work like that.
Click to expand...
If this is the case, then it opens up opportunities to automatically vary the power supplied to the element from zero up to the full 3kw on sunny days, and of course use 'off the shelf' cheap 3kw elements too.
I was basing my opinion of 'theory' rather than 'practice', but pleased it is working out.
 
Yes it does seem that way. I am going to do further tests at a friends house who has a different type of energy meter. And I do like to make sure of these things so I will be repeating my tests under different day light conditions. I have two tanks both with 3 kw heaters fitted this is what is driving me on as it's all there just waiting to be used. The key will now be to use the correct type of measurement coils and accurately plot the slope. This could either be used on a multi output programmable relay ( changing resistances at steps ) or with more work a method of connecting the op amp output to drive the 555 timer circuit that fires the Triac. Any body had any experience of that type of interface? Or which is the best current coils to try?
 
Hi Mac
Just to put your mind at rest, your idea will work. The usage meter measures rms and gives the average across the cycle and not sub cycle. This means that a spike causing a draw from the grid will not be seen by the meter. I have completed a series of tests that confirmed this, so develop away. I am currently working on my system. When completed I will be happy to pass the details on. Good luck with yours.
 
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inie meanie said:
Hi Mac
Just to put your mind at rest, your idea will work. The usage meter measures rms and gives the average across the cycle and not sub cycle. This means that a spike causing a draw from the grid will not be seen by the meter. I have completed a series of tests that confirmed this, so develop away. I am currently working on my system. When completed I will be happy to pass the details on. Good luck with yours.
Click to expand...
Mac,
inie meanie and myself have been discussing this outside outside of the forum, and I also now agree with inie's version, that the meter will average out the power usage.
So this does look the way forward after all.....
 
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Hi Mike
That would be fantastic apart from the fact that you would have to sit there adjusting the pot. I have just bought a triac control unit that will take 0 to 10 volts to control upto 25 amps. If I can provide that voltage from a comparitor circuit measuring the coils. Bingo! Hope to complete soon.
 
Paul.

"Can you expand on this?
Are you using a 1kw or 3kw element, and if 3kw - how are you limiting it's power dump? "

I am using a my original 3kw element. the unit i have purchsed is a Crouzet controller M3, and a thyristor controller.
The M3 controller measures the current from the grid and solar using CT's, then compares the two.
This then sends a 0 - 10 V DC signal that operates the thyrister controller ( 0 v Dc = 0 watts, 10 v Dc = 3000 watts.

The only down side is the way the solar supply is connected to the house fuseboard (by the contractor). It might have to be changed for this system to work.
 
I have fitted a 26" 1kw element, so the power dump is either zero or 1kw.Most of the renta roof installers don't like to wire directly into the fuseboard, as they like their systems to run completely independently (so anything I do won't affect their system), so I was lucky. I did however have to wire a separate feed for the immersion heater, so it didn't use the immersion power in my unit's calculations.
You could probably simply fit a Henley Block between the meter and the fuseboard, to separate the system.

Was calibration of your system difficult?
 
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Paul,

You have hit the nail on the head. One installer (AshadeGreener) fitted a seperate fuseboard and then linked it to the grid via new meter tails and a henley block(mother's house). When i had my solar panels fitted the installers (Homesun) they went directly into my existing fuseboard. (this was quickly changed) Both methods are correct according to th IEE regs.

This forum is about using the surplus available power and using it in an immersion heater. I have just elected to keep my existing immersion heater 3Kw and use a off the shelf controller to run it.
A 2.5 Kw controller is as little as £50
Plc is about £100
CT's are about £70 Pair (probably find cheaper) need them to be 4 - 20ma or 0 - 10v Dc output

Dont suppose anyone know how i can reduce this cost (CT's) ??
 
Millerman said:
Paul,

You have hit the nail on the head. One installer (AshadeGreener) fitted a seperate fuseboard and then linked it to the grid via new meter tails and a henley block(mother's house). When i had my solar panels fitted the installers (Homesun) they went directly into my existing fuseboard. (this was quickly changed) Both methods are correct according to th IEE regs.

This forum is about using the surplus available power and using it in an immersion heater. I have just elected to keep my existing immersion heater 3Kw and use a off the shelf controller to run it.
A 2.5 Kw controller is as little as £50
Plc is about £100
CT's are about £70 Pair (probably find cheaper) need them to be 4 - 20ma or 0 - 10v Dc output

Dont suppose anyone know how i can reduce this cost (CT's) ??
Click to expand...
Try Current Cost Daisy Chain CT Sensor Jaw pack of two | eBay UK at £16.50p!

I use these and they are quite good.
 
Hi Paul,
I am interested in the used of my free lekky to heat water- can you send me details of the 26 inch 1kw heater- who makes it/sells it please?
 
I'm pleased you have located a supplier of a 1 kW immersion heater.
I wish to load up[ my solar panels in the same way. Could you please let me know which company supplies the 1 kW heaters?
 

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