Immersion Heater - PV electricity

[echase]

The unit I am selling is not that expensive and was not that difficult to design. But it did take 100s of hours to perfect the software.

 

[Mark Gee]

Sorry, but I don't understand how putting your heater on for a quarter of an hour every hour is any better than just leaving it on all the time during daylight hours, unless you arrange to switch off your other loads during that time. That's why I assumed you didn't know how it works. I agree that fitting a 1kW heater is less likely to have you drawing from the grid than the standard 3kW one, but having a timer switch it on and off at regular intervals doesn't help, because there is no "averaging" when it comes to how you are charged for your electricity.

No one has mentioned batteries by the way, as far as I remember - the whole point is to use water to store the energy.

Because weather is irregular and changes.As I said originally "what if the sun don't shine?" By simply spreading the "immersion-on-periods" throughout the day you are more likely to coincide with the sun being 'on' eg my strategy would work better than yours on a day when it is cloudy in the morning and sunny in the afternoon. I would also of course refine it to leave periods of no immersion for planning to consume elec for meals/drink/dishwasher/etc

Although your second point is quite correct the thread is about immersion heaters not batteries (or elec car batteries for that matter)

 

[Mark Gee]

Yes, hysterisis has been a problem, and I had to modify my circuit after a few weeks as the relay couldn't cope with the rapid switching.
I have never used, or know anything about micro-controllers, so I can't comment on your suggestion, but good luck.

so get it to turn on the immersion for 5 minutes or 15 minutes! if the sun is shining now you could reasonably expect it to shine for another 5-15 minutes...although obviously it might not. But it probably would.

 

[suntrap]

Because weather is irregular and changes.As I said originally "what if the sun don't shine?" By simply spreading the "immersion-on-periods" throughout the day you are more likely to coincide with the sun being 'on'

No you aren't. If the weather is truly random, and I think we both agree that it is, then pick any time during the day and you will have exactly the same chance of the sun being out.

eg my strategy would work better than yours on a day when it is cloudy in the morning and sunny in the afternoon.

And my "strategy"* would work better than yours on a day when it is sunny in the morning and cloudy in the afternoon. They cancel out. You cannot make any predictions about a random value based on when you sample it.

*I'm inferring that you mean I'm suggesting putting the heater on for one long period in the morning.

I would also of course refine it to leave periods of no immersion for planning to consume elec for meals/drink/dishwasher/etc

That is indeed a sensible use of a timer.

 

[inie meanie]

Because weather is irregular and changes.As I said originally "what if the sun don't shine?" By simply spreading the "immersion-on-periods" throughout the day you are more likely to coincide with the sun being 'on' eg my strategy would work better than yours on a day when it is cloudy in the morning and sunny in the afternoon. I would also of course refine it to leave periods of no immersion for planning to consume elec for meals/drink/dishwasher/etc

Although your second point is quite correct the thread is about immersion heaters not batteries (or elec car batteries for that matter)

Err I think that you TOTALLY missed the point of this blog. It was set up to discuss a way of automatically measuring and controlling the spare power so that it can be used to heat your water for free. Setting a switch for random periods during the day is not controlling anything. It may be cheap to setup but will quickly cost you more than the systems setup by members here.

 

[bradburts]

Hi,
I am about to have a PV system installed. I did not understand the need for the system to be patched in in a particular way so as to make easy measurements etc. Please explain (in simple language!).
I presently have an OWL energy monitor in the external meter cupboard and would expect to attach another current sensor somewhere after the invertor in the loft so don't understand the need for a seperate panel.
Looking back at earlier suggestions; a light dimmer board was thought to be a simple solution, having all needed interference protection etc, but was rejected through having a potentionmeter control.
It may help to note that you could use digipots, digitally controlled potentiometers, to replace the light dimmer potentiometer. You would have to use a microcontroller to control the digipot of course but to my way of thinking the microcontroller circuit has got to be the best way forward with its meter, display and perhaps wireless control capabilities.
I will research and post suitable devices and provide programming examples if converting a light dimmers make electrical sense!
PS
Think the SUNNY Home Manager is Euro 1,300, which is a lot. A small linux wireless microcontroller would cost around £30, software would be technically straight forward but take the usual 'while' to write code for & test.

 

[suntrap]

Hi bradburts - more ideas flowing in! I too have an OWL, given to me as a present. This sort of energy monitor is compromised in that it can't determine power factor, but this shouldn't be a big problem with solar inverters as they have power factor correction. The other annoying thing about it is that the resolution is not great - the reading goes up and down in fairly large steps.

I think that the issue of making easy measurements is that you need a single-core cable (such as a meter tail) to clamp the current transformer round; if it's twin core the opposing currents will cancel and you will get no reading. If your installer uses a separate "garage unit" (small consumer unit) just for your PV installation, there will be meter tails from this which you can use to measure your PV current. My installer put an RCBO in the existing consumer unit (it's split load and there was a spare way on the unprotected side). Although this means there are no meter tails, he pointed out that there is space in the consumer unit to clamp the current transformer around one of the twin and earth cores where the outer insulation is removed, and that's what I have done. The OWL's reading is a bit erratic but it still gives a good indication, which tallies broadly with what the inverter is telling me.

I too am interested in getting an extra current transformer for my OWL, as I noticed the transmitter has three sockets on it (unless that's just for three phase supplies?). I haven't investigated it further yet, though.

 

[Graeme Harrold]

Simples use SMA Inverters. My immersion kicks in when the inverter is at 1.8Kw using the built in multifunction relay. FREE to use, and the additional kit cost about £50

 

[suntrap]

Not so simples - that neither provides proportional control, nor does it take any account of other loads in the house, nor does it allow you to use your existing immersion heater (so if you need to heat your water with 3kW in an emergency, you can't).

I think we need a FAQ for this thread!

 

[Graeme Harrold]

Actually with very little on in the house during the day, its not a problem. Also have the immersion switched between 1.5 and 3kW, so no need to change that either all with an 89p diode.

 

[suntrap]

Yes, it helps if no-one's there to mess up the consumption during the day! A diode will of course not do wonders for the current waveform but I agree is a very easy way to halve the heater's consumption.

 

[bradburts]

Do you have a link to the inverter with the relay?
Its sounds a good solution.
The diode would seem a lot simplier/cheaper than changing the immersion coil (which is where this thread started).
Does the inverter allow other power settings at which the relay switches? 1.8Kw seems quite a high threshold, I only go over 500W when the kettle goes on....
Would there be the problem with chopping out half the wave within a house?

 

[echase]

Hi,

Looking back at earlier suggestions; a light dimmer board was thought to be a simple solution, having all needed interference protection etc, but was rejected through having a potentionmeter control.
It may help to note that you could use digipots, digitally controlled potentiometers, to replace the light dimmer potentiometer. You would have to use a microcontroller to control the digipot of course but to my way of thinking the microcontroller circuit has got to be the best way forward with its meter, display and perhaps wireless control capabilities.

I will research and post suitable devices and provide programming examples if converting a light dimmers make electrical sense!

The device I have designed and am now selling does all this. Send me a private message if interested.

 

[echase]

I too am interested in getting an extra current transformer for my OWL, as I noticed the transmitter has three sockets on it (unless that's just for three phase supplies?). I haven't investigated it further yet, though.

I checked with OWL some weeks ago. It’s for 3 phase, but it just adds the 3 powers together and displays one figure so you can’t use it to display 2 or 3 different things like PV generation and export. Plus it has no idea of the direction of the current so an import and export read the same. The trick in any immersion control device is to discriminate them and my device does that.

 

[pauldreed]

Hi All, as I posted a few days ago, I have been working on version 2 of my solar controller, and it is now complete and fitted, so I thought that I would post a few photos.

The PCB contains the input filter and protection components, as well as the main processor and power supply stabiliser. The LCD display is recycled from an old alarm keypad.

Fitted it all into a small 'Maplins' project box after sawing a few holes in the lid and sides.

All wired up to the power supply and CT's, the first display screen shows the current power state, ie. how much power is being used within the home, and also how much solar power is being generated.

By pressing the button on the front, I can select different screens. This one shows how much power has been used/generated in kwh's. This total is culmative and resets every midnight.

Another screen displays the instantaneous mains voltage and power factor. This works very accurately, and is mostly within the tolerance of 1v to that recorded with a professional test meter.
The voltage measurement is what makes the power calculations much more accurate, instead of assuming that the mains is 240V AC.
As you can see here, it is currently 247V, but sometimes drops to just over 240V.

The small LED just above the button lets me know at a glance what the unit's status is;
1) flashing every 4 seconds - indicates that the unit is functioning, but not enough solar power to activate either relay.
2) steady but lit 25% - indicates that 500w is being unused, and activates relay 1, with a diode in series to limit the load to 500w (yes it does work, I've done the sums on the leccy meter!)
3) steady and lit brightly - 1kw power is now available, so relay 1 is deactivated, and relay 2 activated switching the 1kw load.

The device also provides a 0 - 5v variable output, which could feed a 'Crydom power controller' for progressive power control, which I may purchase at some stage.

 

[suntrap]

Fantastic. You must have spent ages doing that!

You could make the LED flash with every fraction of a kWh produced by the cells, like an electronic meter. Watching the LED flash beats TV any day of the week! When the relays are on, it could inverse flash (flash "off").

Pedantic note: your "w"s (watts) should be capitals :stuart:.

 

[pauldreed]

Pedantic note: your "w"s (watts) should be capitals :stuart:.

You are correct of course, amended accordingly!
The building time wasn't long compared to the learning curve, see post Immersion Heater - PV electricity - Page 3 and you will see that it's developed from that point forward with a lot of help from others, especially Home | OpenEnergyMonitor who have written most of the coding and been inspirational and confidence builders.
With the explosion of renta roof solar installations, similar devices are much in demand and I would encourage interested others to buy a Arduino Uno Arduino - HomePage and have a bash! they are great development circuits, and code is loaded directly from your PC without expensive programmers.
However, 240v is a very different matter, and is best left to the experts.

 

[Andy_WSM]

I am very pleased to say that I have completed my system. I have heated the top part of my tank very quickly. ( only as deep as the length of the immersion heater ) I raised water temp 10 deg before the solar water system had got up to temperature and started to circulate. This was important as some people have said this system won’t work quickly enough and this proved it is much faster than the water type system. Also to confirm that I have had NO interference generated by the triac effecting either the radio or tv (10pcv2425 contains a snubber circuit ). This system, just to confirm doesn’t use an expensive plc and follows the spare power level in an analogue way NOT waiting until there is a spare 1,2 or 3 kw before switching on in steps and thus wasting potential energy. It runs as soon as there is a spare 100 watts and continues to feed all the way up to full power 3kw. All it contains is a small comparison circuit made from 2 opamps ( 741, 75p each ). I triac crydom 10pcv2425 ( just bought one on eBay for £12 ) . A power supply that provides -12 0 +12 vdc, 2 current coils from LEM model AT20B10 ( £ 54 inc vat for the two ). I have fitted and extra dist board with a Henley block to separate the measurements. I now need to find a nice box to put it all in and maybe add a couple of voltage meters form eBay just to make it very easy to see what power is being supplied. The future improvements would now be to add another heating element to the bottom of the tank and a relay circuit to switch between the elements once the top one is up to temp. I hope this will encourage some of you to develop your system. It just goes to show there are several routes to achieve free hot water. :- )

Hi all,

Been liaising with Tony (inie meanie) and purchased a PCB off him for the control circuit he describes here. 2 current coils and crydom triac later and I too have a "spare power recovery" hot water system. I fitted the kit on Tuesday morning and haven't used the immersion heater at premium rates since. Today is a dull, cloudy, intermittent sunshine kind of day - I only have a spare 800w being generated, so to heat my water now would normally cost me 2.2kWh from the grid + the 800w generated locally. By using this cct I can just dump the 800W in for longer until the tank has reached temperature, then any surplus goes to the grid. I've really enjoyed this little project and will continue to enjoy the financial benefits it will bring

 

[echase]

Fantastic. You must have spent ages doing that!

You could make the LED flash with every fraction of a kWh produced by the cells, like an electronic meter. Watching the LED flash beats TV any day of the week! When the relays are on, it could inverse flash (flash "off").

Mine does that and also has an override to turn the immersion on if you want a top up on a sunless day. Some LEDs or an LCD are useful or else you don't know if the unit is working properly.

Looks like we have at least 3 viable solutions here. My one achieves the same aim as the others but I use cheaper components that are just as effective. The Crydom 10pcv2425 is pretty expensive (around £100) unless you find a cheap one somehow, as inie meanie did. So I went for a different approach that does not use such an expensive relay but provides the same function. My whole component cost including the box, current transformer, fuses, relay, processor board, etc. is less than just the 10pcv2425. On the other hand I am charging a modest cost for the design and for assembling it, as I don’t have the kit version available at the moment. My making them up saves you the potentially hazardous work of dealing with the mains side (but some of you are electricians so you don’t mind that).

I don't currently supply an LCD but am providing the connections for one as I do connect one to test and calibrate the units. I’ll make them available later for retrofitting when I get around to writing the full LCD software. LEDs provide much of the info that an LCD could supply.

 

[echase]

2) steady but lit 25% - indicates that 500w is being unused, and activates relay 1, with a diode in series to limit the load to 500w (yes it does work, I've done the sums on the leccy meter!)
3) steady and lit brightly - 1kw power is now available, so relay 1 is deactivated, and relay 2 activated switching the 1kw load.

Have you established that all electricity meters can cope with this half wave rectification? For the positive half cycle you will be importing from grid and exporting for the negative half cycle, or vice versa. As long as the meter successfully averages out the power over a few cycles this is OK but if not it will register an export or import when none really exists (apart from the error between the 2 discrete steps and the real power available for export).