Immersion Heater - PV electricity

[fossy]

I am also very interested in using an immersion heater to dump spare solar PV. But it has been some years since I dabbled in circuit building, another call for a circuit diagram !
I'm stuck on the first hurdle though, detecting current in from solar inverter and in from the grid, comparison. I already use a clamp transformer that sends a wireless signal to a monitor. What it doesn't know is the direction of power. For example on the grid tails it reads 3KW if say the kettle is on (cloudy day), or 3KW on a sunny day sending power to the grid, How does a transformer know the direction of current ?

 

[Millerman]

Fossy Hi .. please read all of the forum pages

 

[shorsfal]

For those of you who are considering the 1kW immersion heater option, I have just completed my install using the Current Cost CT's suggested by paulreed and Picaxe 18M2 controller. My panels are a 4kW installation but as this is a maximum capability I thought it better to use the 1kW heater as it was more likely to be useful.

The CT interface design can be found in post #14 at the following location. The value of R1 needs to be changed to around 47 ohm if using the Current Cost CT's.
Mesuring currents on residentials conexions with my PICAXE - Page 2

The rest of the design uses standard interface circuits described in section 3 of the PICAXE manual to switch a relay on an off depending on the power being generated, if you use a project board the darlington pair in this terface is already on the ULN2803A chip supplied with the board.

The CT's are clamped on the inverter and the house load tails and monitor the generation and house load constantly.

Once the inputs have been calibrated, i.e. noting the values being written to the variables from the READADC10 inputs the code can be written to ensure the heater is switched on when there is sufficient surplus power being generated. I need to do a bit more work on the coding but it is working fine and heats sufficient water for our needs on most days. I switch the heater off after about an hour to avoid overuse of the heater thermostat.

A copy of the code in current use is set out below, it looks much neater in the programmer but most of the TABS have been stripped out by the editor this forum uses;

main:
readadc10 C.0,w0 ;Read C.0 value and write it to w0
readadc10 C.1,w1 ;Read C.1 value and write it to w1
if w0 > 10 then
goto test ;when w0 is greater than 10 goto test sub procedure
endif
if w0 = 0 then
goto byebye ;when w0 is zero goto byebye sub procedure
endif

pause 500 ;wait 0.5 seconds
goto main ;go to start

test: ;make sub procedure called test
readadc10 C.0,w0 ;Read C.0 value and write it to w0
readadc10 C.1,w1 ;Read C.1 value and write it to w1
let w2 = w1 + 3 ;add equivalent of 1kW to value of w1 to evaluate current to power
;heater and write to w2.
if w3 = 3600 then
goto byebye2 ;when w3 is 360 goto byebye2 sub procedure because the heater has been on for an hour
endif

if w0 > 10 and w2 < 8 then
goto sw_on ;when w0 is greater than w2 goto sw_on sub procedure
endif
goto main ;go to start

byebye: ;make sub procedure called byebye
let w3 = 0 ;reset the w3 variable to zero
sleep 130 ;sleep for approx 5 minutes
goto main ;go to start

byebye2: ;make sub procedure called byebye2
sleep 6240 ;sleep for approx 4 hours
let w3 = 0 ;reset the w3 variable to zero
goto main ;go to start
sw_on: ;make sub procedure called sw_on
do
high B.4 ;LED on
high B.5 ;Relay On
high B.6 ;Relay On
pause 1000 ;pause 1 second
inc w3 ;increment variable w3
readadc10 C.0,w0 ;Read C.0 value and write it to w0
readadc10 C.1,w1 ;Read C.1 value and write it to w1
if w0 <= w1 then exit ;exit from loop if generated power is less than load
if w0 < 10 then exit ;exit from loop if w0 falls below 10
loop while w3 < 3600 ;loop while b3 is less than 3600 which should equate to 60 minutes
low B.4 ;LED off
low B.5 ;Relay off
low B.6 ;Relay off

goto main ;loopback to start.

Hope this is of use. Thanks to paulreed who set me off on this project following his original post.

 

[echase]

For some time, since before coming across this thread, I have been designing an immersion controller for my 4kW PV system. I now have it finished and have built a few for family and friends. Seeing as the professional firms have not come up with a sensible product that is affordable please send me a private message if anyone wants to buy one. It is available as a kit or ready built. I am a Chartered Engineer so the design is hopefully quite professional but ultimately the risk is yours if you want to fit one.


It functions similar to the ones mentioned here, i.e. it controls the power into the immersion to ensure that export power is reduced to near zero (as long as the hot water is not up to temperature already). But it uses simpler components to reduce the cost and size, e.g. instead of an expensive PLC (Programmable Logic Controller) it has a small PCB that I make with a microprocessor and has less connections into the house electrics; this does not reduce its performance. It is designed for a 3kW or less immersion but would probably work up 10kW, although I have not tested that. Two or three 3kW immersions in parallel should be OK. It does not rely on an always on external PC either, which would burn lots of watts. No need to purchase an expensive <3kW immersion.

 

[suntrap]

I'm going to need something - I just enquired with npower about registering my PV system and they immediately arranged a date for installing a back-stop meter :-( I know an earlier poster (and also my installer) said that it hadn't happened with them (/clients), but I think the electricity companies are now getting their act together and the likelyhood of your meter not being replaced is diminishing.

 

[echase]

I have a rotary meter of perhaps 10 year old vintage. It does not go backwards or only does so extremely slightly. But when an electricity board electrician came around to look at something else he was most insistent I tell the board that I have PV and get the meter changed. I have not bothered to do so.

 

[suntrap]

Interesting that your analogue meter doesn't go backwards - maybe some of them have mechanical clutches to prevent that?

Are you not claiming a FIT then? As far as I know, the only way of getting it is via your electricity supplier.

 

[pauldreed]

It's good to hear that a number of you have been successful in building these devices, as I am sure that they will pay for themselves many times over!
Mine it still notching up kwh's and even a little bright weather is enough to meet our hot water demands (not today though!).
I too have been busy, and have bought some microprocessors, learnt C++ programming language (albeit basic), and am working on version 2, which is nearing completion.
Version 2 is much more accurate than my old version as it calculates power by measuring both current and voltage 1000 times a second and also compensates for different power factors, has a LCD display which shows useful information via a menu system, multiple outputs, and is upgradeable (I keep thinking of improvements!).
The LCD display shows; instant solar kw, instant mains kw, solar kwh per 24hr period, mains voltage, and mains power factor.
I have a working version on a breadboard, and am just completing one on a PCB to fit into a case. Once it's finished I'll post some photos.

 

[echase]

Yes I believe some have these clutches. After a slight backwards movement (fraction of a turn) my clutch engages.

It’s a question of the left hand not knowing what the right hand is doing. One part of EON pays my FIT payments. But the supply side does not seem to been told that I have PV. Or maybe it’s because the actual utility company who provided the meter is not EON but Southern in my case. My PV supplier says that in some utilities there is an automatic tie up so they will tell you that you need a new meter, particularly if your readings go down.

 

[suntrap]

My Aurora inverter has an RS485 interface whose protocol is actually known, and someone has written Linux software to interrogate it, to get instantaneous output figures as well as the last 8640 readings at 10-second intervals. A USB-RS485 converter is less than £10 from China via eBay (or £60 from the manufacturer!). Not the most elegant or power-efficient for this application but great for general tinkering.

 

[echase]

My Aurora inverter has an RS485 interface whose protocol is actually known, and someone has written Linux software to interrogate it, to get instantaneous output figures .

Does not solve the need to measure consumed power too if controlling the immersion.

 

[suntrap]

Indeed not, but it provides one measurement.

 

[Mark Gee]

Re: Immersion Heater - PV electricity - MISSING THE POINT

What if "the sun don't shine?"

Surely a simple "solution" is to use a long 1kw element and a mechanical timer. With the timer set it to come on for 15 minutes every 'approproate' hour. This will trickle feed and topup the temperature whilst averaging out the time when the system is not generating anything.

Not perfect but a simple compromise. If the system generates more than 1kw then sure it is being 'wasted' but less so for people like me who work fom home a lot.

Total Cost : about £30? and two hours labour.

although not sure where you get a long 1kw element from

 

[suntrap]

I think you are misunderstanding how domestic PV installations work, which happened with some people at the beginning of this thread, before the situation was explained clearly to them.

To summarise, there is no such thing as "averaging out". Your meter does not go backwards when feeding power to the grid*; it only goes forwards when taking power from the grid. Therefore, if the immersion heater ever uses power from the grid, it will cost you and you won't get that cost back.

Hence, you need a system which ensures that the immersion heater is never fed more power than the surplus (generation minus consumption). The simplest way to do this is with a relay that switches it on and off (controlled by determining the surplus, not from a timer), but this is sub-optimal because you will never benefit until there is 1kW surplus. Better is proportional control, which will allow you to use whatever surplus is available, even if less than 1kW. Moreover, as you get to keep your current 3kW immersion heater, you can benefit from a surplus of more than 1kW, and you can use it as originally intended if your boiler fails.

By the way, long 1kW elements were also discussed earlier in this thread. They cost more like £80 than £30.


*in a few cases the meter does go backwards, but electricity companies seem to be becoming more aware of this, and when they know you have PV (which they will, because you have to tell them in order to get the FIT), they will replace your meter with one which doesn't.

 

[Mark Gee]

thank you. I do understand how it works. I appreciate that my idea will sometimes cost me as I import from the grid but sometimes, often, it won't.

Because this thread is so long it seems pretty obvious that there is no easy and cheap and legal technical solution.

My idea is very simple. Any savings of proposed techincal/battery solutions should be compared to such a simple idea to implement rather than the situation where most generated power is exported 'wastefully' to the grid.

If there are any simple/cheap ideas I would REALLY like to know as I am about to get a 4 kWp system and won't use all that I generate.