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Experiments with cycling a kettle on & off have shown that a small amount of averaging takes place in our electricity meter.  A conventional PWM ac power-controller should therefore be OK for matching the power taken by our immersion heater to the spare PV power that is available.  This 'power window' appears to be around 1/1000kWh, so a 'quiet' controller that passes or suppresses entire half-cycles should be fine.  My wacky idea of a variable electronic load can now thankfully be consigned to the Recycle bin.

Being able to 'invest and reclaim' one's precious PV-generated energy at the grid point is really neat.  In less than ideal weather conditions, my PV system would never be able to power our 3kW immersion heater on its own.   The grid is providing an elastic buffer without which our inverter would doubtless complain.  It's rather like trying to vault over a high barrier.  What's needed is the use of a trampoline, and that's what the grid is providing for free :D

but you do not need 3kw to power your immersion heater. if you put 1kw to the element it will still give you a tank full of hot water, it will just take longer to heat up. On cloudy days my solar immersion conroller gives me a tank of warm water using just 600w at the element.

Madmurg: calypso_rae is referring to the need to have a controller to actually be able to dissipate a variable amount of power into the immersion element. There is a long discussion on controller design on the forum here: http://openenergymonitor.org/emon/node/176

madmurg: but you do not need 3kw to power your immersion heater. if you put 1kw to the element it will still give you a tank full of hot water, it will just take longer to heat up. On cloudy days my solar immersion conroller gives me a tank of warm water using just 600w at the element.

With a 3kW immersion heater, there are only two ways to make it take less than 3kw: one is to reduce the level of its ac supply voltage to it; the other is to switch the 240V supply rapidly on and off.  The first approach would require a water-cooled electronic load to be placed in series with the immersion.  Thankfully, after doing some tests with a kettle, it seems that such a drastic approach will not be necessary.  When my SSR finally appears, I therefore look forward to building a conventional controller whereby selected half-cycles of the mains will be fed to the immersion according to how much spare PV is available. 

This approach only works because of the small window of flexibility in our supply meter mains.  A free-standing PV system, if generating only 600W, would presumably be unable to power a 3KW load in this way.

I have built a unit for this. In the unit is a Kemo 4Kw Triac controller http://www.kemo-electronic.de/en/Light-Sound/Effects/Modules/M028N-Power-control-110-240-V-AC-4000-VA.php. Connected to that is a PWM controller http://www.kemo-electronic.de/en/Transformer-Dimmer/Converter/M150-DC-pulse-converter.php.

These in the uk are available from Brocott UK on the web; I have done it this way because these modules are available to anyone who is not to good with electronic circuitry building.

I have split my immersion heater MCB off from the main domestic load in it's own small consumer unit.

Also in the unit is a 30A relay that will operate on and off from a small bank of relays (bought on Ebay) also in the unit. The 30A relay is to completely isolate the Triac control unit when not enough energy is available from the solar.

I have fitted a 9 pin rs232 type connector to the aluminum case it is installed in which carries the PWM signal, 12v for the small relays and signals to operate the small relays and a led to indicate when power is on to the controller.

There is an 9 pin RS232 lead linking the two units so basically, the mains is isolated from the low voltage Arduino based unit. 

This bit all works fine and I would recommend perhaps to do it this way; I will post a pic of the unit next week.

The only thing I cannot get right is the sketch; I have tried using the example voltage and current sketch on here and installed the Emonlib but the sketch just throws up errors when I compile it. I have a 4 line lcd which i have placed into the separate which works fine and should give me all the information I require. Could someone please, please help me with this?

I am mid 50's and some of the sketch I just don't understand, seems harder to take new things in the older you get.

I suppose you could use the arduino based unit to control a storage heater as well as hot water with another power control unit like I mention before. The immersion heater will switch off when it is hot enough via the thermostat so depending on your PV system some spare energy would be available. Monitor the immersion heater as well via a ct.

Mark.

Once a means has been implemented to prevent surplus power from going back into the grid, that power can be then used for any suitable purpose on-site.  On a good day, there should be more than sufficient power to boil up a tankful of DWH, so it makes sense to use any that's left over for heating up a storage heater.  For households with combi boilers, i.e. without a DHW tank, one or more storage heaters would seem the obvious way to go. 

I'm surprised that you would want to measure the immersion heater's feed with another CT.  If you are controlling this supply directly, don't you already know how much power it's getting?

Submitted by calypso_rae on Sat, 21/04/2012 - 22:23.

madmurg: but you do not need 3kw to power your immersion heater. if you put 1kw to the element it will still give you a tank full of hot water, it will just take longer to heat up. On cloudy days my solar immersion conroller gives me a tank of warm water using just 600w at the element.

With a 3kW immersion heater, there are only two ways to make it take less than 3kw: one is to reduce the level of its ac supply voltage to it; the other is to switch the 240V supply rapidly on and off. The first approach would require a water-cooled electronic load to be placed in series with the immersion. Thankfully, after doing some tests with a kettle, it seems that such a drastic approach will not be necessary. When my SSR finally appears, I therefore look forward to building a conventional controller whereby selected half-cycles of the mains will be fed to the immersion according to how much spare PV is available.

This approach only works because of the small window of flexibility in our supply meter mains. A free-standing PV system, if generating only 600W, would presumably be unable to power a 3KW load in this way.

I beg to differ as I have a fully working controller which gives me a tank of hot water everyday without drawing anything from the grid. sometimes if it is cloudy the tank only gets luke warm but on a sunny day it is piping hot. I use a crydom proportional controller to vary the power to the immersion, and it does work with loads as low as 600w.

Well I was thinking that once the water in the Immersion heater tank had got up to temperature and the thermostat had switch off then a CT on the Immersion heater circuit would tell the Arduino to switch excess power to else where such as a small storage heater on another PWM dimmer control such as the one I mentioned from "Kemo". 

@markbeal2:

"Well I was thinking that once the water in the Immersion heater tank had got up to temperature and the thermostat had switch off then a CT on the Immersion heater circuit would tell the Arduino to switch excess power to else where ..."

Fair point.  When the DHW gets up to temperature, the Arduino needs to be informed of this somehow.  Monitoring the current flow in the immersion heater's supply cable would be one way of doing this.  Some logic would be needed to distinguish between the absence of flow due to the thermostat turning off, and the absence of spare power when someone switches the kettle on.

@madmurg

I beg to differ as I have a fully working controller which gives me a tank of hot water everyday without drawing anything from the grid. sometimes if it is cloudy the tank only gets luke warm but on a sunny day it is piping hot. I use a crydom proportional controller to vary the power to the immersion, and it does work with loads as low as 600w.

OK, I think our only difference is one of terminology.  By switching the controlled output rapidly on and off, I would say that your Crydom is changing the supply rather than the load.  But, hey, you have a working system and mine has yet to get off the drawing board ...