excess power from pv

[echase]

come one   people , what is  excellent about a device that burns all the power  (one get FIT payment for)  into hot water ?  Feed the grid as well !


Billi

In summer my gas Rayburn with its long pipes is a very inefficient way to heat hot water so I use the immersion. Rather than sell PV to the grid in daytime and buy it back at 3 times the price in evening I’d like to use my PV directly. Not to mention grid losses.

[billi]

Sure .... sad that you have an inefficient  hot water system

[greentangerine]

come one   people , what is  excellent about a device that burns all the power  (one get FIT payment for)  into hot water ?  Feed the grid as well !


Billi

In summer my gas Rayburn with its long pipes is a very inefficient way to heat hot water so I use the immersion. Rather than sell PV to the grid in daytime and buy it back at 3 times the price in evening I’d like to use my PV directly. Not to mention grid losses.

If you're going to be using the immersion heater anyway since that's the only method you are using to heat the DHW during the summer then just have it switched on during the day instead of the evening.  You don't need any fancy controls and you'll be sure not to export to the grid. 

(The only reason not to do this is if you have Economy 7 of course)

[billi]

every PV unit fed into the grid helps to reduce coal and gas nuclear

[Andy_WSM]

Directly related to this subject, for a project I want to try, does anyone have any links to sites selling lower wattage immersion heaters to fit a standard UK tank?

[Richard Owen]

Directly related to this subject, for a project I want to try, does anyone have any links to sites selling lower wattage immersion heaters to fit a standard UK tank?

Try a ship's chandlers. I've seen a 500W immersion in a boat.

[cj]

Failing that, perhaps a 3kw dimmer thus limiting it to 500W. May play havoc with the waveform though.

[Justme]

Would be a hell of a dimmer to handle 3000watts

[MikeD]

I'm interested in this subject. The obvious way to do it is to use a triac to chop the AC (basically the same as a dimmer), thereby allowing precise control of the power that goes to the heating element. The triac can be turned on for a short period as the mains AC voltage goes through zero, and is turned off before the voltage has gone through the entire cycle. Leave it on for longer the more power you have available.

But if you do this, then you may be drawing in excess of the available PV power while the triac is on (ie, importing), and then exporting again for the part of the cycle when it is off.

So if you're doing this import and export 50 times a second, I guess the question is, will the average electricity meter notice or will it just take an average and show zero in and zero out ?

[cj]

Would be a hell of a dimmer to handle 3000watts

Most only go to 1000W but www.hedcom.fi/files/hedcom/RVE/rve_high_power_dimmers_04.pdf does 5.5kw - 11kw

I think the DNO may be concerned at high power dimmers.

They would need to operate on BOTH halves of the cycle (otherwise you would get a high DC component), so that is 100 times per second, and the meter may respond 'differently', but I doubt it would act as an import and export 100 times per second.

[Andy_WSM]

Directly related to this subject, for a project I want to try, does anyone have any links to sites selling lower wattage immersion heaters to fit a standard UK tank?

Try a ship's chandlers. I've seen a 500W immersion in a boat.

Thanks - that gave me the inspiration to search for the right thing on Google and found a 750W heater for about £40 via eBay in the end - couldn't find it on eBay's search originally - typical!

[echase]

In doing experiments on this I tried pulsing a 3kW immersion with a cycle of 1 second on and 0.1 sec off. The element failed after 10 minutes.  I am assuming that whilst 50Hz is OK, as the thermal time constant is presumably >> 50Hz, when I use a 1 second cycle (1Hz) the element just fatigues rapidly due to it heating up and cooling down too much/quickly. Anyone else had this problem?

I am not suggesting that such a low pulse rate is a good way to do this control of export, It was just a by-product of other experiments.

[cj]

The DNO may have rules about waveform distorting controllers used on hi power items? I would be amazed if thermal changes at that rate caused shock to make it fail. I would firstly suspect it was 'one of those things', or a dud batch, or possibly if the waveform distorts differently on the + and - cycles you end up with a DC element, that could cause vibration(?) If that resonates it might shake itself to bits.

[echase]

Looking at failed one I suspect it was heavily corroded and so pretty much on its last legs anyway. The pulsing could be heard so it just shook itself to bits I think.

[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 professionals have not come up with a sensible product that is affordable 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 on some threads 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; 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 and your utility company might object at more than 6kW. Two 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 <1kW immersion.