I have recently had some solar PV panels installed and am trying to come up with clever ways to use the excess PV being generated on these sunny days, since Meridian only pay 25c for the first 150kWh exported each month. I have come across the very clever chaps at OpenEnergyMonitor (http://openenergymonitor.org/) and in particular one bloke named Robin Emery who develops and sells what he calls the MK2PVRouter (http://www.mk2pvrouter.co.uk/).
The idea is this thing will monitor your feed to the grid using a CT sensor and voltage sensor. As soon as you begin exporting to the grid it will start 'flicking on/off' a specified load, typically a hot water immersion element, just enough to bring your export back to zero. As your solar generation fluctuates this thing will adjust the speed of the HWC element cycling in order to keep your import/export at zero. Very clever stuff!
I have the added complication that I already have a solar hot water system via evacuated tubes. The controller for this system is able to manually boost my HWC at specified times, via a big juicy relay. So mine is configured to boost the HWC at 4am if the cylinder temp is below 50 degrees.
My HWC has two elements, one in the middle (hooked up to the solar H2O controller) and one at the bottom - currently disconnected but which I want my PV router to be controlling, since I want to heat the entire cylinder with this excess solar energy when it is available.
Unfortunately my HWC is not in the same place as my distribution board, and I only have a single 240VAC circuit to the HWC. So I have to be very careful that I don't have both elements on at the same time, since the wiring is not rated for that kind of load.
So attached is my current setup and the proposed setup (a picture is worth a 1000 words and all that!).
The question is this - can anyone see any problems with this design? And secondly does anything know of any SPDT SSRs that would be suitable for this type of application? The switching rate will be very quick - multiple times per second when it is active so it will need to be very robust and fast acting.
Robin is using these SSRs (http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1200241) in his latest iteration but they are only SPST and as you can see from my design I really need SPDT relays.
I am hoping someone one here will be able to point me in the right direction!
EXISTING HWC WIRING DIAGRAM
PROPOSED HWC WIRING DIAGRAM