One of the side issues of the Feed in Tariff with Solar PV is besides the wonderful 43.3p your system will earn from for each unit generated, you are able to use as much of that generated power as you can at your premises without penalty. However, most dwellings will only use around a third of all the available power generated. Most houses are big power users during hours of darkness with lights, TV, etc, PV generated power not much benefit in this case! Even if you are at home all day and running the washing machine, lawn mower, computer, etc this will only scratch the surface of peak generated power of a standard 4kWp system in the summer.
At customer’s request, we put together a test unit a while back and it’s been running faultlessly on test in a customer’s house for some 6 months or so with great effect. They have not needed to use gas to heat their water all summer, beats getting a Solar Thermal system! It will also be of considerable benefit to people who have had 'free' PV systems, only to find minimal financial gain and will be going through a period of disapointment.
The unit is fairly straightforward, and does require you to have a hot water tank with an immersion heater installed. If you have it does the job for very little outlay. There are intelligent units on the market which do the job far more elegantly, such as the EMMA and the Power Router (which we install), but as you can imagine, the purchase cost of such units are into £1000’s rather than this little control circuit which comes in at under £100 in components, or £180 + VAT to buy complete from us.
Before going any further, I strongly insist this is only assembled and installed by qualified personnel who have the knowledge and capability of recognising safe working with electricity and protection of components and wiring.
The Circuit (click on the image above to get full size)
The live output from your inverter is diverted through a Current Sense unit, which is set for a switch point optimum this application. This Current Sense unit has a built in low current relay, which is used to switch a suitable contactor. I would suggest a 10Amp contactor at the very lowest for the sort of use we are looking at.
You will see from the description below, together with reference to the circuit diagram, the ideal location for the completed unit is adjacent to your consumer unit (fuse box). As both the inverter and your immersion heater will be connected directly to this point, it really does make sense.
You can refer to the circuit/ layout diagram for the following:
1) The mains 230v AC output from the inverter needs to have the live conductor diverted through the current sense unit, but otherwise remains connected as normal.
2) The Current Sense built in relay terminal is supplied by a spare 6A MCB in the consumer unit. This relay will switch the contactor. As well as the 6A MCB, it is advisable to provide a 1A inline fuse to protect the relay and contactor coil adequately.
3) The main relay on the contactor will switch the live output from the water / immersion heater in the Consumer unit. To do this will involve diverting the existing immersion heater wiring to route through this unit. Due to regular switching this circuit will now be cycling through an inductive load, it is recommended you change the 16A MCB to a ‘C’ type 16A MCB to avoid nuisance tripping.
At the hot water tank, you will need to disconnect the immersion heater wiring from the wall switch and fit with a 110v yellow plug. Change the immersion switch to a standard switched socket. Plug a 3kVa 110v builder’s site transformer into the socket, and the newly plugged immersion heater cable into the transformer. That’s it!
Normally, your immersion heater runs at 3kW. To make the most of the free to use electricity available, we need to make the heater use less power. By far the easiest way to do this using standard and cheap to buy parts is by a 110v site transformer, available from screwfix, etc, at around £50 each. Once the transformer is in place, this will limit the power consumption of the immersion heater to around 700W. For a standard 200ltr water tank, we have found the immersion running for most of the day will heat the whole tank to around 50 degrees. This obviously assumes the day is bright and sunny all day of course!
On a unit supplied by us we would set the current switch to around 800W, so will only switch your immersion heater on when the inverter is generating more than 800W, ensuring immersion heater usage is powered by the PV system.
It must be mentioned, that although the Current Switch has a built in hysteresis to avoid relay chatter when passing current is marginally over or under the set point, on a day with patchy cloud you will experience the contactor clicking in and out regularly. This can’t really be avoided on a simple circuit as this, but has not proved to be an issue in the 6 months of operation in our test unit.
In essence, the controller is very basic and I will always welcome suggestions to improve the design.