DIY Blog: Solar Water Heating with PV

31st July 2014 in Know How

Energy saving is a key aspect of a Smart Home, and with so much sun recently we thought it was about time to tell you how free electricity from your PV panels can be used for solar water heating.

PV Solar Panels

Did you know that there are over 510,000 domestic PV installations in the UK? All together these are churning out over 2000 Mega Watts on a sunny day. To put this into context: This is 5% of the total power demand in the UK and there are only 5 power stations in the UK that produce as much! Unfortunately the PV panels on our homes produce this electricity during the day when we are at work and need it least… so what to do with this electricity?

The Hot Water Tank – Your Battery

Why sell your electricity to the grid for 4.7p during the day, when you then have to buy it back for 12p in the evening? Instead you should use it yourself, but how? Your hot water tank is like a big battery and you can use the immersion heater to charge it using the free electricity during the day to have it nicely heated up for a hot shower once you get home.

Megaflo Immersion Tank

Switching vs. Dimming

Switching the immersion heater on and off is perfectly adequate if all you want to do is heat the hot water based on a schedule, like most Brits. However if you want to be smart you can dim your immersion heater.
Why? – Most immersion heaters use 3 kW, which means before switching it on your PV panels would need to produce this much, otherwise you pay. So to make the most of the electricity from your roof it is much better to “dim” the immersion heater. This way it can be controlled to use exactly the amount of electricity that you have spare.
How? – All you need is an SSR (a Solid State Relay).

Solid State Relay

The immersion heater is connected to the load terminals of the SSR, 24VDC are connected to power the SSR up from the main power supply of the system and finally a 0-10V output of the Miniserver is used to tell the SSR what level to drive the immersion heater at.

Making it Smart

Of course just having the capability to “dim” the immersion heater does not make the system smart. For smart solar water heating we need information that will allow the Miniserver to automatically decide when to run the immersion heater and at what level. In short we need to know how much power our solar panels are producing, and how much electricity we are consuming with power appliances like our fridge etc, so we can see how much we have spare to heat the water.