For pumped solar panels, i.e. any that are positioned above the hot water tank they're supplying, it's necessary to have an electronic controller to turn the pump on and off as needed (unless, as we discovered with the basic panel, you're prepared to monitor it fairly closely yourself). This is a fairly standard bit of kit, and nicely packaged controllers with graphical displays can be bought for around £90 upwards. Less pretty, but just as effective, versions can be bought for about a third of the price.
The option that appealed to me most (i.e. the cheapest), though, was to make my own. Those nice people at REUK not only sell controllers made to your specification, they also provide instructions for making your own. I'd had this webpage bookmarked for some time, but when I checked back a couple of weeks ago, I saw that they have a new design. Instead of a simple comparator chip, the heart of the new design is an Arduino computer. Having spent much of my childhood
helping Dad rig up various gadgets to our BBC computer, how could I resist having a tiny computer controlling my solar panels? REUK even have a nice tutorial explaining how to do it.
Arduino is an open source project, so whilst the official boards start from €12.00 + tax - which is pretty cheap for a computer - it is possible to get copies that are even cheaper. This is legitimate, provided they don't claim to be the real thing (though I think the Arduino folk would prefer people to do interesting things with their designs, rather than just making cheap copies). There are many variations on the Arduino board available, so I was a bit bewildered by the options. I girded my loins and set about reading, in the hope of learning enough to understand what I needed.
Luckily, there was a shortcut available. My friend Anita was visiting, and commented that her husband Dave would love my solar panels project. Ah... Dave... he's pretty hot on electronics, isn't he?
Does Dave know about Arduinos?
Yes. Why don't you come over for a chat? I did just that, and Dave was not only able to add the voice of experience to my deliberations -
That one doesn't have enough pins... That one's good value, and it has a free cable with it... That LCD package has extra stuff with it, you can get the basic unit much cheaper... and suchlike - he also had several Aduinos, and was happy to lend me one to play with, along with a breadboard (thing for plugging electronic bits into to make temporary circuits) and the half-dozen components specified in the REUK circuit.
I went home feeling much more confident, and the next day I ordered temperature sensors (4 - I might as well get a couple of spares, while I'm at it), a power supply, and a relay from REUK, and an imitation Arduino board from some place in China. I also ordered an LCD display and an SD card socket from the place in China, because it would be kinda cool if I could also display the temperatures and record data, and neither was very expensive.
The items ordered from the UK arrived in a couple of days, and it was a fairly simple matter to plug a couple of temperature sensors into my borrowed Arduino, and the Arduino into my laptop, and get my first Arduino
Sketch working. I was transfixed by the little light on the board, as I warmed up one of the sensors with my fingers to see the light to on, then cooled it by blowing to see the light go off. I could have watched if for hours.
It's all very well watching a little light go on an off - that shows the computer is doing what I want it to - but that doesn't help much with my solar panel. The next step was to rig up a couple of temperature sensors to some long wires (salvaged from the old washing machine. I know wire's cheap, but connecting pieces together gave me useful soldering practice) and put one in the panel and the other in the thermal store (tank). Connecting these up meant moving the control board into the cupboard. This required it to have its own power supply, as I could no longer run it off the computer in there, so I connected that up as well.
I do still need to connect it to the computer if I want to change the program, which proves a little more complicated than you might imagine. The only shelf in the cupboard is rather high, so I have to stand on a chair to reach it. It's also rather cluttered, so there isn't space to put the laptop down, and then I need two hands to connect the micro USB cable to the controller board, preferably without knocking any of the other wires out of the temporary circuit. I had to ask Ian to come and hold the computer for me.
Although it's not very clear in that photo, there are two LEDs on the board, a green one that indicates when the power's on, and a red one that I'd programmed to indicate when the panel is at least 6°C (I've since increased that to 10°C) hotter than the tank. So far so good, but this still required us to switch the pump on an off, and the LED wasn't very easy to see, what with the shelf being so high.
The next step was to connect the controller up to the pump, via a relay. This part was a bit scary, as it brings me into contact with mains voltage (not literally, apart from that time I went to tidy up a stray strand of wire without turning everything off first). A while back, Dad gave me a soldering iron, some solder and a little bit of strip board. I think he meant me to use that to practise on, but waste not, want not, I went straight ahead and used it for the real thing.
A slightly wider view of the controller in the cupboard,
now with relay attached, to the right of the controller.
So there it is, doing its thing. It really does, too! We've had sunshine and showers for the past few days, so I've been able to listen to it turning on and off all on its own, as the sunshine comes and goes.
This part of the project has been almost disappointingly easy. With nice clear instructions from REUK, I haven't had to figure anything out for myself. I'm looking forward to my own Aduino-copy arriving, together with the display and card reader. When I have records of what the temperatures are doing, I'll be able to tweak the settings and see what effect that has. With my spare temperature sensors, I'll be able to look at things like how much heat is lost between the panel and the house, and how much difference insulation makes. In the meantime, though, I must get on with building the next solar panel.