Wednesday, March 23, 2011

New Solar Sensor (again...)

Lately, the tupperware solar sensor that is supposed to measure the available solar power to control my solar water heater has been playing up. Sometimes reading zero in full sunlight or other wonky values that didn't seem right. It also was too directionally sensitive - a feature of the old solar panel used that has micro lenses on it that focus the light - but only when it shines square on to the panel. Also the box itself was possibly causing some shading or variation in light getting through. On closer inspection, the UV had destroyed the tupperware, making it crack and go brittle.

So the search was on for a small sensor that will be weather proof and less directionally sensitive. The local Robert Dyas had the perfect bit of bodgineering raw material... a £1.49 solar garden path LED lantern spike thingy.
It had a little amorphous solar panel on the top and all the bits came apart easily to leave the plastic and stainless steel capsule on its own. I could just rip out the little circuit board with the LED and battery, and connect the wires from the solar panel across a 100 Ohm resistor load in a chocblock (so that the device measures solar power). Then I just had to hot glue up the holes on the base to prevent water getting in. The hole where the LED came out was just the size to fill with a rubber blanking grommet.

Some of the existing holes, that were used for the clear lantern bit to snap on to, were also handy for threading the plastic cable tie through. I mounted the new capsule sensor on a handy hanging basket bracket that was already on the wall when we moved into the house. This, by sheer coincidence, is at the same angle as the main solar panels.
The amorphous panel has no outer window or lens to restrict the angle of light acceptance and amorphous panels are less sensitive to direction anyway, so it seems to give a good reading through the day.

The output Voltage was a little lower than the old sensor, so I had to recalibrate the measuring software on the load controller to get a proper 100% reading in full sunlight. In the end, I decided it would be easier to modify the software to have a user parameter for the sensor scaling, avoiding the need for recompiling the program just to change the value. Amorphous panels put out up to 20% more power when new, but quickly settle down to their usual power when exposed to the sun for a few weeks, so it made sense to change the software to allow tuning the sensor. I even added new today and yesterday counters for the heater run time and estimated kWh of DC power generated.
I've even got the (still working) white LED, solar controller chip, NiMH coin cell and on/off switch gubbins that I can play with. Most of that would be worth at least £2-3 if I'd bought it from Maplins. So the solar panel was actually "free".

Oh, yeah... Like my new multimeter? It's an antique Micronta (Tandy) analogue (but with FET inputs) 1980's test meter that I picked up at a car boot the other week. Nowhere as accurate or convenient as a DMM but looks "retro-cool".
The new sensor has been working better than I expected, and now the water heater works really well.  Getting good heating yields but not cycling the battery much at all.

Here you can see the results of a totally blue sky day on the right of the graphs...  Even on the not so good day, the battery Voltage holds up pretty well with the more accurate solar power availability estimate. Click on the chart below to see it in full size.
Today's trace (on the right) was the first full solar hot water day of the year. You can just see there were no clouds at all by the totally smooth sensor trace.  No gas used today, and 48'C water in the tank :D.