Let the Sun shine!

I have always been interested in solar energy. I once had a high school science project involving heating hot water with the sun, and in college my senior year EE project was to design circuitry for automated temperature control of a small greenhouse. With all this talk about alternative or renewable energy lately, I wanted to find out what it was all about and what it was capable of from a practical point of view. So I decided to implement a small scale photovoltaic project at our house. It would not tie into the grid or home electrical, but instead charge some batteries from which we could run lights and small devices. I am hoping we can run our Christmas tree lights by the power we collect, but that will be a challenge unless I switch to LED lights, because December 21 is the shortest day of the year light-wise and the sun is at it’s lowest angle on the horizon. We shall see.

The first step is location. It must face due South for full exposure, and from what I read it should receive sunlight from 9am to 3pm. Outside that the sun is too far off-axis and does not produce much power. I did not want to mount these on the roof, as that involves support structures, possibly professional installation, and possible leak issues. I couple have put them out in the yard, but then they would be in the way. Also they needed to be close to the house to minimize the length of wire I would need. I also did not want to bury the wire, which was additional expense and effort.

I ended up choosing the front of our deck, because that side of our house is directly South as we specified when we built the house (passive solar design). It took some persuasion to Barbara because she thought they would be ugly and stick way out. I think she has grown used to them by now. Here is the first picture:


These two panels (connected in series for 24 volts to reduce the IR drop) go to a charge controller inside the basement:


And then to a couple gel-type solar batteries rated at 98 Amp-hours each, or roughly 1 KWH (x2) :


I then have those batteries connected to a 1000 Watt AC inverter:


I have a power strip plugged into it which runs straight up through a hole in the floor where our hot water heat pipe goes up, and into our living room. From there we run various stuff off of it – primarily lights but also phone and laptop chargers.

These panels are rated at 115 Watts each, which of course is the maximum they can produce under ideal conditions at peak light direct-on. From the spot checks I have done I am seeing about 95 watts each at peak. The angle to match the height of the sun in the sky is adjustable by the holes in the steel brackets at the bottom. I change it each month. At spring and vernal equinox. it is equal to the latitude of Tewksbury which is 42.61 degrees. At summer and winter solstice it is -15/+15 degrees, so that means 5 degrees a month.


Anyway, on a good sunny day I collect a little over 1 KWH of power. Since my batteries together hold almost twice that, I do have some reserve capacity to both charge into and carry over for cloudy days. One KWH sounds like a fair amount, but it is amazing how quickly it goes. For instance, we have a track light plugged into it which has four 60W bulbs. Running that at full brightness for four hours (like one evening while watching TV or reading) uses up all the power collected that day. One reason is the extreme inefficiency of incandescent bulbs. This experience really drove the point home for me. I recently replaced them with dimmable LED bulbs. Expensive (especially the dimmable ones) but they only use 7 watts each at maximum while producing the same brightness. They are fantastic and barely make a dent in the power draw.

Another example is a laptop. While using it or charging a drained battery those can draw 90 watts or so (depending on the model). Even when charged, they draw some. So if we were to use it for 2-3 hours, have it charge for 2-3 hours, then leave it plugged in it will mostly use up the 1 KWH collected as well. It is not always the instant draw, but the draw over a 24 hour period that adds up.

A big science project, but I have learned a lot and that is the whole point.