Here is my DIY parabolic solar collector as it looked this morning, turned hard east to catch the morning sun just as it came over the trees which partially block my southern sky (you can click any picture to enlarge it). Although the array is turned 50 degrees from vertical it is not quite far enough to fully focus the sun on the collector tubes. It is about 10am.
This is about right, since the heat from the sun is greatest from about 10am all the way though to about 6pm, or about 50 degrees either way from vertical. Also, the Red Rock sensor will not reliably sense light coming from behind it so there isn't much point going for a large range of rotation than about 100 degrees.
You can clearly see the backs of the closest five reflectors are painted white. The rest of the array has the gray finish as it comes from the manufacturer of the acrylic mirrors. You can also see that the backs of some of the reflectors are lit up by the partially focussed light from the adjacent reflector.
As the sun rises just a bit more, there will be no back reflection and the sun's heat will be fully focussed onto the collector tubes.
The very good news is that none of the five mirrors in the foreground that have the white applied to their backs have experienced any degradation due to back heating. They have been in place for about a month and a half now. I've ordered new mirrors to replace the ones that are still gray and which have deteriorated somewhat.
The title of today's post is an opportunity for me to tell you about another strangeness of the system that is a preamble to another improvement that I intend to make.
I have purposefully tried to keep the system as simple as possible. There is no computer required, for example.
There are only two ON-OFF switches to make the system operate, one for the pool pump which starts the flow of water and another for the tracker which enables the tracking circuit to drive the small gear motor that swings the reflectors to follow the sun.
The pool pump is a very hungry motor - one horsepower, about 750 watts. I don't like to leave it running all the time. It does not need to run all the time. But it should be on when the tracker is on otherwise the water in the collectors is stagnant and will boil inside of about 20 minutes. This sometimes causes a blowout of one of the fittings and this is not a good thing.
The tracker does not need to operate all the time either, so I've been switching it off at night. This leaves the array facing west. When the array goes as far as it can, 50 degrees from vertical to the west, it will stop anyway due to a limit switch.
Recently, I've been forgetting to turn off the tracker, or I have been leaving it on purposefully to make sure that it senses the morning sun when it is heeled over west, 100 degrees away.
The good news is that it does sense the morning sun and begins to swing east as soon as the sky begins to lighten. In fact, by the time I am having my first cup of coffee, about 5:30, it has already reached the east limit. The sun has not yet broken above the horizon.
But here's the strangeness: as the sun breaks the horizon in the east, the tracker starts to head west again. It is about 6am and it is heading west. If I let it continue, it will, by about 6:30, be headed fully west again, at the west limit. What is going on?
As I mentioned earlier, there are trees to the south. Until about 9:45, when the sun rises above the tree tops, the Red Rock sensor is in the shade. The western sky looks to it to be brighter than the trees to the east, so it heads west. Although it correctly sees the early first light in the east when the west sky is still dark, it then "makes a mistake" when the sun is rising behind the trees.
Obviously this is a site issue. If I didn't have the trees to the east, the tracker would be working fine. But I'm not going to cut the trees down.
The temporary solution is to switch the tracker off until the sun is above the trees.
The longer term solution is to add a bit of automation: to sense hard sunlight when the sun is above the trees and to turn on both the tracker and the pump.
I will be discussing my approach to this is another post.
Thank you for your interest in my project.