Progress Report - mid August

Thanks once again to all of you who have expressed an interest in my project - a DIY solar tracking parabolic collector water heater which is built, for the most part, from common building materials.

I am now in my fourth year of working on the development of this project. You can perhaps see from the picture that I now have in place a fairly respectable prototype (my fifth) which is used here for heating a swimming pool. (You can click on pictures to enlarge them).

The current demonstration array of collectors (there are 13) is approximately 25 feet in length and about nine feet wide. I estimate from my calculations and measurements that it produces about 50,000 BTU (British Thermal Units) of heat in direct sun.

With few exceptions (the plastic mirrors, the gear motor, the control circuit and the boat paint used protect the wood ribs which form the reflectors) all the materials are common and relatively inexpensive and should be available from most any building supply center, at least here in North America. The special items I have used are relatively low cost and available from national distributors or in the case of the control circuit, can be ordered on the web.

Truthfully this is not a "beginner" easy-to-build, simple project. But if you have patience and a moderate level of skill with common hand tools and if you like unusual things with practical applications, you might be interested in this project.

This year I completely disassembled the previous prototype and modified the support frame to tilt a bit more toward south. Previously the array was at about 10 degress, now it is tilted about 20 degrees. This was done to improve the collection efficiency since a small amount of the focussed sunlight tends to spill out the ends of the collectors if they are not oriented perpendicular toward the sun's latitude. Here it is about 44 degrees so I am only about half way there.

Realistically the payback is pretty small. Probably I have improved collection by only 10% by going to this trouble. The collectors can actually be parallel to the ground and still function well since they are quite long relative to their width (eight feet long and about two foot wide).

There are practical problems with increasing the tilt. Since the reflectors just hang on the collector tubes, they tend to slide down toward the low end of the tube. This has necessitated adding spacers to each at the lower end to keep them in place. Also the tilt tends to stress the lowest rib as it bears part of the weight of the collector that causes it to slide down the collector tube, but no matter, it seems stable enough to continue experimentation and the added friction against the spacers is easily borne by the drive.

You can see the added spacers (the short sections of white PVC pipe at the lower end of the collector tubes in this detail. Also the construction of the cross pipes which carry the fluid from one collector to the next. A small detail, but it seems to have improved the flow rate, is to use larger radius PVC elbows at the corners. I read about about the effect of turbulent versus laminar flow at bends in piping systems that suggests that larger radius bends reduces the resistance to flow and it seems to have worked. All cross pipes (there are 12 with 24 elbows in total, so there are many of them) are now made this way. The larger radius elbows are about twice the price of the the short, tight elbows but seem to be commonly available. You can also see the safety straps which capture the cross pipes in place to the frame so that there is no danger of blowing off the crosspipe if the water boils in the collector tubes, a problem that happened a few times in the past when the pump wasn't turned on as the sun went overhead.

One of the unique features of this design is that the reflectors pivot on the collector tubes rather than being above them as is the usual case, for example the Lutz designed collectors in the Mojave systems. An effort was made to balance the mass of the reflector about the collector tube so that they tend to hang down but very little force is required to swing them about. The implication of this is that only a very small gear-motor is required to swing the whole array of 13 collectors. In this I have been very pleased with the result, but I was dissatisfied with the drive mechanism so in the last couple weeks I have produced a new drive mechanism.

You can see that the reflectors have a small aluminum arm attached to the center rib. A thin stainless steel rod (the drive rod) threads through a series of steel eye bolts all the length of the array. One eyelet is mounted at the bottom of each arm on each reflector. Pushing or pulling the rod swings the reflectors.

A gear-motor (mounted in the box at the right) turns a threaded rod. Previously (you can look in past posts here about "motor drive") I had been using a steel nut riding along this threaded rod to couple the linear motion, through a swing arm, to the end of the drive rod.

Once I had a full complement of reflectors installed however, even though they are largely balanced, the effort required, particularly at the extremes of rotation was increased and the linear motion through the swing arm tended to badly bend and distort the drive rod. What was required was to build a drive system that duplicated the swinging arc of the eyelets, moving the force along an arc (up or down) as the array swung to the extreme end of travel.

I came up with what you see in this picture where the threaded rod again drives a nut but now a couple of bearings attached to the nut move along a channel in an arm which is the same length and located at the same height and in the same plane as traced by the drive rod. I am now able to easily swing the array to 50 degrees off vertical in both east and west directions with a little $50 motor that you can hold in the palm of your hand.

There are some things still to work out, but it is functioning much better than the previous arrangement. There is plenty of torque. I cannot stop the arm from moving with my the full force of my hands, nor slow it down.

I have not made any significant progress on the solar powered DC pumping system or the solar powered UVC sterilization, but these components are still in my plan.

That's all for now.

I will make a serious effort to be more diligent in recording progress here and I do enjoy hearing from you if you chose to write to me. I am sorry that I have not been able to answer all of your emails and requests.

I do plan to make a set of detailed plans and instructions available at a nominal cost (probably about $30) for those who would be interested in receiving them. I have stored all of your requests and will send you personal emails when I have the plans available, probably this fall. I have taken hundreds of pictures, video and I have many notes to consolidate. This will take some time, but I hope that it will help those that have an interest.