Saturday, August 23, 2008

The sensor housing with an important improvement

The solar sensor used here is a small circuit board from Red Rock Energy ( that is housed in a weatherproof enclosure from Leviton ( fastened to an extension of one of the hangers on the most outboard collector, the one that is in the sun earliest in the morning.

Things had been going along quite well since this was installed in this form about two weeks ago. Tracking seemed to be good most of the time from early morning until late in the afternoon.

Of course I was not out there every minute of every day watching how it was doing, but would go out every hour or so on a sunny day to observe how well the collectors were positioned. If they are accurately focussed, the concentrated light beam which is easily visible, will be right down the center of each collector tube. The collectors are all locked together with the metal rod which links them and they are adjusted as part of the initial setup relative to this one which holds the sensor.

What I was noticing was that on some visits the collectors were not well focussed and were off slightly to one side or the other. I would at first try to make a small adjustment to the sensor, but then later that day, or the next day, they would be off again and it did not seem to be a constant amount, or it would be off in the other direction. Once or twice when I tried to make an adjustment, the sensor did not seem to be well behaved at all, and would race off in the opposite of the direction I had wanted. This was bizarre.

Here is what the sensor housing looked like before today. You will notice that there is a message from the manufacturer telling you what to do to open the housing. The letters are raised embossed letters cast into the clear cover when it is molded. Of course it would have been better if there was no lettering on the cover at all and that it would have a smooth clear surface, but that was not the case and I had decided to use it like this.

What appears to have been happening is that the lettering was catching the sun and forming concentrations of light that were fooling the sensor.

You can see how close the lettering is to the two diodes that do the actual tracking. When I took this picture, the sun was above and behind so its rays would pass through or very nearly through the letters. I couldn't take a picture from inside the housing but I could look up through it and the letters had many little facets of light, like tiny diamonds that shone from the sunlight they caught. It wasn't difficult to imagine the sensor being distracted.

Here is the Leviton product I am using as a sensor housing. It is what they call a "Rain Tight While-In-Use Weather Resistant Cover" model 5997-CL. These are intended for use outside to provide a pretty much weather tight shield around a power cord that might be plugged into an outdoor receptacle, for example.

There is a lot to like about this cover. It is inexpensive (about $18). The plastic cover is very clear optically. It is pretty nearly water tight when closed although it could be made a little tighter with the addition of some additional gasket around the cover latch.

But there is the light catching faceted lettering right in the part of the cover that I want the sensor to look through.

I decided to see if I couldn't salvage the cover for use in this solar collector application by first sanding out the lettering and then through a series of progressively finer sandpapers and final polishing try to remove the raised lettering. The cover seemed to be acrylic material so this should be possible. I decided to give the process a try.

Here is what it looked like after careful sanding with 220 grit sandpaper for about five minutes. At this point I thought that this was not going to work and that I had possibly ruined a sensor housing.

I continued with the 220 grit sanding in a circular motion changing direction periodically and then side to side and up and down, trying to vary my pattern often, until the lettering was completely gone. The cover top surface was now completely cloudy like a frosted bulb.

I cleaned it carefully with a clean cloth to remove any abrasive and stepped to 400 grit paper, again varying my sanding pattern. Clean again and step to 600 grit and then to 1200 grit. It still looked very frosted, but very evenly so.

I then used a grinder on which I had mounted a felt buffing disk and loaded the disk with a light coating of plastic polish. After the first pass, I knew that this was going to work and after a few more cleanings and progressively lighter touches to the wheel, again varying the pattern, the cover looked great. It looked just as if it was made without the lettering!

I took the opportunity of having it disassembled to replace the connector which mates with the Red Rock module. Red Rock provides the matching connector but mine had been out in the weather for the last four years and I decided that I'd like to replace it to eliminate a possible source of trouble.

I remounted the sensor housing and the sensor this morning and all seems well so far. I have a pretty good feeling that this was the cause of the wandering behavior, but time will tell.

Here is a final view of the de-lettered sensor housing in place.

This morning also I installed a water flow meter so that I would know the flow rate of the water through the collectors in order to do a thermal calculation which I will present shortly.

Thanks again for your interest in my project.

Thursday, August 21, 2008

About the limit switches

Two electrical switches wired in series with the drive motor prevent motion past which damage to the mechanism would occur. Here is a picture of the West limit switch in the solar heater at it's full West position. The little roller pushes the arm of the switch up to actuate it and stop the motor from turning the drive screw further in the west direction.

All servo controlled systems must have limit switches. It is important that they are reliable and positive acting and they must be weather sealed in the case of the solar collector since they are outside.

Previously I had used magnetically operated switches of the type used in security systems. These are small inexpensive plastic cased switches which are actuated in the presence of a magnet nearby. Initially, they did the job but had become unreliable after years of duty in previous prototypes, so yesterday I replaced them with the industrial limit switches shown here and tested the operation by driving the motor with an external battery (with the limit switches in circuit) to make sure that they functioned correctly. All is good with the new switches.

By unreliable, I mean that the motor had become sluggish or refused to operate at times. This was because the motor current flows through the limit switches which are normally closed (NC) and if the switches do not make good contact, the current to the motor is interrupted. This is not a good thing.

This is how the limit switches are arranged. The illustration is from Duane Johnson's Red Rock web page. He is the designer of the controller I am using. Here is a direct link.

If you can picture a battery connected to the two open wires at the bottom of the top circuit, if the positive is connected to one end and the negative to the other, the motor will go in one direction. If the battery is the other way around, the motor will go in the other direction. In use, the Red Rock controller is connected where the battery would be and does the job of switching the polarity around and deciding when to apply the current.

In the top picture, both limit switches are closed. The solar collectors are somewhere in the middle of their travel and not at the ends. The motor (the circle in the center) can receive current in either direction through both switches. The diodes do nothing as they are shorted by the switches.

In the center picture, the left switch is open and hence current cannot flow through that switch. The motor cannot go any further in that direction. The diode can conduct current but only in one direction, the direction of the arrow. The motor can turn in the direction away from the limit switch only. This would be say at the end of the day and the sun was approaching the horizon. The left switch would be the WEST limit switch and the motor was not to go any further, but was allowed to move EAST when the sun rose in the morning.

The last picture is the other case, where the right switch is open and the motor cannot move any further in that direction, but can go the other way. It's simple but effective.

Here are some of the switches that I've tried.

At the left are two types of magnetically operated alarm type switches. The one on the far right contains a mini reed relay, the one second from the left contains a mechanical switch with a small slab of steel on the swinging arm which is attracted by the external magnet. If you click on the picture to enlarge it, I think that you'll see that it is pretty rusty inside. This one has been outside for four years and has become unreliable.

At the right are two industrial type mechanically operated switches. These are the type that I will be using from now on. Both of these are Chinese made and cost from $20 to $30 each, but I've decided that the price may be worth the reliability.

The alarm type of switches look neat in little plastic packages, but they aren't sealed. In fact they will pop open with a screwdriver so you can see inside. Certainly rainwater, moisture in the air and salt spray can get into these types of switches. I don't think that they are a good choice for long term reliability.

If you decide to use the burglar alarm type switches (which are more widely available and very low cost (a few dollars each), here are some other things that I learned:

Since the full current of your motor passes through both switches, it is important that they be rated for the current used by the motor. Since they are intended for alarm systems where the current is very small, they may not even show the current rating on the packaging. The motor I use draws only about 0.5 Amp and this was in fact the rating of one of the types of switches that I used. Clearly this was marginal and not good practice on my part.

Normally closed alarm switches seemed to be rare, at least they were when I went looking for them. Most that I found were normally open (NO). Only a few had both NO and NC. You definitely need the switch to be normally closed.

One type of switch I tried had the labeling the wrong way around. The NC contacts were really the NO contacts, but maybe that is just case of interpretation by the switch designer. In an alarm application, is the "normal" when the magnet is nearby or when it is not? It is important for you to test switch operation with an ohmmeter to make sure that you are getting what you expect.

Different alarm switches are made in different way. The ones I used the longest were a mechanical switch with a swinging arm that moved between two contacts. There was a spring to hold the arm against the NC contact. Depending on which way I mounted the switch, whether the terminals were oriented up or down, the force of gravity tended to pull down on the arm, pulling it away from the NC contact. Needless to say, this made the switch less reliable, especially after the spring became tired and rusty after three years outside.

Another type of alarm switch was built around a mini reed switch. While this would sound like a good idea, since the switch contacts are enclosed in a sealed glass tube and thus not exposed to the elements, I found that at least with my two samples that one of them was much more sensitive than the other. When I was testing the switchs with a magnet and ohmmeter, the sensitive one would close the switch when the magnet was about 3/4 inch away whereas the other one, the magnet needed to be virtually touching the case of the switch in or for it to operate. This was a little too close for comfort.

Whatever type of switch you use, keep in mind the following:
  • They should comfortably handle the motor current with some margin.
  • They should be robust and weather sealed.
  • They should be normally closed.
  • Test the operation of the limit switches and ensure that they reliably stop the motor at the desired ends of travel.

Thursday, August 14, 2008

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.