Solar Parabolic Trough Concentrator: home experimenter's manual

March 31 2014 Edition

Frequently Asked Questions

None yet

How to build a Tracking Parabolic Solar Collector

March 22 2010 Edition

Frequently Asked Questions

What is the max. temp we can achieve?

Maximum temperature depends upon a number of factors but the main one is the rate of water flow in the collectors. The water heats up as it passes through the collector tubes and the new water coming in cools the collectors. I am heating a swimming pool and have a relatively high rate of flow, about 37 gallons (140 liters) per minute. The pool temperature is 80 degrees F (the inlet temperature). If I slow the water down enough or stop the flow, the water in the collectors will boil in about 15-20 minutes. So the graph of flow versus temperature looks something like this although this is only an approximation. I will be making more measurements this summer to better define the curve:

The question of maximum temperature can be also answered with the stagnation temperature. This is a fault condition where the flow of water stops and it can boil away, leaving the collector without cooling. I recently did some tests of the stagnation temperature. In the case of the insulated collector, the stagnation temperature reached +635F!

Do you heat your home?

No. It could be done but I have not chosen to do it. Here in southern Ontario (at latitude 43 degrees), particularly this year, the sun was noticeable absent most of the winter, disappointingly so. While there were a few bright sunny days and they were very nice indeed, they probably numbered less than 20 during the whole winter. Heat for a pool in summer however is a prime use for this type of system here since there is much more sun available and the cost of heating with fuel has risen dramatically. Before embarking on any large project, you should do some logging of sun days or seek information for your area to make sure that you have adequate solar resource for your needs.

Can you make electricity/steam?

Possibly. Many people are interested in making electricity from steam. I have not done this yet. Making an robust solar collector is only a part of that project. One must have a suitable turbine or engine and supply it with adequate steam. Robert Saunders has done the calculations for what is required and has posted a white paper here.

Did you look at other coatings for the collectors?

Yes. I tried matt black paint but found it hard to apply evenly. Copper does not hold paint well. There are specialized solar absorptive coatings like SOLKOTE but I have not had a chance to try it yet.

What is the reflector material/where do you get it/is it UV stable?

I am currently using 0.060" acrylic mirror but there are other alternatives. I tried a number of different materials and there are four pages in the plan book just about the reflector material. I buy mine from national plastics distributors (I suggest two in the book) to get the best price and I have them do the cutting since they are equipped for it and the edges must be smooth and straight. You can also look in your Yellow Pages under "Plastic - Rod and Sheet" to find potential suppliers near you.

Why aren't the collectors insulated?

Because they don't need to be in my application. Because of the high flow rate, the concentrated solar heat is applied to a very large volume of water. The temperature rise seems small, but I am heating a large amount of water. The pool contains about 50,000 gallons and I must heat it all. Because of the low heat rise through the system, there is not much loss due to radiation or conduction, so the insulation isn't necessary. If I was trying to make steam or wanted higher temperatures, I would have to insulate the entire plumbing run and the collectors.

I have recently begun working on an insulated collector for the parabolic concentrator. This work is new and is not described in the book, but so far, only at the blog.

Can the reflectors be four feet long?

Yes and this is a very good way to build your first prototype. The motor drive, the sensor, the ribs and the hangers are the same but the side channels and the reflective sheets are half the length. The length of the collector tubes would also be shortened by four feet.

1000 watts per section per day or per hour?

A Watt is an instantaneous measure of POWER. Energy has time included. 1000 watts for an hour is 1KWhr, the same as what the power company bills you for.

Do you have test results?

Yes. You can see my test results here. This year I have better test equipment and I hope to improve upon the measurements that I made last year. When I have updated results, I will post them on the blog.

How do you attach the reflectors to the ribs?

The reflector sheets are clamped between the two side channels and held firmly against the parabolic shaped ribs. The sheets are removable if they ever need to be replaced.

Do you offer any parts or a kit?

No, not at this time. I have been considering putting together a kit of some of the little pieces that people may have a hard time finding, but I would appreciate hearing which those are? Also, I am very interested in hearing from people who are NOT in North America about what sorts of things were difficult to find and what substitutions they made, if any.

What's included in the plans?

There is a detailed description of what is included in the plans and ordering information here.

Does it work in the winter?

Not where I am, in the southern part of Ontario, near Toronto (43 degree latitutde). We don't get much sun in the winter.

I am concerned about wind. How well does it stand up?

Very well I think. I too was concerned about wind and we have had some brutal winter storms with high winds of 60 mph or so in the last two winters. The reflectors are light but strong and if parked horizontally when not in use, they don't present a large profile to the wind.

What happens when the sky is overcast?

Output drops dramatically. Since it is a concentrator, it needs to have an unobstructed view of the sun. Clouds cut the output as they pass overhead. An overcast sky will cause output to fall to zero.

How big of a solar array do I need for my pool?

It depends. Please see my answer at the blog:
What size of pool heater do I need?

Can I make the ribs out of aluminum or plastic?

Yes. There are some considerations. Please see my answer at the blog:
Use another material for the ribs?

How does this compare to a traditional flat plate collector?

Of course, the answer must be: the concentrator is way cooler! Seriously, the answer to the question is complex. Leaving aside the economics for a moment, or the perceived difficulty of construction, I did a series of tests this summer comparing the actual measured performance of several different configurations of a DIY (Do It Yourself) flat plate and this parabolic concentrator. You can read about that here: Comparing concentrator to flat plate solar collector. Not surprisingly, both did better when tracking the sun but otherwise they were quite comparable. I think the concentrator had better performance overall and is more versatile. You can decide what is best for you.