Solar Kiln Collector Design

      Keep it simple: as long as the wood is protected from direct sunlight, painting the kiln interior black works as well as more complicated heat-absorbing rigs. July 28, 2006

I am building a solar kiln. How far away is the minimum distance between the solar collector and the absorber?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Usually we use two clear covers that are spaced an inch or so apart... it is not critical. The absorber can then be spaced at any spacing that is desired, as this spacing, with a 2-layer cover, is not at all critical. What is critical is getting the heat from the absorber to the air.

Most solar kilns use the semi-greenhouse design. In this case, much of the absorber is several feet away from the covers. I think we covered this in your earlier postings and encouraged you to use such a design. The "solar collector" is actually the cover and the absorber, so there is no spacing involved. I assume that you meant the spacing between the absorber and the covers.

From the original questioner:
I am going with a single layer of twin wall polycarbonate for a covering. How much space should I leave between the covering and the black colored absorber? I am running the absorber parallel to the covering.

From Professor Gene Wengert, forum technical advisor:
Why run the absorber parallel? If you are using the standard design, just paint the inside of the kiln black and that will work just as well.

From the original questioner:
I thought you were not supposed to let sunlight hit the wood directly. Also I thought that more absorber area would create more heat.

From Professor Gene Wengert, forum technical advisor:
You should put a black screen in front of the wood where the sun might hit it or a black painted sheet of wood on top, etc. as shown in the plans. You do not get more heat when the absorber is large, as the energy coming into the dryer (through the clear covers) is the same. In fact, it is the shadow that the collector makes that determines the amount of solar energy that is intercepted. One kiln design uses collectors that zigzag back and forth a bit, increasing the collector area, but not increasing the shadow. Hence, you have more collector but not any more energy. In fact, since collectors lose a lot of energy, this design is less efficient than the flat roof designs.

From the original questioner:
Maybe I don't understand this, but I thought, for ease of use and to use the absorber to catch the most sunlight, running the absorber parallel to the collector about one foot lower with gaps at the bottom and the top (for air flow over the collector) would be a good design. But most probably I am wrong. If what I am thinking is wrong, can you suggest another design that gives me the most space possible and is easy to use?

From contributor J:
I live in San Diego and have wondered about building a solar kiln. Where can I see some plans for these?

From Professor Gene Wengert, forum technical advisor:
Your design is workable, but it does not need to be that sophisticated. Check the link below for the best design. Solar kiln plans are here at WOODWEB. The most popular design is at the link below.

Solar kiln design

From the original questioner:
First of all, that's the first time that anybody has called my thought sophisticated, so thank you. By what you have said, that design is good to use and if so, is the spacing between the collector and the absorber enough, or too little or too much?

From Professor Gene Wengert, forum technical advisor:
The further that the absorber is away from the cover, the less heat loss, so the better the efficiency. But why make a special absorber when you can just paint all the walls, floor, baffles and fan shrouds black and save the expense of the special absorber?

From the original questioner:
From what I understood from reading your articles, if you're looking in through the covering, everything should be black. Am I incorrect? (Seeing as sunlight degrades lumber.)

From Professor Gene Wengert, forum technical advisor:
Everything should be black. Direct sunlight on the lumber should be avoided, but indirect is okay.

I encourage you to use the standard design (link posted earlier). There have been hundreds constructed with this design; it works very well; it is fairly inexpensive; you do not have to do any engineering work.

From the original questioner:
I had thought that by putting the absorber parallel to the covering and most of the way to the top and to the bottom, it would absorb a good majority of the sunlight and prevent direct sunlight. I have already started construction on a 7.5ft. by 24ft solar off of the south side of a 24x24 garage. I appreciate the designs and to tell you the truth, it is partly based on those designs, but none of the designs were what I really wanted.

From contributor J:
Where are these other designs besides the one that Gene posted the link to?

From Professor Gene Wengert, forum technical advisor:
The designs are in four parts here at WOODWEB. Just because they are cited as a design does not mean that they will work well. Several of the designs in my book reviewing all designs are, in fact, quite poor. I presented them without much comment, except for a comment on the sq ft per bd ft ratio and sometimes a comment on drying time.

Solar Kiln Designs, Part 1
Solar Kiln Designs, Part 2
Solar Kiln Designs, Part 3
Solar Kiln Designs, Part 4

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