A Detached Solar Collector for a Lumber Drying Kiln

A discussion of whether to add a detached solar collector to an insulated drying shed, or to use a dehumidifier instead. January 28, 2014

Question (WOODWEB Member) :
Has anyone built a solar kiln with the collector detached? I have a shed that I could use as a kiln that is already built hence the question. I could build the collector outside of it. Does anyone have any thoughts or ideas? I am in southwest IL - how would the solar kiln work here? I am near St.Louis.

Forum Responses
(Sawing and Drying Forum)
From Contributor U:
I have a solar kiln, but it is the smaller VT design. However, when I built my house I put a standing seam galvanized roof on it. I simply put clear fiberglass 4" above the south facing roof and had a solar collector with very little additional cost. If you put a clear surface about 6" above your existing roof, and open it top/bottom to force air through it, it will act as a collector. Depending on pitch and direction it is facing will determine how efficient it is. Before I built my house, I made 4'x8' collectors and put on my porch roof to determine if it would be effective and I heated bedrooms with no problem.

From Gene Wengert, forum technical advisor:
Detached collectors will work - energy is the same in either case. Most notable designs are the US FPL design that they developed for equatorial latitudes. However, the energy transportation system and cost is high and the collector cost will add cost. In the VT design I developed in 1978, which is widely used, the collector is part of the structure. With your shed you would have to insulate the floor, roof and walls. I have a feeling that using your shed (if it is structurally sound) and try to come as close to the VT design will be the most economical.

From the original questioner:
The shed is very well insulated with 6" walls stuffed full 5/8" ext sheathing tar paper with cedar lap siding and 1/2" sheathing on the inside of the walls. The floor is 3/4" tongue and groove treated 1/2" treated on the bottom of the 8" joist with 8" insulation. It has an 8" ceiling joist, 8" of insulation, and 5/8" sheathing on the underside with 1/2" as the attic side/floor. I have plans if I use this to also add 2" blue board to the inside walls with maybe a layer of 1/2" treated over it just for the moisture.

To be honest the hardest part would be cutting the holes in my shed to duct the heat in an the return out. Up to this point I have been thinking of building a DH type kiln. The solar has just been a thought the last few days. I have a buddy that I haven't seen in some time that has a solar kiln. I am going to try to go see it so I can see how it works in person. Gene, how well would a solar kiln work? I live 30 miles NE of the arch in St.Louis on the IL side on the river.

From Gene Wengert, forum technical advisor:
I suspect you could dry three loads a year with solar, if you had one square foot per ten BF of lumber. However, with this shed I would think that you have a great opportunity for a DH kiln. From a business point, this would make more sense to me.

I am impressed with the specs on this shed. You seem to have done an excellent job in planning and construction. However, with all the moisture from drying lumber it would be critical to have a vapor barrier on the inside of the insulation. Otherwise, moisture will get into the insulation and destroy its properties (with more products). Can you add this barrier to walls, floor and roof? You need it for solar or DH.

With a DH, you will have to cut a small hole for a vent fan that will exhaust any excess heat. I do believe that NYLE has plans that should help you locate the fans, etc. They used to refund the cost if you bought a compressor. Or maybe someone nearby has plans and a unit you could examine in detail.