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Author Topic: Solar Collector  (Read 674 times)

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Offline jaciausa

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Solar Collector
« on: May 26, 2017, 05:26:42 pm »
Question about the sizing of solar kiln.
I have a reefer unit that I would like to make a temporary solar kiln
I would place the collector panels in a framed angled roof area just like the V. Tech on roof of trailer unit
Question: What Sq Foot roof to Volume used in trailer would be the best size ?
I know the board feet is 10 to 1 of roof. I believe this would work by using two openings in collector to get air in trailer unit.
Question: How could the openings be sized properly and the best position on collector (high in collector roof) and low in trailer?
Would this idea work and would it work better placing the collector lower than the trailer so the warmer air would naturally go up and the return air down ? Thanks for any advice.



Offline jaciausa

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Re: Solar Collector
« Reply #1 on: May 28, 2017, 09:11:23 am »
Maybe this idea of separate collector/kiln is not possible? A Nile kiln is my goal for one of my trailer units but designing a collector of solar energy on the top of this trailer seems like a win-win build until I do so. By building just the collector I could get to dry lumber with lot of potential to expand it. Would the separation of the lumber from the collector be possible?

Offline jdonovan

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Re: Solar Collector
« Reply #2 on: May 28, 2017, 02:53:08 pm »
Separate lumber area, and solar collector is certainly possible, but you've got some design challenges.

I'm no kiln expert, but I'll go out on a limb and say natural convection is most likely not going to provide enough air flow through the lumber stack.  So once you need power to create forced flow, its not much more effort to make sure that flow goes through the solar collector, to get the right flow.

The VA-tech design is based on 10:1 as you've stated. That probably is a good place to begin your design, depending on your insulation you may need more/less area. Less is easier, you can keep the vents more open, or put a covering over the collector.

Offline GeneWengert-WoodDoc

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Re: Solar Collector
« Reply #3 on: May 28, 2017, 07:12:46 pm »
Indeed, a separate collector if properly designed, is just fine.  I made the collector and roof together to save on building expense.

The best design for natural air flow requires the collector to be lower than the lumber, but even then, the hot air will accumulate near the top of the kiln and not do much drying.  We need uniform temperature and RH throughout the load to develop uniform final MC.  So, natural air flow will not be effective for lumber drying.  Wind powered, solar powered or electrical fans are needed to move hot air from the collector and to pass air through the load.  Recent posting suggest no less that 750 watts (7 amps) would be the estimated total fan(s) power for 1200 BF.  The requirement is less for air dried lumber.

The 10:1 VT design should be good for your reefer design.

I will be in Story and Marshall County, Forest City, and Amanda, Iowa this summer...are you close?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jaciausa

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Re: Solar Collector
« Reply #4 on: May 29, 2017, 01:34:18 am »
Thanks to both of you for the help with this. I have been studying all the kiln builds on here and really have obtained almost too much knowledge to retain.

I am glad this will work. Actually placing the collector on the roof would be the best solution as far as space and positioning to sun. The trailers are placed somewhat parallel east/ west out in the open on gravel surface. This was only luck, as I had no plans to use them in this way. I am not sure how close they are placed to the best sun for this area. Would that be true east/ west to keep collector square to the south? I probably will not move them, only collector.

I will possibly build them with metal studs, foam, poly vapor barrior and metal siding. I believe a design of collector over hanging the width of trailer enough for the supply from top to bottom with return up the side to bottom would eliminate penetrating the roof. These would look like shed roofs. The “floor” and shed gables would be flat black?

If I use this as the design I would end up with a rather large and tall collector seeing that the trailer is standard width.  Using the side of the trailer that is exposed to the south might be a better option! The vents would have to be higher than the trailer. This might have the problem of the trailer not being oriented correct to the south.


Offline GeneWengert-WoodDoc

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Re: Solar Collector
« Reply #5 on: May 29, 2017, 04:42:27 am »
The area of a collector is the shadow that it makes at solar noon on March 21 or September 21.  This is the 10:1 ratio number.  The best angle is the latitude...zero degrees at the equator and vertical at the North Pole.  A 45 angle is a little too steep for the USA, but is easy and means that it will work better in the cooler months.  So a taller collector that is not wider is not very effective.  A collector on the side that faces due south is likely the best in your case.  Use about a 45 degree angle or slightly flatter.  Ok?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jaciausa

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Re: Solar Collector
« Reply #6 on: May 29, 2017, 09:11:06 am »
Gene, I am located in southeast Iowa. Is it Amana, Iowa and Forest City, Iowa that you are visiting? Amana, Iowa is only 85 miles and Forest City is only 250 miles. What dates are you going to be here?

Offline jaciausa

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Re: Solar Collector
« Reply #7 on: May 30, 2017, 10:55:53 pm »
How much space would be ideal to leave between the walls, ceiling and the lumber package on a solar kiln? The collector will be separate from the kiln box.

Offline GeneWengert-WoodDoc

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Re: Solar Collector
« Reply #8 on: May 31, 2017, 12:26:23 am »
Wall to lumber space is often two feet, but it depends on pile height and width.

The roof space is often where the circulating fans are, so the fan size determines the space needed.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jaciausa

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Re: Solar Collector
« Reply #9 on: May 31, 2017, 09:27:57 pm »
Thanks Gene for that information.
I calculated the following, not sure how to calculate the space at top of stacks or ends. Is there a way to calculate intake air?

The reefer trailer would be the proper width for about 42” lumber package leaving 2 foot on sides.
After deducting 1’ for cart, height of 2 pallets, and about 18 inches for fans would leave 5’ for stickered lumber. About 1000 bd ft. of 8 foot lumber.

That would make the collector about 8 wide 12 foot for collector roof height!
If I used the complete length of unit and 48 feet by 8 collector, 4 lumber packs would be 4000 board feet. Side of trailer would make the tall wall of collector.

Offline GeneWengert-WoodDoc

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Re: Solar Collector
« Reply #10 on: June 01, 2017, 06:25:14 am »
If the collector is larger or taller, and if that means more than about 40 degrees elevation, then that is inefficient.  It is more than the area of the collector.  It is the area perpendicular to the sun at noon, or the shadow it makes at noon on a summer day.  This means that a collector that is 8' wide (north to south) can be about 7' high on the rear wall.

A load of 8' lumber that is 42" wide is 28 bf per layer of 4/4 maximum.  But usually there are a few spaces, so maybe 26 bf.  With 60" height and each layer and sticker being 1-7/8" high, you can have about 34 layers, for a total of just under 1000 bf.  If you assume the 10:1 ration, and use 45 degrees, then the 8' wide and 8' high collector will be collector wIll be 900 sq ft, or a little small.

With 34 layers, and a desired 100 feet per minute velocity through the 8' pile with 3/4" stickers, then you need (3/4)/12 x 8' x 100 x 34 cfm and then add 50% for leaks.  The total is about 2000 cfm.
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jaciausa

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Re: Solar Collector
« Reply #11 on: June 01, 2017, 11:28:44 pm »
Thanks again for your help. The height and width in my prior post is wrong and misleading on my part. An 8 by 12 roof for 96 sq ft of collector roof area was what I was trying to arrive at for initial planning purposes. I will have to proof read somewhat better.

I was trying to arrive at a lineal working design. I am going to only produce 8 feet and 12 feet lumber for sale to standardize. I am actually trying to copy YH design for the packages of lumber, carts, pallets, rail, and etc. His unit is a regular DH Kiln but with collector separate I think I could convert later.

Because the collector is separate, is some of the trouble I am having, as I have not found any thing on the forum like this. I was using the 12’ because that is an available length of the double poly greenhouse cover. The tall wall would be about 8’ 11’ with a 41 degree roof. I could make adjustable with a pivot, but that would take some work to design the end walls. Not sure if I will need the short wall the VT Kiln has.

The tall wall or side of trailer would not need to be built. The floor or ground- I am still trying to come up with something very simple and cheap to build this out of. Hedge, with foam under and down to frost on outside is one thought. I am going to try to upload an end view preliminary plan.

After looking at this I would actually have a complete kiln in the collector its self, except for common wall. I might downsize this.

Offline GeneWengert-WoodDoc

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Re: Solar Collector
« Reply #12 on: June 02, 2017, 09:18:16 am »
Good to see the detailed explanation which seems better.

The key to a solar kiln is having the walls well insulated.  As a collector is somewhat inefficient and also solar is not too strong (a square foot collector can evaporate around one pound of water per day), we want to make sure that as much of the solar energy we collect (hot air) is not lost through the wall being poorly insulated.

The 41 degree angle is fine, but a little steeper will enhance spring and fall performance, and reduce summer performance slightly (but summer is already doing good with the outside heat).  So, I suggest you go for 45 degrees.

I look forward to seeing you plan drawing(s)..
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

Offline jaciausa

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Re: Solar Collector
« Reply #13 on: June 13, 2017, 04:00:48 pm »
I am trying to pick the best solar cover that will cover about 12x 48 feet. I have read most of the solar builds on here and studied the different products. I know the better insulation value and Uv protection from degrade is important.

Does the poly carbonate double wall in clear work for a kiln?
Would a clear greenhouse cover work? What is the best long term product?
We do get hail here at times and heavy snow. I do not really want to insure this. I have found too many products to compare on my own.