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Solar Kiln Project:  

Speed drying firewood with a solar kiln

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Last updated on 11/05/2016
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Anyone searching for supplies for a wood burning stove has a couple of choices. They can buy kiln dried wood which is ideal especially for more sophisticated stoves but is expensive. The other choice is to acquire logs which have been seasoned, meaning they have been air dried. This is a cheaper option but can leave you with wood which has a higher level of moisture than is desirable. By the way when buying wood, you should bring along a moisture meter and a small hatchet. Splitting a sample log and testing the interior surface will tell you what you need to know.

I’ve said you’ve two options but now there is a third which I am currently testing out. Not popular yet here but common enough in the US are solar kilns which dry out wood by trapping the sun’s heat. Essentially these are large, highly insulated boxes, incorporating a sloping double glazed window and equipped with a couple of vents to admit air plus fans to suck out the water vapour hopefully issuing from the soggy logs inside. They work in America but will they work here with our rather overcast weather? 

The design I am going with has thick polystyrene insulation sandwiched between a layer of plywood and wood panels. The window is double glazed with polycarbonate sheeting into which is set two solar powered fans. It is painted black inside better to absorb the sun’s heat. Solar kiln makers normally recommend the glazing be set at the same angle of elevation as the latitude of the site where the kiln is situated. In Belfast that would be 55 degrees. But some research on PV panels suggests, with Ireland’s more cloudy conditions, a more acute angle would help capture the sun’s rays more efficiently. Assuming that would hold also for a solar kiln, I’ve gone for a compromise angle of 45 degrees.

During this summer I will test just how quickly the kiln dries blocks of wood compared with simply leaving them outside in the open air exposed to the sun and wind but protected from the rain. I’m fairly confident the kiln will prove itself. Inside the box the sun will drive up the temperature. You know hot it can get in a car on a relatively cold but sunny day and that’s with single glazing and no proper insulation. The temperature could rise to perhaps 40 degrees inside this sealed box. In those conditions, the water will be driven off. But the experiment will provide the evidence of just how effective it is. 

If it works well after the necessary tinkering and it can be made for a reasonable price, then the solar kiln could be a solution for householders who suspect the wood they’re buying has too high a moisture content but haven’t the months and months it takes to air dry it properly. 

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