Forest Products Notes on...

Solar Kilns for Drying Timber

Using solar kilns to dry timber, using the heat of the sun, is quite feasible and very attractive in certain countries. They require no conventional services (electricity, gas etc.) and are useful to rapidly reduce the moisture content of freshly felled timber and so avoid biodegradation. At their simplest, solar kilns can be a “glass-house” (or UV stabilised transparent plastic sheeting can be used). Solar kilns sometimes use outlying solar collectors to supplement the solar radiation collection. Wood waste might also be used as a supplementary fuel. Often large black bricks are placed within the solar kilns, which absorb heat energy during daylight hours and release in at night to extend the heating period.

Drying times for timber in solar kilns

Compared to Conventional heat and Vent  compartment” kilns, using solar kilns to dry  timber is slow, but faster than air seasoning of timber stacked in the yard:

• Drying “green” moisture content to about 12% moisture content (expressed on a dry wood weight basis) is about 3 times as long as using a conventional kiln.

• Drying “green” moisture content to about  20% moisture content is about  2 times as fast as air seasoning.

In fact when using solar kiln to dry timber the usual process is to air season the timber first down to 60%. Before moving the timber into the solar kiln.

Attractions of using solar kilns to dry timber

• Using solar kilns to drt timber is particularly attractive in tropical regions with high levels of solar radiation but where power sources are limited or non-existent.

• Degradation of stock due to insect and fungal action can be a severe problem in some tropical regions if timber is allowed to remain at elevated moisture contents for extended periods of  time. Solar drying can be used to avoid these degradations with their consequent financial losses.

• Although using solar kilns to dry timber might be slower than if conventional “heat and vent” kilns are used this slower drying can in fact produce higher quality stock by avoiding a number of drying defects which can occur if timber is dried too rapidly: such problems are especially prevalent in many tropical timbers which are especially vulnerable if dried too rapidly.

Furthermore, at night, when a solar kiln cools and the relative humidity consequently rises within the kiln undesirable stresses in the timber are relieved - again aiding the production of higher quality dried timber (this is a form of “re-conditioning” sometimes used in conventional kilning to avoid, or recover from Case Hardened timber).

The NEXT PAGE of the notes starts to address a very important aspect of drying wood - how to avoid causing defects in the dried stock which can seriously reduce the commercial value of the product: read about Drying Defects in Sawn Timber...

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On the PREVIOUS PAGE the interesting and potentially very attractive process of High Temperature Timber Drying is described...

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