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.

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 twice 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 Hardening of timber).

Making a Simple Solar Kiln to Dry Timber

Materials Required

• Clear plastic or polycarbonate sheets (for the roof of the kiln)
• Wooden framing (treated lumber is best for outdoor use)
• Reflective material (e.g. aluminum foil or reflective insulation)
• Hardwaring cloth or wire mesh (for the timber to sit on)
• Thermometer (to monitor the temperature inside the kiln)
• Ventilation holes (for air circulation/ loss of moisture from timber)
• Nails, screws, or nuts and bolts (to assemble)

Decide on the Location

Find a sunny area with little shade (i.e. sunlit).

Build the Frame

Build a box (or lean-to structure) from treated wood. This frame should be about 4-6 feet long, 2-3 feet wide, and 3-4 feet tall (this depends on the size of the timber to be dried). The frame should be angled at approximately 10-15degrees to gain maximum heat from the sun.

The Floor of the Solar Kiln

Use could use hardwaring cloth or wire mesh to make the "floor" of the kiln. In fact this floor should be arranged to be raised a few inches above the ground to avoid moisture contacting the stock and to allow air to circulate around the timber.

Install Reflective Surfaces

The inside of the kiln should be lined with a reflective material (e.g. aluminum foil or reflective insulation). This is to maximize the amount of sunlight that heats the timber.

Make a Transparent Roof

The top of the kiln needs to be made from clear plastic or transparent polycarbonate sheets. Angle this roof to avoid water accumulating.

Provide Ventilation

Small vents at the top and bottom of the kiln to ensure good through airflow and help to allow moisture from the timber to leave the kiln.

Put in the Timber to be Dried

Stack the timber inside the kiln on the wire mesh. It is important that there is space between the pieces of timber to get good air circulation to carry away the moisture from the timber pieces.

Keep checking the Temperature and Humidity in the Kiln

Use a thermometer to monitor the internal temperature. Try to maintain 100-120 Degrees F. (38-49 Degrees C.). It is important that the temperature does not get too high otherwise the timber will become damaged.

Notes

Drying times: These vary depending on species, wood thickness and initial moisture content. A solar kiln might dry thin planks in a few weeks - thicker timbers can take months.

To control the temperature/ humidity at night the kiln can be covered with a tarpaulin (to trap in heat) or prevent rainwater ingress through any leaks in the structure.