Drying timber using a solar-assisted kiln
by T Blaubaum, GCD International and P Philp, CADDET Australia

Originally developed as a way of adding value to a precious native hardwood, a solar- assisted timber kiln is now providing an energy-efficient alternative to conventional drying kilns. Solar-assisted kilns have now been established in every climatic zone in Australia. The solar kiln is particularly valuable for precious timbers which are notoriously difficult to dry using conventional air-drying techniques. This technology reduces both the drying time for timber and the losses caused by uneven drying temperatures, and offers an energy-efficient alternative to conventional drying kilns.


Solar-assisted kiln.

The principle
It is unusual to find a kiln made of plastic. The external cladding of this kiln consists of two skins fabricated from clear, 150 m, UV-resistant polythene kept apart by a small blower. This 'double glazing' reduces heat losses from the kiln at night. The timber is stacked inside the plastic shell and surrounded by a black solar blanket which brings the heat directly to the charge. The frame for the structure is made from stainless steel, which resists the corrosive acids and chemicals inherent in Australian hardwoods and preservative treatments.

The kiln was originally developed and patented by Western Australia's Department of Conservation and Land Management (CALM) as a way of controlling the seasoning of jarrah, a precious native hardwood. GCD International Pty Ltd has been granted the licence to the technology. The company has since developed a supplementary heating and humidification system which controls the performance and rate of drying in the kiln. The operating system is designed to control the temperature and humidity of the charge to achieve the drying schedule required by the sawmiller.

Supplementary heat is added when required by a direct LPG-fired burner system, which can be set up for natural gas or other fuels. The system uses variable-speed fans to control the air flows through the timber stack. Air-flow rates vary according to species chosen, as well as the geometry of the timber in the stack, resulting from the mix of timber thicknesses being dried.

An atomising spray system controls humidity and is used to prevent excessive temperatures being reached on hot days. In some parts of Australia, temperatures of over 45C can be reached in summer. For optimum results, it is important to keep the temperature of the timber fairly low in the early stages of drying. The GCD CALM solar kiln cools the stack by using its humidification system as an evaporative cooler and the fan to vent the resulting hot, humid air to the atmosphere.

The kiln structure is supplied to site in knock-down form, which makes it ideal for remote locations where a more solid structure would be impractical. The heating system is supplied fully built and pre-commissioned so it can be installed in-situ.

Currently, the kilns are being operated in a number of different ways, depending on the requirement of the timber species involved, and the mode of operation of the sawmill. In some cases, the kiln is kept at an even temperature twenty-four hours a day, while other sawmillers allow their timber stack to cool by turning off the supplementary heating overnight. Some timber drying experts believe that allowing the stack to cool by a few degrees overnight allows the timber to relax in a similar way as stress relieving steel.

Projects already established
To date, kilns have been established in all Australian states. The largest application so far has been installed by Hallmark Oaks, in East Gippsland, Victoria. The kiln has a 200 m3 drying chamber capacity and has been custom designed to contain particular arrangements of timber packs. The supplementary heating system is fired by LPG, since the costs of extending the natural gas pipeline to the site, along with connection fees, were commercially unfeasible.

Since the kiln was commissioned in early 1998 it has dried three charges of East Gippsland mixed-species hardwood. On average, it took about nine weeks to reduce the moisture content from 40% to 15% in 180 m3 of 55 mm boards. These charges were all dried during the winter period, which was unusually wet in Australia during 1998. On average, 265,000 litres of LPG were used to dry each charge, at an average cost of $6,500 (where $ is the Australian dollar).

Economics
The method of drying in the solar-assisted kiln allows sawmillers to make substantial savings in fuel costs compared with conventional drying and predrying, as well as achieving substantially improved grade yields compared with air-drying or harsher methods of timber seasoning.

In Australia, some hardwoods are air-dried for up to two years prior to final drying with a total reduction in moisture content from 80% to 12% ready for use. This method of timber drying has been necessary because of the high capital cost of drying in conventional kilns, fuel costs and the difficulty of getting good grade recoveries in 'hard-to-dry timbers' which need to be seasoned slowly. The solar-assisted kiln can reduce the processing time by 4­8 times compared with air-drying, with minimal damage to the timber.

Sawmillers who have used the technology to date have reported almost negligible losses and drying degrade compared with air-drying. Timber dried in the solar kiln can achieve an increase of up to 15% in dry output ready for machining or other processing compared to air-drying. This is of substantial commercial value to the sawmiller, since the hardwood can be valued in excess of $2,000/m2depending on the species of timber.

 

CALM solar-assisted timber drying kiln.

For more information contact the CADDET Australian National Team or Mr Kevin Long at GCD International Pty Ltd; Tel: +61 3 9764 1733; Fax: +61 3 9764 1523.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.