Solar, Vacuum and Radio- Frequency Drying of Wood Forestry 486 Lecture 3-6

Solar Drying Basics Characteristics of solar energy: – Low intensity solar “insolation” (amount of available energy): approx. 428 BTU/hr/ft 2 in the upper atmosphere vs BTU/hr/ft 2 at surface due to interception by clouds etc. – Intermittent (day vs. night; seasonal variation) – Subject to interruption (cloud cover, smoke, etc.)

Solar Drying Basics Classes of solar radiation: – Ultraviolet (UV): < 0.38 μm wavelength, approximately 7% of solar radiation – Visible: μm, 47% – Infrared (IR): > 0.78 μm, 46%. (lower wavelength, e.g., UV=higher energy light)

Solar drying basics… Available energy influenced by angle of incidence; determined by position of sun in the sky and the collector tilt angle A “rule of thumb” for north temperate region is to set collector tilt angle as follows: – Summer, latitude – 10 o – Winter, latitude +15 o

Types of Solar Collectors used for wood drying “Hothouse” – not unlike a plant greenhouse External collectors – Heat transfer medium Air; low “energy volume” Liquid; efficient but more complicated – Flat plate collectors – Concentrating collectors, e.g., parabolic mirror with sun tracking capability

Solar Drying A home-built solar drier can provide quality lumber drying at an accelerated rate 4/4 hardwood may be dried in 6 weeks of good weather 8/4 hardwood may be dried in 15 weeks

Greenhouse-design solar dry kiln

Semi-greenhouse design

Solar dry kiln with external collector

Solar drying attributes Can dry wood 2-3x faster than air drying Can dry to 7-12% final MC Can reduce casehardening by shutting off fans and/or vents at night (increase EMC during evening hours) Largest design I’ve seen was for 50Mbf Most are used by hobbyists or in developing countries (not a bad idea for tropical regions with greater amount of sunlight)

Solar Drying References “Improvements in Solar Dry Kiln Design”, available at “Constructing and Operating a Small Solar-Heated Lumber Dryer” Chen, P.Y.S Design and tests of a 500 bf solar dry kiln. Forest Products Journal 31(3): Tschernitz, J.L., and W.T. Simpson Design for lumber dry kiln using solar/wood energy in tropical latitudes. Drying Technology 4(4): Lumley, T.G., and E.T. Choong Technical and economic characteristics of two solar kiln designs. Forest Products Journal 29(7):49-56.

Solar/Dehumidification Drying References Chen, P.Y.S., W.A. Helmer, and H.N. Rosen Energy efficient lumber dry kiln using solar collectors and refrigeration systems. United States Patent No. 4,432,147. Helmer, W.A., and P.Y.S. Chen Computer simulation of a new method to dry lumber using solar energy and absorption refrigeration. Wood and Fiber Science 17(4):

Additional References on Drying Small Quantities of Lumber Miceli, C.M., H.N. Rosen, and H.A. Spalt Drying small quantities of green hardwoods. U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station Research Paper NC-228. Bois, P.J., E.M. Wengert, and R.S. Boone A checklist for drying small amounts of lumber. U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. Forest Products UtilizationTechnical Report No. 6.

Vacuum Drying of Lumber Use of vacuum permits a low EMC condition at lower temperature than conventional Advantageous for refractory species, thick stock, color maintenance Time and energy savings possible Russian vacuum lumber driers

Vacuum may be used to dry wood at lower temperatures - Example An oven set at 102 o C, operating in a room at 20 o C and 50% RH will have a relative humidity in the oven of 1.1% and corresponding EMC of 0.2% A vacuum oven with an absolute pressure of 0.13 kpa at 50 o C, in the same 20 o C/50%RH room will also have a 1.1% RH and 0.2% EMC. See Siau 1995, p. 13

Vacuum drying… Various means are used for heat transfer, including: – Convective heat transfer…inefficient and ineffective in a partial vacuum. May use alternating vacuum and atmospheric pressure – Steam-heated platens or electrically-heated conductive blankets in contact with lumber (see fig of DKOM) – Radio-frequency energy (see papers by Avramidis et al. for principles of operation)

See pix from Zwick and Lamb pdfs to illustrate vacuum & vacuum/rf kilns…

RFV References Avramidis, S., R.L. Zwick, and J.B. Neilson Commercial scale RF/V drying of softwood lumber. Part I. Basic kiln design considerations. For. Prod. J. 45(5): Avramidis, S. and R.L. Zwick Commercial scale RF/V drying of softwood lumber. Part II. Drying characteristics and lumber quality. For. Prod. J. 46(6): Avramidis, S. and R.L. Zwick Commercial scale RF/V drying of softwood lumber. Part III. Energy consumption and economics. For. Prod. J. 47(1):48-56.