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SOLAR LUMBER DRYING IN THE TEMPERATE COUNTRIES
Matskiv A.I., Poloz A.W. (USUFWT, Lviv,Ukraine)
In article the main solar kiln designs, principles and possibilities of use energy of solar radiation for lumber drying in the temperate countries have been considered.
Background. Drying is the most energy demanding process in solid lumber manufacture and has become increasingly more costly with rising fuel prices. Interest in solar dry kilns was low until the energy concerns of the mid-1970's. Solar energy is the free and abundant energy source throughout most of the world, but the cost of collecting it and the fact that it is a low intensity form of energy often limits the operating temperature and it use.
The principles of solar energy collection. Most solar radiant energy at the Earth's surface is short wave (light) energy. The top of a collector will typically be covered with one or more layers of transparent material, this cover allow short wave radiation to pass through them, but prevent long wave energy from passing back out through them. Under the glazing will be an absorber whose purpose is to absorber incident solar energy. Once the energy is absorbed by the black absorbing surface, it is converted into long wave energy (heat). The space between the glazing and the absorber provides a chamber to circulate the air past the absorber and transfer this heat to the lumber pile. The principles are illustrated in figure 1.
Figure 1. Semi-greenhouse type solar kiln with transparent roof: 1- transparent roof; 2- fan; 3- absorber; 4- flap for exhaust air; 5- flap for fresh air; 6-solid walls; 7- air flow; 8- lumber pile.
Solar kiln designs. Solar kilns can operate by direct solar solar collection (greenhouse type) or by indirect solar collection where the collector is isolated in some way from the drying compartment.
Greenhouse design. A greenhouse dryer typically is a frame structure with transparent glazing on the roof and 3 walls, and solid north wall. The collector then is an integral pert of the dryer. Because the poor thermal insulation properties of glazing, the heat losses through the walls are quite high. Therefore to reduce conduction heat losses this dryers will often use two layers of glazing.
Semi-greenhouse design. This dryers usually has only the roof or the roof and south wall glazed; the other surfaces are opaque and insulated. The semi-greenhouse design reduces the heat losses substantially, thereby resulting in higher dryer temperatures and faster drying.
Opaque wall design. In this , the design, the lumber is placed in a solid, opaque walled and roofed chamber insulated like standard lumber dry kiln. The solar collector is separate from dryer, with hot air being ducted into the kiln from the collector. The dryer can be well insulated, minimizing heat losses, and lends itself to using supplemental heat at night or during cloudy weather.
General characteristics of solar kilns:
S solar kilns dry timber in between one and two thirds of the time required for air drying to 20% moisture content. On average they take 2-3 times as long as conventionally heated kilns to dry timber to 12% moisture content
S solar kilns dry to considerably lower moisture content that is possible with air drying
S the quality of drying is very good, provided humidities are kept high early in the drying cycle for the more difficult timbers. This is because the rate of is slow compared with conventional kilning and cooling during the night provides some degree of "reconditioning" and relaxation of stresses in the timber
S solar kilns are cheap to build and operate
S the initial cost of solar kilns is usually between a tenth and twentieth of the cost of conventional kilns of the same volume capacity
Comparison with air drying.
The advantages of solar kilns over air drying are: S higher temperatures give faster drying
S equilibrium moisture content is much lower and therefore is possible to dry to low enough for any common use. In temperate countries it may not be possible to dry to 12% moisture content in the winter and this reduces the value of solar kilns in these regions. They may be useful for pre-drying in winter and are strong incentives in these region to supplement solar energy S although temperature is very dependent on solar radiation, humidity can be kept high, even if temperature are high. Humidities during the drying process gradually drop as the timber dries. Quality of drying is therefore good.
The main disadvantages are:
S solar kilns are normally more expensive to operate than air drying S more skill is required to operate them well.
Comparison with kiln drying.
Compared with conventional kilns they are:
S slower and temperature cannot be completely controlled but they dry
slowly enough to suit almost any timber S they provide nightly "reconditioning" which tends to give good
quality drying with the more difficult timbers S they require less attention than most conventional kilns and are less
likely to damage timber if poorly operated S they are much less expensive to purchase initially and are almost
always cheaper and easier to operate S they are relatively easy to manufacture.
Conclusions.
> Analysis of world wide and own experience showed that solar drying was superior to air-drying in lumber drying rate, quality, lower final moisture contents.
> Many experimental and prototype solar kilns have been built and tested at a high latitude indicated that to 50 degree latitude it would be good performers and most suitable design is semi-greenhouse design.
> Most suitable period for using solar energy for lumber drying at this countries is summer, however they may be useful for pre-drying in winter. Relatively low operating temperatures make solar drying more attractive for refractory species such as oak.
> At present, solar drying is not widely used in the temperate countries, but solar heated dry kilns is satisfactory for any operation where drying time is not critical.
> The main uses can be small woodworking that not wish to make large capital investment in drying equipment.
