Solar drying

In solar kilns thermal energy comes from solar radiation and can be a reasonable and promising method for almost any wood industry to gain the capacity to dry wood at reduced costs [5-9]. Known past implementations rarely use control embedded in the drying process [11], resulting in poor quality and dry time improvements. However, current instrumentation capabilities allow cost effective control solutions.

In the SECMAD Project, wich means “energy efficient wood drying”, the whole concept of a kiln drier has been reviewed [10,12-14],, namely to reduce its cost and enhance solar energy collection (side walls, double ceiling, etc.). The concept uses natural and mechanical ventilation controlled by an instrumentation and control system, accounting for both internal kiln and external environment conditions.

Figure 1 illustrates the driers prototypes and Figure 2 shows the ventilation and heating concept, with colors indicating the inside conditions of the air: red arrows give indication of direction of hot and dry air while blue ones indicate the direction of cold and humid air. Green arrows indicate the entrance of outside air, being heated by solar air collector at side wall and ceiling. Moisten air is expelled through vents by forced flow, while the fan is turned on. This solar and ventilation dryer is intended to be considerably faster than the traditional open air method and much lesser energy consumption and more cost expensive than conventional kilns.

Two prototypes were installed in two different industries. Prototype I is more appropriate for fresh water charged lumber, which is necessary to remove as fast as possible in order to avoid mould and blue stain when drying softwoods. Prototype II is more appropriate to dry products that need higher temperature and prove easier to remove the water without risk of checks and deformations (poles, agriculture fruits, etc.).

Dimensions of kilns are represented in both figures. Figure 1 prototypes could accept between 40 to 75m3 usable wood volume with equivalent dimensions: 4m inside height; 10 to 20m inside length; 5,60m inside width, but stacking of the wood, ventilation requirements and spaces from the rear wall and from front door gives a actual less wood processing.

Подпись:(Prototype II)

The model of temperature distribution and air circulation is shown in figure 2. The air speed on the ventilators was 13,5 m/s witch provides a volumetric speed of 160m3/min, or in other words, an average of 50 renovations h-1. The average air speed on the wood boards surface was 1,7 m/s on the stacks near the ventilators and 0,7 m/s on the surfaces of stacks more far way.

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Fig. 2 — Heat model concept of lumber kiln dryer (cut sights).