Operating Behaviour of the Whole Dryer

The heat transfer through the cover of the solar dryer depends not only on the used material but also on the air velocity inside and outside the dryer. While the wind velocity of the surrounding air is not predictable for a certain position and time, the air flow distribution inside the solar dryer is quite stable and is only influenced significantly by the position of the air flap. The circulating drying air above the absorber passes in a higher distance to the aluminium sheets at closed air flap than at open air flap, Figure 34. This was expected to reduce the heat transfer from the absorber surface to the drying air which would result in higher absorber temperatures and therefore higher energy losses through heat radiation. However, the affected absorber sheets are also cooled by the drying air that passes below them. In addition, the thermal energy is transferred excellently by the aluminium sheets to

colder areas of the absorber. Therefore, a considerable temperature difference could not be found. Furthermore, the installation of an air duct to eliminate this zone would have caused additional flow resistance and therefore an increase of the electrical energy demand. The other part of the absorber surface was cooled perfectly by the circulating drying air. Turbu- lences existed in the expansion area in front of the timber load on the side of the heat ex- changer. However, this had no negative influence on the air flow distribution in the timber stack.

Air flow

Turbulences

Reduced air velocity

Open air flap

Closed air flap Air flow

Turbulences

Reduced air velocity

Open air flap

Closed air flap

Figure 34: Measured distribution of the air flow inside the solar dryer at open and closed air flap.

The heat transfer coefficient U of the air bubble foil depends not only on the thickness and

thermal conductivity of the plastic foil but also on the convection heat transfer coefficient at its surface. The convection heat transfer coefficient at a plane surface can either be estimated empirically by the mean air velocity or can be calculated by the air velocity, the heat conductivity of the air, the overflowed length and the Nusselt number. The Nusselt number results from the Reynolds number and the Prandtl number. The Reynolds number depends on the air velocity, the overflowed length and the kinematic viscosity of the air. The critical point, where the laminar flow changes in turbulent flow can be estimated by the air velocity, the kinematic viscosity of the air and the critical Reynolds number. These

correlations allowed the determination of a specific U-value depending on the drying air

velocity at every position of the solar dryer.

The aluminium girders of the solar dryer form a barrier to the circulating drying air in a distance of 2.0 m in horizontal and of 1.0 m in vertical direction. This means that the dry- ing air restarts to pass the plane plastic foil close behind a girder. Thereby, an insulating boundary layer is generated that increases with increasing distance from the barrier. This windless air layer reduces the heat transfer from the foil to the drying air which causes a

decreasing U-value. Due to a maximum air velocity of 2.0 m/s and a maximum distance

between two girders of 2.0 m, the critical point was not reached which means that the boundary layer was not destroyed by turbulences. Since the heat transfer coefficient

changes with overflowed distance, an average U-value was calculated for each field be-

tween two aluminium girders by measuring the air velocity every 10 cm close to the foil surface.

Finally, the solar dryer could be subdivided into five different zones with similar velocities of the drying air close to the inner foil surface: the span roof in front of the axial flow fans, at the side of the air flap where the air bubble foil is bent inwards due to a low air pressure, the span roof behind the fans where the foil is curved outwards due to an overpressure, the two gables and the side walls of the dryer, Table 6.

Table 6: Division of the solar dryer in different air flow zones with corresponding aver-

age air velocity close to the foil surface in m/s and heat transfer coefficient U

in W/m²K.

Air flow zone Average air flow velocity U-value

Span roof after the fans 0.9 2.6

Span roof in front of the fans 2.0 3.1

Gable – heat exchanger 0.8 2.8

Gable – air flap 1.0 3.0

Side walls 0.5 2.1

The highest average drying air velocity at the inner dryer cover was measured with 2 m/s in front of the axial flow fans. The lowest air velocity with 0.5 m/s existed at the side walls due to the installed sealing foils. The air velocity was lower on the gable at the side of the

heat exchanger than at the side of the air flap. Altogether, the different air velocities re-

sulted in U-values between 2.1 and 3.1 W/m²K and were therefore noticeable lower than

the cited value of 3.2 W/m²K. However, it must be pointed out that the air flow distribution in the solar dryer is very inhomogeneous and had to be generalised within this calculation method.

Condensation of Moisture Inside the Solar Dryer

The transparent cover of the solar-assisted timber dryer has a significantly lower thermal insulation than that of a high-temperature dryer. The resulting high influence of the ambi- ent air temperature complicates the control of the drying climate since water vapour con- denses at low temperatures on the internal surface of the plastic foil (see also chapter 3.2). The consequential decrease of the relative humidity of the drying air cannot be stopped by humidifying. As a result, thermal energy is consumed to immediately evaporate water that condenses on the plastic cover. Therefore, the air temperature inside the solar dryer has to be adapted to the ambient air temperature and the relative humidity of the drying air.

Figure 35 shows the maximum permissible temperature difference between the drying air and the ambient air for different relative humidities of the drying air. The outside tempera- ture is hereby 20 °C. However, the temperature level at the analysed range does not have a significant influence on the demonstrated temperature differences.

20 40 60 80 100 0 20 40 60 80 % K M axi mum t emp era tu re d iff er enc e ∆T

Relative humidity of the drying air ϕ

Figure 35: Maximum temperature difference between the drying air and the ambient air depending on the relative humidity of the drying air at an ambient air tempera-

For example, a needed relative humidity of the drying air of 75 % allows a maximum tem- perature difference to the ambient air of 11 K. Generally, the higher the relative humidity of the drying air, the lower the temperature difference has to be in order to avoid condensa- tion. This explains why drying schedules for high temperature dryers cannot be applied to the analysed solar dryer. Consequently, the drying air temperature has to be kept low or has to follow the ambient air temperature which results in daily temperature oscillations.