To establish solar kilns as an industrial tool, performance tests must be carried out during different season periods, based on objective data taken from measurement and analysis of several parameters. In this work, the inclusion of permanent measurement and control instrumentation has been adopted, which is an innovative aspect introduced for SECMAD project.

The main objective of the measurements and analysis is to characterize solar kiln performance, regarding operation time and costs under several weather conditions, as well as the related evolution of relevant physical quantities like: inside the kiln chamber, air circulating temperature and relative humidity and moisture contents (MC) of drying wood; at the solar collector zone, air temperature and relative humidity; air temperature and relative humidity on the outside environment;

Wood equilibrium moisture contents (EMC) in the interior and exterior of the kiln are also calculated from these measurements; as they are usual and useful drying indicators in the lumber sector that integrate the contribution of both temperature and relative humidity conditions. Additionally, energy expenditure, solar radiation and air convection can also be measured. Optimization of operational conditions to reduce drying time was considered and can be carried out automatically by the control instrumentation, where a dedicated fuzzy algorithm takes into account the restrictions imposed by quality issues and acts upon the ventilation system, presenting a flexible response to the natural change of exterior conditions (solar radiation, and psychometric conditions of external air, etc.).

Measurements and control can be taken at operator’s defined constant intervals that can lie in the range for one minute to several hours, with a usual value of 15 minutes. Instrumentation system also offers data logging and monitoring capabilities.

2 Results and discussion

A major problem with solar kilns is related with the uncontrollability of environment conditions and the difficulty to repeat them in consecutive drying operations. Therefore, all results are dependent on the specific weather conditions, besides of kiln structure, drying product and control strategy.

However, for similar weather conditions (solar radiation and relative humidity), some common evolution patterns have been noticed. Figures 3 and 4 represent one entire week evolution of temperatures and equilibrium moisture contents in the exterior and in the interior of a solar kiln charged, respectively, with moisture saturated wood and medium dried wood (MC ~ 30%). In both situations, there was sunny weather and the ventilation was switched on during the day period.

Data on these figures were acquired, respectively, during autumn and winter 2006 at INETI Lisbon facility — 38° 46’ N, 9°10’ W.

The efficiency of the kiln, illustrated by EMC values, is lower in the early hours of the morning, but increases with radiation intensity. As can be seen, EMC values present always less amplitude variation inside than outside the solar kiln. Although EMC values at the interior are higher during winter than in autumn, as expected, drying conditions can be quite favourable inside of the kiln during the winter period.

In spite of ventilation, chamber peak air temperatures (around midday) showed to be in the worst case 3-5°C higher than the outside ones. These small increments in temperature are sufficient to low relative humidity and EMC, in a significant way, causing these quantities to attain their lowest levels at this time and promoting wood moisture loss, in a more effective way. In fact, wood moisture loss increases with the decrease of relative humidity, mainly due to the increase of the water vapour pressure differential, which is the key factor for moisture evaporation. In this way, wood works as an additional moisture source inside the kiln, increasing RH and EMC. This effect is more intense if the wood is saturated as in the case of figure 3, where the chamber EMC values are bigger than the exterior ones (in opposite of figure 4, where wood has already attained 30% of moisture content), in spite of the ventilation.

If, in this case, exchange with the exterior is prevented, internal RH and EMC values would attain much bigger values. In such situations air has to be continuously expelled to the exterior and renovated at a high rate, forcing air circulation through the wood-stack.

—— Exterior EMC	Interior EMC
—— Interior Temperature	—— Exterior Temperature

Fig. 4 — Wood temperature and moisture equilibrium content inside and outside of the kiln,

December (winter), one wee

Figure 5 shows another typical evolution pattern of the equilibrium moisture content and temperature during a summer sunny day, both inside and outside the solar kiln, charged with saturated pine wood. Vertical lines correspond to the beginning and end of ventilation period, during which the ventilators were switched on and off.

It can be noticed, that even though the ventilation is on, during the hours of bigger incidence of solar rays, chamber temperature suffers a bigger increase than in autumn or winter, causing a bigger drop in the interior EMC values and offering better drying conditions

Fig. 5 — Typical summer day equilibrium moisture content and temperature
Evolution inside and outside the solar kiln

Global results of an entire drying operation, during the end of autumn and the beginning of winter, using 27 mm thickness boards of pine wood, in a total wood volume of 40 m3, are illustrated in figure 6. Expelled moisture rate was increased by proper exposition of the wood boards to the air. The 40m3 volume of 27 mm thickness boards presented a total surface exposition (two faces for one board) of 3000 m2.

Fig. 6 — Drying process evolution of water loss (left) and moisture content (right).

In the first drying phase, high air circulation was provided, in order to speed as much as possible the drying rate. It should be emphasized that another important reason to quickly reduce wood moisture content at the initial stage, is to prevent the development of mould or blue stain specially when drying softwoods, namely pine. In the illustrated example, at the beginning of the process, an extraction of 75 liters of water per cubic meter was achieved (Figure 6). This represents a total water removal from wood in 24 hours about 3000 liters. This water removal rate dropped in seven days to about 35 liters per cubic meter and a value of 12 % in moisture content was reached in 33 days.

Figure 6 shows some other details of the final stage of the drying operation. Although temperature has not exceeded 30°C the wood moister content (MC) was reduced from 30 % to 15 % in only 7 days. This is quite significant, taking into account that the final drying stage is the slowest in a solar kiln. As can be seen, the EMC during this phase reached an average value of 9 % along the day when temperature increases and relative humidity was at its lowest (RH).

Monitoring data acquired during several runs under variable conditions, in all prototypes, allow the proposal of expected durations for the drying of softwood according to the different weather conditions. Table 1 summarizes the expected drying duration for different board thicknesses and favorable or less favorable conditions, based on the tests carried out during the project.

Table 1 — Expected solar kiln drying duration for softwood (Pinus pinaster, Aitim.), from 120% to 14% M. C. Thickness 27 mm 35 mm Total drying duration with favourable conditions (day) 15 25 Total drying duration with non favourable conditions (day) 45 60

Regarding energy, it must be said that the total amount needed to dry wood is the same whatever the method. For pine wood, the heat energy needed is about 494 kWh / m3. In conventional kilns, this energy is usually obtained from burning residues that can be quite cheap, but it must be remembered that the investment on a boiler is very high and the combustion produces CO2 emissions. In air or solar drying, the use of solar energy greatly reduces the expense and gas emissions. In the case of the present study, the kiln has a low cost structure and only energy spent in electrical ventilation has a cost per operation.

Table 2- Compared energy use in a solar kiln and a conventional kiln drier for softwood (Pinus pinaster, Aitim.), from 120% to 14% M. C. . Electrical energy for ventilation per cubic meter Heat energy per cubic meter Conventional kiln drier 32 kWh / m3 460 000 kcal / m3 494 kWh / m3 Solar kiln drier 28 kWh / m3

3 Conclusions

The present work showed that low cost solar kilns can be competitively used to dry wood, presenting balanced benefits regarding operation time vs. final quality, energy expenditure and gas emissions.

A performance evaluation based on relevant physical quantities measurements and analysis is being carried on, during different season periods and weather conditions, constituting an essential tool to gather knowledge about the process. The results, documenting kiln functionality, strongly contribute to adjust the control to specific needs of products to be dried and to establish confidence in this kind of drying methods for industrial use.

As referred, in the first stage, wood drying requires a great capacity of water extraction, so strong ventilation should be carried, being necessary to extract the moisturized air from the kiln trough entire air renovations (1 to 3 per minute). In these circumstances, during sunny hours the gain in air temperature inside the kiln can be as small as 2 to 5 degrees. Although this apparently low increase

in temperature, the equilibrium moisture content can drop about 5 to 8 %, which is enough to dry wood at a very satisfying rate, even at its final stage.

Electric power reduction can be achieved through ventilation control, according to external conditions and drying level already obtained. It was found out that even forced air convection is always of great benefice, exchange with the exterior can be restricted, most of the time, to day hours (8h per day approximately).

A well designed structure and ventilation control strategy are the key factors for the economic success and drying quality. Even though drying times and energy expenditure have been acceptably low, more improvements can be eventually achieved.

One example is the possibility of allowing internal air recirculation in some circumstances, without exchange with external environment. This will account with another important ventilation effect, that wasn’t addressed in the present work, which is the removal of water vapor accumulated around the wood as a thin skin, whose presence diminishes the vapor pressure differential and favors the occurrence of mould blue stains that can affect final appearance and quality.

Combination of solar drying with conventional layout drying processes, including an adequate active control for optimization purposes, is another open possibility to achieve competitive substitution of fossil sources of energy, with significant decreasing of CO2 emissions, while still requiring low cost investments. Modifications of the industrial process are claimed to be minimal since the system requires no specialised buildings or electrical power, but should be clarified in furtherer works.

Acknowledgments — The information provided in this paper is a result of an applied research and demonstration project, partially financed by EU founds, and included in PRIME program, under the financial management of Innovation Portuguese Agency and managed by INETI.

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