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A binary energy source of a timber-drier.


The Slovak Republic similarly as other countries with the well-developed industry including wood processing industry can easily spend five or more per cent of its national energy consumption on drying processes (Strnad, 2007). Therefore, in order to be able to indicate and determine ways on how to improve operation efficiency, decrease overall costs of drying, achieve satisfactory quality of dried timber, and at the same time implement the above technology in an environmental-friendly way, it is necessary to specify both in a complex way and in detail the currently highly topical process of timber-drying.


The research was focused basically on the design of the kilns, and the aspects of regulation and control have been studied with regard to the fact that the period of solar radiation is determined by natural laws, i.e. its range varies greatly during the day and even season, therefore it is necessary to provide, apart from the solar energy, the heat supply from another energy source in a continual drying kiln to optimise the co-operation of two heat energy sources.


The prototype of the solar kiln placed in the furniture factory is schematically shown in Figure 1. The test runs described in this contribution were carried out in the town of Skalica, 48[degrees]51' Northern latitude, 17[degrees]14' Eastern meridian, 178 m a.s.l. and with approximately 2,100 hours of sunshine per year.

Construction details of the kiln / Structure of the kiln. We used a commercial drier slightly accommodated to be joined to a solar air collector.

The single-row low-capacity kiln has a 7 [m.sup.3] sawn timber volume. The timber stack measures are: the width of 1,500 mm, the height of 1,750 mm, and the length of 6,000 mm. The kiln is designed to dry coniferous and broad-leaved timber at 100[degrees]C max. drying temperature.

Solar air collector. As specific conditions occur during the drying process using solar energy by means of a solar air collector, the first part of our investigation was aimed at a theoretical analysis followed by an experimental analysis of the solar air collector work. An experimental system was designed at the TUZVO to test the characteristics of solar air collectors.


The Solar Kiln Scheme: (A) Drying chamber; (B) Solar collector; (C) External blower to induce air flow through the collector; (D) Hot air discharge to internal fans; (E) Internal fans; (F) Heaters; (G) Wood stack; (H) Aeration stacks.

The heat for the kiln is supplied by the 20 m2 external air heating solar collector facing south and inclined at 45[degrees] to the horizontal for optimum year round performance. Air was circulated over the collector and through the load at 3 [m.s.sup.-1]. Vents and a spray system were used to control the relative humidity in the kiln. The whole control system can be manually checked by switches, but a sensitive and effective equilibrium moisture content (EMC) control system based on control logic is required for performance in the solar kiln to protect wood quality as well as to maintain optimum drying rate and also to prevent energy loss through useless venting. All measured variables inside and outside the kiln were recorded at 15 min. intervals. The ratio of collector area to loading capacity is 2.86 [m.sup.2].[m.sup.-3] which agree with the recommended solar collector ratio in respect to the drying rate and lies in the range between 2 and 7 [m.sup.2].[m.sup.-3] run (Steinmann, 1992).


Solar drying. Test drying runs were carried out using wood species of Pine Monterey (Pinus radiata D. Den.), thickness dimension of 24 mm to assess the kiln by the following criteria:

* Drying time

* Final wood moisture content

* Drying quality

* Supplemental energy

* Temperature and relative humidity conditions in--and outside the kiln

* Efficiency of the drying process.

Solar radiation. Solar radiation recorded during runs was done by the radiation pyrometer HEMI (Sensor 118 SN-8580, Swiss Instrument--HEMI, 100 V = 1 000 [W.m.sup.-2]). Furthermore, the average initial moisture content of the sawn timber to be solar-dried was 60% in the evaluated run. The species dried was Pine radiata, sawn in planks measuring 3,000 mm in length, 100 mm in width and 24 mm in thickness. In the evaluated run the 7.0 [m.sup.3] kiln load was piled using 18 mm square piling sticks and placing the boards edge to edge in each layer. The intensity of casehardening was assessed after allowing the prongs to dry indoors for 8 hours. After 80-hours or 5-day solar drying period, the average moisture content of sawn timber was 20 % in a representative evaluated run. During the run the drying chamber temperature was varying in range of 30-40[degrees]C, but the drying condition expressed by the equilibrium moisture content (EMC) was constant, in the mean average of 10 %.

Air-drying. Simultaneously with the solar drying and using the same species and sizes, air-drying test runs were carried out. The drying quality was evaluated, and both drying time and final wood MC were recorded (Bakos et al., 2000).

Results and discussion. The variables were selected according to a preliminary plan, which would allow conclusions about the two different drying methods, by comparing the results. The variables considered were:

* Species of wood

* Thickness

* Time of year

* Initial and final moisture content.

Comparing the drying time of runs (same thickness--e.g. 24 mm, same species--e.g. Pine, solar kiln and air drying), the conclusion is that solar drying is 6 times faster (80 hours) than air-drying (480-510 hours). Since solar drying is only faster than air-drying below 70 to 80 % wood MC, the initial moisture content is of importance. The influence of the final moisture content is particularly marked below 20 %, as the solar method becomes more profitable the lower the desired final wood MC is. With regard to cracks, splits, honeycombing and final wood MC, the quality of solar dried timber is better than for air-dried stock. The quality is comparable or even superior to that obtained by the conventional kiln drying method. As expected, the efficiency of solar drying improves with an increase of the average external temperature and is more effective during the first stage of the drying process than in the second stage when the wood MC is already below 40 %. Regarding economics it must be pointed out that the supplemental energy consumed (kWh) per kg of evaporated water is only approx. one-tenth to the amount used in a conventional kiln. As the efficiency of the tested solar timber-drying kiln depends strongly on the environmental conditions and mainly on the intensity of the solar radiation collected, the results are only transferable directly to locations of similar latitude.

Solar timber drying kiln control. In a solar kiln, the maximum temperatures are approximately 50[degrees]C, which lasts only for a very short period of time each day, and is only achieved when the wood is partially dry and can endure these temperatures. Temperature control is not necessary in solar wood drying kilns and, therefore, only the relative humidity of the kiln air has to be controlled according to the given drying schedule. The relative humidity inside the solar kiln should be computer controlled in such a way that venting will take place if it rises above the target value as given by the drying schedule and if, at the same time, the absolute humidity of the outside air is less than the absolute humidity of the air inside the kiln. Unless both conditions are satisfied simultaneously, venting will not[degrees]Ccur. This will be to prevent air with too high moisture content from entering the kiln. With this control, venting will be optimised preventing loss of energy (Horbaj et al., 2000). An efficient solar kiln control system giving the best possible quantitative and qualitative results for the final product should ensure that the drying operation is always modulated to match a current timber condition at any moment of a drying period.

Additional heating system. Regarding that the period of solar radiation is determined by natural laws, it is necessary to provide, apart from the solar energy, the heat supply from another energy source in a continual drying kiln. The problem of an additional heating system used in the solar kiln to improve its efficiency was successfully solved at the TU in Zvolen. Several control aspects were investigated. This problem has been solved in an original way of regulating the heating system with a binary source of heat (Viglasky & Novak, 1989). The above regulation makes it possible to optimise the co-operation of two heat energy sources in the kiln in such a way that it maximises the utilisation of solar energy and minimises the heat consumption from conventional energy source, e.g. steam or hot water.


1 The original way of regulating the heating system enables us to optimise the cooperation of two energy sources in kiln in such a way that it maximizes the utilization of solar energy and minimizes heat consumption from conventional energy source.

2 Solar drying is 6 times faster than air-drying.

3 The application of solar energy to wood processing holds an exciting promise of energy savings up to 34 %.

Acknowledgements: This research was partly supported by VEGA grant--the Contract No. 1/0511/08/5 Use of a neuron net technology in diagnostic and reliability of technical systems used in an energy sector. The authors are indebted to the mentioned institution for sponsoring this research work.


Bakos, M. ; Zolotova, I. & Sarnovsky, J. (2000). Remote Labs--Web Based System to Support Education, 6th International Conference on Virtual University, Bratislava, ISBN 80-227-2336-3, pp. 233-237

Horbaj, P. ; Barborak, O. & Andrejcak I. (2006). Transfer energie vetra ako d'alsi podnet pre strojarsku prax, 8rd Conference TRANSFER 06, Trencrn, ISBN 80-8075-1544), pp 165-169

Steinmann, D. E. (1992). The effect of collector area and solar tracking on the performance of a solar lumber drying kiln. In: Proceedings of the 3rd IUFRO International Wood Drying Conference. Vienna, Austria. pp. 283-291.

Strnad O. (2007) ISMS implementation methodology, International Experience Exchange TUV Nord, Essen, ISBN 80-86229-79-3, pp. 236-238

Viglasky, J. & Novak, V. (1989). Technological equipment for optimal exploitation of solar energy by combined heating system. The Invention--Patent No. 255 339, Czechoslovakia (in Slovak).
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Author:Viglasky, Jozef; Langova, Nada; Danihelova, Anna; Suchomel, Jozef; Andrejcak, Imrich
Publication:Annals of DAAAM & Proceedings
Date:Jan 1, 2008
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