Science and technology has played a determining role in reducing energy resources in the last 20 years and the increase in environmental pollution has encouraged the creation of more efficient designs in the field of renewable energy.
Although the practice of drying with solar energy is very old, the first works, fundamentally of a practical nature, have been reported since 1940 . The theoretical and experimental development of the subject is observed from 1960 , where, in addition, the advance in the design of solar dryers is appreciated. Numerical and analytical methods have been used to design solar dryers, using complex and extensive methodologies that must be used by highly qualified personnel. The simplicity of graphic methods gives it dynamism and practicality, even in the age of computing and electronics .
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- 1 The application of dryers.
- 2 To design dryers.
- 3 Principle of operation.
- 4 Solar dryers in Cuba
- 5 Advantages of solar dryers
- 6 Conclusions
- 7 Sources
The application of the dryers.
The application of solar dryers has acquired great importance in the treatment of products to accelerate germination, in thermotherapy of different crops, to eliminate pests and viruses , as well as to dehydrate surplus crops and obtain of basic necessities, at low cost, such as medicines from dry medicinal plants .
Although a certain variety of solar dryers has been developed, from a construction point of view these equipment have similarities, since they all have a drying chamber with adequate insulation and a surface that captures the energy of solar radiation .
The design of this equipment has been aimed at providing geometric solutions for the absorbing surface that increase the performance of the solar collection, together with the use of materials with better optical properties. This has led to the differences in these equipments being found fundamentally in the configuration of the absorber plate.
Depending on the design, the type and composition of the roof material, it will be the temperature of the roof and, consequently, the heat transfer by radiation will determine the value of the energy losses and in the end the performance of the absorber. Although the mechanism of radiant heat transfer predominates, convection must also be considered in the value of energy losses.
To design dryers.
To design solar dryers, methods are used that are fundamentally supported by two elements, which are energy balances and the use of models of the thermal behavior of the system. The algorithms based on the thermal behavior of the system are the most used for their ease and proven efficiency, and the data is obtained from prototype evaluations or by analyzing the behavior of the dryer, using a mathematical model and a computer program.
The design that uses mathematical modeling has been used mainly in solar collectors and not in dryers. The effectiveness of this procedure lies in the simplification hypotheses of the model, and these are presented in the form of differential equations whose solution uses numerical methods and highly complex computer programs.
Different branches of science employ graphic methods for design. A nomogram is considered any graphic representation in one or two planes in which the values of the solutions can be read together with the data. Some of these nomograms are empirical in nature and are obtained from measured values from significant samples, and others are obtained mathematically by defining the functions used in design methodologies.
Designing using nomograms , despite the limitation of determining only the fundamental parameters, such as the area of the absorber plate, the volume of the dryer and the air flow necessary to dry the material, has advantages for its ease of use, speed and precision of the calculation. This work presents a graphic method to design solar dryers. Installations designed using this method work satisfactorily.
As with any conventional dryer, solar dryers base their operation on three overlapping processes. Air heating in the solar dryer is carried out by circulating the air through a heating chamber formed by a transparent cellular polycarbonate cover and a collecting surface made of blackened steel sheets, which together form a body or chamber of absorption of solar radiation entering through the roof, with an intense greenhouse effect. At the start of the process a mass of air with ambient relative humidity is entered into a drying chamber with thermally insulated walls, and its relative humidity decreases rapidly as it circulates through the upper heating chamber. The drying of the material (placed in trays or layers in carrier carts along the drying chamber, located below and along the heating chamber) is carried out by the continuous passage of hot air through the material, which produces a intense process of heat and mass exchange, during which the surface moisture of the material is incorporated into the air by evaporation, to the extent that the air transfers its heat. The successive recirculation at a defined speed, driven by a centrifugal fan and a distribution duct system, through the layers of the product to be dried, will cause that as the relative humidity of the product decreases, that of the air increases,
The renovation is ensured by a device for controlling the humidity of the air that activates the start or stop of two centrifugal fans (one for extraction and the other for injection), which force a certain mass of outside air through a heat exchanger. of the plate type, which in a short time renews the humid and relatively hot air inside the chamber by «fresh» and humid exterior air, now preheated, to once again establish the process that in practice is uninterrupted.
The continuous diffusion of the interior humidity of the material towards the surface by capillarity ensures that at the end of a complete cycle the relative humidity of the entire mass of material will have decreased in a homogeneous way to the acceptable values for commercial use. Drying will be completed when the product reaches the pre-established low humidity according to commercial or use requirements (equilibrium humidity), and which is controlled by direct (humidity measurement device inserted in the dough) or indirect (material control samples) and appropriate laboratory methods).
Solar dryers in Cuba
The drying of products using solar radiation has an important history in Cuba , and among them the successful experimental works of the Center for Solar Energy Research (CIES) , in Santiago de Cuba , between 1984 and 1993 , followed by the Solar Group of the Ministry of Science, Technology and Environment (CITMA ), in Havana , and more recently by the CUBAENERGÍA Solar Energy Group .
From these experiences, several dozen solar dryers of different capacities and characteristics have been developed, corresponding to the applications for which they have been used, such as the drying of woods, medicinal plants and seeds, among others. However, its commercial development has been weighed down by several factors, among which the scarce dissemination of the results and the limited knowledge by potential users of the advantages of solar drying stand out; the tendency to underestimate the results of national investigations; producers’ preference for the ‘fast’ conventional drying process by burning diesel or other fuels, and the misconception that highly automated conventional dryers are more efficient and effective than solar dryers. In the meantime, SecSol, the family of multipurpose solar dryers, is now a reality. The trading companyEcoSol Solar , from the COPEXTEL Corporation , and CUBAENERGÍA , from CITMA , have joined forces to develop a commercial product with high productive performance, without any direct fuel consumption and low electricity consumption, which guarantees uniform drying of a wide range of products throughout a continuous drying cycle, and with a very attractive price. CUBAENERGÍA has contributed the solar drying technology developed and proven during twenty years of research and demonstrations, while EcoSol Solar has made its engineering, marketing structure and technical services available for the development of this product.
Advantages of solar dryers
Advantages of artificial drying or dehydration (in general)
–The quality of the dehydrated product is generally superior when a good process technology is selected. –The drying speed is usually much higher and the drying time decreases considerably, which influences both the quality and the cost of the product. –The sanitary and nutritional conditions (in the case of food) are better because the product is not exposed to the direct action of the sun, rain, dust and insects. –The area used in dehydration is several times less than that used in natural drying. –Dehydration operations are simpler and can be automated. Disadvantages of conventional artificial drying (using fossil and residual fuels ) –High installation cost, both for the drying chamber and for the boiler, furnaces, waste treatment plants or air heaters. –High production cost, in case you consume oil or electricity to heat the air and the product.
There are many advantages of using solar dryers. It is important to emphasize that said method is provided as one more tool to generalize the use of renewable sources, the Sun.
For this purpose, a nomogram is presented, which determines only the fundamental parameters, such as the area of the absorber plate, the volume of the dryer and the air flow necessary to dry the material, it has advantages for its ease of use and speed of calculation.