Session 1. Introduction to agricultural dryers
Step 1. (5 minutes)
Step 2. (15 minutes)
Step 3. (20 minutes)
Step 4. (40 minutes)
Step 5. (10 minutes)
Step 6. (20 minutes)
Step 7. (10 minutes)
Step 8. (10 minutes)
Step 9. (45 minutes)
Step 10. (20 minutes)
Step 11. (10 minutes)
THE POTENTIAL OF SOLAR AGRICULTURAL DRYERS IN DEVELOPING AREAS
Presented to the UNIDO Conference
One of the oldest uses of solar energy since the dawn of civilization has been the drying and preservation of agricultural surpluses. The methods used are simple and often crude but reasonably effective. Basically, crops are spread on the ground or platforms, often with no pre-treatment, and are turned regularly until sufficiently dried so that they can be stored for later consumption. Generally little capital is required on the expenditure of equipment but the process is labor intensive.
There is probably no accurate estimate of the vast amounts of material dried using these traditional techniques. Suffice it to say it is a widespread technology practiced in almost every country of the globe and at nearly every latitude. Diverse products such as fruit, vegetables, cereals and grains, skins, hides, meat and fish and tobacco are dried using these simple techniques.
These technologies have originated in many of the developing countries so there is no major social problem in their acceptance or in the use by the local populations of dehydrated foods for consumption. There are several technical problems, however, with the process. They are:
* Intermittent, affected by cloudiness and rain
In the more advanced segment of the society, whether in developing areas or in industrialized regions, artificial drying has in many cases surplanted traditional sun drying in order to achieve better quality control, reduce spoilage and in general cut down on the losses and inefficiencies engendered by the above difficulties.
The relatively high cost of labor in most industrialized areas and the hitherto, until recently, inexpensive costs of fossil fuels permitted the development of the artificial, generally large scale, drying processes to be evolved. The cost of dehydration was added to the cost of selling the process materials. The advent of higher charges for fossil fuels as well as the danger of depletion and scarcity of these fuels has stimulated renewed interest in solar agricultural dryers.
It is estimated according to the FAO World Book that the amount of agricultural produce dehydrated in 1968 using solar energy amounted to 225 million tons. In that year alone, Australia exported over 72 thousand tons of sun-dried foods worth over 27 million dollars. If all this drying, or even part of it, were to be done using fossil fuels, it would put an even greater strain on our already limited reserves. Over the past three decades, increasing interest has been paid to the development of solar agricultural dryers which make use of known principles of heliotechnology in order to combat some of the principal disadvantages of classical sun drying.
In evaluating technologies which might be amenable to applications in developing areas, one should distinguish between small and large scale operations. In general, small scale systems would be used in those areas where land holdings are not large, with the result that individual farmers, fishermen and herdsman only produce modest amounts of surplus products. The objective is to dehydrate these surpluses for use often only by the family of the producer or for sale in the local market in the immediate vicinity. At times, small scale surpluses of certain products such as peanuts or rice are delivered to central facilities for processing, dehydration and eventual marketing. These systems are generally well established and require a fair degree of organization in the industry. In many instances, these amalgamated handling facilities do not exist. Therefore, in providing an overview of some of the technologies, one must differentiate between the existence of commercial and physical infra-structures within a given locality.
Larger scale systems invariably require the use of an external power source. Where conventional electric power supplies are available, reliable and not excessive in cost, it is logical to utilize these external sources for the operation of fans and blowers, vents and duct baffles in order to increase the efficiency and operating performance of a solar agricultural system. Some dryers are of the portable, powered type, wherein solar air heater collectors are fitted with electrically powered fans (this could be tone using gasoline or diesel engines as well) and are taken directly to the areas of production for in-situ drying. Traditionally, this process was used with fossil fuel, often butane or propane gas, as the energy source. As the price of these systems increases, there has been a tendency to develop systems of this nature relying on solar energy to provide the bulk of the energy required for dehydration. In fact, in some instances, fossil fuels are used to supplement these solar collectors in order to maintain optimum operating conditions in a system partially operated by solar energy.
The other major category applicable for dehydration in the industrialized sectors of developed and developing nations, is to use the roof area of existing buildings as the solar collector, fitting the buildings with suitable blowers, ducts, collectors and often storage mechanisms. In the United States of America, a number of activities along these lines have been developed and interest has been generated in some of the prestigious industrial and academic institutions in the country. An example of this is the project funded by the United States government where solar energy is used as a substitute in dehydration for natural gas. This project is being undertaken by California Polytechnic University and TRW Systems. They indicate that the State of California alone produces annually over 450 million dollars in dried fruit and vegetables. Their system will no doubt become increasingly cost-effective as the cost of fossil fuels and the electricity generated by them continue to escalate. (Ref. Solar EM: -1976, October.) Another system receiving increasing interest in this field both in developed and the developing regions is the use of greenhouses to dehydrate surplus produce. This combined effect of drying and greenhouse operations has much validity and has to be examined for each particular set of circumstances. A number of studies have been undertaken in this regard for specialized crops. Finally, an older but certainly no less valid system has been the use of heat extracted from the underside of roofs. This has proven quite satisfactory in providing some dehydration potential in a number of applications. This is one of the oldest applications in solar agriculture drying.
* Solar Energy Magazine
Technical Characteristics of Solar Agricultural Dryers
There are two principal aspects of this process:
* The solar heating of the working fluid (generally air).
The solar heating aspect can in turn be subdivided into two categories:
* Separate solar air heater collectors using natural or forced convection to preheat the ambient air and reduce its relative humidity.
The sun drying principles have been well described by Lof and others in earlier literature and in some instances are less well understood than commercial dehydration.
A discussion of drying theory is beyond the scope of this paper but a few principles may be advantageously outlined here. These are particularly applicable to direct radiation drying, inasmuch as the principles involved in the drying of materials in various types of opaque enclosures by means of hot air, whether from a solar heater or some other type of heating unit, are well outlined in the drying literature. The first requirement is a transfer of heat to the surface of the moist material by conduction from heated surfaces in contact with the material, or by conduction and convection from adjacent air at temperatures substantially above that of the material being dried, or by radiation from surrounding hot surfaces or from the sun. Absorption of heat by the material supplies the energy necessary for vaporization of water from it, 590 calories per gram water evaporated. Water starts to vaporize from the surface of the moist material when the absorbed energy has increased the temperature enough for the water vapor pressure to exceed the partial pressure in the surrounding air. Steady state is achieved when the heat required for vaporization becomes equal to the rate of heat absorption from the surroundings.
To replenish the moisture removed from the surface, diffusion of water from the center to the surface of the drying material must take place. This may be a rapid or a slow process, depending upon the nature of the material being dried and upon its moisture content at any time. It may thus be the limiting rate in the drying operation, or if moisture diffusion is rapid, the rate of heat absorption on the surface or the rate of vaporization may be the controlling factor. In some very porous materials, vaporization may take place even below the apparent surface of the material, vapor then diffusing through pores in the solid.
In the case of direct radiation drying, part of the radiation may penetrate the material and be absorbed within the solid itself. Under such conditions, heat is generated inside the material as well as at the surface and thermal transfer in the solid is facilitated.
For economic reasons, maximum drying rates are usually desired. Product quality must be considered, however, and excessive temperatures must be avoided in many materials. In addition, because drying occurs at the surface, those materials which have a tendency to form hard, dry surfaces relatively impervious to liquid and vapor transfer must be dried at a rate sufficiently low to avoid this crust formation. Close control of heat transfer and vaporization rates, either by limiting the heat supply or by control of the humidity of the surrounding air, must be provided.
The drying of a product simply by permitting relatively dry air to circulate around it, without the use of any direct or indirect heat source, is known as adiabatic drying. The heat required for vaporizing the moisture is supplied by the air to the solid material, thereby reducing the air temperature while increasing its absolute and relative humidity Because of the low heat capacity of air, in comparison with the high latent heat of vaporization of water, large volumes of air at reasonably low relative humidity must be used in this type of drying process. Air leaving the drier is nearly saturated with water at the wet-bulb temperature. The air supply, at its initial dry-bulb temperature, and humidity is thus cooled and humidified toward its wet-bulb temperature, while the moist solids in contact with this air approach the wet-bulb temperature also.
The foregoing generalization must be somewhat modified if the materials being dried are at all soluble in the water present. Fruits and other agricultural products contain salts and sugars which cause a lowering of the vapor pressure. The surface temperatures of these materials must therefore be higher than the wet-bulb temperature of the air in order for vaporization to take place. This means that the adiabatic drying of these solids requires air at lower relative humidities than do the materials having no solutes in the aqueous phase.
An important property of materials processed by direct radiation drying is their absorptivity for radiation. Fortunately, most solids have relatively high absorptivities but they may change as drying proceeds, the surfaces of the materials becoming less or sometimes more "black" during the process. Also, there may be changes in opacity of the surface of the materials which are partially transparent to some of the wave lengths in the spectrum of the radiant source.
The thermal conductivity of the material is also an important property, particularly if the solids are dried in a layer of sufficient depth to require conduction of heat from particle to particle. If the thermal conductivity is poor, circulation of heated air through and between the particles of moist solid would permit better heat transfer than direct radiation on the surface of a relatively deep bed of particles.
In larger scaled dehydration systems, forced convection, generally powered with an external, non-renewable power source, increases the diffusion transfer of moisture and, if properly applied, increases the rate of dehydration and the quality of the produce. These systems are well documented in the literature.
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