Solar collectors. Drying. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Solar collectors. Drying. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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A solar collector that heats the air can serve as a cheap source of heat for drying crops such as grains, fruits or vegetables. Since solar collectors heat the indoor air temperature by 5-10°C with high efficiency (and complex devices - even more), they can be used for air conditioning in warehouses.

The use of simple and cheap solar collectors to heat the air while drying crops is promising in reducing the huge crop losses in developing countries. Lack of adequate storage conditions leads to significant food losses. Although it is not possible to accurately estimate the extent of crop losses in these countries, some sources put them at around 50-60%. To avoid such losses, growers usually sell crops immediately after harvest at low prices. Reducing losses by drying fresh fruits would be of great benefit to both producers and consumers. In some developing countries, open-air drying is widely used to preserve food. To do this, the product is laid out on the ground, stones, on roadsides or on roofs. The advantage of this method is its simplicity and low cost. However, the quality of the final product is low due to long drying times, contamination, insect infestation and spoilage due to overheating. In addition, achieving a sufficiently low moisture content is difficult and often results in spoilage of the product during storage. The introduction of solar dryers will help improve the quality of dried products and reduce losses.

Various types of small solar devices for drying agricultural products have already been developed in developing countries. In a natural circulation dryer, the solar air heater is either built into the dryer or connected to a drying cabinet or chamber. The air solar collector may be a black mat covered with a plastic shield. Air passes through the mat, heats up and then blows over the fruit or vegetables. Such dryers for fruits, vegetables and spices are applicable in both dry and humid climates. Due to their high productivity, they are mainly used in large farms or cooperative farms to produce high quality products. In developed countries, the solar air heater is mainly built into the southern slope of the roof of the hay barn.

According to the air circulation method, solar dryers use either free (natural) circulation or forced circulation. Dryers with natural circulation do not require the use of a fan. However, their inherent low air flow rate and long drying time lead to low productivity and poor product quality. Therefore, their use is limited to drying small batches of products for family consumption. When large quantities of fresh produce are to be processed for the commercial market, forced circulation dryers must be used. Their main disadvantage is that the fan requires electricity to operate. Since many rural areas in developing countries are not connected to the electricity grid, the use of such dryers is limited. Given the economic difficulties inherent in these countries, the situation is unlikely to change in the foreseeable future. The use of photovoltaic cells to generate the electricity needed to run a fan could give a major boost to solar dryers in developing countries.

Solar air heaters used in developed countries usually consist of a black absorber foil and a transparent plastic film, between which air is pumped by a fan. To increase the area of the collector, the southern slope of the roof of the building is extended so that it touches the ground - then the entire roof becomes a solar collector. Solar greenhouse dryers are used on large farms to dry medicinal and aromatic plants. Using a photovoltaic fan, you can set up the system so that air is forced through the room only in sunlight. Such structures are widely used in the summer cottages of Denmark and Sweden, so that the room is dry throughout the year. Although the solar dryer has many advantages over outdoor drying, weather dependence is the main disadvantage of both methods. In many regions, the weather does not allow the use of solar heat for drying products, since there are rarely many warm and dry days in a row. As a result, the product may deteriorate without having time to dry.

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