Principle and application of solar drying

Solar drying is also one of the main application areas of solar light-heat conversion. It is the instinct of solar heat utilization that mankind has mastered since ancient times to achieve the purpose of drying clothes and agricultural and sideline products by basking in the sun for heating or directly spreading the items that need to be dried in the sun and exposed to the sun. However, this drying method is difficult to prevent the dried material from being contaminated by dust, bacteria and pests. The drying process is also restricted by weather and natural environmental conditions, and the degree of drying and dehydration is not easy to control, and the drying quality is more difficult to obtain due guarantee. Therefore, in addition to meeting the needs of ordinary individuals and small-scale pastoral production and life, the use of solar energy dehydration and drying heat utilization method in the modern industrial and agricultural production process does not have any large-scale production and modern production significance. In order to meet the strict requirements of modern industrial and agricultural production and food sanitation and quarantine, people later had to go from simplicity to complexity and use coal-fired and oil-fired steam boilers to produce high-heat steam to create the high-temperature environment necessary for drying, or directly generate remote electricity through electricity. Infrared radiation is used for fast, controllable and quality-guaranteed drying process of agricultural and sideline native products and food.

With the development of human social production, the cumulative increase of carbon dioxide and other greenhouse gas emissions, the deterioration of the earth’s ecological environment, and energy conservation and emission reduction have become the consensus and obligatory social responsibility of the global international community. In this context, people have raised doubts about the use of high-energy-density, high-value conventional fossil energy or power resources for low-temperature drying; and naturally again think about whether the “inexhaustible, use it” method can still be used. How can “inexhaustible” solar radiant energy solve the problem of low-temperature drying of large quantities of agricultural and sideline native products? The answer is yes. However, this “retrospective action” is definitely not a return to zero in a pure sense. Its starting point has been elevated to a higher level of history and scientific and technological progress.

The so-called drying means that under the boiling point of water (or related liquids), by relatively increasing the air temperature and air flow, reducing the air humidity, using the method of increasing the water content of the air flowing through the object to be dried, and using larger dry hot air Flow or flow rate, the physical method of dehydrating the moisture or other liquid contained in the object to be dried through the evaporation process to remove excess water (or related liquid). In the drying process, the water-containing objects need to pass through the preheating period, the constant drying period, and the decelerating drying period, and finally reach the required equilibrium water content. At this time, if the relevant physical conditions of the drying process are not changed, the drying process ends here, and the dried material enters a dynamic equilibrium state of preservation. Drying is a high-energy-consuming industry in industrial and agricultural production applications in any country. According to some data, drying accounts for about 10% of the country’s total energy consumption. In the drying process, except for a few exceptions, most agricultural and sideline products and foods can only be dehydrated and dried at low or medium temperature: in a sense, this means that solar thermal utilization is fully involved in the drying process. , Provides a rare window of opportunity.

Generally speaking, solar drying devices can be divided into three categories: greenhouse type, convection circulation type and hybrid type according to their specific structure and operation mode. The so-called greenhouse dryer (as shown in Figure 1) is the heat formed by opening a number of channels that allow air to circulate freely in the vertical (or left and right) directions of the greenhouse heat box that can collect solar radiant heat. Box type drying device. This kind of drying device system has a compact structure and strong integrity, but the thermal efficiency of drying is relatively low. The convection circulation type solar dryer (as shown in Figure 2) is more complicated in system structure than the greenhouse type dryer. This type of drying system needs to be equipped with a special solar air collector at the same time. Adjustable forced circulation power equipment and special dehumidification channels and other devices. In practical applications, this system is often widely used in large-scale industrial drying devices, such as tobacco leaf drying, agricultural and sideline product processing, etc. The system can add auxiliary energy heating devices according to production needs, set the air temperature required for the drying process, and reduce air humidity and other auxiliary systems. In actual application, through human operation or automatic control system, the convection circulation fan can be turned on immediately, and the system can be switched to a forced circulation mode to accelerate the drying process. In areas with abundant external sunshine or when the relative humidity of the air is low, the hot air generated by the air collector can also be used to apply the principle of hot bacon for automatic dehumidification and drying or other physical dehumidification methods to carry out convective circulation inside the system Drying; flexibly regulate the dehydration speed and drying effect of the product in a variety of ways. The hybrid solar dryer (as shown in Figure 3) is a combined system composed of a single-stage solar air collector on the basis of the greenhouse type. After adding the first-level air collector, compared with the greenhouse system, it not only increases the heating area, but also greatly increases the temperature of the dry air: at the same time, the hot chimney effect formed by this combination can automatically accelerate The exhaust flow rate of hot and humid air realizes high-efficiency non-assisted power natural circulation drying. To some extent, the hybrid dryer combines the advantages of the greenhouse type and the convection circulation type drying system. The complexity and cost of its system structure are also between the above two systems.

Figure1 Schematic diagram of the principle of greenhouse solar dryer
Figure 2 Schematic diagram of the principle of convection circulating solar dryer
Figure 3 Schematic diagram of hybrid solar dryer principle

Large-scale industrial and agricultural production uses mostly convective circulation drying systems; small household systems use more greenhouse drying systems, or add air collectors to the greenhouse type to form a so-called hybrid drying system.

Solar air collectors that obtain solar radiant energy can be classified into non-permeable, permeable, and focused types according to different heat collection methods. Among them, the non-penetrating type heat collector has a simple structure, but the convection heat exchange between the air layer and the transparent cover plate and the radiation heat loss of the heat absorption plate are difficult to solve, so it is rarely used; the penetration type air collector Because the air heat exchange area is large, and it is easy to form the temperature field distribution of low temperature in the upper layer and high temperature in the lower layer, the heat loss is small, the air flow field is smooth, and the air flow resistance is also small. Although the system structure is more complicated, it is still widely used by people. . In addition to the special purpose of the focusing type, this high-temperature focusing instant drying method is rarely used. On the whole, no matter which type of solar air collector, there is a general problem that the light-to-heat conversion efficiency is far lower than that of solar water collectors. The fundamental reason is that the air quality density is too low, which makes it difficult to improve the heat exchange efficiency.

Solar drying technology is generally widely used in agricultural and sideline products, such as rice, corn, wheat, peanuts, soybeans, coffee and other granular products; fruit and vegetable products: vegetables, grapes, berries, tobacco leaves, tea, medicinal materials, walnuts, nuts and grass, Leaf crops; and forest products such as Chinese medicinal materials, wood, and rubber. Use solar air-conditioning technology to carry out technical transformation of the grain depot. The air flow is heated by the sunlight and heat resources collected from the top of the grain depot every day to form a hot chimney effect on the upper layer of the grain depot; Dispatch the air temperature, flow direction or flow rate in the grain depot to extract air, ventilate, cool, or heat and dehumidify the grain depot, so as to minimize the conventional energy consumption required to maintain the safe operation of the grain depot, so as to save emissions and reduce emissions. Greenhouse gas emissions, reduce the occurrence of mildew, insect pests, chemical pollution and other phenomena in grain stocks: the purpose of scientifically storing grain, ensuring the quality and safety of stored grain, reducing the labor intensity of warehouse management personnel, and improving labor productivity.