Solar collectors. Air collectors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Air collectors have the advantage of avoiding the freezing and boiling problems that fluid systems sometimes suffer from. While a coolant leak in an air manifold is harder to spot and fix, it's less of a problem than a fluid leak. Air systems often use cheaper materials than liquid systems, such as plastic glazing, because the operating temperature is lower.

Air collectors are simple flat-plate collectors and are mainly used for space heating and drying agricultural products. Absorbing plates in air collectors are metal panels, multilayer screens, including those made of non-metallic materials. Air passes through the absorber due to natural convection or under the influence of a fan. Since air is a poorer conductor of heat than liquid, it transfers less heat to the absorber than heat transfer fluid.

Some solar air heaters have fans attached to the absorber plate to increase air turbulence and improve heat transfer. The disadvantage of this design is that it consumes energy to operate the fans, thus increasing the operating costs of the system. In cold climates, the air is directed into the gap between the absorber plate and the insulated back wall of the collector: thus, heat loss through the glazing is avoided. However, if the air is heated by no more than 17°C above the outside temperature, the heat transfer medium can circulate on both sides of the absorber plate without much loss of efficiency.

The main advantages of air collectors are their simplicity and reliability. Such collectors have a simple device. With proper care, a quality collector can last 10-20 years and is very easy to manage. A heat exchanger is not required as the air does not freeze.

However, the use of solar air heaters is still limited to space heating and drying agricultural products, mainly in developing countries. There are several factors limiting the wider use of air collectors on an industrial scale. Among them, a large area of ​​collectors due to the low density and low specific heat of the air coolant compared to the liquid one; the need for a long duct; high demand for electricity to drive air through the collector, as well as the difficulty of storing heat. Regions with relatively low solar radiation and long periods of inclement weather require additional heat; as a result, the cost of investment increases so much that it limits the competitiveness of solar collectors in comparison with traditional heating systems. A potential way to reduce the cost of collectors is to integrate them into the walls or roofs of buildings, as well as to create collectors that can be assembled from prefabricated components.

Operating principle

Air solar collectors can be divided into groups according to the way air is circulated. In the simplest of these, the air passes through the collector under the absorber. This type of collector is only suitable for raising the temperature by 3-5°C due to high heat loss on the surface of the collector through convection and radiation. These losses can be significantly reduced by covering the absorber with a transparent material with low infrared conductivity. In such a collector, the air flow occurs either under the absorber or between the absorber and the transparent cover.

Thanks to the transparent cover, heat radiation from the absorber is slightly reduced, but due to the reduction of convective heat losses, a temperature rise of 20-50°C depending on the amount of solar radiation and the intensity of the air flow. Further reduction of heat loss can be achieved by passing the air flow both above and below the absorber, as this doubles the heat transfer surface area. The heat loss due to radiation is thus reduced due to the reduced temperature of the absorber. However, at the same time there is also a decrease in the absorption capacity of the absorber due to the accumulation of dust if the air flow passes on both sides of the absorber.

Some solar collectors reduce costs by eliminating glazing, metal box and thermal insulation. Such a collector is made of black perforated metal sheets, which allow good heat transfer to be achieved. The sun heats the metal, and the fan draws the heated air through holes in the metal. Such collectors of different sizes are used in private homes. A typical collector measuring 2,4 by 0,8 meters can heat 0,002 m3 of outdoor air per second. On a sunny winter day, the air in the collector heats up by 28°C compared to outside. This improves the quality of the air inside the house, as the collector directly heats the fresh air coming in from the outside. These collectors have achieved very high efficiency - in some industrial applications it exceeds 70%. In addition, they do not require glazing, insulation and are cheap to manufacture.

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