Solar collectors. Tubular evacuated collectors. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Solar collectors. Tubular evacuated collectors. 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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Traditional simple flat plate solar collectors have been designed for use in regions with warm sunny climates. They dramatically lose their effectiveness on adverse days - in cold, cloudy and windy weather. Moreover, weather-induced condensation and humidity will cause premature wear of internal materials, which in turn will lead to system degradation and failure. These shortcomings are eliminated by using evacuated collectors.

Solar collectors. Tubular evacuated collectors

Vacuum collectors heat domestic water where higher temperature water is needed. Solar radiation passes through the outer glass tube, hits the absorber tube, and is converted into heat. It is transmitted by the fluid flowing through the tube. The collector consists of several rows of parallel glass tubes, to each of which is attached a tubular absorber (instead of an absorber plate in flat-plate collectors) with a selective coating. The heated liquid circulates through the heat exchanger and gives off heat to the water contained in the storage tank.

Vacuum collectors are modular, i.e. tubes can be added or removed as needed, depending on the need for hot water. During the manufacture of collectors of this type, air is sucked out from the space between the tubes and a vacuum is formed. Due to this, heat losses associated with the thermal conductivity of air and convection caused by its circulation are eliminated. What remains is radiative heat loss (thermal energy moves from a warm to a cold surface, even in a vacuum). However, this loss is small and negligible compared to the amount of heat transferred to the liquid in the absorber tube. The vacuum in the glass tube is the best possible thermal insulation for the collector - reduces heat loss and protects the absorber and heat pipe from adverse external influences. The result is excellent performance that surpasses any other type of solar collector.

There are many different types of vacuum collectors. In some, another, third glass tube passes inside the absorber tube; there are other designs of heat transfer fins and fluid tubes. There is a vacuum manifold that holds 19 liters of water in each tube, thus eliminating the need for a separate water storage tank. Reflectors can also be placed behind the vacuum tubes to further concentrate solar radiation on the collector.

Factors such as atmospheric pressure and the technical difficulties associated with sealing the collector shell make it very labor intensive to manufacture. To overcome the colossal force of atmospheric pressure, it is necessary to strengthen the transparent shell with a mass of internal supports. The problem of creating an efficient high-vacuum system at a moderate cost has not yet been solved. It makes sense to apply and adapt better technologies developed for the lamp industry with its proven mass production. It is quite feasible in practice to create a tubular evacuated collector and maintain a high vacuum in it, by analogy with electric lamps and TV tubes. To reduce heat loss through the internal gas atmosphere (i.e. convective losses), it is necessary to maintain ideal vacuum insulation of the collector during the entire period of its operation.

In regions with high temperature differences, these collectors are much more efficient than flat collectors for a number of reasons. First, they work well under conditions of both direct and diffuse solar radiation. This feature, combined with the ability of vacuum to minimize heat loss to the outside, makes these collectors indispensable in cold, cloudy winters. Secondly, due to the round shape of the vacuum tube, sunlight falls perpendicular to the absorber for most of the day. For comparison, in a fixed flat collector, sunlight falls perpendicular to its surface only at noon. Vacuum collectors have a higher water temperature and efficiency than flat-plate collectors, but they are also more expensive.

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