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In this type of vacuum collector, the absorber strip is located in an evacuated and pressure proof glass tube. The heat transfer fluid flows through the absorber directly in a U-tube or in countercurrent in a tube-in-tube system. Several single tubes, serially interconnected, or tubes connected to each other via manifold, make up the solar collector. A heat pipe collector incorporates a special fluid which begins to vaporize even at low temperatures. The steam rises in the individual heat pipes and warms up the carrier fluid in the main pipe by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe.
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Sketch of a heat pipe collector
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The pipes must be angled at a specific degree above horizontal so that the process of vaporizing and condensing functions. There are two types of collector connection to the solar circulation system. Either the heat exchanger extends directly into the manifold ("wet connection") or it is connected to the manifold by a heat-conducting material ("dry connection"). A "dry connection" allows to exchange individual tubes without emptying the entire system of its fluid. Evacuted tubes offer the advantage that they work efficiently with high absorber temperatures and with low radiation. Higher temperatures also may be obtained for applications such as hot water heating, steam production, and air conditioning.
How much energy does a solar collector provide?
The efficiency of a solar collector is defined as the quotient of usable thermal energy versus received solar energy. Besides thermal loss there alwas is optical loss as well. The conversion factor or optical efficiency h0 indicates the percentage of the solar rays penetrating the transparent cover of the collector (transmission) and the percentage being absorbed. Basically, it is the product of the rate of transmission of the cover and the absorption rate of the absorber.
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Efficiency graph of solar collector performance
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The heat loss is indicated by the thermal loss factor or k-value. This is given in watt per m² collector surface and the particular temperature difference (in °C) between the absorber and its surroundings. The higher the temperature difference, the more heat is lost. Above a specific temperature difference, the amount of heat loss equals the energy yield of the collector, so that no energy at all is delivered to the solar circulation system.
A good collector will have a high conversion factor and a low k-value.
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Type of Collector
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Conversion Factor
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Thermal Loss Factor in W/m² °C
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Temperature Range in °C
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Absorber (uncovered)
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0,82 to 0,97
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10 to 30
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up to 40
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Flat-plate collector
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0,66 to 0,83
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2,9 to 5,3
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20 to 80
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Evacuated-plate collector
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0,81 to 0,83
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2,6 to 4,3
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20 to 120
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Evacuated-tube collector
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0,62 to 0,84
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0,7 to 2,0
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50 to 120
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Reservoir collector
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about 0,55
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about 2,4
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20 to 70
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Air collector
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0,75 to 0,90
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8 to 30
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20 to 50
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Which collector is suitable for which situation?
The desired temperature range of the material to be heated is the most important factor in choosing the correct type of collector. An uncovered absorber is certainly not suitable for producing process heat. The amount of radiation on that spot, exposure to storms, and the amount of space must all be carefully considered when planning a solar array.
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Graph of efficiency and temperature ranges of various types of collectors (radiation: 1000 W/m²)
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The specific costs of collectors are also important. Evacuated-tube collectors are substantially more expensive (at 511,29 - 1278,23 Euro /m² collector surface) than flat-plate collectors (153,34 to 613,55 Euro /m²) or even plastic absorbers (25,60 to 102,26 Euro /m²). However, a good collector does not guarantee a good solar system. Rather, all components should be of high quality and similar capacity and strength.
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