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>>SOLAR THERMAL SYSTEMS
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 |
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.
Type of Collector |
Conversion Factor |
Thermal Loss Factor in W/m² °C |
Temperature Range in °C |
Absorber (uncovered) |
0,82 to 0,97 |
10 to 30 |
up to 40 |
Flat-plate collector |
0,66 to 0,83 |
2,9 to 5,3 |
20 to 80 |
Evacuated-plate collector |
0,81 to 0,83 |
2,6 to 4,3 |
20 to 120 |
Evacuated-tube collector |
0,62 to 0,84 |
0,7 to 2,0 |
50 to 120 |
Reservoir collector |
about 0,55 |
about 2,4 |
20 to 70 |
Air collector |
0,75 to 0,90 |
8 to 30 |
20 to 50 |
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