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Abbreviations
- Absorptivity or absorptance (α):
-
The fraction of the incoming radiation which is absorbed at a specified wavelength (absorptivity) or summed over a specified spectrum range (absorptance). Absorptivity and absorptance are dimensionless quantities.
- Collector area: Aperture, absorber, and gross area:
-
Net area (of a flat surface) through which the collector receives solar radiation. The gross collector area, the aperture area, or the absorber area can be used. Usually, the performance of the collector is referred to the aperture area. For nonflat areas, a projected area is used for the aperture area, but the full surface area is taken for absorber areas.
- Collector mean temperature (Tm):
-
Defined, in operating terms, as the average value of the inlet and outlet temperatures of the collector (fluid) medium:
$$ {\rm{T_m}} = \frac{{{T_{outlet}} + {T_{inlet}}}}{2}.$$ - Emissivity (ε):
-
Ratio of energy radiated by a particular material at a certain temperature to energy radiated by a blackbody at the same temperature. For a true blackbody, ε = 1, while any real object would have ε < 1.
- Emittance (M):
-
Quantitative ability of a material to release radiative heat per area (W/m2).
- Evacuated tube collector (ETC):
-
Solar collector with many parallel glass tubes each covering an individual long and slim absorber. The glass tubes maintain a vacuum insulation cavity around the absorber.
- Flat plate collector (FPC):
-
Solar collector with a large flat absorber. Optionally, a flat cover (covered flat plate collector) and conventional thermal insulation can be added to reduce thermal losses.
- Heat pipe:
-
Heat transfer construction enabling heat transfer through (closed) convective mass flow. Usually, a critical fluid-gas material is used where gas rises when a fluid is heated below and flows down again when cooled at the top (liquid), thus transferring some sensible but also large amounts of latent heat. Very high heat transfer rates can be sustained through a single, small channel.
- Incident angle modifier (IAM):
-
Dependence of the collector efficiency on the angle between the solar beam radiation incident on a surface and the normal to the collector surface. With increasing incidence angle, reflections of the cover usually increase and the performance of the covered collector drops.
- Operating temperature difference:
-
Difference between the collector mean temperature (Tm) and the ambient temperature (Ta). Usually, the collector performance curve is shown dependent on the operating temperature difference.
- (Solar) Irradiance (G):
-
Irradiated power density (in W/m2) of (solar) radiation. An example is the solar constant, that is, the solar irradiance measured just outside the earth’s atmosphere, which is roughly 1,361 W/m2.
- Spectral-selective coating:
-
Multilayer surface coating with high absorptivity in the wavelength range of solar radiation and low emissivity in the infrared thermal radiation range so that radiative thermal losses of the solar absorber are reduced.
- Solar fraction (Fsav):
-
Ratio of solar energy to total energy used by a heating system. Calculated from the auxiliary energy still needed when using solar energy, divided by the energy needed by a comparable reference system that does not use solar energy:
$$ {F_{sav}} = 1 - \frac{{{Q_{aux}}}}{{{Q_{ref}}}}. $$ - Solar spectrum AM 1.5:
-
Typical spectral distribution of the solar radiation intensity on the earth resulting from transmission through an air mass of 1.5 times (AM 1.5) the thickness of the atmosphere of the earth, which corresponds with incoming solar radiation at an (solar zenith) angle of roughly 45° (to the vertical axis).
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Fortuin, S., Stryi-Hipp, G. (2013). Solar Collectors , Non-concentrating . In: Richter, C., Lincot, D., Gueymard, C.A. (eds) Solar Energy. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-5806-7_681
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