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Published in:
2022 | OriginalPaper | Chapter
Solar Collectors, Non-concentrating
Authors : Stefan Fortuin, Gerhard Stryi-Hipp, Wolfgang Kramer, Korbinian Kramer
Published in: Solar Thermal Energy
Publisher: Springer US
Excerpt
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 aperture/absorber/gross area
Net area (of a flat surface) through which the collector receives solar radiation. Any of these areas can be used for referencing the incoming radiation. For nonflat geometries: a projected flat area is used for the aperture and gross areas, but for absorber areas the full curved surface area is taken into account.
Collector mean temperature (Tm)
Defined, in operating terms, as the averaged value of the inlet and outlet temperatures of the collector heat transfer medium (usually that of water, water/glycol, or air):
\( {T}_m=\frac{T_{\mathrm{outlet}}+{T}_{\mathrm{inlet}}}{2}. \)
Emissivity (ε or e)
The ability of a surface to emit energy by radiation relative to that of a “black body.” For a true “black body”
ε = 1, while any real object would have
ε < 1.
(Radiant) Emittance
Radiated power per surface area (W/m
2). Both overall intensity and spectrum change with surface material and temperature.
Evacuated tubular collector (ETC)
A solar collector which uses double-walled-glass tubes with the gap being evacuated (high vacuum) as thermal insulation.
Flat plate collector (FPC)
A solar collector with a flat absorber sheet. Optionally, a flat cover (covered flat plate collector) and conventional back and side thermal insulation can be added to reduce thermal losses.
Heat pipe
A heat transfer construction enabling heat transfer through (closed) mass flow combined with evaporation and condensation. A liquid is evaporated at the hot side, the vapor travels to the cold side, where it is condenses. The liquid is transferred back to the hot side by gravity or capillary action. Much higher heat transfer rates can be obtained by this kind of device compared to conduction.
Incident Angle Modifier (IAM)
A numerical formula to account for the dependence of the collector efficiency on the angle between the incident beam radiation and the normal to the collector surface.
Operating temperature difference
Difference between the collector mean temperature (
T
m) and the ambient temperature (
T
amb). Usually, the collector performance curve is shown dependent on the operating temperature difference.
(Solar) Irradiance (G)
Radiative power density (in W/m
2) 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/m
2.
Spectral-selective coating
A multilayer surface coating with high absorptivity in the intensive solar radiation spectral range and low emissivity in the infrared thermal radiation range, so that solar radiation is absorbed effectively but radiative thermal losses due to elevated temperatures are reduced.
Solar fraction (Fsav)
The ratio of solar energy contributed to the total energy provided by a heating system. This value is usually calculated from the auxiliary energy fraction still needed by the solar system divided by the energy needed by a comparable reference system that does not use solar energy:
\( {F}_{\mathrm{sav}}=1-\frac{Q_{\mathrm{aux}}}{Q_{\mathrm{ref}}}. \)
Solar spectrum AM 1.5
A spectral distribution of the solar radiation intensity on the earth resulting from transmission through an air mass of 1.5 times the thickness of the atmosphere of the earth (i.e., AM 1.5), which represents the overall yearly average for many major population centers. It corresponds to incoming solar radiation at an (solar zenith) angle of roughly 48 by the solar system divi
