CONCENTRATING COLLECTORS
DEFINITIONS
In
order to be consisting in the use of terms, we will use the phrase
‘concentrating collector’ to denote the whole system. The term ‘concentrator’
will be used only for the optical subsystem which directs the solar radiation
on to the absorber, while the term ‘Receiver’ will normally be used to donate
the subsystem consisting of the absorber, it cover and accessories.
We
will define four terms: Aperture, Area Concentration Ration, Intercept Factor
and Acceptance.
(1) APERTURE
(W):
Aperture is the plane opening of the concentrator through which the solar
radiation passes. For a cylindrical or linear concentrator it is characterised
by the width, while for a surface of revolution, it is characterised by the
diameter of the opening.
(2) AREA
CONCENTRATION RATIO (C): It is the ratio of the effective
area of the aperture to the surface area of the absorber. Values of the concentration
ratio vary from unity (which is the limiting case for a flat plate collector)
to a few thousand for a paraboloid dish. This quantity is also referred to as
the Geometric Concentration Ratio or simply concentration Ratio.
(3) INTERCEPT
FACTOR(γ): The intercept factor (γ) is the fraction of the
radiation, which is referred or refracted from the concentrator and is incident
on the absorber. The value of the intercept factor is generally close to unity.
(4) ACCEPTANCE
ANGLE (2qa): It
is the angle over which beam radiation may deviate from the normal to the
aperture plane and yet reach the absorber. Collectors with large acceptance
angles require only occasional adjustments, while collectors with small
acceptance angles have to be adjusted continuously.
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