Acceptance angle (solar concentrator)
Acceptance angle is the maximum angle at which incoming sunlight can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index in which the receiver is immersed. Maximizing the acceptance angle of a concentrator is desirable in practical systems and it may be achieved by using nonimaging optics.
For concentrators that concentrate light in two dimensions, the acceptance angle may be different in the two directions.
The "acceptance angle" figure illustrates this concept.
The concentrator is a lens with a receiver R. The left section of the figure shows a set of parallel rays incident on the concentrator at an angle α < θ to the optical axis. All rays end up on the receiver and, therefore, all light is captured. In the center, this figure shows another set of parallel rays, now incident on the concentrator at an angle α = θ to the optical axis. For an ideal concentrator, all rays are still captured. However, on the right, this figure shows yet another set of parallel rays, now incident on the concentrator at an angle α > θ to the optical axis. All rays now miss the receiver and all light is lost. Therefore, for incidence angles α < θ all light is captured while for incidence angles α > θ all light is lost. The concentrator is then said to have an (half) acceptance angle θ, or a total acceptance angle 2θ (since it accepts light within an angle ±θ to the optical axis).
Ideally, a solar concentrator has a transmission curve cI as shown in the "transmission curves" figure. Transmission (efficiency) is τ = 1 for all incidence angles α < θI and τ = 0 for all incidence angles α > θI.
In practice, real transmission curves are not perfect and they typically have a shape similar to that of curve cR, which is normalized so that τ = 1 for α = 0. In that case, the real acceptance angle θR is typically defined as the angle for which transmission τ drops to 90% of its maximum.
For line-focus systems, such as a trough concentrator or a linear Fresnel lens, the acceptance angle is one dimensional, and the concentration has only weak dependence on off-pointing perpendicular to the focus direction. Point focus systems, on the other hand, are sensitive to off-pointing in both directions. In the general case, the acceptance angle in one direction may be different from the other.
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