Acceptance angle (solar concentrator)
Encyclopedia
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
.
The concentrator is a lens with a receiver R. On the left, this 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. 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 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 figure "transmission curves" to the left.
Transmission (efficiency) is τ =1 for all incidence angles α<θI and τ =0 for all incidence angles α>θI. However, 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.
However, other errors also affect the acceptance angle. Figure "optical imperfections" on the right exemplifies this.
On the left this figure shows a perfectly made lens with good optical surfaces s1 and s2 capturing all light rays incident at an angle α to the optical axis. However, real optics are never perfect and, on the right, it shows the effect of a badly made bottom surface s2. Instead of being smooth, s2 now has undulations and some of the light rays that were captured before are now lost. This decreases the transmission of the concentrator for incidence angle α, decreasing the acceptance angle. Actually, any imperfection in the system such as:
contributes to a decrease in the acceptance angle of the concentrator. The acceptance angle may then be seen as a "tolerance budget" to be spent on all these imperfections. At the end, the concentrator must still have enough acceptance to capture sunlight which also has some angular dispersion θS when seen from earth. It is, therefore, very important to design a concentrator with the widest possible acceptance angle. That is possible using nonimaging optics
, which maximize the acceptance angle for a given concentration.
Figure "angular aperture of sunlight" on the left shows the effect of the angular dispersion of sunlight on the acceptance angle.
Sunlight is not a set of perfectly parallel rays (shown in blue), but it has a given angular aperture θS, as indicated by the green rays. If the acceptance angle of the optic is wide enough, sunlight incident along the optical axis will be captured by the concentrator, as shown on the left. However, for wider incidence angles α some light may be lost, as shown on the right. Perfectly parallel rays (shown in blue) would be captured, but sunlight, due to its angular aperture, is partially lost.
Parallel rays and sunlight are therefore transmitted differently by a solar concentrator and the corresponding transmission curves are also different. Different acceptance angles may then be determined for parallel rays or for sunlight.
where n is the refractive index of the medium in which the receiver is immersed. In practice, real concentrators either have a lower than ideal concentration for a given acceptance or they have a lower than ideal acceptance angle for a given concentration. This can be summarized in the expression:
which defines a quantity CAP (Concentration Acceptance Product) that must be smaller than the refractive index n of the medium in which the receiver is immersed. The higher the CAP the closer the concentrator is to the maximum possible in concentration and acceptance angle.
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
can be captured by a solar concentrator. Its value depends on the concentration of the optic and the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Nonimaging optics
Nonimaging optics is the branch of optics concerned with the optimal transfer of light radiation between a source and a target...
.
Definition
Figure "acceptance angle" on the right illustrates this concept.The concentrator is a lens with a receiver R. On the left, this figure shows a set of parallel rays incident on the concentrator at an angle α<θ to the optical axis
Optical axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens or microscope.The optical axis is an imaginary line that defines the path along which light propagates through the system...
. 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. 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 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
Transmission curve
For an optical or electronic filter which is described by the fraction of its input that it transmits as a function of frequency or wavelength, the transmission curve or transmission characteristic is the mathematical function or graph that defines the transmission fraction as a function of...
cI as shown in the figure "transmission curves" to the left.
Transmission (efficiency) is τ =1 for all incidence angles α<θI and τ =0 for all incidence angles α>θI. However, 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.
Acceptance angle as a tolerance budget
The acceptance angle θ of a concentrator may be seen as a measure of how precisely it must track the sun in the sky. The smaller the θ, the more precise the tracking needs to be or the concentrator will not capture the incoming sunlight. It is, therefore, a measure of the "tolerance" a concentrator has to tracking errors.However, other errors also affect the acceptance angle. Figure "optical imperfections" on the right exemplifies this.
On the left this figure shows a perfectly made lens with good optical surfaces s1 and s2 capturing all light rays incident at an angle α to the optical axis. However, real optics are never perfect and, on the right, it shows the effect of a badly made bottom surface s2. Instead of being smooth, s2 now has undulations and some of the light rays that were captured before are now lost. This decreases the transmission of the concentrator for incidence angle α, decreasing the acceptance angle. Actually, any imperfection in the system such as:
- tracking inaccuracy
- imperfectly manufactured optics
- imperfectly assembled components
- movements of the system due to wind
- finite stiffness of the supporting structure
- deformation due to aging
- other imperfections in the system
contributes to a decrease in the acceptance angle of the concentrator. The acceptance angle may then be seen as a "tolerance budget" to be spent on all these imperfections. At the end, the concentrator must still have enough acceptance to capture sunlight which also has some angular dispersion θS when seen from earth. It is, therefore, very important to design a concentrator with the widest possible acceptance angle. That is possible using nonimaging optics
Nonimaging optics
Nonimaging optics is the branch of optics concerned with the optimal transfer of light radiation between a source and a target...
, which maximize the acceptance angle for a given concentration.
Figure "angular aperture of sunlight" on the left shows the effect of the angular dispersion of sunlight on the acceptance angle.
Sunlight is not a set of perfectly parallel rays (shown in blue), but it has a given angular aperture θS, as indicated by the green rays. If the acceptance angle of the optic is wide enough, sunlight incident along the optical axis will be captured by the concentrator, as shown on the left. However, for wider incidence angles α some light may be lost, as shown on the right. Perfectly parallel rays (shown in blue) would be captured, but sunlight, due to its angular aperture, is partially lost.
Parallel rays and sunlight are therefore transmitted differently by a solar concentrator and the corresponding transmission curves are also different. Different acceptance angles may then be determined for parallel rays or for sunlight.
Concentration Acceptance Product (CAP)
For a given acceptance angle θ, the maximum concentration possible Cmax is given by:where n is the refractive index of the medium in which the receiver is immersed. In practice, real concentrators either have a lower than ideal concentration for a given acceptance or they have a lower than ideal acceptance angle for a given concentration. This can be summarized in the expression:
which defines a quantity CAP (Concentration Acceptance Product) that must be smaller than the refractive index n of the medium in which the receiver is immersed. The higher the CAP the closer the concentrator is to the maximum possible in concentration and acceptance angle.