EME 812
Utility Solar Power and Concentration

2.3 Concentration Ratio

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The light concentration process is typically characterized by the concentration ratio (C). By physical meaning, the concentration ratio is the factor by which the incident energy flux (Io) is optically enhanced on the receiving surface (Ir) - see Figure 2.4. So, confining the available energy coming through a chosen aperture to a smaller area on the receiver, we should be able to increase the flux.

Cgeo=area of the aperturearea of the receiver=AaAr

In the above equation, Cgeo is called the geometric concentration ratio. It is easy to use, as the areas of the devices are known, although it is adequate only when the radiation flux is uniform over the aperture and over the receiver. Also, please note that for some imaging concentrators, the area of the available receiver surface can be different from the area of the image produced by the concentrator on the receiver. So, if the image does not cover the entire surface of the receiver, we need to use the image area to estimate the concentration ratio.

representation of light concentration: incoming light. Aperture I sub o into concentrator A sub a out as I sub r & into receiver A sub r
Figure 2.4. Schematic representation of light concentration process.
Credit: Mark Fedkin

The concentration ratio can be also represented by the energy flux ratio at the aperture and at the receiver. In this case, it is termed optical concentration ratio Copt (or flux concentration ratio) and can be directly applied to thermal calculations.

Copt=Average flux over the receiverFlux over the aperture (insolation)=1ArIrDArIO

In case the ambient energy flux over the aperture (insolation) and over the receiver (irradiance) is uniform, the geometric and optical concentration ratios are equal (Cgeo = Copt).

The concentration ratios are important metrics used to characterize and rank optical concentrators. Next, we will look at several examples of concentrator designs and see what values of concentration ratios they can provide.

There is a theoretical limit to solar concentration. For circular concentrators - 45,000, and for linear concentrators - 212, based on the geometrical considerations; however, these limits may be unreachable by real systems because of non-idealities and losses. If you are interested in the analytical estimation of the concentration limits, refer to Duffie and Beckman's (2013) book (p.325) for more details. In general sunlight, concentration systems are roughly classified into: low concentration range (C<10), medium concentration range (10<C<100), and high concentration range (C>100). However, only some of the systems provide uniform concentrated light flux (e.g., V-troughs or pyramidal plane reflectors) and can be characterized by a single concentration ratio. Many systems with curved reflecting surfaces (e.g., conical, parabolic, spherical) create a distribution of flux density over the receiver and would rather be characterized by a variable C over the receiver width. In that case, a local concentration ratio (Cl) is the main parameter to characterize the performance of the ideal concentrator:<100),>

Cl=Local intensityIncident intensity=I(y)Iap

where I(y) is determined for any local position y from the center of the produced image, and Iap is the intensity of the incident radiation at the aperture.

*distribution of local radiation intensity over a receiver of a collector. Bell graph w/ peak @ 0. Distance (y)-x axis intensity-I(y) y-axis
Figure 2.5. Example of distribution of local radiation intensity over the receiver of an imaging collector.
Credit: Mark Fedkin after Duffie and Beckman, 2013

In many typical cases of imaging concentrators, the reflectance of the surface (ρ), i.e., the fraction of light radiation reflected from the surface compared to the total incident radiation, is also taken into account. Then the local intensity of the concentrated light, I(y), can be described as follows:

I(y)=IapρCl

Further in this lesson, we will study some examples that use this equation to estimate energy distribution within a concentrated image on the receiver. It would be better to have a specific type of a concentrator to apply these concepts. Please read through the following text to enforce your understanding of the concentration ratios.

Reading assignment

Book chapter: Duffie, J.A. Beckman, W., Solar Engineering of Thermal Processes, Chapter 7: introduction through Section 7.2. pp. 322-327.

This book is available online through the PSU Library system and can also be accessed through e-reserve link on Canvas.

The above referenced sections of the book in part repeat some of the material given here, but may give you more extensive commentary on the basics and probably provide deeper insight how concentration ratio is influenced by other parameters of the system.

After you have completed the above reading assignment, please answer a few self-check questions below.

Check Your Understanding - Multiple Choice Question

As a result light concentration, the energy flux on the receiver:

(a) is decreased
(b) is increased
(c) is kept the same

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ANSWER: B - The radiation energy flux is typically increased due to confining it to a smaller area.

Check Your Understanding - Essay Question

When are the optical and geometric concentration ratios equal?


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ANSWER:

Optical and geometric concentration ratios are equal when the incident concentrated fluxes are uniform (such as in case some planar collectors).

Check Your Understanding - Multiple Choice Question

A sunlight concentration system with a concentration ratio of 150 is generally considered to have:

(a) low concentration range
(b) moderate concentration range
(c) high concentration range

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ANSWER: C - according to classification described in Duffie and Beckman's book, collectors with the concentration ratio above 100 have high concentration range.

Check Your Understanding - Essay Question

If the radiation intensity is distributed unevenly within an image produced by an imaging collector, what parameter is typically used to characterize the concentration performance?


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ANSWER:

Local concentration ratio. It is defined for a small element of the concentrated image and is a function of distance from the center of the image.

Check Your Understanding - True/False Question

True or False: Circular concentrators can reach higher concentration ratios than linear collectors.

(a) True
(b) False

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ANSWER: (a) True. Theoretical limit of the circular collectors is 45,000, while for linear collectors it is 212.