Intelligence Brief
IN-DEPTH: Improving efficiency via a well designed secondary optical element
16 April 2009
A good secondary optical element design can provide additional benefits such as keeping the cell irradiance uniform.
To concentrate the incident radiation, CPV systems require an optical system. This system is generally composed of lenses, mirrors or a combination of both. The materials of such optical systems are significantly cheaper than the photovoltaic materials that they replace.
Optical devices may be simple or consisting of primary and secondary optical devices.
Different concentration devices are being currently used: circular parabolic dishes; parabolic dishes with secondary optical devices; square flat Fresnel lenses; square flat Fresnel lenses with secondary optical devices; linear flat lenses; linear arched lenses and finally linear parabolic reflectors have been developed and implemented at different scales.
The concentration factor (C) is described as the relationship between the average irradiance on the cell surface (GX, in W/m2) and the irradiance input (GI, in W/m2) in the optical concentration device. Thus, C is expressed as C = GX/ GI. It is measured in suns: so, when a cell is said to be at 5 suns, this indicates a level of irradiance of 5000 W/m2 on the cell for a reference irradiance input of 1000 W/m2.
Reflective components are generally employed on low concentration devices. In this case, CPV systems can be based upon plane mirrors, parabolic dishes or V-though mirrors. For medium and high concentration systems, the most implemented optical elements are the refractive devices based on Fresnel lenses which either apply simple refraction or secondary optics.
There are high-efficiency CPV systems fitted with reflective optics elements, too, based on primary and secondary mirrors, or the parabolic dishes used by CPV concentrators.
Most of the systems currently designed are based on Fresnel lenses. A Fresnel lens is a special type of lens that reduces the amount of material required to concentrate the light, by splitting the lens into a set of concentric annular sections known as Fresnel zones. The use of these zones allows keeping the required curvature without increasing the thickness, by means of adding discontinuities between them. An important reduction in thickness can be achieved, but the imaging quality of the lens is reduced. This is commonly known as non-imaging optics.
Going forward, it is being said that low cost materials with high optical efficiency will be incorporated to manufacture new optical devices.
“The focus must be on efficiency first, followed by the cost,” says LPI’s Juan C. Miñano, who is scheduled to speak during the 2nd Concentrated Photovoltaic Summit, to be held on 28-29 April in Toledo, Spain.
This, according to Miñano, means optical efficiency and tolerance angle, spectral irradiance uniformity on the cell and manufacturing simplicity.
“Looking for synergies with the existing low-cost high-throughput optical technologies (such as automotive, optoelectronic, DVD & FPD industries) will always be of high interest for accelerating the learning curve and minimising risks,” says Miñano.
Improving system performance with secondary optical element
Over the years, it has been acknowledged that the requirement of higher concentrations puts excessive pressure on the lens design since the concentrating ability of Fresnel lenses is constrained by the fundamental limits of refractive optics. As a result, further advancements of Fresnel lens technology are typically relying on secondary concentrators which reduce the overall optical efficiency while the performance still remains capped at lower than the desired concentration levels.
Miñano emphasises that point focus Fresnel lenses alone cannot get concentrations above 250x with reasonable tolerance angles and acceptable cell irradiance uniformities.
Adding a secondary concentrator (also called secondary optical element or SOE) can improve the overall system performance a great deal, but not any SOE can be used for this application.
“For instance, LPI has Fresnel lens designs with concentrations above 750x, tolerance angles well above +/-1 deg and nearly perfect irradiance uniformities. As far as we know, there is no other comparable SOE. And we think there is still room for improvement,” he said.
The assertion that the SOE reduces the overall optical efficiency is not accurate or at least can be confusing, asserts Miñano.
Errors associated with manufacturing, assembly and aiming of concentrators cannot be avoided. These errors translate into losses due to the optical mismatches if the tolerance angle is low (i.e., when no SOE is used), he said, adding that these losses in general surpass the optical losses associated with an SOE.
“The false perception as to the effect of the SOE on the efficiency is because when we say that an SOE diminishes the efficiency, we are talking of a single concentrator perfectly aiming at the sun. When we say that the SOE improves the efficiency we are talking of a CPV array under operation. So both things are possible, i.e., even though the optical efficiency of an SOE is below 100%, the overall efficiency of a CPV array can improve through the use of a well designed and manufactured SOE,” said Miñano.
He added, “Moreover, the SOE optical efficiency can be very high because the SOE can be a small element (not much larger than the cell) making it possible to use highly efficient materials (such as anti-reflective coatings or high reflectivity materials), even if the cost per unit of size of these materials is high. The reasoning is the same to support the use of high efficiency cells even if they are expensive. Additionally, a good SOE design can provide additional benefits such as keeping the cell irradiance uniform or providing protection to the cell from the environment. Summarising, I think the use of the well-designed SOE is very desirable.”
The industry has also witnessed the development of concepts such as non-imaging reflective lens concentrators which may enhance the efficiency and utility of many solar concentrating technologies by combining the high collecting power of mirrors with the design flexibility of lenses. The reflective lens uses an array of concave, spaced apart reflectors inclined at relatively sharp angles with respect to the incident sunlight. Each element in the array is designed to reflect the corresponding portion of the incident radiation downward through the space between itself and an adjacent reflector and direct it to a common focus.
On use of such concentrating optical systems, Miñano said this is one type of reflector - used for instance by Solar Systems - and not all reflective CPVs are made this way.
“For instance Solfocus and Boeing reflective systems don’t follow this scheme,” he said. “This scheme is related to the still open question about the size of the unit concentrator in a CPV system. When the unit concentrator is large, as in the Solar Systems solution, we can think of each element of the array having its own design or its own alignment, because each element of this mirror handles as much power as a complete unit of a conventional Fresnel CPV, for instance. It is not clear yet what is the optimum size of a CPV unit.”
Improving efficiency in all parts of the system, be it for solar cells and modules or concentrating optical elements or the tracking system or any other component is a way to make progress towards achieving grid parity. For the optical system, the focus can be on increasing the optical efficiency, redesigning the optical system for new materials and higher concentration ratios, improving the alignment design of lenses and increasing the acceptance angle.
2nd Concentrated Photovoltaic Summit
The second edition of Concentrated Photovoltaic Summit is to be held on 28-29 April in Toledo, Spain.
For more information, click here: http://www.cpvtoday.com/eu09/programme.shtml
Or
Contact: Belen Gallego by email belen@cpvtoday.com
Ends
Related links: optical, fresnel lenses, reflective systems, solar cells
