Triple junction photovoltaic solar cells are expensive, making it desirable to operate them with as much concentration of sunlight as practical. The efficiency of currently available multi junction photovoltaic cells suffers when local concentration of incident radiation surpasses ˜2,000-3,000 suns. Some concentrator designs of the prior art have so much non-uniformity of the flux distribution on the cell that “hot spots” up to 9,000-11,000× concentration happen with 500× average concentration, greatly limiting how high the average concentration can economically be. Kaleidoscopic integrators can reduce the magnitude of such hot spots, but they are more difficult to assemble, and are not suitable for small cells.
There are two main design problems in Nonimaging Optics, and both are relevant here. The first is called “bundle-coupling” and its objective is to maximize the proportion of the light power emitted by the source that is transferred to the receiver. The second, known as “prescribed irradiance,” has as its objective to produce a particular illuminance pattern on a specified target surface using a given source emission.
In bundle-coupling, the design problem consists in coupling two ray bundles Mi and Mo, called the input and the output bundles respectively. This means that any ray entering into the optical system as a ray of the input bundle Mi exits it as a ray of the output bundle Mo, and vice versa. Thus the successfully coupled parts of these two bundles Mi and Mo comprise the same rays, and thus are the same bundle Mc. This bundle Mc is in general Mc=Mi∩Mo. In practice, coupling is always imperfect, so that Mc⊂Mi and Mc⊂Mo.
In prescribed-irradiance, however, it is only specified that one bundle must be included in the other, Mi in Mo. Any rays of Mi that are not included in Mo are for this problem disregarded, so that Mi is effectively replaced by Mc. In this type of solution an additional constraint is imposed that the bundle Mc should produce a prescribed irradiance on a target surface. Since Mc is not fully specified, this design problem is less restrictive than the bundle coupling one, since rays that are inconvenient to a particular design can be deliberately excluded in order to improve the handling of the remaining rays. For example, the periphery of a source may be under-luminous, so that the rays it emits are weaker than average. If the design edge rays are selected inside the periphery, so that the weak peripheral region is omitted, and only the strong rays of the majority of the source area are used, overall performance can be improved.
Efficient photovoltaic concentrator (CPV) design well exemplifies a design problem comprising both the bundle coupling problem and the prescribed irradiance problem. Mi comprises all rays from the sun that enter the first optical component of the system. Mo comprises those rays from the last optical component that fall onto the actual photovoltaic cell (not just the exterior of its cover glass). Rays that are included in Mi but are not coupled into Mo are lost, along with their power. (Note that in computer ray tracing, rays from a less luminous part of the source will have less flux, if there are a constant number of rays per unit source area.) The irradiance distribution of incoming sunlight must be matched to the prescribed (usually uniform) irradiance on the actual photovoltaic cell, to preclude hot-spots. Optimizing both problems, i.e., to obtain maximum concentration-acceptance product as well a uniform irradiance distribution on the solar cell's active surface, will maximize efficiency. Of course this is a very difficult task and therefore only partial solutions have been found.
Good irradiance uniformity on the solar cell can be potentially obtained using a light-pipe homogenizer, which is a well known method in classical optics. See Reference [1]. When a light-pipe homogenizer is used, the solar cell is glued to one end of the light-pipe and the light reaches the cell after some bounces on the light-pipe walls. The light distribution on the cell becomes more uniform with light-pipe length. The use of light-pipes for concentrating photo-voltaic (CPV) devices, however, has some drawbacks. A first drawback is that in the case of high illumination angles the reflecting surfaces of the light-pipe must be metalized, which reduces optical efficiency relative to the near-perfect reflectivity of total internal reflection by a polished surface. A second drawback is that for good homogenization a relatively long light-pipe is necessary, but increasing the length of the light-pipe both increases its absorption and reduces the mechanical stability of the apparatus. A third drawback is that light pipes are unsuitable for relatively thick (small) cells because of lateral light spillage from the edges of the bond holding the cell to the end of the light pipe, typically silicone rubber. Light-pipes have nevertheless been proposed several times in CPV systems, see References [2], [3], [4], [5], [6], and [7], which use a light-pipe length much longer than the cell size, typically 4-5 times.
Another strategy for achieving good uniformity on the cell is the Köhler illuminator. Köhler integration can solve, or at least mitigate, uniformity issues without compromising the acceptance angle and without increasing the difficulty of assembly.
Referring to FIG. 2, the first photovoltaic concentrator using Kohler integration was proposed (see Reference [8]) by Sandia Labs in the late 1980's, and subsequently was commercialized by Alpha Solarco. A Fresnel lens 21 was its primary optical element (POE) and an imaging single surface lens 22 (called SILO, for SIngLe Optical surface) that encapsulates the photovoltaic cell 20 was its secondary optical element (SOE). That approach utilizes two imaging optical lenses (the Fresnel lens and the SILO) where the SILO is placed at the focal plane of the Fresnel lens and the SILO images the Fresnel lens (which is uniformly illuminated) onto the photovoltaic cell. Thus, if the cell is square the primary can be square trimmed without losing optical efficiency. That is highly attractive for doing a lossless tessellation of multiple primaries in a module. On the other hand, the primary optical element images the sun onto the secondary surface. That means that the sun image 25 will be formed at the center of the SILO for normal incidence rays 24, and move towards position 25 on the secondary surface as the sun rays 26 move within the acceptance angle of the concentrator due to tracking perturbations and errors. Thus the concentrator's acceptance is determined by the size and shape of the secondary optical element.
Despite the simplicity and high uniformity of illumination on the cell, the practical application of the Sandia system is limited to low concentrations because it has a low concentration-acceptance product of approximately 0.3°(±1° at 300×). The low acceptance angle even at a concentration ratio of 300× is because the imaging secondary cannot achieve high illumination angles on the cell, precluding maximum concentration.
Another previously proposed approach uses four optical surfaces, to obtain a photovoltaic concentrator for high acceptance angle and relatively uniform irradiance distribution on the solar cell (see Reference [9]). The primary optical element (POE) of this concentrator should be an element, for example a double aspheric imaging lens, that images the sun onto the aperture of a secondary optical element (SOE). Suitable for a secondary optical element is the SMS designed RX concentrator described in References [10], [11], [12]. This is an imaging element that works near the thermodynamic limit of concentration.
A good strategy for increasing the optical efficiency of the system (which is a critical merit function) is to integrate multiple functions in fewer surfaces of the system, by designing the concentrator optical surfaces to have at least a dual function, e.g., to illuminate the cell with wide angles, at some specified approximation to uniformity. That entails a reduction of the degrees of freedom in the design compared to the ideal four-surface case. Consequently, there is a trade-off between the selected geometry and the homogenization method, in seeking a favorable mix of optical efficiency, acceptance angle, and cell-irradiance uniformity. In refractive concentrators and tandem multi-junction cells, chromatic dispersion can cause the different spectral bands to have different spatial distributions across the cell, leading to mismatches in photocurrent density that reduce efficiency, making complete homogenization even more valuable.
There are two ways to achieve irradiance homogenization. The first is a Köhler integrator, as mentioned before, where the integration process is along both dimensions of the ray bundle, meridional and sagittal. This approach is also known as a 2D Köhler integrator. The other strategy is to integrate in only one of the ray bundle's dimensions; thus called a 1D Köhler integrator. These integrators will typically provide a lesser homogeneity than is achievable with in 2D, but they are easier to design and manufacture, which makes them suitable for systems where uniformity is not too critical. A design method for calculating fully free-form 1D and 2D Köhler integrators was recently developed (see References [13], [14]), where optical surfaces are used that have the dual function of homogenizing the light and coupling the design's edge rays bundles.
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