Spectral splitting is widely employed as a way to divide light between different solar cells or processes to optimize energy conversion. Well-understood but less explored is the use of spectrum splitting or filtering to combat solar cell heating. This has impacts both on cell performance and on the surrounding environment. In this manuscript we explore the design of spectral filtering systems that can improve the thermal and power-conversion performance of commercial PV modules.
In designing solar concentrator optics there are many parameters that must be optimized in order to create a useful system, such as compactness, number of elements or interfaces, and acceptance angle, among others. Using geometric optics, tradeoffs between these parameters become inevitable. For example, a lens, trough or dish may be compact but has low tolerance of angular misalignment; angular tolerance can be improved by adding secondary and tertiary optics, but this increases complexity and reduces optical throughput; nonimaging optics such as the CPC offer wide acceptance angles from as single element, but are too long to be practical, in most applications, above low concentrations. These tradeoffs can be avoided by using angle-selective photonic materials to exploit the equivalence between angular restriction and concentration. Recently, broadband angular selectivity in optical films has been demonstrated by the Soljacic group in MIT. In this collaborative work we use this material to experimentally demonstrate two visible-spectrum optical concentrators. We demonstrate that these concentrators are thermodynamically ideal when the material properties are ideal, and describe the material improvements most essential for improving device performance, and discuss how commercial solar concentrator systems could be improved by the use of angular-selective optics
With PV module prices low and flat-plate technology dominating the market, the solar industry has taken a hard turn away from concentrator PV systems, which has faced high cost barriers. Despite this, there may still be space for CPV products and concepts to contribute, but these advantages are best realized by aggressively rethinking the way we design PV concentrators.
Optical spectrum splitting systems that divide light between independent solar cells of different band gaps have received increasing attention in recent years as an alternative to expensive multijunction cells for high-efficiency PV. Most research, however, has focused on dichroic filters and other photonic structures that are expensive to manufacture. This has the effect of transferring the cost of the system from the PV cells to the optics. As a low-cost spectrum splitting approach we designed a prismatic lens that simultaneously splits and concentrates light and can be fabricated by injection molding. We present experimental results of a two-cell demonstration system, and calculations for low-cost configurations of commercial solar cells, enabled by the removal of lattice-matching requirements.
We demonstrate 3D-printed nonimaging concentrators and propose a tracking integration scheme to reduce the external
tracking requirements of CPV modules. In the proposed system, internal sun tracking is achieved by rotation of the
mini-concentrators inside the module by small motors. We discuss the design principles employed in the development
of the system, experimentally evaluate the performance of the concentrator prototypes, and propose practical
modifications that may be made to improve on-site performance of the devices.
We present and analyze a design for a self-tracking solar concentrator based on a switchable-transparency optical element. The switchable element forms a moving aperture that tracks the motion of the sun to admit light into a CPC in which rays are 'recycled,' undergoing many passes through the concentrator to increase the absorption probability. This design has the benefit of not requiring any control of the angular profile of internal radiation, in contrast to other design that rely on total internal reflection to confine and transport the light. Via probabilistic models and rigorous ray tracing, we show that this design can exhibit performance comparable to other self-tracking designs. In particular we demonstrate a system with a 70x geometric concentration ratio and a tracking range of ±20°, achieving optical efficiencies of up to 65%.
We present a novel optical element that behaves as a dynamic aperture capable of tracking a moving light source. The
element is based on a composite material which when heated undergoes reversible transition from an opaque to
transparent state, resulting from a phase transition in one of its components that modifies the microstructure of the
material. The material has been designed to undergo a localized transparency transition at the point of illumination by a
focused beam, activated by the absorption and conversion to heat of a portion of the incident light. As a result of this
mechanism the aperture reactively tracks a moving light spot, such as that created by focusing sunlight onto a surface
during the sun’s apparent motion through the sky. Such an element has been proposed as a solution to the sun tracking
problem of solar concentration, as it allows admission of sunlight into a concentrating light trap over a wide range of
We present a design for a modification of a previously proposed light-trapping solar collector that enables reactive solar tracking by the incorporation of an optically activated transparency-switching material. The material forms an entry aperture whose position reactively varies to admit sunlight, which is focused to a point on the receiving surface by a lens or set of lenses, over a wide range of solar angles. An analytic model for assessing device performance based on statistical ray optics is described and confirmed by raytrace simulations on a model system.
High efficiency concentrator photovoltaic systems are currently based on costly III/V cells and, to offset the high cell capital cost, elevated optical concentrations are used, with consequent reduction in acceptance angles and tight tolerance optics. While this allows for spectacular conversion efficiencies, it does not provide cost effectiveness in a market dominated by low efficiency/low cost technologies. An alternative approach, well known in literature, is based on the combined use of an optical concentrator and a spectral splitting element allowing for the use of separate cells with different spectral responses and, thus, opening the way to a much wider range of possible materials and technologies. While many configurations have been presented during the years, optical efficiency has often been an issue due to the separate action of the concentrating and splitting element. We propose here, as substantial evolution of a previous design , a single injection molded plastic non-imaging optical element embodying both two axes concentration and spectral splitting functions. Based on the specific dispersion characteristics of polycarbonate and on a constructive analytical design procedure, this element allows for optical efficiencies exceeding 80%. Theory, simulations and preliminary experimental results will be presented.
In this article we discuss an emerging concept for non-mechanical solar tracking that can have a significant impact for the design of next generation concentrator photovoltaics systems. Based on the modification of the optical properties of the concentrator elements instead of their mechanical rearrangement, self-tracking concentrators, with recently demonstrated prototypes, could make the mechanical trackers redundant expanding the scope of application of CPV systems. We propose here a new approach to a reactive-tracking system, analyze its underlying physics and discuss initial experimental and simulation results towards the development of a prototype.