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.
High Concentration photovoltaics systems (HCPV) allow for improved efficiency but, due to Etandue conservation, have low optical acceptance. Mechanical tracking is normally employed to maintain the necessary alignment of the system axis with the sun. This, however, prevents HCPV from integration in urban and residential environments. We propose here optofluidic based approaches to achieve a stationary tracking optical concentrator by internal modifications of the system optics based on the manipulation of liquid interfaces or multiphase systems. Transparency induced by phase transitions and electrophoretic driven mechanisms will be discussed. Theoretical framework, multiphysics modeling and preliminary experimental results will be presented.
Surface texturing in thin-film solar cells provides a promising way of addressing the loss components due to reflection and poor light absorption inside the cells. In this work, we study the reflection suppression performance of different submicron-scale periodic surface texturing morphologies through two dimensional (2D) finite-difference time-domain (FDTD) computations. The broadband reflection response is investigated at two interfaces, air/glass and glass/TCO (transparent conductive oxide), for a spectral range of 300-2500 nm. A Drude-Lorentz model is used to account for material dispersion and absorption within the wavelengths of interest. In order to optimize the light trapping performance, numerical simulations of various surface texture structures are compared with those of flat interfaces. Numerical results show a reduction in reflection at the air/glass interface to values below 0.2% for some of the triangular gratings, compared to up to 4% for the non-textured interface. For the glass/TCO interface, reflection decreases to less than half when compared to the non-textured interface, also for triangular gratings. Further structures that replicate perfect multi-layer anti-reflection coatings are also studied. These structures are tuned to cancel specific wavelengths and can create an arbitrary effective index, overcoming the constraint of the limited number of refractive index values available. The best structures obtained for the air/glass and glass/TCO interfaces are combined in one stack, achieving reflectance values at least one order of magnitude below the non-textured air/glass/TCO stack.
The combination of optical concentration, spatial spectral splitting and the use of multiple cells of suitable bandgap, could provide a path for high PV conversion efficiency without requiring the use of monolithically integrated multi junction solar cells. We propose a dispersive point focus single element concentrator and spectral splitting optics coupled with multiple cells employing Cu(InxGa1-x)Se2 cells for the mid wavelengths region. The optical element is designed, taking advantage of the dispersion characteristics of the employed material, to concentrate and provide spatial spectral splitting. The component can be realized injection molding and be mass produced at low cost.
In this paper we present a method of optical characterization of solar concentrators based on the use of a laser beam.
The method, even though constrained by lengthy measurements, gives nevertheless interesting information on local
mirror surface defects or manufacturing defects, like internal wall shape inaccuracies. It was applied to 3D-CPC-like
concentrators and the measurements were supported by optical simulations with commercial codes. The method,
simple to apply, requires just a laser to scan the CPC input aperture following a matrix-like path, at a controlled
orientation of the beam. Maps of optical efficiency as function of the laser beam incidence angle are obtained by
matching the CPC exit aperture with a photodetector with an efficient light trapping. The integration of each map gives
the CPC efficiency resolved in angle of incidence, so curves of optical transmission (efficiency) as function of
incidence angle can be drawn and the acceptance angle measured. The analysis of the single maps allows to obtain
interesting information on light collection by the different regions of CPC input area. It reveals, moreover, how the
efficiency of light collection depends on several factors like surface reflectivity, number of reflections of the single
beam, local angle of incidence, local surface defects, and so on. By comparing the theoretical analysis with the
experimental results, it is possible to emphasize the effects directly related to manufacturing defects.
The optical characterization of a CPC concentrator is typically performed by using a solar simulator producing a
collimated light beam impinging on the input aperture and characterized by a solar divergence (± 0.27°). The optical
efficiency is evaluated by measuring the flux collected at the exit aperture of the concentrator, as function of incidence
angle of the beam with respect to the optical axis, from which the acceptance angle can be derived.
In this paper we present an alternative approach, based on the inverse illumination of the concentrator. In
accordance with this method, a Lambertian light source replaces the receiver at the exit aperture, and the light
emerging backwards at the input aperture is analyzed in terms of radiant intensity as function of the angular
orientation. The method has been applied by using a laser to illuminate a Lambertian diffuser and a CCD to record the
irradiance map produced on a screen moved in front of the CPC.
Optical simulations show that, when the entire surface of the diffuser is illuminated, the "inverse" method allows to
derive, from a single irradiance map, the angle resolved efficiency curve, and the corresponding acceptance angle, at
any azimuthal angle. Experimental characterizations performed on CPC-like concentrators confirm these results. It is
also shown how the "inverse" method becomes a powerful tool of investigation of the optical properties of the
concentrator, when the Lambertian source is spatially modulated inside the exit aperture area.
High-concentration PV systems offer a viable alternative to conventional modules mainly because of the high energy density produced for unit surface of photovoltaic cells. Optimal performances can be obtained by employing simultaneously receivers sensitive to different wavelengths by a proper partitioning of the concentrated solar spectrum. At the same time secondary concentrators with the ability to homogenize the illumination field must be employed to maximize cells performances. The use of a dichroic mirror and compound parabolic concentrator (CPC) homogenizers allows to forecast overall system performances capable of justifying the development of systems for residential applications. Moreover the concentrating systems may open prospects for large scale energy applications.
The optical modeling of a Compound Parabolic Concentrator (CPC), as photovoltaic solar light concentrator, shows that the light distribution on the circular receiver is far from being uniform, as requested for this type of application. The solution lies in the adding of a prismatic optical mixer, which breaks the circular symmetry of the CPC, distributing light over a square surface. This is paid by a ~30% reduction of the original CPC concentration ratio. The CPC-mixer system has been optimized for medium levels of concentration (~100). The optical efficiency of the CPC-mixer concentrator has been evaluated for different values of the walls reflectance, R, and a total loss of ~6% has been found for a 98% reflecting silver coated wall.
Silicon Photovoltaics (PV) compose the workhorse materials platform for contemporary solar cell devices, but their performance Figure-of-Merit (in Watts/$) underscores the compromised impact of this technology as an alternate energy solution. This course will cover 3rd Generation approaches to improving conversion efficiency and materials processing cost and manufacturing yield to create a more cost-competitive industry.
The subjects will be presented in two parts: 1) Context and Limitations: a brief review of 1st and 2nd Generation Si PV, the Shockley-Queisser fundamental limit, and manufacturing constraints; and 2) Technology Solutions: 3rd Generation case studies in spectrum management, concentrator PV, tandem multi-junctions, spectral splitting, intermediate band-gap doping, defect and substrate engineering, and recent "green photonic" solutions to manipulate the flow of incident light using waveguide or sub-wavelength structures. The course objective is a modernized overview of the silicon photovoltaic platform drivers and barriers to efficient design or fabrication.