Spectral Solar Cells

Abstract

A solar concentrator receives sunlight for generating solar power with the concentrator including holographic optical element (HOE) separators for separating sunlight into separated bands, including HOE concentrators for concentrating the separated bands into concentrated bands, including HOE reflectors for reflecting the concentrated bands as reflected bands onto a multiple junction photovoltaic solar cell for generating the solar power with reduced aberrations of the bands for improved conversion of the solar light into the generator solar power, all of which can be constructed in an integrated structure using spacers, waveguides, and a substrate, where the HOEs use chirp Bragg gratings for reducing optical aberrations of the separated, concentrated, and reflected optical bands, with the option of multiple HOE separators for receiving sunlight from various angles of incidence.

Claims

1. A light concentrator for receiving light for generating solar power, the concentrator comprising, a holographic optical element (HOE) separator for separating the light into separated bands, a multiple HOE concentrator for concentrating the separated optical bands into concentrated bands, a multiple HOE reflector for reflecting the concentrated bands as reflected bands, and a photovoltaic (PV) cell for receiving the reflected bands and generating the solar power, the PV cell being a multiple junction PV cell, the PV cell having multiple junctions for receiving the reflected bands.

2. The light concentrator of claim 1 further comprising, a separator spacer disposed between the HOE separator and the multiple HOE concentrator.

3. The light concentrator of claim 1 further comprising, a shield around the PV cell.

4. The light concentrator of claim 1 wherein, a portion of the light passes through the HOE separator as transmitted light.

5. The light concentrator of claim 1 wherein, the HOE separator comprises a plurality of light HOE separators, each of the light HOE separators for receiving the light at respective different angles of incidence of the light.

6. The light concentrator of claim 1 further comprising, a concentrator spacer disposed between the multiple HOE concentrator and the multiple HOE reflectors.

7. The light concentrator of claim 1 further comprising, a waveguide for guiding the concentrated bands to the reflectors.

8. The light concentrator of claim 1 further comprising, a substrate for integrating the light concentrator as an integral structure.

9. The light concentrator of claim 1 further comprising, a substrate for integrating the light concentrator as an integral structure, a portion of the light passing through the substrate as transmitted light.

10. The light concentrator of claim 1 wherein, a number of the separated bands and a number of the multiple HOE concentrators and a number of the concentrated bands and a number of the multiple HOE reflectors and a number of the reflected bands are equal.

11. The light concentrator of claim 1 wherein, a number of the separated bands and a number the multiple HOE concentrators and a number of the concentrated bands and a number of the multiple HOE reflectors and a number of the reflected bands and a number of multiple junctions are equal.

12. The light concentrator of claim 1 wherein, the PV cell is a stacked PV cell having the multiple junctions.

13. The light concentrator of claim 1 wherein, the PV cell is a series of PV cells having respective junctions of the multiple junctions.

14. The light concentrator of claim 1 wherein, the HOE separator and the multiple HOE concentrators and multiple HOE reflectors comprise Bragg gratings.

15. The light concentrator of claim 1 wherein, the HOE separator and the multiple HOE concentrators and multiple HOE reflectors comprise chirp Bragg gratings.

16. The light concentrator of claim 1 wherein, the HOE separator and the multiple HOE concentrators and multiple HOE reflectors comprise Bragg gratings serving to reduce aberrations of the separated bands and the concentrated bands and the reflected bands for improved conversion efficiency of the light into the power.

17. The light concentrator of claim 1 wherein, the light concentrator is a solar concentrator, the power is solar power, the PV cell is a PV solar cell, and the light is sunlight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a diagram of a multidirectional multiple HOE solar concentrator.

[0023] FIG. 2A is a diagram of a multiple HOE stacked solar concentrator.

[0024] FIG. 2B is a diagram of a multiple HOE waveguide solar concentrator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to FIG. 1, a multidirectional multiple holographic element (HOE) solar concentrator received solar sunlight by one or more HOE separators, such as HOE separators .theta..sub.B, .theta..sub.O, and .theta..sub.R for defining a field of view.phi.. The HOE separators .theta..sub.B, .theta..sub.O, and .theta..sub.R are collectively referred to as a multidirectional HOE separator, or simply the HOE separator. The HOE separator allows for the separation of the incident solar sunlight into separated optical bands. The separated bands are directed through a separator spacer to a multiple HOE concentrator having a plurality of individual respective HOE concentrators, each of which are for concentrating the separated bands into respective concentrated bands that may be for example blue, red, and green colored optical wavelength bands. Each of the HOE separators .theta..sub.B, .theta..sub.O, and .theta..sub.R can be multiplexed in the same hologram volume or can be laminated together with each HOE separator separating incidence light at a specific angular incidence. The multiple HOE concentrators may be simply referred to as the HOE concentrator. The HOE concentrator directs the concentrated bands through a concentrator spacer to a multiple HOE reflector likewise having a respective number of HOE reflectors for each of the concentrated bands. The multiple HOE reflectors may be simply referred to as the HOE reflector. The HOE reflector reflects the concentrated bands as reflected bands back through the concentrator spacer and onto a solar cell having a protective solar shield. The solar cell has multiple band gaps for converting the reflected bands into generated solar power. The solar cell may be a stacked solar cell or a series of solar cells. The HOE separator, separator spacer, HOE concentrator, concentrator spacer, HOE reflector, and solar cell are integrated together onto a substrate for constructing an integrated commercial device. A portion of the incident solar sunlight passes through the separator and through the substrate for exiting out of the solar concentrator as transmitted light that is not useful for power generation, and hence, does not illuminate the solar cell. The transmitted light can be infrared, ultraviolet, gamma, and x-ray radiation that could otherwise damage the solar cell. The solar cell can be a photovoltaic (PV) multiple junction solar cell, which may be a thin film solar cell.

[0026] Referring to FIG. 2A, a multiple HOE waveguide stacked solar concentrator includes the HOE separator for separating incident solar sunlight into separated bands that are directed through the separator spacer and to the HOE concentrator that then provides concentrated bands that are propagated along a waveguide to the HOE reflector. The solar cell is a stacked solar cell with a solar shield. The reflected bands are focused to a common point on the stacked solar cell for generating solar power. The substrate may be a glass substrate for communicating the transmitted light and for supporting the HOE separator, the separator spacer, the HOE concentrator, the waveguide, the HOE reflector, and the stacked solar cell.

[0027] Referring to FIG. 2B, a multiple HOE waveguide serial solar concentrator receives incident solar sunlight in the HOE separator generating the separated bands that are directed through the separator spacer to the HOE concentrator. The HOE concentrator generates the concentrated bands that are directed into a waveguide at an appropriate angle for propagation of the concentrated bands towards and to the HOE reflector for generating the reflected bands. The respected bands are respectively focused upon a series of solar cells having respective band gaps for receiving the respective bands, such as blue, green, and red color optical bands that are focused onto the respective solar cells that are all protected by the solar shield. The transmitted light that includes harmful radiation passes through and out of the glass substrate. The waveguide serial solar concentrator can be an integrated device using the supportive glass substrate.

[0028] Referring to all of the figures, the solar concentrator can be used to convert light from any light source provided that the light spectrum falls within the energy band gaps of the solar cells for conversion of light energy into generated electrical energy. The HOE separator, HOE concentrator, and HOE reflector preferably include respective chirp Bragg gratings, not shown, that are designed for reducing aberrations of the separated bands, concentrator bands, and reflected bands. The aberration generation has optical wave dispersion and wavelength shifts. Those skilled in the art can design matching HOE separators, HOE concentrators, and HOE reflectors using chirp Bragg gratings for reducing aberrations of the separator bands, concentration bands, and the reflected bands.

[0029] The principle of operation and applicability of the HOE solar concentrator lies in the recording geometry and processing, which will extend the limited bandwidth of HOEs in comparison to the baseline solar spectrum and quantum efficiency of the solar cells. In general, volume HOEs, while angle and wavelength selective, have limited bandwidths. Because of this selectivity, the recording and reconstruction or readout geometries and wavelengths are different. The HOE separator is a collector system that diffracts the useful portion of a multispectral radiation without dispersion and aberration and couples the separated bands into a spacer or waveguide that propagates the separated bands to a point of concentration, away from the incidence direction, to protect the photovoltaic active cells from natural or man-made space radiation environments.

[0030] The angle and wavelength selectivity of volumetric HOEs provide an opportunity to design an HOE that spectrally and spatially separates an incident multispectral beam into component spectra bands. The HOE separator can be specifically designed to respond only to incident beams from a specific direction. Non-uniform spatial frequencies are inscribed within the volume of the separator hologram, which responds only to beams incident from the directions for which the spatial frequencies were created. All other beams pass through the HOE separator. The HOE separator diffracts the incident beam in accordance to the Bragg condition sin .mu..theta..sub.i+sin .theta..sub.c=.lamda..sub.c/.LAMBDA., where .theta..sub.i is the angle into which the incident beam is diffracted, .lamda..sub.c is the beam incident wavelength, and .LAMBDA. is the spatial period inscribed in the volume of the HOE separator so as to respond to the incident beam from a desired incident direction. The diffracted separator beam is coupled into the waveguide at angle .theta..sub.i and is propagated to a distal point of concentration.

[0031] To cover a wide range of incidence angles, that is a field of view, several HOE separators can be multiplexed in the same volume of the hologram or laminated together, with each multiplexed or laminated HOE separator responding only to a specific beam direction, for example, incident beams .theta..sub.B, .theta..sub.O, and .theta..sub.R. For the solar HOE, advantage is had by designing a multiplexed HOE and PV combination that covers a wide range of incident angles from .theta..sub.B, to .theta..sub.O, to .theta..sub.R of radiation from the sun. This eliminates the need for additional tracking devices to track the sun. Using explicit angular analysis, the recording angles are adjusted, such that, a range of received beams of the same or different wavelengths from various incidence angles are brought to a defined focus, which can be coupled into a waveguide. This focal location can be either a spot or a line image.

[0032] The solar concentrator can have a wide field of view.phi. for passive sun tracking without any additional tracking device. The .theta..sub.B HOE separator only responds to beams from the .theta..sub.B direction, diffracted to the common focus. Similarly, the .theta..sub.O HOE separator is for normal incidence beam direction .theta..sub.O. The .theta..sub.R HOE separator is for beams incident for .theta..sub.R directions. The three multiplexed HOEs may have a total film thickness of approximately 45.mu.m with the sun being tracked within the field of view .phi..

[0033] The capabilities of the band selection holographic optics for generating solar power enable holographic energy conversion of solar power for suitable use in spacecraft missions, as well as alternative energy production both for terrestrial and space power applications. The use of broadband solar HOEs in conjunction with a thin waveguide can efficiently generate solar power. The solar concentrator can be shielded or protected from natural or man-made space environments. The HOE solar concentrator provides an effective approach to protect photovoltaic active components from natural or man-made space environmental radiation threats without moving parts for sun tracking. The solar concentrator can be made cost effectively without polishing. The holographic recording materials are inexpensive, and the cost of fabricating the solar HOE concentrator can be inexpensive.

[0034] The HOE solar concentrator uses non-mechanical components and can be a robust hybrid device with no moving part while rejecting harmful natural or man-made radiation to provide band selected high-efficiency power conversion and generation. For spacecraft power generation, the device can be made lightweight and be thin film with about 15.mu.m in thickness for concentrating aberration compensated bands in holographic optical element volumes that splits the incident solar radiation into component wavelength bands for corresponding PV power conversion. The transmitted light prevents undesirable radiation from exposing the PV cells. The concentrator can provide greater power conversion efficiency at reduced weight and stowed volume, matching selected band spectrum to PV cell band gaps for improved solar cell conversion efficiency.

[0035] The band selection holographic optical solar concentrator can comprise thin waveguides sandwiched between large bandwidth concentrating and reflecting holographic optical elements that are band selection HOEs. The HOE concentrator preferably compensates for aberration and dispersion introduced by the HOE separator. The reflecting HOE redirects diffracted selected concentration bands into the waveguide for remote communication of the concentrated bands. The use of internal reflection and the use of selected solar wavelength bands from the HOE concentrator, with diffraction at an angle greater than the critical angle of the waveguide, enable the concentrated bands to propagate in and through the waveguide, away from the incidence direction and to the band selection HOE reflector, which then directly injects the selected solar radiation bands into the PV cells of appropriate band gap energies.

[0036] The solar concentrator has desirable properties that include high diffraction efficiency, high signal-to-noise-ratio, high angular and spectral filtering selectivity, concentrated optical power, lightweight, thin planar shape, flat optics without required polishing, low cost, wavelength-multiplexing capability, insensitivity to misalignment, and simplicity of fabrication. The HOEs have unique properties that can be exploited to separate the solar spectra received from a wide range of incidence angles, into optical bands, which are brought to a common or serial focus and away from the direction of solar light incidence. The wavelength multiplexing advantage can be realized by multiple HOEs multiplexed in the same holographic volume with each HOE separator responding to a specific incidence angular direction. Thus, the multiplexed multiple HOE separator has advantages of collecting solar radiation from a wide range of incidence angles and provides for passive sun tracking, without any moving parts, without additional tracking devices, and without damage to the PV cells. The lightweight and flexibility of the solar concentrator is uniquely suitable for use in a variety of solar array shapes, sizes, and designs, including multiple junction thin film solar cells. The HOE can be fabricated in any of the commercially available high-efficiency photosensitive holographic recording materials.

[0037] The solar concentrator reduces system costs to mostly the costs of the photovoltaic cells, which are the most expensive part of the solar array. The solar concentrator can be mass-produced. Large format holographic optical elements with high optical quality and diffraction efficiency can be mastered and mass-produced. These can be cut to sizes for lamination along the lengths of the solar arrays. Because of the holographic principle, a large number of HOEs can be multiplexed in the same volume of a thin HOE to meet the requirements for low specific weight and volume of spacecraft. The solar concentrator can be a universal energy conversion device that is amenable to many configurations, shapes, sizes, and designs of solar arrays, and can be integrated with existing multijunction or thin film solar array systems. The solar concentrator can be used to convert power from any light source as a generalized light concentrator. The holographic solar concentrator has applications in a wide variety of fields. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.