We propose a novel design for a lightweight, high-performance space-based solar power array combined with power beaming capability for operation in geosynchronous orbit and transmission of power to Earth. We use a modular configuration of small, repeatable unit cells, called tiles, that each individually perform power collection, conversion, and transmission. Sunlight is collected via lightweight parabolic concentrators and converted to DC electric power with high efficiency III-V photovoltaics. Several CMOS integrated circuits within each tile generates and controls the phase of multiple independently-controlled microwave sources using the DC power. These sources are coupled to multiple radiating antennas which act as elements of a large phased array to beam the RF power to Earth. The power is sent to Earth at a frequency chosen in the range of 1-10 GHz and collected with ground-based rectennas at a local intensity no larger than ambient sunlight. We achieve significantly reduced mass compared to previous designs by taking advantage of solar concentration, current CMOS integrated circuit technology, and ultralight structural elements. Of note, the resulting satellite has no movable parts once it is fully deployed and all beam steering is done electronically. Our design is safe, scalable, and able to be deployed and tested with progressively larger configurations starting with a single unit cell that could fit on a cube satellite. The design reported on here has an areal mass density of 160 g/m2 and an end-to-end efficiency of 7-14 forward to the realization of space-based solar power, a concept once of science fiction.

We present a detailed design treatment for a concentrating photovoltaic mini module subsystem with a specific power of up to 4.1 kW/kg for integration into a space solar power system. Concentrating designs are required to achieve specific power over 1 kW/kg with current high-efficiency III-V multijunction solar cells. The 15 sun, linear concentration concept detailed here reduces the system mass by replacing cell and radiation shield area with ultralight carbon fiber reinforced polymer (CFRP) optics. Reducing the cell size to 1 mm width as well as careful optimization of cell architecture and CFRP material and thickness are critical for maintaining cell temperatures under 100 C despite the concentration. We also describe ultralight multilayer optical coatings to increase the thermal emissivity of the concentrator surfaces and enhance radiative transfer for cell cooling, which is a critical technological component of the total system design.

Spectrum‐splitting photovoltaics incorporate optical elements to separate sunlight into frequency bands, which can be targeted at solar cells with bandgaps optimized for each sub‐band. Here, we present the design of a holographic diffraction grating‐based spectrum‐splitting photovoltaic module integrating eight III‐V compound semiconductor cells as four dual‐junction tandems. Four stacks of simple sinusoidal volume phase holographic diffraction gratings each simultaneously split and concentrate sunlight onto cells with bandgaps spanning the solar spectrum. The high‐efficiency cells get an additional performance boost from concentration incorporated using a single or a compound trough concentrator, providing up to 380X total concentration. Cell bandgap optimization incorporated an experimentally derived bandgap‐dependent external radiative efficiency function. Simulations show 33.2% module conversion efficiency is achievable. One grating stack is experimentally fabricated and characterized.

We provide 20 direct spectra that capture the variation of the solar spectrum composition with intensity. We correlate the value of the air mass, aerosol optical depth at 500 nm, precipitable water, and ozone with the cumulative irradiance for direct sunlight with the use of National Solar Radiation Database (NSRDB). We use the values of these atmospheric parameters to generate spectra that represent their corresponding cumulative irradiance levels with the use of SMARTS multiple scattering and transmission model. By simulation of the performance of a solar cell design under these 20 spectra and combination of the intensity-specific performance with the relative frequency of each irradiance level at a particular location from the NSRDB, we can predict tandem cell energy production across the United States. Through comparison of the energy production of ideal tandem cells with two to ten subcells as predicted by our model to energy production integrated over one year's worth of simulated spectra at nine locations across the United States as well as measured spectral irradiance from NREL's solar observatory, we find the error in our energy production estimate to be under 5% for ten subcells and under 3% for up to five subcells. We demonstrate the utility of the approach with a selection of prospective and ideal multijunction bandgap combinations.

The most feasible pathway to record 50% efficiency photovoltaic devices is by utilizing many (>4) junctions to minimize thermalization and nonahsorption losses. Here we propose a spectrum-splitting design, the polyhedral specular reflector (PSR), that employs an optical architecture to divide and concentrate incident sunlight, allowing the incorporation of more junctions compared with traditional monolithic architectures. This paper describes the PSR design and indicates the requirements to achieve a 50% efficiency module by coupling robust cell, optical, and electrical simulations. We predict that a module comprised of the seven submits with an average external radiative efficiency of at least 3%, an optical architecture capable of a splitting efficiency of at least 88% and 300x concentration, small (<= 1 mu m) metallic fingers for subcell contact, and a state-of-the-art power conditioning system (>98% efficiency) can achieve a module efficiency of 50%, a record for both multijunction cells and modules. We also discuss the flexibility of the design and explore how adjusting the size and type of concentrators can still yield record module efficiencies (>40%).

Changes in the incident spectrum under varying atmospheric conditions can degrade the performance of series connected tandem solar cells that are optimized for performance under the AM1.5D standard spectrum. We present a case study of optimizing series connected tandem solar cells with two to eight subcells for energy production using representative spectra that capture the variation of incident spectra with cumulative irradiance level. Combined with the National Solar Radiation Database information on the frequency of cumulative irradiance levels at locations across the United States, these 20 direct spectra allow us to predict the energy production of spectrum splitting photovoltaic systems across the United States. Series connected tandems optimized to maximize energy production at a local level can generate up to 3% more energy per year than the AM1.5D based design.

We demonstrate the development of a prototype lightweight (1.5 kg/m 2 ) tile structure capable of photovoltaic solar power capture, conversion to radio frequency power, and transmission through antennas.This modular tile can be repeated over an arbitrary area to form a large aperture which could be placed in orbit to collect sunlight and transmit electricity to any location.Prototype design is described and validated through finite element analysis, and high-precision ultra-light component manufacture and robust assembly are described.

We have fabricated a functional prototype of an ultralight power converter tile; a modular building block for a space-based solar power system. The tile is ~10 × 15 cm in area, and weighs 4.5 kg/m 2 . It comprises a photovoltaic (PV) solar energy collector, a radio-frequency (RF) power converter, and an array of transmission antennas. The PV collector subassembly utilizes ~15x, 1D parabolic trough reflective concentrators with triple-junction (3J) solar cells. It has areal mass of ~0.8 kg/m 2 , 74% optical efficiency, and a peak specific power of ~230 W/kg. We demonstrated wireless power transmission over a distance of ~50 cm in our lab. Analysis of the sources of mass and inefficiency suggest a path towards achieving dramatically higher performance with future designs.

We report the design, fabrication, and characterization of ultralight highly emissive structures with a record-low mass per area that emit thermal radiation efficiently over a broad spectral (2 to 30 microns) and angular (0-60°) range.The structures comprise one to three pairs of alternating metallic and dielectric thin films and have measured effective 300 K hemispherical emissivity of 0.7 to 0.9 (inferred from angular measurements which cover a bandwidth corresponding to 88% of 300K blackbody power).To our knowledge, these micron-scale-thickness structures, are the lightest reported optical coatings with comparable infrared emissivity.The superior optical properties, together with their mechanical flexibility, low outgassing, and low areal mass, suggest that these coatings are candidates for thermal management in applications demanding of ultralight flexible structures, including aerospace applications, ultralight photovoltaics, lightweight flexible electronics, and textiles for thermal insulation.

We describe a spectrum splitting solar module design approach using ensembles of 2–20 subcells with bandgaps optimized for the AM 1.5D spectrum. Device physics calculations and experimental data determine radiative efficiency parameters for III ‐V compound semiconductor subcells and enable modification of conventional detailed balance calculations to predict module efficiency while retaining computational speed for a wide search of the design space. Accounting for nonideal absorption and recombination rates due to realistic material imperfections allows us to identify the minimum subcell quantity, quality, electrical connection configuration, and concentration required for 50% module efficiency with realistic optical losses and modeled contact resistance losses. We predict a module efficiency of 50% or greater will be possible with 7–10 electrically independent subcells in a spectral splitting optic at 300–500 suns concentration, assuming a 90% optical efficiency and 98% electrical efficiency, provided the subcells can achieve an average external radiative efficiency of 3–5% and a short circuit current that is at least 90% of the ideal. In examining spectrum splitting solar cells with both series‐connected and electrically independent subcells, we identify a new design trade‐off independent of the challenges of fabricating optimal bandgap combinations. Series‐connected ensembles, having a single set of electrical contacts, are less sensitive to lumped series resistance losses than ensembles where each subcells are contacted independently. By contrast, ensembles with electrically independent subcells can achieve lower radiative losses when the subcells are designed for good optical confinement. Distributing electrically independent subcells in a concentrating receiver module allows flexibility in subcell selection and fabrication, and can achieve ultra‐high efficiency with conventional III ‐V cell technology.

Concentrator photovoltaic systems can provide supplementary shielding against high energy particles. In this paper we compare the radiation environment that the same solar cell would experience in a flat-plate module versus in a parabolic mirror concentrator system. We have observed that the shielding provided by the concentrator system is remarkable. In order to obtain an accurate prediction of the overall shield needed in our concentrator system triple-junction space solar cells have been irradiated on the edge with 350-keV protons at a fluence of 10 12 p + cm -2 . A mild degradation of the open circuit voltage was measured (~70 mV).

We report the design, fabrication and characterization of ultrathin metasurfaces that exhibit wideband 300 K thermal emissivity. The emissive behavior of these structures is almost independent of the emission angle. Our ultralight subwavelength-thickness metasurfaces can be fabricated relatively easily and are excellent candidates for radiative cooling in space applications.

The Space Solar Power Initiative (SSPI) seeks to enable reliable, cost-effective baseload power generation from large-scale solar power stations in space. We propose an ultralight, modular power station, having specific power in the range of 1-10 kW/kg for the photovoltaic (PV) collection subsystem. The building block of the power station is the `tile,' a self-contained element that performs PV energy collection, conversion to radio frequency (RF), and transmission to earth. To minimize PV mass, we select a 1D, 10-20X parabolic trough concentrator geometry, which provides cooling and radiation shielding for the cells, and which folds flat for deployment. Here, we discuss the design, fabrication, and testing of the initial PV tile prototypes.

Lightweight parabolic mirrors for solar concentrators have been fabricated using carbon fiber reinforced polymer (CFRP) and a nanometer scale optical surface smoothing technique. The smoothing technique improved the surface roughness of the CFRP surface from ~3 μm root mean square (RMS) for as-cast to ~5 nm RMS after smoothing. The surfaces were then coated with metal, which retained the sub-wavelength surface roughness, to produce a high-quality specular reflector. The mirrors were tested in an 11x geometrical concentrator configuration and achieved an optical efficiency of 78% under an AM0 solar simulator. With further development, lightweight CFRP mirrors will enable dramatic improvements in the specific power, power per unit mass, achievable for concentrated photovoltaics in space.

Variation of the incident spectrum under real-world illumination conditions can degrade the performance of series connected multijunction solar cells that are optimized for performance under the AM1.5D standard spectrum. Current approaches to correct for this factor and estimate energy production for deployed systems require large amounts of field data and are not useful for evaluating prospective designs. We present a set of 20 spectra that capture the average spectral composition for direct normal irradiance at different cumulative irradiance levels. Combined with the NSRDB information on the frequency of cumulative irradiance levels at locations across the United States, these 20 spectra allow us to predict the energy production of spectrum splitting photovoltaic systems with minimal computation required.

A design for ultra-high efficiency solar modules (>50%) using spectrum splitting is proposed. In the polyhedral specular reflector design, seven subcells are arranged around a solid parallelepiped. Incident light enters the parallelepiped and is directed via specular reflection onto each subcell in order from highest to lowest bandgap. We analyze optical losses due to external concentration and parasitic absorption and optimize the design for >50% module efficiency. We find that moderate concentration designs (90-170x) with a high index parallelepiped and perfect shortpass filters meet target efficiencies and demonstrate an initial design.

Increasing the number of subcells in a multijunction or "spectrum splitting" photovoltaic improves efficiency under the standard AM1.5D design spectrum, but it can lower efficiency under spectra that differ from the standard if the subcells are connected electrically in series. Using atmospheric data and the SMARTS multiple scattering and absorption model, we simulated sunny day spectra over 1 year for five locations in the United States and determined the annual energy production of spectrum splitting ensembles with 2-20 subcells connected electrically in series or independently. While electrically independent subcells have a small efficiency advantage over series-connected ensembles under the AM1.5D design spectrum, they have a pronounced energy production advantage under realistic spectra over 1 year. Simulated energy production increased with subcell number for the electrically independent ensembles, but it peaked at 8-10 subcells for those connected in series. Electrically independent ensembles with 20 subcells produce up to 27% more energy annually than the series-connected 20-subcell ensemble. This energy production advantage persists when clouds are accounted for.

We investigate a spectrum-splitting design, the polyhedral specular reflector, for an ultra-high efficiency module (>50%). The design employs a series of multilayer dielectric stack filters to divide the incident spectrum onto seven independently connected subcells. We optimized the geometry and components of the design through coupled wave-optics, device physics, electrical circuit, and ray tracing models. We show a wide design space where >50% module efficiencies are possible and have chosen a design with a projected 50.8% module efficiency to prototype. Initial efforts show excellent matching of the fabricated optical splitting prism (90.1% splitting efficiency) to the theoretical design (93% splitting efficiency). Additionally, integrating fabricated concentrators yields an optical structure consistent with a 30% efficiency module.