Mark J. O'Neill

A high-performance photovoltaic concentrator array: The mini-dome Fresnel lens concentrator with 30% efficient GaAs/GaSb tandem cells

A high-efficiency, lightweight space photovoltaic concentrator array is described. Previous work on the mini dome Fresnel lens concentrator concept is being integrated with Boeings 30% efficient tandem GaAs/GaSb concentrator cells into a high-performance photovoltaic array. Calculations indicate that, in the near term, such an array can achieve 300 W/m/sup 2/ at a specific power of 100 W/kg. Emphasis of the program has now shifted to integrating the concentrator lens, tandem cell, and supporting panel structure into a space-qualifiable array. A description is presented of the current status of component and prototype panel testing and the development of a flight panel for the Photovoltaic Array Space Power Plus Diagnostics (PASP PLUS) flight experiment.<<ETX>>

Estimating and controlling chromatic aberration losses for two-junction, two-terminal devices in refractive concentrator systems

Although previous studies have measured and calculated solar cell chromatic aberration losses and proposed methods for reducing these by modifying the optics, significant work remains to be done toward understanding how to quantify the losses and how various parameters affect this loss. This paper presents an analytical definition and calculation method for chromatic aberration losses in Ga/sub 0.5/In/sub 0.5/P/GaAs solar cells. The effects of sheet resistance of the midlayers of the cell, total irradiance, incident spectrum, cell width and diode quality factor are studied. A method for measuring the midlayer resistance in finished solar cells is also described.

In-Plane Thermal Conductivity in Thin Carbon Fiber Composites

The objective of this study was to determine whether fiber type, fiber angle, and filler material affected the in-plane thermal conductivity of thin (7-15 mils thickness) carbon fiber composites. Two sets of samples were tested: low thermal conductivity samples made with polyacrylonitrile-based fibers (k = 6.8 W/m .K) in Fiberglast epoxy resin, and high thermal conductivity samples fabricated with coal-pitch-based fibers (k = 620 W/m . K) in cyanate ester resin. Samples were fabricated from 0/90 woven cloths and warped to obtain a range of fiber-pattern angles from 25/ - 25 to 65/ - 65. The filler effect on thermal conductivity was evaluated on additional samples prepared with 10% volume fraction of graphite powder in the matrix. Thermal conductivity of the low thermal conductivity samples was in the range of 15-20 W/m . K and showed up to a 15 % improvement when the angle of the fibers was varied. High thermal conductivity samples showed thermal conductivities between 60 and 150 W/m . K, with an improvement up to 60% when the angle of the fibers relative to the heat flux direction was changed from 0/90 to 25/-25. The samples with graphite powder did not show any enhancement in thermal performance. As potential alternatives, unidirectional tape and eGrafs Spreadershield® foils were also tested, showing good thermal performance.

The stretched lens array (SLA): a low-risk cost-effective concentrator array offering wing-level performance of 180 W/kg and 300 W/m/sup 2/ at 300 VDC

This work presents a new stretched lens array (SLA), as a concentrator array used in the space environment. The recent improvements in the lens, solar cell, photovoltaic receiver, rigid panel structure, and complete solar array wing are described. In addition to excellent durability in the space environment, the SLA provides outstanding wing-level performance.

Inflatable lenses for space photovoltaic concentrator arrays

For 12 years, ENTECH and NASA Lewis have been developing Fresnel lens solar concentrator technology for space power applications. ENTECH provided the point-focus mini-dome lenses for the PASP+ array, launched in 1994. These silicone lenses performed well on-orbit, with only about 3% optical performance loss after 1 year in elliptical orbit, with high radiation, atomic oxygen, and ultraviolet exposure. The only protection for these silicone lenses was a thin-film coating provided by OCLI. ENTECH also provided the line-focus lenses for the SCARLET 1 and SCARLET 2 arrays in 1995 and 1997, respectively. These lenses are laminated assemblies, with protective ceria glass superstrates over the silicone lens. In March 1997, ENTECH and NASA Lewis began development of inflatable Fresnel lenses, to achieve lower weight, smaller launch volume, reduced cost, less fragility and other advantages. This paper summarizes the new concentrator approach, including key program results to date.

Fabrication, installation and initial operation of the 2000 sq. m. linear Fresnel lens photovoltaic concentrator system at 3M/Austin (Texas)

A photovoltaic concentrator system has been designed, developed, fabricated. and installed at the 3M/Austin Center. The two major objectives of the program were to develop a mass-producible, high-performance, low-cost photovoltaic concentrator system and to fabricate and deploy a nominal 300 kW system of the new design. Both of these objectives have been achieved. The authors describe the photovoltaic technology and discuss the fabrication, installation, and initial operation of the 2000 m/sup 2/ system.<<ETX>>

Line-focus optics for multijunction cells in space power arrays

Since 1986, ENTECH has been developing lightweight, high-performance Fresnel lens optics for space photovoltaic concentrator systems. This development work has been technically and financially supported by NASA-Lewis, SDIO, BMDO, Boeing, JX Crystals and AEC-ABLE. A fully functional experimental mini-dome Fresnel lens concentrator solar cell array was onboard the PASP Plus mission launched in August 1994. This array, assembled by Boeing using mechanically stacked multijunction (MSMJ) solar cells, confirmed the high-performance and low-radiation-degradation characteristics predicted for the refractive concentrator approach. Since PASP Plus, ENTECH has developed a line-focus Fresnel lens offering much improved manufacturability, cost, and sun-pointing error tolerance. This paper presents the latest line-focus optical designs customized for use with MSMJ cells and with monolithic multijunction (MMJ) cells.

Balloon and Lear jet testing of SCARLET modules and cells

This paper presents test results from SCARLET (solar concentrator array with refractive linear element technology) experiments performed on several Lewis Research Center Lear jet flights and two JPL balloon flights. The tests were performed in support of the BMDO-sponsored SCARLET II program, which is building a 2.6-kW SCARLET to supply the primary power for the JPL New Millennium Deep Space 1 Mission. The experiments involve TECSTAR dual-junction GaInP2/GaAs/Ge cells flown bare and under two different types of SCARLET lenses. The two types of lenses tested were a developmental design consisting of a monolithic fluoroplastic and the current baseline flight design consisting of ceria-doped microsheet and silicone. Measured lens and total module efficiencies are presented and the flight data are compared to various solar simulator test results.

Component & Prototype Panel Testing of the Mini-Dome Fresnel Lens Photovoltaic Concentrator Array

The mini-dome Fresnel lens concentrator array is a high-efficiency, light-weight space photovoltaic array concept that is being developed by the NASA Lewis Research Center and ENTECH, Inc. The three critical elements of the array concept are the Fresnel lens concentrator, the prismatic cell cover and the photovoltaic call. At previous conferences, the results of gallium arsenide concentrator cells with prismatic cell covers have been presented. The emphasis of the program has now shifted to the mini-dome Fresnel lens concentrator. Prototype concentrator lenses have been fabricated and tested, with optical efficiencies reaching 90%. Work is also progressing on the design and fabrication of the panel structure. In addition, the impact of recent advancements in 30% efficient multi-junction photovoltaic cells on array performance is also discussed.

TACSAT-4 solar cell experiment: Advanced solar cell technologies in a high radiation environment

The TACSAT-4 Solar Cell Experiment will measure the current and voltage characteristics of advanced EMCORE BTJM solar cells thinned to 100 microns and ATJM cells under an 8.5X ENTECH Stretched Lens. TACSAT-4 will fly in a highly elliptical orbit, passing through the electron and proton belts every six hours. This orbit is expected to induce a 25% power reduction in one year due to radiation damage. In addition to demonstrating these new technologies, TACSAT-4 will also demonstrate a radiation hard solar cell measurement circuit designed to a simple interface capable of meeting the standard requirement for future solar arrays built to the AIAA S-122-2007, “Electrical Power Systems for Unmanned Spacecraft.” TACSAT-4 will launch in September of 2009.