Since 1990 thin film optical coatings have taken a prominent role in the development of highly efficient solar power concentrators for future space applications. During the initial development of this coating technology, the Boeing High Technology Center explored various ways of protecting ENTECH's DC93-500 silicone Fresnel lenses from the harsh space environment. ENTECH's mini-dome lenses focused solar energy onto small high-efficiency solar cells for generating electrical power. To protect the silicone lenses from solar UV darkening, one early approach involved a cerium-doped glass cover cemented over the lens. Unfortunately, during launch simulation shock testing the glass lens covers cracked. We next explored the deposition of a UV blocking thin film coating directly to the silicone lens surface. This was a problem of immense proportions analogous to pouring concrete on to the surface of a reservoir filled with "Jell-O." Differential in coefficient of thermal expansion between the DC93-500 silicone and the deposited dielectric optical coating had to be balanced with intrinsic stress of the optical coating materials. Ion Beam Optics' work has culminated, some fifteen years later, in the current coating technology that is being incorporated in the Stretched Lens Array SquareRigger (SLASR). SLASR is designed to replace classic flat panel solar arrays with a lighter, lower cost, and more efficient (30%) concentrator arrays for future space applications. This paper will describe the coating technology and show its performance and benefits for SLASR space power systems. Results from both ground tests and space flight tests will be presented.
Silicone lens materials, baselined for space power applications, were exposed to various components of a Geosynchronous Earth Orbit (GEO) radiation environment to determine the suitability of the material for long-term missions. Sample materials were exposed to electrons, protons, Near Ultraviolet (NUV), and Vacuum Ultraviolet (VUV) radiation. The samples were exposed to individual and to various combinations of these space environmental components. The electron and proton exposure levels were determined from radiation measurements performed in GEO. NUV and VUV radiation exposures were based on solar emissions at zero air mass (AM0). Lens material degradation was determined by the change in optical spectral transmission of the silicone materials. A reduction in the transmittance of the material will reduce the power generating potential of solar cells. The spectral transmission was measured at Marshall Space Flight Center (MSFC), after exposure to space environmental elements including electrons, protons, VUV and NUV. Entech, Inc. conducted performance tests on samples exposed to short duration proton and electron radiation. Results of these tests will be discussed. Minor degradation was witnessed on samples exposed to NUV and VUV light. The largest transmission spectral degradation occurred in the wavelength range below the quantum efficiency of space qualified solar cells. Transmission degradation in the wavelength range of maximum solar cell quantum efficiency was small.
A unique ultra-light solar concentrator has recently been developed for space power applications. The concentrator comprises a flexible, 140-micron-thick, line-focus Fresnel lens, made in a continuous process from space-qualified transparent silicone rubber material. For deployment and support in space, end arches are used to tension the lens material in a lengthwise fashion, forming a cylindrical stressed membrane structure. The resultant lens provides high optical efficiency, outstanding tolerance for real-world errors and aberrations, and excellent focusing performance. The stretched lens is used to collect and focus sunlight at 8X concentration onto high-efficiency multi-junction photovoltaic cells, which directly convert the incident solar energy to electricity. The Stretched Lens Array (<i>SLA</i>) has been measured at over 27% net solar-to-electric conversion efficiency for space sunlight, and over 30% net solar-to-electric conversion efficiency for terrestrial sunlight. More importantly, the SLA provides over 180 W/kg specific power at a greatly reduced cost compared to conventional planar photovoltaic arrays in space. The cost savings are due to the use of 85% less of the expensive solar cell material per unit of power produced. SLA is a direct descendent of the award-winning <i>SCARLET</i> array which performed flawlessly on the NASA/JPL Deep Space 1 spacecraft from 1998-2001.
Transparent polymeric materials are being designed and utilized as solar concentrating lenses for spacecraft power and propulsion systems. These polymeric lenses concentrate solar energy onto energy conversion devices such as solar cells and thermal energy systems. The conversion efficiency is directly related to the transmissivity of the polymeric lens. The Environmental Effects Group of the Marshall Space Flight Center's Materials, Processes, and Manufacturing Department exposed a variety of material to a simulated space environment and evaluated them for change in optical transmission. These materials include Lexan<SUP>TM</SUP>, polyethylene terephalate, several formulate of Tefzel<SUP>TM</SUP> and Teflon<SUP>TM</SUP>, and silicone DC 93 - 500. Samples were exposed to a minimum of 1000 equivalent sun hours of near ultraviolet radiation (250 - 400 nm wavelength). Prolonged exposure to the space environment will decrease the polymer film's transmission and thus reduce the conversion efficiency. A method was developed to normalize the transmission loss and thus rank the materials according to their tolerance to space environmental exposure. Spectral results and the material ranking according to transmission loss are presented. Power loss over time for a typical solar cell was calculated based on degraded transmission of the polymer material.