What's This Project All About?
For several years now, NASA and ENTECH have been developing and refining space-qualified photovoltaic solar arrays that use a refractor concentrator technology. There are advantages to using refractive concentrators instead of reflective concentrators (this will be discussed later). Back in the early 1990's, the first refractive concentrator array was flown on the PASP Plus (Photovoltaic Array Space Power Plus Diagnostics) space mission. The array was composed of ENTECH's "mini-dome" focusing lenses over Boeing's mechanically stacked multi-junction solar cells. The lenses were a point-focus design and were coated to provide protection against the ultraviolet radiation (UV) and atomic oxygen (AO) that is found in space. It was one of a number of small, advanced arrays and it performed very well in a year-long mission that was in a very high radiation environment. It validated both the high performance and radiation hardness of the refractive concentrator approach.
Later, in the mid 1990's, NASA and ENTECH developed a new "line-focus" Fresnel lens concentrator. It was easier to make and more cost-effective than the mini-dome lens concentrator. In 1994, ABLE joined the refractive concentrator team and led the development of the SCARLET (Solar Concentrator Array using Refractive Linear Element Technology) solar array. SCARLET used a silicone Fresnel lens to focus sunlight at 8X concentration onto triple-junction solar cells. When NASA/JPL launched the Deep Space One spacecraft in October 1998, the SCARLET array powered both the spacecraft's electronics and the ion engine. The solar array was the first to fly using triple-junction solar cells as the principle power source for a spacecraft. SCARLET achieved over 200 W/m2 areal power density and over 45 W/kg specific power, the best performance metrics up to that time.
Stretched Lens Arrays
Over the last four years the team has grown to include Auburn University, EMCORE and Ion Beam Optics. The team has developed an ultra-light version of the flight-proven SCARLET array, but with much better performance metrics. The SCARLET array has now evolved into the Stretched Lens Array (SLA). It retains the essential power-generating elements (the silicone Fresnel lens, the multi-junction solar cells, and the composite radiator sheet) while discarding many of the non-power-generating elements (the lens glass arch superstrates, the lens support frames, the photovoltaic receivers support bars, and most of the honeycomb and back face sheet material in the panels). This forms a near-term, low-risk, rigid-panel version of SLA.
The defining feature of the SLA is that it eliminates so many elements of the SCARLET array by using a stretched lens optical concentrator. By using pop-up arches to stretch the silicone Fresnel lens in the lengthwise direction only, these lenses become self-supporting stressed membranes. SCARLET’s glass arches are thus no longer needed, eliminating their complexity, fragility, expense, and mass in the new, patented SLA. With this substantial lens-related mass reduction, the supporting panel structural loads are reduced, making ultra-light panels practical for SLA. This cascading mass-reducing effect of the stretched lenses continues throughout the SLA wing structure, resulting in unprecedented performance metrics.
All three refractive concentrator arrays discussed above (the mini-dome lens, SCARLET and SLA) use Fresnel lens optical elements based on the same symmetrical refraction principle. Solar rays intercept the smooth convex outer surface of the Fresnel lens and each ray is refracted by the curved outer surface one half of the angular amount needed to focus the rays on the solar cell. As the rays exit the prismatic inner surface they are refracted again one half of the amount needed to reach the cell. Each solar ray's angle of incidence at the smooth outer surface equals it's emergence angle at the prismatic inner surface (angle in = angle out). This symmetrical refraction minimizes reflection losses at the two surfaces of the lens and provides maximum optical performance. The multitude of prisms in the symmetrical-refraction lens allows the individual prism angles to be tweaked to tailor the photon flux profile over the solar cell, both spatially and spectrally.
In addition to the near-term, low-risk rigid-panel version of SLA, an advanced version of SLA is also under development. The advanced version is a flexible-blanket SLA, similar to the prototype shown below.
For this SLA version, the lenses form one flexible blanket while the radiator elements, containing the photovoltaic receivers, form a second flexible blanket. Both blankets fold up into a very compact stow volume for launch, and automatically deploy on orbit. One of the most efficient platforms for deploying and supporting the flexible-blanket version of SLA is the SquareRigger platform, developed by ABLE Engineering. The SquareRigger platform was originally developed by ABLE under funding from the Air Force Research Laboratory for use with thin-film photovoltaic blankets in space. However, with the much higher efficiencies achievable with SLA compared to thin-film photovoltaics, the marriage of SLA and SquareRigger provides unprecedented performance metrics.
Initial development of the SLA/SquareRigger technology, including a small prototype demonstrator, has recently been completed by ABLE Engineering, with ENTECH subcontract support, under a NASA Small Business Innovation Research (SBIR) Phase I contract. Additional development, including much larger scale hardware development, is being done under a Phase II SBIR contract. All of this development work is directed toward the SLA/SquareRigger array approach shown schematically below.
Analysis of this type of SLA/SquareRigger system led to the near-term and mid-term performance metric estimates in the table above. Note that SLA/SquareRigger enables giant space solar arrays in the 100 kW to 1 MW class, with spectacular performance metrics (300 to 500 W/kg specific power, 80 to 120 kW/m3 stowed power, and operational voltages above 1,000 V) in the near-term (2010) to mid-term (2015).
In the longer term (2020-2025), with constantly improving solar cell efficiencies and incorporation of new nanotechnology materials into the lens and radiator elements, SLA’s technology roadmap leads to 1,000 W/kg solar arrays, as shown in the image below.
Indeed, SLA is unique among all solar array technologies in its portfolio of attributes, which include world-record-level solar-to-electric conversion efficiency (high W/m2), ultra-light mass density (low kg/m2), spectacular stowed power density (kW/m3), highly scalable power (kW to multi-MW), high-voltage capability (kV), modularity (individual lens/cell building blocks), mass-producibility, and cost effectiveness. SLA’s unique portfolio of attributes matches the critical requirements for space power systems for many of planned NASA’s Exploration missions.