Projects

Entech

Publications

Project Objective: Develop a new 440X, 27%-efficient photovoltaic concentrator module which is a "plug-and-play" replacement for ENTECH's existing, field-proven 21X, 13%-efficient module.  The new module will use advanced multi-junction cell technology to be provided by lower-tier subcontractor Spectrolab, to replace the silicon cell technology used in the existing module.  New color-mixing Fresnel lenses will be required to provide the primary 21X concentration and the secondary 21X concentration, together yielding a 440X overall concentration.  The new module will fit into existing field-proven sun-tracking arrays, including the SunLine 2-module array for small power applications and the SolarRow 72-module array for large power applications.  Due to the anticipated doubling of module and array efficiency, the delivered energy economics for the new systems are expected to be much better than for the existing systems.

Approach/Background:  ENTECH has been involved in photovoltaic concentrator technology for terrestrial applications for the past 23 years.  ENTECH's latest terrestrial concentrator module uses a large (85 cm wide aperture) acrylic lens to focus sunlight at 21X geometric concentration onto air-cooled silicon photovoltaic cells.  This module was developed and refined with the technical and financial support of DOE, NREL, and Sandia, under the Concentrator Initiative and PVMaT programs of the early 1990's.  ENTECH deploys these modules in field-proven two-axis sun-tracking arrays.  For small applications, the two-module SunLine array is used.  For larger applications, the 72-module SolarRow is used.  The performance of ENTECH's terrestrial concentrators has been excellent, as shown by the independent PVUSA project in Davis, California, which compared the long-term performance of leading photovoltaic technologies as measured in side-by-side field testing.  From 1991-1999 (the last year reported by PVUSA), ENTECH's concentrator array outperformed all of the other arrays at PVUSA.

ENTECH has also been involved in photovoltaic concentrator technology for space applications for the past 15 years.  ENTECH's latest space concentrator module uses a small (8.5 cm wide aperture) silicone Fresnel lens to focus sunlight at 8.5X concentration onto radiation-cooled triple-junction photovoltaic cells.  Launched in 1998, the award-winning SCARLET (solar concentrator array using refractive linear element technology) array powered both the satellite and the ion engine on the NASA/JPL Deep Space One probe.  Deep Space One had a successful rendezvous with the comet, Borrelly, on September 22, 2001, resulting in the highest resolution images ever taken of a comet.  To maximize the performance of these multi-junction cell concentrator arrays, ENTECH uses a patented new color-mixing lens approach to minimize chromatic aberration power losses between the multiple junctions of the cell.

The new concentrator module being developed under this NREL-sponsored program represents a convergence of ENTECH's terrestrial and space concentrator efforts over the past two decades.  By using high-efficiency, color-mixing Fresnel lenses with advanced, triple-junction photovoltaic cells, terrestrial module efficiency levels of 27% should be possible in the near term, with much higher efficiency levels achievable in the longer term.  Indeed, last year, NREL tested one of ENTECH's space mini-concentrator modules outdoors and confirmed 27% module-level efficiency.  In October 2001, ENTECH tested the latest space mini-concentrator at over 30% operational efficiency.  To be cost-effective with these high-efficiency cells for terrestrial applications, module concentration ratios must be increased to the several hundred sun range.  Secondary optical elements will be used to focus the sunlight by another 21X to provide the needed high concentration.  Thus, the new 440X module is being developed to cost-effectively provide near-term system performance levels twice as high as achieved by ENTECH's silicon-cell-based PVUSA system, and longer-term system performance levels three times as high as achieved by ENTECH's PVUSA system.

Status/Accomplishments:  Optical, thermal, electrical, mechanical, and cost trade studies have been performed with a goal of optimizing the design of the new module.  To support these studies, outdoor tests of prototype cell assemblies under lens illumination have also been conducted.  These studies and tests indicate that the optical, thermal, electrical, mechanical, and cost goals for the new module should be achievable, provided that the cells can be produced at reasonable cost and can tolerate high output currents and other environmental factors over the 30-year lifetime.  Some unanticipated problems with high cell currents have been experienced to date, but are working (ENTECH, NREL, and the cell suppliers) to overcome these problems.

Perhaps the most encouraging accomplishment to date has been ENTECH's recent (October 2001) outdoor testing of a space mini-concentrator module using the latest space cells from Spectrolab.  This mini-concentrator module provided over 30% net operational efficiency under a variety of conditions in a large number of IV curve measurements taken over several days of outdoor testing. If similar module efficiency levels can be achieved by the final 440X module being developed under this program, the new module should exceed the goals of the program.

Planned FY 2002 Activities:  During the first quarter of FY 2002, the project team hopes to determine the maximum practical operating current level of the multi-junction cells, and select the optical element designs to match this current level.  During the second quarter of FY 2002, Entech shall build, test, and deliver a fully functional prototype module of the new type.  In addition, a SunLine array will be installed at NREL to serve as the sun-tracking platform for the new module, and for a second test module to be delivered without solar cells.  NREL will use the second module to test other cells in the future.

ENTECH High-Performance PV Publications:
"Development of a II-VI-Based High Performance, High Band Gap Device for Thin-Film Tandem Solar Cells," National Center for Photovoltaics Program Review Meeting, Oct. 14-17, 2001 (Lakewood, CO). (PDF 198 KB)

C. S. Ferekides and D. L. Morel