CONCLUSIONS OF THE HUNTSVILLE SPS POWER CONVERSION WORKSHOP J. Richard Williams Associate Dean of Engineering Georgia Institute of Technology Atlanta, Georgia 30332 The National Aeronautics and Space Administration sponsored a workshop on "SPS Energy Conversion and Power Management" February 5-7, 1980. The workshop was hosted by the Johnson Environmental and Energy Center of the University of Alabama in Huntsville. The three topics under consideration were SPS Photovoltaic Power Systems, SPS Solar Thermal Power Systems, and Power Distribution and Management. The photovoltaic working group chaired by Martin Wolf of the University of Pennsylvania examined the current status of SPS photovoltaic R&D and made recommendations for future programs. The examination was carried out from the viewpoint that the SPS will need to become a cost-effective electrical power source in competition with fossil and nuclear fueled base-load plants, as well as with various types of future terrestial solar photovoltaic power systems. A number of important design paramters are interdependent, for example cell efficiency, thickness, and radiation resistance. The development of suitable candidate cell/blanket designs meeting the combined performance/mass/life design parameters, as verified in ground tests, should precede space certification testing and the development of manufacturing methods. The group concluded that adequate resources are available for both the GaAs and silicon photovoltaic systems, and identified performance demonstration issues which need further work. Near term performance goals should be achievement of 16% efficiency in a suitable cell/substrate/cover structure to permit initiation of stability tests. Achieveability of 16% end-of-life efficiency (for GaAs) and 14% end-of-life efficiency (for silicon) after 30 years will need to be demonstrated. Synchronous orbit flight tests should be planned for the post-1986 time frame. Efforts should continue aimed at demonstrating a 25% efficient AMO thin-film cascade solar cell and showing a potential for 35% efficiency. Also alternative concepts leading to 50% conversion efficiency should be pursued because of the enormous advantages of increasing photovoltaic efficiency, thereby reducing the size required for the array. Other major issues are cell encapsulation and blanket integration. The geometry of the blanket submodules, very thin and very large, requires new approaches and innovative techniques for fabrication in space. A critical component of the photovoltaic system is the supporting element or encapsulant to which the active element, the photovoltaic cell, is bonded. The encapsulation material has to provide the structural strength of the blanket and the shielding for the solar cells against the energetic particle radiation of space. Development of encapsulants with simultaneous durability against bombardment by electrons and protons, ultraviolet radiation, and deep thermal cycling for a 30 year period is essential to the program. Appropriate cost studies should also be conducted to ensure that the total array structure (cell, contacts, encapsulant, interconnects) is capable of meeting the SPS cost goals with suitable development and scaling. Since some of the environmental factors are ill-defined and time varying, adequately instrumented on-orbit testing will be necessary to demonstrate the feasibility of achieving cost/performance/lifetime goals. The low mass of the blanket causes very severe temperature cycling during eclipse periods, with consequent stress due to thermal expansion coefficient mismatches in the blanket structure. The extent of these effects can be determined in ground-based thermal cycling tests, and the design, if needed, improved by selection of more suitable materials. The one specific advanced concept recommended by the group for further immediate development is the cascaded or tandem multiple-band-gap solar cell - a concept already being investigated in several materials systems under Air Force
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