Effect of Variable Guide Vanes and Natural Gas Hybridization for Accommodating Fluctuations in Solar Input to a Gas Turbine

Abstract

In recent years, several prototype solar central receivers have been experimentally demonstrated to produce high temperature and high pressure gas capable of driving a gas turbine engine. While these prototype receivers are generally small (<1 MWth), advancements in this technology will allow for the development of solar powered gas turbine engines at a commercial level (sizes of at least several megawatts electric (MWe)). The current paper analyzes a recuperated solar powered gas turbine engine, and addresses engine considerations, such as material limitations, as well as the variable nature of solar input. In order to compensate for changes in solar input, two operational strategies are identified and analyzed. The first is hybridization, meaning the solar input is supplemented via the combustion of fossil fuels. Hybridization often allows for an increase in net power and efficiency by adding heat during periods of low solar thermal input. An alternative strategy is to make use of variable guide vanes on the compressor of the gas turbine engine, which schedule to change the air flow rate into the system. By altering the mass flow rate of air, and assuming a fixed level of heat addition, the operating temperature of the engine can be controlled to maximize power or efficiency. The paper examines how to combine hybridization with variable guide vane operation to optimize gas turbine performance over a wide range of solar thermal input, from zero solar input to solar-only operation. A large material constraint is posed by the combustor, and to address this concern two alternative strategies—one employing a bypass valve and the other a combustor modified to allow higher temperature inlet air—are presented. Combustor modifications could include new materials and/or increased cooling air. The two strategies (bypass versus no bypass) are compared on a thermodynamic basis. It is found that it is possible to operate the gas turbine across the entire range without a significant drop in performance in either design through judicious adjustment of the vanes, though both approaches yield different results for certain ranges of solar input. Finally, a yearly assessment of solar share and thermodynamic performance is presented for a 4.3 MWe gas turbine to identify the overall benefits of the operational strategies. The annualized thermodynamic performance is not appreciably different for the two strategies, so that other factors such as mechanical design, operational issues, economics, etc. must be used to decide the optimal approach.