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Research Papers

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

[+] Author and Article Information
Kyle Kitzmiller

Department of Mechanical Engineering,Combustion and Solar Energy Laboratory,  San Diego State University, San Diego, CA 92182

Fletcher Miller

Department of Mechanical Engineering,Combustion and Solar Energy Laboratory,  San Diego State University, San Diego, CA 92182Fletcher.Miller@sdsu.edu

In Mode 2, sufficient fuel could theoretically be added to raise the temperature of the gas to the design point TIT. However, this would defeat the point of the combustor bypass, since the temperature of the air entering the combustor would be above the maximum value.

J. Sol. Energy Eng 134(4), 041008 (Aug 06, 2012) (12 pages) doi:10.1115/1.4006894 History: Received October 29, 2011; Revised February 23, 2012; Published August 06, 2012; Online August 06, 2012

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.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Recuperated gas turbine cycle with solar receiver and combustor. Note optional combustor bypass.

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Figure 2

Design point efficiency of the recuperated gas turbine engine as a function of pressure ratio

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Figure 12

Efficiency for the operational strategy shown in Fig. 1 for the recuperated gas turbine engine

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Figure 13

Net power output, solar thermal input and fossil fuel input over a summer day for a solar field multiplier of 1.25 for an inline combustor without a bypass

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Figure 14

Net power output, solar thermal input and fossil fuel input over a summer day for a solar field multiplier of 1.25 for an inline combustor with a bypass

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Figure 15

Program flow chart for solving off-design performance of a solar powered recuperated gas turbine

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Figure 3

Off-design efficiency of the recuperated gas turbine engine as a function of VGV angle with constant TIT

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Figure 4

Efficiency of the recuperated gas turbine engine as a function of solar thermal input for two systems, one with no bypass (i.e., no CIT limit), and one with a bypass

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Figure 5

Net power output of the recuperated gas turbine engine as a function of solar thermal input for two systems, one with no bypass (i.e., no CIT limit), and one with a bypass. Also shown are the combustor and turbine inlet temperatures for the bypass case.

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Figure 6

Fossil fuel input of the recuperated gas turbine engine as a function of solar thermal input for two engines, one with a combustor bypass and one without

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Figure 7

Efficiency of the recuperated gas turbine engine without a combustor bypass as a function of solar thermal input for various angles of the variable guide vanes (0 deg shown by the dark black line)

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Figure 8

Net power of the recuperated gas turbine engine without a combustor bypass as a function of solar thermal input for various angles of the variable guide vanes (0 deg shown by the dark black line)

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Figure 9

Efficiency of the recuperated gas turbine engine with a combustor bypass (combustor inlet temperature limited to 850 °C) as a function of solar thermal input for various angles of the variable guide vanes (0 deg shown by the dark black line)

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Figure 10

Net power of the recuperated gas turbine engine with a combustor bypass (combustor inlet temperature limited to 850 °C) as a function of solar thermal input for various angles of the variable guide vanes (0 deg shown by black line)

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Figure 11

Example operational strategy for the recuperated gas turbine engine to keep output power above 3.7 MWe

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