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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

Parametric Thermodynamic Analysis of a Solid Oxide Fuel Cell Gas Turbine System Design Space

[+] Author and Article Information
Brian Tarroja, Jim Maclay, Jacob Brouwer

National Fuel Cell Research Center, University of California, Irvine, CA 92697-3550

Fabian Mueller1

National Fuel Cell Research Center, University of California, Irvine, CA 92697-3550fm@nfcrc.uci.edu

1

Corresponding author.

J. Eng. Gas Turbines Power 132(7), 072301 (Apr 08, 2010) (11 pages) doi:10.1115/1.4000263 History: Received January 19, 2009; Revised August 27, 2009; Published April 08, 2010; Online April 08, 2010

A parametric study of a solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system design is conducted with the intention of determining the thermodynamically based design space constrained by modern material and operating limits. The analysis is performed using a thermodynamic model of a generalized SOFC-GT system where the sizing of all components, except the fuel cell, is allowed to vary. Effects of parameters such as pressure ratio, fuel utilization, oxygen utilization, and current density are examined. Operational limits are discussed in terms of maximum combustor exit temperature, maximum heat exchanger effectiveness, limiting current density, maximum hydrogen utilization, and fuel cell temperature rise. It was found that the maximum hydrogen utilization and combustor exit temperature were the most significant constraints on the system design space. The design space includes the use of cathode flow recycling and air preheating via a recuperator (heat exchanger). The effect on system efficiency of exhaust gas recirculation using an ejector versus using a blower is discussed, while both are compared with the base case of using a heat exchanger only. It was found that use of an ejector for exhaust gas recirculation caused the highest efficiency loss, and the base case was found to exhibit the highest overall system efficiency. The use of a cathode recycle blower allowed the largest downsizing of the heat exchanger, although avoiding cathode recycling altogether achieved the highest efficiency. Efficiencies in the range of 50–75% were found for variations in pressure ratio, fuel utilization, oxygen utilization, and current density. The best performing systems that fell within all design constraints were those that used a heat exchanger only to preheat air, moderate pressure ratios, low oxygen utilizations, and high fuel utilizations.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 2

Fuel cell efficiency versus current density for various fuel utilization conditions, no cathode gas recirculation, constant pressure ratio of 4 and oxygen utilization of 15%

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Figure 3

System efficiency versus current density for various fuel utilization conditions showing applicable constraints (shaded areas)

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Figure 4

System efficiency versus current density for various oxygen utilization conditions, constant pressure ratio of 4, fuel utilization of 0.85, and no cathode off-gas recirculation

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Figure 5

Cathode air flow temperature rise versus current density for various oxygen utilization conditions

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Figure 6

System efficiency versus pressure ratio for various oxygen utilization conditions

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Figure 7

System efficiency versus cathode air flow temperature rise for various fuel utilization conditions—thermal conductance model

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Figure 8

System efficiency versus fuel cell operating temperature for various cathode temperature difference conditions

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Figure 9

System efficiency versus recirculation ratio (ejector) for various oxygen utilization conditions

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Figure 10

Air preheater effectiveness versus ratio of recirculation (ejector) versus oxygen utilization

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Figure 11

System efficiency versus recirculation ratio (blower) for various oxygen utilization conditions

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Figure 1

Pure hydrogen SOFC/GT hybrid system diagram

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Figure 12

Air preheater effectiveness versus ratio of recirculation (blower) versus oxygen utilization

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