Abstract

The goals of a research program recently completed at the University of California, Irvine were to develop analysis strategy for Solid Oxide Fuel Cell (SOFC) based systems, to apply the analysis strategy to tubular SOFC hybrid systems and to identify promising hybrid configurations. A pressurized tubular SOFC combined with an intercooled-reheat gas turbine (SureCell™ cycle) is chosen as the Base Cycle over which improvements are sought. The humid air turbine (HAT) cycle features are incorporated to the Base Cycle resulting in the SOFC-HAT hybrid cycle which shows an efficiency of 69.05% while the Base Cycle has an efficiency of 66.23%. Exergy analysis identified the superior efficiency performance of the SOFC component. Therefore, an additional cycle variation added a second SOFC component followed by a low pressure combustor in place of the reheat combustor of the gas turbine of the SOFC-HAT hybrid. The resulting Dual SOFC-HAT hybrid has a thermal efficiency of 75.98%. The Single SOFC-HAT hybrid gives the lowest cost of electricity (3.54¢/kW-hr) while the Dual SOFC-HAT hybrid has the highest cost of electricity (4.02¢/kW-hr) among the three cycles with natural gas priced at $3/GJ. The Dual SOFC-HAT hybrid plant cost is calculated to be significantly higher because the fraction of power produced by the SOFC(s) is significantly higher than that in the other cases on the basis of $1100/kw initial cost for the SOFC. The Dual SOFC-HAT hybrid can only be justified in favor of the Single SOFC-HAT hybrid when price of natural gas is greater than $14/GJ or if a severe carbon tax on the order of $180/ton of CO2 is imposed while natural gas price remains at $3/GJ.

Highlights

  • The majority of electricity in the U.S is generated by the combustion of fossil fuels to heat either steam or "air" for use in Rankine and Brayton cycles

  • The specific power output defined as the net power developed by the cycle per unit of air entering the system is significantly increased by combining the Solid Oxide Fuel Cell (SOFC) with the humid air turbine (HAT) cycle; as much as a 46% increase is realized

  • One variation considered applies humid air turbine (HAT) cycle features to an SOFC hybrid design

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Summary

Introduction

The majority of electricity in the U.S is generated by the combustion of fossil fuels to heat either steam or "air" for use in Rankine and Brayton cycles. The Humid Air Turbine (HAT) cycle (Rao, 1989) which utilizes generation of "steam" by directly contacting pressurized air with hot water in a counter-current humidifier and circulating the water leaving the humidifier to recover heat rejected in the intercooler and from the stack gas could potentially be applied in this hybrid system to enhance the overall cycle efficiency.

Results
Conclusion

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