Comparative Evaluation of SOFC/Gas Turbine Hybrid System Options

Solid Oxide Fuel Cell [SOFC]/Gas Turbine [GT] hybrid power systems can synergistically exploit the high operating temperature and high electrical generation efficiency of the solid oxide fuel cell and the high power density and simplicity of the gas turbine engine generator. Continued studies at Siemens Westinghouse seek practical system configurations with commercialization potential. Pressurized SOFC [PSOFC]/GT system concepts [directly heated Brayton cycles] can yield electrical power generation at the highest efficiency [circa 70%] {net ac/LHV} with concomitant complexity in configuration, operation and installation. Indirectly heated Brayton cycles utilizing an atmospheric pressure SOFC [ASOFC] can achieve a more modest electrical power generation efficiency [circa 55%] with considerably less complexity. Co-firing of the GT combustor to yield state-of-the-art [SOA] turbine inlet temperature [TIT] can most fully exploit the capability of SOA turbine technology yielding a hybrid system of lesser efficiency, but also of lesser cost ($/kWe). The ideal gas turbine and/or system configuration remains elusive however. Recent studies have focused on the indirectly heated cycle wherein the gas turbine exhaust serves directly as the SOFC oxidant. Consequently, the GT exhaust flow rate and temperature must be compatible with SOFC generator inlet requirements. This compatibility can be difficult to achieve with a state-of-the-art micro gas turbine generator [MTG] that typically operates with relatively low pressure ratio [3 to 4]. Alternatives ranging from the ideal GT to system level feature additions allowing SOA GTs have been analyzed. These alternatives are identified and discussed, and results of a comparative performance and cost evaluation are reviewed.