The continuing increase of greenhouse gases in our atmosphere related to the utilization of fossil energy resources leads to environmental degradation and consequential changes in weather patterns. To reduce the environmental impact of electric power generation, highly efficient and environmentally sensitive energy conversion systems are imperative. Highly efficient electric power systems are crucial for the reduction of carbon emissions from fossil fuels, as well as, in a circular economy where renewable fuels like biogas and/or hydrogen need to be converted to electricity in an efficient manner to maximize resource utilization. Solid oxide fuel cell&nbsp;(SOFC)-gas turbine&nbsp;(GT)&nbsp;hybrid systems have shown to reach exceptional efficiencies of greater than 75%-LHV and present one of the most promising technologies for the future.In this work SOFC-GT hybrid systems are studied in the context of their thermodynamic and economic performance. Before the system performance is evaluated, the SOFC cell design is optimized with respect to thermal gradients and costs. The influence of 9 cell design parameters, such as cell length, rib width and interconnect thickness, and 3 operating parameters, such as degree of pre-reforming, air utilization and operating voltage, are investigated to establish the specific SOFC stack cost on a $2019/kWDC basis. The cost optimum for the cell is at a cell length of 15-20&nbsp;cm and thermal gradients are reduced in shorter cells. Moreover, cell design parameters are identified that synergistically reduce thermal gradients and specific SOFC stack cost.Using this optimized cell design, the SOFC stack is integrated into a SOFC-GT hybrid system. The SOFC-GT hybrid system is investigated at steady state operating condition over a wide range of pressure conditions, fuel utilization factors and operating voltages under consideration of local thermal cell gradients and overall cell temperature differences. The thermodynamic evaluation shows that the highest efficiencies are reached at moderate pressure ratios around 5&nbsp;bar and high operating voltages (0.85&nbsp;V); however, the lowest cost of electricity is obtained at a lower operating voltage of 0.82V and a fuel utilization factor of 0.85. The resulting cost of electricity is 72.60&nbsp;$/MWh.The above-mentioned results relate to an ideal/custom engineered GT to evaluate the full potential of the hybrid system. As of today, GTs are primarily offered for high energy density fuels like natural gas and will have to be operated in off-design when integrated into a SOFC-GT hybrid system. This raises concerns about the GT’s surge margin. A comparison of constant speed operation, variable speed operation and variable inlet guide vane control (for full load operation), shows that the highest surge margin is obtained in the variable inlet guide vane scenario. The variable speed operation shows the lowest surge margin; however, the variable speed operation reaches the highest system efficiency. The lowest cost of electricity is achieved in the constant speed operation scenario, which also has the highest net power output, resulting in a cost-of-electricity of 77.47&nbsp;$/MWh. An increase of 6.7% versus the hybrid system with ideal/custom engineered GT.

Thermo-Economic Analyses of Advanced Solid Oxide Fuel Cell-Gas Turbine Hybrid Systems

Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2021/volume/85109

ASME 2021 Power Conference (POWER2021)

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