Mikhail Granovskii

Exergy Analysis of a Gas Turbine Cycle With Steam Generation for Methane Conversion Within Solid Oxide Fuel Cells

The combination of fuel cells with conventional mechanical power generation technologies (heat engines) promotes effective transformation of the chemical energy of fuels into electrical work. The implementation of solid oxide fuel cells (SOFCs) within gas turbine systems powered by natural gas (methane) requires an intermediate step of methane conversion to a mixture of hydrogen and carbon monoxide. State-of-the-art Ni-YSZ (yttria-stabilized zirconia) anodes permit methane conversion directly on anode surfaces, and contemporary designs of SOFC stacks allow this reaction to occur at elevated pressures. An exergy analysis of a gas turbine cycle integrated with SOFCs with internal reforming is conducted. As the efficiency of a gas turbine cycle is mainly defined by the maximum temperature at the turbine inlet, this temperature is fixed at 1573 K for the analysis. In the cycle considered, the high-temperature gaseous flow from the turbine heats the input flows of natural gas and air, and is used to generate pressurized steam, which is mixed with natural gas at the SOFC stack inlet to facilitate its conversion. This technological design permits avoidance of the generally accepted recirculation of the reaction products around the anodes of SOFCs, which reduces the capacity of the SOFC stack and the entire combined power generation system correspondingly. At the same time, the thermal efficiency of the combined cycle is shown to remain high and reach 65-85% depending on the SOFC stack efficiency. The thermodynamic efficiency of the SOFC stack is defined as the ratio of electrical work generated to the methane oxidized (through the intermediate conversion). For a given design and operating condition of the SOFC stack, an increase in the thermodynamic efficiency of a SOFC is attained by increasing the fuel cell active area. Achieving the highest thermodynamic efficiency of the SOFC stack leads to a significant and nonproportional increase in the stack size and cost. For the proposed steam generating scheme, increasing the load of the SOFC stack is accompanied by a decrease in steam generation, a reduction in the steam to methane ratio at the anode inlet, and an increased possibility of catalyst coking. Accounting for these factors, the range of appropriate operating conditions of the SOFC stack in combination with steam generation within a gas turbine cycle is presented.

Exergy and industrial ecology: an application to an integrated energy system

Exergy analysis can help integrate separate technologies following the principles of industrial ecology. An application of exergy analysis to calculate depletion numbers, which relate exergy destruction and total exergy use, is demonstrated for a gas turbine cycle combined with a hydrogen generation unit. The design includes a Solid Oxide Fuel Cell (SOFC) with internal natural gas reforming and a Membrane Reactor (MR) in place of a combustion chamber. The depletion number for the separate technologies is found to be more than two times greater than for the combined system, implying the latter is more environmentally benign and like an ecosystem.

Economic Aspects of Greenhouse Gas Emissions Reduction by Utilisation of Wind and Solar Energies to Produce Electricity and hydrogen

Implementation of renewable wind and solar energy sources instead of fossil fuels to produce such energy carriers as electricity and hydrogen facilitates reductions in gaseous emissions. Unlike for traditional fossil fuel technologies, greenhouse gas emissions from renewable technologies are associated mainly with the construction of a power plant. With present costs of wind and solar electricity, it is shown that, when electricity from renewable sources replaces electricity from natural gas, the cost of greenhouse gas emissions abatement is about four times less than the cost if hydrogen from renewable sources replaces hydrogen produced from natural gas. When renewable-based hydrogen is used instead of gasoline in a fuel cell vehicle, the cost of greenhouse gas emissions reduction approaches the same value as for renewable-based electricity only if the fuel cell vehicle efficiency exceeds significantly (i.e., by about two times) that of an internal combustion vehicle. It is also shown that when 6000 turbines (Kenetech KVS-33) with a capacity 350 kW and a capacity factor of 24% replace a 500-MW gas-fired power generation plant with an efficiency of 40%, annual greenhouse gas emissions are reduced by 2.3 megatons. The incremental additional annual cost is about

Exergetic Performance Analysis of a Gas Turbine Cycle Integrated With Solid Oxide Fuel Cells

280 million (US). The results provide a useful approach to an optimal strategy for greenhouse gas emissions mitigation.

Economic and environmental comparison of conventional, hybrid, electric and hydrogen fuel cell vehicles

This paper deals with an exergetic performance analysis of a gas turbine cycle integrated with SOFCs with internal reforming. As the efficiency of a gas turbine cycle is mainly defined by the maximum temperature at the turbine inlet, this temperature is fixed at 1573 K for the analysis. In the cycle considered, the high-temperature gaseous flow from the turbine heats the input flows of natural gas and air, and is used to generate pressurized steam which is mixed with natural gas at the SOFC stack inlet to facilitate its conversion. The application of SOFCs provides the opportunity to reduce the exergy losses of the most irreversible process in the system: fuel combustion. Depending on the SOFC stack efficiency, the energy efficiency of the combined cycle reaches 70–80% which compares well to the efficiencies of 54–55% typical of conventional combined power generation cycles. Parametric studies are also undertaken to investigate how energy and exergy efficiencies of the integrated system change with variations in operating conditions. An increase in the efficiency of SOFCs is attained by increasing the fuel cell active area. Achieving the highest efficiency of the SOFC stack leads to a significant and non-proportional increase in the stack size and cost, and simultaneously to a decrease in steam generation, reducing the steam/methane ratio at the anode inlet and increasing the possibility of catalyst coking. Accounting for these factors, likely operating conditions of the SOFC stack in combination with a gas turbine cycle are presented.© 2007 ASME