Thermodynamic sensitivity analysis of hybrid system based on solid oxide fuel cell

This paper mainly studied the solid oxide fuel cell (SOFC)–micro gas turbine (MGT) hybrid power system. The key parameters that greatly influence the overall system performance have been studied and optimized. The thermodynamic potential of improving the hybrid system performance by integrating SOFC with the advanced thermal cycle system is analyzed. The optimization rules of main parameters of SOFC-MGT hybrid power system with the turbine inlet temperature (TIT) of MGT as a constraint condition are revealed. The research results show that TIT is a key parameter that limits the electrical efficiency of hybrid power system. With the increase of the cell number, both the power generation efficiency of the hybrid cycle power system and TIT increase. Regarding the hybrid system with the fixed cell number, in order to get a higher electrical efficiency, the operating temperature of SOFC should be enhanced as far as possible. However, the higher operating temperature will result in the higher TIT. Increasing of fuel utilization factor is an effective measure to improve the performance of hybrid system. At the same time, TIT increases slightly. Both the electrical efficiency of hybrid power system and TIT reduce with the increase of the ratio of steam to carbon. The achievements obtained from this paper will provide valuable information for further study on SOFC-MGT hybrid power system with high efficiency. Copyright © 2010 John Wiley & Sons, Ltd.

Thermodynamic assessment of integrated biogas-based micro-power generation system

The thermal-economic optimization of a combined cycle power plant (CCPP) which can provide 140 MW of electrical power is discussed in this paper. The CCPP is composed of a gas turbine cycle (including, air compressor, combustion chamber, gas turbine), heat recovery steam generator (HRSG), steam turbine, condenser system, and a pump. The design parameters of such a plant are compressor pressure ratio (rAC), compressor isentropic efficiency (ηAC) gas turbine isentropic efficiency (ηGT), and turbine inlet temperature (T3), pinch difference temperature (ΔTpinch), steam turbine inlet temperature (Ta), steam turbine isentropic efficiency (ηST), and pump isentropic efficiency (ηPUMP). The objective function was the total cost of the plant in terms of dollar per second, including sum of the operating cost related to the fuel consumption, and the capital investment for equipment purchase and maintenance costs. The optimal values of decision variables were obtained by minimizing the objective function using sequential quadratic programming (SQP). The effects of change in the demanded power and fuel price on the design parameters werestudied for, 100, 120, and 140MW of net power output.

This paper presents the thermodynamic study of a heat and power system which combines an organic Rankine cycle and a gas turbine (GT) cycle. The required power of the compressor in the GT cycle and pump in the Rankine cycle is provided by the WT which according to the amount of cycle required power. For analysis of the cycle, a simulation has been performed using R123 as the working fluid in the Rankine cycle and air and combustion products in the GT cycle. In this end, effect of various parameters such as wind speed, the angular speed of WT, and the gas turbine inlet temperature as well as the compressor pressure ratio, gas turbine isentropic efficiency, condenser temperature and compressor isentropic efficiency on the total thermal efficiency and total exergy efficiency is calculated and analyzed. Thermal efficiency and exergy efficiency of 21.31% and 23.54% is obtained. Also, it is observed that the greatest exergy destruction occurs in the combustion chamber.

SummaryA design study was undertaken of a 100 kW natural gas solid oxide fuel cell (SOFC)/gas turbine (GT) hybrid system to evaluate the impacts of the reforming process under a broad range of SOFC fuel utilizations. An equilibrium reformer model and a one‐dimensional SOFC model are employed to perform system study for this hybrid, while balance of plant model is built in Ebsion to simulate the associated performance of GT cycle. Fifty design cases with varied external fuel reforming temperatures are analyzed based on different SOFC fuel utilization levels. With the decrease of reforming temperature, optimal fuel utilization in terms of efficiency shift from 90% with external reforming to 60% with internal reforming. Highest efficiency up to 74% was achieved with internal reforming, but this came with 1275 K cathode inlet air temperature and 1673 K turbine inlet temperature, which no current fuel cell and few GTs can tolerate. High fuel pre‐reforming rates provided a higher design flexibility and showed opportunities of efficiency improvement with appropriate thermal integration strategy. And the feasibility of achieving high efficiencies in the hybrid system with thermally integrated external reforming will be explored in our future work.Novelty StatementBoth fuel reforming process and fuel utilization could affect the heat and energy balance in the hybrid gas solid oxide fuel cell (SOFC)‐gas turbine (GT) system. However, the optimal design condition considering different fuel reforming approaches and SOFC fuel utilization levels was not determined in the literature. In addition, appropriate integration strategies, used to balance the heat and power between the two subsystems (SOFC and GT), are imperative under different fuel reforming approaches and SOFC fuel utilization levels. But, the feasibility of those thermal integration strategies has not been examined in existing studies. This work studied the coupling effects of SOFC fuel utilization and fuel reforming process on system performance of a natural gas hybrid SOFC/GT cycle. Highest efficiency up to 74% was achieved with internal reforming, but this came with 1275 K cathode inlet air temperature and 1673 K turbine inlet temperature, which no current fuel cell and few GTs can tolerate. High fuel pre‐reforming rates provided a higher design flexibility and showed opportunities of efficiency improvement with appropriate thermal integration strategy.

In this paper an integrated gasification combined cycle (IGCC) with a high-temperature, pressurized solid oxide fuel cell (SOFC) in the topping cycle and a single-pressure, non-reheat steam bottoming cycle has been conceptualized and analysed. An energy analysis is done for this combined cycle. The study reveals that an overall efficiency above 54 per cent is achievable for this combined cycle, which also offers satisfactory environmental friendliness. A higher pressure ratio across the gas turbine is found to improve the overall efficiency of the combined cycle. The SOFC operating temperature is found to influence the performance of the combined cycle in different ways, depending on the pressure ratios.

High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas

In this paper the operation and performance of a high temperature solid oxide fuel cell (SOFC) stack on biomass syn-gas from a demonstration biomass gasification combined heat and power (CHP) plant is investigated. The objective of this work is to develop a computer simulation model of a biomass-SOFC CHP system, flexible enough for use in industry, capable of predicting system performance under various operating conditions and using diverse fuels. The biomass gasifier is of the dual fluidised bed (DFB) type with steam as the gasifying agent and is operated at atmospheric pressure. The tubular SOFC configuration, developed by Siemens Power Generation Inc (SPGI), is selected. It is considered to be the most advanced design and is approaching commercialisation. The SOFC stack model, developed using the chemical process flowsheet simulator Aspen Plus, is of equilibrium type and is based on Gibbs free energy minimisation. The SOFC model performs heat and mass balances and considers the ohmic, activation and concentration losses for the voltage calculation. Data available in the literature on the SPGI SOFC operating on natural gas is used to validate the model. The system model predicts thermodynamic condition and composition of all internal flow streams, the heat generated by the SOFC stack, voltage (V), current (I) and efficiency. Operating parameters are varied over a wide range, parameters such as fuel utilisation factor (Uf), current density (j) and steam to carbon ratio (STCR) have significant influence. The results indicate that there must be a trade-off between voltage, efficiency and power with respect to j and the SOFC stack should be operated at low STCR and high Uf, within certain limits. SOFC stack operation on biomass syn-gas is compared to operation on natural gas and as expected there is a drop in performance, which is attributed to increased input fuel and air flow due to the lower quality of the fuel gas. The optimum realistic operating conditions with regard to SOFC stack performance are identified. High electrical efficiencies are predicted making these systems very attractive for CHP applications.

In this paper parametric study of a gas turbine cycle model power plant with intercooler compression process was proposed. The power output and the efficiency are simulating with respect to the cycle temperatures and pressure ratio for a typical set of operating conditions. Simple gas turbine cycle calculations with realistic parameters are made and confirm that increasing the turbine inlet temperature no longer means an increase in cycle efficiency, but increases the work done. The analytical study is done to investigate the performance improvement by intercooling. The analytical formula for specific work and efficiency are derived and analyzed. The simulation results shows that increasing turbine inlet temperature and pressure ratio can still improve the performance of the intercooled gas turbine cycle. Key words: Gas turbine, power plant, performance, intercooler.

In conventional gas turbine systems combustion results in high exergy losses (∼30%) of fuel exergy input. Replacing the combustor with a high temperature fuel cell, like the Solid Oxide Fuel Cell (SOFC), will significantly reduce these exergy losses. As the SOFC electrochemically converts the natural gas, exergy losses are far lower (∼10%) compared to combustion. Natural gas entering a SOFC system has to be reformed first to hydrogen and carbon monoxide by steam reforming. Here it is chosen to use the heat generated by the fuel cell to drive the endothermic reforming reactions: internal reforming. The SOFC-GT system has the advantage that both fuel cell and gas turbine technology contribute to power production. In earlier work [1] several fuel cell system configurations with PEMFC, MCFC or SOFC, were analyzed studying the exergy flows. Here is focused on the SOFC-GT configuration, to get a detailed understanding of the exergy flows and losses through all individual components. Several configurations, combining the SOFC with the GT are possible. The selected operating conditions should prevent carbon deposition. Systems studies are performed to get more insight in the exergy losses in these combined systems. Exergy analysis facilitates the search for the high efficient SOFC-GT hybrid systems. Using exergy analysis, several useful configurations are found. Exergy losses are minimized by varying pressure ratio and turbine inlet temperature. Sensitivity studies, of equivalent cell resistance and fuel cell temperature, show that total system exergy efficiencies of more than 80% are conceivable, without using a bottoming cycle.

Combined cycles using Solid oxide fuel cells (SOFCs) are expected to provide very high efficiency. The SOFCs are combined with a Gas turbine (GT) or a Gas turbine combined cycle (GTCC). The major SOFC design parameters greatly impact the combined cycle efficiency because the SOFC still produces a majority of the electricity in the combined cycles. In this paper, the influence of the SOFC’s fuel utilization on the efficiency of the combined cycles is carried out using parametric analysis. It is demonstrated and validated that an optimal fuel utilization exists. Four types of SOFC/GT and SOFC/GTCC combined cycles are analyzed. Each combined cycle is found to have an optimal fuel utilization, which is always lower than that of the SOFC-alone system. The main reason is that the turbine inlet temperature rises and thus GT or GTCC power increases with decreasing fuel utilization because of the increased remaining fuel after the cell stack. The value decreases as the power share of the turbomachinery part increases. The peak efficiencies of the SOFC/GT and SOFC/GTCC were predicted to be over 72 % and 76 %. The optimal fuel utilization corresponding to peak efficiency was 0.8 for the SOFC-alone system, 0.7 for the SOFC/GT system, and 0.6 for the SOFC/GTCC system.

Comprehensive performance assessment of a novel biomass-based CCHP system integrated with SOFC and HT-PEMFC

The objective of this work is the development of a simulation model for a hybrid Solid Oxide Fuel Cell (SOFC)/Micro Gas Turbine (MGT) system, flexible and robust enough, capable to predict the system performance under various operating conditions. The hybrid system consists of a high temperature SOFC, based on a tubular configuration developed by Siemens Power Generation Inc, and a recuperated small gas turbine (GT) validated using data for the Capstone C30. The design and off-design performance of the system is examined by means of performance maps. Moreover, operating parameters such as fuel utilization factor, steam to carbon ratio and current density are varied over a wide range and the influence on system performance is studied. The optimum operating conditions are discussed with regard to overall system performance under part load operation. The results show that high electrical efficiencies can be achieved making these systems appropriate for distributed generation applications.

In recent years improved gas turbine performance through developments in high temperature materials and blade cooling methods has made a positive impact on the combined cycle performance. Transpiration cooling technique has emerged as the most promising technique to improve the gas turbine cycle performance by allowing higher turbine inlet temperatures. This paper concentrates on improving the combined cycle performance by allowing higher turbine inlet temperatures (TIT) using transpiration cooling of gas turbine blades. A four-stage advanced gas turbine coupled with the dual pressure steam bottoming cycle is considered for the performance of combined cycle. Realistic input parameters used in current industrial practice have been considered for this study. The effects of variation of TIT on the performances of topping, bottoming and combined cycle are presented and discussed. For the combined cycle with transpiration cooled gas turbine an increase in TIT from 1600 K to 1800 K exhibits the combined cycle efficiency increase by 2.37 percent and the combined specific work increases by 185.42 kJ/kg. The results indicate that at a TIT of 1800 K the achievable efficiency of combined cycle with transpiration cooled gas turbine is 59.97 percent.

Despite being the most efficient and quiet operation type of fuel cells, solid oxide fuel cells (SOFCs) deal with several constraints in terms of fabrication cost, material selection and durability issues due to their high operating temperature. The high operating temperature of SOFCs limits their stationary and large-scale applications. Moreover, these constraints restrict the commercialization of portable SOFCs. Therefore, the operation temperature of SOFCs must be reduced to overcome the aforementioned problems. However, this task is challenging because the operation temperature mainly affects the material preparation and the stack design to produce the electrical power needed for small-scale applications. This paper provides an overview of the challenges faced by each component such as the materials, the design of stack, fabrication cost and related research in fabricating high power SOFC stacks.