Thermo-economic modeling of a solid oxide fuel cell/gas turbine power plant with semi-direct coupling and anode recycling

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency in order to improve system efficiencies and economics. The SOFC system is semi-directly coupled to the gas turbine power plant, with careful attention paid to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 21.6 MW at 49.2% efficiency. The model also predicts a breakeven per-unit energy cost of USD 4.70 ¢/kWh for the hybrid system based on futuristic mass generation SOFC costs. Results show that SOFCs can be semi-directly integrated into existing GT power systems to improve their thermodynamic and economic performance.     

Study on a novel solid oxide fuel cell/gas turbine hybrid cycle system with CO2 capture

A novel solid oxide fuel cell (SOFC)/gas turbine (GT) hybrid cycle system with CO₂ capture is proposed based on a typical topping cycle SOFC/GT hybrid system. The H₂ gas is separated from the outlet mixture gas of SOFC1 anode by employing the advanced ceramic proton membrane technology, and then, it is injected into SOFC2 to continue a new electrochemical reaction. The outlet gas of SOFC1 cathode and the exhaust gas from SOFC2 burn in the afterburner 1. The combustion gas production of the afterburner1 expands in the turbine 1. The outlet gas of SOFC1 anode employs the oxy‐fuel combustion mode in the afterburner 2 after H₂ gas is separated. Then, the combustion gas production expands in the turbine 2. To ensure that the flue gas temperature does not exceed the maximum allowed turbine inlet temperature, steam is injected into the afterburner 2. The outlet gas of the afterburner 2 contains all the CO₂ gas of the system. When the steam is removed by condensation, the CO₂ gas can be captured. The steam generated by the waste heat boiler is used to drive a refrigerator and make CO₂ gas liquefied at a lower temperature. The performance of the novel quasi‐zero CO₂ emission SOFC/GT hybrid cycle system is analyzed with a case study. The effects of key parameters, such as CO₂ liquefaction temperature, hydrogen separation rate, and the unit oxygen production energy consumption on the new system performance, are investigated. Compared with the other quasi‐zero CO₂ emission power systems, the new system has the highest efficiency of around 64.13%. The research achievements will provide the valuable reference for further study of quasi‐zero CO₂ emission power system with high efficiency. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermo-economic modeling of an indirectly coupled solid oxide fuel cell/gas turbine hybrid power plant

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10 MW gas turbine power plant, operating at 30% efficiency, in order to improve system efficiencies and economics. The SOFC system is indirectly coupled to the gas turbine power plant, paying careful attention to minimize the disruption to the GT operation. A thermo-economic model is developed for the hybrid power plant, and predicts an optimized power output of 20.6 MW at 49.9% efficiency. The model also predicts a break-even per-unit energy cost of USD 4.65 ¢ kWh⁻¹ for the hybrid system based on futuristic mass generation SOFC costs. This shows that SOFCs may be indirectly integrated into existing GT power systems to improve their thermodynamic and economic performance.     

Exergy analysis of a cogeneration plant using coal gasification and solid oxide fuel cell

This paper presents exergy analysis of a conceptualized combined cogeneration plant that employs pressurized oxygen blown coal gasifier and high‐temperature, high‐pressure solid oxide fuel cell (SOFC) in the topping cycle and a bottoming steam cogeneration cycle. Useful heat is supplied by the pass‐out steam from the steam turbine and also by the steam raised separately in an evaporator placed in the heat recovery steam generator (HRSG). Exergy analysis shows that major part of plant exergy destruction takes place in gasifier and SOFC while considerable losses are also attributed to gas cooler, combustion chamber and HRSG. Exergy losses are found to decrease with increasing pressure ratio across the gas turbine for all of these components except the gas cooler. The fuel cell operating temperature influences the performance of the equipment placed downstream of SOFC. Copyright © 2005 John Wiley & Sons, Ltd.     

Thermo-economic optimization of an indirectly coupled solid oxide fuel cell/gas turbine hybrid power plant

Power generation using gas turbine (GT) power plants operating on the Brayton cycle suffers from low efficiencies, resulting in poor fuel to power conversion. A solid oxide fuel cell (SOFC) is proposed for integration into a 10-MW GT power plant, operating at 30% efficiency, in order to improve system efficiencies and economics. The SOFC system is indirectly coupled to the GT, in order to minimize the disruption to the GT operation. A thermo-economic model is developed to simulate the hybrid power plant and to optimize its performance using the method of Lagrange Multipliers. It predicts an optimized power output of 18.9 MW at 48.5% efficiency, and a breakeven per-unit energy cost of USD 4.54 ¢ kW h⁻¹ for the hybrid system based on futuristic mass generation SOFC costs.     

Comparative analysis of power augmentation in air bottoming cycles

The air bottoming cycle (ABC) is a proposed plant configuration in which the steam turbine bottoming cycle in conventional combined power plants is replaced by another gas turbine cycle. Nevertheless, ABC's relatively low efficiency reduces the likelihood of having an ABC power plant in the near future. In this research work, steam injection in the topping cycle combustion chamber and supplementary firing are recommended to improve ABC's performance. Three different configurations of ABC, including simple ABC, ABC with steam injection, and ABC with supplementary firing, are investigated. A thermo-economic analysis is performed to study the effects of the proposed power augmentation approaches thermodynamically and economically. Moreover, optimisation is carried out with the objective of minimising the total cost of the plant for different configurations. Furthermore, a multi-objective optimisation is performed and the results are presented to further understand the trade-off between higher efficiency and lower operating cost.     

Thermodynamic assessment of advanced SOFC-blade cooled gas turbine hybrid cycle

This paper focuses on novel integration of high temperature solid oxide fuel cell coupled with recuperative gas turbine (with air-film cooling of blades) based hybrid power plant (SOFC-blade cooled GT). For realistic analysis of gas turbine cycle air-film blade cooling technique has been adopted. First law thermodynamic analysis investigating the combine effect of film cooling of blades, SOFC, applied to a recuperated gas turbine cycle has been reported. Thermodynamic modeling for the proposed cycle has been presented. Results highlight the influence of film cooling of blades and operating parameters of SOFC on various performance of SOFC-blade cooled GT based hybrid power plant. Moreover, parametric investigation has also been done to examine the effect of compressor pressure ratio, turbine inlet temperature, on hybrid plant efficiency and plant specific work. It has been found that on increasing turbine inlet temperature (TIT) beyond a certain limit, the efficiency of gas turbine starts declining after reaching an optimum value which is compensated by continuous increase in SOFC efficiency with increase in operating temperature. The net result is higher performance of hybrid cycle with increase in maximum cycle temperature. Furthermore, it has been observed that at TIT 1600 K and compression ratio 20, maximum efficiency of 73.46% can been achieved.     

Effects of operating and design parameters on the performance of a solid oxide fuel cell–gas turbine system

We present a steady‐state thermodynamic model of a hybrid solid oxide fuel cell (SOFC)–gas turbine (GT) cycle developed using a commercial process simulation software, AspenPlus?. The hybrid cycle model incorporates a zero‐dimensional macro‐level SOFC model. A parametric study was carried out using the developed model to study the effects of system pressure, SOFC operating temperature, turbine inlet temperature, steam‐to‐carbon ratio, SOFC fuel utilization factor, and GT isentropic efficiency on the specific work output and efficiency of a generic hybrid cycle with and without anode recirculation. The results show that system pressure and SOFC operating temperature increase the cycle efficiency regardless of the presence of anode recirculation. On the other hand, the specific work decreases with operating temperature. Overall, the model can successfully capture the complex performance trends observed in hybrid cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

A critical assessment of thermo‐economic analyses of different air bottoming cycles for waste heat recovery

Steam turbine cycle's low operating temperature makes it suitable for waste heat recovery applications. Even though conventional combined cycles, ie, topping gas turbine and bottoming steam turbine cycles, are thermodynamically efficient, they are not the most economical alternatives for power generation with capacities less than 50 MWe. A recently proposed alternative is to utilize a bottoming gas turbine cycle in form of an air bottoming cycle. In this study, an overview of air bottoming cycle is presented. Based on the discussed studies, it is decided to further evaluate the merits of water injection in the bottoming cycle air stream by using either a humidifier or an air saturator. Thermo‐economic analysis and optimization are performed to evaluate simple and water injected air bottoming cycles against steam bottoming cycles. Results indicate that conventional combined cycles can achieve the highest thermal efficiency of about 48%. While water injected air bottoming cycle with air saturator is the most cost effective combined cycle configuration and most efficient air bottoming cycle with levelized cost of electricity and energy efficiency of 64.41 US$/MWh and 39%–40%, respectively, followed by the water injected air bottoming cycle with humidifier and simple air bottoming cycle with reported levelized cost of electricity of 65.75 US$/MWh, 66.36 US$/MWh, respectively. Steam bottoming cycle has the highest levelized cost of electricity of 68.88 US$/MWh.     

BCHP design for dual phase medical complex

Comprehensive value engineering conducted for a Midwest (USA) developer client focused upon their architect/engineer proposals for a nominal 100,000 m² medical facility comprising under Phase I, two 12 storey medical office towers and adjacent six (6) storey hospital. Client's architect-engineers had originally proposed constructing separate central heating and cooling plants for each of their (i.e., originally designed) Phase I buildings. Phase I plant layout and interim study results demonstrated significant building space and annual owning savings over conventionally designed individual (i.e., in situ) building heating and cooling plant and incorporated gas and steam turbine driven chillers and integrated gas turbine driven synchronous generator resulting in an estimated 2.6 year simple payback. However, subsequent client Phase II project scope expansion undertaken prior to commencing above referenced Phase I BCHP plant construction required major design changes. The subsequent Phase II redesign resulted in the elimination of higher cost heat recovery steam generator which was replaced with a low cost novel hybrid steam generator utilizing a non-toxic, hot-oil energy recovery system; replacement of costly condensing steam turbine with a less expensive combination serial back pressure steam turbine and indirect single stage absorption chiller bottoming cycle; the later sized for the additional Phase II new office tower subsequently required by client all without exceeding Phase I project budgetary criteria.     

Novel and optimal integration of SOFC-ICGT hybrid cycle: Energy analysis and entropy generation minimization

Integrating fuel cells with conventional gas turbine based power plant yields higher efficiency, especially solid oxide fuel cell (SOFC) with gas turbine (GT). SOFCs are energy efficient devices, performance of which are not limited to Carnot efficiency and considered as most promising candidate for thermal integration with Brayton cycle. In this paper, a novel and optimal thermal integration of SOFC with intercooled-recuperated gas turbine has been presented. A thermodynamic model of a proposed hybrid cycle has been detailed along with a novelty of adoption of blade cooled gas turbine model. On the basis of 1st and 2nd law of thermodynamics, parametric analysis has been carried out, in which impact of turbine inlet temperature and compression ratio has been observed on various output parameters such as hybrid efficiency, hybrid plant specific work, mass of blade coolant requirement and entropy generation rate. For optimizing the system performance, entropy minimization has been carried out, for which a constraint based algorithm has been developed. The result shows that entropy generation of a proposed hybrid cycle first increases and then decreases, as the turbine inlet temperature of the cycle increases. Furthermore, a unique performance map has also been plotted for proposed hybrid cycle, which can be utilized by power plant designer. An optimal efficiency of 74.13% can be achieved at TIT of 1800 K and r_p,c 20.     

An integrated power generation system combining solid oxide fuel cell and oxy-fuel combustion for high performance and CO2 capture

An integrated power generation system combining solid oxide fuel cell (SOFC) and oxy-fuel combustion technology is proposed. The system is revised from a pressurized SOFC-gas turbine hybrid system to capture CO₂ almost completely while maintaining high efficiency. The system consists of SOFC, gas turbine, oxy-combustion bottoming cycle, and CO₂ capture and compression process. An ion transport membrane (ITM) is used to separate oxygen from the cathode exit air. The fuel cell operates at an elevated pressure to facilitate the use of the ITM, which requires high pressure and temperature. The remaining fuel at the SOFC anode exit is completely burned with oxygen at the oxy-combustor. Almost all of the CO₂ generated during the reforming process of the SOFC and at the oxy-fuel combustor is extracted from the condenser of the oxy-combustion cycle. The oxygen-depleted high pressure air from the SOFC cathode expands at the gas turbine. Therefore, the expander of the oxy-combustion cycle and the gas turbine provides additional power output. The two major design variables (steam expander inlet temperature and condenser pressure) of the oxy-fuel combustion system are determined through parametric analysis. There exists an optimal condenser pressure (below atmospheric pressure) in terms of global energy efficiency considering both the system power output and CO₂ compression power consumption. It was shown that the integrated system can be designed to have almost equivalent system efficiency as the simple SOFC-gas turbine hybrid system. With the voltage of 0.752 V at the SOFC operating at 900 °C and 8 bar, system efficiency over 69.2% is predicted. Efficiency penalty due to the CO₂ capture and compression up to 150 bar is around 6.1%.     

Exergy based performance analysis of a solid oxide fuel cell and steam injected gas turbine hybrid power system

This paper presents exergy analysis of a hybrid solid oxide fuel cell and gas turbine (SOFC/GT) system in comparison with retrofitted system with steam injection. It is proposed to use hot gas turbine exhaust gases heat in a heat recovery steam generator to produce steam and inject it into gas turbine. Based on a steady-state model of the processes, exergy flow rates are calculated for all components and a detailed exergy analysis is performed. The components with the highest proportion of irreversibility in the hybrid systems are identified and compared. It is shown that steam injection decreases the wasted exergy from the system exhaust and boosts the exergetic efficiency by 12.11%. Also, 17.87% and 12.31% increase in exergy output and the thermal efficiency, respectively, is demonstrated. A parametric study is also performed for different values of compression pressure ratio, current density and pinch point temperature difference.     

High performance SOFC/GT combined power generation system with CO2 recovery by oxygen combustion method

The authors newly propose and investigate two types of carbon dioxide recovering SOFC/GT combined power generation systems in which a gas turbine with carbon dioxide recycle or water vapor injection is adopted as the bottoming cycle. In these systems, fuel gas is first introduced to a SOFC, and its exhaust fuel gas is afterburned by pure oxygen. Carbon dioxide or water vapor is also injected into the combustor to reduce the combustion gas temperature. The obtained combustion gas, which is composed of only carbon dioxide and water vapor, is introduced to a gas turbine in the bottoming cycle. The exhaust gas of the gas turbine preheats the injection gas, and then, carbon dioxide is separated by only cooling and water condensation. It is made clear that the overall efficiency of the system with carbon dioxide recycle reaches 63.87% (HHV) or 70.88% (LHV), and that of the system with water vapor injection reaches 65.00% (HHV) or 72.13% (LHV). These values are sufficiently high, indicating that the proposed systems are worth further research and development.     

Energetic and exergetic analysis of a hybrid combined‐nuclear power plant

Combined‐cycle power plants are currently preferred for new power generation plants worldwide. The performance of gas‐turbine engines can be enhanced at constant turbine inlet temperatures with the addition of a bottoming waste‐heat recovery cycle. This paper presents a study on the energy and exergy analysis of a novel hybrid Combined‐Nuclear Power Plant (HCNPP). It is thus interesting to evaluate the possibility of integrating the gas turbine with nuclear power plant of such a system, utilizing virtually free heat. The integration arrangement of the AP600 NPP steam cycle with gas turbines from basic thermodynamic considerations will be described. The AP600 steam cycle modifications to combine with the gas turbines can be applied to other types of NPP. A simple modeling of Alstom gas turbines cycle, one of the major combined‐cycle steam turbines manufacturers, hybridized with a nuclear power plant from energetic and exergetic viewpoint is provided. The Heat Recovery Steam Generator (HRSG) has single steam pressure without reheat, one superheater and one economizer. The thermodynamic parameters of the working fluids of both the gas and the steam turbines cycles are analyzed by modeling the thermodynamic cycle using the Engineering Equation Solver (EES) software. In case of hybridizing, the existing Alstom gas turbine with a pressurized water nuclear power plants using the newly proposed novel solution, we can increase the electricity output and efficiency significantly. If we convert a traditional combined cycle to HCNPP unit, we can achieve about 20% increase in electricity output. This figure emphasizes the significance of restructuring our power plant technology and exploring a wider variety of HCNPP solutions. Copyright © 2011 John Wiley & Sons, Ltd.     

Perspectives for the direct firing of biomass as a supplementary fuel in combined cycle power plants

An analysis of the performance of a gas turbine–steam turbine combined cycle with supplementary firing has been carried out. Natural gas is fired in the main combustor of the cycle, whereas biomass fuel is considered as the supplementary fuel. Although, supplementary firing is found to reduce the overall cycle efficiency, the low cost of biomass and the CO₂‐neutral attribute of its combustion reduce the specific fuel cost and specific CO₂ emission. The effects of pressure and temperature ratios of the topping cycle and main steam conditions of the bottoming cycle on the performance parameters of the combined cycle have been studied at different degrees of supplementary firing. The topping cycle temperature ratio is found to be the most critical parameter and its low value gives substantial advantages in lowering the fuel cost and CO₂ emission. Marginal advantages are also achieved at higher pressure ratio and better bottoming cycle main steam conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparative studies of augmentation in combined cycle power plants

The attractive features of a combined cycle (CC) power plant are fuel flexibility, operational flexibility, higher efficiency and low emissions. The performance of five gas turbine‐steam turbine (GT‐ST) combined cycle power plants (four natural gas based plants and one biomass based plant) have been studied and the degree of augmentation has been compared. They are (i) combined cycle with natural gas (CC‐NG), (ii) combined cycle with water injection (CC‐WI), (iii) combined cycle with steam injection (CC‐SI), (iv) combined cycle with supplementary firing (CC‐SF) and (v) combined cycle with biomass gasification (CC‐BM). The plant performance and CO₂ emissions are compared with a change in compressor pressure ratio and gas turbine inlet temperature (GTIT). The optimum pressure ratio for compressor is selected from maximum efficiency condition. The specific power, thermal efficiency and CO₂ emissions of augmented power plants are compared with the CC‐NG power plant at the individual optimized pressure ratios in place of a common pressure ratio. The results show that the optimum pressure ratio is increased with water injection, steam injection, supplementary firing and biomass gasification. The specific power is increased in all the plants with a loss in thermal efficiency and rise in CO₂ emissions compared to CC‐NG plant. Copyright © 2013 John Wiley & Sons, Ltd.     

Thermoeconomic modeling and parametric study of hybrid SOFC–gas turbine–steam turbine power plants ranging from 1.5 to 10 MWe

Detailed thermodynamic, kinetic, geometric, and cost models are developed, implemented, and validated for the synthesis/design and operational analysis of hybrid SOFC–gas turbine–steam turbine systems ranging in size from 1.5 to 10 MWe. The fuel cell model used in this research work is based on a tubular Siemens-Westinghouse-type SOFC, which is integrated with a gas turbine and a heat recovery steam generator (HRSG) integrated in turn with a steam turbine cycle. The current work considers the possible benefits of using the exhaust gases in a HRSG in order to produce steam which drives a steam turbine for additional power output. Four different steam turbine cycles are considered in this research work: a single-pressure, a dual-pressure, a triple pressure, and a triple pressure with reheat. The models have been developed to function both at design (full load) and off-design (partial load) conditions. In addition, different solid oxide fuel cell sizes are examined to assure a proper selection of SOFC size based on efficiency or cost. The thermoeconomic analysis includes cost functions developed specifically for the different system and component sizes (capacities) analyzed. A parametric study is used to determine the most viable system/component syntheses/designs based on maximizing total system efficiency or minimizing total system life cycle cost.     

A flexible model integration approach for evaluating tradeoffs between CO2 emissions and cost in solid oxide fuel cell‐based building energy systems

A simulation framework for flexible evaluation of various distributed building energy systems based on the integration of component device simulation models is presented. Device technology models were constructed for a solid oxide fuel cell (SOFC), a gas turbine, a double pipe heat exchanger, and a compressor. A scheme is proposed for defining model interfaces in order to improve the flexibility and accessibility of the models. Based on that scheme, interfaces are defined for each device model. The component device models are integrated to construct system models of (1) a hybrid system combining an SOFC and a gas turbine (SOFC/GT system) and (2) a stand‐alone SOFC system. The integrated model of the SOFC/GT system is then used to carry out a multi‐objective optimization in order to study the tradeoffs between cost and CO₂ emissions of the SOFC system operation for a given electricity demand. Through these analyses, the optimal configuration of the SOFC/GT system and the optimal operation conditions of the SOFC system for the given electricity demand were explored. Copyright © 2005 John Wiley & Sons, Ltd.     

Exergy and Energy Analysis of Combined Cycle systems with Different Bottoming Cycle Configurations

The paper deals with thermodynamic analysis of cooled gas turbine‐based gas‐steam combined cycle with single, dual, or triple pressure bottoming cycle configuration. The cooled gas turbine analyzed here uses air as blade coolant. Component‐wise non‐dimensionalized exergy destruction of the bottoming cycle has been quantified with the objective to identify the major sources of exergy destruction. The mass of steam generated in different configurations of heat recovery steam generator (HRSG) depends upon the number of steam pressure drums, desired pressure level, and steam temperature. For the selected set of operating parameters, maximum steam has been observed to be generated in the case of triple pressure HRSG = 19 kg/kg and minimum in single pressure HRSG = 17.25 kg/kg. Plant‐efficiency and plant‐specific works are both highest for triple‐pressure bottoming cycle combined cycle. Non‐dimensionalized exergy destruction in HRSG is least at 0.9% for B3P, whereas 1.23% for B2P, and highest at 3.2% for B1P illustrating that process irreversibility is least in the case of B3P and highest in B1P. Copyright © 2012 John Wiley & Sons, Ltd.