Performance implications of rapid load changes in carbonate fuel cell systems

The increasing energy demand along with the growing concern for the environmental issues requires a significant research effort on the development of energy systems with high efficiency and low environmental impact. The use of waste derived fuel in fuel cell systems provides the benefits of high efficiency and very low polluting emissions as well as an effective way of waste disposal. In the present work an investigation of the potential use of waste derived fuel gas in Molten Carbonate fuel cell systems has been performed. A detailed description of the simulation model developed by the authors with the Aspen Plus thermodynamic flow sheet code is presented. The basic configuration of the plant uses pure methane as fuel. An improved plant configuration has been proposed which allows an optimal exploitation of all the thermal streams giving as an output high electrical yield and heat at high temperature. The plant is then adapted to different gas blends as fuel: landfill gas and gas from the gasification of MSW (Municipal Solid Waste). The analysis is focused on the comparison between different fuel gases plants in terms of efficiency, feasibility and process requirements. Depending on the fuel characteristics, different solutions are applied to the plant in order to achieve optimal exploitation of the thermal streams. A general conclusion is that the use of low BTU gas results in an efficient and environmentally sound power generation but leads to a reduction of the plant net power output especially in the case of using gas derived from existing gasification plants. A consistent research activity is required to identify waste derived fuel processing systems that can yield gases which allow an optimal exploitation in fuel cell systems.

Early 1996 a three year (1996 - 1998) joint European project was launched under the name {open_quote}Advanced DIR-MCFC Development{close_quote}, aiming at the development of Direct Internal Reforming (DIR) Molten Carbonate Fuel Cell (MCFC) systems for cogeneration applications for the European market. In this project participate: Brandstofcel Nederland BV (BCN), British Gas pic (BG), Gaz de France (GDF), Netherlands Energy Research foundation (ECN), Stork, Royal Schelde and Sydkraft AB. The European Fuel Cell User Group (EFCUG) supports the project as an advisory board. Whereas the US and Japanese programmes are aimed at large-scale demonstrations of the MCFC technology, this project focusses on the development of concepts and technology, required for MCFC systems that will be competative on the cogeneration market. The project partners provide the essential expertise: from end-user, system engineering, stack development up to fundamental material research.

In this paper a coal-fired power plant with CO2 capture by integrating MCFC (molten carbonate fuel cell) system is studied. Aspen Plus software is used to establish the overall system model. The thermal and economic performances of the new hybrid system are investigated and compared with the benchmark system of coal-fired power plant without CO2 capture and the benchmark system with CO2 capture by using the MEA (Mono ethanol amine) method. The effects of the key parameters on the thermal and economic performances of the new hybrid system are studied. The research results show that compared with the benchmark system without CO2 capture, the efficiency of the new hybrid system is increased and the maximum CO2 capture rate can be 96.21%. While ensuring the high CO2 capture rate, the total efficiency of the new system with CO2 capture can be improved by4.05 percent points. Compared with the conventional CO2 capture method with the MEA, the CO2 capture method based on the electrochemical method of MCFC proposed in this paper has the obvious thermal performance advantage, however, its economic performance should be further improved. Achievements in this paper will provide the valuable reference for CO2 capture of coal-fired power plant with low energy consumption.

This paper presents a framework for the system integration and optimization of a molten carbonate fuel cell (MCFC) working under stationary conditions using process integration. Here, the analysis is focused on two systems in terms of the efficiency and process requirements: (i) an MCFC system alone and (ii) an MCFC system integrated with the steam turbine cycle, now onwards referred to as fuel cell combined cycle system for electric power generation. In the first system, a steady state direct internal reforming MCFC system is being simulated using desulphurized natural gas. A heat exchanger network is developed using process integration so that a minimum amount of external thermal energy is provided to the fuel cell system for electric power generation. In the second analysis, a steam turbine system is added to the first (fuel cell) one to form a fuel cell combined cycle system. The procedure for developing a network of heat exchangers and proper integration of the steam turbine system with an optimized minimum temperature difference is discussed. The results of the study elucidate the advantages of properly designed fuel cell combined cycle system to reach power demand with 17% higher efficiency as compared with the system without a combined cycle.

A feasibility assessment of a retrofit Molten Carbonate Fuel Cell coal-fired plant for flue gas CO2 segregation

Optimum start-up strategies for direct internal reforming molten carbonate fuel cell systems

Molten carbonate fuel cells provide a promising technology for the operation of future stationary power plants. In order to enhance service life, a detailed understanding of the dynamical behavior of such fuel cell systems is necessary. In particular, fast load changes shall be simulated, (resp., optimized) without risking material stress due to the extreme temperature differences usually accompanying fast load changes. Fast load changes are important for daily operations in order to react on varying demands. Material stress may lead to irreparable damage of the fuel cell stack. For these contradicting goals, a family of hierarchically ordered mathematical models has been developed with the aim of simulating and optimizing the temporal and spatial dynamical behavior of the gas streams, chemical reactions, and potential fields within the fuel cells. Altogether, the most complicated system, which is investigated in the present paper, results in a Pareto-optimal control problem with constraints in form of a huge system of 28 partial differential algebraic equations and ordinary integro-differential algebraic equations and boundary conditions which are themselves partly given by an ordinary differential algebraic system of dimension 9. The PDEs are of parabolic and hyperbolic type; some are degenerate. Moreover, the variables involved in the different submodels of this fully coupled multiphysical system live on considerably different time scales. Optimal control results are presented for a compromise between sufficiently fast load changes and sufficiently small temperature differences within the cell's solid part by means of a specially tailored formulation of a chain of optimal control problems. This procedure benefits from the different time scales of the state variables and keeps the problem manageable and computable despite its tremendous complexity and scale, although standard numerical methods are employed.

The dynamic simulation and experimental results for a molten carbonate fuel cell power plant are discussed in this paper. Development of the simulation model was motivated by recent interest in the autonomous control of fuel cell systems as distributed sources of electric power. The model replicates a plant design that was a precursor to a 2-MW demonstration fuel cell power plant running on natural gas at the City of Santa Clara, CA, USA. Multiple arrays or stacks of fuel cells used to produce electricity are based on direct fuel-cell (DFC) technology that utilizes internal reforming of natural gas. Simulations illustrate the effects of both the internal reforming and cell electrochemical reactions on electrical transients.

최근 대체에너지에 대한 관심이 높아짐에 따라 수소에너지를 기반으로 하는 차세대 발전 장치인 연료전지 관련 기술 개발이 활발하게 이루어지고 있다. 특히, 고온 연료전지의 대표적인 형태인 용융 탄산염 연료전지(MCFC: Molten Carbonate Fuel Cell, 이하 MCFC)는 전력사업용으로의 높은 가능성을 인정받아 화석연료를 대체할 발전방식으로 평가 받고 있다. 본 연구에서는 Aspen Custom Modeler(<TEX>$ACM^{TM}$</TEX>)에서 평형반응식을 이용하여 스택 모델을 구성한 후, Aspen <TEX>$Plus^{TM}$</TEX>에서 BOP(Balance of Plant) 시스템과 스택을 연결하여 전체 MCFC 발전 시스템의 정상상태를 모사하였다. 모델의 유효성을 입증하기 위해서 전류밀도, 연료이용률, S/C ratio, 재순환 흐름 비와 같은 주요 조업변수에 따른 셀 전압, 전력, 효율 등 시스템의 성능을 분석하였다. 그리고 Aspen <TEX>$Dynamics^{TM}$</TEX>에서 PID제어 방식을 적용하여 제어루프를 구성하였고 부하변화, 설정점 변화, 재순환 흐름비 변화에 따른 각각의 사례연구를 통하여 전체 시스템의 성능변화를 예측하였다. 그 결과 연료이용률과 전류밀도의 변화에 따른 전체 시스템의 최대 발전 효율 및 출력전압을 위한 운전조건을 제안하였다. Recentincreasing awareness of the environmental damage caused by the <TEX>$CO_2$</TEX> emission of fossil fuelsstimulated the interest in alternative and renewable sources of energy. Fuel cell is a representative example of hydrogen energy utilization. In this study, Molten Carbonate Fuel Cell system is simulated by using <TEX>$Aspen^{TM}$</TEX>. Stack model is consisted of equilibrium reaction equations using <TEX>$ACM^{TM}$</TEX>(Aspen Custom Modeler). Balance of process of fuel cell system is developed in Aspen <TEX>$Plus^{TM}$</TEX> and simulated at steady-state. Analysis of performance of the system is carried out by using sensitivity analysis tool with main operating parameters such as current density, S/C ratio, and fuel utilization and recycle ratio.In Aspen <TEX>$Dynamics^{TM}$</TEX>, dynamics of MCFC system is simulated with PID control loops. From the simulation, we proposed operation range which generated maximum power and efficiency in MCFC power plant.

Molten carbonate fuel cells (MCFC) are a promising alternative power source for distributed or residential power plants. Therefore, thermodynamic models are built in an Aspen customer modeler for the externally reformed (ER) MCFC and internally reformed (IR) MCFC. These models are integrated in Aspen Plus™. In this article the performance of internal and external reforming molten carbonate fuel cell systems are investigated. To this end the gas temperature at the anode inlet is varied to be able to exam the effect of operating temperature on the operating conditions for different modes of MCFC systems in a range between 600 and 700°C. It is found that the operating temperature has more effect on the cell voltage of IR-MCFC system compared to ER-MCFC system. Simulations show that the IR-MCFC system is more efficient than the ER-MCFC system. The cycle efficiency is rather independent of the operating temperature for as well ER-MCFC as IR-MCFC systems.

Funding Acknowledgements None. Introduction The adult mammalian heart has a poor regenerative and angiogenic potential. At the same time, both primary cardiac tumors and cardiac metastases are extremely rare. This suggests that shared mechanisms may halt the proliferation of cardiac (both cardiomyocyte and endothelial) and cancer cells. Among the mechanisms that have been claimed to be responsible for the loss of proliferative potential of cardiomyocytes early after birth is a sudden increase in mechanical load. Purpose Here, we investigate whether alterations in mechanical load affect the proliferation of multiple cell types in the heart. Methods Adult heart unloading in vivo was achieved using a mouse model of heterotopic heart transplantation, followed by EdU injection to label proliferating cells. Engineered heart tissues (EHTs) were generated using neonatal cardiomyocytes and fibroblasts, with a modified protocol allowing to finely tune mechanical load. Primary cardiac endothelial cells and multiple types of cancer cells (lung, melanoma and colon cancer) along with adult cardiac endothelial cells were included in EHTs. Results In line with our hypothesis, unloaded, heterotransplanted hearts showed a significant number of proliferating cardiomyocytes and endothelial cells, which were almost absent in native hearts. While cancer cells did not grow in native hearts, they massively proliferated in unloaded hearts. Consistent with in vivo results, mechanical unloading in EHTs significantly increased the percentage of EdU-positive cardiomyocytes, endothelial and cancer cells. In contrast, increasing afterload reduced cell proliferation in all cell types. Mechanistically, we identified Nesprin2 as an essential factor in sensing mechanical load and translating it into a cell proliferation arrest. Conclusions Overall, these data indicate that variations in mechanical load have a dramatic effect on the proliferation of multiple cell types within the heart tissue, including cardiomyocytes, endothelial and cancer cells and that Nesprin2 is required to mediate this effect.Unloading in the heartUnloading in EHT

Fatigue crack growth at elevated temperatures is a complicated problem since widespread creep deformation, in addition to elastic deformation, can accompany crack extension, and since the micromechanisms of crack growth comprise grain boundary cavitation, the striation mechanism, and corrosive effects. In the present paper, the time-dependent crack-tip stress fields in an elastic-nonlinear viscous body are analyzed for variable load. From the results, it is concluded that for slow cycling the C*-integral is the load parameter that determines the crack-tip fields and therefore the crack growth rate. If rapid load variations occur within otherwise slow cycles, the stress intensity factor, ΔKI, also affects the crack-tip fields; ΔKI determines the severity of the stress peaks that follow rapid load changes. For rapid cycling, the time-dependent stressing and straining near the crack tip is governed by ΔKI. If rapid cycling is unbalanced, a time-dependent component is superimposed on the rapidly varying ΔKI-controlled field. This time-independent component can be described by the C*-integral only for R-ratios near unity (that is, for high mean stress and small amplitude). The general case of unbalanced rapid cycling remains unresolved. Characteristic frequencies ω1 are developed to distinguish slow and rapid cycling, and a characteristic time t1 is given to decide whether a given load increase within a cycle is rapid or slow. The stress fields are combined with simple models for the striation and cavitation mechanisms of crack growth. The resulting crack growth rates, da/dN, are compared with a small number of experimental results from the literature.

The sea tests of quick start and rapid load increase were carried out on several types of naval steam turbines by their manufacturers in 1959 and 1960.In the tests, the turbines were acceralated from 12 knots speed to the maximum speed within 25-14 minutes. The temperature of turbine casings and the relative expansion of the casings and the rotors were measured.This paper reports mainly the thermal stress of the horizontal flanges of the high pressure turbine casings, which seemed to be the most important in rapid load increase.The authors conculude that;1) Marine steam turbines are rather tough in rapid load change than usually considered.2) The heat stress at the horizontal flanges of the casings becomes the first limitation against rapid load increse in marine turbines of ordinary design.3) The warming-up time of a few hours is completely unnecessary, and could be saved to several minutes without any trouble even in turbines of ordinary design.

Unit commitment (UC) is the process of committing the most optimal combination of generators in accordance with the power system security, load and cost. This paper proposes solution of UC problem using Forward Dynamic Programming method. The results are generated on two test systems and compared with that of the new and emerging techniques. Day ahead scheduling is done for performing the unit commitment as it is practically realizable and is the most common approach for performing UC. For this, the forecasted load demand of the day ahead is considered. The forecasted data is necessary to be considered for pre-planning the UC so that the units which may not be necessary to operate, can be shut down. Accordingly, the strategies can be developed for any real time discrepancies which may arise in the system network like load increase or decrease, faults etc. All the operational and technical constraints are taken care of during simulation. A spinning reserve is also considered to cater sudden and unexpected load increase. The simulation results obtained on two test systems are analysed and presented.

FUEL CELLS – MOLTEN CARBONATE FUEL CELLS | Overview