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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A comprehensive guide to the modelling and design of solid oxide fuel cell hybrid power plants
This book explores all technical aspects of solid oxide fuel cell (SOFC) hybrid systems and proposes solutions to a range of technical problems that can arise from component integration. Following a general introduction to the state-of-the-art in SOFC hybrid systems, the authors focus on fuel cell technology, including the components required to operate with standard fuels. Micro-gas turbine (mGT) technology for hybrid systems is discussed, with special attention given to issues related to the coupling of SOFCs with mGTs. Throughout the book emphasis is placed on dynamic issues, including control systems used to avoid risk conditions.
With an eye to mitigating the high costs and risks incurred with the building and use of prototype hybrid systems, the authors demonstrate a proven, economically feasible approach to obtaining important experimental results using simplified plants that simulate both generic and detailed system-level behaviour using emulators. Computational models and experimental plants are developed to support the analysis of SOFC hybrid systems, including models appropriate for design, development and performance analysis at both component and system levels.
- Presents models for a range of size units, technology variations, unit coupling dynamics and start-up and shutdown behaviours
- Focuses on SOFCs integration with mGTs in light of key constraints and risk avoidance issues under steady-state conditions and during transient operations
- Identifies interaction and coupling problems within the GT/SOFC environment, including exergy analysis and optimization
- Demonstrates an economical approach to obtaining important experimental results while avoiding high-cost components and risk conditions
- Presents analytical/computational and experimental tools for the efficient design and development of hardware and software systems
Hybrid Systems Based on Solid Oxide Fuel Cells: Modelling and Design is a valuable resource for researchers and practicing engineers involved in fuel cell fundamentals, design and development. It is also an excellent reference for academic researchers and advanced-level students exploring fuel cell technology.
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Veröffentlichungsjahr: 2017
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Table of Contents
Cover
Title Page
Copyright
Preface
Acknowledgements
Chapter 1: Introduction
1.1 World Population Growth, Energy Demand and its Future
1.2 World Energy Future
1.3 Introduction to Fuel Cells and Associated Terms
1.4 Gas Turbines
1.5 Coupling of Microturbines with Fuel Cells to Obtain ‘Hybrid Systems’
1.6 Conclusions
References
Chapter 2: SOFC Technology
2.1 Basic Aspects of Solid Oxide Fuel Cells
2.2 SOFC Types
2.3 Materials for SOFCs
2.4 Different SOFC Geometries
2.5 SOFC Stacks
2.6 Effect of Pressurization for SOFCs
2.7 Fuel Processing for SOFCs
2.8 SOFC Applications in Hybrid Systems
2.9 Aspects Related to SOFC Reliability, Degradation and Costs
2.10 Conclusions
Questions
References
Chapter 3: Micro Gas Turbine Technology
3.1 Fundamentals of the Brayton Cycle
3.2 Turbomachinery
3.3 Recuperative Heat Exchanger
3.4 Bearings
3.5 Conclusions: Commercial Status and Areas of Research
Questions and Exercises
References
Chapter 4: SOFC/mGT Coupling
4.1 Basic Aspects of SOFC Hybridization
4.2 SOFC Coupling with Traditional Power Plants
4.3 Beneficial Attributes Related to SOFC/mGT Coupling
4.4 Constraints Related to SOFC/mGT Coupling
4.5 Design and Off-design Aspects
4.6 Issues Related to Dynamic Aspects
4.7 Main Prototypes Developed for SOFC Hybrid Systems
4.8 Conclusions
Questions and Exercises
References
Chapter 5: Computational Models for Hybrid Systems
5.1 Introduction
5.2 Steady-state Models for Hybrid Systems
5.3 Computational Models for Hybrid Systems: Modelling Steps
5.4 System Modelling
5.5 Results and Discussion
5.6 Dynamic Models
5.7 Model Validation
5.8 Conclusion
Questions and Exercises
References
Chapter 6: Experimental Emulation Facilities
6.1 Experimental Emulation Facilities
6.2 Reduced-scale Test Facilities
6.3 Actual-scale Test Facilities
6.4 Conclusions
Questions and Exercises
References
Chapter 7: Problems and Solutions for Future Hybrid Systems
7.1 The Future of Micro Power Generation Systems
7.2 The Future of Hybrid Systems: Hydrogen as an Energy Carrier
7.3 Future Hybrid Systems: Design, Optimization and Sizing
7.4 Cost Analysis of Hybrid Systems for Power Generation Applications
7.5 Performance Degradation Problems in Solid Oxide Fuel Cells
7.6 Turbomachinery Problems
7.7 Dynamic and Control System Aspects
7.8 CO
2
Separation Technologies for SOFC Hybrid Plants
7.9 Coal and Biofuel for Hybrid Systems
7.10 Conclusions
References
Glossary
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Chapter 1: Introduction
Figure 1.1 Global mean temperature probability changes, for the years 1990–2100 and 1990–2030.
Figure 1.2 First demonstration of a fuel cell by Grove in 1839.
Figure 1.3 Basic fuel cell arrangement.
Figure 1.4 Basic SOFC operation.
Figure 1.5 Tubular SOFC module configuration [17].
Figure 1.6 SOFC performance at 800°C under atmospheric pressure (graph generated using expression provided in [17]).
Figure 1.7 Ideal and actual fuel cell voltage/current characteristics [17].
Figure 1.8 A Turbec T100 microturbine.
Chapter 2: SOFC Technology
Figure 2.1 Schema for a tubular SOFC.
Figure 2.2 Schema for a segmented-in-series tubular SOFC.
Figure 2.3 Schema for a planar SOFC.
Figure 2.4 Stack based on tubular SOFCs.
Figure 2.5 General scheme of an atmospheric SOFC hybrid system.
Figure 2.6 General scheme of a pressurized SOFC hybrid system.
Chapter 3: Micro Gas Turbine Technology
Figure 3.1 Temperature–entropy (left) and pressure–volume (right) thermodynamic diagrams of the Joule-Brayton cycle.
Figure 3.2 Effect of increasing pressure ratio (left) and turbine inlet temperature (right) in a Joule-Brayton cycle.
Figure 3.3 Efficiency (left) and specific work (right) for a quasi-ideal Brayton cycle with and .
Figure 3.4 Efficiency vs. specific work for a quasi-ideal Brayton cycle with and , and constant turbine inlet temperature.
Figure 3.5 Contributions to heat addition and rejection in a quasi-ideal Brayton cycle. Low and high pressure ratio cycles pictured left and right.
Figure 3.6 Breakdown of losses contributing to the thermal efficiency of a quasi-ideal Brayton cycle.
1
Figure 3.7 Energy exchange in a quasi-ideal Brayton cycle with given .
Figure 3.8 Low pressure ratio, recuperative Brayton cycle (left) and high pressure ratio, non-recuperative Brayton cycle (right).
Figure 3.9 Efficiency of the recuperative (solid lines) and non-recuperative (dashed lines) Brayton cycles.
Figure 3.10 Efficiency vs. specific work for a quasi-ideal recuperative Brayton cycle with , and .
Figure 3.11 Brayton cycle with isothermal compression.
Figure 3.12 Ideal Brayton cycle with intercooled compression.
Figure 3.13 Brayton cycle with isothermal expansion.
Figure 3.14 Ideal Brayton cycle with reheat.
Figure 3.15 Quasi-ideal Brayton cycles with intercooling (left) and reheat (right).
Figure 3.16 Quasi-ideal compound recuperative Brayton cycles with compression/expansion stages.
Figure 3.17 Specific work (left) and efficiency (right) of a quasi-ideal compound recuperative Brayton cycle with compression/expansion stages.
Figure 3.18 Efficiency vs. specific work for a quasi-ideal compound recuperative Brayton cycle with , and .
Figure 3.19 Selection of turbomachinery type and attainable total to static efficiency based on specific speed.
Figure 3.20 Specific speed–specific diameter pairs for peak compressor efficiency (Cordier line).
Figure 3.21 Velocity diagrams in a radial compressor impeller.
Figure 3.22 Enthalpy vs. entropy diagram of the radial compressor stage.
Figure 3.23 Velocity diagram at the impeller inlet (hub and shroud) for pre-whirl inflow.
Figure 3.24 Velocity diagram at impeller outlet for different outlet flow (sweep) angles.
Figure 3.25 Dependence of pressure ratio on blade Mach number and outlet blade angle.
Figure 3.26 Trajectory (streamline) of a fluid particle in a vaneless diffuser.
Figure 3.27 Ideal pressure rise coefficient of a vaneless diffuser (diffuser inlet angle ).
Figure 3.28 Impact of the number of diffuser vanes on the total-to-total efficiency and range of a centrifugal compressor.
Figure 3.29 Various diffuser types: (a) straight, (b) curved plate, (c) aerofoil, (d) wedge.
Figure 3.30 Theoretical surge loop in a turbocompressor.
Figure 3.31 Performance map of a centrifugal compressor.
Figure 3.32 Performance map of centrifugal compressor stage: reference fixed geometry unit (top) and unit incorporating variable inlet guide vanes and diffuser vanes (bottom). Adapted from [14] with permission.
Figure 3.33 Enthalpy vs. entropy diagram of the radial inflow turbine stage.
Figure 3.34 Velocity diagrams of a radial inflow turbine.
Figure 3.35 Attainable turbine efficiencies for different velocity ratios.
Figure 3.36 Minimum number of rotor blades for different absolute flow angles at rotor inlet.
Figure 3.37 Attainable total-to-static efficiency depending on flow and loading coefficients.
Figure 3.38 Impact of engine size on component total-to-static efficiency and mass flow rate.
Figure 3.39 Different recuperator geometries. (a) Cross-corrugated, (b) wavy, (c) cross-wavy.
Figure 3.40 Typical arrangement of a plate and fin recuperator (top). Close-up of planar (bottom left) and annular (bottom right) layouts.
Figure 3.41 Different fin geometries in plate and fin recuperators (www.hedhme.com).
Figure 3.42 Specific area and cost for various recuperator technologies.
Figure 3.43 Sketch of a rotary regenerator.
Figure 3.44 Active magnetic bearing schematic.
Figure 3.45 a) Standard Gen I leaf bearing. b) Gen I (left) and Gen III (right) bump bearings [45].
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Figure 3.46 Standard geometry of a Gen III foil thrust bearing [46].
Chapter 4: SOFC/mGT Coupling
Figure 4.1 Electrical efficiency of some fossil-fuel based plants.
Figure 4.2 SOFC hybrid system based on coupling with a steam power plant.
Figure 4.3 SOFC hybrid system based on coupling with a recuperated gas turbine.
Figure 4.4 SOFC hybrid system based on coupling with a combined cycle system.
Figure 4.5 Off-design behaviour related to a hybrid system based on the coupling of a recuperated microturbine with a tubular SOFC.
Figure 4.6 Dynamic and control system aspects related to a hybrid plant, based on the coupling of a recuperated microturbine with a tubular SOFC: fuel cell average temperature and thermal gradient.
Figure 4.7 Dynamic and control system aspects related to a hybrid plant, based on the coupling of a recuperated microturbine with a tubular SOFC: steam-to-carbon ratio (STCR) and differential pressure.
Chapter 5: Computational Models for Hybrid Systems
Figure 5.1 Different modelling approaches.
Figure 5.2 (a) Simple gas turbine layout; and (b) double-shaft gas turbine scheme.
Figure 5.3 Compressor and turbine characteristics.
Figure 5.4 (a) Load characteristics and (b) equilibrium running lines.
Figure 5.5 Equilibrium running line for a free turbine.
Figure 5.6 Equilibrium running line for free turbine.
Figure 5.7 Equilibrium running line for free turbine.
Figure 5.8 Diagram of the hybrid system considered for this modelling activity.
Chapter 6: Experimental Emulation Facilities
Figure 6.1 Layout of the anodic recirculation test rig.
Figure 6.2 Picture of the anodic recirculation test rig.
Figure 6.3 Layout of the cathodic loop test rig.
Figure 6.4 Picture of the cathodic loop test rig.
Figure 6.5 Layout of the surge test rig.
Figure 6.6 Layout of the control test rig.
Figure 6.7 Layout of the emulator by the US Department of Energy – NETL.
Figure 6.8 Layout of the emulator by the University of Genoa – TPG.
Figure 6.9 Picture of the emulator by the University of Genoa – TPG.
Figure 6.10 Layout of the emulator by the DLR.
List of Tables
Chapter 1: Introduction
Table 1.1 Comparison of fuel cell technologies [17]
Table 1.2 Useful relations based on work and heat potentials
Table 1.3 Major manufacturers of fuel cells
Table 1.4 Overview of some groups performing research in SOFC and SOFC-mGT hybrid systems [32, 36]
Chapter 3: Micro Gas Turbine Technology
Table 3.1 Influence of sweep angle on stage performance
Table 3.2 Design requirements of micro gas turbine recuperators
Table 3.3 Commercial micro gas turbines
Chapter 4: SOFC/mGT Coupling
Table 4.1 SOFC/mGT coupling advantages and disadvantages
Table 4.2 Siemens-Westinghouse 220 kWe hybrid system performance
Table 4.3 Data for Question 4.
Chapter 5: Computational Models for Hybrid Systems
Table 5.1 Values of constants used in reforming and water–gas shifting process
Table 5.2 Inlet and outlet percentage flows of the reformer
Table 5.3 Coefficients in equations to evaluate enthalpy, entropy and specific heat capacity
Table 5.4 SOFC parameters for calculation of ohmic losses
Table 5.5 Hybrid system assumptions
Table 5.6 Estimated system performance of the SOFC-GT hybrid system
Hybrid Systems Based on Solid Oxide Fuel Cells
Modelling and Design
Mario L. Ferrari
University of Genova, Italy
Usman M. Damo
University of Manchester, UK
Ali Turan
University of Manchester, UK
David Sánchez
University of Seville, Spain
This edition first published 2017
© 2017 John Wiley & Sons Ltd
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The right of Mario L. Ferrari, Usman M. Damo, Ali Turan and David Sánchez to be identified as the authors of this work has been asserted in accordance with law.
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Library of Congress Cataloging-in-Publication Data
Names: Ferrari, Mario L., 1978- author. | Damo, Usman M., 1984- author. | Turan, Ali, author. | Sánchez, David, 1977 April 14- author.
Title: Hybrid systems based on solid oxide fuel cells : modelling and design / Mario L. Ferrari, University of Genova, Italy, Usman M. Damo, University of Manchester, UK, Ali Turan, University of Manchester, UK, David Sánchez, University of Seville, Spain.
Description: First edition. | Hoboken, NJ, USA : John Wiley & Sons, Inc., 2017. | Includes bibliographical references and index.
Identifiers: LCCN 2017003602 (print) | LCCN 2017004069 (ebook) | ISBN 9781119039051 (cloth) | ISBN 9781119039068 (pdf) | ISBN 9781119039075 (epub)
Subjects: LCSH: Solid oxide fuel cells. | Hybrid power systems-Equipment and supplies. | Renewable energy sources.
Classification: LCC TK2933.S65 F47 2017 (print) | LCC TK2933.S65 (ebook) | DDC 621.31/2429-dc23
LC record available at https://lccn.loc.gov/2017003602
Cover Design: Wiley
Cover Images: (Bottom Image) © Fertnig/Gettyimages; (Top Image) Courtesy of the authors; (Background) © Max Krasnov/Shutterstock
Preface
Even though solid oxide fuel cell (SOFC) technology reached a significant development milestone around 30 years ago, no hybrid system prototypes were built before the 2000 Siemens-Westinghouse plant. Due to the enormous engineering system complexity and cost, SOFC/turbine hybrid plants only attracted substantial research interest at the end of the twentieth century when environmental concerns became very visible and demanding.
Considering the widespread enthusiasm regarding research and development activities for hybrid systems on the eve of the twenty-first century, a ‘partial downsizing’ is now apparent due to several unresolved engineering and sustainability problems and the ever-present, overriding cost and reliability issues. Thus, the forecast plans for commercialization carried out during the past decade seem to have failed to deliver acceptable hybrid system performance under realistic operational conditions, due to the various technological, complexity and cost issues.
Furthermore, comparing the developmental status of hybrid systems with state-of-the-art anticipated performance metrics of the past decade, several publications have now presented newly validated solutions for several of the previously outstanding applied research issues (such as cost decrease, SOFC/turbine coupling and control system development), incorporating significant promising technology improvements. Hence, it is essential to consider that concentrated funding resources are still necessary to profitably combat/resolve all of the technical issues and to reach the required high levels of reliability, high plant operative life and low-cost performance for acceptable commercial adoption on a wider scale. For such reasons, focused efforts and research interests/activities at both academic and industrial levels are absolutely essential. Even if hybrid systems will not be ready for commercialization in a few years, the extremely desirable performance and environmental aspects promised via this technology will be a central pillar for future energy generation and hydrogen economy development.
Due to the promising performance attributes and the recent substantial development of hybrid system technology based on solid oxide fuel cells (SOFCs), the authors have decided to develop this book to produce an updated text targeted at both practicing engineers and academic researchers. In comparison with previously published texts, the authors pay special attention to the latest research and development activities at both the theoretical and experimental levels. Thus, following the discussions of the basic aerothermodynamics and electrochemistry of the primary components (the SOFC stack and microturbine aspects are presented in Chapters 2 and 3), including updated descriptions covering the latest technological improvements and commercialization aspects, an innovative approach is considered to further develop the SOFC/turbine coupling in an individual chapter (Chapter 4). For that reason, special attention is devoted to system constraints, problem/solution details based on the latest academic/industrial research activities, and performance aspects of currently available commercial prototypes. Furthermore, the book presents details regarding hybrid system modelling activities from different points of view including theoretical/computational (Chapter 5) and physically based approaches (Chapter 6). In comparison with previous publications on SOFC based systems, this book devotes large sections and presents detailed discussion on experimental development devices collectively referred to as emulator rigs, as these tools are widely and routinely used to develop rational and profitable configurations covering hybrid systems based on SOFC and gas turbine systems. Currently, these experimental facilities show great potential regarding applied research for such power plants, and the results generated via their use are considered absolutely essential for solving several technical hardware and optimization issues for such hybrid systems.
Finally, various conflicting engineering issues and commercialization potentials to be pursued for the widespread adoption of such innovative and efficient power plants are discussed in Chapter 7, focusing special attention on future perspectives and possible solutions.
M.L. FerrariU.M. DamoA. TuranD. Sánchez
Acknowledgements
The authors would like to thank all the staff of the Thermochemical Power Group (TPG) of the University of Genoa for the shared experience involving theoretical and experimental activities and international collaboration opportunities. A special acknowledgement is due to Prof. Massardo Aristide F. (Director of the TPG) for his essential scientific support. The authors would also like to recognize and thank the fuel cell research group at the US Department of Energy, National Energy Technology Laboratory (NETL), Morgantown WV, US. To Dr Joseph Dawes is due a sincere note of thanks for the wonderful execution of the arduous task of going through the entire manuscript for both technical and language aspects of the material. Mr Ibrahim M. Damo deserves recognition for the redesign/reproduction of many figures in chapters 1 and 5. Also, the authors would like to thank Mr Che-Wei Nien, a graduate of the University of Manchester (MSc Thermal power), for his contribution to Chapter 5 with his thesis. Prof. Sánchez would like to gratefully acknowledge Gonzalo Sánchez-Martínez and José María Rodríguez at the University of Seville for their assistance in editing and largely improving the artwork in Chapter 3 on micro gas turbines.
Chapter 1Introduction
Chapter Overview
World Population Growth, Energy Demand and its FutureWorld Energy FutureIntroduction to Fuel Cells and Associated TermsGas TurbinesCoupling of Microturbines with Fuel Cells to Obtain ‘Hybrid Systems’ConclusionsThe current and future energy scenarios faced by the international community are discussed in this chapter, starting with a brief presentation of the energy landscape and related issues, including the increase in demand and environmental aspects. A list of possible solutions to existing and foreseen problems is presented and discussed, setting the framework to highlight the significant potential of fuel cells for future power generation. Following on from this, the performance characteristics of fuel cells are introduced, including an analysis of their different types and corresponding differential features. Additionally, attention is devoted to hybrid systems based on the coupling of high-temperature fuel cells and microturbines (mGTs).
1.1 World Population Growth, Energy Demand and its Future
A study carried out by the United States Census Bureau (USCB) [1] estimated that the world population exceeded 7 billion on 12 March 2012. Now, at the time of writing in August 2016 with the global population standing at about 7.4 billion [2], this figure is expected to continue rising over the coming decades [2]. As the world population grows, in many countries faster than the global average of 2%, the need for more and more energy is intensifying in a somewhat similar proportion, thus putting pressure on the natural resources available and existing infrastructures. This higher energy consumption is not only due to the growth in world population, but also to the improved lifestyles leading to a greater energy demand per capita (two features that inevitably come together). This is best exemplified by the fact that the wealthy industrialized economies comprise 25% of the world's population but consume 75% of the world's energy supply [3]. A recent study (from ref. [4]) shows that the total world consumption of marketed energy is expected to increase from 549 quadrillion British thermal units (Btu) in 2012 to 629 quadrillion Btu in 2020, and to 815 quadrillion Btu in 2040 – a 48% increase from 2012 to 2040 [4].
Indeed, the landscape of future energy demand and generation projected for the world seems rather bleak, as most nations, including the most developed ones, depend primarily on conventional energy sources such as oil, coal and gas to generate power not only for the domestic and industrial sectors but also for transportation. This dependency results in global warming, contributes to rises in fuel prices that constitute a burden on economies, and can lead to delays in energy production and supply [5, 6]. Furthermore, even if the global production of fossil fuels is currently sufficient to cover the world's needs, the exponential rise in the exploitation rate of this finite, fast-depleting resource would pose a risk to the future energy demand and generation balance [7–9]. In the long run this global dependence on conventional fuel sources for power production will prove problematic because the world will eventually fall short or run out of these resources. Renewable energy sources are often set forth as a feasible alternative to this fossil-fuel dominated world [10], although many of their inherent features, such as their low energy density, intermittency and geographical distribution, pose a number of challenges that remain to be solved today.
1.2 World Energy Future
Due to the heavy reliance of most nations worldwide on fossil fuels for power generation and transportation, the atmospheric concentrations of carbon dioxide and methane have increased by 36% and 148% respectively, compared with pre-industrial levels [11]. These levels are indeed much higher than at any time during the last 800,000 years, the period for which reliable data have been extracted from ice cores. This observation is further confirmed by less direct geological observations that also show that carbon dioxide concentrations higher than today were last seen about 20 million years ago. These findings suggest that the root cause for such high concentrations is anthropogenic, mainly hydrocarbon-based fuel burning (responsible for three-quarters of the increase in CO2 from human activity over the past 20 years) and deforestation [11]. Other environmental factors, including air pollution, acid precipitation, ozone depletion and emission of radioactive substances, are also of concern and raise awareness of the negative impact of human activity on the environment [3].
As a consequence of this massive production of anthropogenic carbon dioxide and other greenhouse gases (trace gases in particular [12]), global temperatures in 2016 were 0.87°C above the long-term 1880–2000 average (the 1880–2000 annually averaged combined land and ocean temperature is 13.9°C), which translates into a warming rate of around 0.61°C/century over the last few decades. In particular, the average temperature of the Atlantic, Pacific and Indian oceans (covering 72% of the Earth's surface) has risen by 0.06°C since 1995. The situation regarding global warming is far from being under control. As stated by the US Department of Energy's forecast, carbon emissions will increase by 54% above 1990 levels by 2015, making the Earth likely to warm by 1.7–4.9°C over the period 1990–2100 (see Figure 1.1). Such observations demonstrate the need for efforts towards alleviating energy-related environmental concerns in the near future [3].
Figure 1.1 Global mean temperature probability changes, for the years 1990–2100 and 1990–2030.
Source: Omer (2008) [3]. Reproduced with permission of Elsevier.
Achieving higher efficiencies and, if possible, the utilization of renewable energies in power generation technologies will be vital steps in mitigating or reducing these environmental problems, whilst meeting the expected rise in energy demand in the future. With increasing fuel prices and significant pressure to reduce emissions, increasing energy efficiency is considered amongst the most feasible and cost-competitive approaches for reducing CO2 emissions. For instance, Britain wastes 20% of its fossil fuel and electricity which, if used efficiently, would translate into a potential £10 billion annual reduction in the collective fuel bill and a reduction of some 120 million tonnes of CO2 emissions [3]. Unfortunately, even if energy is currently recognized globally as being at the centre of the sustainable development paradigm, the industrial and social development paths favour energy consumption rather than conservation [3].
The significant fuel consumption and CO2 emission issues have to link with the fact that conventional thermal power plants (regardless of the type of fuel used) cannot convert all of the thermal energy supply into useful (mechanical) work. In most cases, more than 50% of the heat added to the working cycle is rejected to the environment. Combined heat and power (CHP) installations are able to use a part of this heat, which would otherwise be wasted in a conventional power plant, to raise the overall first law efficiency to values higher than 80% for the best available technology [3]. This concept enables drastic reductions of the primary energy consumption and cost compared with the independent production of both forms of energy (electricity and thermal energy).
Complementary to energy conversion at high efficiency, substituting fossil fuels with renewable energy sources is envisaged as another means to tackle the aforecited social, economic and environmental problems. Renewable energies are broadly regarded as energy sources that are naturally replenished over a short timescale (i.e. in comparison to the lifetime of a human being), such as sunlight, wind, rain, tides, waves and geothermal heat. They have shown the potential to replace conventional fuels in various distinct areas, such as utility-scale electricity generation, hot water production/space heating, fuels for transportation, and rural (off-grid) energy services [13, 14]. Renewable energy sources have the potential to constitute the future energy sector's backbone, despite some evident shortcomings such as low density and inherent intermittency.
According to the REN21's 2014 report [15], renewables contributed 19% to the world's energy consumption in 2012, and 22% to electricity generation in 2013, using both traditional (biomass) and more innovative renewable energy technologies such as solar power, large wind farms and biofuels [10]. The importance of renewable energy sources has been disseminated widely, and several nations worldwide have decided to invest large sums of money in renewable technologies; such is the case in the US with a total investment of more than $214 billion in 2013, whereas other countries like China are following close behind [15].
Hybrid systems based on the coupling of a microturbine with a high-temperature fuel cell are highly regarded as a solution for future power generation due to their high efficiency, ultra-low emissions and their ability to run on fuels such as hydrogen produced from renewable sources. These systems can achieve very high efficiencies: more than 60% electrical efficiency using natural gas (depending on the low heating value). This efficiency is virtually independent of plant size due to the modular nature of these devices. Hybrid systems based on solid oxide fuel cells (SOFCs) are of particular interest, because they have the potential to overcome the main limitations of traditional power plants, and furthermore to meet the hurdles posed for the world's future energy need without worsening environmental issues.
1.3 Introduction to Fuel Cells and Associated Terms
A fuel cell is a device that converts the chemical energy in a fuel into electricity through an electrochemical reaction with oxygen. Hydrogen is commonly used in a fuel cell, but hydrocarbons such as natural gas and alcohols like methanol are also used. In contrast to batteries, fuel cells require a constant source of fuel and oxygen/air to sustain the chemical reaction and thus produce electricity as long as this input flow is supplied [16, 17].
A fuel cell typically consists of an anode (negative electrode), a cathode (positive electrode), and an electrolyte that allows charges to move between the two sides of the fuel cell [16, 17]. Direct current electricity is produced when electrons are drawn from the anode to the cathode. Typical layouts and choices of materials vary between fuel cell types. A classification of layouts/materials used in different types of fuel cells is shown in Table 1.1.
Table 1.1 Comparison of fuel cell technologies [17]
PEFC
AFC
PAFC
MCFC
SOFC
Electrolyte
Hydrated polymeric ion exchange membranes
Mobilized or immobilized potassium hydroxide in an asbestos matrix
Immobilized liquid phosphoric acid in SiC
Immobilized liquid molten carbonate in LiAIO
2
Perovskites (ceramics)
Electrodes
Carbon
Transition metals
Carbon
Nickel and nickel oxide
Perovskite and perovskite/metal cermet
Catalyst
Platinum
Platinum
Platinum
Electrode material
Electrode material
Interconnect
Carbon or metal
Metal
Graphite
Stainless steel or nickel
Nickel, ceramic, or steel
Operating temperature
40–80°C
65–220°C
205°C
650°C
600–1000°C
Charge carrier
H
+
OH
−
H
+
CO
3
−2
O
=
External reformer for hydrocarbon fuels
Yes
Yes
Yes
No, for some fuels
No, for some fuels and cell designs
External shift conversion of CO to hydrogen
Yes, plus purification to remove trace CO
Yes, plus purification to remove trace CO and CO
2
Yes
No
No
Prime cell components
Carbon-based
Carbon-based
Graphite-based
Stainless-based
Ceramic
Product water management
Evaporative
Evaporative
Evaporative
Gaseous product
Gaseous product
Source: US Department of Energy.
1.3.1 Background for Fuel Cells and Thermodynamic Principles
Fuel cells as power systems were first conceived and realized by Sir William Grove in 1839, using the experimental setup shown in Figure 1.2 [18]. His demonstration is the reverse of electrolysis: an electric current is produced through the process of combining hydrogen and oxygen to form water. This process is an ‘electrochemical burning’ reaction (although no real combustion is present in a fuel cell) which consumes hydrogen as fuel and produces electricity instead of heat.
Figure 1.2 First demonstration of a fuel cell by Grove in 1839.
Source: Srinivasan (2006) [18]. Reproduced with permission of Springer.
Despite the fact that a large proportion of those with an interest in fuel cells are professionals with a background in heat engines, it is wise to revisit some fundamental concepts in chemical thermodynamics. To this aim, a brief introduction to the Gibbs potentials is provided below.
Let Γ be a chemical system whose absolute pressure, temperature, volume and entropy are denoted by p, T, v and S respectively. The following four Gibbs potentials are defined to calculate the work yielded by the system: internal energy (U), Helmholtz free energy (A), enthalpy (H), and Gibbs free energy (G), as shown in equations 1.1–1.3.
These Gibbs potentials are used in the analysis of a wide range of processes.
The total work yielded by the system Γ during an infinitesimal process, W, is higher than the amount of work that can actually be employed by a potential user, Wu [19]. The difference between both works is that done by the system on its surroundings:
Based on this consideration, the four state functions previously listed are used to equate the First and Second Laws of thermodynamics applied to a closed system (Eqs 1.5 and 1.6), where work W and heat Q are considered positive when they flow out of and into the system respectively:
Combiningequations 1.21.4 and 1.5 yields the following alternative form of the First Law:
Equations 1.5 and 1.6 can be interpreted in the following terms:
If the process followed by the system Γ does not perform work, internal energy change equals heat added to the system (dU = dQ).
If the process followed by the system Γ does not perform useful work, enthalpy change equals heat added to the system (dH = dQ).
If the system undergoes an adiabatic process:
Total work equals internal energy decrease (dW = −dU).
Total useful work equals enthalpy decrease (dW
u
= −dH).
Equation 1.6 evidences that the entropy gain of the system dS comes about due to the heat added to the system at constant temperature T plus a certain amount of unbalanced entropy change dS'. As stated originally by Clausius, this unbalanced entropy change is either positive in the case of an irreversible process, or null in the case of a reversible process [20]. Again, the combination of equations 1.1–1.3 and the First and Second Laws provide the following useful interpretations:
If the system undergoes an isentropic process:
The total work done by the system equals the internal energy drop minus the energy dissipated to the surroundings (dW = −dU − TdS').
The useful work done by the system equals the enthalpy drop minus the energy dissipated to the surroundings (dW
u
= −dH − TdS').
If the system undergoes an isothermal process:
The total work done by the system equals the Helmholtz free energy drop minus the energy dissipated to the surroundings (dW = −dA − TdS').
The useful work done by the system equals the Gibbs free energy drop minus the energy dissipated to the surroundings (dW
u
= −dH − TdS').
This set of useful thermodynamic relations provides a means to calculate the total and useful work through the four Gibbs potentials in a variety of processes. These relations are summarized in Table 1.2 for clarity.
Table 1.2 Useful relations based on work and heat potentials
Process
−dU
−dA
−dH
−dG
dW = 0
−dQ
dW
u
= 0
−dQ
dQ = 0
dW
dW
u
dS = 0
dW
max
dW
u,max
dT = 0
dW
max
dW
u,max
Despite a thermodynamic analysis utilizing the considerations above showing the promise of the experimental setup shown in Figure 1.2, the current generated is usually very small due to the unfavourable characteristics of the three-phase interface (contact area between the gas, the electrolyte, and the electrode) and the high ion-transport resistance of the electrolyte [21]. These issues have driven the development of fuel cell technology towards flat and porous electrodes with a thin layer of electrolyte, as shown in Figure 1.3. Layouts of this kind result in the contact area being maximized and the resistance kept to a minimum, increasing the current produced [21].
Figure 1.3 Basic fuel cell arrangement.
Source: Larminie et al. (2003) [21]. Reproduced with permission from John Wiley & Sons.
1.3.2 Solid Oxide Fuel Cells (SOFCs)
Solid oxide fuel cells (SOFCs), as the name implies, are completely solid-state entities that use ceramic electrolytes. The development of SOFCs can be traced back to the 1890s when Nernst discovered that stabilized zirconia (ZrO2) could conduct ions at certain temperatures, making zirconia a potentially useful electrolyte [21, 22]. Major manufacturers and their development focuses are listed in Table 1.3. A further investigation was carried out by Baur and Preis in 1943, showing that zirconia could serve as an oxygen-ion conducting electrolyte in fuel cells [22].
Table 1.3 Major manufacturers of fuel cells
Manufacturer
Developments
AFC Energy, Cranleigh, Surrey, United Kingdom
(2006)
.
Alkaline fuel cells.
Apollo Energy Systems, Pompano Beach, Florida, USA
(1966)
.
Develops, produces and markets fuel cell power plants, electrical propulsion systems, and alternative energy generation equipment.
Ballard Power Systems, Burnaby, British Columbia, Canada
(1979)
.
Designs, develops, and manufactures zero-emission proton-exchange-membrane fuel cells. Ballard Power Systems, Inc. is a global leader in PEM (proton exchange membrane) fuel cell technology.
Doosan Fuel Cell America, Sunnyvale, California, USA (HQ)
(2003)
.
A fuel cell manufacturer focusing on the stationary fuel cell and small business markets.
Intelligent Energy, Loughborough, United Kingdom
(2001)
.
Specialists in the development of proton exchange membrane (PEM) fuel cells for application in the automotive, consumer electronics and stationary power markets.
UTC Power, South Windsor, Connecticut
(1958)
.
Produces/develops fuel cells for numerous applications including for use in space and submarines.
SOFCpower SpA
2006
, Trentino, Italy.
For stationary applications with electrical power requirements below 6 kW.
Mitsubishi Heavy Industries (MHI), established in
1914
, Tokyo.
First domestic operation of a combined-cycle system combining SOFC and a micro gas turbine. Maximum power output of up 200 from 21 kW.
Rolls-Royce Fuel Cell Systems (RRFCS),
2002
, Loughborough, UK.
Stationary power generation, applications range from 250 kW to >1 MW.
Hexis AG,
1997
, Switzerland.
For stationary applications with electrical power requirements below 10 kW.
Siemens-Westinghouse, in SOFC/mGT business for more than
3 decades
, USA.
For stationary applications with electrical power requirements above 100 kW.
Today, this type of fuel cell is a high-temperature, solid-state electrochemical conversion device that produces electricity directly from electrochemical (oxidation) reactions. The cell operates at 600–1000°C where ionic conduction of oxygen ions takes place. Commonly, the anode is a Ni–ZrO2 cermet and the cathode is Sr-doped LaMnO3. Not using a liquid electrolyte avoids the attendant material corrosion and/or electrolyte management issues. The high temperature of the SOFC, nonetheless, poses stringent requirements on its materials, and hence the development of low-cost materials and the low-cost fabrication of ceramic structures that still fulfil the technical requirements are now the key technical challenges for the future utilization of SOFCs [17].
The basic operation of a SOFC is sketched in Figure 1.4. Oxygen (O2) is reduced at the cathode–electrolyte interface forming oxygen ions (O=) that are transported to the electrode through the electrolyte. Once at the interface between anode and electrolyte, these react with the hydrogen ions (H+) to form water (H2O) that is disposed of via the exhaust stream. Electrons are released at the anode and flow through the external load to the cathode, where they are used to reduce the oxygen molecules. This operating principle is summarized in Figure 1.4 and through the following reactions [23–25] (Eqs 1.8–1.10):
Figure 1.4 Basic SOFC operation.
Owing to the overriding influence of the reaction shown in Equation 1.9, the reaction represented by Equation 1.10 is not further considered in the analysis regarding the electrochemical reaction set. Thus the overall electrochemical reaction (Eq. 1.11) is obtained by adding equations 1.8 and 1.9:
1.3.2.1 Electrolyte
Currently, the majority of SOFC developers use electrolytes made of zirconia stabilized by a small amount of yttria (3, 8, or 10%), namely YSZ [17]. When the temperature is raised to more than 800°C, such electrolytes become good conductors of oxygen ions and show minimal electrical conductivity (i.e. transportation of electrons) [17–22]. As a main shortcoming, the very high operating temperature of the cell makes material selection very difficult, although the high temperature also provides an exploitable characteristic for hybrid systems.
1.3.2.2 Anode
Present SOFCs use anodes made from zirconia cermet (a mixture of ceramic and metal) [17, 21]. The metal used is nickel, chosen primarily due to its high electrical conductivity and stability under chemically reducing conditions [17, 21]. Nickel is preferred to platinum, the metal of choice in low-temperature fuel cells, due to its much lower cost, with only a small trade-off in transport properties. The zirconia used is both to inhibit the sintering of the metal particles and to provide a thermal expansion ratio close to that of the electrolyte. A well-designed anode should have high electrical conductivity to allow the flow of current, adequate ionic conductivity such that ions come into contact with the fuel flow, and high activity for the electrochemical and fuel conditioning1 reactions [17]. At the same time, the anode should have a high porosity (20–40%) to allow mass transport of gases [17, 21].
1.3.2.3 Cathode
Like the anode, the structure of the cathode has to be porous to facilitate mass transport. Today, strontium-doped lanthanum manganite is widely used in cathodes. Alternative materials include P-type conducting perovskite structures that display mixed ionic and electrical conductivity; these are specifically worthy of consideration for lower temperature operation (about 650°C) as voltage loss becomes significant for the other standard materials [17, 21].
1.3.2.4 Interconnector
An interconnector exists to provide physical connection between neighbouring fuel cells so that a higher current can be produced by operating several fuel cells assembled in parallel. Under normal circumstances the only requirement for the interconnector material is that it has to have very high electrical conductivity. However, complications arise as SOFCs have a very high operating temperature. Problems such as different thermal expansion coefficients, cathode poisoning, and oxidation of metal become serious. For these reasons, at present, ceramic material (lanthanum chromite) is the favoured choice for tubular design SOFCs [17].
The components can be assembled together in various configurations (tubular, planar, etc.) each one of which exhibits advantages and disadvantages. As an example, Figure 1.5 illustrates a conventional cathode-supported tubular layout, which was dominant in SOFC technology in the late 1990s and early 2000s.
Figure 1.5 Tubular SOFC module configuration [17].
Source: US Department of Energy.
1.3.3 Fuel Cell Reactions
The operation of a fuel cell implies a number of electrochemical reactions employing hydrogen and oxygen as fuel and oxidant to generate power (even if some fuel cells are able to operate on different fuels, the main fuel for these reactors is still hydrogen). However, unlike oxygen, which can be easily obtained from air, hydrogen is not found naturally and it must be produced from other hydrogen-containing compounds, typically hydrocarbons and/or water. There are different ways to extract hydrogen from conventional hydrocarbon fuels, amongst which the normal industrial practice is to use fuel reforming [21, 26, 27]:
1.
Water electrolysis.
2.
Steam reforming.
3.
Partial oxidation.
4.
Auto thermal reforming.
5.
Coal gasification.
1.3.4 Fuel Cell Performance
The performance of a fuel cell is often related to fuel cell voltage directly:
where J refers to current density and A to fuel cell active area. Given that the latter parameter is determined during the design and manufacturing process, the voltage of a fuel cell during operation is affected by the variations in current density. Recalling the previous section on the thermodynamic principles of fuel cells, Gibbs free energy changes are used to define the work potential of the cell. In a fuel cell, the ‘external work’ involves driving the flow of electrons around an external circuit. Any work done by a change in volume between inlet and outlet is not harnessed by the fuel cell.
The reference point of zero energy in a fuel cell is normally defined as that of pure elements, in the normal state at standard temperature and pressure (25°C, 0.1 MPa) [21]. If this convention is adopted, then the term ‘Gibbs free energy of formation’, Gf , rather than the ‘Gibbs free energy’, is used (akin to the terms ‘enthalpy of formation’ rather than just ‘enthalpy’ [21]). For an ordinary hydrogen fuel cell operating at standard temperature and pressure (STP), this means that the Gibbs free energy of formation of the input is zero, which is a useful simplification [21]. This simplification (or standardization) is possible because it is the change in energy that is important, meaning that it is the change in this Gibbs free energy of formation, ΔGf , that gives the work potential of the fuel cell, that is, the difference between the Gibbs free energy of the products and the Gibbs free energy of the inputs or reactants [21]:
To make comparisons easier, it is convenient to consider these quantities in their molar-specific form. This indicated by a line over the lower-case letter, for example, is the molar-specific Gibbs free energy of formation for water [21].
The Gibbs potential is defined as:
which is equivalent to:
For fuel cells, it is the change in Gibbs free energy that is responsible for the voltage induced. Therefore the following holds true:
Considering Equation 1.11, it can be observed that for every mole of hydrogen consumed (H2), one mole of oxygen ions is used (O=); one mole of water (H2O) and two moles of electrons (e−) are produced. It thus follows that the amount of electrons produced is then 2·N, N being the moles of hydrogen consumed; this is known as Faraday's law. Assuming one electron carries with it −e charge, the charge flow is −2·N·(−e) which equals to −2F, where F is Faraday's constant which is defined as the electric charge of a mole of electrons.
Upon development of the electrochemical reactions, a voltage difference between electrodes is built up in the cell, which drives the flow of electrons from the cathode to the anode. On the assumption that there are no losses (the ideal case), this is called the Nernst potential and is denoted by E. The electrical work (joules) developed by an ideal fuel cell is then:
where I · t is the flow of electrical charge passing through during a period of time t. This equation can now be linked to the change in Gibbs free energy through the information in Table 1.2:
This ideal potential of a cell operating at constant temperature (E), which yields the maximum electrical work output that the cell can produce, is not attainable in practice. Indeed, the kinetics of the reactions, their activation energy, the limited mass diffusion rate of reactants (to get to the reaction sites) and products (to be evacuated from the reaction sites), and the limited electronic/ionic conductivity of the fuel cell constituents, introduce inefficiencies that manifest as voltage losses. These losses are explained in more detail later in this chapter. The impact of pressure and temperature without these losses (i.e. on the ideal voltage of a fuel cell) are discussion in the next section.
1.3.5 Pressure and Concentration Effects
The foregoing discussion about the ideal cell potential E assumed the selection of a reference pressure and temperature (in this case 25°C and 1 atm), thus utilizing the standard cell potential E0. However, in order to reduce the ionic resistivity of the electrolyte when integrated into hybrid systems, solid oxide fuel cells operate at a much higher temperature, and also typically operate at a higher pressure. This higher operating temperature and pressure means that the above expressions must be applied to Gibbs free energy changes at higher pressure and temperature or, more commonly, a correction must be applied to account for the effects of these two thermodynamic variables and of the concentration of reactants and products (whose proportion is not stoichiometric in a practical case). This correction is a mathematical expression of Le Chatelier's principle.
The correction for temperature can be introduced directly into the standard cell voltage E0
