SOFCs PDF

ADVANCED MATERIALS FOR ENERGY CONVERSION SOLID CELLS SOLID OXIDE FUEL CELLS (SOFC) Ref. 4, Cap. 2 WHAT IS SOFC? A solid oxide fuel cell (SOFC) is an energy conversion device that converts chemical energy of a fuel (such as hydrogen or hydrocarbons) into electricity through a series of...

ADVANCED MATERIALS FOR ENERGY CONVERSION SOLID CELLS SOLID OXIDE FUEL CELLS (SOFC) Ref. 4, Cap. 2 WHAT IS SOFC? A solid oxide fuel cell (SOFC) is an energy conversion device that converts chemical energy of a fuel (such as hydrogen or hydrocarbons) into electricity through a series of electrochemical reactions; no combustion process is involved. HISTORY Faraday’s early investigations of conduction in ceramics in the 1830s Fuel cells were first reported in 1839 by Sir William Grove In the 1890s, Walther Nernst observed the significantly increased conductivity of mixed oxides over their pure constituents that the was first technological implication of ion conduction in solids The 1930s saw the conceptual development of ion conduction through lattice defects by Schottky and Frenkel, the submission of the first SOFC patent through Siemens and Halske NASA began Project Gemini, the first practical application in 1961 EFFICIENCY and OPEN CIRCUIT VOLTAGE (OCV) Other advantages: Electrical efficiency: ≥40% Fuel flexibility Reciprocating engine (≈ 35 %), Noise-free operation Photovoltaics (6-20) % Generating excessive heat Wind turbines (≈25 %). Less pollution Effective reduction of greenhouse gas  WIND-DRIVEN GENERATOR In some electrical power-generating devices, it is very clear what form of energy is being converted into electricity. The input and output powers of a wind-driven generator are simple to understand and calculate. The ratio wind kinetic power-to-mechanical power in the rotor shaft is called power coefficient, Cp overall wind turbine efficiency = (Cp) x (electric machine efficiency)  EFFICIENCY OF FUEL CELL The electrical power and energy output are easily calculated from the well known Formulas“: However, the energy of the chemical input and output is not so easily defined. HYDROGEN/OXYGEN FUEL CELL  HYDROGEN/OXYGEN FUEL CELL The values are negative, which means that energy is released. If there are no losses in the fuel cell, then all this Gibbs free energy is converted into electrical energy. HYDROGEN/OXYGEN FUEL CELL For the hydrogen fuel cell, two electrons pass round the external circuit for each water molecule produced and each molecule of hydrogen used. One mole of hydrogen 2N electrons (N: 6.02x1023) If −e is the charge on one electron, then the charge that flows is: F: the Faraday constant=N e = 96485 C If E is the voltage of the fuel cell, then the electrical work done moving this charge round the circuit is:  THE OPEN CIRCUIT VOLTAGE (OCV) For example, a hydrogen fuel cell operating at 200 °C: It can be applied to other reactions too z is the number of electrons transferred for each molecule of fuel. For example, the methanol fuel cell:  FUEL CELL EFFICIENCY There is a connection between the maximum electromotive force (EMF) of a cell and its maximum efficiency. Electromotive Force (EMF)  EFFICIENCY and THE FUEL CELL VOLTAGE The operating voltage of a fuel cell can be very easily related to its efficiency. If all the energy from the hydrogen fuel, its ‘calorific value’, heating value, or enthalpy of formation, were transformed into electrical energy, then the EMF would be given by 𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 Cell efficiency = 𝐸 𝑎𝑐𝑡𝑢𝑎𝑙 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 Cell efficiency = 𝜇𝑓 𝐸 OPERATIONAL FUEL CELL VOLTAGES Theoretical value of the open circuit voltage (OCV) of a hydrogen fuel cell: CAUSES OF VOLTAGE DROP Activation losses. These are caused by the slowness of the reactions taking place on the surface of the electrodes. Fuel crossover and internal currents. This energy loss results from the waste of fuel passing through the electrolyte, and, to a lesser extent, from electron conduction through the electrolyte. Ohmic losses. This voltage drop is the straightforward resistance to the flow of electrons through the material of the electrodes and the various interconnections, as well as the resistance to the flow of ions through the electrolyte. Mass transport or concentration losses. These result from the change in concentration of the reactants at the surface of the electrodes as the fuel is used. SOFC ADVANTAGES Compared with other fuel cells, SOFC has two particular advantages, owing to its high temperature operation; SOFC allows the use of a variety of fuels ranging from hydrogen to CO to hydrocarbons SOFC produces significant amount of exhaust heat, which can be used in combined heat and power systems (CHP).  SOFC APPLICATIONS AND POWER SYSTEMS Because of their superior electrical efficiency and fuel flexibility, SOFC-based power systems, compared to other fuel cell systems, enable numerous applications at various power levels, from a few-watt to MW size systems. Small SOFC Systems for Residential CHP Applications A major application for SOFCs is at 1–5 kW level to supply combined heat and power (CHP) to residential buildings utilizing natural gas as the fuel. Residential CHP units will probably be the first commercial application of SOFCs.  SOFC APPLICATIONS AND POWER SYSTEMS Large SOFC Systems for Distributed Power Generation Westinghouse100 kW atmospheric power generation system in 1986: The stack consisted of 1152 cells (2.2 cm diameter and 150 cm active length) This system operated for over 36,750 h in USA, Netherlands, Germany, and Italy on desulfurized natural gas at an efficiency of 46 %. Siemens Westinghouse’s 100 kW SOFC cogeneration system  SOFC APPLICATIONS AND POWER SYSTEMS 220 kW pressurized SOFC/gas turbine hybrid system The world’s first demonstration of an SOFC coupled with a microturbine generator and the first demonstration of a pressurized SOFC generator. 3400 h of runtime efficiency of 53 % National Fuel Cell Research Center on the campus of the University of California-Irvine (USA)  SOFC APPLICATIONS AND POWER SYSTEMS Bloom Boxes 100 kW sized SOFC power systems To commercial customers such as Adobe Systems, Bank of America, Cox Enterprises, Coca Cola Company, eBay, FedEx, Google, Safeway, Staples, Walmart, etc. Five 100 kW sized SOFC systems (Bloom Boxes) installed at eBay Headquarters  PORTABLE SOFC POWER SYSTEMS The portable applications generally require power in the range from milliwatts to a few hundred watts. Challenges arising for SOFCs in portable applications stacks must be light short startup time https://rohaltechnologies.com/fuel-cell-natural-gas thermally sustaining Advantege Superior fuel flexibility D350, a back-pack sized 350 W SOFC power generator fueled by propane. SOFC-BASED TRANSPORTATION AUXILIARY POWER UNITS (APU) The challenges for SOFC in APUs Compact size Light weight Short start-up time Mechanical robustness Capability for thermal cycling Delphi’s SOFC APU mounted underneath a Peterbilt’s truck cabin  SOFC DESIGNS Under typical operating conditions, a single cell produces less than 1 V. To obtain high voltage and power from the SOFCs, it is necessary to stack many cells together. Planar SOFC Design Electrolyte-supported Electrode-supported Metal-supported SOFC DESIGNS  PLANAR SOFC DESIGN Advantages Simpler and less expensive manufacturing processes Higher power densities Challenging area in commercializing planar SOFCs High-temperature gastight seals between the components The configuration of gas flow and gas manifold Uniform distribution of temperature and current to reduce thermal stresses  TUBULAR SOFC DESIGN Advantages Not require any high-temperature seals to isolate oxidant from the fuel Very sable performance over long periods of times Challenging area Areal power density is much lower Manufacturing costs are higher The volumetric power density is lower CLASSIFICATION OF SOFCS