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The fuel cell: A technical report. Energy moving into the future Managing global energy supplies is increasingly becoming a key issue for the future of mankind. If present usage levels are sustained, fossil energy resources created over several hundred millions of years will be used up within just a few generations. The future of energy supply lies in opening up renewable energy sources and developing new technologies such as the fuel cell. The fuel cell’s potential For decades, the internal-combustion engine has been a hallmark in the history of the automotive industry and of stand-alone energy supply. To most users, it has so far been the only appropriate solution to drive cars or generate power at remote sites. Fuel cells offer, for the first time, the chance to replace the combustion engine in a number of applications and thereby avoid harmful emissions. For the energy industry, they open up the option of sustainable, resource-saving supply, and – thanks to their ecological soundness – many diverse applications. This includes applications in the mobile sector and all areas of the energy industry. The fuel cell looks back on a long track record. As early as 1839, an Englishman, Sir William Robert Grove (1811 – 1896), constructed the first fuel cell. Its further development proved such an arduous task that Grove’s concept was only used in isolated applications for nearly 100 years. His fuel cells featured electrodes made of platinum sitting in a glass tube with their lower end immersed in dilute sulfuric acid as an electrolyte and their upper part exposed to hydrogen and oxygen inside the tube. This was sufficient to produce a voltage of 1 volt. To turn the fuel cell into a really efficient source of power, substantial technical efforts had to be made. Over 160 years have lapsed since the fuel cell was in- vented. Its true potential as the energy converter of the future has only recently manifested itself. Today, it is on the point of commercial use. Sir William Grove (1811 –1896) constructed the first fuel cell Grove’s historic fuel cell (1839) 2 3 New prospects created by an old idea. Fuel cells can revolutionize the energy sector Fuel cells generate electricity from hydrogen and oxygen – without any harmful emissions and therefore in an extremely environmentally friendly way. Heat is produced in varying amounts and, as a by-product, water. The fuel cell principle. A powerful concept for resource-saving harnessing of energy Operating principle of the fuel cell A proton exchange membrane is coated with a thin platinum cata- lyzer layer and a gas-permeable electrode made of graphite paper. Hydrogen fed to the anode side ionizes into protons and electrons at the catalyzer. The protons pass the catalyzer layer, while the electrons remaining behind give a negative charge to the hydrogen-side elec- trode. During the proton migration, a voltage difference builds up be- tween the electrodes. When these are connected, this difference pro- duces a direct current that can drive an engine, for example. Finally, the protons recombine with the elec- trons and the oxygen into water at the cathode. Besides the recovered electric energy, the only reaction product is water. Additionally, heat is produced by the electrochemical reactions and the contact resistances in the fuel cell, which can be used for space or service water heating. The voltage of a single non-oper- ated cell is about 1.23 V (volts). In operation, this level falls to about 0.6 to 0.7 V under load. As this level is too low for practical applications, a sufficient number of cells is con- nected in series to obtain a usable voltage. They may add up to 800 cells in larger-sized plants. The line-up of cells is equivalent to a stack, and this word has become a technical term generally used for this arrangement. It is characteristic of fuel cells that they generate a DC voltage. To allow practical use, it has to be transformed into an AC signal. This is done by downstream DC/AC converters. 4 5 Hydrogen Oxygen Excess oxygen Excess hydrogen Electrolyte Reaction water CathodeAnode Electrical load How a fuel cell works Benefits of fuel cells Fuel cells convert hydrogen and oxygen into electric energy. At the same time, heat is pro- duced that lends itself to supplying process heat, producing hot water and delivering heat to buildings. If operated as co-generation units (combined heat and power generation – CHP), fuel cells reach energy conversion rates of up to 80 percent and can therefore make a sus- tainable contribution to energy saving. Compared with conventional techniques, the use of fuel cells holds additional promise. This includes high efficiencies even where plant capacity is small, constant efficiency under part load, simple and modular design, low maintenance expenses and a level of hazardous substance emissions so low that it cannot be achieved with any other technique. As hydrogen is directly converted by electro- chemical reactions, the efficiency of fuel cells is – unlike traditional energy conversion processes – not limited. Fuel cells can therefore reach much higher efficiencies than internal-combustion engines. Fuel cells are also effective under part load. Unlike in conventional systems, the efficiency remains largely constant until 50 percent full load. This has merits for plants which are fre- quently operated under part load (e.g. motor vehicles in inner-city traffic). Carbon dioxide emissions (CO2) result from use of carbonaceous fuels. These include all fossil energies such as coal, oil and natural gas. As fuel cells will in the medium and long term use fossil resources (natural gas) as an auxiliary fuel, their use also leads to carbon dioxide emissions. But thanks to combined heat and power generation and the high efficiencies, CO2 emissions will be lower than in conven- tional systems. 50 percent part load Coal-fired plant Efficiency Gas turbine Fuel cell Full load Practically Theoretically NOx CO CHx BHPP FC 50 100 150 200 0 250 300 350 mg/Nm3 (mg / standard cubic meter) Source: HEW Comparison of emissions Compared with established technologies – block heat and power plants (BHPP) or other power plants – fuel cells (FC) boast very low emission levels. Part-load efficiencies of fuel cells The operating parameters of fuel cells look favorable even under part load. As opposed to conventional plants, efficiency remains constant. This qualifies fuel cells for use in units frequently run on part load (e.g. vehicles in inner-city traffic). Five fuel cell technologies are at present being developed.They differ in their electrolyte structure, working temperature and fuel requirements. Their designations refer to the electrolyte used. The fuel cell technique is on the point of commercial use in many areas Fuel cell types and applications. Type Electrolyte Special features Applications SOFC Solid Oxide Solid zirconia Direct power production Central and stand-alone Fuel Cell from natural gas, ceramics CHP generation MCFC Molton Carbonate Molten Complex process control, Central and stand-alone Fuel Cell carbonates corrosion problems CHP generation AFC Alkaline Fuel Cell Aqueous High efficiency, Space operations, potash lye pure H2 and O2 only defense PAFC Phosphoric Acid Phosphoric acid Limited efficiency, Stand-alone CHP Fuel Cell corrosion problems generation PEMFC Proton Exchange Proton exchange High flexibility in operation, Vehicles, stand-alone Membrane Fuel Cell polymer high power density CHP generation membrane (small-scale) 6 7 Solid Oxide Fuel Cell (SOFC) Solid oxide fuel cells are designed for use in all areas of electricity supply. At working temperatures up to 1000 °C, they have potential for highly efficient energy sup- ply. In particular if combined with mod- erately priced gas turbines, SOFC cells can in future also be used to construct small-scale generation plants with effi- ciencies comparable to those of natural gas fired combined-cycle power plants. Mini-plants for residential and small com- mercial applications are being developed by Swiss Sulzer Hexis AG, and larger-sized plants with capacities between 250 kW (kilowatts) and 20 MW (megawatts) by Siemens Westinghouse. SOFC fuel cell with gas turbine Replacing the gas turbine combustion chamber by a fuel cell leads to a hybrid process, raising the gas turbine’s efficiency from less than 30 percent to over 60 percent. Fuel cell Natural gas Waste heat exchanger Compressor Generator Turbine 0.1 1 10 10 20 30 40 0 50 60 70 80 0.01 100 300 1,000 Gas turbineGas engine PEFC PAFC SOFC, MCFC SOFC-, MCFC-Combined-cycle power plant Combined-cycle power plant Future: upper values Present: lower values Electric power output in MW Electrical efficiency in percent Fuel cell efficiency The efficiency of fuel cells is not limited by the Carnot process as hydrogen is directly converted by electrochemical reactions. Fuel cells can therefore attain considerably higher efficiencies than comparable internal-combustion engines. Fuel cell types N2, unconverted O2 and reaction gas Load O2 CathodeAnode Electrolyte Fuel gas Unconverted fuel gas and reaction gas 900 … 1,000 °C 600 … 650 °C 160 … 220 °C 20 … 120 °C 60 … 120 °C AFC OH– PEMFC H+ MCFC CO32– O2– SOFC PAFC H+ O2 O2 CO2 O2 H2O O2 H2O H2 H2O H2 H2 CO2 H2O CO H2 CO2 H2O CO H2 160 … 220 °C 20 … 120 °C N2, unconverted O2 and reaction gas Molten Carbonate Fuel Cell (MCFC) Molten carbonate fuel cells working at temperatures between 500 °C and 600 °C are being developed for industrial appli- cations. They permit electricity genera- tion with high efficiencies (approx. 55 percent) and simultaneous production of process steam, offering excellent potential for industrial combined heat and power generation. In Germany, a prominent developer of this technology is MTU, and in America, Fuel Cell Energy is foremost. Alkaline Fuel Cell (AFC) Alkaline fuel cells are distinguished by a combination of low working temper- atures and high efficiencies. They are favored for niche applications in the space industry or the maritime sector, e. g. to drive submarines. The demands this cell type makes on the purity of hydrogen and oxygen clearly limit the practical scope of use. This fuel cell type is developed by ZeVco (Zero Emission Vehicle Corporation) and others in Germany; worldwide, IFC (International Fuel Cell) and Fuji work on it. Phosphoric Acid Fuel Cell (PAFC) Opportunities for phosphoric acid fuel cells working at temperatures around 200 °C are in combined heat and power production. Administrative buildings, hospitals, indoor pools but also large resi- dential estates are potential applications. Over 180 plants supplied by American manufacturers ONSI are operated world- wide and have in part already been decommissioned after reaching their expected service life. They are proof of the vast interest in using the fuel cell technology. Still, experience with this technique has given rise to some doubts about its further success. They concern the limited potential for saving manu- facturing costs, and technical restrictions resulting from the need to constantly maintain temperatures. Besides ONSI, Japanese companies Fuji and Toshiba work on this method. Proton Exchange Membrane Fuel Cell (PEMFC) Employing fuel cells in the end user mar- ket is seen as a particularly interesting opportunity. About 25 percent of primary energy consumption in Germany are accounted for by space heating and hot water supply, so that use of fuel cells as CHP plants would contribute to energy saving. That temperature levels are rela- tively low in the supply of hot water for space and service water heating opens up a range of applications for low- temperature cells with proton exchange membranes. PEM fuel cells are used in the Berlin demonstration project. With an electric power output of 250 kWel , this is their first commercial-scale application in Europe. The success of this project will be an important prerequisite for marketing planned from 2004 onwards. Energy consumed in Germany to supply heat About 25 percent of the primary energy con- sumption of 492 x 106 tons of hard-coal equiva- lent were used in 1998 to meet space heating and hot water needs. Chemical reactions in each fuel cell type 9 % 34 % 57 % Process heat Hot water Space heating 8 9 Water vapor Shift converter Fuel cell Shift reactionReformer reaction Total Product water Water vapor Anode off-gas (H2) Making hydrogen available Industrial sources are, for example, che- mical industry operations. The excess H2 produced in synthesis gas manufac- ture has already found its way into the energy supply sector, and can in future be used more widely. The same applies to electric energy that is freely avail- able in low-load periods. Electrolyzer outfits can be employed to produce hydrogen out of it and use it in peak load periods to cover electricity needs or supply vehicles with H2. Recovering hydrogen from renewable energies is today still seen as a future option. Economic reasons will continue to reduce it to niche applications for some time. Fossil energy sources such as natural gas and mineral oil appear to be the most promising sources for hydrogen production in the medium term. This requires treatment – so- called reformation – processes to be installed upstream of the fuel cell. Natural gas reformation The future’s name is hydrogen. Hydrogen stores renewable energies Hydrogen is not freely available in nature. Chemical and electrochemical processes will therefore long have to be used to recover it. Fuel cell H2 storage Electrolyzer outfit Electrolyzer outfit Reformer Renewable energy Excess electricity Industrial sources Natural gas reformation Hydrogen production through reformation Reformation is a multi-stage process trans- forming hydrogen-containing energy sources into hydrogen-rich gases. As this process consumes energy, it results in drawbacks to the energetic overall effi- ciency of a fuel cell process. In a first step, natural gas is split in a reformation reaction into a gas mix con- sisting of three parts hydrogen (H2) and one part carbon monoxide (CO). In addi- tion to process heat (+205.8 kJ/mol, i. e. kilojoules per mol), this requires feeding of water vapor as a coreactant. In a sec- ond step, the remaining CO is, with the help of steam, oxidized to carbon dioxide (CO2) in a shift reaction. It releases a fur- ther free hydrogen molecule. The final product is a gas mix consisting of four parts H2 and one part CO2 which can be directly used in the fuel cell. The shift reaction is exothermic, i.e. connected with a release of energy (– 42.3 kJ/mol). This energy can be employed to partly cover the energy demand of refor- mation. PEM fuel cells are normally highly sensitive to CO contained in the fuel gas. Carbon monoxide is regarded as a catalyzer poison. These fuel cells therefore necessi- tate removing residual CO in a third or even fourth treatment stage, which is done in a process called selective oxi- dation. The entire gas treatment process in- volves a 20 to 30 percent loss of energy, which detracts from the efficiency of the fuel cell process. Losses are lower when high-temperature fuel cells are used. They permit internal reformation of the fuel inside the fuel cell, which leads to hardware savings and efficiency advan- tages. But the price paid for these bene- fits is that expensive materials with high temperature resistance have to be used. State of the art Fuel cells have reached a maturity today that allows building complete plants in commercially usable sizes. They serve as demonstration units for their manufac- turers to test and optimize their plant configurations, and help their operators – mainly energy utilities – gain initial ex- perience with this new technology. This is also the purpose of the Berlin fuel cell project. Demonstration projects should not be confused with commercial fuel cell applications. This needs further years of development and operating experience. With a view to mobile applications, this means in particular that a decision has to be made which energy resource will in future be needed – either hydrogen is directly used, or methanol serves as an intermediate solution. For stationary applications, efficient and compact reformers will have to be developed that allow low-cost conversion of natural gas into hydrogen-rich gas. Many companies have committed to working intensively on development. Even well-renowned manufacturers nev- ertheless assume that both mobile and stationary serial products will not be commercially available until after 2004. Methods of hydrogen recovery Design of a PEM fuel cell A fluorine-containing polymer membrane is the outstanding feature of the PEM fuel cell design. Vaporized with a catalytic precious metal and covered by a gas- permeable graphite electrode, it is enclosed by two bipolar plates. PEM fuel cells boast simple design and uncomplicated manufacture. 1 Flow field plate (fuel supply) 2 Hydrogen supply 3 Membrane electrode assembly 4 Air supply 5 One fuel cell – pulled apart 6 Stack – consisting of several cells 6 1 3 42 5 Simple and low-cost A fluorine-containing foil, vaporized with a catalytic precious metal and coated with a gas-permeable electrode made of graphite paper, is enclosed by two bipolar plates made of metal or graphite. Grooves milled or pressed into the plates allow feeding of hydrogen and oxygen or air to the anode and cathode. The simple design of the PEM fuel cell suggests that manufacture will in future be low-cost, permitting mass production as usual e. g. in the automotive industry. Merits of PEM fuel cells PEM fuel cells outperform competing technologies in a number of ways that speak in favor of their good marketing potential. The following aspects are core to its superiority: PEM fuel cells are appropriate for mobile and sta- tionary use.Their versatility in application suggests synergies, with cost benefits lying with stationary applications. PEM fuel cells have the highest power density com- pared with competing technologies, with potential for further development.This allows building small, space-saving systems as required for remote-site applications. PEM fuel cell applications in power supply range from mini-systems generating a few watts via stand- alone units in combined heat and power to applica- tions in stand-by power supply – a universally usable technology. Description of the PEM fuel cell. A variety of applications 200 400 600 800 0 1,000 1,200 Watts / liter 1995 1997199319911989 Development of the power density of PEM fuel cells Project venture and project costs Five energy utilities participate in the Berlin fuel cell project: Bewag as the project leader, Électricité de France (EdF) – Paris, Hamburgische Electricitätswerke AG (HEW) – Hamburg, Hanover-based PreussenElektra Aktiengesellschaft (now E.ON Energie AG, Munich) and Vereinigte Energiewerke Aktien- gesellschaft (VEAG) – Berlin. Canadian manu- facturers Ballard Generation System (BGS) supplied the plant, and ALSTOM delivered the complete system. ALSTOM regards the Berlin project as an important step towards commercial-scale introduction of fuel cells. The company has plans for the future to jointly produce comparable plants in series with its partner Ballard in a new Europe-based company. They will hinge on successful completion of the Berlin demonstration project. Project costs amount to an approximate 3.8 million. The innovative character of the project and its expected favorable impact on the European economy persuaded the European Commission to financially assist this project. It contributes 40 percent of the project