Fuel cells: Basics, Types, applications and technological bottlenecks

Fuel cells have been around longer than most batteries - the principle of the fuel cell was discovered in 1839 by Sir William Grove. They generate electricity from the reaction of hydrogen with oxygen to form water in a process which is the reverse of electrolysis. The fuel cell relies on a basic oxidation/reduction reaction, as with a battery, but the reaction takes place on the fuel rather than the electrodes. The fuel cell produces electricity as long as the cell receives a supply of fuel and can dispose of the oxidized old fuel . In a fuel cell, the anode usually is bathed in the fuel; the cathode collects and makes available the oxidant (often atmospheric oxygen). An ion-conducting membrane separates the two, allowing the reaction to take place without affecting the electrodes. There are six major fuel cell technologies are currently being pursued for different applications each with its own characteristics. Some operate at high temperatures, some use exotic electrode materials or catalysts, all are very complex. Alkali Phosphoric Acid Solid Oxide Molten Carbonate Proton Exchange Membrane PEM Direct Methanol DMFC They have been proposed for a wide range of applications from powering laptop computers, through automotive traction to high power load levelling. The most active developments are currently in the automotive sector where the favoured technology is PEM. This promises a high conversion efficiency of over 60% and an energy density of 120 W/Kg. DMFC do not use Hydrogen fuel with its associated supply problems, but the more convenient liquid Methanol. They are less efficient but offer compact and convenient designs suitable for future consumer electronics applications. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack and the surface area of the PEM. Advantages Fuel cell power is usually proposed as the green, alternative to the internal combustion engine, fuelled only hydrogen and leaving no pollutants other than water. Simple fuel requirements needing hydrogen fuel only, taking their oxygen from the air. No recharging is necessary. No time lost through recharging. (Acts like a perpetual primary cell) So long as fuel provided, the cells can provide constant power in remote locations. Practical fuel cells already have efficiencies as high as 60% Fuel cells deliver maximum efficiency at low power levels.( This is the reverse of the internal combustion engine) For transport applications fuel cell vehicles offer higher "well to wheel" (WTW) efficiencies than conventional internal combustion engines. hortcomings The environmentally friendly credentials of fuel cells overlook the processes needed to generate and distribute the hydrogen fuel. Fuel cells merely shift the pollution from the vehicle to some other location. Today, 98% of hydrogen is produced from fossil fuel sources. According to researchers Andrew and Jim Oswald from Warwick University: To replace petrol and diesel used for road transport in Britain with hydrogen produced by the electrolysis of water would require the building of 100 nuclear power stations or 100,000 wind turbines. If the wind turbines were sited off-shore, this would mean an approximately 10-kilometre-deep strip of wind turbines encircling the entire coastline of the British Isles. If sited on-shore then an area larger than the whole of Wales would have to be given over to wind turbines. A major factor inhibiting market take off is the lack of available infrastructure to provide the hydrogen fuel. Hydrogen fuel can be supplied in pure form in cylinders or the on board cylinders can be refilled at special refueling stations. Despite safety precautions there is still a perception by the general public that hydrogen fuel is unsafe. Alternatively hydrogen can be generated on board, as required, from hydrocarbon fuels such as Ethanol, Methanol, Petrol or Compressed Natural Gas in a process known as reforming. This is not an ideal solution. Reforming generates carbon dioxide as a waste product losing some of the green benefits of fuel cells. It is also expensive and it is like carrying your own chemical plant with you, but it does simplify the fuel supply infrastructure problem, however the fuel could just as easily power an internal combustion engine directly. Even ignoring these problems there are still many shortcomings in using fuel cells for prime motive power. The low cell voltage 0.6 - 0.7 Volts means that the system needs a lot of cells to obtain a normal operating voltage of 200 - 300 Volts to power the drive train motor. Power is generated as required but the process is not reversible within the fuel cell and so, like a primary cell, it can not accept regenerative braking loads. Fuel cells generate electrical energy but they can not store electrical energy. Fuel cells have a low dynamic range and slow transient response which causes an unacceptable lag in responding to calls for power by the driver. A power boost from a battery or from supercapacitors is therefore needed to achieve the desired system performance. Most designs need to work at high temperatures in order to achieve reasonable operating efficiencies. To generate the same efficiencies at lower temperatures requires large quantities of expensive catalysts such as platinum. Low temperature freeze-up of the electrolyte. Electrodes which are prone to contamination. Due to the need to use of exotic materials and complex system designs the system are still very expensive. Theoretically a fuel cell should be all that is needed to power an electric vehicle, however batteries are still needed to support fuel cell systems. Battery Support Batteries are needed in fuel cell vehicle applications for the following functions: During start- up to heat the fuel cell stack and the electrolyte to the optimum working temperature To pump the working fluids through the stack (air, hydrogen, water) To power the reformer if hydrogen is generated on board To provide short term power boosts to compensate for the fuel cell's slow response to sudden power demands (acceleration) To capture regenerative braking energy To power the vehicle's low voltage electrical systems Applications For automotive applications fuel cells are only suited to hybrid applications for providing the base power load with the demand peaks and troughs, and regenerative braking, being accommodated by batteries or booster capacitors. The fuel cell can therefore be dimensioned to work at its optimum working point, providing the average power rather than the peak power requirement permitting significant cost savings. Fuel cells have been used successfully in aerospace applications. Simple low power demonstrator kits are available for education purposes. Perhaps the best applications for fuel cells will be for high power load levelling. Prototypes of Direct Methanol cells are currently being trialled for mobile phone and laptop computer applications. Costs For a true comparison of alternative system efficiencies, costs and benefits, each alternative should be based on the same fuel source. Using oil as the original source of the energy the "well to wheel" cost provides a rational comparison of the energy utilisation efficiency of different systems. But oil is not the only source of energy. Electrical energy used to power electrical vehicles or to produce the hydrogen to feed the fuel cells can be derived from a wide variety of sources. These may include power stations fuelled by oil, or coal or hydro or nuclear power, or renewable resources such as wind, wave and solar power. There can thus be a huge variation in costs and environmental impacts depending on the methods used to supply the necessary fuel. Although many working systems for different applications have been built, practical, cost effective products are still perhaps ten years away. SOFC for stationary power generation Solid oxide fuel cells (SOFC) also operate at high temperatures, from 700°C to 1,000°C, depending on the design and application. There is ongoing research worldwide to establish operating conditions and material sets that could enable both ease of manufacturing and relatively low-cost mass production. For example, SOFCs with high power densities operating at lower temperatures—700°C instead of 1,000°C —have been developed and operated. The lower operating temperature may enable lower costs. A solid oxide fuel cell system has been commercialized as of this writing, but there are a number of companies working to establish systems, including Acumentrics, General Electric, IonAmerica, Rolls-Royce, and Siemens Power Corp. Alone, high-temperature fuel cells show tremendous promise. Through hybridization, high-temperature fuel cells may even achieve even greater efficiency. Hybridization occurs by combining a high-temperature fuel cell with a traditional heat engine such as a gas turbine. The resulting system performs at far higher efficiency than either system alone. Combined with an inherent low level of pollutant emission, hybrid configurations are likely to make up a major percentage of the next-generation advanced power generation systems for a wide range of applications. These efforts will likely be worthwhile given the escalating costs of fossil fuel. Because of the huge potential, some states have taken aggressive steps to be the manufacturing and employment base for fuel cell technology. California has established the Stationary Fuel Cell Collaborative, with a core group composed of state, federal, and non-government agencies to encourage a coordinated strategy. Industry is engaged through an advisory panel. Several years ago, the state of Ohio committed $103 million to establish a manufacturing and employment base for fuel cell technology. All of this activity affirms the strong interest in high-temperature fuel cells as the next generation of electricity and thermal product. Many believe high-temperature fuel cell technology will become an integral strategy for central power production of electricity and transportation fuels and a hybrid configuration is expected to provide hoteling or propulsive power for ships, locomotives, long-distance trucks, and civil aircraft. In all their potential applications—residential, commercial, industrial, or institutional, in distributed generation or in central power plants—high-temperature fuel cells indeed portend a profound change in the manner by which power will be generated in the decades to come.