Fuel Cell education:
The developments leading to an operational fuel cell can be traced back to the early 1800’s with Sir William Grove recognized as the discoverer in 1839. Throughout the remainder of the century, scientists attempted to develop fuel cells using various fuels and electrolytes. Further work in the first half of the 20th century served as the foundation for systems eventually used in the Gemini and Apollo space flights. However,it was not until 1959 that Francis T. Bacon successfully demonstrated the first fully operational fuel cell.
Proton exchange membrane fuel cells were first used by NASA in the 1960’s as part of the Gemini space program, and were used on seven missions. Those fuel cells used pure oxygen and hydrogen as the reactant gases and were small-scale, expensive and not commercially viable. NASA’s interest pushed further development, as did the energy crisis in 1973. Since then, fuel cell research has continued unabated and fuel cells have been used successfully in a wide variety of applications.
Advantages of Fuel Cells
Fuel cell systems are usually compared to internal combustion engines and batteries and offer unique advantages and disadvantages with respect to them. Fuel cell systems offer the following advantages:
Fuel cell systems operate without pollution when run on pure hydrogen, the only by-products being pure water and heat. When run on hydrogen-rich reformate gas mixtures, some harmful emissions result although they are less than those emitted by an internal combustion engine using conventional fossil fuels. To be fair, internal combustion engines that combust lean mixtures of hydrogen and air also result in extremely low pollution levels that derive mainly from the incidental burning of lubricating oil.
Fuel cell systems operate at higher thermodynamic efficiency combustion engines and turbines, convert chemical energy into heat by way of combustion and use that heat to do useful work. The optimum (or “Carnot”) thermodynamic efficiency of a heat engine is known to be: This formula indicates that the higher the temperature of the hot gas entering the engine and the lower the temperature of the cold outlet gas after expansion, the higher the thermodynamic efficiency. Thus, in theory, the upper temperature can be raised an arbitrary amount in order to achieve any desired efficiency, since the outlet temperature cannot be lower than ambient. However, in a real heat engine the upper temperature is limited by material considerations. Furthermore, in an internal combustion engine, the inlet temperature is the operating temperature of the engine, which is very much lower than the ignition temperature. Since fuel cells do not use combustion, their efficiency is not linked to their maximum operating temperature.
As a result, the efficiency of the power conversion step (the actual electrochemical reaction as opposed to the actual combustion reaction) can be significantly higher. The electrochemical reaction efficiency is not the same as overall system efficiency as discussed in Section 4.1.2. The efficiency characteristics of fuel cells compared with other electric power generating systems are shown in figure 4.2.
In addition to having higher specific thermal efficiency than heat engines, fuel cells also exhibit higher part-load efficiency and do not display a sharp drop in efficiency as the power plant size decreases. Heat engines operate with highest efficiency when run at their design speed and exhibit a rapid decrease in efficiency at part load. Fuel cells, like batteries, exhibit higher efficiency at part load than at full load and with less variation over the entire operating range. Fuel cells are modular in construction with consistent efficiency regardless of size. Reformers, however, perform less efficiently at part load so that overall system efficiency suffers when used in conjunction Fuel cells exhibit good load-following characteristics.
Fuel cells, like batteries, are solid-state devices that react chemically and instantly to changes in load. Fuel cell systems, however, are comprised of predominantly mechanical devices each of which has its own response time to changes in load demand. Nonetheless, fuel cell systems that operate on pure hydrogen tend to have excellent overall response. Fuel cell systems that operate on reformate using an on-board reformer; however, can be sluggish, particularly if steam-reforming techniques are used.
When used as an electrical energy generating device, fuel cells require fewer energy transformations than those associated with a heat engine. When used as a mechanical energy-generating device, fuel cells require an equal number of conversions, although the specific transformations are different. Every energy transformation has an associate energy loss so that the fewer transformations there are, the better the efficiency. Thus fuel cells are more ideally suited to applications that require electrical energy as the end product, rather than mechanical energy. Comparative energy transformations for fuel cells, batteries and heat engines are shown in Figure 4-3.
Figure
Fuel cell systems suitable for automotive applications operate at low temperatures (typically less than 212 ºF/100 ºC). This is an advantage in that the fuel cells require little warmup time, high temperature hazards are reduced, and the thermodynamic efficiency of the electrochemical reaction is inherently better. This is a disadvantage in that medium-grade waste heat is harder to expel (especially in hot climates) so that cooling systems must be larger, and the electrochemical reaction proceeds more slowly than at high temperatures. Reformers used in conjunction with fuel cells operate at high temperatures and therefore may require prolonged warmup periods. Fuel cell systems can be used in co-generation applications.
In addition to electrical power, fuel cells generate pure hot water and medium-grade heat, both of which can potentially be used in association with domestic or industrial applications. When this is done, the overall efficiency of the combined systems increases.
Fuel cell systems do not require tuning. Fuel cell systems do not require recharging. Rather, fuel cell systems must be re-fueled, which is faster than charging a battery and can provide greater range depending on the size of the storage tank.
Disadvantages of Fuel Cells
Fuel cell systems suffer the following disadvantages:
Ironically, hydrogen which is of such benefit environmentally when used in a fuel cell, is also its greatest liability in that it is difficult to manufacture and store. Current manufacturing processes are expensive and energy intensive, and often derive ultimately from fossil fuels. An effective hydrogen infrastructure has yet to be established. Gaseous hydrogen storage systems are large and heavy to accommodate the low volumetric energy density of hydrogen. Liquid hydrogen storage systems are much smaller and lighter, but must operate at cryogenic temperatures. Alternatively, if hydrogen is stored as a hydrocarbon or alcohol and released on demand by way of an on-board reformer, the storage and handling issues simplify, but some of the environmental benefits are lost. Fuel cells require relatively pure fuel, free of specific contaminants. These contaminants include sulfur and carbon compounds, and residual liquid fuels (depending on the type of fuel cell) that can deactivate the fuel cell catalyst effectively destroying its ability to operate. None of these contaminants inhibit combustion in an internal combustion engine. Fuel cells suitable for automotive applications typically require the use of a platinum catalyst to promote the power generation reaction. Platinum is a rare metal and is very expensive. Fuel cells must not freeze with water inside. Fuel cells generate pure water during the power generating reaction and most fuel cells suitable for automotive applications use wet reactant gases. Any residual water within the fuel cells can cause irreversible expansion damage if permitted to freeze. During operation, fuel cell systems
generate sufficient heat to prevent freezing over normal ambient temperatures, but when shut down in cold weather the fuel cells must be kept warm or the residual water must be removed before freezing. This normally entails bringing the vehicle into a heated facility or the useof a localized hot air heating device. Fuel cells that use proton exchange membranes must not dry out during use and must remain moist during storage. Attempts to start or operate these fuel cells under dry conditions can lead to membrane damage. Fuel cells require complex support and control systems. Fuel cells themselves are solid state devices, but the systems required to support fuel cell operation are not. Of particular note is the requirement for compressed air; this necessitates a high-speed compressor that imposes a large parasitic load on the overall system. System complexity increases significantly when the fuel cells are operated in conjunction with an on-board reformer.
Fuel cell systems are heavy. Fuel cells themselves are not excessively heavy, but the combined weight of the fuel cells, their support systems and their fuel storage is presently greater than for a comparable internal combustion engine system. Systems that include an on-board reformer are heavier still. Fuel cell systems are generally lighter than comparable battery systems even though the battery systems require less support equipment. System weight will likely continue to decrease as the technology develops. Despite their weight, existing fuel cell prototype vehicles have shown that systems can be made sufficiently compact for automotive use. Fuel cells are an emerging technology. As with any new technology, reductions in cost, weight and size concur- rent with increases in reliability and lifetime remain primary engineering goals.- Basic fuel cell science