Fuel cell system considerations

Keeping the system pressure drop low is a critical issue for practical systems because compression of gases requires energy. The parasitic power losses originating from compressors are known to reduce the efficiency of fuel-cell systems considerably [10]. Steam reforming of liquid fuels is an exception here because only liquid pumps for both fuel and water and no compressor are required to supply... 

The complexity of the overall fuel processing system for carbonaceous fuels indicates the challenge associated with adapting conventional fuels for use in small fuel cell systems. Considerable effort is required in designing and optimizing the system to achieve the requisite miniaturization and low cost in these applications. 

These cost considerations have to rely on some risk analysis if the risks of producing smaller series for stationary markets, which can absorb fuel-cell systems at a specific cost of some 500/kWel are smaller, it seems likely that stationary fuel cells will enter the market earlier than mobile fuel cells in cars at a specific cost of 50 to 80/kWel, but with much larger production series. 

In addition to high-profile fuel cell applications such as automotive propulsion and distributed power generation, the use of fuel cells as auxiliary power units (APUs) for vehicles has received considerable attention (see Figure 1-9). APU applications may be an attractive market because it offers a true mass-market opportunity that does not require the challenging performance and low cost required for propulsion systems for vehicles. In this section, a discussion of the technical performance requirements for such fuel cell APUs, as well as the current status of the technology and the implications for fuel cell system configuration and cost is given. 

The consideration of heat recovery and use in such fuel cell systems requires some consideration of heat generation and transfer within the cells of the system. Direct oxidation of CH4 at the anode of the cell, if possible, would implement the overall process ... 

NFPA 70 National Fire Protection Association 70 is also known as the National Electric Code (NEC). Revisions and addendum s to the code are currently being developed that specifically address fuel cells. Article 690 - Solar Photovoltaic Systems has been targeted for revision to include fuel cells and alternate energy sources systems. This proposal is not expected to be approved since the technological and operational differences between fuel cells and photovoltaic systems is considerable. A new article, currently identified and Article 691, has been proposed and applies to fuel cells for buildings or residential dwellings. This standard addresses the electrical interface between the fuel cell system and a building s electrical distribution panel. 

Apart from hydrocarbons and gasoline, other possible fuels include hydrazine, ammonia, and methanol, to mention just a few. Fuel cells powered by direct conversion of liquid methanol have promise as a possible alternative to batteries for portable electronic devices (cf. below). These considerations already indicate that fuel cells are not stand-alone devices, but need many supporting accessories, which consume current produced by the cell and thus lower the overall electrical efficiencies. The schematic of the major components of a so-called fuel cell system is shown in Figure 22. Fuel cell systems require sophisticated control systems to provide accurate metering of the fuel and air and to exhaust the reaction products. Important operational factors include stoichiometry of the reactants, pressure balance across the separator membrane, and freedom from impurities that shorten life (i.e., poison the catalysts). Depending on the application, a power-conditioning unit may be added to convert the direct current from the fuel cell into alternating current. 

In evaluation of the potential objects which are to define respective options to be taken into a consideration there are 99 objects. In this exercise we will focus our attention on the following hydrogen energy systems Fossil-Reforming-Intemal Combustion Engine System, Natural Gas Steam Reforming-Fuel Cell System, Nuclear Power-Electrolysis-Fuel Cell System, Solar Power-Electrolysis-Fuel Cell System, Wind Power-Electrolysis-Fuel Cell System, Biomass-Reforming-Gas Turbine System. 

Since this evaluation is only demonstration of the method and procedure, it would be inappropriate to make any finale conclusion of the potential selection of the hydrogen system as it was demonstrated in this evolution. But, still it can be concluded that under circumstances demonstrated in this evaluation coal and natural gas with fuel cell systems are presently the most attractive potential systems to be selected among the options under consideration. Also, natural gas and nuclear systems with fuel cell and gas turbine options are potentially acceptable solution for the situation where it can be in accordance with respective local condition. It should be emphasized that the utilization options are not substantially affecting priority of the options under consideration. 

Electrical heating requires power supply by an interim storage device, i.e. a battery. Even though batteries exist as buffer devices in most fuel cell system concepts, their size would need to increase considerably to meet the demands for start-up. Therefore, battery power is a less viable option especially for hydrocarbon reforming systems, where high operating temperatures of the reformer exceeding 600 °C need to be achieved. 

Biofuels being feedstocks result in a high yield of permanent gas (i.e., /6, CO, CO2, CH4, etc), volatiles, char, and condensible hydrocarbons i.e., tar, due to their higher hydrogen/carbon. and oxygen/caibon ratios than older fossil fuels. However, if the tar is allowed to condense or polymerise when the produced fuel gas is cooled down considerable problems with equipment contamination can result. Power generation requires high levels of gas clean up especially in gas turbines and fuel cell systems. 

As desulfurization of gasoline for the fuel cell applications prefers a desulfurization process on-board or on-site at low temperatures without using H2 gas, considerable attention has been paid to developing an adsorptive process under ambient conditions without using H2 gas for the gasoline-based fuel cell systems. 

Naturally, for more powerful fuel cell systems (1-100 kW) such as developed for the power train of cars, the passive approach of the low power systems is not feasible. Powerful systems require fuel cell stacks with power densities of the order of lkWkg to be competitive with internal combustion engines. In such stacks active areas of several hundred square centimeters with current densities of over 1 Acm are required. This leads to the need for forced air supply by a blower or compressor and also for very efficient heat removal as the heat load to be removed is also in the order of 1W cm of active area. Therefore, the complexity of such fuel cell systems increases considerably. Generally, at least four subsystems complementing the fuel cell stack are required ... 

What we have seen is, that electrocatalysts are vital components of fuel cell systems. Much progress has been made over the years in improving their effectiveness both for anode and cathode reactions. There is nevertheless scope for considerable improvement in the performance of the electrocatalysts, particularly at the air cathode, where large activation overpotentials should be overcome. With the anode reaction also, electrocatalysts more tolerant to carbon monoxide should allow the use of less pure hydrogen and stimulate performance. There is much room for further improvement in the design of catalysts for use in fuel cells, by increasing both activity and durability. 

Current Li-ion batteries achieve up to 500 W h L. Thus the overall power density of the fuel cell system must be considerably higher. 

The evaluation was conducted with all the failures subject to consideration as these occurred within the period of monitoring of 41 hours and 27 minutes. The period included both the measurement of particular load characteristics as well as long-term operation at rated power (the supply of electric power into the distribution network via inverter). The NEXA system experienced twelve failures within the period monitored. There were five failures due to the low pressure of fuel at the fuel cell system inlet, four failures due to high stack current and one failure caused by the low stack voltage (Fig. 6) and there were two failures due to causes within the distribution network (or the inverter). The latter failures did not induce the automatic deactivation of the fuel cell system yet only temporary interruption of the supply into the distribution network. 

The author combines a clear and thorough understanding of the theoretical foundations of fuel cells with a sound application of these to the practical considerations and challenges of designing and operating fuel cell systems for a variety of applications. ... 

Progress made in the development of fuel cell drives can be shown by referring to performance data for passenger cars (Figure 1.6). The figure shows that in an early phase of development, the stack performances were comparatively low, certainly due to considerably lower power densities or specific performances of fuel-cell systems. This allowed only a limited driving performance. The range was... 

Assuming that the residue stream can be used for another purpose in the overall system, distillative separation becomes a relevant process in mobile fuel-cell systems for rough desulfurization, provided that it considerably simplifies subsequent desulfurization with further processes. 

Fuel cell systems for residential applications are typically developed for the generation of power and heat, which increases their overall eflEciency considerably, because even low temperature oflf-heat may be utilised for hot water generation, which reduces energy losses considerably. 

Taking into consideration that another two reactors (low temperature shift and preferential oxidation reactors) are required downstream of the high temperature water-gas shift, at least in a conventional fuel processor/low temperature PEM fuel cell system, the calculations demonstrate that it is difficult to pre-heat a chain of reactors without an excessive time demand. Other measures and functionalities become possible when plate heat-exchanger technology is applied, which may help to solve this problem. 

---