Fuel cells reliability

In the area of fuel cells, reliability and availability have much improved. Recent U.S. military experience with phosphoric acid fuel cells found that the mean time between failure (MTBF) was almost 1,800 h and the availability was 67%. This is comparable with the MTBF service intervals for diesel generators. These fuel cells also favorably compare with the service interval needed for a typical gas turbine generation set. Still, much more development is required to obtain a commercially viable product. Today, the typical fuel cell system still requires servicing every 3-4 days to replace its scrubber packs. 

Research on fuel cells reliability in order to use in accumulation systems of electric energy... 

Previous failure mode and effect analysis and fault tree analysis work by the authors has shown that the inherently complex system of a PEMFC assembly can harbor dependencies between multiple failure modes. Therefore in this presented work, Petri-Net simulation has been adopted to develop a more accurate degradation model. Operational parameters such as water content, temperature and current density s effects on the occurrence of failure modes can be modelled through this technique. This work will improve previous fuel cell reliability studies by taking into consideration operating parameters (water content, temperature), ambient weather and fuel cell voltage demand (drive cycles). 

System integration involves numerous miscellaneous development activities, such as control software to address system start-up, shut-down and transient operation, and thermal sub-systems to accomplish heat recovei y, heat rejection and water recoveiy within the constraints of weight, size, capital and operating costs, reliability, and so on. Depending on the application, there will be additional key issues automotive applications, for example, demand robustness to vibrations, impact, and cold temperatures, since if the water freezes it will halt fuel cell operation. 

Dry cells (batteries) and fuel cells are the main chemical electricity sources. Diy cells consist of two electrodes, made of different metals, placed into a solid electrolyte. The latter facilitates an oxidation process and a flow of electrons between electrodes, directly converting chemical energy into electricity. Various metal combinations in electrodes determine different characteristics of the dry cells. For example, nickel-cadmium cells have low output but can work for several years. On the other hand, silver-zinc cells are more powerful but with a much shorter life span. Therefore, the use of a particular type of dry cell is determined by the spacecraft mission profile. Usually these are the short missions with low electricity consumption. Diy cells are simple and reliable, since they lack moving parts. Their major drawbacks are... 

A major problem with the new sustainable energy sources is their reliability. Inherently they will produce electricity as the wind blows and the sun shines. The need for power is not constant either, with peak demands during the day. Hence, ways are needed to store energy that enable release on demand. Synthetic fuels and methanol are candidates, but the most important will be hydrogen. It can be produced conveniently from water and electricity with a reasonably high efficiency of 70 %. Hydrogen is the ideal fuel for fuel cells. 

The major requirement for a reliable hydrogen sensor operation in the fuel cell environment is in 100% condensing humidity Most of the fuel cells have abundant humidity and the sensor needs to operate continuously in humid environments. In some cases, the hydrogen sensor can also be operated at very low temperatures (as low as —40°C). The fuel cells regularly have a cold start, when operated from a very low ambient temperature the sensor needs to attain ambient temperature quickly (<30 s) and continue operation well below ambient temperature before the fuel cell itself reaches the ambient temperature. 

The overall advantages of fuel cells are the low environmental impact, which is one to two orders of magnitude lower than in conventional systems, good part load behavior, easy operation, and low maintenance since no rotating parts are needed. The main disadvantages at the moment are the very high costs and the lack of demonstrated reliability. 

In general, technical developments will lead to a decrease in overall costs of this technology per unit of installed generating capacity (kWe). Some types of fuel cells need to achieve higher power densities per kg weight or m3 most need to increase lifetimes of stacks or other plant components. For smaller applications, the technology must reach a reliable level sufficient to allow the plants to operate unattended. 

R. Loehman et al., Development of Reliable Methods for Sealing Solid Oxide Fuel Cell Stacks, SECA Core Technology Program Review, January 27-28, 2005, Tampa,... 

There have also been revivals of the steam car. Robert McCulloch, the chain-saw millionaire, spent part of his fortune on a steam prototype, called the Paxton Phoenix, between 1951 and 1954. William Lear of Learjet fame, spent 15 million in 1969 on a turbine bus and a 250-horsepower turbine steam car. Both used quiet, efficient steam engines although the bus had reliability problems and poor gas mileage. Lear also tried to enter a steam car into the 1969 Indianapolis 500. The British firm of Austin-Healey was also working on a steam car in 1969. It had four-wheel drive. However, even prosperous entrepreneurs like McCulloch and Lear found that they lacked the means and support structure to successfully mass market a competitive car. Alternative power systems would have to wait until air-quality regulations resulted in some breakthroughs with hybrid and even fuel-cell cars. 

The joint government-industry fuel cell program is aimed at giving the world s power industry a revolutionary new option for generating electricity with efficiencies, reliabilities, and environmental performance beyond conventional electricity generation. 

Since no power is consumed for moving the fuel or the oxidizing agent through the cells, the efficiency of the fuel cell is increased. Operation without pressurization eliminates the moving parts making the fuel cell less expensive and more reliable. Pressurization can also be added for increased performance where the whole assembly is placed under pressure. 

The competition is entrenched in very mature, reliable, low-cost technologies compared to fuel cells and many barriers exist to impede the use of widespread use of small-scale CHP systems. These existing technologies and existing companies can be formidable for the spread of new technologies and new companies. 

The Shell studies imply that fuel cell sales will start with stationary applications to businesses that are willing to pay a premium to ensure highly reliable power without utility voltage fluctuations or outages. This demand helps to push fuel cell system costs below 500 per kW, providing the era of transportation which drives costs to 50 per kilowatt. But, can the high-reliability power market really drive transportation fuel cell demand and cost reductions, especially for proton- exchange membrane (PEM) fuel cells ... 

The overarching drivers for the development of hydrogen technologies are climate change and reductions in oil consumption with additional benefits in emissions reductions. The use of hydrogen in fuel cell vehicles can reduce oil use and carbon plus other emissions in the transportation sector, while hydrogen can enable clean, reliable energy for stationary and portable power applications. 

Discussions with the only US. PAFC manufacturer justified the direct use of the PAFC performance information from the 1994 edition of the Fuel Cell Handbook. There have been only minor changes in cell performance, mostly due to changing the operating conditions of the cell. These are considered within the performance trends shown in this section. The manufacturer has concentrated on improving cell stability and life, and in improving the system components to improve reliability and lower cost. It should be noted that the performance shown in this section is based on information from contracts that the manufacturer had with the Department of Energy or outside institutions. Any new PAFC performance has been accomplished with company funding and is considered proprietary by the manufacturer (1). 

Connection to the utility grid provides many advantages to on-site power producers such as reliability improvement and increase of load factor, as well as giving the electric utilities a chance to improve the supply capability. When a fuel cell power plant is used for electric utility applications, the inverter is the interface equipment between the fuel cell and the electrical network. The inverter acts as the voltage and frequency adjuster to the final load. The interface conditions require the following characteristics for the inverter ... 

The design and optimization of a fuel cell power system is very complex because of the number of required systems, components, and functions. Many possible design options and trade-offs affect unit capital cost, operating cost, efficiency, parasitic power consumption, complexity, reliability, availability, fuel cell life, and operational flexibility. Although a detailed discussion of fuel cell optimization and integration is not within the scope of this section, a few of the most common system optimization areas are examined. 

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