Efficiency system

The system electrical efficiency of a fuel cell is defined as the ratio of the electrical power sent out by the fuel cell system to the chemical energy (i.e., the enthalpy) of the fuel (e.g., H2) received by the fuel cell system. This is the efficiency a user cares abouf fhe mosf, and a fuel cell developer tries every means to achieve the highest number. 

The system efficiency is affecfed by the fuel utilization Tip, the stack efficiency Tis, the system parasitic power losses and the DC-DC converting efficiency line- Th sysfem efficiency is 

The stack efficiency is determined by the average cell voltage. Normally, the average cell voltage is around 0.69 V. Then, the stack efficiency is around 55% (0.69/1.25). 

The DC-DC converting efficiency may range from 85% to 95%, depending on the skill of the manufacturer. A good estimate is 90% most of the time. 

Parasitic power loss is due to the power needs of some fuel cell components, such as sensors, control boards, pumps, fans, blowers (or compressors), solenoid valves, and switches, and due to the power losses when currents pass through certain components such as the diodes and wires. Typically, the sensors, the control boards, the solenoid valves, the switches, the diodes, and the wires consume very little power. The pump for driving the liquid coolant does not consume too much power either. It is the fans (or blowers and compressors) that consume most of the parasitic power. For an air-cooled stack, the total parasitic power loss can be controlled to less than 5% of the stack output power, and for a liquid-cooled stack, the total parasitic power loss can be controlled to less than 10% of the stack output power. 

The efficiency of the total fuel cell system can be calculated by accounting for the generation efficiency of the fuel cell stack, the efficiency of power conversion devices, and the accessory load of the related subsystems. In order to obtain the best overall fuel cell power system efficiency and better economics, a system optimization is required. The optimization involves minimizing the cost of electricity or heat and electric products as in a cogeneration system all the component processes of the system should be integrated into an efficient plant with low capital cost. Often, these objectives are conflicting, so compromises, or design decisions, have to be made. In addition, project-specific objectives, such as desired fuel, environmental emission levels, potential uses of rejected heat, desired output levels, volume (volume/kW) 

The increase in system pressure improves process performance and hence better fuel cell output. The pressurization is a trade-off between the improved performance and/or reduced cell area and the reduced piping volume, insulation, and heat loss compared to the increased parasitic load, capital cost of the compressor, and pressure-rated equipment and design of the cell to withstand higher pressure. 

Similarly, the fuel cell performance at operating current densities increases with increasing temperature owing to reduced mass transfer polarizations and ohmic losses. The increased temperature also yields higher-quality rejected heat. However, temperatures at which the various fuel cells can 

Fuel cell system power optimization options. 

Both fuel and oxidant high utilizations are considered to be desirable because they minimize the required fuel and oxidant flow, for a minimum fuel cost and compressor/blower load and size. However, utilizations that are pushed too high result in significant voltage drops. Optimum utilization requires an engineering trade-off that considers the specifics of each case. 

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The ideal high level heat-transfer medium would have excellent heat-transfer capabiUty over a wide temperature range, be low in cost, noncorrosive to common materials of constmction, nondammable, ecologically safe, and thermally stable. It also would remain Hquid at winter ambient temperatures and afford high rates of heat transfer. In practice, the value of a heat-transfer medium depends on several factors its physical properties in relation to system efficiency its thermal stabiUty at the service temperature its adaptabiUty to various systems and certain of its physical properties. 

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

Some power tubes can be operated without the need for a protective ferrite isolator. One example is the cooker magnetron (700 W) used in modern microwave ovens (57). At higher power levels, such as 25 kW, it is more common to employ a protective ferrite device, particularly in the form of a circulator (58), as shown in Figure 3. This results in a power loss equivalent to a few percentage points in system efficiency. The ferrite circulator prevents reflected power from returning to the power tube and instead directs it into an auxiHary dummy load. The pulling of tube frequency is thus minimised. 

Mixing. Because of the heterogeneous nature of this system, efficient mixing is essential to ensure the intimate contact of the iron, nitro compound, and water soluble catalyst. An agitator which allows the iron to settie to the bottom and the other materials to separate into layers does not function efficientiy. On the other hand, a reaction whose rate is limited by the quaUty of the iron will not be significantly improved by better mixing. 

A Btu meter may be used in the fuel-quality system as an aid in determining turbine system efficiency. A water capacitance probe is used for detection of water in the fuel line. A water-detecting device can be incorporated into the corrosion monitoring system. This monitoring device is based on detection of changes in the dielectric constant of unknown fluid components... 

Estimation of Treatment System Efficiencies and Influent Concentrations... 

Chemical pretreatment is often used to improve the performance of contaminant removal. The use of chemical flocculants is based on system efficiency, the specific DAF application and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeClj or FeSO or aluminum, such as AISO. Organic and inorganic polymers (cationic or anionic) are generally used to enhance the DAF process. 

Increased system efficiency has resulted in higher equipment costs but lower operating costs. A higher initial cost for equipment can often bejustified by the monetary savings from lower energy consumption over the life of the equipment. 

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

In contrast with the AFC, the PAFC can demonstrate reliable operation with 40 percent to 50 percent system efficiency even when operating on low quality fuels, such as waste residues. This fuel flexibility is enabled by higher temperature operation (200°C vs. 100°C for the AFC) since this raises electro-catalyst tolerance toward impurities. Flowever, the PAFC is still too heavy and lacks the rapid start-up that is nec-essaiy for vehicle applications because it needs preheating to 100°C before it can draw a current. This is unfortunate because the PAFC s operating temperature would allow it to thermally integrate better with a methanol reformer. 

Operating costs, in contrast, are more straightforward to determine because they depend on system efficiency, which, in turn, is related to voltage and current density (the current generated per unit area of electrolyte). Fuel savings are expected since the fuel cell operates more efficiently than a heat engine, and there may be lower maintenance and repair costs because fuel cells have fewer moving parts to wear out. 

Jones, T. (1997). Steam Partnership Improving Steam System Efficiency Through Marketplace Partnerships. Proceedings 1997 ACEEE Slimmer Study on Energy Efficiency in Industry, Washington, DC ACEE. 

Further improvements in system efficiency will be difficult to achieve with evolutionary changes. Following are some of the more promising areas of development ... 

The pressurized hybrid cycle provides the basis for the high electric efficiency power system. Applying conventional gas turbine technology, power system efficiencies in the 55 to 60 percent range can be achieved. When the pressurized hybrid system is based on a more complex turbine cycle— such as one that is intercooled, reheated, and recuperated—electric efficiencies of 70 percent or higher are projected. 

Trays are usually designed with F-factor from 0.25 to 2.0 for a turndown of 8 1. Pressure drop per theoretical stage falls between 3 and 8 mm Hg. Note that bubble cap trays are on the high side and sieve trays are on the lower end of the range. Varying tray spacing and system efficiency, the HETP for trays are usually between 24 in. and 48 in. [133]. The C-factor is the familiar Souders and Brown capacity equation. 

System efficiency is influenced by the air plenum chamber and fan housing. Industrial axial flow fans in proper system design will have efficiencies of approximately 75% based on total pressure. Poor designs can run 40%. Speed reducers are about 75% mechanically efficient. 

Many plants do not consider machine or systems efficiency as part of the maintenance responsibility. However, machinery that is not operating within acceptable efficiency parameters severely limits the productivity of many plants. Therefore a comprehensive predictive maintenance program should include routine monitoring of process parameters. 

The use of soluble inhibitors as a means of controlling bimetallic corrosion presents many technical problems. Apart from the fact that this method is limited in applicability to recirculating systems, efficient anodic inhibitors, such as chromates, are frequently quite specific in their action and so certain bimetallic couples, such as the Al-Cu couple in chloride solutions, are... 

Combinations of steam jet ejectors operating in conjunction with mechanical pumps can significantly improve the overall system efficiency, especially in the lower suction pressure torr range of 1 torr to 100 torn They can exist beyond the range cited, but tend to fall off above 200 torr. Each system should be examined individually to determine the net result, because the specific manufacturer and the equipment size enter into the overall assessment. Some effective combinations are ... 

Two main strategies are presently used to suppress immune responses (summarized in Fig. 3). The first focuses on cytokines, the central mediators of the immune system. Efficient inhibition of cytokine production can be achieved by glucocorticoids. Specific anticytokine strategies include the use of monoclonal antibodies, soluble receptors, or receptor constructs. 

Traps that stick open may allow considerable volumes of steam to blow through, thus resulting in a reduction in overall boiler system efficiency. 

Similarly, if the customer refuses to be involved, at least to some degree, in the ongoing water treatment program or fails to take the advice and undertake necessary actions designed to control the program and the boiler system efficiency, the program will again ultimately fail and the contract will be lost. 

With rapid advances in hardware, database management and information processing systems, efficient and competitive manufacturing has become an information-intensive activity. The amounts of data presently collected in the field on a routine basis are staggering, and it is not unusual to find plants where as many as 20,000 variables are continuously monitored and stored (Taylor, 1989). 

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