Efficiency, compressor expander

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

For the high-LOX case, where the refrigeration requirement is 186 Btu/lb-mol (434 kJ/kg-mol) of air ow, the required expander ow of 31% is needed to produce less than 3% of the airflow as liquid. This is a simplistic demonstration of the ability of an ASU to produce liquid. For small amounts of LOX, the ASU is efficient. Beyond 3% of the airflow, the distillation impacts of the high expander flow become prohibitive. Above these rates, the addition of compressors/expanders specifically needed for liquefaction are added. 

IV.E.2 Development and Testing of a High Efficiency Integrated Compressor/ Expander Based on Toroidal Intersecting Vane Machine Geometry... 

A high efficiency motor will be integrated with the TIVM compressor/expander and a prototype compressor/expander/motor unit will be fabricated and delivered to ANL for independent testing. 

One or more full TIVM compressor/expander prototypes will be fabricated by Mechanology and tested across the full operating range. Modifications will be made as necessary to optimize performance. Subsequently, a high efficiency electric motor will be integrated with the TIVM to form a complete compressor/expander/motor (CEM) component. 

Although the second alternative is more efficient and looks promising, it will not be taken into account in this study, as it will influence the combustion process and might be more difficult to retrofit to existing facilities. The calculated equipment costs only consist of the membrane costs and compressors/expanders, but are multiplied with an installation factor. The capital and operational costs adjusted with the parameters used in this study are shown in Tables 2.15 and 2.16. 

Turbomachine adiabatic/isentropic efficiencies CT Compressor Expander ST HP/IP/LP cylinders. 

In order to arrive at numerical results, some further assumptions were made. The compressors have adiabatic stage efficiency. The expanders have 80 efficiency. The liquid pump has 100 efficiency. All heat exchange has a minimum temperature difference of 4°C. 

Note rj, compressor overall efficiency expander overall efficiency expander adiabatic efficiency rj expander mechanical efficiency b, heat-exchanger effectiveness i = mjih, mass in intermediate stream/mass through compressor x = mjm, mass through expander/mass through compressor rhfim, mass liquefied/mass through compressor. 

FIGURE 9-14. Compression power as a function of fuel cell operating pressure for a system with compressor/expander (assuming ambient pressure of 101.3 kPa and ambient temperatme of 20°C, and both compressor and expander efficiency of 70%). 

Mechanical Expanders Reciprocating expanders are very similar in concept and design to reciprocating compressors. Generally these units are used with inlet pressures of 4 to 20 MPa. These machines operate at speeds up to 500 rpm. The thermal efficiencies (actual enthalpy difference/maximum possible enthalpy difference) range from about 75 percent for small units to 85 percent for large machines. 

The turboexpander in combination with a compressor and a heat exchanger functions as a heat pump and is analyzed as follows In Fig. 29-44 consider the compressor and aftercooler as an isothermal compressor operating at To with an efficiency and assume the working fluid to be a perfect gas. Further, consider the removal of a quantity of heat by the tumoexpander at an average low temperature Ti-This requires that it dehver shaft work equal to Q. Now, make the reasonable assumption that one-tenth of the temperature drop in the expander is used for the temperature difference in the heat exchanger. If the expander efficiency is and this efficiency is mul-... 

Hence, the second-law efficiency of the expander-heat-exchanger-compressor system is p p... 

Most ethylene plants operate continuously with the expanders operating at or near design conditions. If necessary, due to their unique design characteristics, radial inflow turboexpanders can accommodate a wide range of process conditions without significant losses in thermal or mechanical efficiency. Expanders may be loaded with booster compressors, gear-coupled generators, dynamometers, or other in-plant mechanical equipment such as pumps. In ethylene plants, turboexpanders are typically used in eitlier post-boost or pre-boost applications. 

In the case of the actual cycle the effect of the turbine compressor (rjc), and expander (rjt) efficiencies must also be taken into account, to obtain the overall cycle efficiency between the firing temperature Tf and the ambient temperature Tamb of the turbine. This relationship is given in the following equation ... 

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