Efficiency second law

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

The first and second law efficiencies are actual heat stored... 

Different ways of formulating exergy efficiency (second law efficiency) are considered. The exergetic COP (i.e., efficiency ratio) used here is as follows ... 

A low second law efficiency is not always realistically improvable. Thus Weber and Meissner (Thermodynamics for Chemical Engineers, John Wiley, New York, 1957) found a 6% efficiency for the separation of ethanol and water by distillation which is not substantially improvable by redesign of the distillation process. Perhaps this suggests that more efficient methods than distillation should be sought for the separation of volatile mixtures, but none has been found at competitive cost. 

The Second Law efficiency, measuring the ratio of actual to ideal output, is therefore given by... 

In answer to the second question raised earlier, the second law efficiency of this system is... 

Analysis of Sub-processes. To determine the locations and magnitudes of the various different consumptions which comprise ac one need only subdivide the system appropriately into sub-systems, and then repeat the foregoing procedure. Thus, in this problem, the consumptions within the boiler can be broken down into. 1) combustion, and 2) heat transfer. Each can be analyzed for its second law efficiency and the amount of exergy it consumes. 

Correspondingly, the second law efficiency for each of these internal processes may be evaluated. If the view is taken that the purpose of the combustor is to convert the (chemical) exergy of the coal into exergy of combustion products. 

A Survey of Typical Results of Second law Efficiency Analyses... 

The methods, approximations and assumptions employed in constructing Figure 6, similar to those used for Figure 5, are described in some detail by Reistad (19, 20). it is clear (20,22, 23) that the Second Law efficiencies shown in Figure 6, especially those for the Residential-Commercial and for the Industrial sectors, are conservatively high. The more recent estimates for the overall tijj of the Industrial sector (2 , 22) are about 8%... 

The system s Second Law efficiency rises as the work/heat ratio increases (Figure 7). This is partially due to improved performance of the turbine, pump, and condenser and the higher temperature steam from the boiler. This considerably decreases the available-energy destruction due to heat transfer in the boiler. Thus, the turbine can take advantage of this for the production of shaft work. 

This means that for the same work/heat ratio, the turbine inlet pressure must also be higher as is clearly demonstrated in Figure 5. Other trends associated with increasing TB include the decreasing of the maximum temperature difference in the condenser, an increase in pump efficiency (Figure 3), and an increase in the system s Second Law efficiency. 

Figure 7. System second law efficiency as a function of work/heat ratio for various hot water supply temperatures at a fuel price of 3 per million Btu.

Available-Energy Analysis. Using standard techniques (7-8) an available-energy analysis was performed on the system operating at a reflux ratio of 1.2, and shows the overall Second-Law efficiency to be 12.9% (9). The relevant equations for this analysis are given in Table II. The reboiler has a Second-Law efficiency equal to AAi/AAst or 74.2%. The tower efficiency, defined as AAfr>.p/(AA], + A ) is 55.7%. Note that it is of little use to... 

Johnson, D. H., "Exergy of the Ocean Thermal Resource and the Second-Law Efficiency of Idealized Ocean Thermal Energy Conversion Power Cycles," Solar Energy Research Institute, Golden, CO, Report No. SERI/TR-252-1420R, Available as NTIS PCA03/MF 01, National Technical Information Service (1982). 

We all widely utilize aspects of the first law of thermodynamics. The first law mainly deals with energy balance regardless of the quality of that part of the energy available to perform work. We define first law efficiency or thermal efficiency as the ratio of the work output to total rate of heat input, and this efficiency may not describe the best performance of a process. On the other hand, the second law brings out the quality of energy, and second law efficiency relates the actual performance to the best possible performance under the same conditions. For a process, reversible work is the maximum useful work output. If the operating conditions cause excessive entropy production, the system will not be capable of delivering the maximum useful output. 

The equality of forces is independent of the individual values of the phenomenological coefficients. This means that the variation of the entropy production rate along the column follows the variation of the phenomenological coefficient /,M. The reversible operation is possible whenX, and X2 approach zero and y increases toward infinity. Therefore, the practical way to improve second law efficiency is to apply the relationship between dX] and dX2. For a constant Jd, we obtain... 

Heat and mass transfer in a distillation column are coupled, and if the temperature field or chemical force is specified in the column, the other force would be defined. Maximum second law efficiency results from minimizing the entropy production rate with respect to one of the forces. For example, if the contribution of mass transfer is dominant, we should try to minimize the change of the entropy production with respect to the chemical force. 

The work performed by the system lowers the temperature of the exhaust gases to -2200 K, where the gases can be effectively utilized by conventional steam-driven power generating plants. Thus, by use of an MHD topping cycle, both the first and second law efficiencies of conventional power plants can be improved. 

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