Thermodynamic Analyses of Cycles

Thermodynamic Analyses of Cycles The thermodynamic quahty measure of either a piece of equipment or an entire process is its reversibility. The second law, or more precisely the entropy increase, is an effective guide to this degree of irreversibility. However, to obtain a clearer picture of what these entropy increases mean, it has become convenient to relate such an analysis to the additional work that is required to overcome these irreversibihties. The fundamental equation for such an analysis is... 

Development in classical thermodynamics is, logically and traditionally, aimed at the analysis of eyeles. Computational efforts impose harsh eonstraints on the kinds and amounts of cycle analyses that can reasonably be attempted. Cycle simulations cannot approach realistic complexity. Even relative sensitivity analyses based on grossly simplified cycle models are computationally taxing compared to their pedagogical benefits. 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

Thermodynamic exergy analyses of gas-turbine cycles show that the major losses occur neither during the compression of air nor during the expansion of hot combustion products, but rather during the combustion reactions. Main reasons for these losses stem from the peak temperature limitations imposed by the materials of construction, coupled with the very high thermodynamic quality of the fuel source. 

In order to clarify these ideas, we need to compare the irreversible entropy productions (or the exergy destruction) in cycles that utilize regenerative heating of compressed air, thermal recuperation in the form of evaporation and superheating of the methanol fuel, and chemical recuperation through either reforming or cracking reaction with methanol. The next section presents such a comparison in a simplified form to illustrate the utility of thermodynamic analyses. 

Due to the intrinsic difficulties of sulfuric acid decomposition, we have introduced two new classes of solar driven TCWSCs by modifying the original S-NH3 cycle. They include (I) 12 metal sulfate-ammonia (MSO4-NH3) based TCWSCs and (II) 3 metal pyrosulfate-ammonia (M2S2O7-NH3) based TCWSCs. Our preliminary experimental results of the ammonia released from the ZnO + (NH4)S04 mixtures show the feasibility of these new cycles. More experiments are currently underway to determine the reaction mechanisms and the nature of the reaction intermediates and products formed. These experimental and thermodynamic analyses are expected to lead to development of a highly efficient, solar driven water splitting cycle. 

The Second Law of Thermodynamics, as was introduced by Clausius and Kelvin, is formulated on the basis of theoretical analyses of reversible cycles established by Carnot (Knacke et al., 1991). From the reversible Carnot cycle, it follows mathematically that the reversible exchange of heat, dQ ev between systems and surroundings, divided... 

Fig. 3. Thermodynamic cycle used to analyse relative binding of ligands Sj and S2 to an enzyme, E. Each state is solvated by a box of explicit water molecules.

A series of UTC were investigated using a combination of thermodynamic analysis and experiments. The thermodynamics of alternative cycles were analysed using Outokumpu s HSC 5.0 Chemistry software computer code version 5.11 (Outokumpu, 2002). 

The performance measures discussed so far have been derived from numerical simulations or experimental data. The correlations given before are useful for a quick estimate, but are specific for the simple configuration considered. Many attempts have been made to derive performance measures from classical cycle analysis. So far, no such analysis heis credibly shown that it is a good representation of the inherent dynamics evident in simulations of PDFs. However, classical thermodynamic cycle analysis may be useful in deriving an upper bound for the performance that can be obtained from engines based on detonative combustion. The results from these analyses must be viewed as representing an ideal detonation wave engine, not necessarily a PDF. 

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