Efficiency exergy

Example 11.12 Exergy efficiency Assume that the oxidation of glucose (G) in living cells produces 38 mol of ATP per mole of glucose (Garby and Larsen, 1995). Estimate the maximum theoretical production and the exergy efficiency. 

The steady composition of the mixture contains 0.01 mol/L glucose and the partial pressures of carbon dioxide and oxygen are 0.07 and 0.21 atm, respectively. The state of the mixture is characterized by the activity ratios (at/a°) = 0.07, 1.0, 0.01, and 0.21 for carbon dioxide, water, glucose, and oxygen, respectively. 

The entropy of reaction for oxidation of glucose at standard conditions is obtained from Table B8 

The entropy of reaction at actual state should be estimated. Assuming that the entropy of mixing is negligible, we have 

Cp is assumed to be a constant. Substituting Eqs. (a) and (b) into the relation of reaction entropy AST = % /yS . 

Equation 13.17 explicitly mentions the useful exergy flows coming out of the process because exergy can be lost in two different ways. First, exergy is lost in any real process as a result of irreversibility in the process itself, and such losses are called internal exergy losses. Second, exergy can be lost via waste streams that are not yet at equilibrium with the natural environment. 

The sustainability parameter r is the efficiency with which the exergy of resources is transferred to the products of the process. 

Examples of such external exergy losses are the release of hot flue gases or high-pressure gas to the atmosphere. Both the internal and the external exergy losses are in principle inefficiencies, and the exergy used efficiently in the process is therefore only the exergy of products and the exergy of waste products, provided they are made useful in other processes. 

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The relation between exergy, sustainability and environmental impact is illustrated in Figure 6. There, sustainability can be seen to increase and environmental impact to decrease as the exergy efficiency of a process increases. Two limiting efficiency cases, as shown in Figure 5, are of practical significance ... 

Figure 6. Qualitative illustration of the linkages between the environmental impact and sustainability of a system or process, and its exergy efficiency...

As exergy efficiency approaches 100%, the environmental impact associated with process operation approaches zero, since exergy is only converted from one form to another without loss. Also sustainability approaches infinity because the process approaches reversibility. 

The exergy efficiency for the overall process can be expressed as Exergy recovered from TES during discharging... 

The exergy efficiencies for the charging, storing and discharging processes,... 

Ozturk, H.H., 2005, Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating, Energy Comers. Mgmt. 46 1523—1542. 

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

Unless there is a specific reason, the supply temperature should be higher in order to increase the exergy efficiency of the heat pumps and hence the overall system, as shown in Figure 82. Other points to be considered in the design include the effect of outdoor conditions on the return temperature of the heat distribution network, the type of users connected to the system, and the characteristics of the heating apparatus. Also, in the heat exchanger... 

Winter, C.-J. (2007). Energy efficiency, no it s exergy efficiency International Journal of Hydrogen Energy, 32 (17), 4109-4111. 

For certain applications, such as the separation of ternary mixtures, divided wall columns may be of interest, [9] which have shown to reduce the energy requirements and increase exergy efficiency. 

As not all thermodynamic data are known, it is not possible to give a good estimate of exergy efficiency of the cycle. But in many cases, a minimal exergy loss can be found, which will eventually give an upper bound efficiency for the exergy efficiency of the cycle (as all exergy losses are cumulative) and therefore for the thermal efficiency of the cycle. 

It is hoped that the illustrative examples presented here will serve to crystallize the basic principles and will motivate the readers to apply exergy efficiency analysis and exergy accounting — taking advantage of these methods for their own practical purposes, while also advancing the state of the art. 

In this way, the exergy loss of each subsystem can be calculated. The overall exergy efficiency r e of the process is defined here in the following way ... 

The total exergy losses consist of configuration limitations due to the design of the column and transportation limitations due to the states of streams. For Exsv > o, the exergy efficiency of column i becomes... 

Example 4.25 Column Exergy efficiency Propylene-propane mixture is a close boiling mixture. A reflux ratio of 15.9 (close to minimum) and 200 equilibrium stages are necessary. Table 4.13 shows the enthalpy and entropies of the saturated feed and saturated products from the simulation results with the Redlich-Soave equation of state. The reboiler and condenser duties are 8274.72 and 8280.82 kW, respectively. The reference temperature is 294 K. The lost work ZTFis obtained from Eq. (4.198) as... 

System Exergy input (MW) Exergy output (MW) Overall exergy loss (MW) Overall exergy efficiency (%) Column exergy losses (MW)... 

The exergy efficiency 17 is a universal measure of process efficiency that accounts for the first and second law principles. 

The above equation expresses the depletion number as a function of a system s structural constants, the exergy efficiency and renewed exergy fraction of the individual resource upgrade processes, and the extent of resource cycling. 

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