Exergy calculations

Once the state points are known round a cycle in a computer calculation of performance, the local values of availability and/or exergy may be obtained. The procedure for e.stimating exergy losses or irreversibilities was outlined in Chapter 2. Here we. show such calculations made by Manfrida et al. [13] which were also presented in Ref. [14]. 

111 Holland, M.J. and Thake, T.F. (1980). Rotor blade cooling in high pressure turbines, AIAA J. of Aircraft 17(6). 412-418. 

19 El-Masri, M.A. (1987a). Exergy analysis of combined cycles Part 1 Air-cooled Brayton-cycic gas turbines. ASME J. Engng Power Gas Turbines 109. 228-23. i. 

1111 Chiesa, P,. Consonni. S.. Lozza, G. and Macchi, E. (1993), Predicting the ultimate performance of advanced power cycles ba.sed on very high temperatures. ASME Paper 93-GT-223. 

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Subsequently, we refer briefly to other comparable studies, including the calculations of exergy losses and rational efficiency. Finally, we show the real gas exergy calculations for two practical plants—[CBT]i and [CBTX]i. 

In Figure 4, the exergy calculation for the H-Coal process by the SPEED is depicted. Several special symbols are used in the Figure. The sequence of the quotation mark implies that the reaction listed below it is the target process. The mark indicates the temperature and pressure of the inlet and outlet streams. 

Klose, E. and Heschel, W., "Calculation of Energy in Chemical Engineering Processes with Particular Consideration of Fuel Engineering 1. Definitions and Exergy Calculation Rules," Energietechnick, 30, No. 8, 295 (1980). 

The equilibrium routes which essentially underlie any fuel chemical exergy calculation involve careful avoidance of all irreversibility, so that the routes can be rather unreal (e.g. Figures A.l, A.2 and A.4). These figures however, bring out the major point that equilibrium concentrations must be used in equilibrium diagrams. Any failure to accept that point results in omission of consideration of circulators, or erroneous sizing thereof. See, for example, Kotas (1995), Figure 2.14, p. 46. 

A.2.15 Circulator Power and Chemical Exergy Calculated at Standard Conditions... 

The addition of, for example KOH, to the water for a strong electrolyte makes a trivial difference to the chemical exergy calculation for hydrogen. The boiling point of the solution is elevated and the steam product expands from a superheat point a little to the left of the saturation line, along the isotherm, making a small difference to the figure of 2.828 in Table A.3. 

Exergy calculations essentially imply the quantitative determination of enthalpy and entropy changes. If the numerical values of enthalpy and entropy are not available, exergy cannot be determined. 

Consider helium at p = 1 bar and T = 1300 K as a system with the surroundings characterized by Po=lbar and To = 298.15 K, then the exergy calculated according to the above equation is the work that would be done by cooling the helium from... 

Reactions often cause extensive exergy loss. Exergy calculations show that the entropy production can cause the loss of considerable potential work due to a reaction. An electrochemical membrane reactor or a fuel cell could reduce exergy loss considerably. 

For the pump, work is supplied in the shaft, which raises the exergy and enthalpy of the feed-water. The energy and exergy calculations from Equations 23.81,23.83, and 23.85 yield... 

During flight - in avionics denoted mission - the ambient temperature is 216.5 K and ambient pressure is 256 mbar. The exergy calculations were therefore adapted to these conditions. 

At outlet, exergies of water in hydrogen and oxygen, and exergies of production of water are same to each other. Exergy calculations of these products are. 

The references given in the end of this Chapter should also be consulted for their content itself, as well as for the references given in them. Finally, for further understanding of exergy calculations, and for some interesting examples, the monograph of Sussman Availability (Exergy) Analysis, is recommended. 

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