Exergetic efficiency

Figure 1. The exergetic efficiency of heat as a function of temperature (reference temperature 0°C).

Others, like energetic and exergetic efficiency are discussed in Dincer et al. (2002). Basically we can distinguish between two kinds of heat transfer fluids with different ranges of heat capacities and heat transfer coefficients ... 

Fig. 2.14 The system efficiency of the ideal and the real fuel cell-heat engine hybrid system with an exergetic efficiency he = 0.7 and the oxidation of hydrogen.

The cell temperature T c is again the temperature T of the process environment. The work wtcc produced by the Carnot cycle CC increases with higher Tpc and the work in/ i crev produced by FC decreases with lower 7jc as already expected. The work wtSyst of the system is independent of 7 if (or nearly independent in the case of the simplified process). The FC operates reversibly in both cases but the Carnot cycle CC does not operate completely reversible in the simplified process caused by the fact that a small part of the waste heat of FC is needed to heat air and fuel. The practical benefit of this combined fuel cell-heat reference cycle is the opportunity for using exergetic efficiencies to describe the operation of real cycles with this very simple model. The needed exergetic efficiency f is defined as... 

The system efficiency r)syst of a hydrogen fuelled combined fuel cell-heat cycle is plotted over the cell temperature lie in Figure 2.14 on the right side. The exergetic efficiency of the fuel cell fc is varied between 0.7 and 1 and the exergetic efficiency of the heat engine he is constant at 0.7. 

The system efficiency jjsyst increases with the cell temperature TFC for all exergetic efficiencies fc < 1 until a maximum is reached. The maximum efficiency jjsyst moves to a higher temperatures for decreasing exergetic efficiencies fc The influence of the Carnot cycle dominates at lower temperatures Tpc and lower exergetic efficiencies fc-... 

The required waste heat is governed by the exergetic efficiency of the heat engine heat sink 7 proccss and the temperature of the heat source Tsofc (T in K). Table 2.4 shows that the relation between Tprocess/ sofc is about 0.4 in the case of the evaporator and about 0.9 in the case of the reformer. The recycling of the anode outlet flow is another interesting option to supply the reformer with steam because no evaporation is needed. 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

Exergetic efficiency r ex for hydrogen production by thermo water-splitting at a temperature Ta can be defined as the quotient of the recoverable work (here equal to the free enthalpy of water formation) divided by this quantity plus the sum of the exergy losses D = Ta AS in the process [Eq. (6)]. Thermal efficiency t T from a heat source at temperature T is linked to this efficiency by transformation of heat into work [Eq. (7)]. 

From these brief considerations an exergetic efficiency of the combustion process can be defined ... 

The exergetic efficiency of the total process can then be evaluated with equations (3a) and (7) as a function of the maximum process temperature. 

Figure 13 shows the exergetic efficiency vs. the maximum temperature for different combustion processes with and without intermediate reactions. As a comparison, the adiabatic, isobaric... 

Figure 13. Exergetic efficiency of methane combustion for different processes as a function of maximum process temperature.

The exergetic efficiency of combustion processes with intermediate reactions is theoretically higher than combustion in usual practice. 

The exergetic efficiency of the two alternatives is shown in relation to the gasification parameters (pressure, temperature). A downstream waste heat boiler has great exergetic advantages particularly at low and medium gasification pressures. 

Figure 6 shows the exergetic efficiency as a function of pressure at various gasification temperatures. In the case of alternative I (waste heat boiler), the efficiency is independent of... 

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