EXERGY ANALYSIS
3.6 GUIDELINES FOR EVALUATING AND IMPROVING THERMODYNAMIC EFFECT ESS
= (0.79 18 MJ/
(0.045
= 27.68 (3.40)
The contribution of the water is negligible in this calculation; the chemical exergy of the ash has been ignored.
3.6 GUIDELINES FOR EVALUATING AND IMPROVING THERMODYNAMIC EFFECT ESS
Several design guidelines are listed in Table 1.1 to assist the concept devel- opment stage of the design process. In the present section additional guide- lines are introduced. Most of these stem from experience with the evaluation and improvement of the thermodynamic effectiveness of thermal systems, and most reflect conclusions evolving from exergy analysis. The guidelines pro- vided here also extend and amplify guidelines presented in Reference 10.
An important guideline for achieving cost-effective thermal systems is to maximize the use of cogeneration when it is feasible. Cogeneration achieves economies by combining the production of power together with process steam generation or heating. This is exemplified by the gas turbine cogeneration system we have considered several times, beginning with Chapter 1. The technical literature is abundant with discussions of various cogeneration al- ternatives and practical applications [ l l , assuring that cogeneration is firmly founded in current practice and will continue to be so in the future.
160 EXERGY ANALYSIS
At some point in the design process it is necessary to deal directly with the issue of thermodynamic inefficiencies. We have previously noted that sources of inefficiency are related to exergy destruction and exergy loss. The primary contributors to exergy destruction have been identified as chemical reaction, heat transfer, mixing, and friction, including unrestrained expansions of gases and liquids. To increase the thermodynamic effectiveness of a par- ticular system, the principal contributors to its inefficiencies not only should be understood qualitatively but also determined quantitatively. Although the relative magnitude of the exergy destructions and losses must be evaluated, extreme precision in such evaluations is seldom required to guide decisions directed at reducing inefficiencies. When evaluating the relative magnitudes of the exergy destruction and loss, therefore, engineers should not hesitate to:
Make reasonable simplifying assumptions Use simplified exergy calculations
Examples of simplified calculations include the appropriate use of ideal-gas property relations with constant specific heats, the incompressible liquid model, and an approximate thermodynamic average temperature.
An important issue encountered in the discussion of Figure 3.9 can be framed as a question: Does the exergy destruction and loss associated with a component reduce capital investment in the overall system and/or fuel costs in another component? If not, it is generally good practice to eliminate or reduce this source of inefficiency.
Design changes to improve efficiency must be done judiciously, however, for as shown in Chapter 8 the unit costs associated with different sources of inefficiency can be different. For example, the unit cost of the electrical or mechanical power required to feed the fuel exergy destroyed owing to a pres- sure drop is generally higher than the unit cost of the fuel natural gas, oil, or coal) required to feed the exergy destruction caused by combustion or heat transfer. As the unit cost attributed to exergy destruction depends on the nature of the source of inefficiency and on the position of the component in the system (Section 8.2. steps taken to reduce inefficiencies should take these aspects into account.
Since chemical reaction is a significant source of thermodynamic ineffi- ciency, it is generally good practice to minimize the use of Combustion. In many applications the use of combustion equipment such as boilers is un- avoidable, however. In these cases a significant reduction in the combustion irreversibility by conventional means simply cannot be expected, for the major part of the exergy destruction introduced by combustion is an inevitable con- sequence of incorporating such equipment. Still, the exergy destruction in practical combustion systems can be reduced by the following:
Minimize the use of excess air Preheat the reactants
3.6 GUIDELINES FOR EVALUATING AND IMPROVING THERMODYNAMIC EFFECTIVENESS 161
In most cases only a small part of the exergy destruction in a combustion chamber can be avoided by such means. Thus after considering options such as these for reducing the exergy destruction related to combustion, efforts to improve the thermodynamic performance should be centered on components of the overall system that are more amenable to betterment by cost-effective conventional means. This illustrates another guideline:
Some exergy destructions and exergy losses can be avoided, others can- not. Efforts should be centered on those that can be avoided.
We have noted previously that nonidealities associated with heat transfer typically contribute heavily to inefficiency. An important general guideline, therefore, is that unnecessary or cost-ineffective heat transfer must be avoided.
Other guidelines specific to heat transfer include the following:
The higher the temperature T at which a heat transfer occurs in cases where T the more valuable the heat transfer consequently, the greater the need to avoid direct heat transfer to the ambient, to cooling water, or to a refrigerated stream. Avoid heat transfer across
The lower the temperature T at which a heat transfer occurs in cases where T the more valuable the heat transfer and, consequently, the greater the need to avoid direct heat transfer with the ambient or a heated stream. Avoid heat transfer across
The lower the temperature level, the greater the need to minimize the stream-to-stream temperature difference.
Avoid the use of intermediate heat transfer fluids when exchanging en- ergy by transfer between two streams.
In the design of heat exchanger networks, consider these additional guidelines:
(a) Try to match streams where the final temperature of one stream is close to the initial temperature of the other.
(b) If there is a significant difference in the heat capacity rates (product of the mass flow rate and specific heat of two streams exchanging energy by heat transfer, consider splitting the stream with the larger heat capacity rate.
(c) Use the pinch method (Section 9.3).
Irreversibilities related to friction, unrestrained expansion, and mixing are generally secondary in importance to those of combustion and heat transfer.
Still, they are not to be overlooked, and the following guidelines apply:
Relatively more attention should be paid to the design of the lower tem- perature stages of turbines and compressors (the last stages of turbines
162 EXERGY ANALYSIS
and the first stages of compressors) than to the remaining stages of these devices.
For turbines, compressors, and motors consider the most thermodynam- ically efficient options.
Minimize the use of throttling; check whether power recovery expanders are a cost-effective alternative for pressure reduction.
Avoid processes using excessively large thermodynamic driving forces (differences in temperature, pressure, and chemical composition). In par- ticular, minimize the mixing of streams differing significantly in temper- ature, pressure, or chemical composition.
The greater the mass rate of flow, the greater the need to use the exergy of the stream effectively.
The lower the temperature level, the greater the need to minimize friction.
These guidelines aim to improve the use of energy resources in thermal systems by reducing the sources of thermodynamic inefficiency. We should always keep in mind, however, that the objectives of thermal design normally include development of the most effective system from the cost viewpoint.
Still, in the cost optimization process, particularly of complex energy systems, it is often expedient to begin by identifying a design that is nearly optimal thermodynamically; such a design can then be used as the starting solution for cost optimization. Further elaboration of this is provided in Chapter 9.
3.7 CLOSURE
In Chapters 2 and 3 we have presented principles of thermodynamics and fluid flow, together with some related modeling and design analysis consid- erations. These principles can play important roles in thermal system design and optimization to be sure. Still, the principles do not suffice, and we must augment them by bringing in fundamentals from other fields. Heat transfer and engineering economics are two important examples. Heat transfer fun- damentals enter the presentation in Chapter 4 to follow and are considered further in the applications of Chapters 5 and 6. Economic analysis enters in Chapter 7 and underpins the discussions of Chapters 8 and 9.
REFERENCES
1. M. J. Moran and H. N. Shapiro, Fundamentals of Engineering Thermodynamics, 2. J. Moran, Availability Analysis: A Guide to Energy Press,
3rd ed., New York, 1995.
New York, 1989.
PROBLEMS 163 of a Project Report for the U.S. Department of Energy, Project
Center for Electric Power, Tennessee Technological University, Cookeville, TN, 1989.
9. Eiserman, P. Johnson, and L. Conger, Estimating thermodynamic properties of coal, char, tar and ash, Fuel Proc. Tech., Vol. 3, 1980, pp. 39-53.
12. H. G . Stoll, Least-Cost Electric Utility Planning, Wiley-Interscience, New York, 1989.
1995.
and Metallurgical Processes, Hemisphere, New York, 1988.
496-506. 3.2 Showing all essential steps, derive the following equations:
(a) Equation 3.5 (f) Equation 3.15 saturated water vapor at (b) saturated liquid water at
Refrigerant 134a at 0.2 and (d) dry at 0.3 400 K.
uate its exergy and discuss.
a stream of matter to be negative? Discuss.
t64
Using the approach of Equations and 3.18, and model I, evaluate the chemical exergy, in for (a) CH,, (b) SO,, (c)
The following flow rates, in pounds per hour, are reported for the product SNG (substitute natural gas) stream in a process for producing SNG from bituminous coal: CH,, 429,684; CO,, 9,093; N,, 3,741; H,,
Verify the exergetic efficiency values provided in Section 3.5.3 for the cogeneration system of Table 3.1.
A gas enters a turbine operating at steady state and expands adiabati- cally to a lower pressure. When would the value of the turbine getic efficiency, Equation be greater than the value of the turbine isentropic efficiency, Equation Discuss.
Methane (CH,) at 1 atm, enters an adiabatic combustion chamber operating at steady state and reacts completely with 140% of the the- oretical amount of air entering separately at 1 atm. The products of combustion exit as a mixture at atm. Calculate an exergetic effi- ciency for this combustion chamber.
Consider a coal gasification reactor making use of the carbon steam process in which carbon at 1 bar, and water vapor at 1 bar, enter the reactor and the product gas mixture exits at 1 bar.
The overall reaction is
C
-
CO+
H,+
The energy required for this endothermic reaction is provided by an electric resistance heating unit. For operation at steady state, determine, in per kmol of carbon (a) the electricity requirement, (b) the exergy entering with the carbon, (c) the exergy entering with the steam, (d) the exergy exiting with the product gas, (e) the exergy destroyed, and (f) the exergetic efficiency.
Methane (CH,) at 1 atm, enters the steam generator of a simple vapor plant operating at steady state and burns completely with 200%
theoretical air also entering at 1 atm. Steam exits the steam
PROBLEMS 165
at 500 expands through a turbine, and exits to the condenser at 1 and a quality of 97%. Cooling water passing through the condenser enters at 77°F and exits at 90°F. The combustion products exit the plant at 1 atm. Pump work can be ignored.
Evaluate the following as a percent of the exergy entering the plant as fuel: (a) the power generated by the turbine, the exergy destroyed in the steam generator.
3.12 For a counterflow heat exchanger, use sketches of the hot and cold stream temperatures (as in Figure 3.7) drawn relative to to discuss Equations 3.32, 3.33, and the case 4 which follows.
Define an exergetic efficiency for the heat-recovery steam generator of the case study cogeneration system. Using data from Table 3.1, eval- uate the exergetic efficiency. Discuss.
3.13