A Broad Look at the Emergy and Exergy as Useful Design Tools and Efficiency Predictors by Robby Whitesell March 29, 2007 Revision 1

In partial fulfillment of the requirements of the course ME 620 Advanced Engineering Thermodynamics D. P. Sekulic

DEPARTMENT OF MECHANICAL ENGINEERING UNIVERSITY OF KENTUCKY

Explore in detail advanced thermodynamic analysis/theory/approach and establish your own position regarding its meaning, correctness, usefulness and relevance for engineering study and/or scientific inquiry Abstract This paper looks at and discusses emergy and exergy on a broad scale. It points out some of the flaws in emergy and emergy analysis, but goes on to say that both emergy and exergy can be useful design tools for designers of various backgrounds. Emergy is designed to be used on a broader scale than exergy and uses transformities to link processes and systems that before had no visible link. For this reason it can be a useful tool in analyzing large scale global systems. Exergy is more useful when applied to individual systems since it is dependent upon a good definition of the reference state. Both have their merits, though emergy may need a little more refining.

Introduction In world where efficiency (not only in the mechanical or thermal sense) is vital to the growth of businesses and the well-being of the world population, it is important to look at ways to model efficiencies of processes and systems that are truly global. Emergy attempts to do this through the use of transformities which are a way to measure the useful exergy that is consumed by a process from a specific source. Because of its broad definition, this concept can be applicable to economic, informational, cultural and many other different types of problems.7 Exergy is a way to quantify the amount of real world work that can be obtained from a system or process.7 Because it is important to create things that are efficient, engineers and designers need better performance guidelines. Theoretical guidelines can be useful, but are often far out of reach of real world systems. Exergy takes into account real world losses (irreversibilities) and helps the designer get a better idea of the expected real world output and performance of a system or process. Both emergy and exergy can provide a way to predict the efficiency of a system. Emergy can look at many types of systems and give predictions for efficiency of energy use and allocation throughout the entire process while exergy provides a better insight into the efficiencies of individual process that would compose a larger system.

This paper will look at both emergy and exergy and show that although emergy may not be as theoretically sound as exergy they both can still be useful.

Introduction and Definition of Exergy

It is well known that ideal or reversible thermodynamic processes can be analyzed to obtain performance boundaries for their real world irreversible counterparts. However, in many cases (e.g. Carnot efficiency, efficiency of an internal combustion engine) the theoretical performance boundary is far greater than actual efficiencies calculated from experimentation. Because of this, using these theoretical performance boundaries in the design of a real world system may not provide sufficient clarity in the solution. Therefore, scientists looked to find other properties or qualities of systems that could

better assist the designer in creating a real world system based on accurate predictions of the system’s performance and efficiency.

The idea of availability was introduced.

Availability is the amount of useful work that can be extracted from a system and takes into account real world losses and irreversibilities.1

Later it became known as exergy.

Exergy can take many forms, e.g. thermodynamic, chemical, kinetic, potential, etc. The total exergy of a system is then the sum of these component exergies.1 By looking at the thermodynamic component (shown below) an important property of exergy becomes clear. e1 = h1 − h0 − T0 ( s1 − s0 )

(1)

The thermodynamic portion of exergy is not entirely dependent upon the system’s enthalpy, but also on its entropy, further demonstrating that exergy takes into account real world irreversibilities.1

Exergy is not simply a property, but rather a co-property of

system and its environment which becomes the reference state. Thus, it is important to properly define the reference state to get an accurate model. The idea of exergy has spawned such analysis tools as Thermo-Economics and Extended Exergy Accounting.1

Introduction and Definition of Emergy

In an effort to further broaden our understanding of processes and their interactions on a more global scale, emergy was introduced. Emergy provides a way to see what kinds and how much exergy was required (directly or indirectly) in a process. For example, a solar emergy is the sum of all inputs of solar exergy directly or indirectly in a process.1 In other words, emergy is a way to describe and quantify the resources that are used up to run processes and make products.6

This concept also applies to processes which do not directly involve thermodynamics. The idea of emergy can be used to quantify culture, information, labor and many other important societal attributes. Because of its multidimensionality, emergy can be used to quantify and analyze global processes seemingly too complicated to analyze as one.6

Emergy does this through what is called transformity. The transformity of a product is defined as the total emergy used to drive the process divided by the exergy of the output.1, 2 The idea of transformity makes emergy unique in that it is path dependent.2 Two systems might start and end at the same state, but the paths they took to get there (emergy) were very different, thus their transformities will be different even though the output is the same.1

In this way it is said that emergy represents an “energy memory”

since the amount of energy used in a process is always accounted through the path the process took to achieve the output.1

Critique of Emergy

Although emergy seems like a good way to link global process, there are some flaws in its concept and usage. If emergy is to be considered a property and theoretically correct, it must address the second law.

Currently, the second law is not addressed in its

definition. As a reminder, the second law states that energy flows from a high level to a low level. Emergy accounts for its flows through transformities which are nothing more than “the energy of one kind required to be transformed to make one unit of energy of another kind”.1 The concept also fails to discuss that two amounts of emergy can have the same value (in emJoules) but clearly are not equal.1 For instance, a Q and a W can originate from the same fuel cell and will clearly have different exergies.1 However, if their transformities remain the same, their emergy values can also be the same for several different combinations of magnitudes of Q and W.1 This, once again goes back to the lack of definition that heat and work are very different.

Next, the idea of transformities is very vague. It is very difficult to measure actual emergy of physical systems, thus there is very little actual data to support published transformity values.1,3

Thus, most transformity values are estimated and thus the

assigned values are left up to the discrimination of the analyst.1,3 For example, the emergy for natural gas is calculated through the average efficiency of coal boilers which can vary greatly depending on the type of coal used (PRB, Bituminous, etc), the technological age of the boiler, the type of boiler, etc.3

Within the realm of emergy supporters there are some discrepancies regarding the definition of available energy. Although they agree it is not Gibbs’ free energy, some say it is not solely exergy because of the definition of exergy requires that the energy be available to perform mechanical work and emergy must take into account all input flows of energy.3

Another highly debated aspect of emergy is the maximum empower principle. It claims that self-organizing systems will attempt to maximize their emergy use or empower.2,3,6 It goes on further to say that systems which maximize empower will survive and those that do not will cease.2,3,6 This, as of now, has not been proven or disproved and will thus remain a topic of great debate, but it is included to here to help demonstrate the problems facing emergy.

Lastly, the mathematical definition(s) of emergy and transformity are not transparent. Equation 2 defines a total solar emergy U:

U = ∑ Ei × Tri

(2)

i

Where Ei is the exergy and Tri is the solar transformity of the ith input flow. The definition of transformity is given below: Tri = U i / Ei

(3)

It is clear that this is a circular definition. Emergy analysts sidestep this issue by assigning the solar transformity, Ts, equal to one.1 This circular definition seems like a point of interest because transformity cannot be defined independent of emergy, making it even harder for emergy analysts to obtain real world data from experiments to better define the transformities for given substances.

Usefulness of Exergy

Although exergy may seem highly specialized and only relative in certain situations, it can be a very useful analysis tool in many ways to many different kinds of system designers. It is important to first note that for an exergy analysis to be successful and as accurate as possible, a good definition and quantification of the reference state or environment is critical because exergy defines available work based on the reference state.4 Because exergy takes into account irreversibilities it is very useful in predicting performance of real world systems and pinpointing actual and potential energy wastes within systems.4 One of the goals of design engineers is to design or create a system that is not only cost effective but also as efficient as it can be given certain constraints. Exergy provides a way for that design engineer to better find a real world efficiency goal and discover critical energy wastes that could hurt the overall efficiency.

Another benefit that stems from an exergy analysis is the exergy efficiency. Like its counterpart, energy efficiency, exergy efficiency can be a useful tool for designers, but it helps take the understanding of the problem one step further. It is known that energy efficiency is defined as:

η = 1 − ( Loss ) /( Input )

(4)

ψ = 1 − ( Loss + Destruction) /( Input ) 4,5

(5)

Exergy efficiency is defined as:

This efficiency helps highlight the need to seriously consider not only losses, but destruction or irreversibilities when designing a system.4,5

This basic equation can be

adapted to any system where the designer must decide what is considered a loss, or what is the important function of the systems and therefore what to optimize.4 This only one of the many types of exergy efficiency. Cornelissen, R.L. describes many different types

of exergy efficiencies and their uses in [5]. Just one is given here as many of the others involve specialized equations for certain cases.

Conclusion

Both emergy and exergy provide a unique and distinct way of addressing various types of problems. Both are well grounded in theoretical thermodynamics, though it is clear that emergy is lacking because it does not take into account the second law in describing emergy flow. It is clear that emergy is a broader concept and tries to encompass more fields of interest than exergy. It is hard to extrapolate exergy analysis into economic, social, cultural or informational problems, yet emergy easily adapts through its use transformities (although their meaning and/or value can sometimes by vague). Emergy is a concept best used to understand the broad scale that so many systems are on today. Emergy allows the analyst to look at, let’s say a power plant, and not only vaguely determine the efficiency of the power generation portion, but also determine the efficiency of capital flow, resource flow, labor use, etc. Although some key aspects of emergy may need further definition and/or revision, its ideas and principles help bring about useful maps like the following:

Figure 1: Basic emergy flow diagram for washing clothes Taken from Cornelissen, R. L page 72

Here, the user can clearly follow the different kinds of energy used to wash clothing. Emergy is a way to bring many processes and systems together, analyze them through a common base (transformities) and help better design the conglomerate system.

Exergy seems more useful to specific systems or parts of a larger system. Exergy can help point our or highlight critical energy losses in a system. Because the best use of exergy will involve a clear reference state definition it is best used on a smaller scale than emergy. It would very difficult to define a reference state, for say an entire power plant, but it would, however, be easy to a define a reference state for the water that enters the plant. It is also important to note that exergy makes little attempt to apply itself to processes outside of thermodynamics like emergy. Thus, in comparing these two the system must be one that both can accurately apply to.

Both emergy and exergy can be useful tools in analyzing very small or very large systems given the right system, reference conditions and correct inputs to each analysis.

References 1

Sciubba and Ulgiati, Emergy and exergy analyses: Complementary methods or irreducible ideological options?, Jan. 1, 2003, pp. 1954-1986 2 Odum, Howard, T., Self-Organization, Transformity, and Information, Science Vol. 242, Nov. 25, 1988, pp. 1132, 1138 3 Hau, Jorge L., and Bakshi, Bhavik R., Promise and Problems of Emergy Analysis, Department of Chemical Engineering, The Ohio State Univeristy, pp. 1-13 4 Dincer, Ibrahim and Cengel, Yunus A., Energy, Entropy and Exergy Concepts and Their Roles in Thermal Engineering, Entropy, Vol. 3, 2001 pages 116-149 5 Cornelissen, R. L., Thermodynamics and Sustainable Development, Enschede, Netherlands, 1997, Chapter 2, Chapter 5 6 Brown, M.T., and Ulgiati, S., Emergy Evaluation of the Biosphere and Natural Capital, AMBIO Vol. 28, No. 6, Sept. 1999, pp. 1-6 7 Odum, Howard T., Emergy Accounting, University of Florida, April 2000 pp. 1-3