We present the relatively less known thermodynamic concept of exergy in the context of ecology and sustainability. To this end, we first very briefly outline thermodynamics as it arose historically via engineering studies. This enables us to define exergy as available energy. An example of applying the concept of exergy to a simple human process is next described. Then we present an exergy analysis of Earth as a flow system, also concurrently describing other necessary concepts. Finally, we briefly comment on the applicability of exergy analysis to ecology and sustainability. Keywords: exergy - energy - work - thermodynamics - heat transfer - flow processes
In addition, the entropy of the ambient medium at temperature T 0 surrounding the heat engine also changes, both together increasing in the real irreversible process that takes place. The minimal increase of entropy of system + surroundings leads to maximum work and hence maximum efficiency η of energy conversion from heat to work. This is given by η = 1 -T 0 /T, called Carnot efficiency, independent of working substance. Existence of this limiting efficiency is a statement of second law of thermodynamics, which can also be expressed as dS (system + surroundings) > 0.
Towards end of 19 th century, empirical thermochemistry was theoretically systematized and incorporated into thermodynamics, thus including chemical energy in addition to mechanical. This needed the concept of chemical potential µ, an intensive property, in combination with number of moles M of the chemical substance being considered. M is an extensive property. Energy conservation had already been extended from classical mechanics to other areas of physics like electromagnetism. Thermodynamics naturally used electric and magnetic potentials and energies when applied to such systems. Efforts to derive the macroscopic principles of thermodynamics from atomic structure of matter gave rise to statistical physics. As physics developed further into relativistic and quantum realms, these methods and results broadened the scope of thermodynamics further. For a more detailed history relevant for exergy, see articles listed at [http://exergy.se], available as pdf files.
Exergy is formed from Greek ex + ergon, meaning “from work”. Some synonyms (from Wikipedia) are: availability, available energy, exergic energy, essergy, utilizable energy, available useful work, maximum (or minimum) work or work content, reversible work, and ideal work. Exergy is that part of energy which is convertible into all other forms of energy. It represents the potential of a system to deliver work in a given environment.
The exergy E of a system of volume V having internal energy U and entropy S, and composed of many substances i (i = 1, 2, …), each amounting to M i moles (and having chemical potential µ 0i in the surroundings), is defined as
relative to surroundings with pressure p 0 and temperature T 0 . H is a derived thermodynamic property called enthalpy. For flow in an open steady state system, it includes, in addition, the kinetic energy (per unit mass) (1/2)ρv 2 of the flow of fluid of density ρ and speed v. For the system, the internal energy U is given by
where T, p and µ i refer to properties of the system. Using this expression for U, exergy E can also be written as
This clearly shows that exergy is measured relative to a reference environment. For more details and relation of exergy to other thermodynamic quantities like Gibbs free energy, Helmholtz free energy, reference may be made to G. Wall (1977 Exergy -a useful concept within resource accounting, available as a pdf file from [http://exergy.se]). The same paper applies exergy to evaluate quality of substances like ores and others relative to Earth’s average environment. This application of exergy has been developed further over the decades. There is as yet no consensus on its unambiguous use for this purposesee references [1]- [3] at the end for a discussion of this aspect.
The concept of exergy has been successfully used in engineering for assessing and improving various types of plants and processes -see references [4]- [16]. These include energy generation, manufacture of consumer goods, acclimatization units for housing, equipment utilized for agriculture, metallurgical processes to extract metals from ores and so on. As an example, a thermal power plant is displayed and discussed briefly below.
—————————————————Energy and exergy flows through a condensing thermal power plant.
The grandest natural process consists of (our current understanding of) birth and evolution of universe in big bang theory on the timescale of 14 Gyr. Formation of Earth as a planet of our solar system occurred about 5 Gyr ago. Geological orogenic (i.e., mountain building) cycles occur every 300 to 500 Myr in Earth’s evolution since its consolidation. There are also natural processes of a few Kyr to decades (e.g., nitrogen cycle and other climatic and ecological cycles), down to 1 yr (hydrologic cycle). In the biosphere, there are processes over millennia, centuries, decades, years, months, weeks, days and shorter, down to milliseconds, and perhaps even shorter cellular (sub)processes in biological cells.
Exergy to drive all natural processes on Earth has its origin in sunlight. This is illustrated in a series of schematic diagrams and figures below, each with brief comments.
————————The Sun-Earthspace system. ————————————Although all energy incident on Earth from Sun is radiated away, quality of incoming energy, measured by its exergy, is much hig
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