







































Ecology, Economy and Society–the INSEE Journal 6(1): 31-51, January 2023 

RESEARCH PAPER 

Exergy Analysis: A Guide to Sustainability? 

Tejasvi Chauhan and Vinod K Gaur 

Abstract: This paper argues for a continuing exploration of Nature’s organizing 
principles that sustain prolonged homeostasis of the earth’s ecosystems punctuated 
by forceful transitions to new emergent states. Ecosystems develop and maintain a 
dynamically stable state by transacting energy and materials with the surrounding 
flows to keep reversing their continual fall to the ground state. Conversely, the 
elevation of any component of the ecosystem above the ground level may be 
regarded as a measure of its functional efficiency. This measure, called exergy, can 
be calculated for an eco-subsystem based on knowledge of the energy and material 
fluxes that thread it and, most importantly, of where the ground level happens to 
be. Admittedly, it is not straightforward to quantify these figures, and the departure 
of assumptions from reality will inevitably translate into errors in the calculated 
exergy figures. However, the variance may be estimated by analysing the results of 
an ensemble of calculations with randomly perturbed input values. Even with these 
limitations, however, a map of exergy losses characterizing different parts of an 
ecosystem has the potential to reveal relative thermodynamic efficiencies for 
appropriate ameliorative interventions. 

Keywords: Exergy, Ecosystem, Thermodynamic limits 

1. INTRODUCTION  

Sustainability is not a value-neutral concern, begging the question as to what 
is it aimed at. But at its heart lies the ambition to prolong the ambient state 
of the earth’s ecosystems, whose future trajectories are inalienably 
conditioned by the current state. Insightful hints to design a path towards 
sustainability, irrespective of the focus of its concern, can perhaps be 
gleaned by looking more incisively at the way ecosystems work. 

 
 Indian Institute of Technology, Bombay, India. chauhan.tejasvi9@gmail.com  
 CSIR - Fourth Paradigm Institute, Bangalore, India. gaurvinod36@gmail.com  

Copyright © Chauhan and Gaur 2023. Released under Creative Commons Attribution © 

NonCommercial 4.0 International licence (CC BY-NC 4.0) by the author.  

Published by Indian Society for Ecological Economics (INSEE), c/o Institute of  Economic 
Growth, University Enclave, North Campus, Delhi 110007.  

ISSN: 2581–6152 (print); 2581–6101 (web).  

DOI: https://doi.org/10.37773/ees.v6i1.914  

mailto:chauhan.tejasvi9@gmail.com
mailto:gaurvinod36@gmail.com
https://doi.org/10.37773/ees.v6i1.914


 Ecology, Economy and Society–the INSEE Journal [32] 

 

Planet earth’s ecosystems are unique in the solar system. They evolved from 
rudimentary forms of self - replicating molecules, feeding off solar radiation 
or energy from the deep earth, growing slowly at first and then more 
deliberately by dispersal and structure formation. The first of these is a 
piece of the universal process that inexorably enlarges the degrees of 
freedom available to a system’s condition, a natural consequence of 
unbiased treatment of all aspects of a situation.  The second process, also 
seen at work in inanimate systems fed by constant energy (such as 
atmospheric convection) has been extensively studied through laboratory 

experiments (for example, the Bénard convection). The results suggest that 
structure formation in constantly powered systems (like ecosystems) greatly 
enhances the rate of energy dissipation, perhaps to hasten the descent to 
equilibrium. The significant fact is that they develop and sustain low-
entropy organized entities in the very act of falling along a gradient. 

Every habitat on earth, including that of humans, belongs to some 
ecosystem of the planet (Figure 1). Ecosystems are a society of living and 
breathing beings much like our own, sustained by throughflows of matter, 
energy, and information exchanged with the surroundings. They grow, 
develop and stay alive by abstracting energy from other sources, such as 
solar radiation and solar-driven cycles of wind and water, to constantly 
restore the driving gradients of their arterial flows. Their potential energies 
keep falling inexorably toward the dead state of equilibrium through the 
very act of flowing. This is the spontaneous process forever at work in the 
universe — a progressive flattening of all gradients. Highlands erode to 
peneplains, gurgling mountain streams slow down to staid meanders, 
concentrated materials disperse into environmental wastes, and energy 
molecules oxidize to inert ones.  

Unless injected constantly with fresh energy, these downhill processes 
would eventually lay all systems to eternal rest (Figure 2). Even when a 
source such as the daily inflow of solar radiation exists, and the landscape 
can sustain thermal, topographic, and material flows (as seen in other 
planets in the solar system) these do not result in a 'Goldilocks world’ 
where the conditions for life are just right. For that to happen, the solar-
driven flows need to be channelled through entities that can transform their 
energy into useful work; entities that would keep reversing the downhill 
processes by assembling inert matter into a chain of energy molecules, and 
transforming wastes to clean the environment and mitigate hazards. This 
functional machinery developed on earth as its ecosystems, made possible 
by the planet’s special endowments: just the right mass to hold an 
atmosphere in its gravitational cage and the right distance from the sun to 
let liquid water exist. In turn, these conditions orchestrated the evolution of 



[33] Chauhan and Gaur 

 

a blue planet, dynamically driven within by plate tectonics and on its surface 
by the hydrological cycle. In time, the latter greened and flowered its land, 
turning it into one of the most spectacular objects in space. 

Figure 1: A Visual Representation of an Ecosystem 

 
Source: Authors 

 
Figure 1 presents an iconic view of an ecosystem. Its several distinctly 
recognizable units are, in fact, mutually supported through invisible streams 
of energy, materials, and information flows. These flows, primarily driven 
by the thermal and electrochemical gradients created by solar radiation, are, 
in turn, channelled through a vast network of energy, material, and 
information transforming devices or ‘eco-engines’. The latter use the work 
potential of their throughflows to maintain their live state and resist the 
universal tendency towards levelling. In the process, they perform critical 
ecosystem services, most notably, they help other eco-engines down the line 
do the same. 

2. ECOSYSTEMS AS A HIERARCHICAL NETWORK OF ECO-
ENGINES 

Since energy can neither be created nor destroyed, but only transformed 
through flows of heat, work, materials, or information, ecosystems are 
essentially constituted as energy transforming devices called ‘engines’. In the 
manner of all dynamical systems with continual energy inputs, which are 
known to maximize the efficiency of energy transformation by developing 



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an orderly structure (for example, Bénard convection cells, as examined in 
Demirel, 2002), ecosystems too develop an optimal network of energy-
transforming flows. They exploit all possible configurations allowable by 
their landforms and the daily and seasonally renewable stocks of energy and 
materials, to maximize their flow density pathways, especially of the miracle 
substance, water. The extraordinary properties of water as a universal 
solvent and its large thermal inertia, further enhance an ecosystem’s 
opportunity space for differentiation and co-development. The result is a 
hierarchically ordered web of material and energy flows and their storages 
that nourish and create the foundational structure of the food chain: soils, 
forests, lakes, wetlands, groundwater, and biodiversity (Allen & Starr, n.d.; 
Müller, 1992; O’Neill et al., 1989; Pahl-Wostl, 1993). 

The invention of steam engines in the seventeenth century was inspired by 
the experiential learning of early civilizations about transforming heat to 
mechanical energy (for example, fire pistons used by Neolithic 
communities) and creation of several ingenious devices that used steam 
power to move shafts. In the nineteenth century, an analytical inquiry in the 
efficiency of energy transformation processes led to an understanding of 
energy quality as distinct from energy quantity. Analyzing the results of a 
thought experiment, Sadi Carnot (Kelvin et al., 1924), showed that whilst 
steam flowing from a high temperature source can be made to perform 
work by moving a piston to drive a wheel, the amount of work (W) 

performed by it was only a certain fraction () of the input steam energy 
(Q). As the energy of flowing steam is progressively consumed in the 
process of performing work, its thermal potential, that is, its temperature, 
gradually reaches equilibrium with the surroundings. Thereupon, the 
residual heat, unable to flow and be transformed any further, simply joins 
the vast and unusable stock of environmental heat. The part of a system’s 
total energy capable of being turned completely into work can thus be 
measured by the elevation of its ambient state above the equilibrium level. 
This is called ‘exergy’, and it is measured in the same unit as energy on a 
scale defined by the zero potential energy of the equilibrium state or a state 
of complete stasis.     

Equivalently, the exergy of a particular system state is the energy required to 
create it from the chemically inert, thoroughly dispersed, zero gradient 
states of its components. For example, the exergy of one mole (a pack of a 

given number of molecules) of biogas (methane) being  832 kJ, means that 
a mole of the gas will produce 832 kJ of energy when allowed to unpack 
itself. This is in fact, equal to the energy stored in every mole of methane 
through the work performed by bio-organisms to i) extract carbon dioxide 
(20 kJ) and water (2.6 kJ) from the environment, ii) bond the extracted 



[35] Chauhan and Gaur 

 

carbon and hydrogen molecules into one of methane (818 kJ), and iii) 
return the residual oxygen (–7.8 kJ) to space. Unlike energy, exergy is not 
conserved, and is, therefore, a more sensible qualifier of the work potential 
of material and energy flows.  

Figure 2: Flow of Energy within an Ecosystem 

 

Source: Authors 

Figure 2 illustrates the ability of systems to perform work through certain 
processes by virtue of the fact that their elevated states are lifted above the 
flat equilibrium state: raised highlands erode to renew soil fertility, 
evaporated sea water is blown by wind to drive the hydrological cycle, inert 
carbon dioxide molecules hydrogenated through photosynthesis form high-
energy glucose molecules, and the osmotic flow of water to plant roots is 
driven by the concentration gradient. Left to their own devices, systems 
possessing potential energies tend to move downhill to the zero gradient 
equilibrium state, consuming the stored exergy that can be utilized to 
perform useful work if channelled through an engine, such as the 
production of electricity from falling water or metabolic energy from the 
oxidation of carbohydrates in the guts of living beings. Ecosystems 
integrate these processes into a biogeochemical cycle by abstracting the 
exergy of inflowing solar energy and outflowing heat energy from the 
earth’s depths to build gravitational and chemical potential energies in the 
form of landscapes and biomass. 



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Ecosystems maintain their health and build reserves for the future by 
channelling the surrounding flows of materials, energy, and information 
into a succession of lower-order streams thereby harvesting the maximum 
of the available exergy (Silow & Mokry, 2010). Exergy extracted from the 
daily and seasonally restored natural flow gradients that drive the 
atmosphere and oceans, and proton flows in plant cells, is consumed to re-
build other less consumptive gradients of the ecosystem. Fertile flood plains 
develop in the wake of diminishing stream power, as do wetlands, where an 
incredible diversity of bio-organisms reduce environmental wastes to 
organic carbon, thus preparing the base of the food chain. As some exergy 
is irretrievably lost along each flow path as dead heat (because of the 
ubiquitous presence of irreversibilities), the hierarchically organized chain of 
eco-engines is driven by a progressively diminishing sequence of residual 
exergies delivered by a preceding one (Figure 3). The entire exergy of solar 
radiation extracted by ecosystems is, thus, used up by a succession of their 
eco-engines to sustain the vital environmental flows and store some of it as 
chemical exergy in the food and fuel molecules for use in the lean season 
(Silow et al., 2011).   

Analogously, one can visualize the same process at work when considering 
a mountain stream that drives large rock masses down a steep valley and 
deposits them on the flattening valley floor on entering the plains (Figure 
3). This happens because the reduced gradient and approaching closeness 
to equilibrium reduces the potential for energy exchange, reducing the 
stream’s power even as the quantity of its flow remains the same. This 
progressive reduction in stream’s power continues apace, depositing finer 
and finer sediments into the biogeochemical marvel of a delta as it levels 
with the sea. Thus, all of the earth’s energy requirements, including those of 
its biosphere and human civilization, are met from the exergy or quality of 
solar energy (Figure 3), not its quantity as in case of a hydroelectric plant 
(which draws from the gravitational exergy of a flowing stream without 
consuming a single drop of water). 

Figure 3 shows how the exergy of solar energy is used by the two principal 
thermodynamic engines of the earth system: the atmosphere-ocean and the 
ecosystem, by harnessing the electromagnetic (thermal and electronic) 
gradients created by the high exergy solar radiation. One is created by the 
earth’s non-uniform heating, and the other by excitation of its otherwise 
inert molecules (photosynthesis). The first converts solar exergy to move 
the atmosphere & oceans, which in turn, drive the hydrological cycle and a 
cascade of progressively smaller engines, notably the nutrient cycle and the 
longer time-scale rock cycle (not shown). The hydrological & nutrient cycles 
collaborate with the photosynthesizing engine to grow and develop the 



[37] Chauhan and Gaur 

 

living world, which continually reverses the various potential gradients to 
maintain its low entropy islands against their ineluctable attrition. Like all 
dynamic systems that evolve into an organized structure, the earth’s primary 
engines too, harness the daily supply of solar radiation to structure the 
earth’s climate and ecosystems. The figure (left middle) shows how the 
exergy of solar radiation is used up in maintaining and developing the living 
earth through a progressively diminishing cascade of energy and material 
transforming Carnot engines, losing some exergy at every stage as heat 
without using any of the energy received from the sun, which at the end of 
the day, is radiated back to space with all its exergy reduced to zero.   

Figure 3: Uses of the Exergy of Solar Energy 

 

Source: Authors 

At the top of the earth’s exergy flow pyramid are two major flow–field 
gradients forged directly by the sun (Figure 3). The first one is the thermal 



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gradient resulting from the non-uniform distribution of temperature on 
earth because of its asymmetry with respect to the sun – earth axis and the 
varying reflectivity of its variegated surface features. The inexorable 
levelling of the thermal gradient mediated, in turn, by convective flows of 
the earth’s fluid spheres keeps the planet at a moderate temperature and 
creates humidity gradients, thereby driving a hydrological cycle. The latter, 
interacting with solid earths near surface texture and topographic gradients, 
creates surface flows of freshwater and its delayed release storages in 
mountain glaciers, soils and in the ground below.   

The second major gradient, collaboratively created with hydrological flows 
and the entrained nutrients, is the electrochemical gradient created by the 
absorption of solar photonic energy by electrons in the green molecules of 
plants and algae (called chlorophyll). The exergy yielded by the flow of 
protons down this gradient hydrogenates the carbon dioxide (CO2) 
molecules plucked from the air, packing part of the flow energy in the 
molecular bonds of the new glucose molecule (C6 H12 O6), which is the 
basic building block of all life. An economically viable replication of the 
process may indeed deliver us one day from the conundrum of global 
warming.   

The earth’s internal thermodynamic engine too builds topographic and 
geochemical gradients through episodic upheavals, subsidence, and volcanic 
effusions, but this involves much longer time scales than the diurnal and 
seasonal flows of energy powered by the sun and the solar energy–driven 
hydrological cycle. However, the former is responsible for sculpting the 
longer-lasting surface features of the earth into smaller biogeophysical units 
in which the annual and seasonal scale solar-powered environmental 
processes operate. 

Ecosystems flourish by flowing their share of fresh water and materials 
along their landscape gradients (principally the gravitational and the 
climatic) and generating a cascade of new ones using the exergy extracted 
from the former. The latter gradients, which are primarily biogeochemical, 
tend to organize themselves into a network of productive subsystems to 
maximize the extraction of all available free energy for conversion into 
biomass and ecological services, which is required by dependent 
communities. The efficiency with which the seasonally replenished natural 
resources of an ecosystem can be harnessed to sustain its well-being and 
productivity is largely determined by the efficiency with which available free 
energy in its various flow systems is harvested. Haphazard anthropogenic 
interventions across the globe have disrupted many ecosystems with self-
organized flows. However, these may yet be rewired to maximize the 



[39] Chauhan and Gaur 

 

productivity of their eco-engines using creative engineering designs 
informed by thermodynamic limits. What are these limits? 

3. THERMODYNAMIC LIMITS 

As explained previously, living systems (Figure A1) survive by being open, 
that is, by exchanging materials and energy with the environment, and by 
using up the latter’s exergies to maintain their well-being, and, thereby the 
functioning of their subsystems. Healthy ecosystems unceasingly deliver 
vital eco-services in the form of energy and material storages for use in 
regular and lean seasons and ensure the availability of clean water and a 
clean environment by transforming wastes. These processes, however, 
proceed in strict accordance with the laws of thermodynamics. The first of 
these laws states that energy is always conserved in any process. This is a 
corollary of the fundamental principle of symmetry established by Emmy 
Noether in 1915, a principle which helps explain the evolving universe. 
Since energy transformation can only be mediated through the exchange of 
heat, matter, information, or work, and the total energy must be conserved, 
we can represent this fact symbolically as: 

Qnet + Enet (matter) + Enet (Info.) − Wnet = (Einternal)syst.   (1) 

The RHS of (1) denotes net additions to a system’s energy stock accrued 
from transformations of flowing streams of heat (Q), materials, and 
information minus the work (W) delivered by the system. Work (W), in 
particular, denotes the energy associated with the displacement of materials 
mediated by force or pressure (such as moving a shaft). The statement 
asserts that the net sum can only appear as a change in the system’s 
molecular structure and its kinetic energy, collectively understood as its 

internal energy (Einternal).    

However, this statement gives no indication of the quality or potential of 
the resulting exergy in terms of performing work. For example, the heat 
energy of a stable atmosphere, which is known to be very large, cannot be 
made to perform any work unless it is made to flow along a temperature 
gradient, which is done by artificially creating a colder reservoir.    

Quantifying energy quality is a concept that is explored in the second law of 
thermodynamics, which was distilled from the analysis of a deeply insightful 
thought experiment by Sadi Carnot in 1824. Assuming the flow to be 
unhurried to allow reversing the process at any instant without any losses 
(such as pushing a bicycle pump infinitely slowly), he proved that the 
maximum useful work (W) extractable from a heat flux (QH) flowing from a 
hot reservoir at temperature TH to a colder one at temperature T0 (to drive 



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a motor, for example), is only a certain fraction () of the total inflow (QH). 
The unhurried, reversible condition specified in his thought experiment was 
used to ensure that no part of the input energy leaked out as waste heat (as 
would otherwise happen, for example, when a bicycle pump is pushed 

energetically). The efficiency () with which heat energy can be transformed 
to work is equal to {1 – (T0 / TH)}, which is always less than 100%, even in 
a slowly transforming reversible process, because, when flowing down a 
thermal gradient with concomitant cooling, a part of it (= Q0) cannot 
produce any work when it reaches equilibrium with the surrounding air at 
T0. He further proved that this limitation was universally true irrespective of 
the nature of the material flowing through the engine, thereby making the 
result applicable to a wide range and variety of energy transformation 
processes. Of equal significance is the fact that he proved that there was no 
way by which this theoretical limit of efficiency could be breached, and 
further, that in a reversible process of energy transformation from state A 
to state B, the quantity, SB = (Q/T)B, remained constant irrespective of the 
path of transformation. Claussius (1824) recognized this quantity as a 
characteristic of the state of a system and called it ‘entropy’—a Greek word 
related to transformation. As explained previously, entropy is inevitably 
generated in any process that transforms heat to work when a part of the 
input energy is reduced to impotence upon reaching equilibrium with its 
surroundings. Indeed, entropy is generated in the transformation of even 
those energies that possess 100% exergy, such as electric energy or 
mechanical work, because of the various dissipative losses involved in all 
physical operations. The ubiquitous presence of irreversibility in the real 
world, such as friction-generated heat, thus makes the entropy of a 
subsequent state of any physical system (SB) greater than the entropy of the 
preceding state (SA). Biological systems and refrigerators transform energy 
to create lower entropy islands locally at the expense of a larger quantum 
being added to the environment. This one-way street, where a system 
constantly evolves into a state of higher entropy, is thus an unexceptional 
rule of energy transformation in the universe. 

Or, dSA→B = (SB – SA) = (QB /TB – QA /TA ) = A →B  d(Q/T) =  A →B  dS   0 (2)                            

The equality holds strictly true for a reversible process where Q = TS, 
which, however, provides a reference for how far removed the efficiency of 
a non-reversible physical system is from the ideal, and how imaginative 
interventions may enable minimizing the gap. Given the thermodynamic 
arbitrage imposed by the second law, we can restate the conservation law 
for reversible processes as:                                                     

TS + Enet (matter) + Enet (Info.) − Wnet = (Einternal)syst.                           (3) 



[41] Chauhan and Gaur 

 

Drawing logical corollaries from the above equation, we can show (vide 
Appendix) that under reversible, ideal conditions, the rate of performing 
useful work, or the power delivery of an open system exchanging heat, 
matter, and information, is provided by (4) below.  

Ẇcv = − d/dt(Ecv − T0Scv) + i=1→n Q ̇i (1 − T0 /Ti) + ṁ (h + h* − T0s)        (4) 

Here, Wcv is the work potential of the system, or its exergy, expressed as the 
net sum of various inflowing exergies (of its internal energy, heat, mass, and 
information).  

However, irreversibilities in real-world systems invariably destroy some of 
the available exergy depending on the degree of their imperfections, 
reducing the actual delivery to less than what was computed from equation 
(3). The quantum of exergy destruction by an open system distilled from (3) 
is given by (5):   

(Exergy destruction rate)cv = T0{d/dt(Sgen)}                         (5) 

The exergy destruction figures of an ecosystem or of its subsystems (which 
can be calculated by applying (5) to the observed data) are an illuminating 
indicator of their respective health. By mapping the exergy destruction 
figures of sub-systems across an ecosystem, we can identify the ones whose 
performance is short of the theoretical limit for reversible processes, and 
thus warrant design interventions. 

4. EXERGY FLOW THROUGH THE EARTH SYSTEM 

The earth system receives a daily supply of highly concentrated radiation 

energy, Qs  1.4 × 1022 joules, or 1.74 × 1017 watts (Kleidon 2012), shone 
by a 6000* K hot sun. Since this radiation is non-uniformly distributed on 
earth, a thermal gradient develops between the equator and the poles, 
driving atmospheric and oceanic flows that keep the earth at an average 

temperature of 300  27* K. Despite this prodigious supply of daily 
radiation, the earth system, on average, is not heating or cooling 
significantly during the day, as it radiates back virtually the entire Qs, after 

abstracting its exergy  Qs = (1 – 300/6000) Qs = 0.95 × 1.74 × 1017 = 
1.65 × 10 17 Watts, to fuel its works and the living world. 

Additionally, the moderate thermal state of the earth, mediated by its 
circulating fluid spheres and constantly hydrated by the wind-driven 
hydrological cycle, promotes photosynthesis. The latter process can 
produce biomass exergy equal to 7.0 × 1020 joules per day at 5% theoretical 
efficiency (Zhu et al. 2008), if every ray of sunlight were to be captured by a 
green plant. Actual biomass production on earth is, however, limited by 



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non-ideal conditions to 70 giga-tonnes/yr. (Popp et al. 2014), providing a 

daily budget of 3.5 x 1018 joules of chemical potential energy that fuels all 
life forms on earth and most of their eco-service requirements.  

The significant point to note here is that in any energy transformation 
process, which is only possible through the exchange of heat, matter, 
information, or work, mediated by a system (or engine), while energy itself 
is conserved, the part available for performing work, that is, its exergy, even 

under ideal conditions, is always less than the original: exergy    energy.  

Thus, while the exergies of various forms of energy have some maximum 

theoretical limits denoted by , the values actually realised are less than  
because of the ubiquitous presence of irreversible processes in far from 
ideal, real-world systems. The latter are determined by the texture of 
specific systems, such as friction-generated heat losses in an electric motor 
despite the 100% theoretical exergy of electrical energy or pollutants-
induced biogeochemical debilitation in the performance of ecosystem 
services. The deficit of an actual state, compared with the maximum 

attainable energy (represented by ) of various system components, may 
thus be regarded as a measure of exergy loss that could be minimized by 
better design.  

The numerical values of the available or designed exergies of eco-
subsystems, can, therefore, prove quite useful, not only in ranking their 
relative contributions to an ecosystem’s well-being, but also in targeting 
potentially ameliorative ones. Furthermore, exergy, being a system’s work 
potential, is a dynamic quantity that increases when energy is stored in it 
and decreases as the system moves downhill in the course of performing 
some work. Its value at any given stage thus measures both its economic 
worth and health. Also, as work potential, it acts as a universal metric (in 
joules) for quantifying the state of any component of a system, whether 
physical, chemical, or biological. 

5. EXERGY OF ECOSYSTEMS 

The exergy of ecosystems is primarily driven by their biomass, which 
includes all life forms. It can be explicitly calculated by unpacking the 
second term, Enet (matter), in (3) above. This can be shown (vide A6f of 
Appendix) to be equal to the sum of the fractional concentration densities 
xq of the endemic chemical and biological species present in a given 

ecosystem, multiplied by their respective weighting factors, q, that is: 

Execo = k=0→n ( k xk)        (6) 



[43] Chauhan and Gaur 

 

The  values for a fair number of commonly occurring species have been 
calculated based on data from various ecosystems, which, in turn, 
determines the efficiency of their exergy use. These values have been 
updated accordingly in newer models (Jørgensen 2002; Jørgensen et al. 
2005). Some interesting applications in the study of the exergy of 
ecosystems can be found in Silow et al. (2011).  

The wide range of ecological services that sustain us are rarely 
acknowledged. What receives even lesser recognition, however, is the role 
played by the immense diversity of life forms in sustaining the perennial 
wheeling of energy and material cycles, which are essentially mediated by 
the biochemical functions ordained by their embodied genetic information. 
As explained above, the growth space available to ecosystems for 
maximizing the harvestable exergy of solar radiation is dependent on the 
number and variety of their eco-engines, that is, their biodiversity. The 
latter also reinforces the resilience of ecosystems when it comes to 
withstanding newly emerging stressors. Eco-engines do this by increasing 
their bio-geographical spreads by creating new niches that suit a 
proportionately larger pool of differentiated genetic traits.  Indeed, some 
studies on exergy analysis of ecosystems (Silow et al., 2011) confirm that the 
movement away from thermodynamic equilibrium during ecosystem 
growth and development coincides with higher levels of organization, 
system configurations, and maximization of exergy use and storage of both 
its chemical and information potential.   

In essence, the pursuit of rational inquiry looks for an organizing principle 
underlying masses of data. The intriguing work of living organisms in 
relentlessly reversing downhill universal processes to maintain a dynamically 
stable state has long engendered the conjecture that life processes are 
governed by some overarching principles. Based on acute observations of 

how the living world sustains itself, Lotka (1921−22), in his papers, 
proposed that in the event of there being higher available exergy than 
utilized, “an opportunity is furnished for suitably constituted organisms to 
enlarge the total energy flux through the system”. This prescient conjecture 
implies a self-directed orientation towards a higher range and level of 
organization and complexity (information potential) in ecosystems that 
would maximize the harvesting of available exergy, given the existence of a 
suitably constituted genetic order. An interesting experiment designed by 
Horowitz and England (2017) showed that a special configuration of atoms 
(Lotka’s suitably constituted genetic order), will start tapping into those 
energy sources, aligning and rearranging to better absorb energy and 
dissipate it as heat. Prigogine and Stengers (1984) state that the growth of 
organized structures, which characterize the growth of all living systems, 



 Ecology, Economy and Society–the INSEE Journal [44] 

 

can only be maintained by exporting a proportionately higher entropy 
energy, produced by higher dissipation. This could, therefore, be a directive 
principle leading the evolutionary trajectory of all dynamical systems. 
Further, the implied principle, in order to be specific, must in some way be 
unique, such as an extremal principle (Kelidon and Lorenz, 2005). Indeed, 
several other indicators of ecosystem health have been proposed, notably 
emergy (Odum 1991), total system throughflow (Patten 1995), and ascendency 
(Robert E Ulanowicz 2000). However, as Fath et al. (2001) argue, they are 
either implied by each other or are complementary. Thus, persuasive as 
these ideas are, they remain to be tested, even though a substantial amount 
of data may now be available in traditional archives of ecological research. 

Exergy analysis of eco-subsystems (by providing a direct measure of their 
efficiency) lights a path to maximize exergy storage for a given set of inputs, 
thereby also maximize the total system throughflow (considered by some as 
an independent goal function).  In our view, therefore, exergy analysis by 
itself or, wherever possible, complementarily with other indicators, offers 
obvious advantages in designing ecosystems and subsystems at various 
scales to serve urgent contemporary objectives such as meeting the decadal 
goals of ecosystem rehabilitation. It also holds high promise of leading to a 
robust theory of ecosystems, further enriching the methodology and 
approach to ecosystem design that would streamline its human dimension. 

APPENDIX 

Exergy Analysis of Ecosystems 

Ecosystems constitute a web of energy-transforming sub-systems (Figure 3) 
that constantly exchange energy, materials, and information with their 
surroundings to maintain their evolving dynamic states above the 
thermodynamically dead state of equilibrium. Their ability to keep 
producing ecosystem goods and services is accordingly measured by their 
dynamically maintained distance (potential) from the downhill flat 
equilibrium state. This equals the total useful work or exergy that a system 
will yield if it were allowed to drift towards the equilibrium state of zero 
exergy when it ceases to exchange any energy or materials with its 
surroundings (Figure 2). Similarly, this potential is equal to the energy 
required to create this system out of the zero gradients everywhere of all 
physical and chemical potentials.   

 

 

 



[45] Chauhan and Gaur 

 

Figure A1: A Schematic of an Open System 

   

 

Source: Authors 

Figure A1 provides a schematic of an open system—an energy and 
material-transforming engine— lying in the field of flowing streams of heat, 
information, and work from which it abstracts exergies depending on its 
specific structure. The application of the principles of thermodynamics to 
the ambient dynamical parameters of the open system enables us to 
calculate the gap between their actual functioning and a possible one, 
thereby providing a diagnostic tool to identify their state of viability and 
design engineering interventions to improve their condition. 

To calculate the exergy produced by such a system, we visualize a 
representative element of it, called the control volume (Figure A1), with 
flowing streams of heat, materials, information, and work. Let Qin (t), min 
(t), Iin (t), and Win (t) denote the heat, mass, information, and work, 
respectively, contained in an imaginary wafer of thickness dx, ready to be 
pushed in such a control volume at the time instant t, by pressure pin. A 
similar wafer containing the respective quantities of Qout, mout, Iout, and Wout 

leaves the control volume, t, seconds later at the time (t + t), pushed out 
by pressure pout. Unlike the heat and work streams, however, the material 
stream carries at least five forms of energy: i) internal energy, ii) the work 
performed by the difference of pressures moving the wafers through the 
system, iii) kinetic energy, iv) gravitational potential, v) and chemical 
potential released by chemical reactions such as the digestion of food. 
Material streams also contain much of the information flowing through the 
system in the form of biodiversity (gene codes) and learned knowledge that 
guides the various organisms to seek food and mates and protect 



 Ecology, Economy and Society–the INSEE Journal [46] 

 

themselves from predators. Thus, the second and third terms in (1) can be 
written as:  

Enet (matter) + Enet (Info.) = [j min{(u + pvsp.) + (cin
2 /2 + gz  +  q0j + I)}in − k 

mout {(u + pvsp.) +(cout
2 /2 + gz  +  q0k  + I)}out] = 1→j m(h + h*)in –   1→k m(h 

+h*)out,                                                                                                        (A1a) 

Here, the summation over j and k denotes the number of inlet and outlet 
portals an open system may have, c is the velocity term in kinetic energy, h 
is the specific enthalpy per unit mass of internal energy u, pvsp is the work 

performed by the inlet and outlet pressures, and h* denotes the sum of the 
kinetic and potential energies of gravitation, information, and chemical 
bonds of the qth chemical species. 

Accordingly, it is instructive to rewrite the conservation equation (1) more 
explicitly by including all forms of energy:   

Qnet + Enet (material) + Enet (Info.) − Wnet =  (Einternal)cv. =  [1→n Qnet + 1→j 

m(h+h*)in − 1→k m(h+h*)out ] – Q0  – Wcv ,                                               (A1a) 

Or, Wcv = –Ecv + 1→n Qnet + 1→j m(h+h*)in − 1→k m(h+h*)out – Q0           
(A1b) 

Here, Wcv is the output work of the system and Q0 is the heat dumped in 

the surroundings, adding entropy equal to Srev. = Q0/ T0 at its equilibrium 
temperature, T0. T0 remains largely unaffected by the various exchanges 
with the system because of its substantially larger thermal reservoir. We use 
this fact to temper the conservation law, otherwise devoid of any sense of 
energy quality, with the limits imposed by the second law. 

According to the second law, the change in entropy Scv within the control 

volume over a time interval t is equal to the net sum of entropies 
associated with the inflowing and outflowing streams of heat and materials 

[{(SQ)in –(SQ)out  +  (ms)in − (ms)out], regarded as ideal reversible 

processes, in addition to the entropy Sgen generated within the system due to 
all departures from the assumed reversibility. Accordingly,                 

Scv = [{Sgen – (Q0/T0)} + {1→n{(Qi/Ti) + 1→j (ms)in − 1→k (ms)out}]       (A2a),  

where capital S refers to the residual entropies of the system as well as those 
generated within, and its lower case to specific entropy (per unit mass).  

Or,  Q0 = T0[–Scv + Sgen + [1→n {(Qi/Ti) + (ms)in – (ms)out]      (A2b) 

Substituting the above value of Q0 into (A1), one obtains the work output 
of the system as:  



[47] Chauhan and Gaur 

 

Wcv = −(Ecv − T0Scv +T0Sgen) + 1→n Qnet (1 − T0 /Ti) + 1→j m (h + h* − T0s) –  

1→k m (h + h* − T0s)                                      (A3a) 

The first term on the RHS is the sum of i) exergy lost from the internal 
energy of the system due to various irreversibilities of energy 

transformation over the time interval t, while the second and third terms 
are the exergies gained by the system from the streams of heat and materials 
through a reversible transformation. Dividing the various difference terms 

in (A3a) by t, the time interval over which the change in the flow regime 
through the system is defined by the entry of an infinitesimally thin wafer of 
their contents and the exit of another, one can express the work output of 
the system by an equivalent power output as: 

Ẇcv = − d/dt(Ecv − T0Scv +T0Sgen) +  Q ̇i (1 − T0 /Ti) + ṁ (h + h* − T0s)   (A3b) 

Since exergy is the work potential of a system assuming reversibility of 
transformations, it is obtained by deleting the T0Sgen term from (A3b), being 
the only one caused by irreversibilities. Thus: 

(Exergy Power)cv = −d/dt(Ecv − T0Scv)+ Q ̇i (1 − T0 /Ti) + ṁ (h + h* − T0s)         
(A3c) 

The difference between (A3b) and (A3c) yields the amount (Ex)Dest of 
exergy lost or destroyed by a given transformer.  

(Ex)Dest rate = d/dt(T0Sgen)                          (A4) 

Since (Ex)Dest should ideally be zero for a perfect system or only marginally 
positive for a healthy one, its value above zero indicates the health of an 
ecosystem. Mapping the numerical values of the exergy destruction of eco-
subsystems, which can be calculated using (A4), provides a powerful tool 
for auditing the functioning of extant ecosystems as well as for testing the 
efficacy of those ecosystems designed for engineering interventions.  

As an example, consider the case of a flowing stream in a steady state, that 
is, d/dt(Ecv) = 0, and all heat exchanges taking place at the environmental 
temperature T0. With these conditions the exergy/mass of a steadily flowing 
stream is given by:  

(Specific Exergy)steady flowing stream = (h + h* − T0s)in − (h + h* − T0s)out               (A5) 

Exergy of Ecosystems 

The work potential of biomass resides in i) energy stored in the chemical 
bonds that hold the molecules of a substance together, ii) information 
contained in the genetic code of bio-organisms that ordain their 
functioning, and iii) the network of biological society that catalyses their 



 Ecology, Economy and Society–the INSEE Journal [48] 

 

collective synergy. Thermodynamic processes involving material 
transformations either perform work to raise the exergy of lower exergy 
inert molecules or extract the energy stored in their chemical bonds by 
breaking them. In either case, the energy transaction alters the chemical 
potential of the molecular assemblage, which is accordingly defined as the 
energy required to add or remove an atom or molecule from a given mass. 

The chemical potential q of a particular species q in a mixture is 
accordingly defined as the partial derivative of the energy of the substance 
with respect to the number of that species, all other species remaining 

constant q = E/nq. The chemical exergy of an ecosystem is hidden in 
the last term of (A3c).   

Therefore, when rewriting this expression in a differential form for a unit 
molar mass and dropping the suffixes, we get:  

dE = d(TS – PV) +  q = 1→n dq                         (A6a) 

Or, (dE – TdS – SdT – Pdv + VdP)  =   dq,                                                                        
which, for isothermal and constant volume processes largely true for 
ecological systems, may be reduced to:                

VdP  =  dq = (RT/P)dP                                      (A6b) 

Integrating (A6b), one obtains:  

q = RT[ log (Pq/Pq0) q0] =  RT  q = 0→n xq [log (xq /xq0) – q0 ]      (A6c)     

where Pq is the partial pressure associated with the flow of the qth chemical 
species = xqP0, xq being the molar fractional concentration of the qth species 

such that xq = 1, and xq0, its value in the zero gradient equilibrium state. 
The summation includes species of inorganic (q = 0) and organic molecules 

(q = 1) and bio-organisms (q  2). The latter, viewed as information exergy 
engines, are driven by both coded information in the genes of different 
communities and network information. 

The biochemical exergy of an ecosystem, which is defined as the work 

potential above the equilibrium state xq q0, is therefore given by:  

Execosystem = RT  q = 0→n  xq log(xq /xq0)    (A6d) 

Values of xq for the inorganic as well as organic molecules in the ecosystem 
can be determined by measuring their fractional values in the laboratory. To 
obtain these with respect to bio-organisms, we must interpret them from 
the perspective of the probability of their occurrence in the environment: xq  
= pq X, where  

X =  q = 1→n  xq, and  X0 =  q = 1→n  xq0                                                         (A6e) 



[49] Chauhan and Gaur 

 

where X0 is the total organic matter per mole, at the equilibrium state, 
which, for an evolving ecosystem, represents a previous state. Assuming, 
however, that there is no substantial transfer of matter during the period, 
we see: 

Execosystem =  XRT  q = 0→n  pq log(pq /pq0) =  q = 0→ n qxq,                        (A6f)   

Jørgensen (2002) indicates how the quantity RT log(pq /pq0) = q, may be 
calculated by measuring the concentration density of the respective species 
in the environment. This calculation is based on available knowledge of the 
genetic structure of endemic communities, and the model assumed for 
transforming information into eco-products. New research on the structure 
of information-controlling genes, for example, was used by Jørgensen et al. 

(2005) to revise Jorgensen’s earlier determinations of q values (Jørgensen 
2002). Researchers continue to update these values for better evaluation of 
ecosystem exergy as new understanding is gained about the microstructure 
of the living world.  

ACKNOWLEDGEMENTS 

The authors wish to thank Sophie Gaur for designing the figures. TC 
acknowledges the support of the Prime Minister’s Research Fellowship 
(PMRF), the Ministry of Education, and the Government of India—
RSPMRF0262. 

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