Energy quality, emergy, and transformity: H.T. Odum’s contributions to quantifying and understanding systems

Abstract

We present in this article, a brief historical overview of the development of the concepts and theories of energy quality, and net energy that were the precursors to emergy. The concepts evolved over decades, beginning in the 1950s with Odum’s work on tracing energy flows in ecosystems. During the 1970s, Odum’s attention was drawn to larger scale systems that included the economies of humans and the concept of net energy. In the 1980s, Odum quantified energy quality and defined it as a “donor-based” evaluation technique. In the 1990s, energy quality was further refined and rigorous definitions for “emergy” and “transformity” were given. The units of emergy were defined as solar emjoules (abbreviated seJ) and the units of transformity were seJ/J. In addition, we provide some insights into the types of processes and systems that have been evaluated using emergy methods.

Full text

Ecological Modelling 178 (2004) 201–213
Energy quality, emergy, and transformity: H.T. Odum’s
contributions to quantifying and understanding systems
Mark T. Browna,∗, Sergio Ulgiatib
aDepartment of Environmental Engineering Sciences, University of Florida, Orlando, FL, USA
bDepartment of Chemistry, University of Siena, Italy
Abstract
We present in this article, a brief historical overview of the development of the concepts and theories of energy quality, and
net energy that were the precursors to emergy. The concepts evolved over decades, beginning in the 1950s with Odum’s work
on tracing energy flows in ecosystems. During the 1970s, Odum’s attention was drawn to larger scale systems that included the
economiesofhumansand theconcept ofnet energy.In the1980s, Odumquantified energyquality anddefined itas a“donor-based”
evaluation technique. In the 1990s, energy quality was further refined and rigorous definitions for “emergy” and “transformity”
were given. The units of emergy were defined as solar emjoules (abbreviated seJ) and the units of transformity were seJ/J. In
addition, we provide some insights into the types of processes and systems that have been evaluated using emergy methods.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Emergy; Energy quality; Transformity
1. Introduction
Probably, the least understood and most criticized of
H.T. Odum’s body of work is his concepts and theories
of energy quality that are embodied in the 35 year
development of the emergy concept. The development
of emergy and its theoretical base cannot be separated
from development of the concept of energy quality.
We do not really know when Odum first considered
that different forms of energy had different “qualities”.
We do know that in the 1950s and early 1960s, he was
tracing energy flows in ecosystems and was probably
reflecting on the differences in the work potential of
energy among sunlight, the currents of water erupting
from Silver Springs (Florida) and the currents bathing
the coral reefs of Eniwetok (Marshall Islands). It was
in his book Environment Power and Society (1971) that
∗Corresponding author.
E-mail address: mtb@ufl.edu (M.T. Brown).
Odum first touched on the concept of energy quality
formally when he stated...
Beginning in the last century man began to develop
an entirely new basis for power with the use of coal,
oil, and other stored-energy sources to supplement
solar energy. Concentrated inputs of power whose
accumulation had been the work of billions of acres
of solar energy, became available for manipulation
by man. (Odum, 1971a)
Concepts of energy quality evolved over the decades
from the early 1970s, where it was primarily a qualita-
tive description of different energy forms to a quantita-
tive method of expressing different forms relative to a
common basis for comparative purposes. Through the
decades, it became clearer and clearer to Odum that
all forms of energy do not have the same ability to do
work and that “quality corrections” were necessary if
one were to compare the different forms with respect
to their differential ability to do work. Reflecting on
0304-3800/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
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202 M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213
these differences, he was one of the first to point out
the fallacies of energy technologies that promised un-
limited energy for society from the sun or from vast
quantities of oil trapped in the western shales, In the
first case, the source was too dilute and the energy
costs of concentration were too high for the source
ever to have a net yield once the cost of the collectors
was subtracted. In the second case, the energy costs
of “liberating” the oil shale, including many energy
costs associated with getting water to the mountain-top
site was so great that again there would be no net
yield.
Odum’s major efforts during the 1970s were aimed
at quantitatively defining energy quality and toward
understanding the net yields of many energy sources,
but it must be understood that it was not just defining
energy sources and their net yields that drove Odum to
explore energy quality. It was his desire to understand
how the biosphere worked from the smallest scales to
the largest, and the fact that his brand of science re-
quired that more than one level or scale of the biogeo-
sphere be considered in order to understand any single
level. In order to combine scales in the same analysis
or to jump from one scale to another as the systems
of interest shifted, it was crystal clear to Odum that a
very different approach to defining energy and ability
to do useful work was necessary. It was apparent that
the ability to do work was dependent on not only the
form of energy but also the system being considered.
2. Energy quality: a historical perspective
Odum recognized principles of energy quality as an
outgrowth of his investigations into the works of the
combined systems of humanity and nature. As a sys-
tems ecologist, he observed energies of many differ-
ent forms at many different scales. He reasoned that
a system organized to use concentrated energies like
fossil fuels cannot process a more dilute energy form
like sunlight, joule for joule. Since the processes of the
biosphere are infinitely varied and are more than just
thermodynamic heat engines, the use of heat measures
for energy that can recognize only one aspect of en-
ergy, its ability to raise the temperature of things, can-
not adequately quantify the work potential of energies
used in more complex processes. The recognition that
resource flows drive processes other than engine-like
ones lead to the concept of energy quality, and even-
tually to the measure of quality now called EMERGY.
Beginning in about 1966, Odum referred to “energy
of one kind” as a common denominator with the name
“energy cost” (Odum, 1967b; Odum, 1971a). In a pre-
sentation to the President’s Science Advisory Com-
mittee, Panel on World Food Supply titled Energetics
of food production, Odum referred to the giant energy
subsidies that were inherent in the green revolution
that made possible “delusions regarding the capac-
ity of science to develop means for feeding growing
populations” (Odum, 1967b). In this same publication,
Odum made that statement more explicit by beginning
the process of quantifying the energy cost of energy
by quantifying the fossil fuel energy required to pro-
duce food. He suggested that there was roughly one
calorie of fossil energy required per calorie of food
delivered through modern agriculture. In a review of a
special issue of Scientific American in 1971 devoted
to “Energy and Power”, Odum suggested that “Sun-
light is dilute energy, and the costs of concentrating
it have already been optimized and yield maximized
by the millions of years of natural selection for this
maximization” (Odum, 1972a).
The first definition of energy quality appears in an
article in AMBIO in 1973:
Energy is measured by calories, btu’s, kilowatt
hours, and other intraconvertable units, but energy
has a scale of quality which is not indicated by
these measures. The ability to do work for man de-
pends on the energy quality and quantity and this
is measurable by the amount of energy of a lower
quality grade required to develop the higher grade.
The scale of energy goes from dilute sunlight up to
plant matter, to coal, from coal to oil, to electricity
and up to the high quality efforts of computer and
human information processing. (Odum, 1973)
Certainly, Odum was thinking about “quality” dur-
ing the early 1970s, and it appears that the first quanti-
tative evaluation of the concept was in 1975. While the
concepts of energy quality were, no doubt, still devel-
oping in 1975 when Odum received the Prize Institute
la Vie in Paris, his acceptance speech titled “Energy
Quality and Carrying Capacity of the Earth” contained
a table of “Energy Quality Factors”, or the kilocalo-
ries of sunlight energy required to make a kilocalorie
of a higher quality energy (Odum, 1976a). It was in

M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213 203
this same speech that Odum unveiled his energy hier-
archy principle and that “energy quality is measured
by the energy used in the transformations” from one
type of energy to the next.
The concept of net energy played an important role
in the development of energy quality and emergy.
Odum was used to the concept of “net production”
in the ecosystems he had studied over the years, and
when applied to the human economy suggested that
an energy source must be able to provide a net contri-
bution to the economy of the larger system in which
it is embedded, i.e. it must provide more energy than
it costs to extract and process it. Odum suggested that
this principle applied to every system at all levels, from
ATP-providing energy to the biochemical reactions in
living systems, to photosynthesis, to the energy ex-
pended by animals as they grazed or chased prey. And
so it was logical that it applied also to the fossil fuels
driving economic sectors and human societies.
In the 1970s, Odum’s work had turned toward larger
scale investigations of the interactions of energy, ecol-
ogy, and economics. This in turn led to the application
of net energy to energy sources for economies and his
suggestion that ... “The true value of energy to soci-
ety is the net energy, which is that after the costs of
getting and concentrating that energy are subtracted”
(Odum, 1973). The concept of net energy as Odum
presented was almost revolutionary, and after a presen-
tation to the US House of Representatives Subcommit-
tee on Energy and Power, and a meeting with Senator
Mark Hatfield of Oregon, a bill was introduced in the
Senate in 1975 for a federal law that made net energy
analysis a requirement of proposed alternative energy
systems. (Odum, 1976b). This legal requirement was
enforced for a while but is now largely ignored.
Odum’s concept of net energy was inextricably
connected to energy quality, since the “true costs of
getting and concentrating energy” included not only
high quality fossil fuel inputs but also human services
and environmental inputs and these inputs required
“quality correction”. Odum presented his concepts of
energy quality linked to net energy at a meeting at
Stanford organized by the National Science Founda-
tion in response to the new law (Odum et al., 1976).
Those present rejected the concept and settled on
net energy strictly defined as the fossil fuel energy
required per unit of fossil energy delivered. Hall
(1986) nearly a decade later defined Energy Return
on Investment (EROI) emphasizing the fossil energy
invested although with an acknowledgement of the
need to include environmental and labor energy inputs
as well for a comprehensive analysis.
3. The emergence of emergy
From 1975 on, Odum’s attention was increasingly
focused on the development of his theory of energy
quality and its quantitative definition. In the latter half
of the 1970s, Odum had several research projects in
south Florida that were investigations of overall carry-
ing capacity of humans and the environmental changes
resulting from human uses. The ultimate purpose of
the projects was to make suggestions for maximiz-
ing economic and environmental vitality of the region
through better environmental management. It was dur-
ing these projects and the very creative atmosphere
that evolved around them that the concept of energy
quality matured into the precursors of emergy. Called
fossil fuel work equivalents (FFWE), the quality of
energy was measured based on a fossil fuel standard
with rough equivalents of 1kcal of fossil fuel equal
to 2000kcal of sunlight. The ratios used to convert
all energy forms to FFWE were called “Energy qual-
ity ratios”. Later termed coal equivalent (CE) calories,
eventually the system of evaluating quality was placed
on a solar basis and termed solar equivalents (SE) in
(Odum, 1977a).
Odum began using the term embodied energy to re-
fer to energy quality differences in terms of their costs
of generation, and a ratio called a “quality factor” for
the calories (or joules) of one kind of energy required
to make those of another in 1980 (Odum and Odum,
1980). The term embodied energy was used by oth-
ers for different ways of thinking and calculating; in
essence not including all energies and not using the
concept to imply quality, so in 1982, Odum switched
to “embodied solar calories” and the quality factors
became transformation ratios. Odum abandoned “em-
bodied energy” altogether in favor of “emergy” a term
suggested in 1983 by David Scienceman, which was
a constriction of embodied energy. Scienceman was
a visiting scholar from Australia, who suggested the
term, and emjoules or emcalories as the unit of mea-
sure to distinguish emergy units from units of avail-
able energy. The term transformation ratio gave way

204 M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213
Table 1
Chronology of development of emergy and transformity and conversions
Years Measures of quality Unit emergy values Units Reference
1967–1971 All energies of higher quality including
wood, peat, coal, oil, living biomass, etc.
expressed in units of organic matter
Direct sunlight equivalent to organic
matter was taken as 1000 solar
kcal/kcal of organic matter
Gram dry wt. O.M.; kcal,
conversion from OM to kcal
=5kcal/g dry wt.
Odum (1971a 1967b)
Recognized energy basis for monetary
payments 10,000kcal of fossil fuels/$
1973–1980 Energy quality of plants, wood, and fossil
fuels were differentiated Direct sunlight equivalents of fossil
fuels =2000 solar kcal/fossil fuel kcal
(first called energy quality factors,
later called solar cost equivalents and
then energy quality ratio)
fossil fuel equivalents (FFE)
and later coal equivalents (CE) Odum et al. (1976)
An energy money ratio was further refined
as ratio of total fossil fuel use (or coal
use) to GNP
25 000 fuel kcal/1973$, 19 000
CE/1975$, revised in 1980 to 11 000
CE per 1980$
Called energy dollar ratio
(CE/$)
1980–1982 Energy quality of earths processes driven
by solar energy recognized, thus solar
energy embodied in winds, rains waves
accounted for.
6800 global solar cal/cal of available
energy in coal Called “embodied energy”
units were global solar
calories.
Odum et al. (1983),Odum (1983)
Energy money ratio 19 600 fuel kcal/$
1983–1886 Recognized that solar energy, deep heat,
and tidal momentum were basis for global
processes total global sources equal to
9.44 E24 solar joules
Embodied solar joules/joule of fossil
fuels =40000 seJ/J, called energy
transformation ratio (ETR)
Called embodied solar
equivalents (seJ) and later
called “emergy” with
nomenclature (seJ)
Odum and Odum (1983)
Embodied solar equivalents per dollar
calculated for different nations seJ/$ in USA economy =2.2E12
seJ/1984$ seJ/$
1987–2000 Further refinements of total energy driving
global processes, embodied solar energy
renamed to EMERGY
Solar emergy/joule of coal energy
∼40000 solar emjoules/joule (seJ/J)
called “Transformity”
seJ/J =transformity Odum (1996)
Emergy money ratio based on sum of
renewable and non-renewable emergy
driving economy divided by GNP
Dollar equivalents of emergy are
called “emdollars” emergy per dollar
in 2000 ∼1.0 E12 seJ/$
seJ/g =specific emergy
seJ/$ =emergy/unit money
2000–present Emergy driving the biosphere reevaluated
as 15.83 E24 seJ/year raising all
previously calculated transformities by the
ratio of 15.83/9.44 =1.68
Solar emergy per joule of coal energy
∼6.7 E 4 seJ/J seJ/J =transformity; seJ/g
=specific emergy Odum et al. (2000)
Emergy per dollar calculation does not
change. Driving energies increased 1.67
times
Emergy per dollar in USA in 2000 ∼
1.67 E12 seJ/$ seJ/$ =emergy/unit money

M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213 205
to ‘’transformity” about the same time. In an appendix
to his book Environmental Accounting (Odum, 1996),
Odum provided a table listing a chronology of nomen-
clature and emergy conversions. The table provides
insight into the development of the emergy concept
and is worth presenting here in slightly different for-
mat and with some additions (Table 1).
Between 1983 and today, the emergy methodology
has undergone continued transformations. It has con-
tinued to mature as each new research project pre-
sented new “theoretical wrinkles” and as they were
explored dissected, and discussed. Always concepts
and theories were explored out-load at our “Systems
Seminar” at the University of Florida that meets ev-
ery Thursday and has for the past 30 some odd years.
It was during these sessions where the entire body
of concepts and theories were first introduced, dis-
cussed, amplified, recycled, and evolved. In many re-
spects, the development of emergy has been a dia-
log between Odum and his colleagues and students
on Thursday afternoons. Thus, the emergy method
developed in a way similar to the information cycle
posited by Odum (Fig. 1), as necessary to maintain
existing, and generate new information through a pro-
cess of natural selection. First, newly generated infor-
mation is selected, tested, and extracted, then copied
and shared, and finally through use, completed the
cycle where it acts through reinforcement to gener-
ate new information. This cycling of information is
necessary to avoid its loss due to second-law depre-
ciation as well as a requirement of new information
generation.
Fig. 1. Diagram illustrating the cycle of generation, selection, sharing, and loop reinforcement through use necessary to maintain and
generate new information. (after Odum, 1996).
4. Emergy and transformity
With his first attempts at defining emergy, and con-
tinuing until his death, Odum used the concept of
an energy hierarchy (Fig. 2) as a means of explain-
ing the work of nature and society that results in
energy transformations. When viewed in totality, the
systems of nature and society are interconnected in
webs of energy flow. His concept was that all en-
ergy transformations of the geo-biosphere could be
arranged in an ordered series to form an energy hier-
archy with many joules of sunlight required to make
a joule of organic matter, many joules of organic mat-
ter to make a joule of fuel, several joules of fuel
required to make a joule of electric power, and so
on.
The maturing of the emergy methodology resulted
in rigorous definitions of terms and nomenclature as
well as the refinement of the methodology of cal-
culating transformities. Given next are definitions of
most important terms used in the emergy methodol-
ogy.
Emergy is the availability of energy (exergy) of one
kind that is used up in transformations directly and
indirectly to make a product or service. The unit of
emergy is the emjoule, a unit referring to the available
energy of one kind consumed in transformations. For
example, sunlight, fuel, electricity, and human service
can be put on a common basis by expressing them
all in the emjoules of solar energy that is required
to produce each. In this case, the value is a unit of
solar emergy expressed in solar emjoules (abbreviated

206 M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213
Fig. 2. Concepts of energy transformation hierarchy. (a) All units view together; (b) units separated by scale; (c) the units as a web
of energy flows; (d) units shown as a transformation series with values of energy flow on pathways; (e) useful power flowing between
transformations; and (f) transformities.
seJ). Although other units have been used, such as
coal emjoules or electrical emjoules, in most cases all
emergy data are given in solar emjoules.
Unit emergy values (emergy intensities) are calcu-
lated based on the emergy required to generate one
unit of output. There are several important types of
emergy intensities, as follows:
Transformity, defined as the emergy input per unit
of available energy (exergy) output. For example,
if 4000 solar emjoules are required to generate
a joule of wood, then the solar transformity of
that wood is 4000 solar emjoules per joule (ab-
breviated seJ/J). Solar energy is the largest but
most dispersed energy input to the earth. The

M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213 207
solar transformity of the sunlight absorbed by
the earth is 1.0 by definition.
Specific emergy, defined as the emergy per unit
mass output, and usually expressed as solar
emergy per gram (seJ/g). Solids may be eval-
uated best with data on emergy per unit mass
for its concentration. Because energy is re-
quired to concentrate materials, the unit emergy
value of any substance increases with concen-
tration. Elements and compounds not abundant
in nature therefore have higher emergy/mass
ratios when found in concentrated form, since
more environmental work was required to
concentrate them, both spatially and chemi-
cally.
Emergy per unit money, defined as the emergy
supporting the generation of one unit of eco-
nomic product (expressed as currency). It is used
to convert money payments into emergy units.
Since money is paid to people for their services
and not to the environment, the contribution to
a process represented by monetary payments
is the emergy that people purchase with the
money. The amount of resources that money
buys depends on the amount of emergy sup-
porting the economy and the amount of money
circulating. An average emergy/money ratio in
solar emjoules/$ can be calculated by dividing
the total emergy use of a state or nation by its
gross economic product. It varies by country and
has been shown to decrease each year, which is
one index of inflation. This emergy/money ra-
tio is useful for evaluating service inputs given
in money units where an average wage rate is
appropriate.
Emergy per unit labor, defined as the amount of
emergy supporting one unit of labor directly
supplied to a process. Laborers apply their work
to the process, and in doing so, they indirectly
invest in it the whole emergy that made their
labor possible (food, training, transport, etc).
This emergy intensity is generally expressed as
emergy per time (seJ/year, seJ/h), but emergy
per money earned (seJ/$) is also used. Indirect
labor required to make and supply the inputs to
a process is generally measured as dollar cost of
services, so that its emergy intensity is calculated
as seJ/$.
Empower is a flow of emergy (i.e., emergy/unit
time). Emergy flows are usually expressed in units of
solar empower (solar emjoules/time: seJ/s, seJ/year).
5. Disciplines and case studies
The emergy concept and the maximum empower
principle (see Hall and Cia, Olsen and Campbell, this
volume) constitute powerful concepts, definitions, and
tools for investigation of systems at all scales, fram-
ing a system’s behavior and sustainability within the
biosphere’s driving forces and evolutionary pattern.
More than an evaluation procedure aimed at just as-
signingnumerical valuestoprocesses,flows,and prod-
ucts, the emergy method is a conceptual framework,
a window through which systems are investigated un-
der a donor-side perspective (i.e. the perspective of
the environmental work required to support a system’s
dynamics). Based on the recognition that “value” has
different meanings depending on the scale and per-
spective of the evaluation, the emergy method assigns
values according to what it takes to drive a process
and make products, under the constraints of maximum
power selection. In Table 2, we list examples of fields
of study and processes where Odum’s emergy theories
have been applied, in order to show the capability of
the approach and its potential for further development.
The Books of Proceedings of the first three Emergy
Conferences (Brown et al., 2001, 2003, 2004) provide
a significant set of theoretical and applied papers, for
further reading.
6. Emergy and other evaluation procedures
Although Odum’s investigations on energy issues
started early in the Fifties, it was his seminal book “En-
vironment, Power and Society” (Odum, 1971a) that
actually originated the discipline of energy analysis
and its uncountable applications to numerous disci-
plines, from environmental sciences to technological
and economic fields of inquiry.
The recognition of the relevance of energy to the
growth and dynamics of all complex systems with and
without humans gave rise to a blooming of diverse
analysis methods, based on accounting and interpret-
ing matter and energy flows, at all scales. In the first

208 M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213
Table 2
Fields of study, emergy projects and references
Emergy and ecosystems (Odum et al., 1999)
Self-organization (Odum, 1970, 1986, 1988)
Aquatic ecosystems (Odum, 1967a;Odum, 1977b;
Odum and Arding, 1991)
Food webs and hierarchies (Brown and Bardi, 2001)
Ecosystem health (Brown and Ulgiati, 2004)
Forest ecosystems (Odum et al., 1995c;
Doherty et al., 1995)
Complexity (Odum, 1987a; Odum, 1994)
Emergy and Information
Diversity and information (Keitt, 1991; Odum, 1996)
Culture, Education, University (Odum and Odum,
1980;Odum et al., 1995b)
Emergy, Agriculture, agricultural products
Food production, agriculture (Odum, 1967b, 1984b;
Ulgiati et al., 1993)
Emergy and energy sources and carriers
Fossil fuels (Brown et al., 1993; Odum, 1996)
Renewable and non-renewable electricity (Odum,
1983; Brown and Ulgiati, 2002)
Hydrologic dams (Brown and McLanahan, 1992)
Biofuels (Odum, 1980a; Odum and Odum, 1984)
Hydrogen (Barbir, 1992)
Emergy and the Economy
National and international analyses (Odum and
Odum, 1983; Odum, 1987b; Brown, 2003;
Cialani et al., 2004)
Trade (Odum, 1984a)
Environmental accounting (Odum, 1996)
Development policies (Odum, 1980b)
Sustainability (Odum, 1973;Odum et al., 1976;
Brown and Ulgiati, 1999)
The prosperous way down (Odum and Odum, 2001)
Emergy and cities
Research on cities (Odum et al., 1995b)
Taiwan spatial organization (Huang, 1998)
Transportation modes (Bayley et al., 1977)
Emergy and landscape development
Empower density (Odum, 1996)
Land development indicators (Brown and Vivas, 2004)
Emergy in landforms (Kangas, 2002)
Emergy and ecological engineering
Restoration models (Prado-Jatar and Brown, 1997)
Reclamation projects (Odum et al., 1981)
Artificial ecosystems: wetlands, ponds...(Odum,
1977b;Odum, 1985)
Emergy, material flows, and recycling
Mining and mineral processing (Odum, 1996)
Recycling pattern in human-dominated ecosystems
(Brown and Buranakarn, 2003)
Table 2 (Continued )
Emergy and thermodynamics
Efficiency and Power (Odum and Pinkerton, 1955;
Odum et al., 1995b)
Maximum Empower Principle (Odum, 1975, 1983;
Hall, 1995)
Pulsing paradigm (Odum, 1982, 1995)
Thermodynamic principles (Giannantoni, 2002, 2003)
Systems modeling
Energy systems language and modeling (Odum,
1971a,b, 1972b)
two decades, since energy analysis has come to the at-
tention of many scientists (1970s and 1980s), studies
were devoted primarily to assessing and demonstrating
the superiority of a given approach compared to oth-
ers (see, for example, IFIAS, 1974 and Slesser, 1998).
As the field matured and new scientific conceptual-
izations developed, (e.g. system thinking, hierarchical
theory, non-linear dynamics, fractal geometry, com-
plex systems analysis), it became increasingly clear
that different approaches were very often required by
the very nature of the problems being dealt with, in or-
der to build a set of complementary descriptions, able
to provide different assessments, narratives, or views
on different space-time scales. This is why, depend-
ing on the goal of the investigation, a large number
of analysts developed tools such as Embodied Energy
Analysis, Exergy Analysis, Material Flow Account-
ing, Life Cycle Assessment, Ecological Footprint, and
Societal Metabolism, among others, each method be-
ing able to answer specific questions about a system’s
performance.
Common features can be explicitly or implicitly
found in the different methods for evaluation, includ-
ing Odum’s emergy. The latter, however, was intended
to account for aspects, which are usually not ac-
counted for by other evaluation methods. Non-emergy
approaches most often evaluate only non-renewable
resources, depending on what human technologies
are able to extract from them (user-side quality). Fur-
thermore, non-emergy approaches do not account for
the free services that a system receives from the en-
vironment (e.g., the photosynthetic activity driven by
the solar radiation, the dilution of pollutants by the
wind, etc.), which are just as much a requirement for
the productive process as are, e.g., fossil fuels nor do
they have an accounting procedure for human labor,

M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213 209
societal services, and information (i.e. for those flows,
which carry negligible energy but are supported by a
huge indirect flow of resources). Emergy includes all
of this, perhaps not perfectly, but in a way to help us
understand that there is a huge network of support-
ing energies necessary to support, e.g., any particular
economic activity in our culture.
More specifically, by expanding the scope of energy
studies to the biosphere’s space and time scales, the
emergy method is able to:
(i) Investigate systems that are outside of human
activities (ecosystems, global biosphere pro-
cesses).
(ii) Focus on the role of the environment in support
of human-dominated processes, both on the re-
source supply side and on the sink side (dilution
or uptake of pollutants).
(iii) Perform a donor-side quality assessment as a
complement of generally used user-side assess-
ments. This provides a measure of how much
the system relies on the biosphere support.
(iv) Evaluate processes that are directly based on
small flows of physical carriers, but are sup-
ported by huge indirect flows of resources, such
as the creation and processing of information.
(v) Expand the time scale of the evaluation, to in-
clude the memory of resource flows converging
to the system.
(vi) Assess the renewability of resources on the ba-
sis of both space and time convergence required
to make them. The transformity quantifies this
renewability in a continuous form, with higher
values corresponding to higher convergence
of environmental work and therefore lower
renewability.
(vii) Evaluate in a quantitative way the (donor) qual-
ity of those resource flows and storages that
have no market (such as fresh water, biodiver-
sity, fertile topsoil) and cannot be evaluated in
monetary terms.
(viii) Assess the environmental impact of processes
based on matching of high quality and low qual-
ity resources.
(ix) Include in the evaluation the emergy supporting
human labor and services.
All of these properties largely expand upon those
of any other evaluation method, provide a powerful
and comprehensive tool for the investigation of sys-
tems on the larger scales of the biosphere, and finally,
help understanding the dynamic interaction between
human-dominated processes and resources and ser-
vices provided for free by nature.
7. The emergy conferences
The emergy method cannot escape undergoing the
information cycle pointed out by H.T. Odum (Fig. 1).
Since information is something that needs to be
copied, shared, tested, and selected to avoid its loss
due to second-law depreciation as well as to allow for
new information to be generated, it is of fundamental
importance that definitions and concepts, methods,
and case studies using the emergy methodology are
also disseminated, tested, and selected by emergy
specialists and by all kinds of interested people, act-
ing in the role of science’s information-processing
specialists. In doing so, errors are found and in time
drop out, and those concepts and methods that work
are reinforced through sharing and selecting.
For this reason, a series of biennial Emergy Synthe-
sis Research Conferences was started in 1999, in order
to gather emergy specialists together and provide the
critical mass for shared information and theoretical
evolution. The response to the calls of the organizing
committee was stronger and more diverse than ex-
pected, indicating that the approach is by itself spread-
ing and reaching research groups and disciplinary
areas far from the original fields of application. In a
way, this was a consequence of Odum’s broad inter-
ests and scientific productivity, which allowed him to
explore links and relations with areas outside systems
ecology and thermodynamics; to embrace and synthe-
size economy, philosophy, social sciences, and policy.
Held every two years in Gainesville on the Univer-
sity of Florida campus, the Emergy Synthesis Confer-
ence has grown steadily from about 35 participants in
1999 to over 90 participants in the January 2004 con-
ference. The proceeding of the conference, published
by the Center for Environmental Policy at the Univer-
sity of Florida (see Brown et al., 2001, 2003, 2004)
has increased in size from a book of 26 papers result-
ing from the 1999 conference to 45 papers published
in the 2004 proceedings. The conference is truly in-
ternational bringing together scientists representing 18

210 M.T. Brown, S. Ulgiati /Ecological Modelling 178 (2004) 201–213
countries from the continents of Asia, Australia, Eu-
rope, and North and South America.
8. Summary comments
The concept of energy quality has been most con-
troversial. While quality has been recognized, some-
what, in the energy literature (Cleveland, 1992) where
different forms of fossil energy are expressed in coal
or oil equivalents, and some researchers have even ex-
pressed electricity in oil equivalents by using first law
efficiencies, there has been wide-spread rejection of
quality corrections of other forms of energy. The idea
that a calorie of sunlight is not equivalent to a calorie
of fossil fuel, or electricity strikes many as prepos-
terous, since a calorie is a calorie is a calorie. Others
have rejected the concept as being impossible since
from their perspective, it is impossible to quantify the
amount of sunlight that is required to produce a quan-
tity of oil. Still others reject it because emergy does
not appear to conform to first law accounting princi-
ples.
In retrospect, there is little debate that the systems
H.T. Odum studied were as varied as the energy
sources that drove them. All of them must have had
an influence on his thinking and the development of
the concepts and theories of emergy. The gigantic
global gyres of tropical lows developing into hurri-
canes during his stint as meteorologist for the Air
Force, the spring boil of Silver Springs, the ocean
currents and waves on the Pacific atoll at Eniwetok,
the freshwater inputs and gulf currents of the Texas
coastline, or the rains and winds of the tropical rain-
forest in Puerto Rico must have had their influence
on his thinking. In each case, Odum translated what
he saw into systems of energy flow and began to
speculate that different forms of energy had differ-
ent abilities to do work ... in terms of not only
amounts of work but also kind of work. He reasoned
that a joule of sunlight was not the same as a joule
of fossil fuel, or a joule of food, and that sunlight
drives photosynthesis but cannot drive an automobile
without significant efforts to concentrate it. These ob-
servations, the quantitative evaluations they fostered,
and the resulting body of theories that are embod-
ied in the emergy approach have been rejected by
some and criticized by many. Yet, we believe that
non-emergy specialists are very likely to find in the
emergy approach the conceptual framework that is
absolutely needed for a reliable investigation of the
interplay of natural ecosystems and human-dominated
systems and processes. The common thread is the
ability to evaluate all forms of energy, materials, and
human services on a common basis by converting
them into equivalents of one form of energy, solar
emergy, a measure of the past and present envi-
ronmental support to any process occurring in the
biosphere.
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