Download emergy: basic concepts and definitions

EMERGY:
BASIC CONCEPTS
AND DEFINITIONS
Enrique Ortega. FEA/Unicamp
San Rafael, Argentina,
10 de agosto de 2012
Environmental Accounting
The increasing awareness of the limited scope
and reliability of monetary accounting methods
creates a need for environmental accounting
methods (EA).
EA methods developed up-to-date had database
problems, related to the unavailability of suitable
inventory data at local and global scales, as
well as reliable intensity factors that can link t
he inventory to measures of the size or impact of
input flows or process output flows.
Intensity factors
The problem with intensity factors (energy,
matter, or emission intensities) is that they are
unavoidably case, location, time, or technology
specific and cannot be considered stable over
time.
Moreover, they are most often affected by nonnegligible uncertainties that are likely to affect
the final results of an evaluation.
Unit Emergy Values
The emergy approach faces similar problems
with its Unit Emergy Values, also named
emergy intensities: transformity, seJ/J; specific
emergy, seJ/g; emergy-to-GDP ratio,
seJ/currency; emergy-to-labor ratio, seJ/time;
emergy/area, seJ/m2; etc.
These values are used to convert input flows or
stocks into emergy values. The reliability of an
emergy assessment depends on factors used
for such a conversion.
Specific state of the system
Unit Emergy Values (UEV) are constrained and
affected by the specific state of the system and
its links to the surrounding environment, so that
it is impossible to use a value that fits all the
situations in a deterministic way.
Therefore, each value is strictly linked to the
process for which it was calculated, so that a
database of Unit Emergy Values (UEVs) is
unavoidably a database of ranges and related
systems (explicated with diagrams).
Unit Emergy Values database
A wide range of EUV for energies, products,
services and information are available in books,
papers, reports and theses worldwide. A critical
selection of these values and their calculation
procedures is a primary task for any solid
emergy evaluation.
The demand for a handbook or revised UEVs
database grows as more and more scientists
around the world conduct emergy studies.
Supporting documentation
Easily accessible, well-defined and meaningful
UEVs and thorough supporting documentation
could improve the entire process of conducting
an emergy evaluation and ensure that a study
uses the highest quality data. Analysts could
complete their work more quickly and
efficiently and have a higher level of confidence
in their results. There are some works made on
that sense: Emergy Folios (University of Florida)
and the transformity database at ISAER site.
Potential is being recognized
There is a growing number of scientific papers
that use emergy methodology to assess natural,
agricultural and industrial processes, published
in international Journals.
Referring only to Elsevier Science Direct
(http://www.sciencedirect.com), the number of
articles was 44 in 2005, 140 in 2006, 92 in 2007,
101 in 2008, 140 in 2009 and 31 in 2010 and xxx
in 2011
Emergy in Scientific meetings
Emergy based papers are presented in
international meetings related to environmental
and economic issues, such as: the International
Biennial Workshop “Advances in Energy Studies”,
the annual International Conference on
“Efficiency, Cost, Optimization, Simulation and
Environment Impact of Energy Systems”, the
annual meeting of “Cleaner Production” and
the Biennial Emergy Research Conference
(http://www.emergysystems.org).
UEVs
The application of the emergy method needs a
large and reliable database of conversion factors
so-called Emergy Intensities or Unit Emergy
Values (UEVs), used to convert the input flows
(energy, matter, money, labor and information)
into flows of emergy driving a process.
Lack of a suitable and constantly updated
database undermines the evaluation process and
weakens any calculated performance indicators.
Efforts towards such a database:
State University of Campinas, Brazil www.unicamp.br/fea/ortega/curso/transformid.htm ;
Tzu Chi University, Taiwan http://www.tabel.tcu.edu.tw/EmergyPubs.zip
University of Florida –
http://www.emergysystems.org
Emergy Folios (2000 and 2002) and
Brown and Arding, Transformities handout (1991)
Two kinds of added work
Emergy values express the unit cost of the direct
and indirect support provided by the biosphere
to the production of a given product or service.
They depend on
(a) the direct ecosphere contribution, and
(b) the human intervention for extraction of
resources and manufacturing.
The two works should be correctly calculated.
Ecosphere work
The ecosphere work (component a) requires that
the biosphere work should be calculated over
long time scales and large spatial scales, which
entails a non-negligible uncertainty on all
calculation steps and assumptions.
The total work displayed by the biosphere is
referred to as the “biosphere baseline” and
recently underwent several recalculation efforts
(Odum, 1996; Odum, 2000).
Human work
The human work (component b) depends on the
previous one as a starting point but also
includes the specific aspects of the investigated
process.
As a consequence, the component (b) is time,
technology, location and society specific, which
leads to values that may change over time in so
requiring a continuous updating effort.
ENVIRONMENTAL AND
MONETARY ACCOUNTING
Monetary accounting is supposed to capture
information on the assets that contribute to a
nation’s wealth, based on the assumption that
safeguarding wealth is indispensable for
maintaining economic vitality (Wackernagel et
al., 2001).
MONETARY FLOWS
In a traditional economic accounting system,
with major economic indicators including Gross
Domestic Product (GDP), Gross National Product
(GNP), Saving Rates, Trade Balance figures, and
so on, monetary accounting links all the
national activities with performance indicators
and expresses these in the form of a single unit
of account: money.
MONETARY ANALYSIS
Monetary analysis provides crucial information
for decision-makers and could be widely
considered among the most important national
economic evaluation frameworks, which allow
international comparisons and help understand
the world’s economic dynamics (IUCN, 1997).
Different scopes
Despite of all the authority generally attributed
to monetary accounting, it has experienced
great problems in dealing with environmental
pollution, resource scarcity, energy crises and
ecological degradation since the 60s (Hecht,
1999) up to the recent monetary storm in the
USA (subprime mortgages) quickly spread to the
rest of developed economies.
Problems
According to Ulgiati et al (2009), the main doubt
about current monetary accounting system is its
missing or misleading relation to
environmental issues:
- environment as a source and
- a sink of resources,
- misuse of the commons
Limitations of monetary analysis
(1) The cost of environmental protection is not yet
clearly addressed, in that money spent in
pollution control increases GDP, even though
the expenditure is not economically productive;
(2) Some ecosystem services are not or cannot be
marketed. For example, the sale price of timber
reflects only the cost of planting, cutting and
distribution but does not reflect the
environmental value and role of forest wood as
standing biomass and related biodiversity;
(3) Environmental services, such as crop pollination
by insects and fertilization by soil biota are
difficult to estimate in economic terms, while
the alternative service replaced, if existing (e.g.,
man-made products or services), does contribute
to GDP;
(4) The national income accounts treat the
degradation of human-made capital (machinery
and equipment) as depletion rather than
income, however, the similar degradation of
natural capital (forests, in particular) is
accounted for as an income (Lange, 2003).
NEW ACCOUNTING EMERGING
Stemming from such monetary accounting
problems, environmental accounting emerged
as an important tool for understanding the role
played by the natural environment in the
economy (IUCN, 1997).
It allows human being to measure their load on
the biosphere and thus strive to live within the
carrying capacity of its ecosystems.
NATIONAL ACCOUNTING
One of the most well-known environmental
accounting frameworks is the adjusted System
of National Accounting (SNA), called System of
Integrated Economic and Environmental
Accounting (SEEA), which attempts to integrate
many of the different methods into a single
organized framework (UNSD, 2003).
SEEA
However, SEEA is not a quick sell, because there
are still technical difficulties and it seems that
not all the countries would like to actually
discover the real environmental-economic
situations.
As an alternative, some other environmental
accounting methods mainly focus on physical
accounting numeraires.
Environmental accounting methods
Ecological Footprint (1996) accounts for
productive land directly or indirectly needed;
Material Flow Accounting (1993; 1998) refers to
the amount of matter diverted from its natural
pathway in support of the economic process;
Embodied Energy Accounting (1974; 1998) is
based on cumulative energy consumption;
Emission Accounting ( 2000; 2009), focuses on a
process emissions
Environmental accounting methods
Emergy Accounting (Odum, 1996; Brown and
Ulgiati, 2004b) considers the global demand for
environmental support from the point of view of
the geo-biosphere.
Those and others environmental accounting
methodologies try to avoid market uncertainty
impact and show the inherent load of human
activities on natural resources.
Global financial worldwide crisis
More recently, the global financial worldwide
crisis of 2008 called for a rethinking about the
monetary accounting system, where it has been
argued that the monetary growth doesn’t
properly account for the real wealth of an
economy, so that currency fails its role as the
unique wealth and quality accounting
numeraire.
GOALS TO BE MEASURED
Filios (1991) pointed out that accounting has to
be adapted appropriately to provide measures
of success in achieving more goals than just
profitability without necessarily having them
quantified under a common denominator such
as money. Such an early warning pertains so
much to the world’s current situation, which
makes it even more meaningful.
CONCERN
Despite the difficulties and controversies about
changing the monetary accounting to more
independent environmental accounting, it is
obvious that interest is growing in research and
practice on environmental accounting to
promote understanding of the interplay of
human activity and the Earth dynamics and
resources.
EMERGY: CONCEPTS AND
DEFINITIONS
Energy quality
While it is true that all energy can be converted
into heat, it is not true that one form of energy is
substitutable for another in all situations.
For instance, plants cannot substitute fossil fuel for
sunlight in photosynthetic production, or humans
cannot substitute sunlight energy for food or water
uptake.
It should be obvious that the quality that makes an
energy flow usable by one set of transformation
processes makes it unusable for another set.
Thus, quality is related to a form of energy and to
its concentration. As a consequence, a higher
quality is somewhat synonymous with higher
concentration of energy and may translate into
greater flexibility (more possible different uses).
Under such a point of view, wood is more
concentrated than detritus, coal more
concentrated than wood, and electricity more
concentrated than coal.
As a consequence, the quality of incoming energy
(concentration, wave-length, pulsing, etc.) makes
it able to drive different forms of complexity in
recipient systems (Ulgiati and Brown, 2009).
Odum (1988, 1994, and 1996) pointed out that in all
processes a large amount of low-quality energy
must be dissipated in order to generate a product
containing a smaller amount of high-quality energy,
in so generating an energy-based hierarchy of
resources and products.
Since it takes resources to make goods and
services, he suggested that the concept of value
should not only consider the receiver’s point of view,
but the donor’s point of view, i.e. the upstream
Nature’s work that cycles and concentrates
resources and makes them available in support of
the self-organization processes of ecosystems and
economies.
Emergy definition
Emergy is defined as the total amount of
available energy (or exergy) of one kind that is
used up directly or indirectly in a process to
deliver an output product, flow, or service’
(Odum, 1996).
The concept developed over a 30 old year period
of time beginning in the early 1970’s and
culminated in the publication of Odum’s book
titled “Environmental Accounting, Emergy and
Environmental Decision Making”.
Solar equivalent Joule (seJ)
According to the emergy theory, different forms of
energy, materials, human labor and economic
services are all evaluated on a common basis (the
environmental support provided by the biosphere)
by converting them into equivalents of only one
form of available energy, the solar kind, expressed
as solar equivalent Joule (seJ).
The concept of “available energy” allows the
analyst to account for all kind of resources used
(minerals, water, organic matter), not only energy
carriers (Gilliland et al., 1978; Gilliland and
Eastman, 1981; Odum, 1996).
In fact, whenever a gradient of a thermodynamic
property (altitude, temperature, concentration,
pressure, chemical potential, etc) is available, it
can be used to support a resource transformation
into work or into another form of resource or
energy. In so doing, the gradient is lowered or
completely used up.
Therefore, all kinds of resources can be converted
into work potential. In so doing, it is possible to
adopt a numeraire (available energy, or exergy)
that applies to all physical resource flows and
calculates emergy flows according to the same
accounting basis, for easier comparison.
Emergy approach
The Emergy approach (Odum, 1988, 1996 and
2007) is a resources evaluation method deeply
rooted in irreversible thermodynamics (Prigogine,
1947; De Groot and Mazur, 1984), and systems
thinking (von Bertalanffy, 1968).
It aims at understanding the global interplay of a
process with its surrounding environment as well
as at calculating indicators of environmental
performance (Ulgiati et al., 1995; Brown and
Ulgiati, 1999; Ulgiati, 2001).
Emergy intensity values
A biological or technological transformation is a
process that converts one or more kinds of
available energy into a different type of available
energy. All such transformations can be arranged
in a series, and the position of an energy flow in
the series is marked by its Emergy Intensity.
The Emergy Intensity (also named Unit Emergy
Value, or UEV) is the emergy driving a
transformation divided by the available energy (or
the mass, the economic value, the information
content, or any other identifying numeraire) of the
transformed product.
Work previously added
The term “intensity” highlights the “convergence”
of environmental support (emergy) to the unit of
product or service, and is synonymous of “Unit
Emergy Value”.
In the emergy nomenclature these terms are
equivalent, while other terms are used when focus
is placed on specific typologies of flow.
In accordance to Brown and Ulgiati (2004b) there
are at least six very important types of emergy
intensities, as follows:
(a) Transformity:
Defined as emergy input per unit of exergy output,
expressed in solar equivalent joules per joule of
output flow (seJ*J-1). The transformities in the
biosphere range from a value equal to one for
solar radiation to trillion of solar emjoules for
categories of shared information (Odum, 1988),
and express three different features:
(a) the environmental support to a product;
(b) the biosphere efficiency of production process;
(c) an energy-scaling factor for items within the
hierarchy of the planet.
High quality = more work added
According to the second law of thermodynamics,
all energy transformations are accompanied by
energy degradation, which represents a measure
of the work done in generating a smaller flow of
higher-quality product.
Solar radiation energy is the largest but most
dispersed available energy input to the Earth: as a
consequence, the solar Transformity of sunlight
was set equal to 1.0 seJ*J-1 by definition (Odum,
1996).
(b) Specific Emergy:
Defined as the emergy per unit mass of output,
and expressed as solar emergy per gram
(seJ*g-1).
Solids may be evaluated best with data on
emergy per unit mass of a given chemical
species times its concentration.
Since available energy is required to
concentrate materials, the unit emergy value of
any substance increases with concentration.
Concentration = more work
Elements and compounds not abundant in nature
therefore have higher emergy per mass ratios
when found in concentrated form, since more
environmental work was required to concentrate
them, both spatially and chemically.
More details, definitions and a database with
several crustal elements can be found in Cohen
et al. (2007).
(c) Emergy per Unit Money:
It is defined as the emergy supporting the generation
of one unit of economic product (expressed as
currency of a given country or as international
reference currency such as euro or dollar;
seJ*currency-1). It is used to convert money flows into
emergy units.
Since money is paid to people for their services
(indirect labor to make a resource available to the
system) and not to the environment, the contribution
to a process represented by monetary payments
translates into the emergy that can be purchased by
that money.
Emergy to dollar ratio
The amount of resources that money buys depends
on the amount of emergy supporting the economy
and the amount of money circulating.
An average emergy per money ratio in solar
emjoules per unit money can be calculated by
dividing the total emergy use of a state or nation by
its Gross Economic Product. It varies by country
and generally decreases over time as a
consequence of inflation accompanying a country’s
economic development. The emergy per money
ratio is useful for evaluating service inputs given in
money units where an average wage rate is
available.
(d) Emergy per Unit Labor:
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 their activity the
support emergy that made their labor possible
(food, technical training, education, transport, etc).
Such an emergy intensity of labor is generally
expressed as emergy per unit time (seJ*year-1
or seJ*h-1), but emergy per money earned
(seJ*currency-1) is also used.
The indirect labor required to make and supply
the input flows (goods, commodities, energy, etc)
to a process is generally measured as dollar cost
of services, so that its UEV is calculated as solar
emjoules per currency.
Unitary Emergy Values (UEV)
(a) Transformity (sej/J)
(b) Specific Emergy (sej/kg)
(c) Emergy per Unit Money (sej/US$)
(d) Emergy per Unit Labor (sej/hour)
(e) Emergy Density (ED) (sej/ha)
(f) Empower (sej/ha.year)
(e) Emergy Density (ED):
It measures the amount of emergy invested on
one unit of land for a specific production
process or development (in units of seJ*m-2 of
land). ED may suggest land to be a limiting
factor for the process or, in other words, may
suggest the need for a given amount of
support land around the system, for it to be
sustainable (Brown and Ulgiati, 2004b).
Renewable density
Higher ED’s characterize city centers, information
centers such as governmental buildings,
universities and research institutions, power plants,
industrial clusters, while lower ED’s are calculated
for rural areas and natural environments (Odum et
al., 1995; Huang et al., 2001).
Renewable and nonrenewable emergy densities
are also calculated separately by dividing the total
renewable emergy by area and the total
nonrenewable emergy by area, respectively.
(f) Empower:
The Emergy per unit time is a measure of power,
indicating the flow rate of a given resource
(seJ*year-1).
The flow of global resources on a process per unit
time affects the development rate of the process,
from the large scale of biosphere to the smaller
scale of economies, farms, individuals and
bacteria.
Energy Network
Figure 1. Concepts of energy
transformation hierarchy.
(a) All units viewed 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;
(f) transformities (Odum,
1996).
Emergy hierarchy
The universe is hierarchically organized (Brown
et al., 2004), with lower levels supporting higher
levels, each of them characterized by increasing
UEVs.
The emergy intensity is therefore a measure of a
system’s hierarchical organization and is
applicable to all kinds of matter, energy or
information flows (Odum, 1996; Figure 1).
Emergy flows of the Biosphere
A baseline to which refer for calculation of basic
Unit Emergy Values of the Earth is a practical need
for emergy accounting.
For this purpose, Odum (1996) places the window
of evaluation around the geobiosphere and
identifies the main energy sources contributing over
a long-run average.
Driving forces
The main driving forces of geobiosphere are: solar
radiation, gravitational, geopotential energy of the
Earth-Moon-Sun (E-M-S) system, and finally
geothermal heat from inside the planet.
The UEV of solar radiation is, by definition, set
equal to 1 seJ/J.
The UEVs of the other main driving forces are
calculated accordingly (based on the effects of
their interaction with solar radiation).
The three main driving forces and the planet
compartments which they affect are fully
interconnected and affect each other. It is
impossible to separate them and their effects.
Therefore, we must identify equations that are
capable to connect some of these effects to the
driving forces, in order to be able to calculate their
own UEVs, within a network of processes that
include the human economic system and the
production and maintenance of storages of globally
shared information (Odum, 1996; p35).
Odum (2000) calculates the UEVs of the
biosphere’s main driving forces as follows.
Solar radiation
A first Equation is one that sets the UEV
(in this case the Transformity) of solar
radiation, TrS, equal to 1:
TrS = 1 seJ/J
Eqn. (1)
Two more Equations can be generated from
considering the processes described in
Figures 2 and 3.
Figure 2. Systems diagram of surface and deep
earth processes that generate heat flow through the
Earth crust, as described in Eqn. (2). Odum (2000).
Figure 3. Systems diagram of gravitational
driving forces that release geopotential energy
through the oceanic system, as described in
Eqn. (3). Odum (2000).
Earth Crust Energy Balance
Based on Figure 2, an equation can be written
linking the heat flow crossing the earth crust
and the driving forces that generate it.
The earth crust is crossed by a heat flow of 13.21
E20 J/yr (Sclater et al., 1980).
Part of it is generated by deep underground
radioactivity (1.98 E20 J/yr) as well as by residual
heat generation from gravitational implosion of
matter towards the inside of the planet (4.74 E20
J/yr).
Solar and gravitacional driving forces
If these two flows are subtracted from total heat flow
crossing the crust, it is possible to calculate the
fraction of heat (6.49 E20 J/yr) generated within the
earth crust by solar and gravitational driving forces
(the Earth/Moon/Sun system, with friction between
oceanic water and crust mass in reciprocal motion).
As a consequence, we can write:
Emergy of solar radiation + gravitational emergy of
the Earth/Moon/Sun system (mainly tide effects) =
Emergy of heat crossing the earth crust (only the
fraction generated by processes other than deep
radioactivity and gravity implosion).
(ES)(TrS) + (ET)*TrT = (EC) *TrH
(39,300 E20 J/yr)(TrS) + (0.52 E20 J/yr)*TrT = (6.49 E20) *TrH
TrS, TrT and TrH are the transformities of solar
radiation, gravitational potential and crustal heat.
ES, ET and EC are the available energies of the
same flows. Their respective values are
3.93E+24 J/yr, 0.52E+20 J/yr, e 6.49E+20 J/yr.
These are experimentally measured values of solar
radiation on land, of heat generated by ocean water
(mainly friction of water and land due to tides), and
heat generated by surface processes (weathering,
fermentation, surface friction, etc).
Oceans’ water thermal and motion forces
The diagram of Figure 3 shows the interaction of
solar radiation and geopotential gradient as well
as deep heat from inside the planet to generate
geothermal gradients and convective motion of
oceans’ water. These driving forces are:
The direct solar radiation, 3.93E+24 J/yr
The gravitational potential energy annually
released by the E/M/S system= 0.52E+20 J/yr
The deep heat from inside, not accounted for in
Equation (2) = 6.72 E20 J/yr (Sum of 4.74 E20 J/yr
of residual “implosion“ heat and 1.98 E20 J/yr from
deep earth radioactivity).
We can therefore write a new equation (Eqn. (3))
for the oceanic system by accounting for the
forces that support its geopotential energy:
Solar Emergy + Gravitational Emergy E/M/S +
Geothermal Emergy =
Oceanic Geopotential Emergy
(that includes thermal, mechanical, and chemical
potentials)
(ES)(TrS) + (ET)*TrT + (EDH)* TrDH = (EO) *TrO
(39,3 E20 J/yr) (TrS) + (0.52 E20 J/yr)*TrT + (6.72E20 J/yr) *TrDH = (2.14 E20 J/yr)*TrO
(39,3 E20 J/yr) (TrS) + (0.52 E20 J/yr)*TrT + (6.72E20 J/yr) *TrH = (2.14 E20 J/yr)*TrT
All symbols have the same meaning than for Eqn. (2).
EDH and TrDH refer to the deep heat generated by
radioactivity and gravitational “implosion”, and
EO and TrO refer to the energy released through the
oceanic system. In particular, the value 6.72E+20 J/yr
was extrapolated from experimental data about
geothermal deep heat (as a fraction of the flow of total
heat crossing the earth crust), while the value
2.14E+20 J/yr was extrapolated from experimental
data about the amount of thermal energy released by
the ocean’s system to geobiosphere (and equal to the
sum of all energy flows the oceanic system receives
annually and in turn releases as heat, e.g. through
evaporation
TrT = TrO
TrDH = TrH
In Eqn. (3) the assumption is made that TrT = TrO
and that TrDH = TrH , since they refer to the same
kind of energy flow (gravitational energy released
as heat by oceans in the first case and heat
crossing the Earth crust in the second case).
Basic biosphere transformities
Solving the system of equations (1), (2) and (3)
provides the values of the basic biosphere
transformities shown in Table 1, column D.
These values, multiplied by the amount of available
energy released in each process (column C)
provides the amount of emergy contributed to the
Earth dynamics through such a process (empower,
column E).
Since all the components interact and are required
for the others, the emergy supporting all internal
pathways is the same.
Table 1. Annual emergy contributions to Global
Biosphere Processes (*) (after Odum et al., 2000).
A
Driving force
B
Units
C
Available energy released
(J/year)
D
Transformity
(seJ/J)
E
Empower
(E+24seJ/year)
Solar insolation
J (a)
3.93E+24
1.0
3.93
Deep Earth heat
J (b)
6.72E+20
1.20E+04
8.06
Tidal energy
J (c)
0.52E+20
7.39E+04
3.84
Total
-
-
-
15.83
(*) Not including non-renewable resources.
Abbreviations:
seJ = solar emjoules;
E24 means multiplied by 1024
Calculations notes
a Sunlight:
solar constant 2 gcal/cm2/min = 2 Langley per
minute; 70% absorbed; earth cross section facing sun
1.27E+14m2.
b Heat release by crustal radioactivity 1.98E+20 J/year
plus 4.74E+20 J/year heat flowing up from the mantle
(Sclater et al., 1980). Solar transformity 1.2E+04 seJ/J
based on an emergy equation for crustal heat as the sum
of emergy from Earth heat, solar input to earth cycles,
and tide (Odum, 2000).
c Tidal contribution to oceanic geopotential flux is
0.52E+20 J/year (Miller, 1966). Solar transformity of
7.4E+04 seJ/J is based on an emergy equation for
oceanic geopotential as the sum of emergy from Earth
heat, solar input to the ocean, and tide following Campbell
(1999) (Odum, 2000).
Climax
After millions of years of self-organization, the
heating transformations by the sun, the
atmosphere, ocean, and land are organized
simultaneously to interact and contribute mutual
reinforcements.
Therefore, the emergy flow supporting each global
process (rain, wind, waves) is the sum of the
emergy contributed by the three main driving
sources (global empower, 15.83 E+24 seJ/yr,
Odum et al., 2000).
Exergy-based corrections
The emergy definition implies that the actual flows
to a process are accounted for as “available energy”
flows (or exergy) (Odum, 1996, page 13, Table 1.1).
Most often, such a definition is not implemented
properly and generates UEVs that are not
consistent with the basic principles of the method as
well as with those values that were, instead,
calculated on the basis of available energy flows
(for example those for minerals calculated by
Gilliland et al., 1978 and by Gilliland and Eastman,
1981).
Inaccurate UEVs of basic flows also affect all the
other flows that are calculated after them.
Although the inaccuracy is not very large in most of
the cases (also considering the uncertainty in
estimates of global flows), the theoretical
inconsistency of the practice with the basic
definitions was pointed out by some authors
(Ulgiati, 2000; Bastianoni, 2007; Sciubba, 2009;
among others).
Bastianoni et al. (2007) suggested an exergy
correction factor in order to account for the
differences arising when flows are expressed by
means of an energy or exergy numeraire.
The exergy of the solar radiation, Exs, depends on
the source (sun) and environment temperatures TS
and To (Petela, 1964), according to Eqn. (4):
Exs= s*[TS4 – (4*TS3*To)/3 + To3/3]
where s is a proportionality constant (StefanBoltzmann constant, 5.6667 x 10-8 W*m-2*K-4).
As a consequence, based on average values for
temperatures (TS=5800 K; To= 255 K) and solar
radiation constant (Es = 1360 W/m2) the solar
exergy value is (Petela, 1964). See Eqn. (5):
Exs= 0.94 * Es
This means that accounting for the solar radiation
in energy terms instead of exergy overestimates
such a flow by about 6%.
Sciubba (2009) noted that “because of the Emergy
hierarchical arrangement of energy flows, this 6%
difference propagates downstream, affecting the
absolute values of all emergy content of material
and immaterial goods in a measure that depends
on the structure of the production process”.
Moreover, after pointing out that exergy values
of Earth’s flows were independently calculated
by Kabelac (2005), Szargut et al. (1988), Chen
(2005) and Hermann (2006).
Sciubba estimated that using energy as a
numeraire to quantify the tidal potential and the
deep heat as in emergy Folio 2 (Odum, 2000)
overestimates the incoming energy by about
28% (Sciubba, 2009).
Using a numeraire that can be applied to all kinds
of inflow is important and should not be further
disregarded.
For the sake of clarity, an input of organic matter
may carry very different work potential depending
on the percentage of water content and its actual
chemical composition: while mass, even if dry
matter, does not properly account for such
differences, chemical exergy does.
Furthermore, an input of water to a process carries
more or less work potential depending on its
temperature; and finally, expressing mass as
grams and energy as joules does not allow any
comparison between the calculated UEVs of mass
and energy flows.
For such a reason, mass and energy numeraires
should be replaced by the exergy numeraire and
all UEVs recalculated accordingly.
This is not only because of the need for more
accurate values, but is mainly aimed at reestablish the consistency with the basic principles
as well as among the different UEVs in our
databases.
Reference:
Criteria for Quality Assessment of Unit Emergy Values
Sergio Ulgiati§, Feni Agostinho*, Pedro L. Lomas#,
Enrique Ortega*, Silvio Viglia§, Pan Zhang°, and Amalia Zucaro§.
§ Parthenope
University of Napoli - Italy
* State University of Campinas (UNICAMP) - Brazil
# Autonomous
University of Madrid - Spain
° Dalian University of Technology - China
Proceedings of 6th Emergy Conference,
University of Florida, 2010