EMERGY: BASIC CONCEPTS AND DEFINITIONS Enrique Ortega. FEA/Unicamp San Rafael, Argentina, 10 de agosto de 2012.

EMERGY: BASIC CONCEPTS AND DEFINITIONS

Enrique Ortega. FEA/UnicampSan Rafael, Argentina,10 de agosto de 2012

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 the inventory to measures of the size or impact of input flows or process output flows.

Environmental Accounting

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 non-negligible uncertainties that are likely to affect the final results of an evaluation.

Unit Emergy ValuesThe 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 systemUnit 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 documentationEasily 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 recognizedThere 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.orgEmergy 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 workThe 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 definitionEmergy 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 valuesA 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 addedThe 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 ratioThe 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.

Unitary Emergy Values (UEV)

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.

(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 densityHigher 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 NetworkFigure 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 forcesIf 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

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).

(39,300 E20 J/yr)(TrS) + (0.52 E20 J/yr)*TrT = (6.49 E20) *TrH

Oceans’ water thermal and motion forcesThe 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:

(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

Solar Emergy + Gravitational Emergy E/M/S + Geothermal Emergy =

Oceanic Geopotential Emergy (that includes thermal, mechanical, and chemical potentials)

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 transformitiesSolving 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).

ADriving force

BUnits

CAvailable energy released(J/year)

DTransformity(seJ/J)

EEmpower (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

Abbreviations: seJ = solar emjoules; E24 means multiplied by 1024

(*) Not including non-renewable resources.

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).

ClimaxAfter 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*TS

3*To)/3 + To3/3]

where s is a proportionality constant (Stefan-Boltzmann 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 re-establish the consistency with the basic principles as well as among the different UEVs in our databases.

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

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