Sunday, December 2, 2012

Definition of Emergy

  


I have been asked to give a definition of the emergy unit.  I would like to do a little more and define emergy itself as well as emergy efficiency.  Further, I will give some examples as they apply to special cases.

Definition (Availability).  Availability (or available energy) is energy [enthalpy, H, or internal energy, U] corrected for entropy, S.  Rigorous definitions of the Gibbs availability function [H – ToS], the Helmholtz availability function [U - ToS], and entropy are given in Appendix I, Fundamentals of Thermodynamics, where the symbols and technical terms employed in this paragraph are explained.  [To is  the  temperature of the environment, usually taken to be the temperature of the coldest body of water or the atmosphere into which the waste heat of a heat engine can be discharged.  For Earth, 300 K will do.  The effect of entropy on the availability function of sunlight is to reduce it by the ratio of the temperature of Earth to the temperature of the Sun – a factor of  about 19/20.  Since the enthalpy of a proton is 4/3 times the energy, the Gibbs availability of sunlight is about 76/60 times the energy.]

Odum’s original definition of emergy.  Odum defined emergy, measured in emjoules, to be the Gibbs availability of the sunlight, measured in joules, required to produce, by an optimal process, (1) fuels; (2) other energy sources such as wind or fresh water in mountain lakes; (3) natural resources such as grass and trees, (4) manufactured objects, (5) human resources; (6) information; and (7) any other objects of economic interest that can be associated with an identifiable quantity of sunlight.  This is a sunlight-based emergy.  It leads to large numbers for the emergies of primary fuels that are known only approximately; therefore, we shall modify the definition slightly to give common industrial energy products emergies that are known precisely and that are close to 1.0 in magnitude.

Definition (Standard Electricity).  In this paper, single-phase, 60 Hz, 110-volt alternating current is taken to be standard electricity.

Definition (Emergy Unit).  My arbitrary – but well-defined – choice for one unit of emergy (1 MU) is 1.0 kilowatt-hours of standard electricity.  Although electrical current carries a small amount of entropy manifest in difference currents, for all practical purposes, that is, for engineering purposes, electricity is pure work.  The availability of electricity is equal to its energy; and, with this choice of emergy unit, the emergy of electrical current is numerically equal to its energy in kilowatt-hours.  The transformity of sunlight, wind, biomass, and other energy products will be less than – but close to – 1.0.

Definition (Transformity).  The transformity of a primary fuel is the number of kilowatt-hours of standard electricity one can obtain from 1 kWhr of the primary fuel by an efficient process, the tradition of reporting the availability of fuels in BTUs per pound or kilocalories per gram mole notwithstanding.  Any unit of energy can be converted to kilowatt-hours.  This is an electricity-based transformity, the units of which are emergy units per kilowatt-hour.

Definition (Emergy).  The embodied energy or emergy of a primary fuel is the Gibbs availability of the fuel in kilowatt-hours multiplied by the electricity-based transformity.  The emergy of anything else is the sum of all the emergy that went into producing it by an efficient process minus the emergies of any by-products formed.  The emergy of an activity is the average rate of expenditure of emergy times the time.  These definitions are easily extended to include the dependence of emergy on location and time.  The concept of nemergy or negative emergy can be introduced to aid in the discussion of environmental damage.

Definition (Emergy efficiency).  Emergy efficiency is emergy out divided by emergy in.  This efficiency is 1.0 for an optimal process because the emergy of the output is defined to be the emergy of the inputs.  For a less than optimal process, the emergy efficiency is the emergy of the inputs to an optimal process over the emergy of the inputs to the process under investigation.  Emergy efficiency lies between zero and one.
The transformity of any fuel can be determined by using it to generate standard electricity by an efficient process.  The most efficient process might be a fuel cell.  Therefore, the emergy of any fuel is the Gibbs availability of the fuel multiplied by the electricity-based transformity.

Balance Equations etc.  

The second and longest part of this paper has been changed sufficiently to bear deletion and replacement by the balance-equations part of On Emergy (Part 1): Definitions and Balance Equations


3 comments :

  1. If someone who reads this has some idea about what I need to do to get Blogger to render MS Word's subscripts so that they look like subscripts rather than the monsters one sees in this article, I would be most appreciative.

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  2. As soon as I get a chance I plan to re-write this article so as to calculate ERoEI rather than ERoEI - 1.0.

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  3. At this late hour, i plan to make good on the previous comment. It is 4:42AM May 20, 2020.

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