Summary Memorandum




RE: Exergyand how it applies to the sustainability of engineered systems

Thismemo provides a brief explanation of the meaning of exergy, and howit applies to the sustainability of engineered systems. Engineersoften conceive the quality of energy as the comprehensible processthat converts energy into work. The amount of work, in this case, isa product of an amount of energy in ideal conditions of conversionusing processes that are reversible. Exergy is therefore, thepotential to produce work. The amount of exergy is a product ofapplying thermodynamics on systems that convert energy. Anunderstanding of exergy makes it possible to understand how itaffects engineered systems. An engineered system is a combination ofsynergic components that work together to perform a specificfunction. For example, a new production process in manufacturing isan engineered system whose sustainability can be affected by exergy.

of the articles

Herman,Weston A: Quantifying global Exergy Resources

Exergyprocesses as a transmission between reservoirs that have differentconditions from the dominant conditions in the biosphere. Exergyresources can be physical, chemical or nuclear form (Hermann, 2006).Exergy flows therefore occur as a set process from primary tosecondary reservoirs, where accumulations and flows on both endsrepresents the typical path of exergy in the terrestrial system. Thisis towards the very end when it is anthropogenic-ally destructed.Understanding the exergy in the biosphere provides a better way toevaluate how resource should be utilized, and types of energytechnologies to be adopted. Thus, it is possible to determine themaximum exergy from any substance from a reference state. Physicalexergy is the reversible work potential brought about by acombination of velocity, gravity (at a defined equilibrium position),pressure, and temperature with a specific reference state. Chemicalexergy originates from electrostatic bond energy, and theconcentration of matter relative to the reference state. Nuclearexergy originates from nuclear reactions that cause a change inpotential energy during the process of beta decay. All the threeexergy reservoirs can provide sufficient energy to sustain thebiosphere. However, the depletive use of resources by increasingglobal populations means that it is no longer negligible. Thus, it islikely to affect natural energy exchanges hence, altering thesustainability of the natural environment.

Chen,G.Q. Energy Consumption of the Earth

Gouy-Stodolalaw explains the earth’s exergy budget, which has direct andessential implication on the sustainability of the earth as anaturally occurring engineered system. The exergy of the earth isdefined in terms of thermal and terrestrial radiation. For there tobe exergy, there must be a balance of the thermodynamic system, whichrepresented by the temperature difference between the sun and thecosmic exergy consumption. Global comic exergy consumption is derivedfrom multiplying the temperature reading of cosmic backgroundmicrowave radiation as well as the global entropy generation. Cosmicenergy is simply energy from the outer space. There are formulae toillustrate the irreversibility of global exergy. An exergy budgetaffects global sustainability as a result of short wave radiationbeing intercepted on the earth’s surface and reflected back to theatmosphere. While 30 per cent of short wave radiation is reflectedback, 69 per cent of it is carried to the outer space through longwave terrestrial radiation (Chen, 2005). Due to its irreversibility,exergy between the earth’s surface and the outer space is a greatconcern for the environmental sustainability of the earth. Thisexplains why artificial engineering systems are made in such a way asto make exergy processes reversible so as to prevent build up ofenergy.


Chen,G. Q. (2005). Exergy consumption of the earth.&nbspEcologicalModelling,184(2),363-380.

Hermann,W. A. (2006). Quantifying global exergy resources.&nbspEnergy,&nbsp31(12),1685-1702.