Exergy account

Sciubba (2003) proposed an approach called the extended exergy accounting (EEA), which calculates the real, resource-based value of a commodity product. The time span of EEA is the whole life of a plant. The EEA includes the exergetics flow sheets for non-energetic costs of labor and environmental remediation expenditures, and hence uses extended exergetic content. It also defines the criterion for an optimum process or operation (Sciubba, 2003). 

Thus, it should be noted that, with exergy accounting, the effect of prospective design changes upon product cost can be estimated accurately, without a complete system re-design and recosting. 

It is hoped that the illustrative examples presented here will serve to crystallize the basic principles and will motivate the readers to apply exergy efficiency analysis and exergy accounting — taking advantage of these methods for their own practical purposes, while also advancing the state of the art. 

Including an ecovector to account for the exergoeconomic costs or environmental impact can extend the thermoeconomic approach. An ecovector is a set of environmental burdens of an operation, and can be associated with input flows it includes information about natural resources, the exergy of these resources, and monetary costs. The external environmental costs associated with the environmental burdens may also be added into the ecovector. Extended exergy accounting includes the exergetics flowcharts for nonenergetic costs of labor and environmental remediation expenditures. 

In the case of a practical heat engine an energy balance and a flow balance are made up based on knowledge of turbine blade, heat exchanger and other characteristics. An exergy account, or entropy balance, then establishes the detail of the losses or irreversibilities. No such losses occur in the equilibrium of Figure A.I. See Figure 2.2 of Barclay (1998). 

Generally, the criteria used in comparing the three electricity-generating systems are complex and difficult to evaluate and sometimes contradictory (e.g. safety sometimes means more expensive). More complex sustainability criteria such as extended exergy accounting or life cycle analysis [13] should be applied for more exact comparisons for all considered systems. 

Accessible work potential is called the exergy that is the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Exergy analysis is essentially a TA, and utilizes the combined laws of thermodynamics to account the loss of available energy. Exergy is always destroyed by irreversibilities in a system, and expressed by... 

Ayres, R.U. Ayres, L.W. Martinas, K. Exergy, waste accounting, and life-cycle analysis. Energy 1998,23,355-363. 

Exergy is a convenient concept if one wishes to assign a quantitative quality mark to a stream or a product. This quality mark expresses the maximum available work or potential to perform work because of its possible differences in pressure, temperature, and composition with the prevailing environment. The physical exergy, Exphys, only accounts for the differences in pressure and temperature the standard chemical exergy, Ex/hf.rn, accounts for the difference in composition with the environment at the environment s pressure and temperature. Thus... 

The concept of cumulative chemical exergy consumption is very useful and accounts for the fact that when a compound (e.g., ammonia) is introduced into a process, its chemical exergy has to be corrected for the exergy consumption accumulated since this compound was manufactured from its natural constituents (air and natural gas in the case of ammonia). 

The second efficiency, %, also accounts for the exergy of the entering flow, whereas the first efficiency, r , does not. Wlost is the same in both calculations. There is much to say for considering r as the value that expresses the performance of the compressor best. r expresses in a meaningful way the performance of the equipment whereas % expresses the quality of the process. 

One year before Ayres publications [7,8], Cornelissen [9] completed his PhD dissertation in which he had combined life cycle analysis with exergy analysis. He called this extension of LCA exergetic life cycle analysis. He explained that ELCA should be part of every LCA because the loss via dissipation of exergy is one of the most important parameters to properly assess a process and measure the depletion of natural resources. Cornelissen even went one step further and extended ELCA to what he called zero-emission ELCA. In this extension of ELCA, the exergy required for the abatement of emissions, that is, the removal and reuse of environmentally friendly storage of emissions, is accounted for. Cornelissen illustrated his ideas with examples of... 

Wall, G. Exergy—A useful concept within resource accounting. Report No. 77-42, Institute of Theoretical Physics, Chalmers University of Technology, Goteborg, Sweden, 1977. 

For an electrolyser, instead of an isentropic coefficient, we take into account the overvoltage necessary for driving the electrolysis (ohmic resistance, anodic-cathodic overvoltage) here the exergy loss is simply connected to this overvoltage by Faraday s law ... 

Eq. (11) becomes for the electrolyser, to take into account the exergy loss for the conversion of heat to work ... 

In this step CuCl needs first to be dissolved in the HC1/H20 matrix. If it is close to the solubility limit, this can be minimised and exergy loss can probably be limited to low values, around 5-10 kj/mol (if not, extra exergy loss should be taken into account). 

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