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Displaced Exergy: the Valuation of Thermal Power

By Paul Shurtleff and William Harvey

ABSTRACT

Conventional exergy analysis is a useful tool to compare the relative efficiency of cycles harnessing geothermal fluids of different fluid properties, as well as to assess the power generation potential of production well flows and power plant process side streams. These side streams could be fluid flows re-injected to the reservoir or low enthalpy fluids discharged from the power generation process. Exergy analysis is also a valuable tool to assess the potentials for improvement of a cycle or plant component. However, exergy analysis on power generation projects has the potential to overlook the value of the process streams and the potential contribution that they can make when the same system is analyzed from a more holistic perspective. This paper discusses the concept of “displaced exergy,” where improved matching of a stream’s utilization to local demands and the prevailing energy resources is shown to result in better conversion efficiencies than conventional exergy analysis would indicate is possible. When viewing projects through this proposed perspective, geothermal plants with flexibility in generating both thermal and electrical energy can achieve much higher effective efficiencies than a conventional exergy analysis would indicate. A higher effective efficiency using displaced exergetic analysis could be possible in geographic locations where fossil fuels are widely used for thermal demands, and where the fossil fuel thermal generation could be substituted by utilizing geothermal fluids.

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