The organic rankine cycle (ORC) as a bottoming cycle
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The expression “bottoming cycle” refers to the power cycle that uses waste industrial heat for power generation by supplementing heat from any fossil fuel.
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to convert low-grade waste heat into useful work has been widely investigated for many years. The CO2 transcritical power cycle, on the other hand, is scarcely treated in the open literature. A CO2 transcritical power cycle (CO2 TPC) shows a higher potential than an ORC when taking the behavior of the heat source and the heat transfer between heat source and working fluid in the main heat exchanger into account. This is mainly due to better temperature glide matching between heat source and working fluid. The CO2 cycle also shows no pinch limitation in the heat exchanger. This study treats the performance of the CO2 transcritical power cycle utilizing energy from low-grade waste heat to produce useful work in comparison to an ORC using R123 as working fluid.
Due to the temperature gradients for the heat source and heat sink the thermodynamic mean temperature has been used as a reference temperature when comparing both cycles. The thermodynamic models have been developed in EES
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EES – Engineering equation solver. The thermodynamic properties for carbon dioxide in EES are calculated by the fundamental equation of state developed by R. Span and W. Wagner, A new equation of state for carbon dioxide covering the fluid region form the triple-point temperature to 1100K at pressures up to 800MPa, J. Phys. Chem. Ref. Data, Vol. 25, No. 6, 1996. http://www.fchart.com/ees/ees.shtml.
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The relative efficiencies have been calculated for both cycles. The results obtained show that when utilizing the low-grade waste heat with the same thermodynamic mean heat rejection temperature, a transcritical carbon dioxide power system gives a slightly higher power output than the organic rankine cycle.