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Investigation of the Criteria for Fluid Selection in Rankine Cycles for Waste Heat Recovery

Author(s): M. Aslam Siddiqi | Burak Atakan

Journal: International Journal of Thermodynamics
ISSN 1301-9724

Volume: 14;
Issue: 3;
Start page: 117;
Date: 2011;
Original page

Keywords: Thermodynamics | Energy | Exergy

The organic Rankine cycle is a promising way for the conversion of low temperature heat to electricity. Different fluids can be used in Rankine cycles for the utilization of waste heat.  The suitability of a certain fluid will depend on its thermodynamic properties as well as on the conditions at which the heat is available, thus it is often unclear if an organic fluid has any advantage compared to inorganic fluids like water. Various substances starting from the refrigerants to high boiling organic liquid have been investigated as possible working fluid for the different temperature ranges at which the waste heat is available. The present communication reports exemplary the results for three different classes of substances, a hydrocarbon (n-heptane), two refrigerants (R245fa) and pentafluoro butane mixture (Solkatherm, SES36), and water in the intermediate temperature range (473 to 773 K) where the exhaust gases of combustion engines may be used as energy source for cogeneration. In this range it turns out that for many conditions, water and heptane are well suited working fluids for cogeneration systems. In the present investigation, the attention was not laid on the cycle efficiency alone, but also on the total exergy usage from an enthalpy stream (e.g. exhausts gas). This is used for defining the total efficiency for the process. The results for the thermodynamic parameter total heat recovery efficiency and the surface area of the heat exchanger have been discussed. diagrams were also used for judging the suitability of a fluid. It turns out that water is well suited for many cases in the intermediate temperature regime.
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