Thermal stability tests on organic working fluids

This work explores the possibility to adopt in organic Rankine cycle (ORC) plants mixtures of water (acting as solvent) plus an organic compound (acting as solute) as the working fluid. Initially an evaluation of the thermodynamic properties of the mixtures is performed, in order to assess their properties, and to point out the molar fractions which entail a near-azeotropic behaviour. Four species from three different classes of chemical compounds are investigated: 2,2,2-trifluoroethanol and n-butanol for alcohols, where the first is fluorinated, acetonitrile for nitrile class and 2-methylpyrazine as a heterocyclic aromatic compound. Simultaneously, the thermal stability of the pure substances considered as the possible solute for the mixtures is experimentally investigated in order to estimate the temperature applicability range. The ORC plant performance, from a low-enthalpy geothermal heat source (hot water stream from 100 to 200 °C), adopting the selected mixtures as the working fluid is finally evaluated, and the analysis includes a preliminary discussion on the turbine design; results are compared with respect to the reference case of a hypothetical plant adopting water as the working fluid.

In this paper we investigate the thermal stability of three representative hydrocarbons used as working fluids in organic Rankine cycles (ORC): n-pentane, cyclo-pentane and toluene. The experimental used method is a “static” one, based on the recording of the pressure during the permanence of the fluid sample at constant temperature and on the measure of the differences in the vapour pressure in comparison with the reference values for the virgin pure fluid. The sample container and the circuit are in stainless steel.