Research On The Thermal Stability Of Working Fluids For Organic Rankine Cycles

China is abundant in the waste heat and renewable resources like geothermal energy,biomass energy and solar energy.The Organic Rankine Cycle(ORC)is one of the most suitable methods to convert these resources into power.The supercritical ORCs with middle and high temperature heat resources(150~350℃)have attracted much interest due to their higher thermal efficiencies and better economy than subcritical ORCs at low temperatures.The thermal stability of working fluid is the primary limitation for the working fluid selection at high temperatures.Thus,the thermal stability of working fluid is studied in this dissertation.This dissertation presented a first order reaction apparent chemical kinetics model method,by which the decomposition concentrations in all ORC conditions could be calculated.The working fluid thermal stability online measurement system was designed considering the fluid characteristics and chemical kinetics experiment requirements.The apparent kinetic parameters were measured experimentally with n-pentane as test fluids.This method can calculate the decomposition concentrations in all conditions and is suitable for all the fluids in theory.Even though the apparent chemical kinetics model method can calculate the decomposition concentrations in all conditions,it is impossible to study all the hundreds of working fluids by this complex method.The decomposition indicators of different kinds of fluids were studied theoretically and experimentally and a rapid experimental method was presented.A working fluid thermal stability rapid experimental system was designed and built.A lot of basic thermal stability experimental data was obtained and the relationships between thermal stability and molecular structures were analyzed.The prediction of the thermal stability by theoretical method is feasible due to the relations between thermal stability and molecular structures.A theoretical prediction method with Gibbs functions and another method with the dissociation energy were presented.The calculation results of these two prediction methods were consistent with the experimental results.Thus,both of the theoretical prediction methods can provide guidance for the thermal stability studies.However,the decomposition concentrations cannot evaluate the thermal stability overall and the influence mechanisms of decomposition products to ORC systems also should be confirmed.The compositions of the decomposition products were first confirmed experimentally.The influence mechanisms were then analyzed with the ORC system model and the main mechanisms of different kinds of fluids were confirmed.The influence extents were also calculated by the ORC model quantitatively.The previous studies are scarce for the compatibility of working fluids with metal materials at high temperatures.Thus,the compatibility of some kinds of working fluids with metal materials like copper and aluminum were studied experimentally in this dissertation.The results showed that the fluorinated fluids had worse compatibility with metal materials at high temperatures and metal materials like copper and aluminum also had catalytic effects on working fluids.