Optimization of ORCs for low grade heat recovery: working fluid selection, methodology and applications
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n this work two different heat sources were considered: geothermal brine at temperatures ranging from 90 to 150°C, and waste heat coming from an aluminum manufacturing plant. A number of 64 pure fluids and 35 mixtures were considered for the geothermal cases, evaluating the behavior of the optimal cycle configuration for each fluid, both in case of power production only and CHP. For the heat recovery case, a number of 72 pure fluids and 30 mixtures was considered. For each fluid, the optimum cycle was achieved by means of an optimization process based on energy targeting and on an evolutionary optimization algorithm. With these, it was able to optimize cycle temperatures, pressures and mass flow rates according to the objective function considered: net power output for the power production cases; sum of the net power output and exergy related to the district heat for the CHP case. Once obtained all the results, best pure fluids were compared with best mixtures for each case. It was showed that mixtures always outperformed pure fluids, especially at lower temperatures, where difference is significant. In the specific case of CHP, mixtures are significantly better in all the heat source temperature range. Analysis of the results of the geothermal power production case, run by pure fluids, showed a relationship between critical to heat source temperature ratio of each fluid (for each source temperature) and second law efficiency. Independently from the temperature of the geothermal brine, the range 0.5-1 of the temperature ratio considered showed always maximum efficiency, meaning that all the fluid that have a ratio inside this range perform better than all the others. Mixtures for all the cases were chosen considering a new selection criteria developed in this thesis, and based on the results already mentioned: all the possible binary mixtures coming from sets of fluids were evaluated on the basis of their critical to heat source ratio. If one mixture, at a certain composition and heat source temperature, showed a ratio in the range 0.5-1, it got one point. The more points a mixture has, the better it is supposed to perform. Criterion was assessed, and it was noted that it is not able to tell the only best working fluid, but actually a relationship between second law efficiency and number of point exists: two fluids with very different number of points will also showed different second law efficiency. Thus this method could be used as preselection method to identify the possible best mixture candidates.