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Nanofluids as working fluids for organic Rankine cycles

Periodic Reporting for period 1 - NanoORC (Nanofluids as working fluids for organic Rankine cycles)

Reporting period: 2017-03-01 to 2019-02-28

Organic Rankine cycles power systems are expected to play a substantial role in the future European energy system, as they contribute to reduce the CO2 emissions and dependence on fossil fuels by converting low temperature heat from renewable energy or industrial waste to electrical power. However, the working fluids that they use exhibit poor environmental properties or safety issues, due to their toxicity or flammability. Consequently, many of them are being phased out. As a result, there is an urgent need for alternative working fluids. The main barrier for the introduction of novel working fluids is the lack of an accurate knowledge of their thermophysical behaviour.
This project addresses the development of predictive models for the thermophysical properties of new working fluids. To this end, the project addresses the investigation of the thermophysical behaviour of novel pure fluids, fluid mixtures, and nanofluids (colloidal suspensions of nanoparticles in fluids). The prediction models rely on the use of group contribution methods.
The generalization of the model for pure fluids to any chemical group was found counterproductive as it implied loss of accuracy. Therefore, the developed predictive models were tailored for specific chemical groups. The research results have demonstrated that the predictions from the developed models for pure halogenated olefins outperform those of equivalent available models.
Concerning mixtures, the project addressed the improvement of mixing models for two types of cubic equations of state for mixtures of commercial refrigerants and hydrofluoroolefins. First, new parameters were fitted for the mixing model of a standard Peng Robinson equation of state. Second, parameters for the more complex mixing model of a volume translated Peng Robinson equation of state were fitted. The performance of these models was evaluated with an extensive set of experimental data.
With regards to nanofluids, a broad literature overview of published thermophysical properties of nanofluids showed that data were scarce, preventing the development of reliable predictive models. To advance the knowledge of the thermophysical behaviour of nanofluids, the first comprehensive database of experimental data of nanofluids was created.
The work performed during the project includes:
• Project plan, coordination and management.
• Development of a data management plan.
• Assessment of the prediction uncertainty of a model that was developed right before the project start for the thermophysical properties of halogenated olefins.
• Multi-screening of halogenated olefins in organic Rankine cycle unit models and assessment of the results considering their molecular structure.
• Assessment of options of novel working fluids for organic Rankine cycle units to be installed on board ships.
• Collection of all the experimental data available for experimental thermophysical properties of mixtures including hydrofluoroolefins.
• Fitting of binary interaction parameters for the use of the Peng Robinson equation of state for mixtures including hydrofluoroolefins.
• Development of parameters for the mixing model of a volume translated Peng Robinson equation of state to represent the behaviour of hydrofluoroolefins with commercial refrigerants.
• Development of the first comprehensive database of thermophysical properties of nanofluids, based on published scientific literature.
• Preliminary evaluation of the potential benefits of using nanofluids as working fluids for organic Rankine cycle units.
• Training activities including a distance course on patenting, specialized course on computational thermodynamic models, leaderships courses tailored for scientists and women, and attendance to conference on entrepreneurship
• Presentation of project results at international conferences
• Supervision of undergraduate and graduate students, and Erasmus interns.
• Outreach activities through: (i) participation in the events and workshops organized by the Marie Curie Alumni Association; (ii) foundation of the Danish Chapter of the Marie Curie Alumni Association; (iii) participation in the Danish program ‘Book a researcher’ and the department Project Open Day
• Organization of workshop to present the results of the project for project and other interested partners, after the termination of the project.
The results and outcomes of the project include:
• Models to estimate the prediction uncertainty of predictive models developed before the project start for the main thermophysical properties of halogenated olefins.
• New parameters for the mixing models for two cubic equations of state for hydrofluoroolefin mixtures.
• A comprehensive evaluation of the potential performance of novel halogenated olefins as working fluids in organic Rankine cycle systems for low- and high-temperature applications.
• The first comprehensive database of thermophysical properties of nanofluids.
The outcomes and results have been disseminated in 4 conference publications and 5 journal publications. The outcomes of the results will be transferred to industry during a specialized workshop on energy fluids after the termination of the project. The outcomes of the project will contribute to bridge the knowledge gap existing on the thermophysical behaviour of novel fluids, including pure novel halogenated olefins, their mixtures, and nanofluids. This knowledge will support decision on future development of novel molecules for refrigerants and working fluids, and their optimization for organic Rankine cycle units.
The progress of the project beyond the state of the art includes the following findings:
1) It is now possible to predict the thermophysical behaviour of novel halogenated olefins, including the prediction uncertainty. The uncertainty of the prediction is larger for molecules containing atypical functional groups containing chlorine.
2) The potential of a wide range of halogenated olefins as working fluids for organic Rankine cycles has been evaluated. Options for low temperature applications may imply a greater content of fluorine, therefore having less flammability but potentially increased global warming potential. The options for higher temperature applications may require the presence of chlorine, for which toxicity should be tested.
3) Cubic equations of state can predict reasonably well the behaviour of binary mixtures of commercial refrigerants and hydrofluoroolefins, except in the cases where the molecule of the refrigerant differs significantly from that of olefins. In those cases, greater deviations of the predicted mixture properties should be expected.
4) The available published thermophysical data on nanofluids is scarce and exhibits high scatter. The first database of these data, developed in the project, will help identifying property trend for fluids and nanoparticles, as well as outliers present in the literature.

The development of reliable property prediction models for novel working fluids and their mixtures will allow predicting their overall system efficiency when used in ORC systems. This will may result in the development of novel working fluids with more environmentally friendly or safety properties, replacing the current working fluids under phase-out schedule. Overall, this will increase the sustainability of the organic Rankine cycle technology, having also a positive impact beyond this field (e.g. refrigeration, heat pumps).
Graphical abstract of the project