The design and analysis of radial inflow turbines implemented within low temperature organic Rankine cycles

White, Martin (2015). The design and analysis of radial inflow turbines implemented within low temperature organic Rankine cycles. (Unpublished Doctoral thesis, City University London)

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Over recent years, with growing concern over climate change, the need for energy which is sustainable, economical and in line with legalisation has led to a substantial surge of interest in organic Rankine cycles (ORC). With the ability to convert low temperature heat sources into power, ORC technology is at the forefront of many sustainable technologies such as biomass, solar, geothermal and waste heat recovery. Despite successful commercialisation for large-scale systems (> 200 kWe), more development is required at the small-scale to realise its potential. For low temperature (< 150_C), low power applications, volumetric expanders are the preferred choice. However, for a 10 kWe system, a well-designed radial inflow turbine could achieve a higher efficiency, and bridge an observed gap between the output powers of existing volumetric expander systems. This thesis investigates the design and analysis of radial inflow turbines for this application.

A thermodynamic ORC model is first developed, which combines cycle analysis with component design. This model is coupled with a multi-objective optimisation, and a novel objective function is developed that considers the trade-o_ between system performance and system complexity. Following a cycle analysis case study, a radial inflow turbine design method for ORC turbines is developed which extends existing ideal gas design methods to be applicable for real gases. Two candidate turbine designs are developed and are validated using computational fluid dynamics (CFD).

For small-scale systems to be economically feasible it is reasonable to assume that the same turbine will be implemented within a number of different systems. This requires o_- design models, and the suitability of using non-dimensional performance maps, obtained using similitude theory, has been investigated using further CFD studies. This has led to the development of a modified similitude theory, suitable for subsonic ORC turbines. This modified similitude theory has been implemented within another thermodynamic model, and the results from a case study show how the same turbine can be effectively utilised within a number of different ORC systems. This is done by selecting a working fluid to match the available heat source.

Overall, this thesis successfully demonstrates the development of modelling methods for small-scale low temperature ORCs utilising radial inflow turbines. This has considered design and o_-design performance models, and ultimately the results demonstrate how the economy of scale of these systems can be improved, aiding in the future commercialisation of the technology.

Item Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Divisions: School of Engineering & Mathematical Sciences > Engineering

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