Modelling and testing of a low temperature solar organic rankine cycle power plant.
Situmbeko, Shadreck Mubiana.
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The post-apartheid era in South Africa has brought with it great challenges regarding electricity supply with demand far outstripping supply mainly due to the refocusing of the economy to foster industrialisation, the mass electrification programme, as well as several years of underinvestment in power generation and distribution infrastructure. Currently South Africa obtains about 77% of her electricity from coal, 6.5% from nuclear and the rest from a mix of diesel, gas and renewables. The government has adopted a diversification policy that encourages growth in electricity generation from renewable resources; this has led to renewed interest in renewables energies research and development as well as investment. By February 2015, government had procured 4 terawatts of renewable energy – electricity from IPP’s while Eskom completed the grid-connected 100 megawatts Sere Wind farm. This current research is a response to the current energy scenario vis-à-vis supply challenges and research space. The research investigated the technical, economic and environmental viability of a small to medium sized low temperature solar thermal organic Rankine cycle (ORC) power plant. Mathematical and computer models were developed for the ORC, and for the cycle components. The ORC model involved 14 working fluids and three plant configurations. The solar field model employed ethylene glycol water as the heat transfer fluid and involved a 9-collector solar field and a 180-collector solar field. An evaporator model was developed based on a counter flow double pipe configuration and a flow boiling process incorporating both convective and nucleate boiling. Ethylene glycol water was placed on the shell side; the tube side fluid was modelled on four candidate working fluids. The condenser model was based on a flow of vapour over a bundle of horizontal tubes. The working fluid was modelled from five organic fluids; the cooling liquid was ethylene glycol water and was placed on the tube side. Preliminary turbine design models for both radial inflow and axial configurations were developed. The designs were presented in terms of geometric and thermodynamic parameters. The initial results have shown that small turbines for low temperature cycles are feasible. The results of the economic and environmental analyses were a negative NPV value (ZAR -126 389.64), six years energy payback period (EPBP) and 426.9 days (1.17 years) carbon payback period (CPBP). Both the EPBP and CPBP values are comparable with similar technologies. A sensitive analysis based on locally designed and produced power block and solar field produced a positive NPV value. The experimental set-up and experimental procedures were successively achieved but the experiments had to be postponed pending supply of a non-defective unit. The data obtained from the manufacturer show the ORC as a viable power supply. The results of these models and simulations have been published in six peer-reviewed journal papers and six peer-reviewed conferences and conference proceedings. Overall, the research can be considered to have been a success judging by the quantity and quality of research output. A post-research proposal is being prepared and covers the outstanding tasks.