Doctoral Degrees (Mechanical and Mechatronic Engineering)

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Browsing Doctoral Degrees (Mechanical and Mechatronic Engineering) by Subject "Air heaters"

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    Performance characteristics of a spiky central receiver air pre-heater (SCRAP)
    (Stellenbosch : Stellenbosch University, 2017-03) Lubkoll, Matti; Von Backstrom, T. W.; Harms, T. M.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: A combined cycle concentrating solar power (CSP) plant provides significant potential to achieve an efficiency increase and an electricity cost reduction compared to current single-cycle plants. A combined cycle CSP system requires a receiver technology capable of effectively transferring heat from concentrated solar irradiation to a pressurized air stream in a gas turbine. The novel spiky central receiver air pre-heater (SCRAP) technology is proposed to provide such a receiver and overcome barriers experienced by developments to date. The SCRAP receiver is a novel metallic receiver technology aimed at preheating an air stream to about 800◦C, either prior to a combustion chamber or alternatively a cascaded secondary non-metallic receiver system, capable of achieving elevated temperatures. The SCRAP receiver is distinguished in shape and functioning from receiver concepts presented to date for the application in Brayton or combined cycles. The receiver is predicted to perform at solar-thermal efficiencies exceeding 80%. The geometric design of the receiver achieves a relatively low radiative heat loss, predicted at about 4% – 5%, whereas the relatively large surface results in vulnerability to convective heat losses. The pressure drop was found to be dependent on the geometries selected but relatively low, compared to existing alternative receiver designs, with system pressure drops below 40mbar achievable. A ray-tracing analysis showed that the flux impinging on the absorber assemblies is in its spatial distribution dependent on the solar field, more specific, the heliostat size and design. A thermodynamic model was developed to investigate the performance characteristics of the SCRAP receiver. The developed thermodynamic computer model was verified against an experimental test setup designed and built at the heat transfer laboratory at Stellenbosch University. Tests with steam heating at nominally 100◦C show good agreement between the experimental results and the modeled predictions, at various air flow rates. Further work on the SCRAP receiver technology is recommended. On the modelingsideworkisproposedondevelopingsolutionsforjetimpingementheat transfer on the spike tip to improve heat transfer while reducing pressure drop. Furtherworkonhelicallyswirledfinsissuggestedtocontributetoincreasedheat transfer characteristics. The receiver showed vulnerability towards convective heat losses; further work to better predict and mitigate these is required. Experiments under solar flux or simulated solar flux should further improve understanding of the technology. Cost effective manufacturing processes need to be developed to satisfy economic suitability of the receiver technology.