Centre for Renewable and Sustainable Energy Studies
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Stellenbosch University has been awarded the responsibility to act as the hub of a Postgraduate Programme in Renewable and Sustainable Energy Studies by the South African National Energy Research Institute (SANERI), a division of the Central Energy Fund (CEF). The overall objective of this initiative is to develop and enhance national capacity in renewable and sustainable energy in support of accelerated and shared economic growth within the area of sustainable energy. This will be achieved by building the human resource capacity, create and disseminate knowledge, and lastly to stimulate innovation and enterprise in the field of renewable and sustainable energy.
Also visit the web page of the Centre for Renewable and Sustainable Energy Studies, Stellenbosch University
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Browsing Centre for Renewable and Sustainable Energy Studies by browse.metadata.advisor "Kroger, D. G."
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- ItemOptimization and control of a large-scale solar chimney power plant(Stellenbosch : University of Stellenbosch, 2007-03) Pretorius, Johannes Petrus; Kroger, D. G.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: The dissertation builds on previous research (Pretorius, 2004) and investigates the optimization and control of a large-scale solar chimney power plant. Performance results are based on a reference location near Sishen in South Africa and a so-called reference solar chimney power plant, with a 5000 m collector diameter and a 1000 m high, 210 m diameter chimney. The numerical simulation model is refined and used to perform a sensitivity analysis on the most prominent operating and technical plant specifications. Thermo-economically optimal plant configurations are established from simulation results and calculations according to an approximate plant cost model. The effects of ambient wind, temperature lapse rates and nocturnal temperature inversions on plant performance are examined. Various new technologies are investigated for the purpose of controlling plant output according to specific demand patterns. The incorporation of vegetation under the collector roof of the plant and the influence thereof on plant performance is also explored. Results indicate that, through the modification of the collector roof reflectance, collector roof emissivity, ground surface absorptivity or ground surface emissivity, major improvements on plant performance are possible. Introducing thermal insulation or double glazing of the collector roof also facilitates substantial enhancements on plant yield. Simulations predict a notable sensitivity to the ground surface absorptivity value, while variable atmospheric temperature lapse rates and windy ambient conditions may impair plant performance significantly. Furthermore, sand is found to be unsuitable as plant ground type and thermoeconomically optimal solar chimney plant dimensions are determined to be generally larger than plant dimensions employed in previous studies. Good dynamic control of solar chimney power output is established, suggesting that a solar chimney power plant can be implemented as a base or peak load electricity generating facility. Lastly, results predict that vegetation, when provided with sufficient water, will be able to survive under the collector roof but the inclusion of vegetation will however cause major reductions in plant performance.
- ItemPerformance characteristics of an air-cooled steam condenser incorporating a hybrid (dry/wet) dephlegmator(Stellenbosch : Stellenbosch University, 2008-12) Heyns, Johan Adam; Kroger, D. G.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.This study evaluates the performance characteristics of a power plant incorporating a steam turbine and a direct air-cooled dry/wet condenser operating at different ambient temperatures. The proposed cooling system uses existing A-frame air-cooled condenser (ACC) technology and through the introduction of a hybrid (dry/wet) dephiegmator achieves measurable enhancement in cooling performance when temperatures are high. In order to determine the thermal-flow performance characteristics of the wet section of the dephlegmator, tests are conducted on an evaporative cooler. From the experimental results, correlations for the water film heat transfer coefficient, air-water mass transfer coefficient and the air-side pressure drop over a deluged tube bundle are developed. During periods of high ambient temperatures the hybrid (dry/wet) condenser operating in a wet mode can achieve the same increased turbine performance as an oversized air-cooled condenser or an air-cooled condenser rith adiabatic cooling (spray cooling) of the inlet air at a considerably lower cost. For the same turbine power output the water consumed by an air-cooled condenser incorporating a hybrid (dry/wet) dephlegmator is at least 20% less than an air- cooled condenser with adiabatic cooling of the inlet air.
- ItemPerformance characteristics of packed bed thermal energy storage for solar thermal power plants(Stellenbosch : University of Stellenbosch, 2010-03) Allen, Kenneth Guy; Kroger, D. G.; University of Stellenbosch. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.ENGLISH ABSTRACT: Solar energy is by far the greatest energy resource available to generate power. One of the difficulties of using solar energy is that it is not available 24 hours per day - some form of storage is required if electricity generation at night or during cloudy periods is necessary. If a combined cycle power plant is used to obtain higher efficiencies, and reduce the cost of electricity, storage will allow the secondary cycle to operate independently of the primary cycle. This study focuses on the use of packed beds of rock or slag, with air as a heat transfer medium, to store thermal energy in a solar thermal power plant at temperatures sufficiently high for a Rankine steam cycle. Experimental tests were done in a packed bed test section to determine the validity of existing equations and models for predicting the pressure drop and fluid temperatures during charging and discharging. Three different sets of rocks were tested, and the average size, specific heat capacity and density of each set were measured. Rock and slag samples were also thermally cycled between average temperatures of 30 ºC and 510 ºC in an oven. The classical pressure drop equation significantly under-predicts the pressure drop at particle Reynolds numbers lower than 3500. It appears that the pressure drop through a packed bed is proportional to the 1.8th power of the air flow speed at particle Reynolds numbers above about 500. The Effectiveness-NTU model combined with a variety of heat transfer correlations is able to predict the air temperature trend over the bed within 15 % of the measured temperature drop over the packed bed. Dolerite and granite rocks were also thermally cycled 125 times in an oven without breaking apart, and may be suitable for use as thermal storage media at temperatures of approximately 500 ºC. The required volume of a packed bed of 0.1 m particles to store the thermal energy from the exhaust of a 100 MWe gas turbine operating for 8 hours is predicted to be 24 × 103 m3, which should be sufficient to run a 25-30 MWe steam cycle for over 10 hours. This storage volume is of a similar magnitude to existing molten salt thermal storage.