Masters Degrees (Mechanical and Mechatronic Engineering)

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Browsing Masters Degrees (Mechanical and Mechatronic Engineering) by Author "Allen, Kenneth Guy"

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    Performance 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.