Performance characteristics of packed bed thermal energy storage for solar thermal power plants

[摘要] 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 perday - some form of storage is required if electricity generation at night or duringcloudy periods is necessary. If a combined cycle power plant is used to obtainhigher efficiencies, and reduce the cost of electricity, storage will allow thesecondary cycle to operate independently of the primary cycle. This study focuseson the use of packed beds of rock or slag, with air as a heat transfer medium, tostore thermal energy in a solar thermal power plant at temperatures sufficientlyhigh for a Rankine steam cycle. Experimental tests were done in a packed bed testsection to determine the validity of existing equations and models for predictingthe pressure drop and fluid temperatures during charging and discharging. Threedifferent sets of rocks were tested, and the average size, specific heat capacity anddensity of each set were measured. Rock and slag samples were also thermallycycled between average temperatures of 30 ºC and 510 ºC in an oven.The classical pressure drop equation significantly under-predicts the pressure dropat particle Reynolds numbers lower than 3500. It appears that the pressure dropthrough a packed bed is proportional to the 1.8th power of the air flow speed atparticle Reynolds numbers above about 500. The Effectiveness-NTU modelcombined with a variety of heat transfer correlations is able to predict the airtemperature trend over the bed within 15 % of the measured temperature dropover the packed bed. Dolerite and granite rocks were also thermally cycled 125times in an oven without breaking apart, and may be suitable for use as thermalstorage media at temperatures of approximately 500 ºC.The required volume of a packed bed of 0.1 m particles to store the thermalenergy from the exhaust of a 100 MWe gas turbine operating for 8 hours ispredicted to be 24 × 103 m3, which should be sufficient to run a 25-30 MWe steamcycle for over 10 hours. This storage volume is of a similar magnitude to existingmolten salt thermal storage.

[发布机构] Stellenbosch University