High temperature packed bed thermal storage for solar gas turbines
Date
2016
Authors
Klein, Peter
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Abstract
Solar powered gas micro-turbines present opportunities for off-grid power generation. Due
to the intermittent nature of the solar energy supply, existing Solar Gas Turbine (SGT) plants
employ hybridisation with fossil fuels to generate dispatchable power. In this work sensible
heat and latent heat storage solutions are investigated as a means of increasing the solar share
of a SGT cycle, thus reducing the consumption of diesel fuel.
The sensible heat storage concept was based on a pressurised packed bed of spherical ceramic
particles, using air as the heat transfer fluid. An axisymmetric, two-phase heat transfer model
of the system was developed, based on the continuous solid phase approach. The model
was successfully validated against experimental data from a packed bed of alumino-silicate
particles over the temperature ranges of gas turbine cycles (350-900 °C and 600-900 °C). The
validated numerical model was utilised to conduct a parametric design study of a six hour
(1.55 MWhth) storage system for a gas micro-turbine. The results show that a high storage
efficiency and high utilisation factor can be achieved when combining sensible heat storage
in alumina with fossil fuel hybridisation, with somewhat lower values without hybridisation.
An analysis of different inventory geometries showed that a packed bed of spherical particles
is best suited to pressurised sensible heat storage.
The latent heat storage concept was based on a pressurised packed bed of Encapsulated
Phase Change Material (EPCM) particles. Sodium sulphate was identified as a suitable phase
change material for the gas turbine cycle. The sensible heat storage model was extended
to account for intra-particle temperature gradients and phase change within the particles.
The intra-particle phase change model was validated against published experimental data
for a single EPCM sphere heated and cooled by convection. The full EPCM storage model
was further successfully validated against experimental data from a packed bed of macro-
encapsulated sodium sulphate particles with alumina shells, up to a temperature of 950 °C.
A comparison of the two storage concepts for a 7 m3 bed shows that a packed bed of en-
capsulated sodium sulphate particles would have a 36% lower energy storage capacity than
a bed of solid alumina particles. This is due to the limited melt fraction in the EPCM bed
when a temperature limit is placed on the base. Increasing the packed bed volume to 10.5 m3
would provide a comparable thermal performance to the 7 m3 solid alumina bed, at a 12%
lower storage mass. A hybrid three-layer packed bed is proposed to increase the volumetric
energy storage density. Modelling shows that this concept could provide a small increase
of 5.3% in the amount of energy discharged above 850 °C, compared to the solid alumina
particles only.
Description
A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, 2016
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Citation
Klein, Peter (2016) High temperature packed bed thermal storage for solar gas turbines, University of Witwatersrand, Johannesburg, <http://wiredspace.wits.ac.za/handle/10539/20989>