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Research Papers

Cost Optimum Parameters for Rock Bed Thermal Storage at 550–600 °C: A Parametric Study

[+] Author and Article Information
Kenneth Allen

Solar Thermal Energy Research Group,
Department of Mechanical and
Mechatronic Engineering,
University of Stellenbosch,
Private Bag XI,
Matieland 7602, South Africa
e-mail: kenallsp@yahoo.com

Lukas Heller

Solar Thermal Energy Research Group,
Department of Mechanical and
Mechatronic Engineering,
University of Stellenbosch,
Private Bag XI,
Matieland 7602, South Africa
e-mail: Lukas_Heller@gmx.de

Theodor von Backström

Mem. ASME
Emeritus Professor
Department of Mechanical and
Mechatronic Engineering,
University of Stellenbosch,
Private Bag XI,
Matieland 7602, South Africa
e-mail: twvb@sun.ac.za

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received February 2, 2016; final manuscript received July 14, 2016; published online September 2, 2016. Assoc. Editor: Prof. Nathan Siegel.

J. Sol. Energy Eng 138(6), 061004 (Sep 02, 2016) (8 pages) Paper No: SOL-16-1061; doi: 10.1115/1.4034334 History: Received February 02, 2016; Revised July 14, 2016

A major advantage of concentrating solar power (CSP) plants is their ability to store thermal energy at a cost far lower than that of current battery technologies. A recent techno-economic study found that packed rock bed thermal storage systems can be constructed with capital costs of less than 10 United States dollar (USD)/kWht, significantly cheaper than the two-tank molten salt thermal storage currently used in CSP plants (about 22–30 USD/kWht). However, little work has been published on determining optimum rock bed design parameters in the context of a CSP plant. The parametric study in this paper is intended to provide an overview of the bed flow lengths, particle sizes, mass fluxes, and Biot numbers which are expected to minimize the levelized cost of electricity (LCOE) for a central receiver CSP plant with a nominal storage capacity of 12 h. The findings show that rock diameters of 20–25 mm will usually give LCOE values at or very close to the minimum LCOE for the combined rock bed and CSP plant. Biot numbers between 0.1 and 0.2 are shown to have little influence on the position of the optimum (with respect to particle diameter) for all practical purposes. Optimum bed lengths are dependent on the Biot number and range between 3 and 10 m for a particle diameter of 20 mm.

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References

Figures

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Fig. 1

Illustration of CSP power plant featuring a rock bed and Rankine cycle. (Reproduced with permission from Allen et al. [25]. Copyright 2014 by Elsevier.)

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Fig. 2

Illustration of air outlet temperature profile during discharge after partial charges (Biv = 0.1, Dv = 10 mm, L = 5 m, and Tc = 550 °C)

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Fig. 3

Plot of minimum LCOE values for a storage capacity of at least 11.9 h. Lines between data points are intended only to link data points for ease of reading.

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Fig. 4

Required bed flow cross-sectional areas corresponding to minimum LCOE for a storage capacity of at least 11.9 h

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Fig. 5

Required bed volumes corresponding to minimum LCOE for a storage capacity of at least 11.9 h

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Fig. 6

Required bed lengths corresponding to minimum LCOE for a storage capacity of at least 11.9 h

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Fig. 7

Plot of minimum LCOE values for a storage capacity of at least 11.9 h (storage capital cost of 25 USD/kWht)

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Fig. 8

Variation of E-NTU predicted temperature profile as a function of time step (Δx = 20 mm)

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Fig. 9

Variation of E-NTU predicted temperature profile as a function of segment size (Δt = 16 s)

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Fig. 10

Subsystem contribution to plant CAPEX

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