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Research Papers

Comparative Analysis of Single- and Dual-Media Thermocline Tanks for Thermal Energy Storage in Concentrating Solar Power Plants

[+] Author and Article Information
Carolina Mira-Hernández

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907-2088
e-mail: cmira@purdue.edu

Scott M. Flueckiger

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907-2088
e-mail: scott.m.flueckiger@gmail.com

Suresh V. Garimella

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907-2088
e-mail: sureshg@purdue.edu

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 July 23, 2014; final manuscript received December 17, 2014; published online January 8, 2015. Assoc. Editor: Nathan Siegel.

J. Sol. Energy Eng 137(3), 031012 (Jun 01, 2015) (10 pages) Paper No: SOL-14-1213; doi: 10.1115/1.4029453 History: Received July 23, 2014; Revised December 17, 2014; Online January 08, 2015

A molten-salt thermocline tank is a low-cost option for thermal energy storage (TES) in concentrating solar power (CSP) plants. Typical dual-media thermocline (DMT) tanks contain molten salt and a filler material that provides sensible heat capacity at reduced cost. However, conventional quartzite rock filler introduces the potential for thermomechanical failure by successive thermal ratcheting of the tank wall under cyclical operation. To avoid this potential mode of failure, the tank may be operated as a single-medium thermocline (SMT) tank containing solely molten salt. However, in the absence of filler material to dampen tank-scale convection eddies, internal mixing can reduce the quality of the stored thermal energy. To assess the relative merits of these two approaches, the operation of DMT and SMT tanks is simulated under different periodic charge/discharge cycles and tank wall boundary conditions to compare the performance with and without a filler material. For all conditions assessed, both thermocline tank designs have excellent thermal storage performance, although marginally higher first- and second-law efficiencies are predicted for the SMT tank. While heat loss through the tank wall to the ambient induces internal flow nonuniformities in the SMT design over the scale of the entire tank, strong stratification maintains separation of the hot and cold regions by a narrow thermocline; thermocline growth is limited by the low thermal diffusivity of the molten salt. Heat transport and flow phenomena inside the DMT tank, on the other hand, are governed to a great extent by thermal diffusion, which causes elongation of the thermocline. Both tanks are highly resistant to performance loss over periods of static operation, and the deleterious effects of dwell time are limited in both tank designs.

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Figures

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Fig. 1

Schematic illustration of a thermocline tank

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Fig. 2

Mass flow rate into (top) or out of (negative) the top of the thermocline during cyclical operation with and without dwell time

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Fig. 3

Schematic illustration of the nonuniform mesh scheme used near the tank wall for the single-medium tank under nonadiabatic wall conditions

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Fig. 4

Instantaneous molten-salt temperature contours and streamlines for the SMT tank with nonadiabatic wall boundary conditions during the cycle without dwell time. The white dashed lines represent the limits of the heat exchange region.

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Fig. 5

Instantaneous molten-salt temperature and velocity fields for the DMT tank with (a) adiabatic and (b) nonadiabatic wall boundary conditions shown at the beginning of the discharge process (t = 13.5 h) under time-periodic operation. The white dashed lines represent the limits of the heat exchange region.

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Fig. 6

Instantaneous molten-salt temperature and velocity fields for the SMT tank with (a) adiabatic and (b) nonadiabatic wall boundary conditions shown at the beginning of the discharge process (t = 13.5 h) under time-periodic operation. The white dashed lines represent the limits of the heat exchange region.

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Fig. 7

Instantaneous molten-salt temperature and velocity fields of the (a) DMT tank and (b) SMT tank with nonadiabatic wall boundary condition during the dwell time when the tank is filled with hot fluid (t = 10.5 h). The white dashed lines represent the limits of the heat exchange region.

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Fig. 8

Effective thickness of the thermocline region inside the DMT tank during the cycles (a) without dwell time and (b) with dwell time. The solid line illustrates the behavior with an adiabatic wall boundary condition; the dashed line illustrates the behavior with a nonadiabatic wall boundary condition.

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Fig. 9

Effective thickness of the thermocline region inside the SMT tank during the cycles (a) without dwell time and (b) with dwell time. The solid line illustrates the behavior with an adiabatic wall boundary condition; the dashed line illustrates the behavior with a nonadiabatic wall boundary condition.

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Fig. 10

Outflow temperature history during discharge of the DMT tank

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Fig. 11

Outflow temperature history during discharge of the SMT tank

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