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Research Papers

# Analysis of Heat Storage and Delivery of a Thermocline Tank Having Solid Filler Material

[+] Author and Article Information
Jon T. Van Lew, Cho Lik Chan, Wafaa Karaki

Department of Aerospace and Mechanical Engineering, University of Arizona, Tucson, AZ 85721peiwen@email.arizona.edu

Peiwen Li1

Department of Aerospace and Mechanical Engineering, University of Arizona, Tucson, AZ 85721peiwen@email.arizona.edu

Jake Stephens

US Solar Holdings LLC, Tucson, AZ 85711

1

Corresponding author.

J. Sol. Energy Eng 133(2), 021003 (Mar 22, 2011) (10 pages) doi:10.1115/1.4003685 History: Received January 05, 2010; Revised February 13, 2011; Published March 22, 2011; Online March 22, 2011

## Abstract

Thermal storage has been considered as an important measure to extend the operation of a concentrated solar power plant by providing more electricity and meeting the peak demand of power in the time period from dusk to late night everyday, or even providing power on cloudy days. Discussed in this paper is thermal energy storage in a thermocline tank having a solid filler material. To provide more knowledge for designing and operating of such a thermocline storage system, this paper firstly presents the application of method of characteristics for numerically predicting the heat charging and discharging process in a packed bed thermocline storage tank. Nondimensional analysis of governing equations and numerical solution schemes using the method of characteristics were presented. The numerical method proved to be very efficient, accurate; required minimal computations; and proved versatile in simulating various operational conditions for which analytical methods cannot always provide solutions. Available analytical solutions under simple boundary and initial conditions were used to validate the numerical modeling and computation. A validation of the modeling by comparing the simulation results to experimental test data from literature also confirmed the effectiveness of the model and the related numerical solution method. Finally, design procedures using the numerical modeling tool were discussed and other issues related to operation of a thermocline storage system were also studied.

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## Figures

Figure 9

Comparison of numerical and analytical results of temperature distribution in the tank after time t∗=4 of a discharge

Figure 10

Comparison of dimensionless temperature distributions in the tank after time t∗=4 of discharge under different numbers of discretized nodes

Figure 11

Dimensionless temperature histories of exit fluid at z∗=1 in the tank for charge and discharge processes

Figure 12

Periodic profile of energy stored in a tank with dimensionless time

Figure 13

Comparison of modeling predicted results with experimental data from Ref. 8

Figure 14

Effect of the ratio of tank height/diameter on the thermal storage efficiency (while keeping the same tank volume)

Figure 15

Effect of the pack bed void fraction on the thermal storage efficiency

Figure 4

One element as a control volume for analysis of energy balance in fluid and rocks. (The direction of z is always identical to the flow direction of fluid.)

Figure 5

Diagram of the solution matrix arising from the method of characteristics

Figure 6

Dimensionless fluid temperatures in the tank every 30 min during heat discharge

Figure 7

Dimensionless temperature distribution in the tank after time t∗=4 of charge

Figure 8

Dimensionless temperature distribution in the tank after time t∗=4 of charge

Figure 1

An ideal thermocline and a heat storage tank having a filler material. (The vertically movable thermal insulation baffle in an ideal thermocline is an imagined perfect insulation layer.)

Figure 2

Qualitative illustration of temperature variation when hot fluid is discharged from the top of a heat storage tank. (tfull is the time that a full tank of pure fluid needs to flow out of the tank, Th is the temperature of the hottest fluid available from solar heat collection field, Tl is the temperature of the coldest fluid that comes from power generation system, and Tcut-off is a temperature below that the fluid is not able to be used for generation of power.)

Figure 3

Qualitative illustration of temperature variation when cold fluid is ejected from the bottom of a heat storage tank during charge

Figure 16

Effect of the rock size on the thermal storage efficiency

Figure 17

Effect of longer charge time on the periodic profile of energy storage in a tank

## Errata

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