Research Papers

Numerical Simulation of Heat Pipe-Assisted Latent Heat Thermal Energy Storage Unit for Dish-Stirling Systems

[+] Author and Article Information
Hamidreza Shabgard

Department of Mechanical Engineering,
University of Connecticut,
191 Auditorium Road,
Storrs, CT 06269
e-mail: h.shabgard@engr.uconn.edu

Amir Faghri

Fellow ASME
Department of Mechanical Engineering,
University of Connecticut,
A. B. Bronwell Building Room 123,
Storrs, CT 06269
e-mail: faghri@engr.uconn.edu

Theodore L. Bergman

Fellow ASME
Department of Mechanical Engineering,
The University of Kansas,
3144B Learned Hall,
Lawrence, KS 66045
e-mail: tlbergman@ku.edu

Charles E. Andraka

Concentrating Solar Technologies,
Sandia National Laboratories,
Albuquerque, NM 87185-1127
e-mail: ceandra@sandia.gov

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING. Manuscript received July 22, 2013; final manuscript received October 18, 2013; published online December 19, 2013. Assoc. Editor: Nesrin Ozalp.

J. Sol. Energy Eng 136(2), 021025 (Dec 19, 2013) (12 pages) Paper No: SOL-13-1210; doi: 10.1115/1.4025973 History: Received July 22, 2013; Revised October 18, 2013

A two-dimensional numerical model is developed to simulate the transient response of a heat pipe-assisted latent heat thermal energy storage (LHTES) unit integrated with dish-Stirling solar power generation systems. The unit consists of a container which houses a phase change material (PCM) and two sets of interlaced input and output heat pipes (HPs) embedded in the PCM. The LHTES unit is exposed to time-varying concentrated solar irradiance. A three-stage operating scenario is investigated that includes: (i) charging only, (ii) simultaneous charging and discharging, and (iii) discharging only. In general, it was found that the PCM damps the temporal variations of the input solar irradiance, and provides relatively smooth thermal power to the engine over a time period that can extend to after-sunset hours. Heat pipe spacing was identified as a key parameter to control the dynamic response of the unit. The system with the greatest (smallest) heat pipe spacing was found to have the greatest (smallest) temperature drops across the LHTES, as well as the maximum (minimum) amount of PCM melting and solidification. Exergy analyses were also performed, and it was found that the exergy efficiencies of all the systems considered were greater than 97%, with the maximum exergy efficiency associated with the system having the minimum heat pipe spacing.

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Fig. 1

Proposed integration of LHTES with a dish Stirling system [1]

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Fig. 2

3D unit modules, (a) comprised of one tube associated with a receiver-to-PCM heat pipe and 4 quarter section tubes associated with PCM-to-engine heat pipes, (b) quarter section of the unit module shown in (a)

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Fig. 3

2D unit modules, (a) Configuration 1 and (b) Configuration 2

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Fig. 4

Physical basis of Configuration 2, (a) the actual physical system with PCM domains associated with input and output HPs identified, and (b) PCM domains separated to construct a 2D model on the sectioning plane A-A

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Fig. 5

Heat pipe-PCM configuration, (a) the physical model including input and output heat pipes and the PCM, (b) the computational domain and identification of boundary locations (not to scale)

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Fig. 6

Average hourly direct normal irradiance data (Aug. 2010 at the El Toro Marine Corps Air Station in California) derived from data provided by NREL [28]

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Fig. 7

Evolution of the temperature field throughout the heat pipe assisted-LHTES system with heat pipe spacing of 0.07 m (temperatures are in °C)

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Fig. 8

Streamlines within the HPs for the system with HP spacing of 0.07 m at 10 a.m.

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Fig. 9

Temporal variations of input solar thermal power to the storage unit and thermal power supplied to the engine by the storage unit, as well as variations of the volume fraction of molt for various heat pipe spacing values

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Fig. 10

Variations of vapor temperature of receiver-to-PCM and PCM-to-engine heat pipes versus time for various heat pipe spacing values

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Fig. 11

Variations of the exergy efficiency of the storage unit for various HP spacing values versus the time of day from sunrise to 1 a.m.

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Fig. 12

Schematic of a section of a heat pipe surrounded by PCM; Computational domain to investigate the heat transfer in a fin-PCM system is enclosed by dashed line

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Fig. 13

Stored thermal energy in the PCM layer and two neighboring half fins obtained with and without considering conjugate and natural convection effects



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