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Research Papers

Improving Discharge Characteristics of Indirect Integral Collector Storage Systems With Multielement Storage

[+] Author and Article Information
A. M. Boies

Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, Rolla, MO 65409-0050

K. O. Homan1

Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, Rolla, MO 65409-0050khoman@umr.edu

1

Corresponding author.

J. Sol. Energy Eng 130(2), 021003 (Mar 10, 2008) (9 pages) doi:10.1115/1.2840569 History: Received April 20, 2004; Revised February 15, 2007; Published March 10, 2008

The desired performance of unpressurized integral collector storage systems hinges on the appropriate selection of storage volume and the immersed heat exchanger. This paper presents analytical results expressing the relation between storage volume, number of heat exchanger transfer units, and temperature-limited performance. For a system composed of a single storage element, the limiting behavior of a perfectly stratified storage element is shown to be superior to a fully mixed storage element, consistent with a more general analysis of thermal storage. Since, however, only the fully mixed limit is readily obtainable in a physical system, the present paper also examines a division of the storage volume into separate compartments. This multielement storage system shows significantly improved discharge characteristics as a result of improved elemental area utilization and temperature variation between elements, comparable in many cases to a single perfectly stratified storage element. In addition, the multielement system shows increased robustness with respect to variations in heat exchanger effectiveness and initial storage temperature.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 4

Simplified schematic of an UPICS system composed of multiple fully mixed storage elements with immersed heat exchangers

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Figure 5

Transient outlet temperature profiles, θo(t), for systems composed of different numbers of storage elements J, each with NTUw,j=1.2 and θsh=1.25. The total storage volume ratio is identical for all systems, Vr=2.74, which for the single fully mixed element gives θo(1)=θm. Curve (1a) denotes the fully mixed single-element limit and curve (1b) denotes the perfectly stratified single-element limit.

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Figure 6

Transient outlet temperature profiles, θo(t), for systems composed of different numbers of storage elements J, each with NTUw,j=1.2 and θsh=1.25. The storage volume ratio Vr varies such that each system has te=1. Curve (1a) denotes the fully mixed single-element limit, and curve (1b) the perfectly stratified single-element limit.

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Figure 7

Transient outlet temperature profiles, θo(t), for systems composed of different numbers of storage elements J. The total storage volume ratio is identical for all systems, Vr=2.74, and NTUw=3.2 for each system. Curve (1a) denotes the fully mixed single-element limit, and (1b) the perfectly stratified single-element limit.

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Figure 8

Transient outlet temperature profiles, θo(t), for systems composed of different numbers of storage elements J. The total storage volume ratio is identical for all systems, Vr=2.74. For each system, NTUw=0.95 and θsh=1.25. Curve (1a) denotes the fully mixed single-element limit, and (1b) the perfectly stratified single-element limit.

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Figure 9

Transient outlet temperature profiles, θo(t), for systems composed of different numbers of storage elements, J. The total storage volume ratio is identical for all systems, Vr=2.74 and NTUw is held constant for each system at 4.75. Curve (1a) denotes the fully-mixed single-element limit, and (1b) the perfectly stratified single-element limit.

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Figure 10

Storage volume ratio Vr necessary to provide te=1 for varying numbers of total elements for a fully mixed system for NTUw=1.2

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Figure 11

Variation of minimum volume ratio for NTUw,j=NTUw∕J. Curve (1a) denotes the fully mixed single-element limit, and curve (1b) the perfectly stratified single-element limit.

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Figure 12

Outlet temperatures θo for varying volume ratios Vr with varying numbers of total elements. In all cases, NTUw=1.2.

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Figure 13

Instantaneous storage temperature for a four-element system at selected times with identical volume ratio and effectiveness for each element

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Figure 14

Extinction time te as both the elemental temperature distribution and elemental volume distribution are varied in a four-element system, where θsh,j=θsh,1+(j−1)Δθs, Vs,j=Vs,1+(j−1)ΔVs, NTUw=1.2, and θ¯sh=1.25.

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Figure 15

Extinction time te as the first elemental volume is varied, Vs,1, for varying elemental temperature distributions with NTUw=1.2 and θ¯sh=1.25

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Figure 1

Simplified schematic of an UPICS system composed of a single fully mixed storage element and an immersed heat exchanger

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Figure 2

Dimensional and dimensionless temperature relationships describing the use of the UPICS system in domestic water heating applications

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Figure 3

Simplified schematic of an UPICS system composed of a single perfectly stratified storage element and a counterflow heat exchanger

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