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Research Papers

Transient and Rate-Dependent Performance of Conventional Electric Storage Water Heating Systems

[+] Author and Article Information
J. W. McMenamy

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

To whom correspondence should be addressed.

J. Sol. Energy Eng 128(1), 90-97 (Jan 18, 2005) (8 pages) doi:10.1115/1.2148974 History: Received April 23, 2004; Revised January 18, 2005

Electric resistance water heaters are relatively simple and are therefore one of the most common water heating configurations. Due to constraints on the allowable instantaneous electrical power draw, most electric water heating systems incorporate a sizable thermal storage component. The inherently unsteady storage component therefore has an overwhelming impact on the system behavior. In this investigation, a residential-scale electric storage water heater was tested across a range of flow rates for both powered and nonpowered discharge processes as well as for charge processes with no throughflow. The flow dynamics internal to the storage volume is shown to be strongly multidimensional and transient, especially when the internal heating elements are energized. Comparison of the measured data to the performance limits of a system with a fully mixed or a perfectly stratified storage element reveals that the conventional system operates relatively near to the fully mixed limit. As a result, there appears to be significant potential for improvements in system performance through reductions in the level of thermal mixing internal to the storage volume.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of dimensional quantities measured by the electronic data acquisition system

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Figure 2

Simplified flow schematic of experimental setup

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Figure 3

Time traces of dimensionless temperature, θ(t), and electrical power input, ψ(t), during charging with no throughflow, φ(t)≡0

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Figure 4

Time traces of dimensionless temperature, θ(t), and fractional flow rate, φ(t), during a discharge process with no electrical energy input, ψ(t)≡0. In this experiment, φ¯=0.15 and ΔTr=0.68.

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Figure 5

Comparison of measured acceptable discharge time, td,*, to limit behaviors as a function of fractional flow rate, φ¯, for the nonpowered discharge process. In all cases, θm=0.675. Curve (1) represents the perfectly stratified limit and curve (2) represents the fully mixed limit.

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Figure 6

Evolution of the average thermocline thickness, δa≡δ̂a∕H, at select flow rates in nonpowered discharge process. In both cases, the thermocline thickness was computed for θe=0.90.

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Figure 7

Time traces of dimensionless temperature, θ(t), and fractional flow rate, φ(t), during a discharge process with simultaneous electrical energy input, ψ(t). In this experiment, φ¯=0.15, ΔTr=0.83, and ψ¯=0.89.

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Figure 8

Time traces of dimensionless temperature, θ(t), and fractional flow rate, φ(t), during a discharge process with simultaneous electrical energy input, ψ(t). In this experiment, φ¯=0.66, ΔTr=0.86, and ψ¯=0.92. The θ8(t) profile is not shown, as it reflected a localized inflow of cold water through a small bleed-off hole in the dip tube.

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Figure 9

Comparison of measured acceptable discharge time, td,*, to performance limits as a function of fractional flow rate, φ¯, for the powered discharge process. The maximum flow rate at which the energy withdrawal rate matches the electrical energy input rate is φ¯*=0.28. In all cases θm=0.675. Curve (1) represents the perfectly stratified limit and curve (2) represents the fully mixed limit for which ψ=1 and ΔTs,r=0.68.

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Figure 10

Time trace of computed instantaneous energy residual, R(t), overlaid with dimensionless internal temperature profiles, θ(t). In this experiment, φ¯=0.32, ΔTr=0.70, and ψ≡0.

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