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Research Papers

A Simulink®-Based Building Load-Ground Source Heat Pump Model Used to Assess Short-and Long-Term Heat Pump and Ground Loop Performance

[+] Author and Article Information
Jonathan L. Gaspredes, Tess. J. Moon

Department of Mechanical Engineering,
University of Texas at Austin,
Austin, TX 78712

Glenn. Y. Masada

Department of Mechanical Engineering,
University of Texas at Austin,
Austin, TX 78712
e-mail: masada@mail.utexas.edu

1Corresponding author.

Manuscript received February 13, 2013; final manuscript received November 19, 2013; published online January 24, 2014. Assoc. Editor: S. A. Sherif.

J. Thermal Sci. Eng. Appl 6(2), 021013 (Jan 24, 2014) (10 pages) Paper No: TSEA-13-1032; doi: 10.1115/1.4026081 History: Received February 13, 2013; Revised November 19, 2013

An integrated building load-ground source heat pump model is developed to capture short-term (30 s) and long-term (10–20 yr) performance of ground source heat pumps with vertical boreholes. The model takes advantage of the built-in computation and organization functions of the simulink®/matlab environment to couple the component building load, heat pump, and ground loop models at every time step. The building load model uses the HAMBASE thermal program and is applicable to residential and commercial buildings. The heat pump model uses manufacturer data and sensible heat corrections to accurately model heat pump operation across a wide range of input conditions. The ground loop model is a combination of Hellstrom's borehole tube model, Eskillson's long-term (>10 yr) g-function ground model and the one-dimensional, short-term (<5 min) numerical ground model by Xu. Fifteen year simulation results for a base case residential house are presented to illustrate the integrated model's ability to predict a wide range of time responses and to illustrate a limiting ground loop sizing criterion that reveals the slow degradation in system performance due ground heating effects. Simulations with varying borehole lengths also illustrate the sensitivity of ground loop sizing on the system's thermal and economic performances. The work emphasizes the importance of proper borehole sizing, design, and placement especially in cooling-dominated climates, where the unbalance of heat loads to the ground cause slowly rising ground temperatures.

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References

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Figures

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Fig. 1

Building, heat pump, and ground loop components of a GSHP system

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Fig. 2

g-functions for typical borehole configurations—data from GLHEPRO 4.0 [9]

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Fig. 3

(Left) q(t) and its piecewise approximation, (right) resulting temporal superposition of q(t)

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Fig. 4

Monthly average water temperature entering ground loop (-GL) and heat pump (-HP), GLHEPRO and IBL-GSHP ground loop models, Jan.–Dec. with full internal loads

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Fig. 5

Base case—building zone 1 and zone 2 air and wall temperatures, June 30, 12:00–15:00

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Fig. 6

Base case—average hourly heat rate rejected to ground loop water by heat pump (positive—cooling. negative—heating), Year 1, Jan–Dec

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Fig. 7

Base case—monthly mean, max and min heat pump EWT, 15 yr, based on hourly averages

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Fig. 8

Base case—annual and peak monthly heat pump electricity used over 15 yr

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Fig. 9

Maximum yearly heat pump EWT for different borehole lengths, based on hourly averages

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Fig. 10

Total annual cooling for different borehole lengths

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