Assessment of Ground Loop Performance in Texas

Photo of Dave Gramlich, Dylan Smith, and Sam Van Metre Students: Dave Gramlich, Dylan Smith, and Sam Van Metre

Sponsor: The University of Texas at Austin

Date: Fall 2009

Requirements:
The team must collect soil data for 10 Texas population centers, and rank the cities on how well the soil can dissipate heat. A common ground loop sizing program, Geodesigner, must be used to run a set of case studies to size loops for various Texas regions. The MATLAB model must model transient effects for a one year period and compute the average heat rate into the ground as well as a temperature profile of the system.

Problem:
The project's goal is to evaluate the performance of geothermal vertical ground loop systems in Texas based on technical and economical feasibility. The team must design, collect data for, and demonstrate a protocol to calculate a ground loop's thermal load. This protocol makes use of MATLAB to solve a finite difference heat transfer model.

Solution:
Three models were developed to meet the requirements and constraints of the project: a bond graph model, a line source theory model, and a finite difference model. The bond graph model yielded the time constant, τ, based on a given city's soil properties. The population regions were then ranked by τ, with a lower value indicating that the ground could dissipate heat faster. A line source theory model was developed in Excel that graphs the distance that heat dissipates from a specified depth line heat source over time. The finite difference model was coded in MATLAB and used both matrix inversion and the Gauss-Seidel iteration method to solve for a temperature profile of a single vertical ground loop circuit over a one year time period. Three case studies were run in Geodesigner for three different cities. \Results from Geodesigner and all three models were compared to draw conclusions and validate the MATLAB finite difference model.

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