A computationally efficient numerical model for heat transfer simulation of deep borehole heat exchangers

Based on discussions at the international workshop on “Thermoactive geotechnical systems for near-surface geothermal energy”, hosted at Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland (http://www.olgun.cee.vt.edu/workshop/), this article attempts to provide a broad overview of the analysis methods used for evaluation of systems that use either boreholes or geo-structures for heat exchange. It identifies commonalities where knowledge transfer from the former to the latter can be made, and highlights where there are significant differences that may limit this cross-fertilisation. The article then focusses on recent developments and current understanding pertaining to the analysis of the thermo-mechanical interaction between a geostructure and the ground, and how this may be incorporated into the geotechnical design of energy geostructures.

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Analysis and design methods for energy geostructures

https://escholarship.org/content/qt52r093vw/qt52r093vw.pdf?t=p8gzyu

Comparative study of the cooling energy performance of variable refrigerant flow systems and variable air volume systems in office buildings

https://escholarship.org/content/qt60q7n0h5/qt60q7n0h5.pdf?t=q56ilx

Energy geostructures: A review of analysis approaches, in situ testing and model scale experiments

A critical component of any ground-source heat pump (GSHP) system is the ground heat exchanger (GHE). For many residential systems and nearly all commercial and institutional systems, the GHE will consist of one or more vertical boreholes with inserted tubing. Most commonly this is a single or double U-tube made of high-density polyethylene, but other configurations are possible. The complex nature of the ground heat transfer and the long time constant of the ground create some challenges in designing vertical borehole GHEs. This chapter covers methods for design of these GHEs.

Vertical borehole ground heat exchanger design methods

This work presents a validation of two common methods for designing vertical ground heat exchangers. Both a simulation-based design tool and the ASHRAE handbook design equation are used to find design lengths for four different real systems, using actual experimental data, including building loads as well as physical parameters as inputs. The measured minimum and maximum ground heat exchanger exiting fluid temperatures were used as the design constraint. The simulation-based design tool predicted the borehole length to within 6% in all cases, while the ASHRAE handbook design equation yielded systems with errors from –21% to 167%. Most of this error can be explained by the way loads are represented in the ASHRAE handbook equation, with differences in the borehole thermal resistance also playing a smaller part. The ASHRAE handbook equation relies on a very simple load representation; although this allows it to be used as a simple hand calculation, it also precludes it achieving acceptable accuracy. It does ...

/pdf/validation-of-vertical-ground-heat-exchanger-design-1yxvhc0gqp.pdf

Laura Southard

Papers

Validation of vertical ground heat exchanger design methodologies

Performance of HVAC Systems at ASHRAE HQ: Part 2

Performance of the HVAC Systems at the ASHRAE Headquarters Building, Part 1: Measured Energy Usage