Analysis and design methods for energy geostructures

Abstract

Based on discussions at the international workshop on “Thermoactive geotechnical systems for near-surface geothermal energy”, hosted at École Polytechnique Fédérale 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.

Introduction

The use of the ground as a means for managing the thermal loads within buildings is a well-established technology and borehole heat exchange systems have been used for several decades, especially following the “oil shocks” of the 1970s.

Worldwide installed Ground Source Heat Pump (GSHP) capacity is estimated to have increased nearly twenty-fold between 1995 and 2010, from about 1854 MW_th to 35,236 MW_th and more than doubled from 15,384 MW_th in 2005 [1]. To the end of 2012, installed capacity of GSHP and Underground Thermal Energy Storage (UTES) systems in Europe, was estimated to total approximately 16,500 MW_th [2]. Lund et al. [1] annualise the growth in this period to a rate of about 20% and Antics et al. [2] suggest that growth within the geothermal energy sector in Europe, which is dominated by GSHP systems, was estimated to be about 30% in the two years to 2015.

While the borehole heat exchange technique is well established, continuing research and development is focussed on reducing installation costs, i.e. speed/ease of installation, improved borehole heat transfer and heat pump efficiency, and more refined models for use in design [3], [4], [5].

The GSHP and UTES installations referred to in the above figures are entirely borehole based systems; increasingly, however, designers and developers are looking to use engineering structures where heat absorber pipes are integrated within structures in contact with the ground, as the means for providing thermal exchange with the ground. These applications have been referred to variously as energy foundations, thermo-active ground structures [6], geothermal piles, heat exchanger piles and energy geostructures [7] – this latter will be used here.

Potentially, this use of energy geostructures (EGS), that are in any case needed to support buildings (i.e. raft/mat foundations, piles) or the ground itself (i.e. retaining walls and tunnels), Fig. 1, can help to facilitate the implementation of GSHP technology on confined urban sites and reduce the initial capital costs of installation, by eliminating borehole construction [6].

The first application of heat exchange via foundation elements was in Austria and Switzerland; shallow foundation elements such as ground bearing slabs and shallow basement walls were first utilised for energy exchange, and these were quickly followed by bearing piles (mid-1980s), diaphragm walls (mid-1990s) and then tunnels (early-2000s) [6], [8]. Subsequently, and in particular since the late 1990s, many projects have been completed in Germany, the United Kingdom, and an increasing number of other countries in Europe and around the World. No collated figures are available; however, the thermal capacity of EGS currently installed is a small fraction of the total shallow geothermal installed – maybe in the range of 100 to 200 MW_th, or less than 1% of total installed capacity.

While this application for heat exchange with the ground is proving compelling on a number of grounds and has already been used in a number of differing configurations, its uptake has been impeded by many of the same factors that have affected the uptake of borehole heat exchangers – initial capital cost, a lack of visibility amongst potential end-users, legislators and design professionals, a need for a more fundamental understanding of material and component response to thermal loading, and a lack of validated analysis and design procedures.

These problems were highlighted and explored during an international workshop on “Thermoactive geotechnical systems for near-surface geothermal energy”, hosted at École Polytechnique Fédérale de Lausanne (EPFL), Switzerland in March 2013 (http://www.olgun.cee.vt.edu/workshop/). About 70 individuals from both research and industrial backgrounds attended the workshop, and this paper results from the discussions relating to the issue of the validation of design tools for energy geostructures, see [9] for a report on this particular session and the same issue of the Journal of the Deep Foundations Institute (DFI) for reports on the other sessions of the workshop.

This article attempts to provide a broad overview of the analysis methods used for evaluation of both borehole heat exchanger (BHE) and EGS heat exchange systems, to identify commonalities where knowledge transfer from the former to the latter can be made, and to highlight where there are significant differences that may limit this cross-fertilization. The article then focusses on recent developments and current understanding pertaining to the analysis of the thermo-mechanical interaction between the geostructure and the ground, and how this may be incorporated into the geotechnical design of EGS.