Improving ground heat exchangers for geothermal heat pump systems with a groundwater-filled borehole and a thermally enhanced double U-pipe

Jasmin Raymond, Jean-Marc Ballard, Claude Hugo Koubikana Pambou, Patrice Kwemo, Jean-François Lavoie

Dans les comptes rendus d’articles de la conférence: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Sustainable Geotechnics

ABSTRACT: Ground heat exchangers (GHEs) used for geothermal heating and cooling systems in North America, more specifically ground-coupled heat pump systems, are commonly made of a single U-pipe installed in a borehole backfilled with grout. This design can be improved to minimize the borehole thermal resistance and increase the system performance. A backfill free GHE with a double U-pipe made of thermally enhanced high-density polyethylene and allowing internal free convection of the groundwater inside the borehole was installed at INRS laboratories to verify this alternative design inspired by Scandinavian methods. A thermal response test was performed to evaluate the borehole thermal resistance. Analysis revealed, in best operating conditions, a borehole thermal resistance equal to 0.05 m K W-1, which is 43 % lower than the thermal resistance inferred for a single U-pipe GHE installed at the same site with the conventional design method.

RÉSUMÉ: Les échangeurs de chaleur géothermique (ÉCG) utilisés pour les systèmes de chauffage et de climatisation en Amérique du Nord, plus précisément les systèmes de pompe à chaleur couplés au sol, sont couramment faits installé dans un forage remblayé avec du coulis. Cette conception peut être améliorée en minimisant la résistance thermique du forage et augmentant ainsi la performance du système. Un ÉCG libre de matériaux de remplissage avec un double tube en U, fait de polyéthylène de haute densité amélioré sur le plan thermique, facilitant la convection naturelle de ldans le forage eption alternative, inspirée des méthodes scandinaves. Un test de réponse thermique a été réalisé pour évaluer la résistance thermique du analyse indique, dans les meilleures 0,05 m K W-1, ce qui est 43 % inférieur à la résistance inférée pour un ÉCG ayant une conception conventionnelle avec un seul tube en U et installé sur le même site. 1. INTRODUCTION Ground-coupled heat pump (GCHP) systems offer an energy-efficient heating and cooling alternative for buildings. GCHP can actually help achieve energy savings on the order of 60-70 % of the heating and 30-40 % of the cooling needs. The installation cost of such system is however important (Robert and Gosselin, 2014), especially when compared to other heating and cooling systems that do not require ground heat exchangers (GHEs). Care should be taken when designing geothermal systems to minimize GHE length and deliver cost-competitive GCHPs. Possible alternatives are to reduce the heating and cooling loads imposed to the ground loop with hybrid technologies, increase distances between GHE and decrease the borehole thermal resistance. Sizing methods, for example that described by Philippe et al. (2010), can yield a 10 to 30 % borehole length reduction with a significant decrease of the borehole thermal resistance. Conventional materials and pipe configuration used by the North American geothermal industry are indeed quite resistive and do not offer an optimal thermal performance. GHE are commonly made with a single U-pipe of high-density polyethylene (HDPE) standing in a borehole filled with a grout composed of silica sand, bentonite and water. Efforts have been made to improve the grout thermal conductivity (Allan and Kavanaugh, 1999; Carlson, 2000; Borinaga-Treviño et al., 2013b, 2013a) or the pipe diameter and configuration (Raymond et al., 2015) to reduce the borehole thermal resistance, but often opting for boreholes filled with solids. In Scandinavia, GHE are commonly filled by groundwater only (Gustafsson and Westerlund, 2011; Gehlin et al., 2016). The idea is that natural convection due to vertical temperature differences in groundwater-filled boreholes can help enhance heat transfer (Spitler et al., 2016). This concept was combined with thermally enhanced pipe made of HDPE containing carbon type nanoparticles (Gosselin et al., 2017) to verify the applicability of Scandinavian GHE design in the St. Lawrence Lowlands, which is the largest geothermal market in Canada (Canadian GeoExchange Coalition, 2012). The objective of this project was, therefore, to evaluate the borehole thermal resistance reduction provided by a groundwater-filled borehole, which was actually improved by using a double thermally enhanced U-pipe. The enhanced GHE was installed at INRS geothermal pilot site, where a conventional GHE made with a single U-pipe and filled with grout had previously been installed and subject to a thermal response test (TRT; Ballard et al., 2016; Raymond et al., 2017). A new TRT was

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