Contents lists available at ScienceDirect Geothermics journal homepage: www.elsevier.com/locate/geothermics Pipe–pipe thermal interaction in a geothermal energy pile Abubakar Kawuwa Sani a, ⁎ , Rao Martand Singh a , Cristina de Hollanda Cavalcanti Tsuha b , Ignazio Cavarretta a a Department of Civil and Environmental Engineering, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, GU2 7XH, UK b Department of Geotechnical Engineering, University of São Paulo at São Carlos, Av. Trabalhador Sãocarlense, 400, São Carlos, SP-13566-590, Brazil ARTICLE INFO Keywords: Ground source heat pumps Thermal pile Numerical modelling Thermal interaction ABSTRACT The use of energy loop(s), fitted into the structural foundation piles, also known as geothermal energy piles (GEPs) is on the rise. This dualizes the role of the piles in meeting the structural performance and the thermal comfort demand of the overlying structure. Heat carrier fluid (HCF) is circulated through the loops, to extract or reject heat energy into the ground, during the space heating or cooling operation. However, this results in thermal interaction between the inlet and outlet leg of the loop. This paper presents a numerical study to investigate the pipe–pipe thermal interaction between the inlet and outlet loop–legs. It was found that factors such as the number of loops, pipe location, soil and concrete thermal conductivity have a significant influence on the magnitude of thermal interaction between inlet and outlet pipes. Similarly, it was found that the central steel bar, used in contiguous flight auger (CFA) piles, contributes towards higher thermal interaction. 1. Introduction The use of pile foundation elements as closed loop heat exchangers, also known as geothermal energy piles (GEPs), thermal piles or pile heat exchangers (PHEs), coupled to a heat pump has long been re- cognized among the most energy-efficient means of achieving space heating and cooling in residential and commercial buildings. The cou- pling process connects the heat pump to the heat exchanger via the use of high density poly-ethylene (HDPE) plastic pipes incorporated into the GEP. Within the GEP element, the HDPE pipes are often installed in a U- shape manner and attached to the structural reinforcement cage prior to lowering of the steel reinforcement cage into the bored hole and concreting. Alternatively, in a contiguous flight auger (CFA) piles, the HDPEs are bunched together around a central steel bar for rigidity and plunged into the fresh concrete prior to concrete setting. Inside the HDPEs, heat carrier fluid (HCF) which consists of water or water plus antifreeze based solution is circulated using the heat pump to transfer heat from the building to the ground via the GEPs during space cooling or vice-versa in space heating operation. The combined system com- prises the ground heat exchanger also known as the primary unit (high density poly-ethylene (HDPE) loops and foundation element), heat pump unit and the secondary unit (which transfer heat from the heat pump to building) and are generally referred to as ground source heat pump (GSHP) system, shown in Fig. 1. In the GSHP system, the major part of the initial capital cost is at- tributed to the GEP element (Lamarche et al., 2010). If the GEP is under designed, the system will fail shortly in the early year(s) of operation. On the other hand, if overdesigned, the system becomes unnecessarily expensive and less energy efficient. Therefore, it is highly imperative to ensure that the GEP is carefully designed, with the HDPE loops ap- propriately situated, to ensure that a cost–effective and efficient unit, which continuously exchange heat with the surrounding soil, is in- stalled. This is achieved by considering the heat transfer within the soil domain as a transient phenomenon, due to its semi-infinite nature and its thermal capacity. Whereas, within the GEP unit, a steady state ap- proach is often assumed, with uniform radial and circumferential temperature distribution, due to the finite dimension of the heat ex- changer. However, numerous studies have shown that the distribution of the radial and circumferential temperature in a GEP are not uniform (Sani et al., 2018a; Loveridge and Powrie, 2014), as a result of factors such as HDPE pipes configuration (shown in Fig. 2), number of loops and their location (Park et al., 2013; Jalaluddin et al., 2011; Hamada et al., 2007; Gao et al., 2008a, b; Mehrizi et al., 2016; Zarrella et al., 2013a, b; Batini et al., 2015). Nonetheless, they are found to offer po- sitive significance towards heat transfer if appropriate number are in- stalled and suitably placed within the GEP. In addition, numerous studies were carried out on improving the heat transfer of the GEP by adding additives to the concrete mix to https://doi.org/10.1016/j.geothermics.2019.05.004 Received 21 September 2018; Received in revised form 9 December 2018; Accepted 7 May 2019 ⁎ Corresponding author. E-mail address: a.sani@surrey.ac.uk (A.K. Sani). Geothermics 81 (2019) 209–223 0375-6505/ © 2019 Elsevier Ltd. All rights reserved. T