Temperature response functions (G-functions) for single pile heat exchangers

Abstract

Foundation piles used as heat exchangers as part of a ground energy system have the potential to reduce energy use and carbon dioxide emissions from new buildings. However, current design approaches for pile heat exchangers are based on methods developed for boreholes which have a different geometry, with a much larger aspect (length to diameter) ratio. Current methods also neglect the transient behaviour of the pile concrete, instead assuming a steady state resistance for design purposes. As piles have a much larger volume of concrete than boreholes, this neglects the significant potential for heat storage within the pile. To overcome these shortcomings this paper presents new pile temperature response functions (G-functions) which are designed to reflect typical geometries of pile heat exchangers and include the transient response of the pile concrete. Owing to the larger number of pile sizes and pipe configurations which are possible with pile heat exchangers it is not feasible to developed a single unified G-function and instead upper and lower bound solutions are provided for different aspects ratios. (172 words)

Long term monitoring of CFA energy pile schemes in the UK

“…This causes the apparent increase in resistance. The implications of the absence of a thermal steady state within the pile are described by Loveridge & Powrie (2013b). Assuming a steady state where none is present can lead to the overestimation of the temperature changes that will occur within the pile and the ground.…”

The Thermal Behaviour of Three Different Auger Pressure Grouted Piles Used as Heat Exchangers

“…However, in pile heat exchangers, depending on their size, this assumption is usually invalid and the concrete or grout may take a numbers of days to reach steady state . As a result, new concrete G-functions have been proposed to allow calculation of the temperature changes within the pile as a function of time (Loveridge & Powrie, 2013). These functions depend on the pile geometry and examples are given in Figure 2, which shows how the resistance of the concrete part of the pile (R c ) increases with Fo.…”

“…Also plotted on Figure 1 is a pair of empirical temperature response functions known as pile G-functions (Loveridge & Powrie, 2013). These give upper and lower bound solutions based on a range of numerical simulations designed to cover a realistic range of common pile heat exchanger geometries.…”

“…The concrete G-function additionally assumes that the pipes are placed near the centre of the pile. Full details of the pile and concrete G-functions are given in Loveridge & Powrie (2013). …”

“…While this technology is now being employed more routinely and is starting to be represented in codes and standards (e.g NHBC, 2010, GSHPA, 2012 there is still scope for improving design and analysis methods (e.g. Bourne-Webb et al, 2013, Loveridge & Powrie, 2013. In particular most thermal design proceeds on the basis of two key input parameters: the pile thermal resistance (R b ) and the surrounding soil thermal conductivity ( g ).…”