A resistive-capacitive model of pile heat exchangers with an application to thermal response tests interpretation

Abstract

• A new finite cylindrical source model with adiabatic surface is developed. • A new resistive-capacitive semi-analytical pile heat exchanger model is developed. • The new model improves the assessment of pile thermal performance. • Neglecting heat capacitance in the pile leads to an underestimation of performance. • The new model is successfully used to interpret a thermal r esponse test.

A New Approach for Characterizing Pile Heat Exchangers Using Thermal Response Tests

“…The investigation was also undertaken with t min * = 3. Regarding set C, previous studies [12] reported λ m between 1.35 and 1.45 W•K•m −1 depending on the values chosen for t min * (from 5 to 7) and t max (from 250 h to 350 h) so this new analysis is consistent with previous work.…”

“…Initially developed for BHE, the model described in Equations ( 1) and ( 3) do not account for the thermal inertia of the backfilling material. Past work shows that this purely resistive model (Figure 2a) is not suitable for pile heat exchangers of large diameters [11,12], since once λ m and R b have been fitted, the temperature change at small time scales (e.g., 1-10 h) is overestimated by several • C, leading to an underestimation of the transfer capacity of the PHE. Moreover, the duration required to thermally load the backfill material t min * is barely compatible with the operational constraints of a construction site.…”

“…Indeed, considering the typical criteria tmin* = 5 [13] and assuming a soil with thermal properties 𝜆 = 1. Despite these limitations, many authors report application of TRT to PHE to determine the piles and ground properties [11,12]. In these studies, the measured fluid temperature is typically interpreted with the ILS approach, which leads to the exclusion of a significant part of the temperature measurements before tmin* reaches 5.…”

“…It has the key limitations that the pipe arrangements in the borehole are predefined (e.g., "pipes close to the borehole wall") and resistances describing the inner thermal transfer must be pre-calculated with a distinct method, such as finite element code. Once the inner Despite these limitations, many authors report application of TRT to PHE to determine the piles and ground properties [11,12]. In these studies, the measured fluid temperature is typically interpreted with the ILS approach, which leads to the exclusion of a significant part of the temperature measurements before t min * reaches 5.…”

“…Set B was undertaken on a pile of 22.5 cm radius at Richmond, Texas, USA, mostly located in saturated sand [15,19]. Set C was a TRT undertaken on a pile of 30.0 cm radius located in London clay [12]. The inner details of the piles are given in Table 1 and illustrated in Figure 3.…”