Evaluation of Subsurface Heat Capacity through Oscillatory Thermal Response Tests

Abstract

Two methods are currently available to estimate in a relatively short time span the subsurface heat capacity: (1) laboratory analysis of rock/soil samples; (2) measure the heat diffusion with temperature sensors in an observation well. Since the first may not be representative of in-situ conditions, and the second imply economical and logistical issues, a third option might be possible by means of so-called oscillatory thermal response tests (OTRT). The aim of the study was to evaluate the effectiveness of an OTRT as a tool to infer the subsurface heat capacity without the need of an observation well. To achieve this goal, an OTRT was carried out in a borehole heat exchanger (BHE). The total duration of injection was 6 days, with oscillation period of 12 h and amplitude of 10 W m−1. The results of the proposed methodology were compared 3-D numerical simulations and to a TRT with a constant heat injection rate with temperature response monitored from a nearby observation well. Results show that the OTRT succeeded to infer the expected subsurface heat capacity, but uncertainty is about 15% and the radial depth of penetration is only 12 cm. The parameters having most impact on the results are the subsurface thermal conductivity and the borehole thermal resistance. The OTRT performed and analyzed in this study also allowed to evaluate the thermal conductivity with similar accuracy compared to conventional TRTs (3%). On the other hand, it returned borehole thermal resistance with high uncertainty (15%), in particular due to the duration of the test. The final range of heat capacity is wide, highlighting challenges to currently use OTRT in the scope of ground-coupled heat pump system design. OTRT appears a promising tool to evaluate the heat capacity, but more field testing and mathematical interpretation of the sinusoidal response is needed to better isolate the subsurface from the BHE contribution and reduce the uncertainty.

Estimation of In-Situ Heat Capacity and Thermal Diffusivity from Undisturbed Ground Temperature Profile Measured in Ground Heat Exchangers.

“…Another common simple practice is to arbitrarily select a literature-based in-situ HC that matches the geological description of materials found in a borehole [7][8][9][10]. However, a study carried out by Giordano et al [11] revealed that typical uncertainty associated to the in-situ TD is about ± 40% when a conventional TRT is analyzed with an approximated HC for common geological materials (1.5 to 3.2 MJ m −3 K −1 ). Therefore, alternative field methods to infer heat storage properties is still need.…”

“…Therefore, alternative field methods to infer heat storage properties is still need. Oscillatory TRT (OTRT) proposed and discussed by, for instance, Oberdorfer [12], and applied by Giordano et al [11], have the potential to evaluate the in-situ TD and HC. Although this methodology is an important step taken towards the in-situ evaluation of this thermal property; it still remains complex because of the analysis of the oscillating temperature response and needs more improvement to significantly reduce uncertainty (currently on the order of ± 15%).…”

“…Other alternatives involve the analysis of ground temperature profiles to characterize geological properties [4,11,[13][14][15][16]. Ground temperature profiles can be acquired with a submersible probe that is lowered into the BHE during or after a TRT⦋15-16⦌.…”

“…The main reason is the time required to continuously measure the ground temperature profiles during several days to a year, in order to record a periodic cycle of heat diffusion in the subsurface [10]. As a matter of fact, field measurements of prefeasibility studies for the design of ground-coupled heat pumps must be conducted within a few days, for instance 2-3 days when considering the heating period of conventional TRT or up to 5 days if the recovery phase of the TRT is included in the analysis [1][2][3][4]10,11,16,33]. Recently, Márquez et al [10] proposed a methodology for the indirect evaluation of the in-situ TD.…”

“…All the aforementioned difficulties in assessing accurately and quickly the in-situ HC can eventually contribute to increasing errors in the design of geothermal heat pump systems and ultimately impact their installation cost [4,11]. Therefore, evaluating the in-situ HC can be useful for simulating the operation of BHEs used for both ground-coupled heat pump and underground thermal energy storage systems [11]. In fact, Giordano et al [11] study indicated that the total drilling length of BHEs calculated when designing a groundcoupled heat pump system can be affected by ± 6-7%, which influences the total system cost by 3-4%.…”