Distributed Thermal Response Tests Using a Heating Cable and Fiber Optic Temperature Sensing

Maria Vélez Márquez,Louis Lamarche,Daniela Blessent,Nataline Simon,Mikael Philippe,Olivier Bour,Jasmin Raymond

doi:10.3390/en11113059

Abstract

Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using heating cable was verified in the field as an option to conduct this heat injection experiment with a low power source and a compact equipment. Two thermal response tests using heating cable sections and a continuous heating cable were performed in two experimental heat exchangers on different sites in Canada and France. The temperature evolution during the tests was monitored using submersible sensors and fiber optic distributed temperature sensing. Free convection that can occur in the pipe of the heat exchanger was evaluated using the Rayleigh number stability criterion. The finite and infinite line source equations were used to reproduce temperature variations along the heating cable sections and continuous heating cable, respectively. The thermal conductivity profile of each site was inferred and the uncertainly of the test was evaluated. A mean thermal conductivity 15% higher than that revealed with the conventional test was estimated with heating cable sections. The thermal conductivity evaluated using the continuous heating cable corresponds to the value estimated during the conventional test. The average uncertainly associated with the heating cable section test was 15.18%, while an uncertainty of 2.14% was estimated for the test with the continuous heating cable. According to the Rayleigh number stability criterion, significant free convection can occur during the heat injection period when heating cable sections are used. The continuous heating cable with a low power source is a promising method to perform thermal response tests and further tests could be carried out in deep boreholes to verify its applicability.

Highlights

Ground coupled heat pump systems, made of a ground heat exchanger (GHE) and a heat pump, are recognized as an efficient and environmentally friendly technology to cool and heat residential and commercial buildings
Thermal conductivity profiles and the errors associated were estimated at two different experimental sites using alternative heating cable methods to conduct the thermal response tests (TRT)
The assessment of free convection in the standing water column suggested the presence of significant convection inside the pipe of the ground heat exchanger during the heat injection period when the heating cable sections were used

Summary

Introduction

Ground coupled heat pump systems, made of a ground heat exchanger (GHE) and a heat pump, are recognized as an efficient and environmentally friendly technology to cool and heat residential and commercial buildings. Thermal response tests (TRTs) have been proposed by Mogensen [3] as an in situ method to assess the subsurface thermal conductivity and the borehole thermal resistance. TRT analysis is usually performed using the infinite line source model, an analytical solution to the heat conduction equation [11,12]. This one-dimensional conductive heat transfer solution describes the mean temperature increment at a radial distance from an infinite source of heat having a constant heat flow rate and assuming a constant temperature at an infinite radial distance from the source

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