Abstract
The correct design of a system of borehole heat exchangers (BHEs) is the primary requirement for attaining high performance with geothermal heat pumps. The design procedure is based on a reliable estimate of ground thermal properties, which can be assessed by a Thermal Response Test (TRT). The TRT analysis is usually performed adopting the Infinite Line Source model and is based on a series of assumptions to which the experiment must comply, including stable initial ground temperatures and a constant heat transfer rate during the experiment. The present paper novelty is related to depth distributed temperature measurements in a series of TRT experiments. The approach is based on the use of special submersible sensors able to record their position inside the pipes. The focus is on the early period of BHE installation, when the grout cement filling the BHE is still chemically reacting, thus releasing extra heat. The comprehensive dataset presented here shows how grout hydration can affect the depth profile of the undisturbed ground temperature and how the temperature evolution in time and space can be used for assessing the correct recovery period for starting the TRT experiment and inferring information on grouting defects along the BHE depth.
Highlights
Nowadays, geothermal heat exchanger facilities are in rapid growth and the research for new instrumentation and surveying methods is in constant evolution in the direction of reliable and low-cost systems.Borehole heat exchangers (BHEs) for ground coupled heat pump (GCHP) applications are the most popular technology in low enthalpy applications, where heat stored in the ground is exploited for heating purposes in building applications
Ground thermal properties are typically inferred during a Thermal Response Test (TRT) experiment, during which a constant flow rate of water is circulated in a pilot borehole heat exchangers (BHEs) while a constant heat transfer rate is supplied to the carrier fluid by some electric heater inside the TRT machine, which is located at the ground surface
Of undisturbed situation later in time) is almost uniform along the BHE depth, suggesting that the heat positionrate from slope andisintercept temperature profile when represented as a function of release perthe unit depth uniform.ofItthe is worth noticing that, provided that the volumetric heat the logarithm of time
Summary
Geothermal heat exchanger facilities are in rapid growth and the research for new instrumentation and surveying methods is in constant evolution in the direction of reliable and low-cost systems. The enOware enterprise developed its integrated sensor (named GEOsniff, [11]), a ball-shaped device able to record temperature and position while travelling along the pipe of a pilot BHE. If a simple and quick way to monitor the heat of hydration effects could be found, it would be possible to complete the BHE test installation with a grouting quality investigation Devices such as submergible sensors could be a good solution for this purpose too. The present work is aimed at developing an experimental technique based on distributed temperature measurements along the depth of pilot BHEs in order to exploit the exothermic hardening reaction of grout for assessing the presence of thermal anomalies along the heat exchanger depth. An additional series of experiments is here discussed for assessing the influence of the hydration heating on TRT experiments, including the estimation of the time needed for assuring a reliable estimation of the ground undisturbed temperature
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