Abstract
The design and field test of a novel sensor system based in autonomous wireless sensors to measure the temperature of the heat transfer fluid along a borehole heat exchanger (BHE) is presented. The system, by means of two specials valves, inserts and extracts miniaturized wireless sensors inside the pipes of the borehole, which are carried by the thermal fluid. Each sensor is embedded in a small sphere of just 25 mm diameter and 8 gr weight, containing a transceiver, a microcontroller, a temperature sensor and a power supply. A wireless data processing unit transmits to the sensors the acquisition configuration before the measurements, and also downloads the temperature data measured by the sensor along its way through the BHE U-tube. This sensor system is intended to improve the conventional thermal response test (TRT) and it allows the collection of information about the thermal characteristics of the geological structure of subsurface and its influence in borehole thermal behaviour, which in turn, facilitates the implementation of TRTs in a more cost-effective and reliable way.
Highlights
In recent years the growth in the number of air conditioning systems driven by ground source heat pumps (GSHP) or geothermal heat pumps (GHP) is estimated between 10% and 30% each year [1]
The most remarkable effect of the use of ground heat exchange through a borehole heat exchanger (BHE) is the reduction of electrical energy consumption, compared with the consumption of the standard technology based on air heat exchange
The sensor system presented in this work contributes to this issue by offering the possibility of measuring BHE thermal properties inside the U-tubes dynamically, while performing the thermal response test (TRT)
Summary
In recent years the growth in the number of air conditioning systems driven by ground source heat pumps (GSHP) or geothermal heat pumps (GHP) is estimated between 10% and 30% each year [1]. The BHE length needed for a given output power greatly depends on subsurface characteristics, such as temperature, particle size and shape, moisture content, and heat transfer coefficients Due to these factors the completion of a thermal response test (TRT), determining ground thermal properties, is very important. The knowledge of the subsurface geological structure may have some importance for a proper sizing of the BHE length in order to minimize the deployment costs, especially for large GHP systems [19] Using this information it could be possible to stop the boreholes before they reach a weakly conductive layer, and use more shorter boreholes instead of fewer long boreholes, increasing the system efficiency and reducing the deployment costs, With this idea in mind, several works have explored alternative methods to perform TRT but measuring the temperature distribution along the U-tubes. Only one probe is required to get the temperature profile, and it can be reused several times to perform new measurements even in different GHP facilities
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