Resumen de Novel method and instruments for the optimal techno-economic sizing of borehole heat exchangers
The thermal response test (TRT) is widely used as a standard test to characterize the thermal properties of the ground near a borehole heat exchanger (BHE). Typical methods to interpret the results apply analytical or numerical solutions which assume that the ground is infinite, homogeneous and isotropic. However, in reality the underground is commonly stratified and heterogeneous, and therefore thermal properties may significantly vary with depth. In this sense and with the intention to overcome standard TRT limitations, this Ph.D. study is focused on developing methods and instruments for the evaluation of the heat transfer behavior of the geological layers surrounding a BHE. This information is key for the optimal energy efficiency and techno-economic sizing of BHE.
In particular, a novel TRT method, called observer pipe TRT (OP-TRT), is proposed based on an additional temperature measurement along an auxiliary pipe. In the last decades, some researchers developed the so-called distributed TRT (DTRT) by measuring the temperature along the length of the heated U-pipe. However, from the studies carried out in this Ph.D. work, the observer pipe demonstrated to amplify the thermal effects produced due to geological layers with different thermo-physical properties, hence requiring less accurate sensors for obtaining more detailed results. Based on this achievement, an inverse numerical solution was developed to parametrize thermal conductivity of geological layers from the measurements along the observer pipe. Basically, the model adjusts thermal conductivity of the geological layers until simulation results fit experimental temperature profile along the observer pipe. The model was developed with a parameter estimation solver for an automatic fitting and more accurate results. Another advantage is that this method only requires two temperature profiles: (1) undisturbed ground (before the TRT) and (2) at the end of the TRT (before stopping the heat injection).
In order to further investigate the proposed method by using higher quality data, a specific instrument (Geowire) was developed to automatically measure the required depth-temperature profiles with high accuracy. The design of the Geowire also covered other features, such as compatibility with TRT equipment and intuitive operation. In addition, an enhanced version of a flowing probe (Geoball) was developed, suitable for both vertical and horizontal pipe arrangements. After laboratory validation tests, the key features of both instruments were evaluated in comparison with new and standard in-borehole instruments for temperature measurements in a test BHE. The main advantage of the proposed instruments over the widespread fiber optics is that they measure the temperature instantaneously (for precise time instants). Moreover, they do not require a dynamic calibration for accurate results while providing higher spatial and temperature resolutions: Geowire (0.5 mm, 0.06 K) and Geoball (10 mm, 0.05 K). Also, they are easier to integrate in existing boreholes and are a potentially more cost-effective solution to measure the distribute temperature.
Finally, the benefits of the proposed method and instruments are demonstrated throughout a DTRT in comparison with fiber optics and with a computer program based on the infinite line source model to estimate the distributed thermal conductivity. The results from the proposed model revealed a highly conductive zone when using data from the Geowire, whereas this was not the case when data from fiber optics were processed.
