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During the MPC hybridGEOTABS project, GEOTER has been working selecting proper elements to monitor the underground temperature and the communication and connection systems to know the evolution with time of the geothermal resources, according to the demand of the buildings. In this context, GEOTER have developed Enhanced Geothermal Response Tests (EGRT) based on usage of a whole new gear for the realisation of an on-site thermal conductivity test and a continuous monitoring of the geothermal probes. It allows us a geothermal field sizing optimisation and will provide us the necessary data oriented to the MPC controller to decide the thermal management of the buildings.
When it comes to sizing the geothermal borehole field, the use of a GRT (Geothermal Response Test) is the most prevalent practice. It consists of an on-site test to determine the thermodynamic parameters of the subsoil. A GRT is used for practical comparison of theoretical data since performing such test allows us to know the effective thermal conductivity (describes the heat transfer through conductivity in the subsoil) and the thermal resistance of the probe (indicates what should be the thermal resistance between the collector circuit and the subsoil for dissipation of power), with the aim to optimise the system and size a field of geothermal probes.
The process of a GRT is as follows:
1) Before the GRT is started, the undisturbed ground temperature must be determined. This can be measured by measuring the temperature of circulated water through the borehole without heating.
2) After this initial measurement, the test starts and the heat transfer fluid is gradually warmed up making it circulate through immersion heaters. Temperature data are provided by two temperature sensors at the top of the borehole and power applied is controlled and collected during the process. Hence, the average thermal conductivity can be obtained.
In one of the study cases, a GRT was performed in Spain (Gijón, Spain) from the 20th to the 23th of March 2018. The nature of the ground in this place is dominated by limestones, loams, clays and sandstones. The borehole was drilled using a direct mud rotary drilling method to achieve the introduction of the geothermal probe with a diameter of 40 mm to the depth of 124 meters, and backfill material with outstanding thermal conductivity was used to fill the borehole. The average thermal conductivity obtained was 2.91 W/(m·K).
In the hybridGEOTABS project, significant improvements in this process have been achieved, by executing more developed, upgraded and detailed GRTs than usual. In a traditional GRT, only flow and return temperatures at the top of the borehole are measured, while an EGRT (Enhanced Geothermal Response Test) enable us to obtain a temperature profile at all the levels of the borehole, measuring accurately how the temperature changes with depth as a function of the flow and the thermal stress of the borehole. The interpretation of the results of an EGRT allows to calculate the effective thermal conductivity and the thermal resistance of the boreholes along the depth of the geothermal probe, centimetre per centimetre. With this data it is possible to find where the optimal areas of the sub-soil are and, even, to know the influence of the different materials or groundwater in the case that these exists.
An EGRT was performed in the same borehole mentioned and it was based on a 12-hour undisturbed ground temperature test, a 48-hour heating test and a 48-hour of temperature recovery test. To make a difference with respect to the traditional GRT, ten different sensors were running all along the length of the geothermal probe while a 130-meter-long heating element were heating the heat transfer fluid. During the EGRT, temperature of the ground was obtained, depending on the depth during the heating-controlled process. Results can be consulted below. In figure 1, the equipment used is shown.
Figure 1. EGRT equipment
In Figure 2, temperature profiles during the undisturbed and heating process are shown. Red curve validates the typical theoretical depth-temperature unaltered profile of the Earth. The rest of colours represent temperatures along the borehole length, which have been represented in different times to verify the temperature changes while a constant heat is injected. With all this information, conductivity along the borehole is known and the optimal depth of the boreholes can be calculated considering the thermal loads of the building which we want to provide heating/cooling and domestic hot water, achieving the length where there is the highest average conductivity, all in ranges that allow to satisfy building demands. Thermal conductivity differences are explained by the different hydrogeological conditions.
Figure 2. Temperature profile during EGRT execution.
In Figure 3, conductivity data obtained along the borehole test is shown. The average thermal conductivity is 2.91 W/(m·K), obviously the same value which was calculated during the GRT performed. Higher values are obtained due to the existence of an aquifer with an important water flow. This is especially reflected between depths between 65 and 85 metres, where the data obtained in test is so high that it isn’t even coherent to provide a numerical value for this thermal conductivity. It’s considered that the flow of water from this area is much more considerable than at 28, 36 or 118-metres depth. Similarly, the lower value obtained is approximately 1.25 W/(m·K), obtained at 55 meters of depth.
Performing a borehole field simulation, EGRT allows to calculate the optimum depth of the boreholes depending on the thermal loads of the building.
Subsequently, three different simulations were carried out to show the importance of knowing the conductivity along the borehole.
1) Simulations performed by engineering, when the conductivity is estimated based on geological and hydrogeological bibliographic studies as well as on the experience.
2) Simulations performed after performing a GRT, when average thermal conductivity is known.
3) Simulations performed after performing an EGRT, when thermal conductivity is known throughout the depth.
The required heat exchanger is longer when conductivity has been obtained from engineering than when has been determined by the GRT, due to the oversizing that used to be done for the estimation of conductivity. Similarly, the required exchanger length is higher when conductivity has been obtained from GRT than when has been determined by the EGRT because it is possible to optimise the length of the boreholes and finally the total metres to drill for the borehole field are lower.
After EGRTs executed during the project, we can affirm that pre-engineering sizing has a reduction of 4-10% of investment in the borehole field performing a GRT and a reduction of 14-16% performing an EGRT. That means, an EGRT has meant a 7% investment reduction compared to the GRT.
In Figure 4, in this particular case in Gijón, conductivities obtained in three simulations are shown, as well as total drilling length necessary according each sizing and total cost of the borehole field.
Figure 4. Average conductivity, total drilling length and borehole field cost in Gijón (case studies: Engineering, GRT, EGRT)
Through an EGRT, areas of high conductivity have been found along the depth, and together with the study through EED software has allowed the optimisation of the global geothermal system. Performing an EGRT is possible to analyse the complete information about the subsoil and decide the best solution considering all the project conditions, always optimising the number of meters to drill. EGRT obtains savings in investment costs enhancing the sizing of the borehole field, increasing the security of supply and the optimum functioning of the HVAC installation.
Since low enthalpy geothermal energy (also called “shallow geothermal application” at depths less than 400 m) is an efficient and abundant renewable energy that can be found almost everywhere in the world, it is logical to investigate better methods to size the system accurately. Field studies were carried out within this project to verify the importance of the conductivity calculation along a geothermal borehole for its correct sizing and economic advantages it offers in this project.
With conductivities obtained with an EGRT and simulations carried out with EED software, the optimal configuration of the geothermal boreholes can be decided. The EGRT allows the design team to make an informed decision on the length of each necessary borehole and reduce the need for unnecessary meters of drilling, therefore reducing the total investment cost of the geothermal installation.