the all-in-one Patent
The knowledge of the thermophysical properties and in particular of the ground thermal conductivity and of the equivalent thermal resistance of the vertical geothermal heat exchanger (Borehole Heat Exchanger, BHE) is of fundamental importance in the sizing phase of the system, guaranteeing its long-term energy efficiency and economic sustainability.
The traditional method for determining the ground and BHE thermal parameters consists of an experimental procedure called Thermal Response Test (TRT). This test involves using a pilot BHE, placed in the ground to be analyzed, in which the carrier fluid, suitably heated above the ground in a controlled manner, is kept in flow circulation for a duration of about 70 hours. During the test, the fluid temperatures are measured and from the measurements analyses, it is possible to deduce the ground thermal conductivity and the effective thermal resistance of the BHE. To carry out this measurement campaign, dedicated experimental equipment is used, the so-called TRT machines, which have a particularly high construction cost (€ 10000-30000 from 2019 estimates).
The proposed invention constitutes an absolute novelty in the market of GCHP applications and refers to the Italian Patent by UniGE n. 102019000023082 ("Method and device for measuring geothermal parameters for sizing and subsequent monitoring of geothermal heat pumps").
The invention is aimed at to carrying out, in a simple and economical way, the measurements of the geothermal parameters without having to resort to the use of either the classic TRT machine or the hydraulic connections for the circulation of fluid in the pipes of the underground exchanger.
The invention represents, to date, a far cheaper solution than all existing technologies that have similar objectives.
The patent relates to a method and a device for determining the thermophysical properties of the ground and of the filling cement mortar (known as “grout”) of the geothermal heat exchangers. The innovative method proposed, called EDDTRT (Electric Depth Distributed Thermal Response Test) provides a set-up phase during which a pilot BHE is equipped with a resistive heating cable and instrumented with one-wire digital temperature sensors at known depths and radial positions relative to the heater cable. Subsequently, the heating cable is powered and the temperature values detected by the sensors are acquired over time. Finally, for each sensor, a temperature / time graph is constructed on a semi-logarithmic scale and the sections of each graph are identified with an almost linear trend: from the slope of these sections it is possible to estimate the thermal conductivity of the grout and of the ground, also highlighting any variations at different depths.
EDDTRT application scheme for estimating the ground thermal conductivity values also in case of different layers along the depth
The EDDTRT method also allows the measurement of the operating parameters of the BHE for monitoring and control of the plant during operation. The continuous acquisition of temperatures along the heat exchangers can be used to control the operation of the heat pump, increase its conversion efficiency, avoid abnormal operating conditions (such as too high or too low ground temperatures), detect unexpected geological situations, such as the appearance or disappearance of groundwater movements among the different ground layers. This monitoring, not available in traditional plant solutions, represents a great added value for the geothermal plant from an economic point of view because it allows optimized use of the overall system in the long term. Finally, compared to similar DTRT systems based on the use of optical fibers, the costs are much lower and the reliability of the measurements is not subject to connection problems between the different sections of the data cable (the optical fiber in the DTRT system).
From an operational point of view, the EDDTRT method involves the use of innovative tubing that has a peripheral axial housing to place the cable with the temperature sensors and a spacer to keep the heating cable in a fixed central position with respect to the pipes. The architecture of the measurement system as conceived in the patent makes use of low-cost digital type sensors (one-wire sensors). The sensor / heater / piping combination therefore constitutes a very compact “all-in-one” element for EDDTRT type measurements.
A) U-tubes of the borehole heat exchanger
B) Extruded tube rib, to contain the instrumented cable with digital temperature sensors
C) Waterproofed digital temperature sensors. They are placed every 0.5-2 m along the heat exchanger
D) Metal spacer that keeps the heater E in a central position, and acts as an elastic element between the pipes
E) Electric heater
3D representation of the "all-in-one" geothermal heat exchanger
Unlike existing methodologies and equipment (TRT and Distributed TRT with optical fibers) which aim for similar measurements, this methodology and related device apply the ILS solution (Infinite Line Source) to recast distributed temperature measurements following localized heating carried out by means of an axial electric cable in the volume occupied by the pipes of the BHE. According to this approach, no additional external equipment is required (the usual trolley-mounted TRT machine) as the integrated pipes with sensors and heater (all-in-one) already carry out all the necessary measurement operations. Furthermore, the method and the relative equipment allow measurements in reduced times and at very low costs compared to traditional solutions. Finally, the system allows the simultaneous estimation of the thermal conductivity of the ground and the filling material of the drilling (grout), measurements not possible with the usual-standard above-ground TRT equipment.
Comparison of overall dimensions for TRT applications: 1) traditional TRT machine connected to the geothermal heat exchanger; 2) TRT machine developed at the University of Genoa-Dime; 3) EDDTRT machine, developed at DIME and based on Arduino like controllers
The realization of an electronic device for reading the digital signal of the temperature sensors, based on the use of micro-personal computers on a miniaturized card, also allows wireless communication, for example towards mobile devices (eg smartphones). The temperature measurement from the sensor set can take place both during heating by the resistive electric cable (for EDDTRT measurements), and during the operations of the BHE field once connected to the heat pump (performance monitoring phase and optimization of operating parameters). During the monitoring phase, the GPRS transmission module connected to the Arduino Uno allows the transmission of the detected parameters to a server. The data received, once interpreted and validated, can be stored and made available in the form of a database for subsequent interrogation and analysis.
Hardware architecture of the prototype and TRT electronic module with Mini Ampere meter
The PoC Experiment
The innovative EDDTRT system has been experimentally validated through the realization of an appropriate scale prototype. In particular, the present Research Group has received funding from Liftt / Compagnia di San Paolo (PoC call) for the validation activity relating to the Italian Patent no. 102019000023082 ("Method and device for measuring geothermal parameters for sizing and subsequent monitoring of geothermal heat pumps").
The scale prototype of the PoC experiment was created to experimentally model the ground around a real heat exchanger for geothermal applications (at Fourier numbers typical to TRT experiments) using a block of rock of appropriate size in terms of Fourier numbers.
PoC experiment: scale modeling of the EDDTRT procedure referred to in the patent.
Through the use of a small-size 3D printer, tubing prototypes were created for the “all-in-one” system on a reduced scale, assembled with spacer, heater and temperature sensor system. The housing for the temperature sensors (in next real pipes) can be made during the extrusion phase of the pipe (typically in polyethylene) by creating a rib on its external surface. The temperature sensors, for the experimental activity of the PoC experiment, were miniaturized using K-type thermocouples.
Creation of the prototype test section using a 3D printer
The measuring apparatus was therefore inserted into a suitably cut and shaped slate sample, representing the ground volume in which the BHE is operating. The geometric and operational parameters were determined through an accurate preliminary evaluation based on dimensionless analyzes and numerical simulations (Comsol MultiPhysics). The dimensions of the slate block (80x80x40 cm) and its mass (about 700kg) require a specific cart that is also suitable for handling.
Data analysis, numerical simulations: mean axial temperature in the volume of slate at different radial distances from the heater. On the abscissa the logarithm of time (image taken from a paper submitted to Energies Journal, September 2021)
Rock sample (slate) with a suitable cart to move the block during the experimental activity.
Realization of the instrumented test section: for needs related to experimentation, a suitable brass pipe of small thickness is placed in correspondence with the 40 mm diameter through the hole
To allow the validation of the procedure described in the Patent and of the “all-in-one” prototype system, the measurements carried out to estimate the thermal conductivity of the slate were compared with some direct measurements. The direct measurements were carried out using the Applied Precision 2114 k-meter instrument at DIME departiment of the University of Genova; the table below presents the results of the measurements on the slate block and on suitably shaped samples of the same rock.
Direct measurements of the thermal conductivity of slate: the Applied Precision 2114 k-meter instrument supplied with the DIME
Table 1 - Measured slate thermal conductivity and heat capacity (as reported in Energies paper )
Similar measurements were carried out using an additional steady state thermal conductivity meter designed and built at DIME (steady state k-meter). These instruments for measuring thermal conductivity, unlike the "all-in-one" system referred to in the Patent, have the limit of being able to be used in the laboratory and only on small samples. The following figures illustrate the design of the thermal conductivity meter and its experimental implementation. The components making up the final setup were designed in PTC Creo and made using the Prusa i3 MK3S 3D printer.
Direct measurements of the thermal conductivity of slate: the steady state thermal conductivity meter designed and built at DIME (steady state k-meter)
Subsequently, using the "all-in-one" scale prototype model, repeated tests were carried out according to the procedure described by the Patent, with a duration of about ten hours each, to which it was necessary to add the transitional period of a few days to restore the rock test section at the original undisturbed temperature.
The tests were carried out in the DIME laboratories at the Genoa and Savona offices (Solar and Geothermal Lab, Cenvis / Unige, Palazzina Oliva, Savona).
The results obtained experimentally demonstrate that the system referred to in the Patent is able to produce measurements that allow an estimate of the ground thermophysical properties with an accuracy comparable to the conventional TRT and DTRT optical fiber systems, using a much more compact, economical and functional technology. In particular, the agreement on the thermal conductivity values resulted to be very good, especially if compared with the thermal conductivity values obtained from the analysis of the temperatures detected by the TK6 sensor of the "all-in-one" prototype (Table 2), sensor placed in the median part of the cylindrical vertical volume that houses the exchanger, a position that allows limiting the edge effects.
Table 2 - Thermal conductivity relative to the experimental test (as reported in Energies paper )
Further details relating to these activities are reported in the paper submitted to the scientific journal Energies:
Morchio, S .; Fossa, M .; Priarone, A .; Boccalatte, A. Reduced Scale Experimental Modeling of Distributed Thermal Response Tests for the Estimation of the Ground Thermal Conductivity. Energies 2021, 14, 6955. https://doi.org/10.3390/en14216955
Thanks to Eng. Andrea Corte who, during his three-year thesis in Mechanical Engineering, contributed to the realization of the experimental apparatus relating to the PoC experiment and the numerous 3D printed components. We also thank Eng. Alessia Boccalatte and Eng. Samuele Memme for his precious suggestions on graphic representations and on setting up the data acquisition system.