DESCRIPTION

Field of invention

The present invention relates to a system and a portable device for determining the advective velocity of the thermal affected zone related to aquifer re- inj ection of hot/cold water.

Background art

Direct measurement of the advective velocity of the heat related to low- enthalpy geothermal systems, e.g. open-loop systems, is an important parameter that fortifies the verification and validation of simulations of the propagation of heat related to aquifer re-inj ection of hot/cold water. Such operations are carried out through the use of dedicated computational software. In addition to being used for research and plant design purposes, such simulations are often mandatory by law in order to obtain the authorization to build open-loop geothermal plants and also to obtain the renewal of the concession for use of the plant.

The application field of the solution described herein is low-enthalpy geothermics, and more specifically systems wherein aquifer re-inj ection of water occurs.

Aquifer re-inj ection of hotter or colder water produces a thermal perturbation, compared to undisturbed thermal (and hydraulic) conditions, resulting in the formation of the so-called Thermal Affected Zone (TAZ).

With reference to Figure 1 , there is shown an example of temperature perturbation in the surroundings of a re-inj ection well. In this case hot water is re-inj ected, which, as can be seen, creates a zone of hotter water near the re- inj ection well.

The thermal affected zone (i.e. the different zones with a temperature gradient) may involve an "external" risk of interference with other systems or particular utilizations of the areas downstream of the return well, and also an "internal" risk of thermal feedback phenomena. An open-loop geothermal system uses a production well from which underground water is extracted, which is made to circulate through a heat exchanger (heat pump); the water is then either re-inj ected in the aquifer through a re-inj ection well located downstream of the plant or discharged into a water body on the surface.

Depending on the mode of operation of the plant (heating or cooling), thermal energy in the form of heat can be either extracted from water taken from the subsoil (winter mode) or yielded to the aquifer (summer mode).

The ATES (Aquifer Thermal Energy Storage) technology is a particular type of thermal accumulation that exploits underground water as a tank by alternatively taking it, according to the season, from two different and sufficiently distant wells dedicated to different purposes, i.e. heating or cooling of buildings. During the summer season, water is extracted from the "cold well" and used for cooling the buildings, and then, once it has acquired heat and warmed up, is delivered through the subsoil into the "hot well". During the winter season, on the contrary, extraction occurs from the "hot well", and the water, after having been used in the evaporator of the heat pump, i.e. after having released heat and cooled down, is delivered into the cold well, ready for the next summer season.

These systems commonly operate in a seasonal mode. The water extracted in the summer is used for cooling buildings, transferring heat from the building to the aquifer by means of a heat exchanger.

Subsequently, the heated water is re-inj ected into the aquifer, which creates a subterranean reservoir of hot water. In the winter, the pumping and re-injection scheme is reversed, so that hot water is extracted from the hot well and can be used for heating buildings (often in combination with a heat pump).

Therefore, an ATES system operates by using the subsoil as a temporary buffer for compensating for seasonal variations in the heating and/or cooling demand.

Replacing heating and cooling systems using traditional fossil fuels with ATES systems may result in lower consumption of primary energy and lower CO2 emissions of a building. This technology can be used to advantage in the presence of low or null aquifer velocity (De Carli et al., 2004).

At present, the advective velocity of heat in an aquifer is calculated mainly through the use of analytical formulae or by numerical modelling.

The choice depends on the actual validity of the analytical models, the real hydrogeological conditions, and the availability of more or less complex numerical modelling means.

It is plain that the use of numerical models implies a higher computational cost and greater operational difficulties.

Therefore, especially for simple systems with two wells only (an extraction well and a re-inj ection well), also known as well-doublets, and wherever the assumptions of validity of the analytical approach are verified, there is a tendency towards the use of an analytical approach.

When the validity conditions are verified, the advective velocity value of the heat is analytically obtained from the undisturbed velocity of the aquifer, designated as V _a and calculated with the following equation (Fetter, 1999): where V _a is the flow velocity of the aquifer, expressed in meters per second [m/s], K is the conductivity of the aquifer, expressed in meters per second [m/s], n _e is the effective porosity of the aquifer, and dh/dl is the hydraulic gradient.

The advective velocity of the thermal affected zone turns out to be lower than the undisturbed velocity of the aquifer by a factor R, called thermal retardation factor, which is given by the ratio between the volumetric thermal capacity of the porous medium (total phase) and the volumetric thermal capacity of water (mobile phase) according to the following equation (Shook, 2001): where p _m Cm indicates the volumetric thermal capacity of the porous medium, pwCw indicates the volumetric thermal capacity of water, and n indicates the total porosity.

The advective velocity of the thermal affected zone is therefore equal to _V p _l lume _

.

As highlighted, in order to compute the advective velocity of heat in an aquifer, both when using the above-mentioned analytical formulae and when using numerical models, which require the knowledge of input hydrodynamic parameters, it is currently necessary, even in the simplest of cases, to experimentally determine the following parameters:

a) porosity and effective porosity of the aquifer,

b) hydraulic conductivity of the aquifer,

c) hydraulic gradient, and

d) thermal capacity of the soil.

In the present state of the art, these hydrodynamic parameters require, in order to be determined, distinct measurement and computation procedures, which are sometimes very time-consuming and costly because they have to be carried out by skilled personnel with specific instruments or in well-equipped laboratories. For each parameter necessary for using the analytical solution for determining the advective velocity of the thermal affected zone, also in order to understand the operational and logistic complexity of the procedures that are necessary for determining such parameters, the following will describe those determination procedures which are most widespread and well-established in the literature and, most importantly, in the operational practice.

In order to determine the parameters a), i.e. porosity and effective porosity of the aquifer, the following values need to be determined.

al ) Porosity of the aquifer = determination by means of laboratory tests or by correlation between aquifer porosity, granulometry and lithology through table values expressed as a range of variation. Laboratory tests involve difficulties in reconstructing existing physical conditions and in obtaining a sample representative of the entire aquifer, while table values are not site-specific. a2) Effective porosity of the aquifer = determination by means of aquifer tests in variable conditions; in addition to the problems listed below, related to the pumping tests required for the conductivity calculation, such tests only allow determining the effective porosity of unconfined aquifers and have a tendency to underestimate the value, unless the tests are conducted for a very long time. As an alternative, multi-well tracking tests may be conducted, which perhaps represent the most reliable, although most expensive, methodology.

Finally, the effective porosity datum can be obtained by correlation with granulometry and lithology; in this case, table values expressed as a range of variation will be obtained, as opposed to a site-specific value.

b) Hydraulic conductivity of the aquifer = determined by constant-rate aquifer test (descent and possible reascent). The constant-rate aquifer test is a pumping test which is generally carried out for the purpose of determining the hydrodynamic parameters of the aquifer, such as hydraulic transmissivity and the resulting hydraulic conductivity. Completion of this test normally requires many hours and high costs, so that it must be accurately programmed. The test requires draining a well (active well) at a constant flow-rate (for at least 24-72 hours) and measuring the piezometric level at control points (piezometers/wells) both during the draining and after pumping, until the initial piezometric level is restored. The control points must affect the same aquifer of the active well, must have known geometric characteristics, and must be located at such a distance from the active well as to allows the measurement of significant level drops. In order to carry out this test, the well must be equipped with a pump and possibly a generator set (if no connection to the electric grid is available), capable of keeping the flow-rate constant throughout the predefined time interval, a continuous-recording flowmeter, one or more well level meters (phreatimeters), and one or more chronometers.

The following will list the logistic problems connected to the execution of the constant-rate aquifer test.

- The first problem relates to the necessity of shutting down the well in order to carry out the flow-rate test and to the fact that it may be impossible or economically or logistically disadvantageous for the owner to shut down the well or any other wells in the vicinity of the well to be tested. Every test should, in fact, be conducted in initial conditions of undisturbed piezometry for at least 24 hours, i.e. in the absence of any draining, clearing or supply of water in the aquifer that might change the static level in the surroundings of the well and/or the observation wells or piezometers. Therefore, the well must not be used for at least 24 hours prior to the execution of the test and for the hours necessary for conducting the test (at least 24-72 hours).

The second problem is related to the implementation of an alternative discharge system for correctly causing the drained waters to flow away from the extraction point, so as to avoid any water supply in the aquifer from the surface, which might adversely affect the test.

Furthermore, for a correct interpretation of the flow-rate tests by analytical formula, it is necessary to regard as valid some assumptions concerning the characteristics of the aquifer and the well involved, which assumptions are often impossible to comply with. Non-compliance with such assumptions will introduce an error in the result.

According to yet another methodology for the calculation of such parameter, the latter is measured in laboratory by means of a permeameter: the intrinsic limitation of this type of measurement lies in the representativeness of the sample and in the difficulty in reconstructing in laboratory stress conditions existing in situ.

Correlation with specific flow-rate may be used as an alternative. The application of this method requires knowledge of the characteristic equation of the active well, which requires the execution of a well test.

Finally, a correlation between hydraulic conductivity and the granulometry or lithology that characterize the aquifer may be used, by means of table values expressed as a range of variation. The value thus obtained is not, therefore, site-specific.

c) Hydraulic gradient = determination by means of hydrogeological maps of the aquifer of interest and piezometric measurements in wells/piezometers involving the aquifer in the area of interest. This requires the planning of a measurement campaign. d) Thermal capacity of the soil = determination by means of laboratory tests. As previously highlighted, the application of analytical solutions, as simple as they may be, or of numerical models, requires experimental measurements that are very demanding in terms of logistics and time. Sometimes the execution of such measurements is hindered by system-related obstacles that cannot be easily overcome.

There is an extensive literature in this field; the following lists the most relevant documents:

- Milnes E, Perrochet P., "Assessing the impact of thermal feedback and recycling in open-loop groundwater beat pump (GWHP) systems: a complementary design tool", Hydrogeology Journal (2013); 21 , 505el4;

- Lund JW, Freeston DH, Boyd TL "Direct application of geothermal energy: 2005 worldwide review", Geothermics (2005), 34: 691 -727. DOI: 10.1016/j .geothermics.2005.09.003 ;

- Larocque M, Mangio A, Razack M, Banton O "Contribution of correlation and spectral analyses to the regional study of a large karst aquifer", Charente, France (1998), J Hydrol 205 :217-23 1 , DOI: 10. 1016/S0022- 1694(97)00155-8;

- Shook MG "Predicting thermal breakthrough in heterogeneous media from tracer tests", Politecnico di Torino, Pag. 5, Geothermics (2001), 30: 573-589, DOI: 10. 1016/S0375-6505(01)00015-3 ;

- Ziwang Y., Yanjun Z., Shuren H., Jianing Z., Xiaoguang L., Bowei C. and Tianfu X., "Numerical study based on one-year monitoring data of groundwater-source beat pumps primarily for heating: a case in Tangshan", China (2016), Environmental Earth Sciences 75, 1070 DOI 10. 1007 /s l2665- 016-5868-y;

- Sterret RJ "Groundwater and wells", Third edition (2007), New Brighton;

- LO RUSSO S., TADDIA G., CERINO ABDIN E., VERDA V., "Effects of different reinjection systems on the Thermal Affected Zone (TAZ) modeling for open-loop Groundwater Heat Pumps (GWHPs)", ENVIRONMENTAL EARTH SCIENCES 75 (2016), pp 75 : 48, D01 : 10. 1007/s 12665-015-4822-8;

- LO RUSSO S., GNAVI L., ROCCIA E., TADDIA G., VERDA V., "Groundwater Heat Pump (GWHP) system modelling and Thermal Affected Zone (TAZ) prediction reliability: influence of temporal variations in flow discharge and injection temperature", GEOTHERMICS (2014), 51 pp.103- 1 12, DOI: 10.1016/j .geothermics.2013.10.008;

- VERDA V., GUELPA E., KONA A., LO RUSSO S., "Reduction of primary energy needs in urban areas through optimal planning of district heating and heat pump installations", (2012) ENERGY 48(1), 40-46, ISSN: 0360-5442, DOI: 10. 1016/j . energy.2012.07.001 ;

- LO RUSSO S., TADDIA G., VERDA V., "Development of the thermally affected zone (TAZ) around a groundwater beat pump (GWHP) system: A sensitivity analysis", (2012) GEOTHERMICS 43, 66-74, Elsevier, ISSN:0375- 6505, DOI: 10. 1016/j . geothermics.2012.02.0012.001 ;

- LO RUSSO S., TADDIA G., BACCINO G., VERDA V., "Different design scenarios related to an open-loop groundwater beat pump in a large building: impact on subsurface and primary energy consumption", (201 1) ENERGY AND BUILDINGS 43, 347-357, Elsevier, ISSN: 0378-7788, DOI: 10. 1016/j . enbuild.2010.09.026;

- LO RUSSO S., TADDIA G., "Advective Heat Transport in an Unconfined Aquifer induced by the Field Inj ection of an Open-Loop Groundwater Heat Pump", (2010) AMERICAN JOURNAL OF ENVIRONMENTAL SCIENCES 6(3), 253-259, ISSN: 1553-345X, DOI: 10.3844/aj essp.2010.253.259;

- BACCINO G., LO RUSSO S., TADDIA G., VERDA V., "Energy and environmental analysis of an open-loop groundwater beat pump system in an urban area", (2010) THERMAL SCIENCE 14(3), 693 -706, Vinca Institute of Nuclear Sciences, ISSN: 0354-9836, DOI: 10.2298/TSCI1003697B

- LO RUSSO S., TADDIA G., "Advective Heat Transport in an Unconfined Aquifer induced by the Field Inj ection of an Open-Loop Groundwater Heat Pump", (2010) AMERICAN JOURNAL OF ENVIRONMENTAL SCIENCES 6(3), 253-259, ISSN: 1553-345X, Politecnico di Torino Pag.6 10.3844/aj essp.2010.253.259,

- LO RUSSO S., CIVITA M., "Hydrogeological and thermal characterization of shallow aquifers in the plain sector of Piemonte region (NW Italy): implications for groundwater heat pumps diffusion", (2010) ENVIRONMENTAL EARTH SCIENCES 60, 703-713, ISSN: 1866- 6280, DOI: 10. 1007/sl2665-009-0208-0;

- LO RUSSO S., TADDIA G., "Groundwater in the Urban Environment: Management Needs and Planning Strategies", (2009) AMERICAN JOURNAL OF ENVIRONMENTAL SCIENCES 5, 494-500, Science Publications, ISSN: 1553-345X, DOI: 10.3844/aj essp.2009 .494.500;

- LO RUSSO S., CIVITA M.V., "Open-loop groundwater heat pumps development for large buildings: a case study", (2009) GEOTHERMICS 38(3), 335-345, ISSN: 0375-6505, DOI: 10.1016/j .geothermics.2008.12.009;

- TADDIA G., Doctoral Thesis: "Low Enthalpy Geothermal Open-Loop Heat Pumps: a suitable tool for thermal energy supply in urban areas" (2015);

- De Carli M., Tonon R., Fellini F., Manente M., Tonon M., Zecchin R. (2004), "Sviluppi nelle pompe di calore: il terreno come sorgente termica", CDA n. 6 Giugno 2004;

- Di Molfetta A. (2002) "Ingegneria degli acquiferi", Politeko Edizioni;

- Fetter, C.W. "Contaminant Hydrogeology. 2nd Edn.", Prentice-Hall (1999), New-Jersey, USA., ISBN: 0- 13-751215-5, pp: 500;

- Shook MG, "Predicting thermal breakthrough in heterogeneous media from tracer tests", Geothermics (2001), 30: 573-589, DOI: 10. 1016/S0375- 6505(01)00015-3.

Obj ect and summary

The invention described herein, through the analysis of monitoring data relating to a period of operation of the plant of at least three months, recorded on an hourly basis, by means of the solution proposed herein, allows obtaining the value of the advective velocity of the heat related to aquifer re-inj ection of hot/cold water. The use of this technique allows computing such parameter in a single step, resulting in extremely evident economical and logistic savings. Brief description of the drawings

Further features and advantages of the invention will be illustrated in the following detailed description, which is provided merely by way of non- limiting example with reference to the annexed drawings, wherein:

Figure 1 shows one example of implementation of a geothermal plant, Figure 2 shows an exemplary graph indicating the trend of the measured temperature, and

Figure 3 shows one example of implementation of a system according to the present description.

Detailed description

The following description will illustrate various specific details useful for a deep understanding of some examples of one or more embodiments. The embodiments may be implemented without one or more of such specific details or with other methods, components, materials, etc. In other cases, some known structures, materials or operations will not be shown or described in detail in order to avoid overshadowing various aspects of the embodiments. Any reference to "an embodiment" in this description will indicate that a particular configuration, structure or feature is comprised in at least one embodiment. Therefore, the phrase "in an embodiment" and other similar phrases, which may be present in different parts of this description, will not necessarily be all related to the same embodiment. Furthermore, any particular configuration, structure or feature may be combined in one or more embodiments as deemed appropriate.

The references below are therefore used only for simplicity' s sake, and do not limit the protection scope or extension of the various embodiments.

Figure 1 shows an example of temperature perturbation in the surroundings of a re-inj ection well. This is a case of re-inj ection of hot water, which, as can be seen, creates a zone of hotter water, designated as zone A, in the vicinity of the re-inj ection well. In particular, zone A propagates farther towards the right- hand part of the figure because of the direction of the aquifer flow, indicated by arrow F. In the example illustrated in the drawing, zone A corresponds to a temperature of 19°C.

In Figure 1 one can also discern three other zones at different temperatures, respectively designated as B, C and D. The zones extend in both directions starting from the re-inj ection well. Zones A, B, C and D have a greater extension in the direction of the water flow, indicated as F. In this example, in particular, zone B corresponds to a temperature of 18°C, zone C corresponds to a temperature of 17°C, and zone D, which is the outermost one, corresponds to a temperature of 16°C.

Still with reference to Figure 1 , there is also a control point, which may be either a second well or a piezometer, located downstream of the re-inj ection well for a second measurement of the parameters.

The data acquired on both sites (re-inj ection well and control point) are collected in a computer PC and analyzed.

Figure 2 shows the results of the measurements at the re-inj ection well and at the control point, and in particular a graph with the analysis of the temperature of both sites. The graph indicated by reference W (Well) concerns the time trend of the temperature detected by a sensor positioned in the re-inj ection well, while the graph indicated by reference CP (Control Point) concerns the time trend of the temperature detected by a sensor positioned at the control point. The solution proposed herein was tested on an "OPEN-LOOP" system, but it may as well apply to ATES (Aquifer Thermal Energy Storage) systems also providing aquifer re-inj ection of water.

The solution proposed herein allows overcoming the drawbacks previously mentioned in the description of the prior art.

In particular, the solution described herein allows overcoming such problems by making it possible to obtain the advective velocity of heat in an aquifer only on the basis of temperature data experimentally measured directly on the geothermal system by using a common temperature measuring and monitoring probe, in a single computational step.

In particular, as shown in Figure 1 , the probes S I and S2 serve to measure the temperatures of the two monitored sites (re-inj ection well and control point).

Such measured data are then sent to a computer PC, which processes and analyzes them.

The results are thus obtained more quickly, resulting in time and cost savings. The simplicity of use of the solution proposed herein also makes the device usable by the plant managing organization itself. As previously highlighted, the application of analytical solutions or numerical models, as simple as they may be, requires the execution of experimental measurements that are very demanding in terms of logistics and time.

In addition, it may sometimes be impossible to conduct the measurements because of system-related obstacles that cannot be easily overcome.

Through the analysis of monitoring data relating to a period of operation of the plant recorded on an hourly basis, it is possible, with the solution proposed herein, to obtain, via statistical support, the value of the advective velocity of the heat related to aquifer re-inj ection of hot/cold water. Typically, the analyzed period of operation is at least three months.

The use of this technique allows computing such parameter in a single step, resulting in evident savings in terms of time, costs and logistics.

According to the solution proposed herein, the datum of the measured temperature is recorded during the normal operation of the well belonging to the open-loop system, thus avoiding the need for turning off the system for a few days. This provides a considerable cost saving.

In particular, the solution proposed herein allows recording the temperature datum related to the operation of the well, and it is therefore not necessary to remove the water with alternative systems, since it will follow the normal cycle and will be re-inj ected in the aquifer through the return well.

According to the prior art, for a correct interpretation of the flow-rate tests by analytical formula it is necessary to regard as valid some assumptions concerning the characteristics of the aquifer and the well involved, which assumptions are often impossible to comply with. Non-compliance with such assumptions will introduce an error in the result.

In the solution proposed herein, no assumptions need to be made as concerns the data analysis methodology.

The portable device proposed herein allows determining, in a statistically rigorous and robust manner through the use of cross-correlation, the advective velocity of the thermal affected zone related to aquifer re-inj ection of hot/cold water, e.g. due to open-loop geothermal systems. The advective velocity of the heat related to aquifer re-inj ection of water is obtained through the analysis of temperature monitoring data measured at the re-inj ection well and at a control point (piezometer or well) located downstream of the plant. Such monitoring data are measured by normal probes on an hourly basis for a period of at least three months.

The advective velocity value obtained by processing the collected data reflects what has been so far obtained through the use of analytical formulae, which however also require the knowledge, i.e. the measurement, of several other hydrodynamic parameters.

This device can be used in low-enthalpy geothermal systems through the use of cross-correlation algorithms.

The use of the device proposed herein allows for direct computation of the advective velocity in a single step on the basis of the measured data. The processing algorithm employed, written in a compilable programming language, can be compiled on different operating systems, such as, for example, Android and Windows.

With reference to Figure 3, it can be seen that the proposed solution comprises three interconnected modules exchanging data with one another.

The first module, referred to as module 1 , consists of means for acquiring the temperature datum, to be positioned within the re-inj ection well, and another similar means to be positioned at the control point located downstream of the re-inj ection zone.

The second module, referred to as module 2, consists of a device to be connected, through a suitable connector, to a means for real-time transmission of the temperature datum to a remote control center consisting of a computer, which represents the third and last module, referred to as module 3.

On such computer there is the algorithm that allows determining the cross- correlation value (CCF), which allows determining the advective velocity value of the heat in the aquifer involved.

The algorithm is based on a first phase of acquisition of the temperature measurements both in the re-inj ection well and at the downstream control point, which are subsequently used in order to compute the cross-correlation value. This value represents the degree of similarity of two signals as a function of time (time shift) or space (displacement) applied to either one of the two. The two temperatures can therefore be considered as two signals having real values x and y which only differ for a shift along the time axis (t). The cross- correlation calculates how much the signal y must be anticipated in order to make it identical to x. The formula essentially anticipates (or delays) the signal y along the time axis t, calculating the integral of the product by every possible shift value. When the two signals coincide, the value of (x*y) is greatest, because when the waveforms are aligned they contribute to the area computation only in a positive way.

The algorithm may be written in any programming language, depending on the operating system of the computer (e.g. Windows platform).

The system thus developed allows determining, in a statistically rigorous and robust manner through the use of cross-correlation, the advective velocity of the thermal affected zone related to aquifer re-inj ection of hot/cold water, for low-enthalpy geothermal systems. Said algorithm determines the time shift of a signal (e.g. the temperature measured in the re-injection well) compared to another signal (the temperature measured at the control point).

Such values reflect what until today has been dealt with through the use of analytical formulae or numerical simulations, which however require the knowledge of several parameters to be obtained through distinct, and also very exacting in terms of time and costs, procedures.

The use of such a statistical approach allows the calculation of the advective velocity of heat through a simple analysis of data obtained by a common temperature measuring and monitoring probe, thereby solving problems related to the determination of the single parameters required for estimating the parameter by analytical formula or numerical simulation.

As far as Piedmont is concerned, the Authorization Body requires that such probes be installed in at least the control piezometer located downstream of the return well.

Therefore, practically all plants have already been equipped with one of the probes capable of acquiring the temperature data necessary for the application of the cross-correlation method developed by the solution proposed herein. This further broadens the field of application of this technique.

There is also the advantage that the direct measurement of the advective velocity of the heat related to aquifer re-inj ection of water can be obtained without having to know or measure any further hydrodynamic parameters.

The system uses the datum relating to the normal operation of the re-inj ection well, so that it is not necessary to turn off the system.

Of course, without prejudice to the principle of the invention, the forms of embodiment and the implementation details may be extensively varied from those described and illustrated herein merely by way of non-limiting example, without however departing from the protection scope of the present invention as set out in the appended claims.