WO2021111373A1

WO2021111373A1 - Method and device for measuring geothermal parameters for dimensioning and subsequent monitoring ground coupled heat pumps

Info

Publication number: WO2021111373A1
Application number: PCT/IB2020/061480
Authority: WO
Inventors: Marco FOSSA
Original assignee: DIMES Dipartimento di Medicina Sperimentale Universita degli Studi di Genova
Current assignee: DIMES Dipartimento di Medicina Sperimentale Universita degli Studi di Genova
Priority date: 2019-12-05
Filing date: 2020-12-04
Publication date: 2021-06-10
Anticipated expiration: 2022-06-05
Also published as: IT201900023082A1; CA3162940A1

Abstract

Object of the invention is a method for determining thermal parameters of a ground adapted to accommodate one or more geothermal probes of a ground coupled heat pump system, which method provides, after a set-up step, in which a geothermal piping (ground heat exchanger) bearing a heating cable of the resistive type and a plurality of temperature sensors at depths and radial positions known in relation to the heating cable are arranged: powering the cable to heat the ground; reading the temperature values detected by the sensors; making, for each sensor, a temperature/time chart on semilogarithmic scale; identifying sections of each chart with an almost linear trend; calculating the slope of these sections; extrapolating thermal conductivity values of the ground from said slopes. Further object of the invention is a corresponding reading device for reading the measures and provides for the implementation of "all in one" heat exchangers equipped with heating cable and sensors.

Description

Method and device for measuring geothermal parameters for dimensioning and subsequent monitoring ground coupled heat pumps

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TEXT OF THE DESCRIPTION

The present invention concerns the sector of ground coupled heat pumps. The ground coupled heat pumps are highly efficient air conditioning systems for buildings and countless evidence in scientific publications position the ground coupled heat pump systems as the most efficient system (in terms of energy saving and reduction of greenhouse emissions) among all those applicable to the air conditioning of buildings for civil and industrial use.

It is estimated that 80,000 (eighty thousand, https://www.eurobserv-er .org/heat-pumps-barometer- 2018/) ground coupled heat pumps are installed in

Europe each year (the data refers to the years 2016 and 2017) and underground heat exchangers of an overall length of between a few hundred meters to tens of kilometers are installed for each heat pump of this type.

The ground coupled heat pumps withdraw the heat needed for the winter air conditioning of buildings from the ground through a series of underground heat exchangers . An example of a geothermal building is the Palazzina Smart Energy Building (SEB) of the UNIGE Campus of Savona (www.energia2020.unige.it).

In summer air conditioning, the underground exchangers release heat to the ground. The ground coupled heat pump systems (GCHP, Ground Coupled Heat Pumps), also named low-enthalpy geothermal plants, exploit the natural heat of the ground with the aid of a heat pump for the heating or cooling of buildings and use geothermal probes (ground heat exchangers) which, in the most common arrangement , are installed vertically in the ground up to typical depths from 80 to 250 meters, such as to draw or disperse heat from or towards the subsoil. The vertical exchangers take on the English name of Borehole Heat Exchangers (BHE).

As known, the ground is characterised by extremely stable temperatures over time starting from about 10 m in depth and can thus be used as a "lower thermal source" for the winter air conditioning and as a "higher thermal source" for the summer air conditioning. The ground temperatures, also while the heat pump is operating, are almost always more favorable (in terms of the energy efficiency of the system) than those of ambient air and, as a result, the ground coupled heat pump is almost always better in terms of Coefficient of Performance (COP) in relation to the normal air vaporizing heat pumps.

The ground coupled heat pump uses a reverse cycle to exchange heat between the ground and the building to be air conditioned. A liquid (typically water and antifreeze) is circulated on the ground side along the ground heat exchangers, by activating a heat withdrawal/release mechanism. An operating fluid (typically water or air) conveys the energy towards and from the building on the building side, depending on the thermal requirements thereof (winter or summer air conditioning). The ground coupled heat pump can also be used for producing hot sanitary water. Thanks to the favorable ground temperatures throughout the year and the proper sizing of the underground heat exchangers, the ground coupled heat pump systems are able to achieve Coefficients of Performance (COPs) in the range of 3.5-5, as seasonal average, thus drastically reducing (in relation to traditional fuel systems or to air vaporizing heat pumps) the energy consumption and polluting and greenhouse emissions . The proper sizing of the underground heat exchangers is in turn linked to the knowledge of the thermophysical properties of the ground (thermal conductivity of the ground kgr) and of the internal thermal resistance of the geothermal probes (Rbhe), the latter is a function of the thermal conductivity (kgt) of the material filling the hole, known as "grout".

Ground coupled heat pumps, combined with the probes in the ground, can completely replace boilers and air conditioners for the heating and cooling rooms and for the production of hot sanitary water.

Both in new constructions and in many renovations, geothermal plants can be installed in every type of public and private buildings, for residential, manufacturing, service or commercial uses.

The geothermal probe is an underground heat exchanger, usually consisting of a double piping U- joined in the lower end part of the bore where pipes have been inserted. The pipes are generally in polyethylene, of an outer diameter of between 25 and 40mm, possibly equipped with spacers to keep the pipes in a reciprocal position. The vertical hole in the ground is generally made for depths of between 80 and 200 m; the piping is inserted in the hole and the borehole is filled by a semi-fluid cement mortar named grout,which provides to ensure that the piping can be in thermal contact with the ground therethrough .

A ground coupled heat pump system generally provides a plurality of geothermal probes , which identify the so-named probe field; the individual underground probes are installed at preset distances (for example 5-8 meters from each other) in the site of thermal exploitation.

One of the problems of the above described geothermal plants concerns the wrong dimensioning of the probe field: a wrong geometry of the probe field (number, interdistance and length of probes) can give rise to progressive variations of ground temperature which are not compatible with the operation conditions of the heat pump. In extreme borderline cases, the wrong dimensioning of the probe field can highly decrease machine COP over years,or even cause conditions in which the machine stays blocked due to temperatures of the operating fluid coming from the ground, which are not compatible with operation parameters of the same machine.

Inefficiencies in the geothermal plant can further result in not being able to meet the energy requirements of the building, in extra costs, in not being able to meet the comfort requirements of the occupants of the building.

The design calculation of the probe field requires a series of input information, among which the thermal requirements of the building throughout the year , the thermophysical properties of the ground, in particular its thermal conductivity kgr, and the thermal resistance of the geothermal probe Rbhe adopted.

The thermal resistance of the geothermal probe Rbhe is a parameter which describes the behavior in terms of the heat exchange, of the heat operating fluid, of the piping walls and of the grout cement mortar filling the remaining space between the piping and hole.

As far as the energy need of the building is concerned, it is possible to resort to theoretical derivation methodologies available in national regulations (e.g. Uni 11300 standards). Experimental in situ investigations are instead necessary for the assessment of the thermal properties of the ground and probe.

The general method of the traditional type for determining thermal parameters of ground and probe is defined Thermal Response Test (TRT) or also GRT (Ground Response Test). The above-ground temperature values of the fluid circulated in the underground probe are recorded in the TRT, whereas a constant heat flow is provided (or subtracted from) to such fluid over time.

The temperature assumed by the fluid (in a time- varying way) depends on the heat flow set, on the thermal characteristics of the ground and on the parts constituting the BHE (piping, grout material). The fluid, by circulating along the entire depth of the exchanger, assumes temperatures (as measured above ground) which describe the thermal properties of the exchanger and ground as averaged over the entire depth: for example, the kgr deducted from a classic TRT test is an average value of the depth of the different thermal properties that the ground can have if it consists (as often occurs) of different layers of rocks and sediments, each with its own characteristics .

The circulation of the fluid in the ground and the administration of heat thereto require a known wheeled machine known as Thermal Response Test Machine in the traditional approach.

This machine, whose first prototypes can be attributed to P. Mogensen and S. Gehlin in Sweden in the 90s, is still a bulky object (1-3 cubic meters of total volume), with a typical weight of 50-150kg and particularly costly from a constructive point of view (20000-60000 € according to 2019 estimates).

The diagram of a machine adapted to make a TRT (traditional) is depicted in Figure 1. A boiler 4, typically electric, is present in such machine but it could also be fueled, where a water flow rate receives an assigned heat flow. Such heat flow should be rigorously constant on the basis of the interpretative model of the measures on which the classic TRT analysis is based. The machine then has a circulation pump 3, hydraulic connections 1 connected to the geothermal probe 2 and above-ground temperature sensors 6 , 7 for measuring the temperature of the water, both delivered and returned from the ground, circulated in the geothermal probe. The control unit 8 reads the temperatures from the sensors 6, 7, the flow rate from the flow rate gage 10 and the electric power delivered to the heater 4 through the gage 9 and provides to estimate the average Kgr and Rbhe values.

For the classic TRT test, it is important to note how it is possible to directly deduce the Rbhe from the measurements, but such value refers to the specific conditions of the classic TRT test (in which, for example, pure water is used and not antifreeze, which has different thermophysical properties) and not to the real operative conditions of the heat pump (which works with water and antifreeze and typically at variable flow rates different from those of the classic TRT test).

However, if the kgt was measured and the geometries of the tubes and the flow rate (actual) of the fluid inside the tubes during the real operation of the heat pump are known, Rbhe can be calculated for each operative condition under which the heat pump is operating.

It should be highlighted that the constant thermal requirement is in practice difficult to meet for many reasons. The typical case concerns the fact that the TRT test (in general) requires long times, in the order of 50-100 hours, during which there are day/night electric voltage fluctuations which make it impossible to ensure a constant heat flow throughout the entire duration of the test when operating with an electric boiler. Further causes of non-constant heat flow are linked to particular operating conditions or to the interruption of the electric power supply, as highlighted in the patent application published with the number WO2017158489, to be considered part of the present description.

Object of the present invention is thus that of allowing to simply and economically measure the geothermal parameters for the dimensioning of the ground coupled heat pump systems without having to rely either on the use of the classic TRT machine or on hydraulic connections and on the circulation of fluid inside the piping of the BHE exchanger.

Further object is to allow to measure the same parameters for the monitoring and control of the operating system.

The invention achieves the object with a method for determining thermal parameters of a ground adapted to accommodate one or more geothermal probes of a ground coupled heat pump system, which method provides, after a set-up step, in which a geothermal piping (ground heat exchanger) equipped with heating cable of the resistive type and a plurality of temperature sensors at depths and radial positions known in relation to the heating cable are arranged: powering the cable to heat the ground; reading the temperature values detected by the sensors; making, for each sensor, a temperature/time chart on semilogarithmic scale; identifying sections of each chart with an almost linear trend; calculating the slope of these sections; extrapolating thermal conductivity values of the ground from said slopes.

If in the ground there is a filling layer of a borehole (grout), the method advantageously provides for identifying at least two sections of each chart with an almost linear trend, calculating the slope in these two sections and extrapolating the values of ground conductivity (kgr) and of the filling layer (kgt) from the values of said slopes.

In order to optimise the estimated values, in an embodiment , the invention provides to use the thermal conductivity values extrapolated at each depth and for each radial position as input for an iterative process that determines the optimum values by minimising a cost function that expresses the deviation of said values in relation to theoretical values obtained from a mathematical model of the ground, for example of the infinite linear source (ILS) type with two thermal resistances.

According to a further aspect, the invention concerns a device for measuring thermal parameters of a ground adapted to accommodate, in boreholes, one or more geothermal probes of a ground coupled heat pump system, the device comprising: a heating cable adapted to be introduced into the ground in a given position, in particular central, in relation to the boreholes; a plurality of temperature sensors adapted to be introduced into the ground at depths and radial positions known in relation to the heating cable; a control/processing unit configured to read the temperature values detected by the sensors over time and, on the basis of the electric power supplied, to determine the values of the conductivity parameters at the various depths and radial distances from the heating cable.

The temperature sensors are typically arranged, at known distances, on one or more strips or cables to be inserted vertically into the ground.

In a preferred embodiment, the sensors are of the one-wire type, i.e. they are distributed along one or more electric cables, typically with two wires, each sensor being independently addressable by the control unit to read the detected temperature.

The device can be of the stand-alone type or advantageously provided in combination with the piping of a probe of an underground exchanger for a ground coupled heat pump (BHE). In this case, the piping can have a peripheral axial housing to accommodate the strip or the cable bearing the temperature sensors thus implementing a very compact device which can also be used when monitoring an operating heat pump system.

The housing for the strip or cable can comprise a ribbing on the outer surface of the piping, preferably obtained during the piping extrusion, typically of polyethylene.

A spacer adapted to keep the heating cable in a fixed position in relation to the piping can be present. In the specific case of a U-shaped piping, the spacer can advantageously be configured to keep the heating cable in a central position in relation to the vertical branches of the piping, such as to ensure a certain degree of symmetry.

Thus, the sensor/heater /heat exchanger combination constitutes an "all in one" element for the ground heat exchange operations and for the measurements of the EDDTRT type and continuous monitoring.

The control/processing unit is a processor or controller adapted to execute program instructions such as to implement one or more steps of the method according to the invention.

Further characteristics and improvements are object of the sub-claims.

The characteristics of the invention and the advantages deriving therefrom will become clearer in the detailed description of the accompanying figures, in which:

Fig. 1 shows the diagram of the machine needed for a traditional TRT measurement.

Fig. 2 shows an exemplified diagram of a device according to an embodiment of the invention.

Fig. 3 shows an exemplified diagram of a device according to another embodiment of the invention with piping integrated with sensor members and a heating element .

Fig. 4 shows, on a semi-logarithmic scale, the temperatures measured during a test with the device according to the invention, by identifying the slope change time t_AB, the slope start and end times t_mA and t_mB and the slopes m_A and me.

Fig. 5 shows the flow diagram of a method according to an embodiment of the invention.

Fig. 6 shows the flow diagram of a method according to another embodiment of the invention.

With reference to figure 2, a device according to an embodiment of the invention comprises: an electric heating cable 1 adapted to be introduced vertically into the ground; a plurality of temperature sensors S_i,j adapted to be introduced into the ground at depths z and radial positions r known in relation to the heating cable 1; a control unit 3 configured to read the temperature values detected by the sensors S_i,j over time and, on the basis of the electric power supplied, to determine the values of the conductivity parameters at the various depths z and radial distances r from the heating cable 1.

The temperature sensors S_i,j are arranged, at known distances, on one or more strips or cables 2 to be inserted vertically into the ground, as shown by way of example in the figure.

The sensors S_i,j can be fully independent, i.e. each being connected to its transmission cable for transmitting the temperature signal detected or distributed along one or more electric cables 2, typically with two wires, to which they are connected in parallel. In this case, each sensor is identified by an address which is used by the control unit to read the temperature value detected by the corresponding sensor.

According to an embodiment, the sensors S_i,j are advantageously integrated in micro-devices adapted to retransmit the digital measurements in one-wire protocol mode or in RS485 protocol mode. These sensors were tested by the present inventor to work with lengths of power/transmission cables of several hundreds of meters and thus compatible with the typical length (along the depth z) of the geothermal BHE probes.

The use of digital sensors of the one-wire or RS485 type has many advantages: low unit cost (cost of the sensor in the order of one euro per sensor), possibility to connect with a two-wire cable of the commercial type, possibility to interrogate the sensors to read the temperature values measured thanks to the digital address each of them is provided with. The temperature sensors S_i,j are generally Ns in number, where Ns is the result of the product of Nz and Nr , which were respectively the number of vertical positions z the sensors are arranged to and the number of radial positions r at which there is a sensor (for that depth z). If, for example, temperature sensors are present every 5 m of depth in an exchanger of an overall length of 100 m and 3 sensors with a different radial distance from the axis of the BHE exchanger are present for each depth, the number Ns is equal to 20x3=60.

The electric cable with heating function 1 is typically adapted to supply heat flows (per unit of length along the depth) in the range of 10-rl00 W/m when connected to a single-phase 220 V power supply. Such heating cable 1 is of the commercial type, consisting for example of copper/nickel, chromium/nickel , iron/chromium/nickel, iron/chromium/aluminum alloys and of an electrical insulating coating with suitable characteristics also in terms of permanent protection from water in immersion mode (i.e. IPX9).

The above-ground control unit 3, which comprises, for example, a microPC or a microcontroller or microprocessor working, for example, with Linux, Android or Windows operative system provided with on-board program memory or outer medium, is intended to read and record the temperature values along the depth of the sensors' strip or cable 2 corresponding to the depth of the exchanger .

The connector for the power supply of the heating cable 1 is also present above ground. The heating cable 1 can possibly be connected to the electric network through an electronic module (i.e. TRIAC) which keeps the supply voltage constant when alternating between day and night.

When the electric heating cable 1 is connected to a supply 101, it delivers a given thermal power which heats the surrounding ground as a function of time, distance and chemical-physical characteristics of the ground itself.

The control unit 3, by reading the temperature detected by the sensors over time, is thus able to reconstruct a thermal map of the soil at many depths and radial distances from the heating cable 1. As will be seen in detail further on, it is possible to extrapolate thermal conductivity parameters of the ground able to allow the proper dimensioning of the geothermal plant to be constructed and, in an advantageous embodiment, the monitoring of the operations of an operating system.

To this end, in the specific example of Fig. 5, the control unit 3 is configured to execute the program instructions to implement the following steps of the method: powering the cable to heat the ground; reading the temperature values detected by the sensors; making, for each sensor, a temperature/time chart on semilogarithmic scale; identifying sections of each chart with an almost linear trend; calculating the slope of these sections; extrapolating thermal conductivity values of the ground from said slopes. If in the ground there is a filling layer of the borehole (grout), the method advantageously provides for identifying at least two sections of each chart with an almost linear trend, calculating the slope in these two sections and extrapolating the values of ground conductivity (kgr) and of the filling layer (kgt) from the values of said slopes.

In a particularly advantageous embodiment, the thermal conductivity values extrapolated at each depth and for each radial position are used as input for an iterative process that determines the optimum values by minimising a cost function that expresses the deviation of said values in relation to theoretical values obtained from a mathematical model of the ground of the infinite linear source (ILS) type with two thermal resistances, which will be discussed in detail in the following section.

Fig. 3 shows an embodiment wherein the sensors' strip or cable 2 is advantageously inserted into a rib 401 obtained on the outer surface of the piping 4, for example during the piping extrusion (typically of polyethylene). Thanks to this detail, it is not only possible to carry out pre-installation measurements but to also control measurements of an operating system if the piping in question is part of the hydraulic circuit of a ground coupled heat pump system using heat exchangers of the BHE type.

In this case, the electric heating cable 1 can advantageously be placed in a central (axial) position in relation to the hole constituting the BHE, as shown in the figure. An appropriate spacer system 402 (at interdistances of a few meters from each other for example) keeps the heating cable 1 in a central position in relation to the hole and piping 4 (with a single or double U) where the geothermal fluid will be circulated when the heat pump will be connected to the BHE exchangers and started. It should be underlined that it is not necessary to circulate the fluid in the piping for the measurements in question.

The spacer shown in Fig. 3, typically of metal, keeps the piping 4 at an assigned distance by acting as a two-arm needle spring. In the end part, each arm is anchored to the piping and stays in position. The central part of the spring, with a single or multiple spirals, is characterised by an inner diameter such as to allow the electric heating cable 1 to be housed therein.

The spacer is preferably placed in a vertical position different than that of the temperature sensors such as to minimise the "thermal bridge" effect on the measurement and to ensure that the sensors can interact only with the conductive Grout and Ground means from a thermal point of view.

METHODOLOGY FOR ANALYSING THE QUANTITIES MEASURED FOR THE CALCULATION OF THE PARAMETERS OF EDDTRT INTEREST The analysis of the temperature data measured for the estimation of the parameters of interest in the present invention is based on the original application of the thermal conduction theory. For this purpose, the Fourier number (which represents a dimensional time) can be introduced, defined as

where α is the thermal diffusivity of the ground, in turn defined as

Here P is the density of the ground and ^c is the specific heat thereof and t is the time.

The physical mathematical model, in relation to which the EDDTRT measurements were interpreted, is based on a series of assumptions which are summarised herein:

The transmission of heat is regulated by thermal conduction only (generalised Fourier's equation);

The heat flow released (or absorbed from) to the ground is constant over time and uniform along the source (heating cable);

The thermophysical properties can vary with the depth of the ground, but are constant in each layer of the ground to which the position of a temperature sensor corresponds;

The conduction can be assumed as monodimensional in the radial direction only;

There are not groundwater circulations significant for the heat exchange.

On the basis of these hypotheses, the solution of the Fourier 's equation becomes the following family of monodimensional solutions:

wherein

is the heat flow per unit of length, T(r_i, Z_j) is the temperature of the conductive means in the generic radial position r_i and in the generic vertical position Z_j . The radial position is referred to the axis of the BHE where the heat flow is generated. The temperature T(r_i, Z_j) is in this case the one measurable by an EDDTRT sensor placed at an assigned depth Z_j in the radial position r_i. ^T∞ is the undisturbed temperature (and initial, before starting the Thermal Test) of the ground, possibly estimated at each depth Z_j . The undisturbed temperature is measured for all sensors before activating the heating cable. is the heat transfer function (Temperature

Response Factor) characteristic of the model adopted and is used to interpret the temperature measurements and to obtain the parameters of interest therefrom. Among the possible transfer functions, the reference adopted herein is the ILS (Infinite Linear Source) solution. Finally, c is a constant which depends on the model used and Fo is the Fourier number.

The ILS model provides the presence of a linear thermal source of infinite length acting at constant heat flow and provides the solution to the problem in terms of radial temperature in each point of the conductive means and at each instant considered.

According to the infinite linear source model, the constant c is worth Aπ and

i_s the function E₁ known as the exponential integral and constituting the Solution to the time-varying conduction problem.

If the auxiliary variable X=1/(4Fo) is introduced , it is possible to express the solution E₁ as a function of X.

In the classic interpretative method (classic TRT), E₁(X) is calculated with a simple two-term expression having the form of: E₁(X)= -γ-1n(X) where g is a constant equal to 0.577216 known as Eulero constant. The approximation here above demonstrates not to be appropriate for representing the function E₁ if Fo<10 and this is a limit to the classic TRT model.

In the present interpretative model referred in the present invention, it is agreed to use a more precise expression of the function E₁ (X), the one suggested by M. Abramovitz and I.A. Stegun, Handbook of Mathematical Functions, 1964. E₁(X)= a₀-1n (X)+ a₁X + a₂X² + a₃X⁴ + a₄X⁴ + a₅X⁵ (0<X≤1) E₁(X)=[1/(X e^x)](b₁X + X²+ b₂)/(c₁X + X²+ c₂) (1≤X<∞) where the constants ao.as are respectively worth:

0.577216 0.999992 0.24991 0.0552 0.00976 0.001079 and the constants b₁, b₂, c₁, c₂ are respectively worth:

2.334733 0.250621 3.330657 1.681534

With the expressions of Abramovitz and Stegun, the function E₁ (X) can be properly estimated at each dimensionless time Fo and not only for the dimensionless times greater than 10 (corresponding to about ten hours, for the usual values of the perforation radius and thermal diffusivity of the ground) .

With reference to Fig. 6, the procedure for analysing and processing the measurements is articulated , in an embodiment, in the following steps:

1) measuring and recording the temperatures T_i,j of all sensors present at the depths Z_j and radial positions r_i with the thermal source (heating cable 1) not active. The measurements of the temperature sensors provide , in this step, the undisturbed temperature T∞,i,j at the different depths and radial positions . The physics of the problem makes it such that, when a thermal source is not active (and never activated in previous periods) , each Z_j must correspond to temperature values invariant with the radius r.

2) Starting the thermal power at the source, by activating the power supply to the heating cable and measuring the current I and applied voltage ΔV to the heating cable for the calculation of the electric power converted into thermal power (or heat flow with equivalent words).

Calculating the specific thermal power (power per unit of length) as:

where H is the overall length of the BHE exchanger measured along the vertical (depth) z.

3) Analysing the experimental temperature trends over time for each depth and radial position.

This preliminary analysis is used to identify whether the temperature trend of the sensor being studied has a trend with different slopes when represented in function of the logarithm of time, as shown in Fig. 4.

The procedure is aimed at estimating the slopes m_A and m_B of the corresponding sections where the temperature profile (with time on a logarithmic scale) takes a linear trend. The analysis of the temperature trend further allows to identify the characteristic times of the initial sloping T_mA, of intersection with the straight lines at a different slope TAB, of the end of second sloping T_mB (which typically coincides with the last instant of the test).

It should be noted that the slopes and times here above are peculiar for each sensor and, thus, there are N_s slopes and N_s times, if N_s is the number of temperature sensors.

It should be underlined that figure 4 is the result of a three-dimensional time-varying finite element simulation aimed at determining the temperature of a sensor (C) housed in the ribbing (B) of the piping (A) of Figure 3. The simulation (carried out ad hoc for this patent proposal) referred to in Figure 4, shows what the theory of the infinite linear source ILS requires to occur in reality. In fact, in a domain where thermal conduction prevails and two contiguous means are present, the "grout" means and the "ground" means, the physics of the problem requires that the temporal evolution of the temperatures in intermediate points is first controlled by the thermophysical properties of the nearest means (kgt) and that the effect of the furthest means (kgr) later becomes prevalent over time.

The above considerations are absolutely true for the furthest means (and thus for the kgr), whereas the kgt estimation is to be expected to be slightly underestimated due to the contiguous presence, at the sensor, of the piping containing liquid at rest, liquid which is characterised by a kfluid conductivity lower than kgt An improvement of the kgt estimation can be achieved by making the EDDTRT test with the piping empty of liquid and only with air in the piping.

The computer simulations have confirmed the evidence exposed above and the possibility to properly estimate the kgt and kgr from the analysis of the evolution of the temperature over time. In the finite element simulation model , the thermal conductivities kgt and kgr are assigned to the domain and are thus known; the temperature resulting in the position (C) when an electric heat flow is supplied in axis to the BHE (heating cable position) allows to properly calculate the kgr (uncertainty 4%) and kgt (uncertainty 10%) quantities according to the procedure referred to in the following point.

4) Applying the procedure for analysing the temperature data for the estimation of the kgt and kgr quantities (thermal conductivities of the grout materials and ground).

The analysis procedure relies on the following input data: temperature measured in undisturbed conditions for the different sensors T∞,i,j, heat flow at the electric heater, temperature at the different sensors Ti,_j following the start-up of the constant heat flow electric heating.

The slopes m_A,i,j and m_B,i,j and the characteristic times t_mA,i,j and t_mB,i,j are initially identified on the basis of the temperatures measured.

Starting from the input data, slopes and characteristic times, an optimum research analysis can advantageously be carried out in order to minimise the deviation (for example mean square deviation) between the measured temperature and the estimated temperature with the ILS model with two "thermal resistances". The deviation to be minimised (sensor by sensor, and thus N_s times) is described by the functions:

where w is an exponent which can be worth 1 or

2 ; for w=2, the deviation is by definition the mean square deviation.

N_mA and N_mB respectively denote the number of temperature measurements for the assigned sensor in the time intervals t_mA<t<t_AB and t_AB<t<t_mB.

The temperatures estimated according to the ILS (TILS) model can be expressed by the following mathematical expression, which implements the model with two thermal resistances, described with different purposes in relation to the present patent, for example, in the scientific paper of the present inventor "The temperature penalty approach to the design of borehole heat exchangers for heat pump applications" , Energy and Buildings 43 (2011) 1473- 1479:

N_mA and N_mB measurements and as many TILS estimations are available for each depth Z_j and radius ri: the minimisation of the deviation occurs by appropriately iterating on k_{i , j} and R_{bgt, i , j} to obtain, in the time interval t_mA<t<t_AB, the values k_{gt, j} and in the time interval t_AB<t<t_mB, the values k_{gr, j ,} and all the corresponding thermal resistances R_{bgt, i , j}. It should be observed that the parameters R_{bgt, i , j} (dimensionally thermal resistances per "unit of length" expressed in [mK/W]) do not represent the overall thermal resistance Rbhe even though they take on its formalism and share its dimensions.

The parameters R_{bgt, i , j} are auxiliary variables of the calculation necessary to reach the proper estimation of k_gr,j and k_{gt, j} with a strategy of deviation mitigation as expressed above.

Once the values k_{gt, j} (and possibly their arithmetic mean or weighed average on the interdistances Δz_j ,Δz_j=(Z_j+1-Z_j-1)/2) have been calculated , the Rbhe can be calculated from the correlations of related literature,which use the kgt together with the geometric parameters of the piping and the convective exchange coefficient related to the operating fluid actually operating (i.e. UNI 11466 Standard).

The procedure described in Fig. 6 ends by obtaining, for each depth at which temperature measurements were available, the corresponding thermal conductivity values of the ground and thermal conductivity of the grout.

It is therefore possible to know the parameters necessary for the dimensioning of the exchanger with this methodology.

When the geothermal plant will be operative and the geothermal exchangers are connected to the heat pump to air condition the building, the present invention will be able to constitute the monitoring system of the geothermal plant,underground part.

This monitoring,which is never available in the traditional system solutions, is able to represent a huge added value for the geothermal plant, both from an economic point of view in terms of operation (the monitoring allows optimised use of the overall geothermal plant) and from a marketing point of view for further implementations (in view of costs comparable to traditional solutions , the EDDTRT equipment provides measurements and estimations of the thermal parameters of the ground that cannot otherwise be deducible).

The monitoring of the temperatures along the exchangers can be used in many activities correlated to the control of the heat pump operation such as to increase its efficiency (COP) of conversion and avoid abnormal or dangerous operating conditions (too high or too low ground temperatures).

The monitoring of the temperatures in the ground further allows to check that no unexpected geological situations , such as for example the appearance or disappearance of groundwater movements in assigned ground layers, occur over time, even over the years.

In its general form, the invention thus concerns a method which comprises the following steps: a) making piping for the circulation of the heat operating fluid in the ground, the piping being equipped with innovative One-Wire/RS485 sensors and appropriate electric heater . The sensors are distributed both in longitudinal direction (along the depth of the ground and thus along the geothermal probe) and in radial direction. Sensors with variable interdistances in depths between 1 and 20 m and in radial direction with interdistances in the order of 0.01m, are provided.

The electric heating cable is of the resistive type, with an appropriate degree of electrical insulation to operate in an environment with high humidity content, and able to supply a heat flow per unit of length of the cable (and of the geothermal probe), between 10 and 100W per meter; b) implementing an electronic device for the reading of the digital signals of the temperature sensors, based on the use of a micro personal computer on a miniaturised board in turn appropriate for wireless communications, for example towards mobile devices (i.e. smartphones); c) measuring the temperatures from a set of multiple temperature sensors of the One-Wire or RS485 type, with digital reading of the temperature values over time, both during the release of thermal energy by a resistive electric cable (for EDDTRT measurements) and during the normal working conditions of the geothermal probe once connected to the heat pump (step of monitoring the performance and optimisation of the operating parameters of the heat pump).

The measurement of the electric power of the heating cable is juxtaposed to these measurements; d) development of an interpretative model to calculate the thermal conductivity values of the ground; e) development of an interpretative model to calculate the thermal conductivity values of the grout filling material, from which to in turn deduce the thermal resistance Rbhe at the current operating flow rate (and fluid) of the operating heat pump with formulas from literature.

The points (d, e) are based on the analysis of the trend of the temperatures detected over time. The overall mathematical model is aimed to properly interpret the measures in relation to time and the radial and longitudinal (in depth) positions of the temperature sensors on the basis of the ILS (Infinite Linear Source) mathematical solution. The ILS solution is, in this case, used within an algorithm for recognizing the parameters by using its complete analytical function (not only the logarithmic approximation as is currently done for the standard TRT analyses) for values of the Fourier number (written number with reference to the radius of the geothermal probe) between 0.1 and 10⁴.

The present invention thus concerns a method and a device to assess the thermophysical properties of the ground, the grout-filling cement mortar and, indirectly, the effective (and current) thermal resistance of vertical geothermal probes used in ground coupled heat pump systems. Unlike existing methodologies and equipment which establish similar measurements , the present methodology and respective device apply the ILS (Infinite Line Source) solution to interpret digital temperature measurements distributed both in depth and in radial direction within the volume occupied by the piping constituting the geothermal probe.

The measuring technique, the interpretative model and the respective measuring device are particularly adaptable for the implementation of geothermal probes consisting of "all in one" piping, where the piping (and/or its supports) has both the sensor part and the thermal source of the electric type. The present invention is thus intended for innovative measurements of the Distributed Response Test type, where, unlike what exists, no additional outer apparatus (the usual wheeled machine of the Thermal Response Test, TRT) is necessary since the integrated piping (all in one) referred to above already carries out all the necessary TRT measuring operations . Moreover, the method and the respective equipment allow measurements in reduced times (and at very low costs) in relation to the traditional solutions .

In addition, the present invention allows the contemporaneous estimation of the thermal conductivity of the filling material of the probe volume (known as "grout") and the thermal conductivity of the ground, measurements which are not possible with the normal above-ground TRT apparatuses .

The present invention thus allows EDDTRT (Electric Depth Distributed Thermal Response Test) measurements; at the same time, the same also allows successive operations of continuous monitoring of the ground temperatures at its different depths throughout the entire useful life of the system.

Claims

1. Method for determining thermal parameters of a ground adapted to accommodate one or more geothermal probes of a ground coupled heat pump system, which method provides, after a set-up step, in which a geothermal piping (ground heat exchanger) is implemented and a device bearing a heating cable of the resistive type and a plurality of temperature sensors at depths and radial distances known in relation to the heating cable is used: delivering a known electric power, preferably but not necessarily constant, to the cable to heat the ground; reading the temperature values detected by the sensors; making, for each sensor, a temperature/time chart on semilogarithmic scale; identifying sections of each chart with an almost linear trend; calculating the slope of these sections; extrapolating thermal conductivity values of the ground from said slopes.

2. Method according to claim 1, wherein in the ground there is a filling layer of a borehole (grout), the method providing for identifying at least two sections of each chart with an almost linear trend, calculating the slope in these two sections and extrapolating the values of ground conductivity (kgr) and of the filling layer (kgt) from the values of said slopes.

3. Method according to claim 1 or 2, wherein the thermal conductivity values extrapolated at each depth and for each radial distance are used as input for an iterative process that determines the optimum thermal conductivity values of the ground by minimising a cost function that expresses the deviation of said values in relation to theoretical values obtained from a mathematical model of the ground of the infinite line source (ILS) type with two thermal resistances.

4. Method according to claim 2 or 3, characterised by comprising the following steps: measuring and recording temperatures (T_i,j) of all the sensors present at the depths Z_j and radial distances r_i with the heating cable (1) not active, in order to acquire the undisturbed temperature (T∞,i,j) at different depths and radial distances; starting the thermal power at the source by activating the power supply to the heating cable; measuring the current I and the applied voltage ΔV to the heating cable for the calculation of the electric power converted into thermal power;

Calculating the specific thermal power as:

where H is the overall length of the BHE exchanger measured along the vertical z; analysing the experimental temperature trends over time for each depth and radial distance; estimating the thermal conductivities of the grout (kgt) and ground (kgr) materials on the basis of said trends , in particular by identifying linearity areas (A, B) with different slopes in temperature/time representations in semilogarithmic scale, and by calculating the slopes (m_A,i,j , m_B,i,j) and the start and end characteristic times (t_mA,i,j , t_mB,i,j) of said areas.

5. Method according to claim 4, wherein the following steps are further provided: carrying out, starting from the input data, slopes and characteristic times, an optimum research analysis in order to minimise the deviation between the measured temperature and the estimated temperature with the ILS model with two "thermal resistances", said deviation being described by the functions :

where w is an exponent that can be 1 or 2, N_mA and N_mB respectively denote the number of temperature measurements for the assigned sensor in the time intervals t_mA<t<t_AB and t_AB<t<t_mB, wherein the temperatures estimated according to the ILS model (TILS) are expressed by the following mathematical expression:

where

is the Fourier number, wherein the minimisation of the deviation occurs by iterating on k_i,j and R_{bgt, i , j} to obtain, in the time interval t_mA<t<t_AB, the values k_{gt, j} and in the time interval t_AB<t<t_mB, the values k_gr,j , and all the corresponding thermal resistances R_{bgt, i , j}-

6. Device for measuring thermal parameters of a ground adapted to accommodate, in boreholes, one or more geothermal probes of a ground coupled heat pump system, the device comprising: an electric heating cable (1) adapted to be introduced into the ground in a defined, particularly central, position in relation to the boreholes, and supplied by a power source of known electric power, preferably but not necessarily constant; a plurality of temperature sensors (S_i,j) adapted to be introduced into the ground at depths and radial distances known in relation to the heating cable; a control unit (3) configured to read the temperature values detected by the sensors (S_i,j) over time and, on the basis of the electric power supplied, to determine the values of the conductivity parameters at the various depths and radial distances from the heating cable (1).

7. Device according to claim 6, wherein the temperature sensors (S_i,j) are arranged, at known distances, on one or more strips or cables (2) to be inserted vertically into the ground.

8. Device according to claim 6 or 7, wherein the sensors (S_i,j) are of the one-wire type, i.e. they are distributed along one or more electric cables (2), typically with two wires, each sensor being independently addressable by the control unit (3) to read the detected temperature.

9. Device according to one or more of preceding claims 6 to 8, characterised by being provided in combination with the piping (4) of a probe of a borehole heat exchanger (BHE), said piping (4) having a peripheral axial housing (401) to accommodate the strip or the cable (2) bearing the temperature sensors (S_i,j).

10. Device according to claim 9, wherein said housing (401) comprises a ribbing on the outer surface of the piping (4), preferably obtained during the piping extrusion, typically of polyethylene.

11. Device according to claim 9 or 10, wherein there is a spacer (402) adapted to keep the heating cable (1) in a fixed position in relation to the piping (4).

12. Device according to claim 11, wherein the piping (4) is U-shaped, the spacer (402) being configured to keep the heating cable (1) in a central position in relation to the vertical branches of the piping (4).

13. Device according to one or more of preceding claims 6 to 12, wherein the control unit (3) is configured for: reading the temperature values detected by the sensors; making, for each sensor, a temperature/time chart on semilogarithmic scale; identifying sections of each chart with an almost linear trend; calculating the slope of these sections; extrapolating thermal conductivity values of the ground from said slopes.

14. Device according to claim 13, the control unit (3) being configured to identify at least two sections of each chart with an almost linear trend, to calculate the slope in these two sections and extrapolate the conductivity values of the ground (kgr) and of the filling layer of a borehole or grout (kgt) from the values of said slopes.

15. Device according to one or more of preceding claims 6 to 14, wherein the thermal conductivity values extrapolated at each depth and for each radial distance are used as input for an iterative process that determines the optimum values by minimising a cost function that expresses the deviation of said values in relation to theoretical values obtained from a mathematical model of the ground of the infinite linear source (ILS) type with two thermal resistances .