WO2017158489A1

WO2017158489A1 - Method and device for assessing thermophysical properties of the ground and effective thermal resistance of geothermal probes

Info

Publication number: WO2017158489A1
Application number: PCT/IB2017/051438
Authority: WO
Inventors: Marco FOSSA; Davide ROLANDO
Original assignee: Erde S.R.L.; Universita' Degli Studi Di Genova
Priority date: 2016-03-14
Filing date: 2017-03-13
Publication date: 2017-09-21
Also published as: ITUA20161616A1

Abstract

Method for assessing thermophysical properties of the ground and effective thermal resistance of a geothermal probe, comprising: a) circulating a heat-transfer fluid in a closed hydraulic circuit (1) comprising at least one geothermal probe (2); b) releasing or subtracting heat flow from the heat-transfer fluid delivered to the geothermal probe; c) measuring the temperatures of the delivered heat-transfer fluid (10) and the returned heat-transfer fluid (11) to/from the geothermal probe; d) calculating the values of thermal conductivity of the ground and the values of thermal resistance of the geothermal probe (2) on the basis of a pattern analysis of detected temperatures versus time; wherein said calculation provides the use of a mathematical model of effect superposition so that the time curve of the average of temperatures detected on the delivery and return sides with respect to the geothermal probe is reconstructed by the infinite line source solution and a summation of contributions relating to a series of thermal pulses having steady value per each time step of the analysis (12), the use of an iterated algorithm (14, 15; 16) being provided for searching said values of thermal conductivity of the ground and the values of thermal resistance of the geothermal probe (2) by minimization cf a target function describing the error between the experimental, temperature curve and the curve reconstructed from the superposition model.

Description

^' & hod and devie® for assessing tfeermo hys cal properties of th g ou d and ef ec ive fcherss l

TEXT OF THE- DESCRIPTION

Object of the present invention is a. method for assessing thermophysioai properties of the ground and effective therm l resistance of g othermal probes, comprising the following steps:

a) circulating a heat- transfer fluid in a closed hydraulic circuit comprising at least one geothermal robe;

b: releasing or subtracting heat flow from the heat-transfer fluid delivered to the geothermal probe;

c) measuring the temperatures of the delivered beat-transfer fluid and the returned heat-transfer fluid to/from the qeothernral probe;

d calculating the values of thermal condcctivity of the ground and the values of therm l resistance of the geothermal probe on the basis of a pattern analysis of detected temperatures versus t ime ,

This type of methods are currently known and used in the area of geothermal plants for dimensioning fields of geothermal probes.

The Ground Coupled Heat Pump systems , also named low-enthalpy geothermai plan s, exploit the natural heat of the ground with the aid of a hea pump for heating or cooling the buildings and use geothermal probes (borehole heat exchangers) installed, in. the most recurring arrangement, vertically in the ground up to depths fros 80 to 150 m_f to draw or dissipate heat from or to the subsoil.

In fact, the ground is characterized by temperatures extremely stable over time and thus can be used as lower ^thermal source" for winter air- conditioning and higher one for sum? nn air- conditioning. Temperatures of the ground, also during the operation of the heat pump, are almost always more favorable {in terms of energy efficiency of the plant) than ambient air ones.

The ground coupled heat pump uses a reverse cycle between the ground and the building to be air- conditioned, On the building side an operating fluid (typically water or air) conveys the energy to and from the building depending on thermal requirements of the latter ( surmter/ointe ) The ground coupled heat pump can also be used for producing sanitary hot water with coefficients of performance higher with respect to conventional solutions, where the reverse machine vaporises ambient air.

Thanks to favorable ground temperatures ail year round and the proper sizing of underground heat exchangers, the ground coupled heat pump systems are able to achieve coefficient* of performance {COP.; in the range 3,5-5 as seasonal average_f thus decreasing the energy consumption and related emissions drastically with respect to conventional fuel system or air vaporizing heat pumps. Correct sizing of underground heat exchangers is in tarn linked to the knowledge of ther ophysical ground properties and inner thermal resistance of qeothermai probes.

Ground coupled heat pumps, combined with probes in the ground, can replace completely boilers and ir-conditioners for heating and cooling rooms and for producing sanitary not water.

Both in new constructions ana in many renovations, geothermal plants can be installed in every type of public and private buildings,- for residential; manuf ctu ing, service or commercial

The geothermal probe is an. underground heat exchanger, usually consisting of a double piping Ό~ joined .in the lower end part of the bore where pipes have been inserted. The vertical bore is made in the ground for depths usually between 80 and 150 m; the piping becomes integral with the bore wall (also from the thermal point of view) thanks to the filling of the free volume with grout suitable for thermal appl icat ions .

Ά plant generally provides a plurality of geothermal probes identifying the probe field; the individual underground probes are installed at predetermined reciprocal distances in the site of thermal exploitation ..

One of the problems of the above described pe-.one a 1 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 he. probe field can highly decrease machine COP over years, or even, cause conditions in which the machine stays blocked due to tempe atures of the operating fluid coming from the ground, oh.; ch are not compatible with operation parameters of the reverse machine.

Hot-sati fied building requirements can also be caused by inefficiencies in the geothermal plant.

The design of trie probe field needs a series of input information, among which thermal building requirements during the year, thermop ysical properties of the ground, thermal resistance of the geothermal probe. For what concerns the energy needs of the building, we can recourse to methods of theoretical derivation (e.g., 11300 Uni regulations}, on the contrary assessment of thermal, properties of ground and probe needs experimental investigations in itu . The thermal resistance of the probe is a parameter describing the behavior, from the heat exchange point of v low, of the heat -trapsi^'er fluid, piping wells and grout filling' the space regaining between piping and poring: this resistance depends primarily from the geometry the piping are arranged (with respect to a horizontal section of the vertical heat exchanger} in the bore, from the thermal conductivity of the grout and the met 1 en regime or the operating fluid in th pipes.

The general method for determining r he too 1 parameter of ground and probe is named RT ~ Thermal Response Test or also GRT , Ground Response Test, .As described above, thermal response means the pattern of temperatures in a fluid circulating in the underground probe to which a steady over time heat flow is supplied (or subtracted) in the overground piping part.

Test is based on the assumption that the heat exchange in the ground is regulated by the Thermal Conduction mechanism onl .

The forced heat flow being the same, a little variation (during time) of the temperature of the operating fluid denotes high thermal conductivity of the ground. On the contrary, when the temperature variation during tim of the fluid circulated in the ground is high, it is correlated to low thermal conductiviti s of the ground. For example, if we consider the case in which the operating fluid is heated, if the ground conductivity is high, this heat flow is released h'transferred" } to the ground with little temperature gradients {conduction Fourier's law; and, therefore, with fluid temperatures closer to the undisturbed ground temperature. Usually, a single pilot geotherrral probe is used for carrying out the TFT,

The classic and literature method of carrying out the TRT provides for a steady heat flow transferred to the operating fluid, In this operative condition the average fluid, empera ure is logged (average of the delivered/returned fluid} and, successively, the development during m is analyzed of such average temperature in accordance with a t iirse-varying conductive heat exchange based on the solution of the infinite line source (IL3 Ingersoii et ai . 1954) and ho ensen ' s working hypothesis (1 83; .

The analytic structure of the particula mathematical model used allows particularly to obtain the thermal conductivity of the ground depending on the slope of the temperatu e line (with respect to the time} when a semilogarithmic scale representation is adopted.

However, in practice the requirement of steady heat flow can be hardly satisfied for rot of reasons. The typical, case is the production of the heat flow transferred to the fluid through electric line by connection to the mains; in this recurring case record, day/night voltage jitters make impossible to guarantee a steady heat flow during the whole test duration {50-100 uninterrupted hours; . Another case of variable heat low is that linked to eiectrrcal connections in case of working anomalies or service interruption,

A farther case of unsteady heat flows is that linked to the generation of heat flow b a refrigerating machine, whose performances (exchanged heat flows} are intrinsically variable with the temperatures of fluids the machine interacts with.

Therefore, in classic TRT method a substantial lack of respect of the main working hypothesis of ILS method (strictly steady heat flow) is found, with consequent errors and doubtfulness about estimation of target parameters.

Not least, there are further important rea sons accoroking to which. a variable heat flow TRT experiment is particularly interesting.

These reasons can be summarised in that rsaking ON/OFF sequences of the thermal power has the following advantages:

an OP/OFF heating experiment allows temperature increases of the ground to be limited, for example in case wherein this temperature is already high due to the presence of geother ai anomalies (thermal regions)

- OS/OFF sequence follows the actual working of the heat pump (and of the underground probes thereof; in the real operation;

The OFF step following the end of the conventional steady heat flow experiment can be used for adding the measured data to search the parameters of the reverse problem, treated herein {as known, in every statistical analysis the sample number decreases the uncertainty! ,

The present invention aims to overcome these drawbacks relating to known methods by a method described at the beginning, wherein said calculation further provides the use of a mathematical model of effect superposition so that the time curv of the average of temperatures detected on the delivery and return aides with respect to the geotherrnai probe is reconstructed by the infinite line source solution and a summation of cont ibutions relating to a series of thermal pulses having steady value pe each time step of the analysis. The use of an iterated algorithm is provided for searching said values of thermal conduct ivi t y of the ground and the values of thermal resistance of the geotherrnai probe by minimization of a target function describing the error between the experimental temperature curve and the curve reconstructed from the .superposition model- In this way it is possible to cope with the variations of the heat flow, since once these variations have been measured, and consequently are known, it is possible to decompose the pattern of the temperature average and reconstruct the signal as he summation of stepped pulses., then to minimise the difference between the measured values and those obtained from the signal reconstruc ion in an iterated process- taking account of target parameters.

This means that the algorithm allows the reconstruction of a curve following the real pattern of what is measured, through the iterated optimization of the target parameters,

in an exemplary embodiment, said steps a) , b and c} are carried out continuously for a time interval typically from 50h to IfOh.

Various studies demonstrated that the duration of the thermal response test must be related to the ther ophysicai properties of the ground with respect to those of the material need for filling the bore (grout;. Horeover, the amount of water in the piping has an important role. If the TR.T duration cannot be set by criteria known a priori to obtain a reiiabie estimation of the target parameters, ex pest analysis of measurement allows verification of the applicability of XLS interpretative model to a convenient sub-interval of the time series of measured temperatures. Also in case of physical properties coincident between ground and grout, the ILS model applied to TRT necessitates not using the measurements for an initial period between 5 to 20 times the Fourier ^fs number referred to ground propert ies »

The Fourier's number^' (denoting a dimensional

F " , ⁽O^Χi^ί

time} is defined as '^;: , where ^a is the thermal k a; ~ di ffusivity of the ground,- in turn defined as -^{: :} . herein, ^ is the ground density and is the specific heat thereof.

The mathematical-physical model. the TRT mea urements being interpreted relating thereto, is based on a series of assumptions summarized herein:

The heat transfer is regulated by the thermal conduction only (generalized Fourier's equation)

The heat flow transferred (or absorbed to the ground is steady over time and uniform along the source (geotno mal probe)

The theraophysical properties of the ground are steady and the medium is homogeneous

The conduction can be assumed as onodimensionai 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 monodimensionai solutions:

wherein *-^· is the hear iiow per length unit, is the temperature at the bore well, T*= is the undisturbed temperature of the ground, Ϊ is the heat transfer function (temperature response factor) peculiar to the adopted model _f is a constant depending on the used model (e, g,, the IL3 model; and P

^* xs the rhnrier' s numoer.

There are three main models for describing the behavior of a single vertical probe. Each of then- refers to a given probe geometry.

The simplest mohe 1 is the already entioned infinite line source (IIS) model,,, wherein the heat exchanger is embodied by an infinite length line immersed in the medium (ground; and subjected, to a steady and uniform neat flow.

Other models are those or infinite cylinder source (ICS) and finite line source FLS; .

In accordance with the infinite line source, the constant ^c ίΐ 1 ^; and ^ is the function ' known as exponential, integral depending from the Fourier's number too.

The use of the effect superposition proposed in the present method allows the use of the 1 hi model also in case the heat flow is not steady over time. The effect superpositi n, in this sense, describes any t ime- arying heat f low function ύ s a variable profile and consisting of a series of steps (stepwise function} .

In a TRT analysis of monodiirensionai type

CMo.gensen, 1983), the thermal interaction between operating fluid and ground is described by a model with two thermal resistances . According to this model, temperatures and thermal resistances at snake f are the temperature ot the heat- transfer fluid ·^' ;to be intended as the average between delivery and

7' .... :·:«'..

r eturn sides, .;.. a , ^ valued overground and time-varying) _f the wail temperature oi the rore T

T

the undisturbed temperature ot trve ground " "^; , the therraal resistance of the geothermai probe '···· and the thermal resistance (time -varying} of the ground R

T

The undisturbed temperature oi the ground ^*«■ is measured before carrying out the Thermal Response Test and just by circulating the heat transfer fluid in th underground probe for a predetermined ime, without heating the fluid. In this ay, the fluid temperature moves to the thermal equilibrium with the ground that, by definition, in every point thereof is in the undisturbed condition. ( ^""■'^·»} (also at the probe periphery., at ::. I .o bore interface) .

When a heat flow is imposed on the operating fluid, the snonodirmansionai radial model w th two resistances provides for the arising of temperature differences at the ends of the thermal resistances at stake, in accordance with the known, relation:

AT )' R

yvhere the heat flo is expressed again per length unit ( [ /mj } .

The temperature difference between the wall bore and the undisturbed ground,- in accordance ith the XLS model, is gi er by;

from which the above rntroduced term thermal resistance of the ground can be deduced:

The other thermal resistance controlling the heat diffusion is the inner thermal resistance (inside the bore) ¾, Therefore, the temperature difference between the fluid and the ground is given by the following expression

4?XK .

O"

In the herein proposed method, the model for the TR analysis with two resistances with XLS transfer function is generalized in case the heat flow is time-varying according to a stepwise function (figure 3) .

f,

In this case, the fluid temperature ' (time-- 5 varyin ; is obtained as:

The equation contains the unknowns ^Wv and ^K , also through the Fourier's number.

The herein proposed mechod adopts a pa r; si 10 searching process through the error analysis between

f

the measures value of ·' and tne one recur: ^:·· v ;:ua:r cd in accordance with the proposed model .

The XLS model use, in combination with the effect superposition applied to a series of heat ):■ flows, has the advantage of being highly simple, since it uses in a new way a model employed as standard in known TRT methods.

The heat flow is typically supplied to the operating fluid (and then to the ground) during0 heating and by mea s of electric resistances.

If calorlmetric measurements (differences between delivered/returned temperatures and estimation of the current specific heat) and the electrical ones allow an effective control of the5 steadiness of the heat flow supplied to the fluid, the proposed method superimposes a series of stepped thermal pulses {heat flows) havinq steady values over tim.e which, de facto, constitute the case of the conventional xperiment in which the heat flow is invar iant during he whole TRT <

In this cay ,. although the me h d allows the analysis or. TRT data also in presence of variations of the heat flow,- the approach i applied in the real case too.

According to a further embodime t _f the heat flow is regulated in accordance w I ι h a predetertvined pattern ,

This allows the TRT to be carried out also with non-steady heat 11ow . but with characteristics predetermined a priori and then applicable in specific situations requiring a process different from the conventional one, in order to simulate real operative conditions of a heat pump (machine on/off; ,< in case of power failure and. system and mains malfunctions' .

The method for simulating real conditions of (discontinuous) operations can be defined as "pulsed flow" method, wherein periods in which the heat flow has a given value alternate with periods in which the heat, flow is null,

in th s sense a puised-TRT execution, possible regarding the data analysis of the present invention (proposed method) , allows the values of ground thermal conducti ity and the values of thermal resistance of the geothermal probe to be estimated in conditions as much as possible similar to the operative conditions of the heat pumps, which will then use the field of geotheiisai probes. Object of 5 the present invention is further a device for assessiiiq theriuophysical properties of ground and geothermal probes, comprising a closed hydraulic circuit comprising at least one qeothei^'mal probe,, at least one circulating pump for the circulation of a

W heat-transfer fluid inside said hydraulic circuit, an electric heater to heat the heat-transfer fluid delivered to the geothermal probe^'y sensors of the temperature of the delivered heat- transfer field and the returned heat-t ansfer fluid with respect to the

:5 seor he mob probe, a recording and retransmitting system for recording and retransmitting the measured data and a calculation code implementing the effect superposition algorithm for calculating the vaiues of thermal conducti ity of the ground and the vaiues of0 thermal resistance of the geothermal probe on the basis of the pattern of detected temperatures versus time. Said, electronic calculating means provides the use of a mathematical model of effect superposition so that the time curve of the average of temperatures5 detected on the delivery and return sides with respect to the geothermal probe is described as effect of a summation of stepped steady thermal pulses per each time instant, the use of an iterated algorithm being provided for the optimisation of said parameters to be detected, on the basis of a search depending on the reconstruct on of the average temperature curve of the operating fluid through the error minimisation with respect to the curve derived from the iTieasu r enients .

The device allows the aforesaid method to he carried out.

.According to an embodiment , said electric heater generates an electric heat flow, measuring means to measure the mains power (by measurements of voltage, current, phase; and regulating means to regulate the heat flow supplied b said heating means being provided, also on the basis of the measurements of the delivery ana return temperatures to/from the qeother al probe nd: the measurements of the supplied electric power (measurements of voltage, current, phase) .

In an embodiment variation, the temperature varying means consist of cooling means to cool the heat - ransfer fluid, such as a refrigeration unit coupled to a heat exchanger thermally contacting the heat -transfer fluid.

In a further embodiment, said heat flow regulating means maintain the heat flow steady over time -

According to an embodiment, heat id ow regulating means to regulate the heat flow in accordance with a redetermined pattern are provided.

In a further improvement ,· said predetermined pattern is of the ON/OFF pulsed or steady flow type.

These and other^' advantages of the present invention will be more apparent from the following descri tion of some exemplary embodiments depicted in: the accompanying drawings, wherein:

fig. 1 is an exemplary embodiment of a device according to the present invention;

fig, 2 depicts a block diagram of an exempiary embodiment of the method object of the present invention;

fiq. 3 depicts an exemplary method of effect superposition to reconstruct the average temperature of the operating fluid;

figs. 4 to 7 depict graphs of possible patterns of the heat flow versus time and the tempera ure of the heat-transfer fluid.

Figure 1 depicts a device for carrying out a TRT according to th present invention, which device comprises a closed hydraulic circuit 1 to which a geothermal probe 2 is connected.

The geothermai probe 2 is an underground heart exchanger consisting of a piping haying double or quadruple pipe and being U-joined at its lower end, inserted in a cylindrical ground bore with 80-150 m typical depth in which the piping are fastened to and integral with the bore wall by specific grouts. The i i g of the single probe can have only one U (two pipes} or a double U (double pipes), but it can also be of coaxial type. The piping can be made: of polye hylene.

The device provides a circulating pump 3 for the circulation of a heat-transfer fluid inside the hydraulic circuit 1,

The heat-transfer fluid can be of any type used in known devices, preferably water, possibly the water being mixed with a.nti- freeze agents.

The device s provided with heating ssans to heat the heat-transfer fluid delivered to the ge.othe.rrQa I probe, preferably one electric heat exchanger 4 connected to the mains ¾ for generating^' a heat flow by electric Joule effect.

Measuring mea s to measure the temperature- of the delivered heat-transfer fluid 6 and the returned heat-transfer fluid 7 with respect to the geot.her.rnal probe 2 and electronic means 8 for recording and retransmitting data and for calculating the values of thermal conductivity of the ground and the values of thermal resistance of the geothermai probe on the basis of the pattern of detected temperatures versus time, are provided.

The device is provided with measuring means S to noasarc the electric powery measuring means 9b to measure the flow rate and regulating means to regulate the heat flow supplied by the electric heat

18 exchanger 4_t on the basis of the delivered, and returned measured temperatures to./from. the geothermal probe and the measu ements of the measured mains power .

When a variation of the neat flow is detected, identifiable by an anomalous variation of both the mains power and the calorirnetric balance relating to the delivery and return sides of the operating; fluid, the heat flow is modified, corrected and returned to predetermined valuer:.

The heat flow regulating means can provide,, for example,. voltage regulators for supplying the electric heat exchanger 4 or TB.1&C devices for controlling the supplied current.

The device works in accordance with the method for assessing theιmophysic.a1 properties of ground and geothermal probes described in figure 2, wherein the heat-transfer fluid is circulated in the closed hydraulic circuit 1 arid ; the geothermal probe 2 and is heated, on the delivery side, to the geothermal probe 2 by the electric heat exchanger 4.

Then measurements of the temperature of the delivered heat transfer fluid 10 by the measuring means 6 and measurements of the temperature of the returned heat transfer fluid 11 by the measuring means 7 are performed, and the calculation of the curve of the average temperature 12 is performed by the calculating electronics means b , by carrying out the arithmetic mean of the temperatures at the delivery and return sides.

Th n a first couple of values, named, attempt values, of thermal ground conducti vrty rid rd^er ai resistance of the geotheroeil probe 13 is: sot through which the iterated process of parameter searching can be started. Through the attempt values a empe ture curve is generated by the ma i homer ca 1 model of effect superposit on 1 approximat ing, oyer time, the curve of the average of the detected del ivery and return cemperatuzes with respect to the geothermal probe in accordance wi th the approach based on : he sum/mat ion of stepped steady thermal poises per each time instant.

Then an iterated algorithm is used for optimizing such values so that the curve reconstructed, on the basis of the calculated values approximates the -measured curve except for an error the algori hm has to minimize.

In each reiteration a comparison 15 is carried out between the calculated curve generated by or resulting from the previous iterative step and the measured curve calculated in step 12,

If the deviation between the measured curve and the generated one onone foe accepted, a modif icat ion 16 of the values of thermal ground conductivity and the vaiues of thermal resistance of the geothermal probe is carried out.. The modification of the values 1.6 allows the generation of a new curve modified with respect the curve of the previous iterative step or the initially generated ore . which modified curve is therefore compared with the measured curve again.

In. a first embodiment variation,, the modification of the parameters 16 is carried out if the deviation value resulting from the comparison exceeds a predetermined value error.

The error between the curves can be calculated, for o-rr e, by the arithmetic mean between the point by point percentage deviation between the calculated curve and the curve measured in accotdan.ee with the following formula;

7

in. which ^■' denotes the temperature measured at f"

the i-th instant and ^' denotes the corresponding c icu 1at ed tempe atere ,

hex^ the curves differ in the predetermined erro ; typically lower than 0.05%,. the values optimized by the last modification are outputted 17.

In this way the algorithm allows a curve to be reconst ucted following the real pattern of what is measured through the iterated optimization of the target parameters. In a second embodijaent variation a conventional optimization algorithm is used., which tends to indefinitely optimize the deviation error between the c rves ,

When the algorithm, generates two values equal to those generated by the previous iterative step, the optimization process is completed and the generated val ues are output ted 16.

In an exemplary embodiment_f the heating of the heat transfer fluid is carried out by electric heat flow of which the electric power and the thermal power are contemporaneously measured for the calor irnet ric balance, and a regulation of its heat flow is provided on the basis of the coupled cower raeasore ents above .

Moreover, the method provides the regulation of the heat flow, which is carried out so that the eat flow is maintained steady over time or in accordance with a predetermined pattern.

Figure 3 shows an example of the effect superposition method, wherein the heat flow per unit length transferred to the operating fluid, and therefore to the ground, is decomposed in a series of stepped components coming one after another.

In the example of figure 4, the heat flow 18 supplied to the heat transfer fluid is stopped after a given time interval, oneteas the heat transfer fluid continues circulating in the gee-thermal probe The behavior of the temperature 19 is clearly affected by the heat flow 18 , arid the curve of the ternperat ure 20 {dotted curve} reconstructed by the optimization algoxithm approximates accurately the curve of e measured temperature 19.

The method object of the present invention allows the temperature profile to be simulated and described also in the period following the stop of the neat flow, in which the temperature decreases. in the example shown in figure the pattern is of the ON/OFF pulsed or a ready flow type, i.e. it alternates periods in which the neat flow has a predetermined non-null value with periods in which the heat flow is null.

The heat flow per unit, length 18 is pulsed by alternating OH type periods with OFF type periods, Such operating condition could be, for example, the one characterising a mains interru tion

Also in this condition in which conventional approaches would have failed,, the method allows an estimation of the parameters to be carried out.

The pattern can. be selected in the most appropriate mode ,· for example ON/OFF periods can correspond to each other or not, and can be equally or differently repeated over time, in this way to simulate a real on/off operation of a heat ump.

A TFT execution according to the present invention with these heat flow features allows the v lues of ground thermal conductivit and the values of thermal resistance of the out fro tool probe to be estimated in conditions as much as possible similar to the operative conditions of the heat purups, which will then use the field of geothermai probes.

Figure 6 shows an example in which the ef oots of the envoi. ron ental conditions are simul a ted, such as the air teraperacnre and the sol r irradiation on the real heat flow supplied to the neat transfer fluid, in particular^" in a situation in which the overground part of piping, in which the operating fluid flows, are not thermally insulated sufficiently, or in which voltage jitters in the mains change the steadiness of the electric power of the heater.

Figure 7 shows an example in which the Thermal Response Test is carried out by cooling the heat transfer fluid through a refrigeration system . In this case, if no logic control units are employed, the heat flew subtracted from the operating fluid is highly t irse-dependeot and typical iy decreases as the temperature of the operating fluid itself decreases,

.As clearly visible _t the generated temperature curve is able to approximate accurately the curve of measured temperature also in this case in which conventional methods may not lead to any reliable estimation of the values of thermal ground conductivity and the values of i. he -Ί'^ ΐ resistance the geother.ip.aI probe .:..

Claims

1. ethod for assessing thermophysi cal properties of the ground and thermal resistance of a geotherrriai probe, eoraprising the following steps:

a) circular..trig a heat-t ansfer fluid in a closed hydraulic circuit (1) comprising at least one qeo dermal probe (2) ;

b) releasing or subtracting heat flow to/from the heat-t ansfer fluid delivered to the geother al probe (-2) ;

c) measuring the temperatures of the delivered heat-transfer fluid {10} and the returned heat- transfer fluid (11) to/from the geothermal probe;

d} calculating the values of thermal conductivity of the ground and the values of thermal resistance of the geothermal probe (2) on the basis of a pattern analysis of detected temperatures versus t ime ;

wherein said calculation provides the use of a mathematic l model of effect uperposi ion so that the time curve of the average of temperatures detected on the delivery and return sides with respect to the geothermal probe is reconst ucted by the infinite line source solution and a summation of contributions relating to a series of thermal pulses having steady value per each time step of the analysis (12), the use of an iterated algorithm. (15, 16, 17) being provided for searching said values of thermal conduc ivi y of the ground and the values of thermal resistance of the geothemai probe by minimi nation of a target function describing the error between the exper imeivtal temperature curve and the curv-e reconstructed from the superposition model .

2, Method according to claim 1, wherein said soa s a) ,,- b) and c; are carried out con inuously for a time interval from 5Oh to 150h_f preferably of about 10Oh .

3,. Method according to claim 1, wherein the time curve of the average T^■'. between: the temperatures detected on the delivery and return sides with respect to the cool derma 1 probe is obtained as;

.4. Method according to c 1 a im i, wherein the heat flow is an electric heat flow of which the electric powe and the thermal power are measured b calor iiaetric balance, a regulation of the heat flow being provided on the basis of said electric and thermal powers,

5. Method according to claim 4, wherein the regulation of the heat flow is carried out so that the heat flow is maintained steady ove time.

6. Method according to clai 1, wherein the heat flow is regulat d i ordance with a predetermined patte n ,

7. Method according to claim 6, wherein said

2.8 prede ermined pattern i of thfe pulsed flow type (17) .

6, Method according to one or more of the preceding claims, wherein the heat riow is subtracted S from he delivered heat-transfer fluid thanks to a ref igeration system,

, Device for carrying out a Thermal Response Test, comprising a closed hydraulic circuit (1) comprising at least one geothermai probe (2) _f at i0 least one circulating pump (3; for the circulation of a heat-tran for fluid inside said hydraulic circuit (1) , varying means (4) to vary the temperature of the heat-transfer fluid delivered to the geothermai prohe {2} , measu ing means to measure the temperature of

IS the delivered heat- transfer fluid (6) and the returned heat-transfe fluid (?) with respect to the geothermai probe (2) and electronic means ;8) for recording and retransmitting the measurements and for calculating the values of thermal conductivity of the0 ground and the values of thermal resistance of the geothermai probe on the basis of the pattern of detected temperatures versus time,

characterized in that

said electronic calculating means (8) provide 5 the use of a mathematical model of effect superposition so that the time curve of the average of temperatures detected on the delivery and return sides with respect to the geo her .al probe {2} is described thanks to an algori hm using a summation, of stepped steady thermal pulses per each time instant, the use of an iterated algorithm being provided for the optimiza ion of said values to be detected, on the basis of a search depending on the reconstruction of the average temperature curve of the operating fluid through the error minimization with respect to the curve derived from the measurements,

10, Device according to claim 9, wherein, said temperature varying means consist of heating means ( ) to heat the heat-transfer fluid and are of electric type and generate an electric heat flow, measuring means 19) to measure the electric power and regulating means to regulate the heat flow supplied by said heating means being provided, on the basis o calorimet ric balances (measuremen, of the delivery and return temperatures to/from the geothermai probe) and electric power (measurements of voltage, current, phase) .

II. Device according to claim 10, wherein said heat flow regulating means maintain the heat flow steady over time.

12. Device according to claim 9y wherein heat flow regulating means to regulate the heat flow in accordance with a predetermined patter are provided.

13, Device according to claim 12, wherein said predetermined pattern is of the pulsed flow type.