Acoustic power modulation in a thermoacoustic machine

Abstract

The invention relates to a thermoacoustic machine (1) comprising a control device (14) configured to modulate the acoustic power generated by one or more acoustic sources (6-9) so that the temperature of an external source (2) connected to one or more thermoacoustic cells (10-13) of the machine (1) reaches or remains substantially the same as a setpoint temperature. Figure for the abstract: Fig. 1

Description

Title of the invention: Acoustic power modulation in a thermoacoustic machine technical field

[0001] The invention relates to the field of thermoacoustic machines.

[0002] The invention is of particular interest for thermoacoustic machines designed to operate in heat pump mode, as distinct from motor mode. In physical terms, heat pump mode corresponds to the use of the mechanical energy of a sound wave to pump energy from a thermal source, also called the pumping source, raise its temperature, and then transfer it to a second thermal source, also called the rejection source, the temperature of the rejection source being therefore higher than the temperature of the pumping source. A heat pump can be used as a heating system, by raising the temperature of the rejection source used as a heating means, or as a cooling system, by lowering the temperature of the pumping source used as a cooling means. Prior art

[0003] In a manner known per se, a thermoacoustic machine is a heat engine in which, according to the physical principles of thermoacoustics, thermodynamic cycles are carried out within a working fluid. In motor mode, these cycles generate mechanical energy in the form of an acoustic wave from a heat input. In heat pump mode, these cycles generate heat pumping using the mechanical energy of the acoustic wave.

[0004] Conventional thermoacoustic machines designed to operate as heat pumps comprise one or more acoustic sources, typically electromechanical actuators or thermoacoustic wave generators, configured to generate an acoustic wave within a waveguide containing the working fluid. This acoustic wave provides, in the form of work, the mechanical energy necessary to simultaneously transfer and raise the temperature of heat drawn from a cold external source to a hot external source.

[0005] In contemporary thermoacoustic machines, such as the one described in US8584471B2, heat transfer is ensured by a thermoacoustic cell arranged in the waveguide. The cell comprises a regenerator and two heat exchangers arranged on either side of the regenerator.

[0006] During the heat pumping process, the thermodynamic cycle implemented The acoustic wave, through the consumption of acoustic work, generates a heat flow from one end of the regenerator to the other, resulting in a temperature gradient along the regenerator. Heat exchangers, on the other hand, ensure heat transfer between the working fluid and a heat transport element, such as a heat transfer fluid connected to a respective external heat source. Specifically, one exchanger transfers heat from a first heat transfer fluid to a working fluid by pumping heat into a pumping circuit through which this first heat transfer fluid circulates. The other exchanger, conversely, transfers heat from the working fluid to a second heat transfer fluid, thus rejecting heat into a rejection circuit through which this second heat transfer fluid circulates.

[0007] In general, the machine is controlled in "on-off" mode, i.e. by supplying the acoustic sources until the temperature of the rejection source - for heating - or of the pumping source - for cooling - reaches a setpoint temperature, after which the control of the sources is interrupted as long as the setpoint temperature is maintained. Description of the invention

[0008] The invention aims to improve the energy efficiency of a thermoacoustic machine, in particular a thermoacoustic machine operating in heat pump mode.

[0009] To this end, the invention relates to a thermoacoustic machine comprising: - a waveguide intended to receive a working fluid, - an acoustic source configured to generate an acoustic wave in such a way as to propagate acoustic energy in the waveguide, and - a thermoacoustic cell comprising a regenerator, a first heat exchanger configured to carry out a heat exchange between the working fluid and a first heat transport element to a first external source, and a second heat exchanger configured to carry out a heat exchange between the working fluid and a second heat transport element from a second external source.

[0010] According to the invention, the machine includes a measuring device for at least one parameter representative of a temperature of the first external source and / or the second external source and a control element configured to modulate the acoustic power of the acoustic source so as to modify the temperature of the first external source and / or the second external source as a function of at least one parameter.

[0011] The invention makes it possible to control the machine by reducing or eliminating interruptions power supply for the acoustic source.

[0012] More specifically, the invention makes it possible to reach a target temperature in the first or second external source and to maintain this temperature in an interval around the target temperature by modifying the acoustic power generated by the acoustic source.

[0013] For example, when the machine is implemented for the purpose of heating a room formed by the first external source, the control element can be configured to reduce the acoustic power to a non-zero value when the room reaches a setpoint temperature and, in the event of a subsequent reduction in the room temperature, to increase the acoustic power by the amount required to reach the setpoint temperature again.

[0014] Similarly, when the machine is implemented for the purpose of cooling a room formed by the second external source, the control device can be configured to reduce the acoustic power to a non-zero value when the room reaches a setpoint temperature and, in the event of a subsequent increase in the room temperature, to increase the acoustic power by the amount required to reach the setpoint temperature again.

[0015] Without limitation, said at least one parameter may be chosen from the following parameters: - a temperature of the first heat transfer element, - a temperature of the second heat transfer element, - a temperature of the first external source, - a temperature of the second external source, - a temperature of the working fluid, and - an acoustic pressure of the working fluid.

[0016] In one embodiment, the acoustic source includes a motor with a moving element, the control member being configured to modify an amplitude and / or a frequency of displacement of this moving element.

[0017] According to a first variant, the motor is an electric motor, for example a linear motor.

[0018] In this first variant, the moving element can be a piston, for example a single or double piston.

[0019] According to a second variant, the motor is a rotary motor.

[0020] In one embodiment, the control element is more specifically configured to modify the amplitude of a voltage and / or current supplying the acoustic source, so as to modulate the acoustic power it generates.

[0021] The acoustic source can also be a thermal thermoacoustic engine.

[0022] Thus, in one embodiment, said thermoacoustic cell is a first thermoacoustic cell, the acoustic source being formed by a second thermoacoustic cell, this second thermoacoustic cell comprising a regenerator, a first heat exchanger configured to carry out a heat exchange between the working fluid and a third heat transport element to a third external source, and a second heat exchanger configured to carry out a heat exchange between the working fluid and a fourth heat transport element from a fourth external source.

[0023] In such a case, the control unit is preferably configured to modify a quantity of heat transported by the third heat transport element and / or the fourth heat transport element.

[0024] Such a modification of the quantity of heat makes it possible to modify the temperature gradient within the second thermoacoustic cell and consequently to modulate the acoustic power that it generates.

[0025] The third and fourth external sources can each be different from both the first and second external sources. Alternatively, the third or fourth external source can be identical to one of the first and second external sources so as to form a three-temperature machine.

[0026] In one embodiment, the third heat transport element and the fourth heat transport element each comprise a heat transfer fluid, the control member being configured to modify a temperature and / or a flow rate of the heat transfer fluid of the third heat transport element and / or the fourth heat transport element.

[0027] The invention also relates to a method for controlling such a thermoacoustic machine.

[0028] This process preferably includes a modulation step which comprises: - a measurement of said at least one parameter, - a comparison of a value of at least one parameter thus measured with a reference value, - control of the acoustic source so as to modulate the acoustic power it generates if these values ​​are different in order to modify the temperature of the first external source and / or second external source.

[0029] Said reference value may be a predetermined value or a previously measured value.

[0030] When the reference value is predetermined, it may be variable and dependent on an external factor such as the daytime or nighttime period or other.

[0031] In one embodiment, the modulation step is repeated over time.

[0032] Said reference value is preferably a setpoint value.

[0033] In other words, the invention can be implemented so that the temperature of the first external source and / or the second external source reaches a setpoint temperature and / or so that this temperature remains the same or close to such a setpoint temperature.

[0034] Other advantages and features of the invention will become apparent from the following detailed, non-limiting description. Brief description of the drawings

[0035] The following detailed description refers to the attached drawings on which:

[0036] [Fig-1] is a schematic view of an installation comprising a thermoa machine acoustic according to a first embodiment of the invention, this machine comprising four acoustic motors and four thermoacoustic heat pumping cells;

[0037] [Fig.2] is a schematic view of an installation comprising a thermoa machine acoustic according to a second embodiment of the invention, this machine comprising two thermal engines and two thermoacoustic heat pumping cells. Detailed description of implementation methods

[0038] Schematically represented in [Fig.1] is an installation comprising a thermoacoustic machine 1, external sources 2 and 3 and a distribution network 4 linking the machine 1 to the external sources 2 and 3.

[0039] The thermoacoustic machine 1 is intended for operation in heat pump mode, in the physical sense of the expression.

[0040] In general, the machine 1 comprises a waveguide 5, four acoustic sources 6, 7, 8 and 9, four thermoacoustic cells 10, 11, 12 and 13, a control unit 14 and a measuring device 15.

[0041] Each of the thermoacoustic cells 10 to 13 includes a regenerator 16, a first heat exchanger 17 and a second heat exchanger 18.

[0042] In this example, the waveguide 5 is a tube defining an internal space, in a closed loop, forming an acoustic waveguide.

[0043] The internal space of the waveguide 5 contains a pressurized working fluid for propagating an acoustic wave. The working fluid can be a monatomic gas, a polyatomic gas such as a mixture comprising helium and argon or another mixture, or a mixture of a gas and a liquid.

[0044] Such a geometry of the waveguide 5, which is in no way limiting, makes it possible to promote the development of a wave with a progressive character and more specifically to obtain locally at the level of the regenerators 16 a wave with a progressive character.

[0045] The waveguide 5 is preferably made of a material such as a metallic alloy or other material that allows the working fluid to be contained under pressure.

[0046] The acoustic sources 6 to 9 and the thermoacoustic cells 10 to 13 are mounted in series along the waveguide 5, in an alternating manner, so that each of the cells 10 to 13 is disposed between two respective of said acoustic sources 6 to 9.

[0047] In this example, each of the acoustic sources 6 to 9 is a linear motor comprising a moving element of the piston type.

[0048] Each of the sources 6 to 9 is configured to generate an acoustic wave in the working fluid, under the action of a displacement of the piston, so as to propagate acoustic energy in the waveguide 5.

[0049] For each of the thermoacoustic cells 10 to 13, the regenerator 16 and the exchangers 17 and 18 are arranged in the waveguide 5 so as to be traversed by the working fluid in order to carry out a thermoacoustic energy conversion.

[0050] The regenerator 16 is a porous structure, that is to say a structure with pores or cavities or openings allowing to increase or maximize the contact surface and therefore of exchange with the working fluid while minimizing pressure losses.

[0051] By way of example, the regenerator 16 of each of the cells 10 to 13 may for this purpose comprise a stack of slats or grids, made of a material having a high heat capacity and a low thermal conductivity, for example a stainless steel or a ceramic material.

[0052] In operation, the regenerators 16 behave like thermal sponges with respect to the working fluid, alternately storing and releasing heat.

[0053] For each of the thermoacoustic cells 10 to 13, the heat exchangers 17 and 18 are arranged on either side of the regenerator 16 so as to be able to carry out, at the ends of the regenerator 16, a heat exchange between the working fluid and a respective heat transport element.

[0054] More specifically, for each of the cells 10 to 13, the first heat exchanger 17 is in this example configured to carry out a heat exchange between the working fluid and a first heat transport element consisting of a heat transfer fluid circulating in a conduit 20 connected to the external source 2, while the second heat exchanger 18 is configured to carry out a heat exchange between the working fluid and a second heat transport element also consisting of a heat transfer fluid circulating in a conduit 21 connected to the external source 3.

[0055] To achieve such a heat exchange, each of the exchangers 17 and 18 may comprise, in a manner known per se, conductive elements forming, for example a stack of fins in contact with the working fluid. Such fins can be made of a conductive metal such as copper or aluminum.

[0056] Thus, the conduits 20 and 21, which form said distribution network 4, allow heat to be transported between the working fluid and, respectively, the sources 2 and 3 which are external to the machine 1, via the heat transfer fluid circulating in these conduits.

[0057] In an alternative embodiment, not shown, the first heat transport element and / or the second heat transport element may not be a heat transfer fluid but a solid element such as fins, part of which forms the, or is connected to, the distribution network 4.

[0058] In one embodiment, the distribution network 4 includes a heat pipe (not shown).

[0059] In this example, the external sources 2 are exhaust sources which together constitute a room to be heated, and the external sources 3 are pumping sources formed by an external space that acts as a thermal reservoir of air or water that is relatively cold compared to the air circulating in the room. In other words, the ducts 20 associated with the thermoacoustic cells 10 to 13 form a parallel configuration in this example. Similarly, the ducts 21 associated with the thermoacoustic cells 10 to 13 form a parallel configuration. Alternatively, a series configuration can be implemented (not shown).

[0060] Sources 2 and 3 thus form thermal reservoirs external to machine 1.

[0061] Of course, machine 1 can also be used not to heat a The room is cooled, but conversely, the room is cooled. Thus, in one variant, the pumping sources 3 can together constitute a room to be cooled, and the discharge sources 2 can be formed by an external space that acts as a thermal reservoir of air or water that is relatively warm compared to the air circulating in the room. The present description applies by analogy to such an implementation variant.

[0062] An example of the operation of the installation of [Fig.1], with a view to heating room 2, will now be described.

[0063] The control member 14 is actuated so as to drive the motors 6 to 9 in order to move their piston according to a periodic function, which in this example is a sinusoidal function of the type ^i(f) — A,sin (2~J t + <p^ avec i le numéro de la source (dans cet exemple, la source 6 a pour numéro i = 1, la source 7 a pour numéro i = 2, la source 8 a pour numéro i — 2 et la source 9 a pour numéro i - 4),‘ le temps, X, la position du piston de la source i, A; l’amplitude du déplacement du piston de la source i, f . la fréquence du déplacement du piston de la source / et la phase associée au déplacement du piston de la source i.

[0064] In a non-limiting manner, it is considered here that / j — / 2~ f 3 ~ / 4 and zi । — A 2 “ A 2 — A.^.

[0065] The displacement of the piston of engine 7 is out of phase with the displacement of the piston of engine 6, the displacement of the piston of engine 8 is out of phase with the displacement of the piston of engine 7, the displacement of the piston of engine 9 is out of phase with the displacement of the piston of engine 8, and the displacement of the piston of engine 6 is out of phase with the displacement of the piston of engine 9.

[0066] In this non - limiting example, the value of this phase difference is respectively = <P2 - <Pr = (P3 ~(P2 = <P4- <P3 = " ^ / 2-

[0067] Such a control of sources 6 to 9 allows acoustic energy to be propagated in the waveguide 5 in a propagation direction producing a progressive or quasi - progressive acoustic wave in the regenerator 16 of each of the thermoacoustic cells 10 to 13 moving in a direction from the exchanger 17 towards the exchanger 18.

[0068] In a manner known per se, such propagation of acoustic energy makes it possible to give rise to a Stirling-type thermal cycle resulting in each of the cells 10 to 13 in a thermoacoustic energy conversion associated with a heat transfer on the one hand from the heat transfer fluid circulating in the duct 21 to the working fluid by the heat exchanger 18 and, on the other hand, from the working fluid to the heat transfer fluid circulating in the duct 20 by the exchanger 17, in other words a pumping of heat from the outside space 3 and a rejection of heat to the room 2.

[0069] Machine 1 thus makes it possible to heat room 2 so that the air it contains reaches a setpoint temperature.

[0070] According to the invention, the machine 1 makes it possible to modify the temperature of the room 2, in particular to increase it when it is below the setpoint temperature and to reduce it when it is above the setpoint temperature, by modulation of the acoustic power generated by the motors 6 to 9.

[0071] In this particular example, the measuring device 15 is configured for this purpose to measure the temperature of room 2.

[0072] The temperature of room 2 thus measured is compared to the setpoint temperature, using a calculation means (not shown) of machine 1.

[0073] If these two temperature values ​​are different, or if the absolute difference of the gap between these two values ​​is greater than a predetermined threshold, for example 1°C, the acoustic power of the motors 6 to 9 is modified, in this case lowered if the temperature of room 2 is greater than the setpoint temperature and increased if the temperature of room 2 is less than the setpoint temperature.

[0074] This process of measuring, comparing and modifying acoustic power constitutes a modulation step that can be repeated continuously over time.

[0075] The machine 1 thus makes it possible to pump heat from the external source 3 to heat the room 2 in such a way as to reach the setpoint temperature and to maintain a temperature equal to or close to the setpoint temperature over time, without it being necessary to interrupt the supply of the motors 6 to 9 when the setpoint temperature is reached and then to re-supply them when the temperature of the room 2 moves away from the setpoint temperature, or at least by reducing the number of interruptions necessary.

[0076] In this non-limiting example, the control member 14 is more specifically configured to modulate the acoustic power of the motors 6 to 9 by modifying the amplitude of their supply voltage so as to modify the amplitude of the displacement of their piston.

[0077] Of course, this acoustic power modulation can be achieved by modifying other control parameters of the motors 6 to 9, for example, the amplitude of their supply current and / or their phase. Furthermore, particularly in embodiments (not shown) in which the motors 6 to 9 are rotary motors, the acoustic power modulation can result from a modification of the frequency and / or the amplitude of the movement of the moving element of the motors 6 to 9. In addition, other parameters can also be modulated in order to modify the temperature of the external sources 2 and / or 3, for example, the flow rate and / or temperature of the heat transfer fluid circulating in the duct 20 and / or in the duct 21 connected to one or more of the thermoacoustic cells 10 to 13.

[0078] Furthermore, the measuring device 15 can be configured to measure one or more parameters other than the temperature of the external sources 2, such as the temperature of the heat transfer fluid circulating in the duct 20 and / or in the duct 21 connected to one or more of the thermoacoustic cells 10 to 13, the temperature and / or the acoustic pressure of the working fluid, and / or the temperature of the external sources 3, in particular when the latter form a room to be refrigerated.

[0079] More generally, the acoustic power modulation of the machine 1, in order to reach a setpoint temperature in the external sources 2 and / or 3, is carried out as a function of one or more parameters representative of the temperature of the external sources 2 and / or 3, including the parameters listed above but not limited to them.

[0080] In alternative embodiments, not shown, the acoustic sources 6 to 9 of machine 1 in [Fig. 1] may comprise actuators other than linear motors, for example rotary motors. Furthermore, the moving element of the acoustic source may be a single or double piston or other.

[0081] Figure 2 illustrates a second embodiment which differs from the embodiment of Figure 1 in that the machine 1 comprises not four but two thermoacoustic heat-pumping cells 11 and 13 and in that it comprises not four acoustic sources but two acoustic sources 30 and 31 which in this example are heat engines. The preceding description applies by analogy to this second embodiment, which is essentially described according to its differences from that of Figure 1.

[0082] With reference to [Fig.2], the acoustic sources 30 and 31 and the thermoacoustic cells 11 and 13 are mounted in series along the waveguide 5, in an alternating manner, so that each of the cells 10 and 13 is arranged between the acoustic sources 30 and 31, and vice versa.

[0083] The acoustic sources 30 and 31 are each formed by a thermoacoustic cell of the same type as the cells 11 to 13 described above. Thus, each of the cells 30 and 31 comprises a regenerator 32, a first heat exchanger 33 and a second heat exchanger 34 which are arranged in the waveguide 5 so as to be traversed by the working fluid in order to carry out a thermoacoustic energy conversion.

[0084] For each of the sources 30 and 31, the first heat exchanger 33 is configured to carry out a heat exchange between the working fluid and a third heat transport element consisting of a heat transfer fluid circulating in a conduit 35 connected to an external source 36. The second heat exchanger 34 is configured to carry out a heat exchange between the working fluid and a fourth heat transport element consisting of a heat transfer fluid circulating in a conduit 37 connected to an external source 38.

[0085] The conduits 35 and 37 form a distribution network 40 distinct from the distribution network 4 of the thermoacoustic cells 11 and 13.

[0086] Furthermore, the external sources 36 and 38, which form thermal reservoirs external to the machine 1, are in this example distinct from the external sources 2 and 3 to which the thermoacoustic cells 11 and 13 are connected. In an alternative embodiment, the external sources 2 and 36 are identical.

[0087] In this example, the external sources 36 are rejection sources containing a relatively cold fluid and the external sources 38 are sources that provide heat through a relatively hot fluid.

[0088] The machine 1 allows to refrigerate / air condition a room made up of the external sources 3 which in this case form pumping sources, the external sources 2 forming discharge sources.

[0089] To achieve this, the control element 14 of the machine 1 is configured here to modify the temperature and / or the flow rate of the heat transfer fluid circulating in the conduits 37 connected to motors 30 and 31, so as to modify the amount of heat transferred by this heat transfer fluid.

[0090] This results in a modification of the thermal gradient between the ends of the regenerator 32 of each of these motors 30 and 31 leading to a modification of the acoustic power generated by the motors 30 and 31.

[0091] In a manner known per se, such a thermal gradient makes it possible to generate and maintain an acoustic wave in the working fluid, so as to propagate acoustic energy in the waveguide 5.

[0092] In this example, the acoustic energy moves along a direction of propagation producing a progressive or quasi-progressive acoustic wave in the regenerator 16 of each of the thermoacoustic cells 11 and 13, going from the exchanger 17 to the exchanger 18.

[0093] The thermoacoustic cells 11 and 13 can thus carry out a thermoacoustic energy conversion as described above in order to pump heat from the room 3 and reject it to the outside space 2, making it possible to cool the room 3 so that the air it contains reaches a setpoint temperature.

[0094] In this example, the measuring device 15 is configured to measure the temperature of room 3, the machine 1 being able to carry out said modulation step by controlling the supply of the thermal engines 30 and 31, in this example by modifying the temperature and / or the flow rate of the heat transfer fluid circulating in the ducts 37.

[0095] Numerous variations can be made to this second embodiment, in particular by applying by analogy the variations of the first embodiment described above. For example, the distribution network 4 and / or 40 may include a heat pipe (not shown). As another example, the sources 30 and 31 and the cells 11 and 13 may be arranged differently relative to each other, with the sources 30 and 31 being able to follow one another and the cells 11 and 13 following one another, in contrast to the alternating arrangement shown in [Fig. 2].

[0096] The embodiments and variants described above can also be combined. For example, machine 1 may include one or more linear and / or rotary motors and / or one or more thermal engines. Machine 1 may obviously include a different number of acoustic sources and / or thermoacoustic cells. Furthermore, the various components of this machine may have structural and / or geometric differences compared to the preceding description.

Claims

Demands

1. Thermoacoustic machine (1) comprising: - a waveguide (5) intended to receive a working fluid, - an acoustic source (6-9;30, 31) configured to generate an acoustic wave so as to propagate acoustic energy in the waveguide (5), and - a thermoacoustic cell (11-13) comprising a regenerator (16), a first heat exchanger (17) configured to effect a heat exchange between the working fluid and a first heat transport element (20) to a first external source (2), and a second heat exchanger (18) configured to effect a heat exchange between the working fluid and a second heat transport element (21) from a second external source (3), characterized in that it comprises a measuring device (15) for at least one parameter representative of a temperature of the first external source (2) and / or the second external source (3) and a control element (14) configured to modulate the acoustic power of the acoustic source (6-9;30, 31) so as to modify the temperature of the first external source (2) and / or the second external source (3) as a function of at least one parameter.;

2. Machine (1) according to claim 1, wherein at least one parameter is selected from the following parameters: - a temperature of the first heat transport element (20), - a temperature of the second heat transport element (21), - a temperature of the first external source (2), - a temperature of the second external source (3), - a temperature of the working fluid, and - an acoustic pressure of the working fluid.

3. Machine (1) according to claim 1 or 2, wherein the acoustic source (6-9) comprises a motor having a moving element, the control member (14) being configured to modify an amplitude and / or a frequency of displacement of this moving element.

4. Machine (1) according to any one of claims 1 to 3, wherein said thermoacoustic cell (11, 13) is a first thermoacoustic cell, the acoustic source (30, 31) being formed by a second thermoacoustic cell, this second thermoacoustic cell- acoustic (30, 31) comprising a regenerator (32), a first heat exchanger (33) configured to carry out a heat exchange between the working fluid and a third heat transport element (35) to a third external source (36), and a second heat exchanger (34) configured to carry out a heat exchange between the working fluid and a fourth heat transport element (37) from a fourth external source (38), the control member (14) being configured to modify a quantity of heat transported by the third heat transport element (35) and / or the fourth heat transport element (37).

5. Machine (1) according to claim 4, wherein the third heat transport element (35) and the fourth heat transport element (37) each comprise a heat transfer fluid, the control member (14) being configured to change a temperature and / or a flow rate of the heat transfer fluid of the third heat transport element (35) and / or the fourth heat transport element (37).

6. Method of controlling a thermoacoustic machine (1) according to any one of claims 1 to 5.

7. A method according to claim 6, comprising a modulation step which includes: - a measurement of said at least one parameter, - a comparison of a value of the at least one parameter thus measured with a reference value, - a control of the acoustic source (6-9; 30, 31) so as to modulate the acoustic power which it generates if these values ​​are different in order to modify the temperature of the first external source (2) and / or the second external source (3).

8. A method according to claim 7, wherein the modulation step is repeated over time.

9. A method according to claim 7 or 8, wherein the reference value is a setpoint value.