GB2524382A - Control unit, heat coupling circuit and method for operating such a heat coupling circuit - Google Patents

Abstract

A heat coupling circuit 15 comprises an expansion engine 90, a first heat exchanger 40, 45, 50, 55 and a second heat exchanger 165, a sensor 350, a steam valve 110, 115, 120, 125 and a bypass 235 comprising a valve 240. The bypass arranged to bypass the section including the steam valve and expansion engine. There may be multiple expansion engines and associated steam valves arranged in parallel. The first heat exchanger may comprise a series of heat exchangers. A control unit 300 for operating the heat coupling circuit, wherein at least one parameter of the heat exchange medium is detected by the sensor, the parameter is compared to first and second predefined threshold values to determine the operation of the steam valve 110 and the bypass valve 240.

Description

Description Title

Control unit, heat coupling circuit and method for operating such a heat coupling circuit The invention relates to a control unit according to Claim 1, to a heat coupling circuit according to Claim 6 and to a method for controlling the heat coupling circuit according to Claim 10.

Prior art

Heat coupling circuits, which have at least one expansion engine, a steam valve arranged on the upstream side of the expansion engine and a first heat *exchanger and a second heat exchanger, are known. The first heat exchanger serves to heat a heat transfer medium arranged in the heat coupling circuit by means of a heat source. The heat transfer medium is evaporated in the first heat exchanger and flows via the steam valve into the expansion engine.

From the expansion engine, the heat transfer medium is led to the second heat exchanger, where it is then liquefied again and is conveyed via a return line and a feed pump arranged in the return line back to the first heat exchanger. In order to control the heat coupling circuit, a control unit which is connected at least to the steam valve is provided.

Disclosure of the invention

The object of the invention is to provide an improved control unit, an improved heat coupling circuit and an improved method for controlling the heat coupling circuit.

This object is achieved by means of a control unit according to Claim 1. Advantages embodiments are specified in the dependent claims.

According to the invention, it has been found that an improved control unit for controlling a heat coupling circuit can be provided by the fact that the control unit comprises a control device, an input, an output and a memory. The control device is connected to the input, to the output and to the memory. The input is connectable to a sensor. The output is connectable to a bypass valve and to a steam valve of the heat coupling circuit. The input is formed to detect a sensor signal of the sensor and provide it to the control device. A first predefined threshold value and a second threshold value different from the first threshold value for a state variable of a heat exchange medium of the heat coupling circuit are stored in the memory. The control device is formed to carry out a comparison of the sensor signal with the first threshold value and with the second threshold value. The control device is formed to provide, in dependence on a result of the comparison, a control signal via the output for controlling the steam valve between an open position and a closed position and a further control signal via the output for controlling the bypass valve between an open position and a closed position.

This configuration has the advantage that the control unit can automatically control the operation of the heat coupling circuit. The control unit can independently avoid an overloading of the expansion engine of the heat coupling circuit and simultaneously keep the efficiency of the heat coupling circuit particularly high.

In a further embodiment, the control device is formed to provide, on an undershooting of the first threshold value by the state variable, the control signal for closing the steam valve. It is thereby avoided that a pressure before the steam valve of the heat coupling circuit drops too low.

In a further embodiment, the control device is formed to provide, on an overshooting of the second threshold value, the further control signal for opening the bypass valve. An overloading of the expansion engines of the heat coupling circuit is thereby avoided.

In a further embodiment, the output is connectable to at least one further steam valve, wherein a third threshold value which is correlated with a predefined first valve position of at least one of the steam valves is stored in the memory, wherein the control device is formed to provide, on an overshooting of the third threshold value by the first valve position, a further control signal for opening at least one of the further steam valves. This increases the efficiency of the heat coupling circuit.

In a further embodiment, a fourth threshold value which is correlated with a predefined second valve position of the bypass valve is stored in the memory, wherein the control device is formed to at least partly close, on an overshooting of the fourth threshold value by the second valve position, at least one of the steam valves. This further increases the efficiency of the heat coupling circuit.

The object is, however, also achieved by a heat coupling circuit according to Claim 6. Advantageous embodiments are specified in the dependent claims.

According to the invention, it has been found that an improved heat coupling circuit can be provided by the fact that the heat coupling circuit comprises a first expansion engine, a first heat exchanger, a second heat exchanger, a first steam valve, a bypass, a sensor and a control unit.

The heat coupling circuit is fillable with a heat transfer medium. The bypass comprises a bypass valve. The first expansion engine is connected to the first heat exchanger and to the second heat exchanger. The first heat exchanger is formed to heat the heat transfer medium and the second heat exchanger is formed to cool the heat transfer medium.

The first steam valve is provided on the upstream side of the first expansion engine between the first expansion engine and the first heat exchanger. The bypass is arranged parallel to the first expansion engine and the first steam valve between the first and the second heat exchanger and is formed to at least partly bypass the first expansion engine in an at least partly opened state of the bypass valve. The control unit is connected to the first steam valve, to the bypass valve and to the sensor. The sensor is formed to detect at least one state variable of the heat exchange medium before the first steam valve and to provide the control unit with a sensor signal corresponding to the detected state variable. The control unit is configured as described above. The heat coupling circuit can thereby be automatically operated by the control unit and at the same time an overloading and underloading of the heat coupling circuit or its expansion engine can be avoided.

In a further embodiment, at least one second expansion engine and at least one second steam valve is provided. The second expansion engine is arranged on the downstream side with respect to the first heat exchanger between the first and the second heat exchanger and is connected to the first and second heat exchanger. The second steam valve is arranged on the upstream side with respect to the second expansion engine and is connected to the control unit. The control unit is formed to control the second steam valve between an open position and a closed position, wherein the control unit is formed to at least partly open, on an exceeding of the third threshold value, the second steam valve. The second expansion engine can thereby be automatically activated and thus an overloading of the first expansion engine avoided. At the same time, the efficiency of the heat coupling circuit is kept particularly high.

In a further embodiment, the control device is formed to compare in a further comparison a first valve position of at least one of the steam valves with a third threshold value and, in dependence on the result of the comparison, to close the second steam valve and to deactivate the second expansion engine. An operation of two or more expansion engines at a poor efficiency is thereby avoided, so that the first expansion engine can be operated by the deactivation of the second expansion engine in a particularly good efficiency range.

In a further embodiment, the control device is formed to compare in a further comparison a second valve position of the bypass valve with a fourth threshold value and, in dependence on the result of the comparison, to open the second steam valve.

It is thereby avoided that heat energy flows via the bypass past the first expansion engine and thereby is operated in a poorer efficiency range.

The object is achieved, however, also by a method according to Claim 10.

According to the invention, it has been found that an improved method for controlling a heat coupling circuit can be provided by the fact that at least one state variable of the heat exchange medium of the heat coupling circuit is detected, wherein the detected state variable is compared in a comparison with a first predefined threshold value and with a second predefined threshold value, wherein, in dependence on a result of the comparison, at least the first steam valve and the bypass valve is controlled between an open position and a closed position. The heat coupling circuit can thereby be automatically controlled.

The invention is explained in more detail below with the aid of figures, in which: Figure 1 shows a schematic representation of a power station; and Figure 2 shows a schematic representation of a flow diagram for representing a method for operating the power station shown in Figure 1.

Figure 1 shows a schematic representation of a power station 10.

The power station 10 has a heat coupling circuit 15 and a heat source circuit 20. The heat source circuit 20 has a heat source 25 and a first feed pump 30. The heat source 25 can be, for example, a thermal solar collector system, an internal combustion engine, a natural gas engine, a biogas system or other heat sources. The heat source 25 is connected via a first connection 35 to the first feed pump 30.

The heat coupling circuit 15 and the heat source circuit 20 have in common a heat exchanger system 40 with a first heat exchanger 45, a second heat exchanger 50, and a third heat exchanger 55. In the heat source circuit 20, on the output side of the first feed pump 30, the first heat exchanger 45 is connected via a second line 60 to the first feed pump 30. The second heat exchanger 50 is connected to the first heat exchanger 45 via a third line 65. On the output side of the second heat exchanger 50, the third heat exbhanger is connected via a fourth line 70. On the output side of the third heat exchanger 55, a fifth line 75 is provided, which connects the third heat exchanger 55 to the heat source 25. A first heat transfer medium 80 flows into the heat source circuit 20. The first heat transfer medium 80 can be, for example, water. The first heat transfer medium flows via the first line 35 to the feed pump 30 and from the feed pump 30 via the second line 60 to the first heat exchanger 45. From the first heat exchanger 45, the first heat transfer medium 80 flows via the third line 65 towards the second heat exchanger 50. From the second heat exchanger 50, the heat transfer medium 80 flows via the fourth line 70 into the third heat exchanger 55, from where it flows via the fifth line 75 back to the heat source 25 again.

The heat coupling circuit 15 has ati expansion engine system 85. In the embodiment, the expansion engine system 85 has a first expansion engine 90, a second expansion engine 95, a third expansion engine 100 and a fourth expansion engine 105. Of course, a different number of expansion engines is also conceivable. The expansion engines 90, 95, 100, 105 have in each case a turbomachine and a generator coupled thereto, which converts a rotational movement of the turbomachine into electrical energy.

On the upstream side of each expansion engine 90, 95, 100, 105, for each expansion engine 90, 95, 100, 105 there is respectively provided a steam valve 110, 115, 120, 125. In front of the first expansion engine 90 there is provided a first steam valve 110, in front of the second expansion engine 95 a second steam valve 115, in front of the third expansion engine 100 a third steam valve 120 and in front of the fourth expansion engine 105 a fourth steam valve 125. The steam valves 110, 115, 120, 125 are connected on the input side to a distributing line 130. The distributing line 130 is furthermore connected on the output side of the first heat exchanger 45. On the output side, the first steam valve 110 is connected via an eighth line 135 to the first expansion engine 90. The second steam valve 115 is connected via a ninth line 140 to the second expansion engine 95. The third steam valve 120 is connected via a tenth line 145 to the third expansion engine 100 and the fourth steam valve 125 is connected on the output side via an eleventh line 150 to the input side of the fourth expansion engine 105. The expansion engines 90, 95, 100, are connected on the output side to a collecting line 155. The collecting line*155 is connected by a twelfth line to a fourth heat exchanger 165. For better efficiency of the fourth heat exchanger 165, which is formed as an air heat exchanger in the embodiment, a plurality of fans 169 are provided, in order to supply the fourth heat exchanger with sufficient air for cooling. On the output side, the fourth heat exchanger 165 is connected via a thirteenth line 170 to a storage container 175. The thirteenth line furthermore has a junction 180, to which a recirculating line 185 is connected.

Connected to the storage container 175 is furthermore a fourteenth line 190, which connects a feed pump system 195 having two second feed pumps 200 to the storage container 175. However, a different number of second feed pumps 200 is also conceivable. On the output side of the second feed pumps 200, a fifteenth line 205 is provided, which connects the second feed pumps 200 to the one shutoff valve 210. On the output side of the shutoff valve 210, a sixteenth line 215 for connecting the shutoff valve 210 to the third heat exchanger 55 is provided. On the output side of the third heat exchanger 55, the third heat exchanger 55 is connected via a seventeenth line 220 in the heat coupling circuit 15 to an input side of the second heat exchanger 50. On the output side, the second heat exchanger 50 in the heat coupling circuit 15 is connected via an eighteenth line 225 to the first heat exchanger 45.

In the heat coupling circuit 15, a second heat transfer medium 230 is provided. The second heat transfer medium 230 can comprise, for example, water, just as the first heat transfer medium 80, and/or a refrigerant. Of course, it is also conceivable for the second heat transfer medium 230 to comprise a different material. Thus, for example, it is conceivable for the second heat transfer medium 230 to comprise additionally or alternatively hydrocarbons, in particular fluorocarbons, as the heat exchange medium.

In the embodiment, the power station 10 is embodied as an ORC power station (Organic Rankine Cycle power station).

However, other embodiments are also conceivable.

At the first heat exchanger 45, furthermore abypass 235 is provided, which has a bypass valve 240. The bypass valve 240 is connected on the output side of the first heat exchanger 45 and is connected via a nineteenth line 245 to the first heat exchanger 45. On the output side, the bypass valve 240 is connected to the collecting line 155.

To control the power station 10, a control unit 300 is provided. In order to control the heat coupling circuit 15, the control unit 300 is connected via a first connection 305 to the first expansion engine 90, via a second connection 310 to the second expansion engine 95, via a third connection 315 to the third expansion engine 100 and via a fourth connection 320 to the fourth expansion engine 105. The connections 305, 310, 315, 320 can be formed as a bus system. Of course, it is also conceivable for the respective expansion engines 90, 95, 100, 105 to be connected directly via separate lines to the control unit 300. The control unit 300 is furthermore connected via a fifth connection 325 to the first steam valve 110. Via a sixth connection 330, the control unit 300 is connected to the second steam valve 115. Via a seventh connection 335, the control unit 300 is connected to the third steam valve and via an eighth connection 340 to the fourth steam valve 125. Via a ninth connection 345, the control unit 300 is connected to the bypass valve 240.

Furthermore, a sensor 350 is provided on the output side of the first heat exchanger 45, which sensor detects as state variable a pressure and/or a temperature of the second heat transfer medium 230 in the sixteenth line 215. The sensor 350 provides a sensor signal corresponding to the detected pressure and/or to the detected temperature. Of course, it is also conceivable for the sensor 350 to be arranged on the distributing line 130, in order to detect there pressure and/or temperature of the second heat transfer medium 230.

The second teed pumps 200 are connected via a tenth connection 355 and an eleventh connection 360, respectively, to the control unit 300.

The control unit 300 has an input 365, an output 366, a memory 375 and a control device 370. The memory 375 is connected via a twelfth connection 380 to the control device 370. Via a thirteenth connection 385, the control device 370 is connected to the input 375 and via a fourteenth connection 386 to the output 366.

The input 365 is connected by a fifteenth connection 390 to the sensor 350. The input 365 can be formed as an interface device for providing a connection between the fifteenth connection 390 and the thirteenth connection 385. It is also conceivable for the input to have an analog-digital converter. The input 365 then provides the control device with a first input signal according1 corresponding to the detected sensor signal. The output 366 serves for connecting the control device 370 to components of the power station 10, in order to control these components by means of control signals.

In the operation of the power station 10, after a start-up operation, which is explained later, at least one of the second feed pumps 200 conveys the second heat transfer medium 230 from the storage container 175 via the fourteenth line 190. The feed pumps 200 force the second heat transfer medium 230 into the fifteenth line 205 and, with an opened shutoff valve 210, into the sixteenth line 215. From the sixteenth line 215, the second heat exchange medium 230 flows on the input side into the third heat exchanger 55 and is heated there for the first time by the first heat exchange medium 80 of the heat source circuit 20. The heated, but still liquid second heat exchanger medium 230 flows via the seventeenth line 220 into the second heat exchanger 50, in which the second heat transfer medium 230 is evaporated. The evaporated heat transfer medium 230 flows on the output side of the second heat exchanger 50 into the first heat exchanger 45, which serves as a superheater. The first heat transfer medium 80 coming directly from the heat source 25 is transferred in the first heat exchanger 45 to the second heat transfer medium 230. In the process, the second heat transfer medium 230 is superheated and flows in the superheated state into the distributing line 130.

In the following, for easier explanation of the operation of the power station 10, it is assumed that the bypass valve 240 is closed and at least one steam valve 110, 115, 120, 125 is at least partly opened. The now vaporous second heat transfer medium 230 flows via the opened steam valve 110, 115, 120, 125 into the associated expansion engine 90, 95, 100, 105. There, the pressure of the second heat transfer medium 230 is reduced by an expansion in the expansion engine 90, 95, 100, 105. Simultaneously, the temperature of the second heat transfer medium 230 decreases. Coming from the expansion engines 90, 95, 100, 105, the second heat transfer medium 230 is collected in the collecting line 155 and led in the form of vapour via the twelfth line 160 to the fourth heat exchanger 165. In the fourth heat exchanger 165, which serves as a liquefier in the embodiment, the second heat transfer medium 230 is cooled and liquefied. In order to accelerate the cooling, air is additionally led to the fourth heat exchanger 165 by the fans 169. In the liquefied state, the second heat transfer medium 230 flows back into the storage container 175, where it is collected.

In the memory 375, a first predefined threshold value S1, a second predefined threshold value 2, as well as a third predefined threshold value S3 and a fourth threshold value 54 is stored. The first threshold value Si correlates in the embodiment with a first pressure value, formed as state variable of the second heat transfer medium 230, of the second heat transfer medium 230. The second threshold value 2 correlates with a second pressure value, formed as state variable of the second heat transfer medium 230, of the second heat transfer medium 230, which is greater than the first threshold value S. The third threshold value S3 correlates in the embodiment with a first valve position, for example an opening angle, of at least one of the steam valves 110, 115, 120, 125. The fourth threshold value 54 correlates in the embodiment with a second valve position, for example an opening angle, of the bypass valve 240.

Furthermore, the third threshold value 53 is assigned a first duration t3 and the fourth threshold value 54 a second duration t4, the durations t3, t being likewise stored in the memory 375. Furthermore, a predefined start-up threshold value 5A and a start-up duration tAg assigned to the start-up threshold value Sp. is stored in the memory 375.

The control device 370 has a first controller 371 and a second controller 372. The first controller 371 has as setpoint the first threshold value S1, as controlled variable the sensor signal and as manipulated variable the first valve position of the steam valves 110, 115, 120, 125. The second controller 372 has as setpoint the second threshold value 2, as controlled variable the sensor signal and as manipulated variable the second valve position of the bypass valve 240. The valve positions are in each case set by means of control signals of the control device 370 to the corresponding sLeam valves 100, 115, 120, and the bypass valve 240, respectively.

Figure 2 shows a flow diagram for controlling the power station 10 shown in Figure 1. Conditions are denoted by diamond-shaped boxes, where on fulfilment of the condition the path of the fulfilled condition is marked by means of a tick and the non-fulfilment of the condition is marked by means of a cross. Figure 2 is explained in more detail in conjunction with Figure 1.

To start up the power station 10, heat energy for heating the first heat transfer medium 80 is provided by the heat source 25. The first heat transfer medium 80 is conveyed via the first feed pump 30 into the first heat exchanger 45. In the first heat exchanger 45, the first heat transfer medium 30 delivers a part of its heat energy to the second heat transfer medium 230. The somewhat cooled first heat transfer medium 80 flows from the first heat exchanger 45 to the second heat exchanger 50 and there delivers a further part of its heat energy to the second heat transfer medium 230. The further cooled first heat transfer medium flows from the second heat exchanger 50 into the third heat exchanger 55 and there heats the second heat transfer medium 230 flowing for the first time into the third heat exchanger 55. The second heat transfer medium 230 is thus preheated in the third heat exchanger 55, where it then flows on into the second heat exchanger 50 and there is further heated. In the start-up operation, the case may occur that the heat source 25 does not yet provide sufficient heat energy, so that in the second heat exchanger 50 the second heat transfer medium 230 is possibly not yet evaporated. If the second heat transfer medium 230 is evaporated in the second heat exchanger 50 and superheated in the first heat exchanger 45, the superheated second heat transfer medium 230 flows into the distributing line 130. The distributing line 130 has in the start-up operation a temperature which is less than a condensation point of the second heat transfer medium 230.

This results in the second heat transfer medium 230 at least partly condensing on the distributing line 130.

In a first method step 500, the control device 370 detects the sensor signal of the sensor 350. Here, the steam valves 110, 115, 120, 125 are closed.

In a second method step 505, the control device 370 compares the sensor signal with the predefined start-up threshold value Sp. stored in the memory 375. compared. If the sensor signal overshoots the start-up threshold value SR stored in the memory 375, the method is continued with the third method step 510. If the sensor signal undershoots the start-up threshold value 5A stored in the memory 375, the method is continued with the first method step 500 and the sensor signal is further detected.

In a third method step 510, a trigger function is started, in which the start-up time tA since overshooting of the start-up threshold value 5A is determined. The determined start-up time tA is compared in a fourth method step 5]5 with the start-up duration tAs stored in the memory 375. If the determined start-up time t since overshooting of the start-up threshold value 5A undershoots the predetermined start-up duration tAs, the method is continued with the third method step 510 and the start-up time tA since overshooting of the start-up threshold 5A is further determined. If the determined start-up time tA since overshooting the start-up threshold value 5A overshoots the predetermined start-up time duration tAs, the method is continued with a fourth method step 515.

It is thereby avoided that the second heat transfer medium 230 condenses on the distributing line 130 and condensed second heat transfer medium 230 flows into the expansion engines 95, 95, 100, 105. This is of great importance insofar as liquid second heat transfer medium 230 can result in damage and/or destruction of the expansion engine system 85.

In the fifth method step 520, the first steam valve 110 is opened in its first valve position as manipulated variable by the first controller 371 and is controlled in its first valve position such that the first threshold value S is applied as setpoint in the distributing line 130. Depending on the heat provision of the heat source circuit 20, the first controller 371 may not be able to adjust the provided heat or the pressure p in the distributing line 130, 50 that the pressure p further rises even with fully opened steam valve(s) 110, 115, 120, 125.

In the sixth method step 525, the control device 370 checks whether the pressure p is less than the second threshold value 2* If this is the case, the method is continued with the fifth method step 520. If the pressure p overshoots the second threshold value s2, the method is continued with a seventh method step 530.

In the seventh method step 530, the second controller 372 is activated. The second controller 372 controls the second valve position as manipulated variable of the bypass valve 240 in such a manner that the pressure p as setpoint corresponds to the second threshold value S2. This is effected by a corresponding control of the second valve position of the bypass valve 240 by the second controller 372.

In an eighth method step 535, which follows the seventh method step 530, the second valve position, for example an open position of the bypass valve 240, is compared with the fourth threshold value S4. If the second valve position overshoots the fourth threshold value S4, the method is continued with a ninth method step 540. If the second valve position undershoots the fourth threshold value S4, then the method is continued further with the seventh method step 530.

In a ninth method step 540, a second trigger function is started and a second time tuE4 since overshooting of the second threshold value S4 by the second valve position is determined.

In a following tenth method step 545, the determined second tine tuE4 since overshooting of the fourth threshold value 54 is compared with the second duration t4 stored in the memory 375. If the determined second duration tuE4 since overshooting of the fourth threshold value S4 undershoots the second predefined duration t4, then the method is continued with the ninth method step 540 and the second time since overshooting of the second threshold value S2 is further determined. If the determined second time tuE4 since overshooting of the fourth threshold value 54 is greater than the predefined second duration t4, the method is continued with an eleventh method step 550.

In the eleventh method step 550, the control device 370 transmits an enabling signal to one of the deactivated expansion engines 95, 100, 105, for example the second expansion engine 95. The selection of the deactivated expansion engine 95, 100, 105 can be made, for example, on the basis of a total running time of the individual expansion engines 90, 95, 100, 105. In the following, for easier understanding, the second expansion engine 95 is thereupon started up. The third or fourth expansion engine 100, 105 could also be started up.

After the starting-up of the second expansion *engine 95, the control device 370 at least partly opens the second steam valve 115 by means of a corresponding control signal.

The opening of the second steam valve 115 is effected by means of a ramp function up to the first valve position of the first steam valve 110, so that a sudden pressure drop in the distributing line 130 is avoided. The second controller 372 is still active and, irrespective of the valve position of the steam valves 110, 115, 120, 125, continues to control the bypass valve in dependence on the pressure p and the controlled variable of second threshold value S2. After conclusion of the ramp function, the first and the second steam valve 110, 115 are controlled in parallel with the same valve position by the first controller 371.

Following the eleventh method step 550, the method is continued with a twelfth method step 555. In the twelfth method step 555, the control device 370 checks whether the pressure p is greater than the second threshold value S2 and greater than the first threshold value S1. If this is the case, the method is continued with the ninth method step 540, and in the tenth and eleventh method step 545, 550 the further steam valves 120, 125 are opened by means of control signals provided by the control device 370, until all the expansion engines 90, 95, 100, 105 are activated.

If the pressure p undershoots the second threshold value 52 and if the pressure p simultaneously overshoots the first threshold value S1, the method is continued with the checking of the condition just mentioned, until this condition is no longer fulfilled.

If the pressure p undershoots the second threshold value S2 and simultaneously the first threshold value S1, the first valve position of the opened steam valve(s) 110, 115, 120, is compared with the third threshold value S3 in a thirteenth method step 560. If the first valve position overshoots the third threshold value 53, a trigger function is started in a following fourteenth method step 565, in which the time tuEJ since undershooting of the third threshold value 53 is determined. In a fifteenth method step, the control device 370 compares the determined time tuE] since undershooting of the third threshold value S3 with the first duration t3 predefined and stored in the memory 375. If the time tuBa undershoots the stored first duration t3, the time tuE3 since overshooting of the third threshold value S is determined by the control device 370 in the tourteenth method step 565.

If the first valve position of the opened steam valves 110, overshoots the third threshold value S3 and the first duration t3, then in a following sixteenth method step 575 one of the opened steam valves 110, 115, 120, 125 is closed and the expansion engine 95, 100, 105, 110 assigned correspondingly to the closing steam valve 110, 115, 120, is deactivated. As a result, the number of expansion engines 90, 95, 100, 105 in operation is reduced, so that the other expansion engines 90, 95, 100, 105 remain activated at their maximum power and thus can be operated at increased efficiency. Thereafter, the method is continued with the fifth method step 520.

The above-described method has the advantage that the power station 10 can switched on, with a modular construction of the expansion engines 90, 95, 100, 105, in dependence on the capacity of the heat source 25, in order thus to react flexibly to the energy provided by the heat source 25.

Furthermore, it is avoided that the power station 10 is operated with its expansion engines 90, 95, 100, 105 in a range in which the expansion engines 90, 95, 100, 105 have a poor efficiency and thus the overall efficiency of the power plant 10 is low. Through the combination of the different threshold values S1, S, 53, S4 with the waiting for predefined durations t3, t4, a continuous activating and deactivating of the expansion engines 90, 95, 100, 105 is avoided, so that possible bearing damage resulting therefrom is avoided.

In the embodiment, the expansion engines 90, 95, 100, 105 are identically formed. Of course, it is also conceivable for the expansion engines 90, 95, 100, 105 to have different power ranges, and in this configuration the threshold values would of course be correspondingly adapted, in order thus to activate or deactivate the respective most favourable expansion engine.

It should be pointed out that the controllers 371, 372 after activation are continuously active and control the respective steam valves 110, 115, 120, 125 and the bypass valve 240 accordingly, in order to achieve the respective predetermined controlled variable. If a plurality of expansion engines 90, 95, 100, 105 are activated, then the first controller 371 controls the steam valves 110, 115, 120, 125 of the activated expansion engines 90, 95, 100, in parallel, that is to say that the steam valves 110, 115, 120, 125 of the activated expansion engines 90, 95, 100, 105 have the same valve position. The steam valves 110, 115, 120, 125 of the deactivated expansion engines 90, 95, 100, 105, however, still remain closed.

In the embodiment, a pressure of 17 bar serves as the first threshold value S. The second threshold value s2 has a pressure of 19 bar, Of course, it is also conceivable that other pressure values for the two threshold values S2 is used. In the embodiment, as predefined durations, 10 minutes is used in each case as duration t3, t4. Of course, other predefined durations are also conceivable depending on the configuration of the expansion engines 90, 95, 100, and of the power station 10. The provision of durations may also be dispensed with. By opening the bypass 235 on the overshooting of the second threshold value S2, when all the steam valves 110, 115, 120, 125 are open, a possible destruction of bearings of the expansion engines 90, 95, 100, 105 is avoided.

Through the stepwise start-up of the different expansion engines 90, 95, 100, 105, a flexible automatic operation of the power station 10 is ensured.

It should be pointed out that additional method steps may, of course, be introduced into the above-described method.

Of course, it is also conceivable for the described method steps to be applied in a different order and/or in parallel.