EP3375990B1 - Model-based monitoring of the operational state of an expansion machine - Google Patents

Description

Field of the Invention

The invention relates to a method for operating a thermodynamic cycle device, in particular an organic rankine cycle device (ORC device) with an expansion machine, and a thermodynamic cycle device that can be operated with the method according to the invention.

State of the art

If a thermodynamic cycle device, for example an Organic Rankine Cycle device, is coupled to a generator or a motor / generator unit in order to feed energy into a power grid, the expansion machine is subjected to rotational speeds due to the grid frequency. The same applies to a coupling with another external device, such as a device with an internal combustion engine, in order to support it.

It has been found that, for example, during a coupling process of the external device, damage to the expansion machine of the thermodynamic cycle device can occur, in particular in the bearing of the rotating elements of the expansion machine. According to the experience of the applicant, this damage occurs when power is effectively supplied to the expansion machine. This applies in particular to screw expansion machines.

Coupling a generator operated with an ORC system is described in EP 1759094 B1 described. The coupling to the power grid takes place when the measured generator speed matches the grid frequency, which therefore implies a power-free coupling. However, this speed measurement represents an additional cost or is even extreme with (semi) hermetic machines complex because the shaft is not directly accessible from the outside. A speed measurement based on the generated voltage is not possible with synchronous generators that are not connected to the mains or are not otherwise magnetized.

Description of the invention

The object of the invention is to avoid the disadvantages mentioned.

The invention describes the solution to the above problem by adding model-based control and / or monitoring to the operation (starting process, normal operation, shutdown process) of the thermodynamic cycle device with the expansion machine.

The solution according to the invention is defined by a method with the features according to claim 1.

The invention thus discloses a method for controlling a thermodynamic cycle device, in particular an ORC device, wherein the thermodynamic cycle device comprises an evaporator, an expansion machine, a condenser and a feed pump and the expansion machine is coupled to an external device in normal operation, and the method comprises the steps of: measuring an exhaust pressure downstream of the expansion machine; and setting a volume flow of the feed pump according to a computer-implemented control model of the thermodynamic cycle device as a function of the measured evaporation pressure and a target speed of the expansion machine as input variables of the control model and with the volume flow of the feed pump as output variable of the control model.

The evaporation pressure downstream of the expansion machine can be measured between the expansion machine and the feed pump, in particular between the expansion machine and the condenser or between the condenser and the feed pump. When measuring between the capacitor and the feed pump, the pressure loss of the condenser can either be neglected or it is known and is taken into account in the regulation.

In the control model (apart from the setpoint speed of the expansion machine), only the measured exhaust steam pressure or a measured value of the exhaust steam pressure corrected by a correction value is used as the input variable. In the case of a measurement of the evaporation pressure between the condenser and the feed pump, a pressure loss of the condenser and / or of pipes between the expansion machine and the measuring point can thus be taken into account and the measured evaporation pressure can be corrected accordingly.

The volume flow of the working medium pumped by the feed pump can be regulated in various ways. Setting the speed of the feed pump is one way of adjusting the volume flow of the feed pump, other options would be a throttle (throttle valve) or a 3-way valve after the pump or an adjustment of the delivery characteristics of the feed pump by adjusting a stator or a piston stroke.

The method according to the invention has the advantage that the measuring point required for the speed measurement according to the prior art can be avoided with the aid of the model-based control within the scope of the present invention.

The method according to the invention can be further developed in such a way that a starting process of the thermodynamic cycle device can include the following steps: regulating the expansion machine into a state in which the target speed of the expansion machine is greater than or equal to a predetermined speed of the external device to be coupled to the expansion machine, wherein the external device to be coupled comprises in particular a generator, a generator / motor unit or a device operated with a separate motor; and then coupling the expansion machine to the external device. If the speeds are the same, there is a power-neutral coupling. If the speed of the expansion device during coupling is (somewhat) greater than a synchronous speed, then the effective performance of the expansion machine is positive and therefore does not damage the bearing.

Another further development is that the following further steps can be carried out: measuring the live steam pressure upstream of the expansion machine; Comparison of the measured live steam pressure with a current model live steam pressure according to the control model; and initiating a shutdown process and / or aborting the starting process if the measured live steam pressure is more than a predetermined amount or more than a predetermined fraction below the model live steam pressure, which depends on the measured exhaust steam pressure.

The live steam pressure upstream of the expansion machine can be measured between the feed pump and the expansion machine, in particular between the evaporator and the expansion machine or between the feed pump and the evaporator. The live steam pressure could, for example, be measured at the outlet of the feed pump / inlet of the evaporator and corrected for the pressure loss of the evaporator and / or the pipelines up to the inlet into the expansion machine.

This can be developed in such a way that during the starting process the expansion machine is only coupled to the external device if the measured live steam pressure is greater than or equal to the model live steam pressure.

According to another development, the following further steps can be carried out: measuring a heat source temperature of a heat source which supplies the thermodynamic cycle device via the evaporator; and performing the starting process only if the measured heat source temperature is greater than or equal to a current model heat source temperature according to the control model.

Another development is that a shutdown process of the thermodynamic cycle device can include the following steps: decoupling the expansion machine from the external device if the live steam pressure and / or the heat source temperature is below a respective one predetermined threshold fall; and opening a bypass line to bypass the expansion machine.

This can be further developed in such a way that the following step is carried out: reducing the volume flow (in particular by reducing the speed) of the feed pump until, according to the control model, a power-neutral or force-free state of the expansion device is achieved in which the power consumed by the expansion device is equal to that of output of the expansion device or the total force acting on the expansion device in the direction of an axis of rotation of the expansion device is zero.

The control model according to the invention can include analytical and / or numerical and / or tabular relationships of the input and output variables.

The above-mentioned object is also achieved by a thermodynamic cycle device according to claim 10.

The thermodynamic cycle device (in particular an ORC device) according to the invention comprises an evaporator, an expansion machine, a condenser, and a feed pump, the expansion machine being coupled to an external device in normal operation; wherein the thermodynamic cycle device further comprises: an exhaust pressure measurement device for measuring an exhaust pressure downstream of the expansion machine; and a control device for setting a volume flow of the feed pump according to a control model of the thermodynamic cycle device, which is stored in a memory of the control device, as a function of the measured evaporation pressure and a setpoint speed of the expansion machine as input variables of the control model and with the volume flow of the feed pump as output variable of the control model. Measuring the evaporation pressure downstream of the Expansion machine can take place at the points mentioned above in connection with the method according to the invention.

The thermodynamic cycle device according to the invention can be further developed in such a way that the control device is designed to carry out the following steps during a starting process of the thermodynamic cycle device: regulating the expansion machine into a state in which the desired speed of the expansion machine is greater than or equal to a predetermined speed of that of the expansion machine to be coupled external device, the external device to be coupled comprising in particular a generator, a generator / motor unit or a device operated with a separate motor; and then coupling the expansion machine to the external device.

According to another development, the thermodynamic cycle device further comprises a live steam pressure measuring device for measuring a live steam pressure upstream of the expansion machine; wherein the control device is designed to compare the measured live steam pressure with a current model live steam pressure according to the control model, and to initiate a shutdown process and / or to abort a start-up process if the measured live steam pressure is increased by more than a predetermined amount or by more than a predetermined fraction is below the model live steam pressure. The live steam pressure upstream of the expansion machine can be measured at the points already mentioned above in connection with the method according to the invention.

Another development is that the thermodynamic cycle device further comprises: a heat source temperature measuring device for measuring a heat source temperature of a heat source that supplies the thermodynamic cycle device with heat via the evaporator; wherein the control device is designed to carry out the starting process only when the measured heat source temperature is greater than or equal to a current model heat source temperature according to the control model.

According to another development, the thermodynamic cycle device further comprises a bypass line as a direct connection between the evaporator and the condenser to bypass the expansion machine; wherein the control device is designed to carry out the following steps during a shutdown process of the thermodynamic cycle device: decoupling the expansion machine from the external device if the live steam pressure and / or the heat source temperature fall below a respective predetermined threshold value; and opening the bypass line by means of a valve in the bypass line.

Another development is that the thermodynamic cycle device further comprises: a coupling for coupling the expansion device to the external device; and / or a transmission for setting a speed ratio from the expansion device to the external device.

The further developments mentioned can be used individually or, as claimed, can be suitably combined with one another.

Further features and exemplary embodiments and advantages of the present invention are explained in more detail below with reference to the drawings. It is understood that the embodiments are not exhaustive of the scope of the present invention. It is further understood that some or all of the features described below can also be combined with one another in other ways.

drawings

Fig. 1

shows an embodiment of the device according to the invention.

Fig. 2

shows forces in the expansion machine.

Fig. 3

shows the performance of the expansion machine depending on its speed.

Fig. 4

shows the performance of the expansion machine depending on the applied pressure ratio.

Fig. 5

shows a control process in the power-pressure ratio diagram.

embodiments

In the following, an ORC process is assumed as an example as a thermodynamic cycle. Figure 1 shows an embodiment 100 of the thermodynamic cycle device according to the invention. The ORC cycle comprises a feed pump 40 for increasing the pressure, an evaporator 10 for preheating, evaporation and overheating of a working medium, an expansion machine 20 for power-generating expansion of the working medium, which with or without clutch 27 is connected to a generator 25 (or a motor / generator Unit) or an external process 26 is connected, a possible bypass 50 for bypassing the expansion machine 20 and a condenser 30 for deheating, condensation and subcooling of the working medium.

In addition, the cycle process device 100 according to the invention comprises an evaporation pressure measuring device 61 for measuring an evaporation pressure downstream of the expansion machine 20. By way of example, the evaporation pressure measuring device 61 is provided here between the expansion machine 20 and the condenser 30. However, it is also possible to arrange them between the condenser 30 and the feed pump, possibly taking into account a pressure loss in the condenser 30 in the form of a correction value for the measured evaporation pressure.

Furthermore, a control device 80 for setting a volume flow of the working medium pumped by the feed pump 40 (for example by setting a rotational speed of the feed pump 40) according to a control model of the thermodynamic cycle device 100 stored in a memory 81 of the control device 80 only as a function of the measured exhaust steam pressure (if necessary corrected by the mentioned correction value) and a target speed of the expansion machine 20 as input variables of the control model and with the Volume flow of the feed pump 40 (eg in the form of the speed of the feed pump 40) as the output variable of the control model.

In the case of coupling a generator 25 (or a motor / generator unit), a coupling switch 28 can also be provided, which couples the generator 25 (or the motor / generator unit) to or disconnects from a power supply system.

The underlying problem of the solution according to the invention is discussed below.

Discussion of the problem

The invention is based on the following problem. If the expansion machine 20 is operated by a motor, ie power is entered, for example, by the generator 25 in motor operation based on a fixed speed setting or by the external process 26, there is a risk of damage since the power flow does not correspond to the design point ("defective operation"). The direction of force on the rotor of the expansion machine (as in Figure 2 ), is based on the force effect of the pressure position of live steam and exhaust steam (depending on the pressure difference across the expansion machine) and the forces due to the power output or power consumption ("transmission force", depending on the pressure quotient via the expansion machine, see also Figure 4 ) certainly. At the operating and thus design point of the expansion machine, these are designed so that the resulting force acts in the direction of the force absorption capacity of the bearing. In the example shown, the expansion machine 20 is a screw expander.

Damage is caused, for example, by abrasion or chip formation from the contact of rotating bodies with the housing, since the force effect is not supported by the bearing ( Figure 2 ). This can also result in a shift in the axial direction and possibly a rotation of the bearing ring due to the relief, which can lead to damage to the bearing.

However, this motor operation occurs automatically when the expansion machine is still at a standstill at the activation point (the current pressure cannot overcome the necessary post-compression) or the speed is below the activation synchronous speed (activation point a) in Figure 3 ). In these points, the expansion machine is accelerated and performance is spent on it. The available performance of the expander is therefore negative.

For better understanding, we speak of post-compression (more precisely: post-compression performance) and post-expansion (more precisely: post-expansion performance). In principle, however, this is a different part of the push-out work (P _AA ), which has to be applied by the expansion machine to push out the medium at the end of the expansion in the chamber of the expansion machine against the evaporation pressure p _AD . This distinction therefore relates to the reference (P _{AA, ref} ), in which the opening pressure of the chamber is equal to the evaporation pressure behind the chamber.

• Für p_Kammer > p_AD : $$P_{\mathit{Nachexpansion}}=P_{\mathit{AA};\mathit{ref}}-P_{\mathit{AA},\mathit{ist}};P_{\mathit{Nachkompression}}=0$$
  
• Für p_Kammer < p_AD : $$P_{\mathit{Nachkompression}}=P_{\mathit{AA};\mathit{ref}}-P_{\mathit{AA},\mathit{ist}};P_{\mathit{Nachexpansion}}=0$$
  
• Für p_Kammer = p_AD : $$P_{\mathit{Nachexpansion}}=0;P_{Nachko\mathrm{m}\mathit{pression}}=0$$
  

The following therefore applies:

• For p _chamber > p _AD : $$P_{\mathit{Further\; Expansion}}=P_{\mathit{AA};\mathit{ref}}-P_{\mathit{AA}.\mathit{is}};P_{\mathit{post-compression}}=0$$
  
• For p _chamber <p _AD : $$P_{\mathit{post-compression}}=P_{\mathit{AA};\mathit{ref}}-P_{\mathit{AA}.\mathit{is}};P_{\mathit{Further\; Expansion}}=0$$
  
• For p _chamber = p _AD : $$P_{\mathit{Further\; Expansion}}=0;P_{NacHkO\mathrm{m}\mathit{pressure}}=0$$
  

For damage-free connection, the expansion machine must therefore at least at a neutral power point at the connection speed (connection point b) in Figure 3 ) or above (connection point c) in Figure 3 ), so that the expansion machine is at least not accelerated or braked, and thus will at least not give any negative power.

No power can be dissipated before the generator or external process is switched on, ie if the steam supply is undefined, the machine could possibly be accelerated uncontrollably until it was damaged.

Knowledge of the current expansion machine speed would in principle be possible with the help of a speed measurement. However, this speed measurement represents an additional cost or can be implemented only with great effort.

The defective condition due to power supply to the expansion machine continues to arise during operation and shutdown if the post-compression performance exceeds the expansion performance due to insufficient pressure levels (see Figure 4 ). This leads to an expansion of the gas in the closed expansion chamber of the expansion machine. After opening, however, the pressure in the chamber is below the level of the evaporation side, which is why the expansion machine partially compresses it when it is pushed out and the medium that has additionally flowed back into the chamber must also push out ("post-compression"). The following applies: $$P_{\mathit{gross}}=P_{\mathit{expansion}}+P_{\mathit{Further\; Expansion}}+P_{\mathit{post-compression}}$$

mit

• p_FD = Frischdampf druck
• p_AD = Abdampf druck

The pressure ratio π is defined as the quotient from live steam pressure to exhaust steam pressure: $$\pi =p_{\mathit{FD}}/p_{\mathit{AD}}$$

With

• p _FD = live steam pressure
• p _AD = exhaust steam pressure

mit

• ρ_FD = Frischdampf dichte
• ρ_AD = Abdampf dichte

In addition to the directly measurable pressure ratio used here, the volume ratio Φ can also be used instead: $$\Phi ={\rho }_{\mathit{AD}}/{\rho }_{\mathit{FD}}$$

With

• ρ _FD = live steam density
• ρ _AD = evaporating density

Both key figures ( π, Φ ) provide the same result in a first approximation.

Solutions to the problem according to the invention starting procedure

Here, the expansion machine 20 is brought to a defined starting point (speed), which prevents damage to the expansion machine when it is switched on. The necessary measurement values of flow and speed of the expansion machine, which can be determined by expensive measuring technology, are avoided by model-based control.

This model-based control is based on the basics of knowledge of the neutral point of the expansion machine (as in Figure 4 shown, P _gross = 0 and thus P _expansion = -P _{post-compression} ). This means that depending on the exhaust steam pressure p _AD, a corresponding live steam pressure p _FD must be reached.

mit

• n_EM = Expansionsmaschinendrehzahl
• V̇_FD = Frischdampf volumenstrom
• V_Kammer = Hochdruckkammervolumen der Expansionsmaschine
• K = Kammerzahl pro Umdrehung

The speed at which the expansion machine is operated in this power-free state also depends on the steam volume flow V̇ _FD supplied: $$n_{\mathit{EM}}={\dot{V}}_{\mathit{FD}}/\left(V_{\mathit{chamber}}*K\right)$$

With

• n _EM = expansion machine speed
• V̇ _FD = live steam volume flow
• V _chamber = high pressure _chamber volume of the expansion machine
• K = number of chambers per revolution

The state of the expansion machine 20 (in particular its speed) can thus be clearly determined by knowing the live steam pressure, exhaust steam pressure and live steam volume flow (depending on the desired connection speed). The equation above to determine the expansion engine speed initially represents the simplest form and can be further improved in accuracy, for example, by correction using a variable-speed leakage volume flow. The electrical power and thus, for example, a state of the thermodynamic cycle can be derived from the expansion machine speed and the other thermodynamic variables.

However, the measurement of the live steam volume flow is a relatively cost-intensive measurement, which therefore has a negative impact on the economy of the overall system.

The live steam mass flow can also be determined relatively easily from the live steam volume flow, which could also be measured in the liquid phase between the feed pump 40 and the evaporator 10. The necessary measuring devices (e.g. Coriolis) are also associated with considerable costs.

mit

• V̇_SP = Volumenstrom durch die Speisepumpe
• V̇_FD = Volumenstrom durch die Expansionsmaschine
• ρ_FD = Dichte des Frischdampfes durch die Expansionsmaschine
• ρ_fl = Dichte des flüssigen Mediums in der Speisepumpe

However, there is also a direct correlation between live steam volume flow and volume flow conveyed by the feed pump 40, which can be determined via the densities: $${\dot{V}}_{\mathit{SP}}={\dot{V}}_{\mathit{FD}}*{\rho }_{\mathit{FD}}/{\rho }_{\mathit{fl}}$$

With

• V̇ _SP = volume flow through the feed pump
• V̇ _FD = volume flow through the expansion machine
• ρ _FD = density of live steam from the expansion machine
• ρ _fl = density of the liquid medium in the feed pump

It should be noted that the live steam density also depends on the position of the evaporation pressure, since it is a function of the live steam pressure (and the live steam temperature). The live steam pressure itself is a function of the exhaust steam pressure in this case of the power-free expansion machine operation. This circumstance (p _FD and V̇ _SP vary) also leads to a static starting behavior with fixed speed specification of the feed pump depending on the evaporation pressure, which depends on the condensation conditions such as heat sink temperature depends on starting with a motor drive (high evaporation pressure p _AD ; under-synchronous until the expansion machine comes to a standstill) or accelerating the expander beyond the permissible speed (low p _AD ).

Furthermore, the necessary pressure difference from the neutral point, which the feed pump 40 must apply, is given as $$p_{\mathit{SP}}=p_{\mathit{FD}}-p_{\mathit{AD}}$$

As a result, the volume flow in the feed pump 40 and the pressure difference which the feed pump 40 must apply are known. By modeling the feed pump 40, a speed point of the feed pump 40 can now be found at which this condition of pressure difference and flow is fulfilled.

This results in a start control which assigns a value for the feed pump speed to each exhaust steam pressure and the associated connection speed (target speed of the expansion machine 20), without additional measuring points being necessary. It is to be mentioned as a disadvantage that the actual values of these important measurement variables are thus represented by a model and actually remain unknown in the system.

1. 1) Ein Versagen der Speisepumpe (Kavitation, Motorschädigung etc.) führt zu einem zu geringeren Druckniveau / Durchfluss als für den schadfreien Betrieb benötigt wird.
2. 2) Ein nicht oder nicht vollständig geschlossener Bypass 50 (Figur 1) oder anderweitiger Abfluss von Kältemittel, der nicht durch die Expansionskammern geführt wird, führt im Zuschalten zu einem zu geringen Druckniveau.
3. 3) Das Temperaturniveau der Wärmequelle liegt unterhalb des notwendigen Niveaus, um das Arbeitsmedium auf dem notwendigen Frischdampfdruck verdampfen zu können.

However, the following mechanisms can still endanger damage-free switching:

1. 1) Failure of the feed pump (cavitation, motor damage, etc.) leads to a lower pressure level / flow than is required for damage-free operation.
2. 2) A bypass 50 that is not or not completely closed ( Figure 1 ) or other outflow of refrigerant, which is not led through the expansion chambers, leads to a too low pressure level when switched on.
3. 3) The temperature level of the heat source is below the necessary level in order to be able to evaporate the working medium at the necessary live steam pressure.

The problems under 1) + 2) can be avoided by additionally monitoring the achieved process variable of the live steam pressure before the connection process. If the pump and the bypass behave regularly, this must correspond to the value determined in the modeling. If it deviates downwards, the start can be terminated without damaging the expansion machine 20.

The problem under 3) is avoided by also using a model of the necessary heat source temperature (T _HW , Figure 1 ) is stored and the starting process is only completed when at least this value necessary for a safe start has been reached or exceeded.

normal operation

In operation, there may be very low pressure differences from p _FD to p _AD if there is no heat supply and poor heat dissipation (e.g. high air temperature / water temperature). It is also possible that this in turn leads to defective operation of the system as in Figure 2 and Figure 4 shown comes. Instead of carrying out a gross power evaluation here, which also has other influencing factors, the chosen model should be used to cause the damage to fall below the required pressure quotient π or volume ratio Φ by monitoring the necessary live steam pressure p _{FD in} relation to the exhaust steam pressure. If a critical threshold value is reached here, the system is shut down in a controlled manner before defective states can be reached. Another option is to monitor the electrical power of the expansion machine. If this falls below a critical threshold value, the system is shut down in a controlled manner.

Depart

In the shutdown program, the temperature on the heat input side of the system is reduced in the desired manner in order to achieve a safe standstill of the system at moderate temperatures. However, this lowering reduces the fresh steam pressure p _FD and thus the pressure quotient π. In extreme cases Damaged operation can also occur here during the downhill run.

To prevent this, the hot water temperature (T _HW ) required for safe operation is also monitored by means of a measuring device 63 and the live steam pressure (p _FD ) is monitored by means of a measuring device 62. If the temperature falls below a defined threshold, the expansion machine is decoupled from the power connection, i.e. neither power is supplied nor supplied, and at the same time the bypass 50 is opened by means of valve 51 in order to reduce the pressure on the live steam side and, if necessary, to let the system continue to run. Switching off based on a live steam pressure depending on the evaporation pressure on the one hand avoids the defective operation and on the other hand that the pressure level is still so high that switching off the expander power connection (decoupling the expansion device) can turn it up in an uncontrolled manner before the pressure can be released via the bypass 50 can mine enough. This safety can also be achieved by gradually reducing the feed pump speed to a value that corresponds to the zero power point from the modeling. This achieves an operating state in which, in the case of a further control of the expansion machine (the expander) 20 or a fault in the bypass opening, the expansion machine 20 is operated at a defined speed below a defective speed without affecting power. Overall, the operating times in the performance-neutral area must also be minimized, since the very low bearing load means that the service life is shortened.

The framework of the control strategy is briefly summarized below and in Figure 5 clarifies:
As a result of the model formation, there is a control device 80 of the feed pump 40, which works without measured values of the expander speed or the flow and contains the low pressure (exhaust pressure) as an input variable in order to regulate to a desired speed of the expansion device 20.

In order to ensure the correct functioning of the feed pump 40 and the bypass 50 (failure in turn leads to defective motor operation), the Live steam pressure and the hot water temperature from the model formation are used as a monitoring variable (falling below the model value means deviation in the system with damage potential).

The illustrated embodiments are merely exemplary and the full scope of the present invention is defined by the claims.

Claims (15)

1. Method for controlling a thermodynamic cycle process apparatus (100), in particular an ORC apparatus, wherein the thermodynamic cycle process apparatus comprises an evaporator (10), an expansion machine (20), a condenser (30) and a feed pump (40), and the expansion machine (20) is coupled to an external apparatus (26) during normal operation, and wherein the method comprises the following steps:

   measuring an exhaust steam pressure downstream of the expansion machine; and

   setting a volume flow of the feed pump in accordance with a computer-implemented control model of the thermodynamic cycle process apparatus as a function of the measured exhaust steam pressure and a target rotational speed of the expansion machine (20) as input variables of the control model and with the volume flow of the feed pump as output variable of the control model.
2. Method according to claim 1, wherein setting the flow rate of the feed pump (40) includes:

   setting the speed of rotation of the feed pump (40); and/or

   setting a throttle valve or a 3-way valve behind the pump; and/or

   setting a conveying characteristic of the feed pump (40), in particular by setting a guide wheel in the case of a centrifugal pump as the feed pump or by setting a piston stroke in the case of a piston pump as the feed pump.
3. Method according to claim 1 or 2, wherein a starting process of the thermodynamic cycle process apparatus comprises the following steps:

   controlling the expansion machine (20) to a state in which the target rotational speed of the expansion machine (20) is greater than or equal to a predetermined speed of the external apparatus to be coupled to the expansion machine (20), the external apparatus (26) to be coupled comprising in particular a generator (25), a generator/motor unit or a device driven by a separate motor; and

   subsequent coupling of the expansion machine (20) with the external apparatus (26).
4. Method according to one of claims 1 to 3, comprising the further steps:

   measuring the live steam pressure upstream of the expansion machine (20);

   comparing the measured live steam pressure with a current model live steam pressure according to the control model; and

   initiating a shutdown process and/or aborting the starting process if the measured live steam pressure is below the model live steam pressure by more than a predetermined amount or by more than a predetermined fraction.
5. Method according to claim 4, wherein during the starting process the expansion machine (20) is coupled to the external apparatus (26) only if the measured live steam pressure is greater than or equal to the model live steam pressure.
6. Method according to one of claims 3 to 5, comprising the further steps:

   measuring a heat source temperature of a heat source supplying heat to the thermodynamic cycle process apparatus (100) via the evaporator (10); and

   performing the start procedure only if the measured heat source temperature is greater than or equal to a current model heat source temperature according to the control model.
7. Method according to one of claims 4 to 6, in case a shutdown process is initiated, wherein the shutdown process of the thermodynamic cycle process apparatus (100) comprises the following steps:

   decoupling the expansion machine (20) from the external apparatus (26) if the live steam pressure and/or the heat source temperature fall below a respective predetermined threshold; and

   opening a bypass line (50) to bypass the expansion machine (20).
8. Method according to claim 7, comprising the further step:
   reducing the volume flow of the feed pump (40) until a neutral or force-free state of the expansion machine (20) is reached according to the control model, in which the power consumed by the expansion machine (20) is equal to the power output by the expansion machine (20) or the total force acting on the expansion machine (20) in the direction of an axis of rotation of the expansion machine (20) is zero.
9. Method according to one of claims 1 to 8, wherein the control model includes analytical and/or numerical and/or tabular relations of the input and output variables.
10. Thermodynamic cycle process apparatus (100), in particular an ORC apparatus, comprising an evaporator (10), an expansion machine (20), a condenser (30), and a feed pump (40), the expansion machine (20) being coupled to an external apparatus (25, 26) during normal operation; further characterized by:

    an exhaust steam pressure measuring device (61) for measuring an exhaust steam pressure downstream of said expansion machine (20); and

    a control device (80) for setting a volumetric flow of the feed pump (40) in accordance with a control model of the thermodynamic cycle process apparatus stored in a storage (81) of the control device (80) as a function of the measured exhaust steam pressure and a target rotational speed of the expansion machine (20) as input variables of the control model and with the volumetric flow of the feed pump (40) as output variable of the control model.
11. Thermodynamic cycle process apparatus according to claim 10, wherein the control device (80) is adapted to perform the following steps during a starting process of the thermodynamic cycle process apparatus:

    controlling the expansion machine (20) to a state in which the target rotational speed of the expansion machine is greater than or equal to a predetermined speed of the external apparatus to be coupled to the expansion machine, the external apparatus to be coupled comprising in particular a generator, a generator/motor unit or a device driven by a separate motor; and

    subsequent coupling of the expansion machine (20) with the external apparatus (25, 26).
12. Thermodynamic cycle process apparatus according to claim 10 or 11 further comprising:

    a live steam pressure measuring device (62) for measuring a live steam pressure upstream of the expansion machine (20);

    the control device (80) being adapted to compare the measured live steam pressure with a current model live steam pressure according to the control model, and to initiate a shutdown process and/or abort a starting process if the measured live steam pressure is below the model live steam pressure by more than a predetermined amount or by more than a predetermined fraction.
13. Thermodynamic cycle process apparatus according to one of claims 10 to 12 further comprising:

    a heat source temperature measuring device (63) for measuring a heat source temperature of a heat source that supplies heat to said thermodynamic cycle process apparatus (100) via said evaporator (10); and

    wherein the control device (80) is adapted to perform the starting process only when the measured heat source temperature is greater than or equal to a current model heat source temperature according to the control model.
14. Thermodynamic cycle process apparatus according to one of claims 10 to 13 further comprising:

    a bypass line (50) as a direct connection between the evaporator (10) and the condenser (30) for bypassing the expansion machine (20);

    said control device (80) being adapted to perform the following steps during a shutdown operation of said thermodynamic cycle process apparatus:

    decoupling the expansion machine (20) from the external apparatus (25, 26) if the live steam pressure and/or the heat source temperature fall below a respective predetermined threshold; and

    opening the bypass line (50) by means of a valve (51) in the bypass line.
15. Thermodynamic cycle process apparatus according to one of claims 10 to 14 further comprising:

    a coupling (27) for coupling said expansion machine (20) to said external apparatus (25, 26); and/or

    a gear for setting a speed ratio from said expansion machine (20) to said external apparatus (25, 26).

Images