METHOD FOR OPERATING A SOLAR-THERMAL PARABOLIC TROUGH POWER PLANT

Abstract

A method for operating an indirectly heated, solar-thermal steam generator and to an indirectly heated, solar-thermal steam generator are provided. A heat transfer medium is used in the solar-thermal steam generator. The supply water mass flow M is predictively controlled by a device for adjusting the supply water mass flow M. To this end, a nominal value Ms is fed to the device. A correction value KT, by which thermal storage effects of stored or withdrawn thermal energy are corrected is taken into account by the nominal value Ms.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2012/051942 filed Feb. 6, 2012 and claims benefit thereof, the entire content of which is hereby incorporated herein by reference. The International Application claims priority to the German application No. 10 2011 004269.5 DE filed Feb. 17, 2011, the entire contents of which is hereby incorporated herein by reference.

FIELD OF INVENTION

The invention relates to a method for operating an indirectly heated solar-thermal steam generator featuring a heat transfer medium, wherein a desired value {dot over (M)}_s for the feedwater mass flow {dot over (M)} is supplied to a device for adjusting the feedwater mass flow {dot over (M)} on the flow medium side. It further relates to an indirectly heated solar-thermal steam generator featuring a heat transfer medium and featuring a device for adjusting the feedwater mass flow {dot over (M)}, and to a solar-thermal parabolic trough power plant comprising an indirectly heated solar-thermal steam generator.

BACKGROUND OF INVENTION

Steadily increasing energy demands and climate change must be addressed by using sustainable sources of energy. Solar energy is one such sustainable source of energy. It does not harm the environment, is available in unlimited supply, and does not represent a burden on future generations.

Solar-thermal power plants represent an alternative to conventional power generation. One power station design already known in this field is the so-called parabolic trough power plant. In this type of power station, thermo-oil is normally used as a heat transfer medium, flowing through the parabolic troughs and absorbing the heat supplied by the sun. The heat absorbed by the oil is normally used in an additional steam generator in order to generate steam. In this case, the heated thermo-oil flows over the steam generator tubes, these being filled with steam, and transfers its heat to the colder steam generator tubes. The steam which is generated in the tubes then drives a conventional steam turbine.

One possible embodiment of the steam generator is based on the once-through principle. The feedwater entering the steam generator is essentially heated, vaporized and superheated as part of a single pass in this case. The superheated medium is supplied directly to the turbine without any further measures (e.g. cooling via additional injection). Consequently, the live steam temperature can only be precisely adjusted by selecting the correct feedwater mass flow {dot over (M)}, and fluctuations in the feedwater quantity are directly linked to fluctuations in the live steam temperature.

The feedwater mass flow {dot over (M)} should preferably be changed at the same time as the heat input into the steam generator via the heat transfer medium, because it is otherwise impossible reliably to avoid deviation of the specific enthalpy of the live steam from the desired value at the outlet of the steam generator. Any such unwanted deviation of the specific enthalpy makes it harder to regulate the temperature of the live steam emerging from the steam generator, and moreover results in significant material stresses and hence a reduced service life of the entire steam generator.

In solar-thermal power plant installations, inaccuracies that are caused by changes in the solar incidence, for example, must be effectively countered by the specification of a specifically adapted desired value for the feedwater mass flow, particularly in the event of a change in the total heat absorption or in the case of load variations.

In solar-thermal energy generation systems in particular, it is generally impossible to assume sufficiently stable system properties that can be directly attributed to a predefined constant solar energy input. Moreover, in the context of such installations configured as indirect steam generators, solar primary output to the parabolic troughs cannot be used as a free parameter to the same extent as is possible in the case of conventionally fired boilers.

SUMMARY OF INVENTION

The object of the invention is therefore to provide a method for operating a solar-heated once-through steam generator, being characterized by a particularly high level of reliability and quality of control, particularly in the case of non-steady operations. Also specified is a solar-thermal steam generator which is particularly suitable for performing the method.

The object of the invention in respect of a method is achieved by the features in the claims.

In this case, the invention is based on the idea of applying a design for predictive or advance mass flow control in the context of an indirectly heated solar-thermal steam generator in order to improve the activation quality when adjusting the feedwater mass flow {dot over (M)}. The essence of the invention in this case is that correction values which are deemed relevant should systematically be taken into account when determining a suitable desired value {dot over (M)}_s for the feedwater mass flow {dot over (M)}. By taking a correction value K_T into account, it is possible to equalize thermal storage effects which occur in the form of storage or withdrawal of thermal energy in the case of non-steady operations in particular.

This type of predictive feedwater flow control makes it possible to minimize both deviations of the specific enthalpy from the desired value at the steam generator outlet and the resulting undesirable levels of temperature fluctuations, in all operating conditions of the steam generator and particularly in transient conditions or in the context of load variations. The necessary desired feedwater values are provided as a function of the current or soon to be expected operating condition in this case, particularly in the context of load variations.

According to an advantageous embodiment of the method, the correction value K_T is used to correct the thermal storage effects of thermal energy that is stored or withdrawn relative to the tube walls of the solar-thermal steam generator. As an alternative or in addition to taking into account the thermal energy in the tube walls, the correction value K_T can also be advantageously used to correct the thermal storage effects of thermal energy that is stored or withdrawn relative to the heat transfer medium.

According to a further advantageous embodiment of the method, the total heat quantity {dot over (Q)} of the solar-thermal steam generator is also taken into consideration when adjusting the desired value {dot over (M)}_s, said total heat quantity {dot over (Q)} being calculated from an enthalpy difference between the enthalpy of the heat transfer medium at the inlet of the solar-thermal steam generator and the enthalpy of the heat transfer medium at the outlet of the solar-thermal steam generator on one hand, and the measured mass flow of the heat transfer medium at the inlet of the solar-thermal steam generator on the other hand.

In this case, the total heat output {dot over (Q)} is advantageously calculated from the product of the enthalpy difference and the mass flow of the heat transfer medium. For the purpose of determining the enthalpies of the heat transfer medium at the inlet and outlet of the steam generator, additional temperature measurements are taken at the corresponding locations on the heat transfer medium side. In order to allow for non-steady effects of the heat conduction through the steam generator tube wall, these measured values for calculating the enthalpies can be slightly delayed in this case, e.g. via a PT3 element.

In order to determine the desired value of the feedwater mass flow {dot over (M)}_s, at least for steady-state operating loads, the total heat output {dot over (Q)} of the heat transfer medium is divided by the warm-up margin of the feedwater (preferred enthalpy increase). In order to determine the warm-up margin of the feedwater, the enthalpies at the steam generator inlet and outlet are required on the water-steam side. For this purpose, in order to determine the inlet enthalpy, the temperature and the pressure on the flow medium side are measured at the steam generator inlet and converted into a corresponding actual enthalpy. At the steam generator outlet, the pressure is likewise captured by means of measurement on the flow medium side, and then converted into a corresponding desired enthalpy value, taking the preferred live steam temperature (desired temperature value) into account. The warm-up margin of the feedwater is the difference between desired enthalpy value at the steam generator outlet and actual enthalpy at the steam generator inlet.

This makes it possible to perform a specifically adapted precontroller-based calculation of the required feedwater quantity on the basis of heat flow balancing, wherein said calculation relates to the current condition of the installation.

In a particularly advantageous development of the method, provision is made for taking a further correction value K_F into account when adjusting the desired value {dot over (M)}_s, said correction value KF being used to correct storage effects of the solar-thermal steam generator on the water-steam side.

Furthermore, the steam generator flow which is specified by the predictive determination of the desired feedwater value can also be corrected by means of overlaid control loops, such that the live steam desired temperature value that is required at the steam generator outlet can actually be achieved with lasting effect. In respect of the corrective control of the precalculated feedwater mass flow, it must nonetheless be taken into consideration that, for reasons of controller stability, this can only be performed very slowly and applying modest controller gain. Significant temporary deviations from the predetermined desired value, which are produced as a result of physical mechanisms following non-steady operation of the solar-thermal steam generator heated by a heat transfer medium, can only be reduced slightly by these corrective control loops, or possibly not at all. The predictive determination of the desired feedwater value must therefore be consolidated by means of additional measures, in order that the temporary deviations from the predetermined desired value can also be minimized during rapid transient operations.

In order to achieve this objective, in addition to the correction value K_T, storage and withdrawal operations on the feedwater side within the steam generator tubes are taken into account by means of a correction value K_F in this particular development of the inventive method. Using both correction values K_T and K_F, it is possible to react appropriately to physical mechanisms which act temporarily on the flow through the steam generator on the water-steam side during non-steady operation and therefore result in deviations, on the flow medium side, of the actual enthalpy at the outlet of the steam generator from the predefined desired value. In order to determine the correction value K_T, it is necessary to distinguish between two different mechanisms.

During non-steady operations, values relating to thermodynamic conditions such as e.g. the live steam temperature, the pressure (and therefore the boiling temperature of the flow medium in subcritical cases) and the feedwater temperature generally change on the flow medium side in the steam generator. As a result of these changes, the material temperature of the steam generator tubes likewise is not constant, and goes up or down according to direction. Consequently, thermal energy is stored in or withdrawn from the tube walls. Compared with the balanced total heat output {dot over (Q)} which the heat transfer medium releases to the feedwater, more or less heat is therefore temporarily available for the steam generation process depending on the direction of material temperature change. In the case of a predefined desired enthalpy value and/or live steam desired temperature value at the outlet of the solar-thermal steam generator, it is therefore essential for this not inconsiderable influence to be taken into account in the control system for the purpose of advance calculation of the necessary desired feedwater mass flow value {dot over (M)}_s.

This physical effect can be reproduced in terms of control engineering by means of a first-order differentiating circuit (DT1 element). An average material temperature of all steam generator tubes must be defined and used as an input signal of the differentiating circuit. For example, the average material temperature here can be determined from the known process variables of live steam temperature, system pressure and feedwater temperature, possibly also taking the measured temperatures of the heat transfer medium into consideration. If this average material temperature now changes, and if the output of the differentiating circuit is multiplied by the mass of the total steam generator tubes and the specific heat capacity of the evaporator material and the reciprocal value of the time constant of the differentiating circuit, the quantities of heat that are stored in or withdrawn from the tube wall can be quantified. By selecting a suitable time constant for this differentiating circuit, the temporal behavior of the described storage effects can be simulated with relative accuracy, such that this additional effect, caused by non-steady operation, of heat of the metal masses being stored or withdrawn can be calculated directly. This is equally applicable to subcritical and supercritical systems.

Alternatively, it is also conceivable to measure the material temperature directly at characteristic locations of the steam generator tubes. In these circumstances, a change in the metal temperature can be taken into account directly. In this case, both the number of differentiating circuits and their corresponding amplification factors (essentially the mass of the steam generator tubes) would have to be adapted to the number of metal temperature measurements. Although this variant involves greater cost in terms of measuring technology, it has the advantage that the quantities of heat being stored or withdrawn are determined more accurately as a result.

In addition to these storage or withdrawal operations in respect of additional heat energy from the material of the steam generator tubes, storage and withdrawal operations in respect of thermal energy of the heat transfer medium are likewise a factor that must not be ignored during non-steady operation of the solar-thermal steam generator. If the average temperature level of the heat transfer medium decreases in the whole steam generator, for example, further heat is released and is additionally absorbed by the steam generator tubes. Compared with the balanced total heat output {dot over (Q)}, more heat output is therefore available for the steam generating process. It follows that less heat is available in the opposite case. These effects must also be taken into account if the necessary desired feedwater mass flow value {dot over (M)}_s is to be predetermined correctly. Here likewise, the additional heat output that is stored or withdrawn can preferably be determined by means of a first-order differentiating circuit (DT1 element) using a suitable time constant. To this end, provision is likewise made for determining an average temperature of the heat transfer medium, for use as an input signal of this DT1 element. The average temperature can preferably be functionally determined from the measured temperatures of the heat transfer medium at the steam generator inlet and outlet. For the purpose of determining this average temperature of the heat transfer medium, it is also conceivable to use further known process parameters, other characteristic values or even additional temperature measurements along the flow path of the heat transfer medium in the steam generator. The output signal of the DT1 element is preferably multiplied by the volume that is occupied by the heat transfer medium in the control volume of the steam generator, the density of the heat transfer medium and the specific heat capacity of the heat transfer medium, and by the reciprocal value of the time constant of the DT1 element. It should be noted in this case that both the density and the specific heat capacity of the heat transfer medium are temperature-dependent, and can possibly be determined using the average temperature that has already been calculated for the heat transfer medium, for example. In these circumstances, the additional heat output is also quantified in this case.

Both of the additional heat quantities from tube wall and heat transfer medium are then added to give the correction value K_T, which must be subtracted from the balanced total heat output {dot over (Q)} in order to determine the desired feedwater mass flow value {dot over (M)}_s.

The second correction value K_F, which correctively acts directly on the desired value of the feedwater mass flow {dot over (M)}_s, also compensates effectively for further disturbing influences, which are produced due to non-steady operation, in the water-steam circuit of the solar-thermal steam generator. In this context, disturbances in the feedwater temperature at the inlet of the solar-thermal steam generator have a significant effect on its flow. Specifically, this means that a decrease in the feedwater temperature is associated with a reduction in the specific volume of the flow medium in the inlet region of the solar-thermal steam generator. This results in a requirement for additional feedwater, which must fill the now unused volume of the steam generator tubes (feedwater is brought in). If the feedwater temperature increases, however, the reverse mechanism is triggered. If the feedwater temperature at the inlet of the solar-thermal steam generator now undergoes changes due to non-steady operation, the resulting storage and withdrawal operations on the feedwater side mean that the inlet and outlet mass flows of the solar-thermal steam generator are not identical. This has a direct influence on the steam generator outlet enthalpy (live steam temperature) which, under these circumstances, may not remain constant even if the heat input remains the same. Therefore the effects of fluctuating feedwater temperatures at the inlet of the solar-thermal steam generator must likewise be balanced out by countermeasures in the form of determining the desired feedwater value (increasing or decreasing the desired feedwater mass flow value {dot over (M)}_s). If an additional first-order differentiating circuit is also used here, it is however possible effectively to reduce the enthalpy fluctuations (fluctuations of the live steam temperature) at the outlet of the solar-thermal steam generator by selecting a suitable input signal (e.g. inlet overcooling of the steam generator, inlet enthalpy of the steam generator or the feedwater temperature itself), an appropriate time constant and a suitable amplification.

In a preferred embodiment of the invention, the solar-thermal steam generator is integrated into a solar-thermal parabolic trough power plant featuring a number of parabolic troughs, by means of which the heat transfer medium is subjected to solar-thermal heating. If the inventive determination of the desired feedwater value is used in oil-heated solar-thermal steam generators, constant live steam temperatures can even be ensured in distinctly non-steady operating conditions, such as increasingly occur in solar-heated power plants (e.g. cloud passage). In addition to a consequently reliable means of control during changeable weather conditions, the availability of the entire power plant installation can be improved by means of a material-preserving design. Moreover, the design according to the invention is also suitable for modular use in a plurality of solar-heated steam generators in a single parabolic trough power plant. The design can also be used without significant changes in combination with other components such as injection coolers, for example.

The heat transfer medium can be a thermo-oil or a salt melt, for example. The use of suitable metal melts as a heat transfer medium is also possible.

The object of the invention in respect of a device is achieved by the features in the claims. It is particularly advantageous in this case if the indirectly heated solar-thermal steam generator is integrated into a solar-thermal parabolic trough power plant and connected to a number of parabolic troughs for the supply of the superheated heat transfer medium, wherein said parabolic troughs can be directly subjected to focused solar incidence. The object of the invention in respect of a device is also achieved by the features in the claims.

In particular, the advantages of the invention consist in being able to correct the desired value determined in the context of predictive mass flow control for the feedwater mass flow {dot over (M)}, by taking into account the temporal dissipation of the enthalpy, temperature or the density of the feedwater at the input of the steam generator, wherein due consideration can also be given to storage and withdrawal operations in respect of thermal energy of the tube material. This makes it possible to determine a specifically adapted desired value for the feedwater mass flow {dot over (M)} with a high level of precision, particularly in the event of load variations or other transient operations.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in greater detail in FIGS. 1,2,3 and 4, in which:

FIG. 1 shows a schematic illustration of an indirectly heated solar-thermal steam generator featuring feedwater flow control for steady-state operation,

FIG. 2 shows a schematic illustration of an indirectly heated solar-thermal steam generator in a development for non-steady operation, featuring predictive determination of a desired value for feedwater mass flow,

FIG. 3 shows a schematic illustration of an indirectly heated solar-thermal steam generator in an alternative embodiment for non-steady operation, featuring predictive determination of a desired value for feedwater mass flow,

FIG. 4 shows a schematic illustration of an indirectly heated solar-thermal steam generator in a particular development for non-steady operation, featuring predictive determination of a desired value for feedwater mass flow.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a schematic diagram of a control circuit for determining a desired feedwater value for steady-state operation of a solar-thermal steam generator 3 in a parabolic trough power plant 1. The indirectly heated solar-thermal steam generator 3, a device for adjusting the feedwater mass flow 5, and a feedwater flow control 11 are illustrated as important components. The circuit is part of a solar-thermal parabolic trough power plant 1, which also features parabolic troughs and a power plant section comprising a steam turbine (not shown).

The solar-thermal steam generator 3 is configured for the supply of thermo-oil 4 as a heat transfer medium, and is connected on the thermo-oil side to an incoming thermo-oil supply line 13 and an outgoing thermo-oil outflow line 14. The solar-thermal steam generator 3 is also connected on the feedwater side to an incoming feedwater supply line 15 and to an outgoing steam line 16. Heat energy can therefore be transferred from the highly heated thermo-oil 4 to the feedwater by means of the solar-thermal steam generator 3, thereby vaporizing the feedwater.

The solar-thermal steam generator 3 is also configured to allow a controlled delivery of feedwater. The device for adjusting the feedwater mass flow 5 comprises a servomotor 18, which activates a throttle valve 19 such that the feedwater quantity or the feedwater mass flow {dot over (M)} that is transported from a feedwater pump 17 in the direction of the solar-thermal steam generator 3 can be adjusted by activating the throttle valve 19 correspondingly. For the purpose of determining a current characteristic value for the supplied feedwater mass flow {dot over (M)}, a measuring device 20 for determining the feedwater mass flow {dot over (M)} through the feedwater line is connected upstream of the throttle valve 19 in the feedwater supply line. The servomotor 18 is activated via a control element 21 which, on the input side, receives a desired value {dot over (M)}_s for the feedwater mass flow {dot over (M)}, said value being supplied via a data line 22, and a current actual value of the feedwater mass flow {dot over (M)} as determined by the measuring device 20. A difference between these two signals indicates a correction requirement, such that a corresponding correction of the throttle valve 19 is effected via the activation of the motor 18 if the actual value deviates from the desired value.

For the purpose of determining a desired value {dot over (M)}_s for the feedwater mass flow {dot over (M)}, the input side of the data line 22 is connected to the feedwater flow control 11, which is so configured as to specify the desired value {dot over (M)}_s for the feedwater mass flow {dot over (M)}.

The desired value {dot over (M)}_s is determined with reference to a heat flow balance in the solar-thermal steam generator 3, and is based on the relationship between the heat flow that is currently being transferred by the thermo-oil 4 into the feedwater in the solar-thermal steam generator 3 on one hand, and a predefined desired value for an enthalpy increase of the feedwater with regard to the preferred live steam condition on the other. The feedwater flow control 11 features a dividing element 23 for the purpose of supplying the desired value {dot over (M)}_s.

The numerator is supplied to the dividing element 23 by a function module 24. The function module 24 calculates the heat output {dot over (Q)} that has been introduced into the solar-thermal steam generator 3, by multiplying the enthalpy difference of the thermo-oil 4 at the inlet and at the outlet of the solar-thermal steam generator 3 by the mass flow of the thermo-oil 4. In order to provide the mass flow of the thermo-oil 4 before it enters the solar-thermal steam generator, the function module 24 is connected to a measuring device 31, which is connected into the thermo-oil supply line 13. In order to provide the enthalpy difference of the thermo-oil 4, the function module 24 is connected to an analysis unit 25.

Connected to the analysis unit 25 are a first module 27 for calculating the enthalpy at the inlet of the solar-thermal steam generator 3 and a second module 28 for calculating the enthalpy at the outlet of the solar-thermal steam generator 3. The first module 27 is connected to a first oil temperature measuring unit 29, which is connected into the thermo-oil supply line 13 at the inlet of the solar-thermal steam generator 3, and the second module 28 is connected to a second oil temperature measuring unit 30, which is connected into the thermo-oil outflow line 14 at the outlet of the solar-thermal steam generator 3. In this case, the inlet of the thermo-oil designates a region of the solar-thermal steam generator 3 in which heat energy of the thermo-oil is not yet transferred to the feedwater. Similarly, the outlet of the thermo-oil is a region of the solar-thermal steam generator 3 in which the heat energy of the thermo-oil is transferred to the feedwater correspondingly. The measurement of the mass flow of the thermo-oil takes place before it enters the generator.

For the purpose of supplying the denominator, i.e. the characteristic value for the preferred enthalpy increase (warm-up margin), the input side of the dividing element 23 is connected to a function module 32. The function module 32 generates the enthalpy difference from a calculated desired enthalpy at the outlet of the solar-thermal steam generator 3 and a measured actual enthalpy of the feedwater before it enters the solar-thermal steam generator 3.

The actual value of the current enthalpy of the feedwater before it enters the solar-thermal steam generator 3 is determined by an analysis unit 33 and transferred to the function module 32. For the purpose of determining measured data, the analysis unit 33 is also connected to a pressure measuring device 35 and a temperature measuring device 36, both of which are connected into the feedwater supply line 15.

The desired enthalpy at the outlet of the solar-thermal steam generator 3 is calculated by a function module 34. This desired value is determined from the preferred live steam temperature (live steam desired temperature value) and the measured pressure at the outlet of the solar-thermal steam generator 3. The data relating to the pressure at the outlet of the solar-thermal steam generator 3 is supplied to the function module 34 by means of a pressure sensor 37.

The enthalpy increase of the flow medium, which increase is required in the solar-thermal steam generator 3 as a function of the preferred live steam condition, is therefore determined by subtraction in the function module 32 and used as a denominator in the dividing element 23. The dividing element 23 calculates the required mass flow signal therefrom.

In an extension of FIG. 1, FIG. 2 shows a control circuit diagram for predictive determination of the desired feedwater value for non-steady operation.

During non-steady operation, values relating to thermodynamic states such as e.g. the live steam temperature, the pressure (and therefore the boiling temperature of the flow medium in subcritical systems) and the feedwater temperature generally change in the steam generator. As a result of these changes, the material temperature of the steam generator tubes likewise is not constant, and goes up or down according to direction. Consequently, thermal energy is stored in or withdrawn from the tube walls. Compared with the balanced heat of the thermo-oil, more or less heat is therefore temporarily available for the steam generation process of the flow medium depending on the direction of material temperature change.

In the case of a predefined desired enthalpy value and/or live steam desired temperature value at the outlet of the solar-thermal steam generator 3, it is therefore essential for this not inconsiderable influence to be taken into account in the control system for the purpose of advance calculation of the required feedwater mass flow. According to the invention, this is effected by means of a correction value K_T. The correction value K_T is a characteristic heat flow value, by means of which the storage and withdrawal effects of the steam generator tubes can be determined. This is equally applicable to subcritical and supercritical systems.

For the purpose of taking the correction value K_T into account, in an extension of FIG. 1, a subtractor 40 is provided in FIG. 2, being connected between the function module 24 and the dividing element 23. The subtractor 40 determines the difference between the heat output {dot over (Q)} that has been introduced into the steam generator (total heat absorption) and the correction value K_T, and forwards the result to the dividing element 23 as a corrected heat quantity that has been introduced {dot over (Q)}_Korr.

The correction value K_T is supplied to the subtractor by a differentiating circuit 41. An average material temperature of all steam generator tubes must be defined and used as an input signal for the differentiating circuit 41. For example, the average material temperature here can be determined from the known process variables of live steam temperature, system pressure and feedwater temperature. If this average material temperature now changes, and if this temporal change (evaluated by the differentiating circuit 41) is multiplied by the mass of the total steam generator tubes and the specific heat capacity of the evaporator material, the quantities of heat that are stored in or withdrawn from the tube wall can be quantified in the form of the correction value K_T. By selecting a suitable time constant of the differentiating circuit 41, the temporal behavior of the described storage effects can be simulated with relative accuracy, such that this additional effect, caused by non-steady operation, of heat of the metal masses being stored or withdrawn can be calculated directly.

FIG. 3 shows an alternative embodiment to that shown in FIG. 2 of the predictive determination of a desired value for the feedwater mass flow. In addition to the differentiating circuit 41, which determines the storage effects in metal masses, a further differentiating circuit 45 is provided for the purpose of determining the storage effects of thermal energy in relation to the thermo-oil. To this end, the differentiating circuit 45 likewise analyzes known process parameters relative to time, e.g. the measured oil temperature at the steam generator inlet or at the steam generator outlet, or an average oil temperature that has been functionally derived from these two measured temperature values. If the output signal of this differentiating circuit 45 is now multiplied by the total oil volume, the density and the specific heat capacity of the oil, it is likewise possible to quantify the heat quantities that are stored or withdrawn relative to the thermo-oil. Using a suitable time constant of the differentiating circuit 45, the temporal behavior of the storage effects in relation to the thermo-oil can be simulated with relative accuracy. This additional effect, caused by non-steady operation, of heat being stored or withdrawn relative to the thermo-oil can therefore be calculated directly.

The output signals of the differentiating circuit 41 and the differentiating circuit 45 are summed in a function element 45 to form the correction value K_T, which is supplied as an input signal to the subtractor 40.

FIG. 4 shows a schematic illustration of an indirectly heated solar-thermal steam generator in a particular development for non-steady operation, featuring predictive determination of a desired value for the feedwater mass flow.

Disturbances in the feedwater temperature at the inlet of the solar-thermal steam generator 3 have a significant effect on its flow. Specifically, this means that a decrease in the feedwater temperature is associated with a reduction in the specific volume of the flow medium in the inlet region of the solar-thermal steam generator 3. This results in a requirement for additional feedwater, which must fill the now unused volume of the steam generator tubes (feedwater is brought in). If the feedwater temperature increases, however, the reverse mechanism is triggered.

If the feedwater temperature at the outlet of the solar-thermal steam generator now undergoes changes due to non-steady operation, the resulting storage and withdrawal operations on the feedwater side mean that the inlet and outlet mass flows of the solar-thermal steam generator 3 are not identical. This has a direct influence on the steam generator outlet enthalpy (live steam temperature) which, under these circumstances, may not remain constant even if the heat input remains the same. Therefore the effects of fluctuating feedwater temperatures at the inlet of the solar-thermal steam generator 3 are likewise balanced out by countermeasures in the form of determining the desired feedwater value (increasing or decreasing the feedwater mass flow). This is effected by means of the correction value K_F.

In an extension of FIG. 3, FIG. 4 further shows an adder 42, which is connected into the data line 22 and corrects the desired value {dot over (M)}_s by the correction value K_F. The adder is a component of the feedwater flow control 11. The correction value K_F is supplied to the adder 42 via a differentiating circuit 43. The input signal of the differentiating circuit 43 in this case comprises data such as e.g. inlet overcooling of the steam generator, inlet enthalpy of the steam generator or the feedwater temperature itself. As a function of this input signal, the differentiating circuit 43 is parameterized using an appropriate time constant and a suitable amplification, in order effectively to reduce the enthalpy fluctuations (fluctuations of the live steam temperature) at the outlet of the solar-thermal steam generator 3.

If the inventive determination of the desired feedwater value is applied in oil-heated solar-thermal steam generators 3, constant live steam temperatures in BENSON mode can even be ensured in distinctly non-steady operating conditions, such as increasingly occur in solar-heated power plants (e.g. cloud passage). In addition to a consequently reliable means of control during changeable weather conditions, the availability of the entire installation can be improved by means of a material-preserving design. Moreover, the design according to the invention is also suitable for modular use in a plurality of solar-heated steam generators in a single parabolic trough power plant. The design can also be used without significant changes in combination with other components such as injection coolers, for example.

By virtue of the method according to the invention, precisely the required feedwater mass flow through the solar-thermal steam generator 3 is supplied at all times, as a function of the available heat output from solar radiation, in order to ensure the required and/or preferred live steam mass flow at the outlet of the solar-thermal steam generator 3 (live steam temperature), even during non-steady operation and in particular in the event of cloud passage through the solar field.

Claims

1-12. (canceled)

13. A method for operating an indirectly heated solar-thermal steam generator featuring a heat transfer medium, comprising:

supplying a desired value {dot over (M)}s for the feedwater mass flow {dot over (M)} to a device for adjusting the feedwater mass flow {dot over (M)},

wherein a first correction value KT is taken into account when setting the desired value {dot over (M)}s for the feedwater mass flow {dot over (M)}, whereby thermal storage effects of thermal energy that is stored or withdrawn relative to the steam generator is corrected.

14. The method as claimed in claim 13, wherein the thermal storage effects of thermal energy that is stored or withdrawn relative to a plurality of tube walls of the solar-thermal steam generator are corrected by means of the first correction value KT.

15. The method as claimed in claim 13, wherein the thermal storage effects of thermal energy that is stored or withdrawn relative to the heat transfer medium are corrected by means of the first correction value KT.

16. The method as claimed in claim 13, wherein a total heat quantity {dot over (Q)} of the solar-thermal steam generator is taken into account when setting the desired value {dot over (M)}s, and is calculated from the product of

an enthalpy difference between a first enthalpy of the heat transfer medium at an inlet of the solar-thermal steam generator and a second enthalpy of the heat transfer medium at an outlet of the solar-thermal steam generator, and

a measured mass flow of the heat transfer medium ahead of the inlet of the solar-thermal steam generator.

17. The method as claimed in claim 13,

wherein a second correction value KF is also taken into account when setting the desired value {dot over (M)}s, and

wherein the second correction value KF is used to correct feedwater quantities that are stored or withdrawn relative to a plurality of steam generator tubes of the solar-thermal steam generator.

18. The method as claimed in claim 17, wherein the second correction value KF is determined using a feedwater inlet overcooling.

19. The method as claimed in claim 17, wherein the second correction value KF is determined using a feedwater inlet enthalpy.

20. The method as claimed in claim 17, wherein the second correction value KF is determined using a feedwater temperature.

21. The method as claimed in claim 13, wherein the solar-thermal steam generator is integrated into a solar-thermal parabolic trough power plant featuring a plurality of parabolic troughs, by means of which the heat transfer medium is subjected to solar-thermal heating.

22. The method as claimed in claim 13, wherein the heat transfer medium is selected from the group consisting of thermo-oil, a salt melt and a metal melt.

23. An indirectly heated solar-thermal steam generator, comprising:

a heat transfer medium; and

a device for adjusting the feedwater mass flow {dot over (M)},

wherein the device is governed with reference to a desired value {dot over (M)}s for the feedwater mass flow {dot over (M)}, and

wherein an associated feedwater flow control is so configured as to specify the desired value {dot over (M)}s by means of the method as claimed in claim 13.

24. The indirectly heated solar-thermal steam generator as claimed in claim 23,

wherein the solar-thermal steam generator is integrated into a solar-thermal parabolic trough power plant and is connected to a plurality of parabolic troughs for the supply of the superheated heat transfer medium, and

wherein the plurality of parabolic troughs are directly subjected to focused solar incidence.

25. The indirectly heated solar-thermal steam generator as claimed in claim 23, wherein the heat transfer medium is selected from the group consisting of thermo-oil, a salt melt and a metal melt.

26. A solar-thermal parabolic trough power plant, comprising:

an indirectly heated solar-thermal steam generator as claimed in claim 23.