Method for operating a directly heated, solar-thermal steam generator

Abstract

A method for operating a directly heated, solar-thermal steam generator is provided. As per the method, a nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)} is conducted to an apparatus for adjusting the supply water mass flow {dot over (M)} wherein, at the adjustment of the nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)}, account is taken of a correction value K.sub.T, by which the thermal effects of storage or withdrawal of thermal energy in an evaporator are corrected.

Claims

1. A directly heated solar-thermal steam generator, comprising: a control element for adjusting the supply water mass flow {dot over (M)}, which is conducted on the basis of a nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)}, wherein the nominal value {dot over (M)}.sub.s is determined by a heat flow balance of an evaporator, wherein the heat flow balance is a ratio of the heat flow transferred into the evaporator and a nominal increase in enthalpy desired with respect to a specified enthalpy nominal value at the evaporator outlet; wherein an associated supply water flow control is configured for adjusting the nominal value {dot over (M)}.sub.s by taking into account a correction value K.sub.T, by which the thermal effects of storage or withdrawal of thermal energy in an evaporator are corrected; and wherein a correction value K.sub.F is further taken into account, by which fluid quantities stored in or withdrawn from the evaporator tubes are further corrected, wherein K.sub.F is determined by fluid-side stored or withdrawn fluid quantities in the evaporator of the solar-thermal steam generator, or from a sum of fluid-side stored or withdrawn fluid quantities in the evaporator and economizer.

2. The directly heated solar-thermal steam generator as claimed in claim 1, comprising a number of parabolic troughs, which can be subjected directly due to focused solar incidence.

3. A solar-thermal parabolic trough power plant with a directly heated solar-thermal steam generator as claimed in claim 1.

4. A method for operating a directly heated, solar-thermal steam generator, comprising: conducting a nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)} to a control element for adjusting the supply water mass flow {dot over (M)}, wherein the nominal value {dot over (M)}.sub.s is determined by a heat flow balance of an evaporator, wherein the heat flow balance is a ratio of the heat flow transferred into the evaporator and a nominal increase in enthalpy desired with respect to a specified enthalpy nominal value at the evaporator outlet; adjusting the supply water mass flow {dot over (M)} using the nominal value {dot over (M)}.sub.s, wherein, the nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)}, further takes into account a correction value K.sub.T, by which the thermal effects of storage or withdrawal of thermal energy in an evaporator are corrected; and wherein a correction value K.sub.F is further taken into account, by which fluid quantities stored in or withdrawn from the evaporator tubes are further corrected, wherein K.sub.F is determined by fluid-side stored or withdrawn fluid quantities in the evaporator of the solar-thermal steam generator, or from a sum of fluid-side stored or withdrawn fluid quantities in the evaporator and economizer.

5. The method as claimed in claim 4, wherein thermal storage effects of thermal energy stored into or withdrawn from the tube walls of the evaporator of the solar-thermal generator are corrected by the correction value K.sub.T.

6. The method as claimed in claim 4, wherein the fluid quantities stored in or withdrawn from an economizer upstream of the evaporator are further corrected by a correction value K.sub.F.

7. The method as claimed in claim 4, wherein the correction value K.sub.F is determined by making use of the supply water intake under-cooling or the supply water intake enthalpy or the supply water temperature or the supply water density.

8. The method as claimed in claim 4, wherein the solar-thermal steam generator comprises a plurality of parabolic troughs, in which supply water is evaporated directly by solar thermal means.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In FIGS. 1 to 4 exemplary embodiments of the invention are explained in greater detail. These show:

(2) FIG. 1 a diagrammatic representation of a directly heated solar-thermal steam generator 3 with supply water flow control for stationary operation.

(3) FIG. 2 a diagrammatic representation of a directly heated solar-thermal steam generator 3 for non-stationary operation with predictive supply water nominal value determination.

(4) FIG. 3 a diagrammatic representation of a directly heated solar-thermal steam generator 3 for non-stationary operation with a further developed predictive supply water nominal value determination.

(5) FIG. 4 a diagrammatic representation with a further development of a directly heated solar-thermal steam generator 3 with predictive supply water nominal value determination, taking account of additional economizer heating surfaces.

DETAILED DESCRIPTION OF INVENTION

(6) FIG. 1 shows a diagrammatic control circuit diagram of a supply water nominal value determination for the stationary operation of a solar-thermal steam generator 3 in a parabolic trough power plant 1. The parabolic trough power plant 1 is not represented in any greater detail. The solar-thermal steam generator is only diagrammatically represented. Solar-thermal steam generators comprise as a rule a number of parabolic trough collectors 13 (or Fresnel collectors respectively), which can be used as evaporator collectors 14, as superheater collectors 9, or as economizer collectors 10. The solar-thermal steam generator 3 represented in FIG. 1 comprises only evaporator collectors 14 and superheater collectors 9. The evaporator collectors 14 are connected to a supply water delivery line 15 for the conducting of supply water.

(7) The solar-thermal steam generator 3 represented in FIG. 1 is additionally in forced-flow operation, in which the supply water is entirely evaporated in the evaporator collectors 13 by solar-thermal direct heating, and is then superheated.

(8) The solar-thermal steam generator 3 is designed for a controlled application of supply water. For this purpose, a supply water pump 17 is connected into the supply water delivery line 15. A choke valve 19 is also connected into the supply water delivery line 15, which is actuated by a servomotor 18. The choke valve 19 and servomotor 18 are integral parts of an apparatus for adjusting the supply water mass flow 5, which also still includes a control element 21, which is provided to activate the servomotor 18, and a measuring facility 20, which determines the supply water mass flow {dot over (M)} in the supply water delivery line 15. The control element 21 is subjected on the input side to a nominal value {dot over (M)}.sub.s supplied by way of a data line 22 for the supply water mass flow {dot over (M)} and is subjected to the current actual value of the supply water mass flow {dot over (M)} determined by way of the measuring facility 20. By differentiating between these two signals, a tracking requirement is conveyed so that when deviating the actual value from the nominal value, a corresponding tracking of the choke value 19 takes place by activating the motor 18.

(9) For the determination of a nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)}, the data line 22 is connected on the input side to the supply water flow control 11, which is configured to specify the nominal value {dot over (M)}.sub.s for the supply water mass flow {dot over (M)}.

(10) The nominal value {dot over (M)}.sub.s is determined on the basis of a heat flow balance of the evaporator of the solar-thermal steam generator 3, by way of the ratio of the heat flow currently being transferred into the evaporator of the solar-thermal steam generator 3 and onto the supply water on the one hand, and, on the other, a nominal increase in the enthalpy which is desired with regard to the specified enthalpy nominal value at the evaporator outlet. For the provision of the nominal value {dot over (M)}.sub.s, the supply water flow control 11 exhibits a divider element 23.

(11) The counter is provided to the divider element 23 from a function module 24. The function module 24 determines the heat output Q transferred into the evaporator heating surface of the solar-thermal steam generator 3 or to the evaporator collector field. For this purpose each evaporator collector 14 of the solar-thermal steam generator 3 is equipped with an appropriate measuring facility. The measured data from the individual evaporator collectors 14 is summated in a function module 25, and, due to the non-stationary heat conduction into the tube walls, is temporally slightly delayed by means, for example, of a PT3 element.

(12) As the denominator, the heat-up range or, respectively, the enthalpy difference of the flow medium in the evaporator collectors 14 is fed to the divider element 23. The enthalpy difference is formed from the enthalpy nominal value at the output of the evaporator collectors 14 and the present enthalpy at the input of the evaporator collectors 14, which is determined by conversion by way of the measured values of pressure and temperature. The actual value of the present enthalpy of the supply water before input into the solar-thermal steam generator 3 is determined by an evaluation unit 33, and transferred to the function module 33. To determine measured data, the evaluation unit 33 is connected to a pressure measuring apparatus 35 and a temperature measuring apparatus 36, both of which are in each case connected into the supply water delivery line 15.

(13) The nominal enthalpy at the output of the evaporator of the solar-thermal steam generator 3 is selected as a function of the state of the system and the evaporator design, and specified as a nominal value. The nominal enthalpy is fed to the function module 32 via a signal transmitter 34. By differentiation in the function module 32, the increase in enthalpy of the flow medium, required as a function of the desired evaporator output state, is determined in the evaporator of the solar-thermal steam generator 3, and then used as a denominator in the divider element 23. The divider element 23 calculates from this the required mass flow signal.

(14) As an extension to FIG. 1, FIG. 2 shows a control circuit diagram of a directly heated solar-thermal steam generator 3 with predictive supply water nominal value determination for non-stationary operation.

(15) With non-stationary processes, thermodynamic state values in the steam generator generally change, such as, for example, the live steam temperature, the pressure (and accordingly, in sub-critical cases, also the boiling temperature of the flow medium), and the supply water temperature. As a result of these changes, the material temperature of the steam generator tubes is also not constant, and becomes greater or smaller depending on the direction. Consequently, thermal energy is stored in the tube walls or withdrawn from the tube walls. Compared with the balanced heat of the thermal oil, depending on the direction of the material temperature change, there is accordingly more or less heat temporarily available for the steam generation process of the flow medium. This can likewise be observed for systems with both sub-critical as well as above-critical steam parameters.

(16) Accordingly, with a predetermined enthalpy nominal value at the evaporator output of the solar-thermal steam generator 3, for the advance calculation of the supply water mass flow required this not insubstantial influence must inevitably be taken into account in the control circuit. According to the invention, this is effected by a correction value K.sub.T. The correction value K.sub.T is a characteristic heat flow variable by means of which the evaporation tube storage and withdrawal effects can be determined equally for sub-critical as well as for above-critical systems.

(17) In order to take account of the correction value K.sub.T, provision is made in FIG. 2, as an extension to FIG. 1, for a subtractor element 40, which is connected between the function module 24 and the divider element 23. The differentiator element 40 forms the difference from the heat output Q (total heat absorption) introduced into the evaporator, which is provided by the function module 24, and the correction value K.sub.T, and forwards the result, as the corrected introduced heat output Q.sub.Korr, to the divider element 23.

(18) The correction value K.sub.T is provided to the subtractor element 40 by a differentiator element 41. For the differentiator element 41, as the input signal, a mean material temperature of all the evaporator tubes is to be defined and used. In this case, for example, the mean material temperature can be determined by way of the values known from the process, the live steam temperature system pressure, and supply water temperature. If this mean material temperature now changes, and if this temporal change (assessed by the differentiator element 41) is multiplied by the mass of the whole of the steam generator tubes and the specific heat capacity of the evaporator material, the heat quantities stored in and withdrawn from the tube wall can be quantified in the form of the correction value K.sub.T. By the selection of a suitable time constant of the differentiator element 41, the temporal behavior of the described storage effects can be recreated relatively precisely, such that this additional effect of the storage and withdrawal of heat from the metal masses, based on non-stationary processes, can be calculated directly.

(19) FIG. 3 shows a diagrammatic representation of a directly heated solar-thermal steam generator 3 in a further development from FIG. 2, with the additional consideration of the correction value K.sub.F.

(20) Disturbances to the supply water temperature at the input of the evaporator of the solar-thermal steam generator 3 have a decisive effect on its throughflow. Specifically, this means that, as the supply water temperature falls, the specific volume of the flow medium in the input area of the evaporator of the solar-thermal steam generator 3 decreases. Due to this process, additional supply water is required, which must top up the volume of the evaporator tubes which is now not exhausted. Consequently, supply water is stored. By contrast, if the supply water temperature rises, the inverse mechanism takes place.

(21) If, as a result of non-stationary processes, the supply water temperature at the input of the evaporator of the solar-thermal steam generator is now subjected to changes, then, with the resultant fluid-side storage and withdrawal processes, the input and output mass flows of the evaporator of the solar-thermal steam generator 3 are not identical. This has an immediate affect on the evaporator output enthalpy, which, under these circumstances, cannot remain constant, even if the heat input is constant. Accordingly, the effects of fluctuating supply water temperatures at the input of the evaporator of the solar-thermal steam generator 3 are likewise compensated by countermeasures of the supply water nominal value determination (increasing or decreasing of the supply water mass flow). This is effected by the correction value K.sub.F.

(22) Taking FIG. 2 as a basis, further represented in FIG. 3 is an adder element 42, which is connected into the data line 22 and corrects the nominal value {dot over (M)}.sub.s by the correction value K.sub.F. The correction value K.sub.F is conducted to the adder element 42 via a differentiator element 43. The differentiator element 43 takes into consideration data such as, for example, input under-cooling of the evaporator, input enthalpy of the evaporator, or the supply water temperature itself. The differentiator element 43 is parameterized with an appropriate time constant and a suitable amplification, in order to effectively reduce the enthalpy fluctuations at the evaporator output of the solar-thermal steam generator 3. In this situation, the differentiator element 43 receives on the input side, for example, the input under-cooling from the evaluation unit 48. The evaluation unit 48 is connected to the pressure measuring apparatus 35 and the temperature measuring apparatus 36, which are already supplying the evaluation unit 33 with measured data.

(23) FIG. 4 shows, in comparison with FIG. 3, an extended circuit arrangement of the solar-thermal steam generator 3, with additional economizer collectors 10.

(24) In order, also with transient processes, to correct the fluid-side storage and withdrawal effects of the parabolic trough collectors 13 used as economizers, a determination of the densities of the flow medium at the input and output of the parabolic trough collectors 13 used as economizers is to be performed. To do this, as well as the differentiator element 43, which corrects the fluid-side storage and withdrawal effects of the evaporator collectors 14, a further differentiator element 44 is provided, by which the fluid-side storage and withdrawal effects of the economizer collectors 10 are corrected. The signals from the differentiator element 43 and the differentiator element 44 are superimposed in an adder element 45, and this sum of both the individual signals forms the correction factor K.sub.F.

(25) The differentiator element 44 in this situation is connected on the input side to a function element 51, in which a mean density of the fluid is determined. To do this, the density of the fluid at the input of the first economizer collector 10 is conducted to the function element 51 via a function module 49, and the density of the fluid at the output of the last economizer collector 10 via a function module 50. The function module 49 is connected for this purpose to a pressure measuring apparatus 55 and a temperature measuring apparatus 56, which are connected into the supply water delivery line 15 upstream of the input of the first economizer collector 10. The function module 50 is connected to the pressure measuring apparatus 35 and the temperature measuring apparatus 36, which are already supplying the evaluation unit 33 with measured data.

(26) The function module 49 and the function module 50 calculate from the pressure and temperature information the fluid densities at the respective measuring points. The function element 51 calculates, by means of a suitable conversion process, a representative density mean value. A change in this density mean value is inevitably an indicator of fluid-side storage and withdrawal effects of the economizer collectors 10. This density mean value is therefore formed in the function element 51, and quantitatively acquired by the differentiator element 44. If a suitable amplification and a suitable time constant are selected for this differentiator element 44, the correction signal generated in this way compensates optimally for the fluid-side storage effects in the economizer. For amplification purposes, use is preferentially made of the complete volume of the economizer collector tubes. As a time constant, use is preferably made of half the passage time of the flow medium through the economizer collectors 10, although this is to be selected in a load-dependent manner.

(27) In addition to a greater stability of the control, this measure also contributes to increasing the quality of control. In this situation, however, account must be taken of the fact that, for the circuit variant in FIG. 4, an adequate input under-cooling must be guaranteed between the economizer collectors 10 and the evaporator collectors 14, in order for the temperature measuring apparatus 36 to be able to supply a valid and evaluatable measurement signal.