Abstract:
A solar thermal power plant, wherein, in an intermediate superheater in the water-steam circuit, steam is heated to a settable setpoint temperature value at the outlet by a heat carrier medium which has been heated solar-thermally, wherein, for the heating of the steam to a set setpoint temperature value, a mass flow of the heat carrier medium entering the intermediate superheater is controlled as a function of a determined enthalpy difference of the heat carrier medium between the entry and exit thereof into and out of the intermediate superheater and as a function of a determined enthalpy difference of the steam between the exit and entry thereof out of and into the intermediate superheater is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Application No. PCT/EP2013/057541, having a filing date of Apr. 11, 2013, based off of DE 102012206466.4 having a filing date of Apr. 19, 2012, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF TECHNOLOGY 
       [0002]    Solar thermal power plants now represent an alternative to conventional electricity generation. One power plant concept already known in this field is the so-called parabolic trough power plant. In this type of power plant, thermal oil is conventionally used as a heat transfer medium, which flows through the parabolic troughs and thereby absorbs the heat provided by the sun. The heat absorbed in this way by the heat transfer medium is subsequently used to generate steam in a steam generator. In this case, the heat transfer medium flows around the steam generator tubes filled with water vapor in the steam generator, in such a way that it delivers its heat to the cooler steam generator tubes. The steam thus generated in the tubes then drives a conventional steam turbine. 
       BACKGROUND 
       [0003]    In order to achieve a system efficiency which is as high as possible, intermediate superheating of the steam is conventionally provided. That is to say, the cooled steam leaving an HD turbine stage is delivered to a further heat exchanger also heated by the heat transfer medium, and brought again to a higher temperature. In contrast to the steam generator described in the introduction, in this intermediate superheater steam now flows around the intermediate superheater tubes, through which the heat transfer medium flows, in such a way that this steam is heated according to the strongly heated tubes carrying the heat transfer medium. Owing to the relatively large volume flows of the intermediate superheater steam, the pressure losses for the steam are kept within acceptable limits with such an intermediate superheater design. For the intermediate superheater physically separated from the steam generator, the amount of heat transfer medium supplied may in this case be regarded as a free parameter. That is to say, the intermediate superheater only needs to be supplied with enough heat transfer medium so that, for a predetermined steam mass flow coming from the HD turbine, the desired final temperature of the intermediate superheater steam is reached. This final temperature can therefore be used as a regulating quantity for the amount of heat transfer medium to be supplied. 
       SUMMARY 
       [0004]    An aspect relates to a method and an apparatus for regulation of the amount of heat transfer medium to be supplied to the intermediate superheater. 
         [0005]    Embodiments of the invention make use of an amount of heat transfer medium adapted to the system state and ascertained predictively. Specifically, this means that, at any given time, the mass flow of the heat transfer medium necessary in order to reach the final temperature of the intermediate superheater steam is ascertained on the basis of system-specific parameters in a predictive way. 
         [0006]    The essential parameter used here is the heat flux required in order to reach the final temperature of the intermediate superheater steam, which is to be transferred from the heat transfer medium, for example thermal oil, to the steam in the intermediate superheater by means of correspondingly arranged tubes. To this end, in order to ascertain this heat flux, the temperature and the pressure of the steam at the entry of the intermediate superheater are measured and converted into an associated actual entry enthalpy. At the exit, an associated setpoint exit enthalpy is likewise determined with measured steam pressure and a desired temperature setpoint value to be adjusted. If the actual entry enthalpy is subtracted from the setpoint exit enthalpy and the difference is subsequently multiplied preferably by the measured steam mass flow, although another parameter characterizing the steam mass flow may also be used, then the heat flux required for heating the steam is known. If the entry and exit temperatures as well as their associated pressures of the heat transfer medium are likewise measured, and with known substance values of the heat transfer medium are converted into associated enthalpies, then here again it is possible to determine an enthalpy difference of the heat transfer medium between the entry and exit. If the heat demand calculated for the steam is divided by this enthalpy difference of the heat transfer medium, then the required mass flow of the heat transfer medium is known at any given time for the steady system state. 
         [0007]    Preferably, by using the method according to embodiments of the invention, or the apparatus according to embodiments of the invention, it is also possible to regulate the final temperature of the intermediate superheater with the least possible fluctuations even during very unsteady system states of the parabolic trough power plant. Since system-specific parameters, for example measurement quantities, may also be taken into account appropriately as additional design features of the intermediate superheater design, the mass flow requirement for the heat transfer medium may be precalculated for any given system state at any time. In comparison with pure regulation of the final steam temperature, which can only react in the event of changes in this steam temperature, with embodiments of the invention, an externally imposed perturbation (for example a change in the steam mass flow) can already be detected and corrected promptly at its start. The method according to embodiments of the invention, and the apparatus according to embodiments of the invention, already react predictively to an expected change in the exit temperature of the intermediate superheater, and therefore already counteract such a change in advance. 
         [0008]    Preferably, the method according to embodiments of the invention and the apparatus according to embodiments of the invention are integrated into a solar-thermal parabolic trough power plant with an intermediate superheater so as to ensure steam temperatures which are as constant as possible at the exit of the intermediate superheater even for very unsteady operating states, such as occur with increased frequency in solar-heated power plants (for example as a result of cloud movement). Besides a procedure which is therefore reliable and stable in the event of changing weather conditions, the availability of the overall power plant system can be improved by a concept which is economical in terms of material. Preferably, embodiments of the invention are employed in solar thermal power plants in which thermal oil is used as the heat transfer medium. The concept according to embodiments of the invention may, however, in principle also be used in systems with different heat transfer media, assuming that the system has a separate intermediate superheater, not integrated into the steam generator, as a further heat exchanger. Furthermore, the concept according to embodiments of the invention are also suitable without significant modifications for use in combination with other components, for example final injection coolers or comparable measures for stabilizing the feed water mass flow in the steam generator of a solar thermal parabolic trough power plant. 
     
    
     
       BRIEF DESCRIPTION 
         [0009]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0010]      FIG. 1  schematically shows a regulating concept; and 
           [0011]      FIG. 2  schematically shows an enhanced regulating concept. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  schematically shows a possible regulating concept for the steady state operation of a solar thermal parabolic trough power plant. Represented here are the intermediate superheater Z, a regulating device K for adjusting and correcting the mass flow of the heat transfer medium W, and a corresponding mass flow setpoint value control device for driving and therefore controlling the regulating device K as a function of an ascertained enthalpy difference of the heat transfer medium W between its entry and exit into and out of the intermediate superheater Z and an ascertained enthalpy difference of the steam D between its exit and entry out of and into the intermediate superheater Z. 
         [0013]    The intermediate superheater Z is connected on the steam side to corresponding lines for conveying the steam D, and on the heat transfer medium side to corresponding tubes for conveying the heat transfer medium W. The regulating device K for adjusting the mass flow of the heat transfer medium W comprises a motor actuator, a throttle valve driven by the motor actuator, and a measuring device arranged before the throttle valve for ascertaining the respective current mass flow of the heat transfer medium W. Together with a correspondingly formed regulating element, the measuring device, the motor actuator and the throttle valve form a control loop for modifying the currently adjusted mass flow of the heat transfer medium W according to a predetermined mass flow setpoint value. 
         [0014]    The regulating device K is controlled by a mass flow setpoint value control device, which specifies the desired mass flow setpoint value. To this end, the mass flow setpoint value control device is formed according to embodiments of the invention so that, during operation of a solar thermal power plant in which steam D is heated in the intermediate superheater Z in the water/steam circuit by a solar-thermally heated heat transfer medium W to an adjustable setpoint temperature value at the exit, in order to heat the steam D to an adjusted setpoint temperature value, a mass flow of the heat transfer media W entering the intermediate superheater Z is correspondingly modified as a function of an ascertained enthalpy difference of the heat transfer medium W between its entry and exit into and out of the intermediate superheater Z and an ascertained enthalpy difference of the steam D between its exit and entry out of and into the intermediate superheater Z. 
         [0015]    To this end, the mass flow setpoint value control device comprises a first and a second module  10  and  11  for ascertaining the enthalpy of the heat transfer medium W at the entry and exit, as well as a third and a fourth module  20  and  21  for ascertaining the enthalpy of the steam D at the entry and exit. This determination is carried out on the basis of measurement values of correspondingly arranged pressure sensors WP10, WP11, DP20 and DP21, and correspondingly arranged temperature sensors WT10, WT11 and DT20, for measuring the pressure and the temperature both of the steam D and of the heat transfer medium W. These sensors are preferably arranged directly at the entry and exit of the steam D and of the heat transfer medium W into the intermediate superheater Z, so as to be able to ascertain as accurately as possible the system-specific parameters currently prevailing in the intermediate superheater Z. 
         [0016]    Since the amount of heat transfer medium supplied can be regarded as a free parameter in an intermediate superheater Z physically separated from the steam generator, the intermediate superheater Z only needs to be supplied with as much heat transfer medium W as is necessary in order to reach a desired setpoint temperature of the intermediate superheater steam. This setpoint temperature at the exit of the steam D from the intermediate superheater Z is then intended to be used as a regulating quantity for adjusting the optimal mass flow of the heat transfer medium W. In order to adjust this optimal setpoint temperature, a controlling element  22  is therefore provided, by which a selected setpoint value can be specified for the fourth module  21 . 
         [0017]    Furthermore, the mass flow setpoint value control device comprises a first subtractor element  24  for subtracting the ascertained steam entry enthalpy from the ascertained steam exit enthalpy, as well as a second subtractor element  12  for subtracting the ascertained heat transfer medium entry enthalpy from the ascertained heat transfer medium exit enthalpy. With a fifth module  23 , a parameter characterizing a mass flow of the incoming steam D is ascertained. By means of a multiplier element  25 , this characteristic parameter is multiplied by the difference from the first subtractor element  24 , and in a subsequent divider element  30  the product from the multiplier element  25  is divided by the difference from the second subtractor element  12 . The result of this divider element  30  is delivered as an ascertained mass flow setpoint value, then as a regulating quantity to the regulating device K for modifying the currently adjusted mass flow of the heat transfer medium W. 
         [0018]      FIG. 2  shows another configuration according to embodiments of the invention, in which the amounts of heat stored in or released from the tube material of the intermediate superheater Z, as well as the amounts of heat stored in or released from the heat transfer medium located in the intermediate superheater Z, are additionally taken into account for the unsteady state case. Depending on whether heat is stored in the system (consisting of tube material and heat transfer medium) or released from the system, a greater or lesser heat supply by the heat transfer medium is consequently necessary compared with the heat flux determined for the quasi-steady state. As a result, the flow of the heat transfer medium through the intermediate superheater Z needs to be adapted. In order to ascertain the stored or released amounts of heat of the intermediate superheater tubes, the characteristic temperature parameter ascertained for the material may in this case be used. This may, for example, be the average material temperature of all the tubes. As a result of a change in this average material temperature, the heat flux stored in the tube material or released from the tube material could be quantified in more detail by suitable measures, and appropriately taken into account when the required heat transfer mass flow is ascertained. To this end, as represented in  FIG. 2 , a sixth module  50  is provided for taking into account thermal energy stored in or released from tube walls of the intermediate heater Z, the output value of which is added to the product from the multiplier element  25  by an adder element  60  before the divider element  30 . Preferably, the change in the average material temperature of the tube material is in this case to be evaluated by means of a first-order differencing element. By the selection of a suitable time constant Tm and a suitable gain Km of this differencing element, an approximately exact precalculation of the amounts of stored heat is possible. 
         [0019]    In order to ascertain the stored or released amounts of heat of the heat transfer medium, the procedure to be adopted is similar. To this end, a seventh module  55  may be provided for taking into account a thermal energy stored in or released from the heat transfer medium W, the output value of which is added to the product from the multiplier element  25  by an adder element  60  before the divider element  30 . Preferably, here again the change in the average temperature of the heat transfer medium is to be evaluated by means of a first-order differencing element. By the selection of a suitable time constant Tw and a suitable gain Kw of this differencing element, an approximately exact precalculation of the amounts of stored heat is thus possible. 
         [0020]    For the gain of the two differencing elements, the product of the mass and heat capacity is preferably to be used in the first case of the tube material, and in the second case of the heat transfer medium. In addition, this product is also to be divided by the time constant of the associated differencing element. The time constant of the two differencing elements may be different, and is preferably to be coupled to the flow time of the steam, of the heat transfer medium or a suitable combination of the two quantities. If the heat requirement calculated for reaching the final steam temperature is corrected by these two heat fluxes occurring for the unsteady state case, then a corresponding adaptation of the required mass flow of the heat transfer medium W is carried out by the regulating method described here, so that even in the unsteady state case the intermediate superheater steam exit temperature can be regulated with the least possible fluctuations. 
         [0021]    If need be, additional regulation  70  may be superordinated to this method with predictive nature, as represented in  FIG. 2 , which regulation, in the event of a steady state deviation of the steam temperature ascertained by means of a temperature sensor DT 70  at the exit of the intermediate superheater Z from the temperature setpoint value specified by the controlling element  22 , constantly eliminates this deviation. It is, however, to be taken into account that this superordinate regulation  70  may only intervene correctively, and must therefore have a relatively slow regulating behavior in relation to the overall regulating task.