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United States Patent Application 20170045265
Kind Code A1
EICKHOFF; Martin February 16, 2017

METHOD FOR OPERATING A SOLAR THERMAL POWER PLANT, AND SOLAR THERMAL POWER PLANT

Abstract

A method for operating a solar thermal power plant comprising multiple solar radiation receivers operated using a molten salt as the heat transfer medium, wherein each solar radiation receiver comprises a reflector device and an absorber tube, includes: preheating of the absorber tubes, in the state in which said tubes are empty of the molten salt, to a temperature T by concentrating solar radiation on the absorber tubes by means of the reflector devices, wherein the temperature T is greater than or equal to the melting temperature of the salt; after reaching the temperature T: introduction of the molten salt into the absorber tubes and recirculated conduction of the molten salt through the absorber tubes while simultaneously repositioning the reflector devices depending on the position of the sun; on ending the operation: release of the molten salt out of the absorber tubes.


Inventors: EICKHOFF; Martin; (Tabernas, ES)
Applicant:
Name City State Country Type

DEUTSCHES ZENTRUM FUR LUFT- UND RAUMFAHRT E.V.

Koln

DE
Assignee: DEUTSCHES ZENTRUM FUR LUFT- UND RAUMFAHRT E.V.
Koln
DE

Family ID: 1000002265448
Appl. No.: 15/118600
Filed: February 5, 2015
PCT Filed: February 5, 2015
PCT NO: PCT/EP2015/052357
371 Date: September 19, 2016


Current U.S. Class: 1/1
Current CPC Class: F24J 2/30 20130101; F24J 2/4649 20130101; F24J 2/34 20130101
International Class: F24J 2/30 20060101 F24J002/30; F24J 2/46 20060101 F24J002/46; F24J 2/34 20060101 F24J002/34

Foreign Application Data

DateCodeApplication Number
Feb 13, 2014DE10 2014 202 633.4

Claims



1-19. (canceled)

20. A method for operating a solar thermal power plant comprising a plurality of solar radiation receivers operated using a molten salt as the heat transfer medium, wherein each solar radiation receiver has a reflector device and an absorber pipe, the method comprising the following steps: preheating in a state emptied of molten salt, the absorber pipes to a temperature T by concentrating solar radiation on the absorber pipes by means of the reflector devices, wherein the temperature T is higher than or equal to the melting temperature of the salt; introducing the molten salt into the absorber pipes and conveying the molten salt through the absorber pipes in a recirculating manner, while at the same time adjusting the reflector devices according to the position of the sun; discharging the molten salt from the absorber pipes.

21. Method of claim 20, wherein the reflector devices are defocused when operation is ended.

22. Method of claim 20, wherein when preheating the absorber pipes, a power control of the reflector devices is performed in dependence on the position of the sun and the weather conditions.

23. Method of claim 20, wherein discharging the molten salt from the absorber pipes is effected at least in part due to gravity.

24. Method of claim 20, wherein a secondary heat transfer medium, preferably an inert gas, preferably nitrogen, is fed through the absorber pipes, preferably in a recirculating manner, during the preheating of the absorber pipes.

25. Method of claim 24, wherein for discharging the molten salt from the absorber pipes, the secondary heat transfer medium is introduced into the absorber pipes at a pressure higher than ambient pressure, wherein the secondary heat transfer medium presses the molten salt out of the absorber pipes.

26. Method of claim 20, wherein when discharging the molten salt from the absorber pipes, the molten salt is at least in part pumped out of the absorber pipes.

27. Method of claim 22, wherein during the preheating of the absorber pipes, the reflector devices are focused only partly on the absorber pipes or are periodically focused and defocused.

28. Method of claim 20, wherein when being discharged from the absorber pipes, the molten salt is conducted into at least one thermally insulated storage tank.

29. Method of claim 20, wherein when the molten salt is introduced into the absorber pipes, the secondary heat transfer medium is conducted into at least one secondary storage tank.

30. Method of claim 25, wherein the secondary heat transfer medium remains in the absorber pipes when the power plant is not in operation.

31. Solar thermal power plant for operation using a molten salt as the heat transfer medium and comprising: a plurality of solar radiation receivers, each having a reflector device and an absorber pipe through which the heat transfer medium can be conducted, wherein the absorber pipes are arranged with a gradient in the direction of the at least one storage tank for the molten salt.

32. Solar thermal power plant of claim 31, wherein the absorber pipes of a plurality of reflector devices are connected to form an absorber pipe chain and form a continuous gradient.

33. Solar thermal power plant of claim 31, wherein a supply line for a secondary heat transfer medium opens into an upper end of an absorber pipe or an absorber pipe chain.

34. Solar thermal power plant of claim 32, wherein the solar radiation receivers are arranged in loops, wherein each respective loop is formed by two parallelly arranged absorber pipe chains with the associated reflector devices, and wherein a transverse connection connects the upper ends of the absorber pipe chains.

35. Solar thermal power plant of claim 34, wherein the supply line for a secondary heat transfer medium opens into the transverse connection.

36. Solar thermal power plant of claim 33, wherein by at least one secondary storage tank for the secondary heat transfer medium.

37. Solar thermal power plant of claim 36, wherein the at least one secondary storage tank is formed by at least one storage tank for the molten salt.

38. Solar thermal power plant of claim 33, wherein a pump or a compressor may be arranged in the supply line for the secondary heat transfer medium, by means of which the secondary heat transfer medium can be introduced into an absorber pipe or an absorber pipe chain at a pressure higher than ambient pressure
Description



[0001] The present invention relates to a method for operating a solar thermal power plant comprising a plurality of radiation receivers that are operated using a molten salt as the heat transfer medium, as well as to such a power plant.

[0002] In known solar thermal power plants sunlight is used to heat a heat transfer medium by reflecting the sunlight onto an absorber via reflectors, the heat transfer medium flowing through the absorber. The heat transfer medium may for example be a thermal oil or water. The thermal energy of the heat transfer medium is then either used immediately, e.g. for power generation, or a short-term heat storage is performed. It is further known to operate such power plants with a molten salt. The use of a molten salt is particularly useful, since high operating temperatures can be reached, resulting in very high levels of process efficiency. Further, liquid salts are a very economic thermal storage medium.

[0003] In particular line-focused solar thermal power plants, in which elongated absorber pipes are provided onto which the reflectors reflect sunlight linearly, are operated using such thermal transfer media. However, the use of molten salt is disadvantageous in that the liquid salts remain in the absorber pipes in times of insufficient solar irradiation, e.g. at night or during bad weather periods, and thus there is a risk that the salt will freeze. Frozen absorber pipes can only be thawed with great effort, and the volume changes of the salt caused by the change of phase result in the risk of damages to the absorber pipes. This is due to the fact that the liquid phase of the molten salt has a larger volume than the solid phase.

[0004] If the molten salt in a conventional absorber pipe has frozen and if it is attempted to thaw the same at one location by heat input, the salt becoming liquid, which is enclosed by solid salt, generates enormous pressure on the inner walls of the absorber pipe, and the absorber pipe is in danger to burst. Therefore, frozen absorber pipes must be thawed stepwise starting from the still liquid side, which is very troublesome and time-consuming. For this reason, the molten salts used are often heated by means of fossil fuels or electricity in order to protect them from freezing. However, additional heating is expensive in particular in the long bad weather periods or in winter and results in compromised efficiency.

[0005] Therefore it is an object of the present invention to provide a method for operating a solar thermal power plant, as well as a solar thermal power plant, which allow doing without the problematic additional heating.

[0006] The method of the present invention is defined by the features of claim 1. The solar thermal power plant is defined by the features of claim 14.

[0007] The present method for operating a solar thermal power plant comprising a plurality of solar radiation receivers operated using a molten salt as the heat transfer medium, wherein each solar radiation receiver has a reflector device and an absorber pipe, comprises the following steps: [0008] in a state emptied of molten salt, preheating the absorber pipes to a temperature T by concentrating solar radiation on the absorber pipes by means of the reflector devices, wherein the temperature T is higher than or equal to the melting temperature of the salt;

[0009] after the temperature T has been reached: [0010] introducing the molten salt into the absorber pipes and conveying the molten salt through the absorber pipes in a recirculating manner, while at the same time adjusting the reflector devices according to the position of the sun;

[0011] when ending operation: [0012] discharging the molten salt from the absorber pipes.

[0013] Thus the method of the present invention provides that, when the operation of a solar thermal power plant is ended, the absorber pipes of the solar radiation receiver are emptied so that no molten salt remains in the absorber pipes. It is thereby avoided that the molten salt threatens to solidify and causes a time- and energy-consuming melting of molten salt in the absorber pipes and a costly heating at night tie becomes obsolete, respectively. The cost-intensive constant heating of large solar fields during night time or during days of bad weather, e.g. during the winter, is therefore not necessary so that a significant saving of costs is achieved. By preheating the absorber pipes by means of the reflector devices before the molten salt is introduced into the absorber pipes, it is prevented in an economic manner that the molten salt freezes instantly when it is introduced due to the absorber pipes being too cold and that a so-called salt plug formation occurs. Since solar radiation is used to heat up the absorber pipes, no external costly energy source is required for heating the absorber pipes.

[0014] In addition, discharging the molten salt from the absorber pipes has the advantage that, in a state empty of molten salt, damaged absorber pipes can readily be replaced with new pipes. Thus, the maintenance of the solar thermal power plant is significantly facilitated.

[0015] Preferably it is provided that, when the operation is ended, the reflector devices are defocused. It is thereby prevented that the absorber pipes in the state empty of molten salt are irradiated with solar radiation also after the operation has been ended and before the preheating phase and that the pipes may threaten to overheat.

[0016] In a preferred variant of the method according to the present invention it is provided that, when preheating the absorber pipes, a power control of the reflector device is performed in dependence on the position of the sun and/or the weather conditions. With reflector devices in the form of parabolic trough collectors, for example, the preheating process is not critical in the morning hours due to the position of the sun and the resulting relatively low solar radiation so that the reflector device can be focused directly on the absorber pipe. Even when the sky is cloudy, the diffuse radiation is sufficient to achieve the greater part of the preheating. If the preheating process is performed during the time of the midday sun, the solar radiation is too strong so that inadmissible temperature gradients can occur in the absorber pipes. For this reason it is necessary to perform power control so that less solar radiation is reflected onto the absorber pipes. Therefore, the focus of a reflector must not be directly on the absorber pipe but must be slightly shifted with respect to the absorber pipe so that only a part of the reflected solar radiation reaches the absorber pipe. After the absorber pipes have been preheated and the molten salt has been introduced, the reflector device can be focused fully on the absorber pipe. In particular, it is possible to focus the reflector device only partly on the absorber pipes, e.g. on the edge zone, during the preheating of the absorber pipes, or they may be periodically focused and defocused. Thereby, it is achieved that the absorber pipes can be specifically preheated to the desired temperature T without the possibility of temperature related damages caused by excessive temperature gradients in the absorber pipe walls.

[0017] Preferably, discharging the molten salt from the absorber pipes is effected at least in part due to gravity. For this purpose, it is possible to provide absorber pipes inclined in one direction, for example, whereby a gradient towards one end of the absorber pipe is obtained and the molten salt can flow from the absorber pipe due to the gradient. Of course, it may also be provided to additionally provide pumps that support the discharge of the molten salt.

[0018] In a particularly preferred variant of the method of the present invention it is provided that a secondary heat transfer medium is fed through the absorber pipes during the preheating of the absorber pipe. This is preferably done in a recirculating manner. The secondary heat transfer medium may for example be an inert gas, preferably nitrogen. The secondary heat transfer medium passed through the absorber pipes causes cooling in hot regions of the absorber pipes, so that the risk of an overheating of some regions of the absorber pipes during preheating is reduced. The secondary heat transfer medium may further be used to transport the thermal energy transferred to the heat transfer medium by the cooling to cold regions of the absorber pipes so that these are heated. In this manner it is possible to reduce temperature gradients between hot regions of the absorber walls and cool regions of the absorber walls. Especially if the secondary heat transfer medium is fed through the absorber pipes in a recirculating manner, the heat can be distributed uniformly via the pipe lines of the entire power plant during preheating. It is thereby also achieved that all regions of the absorber pipes reach the desired temperature T so that the molten salt does not threaten to freeze in individual regions of the absorber pipes.

[0019] In molten salt tanks that are typically used in solar thermal power plants, a buffer of inert gas, e.g. nitrogen, is often used to avoid corrosion caused by air inclusion and to reduce ageing of the molten salt. The inert gas, e.g. nitrogen, is therefore already present in the system of the power plant and can therefore be used in an advantageous manner during the preheating process.

[0020] When feeding an inert gas through the absorber pipes during preheating, it is often necessary to move the reflector devices not fully into focus so that the introduction of thermal energy into the absorber pipes is reduced, since a relatively poor thermal transfer exists between the absorber pipes and the gas flow. Thus it is possible to avoid high local temperature gradients inside the absorber pipes. Of course, it is also possible to periodically focus and defocus the reflector devices.

[0021] It may preferably be provided that, when the molten salt is discharged from the absorber pipes, the secondary heat transfer medium is introduced into the absorber pipes at a pressure higher than ambient pressure, wherein the secondary heat transfer medium presses the molten salt out of the absorber pipes. The discharge of the molten salt from the absorber pipes can thereby be accelerated and, for example, a gravity-related flow of molten salt from the absorber pipes can be supported. Thus, the process of discharging the molten salt is significantly shortened.

[0022] Basically, it may also be provided that the molten salt is pumped at least in part from the absorber pipes when the molten salt is discharged from the absorber pipes. This may be performed as an alternative or in addition to the molten salt being discharged by the effect of gravity or by pressing.

[0023] Preferably it is provided that, when being discharged from the absorber pipes, the molten salt is conducted into at least one thermally insulated storage tank. It can thus be achieved that the thermal energy contained in the liquid molten salt can be stored at least in part during the operating pause of the solar thermal power plant. The storage tank may for example be a cold salt tank already existing in conventional power plant circuits. It is also possible to first direct the salt into separate storage tanks and from there into larger main salt tanks such as into a cold salt tank, for example.

[0024] According to a variant of the method of the present invention it is provided that the secondary heat transfer medium is directed into at least one storage tank when the molten salt is introduced into the absorber pipes. The secondary heat transfer medium can thus be stored in an advantageous manner in order to use it again during the preheating process or during the discharge of the molten salt from the absorber pipes. It may in particular be provided that the secondary storage tank is formed by the thermally insulated storage tank for the molten salt. This is particularly advantageous if an inert gas, e.g. nitrogen, is used as the secondary heat transfer medium, since this inert gas is already used as a gas buffer in the thermally insulated storage tank for the molten salt so as to prevent corrosion or the ageing of the molten salt. When introducing the molten salt from the storage tank into the absorber pipes, an approximately comparable volume of molten salt is taken from the thermally insulated storage tank, which volume is fed into the storage tank as inert gas. Further, the use of the thermally insulated storage tank for molten salt as a secondary storage tank has the advantage that possible remainders of inert gas which may at first remain in the absorber pipes are entrained by the molten salt during the recirculation of the molten salt through the absorber pipes and thus get into the thermally insulated storage tank or, for example, into a hot salt storage tank in which the inert gases slowly rise and can merge with the corresponding inert gas buffer of the storage tank.

[0025] It may be provided that the secondary heat transfer medium remains in the absorber tubes when the power plant is not in operation. This is advantageous in particular when the inert gas used for the inert gas buffers of the storage tanks included in the system serves as the secondary heat transfer medium, since in this case no further medium, such as air, for example, gets into the system which would result in an additional discharge of this medium from the system. In particular it may be provided that the secondary heat transfer medium is subjected to a high pressure and remains in the absorber pipes. The high pressure may be up to 15 bar for example.

[0026] It may also be provided that, during preheating, the secondary heat transfer medium is recirculated under a high pressure, e.g. up to 15 bar. This has the advantage of a lower pressure loss upon recirculation and of a higher density of the secondary heat transfer medium, whereby the neat transfer behavior and the heat transport capacity are improved.

[0027] The invention further relates to a solar thermal power plant for operation using a molten salt as the heat transfer medium and comprising a plurality of solar radiation receivers, each having a reflector device and an absorber pipe through which the heat transfer medium can be conducted. The solar thermal power plant of the present invention is characterized in that the absorber pipes are arranged with a gradient in the direction of the at least one storage tank for the molten salt. Thus it is achieved that the molten salt used for the normal operation of the solar thermal power plant can be discharged from the absorber pipes in a simple manner so that it is prevented that the molten salt solidifies in the absorber pipes during an operating pause of the solar thermal power plant. In this manner it is prevented that a complicated thawing of the molten salt becomes necessary when restarting the solar thermal power plant or that a complicated and costly heating of the molten salt becomes necessary during the operating pause. The gradient advantageously allows the molten salt to flow into a storage tank under the effect of gravity.

[0028] Within the framework of the invention, the feature "gradient in the direction of the at least one storage tank" also includes variants where a lower end of the absorber tubes is directed into another direction than to the storage tank, while still having a pipe connection between the lower end of the absorber pipe and the storage tank via which the molten salt gets into the storage tanks. Thus, the direction of the gradient is the flow direction along the flow path towards the storage tanks. The absorber pipes may for example be arranged under an angle .alpha. of up to 10.degree. with respect to the horizontal plane.

[0029] Preferably it is provided that the absorber pipes of a plurality of reflector devices are joined to form an absorber pipe chain and form a continuous gradient. Thus a long line of absorber pipes is formed through which, due to the continuous gradient, the molten salt can be discharged from the absorber pipes in an advantageous manner. For example, the absorber pipes can be formed as partial pipes of a continuous pipe forming the absorber pipe chain. In this embodiment a plurality of reflector devices is assigned to the pipe forming the absorber pipe chain. Thus, a plurality of solar radiation receivers shares a common continuous pipe.

[0030] Preferably it is provided that a supply line for a secondary heat transfer medium opens into an upper end of an absorber pipe or an absorber pipe chain. In this manner it is possible to introduce a secondary heat transfer medium when discharging the molten salt or to introduce said medium into emptied absorber pipes, the medium being fed through the absorber pipes for example in a circulating manner during the heat-up phase of the absorber pipes.

[0031] It may be provided that the solar radiation receivers are arranged in loops, where each loop is formed by two parallelly arranged absorber pipe chains with the associated reflector devices, and wherein a transverse connection connects the upper ends of the absorber pipe chains. Such an arrangement has proven to be particularly advantageous, since the arrangement of the solar radiation receivers with absorber pipes having a gradient can be manufactured in a constructively simple manner. The loops of the solar radiation receivers are thus arranged such that in normal operation of the power plant the molten salt is first conducted up the gradient through an absorber pipe chain and flows with the gradient in the second absorber pipe chain after having flown through the transverse connection. Thereby, it is possible to keep the pumping power relatively low, since greater pumping power is necessary only for conveying the molten salt against the gradient, while the return flow of the molten salt is assisted by the effect of gravity. When the molten salt is discharged, the molten salt flows in the same direction in the parallelly arranged absorber pipe chains, i.e. in the first absorber pipe chains it flows against the flow direction of the normal operation. An arrangement in loops has the advantage that the molten salt can be discharged relatively quickly from a loop, since this may be effected at the same time in both parallelly arranged absorber pipe chains.

[0032] It may be provided that the supply line for a secondary heat transfer medium opens into the transverse connection. In this manner the heat transfer medium can be introduced simultaneously into both parallelly arranged absorber pipes of a loop.

[0033] The secondary heat transfer medium may basically also be used to press the heat transfer medium out of the absorber pipes.

[0034] The solar thermal power plant of the present invention may comprise at least one secondary storage tank for the secondary storage medium. The at least one secondary storage tank may be formed for example by at least one storage tank for the molten salt. This is advantageous in particular when an inert gas is used as the secondary heat transfer medium, which inert gas is used as a gas buffer in the storage tank for the molten salt. Thus, it is possible to avoid the technologic effort for a separate secondary storage tank.

[0035] A pump, a blower or a compressor may be arranged in the supply line for a secondary heat transfer medium, by means of which the secondary heat transfer medium can be introduced into an absorber pipe or an absorber pipe chain at a pressure higher than ambient pressure. In this manner the secondary heat transfer medium can be transported in an advantageous manner to the upper end of the absorber pipe or the absorber pipe chain and, in addition, the secondary heat transfer medium can press the molten salt out of the absorber pipes or the absorber pipe chains as the molten salt is discharged. Thereby, the gravity-related discharge of the molten salt is supported and accelerated.

[0036] The solar radiation receivers may be designed as parabolic trough receivers or Fresnel receivers.

[0037] The solar thermal power plant of the present invention may in particular be operated using the above described method.

[0038] The following is a detailed explanation of the invention with reference to the accompanying sole FIGURE.

[0039] The sole FIGURE shows a schematic illustration of a solar thermal power plant 100 according to the present invention. The solar thermal power plant 100 is operated using a molten salt as the heat transfer medium.

[0040] The solar thermal power plant 100 has a plurality of solar radiation receivers 1 that each comprise a reflector device 3. In the embodiment illustrated the solar radiation receivers 1 are designed as parabolic trough collectors so that the reflector devices have a parabolic shape. The solar radiation receivers 1 each comprise an absorber pipe 5. A plurality of solar radiation receivers 1 (four in the embodiment illustrated) are arranged one after the other in a row, the absorber pipes 5 forming an absorber pipe chain. In the embodiment illustrated the absorber pipe chain is designed as a continuous pipe so that the absorber pipes 5 are each partial pipes of the continuous pipe. Two rows of solar radiation receivers with a respective absorber pipe chain are arranged in parallel to each other and are connected to each other at one of their ends by means of a transverse connection 7 so that the solar radiation receivers 1 form a loop 9. The solar thermal power plant 100 may comprise a plurality of these loops 9 of solar radiation receivers 1, although only one loop 9 is illustrated for the sakes of clarity.

[0041] The loop 9 of solar radiation receivers 1 illustrated is arranged to be inclined so that the transverse connection 7 connects two upper ends of the absorber pipe chains. In other words: The absorber pipes 5 of the absorber pipe chains show a gradient. The lower ends of the absorber pipes 5 or the absorber pipe chains are connected with a storage tank 1 for the molten salt. The absorber pipes 5 thus have a gradient in the direction of the storage tank 11. The solar thermal power plant 100 further has a hot salt tank 13. In normal operation the storage tank 11, which preferably is thermally insulated, forms a so-called cold salt tank. The molten salt is conveyed, for example by means of pumps not illustrated herein, from the storage tank 11 through the loop 9 of solar radiation receivers 1 and is heated by the solar radiation reflected from the reflector devices 3 onto the absorber pipes 5. Thereafter, the molten salt is fed into the hot salt tank 13. From the hot salt tank 13 the molten salt is directed to a heat exchanger not illustrated herein, via which the thermal energy may be transferred for further exploitation, for example to a steam turbine process including the generation of electricity. Then, the molten salt is returned into the storage tank 11. During normal operation the molten salt is thus recirculated through the solar radiation receivers, with the molten salt possibly being heated from a temperature of 290.degree. C. to a temperature of about 550.degree. C., for example.

[0042] The storage tank 11 and the hot salt tank 13 each include a nitrogen buffer 15, whereby corrosion by air inclusions and the ageing of the molten salt are avoided.

[0043] When the operation of the solar thermal power plant 100 is ended, the molten salt is discharged from the absorber pipes 5 and is conducted into the storage tank 11. Here, due to the gradient, the molten salt flows towards the storage tank 11 under the effect of gravity. In the embodiment illustrated, when the molten salt is discharged, the molten salt flows through the front solar radiation receivers 1, seen in the normal flow direction (i.e. the flow direction in normal operation), in a direction opposite to the normal flow direction of the molten salt, whereas the rear solar radiation receivers 1, seen in the normal flow direction, are discharged in the normal flow direction. The normal flow direction of the molten salt is indicated by arrows. The solar radiation receivers 1 in a loop 9 are thus very quickly emptied of molten salt by making the molten salt flow simultaneously from the parallelly arranged absorber pipe chains.

[0044] The absorber pipes 5 or the absorber pipe chains are preferably inclined under an angle .alpha. of 10.degree. with respect to the horizontal plane.

[0045] The solar thermal power plant 100 further comprises a supply line 17 opening into the transverse connection 7 at the upper end of the absorber pipe chains. A secondary heat transfer medium can be introduced into the absorber pipes 5 via the supply line. The secondary heat transfer medium may be nitrogen for example. In this regard, nitrogen from the nitrogen buffer 15 of the storage tank 11 can be used. Preferably the secondary heat transfer medium is introduced into the absorber pipes 5 while the molten salt is discharged. Since nitrogen from the nitrogen buffer 15 is displaced from the storage tank 11 as the molten salt is discharged from the absorber pipes, the same can advantageously be conducted into the absorber pipes 5 via the supply line 17. The supply line 17 may be provided for example with a compressor or a pump arranged therein, via which the secondary heat transfer medium is introduced into the absorber pipes 5 at high pressure. Thereby, the secondary heat transfer medium can be of assistance in discharging the molten salt, the molten salt pressing the secondary heat transfer medium out of the absorber pipes 5.

[0046] To assist in discharging, it is further possible to use a pump not illustrated herein.

[0047] During the operating pause of the solar thermal power plant which for example occurs at night or in periods of bad weather or for maintenance purposes, the secondary heat transfer medium remains in the absorber pipes 5.

[0048] When the solar thermal power plant is started up, first the secondary heat transfer medium is recirculated through the solar radiation receivers 1. From the defocused position assumed during the operating pause, the reflector devices are focused onto the absorber pipes 5 to preheat these for the regular operation. The recirculation of the secondary heat transfer medium distributes the heat relatively uniformly in the absorber pipes 5 so that excessive temperature gradients are avoided that could cause damage to the absorber pipes 5. Depending on the intensity of the solar radiation it may be necessary that the reflector devices are not fully focused on the absorber pipes 5 during the preheating phase, but that a so-called "reduced focusing" is performed, wherein the focus only affects an edge portion of the absorber pipe, for example. The heat input can thus be reduced. As an alternative it may also be provided that the reflector devices are alternately focused and defocused.

[0049] As soon as the absorber pipes have a temperature T that is higher than or equal to the melting temperature of the salt, the introduction of the molten salt into the absorber pipes 5 can be started. In doing so, the molten salt displaces the secondary heat transfer medium and presses the same into a secondary storage tank. In the embodiment illustrated the secondary storage tank is formed by the storage tank 11, wherein the nitrogen used as the secondary heat transfer medium is stored in the form of the gas buffer contained in the storage tank 11.

[0050] Due to the absorber pipes 5 being preheated to the temperature T it is avoided that the molten salt introduced into the absorber pipes 5 threatens to solidify.

[0051] Thereafter, the regular operation of the solar thermal power plant 100 can be performed, in which the molten salt is recirculated through the solar radiation receivers 1.

[0052] During the preheating process of the absorber pipes the secondary heat transfer medium can be recirculated through the absorber pipes flowing in the normal direction of flow or it can be conducted via the supply line 17 so that the parallel absorber pipe chains of the loop 9 are flown through in parallel.

[0053] The solar thermal power plant 100 of the present invention or the present method for operating a solar thermal power plant 100, respectively, is advantageous in that the molten salt does not remain in the absorber pipes during operating pauses and that the risk of the molten salt solidifying is avoided and a complex heating of the molten salt during the operating pauses becomes obsolete. By discharging the molten salt into a thermally insulated storage tank 11 it becomes possible to temporarily store a great part of the thermal energy contained in the molten salt during the operating pauses so that the energy can be used again when the solar thermal power plant 100 is started again. Only in case of longer operating pauses, for example in bad weather periods or in winter, is heating necessary before starting the power plant, if the molten salt solidifies in the storage tank 11.

[0054] In the solar thermal power plant 100 of the present invention or the present method it may be provided that discharging the molten salt from the absorber pipes is effected before each operating pause. However, it may basically also be provided that the molten salt is discharged from the absorber pipes if the operating pause is planned for a longer predetermined period. For example, the daily emptying process of an entire solar field before a nightly operating pause may be too troublesome if the night hours are rather short in summer. Therefore, the method of the present invention may also be implemented only in longer operating pauses, e.g. during bad weather periods or for maintenance periods.

[0055] The method of the present invention may for example be implemented only for parts of a power plant if repair work is required there.

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