Abstract:
A solar thermal power plant includes a solar radiation receiver mounted on a tower surrounded by a heliostat field to receive solar radiation reflected by heliostats forming the heliostat field. The power plant includes a power generation circuit including a steam turbine for driving an electrical generator to produce electrical power, and water in the power generation circuit is capable of being heated directly by solar radiation reflected onto the solar radiation receiver by the heliostat field to generate steam to drive the steam turbine. The power plant also includes an energy storage circuit including a thermal energy storage fluid, such as molten salt, which is capable of being heated directly by solar radiation reflected by the heliostat field. A heat exchanger is also provided for recovering thermal energy from the thermal energy storage fluid in the energy storage circuit; the recovered thermal energy may then be used to generate steam to drive the steam turbine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT/EP2012/058970 filed May 15, 2012, which in turn claims priority to European application 11166996.6 filed May 20, 2011, both of which are hereby incorporated in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to the field of concentrated solar power (CSP). Embodiments of the present invention relate in particular to a solar thermal power plant that utilises concentrated solar power to generate electricity. 
       BACKGROUND 
       [0003]    Concentrated solar power (CSP) involves the use of lenses, mirrors or other optical apparatus to focus solar radiation from a large incident area onto a small area. The energy from the solar radiation is then used to generate electrical power. Concentrated solar power has the potential to become an important energy source in the future. 
         [0004]    There have been many proposals for concentrated solar power technology. The technology believed to have the most potential for providing high efficiency power generation is the central receiver solar thermal power plant. This technology involves the use of a solar radiation receiver, mounted atop a tower, to receive solar radiation that is reflected to be incident upon it by an array of tracking reflectors located in a solar field around the tower. The tracking reflectors are typically heliostats and the array of heliostats is commonly referred to as a heliostat field. 
         [0005]      FIG. 1  is a diagrammatic illustration of a conventional direct steam concentrated solar thermal power plant in which solar radiation is reflected by a heliostat field  2  to be incident upon a solar radiation receiver  4  mounted atop a tower  6 . The reflected solar radiation directly heats water circulating in a power generation circuit  8 . This generates superheated steam which is used to drive a steam turbine generator set  10 , and thereby generate electrical power, in a well-known manner using the Rankine cycle. In addition to the steam turbine generator set  10 , the power generation circuit  8  includes an air-cooled condenser  12  and a feed water heater  14 . 
         [0006]    Direct steam concentrated solar thermal power plants can only operate effectively during daylight hours at times when the available solar radiation reflected to be incident upon the solar radiation receiver  4  is sufficient to generate superheated steam at the required pressure and temperature in the power generation circuit  8 . This is because the high pressure and high temperature steam cannot be stored easily for later use. 
         [0007]    In order to overcome this drawback, direct steam concentrated solar thermal power plants with energy storage capability have been proposed. These plants use high specific heat capacity thermal energy storage fluid, typically a molten salt or a mixture of different molten salts, for energy storage. Thermal energy is stored during a charging cycle by heating the molten salt and the thermal energy is subsequently recovered during a discharging cycle to heat water, and thereby generate steam, in a power generation circuit. The generated steam is then used to drive a steam turbine generator set to generate electrical power. 
         [0008]    A first heat exchanger is needed to transfer thermal energy from steam not used to drive the steam turbine to the molten salt during the charging cycle. A second heat exchanger is also needed to recover thermal energy from the hot molten salt, and transfer it to the power generation circuit, during the discharging cycle. There are, therefore, multiple heat transfer stages, which reduces the efficiency of this type of solar thermal power plant. In particular, as the steam cools during the charging cycle, it undergoes a phase change whereas the molten salt does not. The amount of thermal energy transfer to the molten salt is, thus, limited thereby limiting the maximum attainable temperature of the molten salt and resulting in what is known as a ‘pinch point loss’. Consequently, when thermal energy is recovered from the hot molten salt to generate steam in the power generation circuit, the generated steam attains a significantly lower temperature and pressure than the steam originally used to heat the molten salt. This significantly reduces the efficiency of this type of solar thermal power plant. Furthermore, the lower steam pressure in the power generation circuit may be insufficient to run the steam turbine generator set at full load, meaning that the power generation requirement cannot be met. 
         [0009]    Another type of concentrated solar thermal power plant utilises a solar radiation receiver to directly heat a high specific heat capacity thermal energy storage fluid such as molten salt. In this type of plant, thermal energy is recovered from the molten salt to heat water in a power generation circuit, and thereby generate steam to drive a steam turbine generator set, irrespective of the prevailing daylight conditions. This type of solar thermal power plant is generally less efficient than a direct steam concentrated solar thermal power plant during daylight hours because the steam in the power generation circuit is generated at all times through indirect heating, by recovering thermal energy from the molten salt in a heat exchanger. Furthermore, this type of solar thermal power plant is generally less attractive than a direct steam concentrated solar thermal power plant because its construction is more complex (and hence more costly) and the technology is still at a relatively early stage of development. 
         [0010]    It would, therefore, be desirable to provide a solar thermal power plant having improved efficiency and operational flexibility. 
       SUMMARY 
       [0011]    According to one aspect of the present invention, there is provided a solar thermal power plant comprising:
       a tower;   a plurality of heliostats surrounding the tower and forming a heliostat field;   a solar radiation receiver mounted on the tower to receive solar radiation reflected by the heliostat field;   a power generation circuit including a steam turbine for driving an electrical generator to produce electrical power, water in the power generation circuit being capable of being heated directly by solar radiation reflected onto the solar radiation receiver by the heliostat field to generate steam to drive the steam turbine;   an energy storage circuit including a thermal energy storage fluid capable of being heated directly by solar radiation reflected by the heliostat field; and   a heat exchanger for recovering thermal energy from the thermal energy storage fluid in the energy storage circuit.       
 
         [0018]    The solar radiation receiver and associated power generation circuit provides highly efficient direct steam generation during daylight hours, particularly during sunny conditions when the solar radiation reflected by the heliostat field onto the solar radiation receiver is sufficient to generate superheated steam in the power generation circuit. Thermal energy can also be simultaneously stored in the energy storage circuit during a charging cycle, for subsequent recovery during a discharging cycle, thereby increasing operational efficiency and flexibility relative to a conventional direct steam solar thermal power plant of the type described above. Because the thermal energy storage fluid circulating in the energy storage circuit is heated directly by solar radiation, rather than indirectly from steam circulating in a heating circuit, a heat exchanger is not needed to transfer heat from the steam to the thermal energy storage fluid and this results in a significant improvement in the operational efficiency of the solar thermal power plant. Additionally, the heat exchanger that is provided to recover thermal energy from the thermal energy storage fluid is significantly smaller than the heat exchanger employed in a dedicated molten salt power plant of the type described above. 
         [0019]    Thermal energy recovered from the thermal energy storage fluid during the discharging cycle can be used for any purpose. 
         [0020]    The recovered thermal energy can most conveniently be used to generate steam for the power generation circuit of the solar thermal power plant at times when insufficient solar radiation is reflected onto the solar radiation receiver to generate steam, typically superheated steam, at the required temperature and pressure in the power generation circuit, for example during non-daylight hours or cloudy conditions. Accordingly, the heat exchanger may be arranged to generate steam for the power generation circuit and, more particularly, may be arranged to transfer the recovered thermal energy to the power generation circuit to support steam generation in the power generation circuit. The recovered thermal energy transferred to the power generation circuit may heat the fluid, namely water or steam, circulating in the power generation circuit to provide steam at a desired temperature and pressure. The steam generated indirectly using the thermal energy recovered by the heat exchanger can be used in the reheat cycle of the steam turbine, in the high pressure stage of the steam turbine, to power the complete operating cycle of the steam turbine or to preheat feed water for the steam turbine. It will be appreciated that use of the recovered thermal energy in this way significantly improves the operational efficiency and flexibility of the solar thermal power plant. 
         [0021]    The recovered thermal energy can be used at times when sufficient solar radiation is reflected onto the solar radiation receiver to generate steam at the required temperature and pressure to drive the steam turbine, for example to support the start-up of the solar thermal power plant, to reduce the plant start-up time, during transient operation of the solar thermal power plant or to precondition one or more plant components, for example the solar radiation receiver. 
         [0022]    The recovered thermal energy could conceivably be supplied to a hybrid power plant or a desalination plant, typically located near to the solar thermal power plant. 
         [0023]    The thermal energy storage fluid is typically a liquid. The thermal energy storage liquid may be a molten salt, which may for example be capable of being heated to a maximum operating temperature in the region of 580° C. for the effective storage of thermal energy. The molten salt may be a nitrate salt or a carbonate salt, although other forms of molten salt are entirely within the scope of the present invention, e.g., a mixture of salts. 
         [0024]    The energy storage circuit may include two fluid storage locations, comprising two tanks for the thermal energy storage fluid, one of which may be a high temperature fluid storage tank and the other of which may be a low temperature fluid storage tank. The energy storage circuit may alternatively include both fluid storage locations in a single thermocline fluid storage tank, for example with high temperature fluid at the top and low temperature fluid at the bottom, although these single tank storage solutions are still under development. The heat exchanger may be positioned in the energy storage circuit between the high temperature and low temperature fluid storage locations, enabling thermal energy to be recovered from the thermal energy storage fluid as it circulates in the energy storage circuit from the high temperature fluid storage location to the low temperature fluid storage location. 
         [0025]    In one configuration of the solar thermal power plant, the thermal energy storage fluid may be heated directly by solar radiation reflected onto the solar radiation receiver mounted on the tower. This configuration employs a single radiation receiver mounted atop the tower. Accordingly, the solar radiation receiver may include one or more first receiver panels for receiving solar radiation reflected by the heliostat field and transferring thermal energy provided by the reflected solar radiation directly to the fluid, namely water or steam, circulating in the power generation circuit. The solar radiation receiver may also include one or more second receiver panels for receiving solar radiation reflected by the heliostat field and transferring thermal energy provided by the reflected solar radiation directly to the thermal energy storage fluid circulating in the energy storage circuit. 
         [0026]    In another configuration, the solar thermal power plant may include a further solar radiation receiver which may receive solar radiation reflected by the heliostat field and transfer thermal energy provided by the reflected solar radiation directly to the thermal energy storage fluid circulating in the energy storage circuit. 
         [0027]    The further solar radiation receiver can be mounted on the tower so that it receives solar radiation reflected by the heliostat field surrounding the tower. Since the solar radiation receiver for the power generation circuit and the further solar radiation receiver for the energy storage circuit are mounted on the same tower, only a single tower is needed. 
         [0028]    The solar thermal power plant may include a further tower on which the further solar radiation receiver may be mounted. The heliostat field may surround the tower and the further tower. This may simplify the construction of the solar thermal power plant because the further solar radiation receiver for the energy storage circuit is typically smaller than the solar radiation receiver for the power generation circuit. This results in a consequential and beneficial reduction in the size of the further tower. 
         [0029]    The position of a subset of the plurality of heliostats in the heliostat field may be adjustable to selectively direct the solar radiation to provide thermal energy either directly to the power generation circuit or directly to the energy storage circuit. The operational efficiency of the solar thermal power plant can, thus, be optimised according to requirements. 
         [0030]    When the solar thermal power plant includes a solar radiation receiver having one or more first and second receiver panels, the position of a subset of the plurality of heliostats in the heliostat field may be adjustable to selectively direct solar radiation onto either the first receiver panel(s) or the second receiver panel(s). 
         [0031]    When the solar thermal power plant includes a solar radiation receiver and a further solar radiation receiver, the position of a subset of the plurality of heliostats in the heliostat field may be adjustable to selectively direct solar radiation onto either the solar radiation receiver or the further solar radiation receiver. This selective adjustment of the subset of heliostats can be provided when the further solar radiation receiver is mounted on the same tower as the solar radiation receiver or on a further tower. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a diagrammatic illustration of a known direct steam concentrated solar thermal power plant; 
           [0033]      FIG. 2  is a diagrammatic illustration of a first configuration of a solar thermal power plant according to the present invention; 
           [0034]      FIG. 3  is a diagrammatic illustration of a second configuration of a solar thermal power plant according to the present invention; and 
           [0035]      FIG. 4  is a diagrammatic illustration of a third configuration of a solar thermal power plant according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings. 
         [0037]    Referring to  FIG. 2 , there is shown a first configuration of a solar thermal power plant  20  comprising a tower  22  and a heliostat field  24  surrounding the tower  22 . The heliostat field  24  comprises a plurality of individual tracking heliostats  26  and the position of each heliostat  26  is adjusted by a suitably programmed computer controlled tracking system to track the movement of the sun. A solar radiation receiver  28  is mounted on top of the tower  22  and solar radiation is reflected by the heliostat field  24  to be incident upon the solar radiation receiver  28 . 
         [0038]    The power plant  20  includes a direct steam power generation circuit  30  in which steam is generated directly by thermal energy arising from the solar radiation reflected by the heliostat field  24  to be incident upon the solar radiation receiver  28 . More particularly, water circulating in the power generation circuit  30  is heated by the thermal energy, thereby producing superheated steam at a pressure in the region of 120 to 175 bar and a temperature in the region of 565° C. The superheated steam is circulated through a power generating system  32  to thereby generate electrical power, and the power generating system  32  typically comprises a steam turbine generator set through which the superheated steam is expanded, an air-cooled condenser and a feed water heater. 
         [0039]    In order to directly heat the water circulating in the power generation circuit  30 , the solar radiation receiver  28  includes a plurality of first receiver panels  34  arranged circumferentially around the solar radiation receiver  28 . Each of the first receiver panels  34  comprises a plurality of parallel vertically arranged, small-diameter, tubes which are connected at their upper and lower ends by headers. The first receiver panels  34  are connected in series. Water circulating in the power generation circuit  30  flows through the tubes in the first receiver panels  34  where it is directly heated by the thermal energy arising from the solar radiation reflected by the heliostat field  24  to be incident upon the first receiver panels  34 . Although multiple circumferentially arranged first receiver panels  34  may be preferred, a single first receiver panel  34  could be provided. 
         [0040]    In addition to the direct steam power generation circuit  30 , the power plant  20  includes an energy storage circuit  36  which utilises molten salt, e.g., a nitrate salt or a carbonate salt, or a salt mixture, such as a mixture of 60% sodium nitrate and 40% potassium nitrate, to store thermal energy. Like the water circulating in the power generation circuit  30 , the molten salt is heated directly by thermal energy arising from the solar radiation reflected by the heliostat field  24  to be incident upon the solar radiation receiver  28 . 
         [0041]    In order to directly heat the circulating molten salt, the solar radiation receiver  28  includes a plurality of second receiver panels  38  arranged circumferentially around the solar radiation receiver  28 . Each of the second receiver panels  38  comprises a plurality of parallel vertically arranged, small-diameter, tubes which are connected at their upper and lower ends by headers. The second receiver panels  38  are also connected in series and form a second fluid circuit to the series-connected first receiver panels  34 . Molten salt circulating in the energy storage circuit  36  flows through the tubes in the second receiver panels  38  where it is directly heated by thermal energy arising from the solar radiation reflected by the heliostat field  24  to be incident upon the second receiver panels  38 . The first and second receiver panels  36 ,  38  are typically alternately arranged around the solar radiation receiver  28  in the circumferential direction. Although multiple circumferentially arranged second receiver panels  38  may be preferred, a single second receiver panel  38  could be provided. 
         [0042]    The energy storage circuit  36  includes an insulated hot salt storage tank  40  and a cold salt storage tank  42 . Molten salt that has been heated directly, during a charging cycle, by solar radiation reflected onto the second receiver panels  38  is pumped to the hot salt storage tank  40 . The molten salt is typically heated to a storage temperature in the region of 580° C. A heat exchanger  44  is arranged between the two storage tanks  40 ,  42  to recover heat from the hot molten salt as it circulates from the hot salt storage tank  40  to the cold salt storage tank  42  during a discharging cycle. The heat exchanger  44  is operatively associated with the power generation circuit  30  and heat recovered from the molten salt by the heat exchanger  44  through steam generation is transferred to the power generation circuit  30  where it can be used to support electrical power generation. 
         [0043]    The solar thermal power plant  20  can be operated in different modes depending on the prevailing power generation requirements and the amount of solar radiation that is available. In order to vary the operating mode, a subset of the plurality of heliostats  26  in the heliostat field  24  is controllable by the computer controlled tracking system so that the heliostats  26  can reflect solar radiation to be incident upon either the first receiver panels  34  associated with the power generation circuit  30  or the second receiver panels  38  associated with the energy storage circuit  36 . Some of the heliostats  26  are arranged so that they are dedicated exclusively to reflecting solar radiation to be incident only upon the first receiver panels  34  associated with the power generation circuit  30 . 
         [0044]    During daylight hours, the power plant can be operated in a hybrid generation/storage operating mode provided that there is a sufficient amount of solar radiation. In this hybrid operating mode, the position of at least some of the subset of heliostats  26  is controlled to direct solar radiation onto the second receiver panels  38  associated with the energy storage circuit  36 . In this hybrid operating mode, steam is generated directly in the power generation circuit  30  by the solar radiation that is reflected onto the first receiver panels  34  by the dedicated heliostats  26  and is employed by the power generating system  32  to provide immediate generation of electrical power. Furthermore, the molten salt circulating through the second receiver panels  38  in the energy storage circuit  36  is heated by the solar radiation reflected onto the second receiver panels  38  during a charging cycle and the heated molten salt is pumped to the hot salt storage tank  40 . In this hybrid operating mode, the power plant  20  generates electrical power via the power generation circuit  30  and power generating system  32  and at the same time stores thermal energy via the energy storage circuit  36 . In both cases, the solar radiation reflected by the heliostat field  24  provides direct heating of the water circulating in the power generation circuit  30  and the molten salt circulating in the energy storage circuit  36 , thereby maximising the efficiency of the power plant  20 . 
         [0045]    If there is insufficient solar radiation during daylight hours to operate the power plant  20  in the hybrid generation/storage operating mode, it can be operated in a generation only mode in which the position of the subset of heliostats  26  is controlled to direct solar radiation onto the first receiver panels  34  associated with the power generation circuit  30 . This ensures that heliostat field  24  directs all of the available solar radiation to the immediate production of steam in the power generation circuit  30 . 
         [0046]    At times when there is insufficient solar energy to directly heat the water circulating in the first receiver panels  34  to provide steam at the desired pressure and temperature for efficient operation of the power generating system  32 , the power plant  20  can be operated in an energy recovery mode to support the generation of steam in the power generation circuit  30 . In this operating mode, the heat exchanger  44  is used to recover thermal energy from hot molten salt during a discharging cycle as the hot molten salt flows from the hot salt storage tank  40  to the cold salt storage tank  42 . This operating mode can be used during non-daylight hours when solar radiation is not available to generate steam in the power generation circuit  30 . It can also be used during daylight hours if there is insufficient solar radiation to provide steam at the desired pressure and temperature in the power generation circuit  30 , for example during overcast conditions. 
         [0047]    Referring to  FIG. 3 , there is shown a second configuration of a solar thermal power plant  46 . The power plant  46  shares many features in common with the power plant  20  of  FIG. 2  and corresponding components are, therefore, designated with corresponding reference numerals. 
         [0048]    The power plant  46  includes two solar radiation receivers  48 ,  50 , both of which are mounted on top of the same tower  22 . The first solar radiation receiver  48  is associated with the power generation circuit  30  and includes one or more receiver panels onto which solar radiation is reflected by the heliostat field  24 . Water circulating through the first solar radiation receiver  48  is, thus, directly heated to generate steam for the power generating system  32 . The second solar radiation receiver  50  is associated with the energy storage circuit  36  and includes one or more receiver panels onto which solar radiation can be reflected by the heliostat field  24 . Molten salt circulating through the second solar radiation receiver  50  is, thus, directly heated during a charging cycle and subsequently pumped to a hot salt storage tank  40 . Heat can subsequently be recovered from the hot molten salt, via a heat exchanger  44 , as described above in connection with the power plant  20  of  FIG. 2 . 
         [0049]    The first solar radiation receiver  48 , associated with the power generation circuit  30 , is mounted below, and larger than, the second solar radiation receiver  50  associated with the energy storage circuit  36 . It should, however, be understood that the relative positions and dimensions of the first and second solar radiation receivers  48 ,  50  will in practice be selected based on a number of factors, including structural and thermal considerations and the energy storage requirements of the power plant  46 . For example, the positions of the first and second solar radiation receivers  48 ,  50  could be reversed or they could be arranged side-by-side. 
         [0050]    The power plant  46  can be operated in different modes as described above in connection with the power plant  20 . Like the power plant  20 , the operating mode of the power plant  46  can be varied by controlling the position of a subset of the plurality of heliostats  26  in the heliostat field  24 . This enables solar radiation to be reflected exclusively onto the first solar radiation receiver  48  during a generation only operating mode or onto both the first and second solar radiation receivers  48 ,  50  during a hybrid generation/storage operating mode. The heat exchanger  44  is used to recover energy from the hot molten salt during the energy recovery mode. 
         [0051]    Referring to  FIG. 4 , there is shown a third configuration of a solar thermal power plant  52 . The power plant  52  shares many features in common with the power plants  20  and  46  of  FIGS. 2 and 3  and corresponding components are, therefore, designated with corresponding reference numerals. 
         [0052]    Like the power plant  46  of  FIG. 3 , the power plant  52  employs two separate solar radiation receivers  54 ,  56 . The first solar radiation receiver  54  is mounted on top of a first tower  58  and is associated with the power generation circuit  30 . The first solar radiation receiver  54  includes one or more receiver panels onto which solar radiation is reflected by the heliostat field  24  to directly heat water circulating through the first solar radiation receiver  54 . The second solar radiation receiver  56  is mounted on top of a second tower  60 , spaced apart from the first tower  58 , and is associated with the energy storage circuit  36 . The second solar radiation receiver  56  includes one or more receiver panels onto which solar radiation is reflected by the heliostat field  24 . Molten salt circulating through the second solar radiation receiver  56  is, thus, directly heated during a charging cycle and subsequently pumped to a hot salt storage tank  40 . Heat can subsequently be recovered from the hot molten salt, via a heat exchanger  44 , as described above in connection with the power plants  20 ,  46  of  FIGS. 2 and 3 . 
         [0053]    The power plant  52  can be operated in different modes as described above in connection with the power plants  20 ,  46 . The daytime operating mode of the power plant  52  can be varied by controlling the position of a subset  62  of the plurality of heliostats  26  in the heliostat field  24 . This enables solar radiation to be reflected by the subset  62  of heliostats onto the first solar radiation receiver  54  mounted on the first tower  58  during a generation only operating mode or onto the second solar radiation receiver  56  mounted on the second tower  60  during a hybrid generation/storage operating mode. Some of the heliostats  26  in the heliostat field  24 , shown in  FIG. 4  as a subset  64 , are arranged so that they exclusively reflect solar radiation onto the first solar radiation receiver  54  mounted on the first tower  58 . The heat exchanger  44  is used to recover energy from the hot molten salt during the energy recovery mode. 
         [0054]    Although embodiments of the present invention have been described in the preceding paragraphs, it should be understood that various modifications may be made to those embodiments without departing from the scope of the present invention. 
         [0055]    For example, although the heat exchanger  44  is arranged so that thermal energy recovered from the hot molten salt is transferred to the power generation circuit  30  to support steam generation, the recovered heat could be used for other purposes such as those described earlier in this specification. 
         [0056]    In the power plant  20  of  FIG. 2 , the first and second receiver panels  34 ,  38  could be arranged to be adjacent to each other in the vertical direction, rather than the circumferential direction. 
         [0057]    Although the thermal energy storage fluid is typically a molten salt, other thermal energy storage fluids having a high specific heat capacity could be employed. 
         [0058]    In order to simplify the construction of the solar thermal power plants  20 ,  46 ,  52  described above with reference to  FIGS. 2 to 4 , the separate hot salt and cold salt storage tanks  40 ,  42  could be replaced with a single thermocline molten salt storage tank. In a thermocline molten salt storage tank, hot molten salt is typically stored at the top of the tank and cold molten salt is typically stored at the bottom of the tank. In this case, the heat exchanger  44  would be positioned between the hot and cold molten salt storage locations, so that heat can be recovered from the hot molten salt as it flows from the hot molten salt storage location at the top of the tank to the cold molten salt storage location at the bottom of the tank.