Abstract:
A steam power plant and method for operation the steam power plant is provided, that comprises: a main water-steam-cycle with a high pressure (HP) steam turbine, an intermediate pressure (IP) steam turbine and a low pressure (LP) steam turbine, a condenser, and a feed water tank, wherein low pressure heaters are arranged between said condenser and said feed water tank and wherein a plurality of high pressure heaters are arranged downstream of said feed water tank, whereby said low pressure heaters, said feed water tank and said plurality of high pressure heaters are supplied with steam from a plurality of extractions at said steam turbines.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments of the present invention relate to a steam power plant and a method for operating a steam power plant. 
         [0002]    Embodiments further refer to a steam power plant for conducting said method. 
       BACKGROUND 
       [0003]    In the past various attempts have been made to store energy in a steam power plant in order to use it during certain operational conditions. 
         [0004]    One attempt suggests a steam power having, parallel to the low pressure preheater passage, a heat reservoir, which is loaded with preheated condensate in weak-load times. This preheated condensate is taken from the heat reservoir for generating peak-load and inserted downstream of the preheater passage into the condensate line and the feed water tank, respectively. Thus it is possible to quickly control the power generation of the power plant in a wide range without significantly having to change the heating output of the boiler of the steam generator. A steam power plant equipped according to embodiments of the invention can thus be operated with bigger load modifications and also provide more control energy. 
         [0005]    In another attempt, a steam power plant has, parallel to the low-pressure passage, a heat reservoir which is loaded with preheated condensate in weak-load times. This preheated condensate is taken from the heat reservoir for generating peak-load and inserted downstream of the low-pressure preheater passage into the condensate line and the feed water tank, respectively. An additional heat exchanger is provided to increase the temperature of the hot water sent to the storage. Thus it is possible to quickly control the power generation of the power plant in a wide range without significantly having to change the heating output of the boiler of the steam generator. A steam power plant equipped according to embodiments of the invention can thus be operated with bigger load modifications and also provide more control energy. 
         [0006]    In yet another attempt, the integration of hot water energy storage is parallel to the HP feed water preheaters. In this case the storage is at higher temperatures and pressures than in previous attempts mentioned before. 
         [0007]    In yet another attempt, a method involving extracting a portion of a steam mass flow from a boiler connected to a water-steam circuit of a steam turbine into an external storage. The steam is released from the external storage and supplied to the steam turbine process when needed. The steam is extracted into the external storage, when the power plant is operated at partial load or when a rapid power reduction is required. The steam turbine is operated in modified variable pressure, and the boiler is filled with the steam while the steam is released from the external storage. Here, during charging the storage is fed by steam from the boiler. 
         [0008]    For power plants, where no storage is already installed, one of the above attempts may provide the best solutions. But when a power plant has already a steam storage installed but not dedicated to use it to provide additional power, it is the most cost-effective solution to integrate the existing steam storage in a different way. 
       BRIEF DESCRIPTION 
       [0009]    It is an object of the present invention to provide a method for operating a steam power plant, which is capable of storing energy in order to utilize the fluctuation of the electricity price to earn additional revenues (arbitration). 
         [0010]    It is a further object of the present invention to provide a steam power plant for conducting said method. 
         [0011]    These objectives are obtained by a method and a steam power plant as disclosed herein. 
         [0012]    The inventive method for operating a steam power plant is based on a steam power plant comprising: a main water-steam-cycle with a high pressure (HP) steam turbine, an intermediate pressure (IP) steam turbine and a low pressure (LP) steam turbine, a condenser, and a feed water tank, wherein low pressure heaters are arranged between said condenser and said feed water tank and wherein a plurality of high pressure heaters are arranged downstream of said feed water tank, whereby said low pressure heaters, said feed water tank and said plurality of high pressure heaters are supplied with steam from a plurality of extractions at said steam turbines. 
         [0013]    The inventive method comprises the steps of: providing a steam storage tank within said steam power plant, storing during a first operation period of said steam power plant steam in said steam storage tank, and discharging during a second operation period of said steam power plant steam stored in said steam storage tank into the main water steam cycle to save steam extracted from said plurality of extractions at said steam turbines. 
         [0014]    An embodiment of the inventive method is characterized in that during said first operation period steam extracted from said high pressure (HP) steam turbine is stored in said steam storage tank, a first of said plurality of high pressure heaters is supplied with steam extracted from said intermediate pressure (IP) steam turbine, and steam is discharged into said first of said plurality of high pressure heaters from said steam storage tank during said second operation period of said steam power plant. 
         [0015]    Said steam discharged from said steam storage tank into said first of said plurality of high pressure heaters during said second operation period may be superheated with steam extracted from said high pressure (HP) steam turbine. 
         [0016]    Alternatively, said steam discharged from said steam storage tank into said first of said plurality of high pressure heaters during said second operation period may be superheated with hot reheat steam, which is available at the inlet of said intermediate pressure (IP) steam turbine. 
         [0017]    Alternatively, said steam discharged from said steam storage tank into said first of said plurality of high pressure heaters during said second operation period is superheated with steam, which is extracted from said intermediate pressure (IP) steam turbine for supplying said first of said plurality of high pressure heaters. 
         [0018]    Another embodiment of the inventive method is characterized in that during said first operation period steam extracted from said high pressure (HP) steam turbine is stored in said steam storage tank, said feed water tank is supplied with steam extracted from said intermediate pressure (IP) steam turbine, and steam is discharged into said feed water tank from said steam storage tank during said second operation period of said steam power plant. 
         [0019]    Said steam discharged from said steam storage tank into said feed water tank may be superheated with steam extracted from said high pressure (HP) steam turbine. 
         [0020]    Alternatively, said steam discharged from said steam storage tank into said feed water tank may be superheated with hot reheat steam, which is available at an inlet of said intermediate pressure (IP) steam turbine. 
         [0021]    Alternatively, a first of said plurality of high pressure heaters is supplied with steam extracted from said intermediate pressure (IP) steam turbine, and said steam discharged from said steam storage tank into said feed water tank may be superheated with steam, which is extracted from said intermediate pressure (IP) steam turbine for supplying said first of said plurality of high pressure heaters. 
         [0022]    Alternatively, said steam discharged from said steam storage tank into said feed water tank may be superheated with steam extracted from said intermediate pressure (IP) steam turbine for being supplied to said feed water tank. 
         [0023]    A steam power plant according to embodiments of the invention for conducting said inventive method comprises: a steam-water-cycle with a high pressure steam turbine, an intermediate pressure steam turbine and a low pressure steam turbine, a condenser, and a feed water tank, wherein low pressure heaters are arranged between said condenser and said feed water tank and wherein first and second high pressure heaters are arranged downstream of said feed water tank, whereby said low pressure heaters, said feed water tank and said high pressure heaters are supplied with steam from a plurality of extractions at said steam turbines. 
         [0024]    A steam storage tank with an input for receiving steam and an output for discharging steam is provided at said steam power plant, that said input of said steam storage tank is operationally connected to a steam extraction at said high pressure steam turbine, and that said output of said steam storage tank is operationally connected to said first high pressure heater. 
         [0025]    Another steam power plant according to embodiments of the invention for conducting said inventive method comprises: a steam-water-cycle with a high pressure steam turbine, an intermediate pressure steam turbine and a low pressure steam turbine, a condenser, and a feed water tank, wherein low pressure heaters are arranged between said condenser and said feed water tank and wherein first and second high pressure heaters are arranged downstream of said feed water tank, whereby said low pressure heaters, said feed water tank and said high pressure heaters are supplied with steam from a plurality of extractions at said steam turbines. 
         [0026]    A steam storage tank with an input for receiving steam and an output for discharging steam is provided at said steam power plant, that said input of said steam storage tank is operationally connected to a steam extraction at said high pressure (HP) steam turbine, and that said output of said steam storage tank is operationally connected to said feed water tank. 
         [0027]    Especially, steam extracted from said steam storage tank may be superheated with steam extracted from said high pressure (HP) steam turbine or hot reheat steam, which is available at the inlet of said intermediate pressure (IP) steam turbine, or steam, which is extracted from said intermediate pressure (IP) steam turbine for supplying said first of said high pressure heaters. 
         [0028]    Furthermore, steam extracted from said steam storage tank may be superheated with steam extracted from said intermediate pressure (IP) steam turbine for being supplied to said feed water tank. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Embodiments of the present invention will now be explained more closely and with reference to the attached drawings. 
           [0030]      FIG. 1  shows a prior art basic water-steam-cycle arrangement; 
           [0031]      FIG. 2  shows a steam storage integration at a high pressure heater, in a water-steam-cycle arrangement as shown in  FIG. 1  according to an embodiment of the invention; and 
           [0032]      FIG. 3  shows a steam storage integration at the feed water tank, in a water-steam-cycle arrangement as shown in  FIG. 1  according to another embodiment of the invention; 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The main objective is the integration of thermal energy storage (steam storage) into a steam power plant. During discharging of the steam storage tank, the steam is fed to the main water-steam cycle to save extraction steam. By doing this, the power output of the plant can be increased. 
         [0034]    The basis is a prior art steam power plant shown in  FIG. 1 . The steam power plant  10  of  FIG. 1  comprises a high pressure (HP) steam turbine  11 , intermediate pressure (IP) steam turbines  12  and low pressure (LP) steam turbines  13 , which drive a generator  14 . Life steam  25  is supplied to high pressure steam turbine from a boiler (or heat recovery steam generator HRSG) not shown. After expansion in high pressure steam turbine  11  steam is fed back to cold reheat  24  of the boiler. Hot reheat  26  steam from the boiler is then supplied to intermediate pressure (IP) steam turbines  12  the exits of which are connected to the inlet of low pressure (LP) steam turbines  13 . 
         [0035]    Steam from the low pressure (LP) steam turbines  13  flows into condenser  15 . The resulting condensate is pumped by condensate pump  16  through heat exchanger  17  and a series of low pressure heaters (LPH)  18  to feed water tank  19 . From feed water tank  19  a feed water pump  20  pumps feed water through high pressure heaters (HPH)  21   a  and  21   b  and desuperheater (DeSH)  22  to an economizer  23  of a boiler/heat recovery steam generator (not shown). 
         [0036]    The low pressure heaters  18  are supplied with steam extracted at various points of low pressure steam turbines  13  and intermediate pressure steam turbines  12  (extractions E 1  to E 4 ). Feed water tank  19  receives steam from extraction E 5  of intermediate pressure steam turbines  12 , while first high pressure heater  21   a  and desuperheater  22  are connected to extraction E 6  of intermediate pressure steam turbines  12 . Second high pressure heater  21   b  receives steam from extraction E 7 , i.e. directly from the outlet of high pressure steam turbine  11 . 
         [0037]    An HP extraction is not shown in the drawing of  FIG. 1 , but can also be possible. 
         [0038]    Now, in general, the higher the pressure of the extraction steam, the longer is the path in the steam turbine where the steam can deliver “work”. If the mass flows were similar, this would be true. But for a steam storage, the lower the minimum pressure, the more mass can be extracted from the storage and so, integration of a steam storage at a lower stage can result in even higher electrical power output increase. 
         [0039]    When the maximum storage pressure is the cold reheat (CRH) pressure at  24 , the storage cannot be connected to the second high pressure heater  21   b  (in  FIG. 1 ), as the pressure decreases when extracting steam from the storage. Therefore, the first possible feed water preheater in descending order is the first high pressure heater  21   a . If there are several high pressure feed water preheaters, the storage can be connected to either of them, which has a pressure lower than the storage pressure. 
         [0040]    Depending on the source of superheating steam, it can occur that the steam pressure from the storage is a little bit below the original extraction pressure (depending on pressure drops in the system). 
         [0041]      FIG. 2  now shows an embodiment of the invention, where a steam storage tank  27  is integrated at high pressure heater  21   a.    
         [0042]    If this high pressure heater  21   a  is connected to the IP steam turbine  12  (extraction E 6 ), it will have a high temperature (approx. 400° C. and more) and a pressure lower than the cold reheat pressure at 24 (approx. 25 bars). 
         [0043]    There are different ways to superheat the steam from storage tank  27 . 
         [0044]    According to a first superheat option  29  (valve  30 ), the steam from storage tank  27  can be superheated with cold reheat  24  from the exit of high pressure steam turbine  11 . 
         [0045]    According to a second superheat option  31  (valve  32 ), the steam from storage tank  27  can be superheated with hot reheat  26 , i.e. steam supplied to the inlet of intermediate pressure steam turbines  12 . 
         [0046]    According to a third superheat option  33  (valve  34 ) the steam from storage tank  27  can be superheated with steam from extraction E 6  at intermediate pressure steam turbine  12  to high pressure heater  21   a . Further valves  28 ,  35  and  36  are provided to complete the described functionality. 
         [0047]    If the degree of superheating of the steam is rather low, it only makes sense to shut off the desuperheater  22  of that high pressure preheater  21   a  and introduce the steam from the storage tank  27  directly at the condensing part. If there is no non-return valve between desuperheater  22  and the condensing part (valve  35 ), it must be retrofitted. Third superheat option  33  has the highest storage efficiency of the three superheating variants explained above. 
         [0048]    Furthermore, a throttle valve (valve  28 ) controls the pressure to the pressure of the high pressure heater  21   a.    
         [0049]    Another embodiment of the invention is shown in  FIG. 3 . According to  FIG. 3  the steam storage tank  27  is integrated at feed water tank  19 . When integrating the steam from the storage tank  27  at feed water tank  19 , which is at a pressure level of approx. 10 bars, more steam can be extracted from the storage tank  27 . A throttle valve  28  downstream the storage tank  27  will also be necessary. 
         [0050]    To superheat the storage steam, the three superheating options explained above are possible. 
         [0051]    A fourth option  39 , using steam from the extraction E 5  of the feed water tank  19  is possible. This solution delivers a higher electrical power increase, but has slightly slower storage efficiency than when integrating at high pressure heater  21   a.    
         [0052]    The throttle valve  28  controls the pressure to the feed water tank pressure. With a closed stop valve  38 , the original extraction steam flow cannot enter the feed water tank  19 . Valves  37  and  40  are provided to complete the described functionality. 
         [0053]    Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, the present disclosure can be embodied in other specific forms. For example, the exemplary power plant may only has two low pressure heaters, and/or a feed water tank connected to a lower extraction, and/or more than two high pressure heaters. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.