METHOD FOR OPERATING A STEAM POWER PLANT AND STEAM POWER PLANT FOR CONDUCTING SAID METHOD

BACKGROUND OF THE INVENTION

The present invention relates to a steam power plant. It refers to a method for operating a steam power plant according to the preamble of claim 1.

It further refers to a steam power plant for conducting said method.

PRIOR ART

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.

Document EP 2 333 254 B1 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 resp. the feed water tank. 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 the invention can thus be operated with bigger load modifications and also provide more control energy.

Document EP 2 589 761 A1 describes an extension to EP 2 333 254 B1 mentioned before. Again, 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 resp. the feed water tank. 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 the invention can thus be operated with bigger load modifications and also provide more control energy.

Document EP 2 589 760 A1 describes the integration of hot water energy storage parallel to the HP feed water preheaters. In this case the storage is at higher temperatures and pressures than in EP 2 333 254 B1 mentioned before.

Document DE 10 2012 213 976 A1 discloses 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.

For power plants, where no storage is already installed, EP 2 333 254 B1 and EP 2 589 761 A1 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.

SUMMARY OF THE INVENTION

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).

It is a further object of the present invention to provide a steam power plant for conducting said method.

These objectives are obtained by a method according to Claim 1 and a steam power plant according to Claims 12 and 13.

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.

The inventive method comprises the steps of: (a) providing a steam storage means within said steam power plant, (b) storing during a first operation period of said steam power plant steam in said steam storage means, and (c) discharging during a second operation period of said steam power plant steam stored in said steam storage means into the main water steam cycle to save steam extracted from said plurality of extractions at said steam turbines.

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 means, 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 means during said second operation period of said steam power plant.

Said steam discharged from said steam storage means 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.

Alternatively, said steam discharged from said steam storage means 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.

Alternatively, said steam discharged from said steam storage means 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.

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 means, 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 means during said second operation period of said steam power plant.

Said steam discharged from said steam storage means into said feed water tank may be superheated with steam extracted from said high pressure (HP) steam turbine.

Alternatively, said steam discharged from said steam storage means 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.

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 means 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.

Alternatively, said steam discharged from said steam storage means 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.

According to another embodiment of the inventive method said steam storage means is a steam storage tank.

A steam power plant according to 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.

It is characterized in that a steam storage means 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 means is operationally connected to a steam extraction at said high pressure steam turbine, and that said output of said steam storage means is operationally connected to said first high pressure heater.

Another steam power plant according to 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.

It is characterized in that a steam storage means 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 means is operationally connected to a steam extraction at said high pressure (HP) steam turbine, and that said output of said steam storage means is operationally connected to said feed water tank.

Especially, means may be provided to superheat steam extracted from said steam storage means 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.

Furthermore means may be provided to superheat steam extracted from said steam storage means with steam extracted from said intermediate pressure (IP) steam turbine for being supplied to said feed water tank.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1

shows a prior art basic water-steam-cycle arrangement;

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

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 OF DIFFERENT EMBODIMENTS OF THE INVENTION

The main objective is the integration of thermal energy storage (steam storage) into a steam power plant. During discharging of the steam storage means or 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.

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.

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).

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 E1 to E4). Feed water tank 19 receives steam from extraction E5 of intermediate pressure steam turbines 12, while first high pressure heater 21 a and desuperheater 22 are connected to extraction E6 of intermediate pressure steam turbines 12. Second high pressure heater 21 b receives steam from extraction E7, i.e. directly from the outlet of high pressure steam turbine 11.

An HP extraction is not shown in the drawing of Fig. 1, but can also be possible.

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.

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.

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).

Fig. 2 now shows an embodiment of the invention, where a steam storage means with storage tank 27 is integrated at high pressure heater 21 a.

If this high pressure heater 21 a is connected to the IP steam turbine 12 (extraction E6), 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).

There are different ways to superheat the steam from storage tank 27.

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.

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.

According to a third superheat option 33 (valve 34) the steam from storage tank 27 can be superheated with steam from extraction E6 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.

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.

Furthermore, a throttle valve (valve 28) controls the pressure to the pressure of the high pressure heater 21 a.

Another embodiment of the invention is shown in Fig. 3. According to Fig. 3 the steam storage means with 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.

To superheat the storage steam, the three superheating options explained above are possible.

A fourth option 39, using steam from the extraction E5 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. 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.

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.

LIST OF REFERENCE NUMERALS

10

steam power plant

10a,b

steam power plant

11

high pressure (HP) steam turbine

12

intermediate pressure (IP) steam turbine

13

low pressure (LP) steam turbine

14

generator

15

condenser

16

condensate pump

17

heat exchanger

18

low pressure heater (LPH)

19

feed water tank

20

feed water pump

21 a,b

high pressure heater (HPH)

22

desuperheater (DeSH)

23

(to) economizer

24

(to) cold reheat

25

(from) life steam

26

(from) hot reheat

27

steam storage tank

27a

input (steam storage tank)

27b

output (steam storage tank)

28,30,32

valve

29,31,33,39

superheat (SH) option

34-38,40

valve

E1-E7

extraction (of steam)