WO2013013682A1

WO2013013682A1 - Arrangement and method for load change compensation at a saturated steam turbine

Info

Publication number: WO2013013682A1
Application number: PCT/EP2011/003707
Authority: WO
Inventors: Marco OCHS; Patrick MEADE VARGAS; Tobias SCHÖNBERG
Original assignee: Abb Technology Ag
Priority date: 2011-07-23
Filing date: 2011-07-23
Publication date: 2013-01-31

Abstract

An arrangement for load change compensation at a saturated steam turbine comprises the saturated steam turbine (1) and a saturated steam generator (2), a generator outlet (5) of which is connected via a first pipe (8) to an inlet (6) of a throttle (3), where a second pipe (9) connects an outlet (7) of the throttle (3) to a turbine inlet (4) of the saturated steam turbine. In order to reduce the thermal stress on the saturated steam turbine due to a pressure and temperature drop in the saturated steam at decreased load, a superheating heat exchanger (20) is connected between the outlet (7) of the throttle (3) and the turbine inlet (4), and a branch pipe (19) branches off from the first pipe (8) before the inlet (6) of the throttle (3) and leads to a secondary inlet (21) of the superheating heat exchanger (20). Thereby, the flow of the saturated steam is divided. One part of the steam flow is condensed inside the superheating heat exchanger (20) at constant temperature. The heating energy released during the condensation is used to increase the temperature of the other part of the steam flow.

Description

Arrangement and method for load change compensation at a saturated steam turbine

Description

The invention relates to an arrangement for load change compensation at a saturated steam turbine comprising the saturated steam turbine and a saturated steam generator, a generator outlet of which is connected via a first pipe to an inlet of a throttle, where a second pipe connects an outlet of the throttle to a turbine inlet of the saturated steam turbine. The invention further relates to a method to compensate for a load change at a saturated steam turbine, where saturated steam is generated by a saturated steam generator and the saturated steam is used to drive the saturated steam turbine.

Saturated steam turbines are used to convert energy which is stored in pressurized steam into a rotary motion, i.e. into mechanical work. They are used in power plants, such as nuclear power plants or concentrated thermal solar power plants. The term saturated steam means hereby dry saturated steam which is steam being just at the brink between vapour and wet steam. At a specific pressure, steam has a corresponding so called saturation temperature, which is the point where vapour starts to condensate into water droplets. This is the point where vapour becomes dry saturated steam. The commonly used working fluid in saturated steam turbines is water.

In Fig. 1 , a simplified block diagram of basic elements used in the steam cycle of a power plant, in particular a solar power plant is shown as an example. A saturated steam generator 2 is connected via its generator outlet 5 and via a first pipe 8 to an inlet 6 of a throttle 3. A second pipe 9 connects an outlet 7 of the throttle 3 to a turbine inlet 4 of a saturated steam turbine 1 , where the saturated steam turbine 1 is a high pressure turbine. A turbine outlet 10 of the saturated steam turbine 1 is connected to an inlet of a re-heater 13, an outlet of which is connected to an inlet of a low pressure steam turbine 11. The saturated steam turbine 1 and the low pressure steam turbine 11 are arranged on the same shaft 16 with an electrical generator 17. An outlet of the low pressure steam turbine 11 is connected to an inlet of a condenser 15, an outlet of which is connected to an inlet of a primary pump 12. An outlet of the primary pump 12 is connected via a feed water pipe 25 to an inlet 26 of the saturated steam generator 2. In the pipes, water is flowing and steam or vapour is circulating, respectively.

The basic functional principle of the elements of the steam cycle of Fig. 1 can be understood from the corresponding T-s-diagram depicted in Fig. 2, with T being the temperature and s being the entropy of the water at a certain location within the cycle. The current location of the water is indicated with the numbers in the diagram, which correspond to the indices in Fig. 1. The bell shaped line in Fig. 2 separates areas of different states of aggregation of water. To the left of the line, water is in its liquid state. From left to right, more and more water vaporizes, until pure vapour is present to the right of the line. Accordingly, the area below the bell shape is the area of wet steam, where wet steam is steam containing differing percentage of water drops. The right-hand part of the bell shaped line indicates where wet steam turns into vapour, i.e. the line is the saturated steam line.

Beginning the description of the cycle in the lower left corner, the primary pump 12 increases the pressure of liquid water above a so called live-steam working pressure P_ev. Then, the water is heated in the saturated steam generator 2 at constant pressure until it reaches its boiling point at the evaporating or life-steam temperature T_ev, where T_ev is a function of P_ev, and the heating is even continued further on. At the evaporating temperature T_ev, the water starts to absorb heating energy without increasing its temperature anymore. Instead, it changes its state of aggregation from liquid into gaseous, until it becomes saturated steam when the horizontal line crosses the right hand side of the bell shaped line. At this point, the saturated steam is conducted through turbine 1 , where it expands and produces mechanical work. As a result, the pressure of the steam decreases from live-steam working pressure to an intermediate working pressure, which may be called reheated-steam pressure, its temperature drops correspondingly and the percentage of water droplets in the steam increases, i.e. it becomes wet steam. Then, the low pressure steam enters the re-heater 13, where it starts absorbing heating energy again until it turns into saturated steam again. The heating is continued so that the saturated steam increases its temperature at constant low pressure and turns into vapour. The low pressure vapour arrives at the low pressure steam turbine 11 , where it expands to a further reduced pressure, also called condenser pressure, and abruptly drops its temperature, thereby becoming wet steam again. In the condenser 15, the wet steam condenses at constant temperature and turns into liquid water, before it is pumped again by primary pump 12 from very low to high pressure.

In the cycle described with respect to Fig. 2, which is also known as Rankine cycle, the power plant is operated at full load. Accordingly, throttle 3 is fully open. Another case of operation where the power plant operates at partial load is depicted in Fig. 3. There is only one difference to the cycle of Fig. 2, which is that throttle 3 is partially closed so that the flow of the saturated steam coming from the saturated steam generator 2 is reduced. Thereby, the inlet pressure to the saturated steam turbine 1 is decreased. This, however, causes a temperature drop due to an isenthalpic change of pressure, as can be seen from Fig. 3, where the temperature decrease occurs at constant specific enthalpy. The resulting temperature gradients affect the material in thick-walled components, especially in saturated steam turbine 1 , and lead to mechanical stress which significantly reduces the components' lifetime.

This problem is particularly immanent in thermal solar power plants where not only load drops occur frequently but also comparatively abruptly whenever the sun is shadowed and accordingly direct insolation is lost. The sudden load drops lead to high thermal transients with the consequences of thermal stress in the system as described above.

However, as is generally known, it is one of the main goals for the future to increase the integration of renewable energy in the generation of electrical power. Currently, concentrated thermal solar power gains more and more market share and becomes commercially attractive. Two different concepts compete on the market, the so called parabolic trough and the Fresnel technique, where both work with the above described water-steam cycle. With the parabolic troughs, sunlight is concentrated by parabolic mirrors. With the Fresnel technique, numerous flat mirrors are used to concentrate the solar rays. The concentrated solar rays are then used to generate saturated steam by either focusing them onto a receiver-pipe where oil flows through, which then transfers the heat to the water-steam cycle via heat exchangers, or by directing them onto a receiver-pipe in which water is directly converted to saturated steam. The second alternative is cheaper, however, the design and control of such a direct-steam-generating solar field is more challenging.

In case of a sudden shadcwing by clouds for example, the primary heat source stops producing steam. In order to prevent an immediate plant shutdown, one or more steam accumulators are installed in the plant as a buffer storage system to hold a minimum amount of saturated steam in order to bridge a load drop for a short period of time. In other words, during the shadowing, the steam accumulators take over the function of the saturated steam generator 2, until insolation is available again and the above described receiver-pipe arrangements work as saturated steam generator 2 again. During steam provision by the accumulators, the inlet pressure of the saturated steam turbine 1 has to be reduced via throttle 3. This results in the above described temperature drop and thermal stress for system components.

In the article "Buffer storage for direct steam generation" by Steinmann and Eck, Solar Energy 80 (2006), pp. 1277-1282, various steam accumulator concepts used in solar thermal systems are described. Among others, different so called constant pressure concepts are presented where the steam pressure drop associated with the discharging of the steam accumulator is avoided. Since the pressure gradients are reduced, the temperature gradients are reduced as well, thereby limiting the thermal stress.

It is an object of the invention to present another arrangement and method to reduce the thermal stress associated with the load change at a saturated steam turbine. This object is achieved by an arrangement and a method according to the independent claims.

The arrangement comprises a saturated steam turbine, a saturated steam generator, a generator outlet of which is connected via a first pipe to an inlet of a throttle, where a second pipe connects an outlet of the throttle to a turbine inlet of the saturated steam turbine.

According to the invention, the arrangement further comprises a superheating heat exchanger which is connected between the outlet of the throttle and the turbine inlet as well as a branch pipe branching off from the first pipe before the inlet of the throttle and leading to a secondary inlet of the superheating heat exchanger.

In this arrangement, saturated steam is generated by the saturated steam generator and used to drive the saturated steam turbine, as is known from the art. The invention lies now in that in case of a load reduction, the flow of the saturated steam is divided by means of the branch pipe into a first steam flow and a second steam flow before it reaches the saturated steam turbine. The second steam flow is led through the branch pipe to the superheating heat exchanger, inside which the steam of the second steam flow is condensed at constant temperature and turns into water, thereby forming a second water flow. The first steam flow inside the first pipe is throttled resulting in a pressure and temperature drop of the first steam flow. The temperature drop is compensated for by leading the first steam flow after the throttle to the superheating heat exchanger, inside which it is superheated by the heating energy released during the condensation of the second steam flow. In other words, the first steam flow is the primary medium in the superheating heat exchanger, entering the superheating heat exchanger through a primary inlet and leaving it through a primary outlet, while the second steam flow is the secondary medium used to increase the temperature of the first steam flow. Since the first steam flow contains saturated steam, further heating turns it into superheated steam. Afterwards, the superheated first steam flow is led to the saturated steam turbine where it arrives with the same temperature or with only a moderately changed temperature compared to the temperature of the saturated steam before the load change. As a result of the invention, the temperature at the inlet of the saturated steam turbine is approximately maintained even during load changes, thereby avoiding the occurrence of thermal stress in the saturated steam turbine as well as in other thick-walled components in the arrangement. Since the first steam flow is superheated, i.e. very dry, the erosion of the components in the arrangement which is associated with water droplets is reduced.

The invention and its embodiments will become apparent from the example and its embodiments described below in connection with the appended drawings which show:

Fig. 1 a simplified block diagram of the known steam cycle in a power plant,

Fig. 2 a temperature-entropy diagram corresponding to Fig. 1 , for operation at full load, Fig. 3 a temperature-entropy diagram corresponding to Fig. 1 , for operation at partial load,

Fig. 4 a section of the block diagram of Fig. 1 including the elements according to the invention,

Fig. 5 a temperature-entropy diagram corresponding to Fig. 4, for operation at partial load,

Fig. 6 a section of the block diagram of Fig. 1 including an embodiment of the

invention, and

Fig. 7 a temperature-time diagram without and with the invention.

Fig. 4 shows a section of the block diagram of Fig. 1 , with the saturated steam turbine 1 , the saturated steam generator 2 and the throttle 3 in between the two. According to the invention, a superheating heat exchanger 20 is arranged with its primary flow path in the second pipe 9, i.e. between the outlet 7 of the throttle 3 and the turbine inlet 4 of the saturated steam turbine 1. As was already described in connection with the state of the art and Figs. 1 , 2 and 3 above, the water or steam inside the pipes, respectively, goes through a cycle process.

In the following, the operation of the arrangement of Fig. 4 is described for the case of a load reduction in the overall power plant, which may in the example of a solar power plant be the result of shadowing by clouds. The corresponding cycle process for the power plant which includes the arrangement of Fig. 4 is depicted in the temperature-entropy diagram of Fig. 5. The only difference with the known art of Fig. 3 occurs between the throttle 3 and the saturated steam turbine 1.

The steam which leaves the saturated steam generator 2 at its generator outlet 5 is saturated steam at an evaporating or life-steam temperature T_ev and a life-steam working pressure P_ev, which is depicted as the right-hand end of the horizontal line in the T-s diagram of Fig. 5. Before this saturated steam, which flows through the first pipe 8, arrives at the inlet 6 of the throttle 3, the steam is divided into a first steam flow and a second steam flow, where the first steam flow continues to flow inside the first pipe 8 towards the throttle 3 and the second steam flow is branched off from first pipe 8 and led through a branch pipe 19 towards a secondary inlet 21 of the superheating heat exchanger. Throttle 3 causes an isenthalpic change of pressure in the first steam flow, which leads to a temperature drop in the first steam flow, as can be see τ from Fig. 5. Inside the superheating heat exchanger, the second steam flow is condensed at constant temperature, thereby turning the saturated steam into water at about the same temperature level. The heating energy released during the condensation is transferred inside the superheating heat exchanger to the first steam flow which is thereby heated again and is turned into superheated steam, which is illustrated by Fig. 5, where the cycle line leaves the saturated steam line. The temperature drop of the first steam flow is compensated for, so that the first steam flow almost reaches T_ev again. Then, the first steam flow enters the saturated steam turbine 1 , expands and produces mechanical work. The remaining parts of the steam cycle of Fig. 5 correspond to Fig. 3.

The steam temperature over time at the inlet of the saturated steam turbine 2 is depicted in Fig. 7. Before the point in time tO, the power plant is operated at full load and the saturated steam has life-steam temperature T_ev, as is shown by line 29. At tO, the load demand decreases abruptly and the saturated steam in the first pipe 8 is throttled. The solid line 31 illustrates the resulting temperature drop for a common power plant without superheating heat exchanger. In contrast to that, the dotted line 30 shows that the temperature is maintained at the same level before and after the load reduction and that it is decreased only gradually afterwards, thereby considerably reducing the thermal stress on saturated steam turbine 2, when a superheating heat exchanger is included.

In an embodiment of the invention, a further heat exchanger 23 is arranged inside the feed water pipe 25 towards the inlet 26 of the saturated steam generator 2. A secondary inlet 27 of the further heat exchanger 23 is connected to a secondary outlet 22 of the superheating heat exchanger 20. The second water flow is accordingly led as secondary medium to the further heat exchanger 23, where its heat is used for heating feed water inside feed water pipe 25. The feed water is then led to the saturated steam generator 2.

In a further embodiment of the invention, an auxiliary pump 24 is connected either between a secondary outlet 22 of the superheating heat exchanger 20 and an injection point in the feed water pipe 25 (not shown) or between a secondary outlet 28 of the further heat exchanger 22 and the injection point in the feed water pipe 25. The auxiliary pump 24 is used to increase the pressure of the second water flow to approximately the same pressure level as has the feed water. The pressurized second water flow is then added to the feed water in feed water pipe 25.

Claims

1. Arrangement for load change compensation at a saturated steam turbine comprising

• the saturated steam turbine (1),

• a saturated steam generator (2), a generator outlet (5) of which is connected via a first pipe (8) to an inlet (6) of a throttle (3), where

• a second pipe (9) connects an outlet (7) of the throttle (3) to a turbine inlet (4) of the saturated steam turbine (1),

characterized by

• a superheating heat exchanger (20) connected between the outlet (7) of the throttle (3) and the turbine inlet (4),

• a branch pipe (19) branching off from the first pipe (8) before the inlet (6) of the throttle (3) and leading to a secondary inlet (21) of the superheating heat exchanger (20).

2. Arrangement according to claim 1 , where a further heat exchanger (23) is arranged inside a feed water pipe (25) towards an inlet (26) of the saturated steam generator (2), with a secondary inlet (27) of the further heat exchanger (23) being connected to a secondary outlet (22) of the superheating heat exchanger (20).

3. Arrangement according to claim 1 or 2, where an auxiliary pump (24) is connected between either a secondary outlet (22) of the superheating heat exchanger (20) or a secondary outlet (28) of the further heat exchanger (23) and an injection point in the feed water pipe (25).

4. Arrangement according to any of the previous claims, where the arrangement is part of a solar power plant.

5. Method to compensate for a load change at a saturated steam turbine, where

• saturated steam is generated by a saturated steam generator (2),

• the saturated steam is used to drive the saturated steam turbine (1), characterized in that in case of a load reduction,

• the flow of the saturated steam is divided before the saturated steam turbine (1) into a first steam flow and a second steam flow, where • the second steam flow is led to a superheating heat exchanger (20), inside which the steam is condensed at constant temperature turning it into a second water flow,

• the first steam flow is throttled, then led to the superheating heat exchanger, inside which it is superheated by the heating energy released during the condensation of the second steam flow, and afterwards led to the saturated steam turbine (1).

6. Method according to claim 5, where the heating energy of the second water flow is additionally used in a further heat exchanger (23) for heating feed water for the saturated steam generator (2).

7. Method according to claim 5 or 6, where the pressure of the second water flow is

increased by an auxiliary pump (24) to the same pressure level as the feed water for the saturated steam generator (2).