U.S. patent application number 14/383496 was filed with the patent office on 2015-02-05 for method for improving thermal-cycle yield in nuclear power plants.
The applicant listed for this patent is SENER, INGENIERIA Y SISTEMAS, S.A.. Invention is credited to Irune Gutierrez Larranaga, Borja Herrazti Garcia, Antonio Lopez Garcia.
Application Number | 20150033742 14/383496 |
Document ID | / |
Family ID | 48325747 |
Filed Date | 2015-02-05 |
United States Patent
Application |
20150033742 |
Kind Code |
A1 |
Herrazti Garcia; Borja ; et
al. |
February 5, 2015 |
METHOD FOR IMPROVING THERMAL-CYCLE YIELD IN NUCLEAR POWER
PLANTS
Abstract
The present invention relates to a method for increasing the
efficiency of electric power generation in pressurized water
nuclear power plants, comprising steps of superheating a main steam
and reheating the reheated steam by means of an auxiliary circuit,
where the streams for the superheating and the reheating work in
parallel.
Inventors: |
Herrazti Garcia; Borja;
(Tres Cantos (Madrid), ES) ; Lopez Garcia; Antonio;
(Tres Cantos (Madrid), ES) ; Gutierrez Larranaga;
Irune; (Tres Cantos (Madrid), ES) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SENER, INGENIERIA Y SISTEMAS, S.A. |
Vizcaya |
|
ES |
|
|
Family ID: |
48325747 |
Appl. No.: |
14/383496 |
Filed: |
March 8, 2013 |
PCT Filed: |
March 8, 2013 |
PCT NO: |
PCT/ES2013/070148 |
371 Date: |
September 5, 2014 |
Current U.S.
Class: |
60/653 ; 122/441;
122/479.7; 122/488 |
Current CPC
Class: |
F01K 3/183 20130101;
F01K 3/265 20130101; F01K 3/181 20130101; F22G 1/16 20130101; F01K
3/185 20130101; Y02E 30/00 20130101; F01K 7/223 20130101; F01K
11/02 20130101; F01K 3/06 20130101; F22D 1/32 20130101; F01K 5/02
20130101 |
Class at
Publication: |
60/653 ;
122/479.7; 122/488; 122/441 |
International
Class: |
F01K 11/02 20060101
F01K011/02; F22D 1/32 20060101 F22D001/32; F01K 7/22 20060101
F01K007/22; F22G 1/16 20060101 F22G001/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2012 |
ES |
P201230351 |
Claims
1. Method for increasing the efficiency of electric power
generation in pressurized water nuclear power plants, comprising
the following steps: a. the saturated or slightly wet steam
originating from the steam generator (SG) is superheated before
entering a steam turbine (ST) with several bodies; b. the steam
reheated with steam from a high pressure (HP) turbine extraction,
is again reheated using live-steam from the reactor; c. the steam
reheated in the preceding step is again reheated, exchanging heat
with a thermal fluid at a higher temperature; d. the reheated steam
of step c is expanded in the low (LP) body of the steam turbine; e.
the expanded steam of step d is condensed and the condensed water
is recirculated to the steam generators after heating with water
steam originating from turbine extractions characterized in that
the superheating in a and the reheating in c are performed by means
of an auxiliary thermal fluid circuit with the streams for the
superheating and the reheating working in parallel.
2. Method according to claim 1, characterized in that in steps a
and c the exchange with the thermal fluid is performed by means of
pressurized water and at a higher temperature, where the water
originates from a second auxiliary circuit which diverts part of
the water from the reactor to an exchanger.
3. Method according to claim 1, characterized in that the energy
source or sources used for the superheating and the reheating of
steps a and c is/are external to the power plant.
4. Method according to any of claim 3, where the energy source or
sources is/are a renewable source.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for being applied
in the energy industry, and more specifically in nuclear power
plants intended for generating electricity in which the fluid of
the primary reactor cooling circuit is water (or heavy water). Said
method is applicable in those nuclear power plants the primary
circuit of which works with high temperature pressurized water
(PWR) and in power plants with boiling water (BWR).
BACKGROUND OF THE INVENTION
[0002] Today, in most nuclear power plants the reactors of which
are cooled by water, all the thermal energy generated by the
reactor is used for generating saturated steam either in the steam
generators or in the reactors themselves depending on the reactor
type.
[0003] Saturated or slightly wet steam having a relatively high
pressure (generally between 55 and 78 bars), referred to as main
steam or live-steam, is generated for being expanded in a steam
turbine which is usually a condensation turbine with reheating and
several bodies. The steam is expanded within the turbine
transferring part of its thermal energy, producing mechanical
energy which is in turn converted into electric energy by means of
a generator coupled to the shaft of the turbine. A cycle having
these features can be seen in FIG. 1. The expansion process is
divided into two phases. In the first phase, the steam is expanded
in the high pressure (HP) body of the turbine, from which it exits
with a moisture content of the order of or greater than 10%.
Passing the steam through a moisture separator (MS) is common
practice to remove most of the moisture. After the moisture
separator, the steam is reheated to achieve a temperature higher
than the saturation temperature (between 50 and 80.degree. C.
higher) before expanding it in a second phase in the medium or low
pressure (LP) body. In most power plants today, the reheating
between the two expansion phases is done in two steps. In the first
step, the first reheater (RH1), which is located downstream from
the outlet of the high pressure turbine and from the moisture
separator is a steam-steam exchanger fed by a high pressure turbine
extraction. In the second step, in a second reheater (RH2) also of
the steam-steam type, the steam exiting the first reheater with a
moderate fraction of live-steam is again reheated. The moisture
separator, the first reheater and the second reheater are
integrated in a single equipment made up of an outer shell
containing therein the moisture separator and the tubular bundles
of the reheaters through which the live-steam and the steam from
the turbine extraction circulate. The steam originating from the
first expansion in the turbine circulates through the inside of the
shell and through the outside of the tubular bundles of the
reheaters. The steam at the outlet of the reheating is referred to
as a reheated steam or simply as reheated.
[0004] The objective of reheating in these power plants is to
obtain a lower moisture content in the last steps of expansion in
the low pressure turbine, providing better protection against the
formation of high speed droplets which would damage the blades of
the turbine, thus reducing the availability thereof. Furthermore, a
small increase in the efficiency of the thermal cycle is
achieved.
[0005] Once expanded, the steam is condensed in a condenser cooled
by a relatively cold fluid, either seawater, river water or water
from the cooling towers, depending on the characteristics and
location of the power plant.
[0006] In most power plants, the condensed steam (or simply, the
condensate, as it is commonly known) is pumped to the degasser
after passing through the condensate preheaters (three or four,
depending on the power plant), which are shell-tube exchangers
connected in cascade and fed by different turbine steam
extractions, increasing the temperature of the condensate. The
degasser where the non-condensable gases dissolved in the
condensate are removed is also fed by a turbine steam extraction.
Feed water is again pumped from the degasser to the reactor or the
steam generator, depending on the type of power plant, by means of
the feed water pumps after passing through the feed water
preheaters (two or three, depending on the power plant), which are
also shell-tube exchangers connected in cascade and fed by the
turbine steam extractions.
[0007] The objective of both feed water and condensate preheaters
is to produce a more regenerative and therefore more efficient
cycle.
[0008] One of the main features of most of the nuclear power plants
the reactors of which are cooled by water is that due to the
limitations inherent to reactor design, the live-steam is at
limited pressures and at the corresponding saturation temperature.
For example, in pressurized light water reactors, pressure and
temperature values of between 55 and 78 bars and between 270 and
293.degree. C., respectively, are common. Therefore, the yield of
the thermal cycle of these nuclear power plants is less than that
of a modern fossil fuel power plant (a difference of more than 10
percentage points).
[0009] As a result, the nuclear power plants require wet steam
turbines because, since they do not have superheated steam at the
inlet of the high pressure body and provide a low reheating in the
low pressure bodies, they operate with steam that reaches a higher
moisture content. The water droplets contained in the steam cause a
drop in the yield of the steam turbine as they hit against the
blades, in addition to the erosion of those blades, and the high
vibrations and stresses in the last steps of expansion. On the
other hand, since the yield of the thermal cycle is lower, these
turbines operate with higher steam mass flow rates than the
turbines of a cycle with high superheating and reheating (the steam
has less thermal energy per unit of mass) in order to generate high
electrical outputs in the power plant. The higher the steam flow
rate, the longer the blades of the last step of expansion must be
so that the losses in unloading due to the speed of the steam are
not increased. This results in high mechanical stress values in the
blades due to the greater moment of inertia. To prevent same, steam
turbines usually have two or three low pressure bodies (where the
volumetric flow rates are higher, and the blades are therefore
longer) of symmetrical dual flow turbine connected in tandem.
Normally in addition to steam, water is extracted in the last steps
of expansion (and sometimes also in the high and medium pressure
bodies) by means of internal moisture separators which are used to
feed the condensate preheaters that work at a lower temperature.
All these problems arise despite the fact that to reduce the
effects of the speed of the water droplets, it is common practice
to reduce the rotational speed of the turbine to 1800 or 1500 rpm,
depending on the frequency of the grid to which the power plant is
connected.
[0010] In some nuclear power plants with pressurized water
reactors, in order to reduce the size of the steam generators (one
of the largest and most expensive pieces of equipment in these
installations), the decision is made to reduce the steam generating
pressure by increasing the difference between the temperature of
the water exiting the reactor and the temperature of the steam
exiting the steam generator, thus reducing their cost but
sacrificing the yield of the thermal cycle.
[0011] Ultimately, the analysis of the state of the art of nuclear
power plants the reactors of which are cooled by water shows that
more efficient thermal cycles would be desirable in order to
increase the efficiency of the power plant and to reduce the
moisture content of the expanded steam to thus increase the service
life and the availability of steam turbines and simplify their
design.
[0012] For this purpose, methods have been developed which attempt
to improve the yield of the thermal cycle by superheating or
reheating the steam by means of different methods. Therefore, for
example, patent GB 1,029,151 develops for PWR power plants a method
of superheating the main steam and eventual reheating of the
reheated steam by means of using part of the energy of the primary
circuit through an auxiliary circuit with thermal fluid. Said
thermal fluid in turn transfers the received heat to an exchanger
which superheats the main steam. An exchanger which reheats the
reheated steam before the second expansion in the low pressure
turbine is placed in series with the aforementioned exchanger.
However, in this manner the superheating of the steam before the
high pressure body takes priority, whereby an optimum increase in
the yield is not achieved.
[0013] According to the configuration described in patent GB
1,029,151, the temperature increase available for superheating the
steam is determined by the temperature difference between the
primary circuit and the main steam. This difference is relatively
small whereby, even by including the necessary temperature jumps in
the exchangers of the auxiliary circuit (primary circuit
exchanger--auxiliary circuit and auxiliary circuit exchanger--main
steam), the increase in steam temperature, and thus the main
advantages of superheating and reheating, are limited: the increase
in the efficiency of the cycle, and reduction in moisture content
at the exhaust outlet of the high pressure turbine, preventing the
formation of high speed droplets which would damage the blades.
OBJECT OF THE INVENTION
[0014] The object of the invention is to overcome the technical
problems mentioned in the preceding section. To that end, the
invention proposes a method for increasing the efficiency of
electric power generation in pressurized water nuclear power plants
comprising the steps of: superheating the saturated or slightly wet
steam originating from the steam generator before entering a
turbine; reheating said steam with a high pressure turbine
extraction and reheating it again using live-steam from the
reactor; the steam reheated in the preceding step is again
reheated, exchanging heat with a thermal fluid at a higher
temperature; this reheated steam is expanded in the low body of the
steam turbine, is then condensed and recirculated to the steam
generators after heating with water steam originating from turbine
extractions. The superheating in the first step and the reheating
before the low turbine are performed by means of an auxiliary
thermal fluid circuit, with the streams for the superheating and
the reheating working in parallel. The exchange with the auxiliary
circuit can be performed by means of pressurized water (at a higher
temperature) coming in turn from a second auxiliary circuit which
diverts part of the water from the reactor to an exchanger.
Alternatively, the energy source or sources used for the
superheating and reheating auxiliary circuit can be external to the
power plant, renewable sources being preferred (but not
necessary).
[0015] As a result of the parallel configuration of the
superheating and reheating exchangers, the temperature of the
auxiliary circuit available for the reheating is higher with
respect to a series configuration such as that described in the
state of the art.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For the purpose of aiding to better understand the features
of the invention according to a preferred practical embodiment
thereof, a set of drawings is attached to the following description
in which the following has been depicted with an illustrative
character:
[0017] FIG. 1 shows the schematic configuration of a pressurized
light water (PWR) nuclear power plant.
[0018] FIG. 2 shows a configuration according to the invention in
which part of the thermal energy of the reactor is used for
superheating the main steam and reheating the reheated steam.
[0019] FIG. 3 is a diagram of the configuration of a PWR power
plant according to an embodiment which directly uses the thermal
fluid from a solar field for superheating the main steam and
reheating the reheated steam.
[0020] FIG. 4 shows a diagram of the PWR configuration according to
an embodiment which uses the energy from a solar field by means of
an auxiliary pressurized water circuit for superheating the main
steam and reheating the reheated steam.
[0021] FIG. 5 shows a configuration in which the energy originating
from two different heat sources located in series is used for
superheating the main steam and reheating the steam reheated by
means of a single auxiliary circuit.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The method of the invention comprises a step within the
thermal cycle which consists of increasing the degree of reheating
the steam such that the yield of the medium and/or low pressure
body of the steam turbine increases, further reducing the moisture
contained in the steam in the last steps of the expansion, with the
subsequent effect of reducing the high speed impact of the droplets
on the blades. Associated problems of vibrations and wear in the
blades are thus reduced and the availability of the steam turbine,
and accordingly the availability of the entire nuclear power plant,
increases. The method also comprises a step of superheating the
steam as it reaches the steam turbine, therefore increasing the
thermal energy of the live-steam per unit of mass and the yield of
the thermal cycle.
[0023] With External Heat Source
[0024] In a first aspect, the invention is applicable when one or
several energy sources external to the actual reactor of the power
plant with a hot spot temperature higher than that of the hot water
of the primary exiting the reactor at a high pressure are available
or can be built in the vicinity of the plant. These energy sources
could be renewable sources, fossil fuel sources or even nuclear
sources.
[0025] In this case, the method for increasing the energy
efficiency of the heat cycles of the nuclear power plants comprises
the following steps (FIG. 3): [0026] a) The saturated or slightly
wet steam produced in the steam generator, after the feed water is
heated in a regenerative cycle through which it is pumped under
pressure to the steam generator, is superheated in a superheater
(SH1) by means of a heat exchange fluid which is in turn heated
from one or several external heat sources. The fluid can be
pressurized water or any other fluid, provided that it is at a
higher temperature than the saturated live-steam. [0027] b) The
superheated steam is expanded in the high pressure body of the
steam turbine, steam with an intermediate or low pressure and a
moisture content generally less than 10% being obtained. [0028] c)
The steam obtained in step b is dried by means of at least one
moisture separator (MS). [0029] d) The steam with intermediate
pressure dried in step c is reheated in a first reheater (RH1) fed
by a high pressure turbine extraction. [0030] e) The steam reheated
in the first instance in step d is additionally reheated in an
exchanger (RH2) in which part of the live-steam diverted before the
superheater (SH1) of step a) acts as a hot fluid. [0031] f) The
steam reheated in the second instance is additionally reheated in a
thermal fluid--steam exchanger (RH3 in FIG. 3), in which the
heating fluid is a thermal fluid (the thermal fluid of step a which
in turn receives the energy from the external sources (similar to
or different from the sources in paragraph a). [0032] g) The
reheated steam obtained in step f is reheated in the low pressure
body of the steam turbine directly. Alternatively, the expansion
can be done in a medium pressure body at the outlet of which it
passes to the low turbine, in which case the inlet pressure in this
low turbine is less than that of the turbines lacking this step of
medium pressure. All this depends on the configuration of the
turbine with which the power plant will be equipped. [0033] h) The
condensed water from the secondary used for generating the
saturated steam of step a is heated by means of using a plurality
of turbine steam extractions for exchanging heat with said water of
the secondary and heating the water before being pumped to the
steam generator or generators. [0034] i) Once the heat exchange
fluid has transferred its heat in the exchangers (SH1 and RH3), it
returns to the external heat sources (one or several sources) where
thermal energy is again obtained.
[0035] The energy provided from outside the nuclear power plant for
improving steam quality by means of superheating and reheating the
steam can be obtained from one of the sources mentioned below or
from other similar sources: [0036] 1) Renewable energies: solar
thermal energy with parabolic trough collectors or a central tower
with heliostats or other similar systems, being able to use heat
storage systems such as solid or molten salts, energy originating
from solid urban waste, energy originating from either forest
biomass or biomass from crops for that purpose, solid, liquid and
gas fuels originating from biomass, geothermal energy, etc. [0037]
2) Non-renewable energies: Energy originating from industrial
processes: refineries and other chemical industries, iron and steel
mills, thermal power plants for generating electricity with the
simultaneous production of thermal energy; plants intended for
generating thermal fluid from fossil fuels. [0038] 3) Nuclear
energy, including energy from reactors known as fast-breeder
reactors, in which various fluids (helium, liquid sodium, etc.)
which allow achieving high temperatures are used as coolant.
[0039] A difference and an improvement with respect to the prior
art which uses external heat sources for superheating or reheating
the steam is the use of an auxiliary circuit of any thermal fluid
which allows connecting several heat sources in parallel or in
series, and which maintains the nuclear circuit isolated from the
external heat source or sources. Furthermore, this configuration
allows the inclusion of heat sources originating from renewable
energies, such as solar energy or biomass.
[0040] Without External Heat Source
[0041] In the event that a heat source external to the actual
reactor is not available, the invention will only be applicable to
nuclear power plants with pressurized water reactors (PWR), with
water or heavy water. In that case, the method for increasing the
energy efficiency of the heat cycles of the nuclear power plants
with such pressurized water reactors comprises the following steps
shown in FIG. 2: [0042] a) Part of the water of the primary which
is pressurized and heated by heat transfer in the reactor is
directed to a water exchanger of the primary-auxiliary thermal
fluid (A-A) located in the containment building, the rest is
directed to the steam generators, where it circulates through a
bundle of tubes for exchanging thermal energy with the water of the
secondary originating from the steam turbine after being heated in
a regenerative cycle, from which it is pumped under pressure to the
steam generator to convert it into a saturated or slightly wet
steam. [0043] b) The auxiliary thermal fluid which is heated in the
exchanger mentioned in the preceding step (A-A) is divided into two
streams: the first one superheats the main steam (see point c), and
the second one reheats the reheated steam (see point h) [0044] c)
The saturated or slightly wet steam generated in the steam
generator is superheated by means of an exchanger (SH1), in which
the hot fluid is the auxiliary thermal fluid originating from the
exchanger located in the containment building water of the
primary--thermal fluid (A-A). [0045] d) The superheated steam is
expanded in the high pressure body of the steam turbine, steam with
an intermediate pressure with moisture that can reach the order of
10% in the last step of expansion being obtained. [0046] e) The
steam obtained in step d is dried by means of at least one moisture
separator. [0047] f) The steam with intermediate pressure dried in
step e is reheated in a first reheater fed by a high pressure
turbine extraction (RH1). [0048] g) The reheated steam in step f)
is additionally reheated in a second reheater fed with live-steam
originating from the steam generator or generators (RH2). [0049] h)
The steam reheated in the second instance is again reheated in a
third reheater (RH3), heated by means of the second stream of
auxiliary thermal fluid of point b. [0050] i) The reheated steam
obtained in step e is expanded in the low pressure body of the
steam turbine directly. Alternatively, the expansion can be done in
a medium pressure body at the outlet of which it passes to the low
turbine, in which case the inlet pressure in this low turbine is
less than that of the turbines lacking this step of medium
pressure. [0051] j) The condensed water of the secondary used for
generating the saturated steam of step a) is heated by means of
using a plurality of turbine steam extractions for exchanging heat
with said water of the secondary and heating the water before being
pumped to the steam generator or generators. [0052] k) Once the
auxiliary thermal fluid of points c and h has transferred its heat
to the live-steam and to the reheated steam, respectively, it
returns to the water exchanger of the primary-auxiliary thermal
fluid (A-A) to be heated again.
[0053] By way of example, the conventional configuration of a
pressurized light water nuclear power plant (FIG. 1) is compared
with the configuration of a pressurized light water power plant
according to the invention, with an external solar type heat source
in the auxiliary circuit (FIG. 4).
[0054] In the conventional configuration, the main saturated steam
(78 bars) is expanded in the high pressure body of the steam
turbine to a pressure of 11 bars, reaching a moisture content of
15.2% at the outlet. After passing through the moisture separator,
a first reheating with high pressure turbine extraction steam and a
second reheating with main steam, the steam reaches the conditions
of 10.47 bars and a temperature of 278.5.degree. C. before being
introduced in the low pressure steam turbine where it is finally
expanded to the pressure of 0.05 bars, with a moisture content of
13.2%.
[0055] According to an implementation of the invention, the
superheating and reheating of the steam will be performed by means
of respective heat exchangers through which high pressure hot water
streams (325.degree. C. and 140 bars) originating from the
exchangers from the adjacent solar field will circulate.
[0056] The pressurized water stream which reaches the superheater
(SH in FIG. 4) transfer its heat to the main steam, reducing its
temperature to 298.degree. C. The main steam achieves a
superheating of 7.degree. C. before entering the high pressure body
of the steam turbine where it is expanded to a pressure of 11.2
bars, achieving a moisture content of 13.5%. After the moisture
separator, a first reheating with extraction steam of the high
pressure body of the steam turbine and a second reheating with main
steam, the steam is again reheated in a third reheater with the
other pressurized water stream, achieving the steam conditions of
10.47 bars and 300.5.degree. C. The steam is then expanded in the
turbine to the pressure of 0.05 bars, achieving a moisture content
of 12.2%.
[0057] Once the pressurized hot water streams transfer their heat,
they are mixed and returned to the solar heat source where they are
heated by means of thermal fluid from the adjacent solar field,
achieving the temperature of 325.degree. C. again.
[0058] In summary, a table with the most significant parameters of
the two configurations is shown:
TABLE-US-00001 Units Conventional Invention Reactor type PWR PWR
Reactor thermal power MWt 4,300 4,300 Pressure of main bar 75.6
75.6 steam at the inlet of the steam turbine Temperature of main
.degree. C. 291 298.7 steam at the inlet of the steam turbine Titer
of the %/.degree. C. 99.8% 7.7.degree. C. steam/superheating of the
steam Moisture after the % 15.2 13.5 first expansion Pressure of
reheated bar 10.47 10.47 steam at the inlet of the steam turbine
Reheated steam .degree. C. 278.5 300.5 temperature Superheating of
the .degree. C. 96.6 118.6 steam Moisture content after % 13.2 12.2
the second expansion Electrical output in MWe 1,671.6 1,746.9 the
generator of the steam turbine Thermal power provided MWt NA 178
from the adjacent solar field Gross yield of the power % 38.87%
39.01% plant considering all the heat sources Additional power from
MWe NA 75.3 the external energy Gross yield of the % NA 42.3%
additional external (solar) energy
[0059] Based on Table 1, it is directly deduced that a slight
increase in the degree of superheating, both of the main steam and
of the reheated steam, results in the technical effects mentioned
above. On one hand, it increases the overall yield of the plant, in
this case by 0.36% (0.14 point difference); and on the other, it
reduces the moisture at the exhaust outlet, both in the high
pressure turbine and in the low pressure turbine, thus improving
the yield thereof and reducing the effect of erosion of the
blades.
[0060] The parallel configuration of the two pressurized water
streams allows both the superheated steam and the reheated steam to
reach the maximum available temperature of the external source
which is close to 300.degree. C. with the necessary temperature
jumps of the auxiliary intermediate exchangers. With a series
configuration, first passing through the superheater (in which the
same steam temperature would be achieved), the temperature of the
pressurized water stream would drop, therefore, the maximum
temperature available for the reheater would be less. This would
cause the temperature of the steam reheated before entering the
turbine to be lower than 300.degree. C., and therefore the increase
in efficiency and the reduction in moisture will also be less.
* * * * *