Patent Application
20150033742
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).
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.
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
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.
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.
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.
The objective of both feed water and condensate preheaters is to produce a more regenerative and therefore more efficient cycle.
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° 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).
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.
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.
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.
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.
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.
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).
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.
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:
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.
With External Heat Source
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.
In this case, the method for increasing the energy efficiency of the heat cycles of the nuclear power plants comprises the following steps (
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:
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.
Without External Heat Source
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
By way of example, the conventional configuration of a pressurized light water nuclear power plant (
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° 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%.
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° C. and 140 bars) originating from the exchangers from the adjacent solar field will circulate.
The pressurized water stream which reaches the superheater (SH in
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° C. again.
In summary, a table with the most significant parameters of the two configurations is shown:
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.
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° 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° C., and therefore the increase in efficiency and the reduction in moisture will also be less.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201230351 | Mar 2012 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2013/070148 | 3/8/2013 | WO | 00 |
