Patent Application
20150204315
The invention pertains to the field of methods of operation at thermoelectric solar plants and within these methods those specially designed to make use of the plant even when there is no sunshine (transient or night periods).
There are several possible classifications for the steam turbines. One depends on the type of steam for which they have been designed, such as saturated steam turbines or superheated steam turbines. The other depends on the steam pressure at the turbine inlet. If the cycle has two turbines, the first will be high-pressure and the second will be low-pressure. If there is also an intermediate turbine, it is to be called medium-pressure. These two classifications are therefore independent and all the possible combinations exist between them, such as high-pressure superheated steam turbines or high-pressure saturated steam turbines.
The difference between superheated steam and saturated steam turbines is construction-related and notable, because internally they differ in important ways in terms of material, size, length and orientation of the blades and even in the size of these turbines (being higher in superheated steam turbines than in the saturated steam turbines at the same mass flow). The first blades of the superheated steam turbines are designed to withstand very high temperatures while the last blades are designed to prevent damage when the steam starts to have humidity (as occurs with saturated vapour). For saturated steam turbines the humidity is higher in all the blades, so that this is taken into account in the manufacturing material, especially in the last crowns where the blades are reinforced with layers of chromium. For this reason, until now if a cycle used saturated steam, it was imperative to use saturated steam turbines and if it used superheated steam, it was necessary to use superheated steam turbines.
As a result, when until now the literature described a specific power cycle, the turbines tended to be designated by the inlet pressure of the steam (high-, medium- or low-pressure), obviating in many cases the designation according to the steam type that feeds them as this is implied by the properties of the fluid.
In power cycles in which the inlet fluid to the high-pressure turbine is superheated steam, it is common to superheat the high- and low-pressure (or medium-pressure) turbines, although at the outlet of the high-pressure turbine, the steam continues to have a minimum superheating. This is done because the cycle efficiency is increased significantly as the superheating of the steam is increased (higher temperature and higher pressure in the steam increases the enthalpy difference in expansion and therefore the thermal efficiency of the Rankine cycle).
Several different patents feature different optimised Rankine cycle configurations in order to increase the efficiency of said cycle. These patents are detailed below.
The U.S.20100071366 patent involves the generation of saturated steam, solar superheating and the sending of this superheated steam to a superheated steam turbine. It mentions the possibility of sending the saturated steam directly to the turbine, but that this would require changing the turbine to a saturated steam turbine, since the superheated steam turbine could not be used.
In the cycle in the U.S.20090260359 (A1) patent, superheated steam is generated and sent to a high-pressure turbine. The steam at the outlet of this turbine is reheated with solar input and sent to the low-pressure turbine. The possibility of sending the steam directly to the low-pressure turbine without passing through the superheater is also mentioned. Since in both modes of operation described both turbines always receive superheated steam, the turbines used are superheated steam turbines, for the construction-related reasons explained above.
In the WO2011119413 (A2) patent, the turbines are fed only with superheated steam. A heat transfer fluid is used to superheat the steam prior to its introduction into the high-pressure turbine, and it is reheated before being placed into the low-pressure turbine.
In the U.S. Pat. No. 5,754,613 patent, a steam cycle for a nuclear power plant is proposed. In this cycle, saturated steam is fed into a high-pressure turbine, at the outlet of which this steam is superheated in a reheater in order to feed a low-pressure turbine with superheated steam. In the cycle described in the U.S.20060248897 (A1) patent, only superheated steam is used. This is a combined cycle, in the steam cycle of which the high-pressure turbine is fed with steam that has been superheated with the exhaust fumes. After expanding in this turbine, the steam is superheated again using gas in order to introduce it into the low-pressure turbine.
As in the previous patent, in the cycle described in the U.S. Pat. No. 5,111,662 (A) patent, only superheated steam is used. This is also a combined cycle, in the steam cycle of which the high-pressure turbine is fed with steam that has been superheated with the exhaust fumes from the gas cycle. After expanding in this turbine, the steam is superheated again using gas in order to introduce it into the low-pressure turbine.
The different configurations proposed in these patents, which are applicable to all types of electricity generating plants (including solar plants), feature high-pressure turbines into which either saturated or superheated steam enters (normally with over 100° C. of superheating). However, in none of the patents is the possibility mentioned of feeding the same turbine with both types of steam depending on the operating mode, but rather that the same turbine is always powered with saturated steam or superheated steam. In the U.S.20100071366 patent, in which the option to feed the turbine with saturated steam instead of superheated steam is presented, it is explained that for this operation the type of turbine would need to be changed.
In conventional power plants, the plant configuration is designed for only one of the steam conditions (either saturated or superheated) and the fuel is a controllable variable, so it is easy to generate steam which always has the same characteristics of pressure and temperature.
Thermoelectric solar power plants, however, need to adapt to the fact that the solar resource is limited, unlike conventional power plants where fuel is continuously available. To do this, it has different storage systems that allow operation during transient or night periods, when the solar resource is not available. It is well known that it is very inefficient to store energy in the form of superheated steam due to its high specific volume, which causes it to require a larger area for heat transfer, and that more expensive materials are required due to the high temperatures. For these reasons, it is advisable for energy to be stored either in the form of pressurised saturated liquid or using another fluid (typically molten salts).
One of the drawbacks to storing energy using another fluid is that it drives up the price and complicates the operation of the power generation plant due to the need to install new systems for the operation of said fluid (heat exchanger etc.). Moreover, since only steam can be generated in steam cycle turbines, if the stored energy is contained in another fluid, this energy needs to be re-transferred to the water to generate the steam which is to be sent to the turbine. This energy exchange between water and the other fluid always involves an inherent loss of energy (heat loss) and exergy (loss of pressure and temperature).
To avoid using the other fluid as a storage medium, it is possible to store the excess energy as pressurised saturated liquid.
Since the storage is performed on pressurised saturated liquid and the target cycle is a high efficiency cycle, a Rankine cycle at high pressures and temperatures (superheated steam turbine), until now solutions and possible ways of using this type of storage efficiently and easily have been sought.
One example of how to efficiently perform this use is described in the ES 2350221 patent. The saturated liquid is stored at high pressure (typically 100 bar) and is expanded and cooled (typically to 40 bar and 250° C.) resulting in saturated steam, which is then superheated to the minimum superheat degree required by the turbine until a stream of about 40 bar and 300° C. is obtained. The heat for superheating is provided by another stream of saturated steam at around 100 bar and 310° C., also from the same storage system. With the configuration described above, the cycle efficiencies achieved with the conditions indicated are about 35% during the accumulator discharge stage (unloading of storage tanks), because the stream entering the high-pressure turbine is only about 40 bar and the stream flowing into the medium-pressure turbine is about 8 bar. In addition, the system operation in this mode is quite complex since it is necessary to superheat the stream entering the high pressure turbine and then the outlet stream if its entry is desired later in a medium- or low-pressure turbine.
Therefore, the present invention aims to provide a less complex method of operation, and also raise cycle efficiency above 30% during the accumulator discharge stage. For this, saturated steam will be used directly from the high-pressure superheated steam turbine.
The invention consists of a method of operating a thermoelectric solar power plant that allows operation of the high-, medium- and low-pressure superheating turbines with both superheated steam and saturated steam. The plant has to have energy storage tanks (also called accumulators) of saturated liquid at high pressure.
As mentioned above, in thermoelectric solar power plants for electricity generation, being able to generate superheated steam and saturated steam in the same plant would reduce costs and solve many operational and optimisation-related problems because it is possible to store the required energy in the form of pressurised saturated fluid (capable of feeding accumulators with both superheated and saturated steam), without needing to use other fluids which, although they allow the generation of superheated steam, complicate the operation of the plant, increasing its costs and decreasing the overall yields since in each heat transfer from one fluid to another there are always energy losses.
This invention describes the method of operation of a thermoelectric solar power plant that has at least one superheated steam turbine and preferably two or three turbines (of high- and low-pressure or high-, medium- and low-pressure), with at least the high pressure turbine using superheated steam.
It also offers saturated steam generation solar power systems, superheated steam generation solar power systems, a solar reheater or the use of input from a conventional source (an intermediate plant between a high- and low-pressure turbine which enables us to raise the steam temperature at the outlet of the high-pressure turbine) and a set of storage tanks (accumulators).
The plant configuration described here is based on a Rankine superheated steam cycle. The method of operation under normal conditions, i.e. when operating with sunshine, causes the high-pressure turbine inlet (typically over 100 bar) to have superheated steam temperatures above 500° C. At the outlet of the high-pressure turbine, slightly superheated steam is obtained (typically about 20-25 bar) which is taken to the reheater (where again power is lost to the fluid burning the fuel or with the solar field) to bring the fluid back up to 500° C. or more. The fluid in these conditions is taken to the medium- and low-pressure turbine to produce electricity after being optimally expanded to a pressure of about 100 mbar.
The operation method proposed below refers to the mode of operation of storage discharge, i.e. transient operation (clouds) or absence of sunshine (night time). The new operating configuration proposes generating saturated steam directly in the high-pressure superheated steam turbine at a pressure of 20 bar or higher and temperature corresponding to that of the water saturation at the given inlet pressure (preferably 90 bar and 300° C.) coming from the storage system. Prior to entering the high-pressure superheated steam turbine, the saturated steam is passed through a moisture separator.
The outlet stream of the high-pressure turbine is at a lower pressure than the inlet stream (preferably 20 bar or higher) and with some moisture (saturation state). This stream could be taken to a medium-pressure saturated steam turbine or superheated in a reheater to temperatures between 280 and 350° C., depending on the pressure in the storage tanks, and introduced into a medium-pressure superheated steam turbine to produce electricity. Superheating is performed by exchanging heat in a reheater with another saturated steam stream at the same pressure or higher than the inlet stream of saturated steam into the high-pressure superheated steam turbine, so that the final superheating obtained is at least 50° C. (superheating stream pressures of preferably 90 bar or higher). In the event that said steam stream should proceed from the storage system, the choice of the superheating temperature is a function of the pressure of saturated liquid in the storage tanks, since this pressure in turn determines the pressure (and temperature) of the steam stream with which the steam sent to the second turbine is superheated. In this case, the saturated steam stream with which it is superheated is at a lower pressure than the pressure inside the tank. Another possibility for this superheating is the use of an auxiliary boiler.
Finally, the steam leaving the medium-pressure turbine may be taken to a low-pressure saturated or superheated steam turbine.
The advantages provided by the method of operation proposed for thermoelectric solar power plants are:
This new method of operation optimises the cost of the plant, cycle efficiency and the operation of the plant when operating under normal conditions and when harnessing the steam coming from the storage system or during the operation in solar mode with transient periods.
To complement the preceding description and in order to aid a better understanding of it, a set of drawings has been attached which, as a non-limiting illustration, show the following:
To achieve a better understanding of the invention, below is a description of two modes of operation for a tower thermoelectric solar plant. First, normal plant operation is described, i.e. operation in periods with sunshine, reflected in
This preferred embodiment is a tower thermoelectric solar plant of 400 MWe.
The method of normal operation, namely with sunshine, of a state-of-the-art plant as shown in
As for the method of the invention, the general steps, taking into account that there is at least one high-pressure superheated steam turbine, are:
If there is also a medium-pressure superheating turbine:
If instead of being superheated, the medium-pressure turbine uses saturated steam, the superheating stage would be avoided, leaving the following:
If, in addition, the plant has a low-pressure turbine following the medium-pressure turbine, the last stages would be:
Below is a description of a preferred operating method in the absence of sunshine or during accumulator discharge, with the aid of
Thus, during the operation mode saturated steam is introduced from accumulators into superheated steam turbines which in normal operation mode receive superheated steam.
| Number | Date | Country | Kind |
|---|---|---|---|
| P201200750 | Jul 2012 | ES | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/ES2013/000171 | 7/11/2013 | WO | 00 |
