ES2555531A1 - Solar thermal plant with combined supercritical steam generator (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Solar thermal plant with combined supercritical steam generator, comprising: - a receiver (3) of solar thermal concentration, - a circuit through which a first heat transfer fluid fc1 circulates, passing through the receiver (3) of solar thermal concentration - a circuit for circulating a second fluid heat carrier fc2, so that the temperature is less than fc2 fc1 temperature, - a heat exchanger (4) between both circuits - a train (7) for supercritical steam generation, with inputs for fc1, fc2 and a working fluid ft, and comprising several heat exchangers, and - a circuit corresponding to the working fluid ft, with at least one turbine (8) working with the supercritical steam generated in the supercritical steam generation train (7), and a condenser (9), with the optimum temperature ranges of fc1 and fc2, and there is an overlap between them. (Machine-translation by Google Translate, not legally binding)

Description

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a) Towards the hot storage tank of FC2 where it is stored at Tc2

b) Towards the supercritical steam generation train, where energy is communicated to the FT working fluid in the low temperature superheater and in the low temperature superheater in parallel, to join at the outlet of both and circulate through the evaporator and, then from the economizer, reducing its temperature to Tf2 at the outlet of the economizer, continuing to the cold storage tank of FC2.

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Water from the condenser is pumped and sent to the supercritical steam generation train

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 The water is preheated by a series of pre-heaters, so that it reaches a temperature Tp.

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 Water flows successively through the economizer, the evaporator, the low temperature superheater and the high temperature superheater, to reach supercritical conditions of pressure and temperature at the outlet of the high temperature superheater.

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Water is sent to the high pressure turbine, where it performs work by rotating the turbine around its axis.

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 The water leaves the high pressure turbine and is sent back to the supercritical steam generation train, where it circulates successively through the low temperature reheater and high temperature reheater, obtaining superheated steam.

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 The superheated steam is turbinated in a second turbine, expanding and performing work again, until finally reaching the condenser.

The fact that each of the heat transfer fluids yields its energy in a different range of temperatures allows to reach higher temperatures and final pressures in the steam, which translates into an increase in the efficiency of the thermodynamic cycle.

The use of the two heat transfer fluids gives the system greater freedom for the design, due to the fact that both heat transfer fluids can operate in different temperature ranges and are coincident in a part of said ranges. This allows some flexibility to optimize the thermal storage system based on the costs of each of the heat transfer fluids.

The fact of working with a supercritical cycle allows to obtain a cycle performance several points above the performance of subcritical cycles, (it increases from 42% to more than 48%); This implies an increase in production close to 15%. The fact of increasing the performance in the cycle allows to go to smaller sizes in the equipment used in the plant, that is, to achieve the same thermal power of the plant than with a subcritical cycle, with the supercritical cycle we can use smaller receiver sizes , as well as smaller solar field size and smaller number of heliostats for a given electrical power. This means significant savings not only in cost, but also in space, so solar plants can be built on smaller farms.

Likewise, having two circuits with heat transfer fluids that generate steam gives the system some redundancy, allowing the operation of the plant at partial load in the event that one of the previous circuits fails.

Other features and advantages of the present invention will be apparent from the following detailed description of an illustrative and non-limiting embodiment of its object in relation to the accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1: shows a diagram of a solar thermal plant according to an embodiment of the invention.

Fig. 2: shows a detailed scheme of the combined supercritical steam generation train of the thermosolar plant of the invention.

Fig. 3: shows the evolution of the temperatures of one of the heat transfer fluids of the invention, of a state-of-the-art fluid and of the FT working fluid in the work cycle.

The references in figures 1 and 2 correspond to the following elements:

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Cold salt storage tank (first FC1 heat transfer fluid)

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Hot salt storage tank (first heat transfer fluid FC1)

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Solar molten salt receiver (first FC1 heat transfer fluid)

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Heat exchanger sales-salts (first heat transfer fluid FC1 - second heat transfer fluid FC2)

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(FC2) is heated from the first in a heat exchanger 4. Both fluids will yield their thermal energy to the working fluid (FT) of the system, which is preferably water, and will do so in a steam generator. However, each of the heat transfer fluids will yield its energy in a different temperature range, thus allowing higher temperatures and final pressures to be reached in the steam, which translates into an increase in the efficiency of the thermodynamic cycle. Next, the transformation of the thermal energy communicated to the steam into mechanical energy will take place, and subsequently into electrical energy thanks to the movement of the axis of a turbine 8, integral to that of the corresponding generator. These high temperatures and pressures (supercritical) in the steam allow to reach a yield of the Rankine thermodynamic cycle higher than those currently obtained.

The first FC1 heat transfer fluid circulating through the receiver 3 is heated in a certain temperature range, this range being between 450 ° C and 700 ° C, lower than that of conventional molten salt receivers operating from 290 ° C to 565 ° C. This minor gradient

or temperature difference in the longitudinal direction of the tubes of the receiver 3 causes a greater homogenization of the temperatures in each of the receiver panels 3 and therefore the thermal stresses of the material are lower.

In addition, this first heat transfer fluid FC1 allows thermal energy to be stored at a very high temperature (650 ºC - 700 ºC) because it has a high calorific value equal to or greater than 1.54 kJ / kgC at 565ºC of state fluids. art, and equal to or greater than the second FC2 heat transfer fluid by 20%.

On the other hand, the second heat transfer fluid FC2 works in a different temperature range from the first heat transfer fluid FC1, allowing energy storage at (450 ºC - 565 ºC) and, consequently, the optimization of energy storage.

The use of both heat transfer fluids gives the described system greater freedom for the design, because both fluids, being able to operate in different temperature ranges, are also coincident in a part of said range. In the case of the first heat transfer fluid FC1 of the invention, it operates in an optimum range of 450 ° C-700 ° C, while the second heat transfer fluid FC2 of our invention operates optimally in a temperature range of 290 ° C-565 ° C, coinciding with both in a temperature range in which both heat transfer fluids can be operated (450ºC565ºC). This overlap gives us some flexibility to optimize the thermal storage system at the time of plant design based on the cost of

each of the reagents ($ / kWht), since depending on the cost of the fluids used, more or less of one or another fluid can be used in the system. With this in mind, and that the storage temperature of the cold storage tank 1 of the first heat transfer fluid FC1 and the storage temperature of the hot storage tank 6 of the second heat transfer fluid FC2 are coincident, we can vary their value depending on which Be the most expensive fluid.

In the case, for example, in which the first heat transfer fluid FC1 is of a higher cost than the second heat transfer fluid FC2, the temperature difference between the cold tank 1 and the hot tank 2 of the fluid FC1 would be less than the difference of temperature between the cold tank 5 and the hot tank 6 of the FC2 fluid (for example 565 ° C for the cold tank 1 and 700 ° C for the hot tank 2 of the FC1 fluid, compared to 250 ° of the cold tank 5 and 565 ° C of the hot tank 6 of the FC2 fluid, so that less amount of FC1 fluid is used than FC2 fluid in the system because it is more expensive, on the contrary, in the case that FC1 fluid has a lower cost than FC2 fluid the temperature difference between the cold tank 1 and the hot tank 2 of the FC1 fluid would be greater than the temperature difference between the cold tank 5 and the hot tank 6 of the FC2 fluid, thus using less amount of the FC2 fluid than the FC1 fluid (cold tank 1 at 450 ° Cand hot tank 2 at 700 ° C for fluid FC1, and cold tank 5 at 290 ° C and hot tank 6 at 450 ° C for fluid FC2).

The combination of these fluids in the supercritical steam generation train 7 allows finally working with a supercritical steam cycle of very high efficiency, greater than 48%. In particular, a cycle of these characteristics requires higher working pressures and temperatures than usual (above 240 bars, preferably in a pressure range of 260 - 350 bars and in a temperature range of 620 ºC - 680 ºC ), which, with the typical steam generation train configuration with conventional fluids, would not be optimal from a point of view of the temperature difference at the inlet and outlet of the SGS (steam generation system).

Specifically, as we can see in Figure 3, the temperature difference (T1-T3) between a heat transfer fluid of the state of the art and the working fluid FT (preferably, water) at the outlet of the economizer 70, and the temperature difference (T2-T3) between a fluid of the invention and the water at the outlet of the economizer 70, is much greater for the case of the state of the art fluid, which implies that in that case, the use of said fluid is not being optimized. exchanger 4, having to use a larger amount of fluid from this

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of the art to reach the same working fluid temperature FT as in the case of our invention with one or more of the fluids mentioned FC1, FC2.

So that all the advantages explained above can be combined, some key equipment is incorporated in the operation of the plant:

- A heat exchanger 4 between both heat transfer fluids FC1 and FC2.

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 Four types of energy storage tanks, two cold tanks 1, 5 and two hot tanks 2, 6. The storage is done with two different FC1 and FC2 heat transfer fluids, both of which have a good heat capacity (preferably equal to or greater than 1.54 kJ / kgC at 565 ° C).

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A shared supercritical steam generation train 7 (with two heat transfer fluids FC1 and FC2 and a working fluid FT).

The normal operation of the plant would then be as follows:

The first heat transfer fluid FC1 is pumped from its cold storage tank 1 where it can be found at a temperature Tf1 (cold temperature of the FC1 fluid) between 450 ° C and 565 ° C to the top of the tower, where it collects the concentrated radiation and raises its temperature up to Tr (receiver temperature) between 650 ºC - 700 ºC. Subsequently, this FC1 fluid is circulated through three different paths: one towards the supercritical steam generator, another towards the heat exchanger 4 with the second heat transfer fluid FC2 and finally, another towards the hot storage tank 2 at very high temperature Tc1 ( hot fluid temperature FC1) between 650 ºC and 700 ºC.

On the other hand, the second heat transfer fluid FC2 housed in its corresponding cold tank 5 at a Tf2 (cold temperature FC2 fluid) of 290 ºC will be pumped to circulate it through the heat exchanger 4 where it will collect the energy transferred by the first fluid Heat carrier FC1, raising its temperature to the desired value Tc2 (hot fluid temperature FC2) between 450 ºC - 565 ºC. Next, it will follow two paths: one towards the supercritical steam generator and another towards its hot storage tank 6.

In this way, the necessary energy will be stored to allow the production of electricity to continue after the sun has set.

The proposed plant configuration has the following elements:

- Cold storage tank 5 in salts (second FC2 heat transfer fluid)

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- Hot storage tank 6 in salts (second FC2 heat transfer fluid)

- Cold storage tank 1 in salts (first FC1 heat transfer fluid)

-Hot storage tank 2 in salts (first heat transfer fluid FC1)

- Solar receiver 3 of molten salts (first heat transfer fluid FC1)

- Heat exchanger 4 salts-salts (first heat transfer fluid FC1 - second heat transfer fluid FC2)

-Train 7 of combined supercritical steam generation, steam-salts (first heat transfer fluid FC1 - steam; second heat transfer fluid FC2 - steam)

In a preferred embodiment, the configuration would be that shown in Figure 1.

In said figure 1 it is observed that at the exit of each of the cold storage tanks 1, 5 and at the exit of each of the hot storage tanks 2, 6 there is a pump 10, allowing these pumps 10 to pump the first FC1 heat transfer fluid and the second FC2 heat transfer fluid in their respective circuits.

Figure 2 shows the detail of what the combined supercritical steam generation train 7 would be like. It is described below how the system would work together.

First, the molten salts (FC1 fluid) stored at a Tf1 temperature of 550 ° C in a tank 1, are pumped up the tower where a solar receiver is located

3. Said solar receiver 3 is preferably constituted by panels formed in turn by vertical tubes. The solar radiation is concentrated on the outer surface of these tubes, communicating the power necessary to undertake the desired temperature jump in the FC1 fluid, reaching 650 ºC (Tr) at the outlet of the receiver 3. After this, the FC1 fluid already Hot is circulated to three different teams. On the one hand, it is sent to the heat exchanger 4, where it will be cooled to approximately 550 ° C (Tf1), to be then redirected to the cold storage tank 1. On the other hand, it is sent to the supercritical steam generation train 7 entering our preferred embodiment at 650 ºC, where it communicates energy in two of the equipment: the high temperature superheater 73 and the high temperature superheater 75, also cooling down to 550 ºC (Tf1) to be also redirected to the cold storage tank 1. By Finally, the third of the paths behind the solar receiver 3 is to send part of this FC1 fluid directly to the hot storage tank 2, where it will be stored at 650 ºC to dispose of it when there is no contribution

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Claims (1)

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