THERMAL-ENERGY-STORAGE TANK WITH INTEGRATED STEAM GENERATOR

A thermal-energy storage tank (1) comprising a containing structure (2) designed to house a store of thermovector fluid in the liquid state, a regenerating circuit (6) designed to draw the thermovector fluid from a bottom of the containing structure (2) and, once heated outside the tank, to deposit it in a surface portion of the store of thermovector fluid, at least one steam generator (13) comprising a heat exchanger (14, 16b) with vertical extension housed within the containing structure (2) and having at least one top opening (18) designed for inlet of the thermovector fluid and a bottom opening designed for outlet of the thermovector fluid (15).

BEST MODE FOR CARRYING OUT THE INVENTION

Designated as a whole by1inFIG. 1is a thermal-energy storage tank according to the present invention.

The tank1comprises a containing structure2, which has a number of openings (a first opening3and a second opening4are shown in the figure) made in a top cap5thereof.

The tank1comprises a regenerating circuit6for heating a thermovector fluid and for thermal storage, which in the case in point is constituted by a mixture of molten salts constituted for 60% by sodium nitrate and for 40% by potassium nitrate, but which can be even constituted by other types of mixtures of salts or fluid a with high thermal capacity.

In what follows, by the term “cold molten salts” is meant the mixture defined above at a temperature of approximately 290° C., whilst by the term “hot molten salts” is meant the mixture defined above at a temperature of approximately 550° C.

The regenerating circuit6in turn comprises an intake pump7, the intake mouth of which8is housed in the proximity of the bottom of the containing structure2, a pipe9for intake of the cold molten salts arranged so as to traverse the first opening3and of which a first part9ais housed inside and a second part9bis housed outside the containing structure2, a pipe10for pouring the hot molten salts, which is also arranged so as to traverse the first opening3and of which a first part10ais housed outside and a second part10bis housed inside the containing structure2, and means for heating the molten salts designated as a whole by11and set between the intake pipe9and the pouring pipe10.

The heating means11can be identified, by way of example, by a solar concentrator (e.g., a field of linear parabolic mirrors) or with a biomass boiler.

In particular, the intake pipe9, the pouring pipe10, and the set of electrical components of the pump7traverse a flange set so as to close the first opening3to guarantee insulation from outside of the molten salts set inside the containing structure2.

Moreover, by the term “cold molten salts” is meant the mixture defined above at a temperature of approximately 290° C., whereas by the term “hot molten salts” is meant the mixture defined above at a temperature of approximately 550° C.

The molten salts arrange themselves within the containing structure2according to a temperature gradient, whereby the cooler molten salts arrange themselves on the bottom of the containing structure and the hotter molten salts arrange themselves on the surface of the store of molten salts present in the containing structure2.

Finally, the tank1comprises a steam generator13housed inside the containing structure2.

As illustrated inFIGS. 2aand2b, the steam generator13comprises a cylindrical shell14, practically completely immersed in use in the molten salts, a diffuser15connected in a fluid-tight way to a bottom end14aof the shell14, and three water/steam tubes16. Each of the tubes16is constituted by a delivery portion16a,from which the under-cooled water enters at a temperature of 240° C., a heat-exchange portion16bwith helical conformation, housed inside, and coaxial with, the shell14, and a return portion16c,through which the superheated steam comes out at a temperature of 520° C.

On account of their relative arrangement, visible inFIG. 2ain cross-sectional view are a portion16aand a portion16c, whereas visible inFIG. 2bin cross-sectional view are two portions16a.

The portions16care visible both entirely and in cross-sectional view inFIG. 2a.

The respective delivery portions16aand return portions16ctraverse a flange17set for closing the second opening4in order, as described above in connection to the flange12, to guarantee insulation of the molten salts set inside the containing structure2from the outside world.

Unlike what has been described above, the various delivery portions16aand return portions16cof the tubes16can be replaced, respectively, by a single delivery header and a single return header. In other words, the tube nest, in which there takes place heat exchange with the molten salts, has a first end that branches off from the delivery header for the under-cooled water and a second end that converges in a return header for the steam. In this way, both the delivery header and the return header traverse the flange17.

In the shell14a plurality of openings18are made in the proximity of a top end14bof the shell14itself. As will be described in what follows, the steam generator13will be housed in the containing structure2in such a way that the openings18will be at the level of the hot molten salts, and the diffuser15will be at the level of the cold molten salts.

In this way, the hot molten salts enter the shell14through the openings18and exit therefrom as cold molten salts from the diffuser15after having performed their function as primary fluid in the heat-exchange process.

Basically, the ensemble constituted by the shell14and the heat-exchange portions16bdefines as a whole a tube-nest heat exchanger and shell.

In the specific case, in each of the tubes16the delivery portion16aextends linearly from the outside of the containing structure2as far as the bottom end14aof the shell14, the heat-exchange portion16bextends in a helical conformation from the bottom end14aof the shell14as far as the openings18, and the return portion16cextends in a linear conformation from the openings18and exits from the containing structure2through the flange17to reach the utilizer provided for the production of energy, such as, for example, a turbine.

Hence, the hot molten salts enter through the openings18within the shell14and surround the three heat-exchange portions16bto perform thermal exchange with the water that flows therein. In particular, the molten salts flow from top downwards yielding heat to the water that flows in countercurrent from the bottom up along the heat-exchange portions16b.The molten salts, during the process of heat exchange, are gradually cooled, thus becoming more heavier and descending towards the diffuser15to come out therefrom at a temperature equal to the temperature of the molten salts present at the level where the diffuser15itself gives out.

In order to guarantee the flow of the molten salts from the openings18to the diffuser15, at least in a first initial operating step, the steam generator13comprises a component, which, by promoting motion of the molten salts, contributes to onset of the natural circulation thereof. According to the example illustrated inFIG. 2a, said component is a propeller19housed upstream of the diffuser15and moved by an actuation rod20, which extends axially with respect to the helical heat-exchange portions16band then outside the containing structure2through the flange17.

The steam generator13further comprises a first thermal-insulation cladding21set around the shell14and a second thermal-insulation jacket22set inside the heat-exchange portions16band outside the delivery portions16a.

The thermal-insulation jackets21and22are obtained by filling the annular gap that they create with alumina balls of 5 mm in diameter immersed in stationary molten salts. The balls of alumina, which is a good thermally insulating material to which the stationary molten salts are equivalent as regards thermal conductivity, have the purpose of improving prevention of flow of the molten salts themselves. The purpose of the insulation jackets is to insulate, respectively, the molten salts that flow within the shell14, in order to guarantee heat exchange thereof exclusively with the water that flows within the heat-exchange portions16b,and the water that flows in the delivery portions16a,so that the water will not undergo a heating process during its inlet path.

The tank forming the subject of the present invention guarantees the production of superheated steam by means of a steam generator without the need to provide external circuits for inlet and outlet of the thermovector fluid.

In fact, the stratification of the molten salts, as thermovector fluid, guarantees the possibility of using a single containing structure for the hot molten salts and for the cold molten salts without the heat of the former being yielded by convection to the latter. It is thus possible to exploit the stratification in temperature of the molten salts immersing therein the steam generator with vertical development in a position such as to receive in the generator the molten salts at the level in which these are hot and release them at the level in which they are cold.

The capacity of stratification of the thermovector fluid plays an essential role also inside the steam generator. In fact, the molten salts penetrate into the shell14through the openings18and, by natural circulation in so far as they cool down and become heavier, traverse with a motion from top downwards the shell14itself yielding heat to the water that flows in countercurrent in the heat-exchange portions16b. Within the steam generator13there is thus no need to force in any way advance of the molten salts in so far as this takes place in a natural way.

In order to prevent any convective motion that might disturb the stratification referred to above, the steam generator must be designed in such a way that the molten salts will exit from the steam generator with a temperature equal to the temperature present at the level of the diffuser from which they physically come out.

The diffuser15must be designed in such a way as to minimize the movements of the fluid stored on the bottom of the containing structure2to prevent the possible onset of undesirable motions of mixing of the thermal stratification in the stored mass.

The only external circuit provided by the tank forming the subject of the present invention is constituted by the regenerating circuit6whereby a constant production of hot molten salts is guaranteed. In fact, the cold molten salts are taken from the bottom of the containing structure2to undergo a heating treatment provided, for example, by means of a set of solar concentrators, and are then fed back into the containing structure2to be deposited almost on the surface of the molten salts, i.e., where these are hotter.

The tank according to the present invention can comprise a plurality of steam generators which are independent of one another and are housed in one and the same containing structure. In this way, using always a single tank in which the thermovector fluid stratifies in temperature, the production of superheated steam, and, consequently, the energy that it is possible to produce with said steam is multiplied.

Another variant as compared to what has been illustrated above consists in the possibility of arranging the steam generator outside the containing structure2, to which it is in any case connected in order to receive therefrom the thermovector fluid at a high temperature and to yield thereto the thermovector fluid that has cooled following upon heat exchange. Also in this case, the cooled thermovector fluid will be introduced into the structure2in a region corresponding to other stratified thermovector fluid having the same temperature.

The tank of the present invention is then provided with particular structures (“traps”) set on its internal walls, in order to trap the molten salt in the vicinity of the walls themselves and prevent them from flowing, thus exploiting the good thermal-insulation properties of the stationary molten salts to provide a heat shield that will minimize the losses of heat towards the outside of the tank itself.

As may be immediately appreciated by a person skilled of the sector, in this way it will be possible to produce a high amount of energy with running and maintenance costs that are considerably reduced as compared to plants according to the known art and, at the same time, it is possible to reduce drastically the necessary overall dimensions.

As emerges clearly, some peculiarities of the tank forming the subject of the present invention as described above cannot constitute a limitation of the patent right sought.

As emerges from the above description the thermal-energy storage tank forming the subject of the present invention enables reduction both of the number of components (a single tank and a single molten-salt circuit) and of the pressurized apparatuses, consequently reducing both the plant costs and the corresponding problems of maintenance and/or replacement. In addition, it guarantees a greater compactness of the plant as a whole, thus leading to a better use of the necessary spaces.