WO2011055307A2

WO2011055307A2 - Apparatus for storage and transmission of thermal energy

Info

Publication number: WO2011055307A2
Application number: PCT/IB2010/054973
Authority: WO
Inventors: Mario Magaldi
Original assignee: Magaldi Industrie S.R.L.
Priority date: 2009-11-03
Filing date: 2010-11-03
Publication date: 2011-05-12
Also published as: ITRM20090565A1; WO2011055307A3

Abstract

A heat energy storage device (1 ) based on a storage means of graphite and having a receiving cavity (13) inside which a radiation of solar origin is concentrated by heliostats, said device (1 ) further comprises tube bundles (17) extending inside said storage means (12) and apt to receive a heat exchange operating fluid. An apparatus for storage and transmission of heat energy of solar origin comprising a plurality of storage devices and of heliostats (40).

Description

APPARATUS FOR STORAGE AND TRANSMISSION OF THERMAL ENERGY

DESCRIPTION

Field of the Invention

The present invention refers to a system for storage of heat energy, in particular of solar origin, and reuse of the same for electric energy production.

Background of the Invention

The known art envisages storage and reuse of solar energy concentrated by stationary or sun-tracking heliostats inside a receiver made of a block of material having high thermal conductivity, associated to a suitably oriented cavity at which the concentrators (heliostats) are aimed and to a heat exchanger for transfer of accumulated heat to a suitable operating fluid, the latter in turn associated to an electric energy generator.

In a system for storage of solar energy in a graphite block, temperatures of the order of 1000 - 2000 °C may be reached.

The temperature upper limit is however determined by the capacity of the exchanger, usually made of metal tubes in which the operating fluid (or carrier fluid, typically water) flows in a liquid state or in a high-temperature vapour state.

In connection to the temperature difference between the fluid being inlet and the exchanger tubes, the thermodynamic conditions of the fluid can vary so fast as to create marked stresses in the metal of the tubes (thermal and mechanical shocks) and such as to subject heat exchangers to extreme physical conditions, with the risk of excessive internal tensions and entailed ruptures.

Moreover, continuity of the flow rate of heat removed from the storage unit is not easy to realize, the storage step being linked to the pattern of atmospheric conditions.

A further drawback of the known systems is then represented by the constructive complexity of the integrated assembly of graphite block - tube bundles.

In general, moreover, the known systems are not optimized in terms of efficiency of exploitation and conversion of the solar heat energy being inlet. Again, as mentioned above, the known systems are scarcely versatile in terms of ability to adapt to downstream energy needs.

Summary of the Invention

Hence, the technical problem underlying the present invention is to obviate the drawbacks mentioned above with reference to the known art.

The present invention provides a device for storage of heat energy of solar origin, an apparatus for storage and transmission of such energy, a related plant for the production of electric energy (power plant) and an associated method for storage of concentrated solar energy and transfer of said energy in the form of heat to an operating fluid utilized in connection to an electric energy generator.

The above-mentioned problem is solved by a device and an apparatus according to the independent claims reported hereinafter.

Preferred features of the invention are set forth in the dependent claims thereof. An important advantage of the invention lies in that it allows to efficiently and reliably carry out a storage of heat energy of solar origin, minimizing the heat stresses of the exchangers, simplifying the step of producing of the storage unit and increasing the system availability in a way less dependent both on atmospheric conditions and the electric energy generator fed by the operating fluid.

In particular, the making of the graphite block as a mixture of carbonaceous particulate and/or graphite micropowders and graphite fragments of specific grain size allows a reduction of the times and a simplification of the steps of manufacturing the storage unit, with an extreme constructive simplicity of the block - tube bundles assembly, and considerably increases also the heat flow transferred to the fluid. In connection to this latter aspect, such implementation modes determine an increase of efficiency of the heat exchange between the storage unit and the operating fluid thanks to the greater contact surface of the storage means with the exchanger tubes, carried out by the use of carbonaceous particulate and/or graphite micropowders as filler for the spheres/graphite fragments mixture. The carbonaceous particulate and/or the graphite micropowders perform the function of filler of interstitial gaps, realizing thermal continuity between the particles of the mixture of graphite spheres/fragments. The efficiency increase of the heat exchange between accumulator and operating fluid is also obtained, according to a preferred variant of the invention, by use of inert gas recirculated inside the storage unit itself.

Moreover, the use of the mixture of carbonaceous particulate and/or graphite micropowders and of graphite fragments exhibiting decreasing grain size allows to use receiving cavities and heat exchangers of various geometry (cylindrical, cubic, spherical, or other), obtaining the dual result of ensuring at all times a broad contact surface between storage means and heat exchanger and simplifying the manufacturing of the carter-storage means-heat exchanger assembly as a whole. Furthermore, the mixed solution of carbonaceous particulate and graphite fragments compensates for the different thermal expansions between the (metal) tube exchanger and the material surrounding it.

Moreover, the invention allows, thanks to the possibility of a selective aiming of the heliostats and the presence of a secondary reflector at the inlet of the receiving cavity of the storage unit, maximum exploitation and storage of solar energy and drastic reduction of efficiency losses.

Moreover, the continuous feeding of inert gas between graphite block and tube bundles allows to make the system extremely versatile even with respect to downstream energy needs.

Alike versatility is possible also thanks to the arrangement in series - with increasing temperatures - of the storage units, which arrangement further allows to drastically limit thermal stresses, and therefore mechanical ones, on the tube bundles, and increase reliability and operating continuity of the equipment, with the possibility of increasing the operating temperature of the storage unit.

Other advantages, features and the operation modes of the present invention will be made apparent in the following detailed description of some embodiments thereof, given by way of example and not for limitative purposes.

Brief description of the figures

Reference will be made to the figures of the annexed drawings, wherein:

- Fig. 1 shows a schematic perspective view of a preferred embodiment of a storage device according to the invention, made of a mixture of carbonaceous particulate and/or graphite micropowders and graphite spheres/fragments exhibiting a decreasing, preferably bimodal grain size distribution;

Fig. 1A shows a magnification of the mixture of carbonaceous particulate and/or micropowders and spheres/graphite fragments of the device of Fig. 1 ;

Fig. 2 shows a schematic sectional front view of the device of Fig. 1 , highlighting recirculation of a stream of inert gas on the storage unit;

Fig. 2A shows an enlarged detail of an exchange tube of the device of Fig. 2, in a perspective view and in cross-section;

Fig. 3 shows a schematic sectional view of the device of Fig. 1 , equipped, according to one of the aspects of the invention, with a parabolic reflector/concentrator at the inlet of the receiving cavity;

Fig. 3A shows the same view of Fig. 3, in which it is highlighted also the operation of the parabolic reflector/concentrator in association with the heliostats;

Fig. 4 shows part of an apparatus for storage and transmission of solar heat energy according to a preferred embodiment of the invention, which incorporates two storage devices according to Fig. 1 , thermally connected in series and each with its own receiving cavity, and provides a recirculation of inert gas;

Fig. 4A shows part of an apparatus for storage and transmission of solar heat energy according to a different embodiment of the invention, which incorporates two storage devices according to Fig. 1 , thermally connected in series and having a common receiving cavity;

Fig. 5 shows a variant of a storage module of the apparatus of which at Fig. 4, based on the storage device of Figs. 1-3 and in a configuration with a back-up storage unit and a reflector/concentrator field having variable orientation; and

Fig. 6 shows a general diagram of a process for production of electric energy in a plant fitted with an apparatus according to a preferred embodiment of the invention.

Referring initially to Fig. 1 , a storage device according to a preferred embodiment of the invention is generally denoted by 1. The device 1 comprises first of all a main body 12, which serves as storage means and is made of a material exhibiting high conductivity and thermal capacity, so as to be able to perform a rapid diffusion of heat therein and maximize the amount of stored heat.

The device 1 comprises a receiving cavity 13, obtained into the storage means 12 and inside which solar rays are concentrated by means of stationary or sun-tracking heliostats. Preferably, the walls of the cavity 13 have a metal coating 131.

The storage means 12 is surrounded by a metal casing - or carter - 14, thermally insulated thereinside so as to reduce to the minimum heat dispersion from the storage means 12 to the external environment. In particular, in Figure 1 an insulating layer 15 is depicted, arranged on the internal faces of the carter 14, i.e. on those in contact with the means 2.

In the present example, the device 1 has a generally cubic or parallepiped geometry.

Fig. 1 also shows a cylindrical body 16 serving as gas-liquid separator of the carrier fluid. The function of the cylindrical body is known to the art, therefore its description will not be carried on further.

Inside the storage means 12, tube bundles of a heat exchanger associated to the means 1 are arranged, which receive an operating (or carrier) fluid apt just to receive heat from the storage means 12. By way of example, one of said tubes is denoted by 17 in Fig. 2.

According to an aspect of the invention, and always referring to Fig. 1 , the storage means 12 is obtained as a vibro-compacted mixture made of graphite spheres, or other geometric shapes of graphite fragments, of suitable decreasing size, preferably with a bimodal grain size distribution, with addition of carbonaceous particulate and/or graphite micropowders. This solution allows to obtain a thermal diffusivity of the end storage means very close to that of a single (massive) block of graphite, with the advantages of remarkably simplifying the constructing and machining step with respect to the massive graphite block, which should provide machining for the positioning seats of the exchanger tubes.

The specific particle embodiment considered herein allows, in the step of producing the device 1 , to position the heat exchanger tubes inside the metal carter 14, and subsequently introduce the mixture of carbonaceous particulate and graphite fragments. The whole is then subjected to vibration in order to allow compacting of said mixture inside the metal carter 14 and around the tubes positioned therein. Filling and vibro-compacting are carried on until complete filling of the volume available in the metal carter 1 .

With this procedure, it is ensured that the tubes of the exchanger have many more points of contact with the storage means 2, with respect to the case in which the seats of the tubes are machined into the continuous block of graphite. In fact, in this latter case a minimum tolerance of the machining would lead to zones of the tube surface not in contact with the storage means, lowering quite significantly the amount of heat locally ceded.

In order to prevent extremely violent oxidative phenomena at the operating temperatures of the device 1 (even of 1000°C), typically air evacuation from the internal environment of the same, or inletting of inert gas therein is provided.

On the basis of an aspect of the invention and referring to Fig. 2, in the system considered herein, inside the storage device 1 continual fluxing of inert gas (preferably Nitrogen or Helium) is provided by a suitable system for supplying said gas, system generally denoted by 20 and equipped with a fan or equivalent means 21. Particularly advantageous is the sending of said gas at the zone of the tubes 17 of the exchanger, or preferably in a suitable annular section concentric to said tubes. By varying the crossing velocity of the inert gas, particularly when fluxed in the air space between tubes 17 and storage means 12, the overall heat exchange coefficient between storage means 12 and operating fluid can be controlled and modified.

In fact, though with a nonlinear dependence, the increase of gas velocity modifies the Reynolds number, and therefore the convective exchange coefficient, increasing the amount of thermal power exchanged. This effect becomes particularly useful for the adjustment of the amount of heat transferred from the storage means to the operating fluid, given the conditions of solar radiation depending on the requested load. The control of the velocity of crossing the storage means by the inert gas therefore allows a greater flexibility of adjustment of the overall heat exchange between the storage means 12 itself and the tubes 17 of the exchanger.

In a preferred configuration, the inert gas is also recirculated in the storage device 1 and therefore, by reusing heat subtracted from the same device , the gas acts as heat regenerator. More in detail, by matching the inletting of inert gas to the zone of the device 1 where exchanger tubes are colder because they receive the fluid to be heated, and, on the contrary, matching the outletting of gas from the device 1 to the zone where the tubes are hotter, heat removed by outlet gas recirculated to the inlet point is ceded to the initial section of exchanger at a lower temperature, preheating the tubes themselves and making, as mentioned, a heat regenerator.

Thus, plant operation, based on storage devices having adjusted heat exchange, is adapted to various possible operating conditions that can occur during the day or in different seasons.

Moreover, the use of inert gas recirculated on the storage unit improves heat exchange between the graphite spheres/fragments during the heating step, particularly in the absence of graphite micropowders or carbonaceous particulate in the mixture of the storage means.

In connection to this aspect of the invention, it will be understood that it also provides a method for storage of heat energy in a storage means as described above, which method provides the continuous circulation of inert gas and the control of the flow thereof so as to vary the overall heat exchange coefficient between said storage means and the operating fluid and inside the storage means itself.

Greater effect on the heat exchange, also in association to the use of inert gas in the annular seat of the tubes of the exchanger, is had by the use of finned tubes, one of which depicted by way of example in Figure 2A and having a substantially star-shaped cross section.

Again in a preferred configuration of the apparatus, the tube bundle constituting the heat exchanger has the junction points in the zone far from the receiving cavity 13 or anyhow not immersed in the storage means 12, but rather adjacent to the insulating surface external to such means.

On the basis of another aspect of the invention to which Figures 3 and 3A refer, to the storage device 1 a secondary reflector/concentrator 30 is associated, positioned at the inlet of the cavity 13, therefore around the opening of the metal carter 14 which allows access of radiation concentrated by the heliostats, the latter each denoted by 40.

Such secondary reflector 30, thanks to a suitably shaped internal mirror surface, e.g. with a parabolic or hyperbolic profile, allows to recover part of the reflected radiation which would not reach the cavity 13. In fact, part of the radiation reflected by the heliostats, for reasons due to imperfections of the surfaces and/or in the aiming thereof, does not penetrate the opening of the cavity 13, and would therefore be lost.

A possible alternative would consist in making a broader opening of the cavity: this solution however would significantly increase radiation from the cavity itself to the external environment, with the result of losing anyhow a consistent part of the incident power. The use of a secondary reflector also allows to relax design constraint on precision in the curvature of heliostats, which varies the size of the beam reflected on the receiver.

Moreover, the use of said secondary concentrator allows to employ plane heliostats, of area no greater than the opening surface. This aspect has a great influence on the total cost of the technology: plane mirrors are very inexpensive, and heliostat expenses typically represent over one-half of the total cost of a plant.

The orientation of the concentrator follows the orientation and the position of the cavity which is aimed in favour of the heliostat field, as depicted in Fig. 5.

Referring now to Figure 4 and on the basis of a further aspect of the invention, when temperatures occurring in the storage device 1 are so high as to cause thermal stresses to the exchanger pipelines, a particularly advantageous apparatus configuration consists in an organization of plural storage devices of the described type, thermally arranged in series.

In particular, such arrangement causes the devices in series to receive heat so as to obtain maximum exchange efficiency and allow the fluid to reach the higher temperature in a heating series in subsequent stages.

Fluid heating takes place by crossing two or more storage devices connected in series - denoted by 1 and 10 in Fig. 4 - and having increasing operating temperatures. Therefore, the amount of concentrated solar energy, the volume of the related storage means, the density (efficiency) of the exchanger associated to each storage device and the control logic of the same are realized so as both to reduce thermodynamic stress of the exchanger tubes and maximize system efficiency.

Therefore, the solution to the problem of the thermal and mechanical stress of the tube bundles associated to the storage devices is singled out in a fractionated heating, i.e. in the subdividing of the overall thermal head of the fluid, useful to the thermodynamic cycle, into subsequent thermal stages, thereby obtaining for each stage a suitable temperature delta between the fluid being inlet into the exchanger and the tubes of the same exchanger, limiting, up to nullifying, stresses to metal tubes.

In the configuration of Fig. 4 - in which "L" denotes the liquid phase and "V the vapour phase of the operating fluid - each storage device 1 , 10 is equipped with its own respective receiving cavity 13, 130.

In said configuration, the two (or more) storage devices in series make, as a whole, a storage apparatus consisting of two (or more) distinct volumes of graphite (storage means), having the features described in the foregoing, equipped each with its own cavity and related heating wall.

In such a case, the temperature, and therefore the amount of heat energy stored in each volume of graphite, depends on the combination of factors: surface of heliostats/mirrors aiming at the cavity, atmospheric conditions and aiming duration.

In an alternative configuration shown in Fig. 4A, the storage devices thermally arranged in an increasing series may have a common receiving cavity, with a same thermally insulated metal carter. In such a case, the common cavity heats the provided zones of the storage unit to a different temperature.

Therefore, inside the single metal carter two or more volumes of graphite are made, thermally insulated therebetween and each one in contact with a respective portion of the heating surface of the cavity. The extension of the heating surface (of the cavity) in contact with each volume of graphite will be sized according to the operating temperature that is to be reached at the end of the heating for the determined storage volume. Likewise, the end temperature of each volume of graphite is proportional to the cavity surface and to the graphite mass in contact therewith.

Optionally, in this case, the fraction of energy reflected and re-emitted into the environment can be reduced thanks to the geometry of the internal surfaces of the cavity and/or to suitable lenses placed at the inlet to the cavity, allowing a localized "greenhouse effect". The effect may be amplified by resorting to varnishing of the irradiated surface of the cavity with materials suitable for retaining incident heat.

In both of the above-described cases - i.e., serial storage apparatus with single receiving cavity or dedicated cavities - the different storage volumes are thermally insulated therebetween, and the necessary adjustment of flow rate, temperature and pressure of the vapour circulating in the various volumes at increasing temperatures could be obtained through use of a separator or cylindrical body 160.

In the above-indicated context, the invention also provides a method for storage and transmission of heat energy of solar origin concentrated by heliostats, providing the thermal arrangement in series of storage means as described above, so that it be sequentially crossed by the tube bundles, the overall configuration being such that said storage means is apt to assume increasing temperature, with respect to the crossing direction of the operating fluid, following heliostat irradiation.

Referring now to Fig. 5, it is depicted a typical module of an apparatus for storage and transmission of heat energy according to a further preferred embodiment of the invention.

Such module comprises a primary storage device 1 , made as already described and positioned on a tower 5 at a suitable height.

The apparatus provides a plurality of heliostats 40, already introduced above, realizing a dedicated mirror field.

For a further margin of independence from atmospheric conditions, the module considered herein provides at least one further back-up storage device 20 positioned on the same tower 5. Such back-up storage unit(s) 20 is/are prearranged for receiving incident radiation in case of saturation of the heat storage capacity of the main device 1. Such prearrangement is particularly advantageous in case of temporary unavailability of the electric generator associated to the storage apparatus. To allow fluid transit in the additional devices 20 as well, it is provided that the pipelines of the exchangers connected to the main device 1 be adequately sectioned by means of automatic valves arranged in line.

The choice of reserving back-up (ancillary) storage units is extremely advisable, as allowing to provide further receivers without excessive burdening in connection to the cost of a complete module.

The placement of a further storage unit on the same tower provided for the main storage system allows, in the face of a modest cost increase, the storage of an energy fraction that would not be utilized, and that therefore would be lost in case of saturation of the heat storage capacity of the main apparatus. A saturation event can occur for three main reasons, i.e. for: peaks of incident solar power (above all in summertime), plant maintenance downtimes, and impossibility or non-convenience to inlet electric energy in the distribution network.

In presence of such an event, it is provided that the orientation of the heliostats dedicated to the saturated heliostat be changed and directed to the cavity of the back-up storage unit.

On the basis of a further aspect of the invention - and particularly in the above- considered case of storage devices connected in series in order to carry out the heating of the fluid in increasing thermal stages, or in case of back-up storage devices - it is provided the possibility of changing the orientation of the heliostats dedicated to each storage device, upon reaching the preset temperature, to another device of the same module or of another module still in the heating stage.

Then, it will be possible to operate heliostat orientation as needed, i.e. upon reaching the preset temperature, and change the orientation of the mirrors to another storage unit of the same or another module.

Heliostat aiming logic takes into account the various operating parameters and pursues best plant efficiency.

In a further preferred configuration, it is provided that the apparatus comprised of a suitable number of modules according to plant power be equipped with one or more storage modules or devices for preheating the operating fluid. Such preheating modules or devices constantly keep a reserve of high-temperature water under pressure (no less than 200°C at 40 bars) to enable daily start-up and until vapour becomes available downstream of the initial transient. The use of such preheater is limited to the start-up times. Optionally, the preheater may be integrated in each module.

The present invention has been hereto described with reference to preferred embodiments thereof. It is understood that other embodiments might exist, all falling within the concept of the same invention, as defined by the protective scope of the claims hereinafter.

Claims

1. An apparatus for storage and transmission of heat energy of solar origin concentrated by heliostats, comprising:

- a plurality of storage means (1 , 10), each made of a material comprising graphite;

- tube bundles ( 7), extending inside said storage means ( 2) and apt to receive a heat exchange operating fluid,

- a plurality of heliostats (40);

- means for controlling the orientation of said heliostats, apt to modify the storage means irradiated by one or more of them.

2. The apparatus according to claim 1 , comprising at least one module comprised of a main storage means (1) and a back-up storage means (20), which can be selectively irradiated.

3. The apparatus according to claim 1 or 2, wherein said storage means (1 , 10) is thermally arranged in series, it being sequentially crossed by said tube bundles

(17), the overall arrangement being such that said storage means (1 , 10) is apt to assume increasing temperature, with respect to the crossing direction of the operating fluid, following heliostat irradiation.

4. The apparatus according to the preceding claim, wherein said storage means has a common receiving cavity.

5. The apparatus according to any one of the preceding claims, comprising at least one heat energy storage device (1) having a receiving cavity (13) inside which a radiation of solar origin is concentrated by heliostats, said device (1) comprising a storage means (12), in the form of 25 a vibro-compacted mixture of graphite fragments and carbonaceous particulate and/or graphite micropowders.

6. The apparatus according to the preceding claim, wherein said graphite fragments exhibit a bimodal grain size distribution.

7. The apparatus according to claim 5 or 6, wherein said graphite fragments are substantially spherical.

8. The apparatus according to any one of the preceding claims, comprising at least one heat energy storage device (1), having a receiving cavity (13) inside which a radiation of solar origin is concentrated by heliostats, said device (1) comprising means (20) for circulating inert gas, apt to cause a continuous flow of inert gas at the external shell of said tube bundles (17) and to control the velocity of said inert gas so as to vary the overall heat exchange coefficient between said storage means (12) and the operating fluid.

9. The apparatus according to the preceding claim, wherein the overall arrangement is such that said circulating means (20) is apt to make the inert gas flow into an interspace between said tube bundles (17) and said storage means (12).

10. The apparatus according to the preceding claim, wherein the overall arrangement is such that the inert gas is recirculated inside said storage means (12).

11. The apparatus according to claim 9 or 10, wherein the overall arrangement is such that the inletting of inert gas into the device (1) is performed at a zone upstream of the thermal exchange between tube bundles (17) and storage means (12) and the outletting of inert gas is performed at a zone downstream of said heat exchange.

12. The apparatus according to any one of the preceding claims, comprising at least one heat energy storage device (1) having a receiving cavity (13) inside which a radiation of solar origin is concentrated by heliostats, said device (1) comprising a reflector/concentrator (30), arranged at the inlet of said cavity (13) and positioned or positionable so as to reflect thereinside a reflected or residual solar radiation.

13. The apparatus according to any one of the preceding claims, comprising a thermally insulated metal carter ( 4) housing one or more of said storage means (12).