Patent Publication Number: US-2013239950-A1

Title: Device, plant and method with high level of energy efficiency for storing and use of thermal energy of solar origin

Description:
FIELD OF THE INVENTION 
     The present invention relates to an industrial plant based on the use and storage of solar energy, to a device for storing and releasing solar thermal energy suitable for use within said plant and to a related method. 
     BACKGROUND OF THE INVENTION 
     It is known the use of solar energy concentrated through heliostats, fixed or by tracking. It also known the possibility of storing not used heat in solid materials with high thermal conductivity (typically graphite) for later use. For the exploitation of heat is normally used a heat exchanger which can also be immersed in the storage material and crossed by a working fluid—typically water, steam or other carriers—capable to absorb and transport heat energy contained therein. 
     As storage systems, such as graphite, are able to reach high temperatures (even 2000° C.), the limit of technology is set out by the thermal resistance of the metal tube bundles in charge for heat removal. 
     In addition, the storage in solids does not allow—for the thermal inertia of the same and for the low thermal diffusivity values—to follow the trend of the required power as atmospheric conditions and day-night cycles vary. 
     Based on the foregoing, therefore, a difficulty of the known systems is related to the low efficiency due to the limited maximum temperatures reachable and their lack of flexibility to follow the trend of the load as atmospheric conditions vary. 
     SUMMARY OF THE INVENTION 
     The technical problem underlying the present invention is therefore to overcome the drawbacks mentioned above with reference to the state of the art. 
     The above problem is solved by a device according to claim  1  and by a method according to claim  17 . 
     Preferred features of the invention are object of the dependent claims. 
     An important advantage of the invention is that it allows to efficiently and reliably produce a storage of solar thermal energy, minimizing thermal stress of the heat exchangers and increasing the efficiency of the heat exchange to the working fluid, through the use of a fluidisable granular bed that can play the dual role of exchange system and storage system of the collected heat. The basis of such use are the favourable features of the heat transfer of fluidised beds and the efficient heat transport due to the mobility of the granular phase. Both these features are linked to the possibility of giving a solid granular rheological behaviour that can be assimilated to that of a liquid, thanks precisely to its fluidisation. 
     Synergistically with the foregoing, the separation of the storing step of the heat received from the concentrated solar radiation from that of releasing of such heat and the resulting potential energy production—i.e. generally of heat transfer to the working fluid—allows to dramatically improve the versatility and the overall efficiency of the plant. 
     In a preferred embodiment, such separation is made possible by separating the fluidisation gas circuit from that of the working fluid. 
     In a particularly preferred embodiment, the device comprises two fluidisable beds, one essentially in charge of storing and a second, which receives heat from the first, primarily in charge of exchange with the working fluid. Also according to this embodiment, the fluidisation of beds is made by using air taken from the environment as fluidising gas. 
     To achieve maximum energy recovery, the hot air outlet from fluidised beds is sent to an air-air exchanger where it releases its heat to the cold fluidisation air drawn from the environment. 
     During the storing step the first bed receives heat from a field of heliostats through a receiver and is held in fluidisation conditions by air that is heated inside the air to air heat exchanger mentioned above. 
     In case be necessary to keep the stored heat at a step with no energy production, the first storage bed is kept at rest. 
     During a step of energy production, the now fluidised storage bed will exchange heat with the generation fluid bed adjacent to it and possibly separated by septums, preferably of metallic type. Inside the generation bed it is immersed the tube bundle crossed by the working fluid. Also in this case, the fluidisation air will be preheated using the air-air exchanger. Operating in this way, the fluidisation air compression system will be operated at room temperature as well as the filtration system of the air outlet from the fluidised bed where the presence of elutriate solid particulate is possible. 
     Having concentrated the heat exchange to the benefit of the working fluid in the fluidised generation bed will, because of high heat transfer coefficients of fluidised beds, minimize the surfaces of the tube bundle and then the use of precious materials for the manufacture of the same. 
     Briefly, the system can be substantially divided into three bulks, namely a bulk for storage (the first fluidised bed, in the embodiment shown above), a bulk for exchange towards the working fluid (the second fluidized bed, in the embodiment shown above) and a bulk for heat recovery (heat exchanger air/air still in the preferred embodiment above mentioned), such bulks being linked to one another allowing to achieve a system that can operate with great flexibility and efficiency. 
     Other advantages, features and modality od use of the present invention will be apparent from the following detailed description of some embodiments, presented by way of example and not for imitative purposes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       It will be referred to the figures of the annexed drawings, wherein: 
         FIG. 1  shows a plant layout according to a preferred embodiment of the invention; 
         FIG. 1   a  shows a schematic section/front view of the layout of a part of the plant of  FIG. 1 , which incorporates a tower structure with air/air heat exchanger and shows a partitioning of a cash distribution, a fluidisation gas inlet circuit and its path; 
         FIG. 2  shows a plan section of the portion of plant of  FIG. 1   a , showing the arrangement of tubes of a heat exchanger that receives the working fluid and the partitioning of a first bed of fluidisable particles that acts as a means of storing and a second bed of fluidisable particles committed to heat exchange with the working fluid, and 
         FIGS. 3 and 3   a  refer to a release and storage device usable in the plant of  FIG. 1 , which device uses fuel gas as auxiliary energy source and it is shown respectively in side section and plan view. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     With reference initially to  FIGS. 1 and 1   a , an electrical power production plant from concentrated solar radiation according to a preferred embodiment of the invention is globally denoted by  100 . 
     The plant  100  in its turn incorporates one or more devices  1  for storing thermal energy received by the concentrated solar radiation and for releasing such energy to a working fluid, the latter being typically water or steam. 
     The device  1  is indeed apt to store the thermal energy that originates from a solar radiation conveyed/concentrated on it for example by tracking heliostats. 
     The device  1  comprises a containment casing  2 , preferably made of metal, thermally insulated to minimize heat loss to the surrounding environment. 
     The casing  2  may contain one or more receiving surfaces  20  upon which solar radiation is concentrated. 
     At the base of the case  2  a feeding inlet  21  for a fluidising gas is obtained, whose role will be clarified shortly. At the inlet  21  there is a distribution septum, or distributor, for simplicity&#39;s sake also identified by  21 , of such fluidisation gas, apt to enable a uniform entry of the latter and at the same time providing support for one or more beds of fluidisable particles  3 ,  30  described below. 
     Within the casing  2  there is a storage zone, in the form of a first bed of fluidisable particles  30  suitable for thermal storage according to favourite features described below. In the present embodiment, the area of the storage bed  30  is placed immediately at the receiving surfaces  20 , close to the inner surfaces of the case  2 , so as to be directly affected by the solar radiation concentrated upon said receiving surfaces  20 . 
     Also within the casing  2  and in the most central zone of the device  1  is then provided a second bed of fluidised particles  3 , which may be continuous with or separate from the storage bed  30  and whose function is to release the thermal energy stored in the latter to the working fluid, as better explained later. To this end, within the bed of particles  3 , or close to this, heat exchange elements are arranged—and in particular tube bundles  4  of a heat exchanger—which are flown, when in use, by the working fluid. 
     In a preferred further embodiment, the two beds of particles  3 ,  30  may be adjacent portions of the same bed selectively fluidisable. 
     As mentioned, the inlet  21  of the device  1  is capable of allowing the feeding within the casing  2 —and specifically through the base of beds of particles  3 ,  30 —of a fluidisation gas, which in this preferred configuration is air. In particular, the overall arrangement is such that the gas pushed through the distribution septum  21 , can move the particles of the bed  3  and/or  30  so as to generate a corresponding flow/motion of particles suitable for the mutual heat exchange between particles of the same bed  30  and/or between the latter and the inner part of the receiving surfaces  20  or between particles of portions of the bed adjacent to each other, between particles of two beds  3 ,  30  or between the particles of the bed  3  and the tube bundles  4 . 
     The position of the tube bundles  4  with respect to the bed of particles  3 , or rather the exposure of the surfaces of the tubes with respect to the bed of particles, is such as to maximize the rate of heat transfer, the latter being proportional to the product of the heat exchange coefficient and the surface affected by the same heat exchange. 
     The tube bundles  4  can be totally or partially immersed in the bed of particles  3  or facing it. The choice lies in the modality of handling utilised for the device and the minimum and maximum heights of the bed of particles as a function of the fluidisation air velocity: as the velocity increases, the surface of the tube bundle affected by the heat exchange increases. For enhanced versatility of the plant, a means to vary the fluidisation air velocity, and therefore its flow, is in fact provided. 
     Therefore, varying the crossing velocity of the fluidising gas, the overall heat transfer coefficient between the fluidised bed and the exchange surface can be monitored and modified, resulting in regulation flexibility of the amount of transferred thermal power. 
     It can thus be obtained a substantial reduction of heat transfer in the absence of fluidisation or with gas velocity less than the minimum fluidisation velocity. 
     In addition, it is possible to vary the height of the first storage bed  30  and/or the release bed  3  by varying the amount of particle charge of the bed, such variation can be operated when heated, during operation, using a suitable exhaust and loading system of particles associated with each device  1  or groups of similar devices. Acting on the height of beds  30 ,  3  both in terms of particulate material present in the bed when fixed, either by varying the fluidisation velocity, it allows more levers of action that make the handling of device  1  and then the system  100  extremely flexible. 
     As best illustrated in  FIG. 1   a , in the present example the device  1  has or is associated with an elevated tower structure  70 . At the central zone of the latter is located an exchanger gas/gas  7 , in the present example of air/air type, which extends vertically within the support structure of the device itself. 
     The environment inside the device  1  is communicating with the air-air exchanger  7 , in the example at its central zone. In particular, the base of the device  1  communicates with the section of the exchanger in which hot air outlet from the bed of particles  3  and/or  30  flows, while section  7  of the air-air exchanger from which the preheated environment air exits is connected to the distribution septum  21  at the base of particles bed  3 ,  30 , through a manifold or air chamber  14  which contributes to uniform the air flow inlet in the device  1 . 
     In this way, the exchanger  7  allows to preheat in countercurrent environment air inlet in the distributor  21  at the expense of the fluidisation hot air outlet from the particles bed of  3  and/or  30 , and then to recover the heat content of fluidisation air outlet from the bed. 
     As said, the fluidisation air circuit requires cold environment air to be forced by a means of forced circulation, in particular one or more blower/compressor  8 , inside the air-air exchanger  7  and to be preheated along the way at the expense of the fluidising hot air which, leaving the particles bed  3 ,  30 , is forced in countercurrent inside said exchanger  7 . The preheated environment air reaches the air chamber  14  and the distribution septum  21  through a feeding manifold circuit  142 . The air oulet from the particles bed  3 ,  30 , cooled after having passed through the air-air heat exchanger  7 , is fed, through a downflow pipe  5 , a dust separator  6 , or dust exhaust, and it is then expelled to external environment. 
     Preferably, the dust separator  6 —typically of inertial impactors—type or equivalent devices having low pressure drop and cycloidal operation—is located at the base of the structure of the device  1  in line with the down flow pipe  5  and provides then to dedust the fluidisation air from any elutriato out of the particles of beds  3 ,  30 . 
     As already mentioned, the particles bed  3  can be physically separated from the particles bed  30  in case through septums  41 , however on the whole displaying a modular structure allowing a selective fluidisation of the bed zones. In general, device  1  allows a selective and/or differentiated fluidisation of one or more portions of particles bed  3  and  30  and/or a selective and/or differentiated fluidisation of the beds or portions thereof. 
     In the present example, such selective fluidisation is achieved through a compartmentalization of both the air chamber  14 , by means of septums  141 , and the feeding circuit  142  by means of valves  143 , allowing the feeding of air only at portions of bed  3  and/or  30  selectable according to specific needs of storage or of steam/power production. 
     It is thus possible to operate portions of the bed as thermal switches that close the heat transfer circuit only if fluidised. Said monitored and selective fluidisation of zones of the bed granular means ensures heat extraction continuity and plant&#39;s flexibility with respect to the energy demand downstream. 
     Moreover, in the present example it is provided a partitioning of the air chamber  14  also at an area of the bed adjacent to receiving surfaces  20 . This configuration allows, in the absence of solar radiation, to significantly reduce heat loss stored in the first bed  30  towards the external environment through the same receiving surfaces  20 . This partitioning is as well operated by means of the aforementioned automatic valves  143  timed and/or handled by a signal from a irradiance sensor (solarimeter for example, pyranometer or equivalent equipments) associated with each device  1  or with groups of devices. 
     The regime of fluidisation of the bed particles is preferably ebullient or in any case such to maximize the heat transfer coefficient. 
     The choice of particle material for storage and release beds  3  and  30  is based in particular on the poor aptitude for abrasion and fragmentation, in response to the need to minimize the phenomenon of bed particles elutriation so as to limit the production and transportation of fines in the fluidisation air. Based on these considerations, a preferred configuration favours the use, for bed particles, of granular material inert to oxidation, with a regular shape, preferably spherical and/or preferably having the size of the order of 50 to 500 microns, and such that said dimensions be preferably native, that is not resulting from the aggregation of smaller particles. 
     In the example in  FIG. 1 , the working fluid is water in a liquid state which receives during the crossing of the heat exchanger  4  thermal energy transferred from the particles of the bed  3  to become superheated steam. Said steam in pre-determined conditions of temperature and pressure is then utilised to produce electricity expanding in a steam turbine associated with a generator  10 . 
     As shown in  FIG. 1 , the working fluid circuit provides ducts  90  that define the tube bundles  4  within the device  1 , and in the present example is provided the aforementioned steam turbine  10  connected to an electricity generator, a condenser  11 , a getter  40  with a bleed in turbine  40 , a supply pump  12 , an extraction pump  120  or equivalent means to those just mentioned. 
     The described configuration allows the remarkable advantage of separating the process of thermal energy storage from the steam generating process. 
     In the storing step, solar energy is concentrated at the receiving surfaces  20  and through fluidisation of the storage bed  30 , or part thereof, the thermal energy is transferred precisely from the surfaces  20  to the particles of bed  30 . As said this step is independent from the production phase. In a regime of sole storing only the first bed  30  is fluidised. 
     In the production phase the second release bed  3  is also activated, so that the heat transfer occurs from the storage bed  30  to the particles of the release bed  3 , from these to the tube bundles  4  and then to the working fluid flowing in the latter. 
     Therefore, the working fluid which crosses the tube bundles  4  receives from the second bed  30  thermal energy stored by the first bed  30 , where heat transfer takes place by activating the beds  3 ,  30  that is by fluidising particles of the zones of bed  30  and  3 . The thermal energy transferred to the working fluid can also be used for industrial purposes different from the example herewith considered. 
     In particular, in the above description it has been referred as a way of example to the application of the device to an electrical power production plant, stand-alone. It will nevertheless be understood that the possible applications of the device are large and related to the production of steam or heat for industrial plant like power plants, desalination, district heating, and so on. 
     With this configuration, even in the absence of solar energy—such as at night—it is ensured continuity of operation and delivery of steam and thus heat output from the device  1   
     In particular, the size of the device  1 , the particle beds  3 ,  30 , the surfaces of the tube bundles  4  and the speed range of the fluidisation gas, may be such to assure storing of thermal energy during sunshine hours and release of the same during night hours to the heat exchanger through fluidisation of particles of beds  3 ,  30 . 
     In addition, as previously mentioned, using a modular structure of the fluidised bed and modulating for each section and the fluidisation velocity of the particles themselves, as well as the height of the particles bed, it is possible to adjust the amount of thermal energy transferred to the tube bundles, by choosing to dedicate one or more sections to the heat transfer or to the storing through a selective and/or differentiated fluidisation of said sections, ensuring continuous operation of the plant. 
     Moreover, in the eventuality of plants that provide a variety of devices  1 , the ability to adjust for each device the amount of heat transferred to the working fluid is necessary to maintain a constant temperature and pressure of the steam produced, and it allows the advantage of maintaining constant, decreasing or increasing the temperature of the working fluid or, at the same temperature, increasing the flow of working fluid. 
     The sizing of the devices  1  and the operational logic can be coordinated to achieve a given energy output even in the absence of solar radiation. 
     It is also possible, depending on the needs of the plant downstream, to manage single devices in such a way that, within 24 hours, part of these work only for storing and part in production or to vary the fluidisation velocity of each unit resulting in overall changes of the thermal power produced. For a given stored thermal energy, this allows to release it quickly to higher power or for extended periods at lower power. 
     In a different embodiment (not shown), it is provided the presence of a partialisation system, a sort of shutter, of the receiving surfaces  20  by means of insulating septums. Said septums, whose activation can be automated and follow the sun trajectory, allow to isolate the portion of the storage bed  30  corresponding to the obscured zone of surface receiver  20  and thus prevent re-radiation to the outside by said portion of the bed when the same receiving surface  20  is not affected by the incident radiation. 
     According to another preferred embodiment of the invention that can be used in combination with all other configurations above described, it is provided the use of gaseous fuel within the fluidised bed, to compensate for the prolonged absence of sun exposure and/or to ensure the achievement of a certain power level according to the needs downstream the production plant. 
     An important advantage derives from the possibility of burning said gaseous fuel directly into the fluidised bed. Usually, for devices of prior art this operation is carried out in production units separated from the main manufacturing plant. 
     This configuration is schematically shown in  FIGS. 3 and 3   a , which shows a possible circuit for the fuel gas  15 - 151 . The latter is such as to provide inlets to each sector of the air chamber  14  in which the pre-mixing of fuel gas with the preheated fluidising air takes place or in the distribution circuit of the fluidising gas  151 . 
     In the presence of combustible gas, the device  1  is equipped with one or more torches  22  inserted into the environment of the device  1  for the trigger of combustion and to ensure the system from any dangerous accumulation of gas inside the device, and one or more rupture discs  222  on the case  2 . These expedients—like others that may be applicable—are aimed at preventing the risk of explosion. 
     As for the burning of gas per se, this is known technique and it won&#39;t be described further in the following. 
     The use of this additional configuration is made even more convenient by the fact that the regulations governing the production of energy from renewable sources admit that a minimum amount, usually less than or equal to 15% of the rated power, be produced by burning fuels fossils. 
     It will be better understood at this point that the invention has considerable advantages in terms of:
         sizing of the plant, and in particular of storage and release devices and of the relative structure that are very compact—obviously deriving from the preferred configuration in which the air/air exchanger extends in height within the support structure of the device itself;   sizing and operation of the compressor-blowers  8  which elaborate a cold fluid, that environment air;   sizing and operation of dust exhaust  6  that like the blower-compressor  8  works with exhausted fluidisation air of at low temperature (about 100° C.);   sizing of the tube bundles  4 , which are being drastically downsized the steps of preheating evaporation and overheating of the working fluid being assigned to the heat exchange within the bed with typical coefficients of 300 to 500 W/m2K.       

     It will be finally understood how the invention provides also a method of storage and heat exchange as defined in the claims that follow and presenting the same preferred features as set forth above in relation to the preferred forms and various embodiments of the device and the plant of the invention. 
     The present invention has been described so far with reference to preferred embodiments. It is intended that there may be other embodiments which refer to the same inventive concept, that may fall within the scope of the appended claims.