WO2009147651A2 - A solar energy generator - Google Patents

Abstract

A solar energy generator comprises a system (10) for concentrating solar rays (R), means (12) for absorbing and accumulating the heat of the solar rays concentrated by the concentrating system (10), and a heat utilization plant (14) operatively connected to the absorbing and accumulating means (12). The absorbing and accumulating means (12) comprise a hollow body (20) with a chamber (22) which is substantially isothermal at high temperature (greater than about 800 "C) and which is utilized in a high-efficiency thermodynamic cycle, for example, a gas cycle of the gas turbine type or a steam cycle for the generation of electricity with zero emissions. The hollow body (20) houses a plurality of capsules (30) containing a two-phase material or compound with a phase transition temperature substantially in the region of the range of temperatures that can be reached inside the chamber (22). The solar-ray concentrating system (10) comprises a plurality of concentrating mirrors (16) and collimating mirrors (18) with two stages so as to reflect the radiation towards the absorption and accumulation means (12).

Description

A SOLAR ENERGY GENERATOR

The present invention relates to a solar energy generator.

The invention has been developed with particular regard to a generator of electrical and/or thermal energy by the concentration of solar rays . In this field, there are known systems of various types in which a series of mirrors, otherwise known as heliostats or coelostats, concentrates the solar radiation onto a central receiver containing a fluid with a high heat absorption capacity such as, for example, mineral oil or molten salts. This fluid is heated, reaching high temperatures, for example, even of the order of 400- 600 "C, and is then used, for example, by being passed through a heat exchanger to produce steam which in turn is used in conventional manner to supply a steam turbine and, in short, to produce electrical energy.

A disadvantage of the systems of the prior art is that the central receiver reaches very high temperatures such as to dissipate some of the total amount of energy collected by the mirrors to the exterior by radiation, resulting in a significant drop in yield.

Moreover, the functional capacity of the systems of the prior art is closely linked to the presence of solar radiation and is zero at night and considerably reduced on cloudy days. This leads to a degree of uncertainty in the utilization of known systems, which has a negative impact where a certain functional capacity is required, for example, for the production of electrical energy.

The systems of the prior art also involve other costs and inefficiencies of various types, for example, due to the fact that the relative orientations of the position of the sun, of the mirrors, and of the central receiver are optimal only at certain times of day whereas, for most of the time, the solar radiation strikes the mirrors in a direction that is inclined to the perpendicular, resulting in a reduction in the radiation that is collected and reflected towards the central receiver. To reduce this effect, it is necessary to arrange the central receiver on high towers but these are expensive and have a great environmental impact.

An object of the present invention is to provide a solar energy generator which operates at high temperatures, which is more efficient and economically advantageous than the generators of the prior art, and which intrinsically permits the absorption and storage of energy for utilization even when there is a lack of solar radiation. Another object of the present invention is to provide an energy generator which, although it can be used with high-efficiency cycles, can be produced relatively simply, is easy and inexpensive to manage and maintain, and is reliable over time, even under difficult conditions of use.

To achieve the objects indicated above, the subject of the invention is a system for generating energy from the sun the characteristics of which are given in the appended claims .

A particularly advantageous characteristic is that the solar energy is concentrated directly into a reservoir at high temperature which constitutes a significant difference with respect to the known techniques that are currently used.

In the energy generator according to the present invention, the solar radiation is absorbed and stored in a receiver/reservoir at high temperature, which is utilized in a high-efficiency thermodynamic cycle, for example, but in non-limiting manner, a gas cycle of the gas turbine type or a steam cycle, preferably with supercritical parameters, for the generation of electricity with zero emissions. In a simpler application, the generator of the present invention can be used as a purely thermal source, for example, but in non-limiting manner, for heating water for domestic and industrial use with capacity to cope with day/night cycles, possibly combined with a system for heating and cooling environments. In this case, the characteristics of the present invention mean that the efficiencies and continuity of operation that are achieved are markedly better than those that can be obtained with the prior art available.

Another application of the present invention is offered by the capability to provide a very high-temperature absorber for the production of hydrogen.

In a preferred but non-limiting embodiment of the invention, the generator of energy by the concentration of solar rays comprises three basic functional units: a system for concentrating solar rays, means for absorbing and accumulating heat, and a plant for utilizing the heat. The solar-ray concentrating system preferably comprises a plurality of concentrating or collimating mirrors and, more preferably but in non-limiting manner, is constructed in two successive concentration stages by virtue of the use of a set of concentrating mirrors, also known as coelostats, which concentrate the solar radiation onto a collimating mirror which reflects the radiation towards the heat absorber/accumulator . The solar-ray concentrating system may comprise one or more sets of coelostats with respective collimating mirrors . When there are several sets of coelostats and respective collimating mirrors, the various collimating mirrors can be used pointing at the same absorber.

In greater detail, in a preferred embodiment, the two-stage focusing and concentrating system is composed of modules each of which comprises a concentrating stage and a focusing and collimating stage. The first, concentrating stage comprises a predetermined number of parabolic mirrors, or flat mirrors combined to form a larger, approximately parabolic mirror, which collect the solar radiation and concentrate it by focusing it in the direction of the second, focusing and collimating stage. The second stage is preferably, but in non-limiting manner, formed by a slightly convex mirror or similar optical devices which collect and deflect the incident radiation coming from the first stage, directing it towards an inlet hole of the heat absorber/accumulator.

The two stages are motorized so as to follow the movement of the sun during the day to permit more efficient utilization of the solar radiation. The system of mirrors of the first stage can thus in fact be oriented so that it always has optimal exposure to the solar rays which always fall on it almost perpendicularly, whilst the second stage is dedicated to the transmission of the radiation to the absorber/accumulator which can thus be mounted on a much lower tower than in the systems of the prior art. This two- stage arrangement is also particularly advantageous for low power applications in which the heat generated is used to drive small gas turbines.

A factor which is particularly characteristic of the solar energy generator according to a preferred embodiment of the present invention is the high-temperature solar radiation absorber and accumulator which has characteristics that are particularly advantageous with regard both to the type of absorption and to the specific thermal accumulation system which is based on a technique that provides for the use of encapsulated molten salts.

In a preferred embodiment, the solar radiation absorber comprises a hollow body in which the cavity is practically completely insulated from the exterior with the exception of the hole or holes for the inlet of the radiation and for the inlet and outlet of the cycle process fluid. The solar radiation is collected and trapped in the cavity of the absorber to produce a reservoir with a high internal temperature but with relatively cold external walls so as not to dissipate energy to the exterior by radiation.

In greater detail, the innovative concept underlying the solar energy absorber is based on the provision of a hollow body with one or more inlet holes through which high- intensity radiation enters and is then trapped by multiple reflections on the internal walls of the hollow body so that only a negligible fraction of energy can escape to the exterior. Inside the cavity there is a reflection system for distributing the radiation so as to prevent excessive heating of the radiation inlet zone. The internal surface of the cavity is reflective so as to trap the radiation and to prevent the dissipation of heat towards the exterior. The cavity is insulated from the exterior preferably by means of a vacuum chamber between two shells, that is, an inner shell and an outer shell. The insulation may further or alternatively comprise an insulating layer, for example, mineral wool or ceramic fibre. The salts system for absorbing and accumulating heat is arranged inside the cavity so that the cavity constitutes a high-temperature isothermal reservoir.

The temperature which can be reached and maintained in the cavity may reach and exceed 750-800 "C so as to permit highly efficient thermodynamic cycle applications . For high- temperature applications, the cavity is preferably filled with nitrogen to improve the resistance of the materials to the environment at high temperature. A heat exchanger with the working- fluid means is arranged in the central area of the cavity, along its axis, and is used for the thermal cycle for utilizing the energy derived from the accumulated heat. The exchanger is preferably of the type with a single tube or multiple tubes arranged parallel to an axis of the hollow body.

The cavity of the hollow body of the absorber and accumulator houses a large number of capsules or tubes, meaning closed containers, preferably but in non-limiting manner of spherical, ovoid, tubular, cylindrical, or similar shape, which are made of a material, for example, steel, suitable for the high temperatures that can be reached inside the absorber and accumulator and which contain a compound that has at least two-phase behaviour at the working temperatures of the absorber and accumulator such as, for example, a high- melting salt. The size of the capsules or tubes is selected on the basis of the optimization of the salt/steel weight- ratio parameters, production costs, ease of fluidization and/or mixing by the gas circulating in the absorber and, above all, on the heat-exchange efficiency. In fact, the size of the capsules or tubes of salt, which are preferably but in non-limiting manner of limited size, permits a particularly efficient exchange with the gas by virtue of the large surface area involved. The salts also permit absorption and maintenance of the temperature during the night and during variations in the degree of exposure to the sun, also by virtue of the latent heat of fusion of the salts .

In a preferred embodiment, the distribution of the tubes full of salt is such as to optimize the absorption of radiation and, as far as possible, to limit the space required and hence the volume of the cavity of the hollow body. A typical but not thereby limiting spatial distribution provides for the radiation to be able to penetrate from the wall of the cavity through the first row of tubes whereas the distribution of tubes then becomes gradually denser towards the axis of the absorber as the tubes are arranged closer and closer to the heat exchanger with the working fluid means. Naturally, the spatial distribution of the tubes of salt inside the hollow body can be modified and optimized according to the specific application and to energy storage or peak supply requirements .

The system of salts which are confined, that is, enclosed in sealed containers, is a heat storage and exchange system in which the heat is absorbed and released by virtue of the liquefaction and of the solidification of the salts so as to increase the storage capacity and permit a constant temperature in the reservoir. Since the salts are enclosed and confined in a tube or capsule of some shape, the liquefaction and solidification can take place without undesired effects and above all without contamination of the working fluid means . The concept of the confinement of the salts in small tubes, spheres, capsules or the like has the effect of providing a large heat exchange surface area which optimizes the absorption and the release of heat and renders them fully effective. Typically, the tubes, capsules or the like are filled with salts, topped up with nitrogen and sealed. Since the tubes or capsules are not subject to significant mechanical stresses but are limited to containing the salts in a leaktight manner, the steel of the tubes or capsules can be replaced by or covered with alternative materials such as glass-based or ceramic-based materials so as to withstand their high temperatures and resist corrosion.

The salt or salts used for the application in the confined, two-phase heat exchanger are selected according to the application and the temperature at which the exchange of heat must take place. In high-temperature energy generation applications, common salts such as NaCl may be used.

The heat utilization cycle may be a conventional water/steam cycle or a simple regenerative or combined closed gas turbine cycle. In the present invention, the basic difference with respect to conventional steam cycles with steam or gas that are generally used in production applications is connected with the fact that there is no combustion since the heating of the process fluid takes place by means of the solar thermal absorber and accumulator. The most direct consequence and the clearest advantage of this characteristic is that the production of energy by the generator of the present invention takes place with zero emissions. Another considerable advantage is that the process fluid is and remains clean, that is, it is not contaminated by agents resulting from combustion. The consequence is that there is no attack by corrosive particles and agents on the tubes of the exchanger in the case of a steam cycle or on the blades of the expander in the case of a gas turbine cycle. This latter fact not only enormously lengthens the life of the components but also permits the use of higher temperatures, with greater efficiency, opening the way for higher-powered applications and uses than have been possible up to now with the generators of the prior art.

As mentioned above, another application of the present invention is offered by the capability to provide a very high-temperature absorber for the production of hydrogen in accordance with techniques which are known but up to now have been little used or expensive to use owing to the lack of adequate high-temperature sources.

Further characteristics and advantages will become clear from the following description of a preferred embodiment given with reference to the appended drawings which are provided purely by way of non-limiting example, and in which:

Figure 1 is a schematic view of the solar ray concentration and absorption section of a solar energy generator according to the present invention, Figure 2 is a schematic section through an absorber- accumulator of a solar energy generator according to the present invention,

Figure 3 is a schematic view of a variant of the solar energy- generator according to the present invention with an absorber-accumulator shown in section in order better to illustrate its interior,

Figure 4 is an enlarged view of the section through the absorber-accumulator of Figure 3 , and

Figure 5 is a view taken on the arrow V of Figure 4.

With reference now to the drawings, a solar energy generator according to the present invention comprises three basic functional units: a system 10 for concentrating solar rays R, heat absorbing and accumulating means 12, and a heat utilization plant 14.

The solar-ray concentrating system 10 preferably comprises a plurality of concentrating mirrors 16, also known as coelostats, which concentrate the solar radiation by virtue of their shape and their arrangement. There are various known types of concentrating mirrors 16 and arrangements thereof which are suitable for effectively concentrating the solar radiation towards a predetermined focal point . In the specific case of the preferred embodiment of the present invention, the solar-ray concentrating system 10 is preferably a two-stage system. In other words, one or more arrays of concentrating mirrors 16 concentrate the solar rays onto one or more corresponding collimating mirrors 18 which reflect the radiation towards the heat absorption and accumulator means 12. Altogether, in the first, solar radiation concentrating stage formed by the concentrating mirrors 16, and in the second, collimation concentrating stage formed by the collimating mirror or mirrors 18, it is possible to reach very high radiation concentration factors, for example, even greater than 10000 times at the inlet to the absorption and accumulator means 12.

The concentrating mirrors 16 and the collimating mirrors 18 may adopt various configurations and, in particular, may individually be concave, for example, parabolic, to concentrate and collimate the solar rays, respectively, towards a focal point or, for constructional simplicity and economy, may be formed by the combination and juxtaposition of a predetermined number of smaller, flat mirrors arranged in a generally concave configuration, for example, approximating to a parabola, or a slightly convex configuration for the collimators. In a preferred typical configuration, the concentrating mirrors 16 or coelostats are composite and are constituted by a predetermined number, preferably but in non-limiting manner, from 10 to 20, of converging flat mirrors .

The concentrating mirrors or coelostats 16 are motorized and are controlled automatically to follow the diurnal arc of the sun, keeping their own axis, on which the concentration focus is disposed, on the axis of the direction of the sun, whilst the one or more collimating mirror/s 18 are oriented to deflect the solar rays towards the absorber and accumulator means 12. Several coelostat/collimating units may be used, pointing to the same absorption and accumulation means .

The absorption and accumulation means 12 comprise a hollow body 20 with an internal cavity or chamber 22 which is practically completely insulated from the exterior except for a hole (or several holes) 24 for the inlet of the radiation coming from the solar-ray concentration system 10, in particular from the collimating mirrors 18 and holes or openings 26 and 28 for the inlet and outlet of the cycle process fluid, respectively. The chamber 22 is substantially- isothermal at high temperature and very high temperatures, for example of about 850° C or, more generally, greater than about 750-800¹C, may be reached therein. A further mirror (not shown in the drawing) , for example, a conical collimating mirror, may be provided in the region of the radiation inlet hole 24 to compensate for possible errors of alignment .

In order to store the heat and homogenize the temperature inside the hollow body 20, the chamber 22 houses a large number of capsules 30 which, preferably but in non-limiting manner, are spherical and made of steel suitable for the high temperatures that may be reached inside the absorber and accumulator and which contain a high melting salt of generally known type or some other two-phase compound selected according to the temperatures of use of the invention. A gas circulates inside the hollow body 20 (arrow G of Figure 2), passing through the chamber 22 and fluidizing and mixing the capsules 30 so as to homogenize the temperature inside the chamber 22. The gas inside the heat absorber and accumulator 12 may circulate naturally by convection but may also be force-circulated to ensure greater uniformity, for example, when heat is being released during the night. The capsules 30 may also be partially fluidized and mixed by the same process fluid (for example, air/nitrogen, helium or other gas mixtures) that is used in the heat utilization plant 14.

The small size of the capsules 30 permits a particularly efficient exchange with the gas by virtue of the large surface area involved. The salts also permit absorption and temperature maintenance during the night and during variations in the degree of insulation, also by virtue of the latent heat of fusion of the salts. Inside the chamber 22, a distributor/reflector 50, preferably with multiple cusps (visible in the variant of Figures 3-5), prevents a hot spot in the vicinity of the radiation inlet zone 24 which would lead to greater radiative re-emission losses. The heat utilization cycle may be a conventional water/steam cycle or a simple regenerative or combined closed gas turbine cycle. The exchange of heat with the process fluid takes place within the absorber and accumulator means 12. If the process fluid does not itself pass through the plurality of capsules 30 inside the chamber 22, a heat- exchanger device 32 is provided. In the case of a water/steam cycle, the exchanger 32 comprises a set of tube banks 34, which are preferably made of steel and are arranged inside the hollow body 30 forming a generator of steam for use in a turbine 36, preferably but in non-limiting manner with supercritical parameters to achieve greater efficiency. In applications with a gas cycle, the process fluid may be heated directly in the absorber, with or without the interposition of exchange tubes, according to pressure parameters, after the compression stage, reaching the temperature of admission to the expander with a substantially isobaric transformation.

In the preferred embodiment, a solar energy generator according to the present invention can be developed with modules having a size of from 0.2 to 5 MWeI.

Purely by way of example, the size of the solar energy generator according to the present invention is selected by estimating a thermal cycle efficiency and evaluating the solar power coefficient usable: the latter coefficient is greatly affected by the installation site and takes account of the day/night cycle, of the mean insulation levels, of the efficiency of the mirror concentration system, etc. From this data it is possible to derive the necessary mirror area and, on the basis also of the requirements for operation "in the dark", that is, with consumption and autonomy without sunlight or with low and very low insulation, the size and quantity of capsules 30 necessary to form the thermal flywheel, that is, the desired degree of heat accumulation and the required thermal exchange. In the example of a pilot plant for providing 100 kW continuously, day and night, with a cycle efficiency of about 45% and an insulation coefficient of 15%, about 1000 m² of concentrating mirrors 16 and a chamber 22 containing about 80-100 tons of capsules 30 are required.

As mentioned in the preamble to this description, in a simpler application, the solar energy generator of the present invention can be used purely as a heat source (for example, for heating water for domestic and industrial use with the capacity to cope with day/night cycles and optionally combined with a heating and cooling system) . Applications in the field of the production of hydrogen by the implementation of very high-temperature absorber means 12 are also provided for.

Figures 3 to 5 illustrate a variant of the energy generator described above. This variant is characterized and differentiated from that described above basically by the provision of a plurality of tubes 330 as leaktight containers of the two-phase compound, in particular salt, in a closed nitrogen chamber, which determines the heat accumulation behaviour. Also visible are the reflectors 50 which preferably have multiple cusps and which prevent excessive heating in the zone of maximum concentration of the solar radiation.

In greater detail, the absorber-accumulator 12 comprises a cylindrical tower thermally insulated by means of a space or chamber which is under vacuum and/or contains an insulating material such as rock wool or the like. Inside the absorber- accumulator 12 the tubes 330 are arranged in parallel arrays more spaced apart towards the exterior but closer together in a centrally axial position in the vicinity of a central heat exchanger through which the working fluid flows .

Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated, without thereby departing from the scope of the present invention.

Claims

1. A solar energy generator, comprising a system (10) for concentrating solar rays (R) , means for absorbing and accumulating the heat of the solar rays concentrated by the concentrating system (10), and a heat utilization plant (14) operatively connected to the absorbing and accumulating means (12), characterized in that the absorbing and accumulating means (12) comprise a hollow body (20) with a chamber (22) which is substantially isothermal at high temperature.

2. An energy generator according to Claim 1 in which the heat utilization plant (14) comprises means for implementing a high-efficiency thermodynamic cycle selected from a gas cycle of the gas turbine type, a steam cycle, and a steam cycle with supercritical parameters, for the generation of electricity with zero emissions.

3. An energy generator according to Claim 1 or Claim 2 in which the absorbing and accumulating means (12) can reach temperatures greater than approximately 800¹C.

4. An energy generator according to any one of Claims 1 to 3 , characterized in that the hollow body (20) houses a plurality of capsules or tubes (30, 330) containing a two-phase material or compound with a phase transition temperature substantially in the region of the range of temperatures that can be reached inside the chamber (22) that is subject to irradiation by the solar rays concentrated by the concentration system (10) .

5. An energy generator according to Claim 4, characterized in that the two-phase material or compound is sodium chloride.

6. An energy generator according to Claim 4, characterized in that the capsules or tubes (30, 330) are hermetically sealed.

7. An energy generator according to Claim 5, characterized in that the capsules or tubes (30, 330) contain nitrogen.

8. An energy generator according to any one of the preceding claims in which the solar ray concentrating system (10) comprises a plurality of concentrating mirrors (16) and collimating mirrors (19) constructed in two successive concentration stages, the concentrating mirrors (16) concentrating the solar radiation onto at least one collimating mirror (18) which reflects the radiation towards the absorption and accumulation means (12) .