WO2013168074A1

WO2013168074A1 - Concentration solar thermodynamic plant

Info

Publication number: WO2013168074A1
Application number: PCT/IB2013/053611
Authority: WO
Inventors: Ernesto ROCCARO; Carla Lazzari
Original assignee: Eni S.P.A.
Priority date: 2012-05-11
Filing date: 2013-05-06
Publication date: 2013-11-14
Also published as: ITMI20120809A1

Abstract

A concentrating solar plant with linear collectors comprising a reflector for reflecting and concentrating solar radiations along a concentration line; a receiver tube positioned along said concentration line, heated by said solar radiations. Said receiver tube comprises an absorber tube in which a heat transfer fluid flows, preferably of the gaseous type, and has a first portion of the mantle facing the sun and a second portion opposite to the first portion. Said absorber tube is made of a metallic material and comprises at least one septum made of a metallic material, positioned longitudinally inside said absorber tube and constrained to it so as to form a thermal contact between said first and second portion of the mantle. Alternatively, said concentrating solar plant with linear collectors comprises an absorber tube having a monolithic structure made of a metallic material comprising, in its interior, a plurality of longitudinal channels having a transversal form and arranged so as to form at least one communication area between said first and second portions of the mantle which extends along the whole length of the absorber tube.

Description

THERMODYNAMIC SOLAR CONCENTRATION PLANT

The present invention relates to a concentrating sola plant for producing energy through the conversion of solar radiation to heat at a high temperature.

More specifically, the present invention relates to a concentrating solar plant of the type with linear collectors .

An example of a concentrating solar plant is that of the linear parabolic type, in which direct solar radiation is concentrated through linear parabolic reflecting systems, called parabolic troughs, on a straight receiver tube, generally situated in the focus of parabolas, in which a suitable heat transfer fluid flows. Said parabolic troughs are also able to follow the movement of the sun to optimize the incidence angle .

A further type of linear collector solar plant is that with Fresnel collectors. This plant comprises linear reflectors which concentrate solar radiation on a receiver tube situated in a fixed horizontal position above the collectors.

Said reflectors are capable of rotating along the longitudinal axis so as to follow the movement of the sun, maintaining and concentrating the solar radiation constantly reflected on the receiver tube.

The concentrated solar irradiation increases the thermal energy collected by the tube favouring its heating and consequently heating the heat transfer fluid that flows inside the tube. In common linear solar collector plants, the heated heat transfer fluid present in the receiver tube is used for a series of applications which exploit its thermodynamic state, for example applications connected with the conversion of thermal energy into electric energy through a conventional thermoelectric plant.

Each linear collector represents the independent base unit of a concentrating solar plant, generally called solar field, and mainly consists of a reflector (mirror), a receiver tube, a metallic supporting structure and a movement system which comprises the motor, sensors and controls.

Receiver tubes represent the most delicate element of solar technology and generally comprise a metallic tube, commonly called absorber tube, in which the heat transfer fluid flows.

These receiver tubes generally have a uniform thickness in order to resist the operating pressures generated by the heated transfer fluid, and are generally internally smooth to avoid pressure drops of the heat transfer fluid.

The absorber tube is sometimes coated with a layer of spectrally selective material which increases its absorption of solar radiation.

Occasionally, the metallic absorber tube is encapsulated in a coaxial outer glass tube which protects it and isolates it.

A vacuum can be applied in the interspace defined between the absorber tube and glass tube to prevent degradation of the coating and reduce dispersions.

Receiver tubes commonly comprise elastic bellows which allow the absorption of axial expansions that take place in the absorber tube due to variations in temperature, and joints that allow the receiver tube to rotate with respect to the other components of the structure .

The systems available in the state of the art with linear collectors use a mixture of organic compounds as heat transfer fluid, normally diathermic oil consisting of diphenyl oxide and biphenyl . The synthetic oil has a solidification temperature equal to 12 °C and a maximum operating temperature close to 400°C.

One of the problems linked to the use of oil in linear collectors, especially in those of the parabolic type that allow higher temperatures to be reached, is associated with the thermal degradation that the oil has at its maximum operating temperature: at 400 °C, a pyrolytic decomposition of the heat transfer fluid takes place with the formation of hydrogen, carbon monoxide and light hydrocarbons.

The oil must therefore be periodically substituted and it may happen that the possible vacuum created between the outer glass tube and absorber tube is progressively reduced due to the permeation of hydrogen from the oil.

Together with these solar plants that use synthetic oils as heat transfer fluid, technologies using water/steam directly in the receiver tubes are also in the process of being developed. In this type of technology, called Direct Steam Generation (DSG) , the water is heated in the receiver tubes of the collectors and converted to overheated steam which is expanded directly in the turbine connected to the current generator (temperature of about 400°C and about 100 bar) . The substitution of oil with steam involves lower investment and operating costs, a higher efficiency and a reduction in environmental risks and fires.

The main technical difficulty of DSG plants is connected with the flow regime which is established inside the absorber tube. A stratified liquid-steam flow can lead to a non-homogeneous distribution of the temperature along the circumference of the tube itself causing mechanical stress which at times can be very dangerous .

DSG systems were the object of DISS European projects (1996-1998) and DISS-2 (1999-2001) during which a "DSG test loop" was projected and constructed at the Plataforma Solar de Almeria (Spain) consisting of a single row of 300 kWth collectors, divided into two sections, one for evaporation of the water and the other for producing overheated steam (T>400°C) .

A further technical solution consists of the use of mixtures of salts as heat transfer fluid. These inorganic fluids have a maximum operating temperature ranging from 500°C to 550°C, depending on the salts used, a vapour pressure which at the maximum temperature is a few mbar and a high thermal capacity. The salts most commonly used are eutectic mixtures of nitrates, for example NaN0₃ 60% by weight - KN0₃ 40% by weight .

The main problem in the use of these mixtures of salts as heat transfer fluid in the solar field lies in their high melting point, which requires that their temperature be maintained at above 250-260 °C with energy consumption for keeping the mixture liquid during long periods of limited or zero solar irradiation.

This type of solar plant requires particular attention in the projecting and management phases in order to avoid solidification of the salts in the pipes of the plant. Furthermore, significant quantities of energy are required in the start-up phase for the melting .

Considerable developments have been made in the last 25 years in the use of molten salts together with various demonstrative experiments. Among these one of the most significant examples was that effected at the combined cycle electric power plant of Priolo Gargallo (Sicily) owned by ENEL, which was integrated with a solar field which uses a mixture of molten salts, consisting of sodium and potassium nitrates (Hitec® Solar Salt), as both heat transfer fluid within the solar field and as thermal storage (Enea technology) .

Solutions of solar plants with solar collectors using gaseous heat transfer fluids have recently been proposed. The use of gas overcomes the limitations on the operating temperatures typical of other heat transfer fluids.

Gas allows high thermodynamic performances to be obtained due to the absence of intrinsic limitations on the maximum temperature that can be reached. Furthermore, the use of a gas as heat transfer fluid improves the safety and environmental impact of solar collector plants.

In spite of this, the use of gas as heat transfer fluid requires higher pressure values due to the low density .

Furthermore, as thermodynamic plants using gas can reach higher temperatures, the materials forming the receiver tube can undergo higher circumferential and axial thermal stress.

Experimentations in this respect are being carried out in Spain at the Plataforma Solar de Almeria (PSA) , where the use of pressurized gas (CO₂) is being tested, and in Italy within the ESTATE project developed by a collaboration between the University of Cagliari and the Centro di Ricerca, Sviluppo e Studi Superiori (Research, Development and Higher Study Centre) in Sardinia .

In particular, one of the main problems connected with parabolic solar trough plants is represented by the fact that the receiver tube is irradiated by solar radiation only on a fraction of its surface; the solar radiations can, in fact, only be concentrated on the surface portion facing the parabolic reflector, whereas the remaining surface portion is heated by direct solar radiation .

The fact that the receiver tube is heated by concentrated solar radiations only on a portion of its surface causes a thermal stratification phenomenon of the heat transfer fluid flowing internally and a difference in temperature between the two portions of the tube, which causes thermal distortions of the tube itself .

This drawback also occurs in plants of the Fresnel type as the side of the receiver tube opposite the sun is irradiated by reflected solar radiations, whereas the side facing the sun is irradiated by a part of the same radiations which is further reflected towards the tube by a reflecting element situated above the tube. This difference in irradiation between the two portions of receiver tube, generates a thermal stratification phenomenon of the heat transfer fluid and a difference in temperature between these two portions.

In particular, the heatings localized on a portion of the tube cause a greater thermal energy transfer from this side, causing a local variation in the density of the heat transfer fluid and therefore the other thermodynamic and relative physical properties. This phenomenon mainly takes place when the heat transfer fluid is gaseous. In this case, the difference in temperature between the two portions of the absorber tube can exceed 100°C.

In particular, parabolic reflectors reach concentration ratios of solar radiation 50-100 times higher than direct radiation.

For this reason, the hot side can have a much higher temperature than the cold side of the absorber tube, even if the heat is transferred along the mantle by conduction.

This phenomenon increases using heat transfer fluids having a low density or low thermal capacity, for example using gas.

Various solutions for solving this problem are known in the state of the art.

An example is described in the document WO 2011/055401, in which a concentrating solar plant provides for a plurality of fins arranged inside the absorber tube for increasing the thermal exchange of the convective type.

This type of absorber tube reduces the thermal stratification phenomenon of the heat transfer fluid, improving the heat transmission by convection, i.e. by increasing the convective exchange between the hot wall of the tube and the fluid, with the double effect of obtaining an increase in the temperature of the fluid and a heat transmission towards the portion of absorber tube not exposed to concentrated solar rays.

In the state of the art, it is also known that the same effect can be obtained by increasing the surface roughness inside the tube, in order to increase the turbulence of the fluid and therefore increase the heat exchange both within the fluid and between the fluid and inner surface of the tube.

A further solution is represented by introducing a series of helicoidal generators on the internal wall of the tube, i.e. helix-wound strips, in order to increase the turbulence and continuously renew the portion of fluid in contact with the portion of absorber tube heated by concentrated solar rays, thus preventing the development of a stratified flow (Massidda, L., and Varone, A., 200X, A Numerical Analysis of a High Temperature Solar Collecting Tube, Using Helium as an Heat Transfer Fluid, Internal Note XX, CRS4)

In spite of this, the above solutions only partly solve the problem of the temperature difference generated between the cold part and hot part of the absorber tube during the solar irradiation phase.

The above solutions reduce the difference in temperature between the two portions of the absorber tube, exploiting the convective motions of the heat transfer fluid, in the case of a heat transfer gas, however, the heat exchange between the two portions of tube is limited, only partly solving the problem.

A further solution to the problem of the thermal stratification of the heat transfer fluid is described in patent US 4583519, in which, absorber tubes internally divided by one or more septa are used in a solar tower plant.

In particular, the absorber tube is divided by one or more septa positioned orthogonally to the direction of the solar rays, so as to form a plurality of passage channels for the heat transfer fluid inside the absorber tube.

The passage channels created can have a variable transversal form and dimensions proportional to the heat absorbed by the relative portion of wall of the absorber tube.

In this solution, the thermal stratification phenomenon of the heat transfer fluid is mitigated by passing flows of heat transfer fluid in the different passage channels, having different rates depending on the heat absorbed by the specific passage channel.

Thanks to the differentiated passage of the heat transfer fluid in the passage channels of the absorber tube, the above solution solves the problem of thermal stratification of the heat transfer fluid, without solving the problem, however, of the thermal difference between the hot portion and cold portion of the absorber tube, leaving the problem of thermal distortions of the receiver tube unsolved.

An objective of the present invention is to overcome the drawbacks mentioned above and in particular to conceive a concentrating solar plant with linear collectors which reduces the difference in temperature that is generated between the portion of absorber tube facing the sun, hereinafter called "cold portion", and the opposite portion heated by reflected and concentrated solar radiation, hereinafter called "hot portion".

A further objective of the present invention is to provide a concentrating solar plant with linear collectors which is particularly suitable for the use of a gaseous heat transfer fluid.

Another objective of the present invention is to provide a concentrating solar plant with linear collectors which is simple to produce and assemble and which can be produced, with particular reference to the receiver tube, at reduced costs.

Yet another objective of the present invention is to provide a thermodynamic plant with linear collectors having a greater efficiency and capable of reaching higher temperatures of the heat transfer fluid at the outlet of the plant.

A further objective of the present invention is to provide a concentrating solar plant with linear collectors which is safe and reliable with time.

A first object of the present invention relates to a concentrating solar plant with linear collectors comprising a reflector for reflecting and concentrating solar radiations along a concentration line; a receiver tube positioned along said concentration line heated by said solar radiations, comprising an absorber tube in which a heat transfer fluid flows, having a first portion of the mantle facing the sun and a second portion opposite to the first portion, wherein said absorber tube is made of a metallic material and comprises at least one septum made of a respective metallic material, positioned longitudinally inside said absorber tube and constrained to it so as to achieve a thermal contact between said first and second portion of the mantle.

In particular, according to the invention, the number of septa is less than five, preferably two or three .

Furthermore, according to the invention, said receiver tube can comprise and outer tube permeable to solar radiations which encapsulates said absorber tube.

Said heat transfer fluid can be a gas, preferably helium, or carbon dioxide, or air.

According to the invention, the metallic material of said absorber tube and/or the metallic material of said septum can be stainless steel or copper or a copper alloy.

Furthermore, according to the invention, said septa can be parallel to each other and aligned with the direction of the solar radiations.

Said plant can also comprise a coating layer of a spectrally selective material, with a high absorbance of solar radiations and a low heat emission within the infrared, covering the outer mantle of said absorber tube .

A second object of the present invention relates to a concentrating solar plant with linear collectors comprising a reflector for reflecting and concentrating solar radiations along a concentration line; a receiver tube positioned along said concentration line heated by said solar radiations, comprising an absorber tube in which a heat transfer fluid flows, having a first portion of the mantle facing the sun and a second portion opposite to the first portion, wherein said absorber tube has a monolithic structure made of a metallic material comprising, in its interior, a plurality of longitudinal channels having a transversal form and arranged so as to form at least one communication area between said first and second portions of the mantle which extends along the whole length of the absorber tube.

According to the present invention, said heat transfer fluid can be a gas.

Furthermore, according to the invention, said longitudinal channel of said plurality of longitudinal channels has a circular transversal form, or said plurality of longitudinal channels has an overall honeycomb transversal form.

In particular, according to the invention, the metallic material of said absorber tube can be stainless steel or a copper alloy.

Said concentrating solar plant with linear collectors can have linear parabolic troughs or alternatively linear Fresnel collectors.

The characteristics and advantages of the concentrating solar plant with linear collectors according to the present invention will appear more evident from the following illustrative and non- limiting description, referring to the enclosed schematic drawings, in which:

- figure la shows a schematic view of a concentrating solar plant with linear collectors;

- figure lb shows the section of a linear collector of the parabolic type;

- figure lc shows the section of a receiver tube according to an embodiment of the present invention;

- figure 2a shows the section of an absorber tube of a concentrating solar plant with linear collectors according to a first embodiment of the present invention;

- figure 2b shows the section of an absorber tube of a concentrating solar plant with linear collectors according to a second embodiment of the present invention;

- figure 2c shows the section of an absorber tube of a concentrating solar plant with linear collectors according to a third embodiment of the present invention;

- figure 3a shows the section of an absorber tube of a concentrating solar plant with linear collectors according to a fourth embodiment of the present invention;

- figure 3b shows the section of an absorber tube of a concentrating solar plant with linear collectors according to a fifth embodiment of the present invention.

With reference to figure 1, this shows a concentrating solar plant with linear collectors of the parabolic type, indicated as a whole with 100, for heating a heat transfer fluid flowing in a plurality of receiver tubes 102, said heat transfer fluid arriving through a supply tube 103 from a feeding system (not shown) , and once heated, the heat transfer fluid being sent by means of a return tube 104 to a utilization system (not illustrated) positioned downstream of the solar plant .

Said concentrating solar plant with linear parabolic collectors can comprise a plurality of parabolic reflectors suitable for reflecting and concentrating solar radiations 105 in their focus.

Receiver tubes that can be heated either directly or reflected by solar radiations, can be positioned in the focus of said parabolic reflectors.

With particular reference to figure lb, said direct solar radiations 105' are those that irradiate the portion of tube exposed to the sun, whereas the reflected solar radiations 105'' are those that irradiate the tube in its portion that faces the parabolic reflector 101 and which, without the same, would be in the shade.

The parabolic reflector 101 can be a highly reflecting surface, generally a mirror, oscillating with respect to a longitudinal axis and having the section in the form of a parabola.

Said parabolic reflector 101 is capable of reflecting solar radiations 105 by concentrating them on the receiver tube 102.

The receiver tube 102 normally comprises an absorber tube 106 and an outer tube 108 permeable to solar radiations 105 that surrounds the absorber tube 106.

The solar radiations 105, whether they be direct 105' or reflected 105'', pass through the outer tube 108 irradiating the absorber tube 106.

A heat transfer fluid, preferably a heat transfer gas such as helium, carbon dioxide or air, flows in the absorber tube 106.

In a preferred embodiment, said absorber tube 106 is made of a metallic material, preferably stainless steel, copper or a copper alloy.

The absorber tube 106 can be covered by a coating layer 107 made of a spectrally selective material with a high absorbance of solar radiations, preferably higher than 94%, and a low heat emission within the infrared, preferably lower than 15% at 350°C.

Said outer tube 108 permeable to solar radiations encapsulates said absorber tube 106, and the two tubes can be positioned coaxially.

In particular, the internal diameter of the outer tube 108 is greater than the outer diameter of the absorber tube 106 so as to create an interspace between the two tubes .

The outer tube 108 can be produced with materials transparent to solar radiations, allowing the same to filter through the outer tube 108 and reach the absorber tube 106.

Said outer tube 108 is preferably made of high- transmittance borosilicate glass and has the function of insulating the absorber tube 106. Said outer tube can also receive an anti-reflection treatment.

Said outer tube 108 is connected to the absorber tube 106 by means of metallic bellows (not shown) whose function is to further compensate any possible thermal expansions between the absorber tube 106 and outer tube 108, due to temperature gradients.

A vacuum can be produced in the interspace created between the absorber tube 106 and outer tube 108, for reducing thermal dispersions, protecting the coating surface 107 of the absorber tube 106 if contact with the air jeopardizes its performances, and reducing heat losses which take place by convection between the two tubes when they are at a high temperature.

The vacuum in the interspace is preferably created at a pressure lower than 1 Pa, even more preferably at 10^~4 mmHg.

In a particular embodiment of the concentrating solar plant, absorbing elements (not illustrated), generally known as "getters", i.e. metallic elements that absorb gaseous molecules, are installed in the vacuum interspace.

In particular, if the heat transfer fluid is oil, said absorbing elements absorb hydrogen coming from the partial decomposition of the high-temperature heat transfer oil, which permeates into the vacuum interspace with time.

As the absorber tube 106 is irradiated along a first portion of the mantle by direct solar radiations 105' and along a second portion of the mantle by reflected solar radiations 105'', it can be heated non- homogeneously .

This fact can cause uneven thermal expansion phenomena of the material forming the absorber tube 106.

This phenomenon particularly arises when a heat transfer fluid having a low thermal capacity, for example a gas, is flowing in the absorber tube 106.

The thermal exchange which would normally take place between the absorber tube 106 and heat transfer fluid, can in fact also occur between the heat transfer fluid and absorber tube 106, in the "cold" portion of the absorber tube 106.

When a gaseous-type heat transfer fluid is used in the concentrating solar plant, however, the low thermal capacity of the fluid and limited heat exchange coefficients between the absorber tube 106 and heat transfer gas, increase the problem of strong thermal gradients between the different portions of the absorber tube 106.

The strong thermal gradients can create thermal deformations in the absorber tube 106 and cause risks of misalignment of the receiver tube 102 from the focal axis or even breakage of the outer tube 108.

The maximum temperature values in the absorber tube 106 and temperature gradient in a section of the same, are important factors for producing the concentrating solar plant 100 and for the structural stability of the parabolic collector.

It is therefore fundamental to increase the heat transmission in the absorber tube 106 along the direction Y, parallel to the solar rays 105, in order to reduce the maximum temperature and radial temperature gradient in the tube, thus obtaining a more uniform temperature distribution in the absorber tube 106 and heat transfer fluid.

According to the first object of the present invention, said absorber tube 106 of said concentrating solar plant 100 with linear parabolic collectors comprises a metallic septum 109 in its interior which allows heat exchange by conduction between the hot portion of the tube 106, i.e. that irradiated by reflected radiations 105'', and the cold portion, i.e. that irradiated by direct radiations 105' .

Said metallic septa 109 are metallic inserts positioned longitudinally inside said absorber tube 106 and constrained to it so as to create a thermal contact between the hot portion and cold portion of the mantle.

In particular, there are preferably one, two or three metallic septa 109 arranged inside the absorber tube 106, to increase the above heat exchange, as illustrated in figures 2a, 2b and 2c.

The metallic material for producing said metallic septa 109 may preferably be selected from stainless steel or copper, due to the optimum relationship between mechanical performances, thermal capacity and cost. In particular, copper has a high thermal conductivity (390 w/m/K at 150°C) and thermal expansion coefficients comparable with those of steel.

Due to the high operating temperatures (higher than 500°C) and high pressures, said metallic septa 109 can more preferably be made of a copper alloy in order to increase their mechanical resistance, for example the alloys AMPCOLOY® of AMPCO METAL or Cu-Be alloys of Smiths Metal Centres Ltd.

Said metallic septa 109 preferably have a thickness of 3 mm and said absorber tube 106 has a substantially cylindrical form with a thickness of the mantle of 3 mm .

Said absorber tube 106 can be produced with the same materials selected for producing said metallic septa 109.

Said metallic septa 109 have the additional advantage of increasing the exchange surface between the heat transfer fluid and absorber tube 106.

Said metallic septa 109 can be installed in the absorber tube 106 so as to create an interference along the longitudinal profile of the septum thus creating a rigid structure capable of resisting brusque variations in the pressures inside the tube.

According to the second object of the present invention, said absorber tube 106 of said concentrating solar plant 100 with linear parabolic collectors has a metallic monolithic structure, preferably having an outer cylindrical form, and having a plurality of longitudinal channels 110 in its interior, in which the heat transfer fluid flows.

Inside its monolithic structure, said absorber tube 106 has at least one longitudinal communication area 111, between said longitudinal channels 110, between said first portion of the mantle and said second portion of the mantle.

Said longitudinal channels 110 extend for the whole length of the absorber tube 106, crossing it from one end to the other.

The heat can be transferred by conduction along said communication area 111, from said first portion to said second portion of the absorber tube 106, facilitating a thermal re-equilibrium of the absorber tube 106.

Said longitudinal channels 110 can have a varying section in relation to the operating variables, for example pressure or flow-rate of the heat transfer fluid. They can also have a varying transversal form, i.e. section, in relation to the nature of the heat transfer fluid, depending on whether it is liquid or gaseous .

The transversal form and arrangement of said longitudinal channels 110 are such as to optimize the heat transmission along the direction Y and passage of the heat transfer fluid through the absorber tube 106.

Said longitudinal channels are preferably arranged symmetrically with respect to the direction Y, so as to produce a communication area 111 inside the absorber tube 106 substantially aligned with the axis Y. Said longitudinal channels 110 preferably have a circular transversal form.

In a preferred version, there are four of said longitudinal channels 110 having a circular form and they are symmetrically arranged in pairs with respect to the direction Y, as illustrated in figure 3a.

In a further preferred embodiment, said channels 110 are positioned so that any section orthogonal to the axis of the absorber tube 106, has a honeycomb form, in which the walls of the various cells of the honeycomb, forming the communication area 111, are parallel or perpendicular to the direction Y, as illustrated in figure 3b.

Said absorber tube 106 can be produced by extrusion or by the perforation of a solid metallic bar.

Said longitudinal channels 110 have the further advantage of increasing the exchange surface between the heat transfer fluid and absorber tube 106.

Said absorber tube 106 can preferably be produced with a metallic material selected from stainless steel, copper or a copper alloy, for example the alloy AMPCOLOY® of AMPCO METAL or one of the Cu-Be alloys of Smiths Metal Centres Ltd.

Said metallic monolithic structure of the absorber tube 106 is preferably produced with a metallic material having a high thermal conductivity.

Said channels are preferably at a distance of at least 3 mm from each other and at least 3 mm from the outer surface of the absorber tube 106. A further embodiment of the thermodynamic solar plant with linear collectors can comprise linear collectors of the Fresnel type, in which the absorber tube of said collectors corresponds to the characteristics described for both objects of the present invention.

The various embodiments of said metallic monolithic structure of the absorber tube 106 advantageously allow an effective heat exchange along the communication area 111 between said first and second portion of the absorber tube 106.

The above advantage can also be found in the absorber tube 106 comprising one or more septa, as the thermal exchange in these between one portion and another of the tube takes place more efficiently by conduction .

Advantageously said concentrating solar plant is particularly suitable for the use of a heat transfer fluid of the gaseous type, improving the overall efficiency of the plant due to the higher temperature that can be reached, and avoiding the drawback of phase changes of the heat transfer fluid at low temperatures. Said plant also improves safety from an environmental point of view.

In the case of the use of a heat transfer gas, the concentrating solar plant 100 with linear collectors can also comprise a direct thermal storage system with solid elements (cement, ceramics, etc.) which also allows a significant reduction in storage costs. The absorber tube according to both objects of the present invention can be advantageously and easily produced by means of common mechanical processings known in the state of the art.

The concentrating solar plant with linear collectors of the present invention thus conceived can in any case undergo numerous modifications and variants, all included in the same inventive concept. The protection scope of the invention is therefore defined by the enclosed claims.

Claims

1) A concentrating solar plant (100) with linear collectors comprising:

- a reflector (101) for reflecting and concentrating solar radiations (105) along a concentration line;

- a receiver tube (102) positioned along said concentration line, heated by said solar radiations

(105) , comprising:

- an absorber tube (106), in which a heat transfer fluid flows, having a first portion of the mantle facing the sun and a second portion opposite to the first portion;

characterized in that said absorber tube (106) is made of a metallic material and comprises at least one septum (109) made of a respective metallic material, positioned longitudinally inside said absorber tube

(106) and constrained to it so as to form a thermal contact between said first and second portion of the mantle .

2) The concentrating solar plant (100) with linear collectors according to claim 1, wherein the number of septa (109) is less than five.

3) The concentrating solar plant (100) with linear collectors according to claim 2, wherein there are two or three septa (109) .

4) The concentrating solar plant (100) with linear collectors according to any of the previous claims, wherein said receiver tube (102) comprises an outer tube (108) permeable to solar radiations (105) which encapsulates said absorber tube (106) .

5) The concentrating solar plant (100) with linear collectors according to any of the previous claims, wherein said heat transfer fluid is a gas. 6) The concentrating solar plant (100) with linear collectors according to claim 5, wherein said heat transfer gas is helium or carbon dioxide or air.

7) The concentrating solar plant (100) with linear collectors according to any of the previous claims, wherein the metallic material of said absorber tube (106) is stainless steel or copper or a copper alloy .

8) The concentrating solar plant (100) with linear collectors according to any of the previous claims, wherein the metallic material of said septum (109) is stainless steel or copper or a copper alloy.

9) The concentrating solar plant (100) with linear collectors according to any of the previous claims, wherein said septa (109) are parallel to each other and aligned with the direction of the solar radiations (105) .

10) The concentrating solar plant (100) with linear collectors according to any of the previous claims, comprising a coating layer (107) of a spectrally selective material, with a high absorbance of solar radiations (105) and a low heat emission within the infrared, covering the outer mantle of said tube absorber (106) .

11) A concentrating solar plant (100) with linear collectors comprising:

- a receiver tube (102) positioned along said concentration line heated by said solar radiations

(105), comprising:

characterized in that said absorber tube (106) has a monolithic structure made of a metallic material comprising, in its interior, a plurality of longitudinal channels (110) having a transversal form and arranged so as to form at least one communication area (111) between said first and second portions of the mantle which extends along the whole length of the absorber tube (106) .

12) The concentrating solar plant (100) with linear collectors according to claim 11, wherein said heat transfer fluid is a gas.

13) The concentrating solar plant (100) with linear collectors according to claim 11, wherein said longitudinal channel of said plurality of longitudinal channels (110) has a circular transversal form.

14) The concentrating solar plant (100) with linear collectors according to claim 11, wherein said plurality of longitudinal channels (110) has an overall honeycomb transversal form.

15) The concentrating solar plant (100) with linear collectors according to any of the claims from 11 to 14, wherein the metal material of said absorber tube (106) is stainless steel or copper or copper alloy .