WO2016128863A1

WO2016128863A1 - A method and a system for generating steam in a planar structure using solar radiation or another radiation

Info

Publication number: WO2016128863A1
Application number: PCT/IB2016/050569
Authority: WO
Inventors: Pietro ASINARI; Eliodoro Chiavazzo
Original assignee: Politecnico Di Torino
Priority date: 2015-02-12
Filing date: 2016-02-04
Publication date: 2016-08-18
Also published as: ITUB20150481A1

Abstract

The invention refers to a method for generating steam (V) that provides, in particular, to direct a solar radiation or another radiation (R) onto a planar structure (100), and specifically onto a support (2) of said planar structure (100) comprising a material (4) absorbent of said radiation (R), so that the thermal energy released by said radiation (R) causes the evaporation of a fluid (F) that flows as a thin film into a thin cavity (3) of said planar structure (100), with consequent production of steam (V); the invention also refers to the relevant system for generating steam (V) that uses said method and that comprises at least one planar structure (100) comprising, in turn, an upper element (1), a support (2) comprising a material (4) absorbent of solar radiation or another radiation (R), a thin cavity (3) between said upper element (1) and said support (2) inside which a fluid (F) flows as a thin film, onto said planar structure (100) being directed said radiation (R) reflected by at least one reflective device (16) facing said planar structure (100), said system further comprising at least a duct (13) through which is discharged the steam (V) produced by the evaporation of said fluid (F) due to the thermal energy released by said radiation (R). The method and the system according to the present invention are preferably and advantageously employed for generating steam and its subsequent condensation in purification and/ or desalination plants for sea, river, swamp or rain water; the present invention is particularly suitable for use in remote regions and for so-called "off-grid" applications.

Description

Ά method and a system for generating steam in a planar structure using solar radiation or

DESCRIPTION

TECHNICAL FIELD

The present invention refers to the energy sector and, specifically, to the generation of steam.

More precisely, the present invention refers to a method, and to the relevant system, for generating steam in a planar structure by means of solar radiation or other radiation.

Even more precisely, the present invention refers to a method, and to the relevant system, for generating steam in a planar structure by means of solar radiation or other radiation, wherein the fluid that is made to evaporate is in the form of a thin film.

The present invention is preferably and advantageously applied when employed for generating steam and its subsequent condensation in purification and/ or desalination plants for sea, river, swamp or rain water; the present invention is particularly suitable for use in remote regions and for so-called "off-grid" applications.

PRIOR ART

Solar steam generators, or evaporators, are known that are generally formed of a closed container equipped with a transparent cover able to let the solar radiation pass through and, at the same time, to ensure a cold surface for condensation; examples of such solar evaporators, in their different variants, are described in the scientific publication V. Sivakumar, E.Ganapathy Sundaram, "Improvement techniques of solar still efficiency: A review", Renewable and Sustainable Energy Reviews 28, pp. 246-264, 2013.

In particular, the evaporation occurs by means of a free surface or a porous matrix, in contact with humid air trapped inside the evaporator and subject to natural convection. Such a configuration has some inherent limitations: (a) the radiation must pass through a transparent medium, which absorbs a not negligible amount of solar radiation, dispersed to the environment; (b) the traditional devices are subject to losses due to convective flows external to the evaporator; (c) in the traditional evaporators, the ratio of the surface subject to the solar radiation to the volume of water contained in the evaporation chamber is quite low; (d) vice versa, the ratio of the dispersing surface towards the environment to the same volume of water in the chamber is quite high, resulting in a low energy efficiency.

Methods and systems for generating steam by exploiting solar radiations or other radiations are also known; examples of such methods and systems can be found in the International application published at no. WO 2004/035168 A2, in the US Patent application no. US 2008/ 0164135 Al and in the International application published at no. WO 2014/172859 Al.

The International application published at no. WO 2004/035168 A2 describes a method for the desalination of water by means of solar energy and, in particular, among the several proposed solutions, it provides for the handling of a thin film of salt water laid onto a belt conveyor.

The solution described in such document, though addressing the technical problem of generating steam by exploiting solar radiations, however is not able to guarantee the optimal thickness of the thin film laid onto the belt conveyor and to control the fluid flowrate in the thin film, independently from the sliding speed of the belt; furthermore, the belt of the conveyor is subject to optical wearing, since, besides fulfilling the function of absorbing the solar radiation, it must also ensure the mechanical handling; furthermore, the handling of the fluid is entirely entrusted to the drive of the belt conveyor and can not use other phenomena, such as the capillarity in the thin gaps; finally, movement elements and/ or actuators necessarily are required for carrying out the described method.

The aforesaid International application published at no. WO 2004/035168 A2 also shows the use of a planar structure for generating steam by exploiting solar radiations or other radiations; however, said planar structure does not allow to realize thin films confined between two parallel flat surfaces, being substantially a mobile belt, moved by two rollers, onto which a thin layer of salt water is deposited. Therefore, in this document, the thin film, and in particular its thickness, is uncontrollable beyond certain flowrates, namely when the deposited drops are subject to coalescence.

The US Patent application no. US 2008/0164135 Al and, likewise, the International application published at no. WO 2014/172859 Al describe generic paraboloid concentration systems for the treatment of water; however, such solutions address neither the reduction of losses by the receiver, nor that of its thermodynamic efficiency for the purposes of the production of steam.

Methods and systems for generating of steam are also known in which the fluid evaporates in the form of a thin film; an example of such methods and systems is shown in the Canadian Patent no. CA 2 347 456 C that describes the realization of the process of evaporation in a thin film through the use of canvases, fabrics or similar hydrophilic materials. However, the aforesaid technical solution necessarily requires the use of materials with non-repeatable performances and not well controllable, as they are based on a substantially disordered microscopic structure. Moreover, these materials are subject to wear and degradation over time as a result of continuous operation. Finally, also in this case, the evaporating fluid flowrate is closely linked to the handling speed of the canvas.

Therefore, the need of generating steam by means of solar receivers for concentration systems, able to maximize the evaporating fluid flowrate and consequently the exploitation of the solar energy, remains unsatisfied.

Furthermore, the need of effectively limiting the thermal losses towards the environment and of reducing the optical losses remains unsatisfied.

In brief, up to the present time, to the Applicant's knowledge, there are not known solutions allowing to carry out evaporation in thin films, by exploiting the best optical materials available today, for the reflection of radiation and for the selective absorption/ emission of the solar radiation in the field of the spectrum of interest for thermal applications, while simultaneously avoiding the optical transmission and reducing the thermal losses towards the environment; therefore, the Applicant, with the method and the system according to the present invention, intends to remedy such lack.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the drawbacks of the known prior art related to the steam generation by means of a solar radiation or another radiation. It is also an object of the present invention to overcome the drawbacks of the known prior art related to the steam generation starting from a fluid in the form of a thin film.

More precisely, the present invention intends to solve the problem of the thin film evaporation by means of a solar radiation or another radiation, by exploiting the best optical materials available today, for the reflection and the selective absorption/ emission of the solar radiation, while simultaneously avoiding the optical transmission and minimizing the thermal losses towards the environment. In particular, an object of the present invention is to provide a method for the generation of steam by means of a solar radiation or another radiation in a planar structure within which the evaporating fluid flows in the form of a thin film, possibly exploiting the capillarity phenomenon.

Furthermore, an object of the present invention is to provide a system for the generation of steam by means of a solar radiation or another radiation comprising a planar structure within which the fluid that is made to evaporate flows in the form of a thin film, possibly exploiting the capillarity phenomenon.

The aforesaid and other objects and advantages of the invention, as they will appear from the following description, are achieved with a method for generating steam in a planar structure using a solar radiation or another radiation like the one according to claim 1.

Moreover, the aforesaid and other objects and advantages of the invention are achieved with a system for generating steam in a planar structure using a solar radiation or another radiation like the one according to 8.

Preferred embodiments and variants of the method and the system of the present invention are the subject-matter of the dependent claims; in particular, in a preferred and advantageous embodiment, the method and the system according to the invention provide for the combination with a device suitable to condense the produced steam, the obtained condensate being collected and subsequently utilized. In a further embodiment, the method and the system according to the present invention are employed in purification and/ or desalination plants for sea, river, swamp or rain water.

It is understood that all the appended claims form an integral part of the present description and that each of the technical features therein claimed is possibly independent and autonomously usable with respect to the other aspects of the invention.

It will be immediately evident that countless modifications (for example relevant to shape, sizes, arrangements and parts with equivalent functionality) could be brought to what described without departing from the scope of the invention as claimed in the appended claims.

Advantageously, the technical solution according to the present invention that carries out the generation of steam in a planar structure, allows:

- the use in remote or so-called "off-grid" applications thanks to the fact that it does not require any electrical energy;

- the use of a thin planar structure, per unit volume of water in an evaporation chamber, greatly reduces the dispersing surface through which the thermal flows pass towards the environment;

- the use of a thin planar structure, per unit volume of water in an evaporation chamber, maximizes the surface subject to the solar radiation;

- in the inverted configuration (i.e. with the radiation directed from downwards to upwards), it allows to effectively prevent, by means of suitable barriers, the external convective flows;

- the inverted configuration, then, can be well adapted to the use of reflective solar concentrators, typically having higher efficiencies than the transmission ones; and - it does not require the use of transparent materials, thus increasing the optical efficiency.

Further advantageous features will appear more evident from the following description of preferred but not exclusive embodiments, merely given by way of explanatory and not limiting example.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described hereinbelow by means of some preferred embodiments, given by way of explanatory and not limiting example, with reference to the accompanying drawings. These drawings illustrate different aspects and examples of the present invention and, where appropriate, similar structures, components, materials and/ or elements in different figures are denoted by similar reference numbers.

FIG. 1 is a flow chart showing the steps of the method for generating steam in a planar structure using solar radiation or another radiation for the thin film regenerative condensation according to the present invention; and

FIG. 2 is a schematic representation of a general embodiment of the system for generating steam in a planar structure using solar radiation or another radiation for the thin film regenerative condensation according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible of various modifications and alternative constructions, some preferred embodiments are shown in the drawings and will be described in detail hereinbelow.

It should be understood, however, that there is no intention to limit the invention to the specific illustrated embodiment but, on the contrary, the invention intends to cover all the modifications, alternative constructions and equivalents that fall within the scope of the invention as defined in the claims.

In the following description, therefore, the use of "for example", "etc." and "or" denotes non-exclusive alternatives without limitation, unless otherwise indicated; the use of "also" means "among, but not limited to", unless otherwise indicated; the use of "includes / comprises" means "includes / comprises, but not limited to", unless otherwise indicated. The method and the system of the present invention are based on the innovative concept of using and converting radiative energy, with specific reference to solar energy.

An important feature of said method and system resides in the fact that they do not require any electrical energy and, therefore, they are particularly suitable - although certainly not limited - to remote or so-called "off-grid" applications.

In the present description, the term "solar radiation or another radiation" means any electromagnetic radiation; more precisely, the term "solar radiation or another radiation" means the radiation directly or indirectly received from the sun or from any other hot body, industrial device or thermal refuse of an industrial process.

In the present description, the term "thin film" means a thin layer of a fluid confined in the thin cavity between two surfaces.

In the present description, the term "planar structure" means a pair of flat surfaces having any shape able to form a thin cavity within which a thin film of fluid, which completely fills the thin cavity itself and is possibly subject to the capillarity phenomenon, is housed. One of the two flat surfaces is made or is in good thermal contact with an optical material with properties of absorption and selective emission of radiation, having the purpose of maximizing the amount of energy absorbed by the structure itself. In the present description the term "planar cell" is intended to have the same meaning of the term "planar structure" and it is indifferently used to denote the same element.

With reference to Fig. 1, the method for generating steam V in a planar structure 100 using solar radiation or another radiation R according to the present invention comprises the steps of:

a. providing at least one planar structure 100 comprising:

an upper element 1,

^■ a support 2,

^■ a thin cavity 3 between said upper element 1 and said

support 2,

■ a hole 5, and ^■ at least one duct 13,

wherein said hole 5 is positioned on the face of said upper element 1 opposite to said thin cavity 3 and wherein the face of said support 2 opposite to said thin cavity 3 comprises a material 4 absorbent of solar radiation or another radiation R

(step 101);

b. providing at least one tank 8 containing a fluid F (step 102); c. providing at least one reflective device 16 of solar radiation or

another radiation R (step 103);

d. connecting said tank 8 containing said fluid F, through a

connecting tube 6, to said at least one planar structure 100 in correspondence of said hole 5 (step 104);

e. making said fluid F flow into said thin cavity 3, thus obtaining a

thin film of said fluid F (step 105);

f. positioning said at least one planar structure 100 facing said at

least one reflective device 16 so that said face of said support 2, opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R, faces and corresponds to the reflecting point/s P of said at least one reflective device 16 (step 106);

g. causing the evaporation of said fluid F and the production of

steam V by releasing the thermal energy of said solar radiation or another radiation R onto said planar structure 100, specifically onto said face of said support 2 opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R (step 107); and

h. discharging said steam V through said at least one duct 13 (step 108).

The method according to the present invention can further, optionally, comprise the following steps: i. providing at least one cold surface 15 (step 109);

j. directing said steam V, discharged through said at least one duct 13, towards said at least one cold surface 15, onto which the condensation thereof occurs (step 110); and

k. collecting the produced condensate for a subsequent use (step

111).

It is intended here to specify that said absorbent material 4 of solar radiation or another radiation R is a high efficiency absorbent material, this meaning a material suitable to absorb 95% of the incident solar energy and to convert the 90% thereof into thermal energy, dispersing the difference in the form of radiation towards the environment.

Thanks to the use of said high efficiency absorbent material, the planar structure 100 acts as an almost ideal optical absorber, that is with the absorption coefficient of 90% with respect to the incident radiation.

Said material 4 can be used to cover the face of said support 2 opposite to said thin cavity 3; as an alternative, said material 4 forms a coating layer of the face of said support 2 opposite to said thin cavity 3.

In the case said 4 material is used to cover the face of said support 2 opposite to said thin cavity 3, preferably it is a covering of suitable oxides able to guarantee the necessary selective coating properties; more preferably it is a layer of titanium dioxide; even more preferably it is a layer of commercial titanium dioxide explicitly developed for solar applications, such as for example the TINOX material produced by the German company ALMECO GmbH.

Always in the case said material 4 is used to cover the face of said support 2 opposite to said thin cavity 3, preferably it is applied on said support 2 by means of physical vapor deposition (PVD, Physical Vapor Deposition).

Always in the case said material 4 is used to cover the face of said support 2 opposite to said thin cavity 3, preferably it has a thickness ranging between less than one μιη and a few tens of μιη, more preferably equal to 1-2 μιη.

In the case said material 4, instead, forms a coating layer of the face of said support 2 opposite to said thin cavity 3, preferably it is a covered metal sheet; more preferably it is a sheet of covered copper or aluminum; even more preferably it is a sheet of copper or aluminum covered with titanium dioxide, for example a sheet of TINOX material produced and marketed by the German company ALMECO GmbH.

Always in the case said material 4 forms a coating layer of the face of said support 2 opposite to said thin cavity 3, preferably it has a thickness ranging between 0.1 mm and 2 mm, more preferably equal to 0.2 mm.

Always in the case said material 4 forms a coating layer of the face of said support 2 opposite to said thin cavity 3, preferably it is fixed on said support 2 by gluing, welding and localized melting.

Preferably, said upper element 1 is hydrophilic, in order to distribute the fluid F in the thin cavity 3 by capillarity; more preferably it is hydrophilic and with a low thermal conductivity (for example, not greater than about 1 W/(m*K)); even more preferably it is glass.

It is intended here to specify that "upper" is considered the element of the planar structure 100 placed in connection with the tank 8 and opposite to the support element of the absorbent material 4.

Preferably, said support 2 is made of sufficiently conductive materials from the thermal point of view (for example, with thermal conductivity of the order of 300 W/ (m*K)); more preferably it is made of metal.

It is intended here to specify that a first face of the support 2 is in contact with the fluid F confined in the thin cavity 3, while a second face of the support 2 is covered or is in contact with the absorbent material 4.

Preferably, said thin cavity 3 has a variable size between 100 μιη and 1,000 μιη, preferably equal to 300 μιη.

Preferably, said hole 5 has a diameter ranging between 4 mm and 50 mm, preferably equal to 10 mm.

Since the method according to the present invention is preferably applied in purification and/ or desalination processes, said fluid F is preferably contaminated or salt or brackish, sea, river, swamp or rain water. Since, however, other applications of the method according to the present invention are possible, said fluid F may be of any type depending on the specific needs.

It is wanted here, in particular, to highlight the existence of industrial processes different from the purification and/ or desalination and using small steam flowrates; for example, steam is used as an antioxidant process gas during the annealing step of the copper wire in the wire enameling processes for the windings of electric motors; another example is given by the agri-food field, in the steam food cooking; furthermore, the use of steam in small quantities is present in the paper industry, for separating the cellulose from the lignin.

In the above-cited processes, said fluid F will preferably be contaminated or salt or brackish, sea, river, swamp or rain water.

Said fluid F will preferably have a temperature between 5 °C and 40 °C, will preferably be at the room temperature of about 20 °C.

As mentioned above, the fluid F flows from the tank 8, through the connecting tube 6 and the hole 5, in the thin cavity 3; preferably, the flow of fluid F coming from the tank 8 is adjusted through a valve 7 mounted on the connecting pipe 6 and/ or a septum made of porous material, preferably hydrophilic, 14 inserted into the connecting tube 6.

It is wanted here to highlight that, although a water feed system from said tank 8 by gravity has been shown so far, i.e. by means of an elevated tank, in accordance with the concept of autonomy from any electrical auxiliaries underlying the present invention, it will be apparent to the expert of the field that a water feed system from said tank 8 equipped with a pump, preferably a peristaltic one considering the flowrates involved, can also be provided.

Preferably, said thin film of said fluid F present in said thin cavity 3 has a thickness ranging between 100 μιη and 1,000 μιη, preferably equal to 300 μιη; preferably, said thin cavity 3 is realized by inserting a thin frame or a thin spacer between said upper element 1 and said support 2.

Preferably, said reflective device 16 of solar radiation or another radiation R is a punctual or linear or parabolic concentrator or a concentrator through Fresnel mirrors; more preferably, said reflective device 16 is a parabolic mirror in which the radiation R is collected and concentrated, and then redirected towards the planar structure 100, and more precisely towards the face of said support 2 opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R, said face resulting facing and in correspondence of the reflective point/ s P of said reflective device 16.

In this way, the planar structure 100 appears to be an almost ideal optically black cavity, and it is therefore not necessary to use expensive optically absorbent materials.

It is to be noted that, although the inverted configuration herein illustrated - i.e. with the light or radiation source coming from downwards - is the preferred one because it allows the use of concentrators through mirrors or other reflective means, the expert of the field is certainly able to realize the non-inverted configuration - i.e. with the face of said support 2 opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R positioned upwards and the light concentrated, for example, with a Fresnel or convex lenses system that however, typically, are characterized by lower optical efficiencies.

Preferably, said solar radiation or another radiation R has an intensity ranging between 100 W/m² and 2,000 W/m², preferably equal to 600 W/m².

In this way, the fluid F is heated in the thin cavity 3 and is discharged, at the state of vapor V, through the duct/ s 13.

Preferably, said at least one duct 13 has a diameter ranging between 1 mm and 10 mm, preferably equal to 5 mm.

Preferably, the evaporation of said fluid F produces a quantity of steam V (defined with respect to a plane orthogonal to the solar rays) ranging between 0.1 kg/h/m² and 2.0 kg/h/ m², preferably equal to 0.6 kg/h/ m².

Optionally, said steam V is directed towards a cold surface 15 where it condenses, and the condensate thus obtained is collected and subsequently used.

Preferably, the condensation of said steam V produces an amount of condensate between 50 % and 100 % of the produced steam flowrate. Preferably, said condensate is fresh and/ or drinking water.

For a greater thermal efficiency, i.e. in order to limit the losses towards the environment and, consequently, to increase as much as possible the light/heat/ steam conversion, a thermally insulating material 9 can be possibly used, having a low thermal conductivity value (for example, in the order of 0.04 W/ (m*K)); said thermally insulating material 9 can be placed in part on said connecting pipe 6 and in part on said planar structure 100.

According to an alternative variant (not shown), the thermally insulating material 9 may be replaced by a coating comprising a compartment in which the vacuum has been achieved, so as to ensure the lowest possible thermal transmission (as it happens in the Dewar flasks); according to another alternative variant (not shown), the thermally insulating material 9 may be replaced by a coating comprising a compartment filled with a phase change material (Phase-change material - PCM), such as for example paraffin waxes, so to accumulate the thermal heat dispersed from the planar cell and then make it available again in periods of lower solar radiation and/ or in absence of radiation; according to a further alternative variant (not shown), the thermally insulating material 9 may comprise a multi-layer coating, i.e. with a first compartment filled with a phase change material, in direct contact with the upper element 1, and a second compartment in which the vacuum has been achieved, placed even more upwardly i.e. on the opposite side with respect to the upper element 1, able to guarantee the maximum thermal insulation.

It is here important to notice that between the abovementioned solutions, those that employ a phase change material allow to significantly lengthen the operation period of the planar cell according to the present invention, consequently obtaining an overall lengthening of the useful life of the plant to which such planar cell is associated.

It will be apparent to the expert of the field that the insulating coating 9 may also be realized by means of other combinations of the described solutions; furthermore, different solutions allowing to optimize the thermal balance, which have equivalent and known to the expert of the field functions, are included in the scope of the present invention.

Said thermally insulating material 9 can also be used to make walls around said planar structure 100, developing downwards (by mere way of example, FIG. 2 shows that such walls form a conical frustum at whose top the absorbent material 4 is located); this expedient allows to create a stagnation zone around the hot part of the support 2 / absorbent material 4 set, which is able to prevent the upwards convective thermal flows.

Thanks to the expedients implemented in the method according to the present invention, the conduction, convection and optical thermal losses are respectively reduced of about 10 %, about 10 % and about 20 % (compared to a transmission optical system).

Consequently, very high conversion efficiencies of the radiative energy into steam are reached, preferably in the order between 70 % and 80 % .

According to a much more efficient embodiment, the condensing cold surface 15 may be replaced with a heat regenerator or recovery device that, by exploiting the latent heat of condensation, carries out a pre-heating and/ or a partial pre-evaporation of the fluid F at the entrance of the hole 5; this type of regenerator was studied by the same Applicant and it forms the subject-matter of a separate but simultaneous Italian Patent application entitled "A device for the thin film regenerative condensation, and the method thereof".

The method according to the preferred embodiment of the present invention provides for the application to a desalination plant; such preferred embodiment of the method according to the present invention is described hereinbelow in more detail and specifically with reference to an example, which is to be understood as illustrative but not limitative of the present invention; the following example has been developed on the basis of experimental data.

Example: generation of steam in a planar structure by means of a solar radiation. A summer day, in which the solar radiation is equal to 500 W/ m², and the use of a punctual parabolic concentrator (parabolic dish) with a capture surface of 0.04 m² (defined on a plane orthogonal to the solar rays) are considered. A planar cell, such as that proposed in this invention, having a square size of 3 cm for 3 cm and a thin cavity thickness of 200 μιη, appears to be able to produce approximately 27 g/h of steam, which correspond to approximately 0.68 kg/h/m². Consequently, depending on the quality of the condensing cold surface 15, approximately 0.61 kg/h/m² of condensate, which corresponds to an overall thermodynamic efficiency of about 80 % .

Making now reference to Fig. 2, it is observed that a system for generating steam V in a planar structure 100 using solar radiation or another radiation R, comprises:

at least one planar structure 100 comprising, in turn:

an upper element 1,

a support 2,

a thin cavity 3 between said upper element 1 and said support 2, a hole 5, and

at least one duct 13,

wherein said hole 5 is positioned on the face of said upper element 1 opposite to said thin cavity 3 and wherein the face of said support 2 opposite to said thin cavity 3 comprises a material 4)absorbent of solar radiation or another radiation R;

at least one tank 8 containing a fluid F connected, through a connecting tube 6, to said at least one planar structure 100 in correspondence of said hole 5, so that said fluid F can flow into said thin cavity 3 therein forming a thin film; and

at least one reflective device 16 of solar radiation or another radiation

R facing said at least one planar structure 100 so that said face of said support 2, opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R, faces and corresponds to the reflecting point/ s P of said at least one reflective device 16, so that the thermal energy released by said solar radiation or another radiation R onto said planar structure 100, specifically onto said face of said support 2 opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R, causes the evaporation of said fluid F and the production of steam V, which is discharged through said at least one duct 13.

The system according to the present invention can further comprise, optionally:

at least one cold surface 15 towards which said steam V, discharged through said at least one duct 13, is directed and onto which the condensation thereof occurs; and

- collecting means for the produced condensate to be subsequently

used.

The system according to the present invention can further comprise:

a septum of a porous, preferably hydrophilic, material 14 inserted inside said connecting tube 6,

a thermally insulating material 9 placed in part on said connecting tube 6 and in part on said planar structure, and

a control valve 7 positioned on said connecting tube 6.

Similarly to what above exposed for the method according to the present invention, preferably:

- said material 4 absorbent of solar radiation or another radiation R is a high efficiency absorbent material, which can be used to cover the face of said support 2 opposite to said thin cavity 3 (in the case preferably it is a covering of suitable oxides able to guarantee the necessary selective coating properties, more preferably it is a layer of titanium dioxide, even more preferably it is a layer of commercial titanium dioxide explicitly developed for solar applications (for example, the TINOX material produced by the German company ALMECO GmbH); preferably it is applied on said support 2 by means of physical vapor deposition (PVD, Physical Vapor Deposition); preferably it has a thickness ranging between less than one μιη and a few tens of μιη, more preferably equal to 1-2 μιη) or it can form a coating layer of the face of said support 2 opposite to said thin cavity 3 (in the case preferably it is a covered metal sheet, more preferably it is a sheet of covered copper or aluminum, even more preferably it is a sheet of copper or aluminum covered with titanium dioxide; preferably it has a thickness ranging between 0.1 mm and 2 mm, more preferably equal to 0.2 mm; preferably it is fixed on said support 2 by gluing, welding and localized melting);

- said upper element 1 is hydrophilic, in order to distribute the fluid F in the thin cavity 3 by capillarity; more preferably it is hydrophilic and with a low thermal conductivity (for example, not greater than about 1 W/ (m*K)); even more preferably it is glass;

- said support 2 is made of sufficiently conductive materials from the thermal point of view (for example, with thermal conductivity of the order of 300 W/ (m*K)); more preferably it is made of metal;

- said thin cavity 3 has a variable size between 100 μιη and 1,000 μιη, preferably equal to 300 μιη;

- said hole 5 has a diameter ranging between 4 mm and 50 mm, preferably equal to 10 mm;

- said at least one duct 13 has a diameter ranging between 1 mm and 10 mm, preferably equal to 5 mm;

- said fluid F preferably has a temperature between 5 °C and 40 °C, more preferably it is at the room temperature of about 20 °C;

- said thin film of said fluid F present in said thin cavity 3 has a thickness ranging between 100 μιη and 1,000 μιη, preferably equal to 300 μιη; preferably, said thin cavity 3 is realized by inserting a thin frame or a thin spacer between said upper element 1 and said support 2;

- said reflective device 16 of solar radiation or another radiation R is a punctual or linear or parabolic concentrator or a concentrator through Fresnel mirrors; more preferably, said reflective device 16 is a parabolic mirror in which the radiation R is collected and concentrated, and then redirected towards the planar structure 100, and more precisely towards the face of said support 2 opposite to said thin cavity 3 and comprising said material 4 absorbent of solar radiation or another radiation R, said face resulting facing and in correspondence of the reflective point/ s P of said reflective device 16;

- said solar radiation or another radiation R has an intensity ranging between 100 W/m² and 2,000 W/m², preferably equal to 600 W/m²;

- the evaporation of said fluid F produces a quantity of steam V (defined with respect to a plane orthogonal to the solar rays) ranging between 0.1 kg/h/m² and 2.0 kg/h/ m², preferably equal to 0.6 kg/h/ m².

Preferably, said cold surface 15, onto which said steam V condensates, is formed of a conical sloping, with a smooth surface able to let the drops of condensate glide and cause them to break away from the bottom edge, which may be shaped in a spiral to facilitate the collection of the condensate itself.

Preferably, the condensation of said steam V produces an amount of condensate between 50 % and 100 % of the produced steam flowrate, depending on the quality of the condensing cold surface 15.

Preferably, the obtained condensate is collected in a circular gutter or in another suitable collection means able to receive the drops breaking away from the bottom edge of the cold surface 15.

Preferably, the condensate is fresh and/ or drinking water and it is used for domestic use.

Preferably, said septum of porous material, preferably hydrophilic, 14 is made of fabric; more preferably it is made of hydrophilic cotton.

Preferably, said septum of porous material, preferably hydrophilic, 14 is inserted inside said connecting tube 6 close to said hole 5; in an alternative version (not shown), said septum of porous material, preferably hydrophilic, 14 extends up inside the thin cavity 3, thus improving the distribution of the fluid F in the thin cavity 3 by capillarity; in a further alternative version (not shown), said septum of porous material, preferably hydrophilic, 14 completely fills the thin cavity 3, thus further improving the distribution of the fluid F in the thin cavity 3 by capillarity.

Preferably, said thermally insulating material 9 is a polymeric material; more preferably it is polystyrol (polystyrene); if costs permit, even more preferably said thermally insulating material 9 is replaced by a coating comprising a compartment in which the vacuum has been obtained (to reduce the thermal losses) or a compartment that has been filled with a phase change material (in order to accumulate and then recover the thermal dispersions).

Preferably, said thermally insulating material 9 is placed on the portion of said connecting tube 6 comprising said septum of porous material 14.

Preferably, said control valve 7 is of the type with a precise regulation of flow and is positioned on said connecting tube 6 close to said tank 8.

Preferably, said tank 8 is placed in an elevated position with respect to said planar structure 100, so as to realize a gravity water feed system, in accordance with the concept of autonomy from any electrical auxiliaries underlying the present invention; as an alternative, however, a pump, preferably a peristaltic one, can be provided to make said fluid F flow from said tank 8 into said thin cavity 3.

Similarly to the method according to the present invention, also the system according to the present invention can be preferably applied to purification/ desalination plants; however, it will apparent to the expert of the field that the present steam generation system can be used in other types of plants such as, for example, the enameling plants of the copper wire used for the windings of electric motors (specifically during the annealing step of the wire, in which steam is used as an antioxidant process gas), the food cooking lines (for example, biscuits) and the paper production plants (specifically for separating the cellulose from the lignin).

As it can be deduced from the above, the innovative technical solution herein described has the following advantageous features:

- confinement of the water to be treated within a thin film in order to significantly reduce the residence time and the conductive thermal losses towards the outside;

- inverted configuration (i.e. with the radiation directed from downwards to upwards) that allows the use of concentrators through mirrors or other reflecting means instead of lenses that, typically, are characterized by lower optical efficiencies; - the same inverted configuration also allows you to create appropriate barriers to prevent the convective air motions near the hot surface of the support 2 / absorbent material 4 set, so as to reduce the energy losses;

- significant reduction of the dispersing surface through which the thermal flows pass towards the environment thanks to the use of a thin planar structure, for unit of volume of water in the evaporation chamber;

- maximization of the surface subject to the solar radiation always thanks to the use of a thin planar structure, for unit of volume of water in the evaporation chamber;

- increase of the optical efficiency since it does not require the use of transparent materials; and

- thanks to the intrinsic simplicity and robustness and since it does not require any electrical input, the method and the device according to the present invention are particularly suitable for use in remote regions and for so-called "off-grid" applications.

From the description hereinabove it is clear, then, how the described method and system allow to reach the proposed objects.

It is equally evident, to an expert in the field, that changes and variants can be brought to the solution described with reference to the attached figures, however without departing from the teaching of the present invention and from the scope as defined in the appended claims.

Claims

A method for generating steam (V) in a planar structure (100) using solar radiation or another radiation (R), comprising the following steps:

a. providing at least one planar structure (100) comprising:

^■ an upper element (1),

^■ a support (2),

^■ a thin cavity (3) between said upper element (1) and said support (2),

^■ a hole (5), and

^■ at least one duct (13),

wherein said hole (5) is positioned on the face of said upper element (1) opposite to said thin cavity (3) and wherein the face of said support (2) opposite to said thin cavity (3) comprises a material (4) absorbent of solar radiation or another radiation (R) (step 101);

b. providing at least one tank (8) containing a fluid (F) (step 102); c. providing at least one reflective device (16) of solar radiation or another radiation (R) (step 103);

d. connecting said tank (8) containing said fluid (F), through a connecting tube (6), to said at least one planar structure (100) in correspondence of said hole (5) (step 104);

e. making said fluid (F) flow into said thin cavity (3), thus obtaining a thin film of said fluid (F) (step 105);

f. positioning said at least one planar structure (100) facing said at least one reflective device (16) so that said face of said support (2), opposite to said thin cavity (3) and comprising said material (4) absorbent of solar radiation or another radiation (R), faces and corresponds to the reflecting point/ s (P) of said at least one reflective device (16) (step 106);

g. causing the evaporation of said fluid (F) and the production of steam (V) by releasing the thermal energy of said solar radiation or another radiation (R) onto said planar structure (100), specifically onto said face of said support (2) opposite to said thin cavity (3) and comprising said material (4) absorbent of solar radiation or another radiation (R) (step 107); and h. discharging said steam (V) through said at least one duct (13) (step 108).

2. A method according to claim 1, further comprising the following steps:

i. providing at least one cold surface (15) (step 109);

j. directing said steam (V), discharged through said at least one duct (13), towards said at least one cold surface (15), onto which the condensation thereof occurs (step 110); and

k. collecting the produced condensate for a subsequent use (step 111).

3. A method according to claim 1 or 2, wherein said upper element (1) is hydrophilic and wherein said material (4) absorbent of solar radiation or another radiation (R) is used to cover the face of said support (2) opposite to said thin cavity (3) or is a coating layer of the face of said support (2) opposite to said thin cavity (3).

4. A method according to any of the preceding claims, wherein said fluid (F) is contaminated, salt or brackish water.

5. A method according to any of the preceding claims, wherein said thin film of said fluid (F) present in said thin cavity (3) has a thickness ranging between 100 μιη and 1,000 μιη, preferably equal to 300 μιη.

6. A method according to any of the preceding claims, wherein said solar radiation or another radiation (R) has an intensity ranging between 100 W/m² and 2,000 W/m², preferably equal to 600 W/m².

7. A method according to any of the preceding claims, wherein the evaporation of said fluid (F) produces a quantity of steam (V) (defined with respect to a plane orthogonal to the solar rays) ranging between 0.1 kg/h/ m² and 2.0 kg/h/ m², preferably equal to 0.6 kg/h/ m².

A system for generating steam (V) in a planar structure (100) using solar radiation or another radiation (R), comprising:

at least one planar structure (100) comprising, in turn:

an upper element (1),

a support (2),

a thin cavity (3) between said upper element (1) and said support (2),

a hole (5), and

at least one duct (13),

wherein said hole (5) is positioned on the face of said upper element (1) opposite to said thin cavity (3) and wherein the face of said support (2) opposite to said thin cavity (3) comprises a material (4) absorbent of solar radiation or another radiation (R);

- at least one tank (8) containing a fluid (F) connected, through a connecting tube (6), to said at least one planar structure (100) in correspondence of said hole (5), so that said fluid (F) can flow into said thin cavity (3) therein forming a thin film; and

at least one reflective device (16) of solar radiation or another radiation (R) facing said at least one planar structure (100) so that said face of said support (2), opposite to said thin cavity (3) and comprising said material (4) absorbent of solar radiation or another radiation (R), faces and corresponds to the reflecting point/s (P) of said at least one reflective device (16), so that the thermal energy released by said solar radiation or another radiation (R) onto said planar structure (100), specifically onto said face of said support (2) opposite to said thin cavity (3) and comprising said material (4) absorbent of solar radiation or another radiation (R), causes the evaporation of said fluid (F) and the production of steam (V), which is discharged through said at least one duct (13). A system according to claim 8, further comprising:

- at least one cold surface (15) towards which said steam (V), discharged through said at least one duct (13), is directed and onto which the condensation thereof occurs; and

- collecting means for the produced condensate to be subsequently used.

A system according to claim 8 or 9, further comprising:

a septum of a porous, preferably hydrophilic, material (14) inserted inside said connecting tube (6),

a thermally insulating material or hollow cover (9) placed in part on said connecting tube (6) and in part on said planar structure (100), and

- a control valve (7) positioned on said connecting tube (6).