WO2017162252A1

WO2017162252A1 - Direct flow thermal absorber and method therefore

Info

Publication number: WO2017162252A1
Application number: PCT/DK2017/050082
Authority: WO
Inventors: Jens William Larsen
Original assignee: Polycsp Aps
Priority date: 2016-03-22
Filing date: 2017-03-21
Publication date: 2017-09-28
Also published as: WO2017162252A8

Abstract

Pressure-formed thermal absorber means (10) configured with a surface coating (130) and a method (400) therefore. The invention is achieved by a method (400) for making a pressure-formed thermal absorber means (10) configured with a surface coating (130) comprising one act of providing (410) at least two joinable sheets (30). The method (400) comprises an additional act of arranging (420) the at least two joinable sheets (30) substantially flat on top of each other with a bottom outer sheet (32) and a top outer sheet (34) to form a thermal absorber panel (20) configured with a first surface (22) and a second surface (24). The method (400) comprises yet an additional act of jointing (430) at least two sheets (30) by high pressure joints (50) in a closed loop (520) encircling provided inlet(s) (60) and outlet(s) (70). The method (400) comprises yet an additional act of coating (470) at least one of the first surface (22) or the second surface (24) of the thermal absorber panel (20) which surfaces are pre-polished (160). Furthermore, the method (400) comprises yet an additional act of applying (440) a high pressure fluid (48) to inlet(s) (60) and/or outlet(s) (70), thereby forming at least one flow channel (40) connecting an inlet (60) to an outlet (70).

Description

[Direct flow thermal absorber and method therefore]

Field of the Invention

The present invention relates to pressure-formed thermal absorber means configured with a surface coating and a method therefore. The invention may be achieved by a method for making a pressure-formed thermal absorber means configured with a surface coating comprising one act of providing at least two joinable sheets. The method comprises an additional act of arranging the at least two joinable sheets substantially flat on top of each other with a bottom outer sheet and a top outer sheet to form a thermal absorber panel configured with a first surface and a second surface. The method comprises yet an additional act of jointing at least two sheets by high pressure joints in a closed loop encircling provided inlet(s) and outlet(s). The method comprises yet an additional act of coating at least one of the first surface or the second surface of the thermal absorber panel which surfaces are pre-polished. Furthermore, the meth- od comprises yet an additional act of applying a high pressure to inlet(s) and/or outlets), thereby forming at least one flow channel connecting an inlet to an outlet.

Background of the Invention

Thermal absorbers are used in a broad range of industries typically for cooling or heat- ing, thermal absorbers may for example be used in the food industry and medical industry for cooling. Furthermore, today thermal absorbers are also widely used in solar absorber systems such as concentrated solar power (CSP) systems. Here the thermal absorbers are used with a solar selective coating which converts solar radiation into heat through photo-thermal conversion. Typically, thermal absorbers comprise flow channels and thus transmit the heat to or from the fluid from or to the thermal absorber depending on the application.

Planar or flat traditional thermal absorbers are often constructed as metal sheets mounted on tubes where the tubes are used as flow channels. One use of a thermal absorber is that the metal sheet absorbs the heat, which is then transmitted through the construction to the tubes and further on to the fluid flowing in the flow channels comprised by the tubes. The planar traditional thermal absorbers have limited energy effi- ciency due to the indirect heat transmission from the metal sheet to the fluid through the tubes.

To increase the energy efficiency, improvements in form of changes in construction materials have been suggested, for example copper and aluminium. Copper has high heat conductivity and is therefore a highly valuable material for heat transmission systems. However, copper is a limited resource resulting in high prices and copper installations are to a higher and higher extent becoming a subject for theft. The heat conductivity of aluminium is at a reasonable level for use in thermal absorber construc- tions. However, the corrosion rate of aluminium is not ideal for this use resulting in limited lifetime.

Planar traditional thermal absorbers may be used in solar absorber systems and thus the metal sheet should be coated with a solar selective coating. Today, the metal sheets are often coated in coil form, then cut and straightened into sheets to be mounted by welding with the tubes constituting the flow channels. One problem with this method is that the selective coating may degrade or be spoiled or broken where the tubes are joined to the metal sheets. Today, thermal absorbers used for solar absorber systems, for example concentrated solar power systems often comprise tubes coated with a solar selective coating to achieve a direct heat transmission from the coating to the fluid through the tube wall. The planar traditional thermal absorbers are also used as solar absorbers, however, with limited energy efficiency due to the indirect heat transmission.

Direct flow thermal absorbers are a different type of planar thermal absorbers with direct heat transmission. These are often referred to as pillow-plate absorbers. The pillow-plate absorbers are typically constructed in stainless steel or steel and comprise two sheets welded together to comprise flow channels, where the fluid is in direct con- tact with the sheet to be heated or cooled. Pillow-plate absorbers are often constructed by a method where two or more metal sheets are pressure or emboss formed and afterwards welded together. Direct flow thermal absorbers are for example used in heat exchangers for heating or cooling purposes and the typical thickness of the sheets are 0.8-2 mm because of the welding process and because of the high static pressures arising in heat exchangers during use.

Because of the material's thickness resulting in low heat conductivity and high mate- rial costs the pillow-plates available today are not commercially attractive to be used in solar absorber systems and thus are not an attractive alternative to the traditional planar thermal solar absorber.

Recently, pillow-plate absorbers constructed by welding two metal or plastic sheets together and then forming a set of channels by applying a high pressure fluid to the construction have been disclosed.

Construction of pillow plate absorbers using the above method calls for a balance between the thickness of the sheets, the strength of the welding and the pressure applied for expanding the construction to achieve a stiffness of the construction and a form of the construction durable for the pressure and environmental conditions to which the construction will be exposed during use.

US2013276776 discloses such a pillow-plate solar heat absorber for a solar collector. The absorber is formed by using a fluid pressure of 20-60 Bar depending on the plate thickness and a press with a die comprising a pattern of the resulting absorber. The die thus comprises the pattern of the welding and of the channel(s). The applied press is applying a pressure of at least 200 tons per m² of the entire absorber. Thus for large- scale constructions of solar heat absorbers of for example 15 m², a press with capacity of at least 3000 ton is required. Presses providing forces on this scale and above are expensive and heavy machinery which may add significant costs to the final product. Dl further discloses a production step of coating the metal surface using paint - often matt black - that absorbs as much as possible of the solar radiation. However, this production step subsequently requires a delay/storage step for drying of the coating layer after the absorbers have been coated. Furthermore, the efficiency and lifetime of such coatings may be limited. D2 US2015168017 also discloses such a pillow-plate absorber used in a solar collecting panel for solar water heating systems intended for household installation for example on a roof. One of the benefits of the disclosed solar colleting panel is that it may be reduced in size compared to traditional systems.

The solar collecting panel is formed using thin metal sheets between 0.3 and 0.6 mm, and using a significant lower pressure of 160 psi (-11 Bar). An additional benefit of this system is that it may be operated at elevated pressures, such as those provided commonly by municipal water delivery systems.

Similar to US2013276776, US2015168017 also discloses that the surface of the solar collection panels facing the sun can be coated with a material that contributes to the solar collection panel operating with increased efficiency - here a siliconized paint, which may be the same as that proposed by US2013276776. However, a further con- struction comprising a casing including a cover for the solar collection panel is disclosed to encapsulate the solar collection panel and protect the coated surface of the solar colleting panel.

Yet another example of a pillow-plate absorber constructed by welding two metal or plastic sheets together and then forming a set of channels by applying a high pressure fluid is disclosed in US2016116187. This solar thermal collector differs in construction from US2013276776 and US2015168017 by comprising a plurality of parallel channels and two connecting tubes, wherein the channels are connected between the connecting tubes and the connecting tubes serve as inlets/outlets. Thus, a construction comprising the manifold is achieved avoiding some of the drawbacks arising for mounted manifolds.

Similar to US2013276776 the channels are formed by placing the welded slabs in a mold and then filling a high-pressure fluid in between the first slab and the second slab. The shape of the formed construction corresponds to the grooves of the mold as the slabs - except for the stripe-shaped connecting portions - are confined by the grooves of the mold. Thus, the disclosed method may require an applied pressure on the same scale as in US2013276776. However, US2016116187 suggests using sheets of plastic to overcome some of the drawbacks of using metal sheets to achieve easier and faster shaping because of the flexibility and manufacturability of plastics, and to take advantage of the fact that plastic generally is light in weight and low in cost.

Again a solar selective coating is suggested to be formed on the outer surface of one of the slabs. No details of such coating are disclosed except that the coating may be formed on the surface after welding.

Object of the Invention

It is an objective to overcome one or more of the before mentioned shortcomings of the prior art. Description of the Invention

An object of the invention may be achieved by a method for making a pressure- formed thermal absorber means configured with a surface coating comprising one act of providing at least two joinable sheets. The method comprises an additional act of arranging the at least two joinable sheets substantially flat on top of each other with a bottom outer sheet and a top outer sheet to form a thermal absorber panel configured with a first surface and a second surface. The method comprises yet an additional act of jointing at least two sheets by high pressure joints in a closed loop encircling provided inlet(s) and outlet(s). The method comprises yet an additional act of coating at least one of the first surface or the second surface of the thermal absorber panel which surfaces are pre-polished. Furthermore, the method comprises yet an additional act of applying a high pressure fluid to inlet(s) and/or outlet(s), thereby forming at least one flow channel connecting an inlet to an outlet. The act of coating is performed by a vacuum deposition process. One effect of this embodiment may be that thermal absorber means with direct heat transmission are achieved. This is advantageous in regard to obtaining a high energy transmission.

Another effect of the embodiment is that thermal absorber means with direct heat transmission may be achieved using a method with a minimum of handling and storage. This may be advantageous in regard to reduced costs of the final product. The reduced costs may be both expenses to manual labour, short production time, streamlined production and/or reduced storage capacity. The handling may be reduced due to that the alignment and jointing process may be performed in a single process, the pressure-forming process includes that the thermal absorber means are pressure-tested to the applied pressure of the high pressure fluid and a vacuum process. The process may be performed without storing the elements between the processes. Furthermore, the required storage of the sheets prior to making the thermal absorber means may be reduced because the final shaping of the absorber means are first performed during the process of making the absorber means.

Yet another effect of this embodiment is that the vacuum deposition process may be described as a dry process meaning that the coating is dry when the coating has been deposited thereby avoiding a delay/storage step in the process after the act of coating for drying of the coating layer.

The sheet material or materials may be chosen to be joinable by high pressure joints. The jointing may be achieved by gluing, melting, welding or combinations hereof. However, the jointing may also be performed in other ways not mentioned here.

High pressure joints may be referred to as joints with strength high enough to withstand the pressure arising during the pressure expansion of the flow channel. High pressure joints may also be referred to as joints with strength withstanding the stress and/or strain arising in the construction during use.

The strength of the high pressure joints may be dependent on the material or materials of the sheets. The strength of the high pressure joints may be dependent on the stress and/or strain arising in the construction during use.

The sheet material or materials may be chosen in consideration of achieving a high heat conductivity, corrosion resistance, temperature stability and jointing abilities. The material of the individual sheets may be the same for all sheets constituting the ther- mal absorber panel, or different materials may be chosen. Using different materials may have the effect that one sheet is deformed at a different rate than another sheet with the advantage of achieving for example one flat surface and one curvy surface of the thermal absorber panel. Using the same material for all sheets may have the effect of achieving less complex joints and structures than those required when using differ- ent kinds of materials, as the properties of the sheets are comparable for example heat expansion, chemical components in the surface and surface adhesion properties. One advantage of this may be the use of well-proven jointing techniques. Another advantage could be to achieve joints withstanding higher pressures because of the simi- lar properties of the sheets during jointing and during use.

The sheet material or materials may for example be selected from the group consisting of stainless steel, mild steel, aluminium, high temperature polymer, high temperature polymer blends or combinations thereof. However, the materials may also be chosen amongst other appropriate materials not mentioned here.

In addition the sheets may also have different thicknesses. One effect may be to combine the abovementioned effect of achieving different deformation properties of the sheets but with the same material properties which may be important in regard to jointing ability/ties according to the above-mentioned advantages.

An additional effect of this embodiment may be that the surface coating acts as a protective layer for the thermal absorber means thereby prolonging the lifetime of the pressure-formed thermal absorber means. Especially, the efficiency and lifetime of vacuum deposited coatings may be increased compared to a simple matt black paint or siliconized paint applied to the surface.

Yet another effect of this embodiment may be that a vacuum deposited coating may be designed with special properties and thus be a functional surface coating.

The vacuum deposition process may for example be selected from the group consisting of: physical vapour deposition (PVD), reactive magnetron sputtering, chemical vapour deposition (CVD), plasma enhanced chemical vapour deposition (PECVD), electron beam deposition and cathodic arc evaporation. Furthermore, vacuum deposi- tion processes may include sputtering deposition process which may for example be performed using pulsed DC sputtering, HIPIMS (High Power Impulse Magnetron Sputtering) or RF sputtering. However, the vacuum deposition process is not limited to these examples just as the sputtering deposition may be performed using other techniques. A further effect of this method is that the surface coating may be deposited using well- controlled deposition processes by which thin layers of surface coatings may be applied. This may be advantageous in regard to functional coatings which may include multiple layers where the thickness is an important characteristic for achieving the right functionalities.

Another effect may be that the surface coating may be deposited on a clean surface, where clean surface refers to a surface, which is cleaned under vacuum and thus do not contain contaminants. This may be advantageous in regard to obtaining better adhesion of the surface coating to the surface. A further advantage may be that multiple layers may be deposited while the surface of the pressure-formed thermal absorber means or the recent deposited layer is, at all time kept under vacuum condition, thereby avoiding contaminants in the coating and between the layers.

Yet another effect may be that, if multiple processes are required for performing the surface coating, the processes may be performed in an in-line process. One advantage is that the pressure-formed thermal absorber means is kept in a controlled vacuum environment throughout the deposition process and during the time between each deposition process, thereby preventing oxidations of the surfaces that subsequently constitute the interface between the individual layers of the solar selective coating.

In one aspect the vacuum deposition process may be a sputtering process. This is advantageous in regard to the fact that the sputtering process may be done as an in-line deposition process.

In an in-line sputtering deposition process the workpiece may be moved in a continuous movement through one plasma region in which one layer is deposited onto the next plasma region where another layer is deposited and so forth. Alternatively, sever- al in-line sputter zones are arranged in series and moved across the workpiece. In either case, the gas composition and plasma intensity may be controllable for a stable deposition rate during the process to ensure a layer of uniform material composition and uniform thickness on the entire workpiece. One advantage is that the workpiece, here the pressure-formed thermal absorber means may be kept in a controlled vacuum environment throughout the deposition process and during the time between each deposition process, thereby preventing oxidations of the surfaces that subsequently constitute the interface between the individu- al layers of the solar selective coating.

Another advantage is that magnetron and plasma regions may be kept relatively small compared to the size of the workpiece, thereby reducing the cost of production facilities because the equipment's "core" in form of sputtering zones with magnetrons, gas inlet channels, sensors and so forth.

The challenge of controlling the process parameters of the deposition process over a large area may also be reduced due to the movement of the workpieces in relation to the deposition regions.

In one aspect the absorber layer(s) may comprise a ceramic and/or metallic composition which may be deposited using a vacuum deposition process.

In one aspect the absorber layer may comprise ceramic and/or metallic materials com- prising both ceramic compositions and ceramic metallic CERMET compounds which may be deposited using a vacuum deposition process.

In one aspect pressure-formed thermal absorber means comprises thin stainless steel sheets and the jointing act is performed using a welding process. The stainless steel sheets may be chosen with a thickness below 0.8 mm, preferably in the range 0.1 mm to 0.6 mm, even more preferably in the range 0.2 to 0.5 mm to achieve a high heat conductivity and low material use. One advantage of these effects may be that the pressure-formed thermal absorber means is profitable to be used as pressure-formed thermal solar absorber in thermal solar systems.

The welding of thin plates exposes the risk of overheating the plates causing deformation of the joints and/or sheets afterwards. Overheating may happen due to the limited quantity of material to absorb and distribute the heat of the welding process. Es- pecially automatic welding may expose a challenge. Thus, a special welding method should be used wherein the following provision may be taken:

The clamping of the plates may need to be with elevated forces/stresses compared to well-known clamping methods used in industrial welding processes. - The distance from the clamping to the welding seam should be kept short to avoid shift of position of the plates.

A larger part of the total surface of the thermal absorber panel needs to be clamped at the same time.

The sheets to be welded should be in close contact.

- A high welding speed is used.

The special welding method and the provision to be considered will be elaborated on later. In one aspect at least one inlet and at least one outlet is placed in the top sheet.

In another aspect at least one inlet and at least one outlet is placed in the bottom sheet. In yet another aspect at least one inlet is placed in the bottom sheet and at least one outlet is placed in the top sheet.

In yet another aspect at least one outlet is placed in the bottom sheet and at least one inlet is placed in the top sheet.

In yet another aspect at least one inlet and/or one outlet is placed in the side of the thermal absorber panel between two sheets of the thermal absorber panel.

A further object of the invention may be achieved by a method for making a pressure- formed thermal absorber means configured with a surface coating wherein the method act of jointing by high pressure joints comprises at least one further pattern forming one or more flow channels from inlet(s) to outlet(s), which at least one further pattern is comprised within the closed loop joint. A further effect of this embodiment may be that the pressure-formed thermal absorber means may be designed with multiple flow channels. The multiple flow channels may be separated throughout the thermal absorber panel from inlet to outlet. The multiple flow channels may alternatively alternate from joining and separating throughout the thermal absorber panel. Another effect of this embodiment may be that the pressure- formed thermal absorber means is designed with a single flow channel designed with a travel distance of the fluid to achieve a given surface contact area through the thermal absorber panel. This may have the advantage of designing the flow through the thermal absorber panel according to the heat transmission level desired for the appli- cation. A further advantage may be to design the flow channel or channels according the specific areas of the thermal absorber panel to be cooled or heated at a different rate than others.

An additional effect may be that the joints may be positioned with reduced distance between adjacent joints which may be advantageous in regard to achieving a thermal absorber means for higher internal static pressure use. The internal pressure of thermal absorber panel may be high due to the elevated temperatures of the fluid comprised in the panel during use. A further object of the invention may be achieved by a method for making a pressure- formed thermal absorber means configured with a surface coating wherein at least one of the joinable sheets comprises at least one imprint prior to jointing.

The imprints may be fully formed or partly formed imprints. The imprint may com- prise flange(s) for the inlet(s)/outlet(s). The imprint may comprise one or more structure or profile of individual flow channels. The imprints may be performed by a deep drawing process or similar methods.

A further effect of this embodiment may be that the inlet(s) and outlet(s) may be placed and aligned with the sheets during the act of arranging the at least two joinable sheets substantially flat on top of each other. This may be advantageous in regard to jointing the inlet(s) and outlet(s) to the sheets in the same process of aligning and jointing the sheets. Hereby the handling of the absorber panel during production may be further reduced.

Yet a further effect may be that individual flow channels of the thermal absorber panel may be configured with an extra depth or imprinted profile(s) in the wall of the flow channel. This may be advantageous in regard to achieving an improved overall mechanical stiffness of the absorber panel. An improved mechanical stiffness may im- prove the lifetime of the panel because of reduced risk of fatigue in the panel. Furthermore, the improved mechanical stiffness of the panel may improve the lifetime of the other components of the complete installation. A complete installation may for example be a solar collector which may include thermal absorber panel(s), insulation, cover, back plate, cabinet, mounting means amongst other. A complete installation may be installations used in other ways but which at least includes the thermal absorber panel. E.g. for solar collectors, the absorber panels with improved stiffness may bring reduced risk for breaking other components in the collector such as glass cover, insulation materials and cabinet.

Improving the mechanical stiffness may give the effect that the number of mounting points may be reduced, which may improve energy efficiency of the absorber panel further because each mounting point may bring heat losses. Furthermore each mounting point brings increased production and mounting costs. To allow for only a few connection points to transfer the support forces to the absorber panel, the panel may need to have sufficient mechanical stiffness to avoid that the absorber touches or deforms other parts of the complete installation comprising the absorber panel. Furthermore with an enhanced absorber stiffness, the requirements to the mechanical stiffness of the other parts of the complete installation may be reduced. This may provide for less costly and less heavy installations. The challenge of reduction of deflections of the absorber increases, when the area of the absorber increases; so especially for extra large absorbers the mechanical stiffness of the absorber panels is important. A further object of the invention may be achieved by a method for making a pressure- formed thermal absorber means configured with a surface coating comprising a further act of placing the jointed sheets in a press. The press applies a surface pressure on the jointed sheets in an area along the joints which joints the sheets and across the joints which joints the sheets in a width of up to eleven times the width of the joints, preferable in a width of up to nine times the width of the joints, or more preferably in a width of up to five times the width of the joints.

In one aspect the further act of placing the jointed sheets in a press applying a surface pressure on the jointed sheets, the surface pressure is applied in an area along the joints which joints the sheets and across the joints which joints the sheets in a width of up two times a heat affected zone (HAZ).

A further effect of these embodiments is that the pressure may only be applied to a part of the total area of the joint sheets as the pressure may only be applied in a trail along the joint(s) and a width across the j oint(s) extending to each side of the joint(s).

This may be advantageous in regard to achieving that a press with less pressure capacity may be used compared to the capacity required by a press applied for forming the imprint on the entire surface and thus a press applying pressure to the entire surface of the absorber, which is typically used in the process of pressure forming. Depending on the number of channels, and thus the length of the joints and how close the joints are placed, the required capacity of the press may be reduced to 1/5 or even down to 1/10 compared to that of a press applying pressure to the entire surface of the absorber. Generally, the price of comparable presses but with different pressure capacity decreases with decreased pressure capacity. This price dependence may be due to that presses with a high pressure capacity require more heavy constructions to withstand the large external and internal forces. Furthermore, the quantity of high-capacity presses produced may decrease as the capacity increases which is often seen depicted in the price. Thus, reducing the pressure capacity may significant reduce cost of the final product. These advantages may be even more pronounced for large-scale absorbers. Here the typical process of applying pressure to the entire surface of the absorber may require even heavier presses as the required pressure capacity increases. In one aspect the further act of placing the jointed sheets in a press, the press comprises a base tooling plate and two dies. The two dies are configured as a pressure-active die having a pattern according to the area of the applied surface pressure according to the description above and a pressure-passive die which defines the maximum height of the channels.

The pressure-active die is connected to a press and thereby a pressure may be applied to the joint sheets in a pattern comprising at least the pattern of the joints that joint the sheets. The pressure-active die may both provide a surface pressure and clamp (hold) the joint sheets in place during the act of pressure-forming the absorber by applying a high pressure fluid.

The pressure-passive die is placed between the joint sheets and the upper part of the pressure-active die with the pattern of the pressure-active die extending through the pressure-passive die. The pressure-passive die and the base tooling plate may be clamped to each other with a fixed distance between them by simple mechanical clamps. The pressure-passive die may be configured with flat surfaces facing the joint sheets. The pressure-passive die may be configured with structures of channels facing the joint sheets.

A further effect of using a two-part die comprising a pressure-active die and a pressure-passive die is that the pressure applied by the press is reduced significantly compared to using a single-part die comprising the complete pattern of the absorber. Using a single part die may require using a press with a pressure capacity of 200 tons/m². For absorbers of for example 15 m^" a press with a minimum pressure capacity of 3000 tons may bebe required. While a press with a pressure capacity of minimum 12000 tons presses may be required for absorbers of 3m x 20m. Presses providing forces on this scale and above requires heavy constructions and may dramatically raise costs on the final product. Thus, an advantage of using a two-part die may be that a smaller press may be used thereby reducing the costs of the final product.

Yet a further object of the invention may be achieved by a method for making a pressure-formed thermal absorber means configured with a surface coating wherein the provided inlet(s) and outlet(s) are provided in one sheet and arranged with an inlet flange to the individual inlet(s) and an outlet flange to the individual outlet(s). Additionally, wherein the method act of jointing by high pressure joints comprises jointing the inlet flange(s) and outlet flange(s) to the sheet comprising the inlet(s) or outlet(s) and wherein the jointing is performed from one side of the thermal panel in one pro- duction step.

A further effect of this embodiment is that the pressure-formed thermal absorber means comprises one surface, which we may call the first surface, comprising the inlet and outlet flanges. The first surface may also comprise possible areas on the surface weakened due to the jointing process. The second surface of the pressure-formed thermal absorber means may also comprise possible areas on the surface weakened due to the jointing process, however, weakened by indirect contact to the jointing process. This may be advantageous in regard to the fact that the pressure-formed thermal absorber means comprises a primary surface without any mechanical connections. This may be advantageous in regard to applying or depositing a homogeneous and coherent surface coating across the second surface. This may be the used as the primary surface of the pressure-formed thermal absorber means exposed to the environmental influence for intended use.

Welding of thin sheets comprises a broad range of challenges. This is elaborated on here.

Welding two plates together e.g. with laser doing a keyhole welding, requires that the two plates are in contact in the places to be welded. By the term "in contact" is meant that at any place the distance between the surfaces of the sheets facing each other is 1/10 of the total sheet thickness. For example for two sheets of 0.3 mm the max distance is 0.06 mm. Preferably, the distance between the surfaces of the sheets facing each other is 1/20.

One challenge is related to the thermal input during welding. The thermal input may cause local expansion of the relatively thin sheets and cause the sheets to deform, wrinkle and/or make micro folds. This may be avoided by clamping the sheets with high forces very close to the welding seam during the welding process, thereby avoid- ing the plates to form wrinkles and folds and to loose contact between the plates. In case of deformation of the thin sheets during welding, problems may arise with melted welding pools, which flow out of the plate surface and causes holes.

Another challenge is that the period of the melted metal or welding pools' existence needs to be very short in order to avoid heat to distribute or be transferred out into the thin sheets, thereby causing extra challenges regarding clamping. The reduction of heat input may be enabled by doing welding with relatively high intensity and with high welding speed. For example for thin stainless steel plates doing a keyhole laser welding an intensity of 5-50 KW/mm² may be used in combination with a welding speed of 100 mm/s to 1000 mm/s.

For example the welding of two stainless steel plates of 0.3 mm thickness should re- suit in a welding seam width of approximately 1.5 times the total thickness of 0.6 mm, this being approximately 0.9 mm. To enable this, the width of the focused laser beam needs to be approximately half the width of the welding seam thus, a laser beam diameter of 0.5 mm with an area of the focused laser beam of approximately 0.2 mm². Experience shows that the acceptable range of intensity for the welding power for keyhole laser welding is in the range 5-50 KW/mm², which for the estimated focus area corresponds to a welding power of 1 - 10 KW.

The welding speed with a 1 KW power is approximately lOOmm/s, and power and speed should be increased approximately linear.

Related to the need for high speed during welding are the challenges of ramping up and ramping down both speed and laser power in a controlled way. In practice, this may bring limitations for the length of the welding during ramp up and ramp down, where these parts of the welding are not at an acceptable quality. The realistic length for ramp up and ramp down may be in the range of 5 to 10 mm, in some cases up to 15 mm, and this means that doing patterns of relatively short lengths of welding causes lack of efficiency in the production. If e.g. a welding pattern has linear elements or circular elements of welding seams in the range of 20 mm, the time and space on the sheet used for ramping up and ramping down may be similar or longer than the time and length for the well-defined welding - well-defined welding meaning welding with defined characteristics such as width, depth and strength.

These challenges related to welding of relatively thin plates should be considered when designing the pattern forming one or more flow channels. The clamping of the plates may need to be with elevated forces/stresses compared to well-known clamping methods used in industrial welding processes. This may be done to avoid the sheets to shift position relative to each other due to thermal expansion. The distance from the clamping to the welding seam should be kept short again to avoid shift of position of the plates. The distance should be comparable to the sheet thickness - preferably below ten times the sheet thickness. To enable efficient welding of the thin plates a certain area of the surface or the total surface of the thermal absorber need be clamped at the same time. This may be achieved by only leaving enough undamped space along all welding seams for the laser beam to have access to the thermal absorber surface.

The welding of thin sheets widens the group of materials from which the sheet material may be chosen. As the sheet thickness decreases the sheets may comprise the properties sought for a given application. The advantage of this may be that cheaper mate- rials, than what would be chosen today, can be used. For example if the corrosion properties of stainless steel are sought for a given application but stainless steel has previously been deemed out because of a low thermal transmission rate due to the traditional used thicknesses, this may now be a realistic and useable choice because of decreased sheet thickness. Previously in this situation, the choice may have been to use copper instead, which is an extensively more costly material.

Typically, the dimensions of thermal absorber means can be from a limited size of few square centimeters for example 0.02m x 0.02m up to many square meters for example 3m x 20m.

An object of the invention is achieved by a pressure-formed thermal absorber means configured with a surface coating obtained by a process of method acts described above wherein at least one flow channel is a pressure expanded flow channel and that the surface coating is a vacuum deposited coating.

According to the above, in one embodiment of a pressure-formed thermal absorber means configured with a surface coating, the pressure-formed thermal absorber means comprises at least two joinable sheets which form a thermal absorber panel configured with a first surface and a second surface, and a bottom outer sheet and a top outer sheet. The joinable sheets are joint be high pressure joints in a closed loop encircling provided inlet(s) and outlet(s). Furthermore, the thermal absorber panel is configured with at least one coated surface with a surface coating, which surface is at least one of the first surface or the second surface of the thermal absorber panel, and which surface is pre-polished. The pressure-formed thermal absorber means comprises at least one pressure-formed flow channel, and a surface coating which surface coating is a vacuum deposited coating.

The effects and advantages of this embodiment are in line with those already pointed out and previously described including the following effects:

• The surface coating may not be degraded due to heating during the jointing process.

• Areas on the surface weakened due to the jointing process may be covered by the surface coating.

• The pressure-formed thermal absorber means may be designed with multiple flow channels.

• The pressure-formed thermal absorber means may be designed with a single flow channel for a travel distance of the fluid for a given fluid-to-thermal absorber panel surface contact area.

The advantages may be captured as:

• Achieving the surface properties intended by use of a surface coating.

• The surface coating acts as a protective layer.

• The flow through the thermal absorber panel may be designed according to the heat transmission level desired for the application.

• The flow through the thermal absorber panel may be designed in order to achieve heat transmission in specific areas of the thermal absorber panel at a different rate than in other areas.

• The pressure-formed thermal absorber means comprises a primary surface without any mechanical connections.

• A homogeneous and coherent surface coating may be achieved on the primary surface of the pressure-formed thermal absorber means. Furthermore, the flow channel or channels may be designed in a pattern to achieve improved stability and stiffness of the overall absorber panel. The pattern may include that one or more sections of the flow channel(s) has (have) different depths. In one aspect the surface coating is a pre-jointing applied surface coating.

In another aspect the surface coating is a post-jointing applied surface coating.

The vacuum deposited coating may be deposited at any point along the method for making a pressure-formed thermal absorber means configured with a surface coating.

As previously described one effect of using a vacuum deposited coating is that the deposition process may be described as a dry process meaning that the coating is dry when the coating is deposited thereby avoiding delay/storage step in the process after the act of coating for drying of the coating layer.

A further effect of using a vacuum deposited coating is that the coating may have a high heat resistance and a high flexibility. This may be advantageous in regard to reduced risk of degradation or rupture of the coating during the fabrication Using a vacuum deposited coating with a high heat resistance may accommodate for depositing the coating on one surface forming the first surface of the thermal absorber panel before jointing the joinable sheets even if a heat dissipating jointing process is used. The heat dissipating jointing process may be performed via the surface forming the second surface of the thermal absorber panel. One example, but not limited to this, may be a welding process performed on the one surface after the other surface already may be coated.

An object of the invention is achieved by a pressure-formed thermal absorber means configured with a surface coating, wherein the height of at least one section of a flow channel is different from the height of at least one adjacent section of a flow channel.

A further effect of this embodiment may be that individual flow channels or sections of the flow channel(s) of the thermal absorber panel may be configured with extra depth. This may be advantageous in regard to achieve an improved overall mechanical stiffness of the absorber panel. An improved stiffness may improve the lifetime of the panel because of reduced risk of fatigue in the panel. The improved mechanical stiffness of the panel may also be advantageous in regard to improved lifetime of additional part comprised in a complete installation An object of the invention is achieved by a pressure-formed thermal absorber means configured with a surface coating, wherein at least one section of a flow channel comprises a stiffness enhancing profile.

A further effect of this embodiment may be that individual flow channels of the ther- mal absorber panel may be configured with profile(s) in the wall of the flow channel. This may be advantageous in regard to achieve an improved overall mechanical stiffness of the absorber panel. An improved stiffness may improve the lifetime of the panel because of reduced risk of fatigue in the panel. Improving the mechanical stiffness may give the effect that the number of mounting points may be reduced, which may improve energy efficiency of the absorber panel further because each mounting point may bring heat losses. Furthermore each mounting point brings increased production and mounting costs. To allow for only a few connection points to transfer the support forces to the absorber panel, the panel may need to have sufficient mechanical stiffness to avoid that the absorber touches or deforms other parts of the complete installation comprising the absorber panel. Furthermore with an enhanced absorber stiffness, the requirements to the mechanical stiffness of the other parts of the complete installation may be reduced. This may provide for less costly and less heavy installations. The challenge of reduction of deflections of the absorber increases, when the area of the absorber increases; so especially for extra large absorbers the mechanical stiffness of the absorber panels is important.

In one embodiment of the pressure-formed thermal absorber means configured with surface coating, the surface coating comprises an infrared reflective coating. A further effect of this embodiment is that the infrared reflective coating may reflect the heat from the fluid in the flow channels of the pressure-formed thermal absorber with the advantage of reducing heat emission from the fluid to the surroundings.

In one embodiment of the pressure-formed thermal absorber means configured with a surface coating, the surface coating comprises an anti-fouling coating.

In one aspect the antifouling coating may be applied on top of an anti-reflective coating or other functional coatings.

An antifouling coating may reduce adsorption of gasses and liquids to the surface and may reduce adsorption to the surface of elements in these gasses and liquids. Thus, one effect of this embodiment is that the surface may be more resistant to nonspecific protein adsorption.

Furthermore, an effect of this embodiment is that the surface may be more resistant to bacterial growth on the surface.

A further effect of this embodiment is that the level of adhesion to the surface of various elements from gasses and liquids, being in contact with the surface, may be reduced.

In general, this is advantageous in regard to reducing or avoiding build-up of impurities which may reduce the function of the surface coating or reduce the heat transmission properties. A build-up of impurities may further damage the surface due to increased corrosion rate.

In one embodiment the pressure-formed thermal absorber means configured with a surface coating further comprises an insulation structure mounted on one of the first surface or the second surface of the thermal absorber panel.

A further effect of this embodiment is to reduce the heat loss to the surroundings to achieve an increased energy efficiency of the thermal absorber panel. A further effect may be that the insulation structure provides for an increased overall mechanical stiffness to the thermal absorber panel.

An improved mechanical stiffness may improve the lifetime of the panel because of reduced risk of fatigue in the panel. Improving the mechanical stiffness may further reduce the number of mounting points for improved energy efficiency of the absorber panel because each mounting point may bring heat losses. Furthermore, reducing the number of mounting point may bring reduced production and mounting costs. One object of the invention may be achieved by a method for making a pressure- formed thermal solar absorber wherein the surface coating is a solar selective coating and the coating act comprises the following acts of depositing a solar selective coating onto the thermal absorber panel:

depositing an adhesion layer on at least one pre-polished surface;

- depositing at least one absorber layer one layer at a time; and

depositing at least one anti-reflection layer one layer at a time,

in a sandwich construction configured with the adhesion layer deposited onto the surface^), the absorber layer(s) deposited onto the adhesion layer and the anti-reflection layer(s) deposited onto the absorber layer(s).

Solar selective coating is defined as a coating having high absorption of electromagnetic radiation at the solar spectrum wavelengths and low thermal emittance in the thermal infrared wavelength range defined by its temperature of operation. The solar selective coating may be defined with a solar selective surface, which is the free surface of the anti-reflection stack. The free surface is the surface facing the ambient surroundings and opposite to the surface towards the absorber stack.

A further effect of this embodiment is that the solar selective coating is configured to be spectrally selective absorbing with high absorption of solar radiation and low emission of absorbed energy as infrared radiation (heat) from the anti-reflection stack. This is advantageous in regard to converting solar radiation into heat - sometimes referred to as photo-thermal conversion. The thermal emittance is low to the ambient surroundings but may be high to the pressure-formed thermal absorber means. Using a pressure-formed thermal absorber means in combination with a solar selective coating gives the effect of direct thermal transmission from the solar selective coating to the fluid through a sheet of the thermal absorber panel. The efficiency of heat trans- fer depends on the fluid and the contact from the thermal absorber panel to the fluid. Due to the construction of the pressure-formed thermal absorber a large contact area of thermal absorber panel-to-fluid may be achieved. The advantages of an effective heat transfer may be an increased heat energy yield. Furthermore, an effective heat transfer may result in improved lifetime due to the fact that overheating or prolonged use at high temperatures is avoided.

An object is achieved by a pressure-formed solar absorber obtained by a process of method acts comprising the method for making a pressure-formed thermal solar absorber wherein the surface coating is a solar selective coating and the coating act comprises the following acts of depositing a solar selective coating onto the thermal absorber panel:

depositing an adhesion layer on at least one pre-polished surface;

depositing at least one absorber layer one layer at a time; and

depositing at least one anti-reflection layer one layer at a time,

in a sandwich construction configured with the adhesion layer deposited onto the surface^), the absorber layer(s) deposited onto the adhesion layer and the anti-reflection layer(s) deposited onto the absorber layer(s) and wherein at least one flow channel is a pressure expanded flow channel and that the surface coating is a vacuum deposited coating.

The further effects and advantages of this embodiment are in line with those already pointed out and previously described including the following effects:

• The solar selective coating may be configured to be spectrally selective absorbing with high absorption of solar radiation and low emission of absorbed energy as infrared radiation (heat) from the anti-reflection stack.

• A pressure-formed thermal absorber means in combination with a solar selective coating may result in direct thermal transmission from the solar selective coating to the fluid through a sheet of the thermal absorber panel. The advantages may be captured as an increased heat energy yield and improved lifetime due to the fact that overheating or prolonged use at high temperatures is avoided because of effective heat transfer to the fluid.

In one embodiment of a pressure-formed solar absorber obtained by the process of method acts described above, the adhesion layer comprises a metallic layer comprising a refractory metal and dope-material, which dope-material comprises a metal or metalloid and which metallic layer is configured with an amorphous disordered structure.

The adhesion layer may have multiple functions:

• High IR reflection - in order to achieve high emission characteristics for the solar selective coating

• High corrosion resistance - so that for example a pinhole or scratch in the upper coating layers will not start corrosion in the adhesion layer and consequently release the solar selective coating in large areas.

• Diffusion barrier at the operational temperature levels - in order to reduce the diffusion of elements from the substrate into the absorber layers and thus to achieve low degradation of optical properties during the lifetime thereby obtaining improved performance of the solar selective coating.

• Good adhesion to the substrate and enabling good adhesion to the first absorber layer.

The refractory metals share properties such as a high melting point above 2000°C, high hardness at room temperature, and they are chemically inert and have a relatively high density. The refractory metals may be selected from the group consisting of: Molybdenum (Mo), Niobium (Nb), Tantalum (Ta), Tungsten (W) and Rhenium (Re).

One effect of this embodiment is that the dope-material may provide cathodic protection to the metal comprised in the adhesion layer. The refractory metal may hereafter also be referred to as the main metal. The dope-material may be a metal or metalloid, which has high affinity to oxygen, and which has such characteristics that stable and dense oxide layers will be formed in the ranges of potential and pH, where the refrac- tory metal will corrode in the actual application. This may be advantageous in regard to achieving improved corrosion protection and thus a lower corrosion speed.

For a certain level of dope-material the corrosion rate relates to the structure of the alloy. The highest reduction of corrosion rate is when the alloy is in an amorphous disordered structure, where the elements are distributed completely uniformly and where there are no intermetallic chemical connections/interfaces in a crystallized structure. This may be further advantageous in regard to achieving improved corrosion protection and thus reduced corrosion speed.

The additional corrosion protection/corrosion control function is important in medium- and high temperature absorbers (>80° C) placed in an atmospheric environment (as opposed to being placed in a vacuum environment), and are especially important when the absorbers are placed in high-corrosion environments, e.g. seaside environ- ments.

Solar selective coating is defined as a coating having high absorption of electromagnetic radiation at the solar spectrum wavelengths and low thermal emittance in the thermal infrared wavelength range defined by its temperature of operation.

The solar selective coating may be defined with a solar selective surface, which is the free surface of the anti-reflection stack. The free surface is the surface facing the ambient surroundings and opposite to the surface towards the absorber stack. One effect of this embodiment is that the solar selective coating is configured to be spectrally selective absorbing with high absorption of solar radiation and low loss of absorbed energy as infrared radiation (heat) out through the anti-reflection stack. This is advantageous in regard to converting solar radiation into heat - sometimes referred to as photo-thermal conversion. The thermal emittance is low to the ambient environ- ment but may be high to the substrate.

In one aspect the solar selective coating may be used in different environments. Thus, the solar selective coating may be a non-vacuum-use solar selective coating but may also be used in protective conditions such as vacuum and inert gas. In one embodiment of a pressure-formed solar absorber obtained by the process of method acts described above, the adhesion layer comprises a metallic layer comprising molybdenum and titanium with layer thickness in the range 30 nm to 500 nm, preferably in the range 80-200 nm, even more preferably in the range 110-130 nm. The adhesion layer comprises a metallic layer comprising 85-99% (w/w) Mo and 1- 15% (w/w) Ti, preferably in the range 90-97%) (w/w) Mo and 3-10% (w/w) Ti, even more preferably in the range 95-96%) (w/w) Mo and 4-5% (w/w) Ti. The adhesion layer may comprise a metallic surface comprising molybdenum (Mo) and titanium (Ti) and/or an alloy thereof. The adhesion layer comprising a metallic layer comprising Mo and Ti will throughout the description and claims also be described by and referred to as a MoTi adhesion layer or MoTi layer regardless of the structure.

In one aspect the solar selective coating may be used in ambient surroundings. Thus, the solar selective coating may be a non-vacuum-use solar selective coating but may also be used in protective conditions such as vacuum and inert gas. One effect of the MoTi layer as adhesion layer is that it constitutes a good reflector in relation to IR radiation or thermal emittance from the substrate side. Molybdenum has a high reflectivity at wavelengths above 2 μιη. This is advantageous in regard to achieving good solar selective properties for the coating as the thermal emittance from the substrate is reduced.

Molybdenum exhibits good properties in terms of adhesion to substrate materials conventionally used in solar absorbers. Thus, another effect of the MoTi layer as adhesion layer may be that it constitutes a good adhesion or bonding layer for the absorber stack. Good adhesion properties may prevent the absorber stack from loosening from the substrate and thus an advantage of this is an increase in lifetime of the solar selective coating.

Yet another effect of the MoTi layer as adhesion layer is that it constitutes a diffusion barrier in relation to the absorber stack. A diffusion barrier or barrier layer inhibits or reduces the movement of material across the barrier or barrier layer. Mo is very stable at high temperatures, and therefore acts as a good barrier against diffusion of material components between the substrate and the absorber and anti-reflection stacks. This is advantageous in regard to the fact that the optical properties of the absorber stack and the anti-reflection stack are maintained due to reduced contamination. Furthermore, deterioration of the solar selective properties may be reduced considerably compared to prior art due to reduced diffusion, thereby, maintaining long-term stability of the coating and thus, maintaining the desired properties to prolong the lifespan of the coating.

Molybdenum generally has good corrosion properties. In addition titanium is particularly characterized by the material's good corrosion properties, and thus an additional effect of the MoTi layer as adhesion layer may be that the MoTi layer is highly resistant to corrosion. This is essential for the corrosion resistance of the entire solar selective coating and is advantageous in regard to acting as a corrosion protection of the substrate.

The problem of combining a molybdenum adhesion layer with an absorber stack comprising titanium and aluminium is that the standard electrochemical potential of mo- lybdenum is much higher than the electrochemical potential of titanium and aluminium. This causes the molybdenum adhesion layer to act as a cathode for the absorber stack and increases the corrosion speed of any outside agents. By adding a small fraction of titanium to the molybdenum layer, the electrochemical potential of the layer will be lowered, thus making the relative corrosion potential smaller, resulting in a lower corrosion speed.

Especially the combination of the above-mentioned effect is advantageous in regard to achieving an adhesion layer which serves a as a good reflector, a corrosion resistant layer and a diffusion barrier with excellent adhesion properties. These features are especially important in harsh environmental surroundings and when operated at high temperatures.

The adhesion layer of the solar selective coating thus also acts as an IR-reflector layer and a diffusion barrier. In one aspect the MoTi adhesion layer may contain a small contribution of silicon (Si), yttrium (Y) and/or niobium (No). The effect of adding these materials to the MoTi adhesion layer may be to modify the larger MoTi structures with the advantage of obtaining a similar or increased corrosion resistance and with the additional effect of obtaining a similar or increased long-term stability of the optical properties of the solar selective coating.

In general the thickness of the individual layers may be up to 5000 nm. However, the benefit of increasing the layer thickness should be considered in comparison with the cost of the materials.

A further effect of this embodiment is that a sufficient thickness of the adhesion layer is achieved for covering the substrate and to achieve the abovementioned effects of the adhesion layer with the advantages also described above. A further advantage is that excessive costs to adhesion layer materials are prevented.

One effect of this embodiment is that a sufficient amount of Ti may be added to the MoTi metallic composition to achieve improved corrosion resistant properties caused by titanium with the advantages, as previously mentioned, of improved corrosion resistance of the entire solar selective coating in regard to acting as a corrosion protection of the substrate.

In one aspect the pressure-formed thermal absorber means may be configured with a surface coating on one surface, the first surface of the thermal absorber panel, comprising a solar selective coating, and a thermal infrared reflective coating on the opposite surface, the second surface of the thermal absorber panel. Thus, in one embodiment of the pressure-formed thermal solar absorber a surface coating is deposited on the first surface, which surface coating comprises a solar selective coating, and a sur- face coating is deposited on the second surface, which surface coating comprises an infrared selective coating.

In one aspect the thermal infrared reflective coating may be comparable to the features of the adhesion layer such as materials, thicknesses and deposition method. One effect of combining a solar selective coating on one side with a thermal infrared reflective coating on the opposite side of the thermal absorber panel may be to achieve a low thermal emittance to the surroundings from the fluid because of a low thermal emittance to the surroundings from the fluid through the thermal infrared reflective coating and because the solar selective coating is configured - as previously described - to convert solar radiation into heat with a low thermal emittance to the ambient surroundings but with a high thermal emittance to the pressure-formed thermal absorber means. This has the advantage of a high energy yield from the thermal absorber means configured with a surface coating.

One object of the invention may be achieved by a method for making a pressure- formed thermal solar absorber comprising an adhesion layer which comprises a metallic layer comprising molybdenum and titanium with layer thickness in the range 30 nm to 500 nm, preferably in the range 80-200 nm, even more preferably in the range 110-130 nm. The adhesion layer comprises a metallic layer comprising 85-99% (w/w) Mo and 1-15% (w/w) Ti, preferably in the range 90-97% (w/w) Mo and 3-10% (w/w) Ti, even more preferably in the range 95-96%) (w/w) Mo and 4-5% (w/w) Ti, wherein the adhesion layer is deposited onto the surface of the thermal absorber panel comprising the acts of:

• providing a base pressure of < 1E-4 mbar;

• providing a surface temperature of the thermal absorber panel above 50°C, preferably above 100°C, even more preferably above 150°C;

• providing a process pressure of < lE-1 mbar by providing a protective atmosphere to the process chamber of instrument grade argon gas prior to deposition of the adhesion layer by a vacuum deposition process; and

• performing the vacuum deposition process.

The base pressure is the pressure provided in the vacuum chamber prior to the deposition processes. A further effect of this method is that the adhesion layer may be deposited using a sputtering process. This is advantageous in regard to the fact that the sputtering process may be done as an in-line deposition process. In an in-line sputtering deposition process the workpiece may be moved in a continuous movement through one plasma region in which one layer is deposited onto the next plasma region where another layer is deposited and so forth. Alternatively, several in-line sputter zones are arranged in series and moved across the workpiece. In either case, the gas composition and plasma intensity may be controllable for a stable deposition rate during the process to ensure a layer of uniform material composition and uniform thickness on the entire workpiece.

One advantage is that the workpiece is kept in a controlled vacuum environment throughout the deposition process and during the time between each deposition pro- cess, thereby preventing oxidations of the surfaces that subsequently constitute the interface between the individual layers of the solar selective coating.

Another advantage is that magnetron and plasma regions may be kept relatively small compared to the size of the workpiece, thereby reducing the cost of production facili- ties because the equipment's "core" in form of sputtering zones with magnetrons, gas inlet channels, sensors and so forth.

In one aspect the absorber layer(s) comprise(s) a ceramic and/or metallic composition which may be deposited using a vacuum deposition process. In one aspect the ceramic and/or metallic composition may comprise elements selected from the group consisting of: aluminium, nitrogen, titanium, oxygen or combinations hereof. In one aspect the absorber layer may comprise ceramic and/or metallic materials comprising both ceramic compositions and ceramic metallic CERMET compounds which may be deposited using a vacuum deposition process.

Such absorber layers are characterized by being high temperature stable, having high oxidation resistance and good absorption properties for wavelengths below 2.5 μιη.

These absorber layers may comprise titanium aluminium nitride or titanium aluminium oxynitride. For example the absorber layers may include TiAIN, TiAINO, TiN but not limited to these examples.

In one embodiment the anti-reflection layer(s) comprise(s) a ceramic composition which may be deposited using a vacuum deposition process.

In one aspect the ceramic composition may comprise elements selected from the group consisting of: silicon nitride(s), silicon oxide(s), aluminium nitride(s), aluminium oxide(s), titanium oxide(s) or combinations hereof. For example the anti-reflection layers may include Si₃N₄, SiO, Si0₂, A1₂0₃, ΑΓΝ, TiO, Ti0₂ but not limited to these examples. In one aspect the anti-reflection layer(s) may comprise inorganic compositions with a refractive index below 2 which may be deposited using a vacuum deposition process.

Such anti-reflection layers are characterized by having a high transmission for wavelength below 2.5μιη and also being highly resistant towards oxidation and humidity and corrosion resistant.

In addition, one effect of the above-mentioned embodiments with the described absorber layers in combination with the anti-reflective layers may have optical properties resulting in high optical absorption. This is advantageous in regard to increasing the energy yield of the solar selective coating.

In one aspect the absorber layer(s) may be deposited using a sputtering process.

In one aspect the anti-reflection layer may be deposited using a sputtering process. Existing solar selective coatings often require several processes, one for each layer or comprised in the solar selective coating. The embodiments described above for depositing the adhesion layer, the absorber stack and the anti-reflection coating may all be deposited by methods comprising substantially the same acts and thus the same environment parameters in the process chamber. One effect of these embodiments is that the solar selective coating may be made in an in-line deposition process with the advantages just described previously.

In one aspect the pressure-formed thermal absorber means may be bent.

In one aspect the pressure-formed thermal absorber means may be rolled into a tube. In one aspect the pressure-formed thermal absorber means may be shaped with a wavy structure.

The effect of shaping the pressure-formed thermal absorber means may be that the device may be implemented in a wide range of applications with the advantage of ex- tending the use of the device but also with the advantage of shaping the device according to the energy source for which it is used.

Description of the Drawing

Figure 1 illustrates a pressure-formed thermal absorber means.

Figure 2 illustrates two embodiments of planar thermal solar absorbers.

Figure 3 illustrates a method for making a pressure-formed thermal absorber means. Figure 4 illustrates the acts of depositing a solar selective coating comprised in the method for making a pressure-formed solar absorber. Figure 5 illustrates the acts of depositing an adhesion layer onto a surface of the thermal absorber panel for making a pressure-formed solar absorber.

Figure 6 illustrates a thermal absorber panel.

Figure 7 illustrates embodiments of joints configured to form one or more flow channels in a thermal absorber panel.

Figure 8 illustrates an embodiment of jointing a flange to an inlet or outlet of one sheet and jointing the sheets together for making a pressure-formed thermal absorber means.

Figure 9 illustrates a substrate surface with surface roughness comprising micro and macro roughness (fig. 9A) and macro roughness (Fig. 9B).

Figure 10 illustrates a four-layer stack (fig. 10A) and a three-layer stack (fig. 10B) solar selective coating deposited on a thermal absorber panel surface.

Figure 11 illustrates the calculated reflectance (A) versus wavelength for a four-layer solar selective coating and the Solar insolation AM 1.5 spectrum (B) versus wavelength.

Figure 12 illustrates different constructions of pressure-formed thermal absorber means.

Figure 13 illustrates different configurations for applying surface pressure to the jointed sheets during the act of forming pressure-formed thermal absorber means.

Figure 14 illustrates forces acting on a thermal absorber panel.

Figure 15 illustrates one embodiment of a thermal absorber panel comprising flow channels with stiffness enhancing profiles. Figure 16 illustrates one embodiment of a thermal absorber panel comprising deep flow channels.

Figure 17 illustrates one embodiment of a thermal absorber panel integrated with insu- lation material and a back plate.

Detailed Description of the Invention

No Item

10 Pressure-formed thermal absorber means

20 Thermal absorber panel

22 First surface

24 Second surface

26 Imprint

30 Joinable sheets

32 Bottom outer sheet

34 Top outer sheet

36 Sheet thickness

38 Jointed sheets

40 Flow channel

42 Pressure expanded flow channel

44 Flow channel first end

46 Flow channel second end

48 High pressure fluid

50 High pressure joints

52 Closed loop joint

54 Pattern

55 Linear welding elements

56 Circular welding elements

58 Heat affected zone (HAZ)

60 Inlet

62 Inlet flange

70 Outlet 72 Outlet flange

80 Sheet surface

82 Surface topology

84 Micro roughness

86 Macro roughness

90 Pressure-formed thermal solar absorber

100 Solar selective coating

102 Adhesion layer

104 Absorber layer

106 Semi-absorber layer

108 Anti-reflection layer

110 Layer thickness

112 Layer material

114 Three-layer coating

116 Four-layer coating

118 Sandwich construction

120 Refractory metal

122 Dope-material

124 Amorphous disordered structure

130 Surface coating

132 Vacuum deposited coating

134 Infrared reflective coating

136 Anti-fouling coating

150 Flow

160 Pre-polished

170 Base pressure

172 Process pressure

174 Surface temperature

180 Vacuum deposition process

190 Solar insolation

200 Tube

202 Flat

204 Bended 210 Planar thermal solar absorber

212 Thermal absorber plate

214 Thermal absorber tube

220 Temperature

222 Thermal absorber surface temperature

224 Fluid temperature

300 Press

310 Surface pressure

320 Die(s)

322 Pressure-active die

324 Pressure-passive die

326 Clamping means

328 Base tooling plate

340 Stiffness

342 Deflection

344 External forces on the thermal absorber structure

346 Support forces

348 Stiffness enhancing profile

350 V-profile

352 Flow channel height

354 Flow channel, long direction

356 Flow channel, short direction

358 Deep flow channel

360 Riser tube

362 Riser tube height

364 Deep flow channel height

380 Insulation material

382 Back plate

384 Back plate adhesive

386 Insulation structure

400 Method

410 Providing

420 Arranging 430 Jointing

440 Applying

450 Depositing

460 Performing

470 Coating

480 Placing

In figure 1 a pressure-formed thermal absorber means is illustrated. The illustrated embodiment comprises two sheets 30, a bottom outer sheet 32 and a top outer sheet 34, to form a thermal absorber panel 20 with a first surface 22 and a second surface 24. The top outer sheet 34 comprises an inlet 60 and an outlet 70. The two sheets 30 are joined by high pressure joints 50 in a closed loop 52 and thus the closed loop joint encircles inlet 60 and outlet 70. An inlet flange 62 and an outlet flange 72 are connected to the thermal absorber panel through the inlet 60 and outlet 70. Between the two sheets 30 and encircled by the joint 52 a pressure expanded flow channel 42 is comprised.

Figure 2 illustrates two embodiments of planar thermal solar absorbers. In figure 2A a segment of a traditional planar thermal solar absorber 210 is illustrated and in figure 2B a pressure-formed thermal solar absorber 90 is illustrated. The traditional planar solar absorber 210 illustrated here comprises thermal absorber tubes 214 mechanically connected to a thermal absorber plate 212. Only a small segment of a traditional planar thermal solar absorber 210 is illustrated which comprises a single tube. The traditional planar thermal solar absorber 210 may comprise a multiple number of the illustrated segments. The typical distance between the mechanical connection points of the tubes to the thermal absorber plate 212 are in the rage 150 mm. The tubes 214 are configured with flow channels 40 and surface of the thermal absorber plate 212 facing away from the tubes 422 comprises the surface 80 for the solar selective coating 100 and thus the surface facing the sun, illustrated by solar insolation 190 onto the surface. The tubes may be double walled tubes comprising two tubes within each other thereby forming two flow-channels: one in the inner tube and one comprised between the two tubes. The channel - the outer channel - comprised between the two tubes may be used for fluid for heat transfer while the inner most often may not be used as a flow- channel. By using the outer channel a large fluid-to-tube-surface area is obtained.

Figure 2B illustrates a pressure-formed thermal solar absorber 90 comprising two sheets - a bottom outer sheet 32 and a top outer sheet 34 joined by high-pressure joints. The pressure-formed thermal solar absorber 90 constitutes flow channels 40 comprised between the two sheets 32,34. The sheet surface 80 of the top outer sheet 34 facing towards the solar insolation 190 may constitute the surface 80 for a solar selective coating.

The thermal transfer from the planar thermal solar absorber 210 in figure 2 A may be lower than the thermal transfer obtainable by the pressure-formed thermal solar absorber 90 in figure 2B because of the direct contact of the fluid with the top outer sheet 34 comprising the solar selective coating 100. For the traditional planar thermal solar absorber 210 the thermal absorber plate 212 comprising the solar selective coating 100 is only connected in the areas connecting the tubes to the plate 212 and furthermore, the thermal contact to the fluid is thus indirect from the plate to the fluid through the tube walls. This is illustrated by the temperature 220 curves in each figure, wherein the thermal absorber surface temperature 222 and the fluid temperature 224 are illustrated as a function of distance across the sheet surface.

For the pressure-formed thermal solar absorber 90 in figure 2B, a flat substantially flat temperature curve across the surface 80 for both the surface temperature 224 and the fluid temperature 224 may be achieved. This may be achieved because of a substan- tially even thermal transfer rate across the surface 80 from the top outer sheet to the fluid, due to the large fluid-to-top outer sheet contact area. The surface temperature 222 is higher than the fluid temperature 224.

In figure 2A the temperature curves across the surface 80 shows a different situation. The thermal absorber surface temperature 222 may increase as the distance to the mechanical connection point of the tube increases, while a substantially constant fluid temperature 224 is achieved. This may arise because of the lower thermal transfer from the top outer sheet to the fluid due to the indirect and small fluid-to-top outer plate contact area. Figure 3 illustrates one method 400 for making a pressure-formed thermal absorber means 10. The joinable sheets 30 are provided 410 and arranged 420 to lie substantially flat on top of each other. The sheets are arranged 420 with a bottom outer sheet 32 and a top outer sheet 34 to form a thermal absorber panel 20. See figure 1 for the mechanical parts. The sheets 30 are joined 430 by high pressure joints 50 in a closed loop 52. The closed loop 52 as seen in figure 1 encircles the provided inlet(s) 60 and outlets) 70. The sheets 30 may be further joined 430 by high pressure joints 50 within the closed loop 52 whereby a pattern 54 (an example is illustrated in figure 6) is creat- ed. This pattern 54 may define a flow channel whereby the fluid in the flow channel is guided from inlet to outlet to achieve a longer way of travelling and thus a larger contact area for the fluid with the sheets from inlet to outlet. The pattern 54 may also define multiple flow channels 42. In the illustrated embodiment the thermal absorber panel 20 is coated 470 on one surface or on both surfaces after the sheets are joined.

A high pressure is applied 440 to the thermal absorber panel 20 to form the flow channel(s). The pressure-formed thermal absorber means 10 may comprise one or more inlets and one or more outlets. Furthermore, the pressure-formed thermal absorber means 10 may comprise one or more flow channels. The high pressure provid- ed to the thermal absorber panel 20 to form the flow channel(s) 42 may thus be provided 410 to one inlet, multiple inlets, one outlet, multiple outlets or any combination of this.

The coating 470 may be performed as illustrated here before applying the high pres- sure to form flow channel(s). However, the coating 470 may be performed before jointing, or after jointing and after applying the high pressure to form flow channel(s).

Figure 4 illustrates the acts of depositing a solar selective coating 100 onto the thermal absorber panel 20 comprised in a method 400 for making a pressure-formed thermal solar absorber 10.. The illustrated acts may be added to the method illustrated in figure 3 before applying high pressure to the thermal absorber panel. Alternatively, the illustrated acts may be added to the method illustrated in figure 2 after applying high pressure to the thermal absorber panel and thus, after pressure forming the flow channels). The solar selective coating 100 is deposited on a pre-polished substrate, here a pre-polished 160 surface is provided 410 by a pre-polished surface of the sheets comprised in the thermal absorber panel 20 and which surface constitutes the first 22 or second 24 surface or the first and second surface of the absorber panel 20. The adhesion layer 102 is deposited 450 onto the pre-polished 160 surface. The absorber layer 104 is deposited 450 onto the adhesion layer. In case the absorber layer 104 comprises multiple layers the individual layers are deposited one layer at a time. The anti- reflection layer 108 is deposited 450 onto the absorber layer 104. In case the anti- reflection layer 108 comprises multiple layers the individual layers are deposited one layer at a time.

Figure 5 illustrates a method 400 for depositing the adhesion layer 102 onto the surface of the thermal absorber panel 20. The adhesion layer 102 is deposited by a vacuum deposition process 180 and the method 400 comprises several acts. A base pressure 170 is provided 410 and a temperature 174 of the thermal absorber panel surface is provided 410. Prior to deposition of the adhesion layer by the vacuum deposition process 180 a process pressure 172 is provided 410 by adding a protective atmosphere to the process chamber. The deposition is performed 460 by a vacuum deposition process 180. Figure 6 illustrates a thermal absorber panel 20 comprising two sheets 30. In a perspective view figure 6A illustrates the parts of a thermal absorber panel 20 consisting of two sheets 30 with sheet thickness 36: a top outer sheet 34 and a bottom outer sheet 32. The sheets may be of the same thickness but may also differ depending on the application. In the illustrated embodiment one inlet 60 and one outlet 70 are com- prised in the top outer sheet 34. The two sheets are arranged substantially flat on top of each other to form a thermal absorber panel 20. In figure 6B the joined thermal absorber panel 20 is illustrated from a top view. The inlet 60 and outlet 70 are placed in an orthogonal manner across from each other and the sheets 30 are joined by high pressure joints 50 in a closed loop 52 encircling provided the inlet 60 and the outlet 70. In this embodiment the closed loop joint is on the periphery of the thermal absorber panel 20.

Figure 7 illustrates embodiments of thermal absorber panels 20 comprising patterns 54 of high pressure joints 50 to shape the flow channels such that the flow channels are configured to form the flow through the thermal absorber panel from inlet 60 to outlet 70. Figure 7 A illustrates a pattern comprising of linear elements substantially parallel to the long side of the thermal absorber panel 20. The linear elements are placed in four rows with 14, 13, 13 and 14 elements in each row but with the elements of each row shifted compared to the adjacent row. At the inlet 60 and the outlet 70 a linear element is placed perpendicular to the linear elements comprised in the four rows. The pattern may provide for a flow through the thermal absorber panel 20 guided substantially linear with the long side of the panel through the four rows but guided in direction of the outlet due to the shift between the rows. Furthermore, the perpendicular linear elements at the inlet 60 and the outlet 70 may influence on the turbulence which may arise due to the flow distribution at the end of the linear elements forming flow channels ending out close to one side of the closed loop. In figure 7B four additional linear elements are added to the pattern. These elements are placed in a zig-zag pattern between the four rows and substantially perpendicular to the linear elements com- prised in the rows. This pattern may shape the flow direction further, which is illustrated in figure 7C. In figure 7C the overall flow 150 is illustrated to move in a zig-zag direction according to the zig-zag pattern from the flow channel first end 44 to the flow channel second end 46. Figure 8 illustrates an embodiment of jointing an inlet flange 62 to an inlet 60 comprised in one sheet 30 and jointing the sheets 30 together for making a pressure- formed thermal absorber means 10. Only part of the pressure-formed thermal absorber means 10 is illustrated and the embodiment is illustrated as a cross-sectional side view. The joinable sheet 30 comprising the inlet 60 comprises imprints prior to weld- ing. The imprint is a fully embossed flange to which the inlet flange 62 is jointed. In this embodiment the jointing is performed by a welding process and thus, the welding seams constituting the high pressure joints 50 are illustrated (the welding seams are illustrated by the bold, filled triangles). The welding of an upper part to an adjacent lower part may be done through the upper part and thus, the welding of the illustrated embodiment may be performed from the side comprising the inlet flange 62 as illustrated in figure 8A. Figure 8B illustrates one embodiment of a pressure-formed thermal absorber means 10. The embodiment is seen from the side. One inlet 60 and one outlet 70 configured with one inlet flange 62 and one outlet flange 72 are comprised in the bottom outer sheet which also comprises the second surface 24. The top outer sheet comprises the first surface 22.

Figure 9 illustrates a sheet 30 comprising a surface 80 and the surface topology 82. Figure 9A illustrates a sheet surface 80 comprising micro roughness 84 and macro roughness 86. Figure 9B illustrates a polished sheet surface 80 comprising only macro roughness 86 after a polishing treatment. The sheet surface 80 may be polished using for example ion etching. A raw but also a pre-polished 160 sheet surface may comprise tips and edges of nano and micro sizes. By polishing the sheet surface 80 the surface structure may be changed to comprise rounded and smoothed tips and edges upon which the solar selective coating 100 may be deposited. Figure 10 illustrates solar selective coatings 100 deposited on a sheet 30. Figure 10A illustrates a four-layer coating 1 16 comprising an adhesion layer 102, an absorber layer 104, a semi-absorber layer 106 and an anti-reflection layer 108. The solar selective coating 100 constitutes a sandwich construction 118, configured with the adhesion layer 102, deposited on a sheet surface 22, the absorber layer 104 deposited on the adhesion layer 102, the semi-absorber layer 106 deposited on the absorber layer 104 and the anti-reflection layer 108 deposited on the semi-absorber layer 106.

Figure 10B illustrates a three-layer coating 114 comprising an adhesion layer 102, an absorber layer 104, and an anti-reflection layer 108. The solar selective coating 100 constitutes a sandwich construction 118 configured with the adhesion layer 102 deposited on a sheet surface 22, the absorber layer 104 deposited on the adhesion layer 102, and the anti-reflection layer 108 deposited on the absorber layer 104.

The individual layers of the selective coatings may be described by a layer thickness 110 and layer material 112 with a corresponding refractive index. The interfaces between the layers may be described by boundary conditions by which reflectance and absorbance of incident radiation on the solar selective coating 100 may be calculated through the coating using classical optical theory. Figure 11 illustrates the calculated reflectance (A) versus wavelength for a four-layer solar selective coating and the Solar insolation AM 1.5 spectrum (B) versus wavelength. The four-layer solar selective coating comprises a 120 nm thick adhesion layer comprising a metallic layer comprising 95% Mo (w/w) and 5% Ti (w/w), a 70 nm thick titanium aluminium nitride absorber layer, a 30 nm thick titanium aluminium oxynitride semi-absorber layer, and a 80 nm thick silicon oxide(s) anti-reflection layer.

The reflectance of the four-layer solar selective coating is calculated using a matrix formalism algorithm based on boundary conditions and based on input of the reflective indices of individual layer materials 112 and sequence of the individual layers at a temperature of 350°C. The four-layer solar selective coating is calculated to obtain an optical absorption of 93% of the Solar insolation AM 1.5 spectrum. For this invention the solar selective coating may be chosen amongst conventional coatings already used today or any appropriate coatings for the intended application.

Figure 12 illustrates different constructions of thermal absorber panels 20 for pressure-formed thermal absorber means 10.

In figure 12A one embodiment of a thermal absorber panel 20 comprising a pattern 54 of high pressure joints to shape the flow channels such that the flow channels are configured to form the flow through the thermal absorber panel from inlet 60 to outlet 70. The pattern is configured by linear welding elements 55 comprising the pattern and thus, the flow channels 40. In figure 12B a similar pattern 54 is illustrated, however, this pattern is configured by circular linear welding elements 56 and thus the flow may be between the individual circular welding. If compared to figure 12 A the flow in figure 12B may be said to be across the channels shaped in figure 12A by the linear welding elements 55. For both figures the inlet 60 and outlet 70 are comprised in one sheet and thus on the same side of the panel 20 but orthogonal to each other.

In figure 12C a bended 204 thermal absorber panel 20 is illustrated. The welding pattern shaping the flow channels comprises linear welding elements 55 like in figure 12 A. The inlet 60 and outlet 70 are comprised in one sheet and thus on the same side of the panel 20 but orthogonal to each other. Figure 12D also illustrates a bended 204 thermal absorber panel 20 but bended to such a degree that a tube is formed. The welding pattern shaping the flow channels comprises circular welding elements 56 as in figure 12B. The inlet 60 and outlet 70 are not illustrated in this figure.

Figure 13 illustrates four different configurations for applying surface pressure 310 to the jointed sheets 38 during the act of forming pressure-formed thermal absorber means. Pressure-forming by applying a high-pressure fluid to the structure may also be referred to as inflated or embossed. For all for configurations the sheets are jointed (here illustrated as welded) before forming and the jointed sheets 38 are placed on a base tooling plate 328. The pillow-plate absorber is said to be double embossed if it is free to inflate to both sides and single embossed if one of the sheets is not significantly deformed during the pressure forming process. Figure 13 A illustrates an embodiment of a traditional method for forming a pillow- plate absorber by a die 320 which defines the overall shape of the absorber when it is pressure-formed. The die 320 is pressure passive die 324. The die and the base tooling plate 328 are clamped together using clamps 326 along the periphery of the die 320 and the base tooling plate 328. The die 320 and the base tooling plate 328 are arranged with a distance between them thereby defining the maximum height of the channels. In this embodiment the pillow-plate absorber is said to be double embossed as it is free to inflate to both sides. Thus, the channels/pockets in the jointed sheets 38 inflate/expand between the die 320 and the base tooling plate 328, and the height of the channels/pockets is limited by the two parts as these two parts are clamped to a fixed distance by simple mechanical clamps 326. This is the traditional method used when inflating pillow plates where no press 300 is required. This method may also be used for inflation of single embossed pillow plates when one sheet is significantly thicker than the other (e.g. 2 - 3 times thicker), as the thin sheet may be deformed while the thick plate does not deform significantly.

Figure 13B illustrates an embodiment of a method for forming a pillow-plate absorber by a pressure-active die 322. The die 320,322 and the base tooling plate 328 are placed in a press 300. The pressure-active die 322 defines the overall shape of the absorber when it is pressure-formed. The pressure-active die 322 further comprises a pattern which pattern defines the area where a surface pressure 310 is applied before inflating/expanding the jointed sheets 38. The pattern is that of the joints that joint the sheets. The area of the pattern may comprise at least the width and length of the joints. The area defined by the pattern may be greater comprising a further trail along the joint(s) extending to each side of the joint(s). The surface pressure is the pressure applied by the press. The pressure is distributed to the joint sheets by the pressure-active die 322. The method illustrated here is for a single embossed absorber (where one plate remains flat) as the base tooling plate 328 is flat. The pressure-active die 322 may also act as a clamping mechanism which clamps the jointed sheets 38 during the pressure-forming process. As the jointed sheets 38 are inflated/expanded, the pressure-active die 322 defines the overall shape of the absorber, and at the end of the process, the pressure applied by the press is not only applied to the area defined by the pattern of the die but also by the upper part of the die 322. Therefore the pressure applied by the press should be the total pressure applied to the entire surface of the absorber. Using a method illustrated in figure 13B may require using a press with a pressure capacity of 200 tons/m^A2

Figure 13C illustrates an embodiment of a method for forming a pillow-plate absorber using a two-part die 320 comprising a pressure-active die 322 and a pressure-passive die 324. The two-part die 320 and a base tooling plate 328 are placed in a press 300. This method is a combination of the methods illustrated in figure 13A and figure 13B. The pressure-passive die 324 and the base tooling plate 328 are clamped together using clamps 326 along the periphery of the pressure-passive die 324 and the base tool- ing plate 328. The pressure-passive die 324 and the base tooling plate 328 are arranged with a distance between them thereby defining the maximum height of the channels. Thus, the channels/pockets in the jointed sheets 38 inflate/expand between the die 320 and the base tooling plate 328, and the height of the channels/pockets is limited by the two parts as these two parts are clamped to each other with a fixed dis- tance by simple mechanical clamps 326.

Contrary to the method illustrated in figure 13B, the pressure-active die 322 does not define the overall shape of the absorber when it is pressure-formed. As described above, the pressure-active die 322 comprises a pattern which pattern defines the area where a surface pressure 310 is applied before inflating/expanding the jointed sheets 38. The pattern is that of the joints that joint the sheets. The area of the pattern may comprise at least the width and length of the joints. The area defined by the pattern may be greater comprising a further trail along the joint(s) extending to each side of the joint(s). The surface pressure is the pressure applied by the press. The pressure is distributed to the joint sheets by the pressure-active die 322.

The pressure-active die 322 may also act as a clamping mechanism which clamps the jointed sheets 38 during the pressure-forming process.

Contrary to the method illustrated in figure 13B, the pressure-active die 322 may only have contact with the jointed sheets 38 at the area of the pattern and thus only apply surface pressure to this area through the entire pressure-forming process. The method illustrated here is for a single embossed absorber (where one plate remains flat) as the base tooling plate 328 is flat.

Figure 13D also illustrates an embodiment of a method for forming a pillow-plate absorber using a two-part die 320 comprising a pressure-active die 322 and a pressure- passive die 324. Thus the description of the method illustrated in figure 13 C is also descriptive for this embodiment. The only difference is that the base tooling plate 328 only supports the jointed sheets 38 in selected areas which may include the areas comprising the joints. Thus, the method illustrated here is for a double embossed absorber where the base tooling plate 328 may define the overall shape of the pillow- plate absorber in one direction, and the pressure-passive die 324 may define the overall shape of the pillow-plate absorber in the other direction.

Figure 14 illustrates forces acting on a thermal absorber panel 20. The embodiment of a thermal absorber panel 20 illustrated here in figure 14A corresponds to that illustrat- ed in figure 7A. The thermal absorber comprises patterns of high pressure joints 50 to shape the flow channels such that the flow channels are configured to form the flow through the thermal absorber panel from inlet 60 to outlet 70. Figure 14A illustrates the thermal absorber panel 20 seen from a top view. Figure 14B illustrates a side view of the thermal absorber seen from a side view in the short direction. The short direc- tion runs across the thermal solar panel in a direction parallel to B-B direction. Figure 14C illustrates a side view of the thermal absorber seen from a side view in the long direction. The long direction runs across the thermal absorber panel 20 in a direction parallel to A-A direction. Some of the forces acting on a thermal absorber panel 20 when mounted are support forces 344 and external forces on the thermal absorber structure 344. The external forces 344 may include gravitational forces and inertia forces. The external forces 344 are not limited to these mentioned here and may include additional forces. In this embodiment the support forces 344 are illustrated as employed along the side of the panel 20. The external forces 344 may be evenly distributed across the panel 20. This may result in a defection 342 of the panel 20. By improving the stiffness of the panel 20 this deflection may be limited. The optimal thermal absorber panel 20 should have as few mounting points as possible as each mounting point gives rise to heat los- es and brings increased production costs.

Figure 15 illustrates one embodiment of a thermal absorber panel 20 comprising flow channels 20 with stiffness enhancing profiles 348. Aspects of the thermal absorber panel 20 and the directions described in figure 14A may pertain to the details dis- closed in this embodiment. The stiffness enhancing profiles 348 may be any shape, which increases the stiffness of the panel 20. Here, V-shaped profiles 350 are used. The number of stiffness enhancing profiles 348 imprinted in the individual flow channels may differ across the thermal absorber panel 20. The illustrated embodiment further comprises deep flow channels 358, which are extra deep with a height 364. The deep flow channels 358 may act as stiffness enhancing structures. The stiffness enhancing profiles 348 and the deep flow channels 358 may be partly or fully imprinted in the joinable sheets prior to starting the process of making the pressure-formed thermal absorber means. The imprints may be performed by a deep drawing process or similar methods.

Three sections of the thermal absorber panel 20 are illustrated as enlarged illustrations I, II, and III and illustrated from a side view. Enlarged illustration I illustrates a flow channel 40 in the short direction 356 of the thermal absorber panel 20 (direction B-B in figure 14A). The flow channel 356 with the height 364 comprises two stiffness enhancing profiles 348. The part of the flow channel in the short direction 356 illustrated in this enlarged part is connected to a number of flow channels in the two flow channels in the long direction 354. The flow channel in the short direction 356 is configured with a deep flow channel 358 with height 364. The flow channels in the long direction 354 have more shallow profile with a height 352. Enlarged illustration II illustrates a flow channel 40 in the long direction 354 of the thermal absorber panel 20 (direction A-A in figure 14A). The flow channel 354 comprises two stiffness enhancing profiles 348. These profiles may be different in shape or size compared to other stiffness enhancing profiles 348 placed otherwise in the structure. The flow channel in the long direction 354 is a deep flow channel 358 with height 364. The joinable sheets 30 are jointed on both sides of and in a direction along the flow channel 354.

Enlarged illustration III illustrates a flow channel 40 in the short direction of the thermal absorber panel 20 (direction B-B in figure 14A). This flow channel may also be referred to as a riser tube 360 as this channel is connected to the outlet (the flow channel connected to the inlet may also be referred to as riser tube). In this embodiment the riser tube 360 does not comprise any stiffness enhancing profiles 348. The part of the riser tube 360 illustrated in this enlarged part is connected to a flow channel in the long direction. The riser tube 360 is configured as a deep flow channel and thus, the riser tube height 362 may be referred to as deep flow channel height 364. The flow channel in the long direction illustrated in this enlarged part has a more shallow profile with a height 352. The joinable sheets 30 are jointed on one side of and in a direction along the flow channel 40 comprising the riser tube 360. The pattern of inflated flow channels in the two directions combined with stiffness enhancing profiles 348 imprinted in the walls of the flow channels may bring a mechanical structure of the thermal absorber panel 20 comprising an enhanced stiffness. The depth of the flow channels may have high impact on how much the flow channels contribute to the overall stiffness of the panel. Figure 16 illustrates one embodiment of a thermal absorber panel 20 comprising deep flow channels 358. Aspects of the thermal absorber panel 20 and the directions described in figure 14A may pertain to the details disclosed in this embodiment. The flow channels 40 are configured to comprise a number of flow channels with extra depth, illustrated by the flow channels marked with Xs. The deep flow channels 358 are comprised in two directions - in the short direction (B-B in figure 14 A) and in the long direction (A- A in figure 14 A).

Flow channels of extra depth dimensions in each of the two directions (lengthwise and across) may improve the overall stiffness of the absorber panel 20. With "extra depth" means that the depth is not optimized for heat transfer nor for differential pressure, but for contributing to overall mechanical stiffness of the absorber panel 20. The overall mechanical stiffness may be dependent on the placement and length of the deep flow channels 358. The deep flow channel(s) 358 may be placed along the length of the absorber panel 20 (flow channels in the long direction 354) and/or across the absorber panel 20 (flow channels in the short direction 356). In this embodiment at every position along the lengthwise direction, there are two or more deep flow channels 358,354 running in the lengthwise direction and at any position along the crosswise direction there are two or more deep flow channels 358,356 running across the absorber.

Figure 17 illustrates one embodiment of a thermal absorber panel 20 integrated with an insulation structure 386. Aspects of the thermal absorber panel 20 and the directions described in figure 14A may pertain to the details disclosed in this embodiment. In figure 17A the embodiment is seen from a top view and in figure 17B the embodiment is seen from a side view. The thermal absorber panel 20 is integrated with an insulation structure 386 on one surface of the thermal absorber panel 20. The insulation structure 386 comprises an insulation material 380 a back plate adhesive 384, and a back plate 382 in a sandwich structure. The back plate adhesive joints the back plate 382 to the insulation material 380. In the case where the thermal absorber panel 20 is used as a thermal solar absorber the insulation structure is always mounted at the back side of the absorber panel 20 (The surface opposite to the surface comprising the solar selective coating and facing the sun). The insulation structure 386 may be mounted onto the absorber by use of high temperature glue/adhesive. A sandwich structure of: absorber panel 20 - adhesive - insulation material 380 - adhesive 384 - back plate 382 may have extremely high stiffness due to the relative large thickness dimensions of the insulation structure 386 which may be in the range of 50mm - 150mm. However, the thickness dimensions of the insulation structure 386 is not limited to this range and may depend on which material is used as the insulation material 380 and the material of the back plate 382. The mechanical and insulating properties of the materials must be balanced to achieve the overall stiffness and insulation required for a specific application.

Claims

1. A method (400) for making a pressure-formed thermal absorber means (10) configured with a surface coating (130) comprising acts of:

providing (410) at least two joinable sheets (30);

- arranging (420) at least two joinable sheets (30) substantially flat on top of each other with a bottom outer sheet (32) and a top outer sheet (34) to form a thermal absorber panel (20) configured with a first surface (22) and a second surface (24);

- jointing (430) at least two sheets (30) by high pressure joints (50) in a closed loop (52) encircling provided inlet(s) (60) and outlet(s) (70);

coating (470) at least one of the first surface (22) or the second surface (24) of the thermal absorber panel (20) which surfaces are pre-polished (160); and applying (440) high pressure fluid (48) to inlet(s) (60) and/or outlet(s) (70), thereby forming at least one flow channel (40) connecting an inlet (60) to an outlet (70) and characterized in that the act of coating is performed by a vacuum deposition process (180).

2. A method (400) for making a pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claim 1 wherein the method act of jointing (430) by high pressure joints (50) comprises at least one further pattern (54) forming one or more flow channels (40) from inlet(s) (60) to outlet(s) (70) which at least one further pattern (54) is comprised within the closed loop joint (52).

3. A method (400) for making a pressure-formed thermal absorber means (10) config- ured with a surface coating (130) according to claim 1 or 2 wherein at least one of the joinable sheets (30) comprises at least one imprint (26) prior to jointing (430).

4. A method (400) for making a pressure-formed thermal absorber means (10) configured with a surface coating (130) according to any of the preceding claims 1 or 3 comprising a further act of placing (480) the jointed sheets (38) in a press (300) applying a surface pressure (310) on the jointed sheets (38) in an area along the joints (50) which joints the sheets (30) and across the joints (50) which joints the sheets (30) in a width of up to eleven times the width of the joints (50), preferable in a width of up to nine times the width of the joints (50), or more preferably in a width of up to five times the width of the joints (50).

5. A method (400) for making a pressure-formed thermal absorber means (10) config- ured with a surface coating (130) according to any of the preceding claims 1 or 3 comprising a further act of placing (480) the jointed sheets (38) in a press (300) applying a surface pressure (310) on the jointed sheets (38) in an area along the joints (50) which joints the sheets (30) and across the joints (50) which joints the sheets (30) in a width of up two times a heat affected zone (58).

6. A method (400) for making a pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claim 4 or 5 wherein the press comprises a base tooling plate (328) and two dies (320), wherein the two dies are configured as:

-a pressure-active die (322) having a pattern according to the area of the applied surface pressure according to claims 3 or 4, and

- a pressure-passive die (324) which defines the maximum height of the channels (40).

7. A method (400) for making a pressure-formed thermal absorber means (10) config- ured with a surface coating (130) according to any of the preceding claims wherein

- the provided inlet(s) (60) and outlet(s) (70) are provided in one sheet (30) and arranged (420) with an inlet flange (62) to the individual inlet(s) (60) and an outlet flange (72) to the individual outlet(s) (70); and

- the method act of jointing (430) by high pressure joints (50) comprises jointing the inlet flange(s) (62) and outlet flange(s) (72) to the sheet comprising the inlet(s) (60) or outlet(s) (70) and wherein the jointing (430) is performed from one side of the thermal panel (20) in one production step.

8. A pressure-formed thermal absorber means (10) configured with a surface coating (130) obtained by a process of method acts according to claims 1 to 7 characterized in that at least one flow channel (40) is a pressure expanded flow channel (42) and that the surface coating (130) is a vacuum deposited coating (132).

9. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claim 8 wherein the surface coating (130) is a pre-jointing applied surface coating (130).

10. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claim 8 wherein the surface coating (130) is a post-jointing applied surface coating (130).

11. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to any of claims 8 - 10 wherein the height (352) of at least one section of a flow channel (40) is different from the height of at least one adjacent section of a flow channel (40).

12. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to any of claims 8 - 11 wherein at least one section of a flow channel

(40) comprises a stiffness enhancing profile (348).

13. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claims 1-12 characterized in that the surface coating (130) com- prises an infrared reflective coating (134).

14. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claims 1-13 characterized in that the surface coating (130) comprises an anti-fouling coating (136).

15. A pressure-formed thermal absorber means (10) configured with a surface coating (130) according to claims 1-14 comprising an insulation structure (386) mounted on one of the first surface (22) or the second surface (24) of the thermal absorber panel (20).

16. A method (400) for making a pressure-formed thermal solar absorber (90) comprising a pressure-formed thermal absorber means (10) according to any of the preceding claims wherein the surface coating (130) comprises a solar selective coating (100) and the act of coating (470) comprises the following acts of depositing a solar selective coating (100) onto the thermal absorber panel (20):

depositing (450) an adhesion layer (102) on at least one pre-polished surface

(22,24);

- depositing (450) at least one absorber layer (104) one layer at a time; and

depositing (450) at least one anti-reflection layer (108) one layer at a time, in a sandwich construction (118) configured with the adhesion layer (102) deposited onto the surface(s) (22,24), the absorber layer(s) (104) deposited onto the adhesion layer (102) and the anti-reflection layer(s) (108) deposited onto the absorber layer(s) (104).

17. A pressure-formed thermal solar absorber (90) obtained by a process of method acts according to claim 16 wherein the surface coating (130) comprises a solar selective coating configured with an adhesion layer (102) deposited onto the surface(s) (22,24), absorber layer(s) (104) deposited onto the adhesion layer (102) and anti- reflection layer(s) (108) deposited onto the absorber layer(s) (104).

18. A pressure-formed thermal solar absorber (90) according to claim 17 characterized in that the adhesion layer (102) comprises a metallic layer comprising a refracto- ry metal (120) and dope-material (122), which dope-material (122) comprises a metal or metalloid and which metallic layer is configured with an amorphous disordered structure (124).

19. A pressure-formed thermal solar absorber (90) according to any of claims 17 or 18 characterized in that the adhesion layer (102) comprises a metallic layer comprising molybdenum and titanium:

- with layer thickness (110) in the range 30 nm to 500 nm, preferably in the range 80- 200 nm, even more preferably in the range 110-130 nm;

- comprising 85-99% (w/w) Mo and 1-15% (w/w) Ti, preferably in the range 90-97%) (w/w) Mo and 3-10% (w/w) Ti, even more preferably in the range 95-96%) (w/w) Mo and 4-5% (w/w) Ti.

20. A pressure-formed thermal solar absorber (90) according to any of claims 17 or 19 wherein - a surface coating (130) is deposited on the first surface (22), which surface coating (130) comprises a solar selective coating (100), and

- a surface coating (130) is deposited on the second surface (22), which surface coating (130) comprises an infrared selective coating (134).

21. A method (400) for making a pressure-formed thermal solar absorber (90) according to any of claim 17 - 20 wherein the adhesion layer (102) is deposited (450) onto the surface (22,24) of the thermal absorber panel (20) comprising the acts of:

providing(410) a base pressure (170) of < 1E-4 mbar;

- providing (410) a surface temperature (174) of the thermal absorber panel above 50°C, preferably above 100°C, even more preferably above 150°C; providing (410) a process pressure (172) of < lE-1 mbar by providing (410) a protective atmosphere to the process chamber of instrument grade argon gas prior to deposition (450) of the adhesion layer (102) by a vacuum deposition process (180); and

performing (460) the vacuum deposition process (180).