The present invention relates to a system for concentrating sunlight, an optical element for concentrating solar irradiation, and method and apparatus for manufacturing thereof

BACKGROUND OF THE INVENTION

In many optical applications, concentrators or lenses are used to focus light. Many types of lenses have been demonstrated, classical refractive lenses, concave reflector lenses, flat lenses etc. In the areas of Concentrated Solar Power (CSP) or Concentrated Photo Voltaics (CPV) in particular, the concentrators have failed to provide all the desired aspects, namely simultaneous precision, low weight, durability, large deflection angles, low energy loss and especially low cost.

What we disclose here is a design, implementation and manufacturing method of such an optical element fulfilling all the desired aspects mentioned above.

OBJECT OF THE INVENTION

It may be seen as an object of the invention to provide an improved method for making solar concentrators by industrial polymer roll-to-roll manufacturing processes.

It may be seen as a further object of the invention to reduce losses in Fresnel based solar concentrators at large deflection angles.

It may be seen as a further object of the invention to provide a planar solar concentrator capable of deflecting normal incident sunlight at large angles, thus allowing the solar plant to be constructed closer to the ground.

It may be seen as a further object of the invention to reduce the weight of solar concentrators.

It may be seen as a further object of the invention to simplify the construction of the solar field and reduce the number of solar receivers in a solar plant.

It may be seen as a further object of the invention to simplify the construction of the solar field by allowing for a planar field. It may be seen as a further object of the invention to enhance durability, simplify maintenance and reduce barriers towards replacement of the solar concentrators.

It may be seen as an object of the invention to provide an improved method for producing large areas of solar concentrator foils at either a throughput rate larger than today's state-of-the-art, at a substantially lower cost than the cost associated with today's state-of-the-art processes.

It is a further object of the invention to provide an alternative to the prior art. DESCRIPTION OF THE INVENTION

In CSP or CPV applications, incident solar irradiation is focused by the means of a reflector onto a solar receiver, capable of transferring heat to a power generation system, typically using a thermal Rankine process or by other means. In accordance with Carnot's law, the power generation process does in general become more effective if the solar receiver can be heated to higher temperatures. Typically parabolic mirror are used to reflect sunlight into the focal point of the parabola, in which the solar receiver is placed. However, due to the parabolic shape, there is a practical and economical limit of the width of the parabolic mirrors, which today is about 6-7 meters. Alternatively, reflectors using linear Fresnel lenses are used. Fresnel lenses have been known for a long time in the literature. Fresnel lenses consist of multiple discrete segments located in the same plane, but being tilted out-of-plane. There are two main types of Fresnel lens: imaging and non-imaging. Imaging Fresnel lenses use curved segments and produce sharp images, while non-imaging lenses use flat segments, and do not produce sharp images. For a thorough description of Fresnel lenses, see e.g. [1].

Fresnel lenses have been used in CSP applications, where large (on the order of centimeters to lO's of centimeters) free-space reflectors are tilted relative to a focal point with the purpose of maximizing the amount of collected solar irradiation. The reflectors typically consist of glass mirrors, where metal is evaporated onto one side of a glass plate. This approach typically results in relatively heavy constructions, with a weight on the order of 11 kg of steel and 18 kg of glass per square meter of reflector [2]. The main challenge towards employing classical linear Fresnel reflectors is that the losses approach 100% as the deflection angle approaches 90 degrees, which is the case for reflectors positioned farthest from the point of focus. This increasing loss limits the width from which the solar irradiation may be collected, ultimately limiting the power and achievable temperature of the solar receiver. Furthermore, as the Fresnel lenses are typically placed horizontally, there is a large shadow effect when the sun angle is low. If one could reduce the losses and the shadow effect of Fresnel elements, this would be desirable, and allow for higher temperatures in the power generation cycle, and thus lead to higher conversion efficiency, and hence cheaper electrical power. If one could further reduce the cost, weight and material usage of the reflectors, this would be economically and environmentally desirable.

We have demonstrated that a micro-structured and transparent polymer foil, that comprises a uniform linear Fresnel grating on a planar surface, comprising microstructures with a height between 5 and 200 prn on the one side and an anti- reflective nanostructure or coating on the other side, the microstructures being coated with a thin layer of aluminum or silver, may be used as a reflector concentrating the sunlight in a more efficient way than conventional Fresnel elements.

The invention and its use is sketched in figure 1. The optical element consists of a polymer foil ( 1) comprising linear Fresnel gratings on the back side, which is coated by a thin layer of metal (2) and an optional anti-reflective structure on the front side (3) and an optional back side protection and/or adhesive for easier mounting of the elements (4). The incident solar irradiation (5) is deflected as a result of three optical processes. First, a normal incident transmission from air to the transparent foil takes place without altering the angle of the light, the reflection preferably being minimized by the anti-reflection coating or structure, secondly, the Fresnel structures reflect the light at an angle of twice the tilt angle of the element, and thirdly, a refraction from the higher refractive index foil to the lower refractive index air further increases the angle of the outgoing light (6), the internal reflection preferably being minimized by the above mentioned anti reflective coating or structure. Due to this third refraction, the tilt angle of the Fresnel elements can be kept at a low level, on the order of 15-21 degrees, while still deflecting the outgoing light to angles within 10 degrees to parallel to the element, where a conventional free-space Fresnel reflector element would need a tilt angle of 40-45 degrees. However, lower deflection angles may also be used, and the demonstrated optical element can have Fresnel element tilt angle of e.g. 18 degrees to reflect the light to approximately 30 degrees relative to the foil. By arranging the foil in e.g. a concentric planar pattern, all elements will deflect light through a line in the center, perpendicular to the plane. In this case the length of the receiver would need to be half of the radius of the concentric pattern. Using the disclosed invention, fewer solar receivers may be required, the reflectors will be orders of magnitude cheaper to manufacture, and hence a better total economy of the installation will be possible.

The theoretical efficiency of a free-space Fresnel reflector and the disclosed optical element as function of the deflection angle relative to the incident light is shown in figure 2, the disclosed invention being described by the black line and a free- space Fresnel reflector being described by the gray line. The x-axis describes the deflection angle where 0 is direct reflection and 90 is deflection along the foil plane, and the y-axis describes the portion of the incident light being deflected and hence not being lost due the shadow effect of neighboring gratings.

Another key element in the proposed element is the anti reflection layer, either consisting of a coating or a structured polymer layer. A fraction of the normal incident light will be reflected, without the layer it will be on the order of 4-5%, and with the coating this loss can be reduced to below 1%. When the light is to cross the polymer-air boundary where the above mentioned refraction takes place, the losses will in general be much higher, and can, in the case of no anti reflective layer, be calculated using Fresnel equation's. Depending on the polarization of the light and especially the refracted angle, losses will approach 100% when the refracted angle approaches 90 degrees to normal incidence. By incorporating the above mentioned anti reflective layer, our modeling shows that the losses may be reduced significantly, thereby either increasing the refracted angle (at constant loss) or decreasing the loss (at constant refracted angle), or a combination thereof.

Due to the small thickness of the disclosed optical element, and especially of the incorporated Fresnel structures, it may be manufactured using standard roll-to-roll processes such as extrusion coating, where a carrier foil is laminated to a film melt which is being structured or coated using a structured cooling roller. The structured cooling roller may be manufactured using nickel sleeve technology, or by imprinting a pattern in an imprint layer on the surface of a conventional roller. The use of roll-to-roll processes, when compared to conventional metal coated glass mirrors, will have the potential to reduce the cost per square meter from the range of 200$ [2] to the range of 1-2$ per square meter, similar to the cost of traditional packaging foils.

The inventive step of the disclosed optical element is the combination of the micro Fresnel elements with a subsequent dielectric-air refraction, greatly improving the efficiency of the element at large deflection angles, from a few percent to around 80%, combined with a layout of the foil on a planar or slightly curved structure to ensure a common focal line or focal point. The inventive step is further realized by adding a structural anti-reflective layer, which allows refraction to take place without a high degree of internal reflection, which would otherwise significantly lower the efficiency of the proposed optical element. The efficiency of structural, refractive index gradient anti-reflective layers is almost independent of

wavelength and incident angle compared to multilayer, dielectric anti-reflective films, and is therefore especially well-suited for this application, where broadband visible sunlight is to be transmitted over the interface at a near-critical angle. Furthermore, the concept of having a planar structure, deflecting normal incident solar irradiation in a direction close to being parallel to the planar structure will significantly simplify construction and lower cost of solar plants, allowing for a more scalable way of concentrating solar irradiation. The height and weight of today's state-of-the-art solution, parabolic mirrors, scales with the width squared, thereby limiting the area which may be concentrated onto one solar receiver. The weight of the disclosed invention scales with the width, and the height is constant, thereby allowing for (in principle) arbitrarily large solar fields using only one solar receiver.

Furthermore, contributing to the inventiveness and especially the industrial applicability, all the processes for manufacturing the disclosed element may be performed in roll-to-roll setups, thereby significantly lowering the cost of the proposed element. Even further, the low cost of the roll-to-roll manufactured optical element makes it economically feasible to replace the foil when it is degraded by the environment, whereas parabolic trough mirrors made of glass will be too expensive to replace and therefore suffer continuous reduction of efficiency as the mirror surface is degraded by abrasion from dust and other factors. A further advantage of the disclosed optical element lies in the inherent protection of the mirror structure beneath the dielectric layer, thus avoiding further protective layers apart from an optional back coating.

SUBSTITUTE SHEET (RULE 26J Furthermore, the inventive step permits the desirable transition from parabolic trough mirrors to flat reflectors, reducing the complexity of manufacturing by a large amount and therefore enabling on-site assembly of reflector structures.

The invention furthermore relates to an optical element, comprising at least the following parts:

- a polymer foil,

- said polymer foil having a lower interface comprising a linear Fresnel lens microstructure

- said linear Fresnel lens microstructure being coated with a reflective metal layer - said optical element being used for deflecting solar irradiation by allowing the sunlight to first pass through said polymer foil, then being reflected by said metalized Fresnel lens microstructure and then being refracted when passing out of the said foil into the air

The invention furthermore relates to a system for collecting solar energy comprising at least the following parts:

- an optical element, according as described above

- arranging a multitude of said optical elements in a pattern providing a volume where the solar intensity is at least 10 suns

- collecting the energy in said volume by the means of a receiver converting the solar irradiation into heat or electricity or a combination thereof.

The invention furthermore relates to an optical element where the Fresnel microstructure has a height between 5 and 300 μιτι, and a constant slope between 15 and 21 degrees relative to the foil plane.

The invention furthermore relates to an optical element where the sun-facing side of the said polymer foil further comprises an anti reflection layer reducing the reflection at normal incidence by more than 50%, more preferably more than 75% and most preferably by more than 90%.

The invention furthermore relates to an optical element where the said anti reflection layer consists of an index gradient anti reflective layer.

SUBSTITUTE SHEET (RULE 28) The invention furthermore relates to an optical element where the total thickness of the said optical element is less than 200 μιτι, more preferably less than 100 μιτι and most preferably less than 75 pm.

The invention furthermore relates to an optical element where the said Fresnel microstructure in said optical element is manufactured by a roll-to-roll extrusion coating process with the said reflective metal layer being formed by a subsequent roll-to-roll vacuum metallization step and the anti reflective layer is formed by a further subsequent roll-to-roll extrusion coating process with a throughput of at least 5 m2/min.

The invention furthermore relates to a system where the said multitude of optical elements comprise a total area of more than 10 square meter.

The invention furthermore relates to a system where the said multitude of optical elements are arranged on a planar structure which is actuated to be constantly perpendicular to the sun, thereby deflecting the sunlight into a common focal line being perpendicular to the planar structure.

The invention furthermore relates to a system where the said multitude of optical elements are arranged in a concentric manner on a slightly curved structure which is actuated to have at least a part of the curved surface to be constantly perpendicular to the sun, thereby deflecting the sunlight into a common focal point being perpendicular to the planar structure.

By polymer foil is meant a flexible sheet of polymer materials, consisting of one or more layers of polymeric materials, The thickness of polymer foils is typically in the range of 20 to 200 pm, but thinner or thicker foils may be found.

By extrusion coating is meant the process of coating a foil in a continuous roll-to- roll process, as described in the literature, see e.g. Gregory, B. H., "Extrusion Coating", Trafford, 2007, ISBN 978-1-4120-4072-3.

By anti reflective layer is meant a layer of material at an interface that decreases the reflection coefficient of light travelling across the said interface. Multilayer films is one method to make anti reflective coatings, and structural gradient refractive index surfaces is another. The latter method takes advantage of nanostructures at the surface defined in the solid side of the interface, in this context would be the polymer foil. The nanostructures have a different filling ratio as function of elevation from the surface, e.g. pyramidal structures, and are smaller than the wavelength of light. The nanostructures for refractive index grating may be produced via extrusion coating or hot-embossing using a master structure. A typical embodiment is to make 300 nm-1000 nm high structures, with a pitch of e.g. 200 nm, where the structures are wider at the bottom. This method can reduce normal incident reflection by as much as 20 times.

By Fresnel structures is meant locally planar micro structures inclined at an angle relative to the macroscopic surface plane. There are two main types of Fresnel lens: imaging and non-imaging. Imaging Fresnel lenses use curved segments and produce sharp images, while non-imaging lenses use flat segments, and do not produce sharp images. See e.g. [1] for a thorough review and explanation about non-imaging Fresnel lenses.

By linear Fresnel lens microstructure is meant a Fresnel structure being linear or almost linear, thereby not focusing the light in the direction parallel to the microstructure. All structures that are fabricated will deviate from the desired design, hence for these purposes we will define a linear Fresnel lens

microstructure as a microstructure where the macroscopic layout radius is larger than 1 meter.

By index gradient anti reflective layer is meant a structured material where the effective refractive index changes spatially continuous from the unstructured bulk material to the medium, typically air, which it is in contact with. To obtain the anti reflective properties of such a layer, the structures in the material needs to be smaller than the wavelength of light, and typical dimensions in the plane of the surface is ranging from 50 to 300 nm, with heights ranging from 300 nm to 1 μιτι.

By slightly curved means a surface with a deviation from a planar surface of maximum 10% of the width of the surface. By slightly curving the surface, the optical element will change its deflection angle, and hence the light can be deflected to a common point instead of a common line, thus generating a higher solar flux.

All of the features described may be used in combination in so far as they are not incompatible therewith.

SUBSTITUTE SHEET (RULE 26} BRIEF DESCRIPTION OF THE FIGURES

The method and apparatus according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 shows the optical element. The optical element consists of a polymer foil (1) comprising linear Fresnel gratings on the back side, which is coated by a thin layer of metal (2) and an optional anti-reflective structure on the front side (3) and an optional back side protection and/or adhesive for easier mounting of the elements (4). The incident solar irradiation (5) is deflected as a result of three optical processes. First, a normal incident transmission from air to the transparent foil takes place without altering the angle of the light, the reflection preferably being minimized by the anti-reflection coating or structure, secondly, the Fresnel structures reflect the light at an angle of twice the tilt angle of the element, and thirdly, a refraction from the higher refractive index foil to the lower refractive index air further increases the angle of the outgoing light (6), the internal reflection preferably being minimized by the above mentioned anti reflective coating or structure.

Figure 2 shows the concentric layout of long strips of foil (1), thereby focusing light on a central line (2) being perpendicular to the plane.

Figure 3 shows the theoretical efficiency (y-axis) of the proposed element (black line) relative to a conventional free-space Fresnel reflector (gray line) as function of the deflection angle (x-axis).

Figure 4 shows a side view of a second embodiment of the arrangement of the optical element. By slightly curving the plane on which the optical elements are mounted, the light will be deflected into a much smaller point-like volume. The volume will be defined by the divergence angles of the deflected light, which in the in-plane direction will be around 0.55 degrees (the size of the solar disc) and in the off-plane direction around 3 degrees (amplification of the solar disc size due to the refraction). Using a slightly curved plane, the solar intensity may be increased from 50-100 suns in the planar arrangement to 500-1000 suns in the slightly curved arrangement. DETAILED DESCRIPTION OF AN EMBODIMENT

In one embodiment a sheet of aluminum is diamond ruled to form a Fresnel grating with uniform angle of 18 degrees and a height of 40 pm. The sheet is bend around a roller and fixed using screws. The roller is then used in an extrusion coating process, where a 30 pm thermoplastic cyclic olefin copolymer (COC) foil is coated with a 50 pm layer of thermoplastic COC which is structured during contact with the structured cooling roller, yielding a first structured foil . The first structured foil is placed in a roll-to-roll vacuum metallization plant, where 20 nm of aluminum is evaporated onto the structured side of the first structured foil . The coated foil is then further extrusion coated with a 40 pm layer of COC in order to protect the aluminum coating from oxidation by the ambient air. This foil is then further added an anti reflective layer on the upper surface using an UV- curable lacquer, being applied in a roll-to-roll process, using a shaping roller mounted with a nickel master sleeve comprising the inverse of the anti-reflective structure. The final foil is then cut out and applied in a concentric pattern onto large, tiltable and planar plates, capable of tracking the sun, thereby being able to focus the incident sunlight onto a solar receiver which heats a medium driving a steam engine, driving a generator, thus producing electricity.

In another embodiment, a nickel master structure sleeve comprising Fresnel structures with a tilt angle of 15 degrees and a height of 10 pm is mounted on a roller, and used in a roll-to-roll hot embossing process, where a 30 pm cyclic olefin copolymer (COC) foil is embossed, yielding a first structured foil . The first structured foil is placed in a roll-to-roll vacuum metallization plant, where 100 nm of silver is evaporated onto the structured side of the first structured foil. The coated foil is then further extrusion coated with a 15 pm layer of COC in order to protect the silver coating from oxidation by ambient air. This foil is then further added an anti reflective layer on the upper surface using an extrusion coating process using a nickel master comprising a gradient refractive index structure. The final foil is then cut out and placed in a concentric pattern and applied onto large, tiltable and planar plates, capable of tracking the sun, thereby being able to focus the incident sunlight onto a concentrated photovoltaic cell with water cooling, thus generating electricity and hot water.

In another embodiment a sheet of brass is diamond ruled to form a Fresnel grating with uniform angle of 18 degrees and a height of 40 pm . The sheet is bend around a roller and fixed using screws. The roller is then used in an extrusion coating process, where a 30 pm PET foil is coated with a 50 pm layer of thermoplastic PP which is structured during contact with the structured cooling roller, yielding a first structured foil. The first structured foil is placed in a roll-to- roll vacuum metallization plant, where 100 nm of silver is evaporated onto the structured side of the first structured foil. The coated foil is then further extrusion coated with a 40 pm layer of PP in order to protect the silver coating from oxidation by the ambient air. This foil is then further added an anti reflective layer on the upper surface using an layer of PP, being applied in a roll-to-roll extrusion process, using a shaping roller mounted with a nickel master sleeve comprising the inverse of the anti-reflective structure. The final foil is then cut out and applied in a concentric pattern onto large, tiltable plates with a width of 10 meters with a curvature of 5%, capable of tracking the sun, thereby being able to focus the incident sunlight onto a solar receiver being 40*40 cm which heats a salt melt which is passed through a heat exchanger where the heat is used to generate steam driving a steam engine, driving a generator, thus producing electricity.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

All patent and non-patent references cited in the present application are also hereby incorporated by reference in their entirety. REFERENCES

[ 1]. Nonimaging Fresnel Lenses: Design and Performance of Solar Concentrators (Springer Series in Optical Sciences, By Ralf Leutz and A. Suzuki ISBN-13: 978- 3540418412

[2] Fresnel CSP: a startling technology for solar generation, Power Engineering International, 01/05/2009,

http://www.powerengineeringint.com/articles/print/volume- 17/issue- 5/features/Fresnel-csp-a-startling-technology-for-solar-gene ration.html

[3] "NaPaNIL - Library of Processes", H. Schift (Ed.)