EP2268981A2

EP2268981A2 - Solar energy reflector

Info

Publication number: EP2268981A2
Application number: EP09753750A
Authority: EP
Inventors: Lionel Ventelon; Olivier Bouesnard
Original assignee: AGC Glass Europe SA
Current assignee: AGC Glass Europe SA
Priority date: 2008-04-18
Filing date: 2009-04-17
Publication date: 2011-01-05
Also published as: AU2009253324B2; CN102007347A; AU2009253324A1; IL208736A0; WO2009144095A3; WO2009144095A2; US20110220098A1; CN102007347B

Abstract

Solar energy reflectors (1) according to the present invention comprise a mirror (5) laminated to a support (6) by means of a bonding material (9) comprising a foam tape.

Description

Solar energy reflector

This invention relates to solar energy reflectors and to processes for their manufacture.

The reflectors of this invention may be used in solar energy or heating installations, for example concentrating solar power plants. Such installations use the solar energy to first generate heat, which later can be converted into electricity or used for steam production. Concentrating solar power plants wherein reflectors according to the present invention may be used comprise, for example, parabolic trough power plants, central tower power plants (also called heliostat power plants), dish collectors and Fresnel reflector power plants. Solar energy reflectors according to the present invention may be used in such installations as flat or curved mirrors.

Solar energy reflectors may be produced by forming a laminate comprising a thin mirror bonded to a supporting sheet having substantially the same surface dimensions as the mirror. Maximum reflectivity for the mirror may be obtained if it is thin, so that less solar energy is absorbed when passing through the substrate of the mirror. However thin mirrors may be poor in terms of mechanical resistance, therefore it may be necessary to laminate them on a supporting substrate, for example a metallic sheet. The mirror and the supporting substrate then form a single laminated structure. Alternatively, solar energy reflectors may be produced by using a mirror adapted to offer enough mechanical resistance for the mirror to stand without a supporting sheet, e.g. a mirror having a greater thickness. Supporting means may be attached to the mirror for fixation in the solar energy installation and /or may help in maintaining the curved shape of the mirror.

It is known to laminate mirrors to their support to provide a laminated assembly, by means of epoxy resins, silicone-based adhesives, polyurethane adhesives, hot-melt adhesives, acrylic resin based adhesive or a polyvinylchloride bonding layer. It is generally found advantageous for the bonding material to comprise an acrylic resin. Epoxy, silicone and polyurethane resins, in certain circumstances, may release chemical species which may attack a paint layer of the mirror and finally corrode the silver layer; epoxy resins may create tensile stress between the paint and silver layers of the mirror when cross-linking takes place, which may cause detachments to occur in the mirror structure; hot-melt adhesives may lose at least part of their elasticity when exposed to higher temperatures, which may cause the mirror to detach from the supporting sheet; some silicone based materials (e.g. structural silicone) may be too rigid when the metallic sheet dilates when subjected to differences in operating temperatures, which may cause the paint and silver layers of the mirror to be pulled out; silicone may create planarity defects during lamination; polyurethane resins may not be sufficiently resistant to UV rays. Acrylic resins may show advantageous properties in terms of chemical neutrality, resistance to UV rays, and flexibility and elasticity. The bonding material may alternatively comprise EVA (ethylvinylacetate) or other acetate-based polymer film, PVB (polyvinylbutyral), TPU (thermoplastic urethane) or ionomer-based films. Such materials may show advantageous properties in terms of chemical neutrality, resistance to UV rays, and flexibility and elasticity. Other known bonding materials may comprise an acrylic pressure-sensitive adhesive. This may be provided as a transfer tape wherein the adhesive is present between two supporting release sheets which are intended to be removed. Alternatively, it may be provided as a supported tape wherein the adhesive is provided on both side of a supporting sheet which is intended to stay. Such a supported tape may, for example, comprise a core which consists essentially of a PET film or a polyester foil, surrounded by layers consisting essentially of the acrylic pressure-sensitive resin.

According to one of its aspects, the present invention provides a solar energy reflector as defined by claim 1. Other claims define preferred and/or alternative aspects of the invention.

Solar energy reflectors according to the invention comprise a mirror laminated to a support by means of a bonding material comprising a foam tape, so that the mirror and support form a laminated assembly, i.e. a single laminated structure constructed in such a way that the bonding material units together the mirror and the support. Preferably the bonding material consists essentially of the foam tape. This may provide advantageous properties including one or any combination of:

. adhesion directly after application may be greater than with an identical adhesive not supported by a foam carrier;

. thickness and flexibility of the foam tape may provide an effective bonding surface which is greater than for example a typical thin and more rigid supported tape of PET with acrylic resin. Indeed, a thin and more rigid bonding material, contrary to foam, may not adapt to irregularities of planarity in the mirror and/or the support and consequently may not adhere correctly over its whole surface, creating air bubbles trapped between the mirror and/or the support and the bonding material. These defects are known as "popping"; they may create optical defects in the reflector. The present invention may reduce or avoid the risk of popping;

. elasticity of the foam tape may allow for slight relative parallel displacement of the bonded faces of the mirror and support, for example in consequence of flexing of the laminate or differential thermal expansion of the mirror and the support; . adequate bond strength and high degree of water or moisture resistance of the foam tape may provide good corrosion resistance to the reflector; . good resistance to UV rays.

The foam tape may advantageously comprise, or preferably consist essentially of, a foam carrier coated on its two main surfaces with an adhesive, i.e. a foam core surrounded by layers of adhesive material.

Preferably, the foam carrier comprises, or more preferably consists essentially of, at least one material selected from the group consisting of polyethylene, urethane, vinyl, neoprene, EPDM and polyester. Polyethylene may be preferred for its low cost. We have found that, more than the adhesive coated on the foam carrier, it seems to be the mechanical resistance of the foam itself which determines the resistance of the reflector against delamination. Cohesion of the reflector may be controlled by selecting appropriate foam carriers. Advantageously, the foam carrier may consist essentially of polyethylene and may have a tensile strength according to DIN 53455 (measured on a 15 mm broad sample) of at least 5, preferably at least 8 or more preferably at least 10 N/mm²; its tensile strength may be less than or equal to 50, preferably less than or equal to 40, more preferably less than or equal to 30, or still more preferably less than or equal to 25 N/mm². It may be advantageous for the polyethylene foam carrier to have an elongation according to DIN 53455 of at least 80, preferably at least 100, more preferably at least 200, or still more preferably at least 240 %; elongation may be less than or equal to 500, preferably less than or equal to 450 or more preferably less than or equal to 400 %. The foam carrier may have a thickness of at least 0.2, preferably at least 0.4 mm; thickness may be less than or equal to 3.5 mm, preferably less than or equal to 2 mm.

Advantageously, the foam carrier consists essentially of a closed cell foam. This may help provide good water, water vapour and moisture sealing and consequently help provide good durability to the reflector.

Preferably, the adhesive coating the foam carrier comprises acrylic- based or structural adhesives; it may comprise, or more preferably consist essentially of, at least one material selected from the group consisting of acrylic, silicone, polyurethane, epoxy, MS polymer and rubber. It may be preferred for the adhesive to consist essentially of acrylic. Acrylic resins may show advantageous properties in terms of chemical neutrality, resistance to UV rays, and flexibility and elasticity. They may further offer a rapid adhesive tack. In particular embodiments, the adhesive may be different on both sides of the foam carrier.

In preferred embodiments of the invention, the bonding material is present on substantially the whole surface of the support facing the mirror. This may provide good adhesion and resistance to delamination and/or may reduce the risk of water entering the space between the support and the mirror, which could increase the risk of corrosion of the mirror.

Prior art mirrors used in solar energy reflectors have generally been produced as conventional domestic mirrors used for interior applications, i.e. as follows: a sheet of flat glass (float, soda-lime glass) was first of all polished and then sensitised, typically using an aqueous solution of SnCl₂; after rinsing, the surface of the glass was usually activated by means of an ammoniacal silver nitrate treatment, and a silvering solution was then applied in order to form an opaque coating of silver; this silver coating was then covered with a protective layer of copper and then with one or more coats of leaded paint in order to produce the finished mirror. The combination of the protective copper layer and the leaded paint was deemed necessary to provide acceptable ageing characteristics and sufficient corrosion resistance.

More recently, mirrors were developed which dispensed with the need for the conventional copper layer, which could use substantially lead-free paints and yet which still had acceptable or even improved ageing characteristics and corrosion resistance. For example, US patent number 6,565,217 describes embodiments of a mirror with no copper layer which comprises in the order recited: a vitreous substrate; both tin and palladium provided at a surface of the vitreous substrate; a silver coating layer on said surface of the substrate; tin present at the surface of the silver coating layer which is adjacent to an at least one paint layer; and at least one paint layer covering the silver coating layer. Such mirrors provided a significant advance with respect to conventional coppered mirrors.

Solar energy reflectors according to the present invention preferably comprise a mirror which is free of a copper layer and comprises a glass substrate, a silver coating layer provided at a surface of the glass substrate and at least one paint layer covering the silver coating layer. Preferably, the mirror is laminated to the support so that the at least one paint layer is facing the support. The silver coating layer provides the reflective layer of the mirror (which reflects the sun rays that pass through the glass sheet). The at least one paint layer may provide a protection for the silver layer from possible chemical attacks by the bonding material, and a surface to which the bonding material can adhere.

Solar energy reflectors according to the invention may comprise an edge protection provided on the edges of the mirror. This may help protect the exposed edges of the silver layer against corrosion.

The support may consist essentially of at least one material selected from the group consisting of metallic materials, plastic materials, composite materials and glass. Preferably, it is made of steel, stainless steel, galvanised steel, painted steel, or aluminium. In preferred embodiments, the support is a sheet having substantially the same surface dimensions, i.e. same length and same width, as the mirror. This includes embodiments wherein the support may be slightly greater in size than the mirror, and the mirror may be bonded to the support such that projecting margins, of for example 5 mm, may extend beyond the periphery of the mirror.

Preferably, the thickness of the support, when it is a metallic sheet, may be greater than 0.5 mm or 0.6 mm and less than 1 mm or 0.9 mm; it may preferably be around 0.7 or 0.8 mm.

Advantageously, the thickness of the mirror, when it is laminated on its whole surface to the support, may be greater than 0.9 mm or 1 .1 mm; it may be less than 2 mm or 1.5 mm; it may preferably be around 0.95 or 1.25 mm. Such thin and flexible mirrors may be used in applications were curved reflectors are needed. Curved reflectors may also be manufactured with thicker mirrors which are not laminated to a support on their whole surface; in that case, the thickness of the mirrors may be greater than 2 mm or 2.5 mm; it may be less than 5 mm or 4.5 mm. When flat reflectors are used, the total thickness of the mirror may be greater than 2 mm or 2.5 mm; it may be less than 6 mm or 5 mm.

Preferably, the silver coating layer of the mirror has a thickness of at least 80 nm, at least 100 nm, more preferably at least 120 nm, or at least 140 nm; its thickness may be less than 200 nm, preferably less than 180 nm. The layer of silver may contain between 800 and 2000 mg/m² of silver, preferably between 1400 and 1800 mg/m² of silver. These values offer a good compromise between a good energetic reflectance value for the reflector and an acceptable cost of production. Preferably, the glass substrate of the mirror is made of extra-clear glass, i.e. a glass with a total iron content expressed as Fe₂θ₃ of less than 0.02% by weight. This also may favour a good energetic reflectance value for the reflector.

In one preferred embodiment of mirrors for solar energy reflectors according to the invention, the paint layer or at least one of the paint layers applied over the silver layer is lead -free or substantially lead -free. This is advantageous in that lead is toxic and its avoidance has environmental benefits. Substantially lead-free means herein that the proportion of lead in the paint is significantly less than the proportion of lead in leaded paints conventionally used for mirrors. The proportion of lead in a substantially lead-free paint layer as herein defined is less than 500 mg/m², preferably less than 400 mg/m², more preferably less than 300 mg/m². The proportion of lead in a lead-free paint layer as herein defined is less than 100 mg/m², preferably less than 80 mg/m², more preferably less than 60 mg/m².

The finished reflector may have an energetic reflectance according to standard ISO 9050:2003 of greater than 90%, preferably greater than 92%. The energetic reflectance may be less than 97% or less than 96%.

Embodiments of the invention will now be further described, by way of example only, with reference to figures 1 to 2 and to examples 1 to 5, along with comparative examples 1 to 4.

Figure 1 is a schematic cross-section of a solar energy reflector according to the invention. Figure 2 is a schematic view of a curved solar energy reflector according to the invention. Figure 3 is a schematic view of a dynamic shear test. Figures are not drawn to scale.

Figure 1 shows a solar energy reflector (1 ) which comprises a mirror (5) laminated to a support in the form of a sheet (6), for example of metal, by means of a bonding material (9) consisting essentially of a foam tape. The mirror comprises a glass substrate (2), a silver layer (3) and at least one paint layer (4). The foam tape (9) comprises a foam carrier (7) coated on its two main surfaces with an adhesive (8, 8').

Figure 2 shows a curved solar energy reflector (10) which comprises a mirror (5) laminated to a support in the form of two distinct profiles (60, 60'), for example of metal, by means of foam tapes (90, 90').

Figure 3 shows a sample comprising a mirror (5), a tape (99) and a steel support (6) subjected to a dynamic shear test. The arrows show directions of movement. Examples 1 to 4 and comparative examples 1 to 3 (not in accordance with the present invention) report adhesion tests measurements made on bonding materials adhered to mirrors of the type MNGE^®, i.e. mirrors with no copper layer commercialised by AGC Flat Glass Europe SA. Results are given in Tables I and II. AF means adhesive failure and CF means cohesive failure.

In example 1 , the bonding material is a foam tape having a polyethylene foam carrier of 0.8 mm thickness coated with a pure acrylic adhesive. Example 2 has the same bonding material as example 1 except that the foam carrier has a thickness of 1.6 mm. Example 3 has the same bonding material as example 1 except that the foam carrier has a thickness of 2 mm. Example 4 has the same bonding material as example 1 except that the foam carrier has a thickness of 3.2 mm.

In comparative example 1 , the bonding material is an acrylic transfer tape; the acrylic adhesive of this transfer tape is identical to the acrylic adhesive of examples 1 -4. In comparative example 2, the bonding material is an acrylic supported tape of 75 μm thickness comprising a PET support of 12 μm thickness; the acrylic adhesive of this supported tape is of a lower quality in terms of adhesion than the acrylic adhesive of examples 1 -4. In comparative example 3, the bonding material is an acrylic supported tape of 130 μm thickness, comprising a PET support of 12 μm thickness; the acrylic adhesive of this supported tape is identical to the acrylic adhesive of examples 1 -4.

The peel test measures the strength required to pull apart a bonded surface. Peel test measurements were made on mirrors at 180° after 20 minutes and 24 hours without load. Results of these tests (Table I) show best results for foam tapes and for an acrylic supported tape (comp. ex. 3).

The static shear test shows the ability of the bonding material to withstand a fixed load over time. For these tests, adhesives were placed on the mirror and left during 1 hour at 23°C for some samples and 80⁰C for others; then a weight of 1 kg was fixed to the sample. The adhesive surface was 12.5 x 12.5 mm. Best results (see Table I) are obtained for foam tapes, especially at room temperature. At higher temperature, thinner foam tapes seem to give better results. Comparative examples show that acrylic transfer or supported tapes give inferior results.

TABLE I

The dynamic shear test was realised as follows (see figure 3). A sample of mirror (2.5 x 10 x 0.4 cm) and another of painted galvanised steel (2.5 x 6.5 cm) were cleaned and dried. A piece of tape of 1 .5 x 2.5 cm was applied on the mirror and pressed with a roller of 1 kg. The steel sample was then applied on the tape and pressed with the roller. The assembly was then left to polymerise during 2 days at room temperature and humidity. Initial values were measured after these 2 days and other measurements were made after 1 week of ageing at room temperature. Shear forces were applied to the samples until failure, with a traction speed of 5 mm/min and at room temperature. Three or four measurements were taken for each example and comparative example and mean values and standard deviations of these results are given in Table II. This shows the advantage of the foam tapes over an acrylic supported tape and especially the reproducibility of the results for the foam tapes (lower standard deviations).

TABLE Il

In example 5, a mirror of 40 cm x 60 cm is laminated to a supporting sheet of steel having substantially the same surface dimensions as the mirror, with a foam tape having a polyethylene foam carrier coated with acrylic adhesive. The assembly is curved. It is placed in a static oven at a constant temperature of 80⁰C during 1 month. After this treatment, no haze is visible on the mirror and no peeling-off or detachment is observed. In comparative example 4, the same assembly and test are made except that the bonding material is an acrylic transfer tape. Haze is visible on the whole surface of the mirror, in particular on the edge portions thereof, and peeling-off of the mirror silver layer is observed on the edges.

Claims

1. Solar energy reflector comprising a mirror laminated to a support by means of a bonding material, wherein the bonding material comprises a foam tape.

2. Solar energy reflector according to claim 1 , wherein the foam tape comprises a foam carrier coated on its two main surfaces with an adhesive.

3. Solar energy reflector according to claim 2, wherein the foam carrier consists essentially of at least one material selected from the group consisting of polyethylene, urethane, vinyl, neoprene, EPDM and polyester.

4. Solar energy reflector according to claim 2 or 3, wherein the foam carrier consists essentially of a closed cell foam.

5. Solar energy reflector according to any of claims 2 to 4, wherein the adhesive comprises at least one material selected from the group consisting of acrylic, silicone, polyurethane, epoxy, MS polymer and rubber.

6. Solar energy reflector according to any of claims 2 to 5, wherein the foam carrier consists essentially of polyethylene and has a tensile strength according to DIN 53455 within the range 5 to 50 N/mm².

7. Solar energy reflector according to any of claims 2 to 5, wherein the foam carrier consists essentially of polyethylene and has an elongation according to DIN 53455 within the range 80 to 500 %.

8. Solar energy reflector according to any of claims 2 to 7, wherein the foam carrier has a thickness within the range 0.2 to 3.5 mm.

9. Solar energy reflector according to any preceding claim, wherein the bonding material is present on substantially the whole surface of the support facing the mirror.

10. Solar energy reflector according to any preceding claim, wherein the mirror is free of a copper layer and comprises a glass substrate, a silver coating layer provided at a surface of the glass substrate and at least one paint layer covering the silver coating layer.

11. Solar energy reflector according to any preceding claim, wherein the mirror is laminated to the support so that the at least one paint layer is facing the support.

12. Solar energy reflector according to any preceding claim, wherein the support consists essentially of at least one material selected from the group consisting of metallic materials, plastic materials, composite materials and glass.

13. Solar energy reflector according to any preceding claim, wherein the support is a sheet having substantially same surface dimensions as the mirror.

14. Use of a foam tape as bonding material for laminating a mirror to a support in a solar energy reflector.