METHOD FOR MANUFCTURING A BENDED GLASS SHEET , MANUFATURING SYSTEM FOR EXECUTING THE METHOD , BENDED MIRROR ASSEMBLY WITH THE BENDED MIRROR AND USE OF THE BENDED MIRROR OR THE ASSEMBLY WITH THE BENDED MIRROR

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a method for manufacturing a bended glass sheet and a manufacturing system for executing the method. Moreover a bended mirror and an assembly with the bended mirror are described.

2. Description of the Related Art

Solar energy is one of the most promising sources for renewable energy. Conventional thermal solar systems of a solar field are based on concentrating the light of the sun by concentrating mirrors to a collecting element that transfers the heat by a heat transfer fluid (HTF) to a heat exchanger. Water that passes through the heat exchanger becomes steam. The steam runs into a turbine. The turbine is coupled to a generator which produces electricity .

The concentrating mirrors are the key element of the solar field. The costs of the mirrors are 20% - 30% of the entire costs of a solar field.

One of the main costs of the mirrors has its origin from forming a glass sheet (glass plate) with a precise parabolic shape. The bending of the glass sheet is usually done in a hot furnace using a high temperature.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for bending a glass sheet, which is cheaper and more accurate compared to the state of the art. It is another object of the invention to provide a manufacturing system for the execution of the method.

It is a further object of the invention to provide a bended mirror. An additional object of the invention is to provide an assembly with the bended mirror. The bended mirror or the assembly with the bended mirror should applicable for the concentrating sunlight .

These objects are achieved by the inventions specified in the claims.

The present invention provides method for manufacturing a bended glass sheet with a permanent glass sheet bending, wherein the method comprises the following steps: a) Providing a tempered flat precursor glass sheet with a glass sheet thickness selected from the range between 2 mm and 10 mm; b) bending the tempered flat precursor glass sheet with the aid of an external bending force such, that a bended glass sheet with a glass sheet bending results; and c) fixing the glass sheet bending of the bended glass sheet such, that the permanent glass sheet bending results; wherein the bending the tempered flat precursor glass sheet is executed at a bending temperature which is selected from the range between 0°C and 70 °C. In a preferred embodiment the bending temperature is selected from the range between 0°C and 50 °C and particularly from the range between 0°C and 40°C.

The method is used for manufacturing a bended mirror with a permanent mirror bending. Thereby before, while and/or after the bending the tempered flat precursor sheet and/or before, while and/or after the fixing the sheet bending at least one mirror coating is attached to a glass surface such, that the permanent glass sheet bending results in the permanent mirror bending. For instance, the tempered glass sheet is coated with a multilayer stack with a reflecting layer containing Silver. Alternatively to Silver or in combination with Silver the reflecting layer contains Aluminium or other suitable materials. This coated glass sheet is brought into the desired parabolic shape by the described bending process. In view of a solar field application it is advantageous that instead of 2 or 3 mirrors one "long" mirror can be used. This reduces the costs and improves the optical performance of the concentrator of sunlight. Moreover a manufacturing system for executing the method with at least one fixing device for the fixing the glass bending is provided .

Additionally a bended mirror manufactured by the method is given as well as a mirror assembly comprising at least one such bended mirror and at least one mirror bending fixing device for fixing the mirror bending such that the permanent mirror bending results. The mirror can be a front surface mirror or a back surface mirror. The permanent glass sheet bending and the permanent mirror bending have a parabolic shape.

Finally a use of the bended mirror or the mirror assembly for concentrating sunlight is provided. By the bended mirror the sunlight is directed to a sunlight receiver (collecting element) . The glass sheet is tempered. This means, that the glass sheet is pretreated by an application of heat (preheated) and/or by an application of a chemical reagent. Additionally a fast cooling can be carried out. These treatments lead to glass sheet internal forces. Internal pressures between 50 MPa and 150 MPa result.

For instance tempered glass is made by taking a very hot, nearly molten sheet of glass and cooling it very rapidly (usually by exposing it to a blast of cold air) . This instantly cools and freezes the outer surfaces, while the inner part is still nearly molten. Then, as the glass sheet fully cools, the inner part shrinks, but since the outer surfaces were already cold and hard, the shrinking inner part causes the outer surfaces to be stressed in compression. So once totally cool, the inner part is "normal" glass, but the outer surfaces are highly stressed in compression. When a bending of a such tempered glass is carried out, the outer surface which normally "sees" tension is already in compression and by this resists the tensile stresses. This increases cracking and breaking resistance.

For instance the glass sheet thickness is 2 mm, 3 mm, 4 mm or 5 mm. Due to these glass sheet thicknesses the tempered glass sheet is flexible and a "cold" bending of the tempered glass sheet can be carried out. No furnace or heating process is necessary. At the most a heating process with low heat for releasing mechanical stress within the glass sheet has to be carried out. The avoidance of a hot bending reduces the costs connected with the

manufacturing a bended glass dramatically. Additionally, due to the fact, that the glass sheet is tempered, the probability for a breaking of the glass sheet while the bending process is significantly reduced. The external force is caused by at least one power source selected from the group consisting of vacuum, air pressure, mechanical moment force, gravitation and distributed pressure force. For instance the tempered flat precursor glass sheet is fixed at edges of the precursor glass sheet. Point pressure at the edges of the flat glass plate is carried out. Due to the flexibility of the tempered flat precursor glass sheet a center of the tempered flat precursor glass sheet can be moved. The bending of the precursor glass sheet is carried out. Alternatively the center of the flat precursor glass sheet is fixed and the edges of the precursor glass sheet are moved. In both cases a relative movement of the center and of the edges of the precursor glass sheet to each other takes place. For instance the flat precursor glass sheet is held horizontally. Based on a self weight caused by gravitation the bending of the precursor glass sheet occurs by a downward movement of the center of the precursor glass sheet. Alternatively an external force device for the application of the external force can be used. This device can be coupled to the edges and/or to the center of the flat precursor glass sheet.

Another example is a movement of the center of the flat precursor glass sheet in comparison to the edges of the flat glass sheet with the aid of gas pressure and/or with the aid of vacuum. Point pressure on the edges and/or on the center of precursor glass sheet is caused by the air pressure and/or vacuum. The gas can react with glass material of the precursor glass. The gas can be inert for the glass material, too. No chemical reaction occurs. For instance, the gas is pure nitrogen. Gas mixtures like air are possible, too. The bending the flat precursor glass sheet and/or the fixing the glass sheet bending are carried out with the aid at least one holder for holding the glass sheet. The glass sheet is held (fixed) in a precise required position. In a preferred embodiment a holder is used comprising at least one holder frame, wherein the bending the flat precursor glass sheet and/or the fixing the glass sheet bending of the bended glass sheet are carried out with the aid of the holder frame. For instance, by the holder frame the glass sheet is just held on its edges.

In an alternative embodiment a holder is used comprising at least one holding template with a bended template surface, which is inverse shaped compared to the glass sheet surface bending of the glass sheet bending, and wherein for the bending the tempered flat precursor glass sheet the tempered flat precursor glass sheet and the template surface of the template are brought together and/or for the fixing the glass sheet bending the glass sheet bending surface and the template surface are brought together. The template surface has the function of a supporting surface for the glass sheet. Via the template surface the external bending force is coupled into the glass sheet. Moreover the template surface provides the shape of bending of the glass sheet.

In a further preferred embodiment the fixing the glass sheet bending comprises an attaching at least one adhesive bond material onto a glass sheet bending surface of the glass sheet bending. The attaching the fixing material can be carried out before, while or after the bending process. In a preferred embodiment the adhesive bond material is selected from the group consisting of glue and/or metal. Moreover the adhesive bond material can partially or completely cover the bending surface.

Following advantages are connected with the invention: - By using a tempered glass sheet a "cold" bending is possible. A high temperature forming process is not necessary.

- By making the high temperature glass forming process unnecessary the proposed process reduces the costs for the mirrors dramatically.

- The proposed solution reduces dramatically the mirror shipment costs since a flat mirror needs less space than a bended mirror.

- A quality of the coating of a mirror (its optical reflecting surface quality) is improved since the coating process is easier and more precise to be performed on a flat glass surface than on a cured surface.

- The costs for silvering and coating of the mirror optical reflecting surface are reduced.

- Additionally accessories, e.g. expensive furnaces are not necessary. The costs of installation and future operational and maintenance costs of these accessories are saved.

- For the concentrating the sunlight just one mirror can be used. This reduces the costs and improves the optical performance of the mirror.

BRIEF DESCRIPTION OF THE DRAWING

Further features and advantages of the invention are produced from the description of an exemplary embodiment with reference to the drawings. The drawings are schematic.

Figure 1 shows the method for manufacturing a cold bended mirror in a sideways view. Figure 2 shows a mirror assembly of a cold bended mirror and a mirror bending fixing device. Figure 3 shows details of the bending method.

DETAILED DESCRIPTION OF THE INVENTION

A method for manufacturing a bended mirror 1 with a permanent mirror bending 11 is described. The mirror comprises a tempered glass sheet 12 with a glass sheet thickness 120 of about 5 mm. On a glass surface of the glass sheet a reflecting coating 13 is attached. The reflecting coating comprises a multilayer stack with at least one reflecting layer with Silver. The mirror is a front surface mirror. Alternatively the mirror is a back surface mirror .

First a tempered flat precursor glass sheet is provided.

Therefore a flat tempered glass sheet (glass sheet thickness 5 mm) is coated with the reflecting coating. After that the bending the tempered flat precursor glass sheet with the aid of an external bending force 20 is carried out (Figure 3) . The bending is preformed at room or surrounding temperature (about 20 °C) . The external bending force which is caused by gravitation or by air pressure results in point pressure at the edges of the precursor sheet .

By the bending a bended glass sheet 121 with a glass sheet bending 122 results. The next step is the fixing the glass sheet bending of the bended glass sheet. The permanent glass sheet bending results. Altogether a bended mirror with a permanent parabolic mirror bending is obtained. For the bending of the mirror a holder 14 is used. After the bending the holder and the mirror remain assembled to each other. The mirror assembly with the bended mirror and with the holder results (figure 2) . The holder has the function of the mirror bending fixing device.

The bended mirror and the mirror assembly with the bended mirror respectively are used in a solar field for the concentrating sunlight .