The invention refers to an optical concentration system according to the preamble of independent claim 1 and to a concentrator solar energy assembly according to the preamble of independent claim 9.

Optical concentration systems and concentrator solar energy assemblies are known in the prior art. They can be used to concentrate sunlight onto target areas. In the target areas, highly efficient photovoltaic elements such as solar cells or solar thermal absorber elements can be placed. In the concentration systems as known in the prior art, optical systems are used which are either expensive, laborious to manufacture, lose efficiency due to strongly inhomogeneous illumination, or lose a large portion of the light due to absorption and/or reflection and therefore have a low efficiency.

It is therefore an object of the invention to provide an optical concentration system and a concentrator solar energy assembly which is easy to produce and which provides a high yield of concentrated light as well as homogenizes the illumination on the target, i.e. the photovoltaic element.

For the optical concentration system as mentioned above, this object is achieved by an optical concentration system according to independent claim 1.

For the concentrator solar energy assembly, the object is achieved by a concentrator solar energy assembly according to independent claim 9.

The solution according to the invention, allows at least a part of the light which is directed from the first optical element towards the target area to reach the target area unobstructed. Therefore, the optical concentration system can provide a high efficiency for transporting light to the target area. The light cone as mentioned in independent claim 1 refers to the shape of the light beam after passing the first optical element. Since the first optical element concentrates the incoming light, the resulting light beam tapers towards the target area. The boundaries of the light beam may be shaped as a cone, at least in the space close to the target. It may be noted that, if the first optical element has a square aperture, the light beam may be shaped pyramidal at least in the space close to the first optical element. For the sake of convenience, the light which is directed from the first optical element towards the target area is named as light cone in the following.

The light which is directed from the first optical element towards the target area is homogenized, because the first optical element is a multi-focal element. The second optical element reflects light to a region of the target area which is outside the center of the target area or to a border region of the target area. It may therefore serve as a second homogenizer. The light which is concentrated from the first optical element towards the target area generally has a distribution in which the intensity of the light is the highest in the center of the target area. When the reflected light then superimposes with the light outside the center of the target area, the resulting distribution gets homogenized.

To summarize, the first optical element and the second optical element, which each serve as homogenizers, form a double homogenizing system. This solution leads to a homogeneous illumination of the target area and, at the same time, a negligible loss of light, especially compared to a system which uses a point focus created by the first optical element.

Further, advantageous improvements are described in the dependent claims.

Additional to the improvements as mentioned in the dependent claims, further advantageous improvements will be described in the following. The improvements as mentioned in the dependent claims and the additional improvements may be combined independently of each other, depending on whether a particular advantage of a particular improvement is needed in a specific application

According to a first advantageous improvement, the first optical element may be adapted to irradiate an area which exceeds the target area. In this case, it is ensured that the whole target area is illuminated. The portion of light which exceeds the target area can be directed by the second optical element towards the target area. In this case, the majority of the light which is part of the light cone reaches the target area.

In a preferred embodiment, the first optical element is a refractive element, such as a Fresnel lens, in particular a color mixing Fresnel lens. However, the invention is not restricted to a Fresnel lens. Any other suitable multi-focal optical element can be used. For example, the first optical element can be a total internal reflection element or a mirror.

According to another advantageous embodiment, the first optical element may have a surface with an overall planar shape. The first optical element may be part of an array, for example a Fresnel lens array.

In order to reflect the majority of light which exceeds the target area, the second optical element may completely surround the target area. It may also be possible that the second optical element only partially surrounds the target area. The second optical element may taper towards the target area and/or may be funnel-shaped. In the following, the invention and its improvements are described in greater details using an exemplary embodiment and with reference to the figures. As described above, the various features shown in the embodiment may be used independently of each other in specific applications.

In the following figures, elements having the same function and/or the same structure will be referenced by the same reference signs.

In the drawings:

Fig. 1 shows a schematic cross-sectional view of a concentrator solar energy assembly with an optical concentration system according to the invention;

Fig. 2 schematically shows a light distribution achieved by the optical concentration system as shown in Figure 1.

Figure 1 shows an optical concentration system 1 comprising a first optical element 3 and a second optical element 5. The optical concentration system 1 forms together with a photovoltaic element (solar cell 7) a concentrator solar energy assembly 9.

In the following, the optical concentration system 1 and the concentrator solar energy assembly 9 are described in an order which follows the path of light which illuminates the optical concentration system 1 along an optical axis O. Incoming light 1 1 illuminates the first optical element 3. The incoming light 1 1 is in general sunlight and can therefore be seen as a bunch of parallel light rays. Preferably, the first optical element 3 is arranged in a way that the incoming light 1 1 illuminates it with a normal incidence. This is indicated in Figure 1 with the right angles. In order to keep a normal incidence, a concentrator solar energy assembly 9 can be provided with an auto-tracking system (not shown) following the relative movement of the sun.

The first optical element 3 is adapted to at least partially focus the incoming light 1 1 in a way that a light cone 13 or a light beam shaped as a truncated pyramid is formed. The first optical element 3 is just shown schematically with a rectangular cross-section. It may have any applicable shape. However, it is preferred that at least the outer surface 15 of the first optical element 3 has an overall planar shape. The planar shape may facilitate cleaning of the first optical element 3 and may allow a compact structural form, especially if several optical concentration systems 1 are combined in order to form an array.

The first optical element 3 is preferably formed by a Fresnel lens. However, also other optical elements may be used, for example an element which uses total internal reflection. More preferably, the first optical element 3 is formed by a color mixing multi-focal Fresnel lens. The light cone 13 illuminates the target area 17 which is shown as a dashed line in Figure 1 . A solar cell 7 or a solar thermal element may be placed in the target area 17. If a second optical element 5 was not present, the light cone 13 would illuminate an area with at least a width 19 in the shown cross-section which is larger than the width 29 of the target area 17. Preferably, the largest portion of the light cone 13 reaches the target area 17 unobstructed.

Adjacent to the target area 17, the second optical element 5 is placed. The second optical element 5 preferably completely surrounds the target area 17 circumferential around the optical axis O. Alternative, the second optical element 5 may only partially surround the target area 17. The second optical element 5 is preferably shaped as the surface of a truncated cone or a truncated pyramid 30. It may taper towards the target area 17 and may surround the target area 17 with its smaller diameter end 32. The second optical element 5 preferably uses external reflection to reflect light towards the target area 17.

The second optical element 5 is adapted to reflect especially parts of the light cone 13 which would exceed the target area 17 if the second optical element 5 was not present. The second optical element 5 is arranged in a way that the light, which is reflected by it onto the target area 17, is preferably directed onto a region 34 of the target area 17 which is outside of the center 23 of the target area 17 and close to the borders of same. Benefits of this arrangement are described in further detail with respect to Figure 2. In order to reflect the light as described, an opening angle 25 of the second optical element 5 may be larger than an opening angle 27 of the light cone 13.

Figure 2 schematically shows a resulting light distribution 31 as achieved with the optical concentration system 1 as described with respect to Figure 1. A light distribution 29 in the plane of the target area 17 which would result without the second optical element 5 is represented by the solid line. The light distribution 31 that results from the usage of the optical concentration system 1 which comprises a second optical element 5 is shown by the dashed line.

The shape of the light distribution 29 depends on the properties of the first optical element 3. The light distribution 29 can generally be described with a bell-shaped or a Gaussian-like function. In the center 23 of the target area 17, the light intensity has a maximum and it falls with increasing distance from the center 23. For comparison, the target area 17 is indicated in Figure 2. Without a second optical element 5, the intensity rapidly decreases perpendicular to the optical axis O in the direction of the limits of the target area 17, and tails 33 of the distribution do not reach the target area 17 and therefore do not contribute to the collected light.

In contrast to this, the optical concentration system 1 with the second optical element 5 provides an advantageous light distribution 31. The tails 33 of the distribution 29 are reflected by the second optical element 5 mostly onto the region 34 of the target area 17. There the light may superimpose with light of the distribution 29 which is presented by the region 35 of the distribution 29. The region 35 of the distribution 29 is the light that illuminates the region 34 of the target area 17 and is therefore the part of the distribution which is located between the maximum of the distribution 29 and the tails 33. The resulting light distribution 31 has two advantages over the light distribution 29:

First, the light distribution 31 is more homogenous since the decrease towards the limits of the target area 17 is lower than for the distribution 29. Second, the Integral of the distribution 29 over the target area 17 and therefore the amount of collected light is larger than for the distribution 29, since the tails 33 now also contribute to the collected light.

REFERENCE NUMERALS

Optical concentration system

First optical element

Second optical element

Photovoltaic element

Concentrator solar energy assembly

Incoming light

Light cone

Outer surface

Target area

Width of the light cone

Width of the target area

Center of the target area

Opening angle of the second optical element Opening angle of the light cone

Light distribution without second optical element Truncated cone

Light distribution with second optical element Smaller diameter end of truncated cone

Tails of the distribution

Region on the target area

Region of the light distribution

Optical axis