WO2011064771A1

WO2011064771A1 - Combined solar photovoltaic optical system and method

Info

Publication number: WO2011064771A1
Application number: PCT/IL2010/000979
Authority: WO
Inventors: Ishai Ilani; Dror Yachil
Original assignee: Ishai Ilani; Dror Yachil
Priority date: 2009-11-25
Filing date: 2010-11-23
Publication date: 2011-06-03

Abstract

Solar photovoltaic optical methods and systems for generating electricity from solar radiation, including at least one primary mirror adapted to receive incident solar radiation, the primary mirror disposed at least one of a) along a side of a photovoltaic panel and b) at a corner of the photovoltaic panel; and a secondary mirror disposed in at least one of a vicinity of a focal point focal, a vicinity of a focal line; and a vicinity of a focal plane of the primary mirror, the secondary mirror adapted to highly concentrate reflected solar radiation received from the primary mirror and to convey the highly concentrated solar radiation as homogenously concentrated solar radiation to the photovoltaic panel; and the photovoltaic panel for generating electricity from both the homogenously concentrated solar radiation received from the secondary mirrors and direct solar radiation received from the sun.

Description

COMBINED SOLAR PHOTOVOLTAIC OPTICAL SYSTEM AND

METHOD

FIELD OF THE INVENTION

The present invention relates generally to systems for capturing solar energy, and more specifically to systems and methods for efficient solar energy capture and conversion to electricity.

BACKGROUND OF THE INVENTION

The need for developing energy from renewable sources is nowadays widely recognized.

Preventing global warming by reducing emissions is cited by green activists as one major reason for the need to produce clean energy.

Even from a pure economic perspective, the souring prices of oil during 2007-2008, was a big reminder that there is a need to reduce the dependence on oil, especially since most of the oil resources are under control of extreme and unstable countries.

Many initiatives are now in deployment and research in many areas from developing of bio-fuels from corn, sugar-cane or algae, through generation of clean electricity via wind, solar or hydro-electric plants, and initiative are under way for electric cars, which will replace the 100 year old fossil fuel-driven cars.

Tremendous effort, research and money have been invested in the search for new energy sources. The result is a multitude of ingenious technologies, which when combined together may some day provide a comprehensive set of energy generation, which can replace the traditional systems of today that rely heavily on fossil fuels.

From the global economical perspective, energy from renewable sources plays a significant role. The authors of this invention believe that the stabilization of oil prices since late 2008 is partially due to the fact that renewable energy is now a real alternative to energy generated by fossil fuels, and at prices of 200$ per barrel can become economical even from a consumer point of view.

However, not even one of the new energy generation technologies is economical per se at the time of writing of this application. The incentive for the massive deployment of renewable energies comes from government subsidies and from regulations which set a goal for a certain percentage of energy generation to come from renewable sources.

The main effort in the area of renewable energies is to reduce the price of energy generation until a point of "grid parity" is reached. The "grid parity" point isn't very well defined, but as a number of reference, a price of 1$ per KW peak for photovoltaic solar systems is considered a "magic number".

Over the last few decades, photovoltaic (PV) cells have been used to convert solar radiation into electricity. Despite the huge investments in this field PV cells have not yet been introduced in most electricity grids to generate electricity. The cost of the PV cell production is very high making the electricity produced therefrom too expensive to compete with the current conventional sources of energy, such as natural gas and coal.

Various methods of concentration of the solar radiation using collectors have been developed to reduce the surface area of the PV cell: energy ratio.

Some patent publications in the field of solar panel arrays include:

US3118437, to Hunt, describes a means for concentrating solar radiation.

WO08046187A describes an improved solar concentrating system (100) uses a two-stage arrangement of mirrors wherein the rays of the sun are reflected and concentrated to a point focus. The solar concentrator (100) may be used to increase the temperature of a substance such as metal, for use in a variety of applications including the melting of metals in a foundry furnace. The solar concentrating system (100) comprises at least two single-curved parabolic mirrors (10, 20) connected in an operable arrangement. The rays of the sun are reflected from a first single-curved parabolic mirror (10) to a second single-curved parabolic mirror (20). The plane of symmetry of the second single-curved parabolic mirror is arranged substantially orthogonal to the plane of symmetry of the first single-curved parabolic mirror thereby concentrating the rays of the sun to a point focus.

There is still a need to provide systems and methods to efficiently and economically form electricity from solar radiation. There is a further need for solar photovoltaic systems which are less costly to manufacture than the prior art systems and operate effectively.

SUMMARY OF THE INVENTION

It is an object of some aspects of the present invention to provide systems and methods for generating electricity from solar radiation.

In some embodiments of the present invention, improved methods and apparatus are provided for generating electricity from solar radiation employing combined solar-photovoltaic optical systems.

In other embodiments of the present invention, a method and system is described for providing electricity from solar radiation, the system comprising at least one photovoltaic panel, at least one primary mirror and at least one secondary mirror adapted to highly concentrate solar radiation reflected from the primary mirror and to convey the highly concentrated solar radiation as homogenously concentrated radiation to the photovoltaic panel, whereby electricity is generated.

According to some embodiments of the present invention, there are provided combined solar photovoltaic optical systems and methods for generating electricity from solar radiation and methods, the systems comprising at least one an optical arrangement for homogenously concentrating solar radiation, the optical arrangement comprising at least one primary mirror adapted to receive solar radiation, the primary mirror being disposed along a side of the photovoltaic panel; and a secondary mirror disposed in a focal plane of the primary mirror, the secondary mirror adapted to highly concentrate reflected solar radiation from the primary mirror and to convey the highly concentrated solar radiation as homogenously concentrated radiation to the photovoltaic panel.

In the present invention, the solar radiation is distributed substantially homogenously on the photovoltaic panel array. One differentiator between the present invention and prior art WO08046187A, mentioned hereinabove is that in the prior art there are two stages, namely, a first stage which concentrates solar radiation to some degree and the second improves the concentration. In contrast, in the present invention, the first stage concentrates solar radiation, while the second spreads the radiation in a homogeneous fashion (the sum is homogeneous even though the radiation from each mirror is not) onto the photovoltaic panel array. Thereafter, the photovoltaic panel array is adapted to generate electricity from at least one of the concentrated solar radiation received from the optical arrangement and direct solar radiation received from the sun.

There is thus provided according to an embodiment of the present invention, a combined photovoltaic-optical system for generating electricity from solar radiation, the system including;

a. at least one optical arrangement for concentrating solar radiation, the optical arrangement including at least one;

i. a primary mirror adapted to receive incident solar radiation, the primary mirror disposed at least one of a) along a side of the photovoltaic panel and b) at a corner of the photovoltaic panel; and

ii. a secondary mirror disposed in at least one of;

a) a vicinity of a focal point focal;

b) a vicinity of a focal line; and

c) a vicinity of a focal plane;

of the primary mirror, the secondary mirror adapted to highly concentrate reflected solar radiation received from the primary mirror and to convey the highly concentrated solar radiation as homogenously concentrated solar radiation to the photovoltaic panel; and

b. a photovoltaic panel for generating electricity from both;

i. the homogenously concentrated solar radiation received from the secondary mirrors; and

ii. direct solar radiation received from the sun.

Additionally, according to an embodiment of the present invention, the combined photovoltaic-optical system further includes a solar tracking system for moving the at least one optical arrangement along at least one axis to track the movement of the sun.

Furthermore, according to an embodiment of the present invention, the solar tracking system is constructed to retain the at least one primary mirror perpendicularly to the sun.

Further, according to an embodiment of the present invention, the at least one optical arrangement is adapted to concentrate the solar radiation by a factor of at least two.

Yet further, according to an embodiment of the present invention, the at least one optical arrangement includes at least four primary mirrors.

Moreover, according to an embodiment of the present invention the at least one optical arrangement includes at least four secondary mirrors.

Additionally, according to an embodiment of the present invention, the at least one optical arrangement includes at least twelve primary mirrors.

Furthermore, according to an embodiment of the present invention, the at least one optical arrangement includes at least twelve secondary mirrors.

Further, according to an embodiment of the present invention, the factor is at least three. In some further embodiments, the factor is at least five.

Moreover, according to an embodiment of the present invention, the at least four primary mirrors are disposed such that a central point of all the at least four primary mirrors fall in a common plane. Additionally, according to an embodiment of the present invention, the photovoltaic panel is disposed to receive light orthogonally from the sun.

Furthermore, according to an embodiment of the present invention, the secondary mirrors are adapted to convey the homogenously concentrated solar radiation towards the photovoltaic panel with a homogeneity over a receiving surface of the photovoltaic panel of at least 90%. According to some further embodiments, the homogeneity is at least 95%.

Additionally, according to an embodiment of the present invention, a utilization of the primary mirror is at least 90%. According to some embodiments of the present invention, the utilization of the primary mirror is at least 95%.

Moreover, according to an embodiment of the present invention, the secondary mirror is a selective mirror designed to pass only a part of a spectrum of the solar radiation to the photovoltaic panel.

Additionally, according to an embodiment of the present invention, a distance from the primary mirror to the secondary mirror is approximately the same as a length of a side of the primary mirror.

Moreover, according to an embodiment of the present invention, the primary mirror is a section of a parabaloid. According to other embodiments, the primary mirror is formed from a section of a sphere. According to yet further embodiments, the primary mirror is formed as a section of a parabolic trough. According to some other embodiments, the primary mirror is formed as a section of a cylinder.

Additionally, according to an embodiment of the present invention, the secondary mirror has a surface area of less than 10% of the corresponding primary mirror. According to yet further embodiments, the secondary mirror has a surface area of less than 5% of the corresponding primary mirror.

Additionally, according to an embodiment of the present invention, the primary mirror is disposed in a plane that is parallel to the photovoltaic panel.

Moreover, according to an embodiment of the present invention, the primary mirror is disposed below the plane of the photovoltaic panel, thereby forming an air gap between the primary mirror and the photovoltaic panel.

Furthermore, according to an embodiment of the present invention, the secondary mirror is disposed above a corner of the photovoltaic panel.

Additionally, according to an embodiment of the present invention, the secondary mirror is disposed above and along a side of the photovoltaic panel.

There is thus provided according to a further embodiment of the present invention, a method for generating electricity from solar radiation, the method including;

a) reflecting received solar radiation from the sun from a first reflective surface onto a second reflective surface to form highly concentrated solar radiation;

b) passing the highly concentrated solar radiation from the second reflective surface to a photovoltaic panel thereby generating homogeneous concentrated radiation; and

c) converting both;

i. the homogeneous concentrated radiation; and

ii. direct solar radiation received from the sun on the photovoltaic panel;

into electricity at the photovoltaic panel.

Additionally, according to an embodiment of the present invention, the homogeneous concentrated radiation is concentrated by at least three times the received solar radiation. According to some further embodiments, the homogeneous concentrated radiation is concentrated by at least five times the direct solar radiation.

Furthermore, according to an embodiment of the present invention, the homogeneous concentrated radiation has a homogeneity over a receiving surface of the photovoltaic pane; of at least 90%. According to some further embodiments, the homogeneity is at least 95%.

a) placing a combined photovoltaic-optical system as described herein at an outdoor location such that the primary mirror and the photovoltaic panel are disposed in a direct path from the sun to receive the solar radiation; and

b) generating electricity from the solar radiation employing the system.

Additionally, according to some further embodiments of the present invention, the photovoltaic panel receives concentrated solar radiation from the at least one secondary mirror with a homogeneity of at least 90% over the photovoltaic panel.

The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings:

Fig. 1A is a simplified schematic illustration showing a prior art dual axis photovoltaic solar tracking system;

Fig. IB is view of the system of Fig 1A from another angle;

Fig. 2 is a simplified schematic illustration showing a prior art dual axis high concentration photovoltaic solar tracking system;

Fig. 3 is a simplified schematic illustration showing a prior art low concentration photovoltaic solar tracking system;

Fig. 4 is another simplified schematic illustration showing a prior art low concentration photovoltaic solar tracking system employing a bifacial photovoltaic panel;

Fig. 5 is a simplified schematic illustration showing a low concentration photovoltaic solar tracking system, in accordance with an embodiment of the present invention; Fig. 6 is a simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system, in accordance with an embodiment of the present invention;

Fig. 7 is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system, in accordance with an embodiment of the present invention;

Fig. 8 is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system with mirrors disposed at corners of a photovoltaic panel, in accordance with an embodiment of the present invention;

Fig. 9A is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system with mirrors disposed along the sides of a photovoltaic panel, in accordance with an embodiment of the present invention;

Fig. 9B is a ray diagram of the incident sun rays on primary and to secondary mirrors of the system of Fig. 9 A;

Fig. 9C is a ray diagram of the reflected sun rays from secondary mirrors and onto photovoltaic panels of the system of Fig. 9 A;

Fig. 10 is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system with mirrors disposed along a perimeter a photovoltaic panel, in accordance with an embodiment of the present invention;

Fig. 11 is a graphical display of a simulated calculation of radiation distribution against a two dimensional photovoltaic surface, based on the geometry of the system of Fig. 10, in accordance with an embodiment of the present invention;

Fig. 12 is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system with mirrors disposed below the plane of a photovoltaic panel, in accordance with an embodiment of the present invention; and

Fig. 13 is another simplified schematic illustration showing an optical element of a dual axis low concentration photovoltaic solar tracking system, the optical element comprising a gap between a photovoltaic panel and a primary mirror in accordance with an embodiment of the present invention.

In all the figures similar reference numerals identify similar parts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that these are specific embodiments and that the present invention may be practiced also in different ways that embody the characterizing features of the invention as described and claimed herein.

Definitions and Conventions

Unless defined otherwise herein, all terms used herein are defined by their common dictionary definitions.

By solar radiation is meant any light energy emitted by the sun in the visible and non visible spectral range of wavelengths, as is known in the art.

By "focal point" is meant a point where light rays originating from the sun and reflected by the primary mirror pass in the vicinity of the point

By "focal line" is meant a line where light rays originating from the sun and reflected by the primary mirror pass in the vicinity of the line

By "in the vicinity of is meant that the distance between the reflected light rays and the focal point (focal line) is less than 10% of the distance between the focal point (focal line) and the primary mirror

By "highly concentrated solar radiation" is meant radiation which is reflected by the primary mirror toward the secondary mirror, typically resulting in radiation which is more than 10 times the direct radiation of the sun.

By "homogenously concentrated solar radiation" is meant the total radiation which is reflected by the all secondary mirrors toward the photovoltaic panel, resulting in radiation where the difference between the lowest concentration point and the highest concentration is less than 10%.

In this description a conventions is used in the figures. The photo-voltaic array is illustrated by rectangles with a coarse grid on them (see Fig. 5 hereinbelow for example), such that each single rectangle 501 can be seen as representing a single photovoltaic unit, all such units forming a photovoltaic panel 502.

The primary mirrors which are generally curved mirrors are represented by a fine grid. The grid is for illustration purposes only. The actual primary mirrors will probably be continuous curved mirrors with no grid at all.

The secondary mirrors are represented by rectangles with no grid at all.

In order to understand the present invention we will begin by describing a prior art tracker, shown in Fig. 1A and Fig. IB. Fig. 1A is a simplified schematic illustration showing a prior art dual axis photovoltaic solar tracking system 100 comprising a photovoltaic panel array of a plurality of photovoltaic panels 102 supported by a mechanical support 108. Fig. IB shows the same system 100B from the rear side and shows the connection of the photovoltaic panel array 102B to the mechanical support 108B.

The disadvantages of prior art system 100 include, arise from the fact that a typical photovoltaic array is made of silicon either monocrystalline silicon or polycrystalline silicon. In any case silicon is expensive and heavy relative to reflecting optics, whether reflecting optics is made of glass mirrors or other types of reflecting materials which are now entering the solar market. The fact that silicon is heavier than reflectors also projects on the cost of the tracker, so altogether a system which is made from silicon alone is more expensive than necessary, and significant cost savings can be made by replacing some of the silicon by non-expensive optics.

Prior art dual axis photovoltaic solar tracking system 100 uses only photovoltaic material, without any optical devices such as mirrors or lenses for concentration. These typical prior art tracking systems may be mounted on a single axis or dual axis tracker, or even on static mechanical structures. However, it is commonly accepted that the performance of a single axis tracking system is about 20% higher than a static system, and dual axis trackers can achieve 40% advantage in performance over static systems. A dual tracking system can track the sun and maintain a constant 90° angle between the sun's position and the PV plane. An illustration of a prior art dual axis tracking PV system is depicted below in Fig. 2.

Reference is now made to Fig. 2, which is a simplified schematic illustration showing a prior art dual axis high concentration photovoltaic solar tracking system 200.

Dual axis photovoltaic solar tracking system 200 comprises mirror units 204 disposed such that a photovoltaic receiver 202, placed facing the mirror units, by means of a photovoltaic receiver mechanical support 208, receives reflected radiation from the mirror units, which was received by the mirror units as incident solar radiation. The solar radiation is thereby concentrated towards a single focal point, and the photovoltaic receiver 202 of photovoltaic material is placed at the focal point.

In prior art system 200, the photovoltaic receiver 202 has a small surface area and gets extremely hot, with a limited surface area for cooling System 200 belongs to the category of High Concentrated Photo- Voltaic (HCPV). Typically the concentration ratio for systems in this category is 100 and higher. The receiver 202 cannot be made of silicon, but requires special PV cells known as multi- junction PV cells, which are made of Germanium or Gallium Arsenide. These types of cells do not degrade their performance with heating and they can absorb and convert sunlight in the intensity of 100 times the direct radiation of the sun and convert it to electricity in an efficient manner. For such type of PV cells the architecture of system 200 is close to optimal. The active part of the PV receiver 202 faces the paraboloid mirrors 204, and does not receive direct radiation from the sun. However, its area is less than 1% of the area of the paraboloid mirrors 204, so there is practically no degradation of performance due to the loss of the direct radiation upon the receiver 202. The present invention is related to silicon PV, where heating of the silicon above 70°-80° can degrade the PV performance significantly. As a consequence concentration ratios should be limited to ~5 times the natural radiation of the sun. Under these conditions it becomes very important that the PV will receive not only the reflected radiation but also direct radiation.

Reference is now made to Fig. 3, which is a simplified schematic illustration showing a prior art low concentration photovoltaic solar tracking system 300. Low concentration photovoltaic solar tracking system 300 comprises a centrally disposed photovoltaic panel 302 with a mirror unit 304 disposed at an angle on each longitudinal side of the photovoltaic panel. Low concentration photovoltaic solar tracking system 300 is typically supported on a mechanical support 308.

In order to achieve a concentration factor of 2, the system 300 uses two mirror units 304 of reflective material for each unit of photovoltaic panel 302, comprising photovoltaic material. This is due to the fact that the projection of the mirrors into the photovoltaic panel plane should have at least the same area as the photovoltaic panel 302 array. Since the angle between the mirror and the PV plane is around 60°, this causes the total area of the mirrors to be twice the size of the photovoltaic panel 302.

Low and medium concentration photovoltaic systems (L-CPV, M-CPV), such as those shown in Figs. 3 and 4 typically use monocrystalline silicon or polycrystalline silicon cells, achieving conversion efficiencies between 15% - 22%. A typical example of an L-CPV system is the V-shaped architecture is shown in Fig. 3 hereinabove. In this architecture photovoltaic unit 302 is surrounded by tilted mirror units 304 such that the radiation incoming into the mirrors is reflected toward the PV array and increases the radiation on the PV array.

The disadvantages of prior art system 300 include, inter alia, that in order to generate a concentration factor of 2 system 300 uses 2 units of reflector material for each unit of PV material, thus the total amount of material used for this system is high. Moreover the V-shape profile of the system requires tough mechanical support for both the size and weight of the system and for making it resilient to gusts of wind. Reference is now made to Fig. 4, which is another simplified schematic illustration showing a prior art low concentration photovoltaic solar tracking system 400 employing a bifacial photovoltaic panel 402. Mirror units 404 are supported by a low concentration photovoltaic solar tracking system mechanical support 408, which also holds the photovoltaic panel 402 centrally above the mirror units.

The mirror units have a surface area of around four times that of the photovoltaic panel 402. Together with the direct radiation on the bifacial photovoltaic panel 402 system 400 should theoretically provide a five times concentration of the impingent solar radiation on the mirror units. However, in practice, the efficiency is reduced due to the lack of cooling of the bifacial photovoltaic panel 402.

System 400 avoids the disadvantage of having the photovoltaic panel receive only reflected radiation by using bifacial silicon photovoltaic panels. This is an advantage in terms of radiation received by the photovoltaic panel. However, bifacial PV is more complex and expensive than ordinary PV. Moreover since the photovoltaic panel receives radiation on both sides there is no room for attaching cooling mechanisms on the photovoltaic panel. As a result the photovoltaic panel heats up and its performance degrades (recall system 400 belongs to the LCPV category, which use silicon photovoltaic panels). Another disadvantage of system 400 is the need for the mechanical support for the large and heavy photovoltaic panel 402, which is mounted at a significant distance from the mechanical support axis 408 and the primary mirrors 404.

Figure 5-10 below are illustrations of various implementations of systems designed according to the present invention. The systems differ in some aspects of the implementation but share some common principles:

1. All systems use ordinary one sided silicon PV panels or other material equivalents.

2. All systems use a 2 stage reflection where the first stage is a highly concentrating stage, while the 2^nd stage spreads the radiation homogeneously upon the PV. 3. In all systems both the reflective material (the mirrors) and the PV are utilized with very high efficiency. The utilization of an element (PV or mirror) is defined as the ratio P/A where P is the projection of the element on a plane perpendicular to the sun and A is the total area of an element. However, if the receiving side of the element is not facing the sun its utilization is considered as 0%. For example the PV utilization in Fig.l is 100% since the PV is on a plane perpendicular to the sun. The utilization of the PV in Fig. 2 is 0%, but its area is less than 1% of the paraboloid mirrors and the utilization of the paraboloid mirrors is > 95%. The utilization of the mirrors in Fig.3 is -50%. For the systems of the present invention the PV are always utilized at 100% or very close while the primary mirrors are utilized at 95% and higher. Even if the secondary mirrors are considered together with the primary mirrors the utilization of the reflective part of the systems is > 90%.

4. The focal point of the systems of the present invention are located at a height which is relatively low. By relatively low is meant that the focal point height is approximately equal to the length of an edge of the PV panels array, or an edge of a primary mirror element. It is very easy to compute an array of primary and secondary mirrors which will transfer the radiation homogeneous on the PV array if the focal point is allowed to be at great height (10 times the PV edge). However such systems would be very expensive and practically impossible. The present invention achieves the goals of homogeneous radiation with relatively low focal points.

5. The optics of the system is kept to be very simple. The primary mirrors are preferably sections of paraboloids, parabolic troughs, spheres and cylinders, while the secondary mirrors are preferably ordinary flat mirrors.

Reference is now made to Fig. 5, which is a simplified schematic illustration showing a low concentration photovoltaic solar tracking system 500, in accordance with an embodiment of the present invention.

Low concentration photovoltaic solar tracking system 500 comprises at least one photovoltaic panel 502, comprising rectangular units 501, for example, with primary mirror units 504 disposed along two opposing sides of the at least one photovoltaic panel 502. Low concentration photovoltaic solar tracking system 500 is supported by a low concentration photovoltaic solar tracking system mechanical support 508. The mechanical support may include a mechanical apparatus (not shown) for tracking the sun's movement, which may be uni-axial or biaxial, as is known in the art.

Above each primary mirror unit there is placed a secondary mirror unit 506 at the focal point of the primary mirror unit. The secondary mirror units are held in place by a secondary mirror units' mechanical support 512. According to the embodiment shown in Fig. 5, there are four primary mirror units (two on each side of a photovoltaic panel 502), which convey reflected light to the secondary mirror units. The secondary mirrors locations and angles are designed such that the total reflected light from all the secondary mirrors onto the photovoltaic panels 502 will be spread essentially homogeneous, (even though the radiation from each paraboloid mirror is not homogeneous) thereby obtaining approximately a doubling of the radiation received by the photovoltaic panel 502.

The sun 520 emits incident rays 522 which impinge onto primary mirrors

504. Incident rays 522 are reflected as first reflected rays 524 by the primary mirrors onto corresponding secondary mirrors 506 as can be seen in Fig. 5. The first reflected rays are reflected as secondary reflected rays 526 onto photovoltaic panel 502.

It should be understood that the examples provided herein should not be deemed as limiting. The combined solar photovoltaic optical systems and methods of the present invention can be applied in many ways, and at different orders of magnitudes of panel/mirror surface areas.

One embodiment of a tracking system constructed according to the present invention will comprise of an array of PV modules for generating electricity mounted at the center of the tracking system. In one specific example photovoltaic panel 502 (which may be one panel or an array of panels) is of as size of 2x2 meters. The projection of the primary mirror units 504 on the plane perpendicular to the sun comprise, according to this example four one by one meter squares. The total area of each of the primary mirrors is less than 1.02 m², thus the total area of all 4 mirrors is less than 4.08 m². Note that the minimal requirement for achieving a concentration factor of 2 is that the primary mirrors will be at least 4 m², thus with the embodiment of the present invention the system is close to the theoretical limit. According to one example, the size of the secondary mirror units 506 is approximately 0.2x0.2 meters, and they are preferably located at or very near the focal point of the primary mirrors. The focal point for this embodiment is at a height of approximately 1.2 meters above the base of the primary mirror.

In order to describe the systems of Figs 5-10 accurately as possible, we shall use mathematical notation. The following convention will be used in describing all the figures. A Cartesian axis system will be used, where the sun's position is on the z-axis (at a very large height), the photovoltaic plane will be at or parallel to the xy-plane.

The primary mirrors in all the systems are preferably either sections of paraboloids and spheres (for point focus systems), and sections of parabolic troughs or cylinders (for line focus systems). However it should be noted that the essential property of the primary mirror is that it highly concentrates light to the close vicinity of a focal point (for point focus systems) or a focal line (for line focus systems).

According to a preferred embodiment of system 500, the primary mirrors 504 are sections of a paraboloid.

In mathematical formula, each of the paraboloids is defined by the equation

where x is in the range J ,≤x≤x₂ , y is in the range y_l≤y≤y₂ , and the focal point of the paraboloid is at (x₀,y₀,f + z₀) . In the example of system 500 the focal point is straight above the center of the paraboloid center. This translates to

X H™ X V ~f~ V

J ₀ = ^{1 2} , y₀ = ¹ ^ ² . If the projection of the paraboloid on the xy-plane is a square (as is the case for Fig. 5), then x₂ - x_x = y₂ -y · To be more specific, the upper right paraboloid in Fig. 5 may be defined by the parameters x_x = \,x₂ = 2 , y_l = 0,y₂ = l , x₀ = 1.5, y₀ = 0.5 , and / = 1.2 . The base point of the primary mirror is at the point (x₀,y₀,z₀) , which is the lowest point of the paraboloid. Preferably all base points of all the primary mirrors 504 fall in a common plane. This plane is typically parallel to the photovoltaic panel plane and perpendicular to the sun. According to one embodiment, primary mirror units 504 are paraboloid.

The secondary mirror 506, or secondary mirror array, will be mounted near the focal point of the paraboloid primary mirror unit 504, and the secondary mirror unit 506 is tilted. One preferred method for defining the tilt would be such that a ray of light shinning upwards (perpendicular to the plane of the photovoltaic panel 502) from the center of the primary paraboloid mirror unit 504 would be reflected by the secondary mirror 506 towards a predefined point (not shown), preferably a point near the center of the photovoltaic panel 502.

According to another embodiment, primary mirror units 504 are taken as a section of a sphere, with radius R approximately twice the focal length / of the paraboloid of the first embodiment ( R « 2/ ).

In mathematical formula, each of the spheres is defined by the equation z = z₀ - lR² -(x - x₀)² -(y -y₀)² (2) where x is in the range x_x < J < x₂ , y is in the range y_l≤ y≤ y₂ , R is the radius of the sphere, which in this case be approximately If ( R « 2/ ), and the center point of the sphere is (x₀,y₀,z₀) In the example of Fig. 5 z₀ can be set to z₀ = R and x₀ = *' ^{+ Xl} , y₀ = ⁺ ·^² ,. If the projection of the spherical section on the xy-plane is a square (as is the case for Fig. 5), then J ₂ -x = y₂ -y_\ · To be more specific, the upper right sphere in Fig. 5 may be defined by the parameters x_x = l,x₂ = 2,y_x - 0,y₂ = 1 , x₀ = 1.5,.y₀ = 0.5 , and R = 2A . f = 1.2. The base point of the primary mirror is at the point (x₀,y₀,z₀ -R) , which is the lowest point of the spherical section. Preferably al base points of all the primary mirrors 504 fall in a common plane. This plane is typically parallel to the photovoltaic panel plane and perpendicular to the sun.

Since the whole system is mounted on a tracker 508, the sun will always shine on the primary mirror units and photovoltaic panel at an angle of around or exactly 90° to the photovoltaic panel 502, which is also the plane of primary mirror units 504. Thus the angle between the sun and the photovoltaic panel 502 PV and the primary mirror units 504 is always very nearly constant. This removes a need for moving the mirrors, and enables the use of mirrors fixed to their mechanical support 508. This both simplifies system 500 and reduces its cost.

Another variant to the system of Fig. 5 is shown below in Fig. 6. Fig. 6 shows a dual axis low concentration photovoltaic solar tracking system 600. The basic elements of the system of Fig. 6 are the same as the basic elements of the system of Fig. 5. The difference between the systems is that in the system of Fig.6 12 primary and secondary mirrors are surrounding the photovoltaic panel array from all sides, while in the system of Fig. 5 only 4 primary and secondary mirrors are used. And they are deployed on the sides of the photovoltaic panel only.

Dual axis low concentration photovoltaic solar tracking system 600 comprises at least one photovoltaic panel 602 with primary mirror units 604 disposed around all sides of the at least one photovoltaic panel array 602. Dual axis low concentration photovoltaic solar tracking system 600 is supported by a low concentration photovoltaic solar tracking system mechanical support 608. According to the embodiment shown in Fig. 6, there are twelve primary mirror units surrounding two photovoltaic panels 602. Additionally, disposed above each of the twelve primary mirror units is one secondary mirror unit 606. A secondary mirror unit mechanical support 612 holds each secondary mirror in a focal plane of the primary mirror.

According to one embodiment, photovoltaic panel array 602 is of a size of

2x2 meters and is surrounded by twelve photovoltaic panels 602, each of them consisting of a paraboloid section (or a spherical section) mounted on a l x l meter plane surface, and near a focal point (not shown) of each paraboloid, there is small secondary mirror 606 of approximately 0.2x 0.2 meters tilted at a predefined angle, for example in the range of -5 to +5 degrees toward the PV array.

Since the whole system is mounted on a tracker, the sun will always shine on the tracker system in a 90° to the PV plane, thus the angles between the sun and the PV and the mirrors is always constant. This relaxes the need for moving the mirrors, and enables the use of mirrors fixed to their base. This both simplifies the system and reduces its cost.

In the examples described above all the mirrors are fixed, and there is no need for additional motors. However an alternative embodiment may include an adjustable array of mirrors at the focal point to allow for adaptation of the system.

Yet another embodiment may implement adjustable mirrors at the primary array (the paraboloid) or at both the primary array and secondary array.

Reference is now made to Fig. 7, which is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system 700, in accordance with an embodiment of the present invention. Dual axis low concentration photovoltaic solar tracking system 700, comprises a photovoltaic panel 702 with four primary mirror units 704 (one on each side of the panel). The panel and mirror units 704 are mounted on a dual axis low concentration photovoltaic solar tracking system mechanical support 708. Additionally, each primary mirror unit 704 has a secondary mirror unit 706 mounted above it and supported by its own secondary mirror units' mechanical support 712. The main difference between the system of Fig. 5 and the system of Fig. 7 is that in the system of Fig. 7 only a single primary and secondary mirror is mounted on each side of the photovoltaic panel. The primary array may be either a section of a paraboloid or a section of a sphere (as in Fig. 5). Another difference is that the focal point of the primary mirror is right above the border between the photovoltaic panel and the primary mirror (exact parameters appear in the next sections)

According to one embodiment, the dimensions of this system are as follows from the mathematical description of the system: The photovoltaic panel is a rectangle with corners at: (-a,-b) ,

(a,-b) (a,b) (-a,b) . In the example of Fig. 7 the rectangle is a square with a = b = \

Each of the paraboloids is defined by the equation z = z₀o ₊ ⁽ - °⁾² ₄ ⁺ - >⁾² (3) where J is in the range .Xj < x≤ x₂ , y is in the range y_x < y < y₂ , and the focal point of the paraboloid is at (x₀,y₀,f + z₀) . For the primary mirror on the right side of the photovoltaic panel x_x = \,x₂ = 2 , y_x = -\,y₂ = \ J ₀ = JC, = 1 ,

If the primary mirrors are a section of a sphere then they will be defined by the following parameters.

Each of the spheres is defined by the equation

where x is in the range x_x≤ x≤ x₂ , y is in the range y_x≤y≤y₂ , R=2f is the radius of the sphere, and the center point of the sphere is (x₀,y₀,z₀) . For the primary mirror on the right side of the photovoltaic panel χ_ί = \,x₂ = 2, y_l = -l,y₂ = 1 x₀ = x_l = 1 , y₀ = ^¹ ^² = 0 , and z₀ = R .

Reference is now made to Fig. 8, which is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system 800 with primary mirrors 804 disposed at corners 805 of a photovoltaic panel 802, in accordance with an embodiment of the present invention. Additionally, each primary mirror unit 804 has a secondary mirror unit 806 mounted above it and supported by its own secondary mirror units' mechanical support 812. The panel 802 and mirror units 804 are mounted on a dual axis low concentration photovoltaic solar tracking system mechanical support 808. System 800 can be defined by the following parameters:

The photovoltaic panel is a rectangle with corners at: (-a,-b) , (a,-b) (a,b) (-a,b) . In the example of Fig. 8 the rectangle is a square with a = b = \

Each of the paraboloids is defined by the equation (*-*₀)² ₊ 0^o)²

o ₄ where x is in the range j≤x≤x₂ , y is in the range y_l≤y≤y₂ , and the focal point of the paraboloid is at (x₀,y_Q,f + z₀) . For the primary mirror on the right side of the photovoltaic panel J ,

yo = y\ = ¹ · If the primary mirrors are a section of a sphere then they will be defined by the following parameters.

Each of the spheres is defined by the equation z = z₀ -^R² - (x -x₀)² - (y -y₀)² (6) where x is in the range J , < x≤ x₂ , y is in the range y_l≤ y≤ y₂ , R=2 is the radius of the sphere, and the center point of the sphere is (J ₀ , ^₀ , Z₀) . For the primary mirror on the right side of the photovoltaic panel x_l=\,x₂=2.5, y ⁼¹.½ = ²-⁵ ₀=*i⁼¹> yo=y\=^l and ^zo=^R-

Turning to Fig. 9A, there can be seen a dual axis low concentration photovoltaic solar tracking system 900 with primary mirrors 904 disposed along the sides of a photovoltaic panel 902, in accordance with an embodiment of the present invention.

The panel 902 and mirror units 904 are mounted on a dual axis low concentration photovoltaic solar tracking system mechanical support 908.

Additionally, each primary mirror unit 904 has a secondary mirror unit 906, of a long and thin rectangular shape mounted above it and supported by its own secondary mirror units' mechanical support 912.

The system 900 is similar to system 700 of Fig 7, with the following difference. The primary mirrors are not sections of paraboloid mirrors or spheres but rather sections of parabolic troughs and cylinders. The focus of the system 900 is a line focus and not a point focus like the previous systems.

According to one embodiment, this system can be described by the following parameters:

The photovoltaic panel is a rectangle with corners at: (-a,-b), (a,-b) (a,b) (-a,b) . In the example of Fig. 9 A the rectangle is a square with a=b=\

The parabolic troughs on the right side and the left side are defined by the equation o ₄ where x is in the range x_x≤ x≤ x₂ , y is in the range y_l≤ y≤ y₂,f is the focal height of the paraboloid. For the primary mirror on the right side of the photovoltaic panel x_l=\,x₂=2.5, y_x = -l,y₂ = 1 , x₀=x_l=l. The focal line of the right hand side parabolic trough is along the line (l,y,/ + z₀). The secondary mirror corresponding to the right hand side primary mirror is of dimensions approximately 0.2x2 meters and is mounted along the focal line.

If the primary mirrors are a section of a cylinder then they will be defined by the following parameters.

The cylinders on the right and left hand sides are defined by the equation

where x is in the range x_l≤ x≤ x₂ , y is in the range y_x≤y≤y₂ , R=2f is the radius of the cylinder, and the center line of the cylinder is (x₀,y,z₀) . The secondary mirror corresponding to the right hand side primary mirror is of dimensions approximately 0.2x2 meters and is mounted along the focal line.

For the primary mirror on the right side of the photovoltaic panel JC_| = l,x₂ = 2.5, y_x = \,y₂ = 1 , x₀ = x = \ and z₀ = R .

The other primary and secondary mirrors are quite obviously defined from the right hand side mirrors.

Reference is now made to Fig. 9B, which is a ray diagram of the incident sun rays 922 onto primary mirrors 904 reflected onto secondary mirrors 906 of the system of Fig. 9A..

Primary mirror 904 comprises a plurality of rectangles 934 (may be real or virtual). The sun 920 emits incident rays 922 which impinge onto rectangle 934 of a specific primary mirror 904. Incident rays 922 are reflected as first reflected rays 924 from rectangle 934 by the primary mirrors onto a corresponding area 936, such as a rectangle of a corresponding secondary mirror 906 as can be seen in Fig.

9B.

Reference is now made to Fig. 9C, which is a ray diagram of the first reflected sun rays 924 from secondary mirrors 906 and onto photovoltaic panels 902 of the system of Fig. 9A.. The first reflected rays 924 are reflected as secondary reflected rays 926 onto photovoltaic panel 902, such that area 936 of the secondary mirror is reflected to a corresponding area 932 of the photovoltaic panel 902.

Reference is now made to Fig. 10, which is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system 1000 with mirrors 1004 disposed along a perimeter a photovoltaic panel 1002, in accordance with an embodiment of the present invention.

The panel 1002 and mirror units 1004 are mounted on a dual axis low concentration photovoltaic solar tracking system mechanical support 1008.

Additionally, each primary mirror unit 1004 has a secondary mirror unit 1006, of a long and thin rectangular shape mounted above it and supported by its own secondary mirror units' mechanical support 1012.

The system 1000 of Fig. 10 is a combination of systems 800 and 900. It has 4 primary mirrors along the edges, which are parabolic trough or cylinder sections, and 4 primary mirrors along the corners which are paraboloid or sphere sections.

Fig. 11 is a graphical display of a simulated calculation of radiation distribution against a two dimensional photovoltaic surface, based on the geometry of the system of Fig. 10, in accordance with an embodiment of the present invention.

The system 1000 of Fig. 10 collects sun radiation from a square of 5><5 meters (2x2 meters photovoltaic panels surrounded by the mirrors whose dimensions are given above). Altogether the total receiving area is 25 m², while the photovoltaic panel size is 4 m². In order to make use of all of the sun's radiation it is therefore required that the radiation on the photovoltaic panel from both the direct radiation and the reflected radiation will be approximately 6 times the direct radiation, (1 for direct + 5 for reflected radiation). Fig. 11 plots the reflected radiation from the mirrors upon the photovoltaic panel, and it can be seen that this radiation distribution is homogeneous and approaches the value of 5 times the direct radiation. In generating the simulation perfect mirrors were assumed with 100% reflection and no losses. In practice -10% loss may occur in the reflected radiation.

Thermal Issues

One of the major issues that concentrating systems must deal with is the issue of heat.

Silicon PV degrades significantly with increase in temperature, so the benefits from concentrating radiation may be overcome by the loss in conversion efficiency

For a system built according to the present invention, heat is generated at two points:

a) On the PV

b) At the focal point (or focal line)

The heat at the focal point (line) causes no harm or degradation to the system's performance, but rather it may be used to generate even more output from the system either in the form of hot water or additional electricity.

Another aspect of the present invention allows for preventing heating of the

PV array. This may be done by setting the secondary mirrors at the focal points (or focal lines) to be selective mirrors, which will absorb the unwanted parts of the spectrum (e.g. IR), and pass only those parts of the spectrum, which can be converted efficiently to electricity. This will prevent heating of the PV, at the expense of heating the secondary mirrors. However, as explained above the secondary mirrors can withstand high temperatures. Moreover, the excessive heat at the secondary mirrors may be used to generate additional (thermal) energy. The added cost of using selective mirrors is marginal, since according to the present invention the area of the secondary mirrors is small. In contrast, a V-shaped system as the system 300 of Fig. 3 which will use selective mirrors will have to use the entire mirror as a selective mirror and this will increase the cost significantly.

Another aspect of the present invention allows for cooling of the photovoltaic panel by letting natural airflow beneath the photovoltaic panel. This is done by lowering the primary and secondary mirrors of the systems presented above (Figs 5-10).

According to this aspect of the invention, the primary mirrors act as a collector area for the wind blowing on them, and the wind can blow across the primary mirrors and beneath the PV array, thus cooling the PV array. Additional tracks may be added in order to generate a spiral effect and cause the wind to blow in a circular pattern beneath the PV array.

Optionally, it may be possible to use this wind for the generation of additional electricity.

Another advantage of this configuration is that the focal points for this configuration are lower than for the other configurations, and this can simplify the mechanic design of the system.

This aspect of the invention is illustrated in Fig 12, which is a system similar to the system 900 of Fig. 9A, but with the primary and secondary mirrors lowered, so that a gap 1214 is generated.

Fig. 12 is another simplified schematic illustration showing a dual axis low concentration photovoltaic solar tracking system 1200 with primary mirrors 1204 disposed below the plane of a photovoltaic panel 1202, in accordance with an embodiment of the present invention.

The mirror units 1204 are mounted on a dual axis low concentration photovoltaic solar tracking system mechanical support 1208, below the level of the panel 1202, thereby forming a gap 1214. According to some embodiments, this gap may be a) vertical only; b) horizontal only; and c) vertical and horizontal. Additionally, each primary mirror unit 1204 has a secondary mirror unit 1206, of a long and thin rectangular shape mounted above it and supported by its own secondary mirror units' mechanical support 1212.

The system shown in Fig. 12 allows for cooling of the photovoltaic panel 1202 (or PV array), by allowing, or even guiding, wind to flow on the PV panel (on a lower and/or upper side thereof). According to this embodiment, system 1200 deploys the primary mirrors below the PV panel. According to another aspect of the present invention the gap 1214 can be not only in the vertical direction but also in the horizontal direction. According to this aspect of the invention the secondary mirrors would be deployed on top of the horizontal gap, and thus will avoid shading the primary mirrors or the photovoltaic panels. A view from above onto one photovoltaic panel, one primary mirror and one secondary mirror deployed according to this aspect is illustrated in Fig. 13.

Reference is now made to Fig. 13, which is another simplified schematic illustration showing an optical element of a dual axis low concentration photovoltaic solar tracking system 1300, the optical element comprising a first gap 1316 between a primary mirror unit 1304 and a secondary mirror unit 1306, and a second gap 1314 between the secondary mirror unit and a photovoltaic panel 1302, in accordance with an embodiment of the present invention.

The references cited herein teach many principles that are applicable to the present invention. Therefore the full contents of these publications are incorporated by reference herein where appropriate for teachings of additional or alternative details, features and/or technical background.

It is to be understood that the invention is not limited in its application to the details set forth in the description contained herein or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Those skilled in the art will readily appreciate that various modifications and changes can be applied to the embodiments of the invention as hereinbefore described without departing from its scope, defined in and by the appended claims.

Claims

1. A combined photovoltaic-optical system for generating electricity from solar radiation, the system comprising:

a. at least one optical arrangement for concentrating solar radiation, the optical arrangement comprising at least one:

i. a primary mirror adapted to receive incident solar radiation, the primary mirror disposed at least one of a) along a side of a photovoltaic panel and b) at a corner of said photovoltaic panel; and

ii. a secondary mirror disposed in at least one of:

a) a vicinity of a focal point focal;

b) a vicinity of a focal line; and

c) a vicinity of a focal plane;

of said primary mirror, the secondary mirror adapted to highly concentrate reflected solar radiation received from said primary mirror and to convey said highly concentrated solar radiation as homogenously concentrated solar radiation to said photovoltaic panel; and

b. said photovoltaic panel for generating electricity from both:

i. said homogenously concentrated solar radiation received from said secondary mirrors; and ii. direct solar radiation received from the sun.

2. A combined photovoltaic-optical system according to claim 1, further comprising a solar tracking system for moving said at least one optical arrangement along at least one axis to track the movement of the sun.

3. A combined photovoltaic-optical system according to claim 2, wherein said solar tracking system is constructed to retain the at least one primary mirror perpendicularly to the sun.

4. A combined photovoltaic-optical system according to claim 1 , wherein said at least one optical arrangement is adapted to concentrate said solar radiation by a factor of at least two.

5. A combined photovoltaic-optical system according to claim 4, wherein said at least one optical arrangement comprises at least four primary mirrors.

6. A combined photovoltaic-optical system according to claim 5, wherein said at least one optical arrangement comprises at least four secondary mirrors.

7. A combined photovoltaic-optical system according to claim 6, wherein said at least one optical arrangement comprises at least twelve primary mirrors.

8. A combined photovoltaic-optical system according to claim 7, wherein said at least one optical arrangement comprises at least twelve secondary mirrors.

9. A combined photovoltaic-optical system according to claim 8, wherein said factor is at least three.

10. A combined photovoltaic-optical system according to claim 9, wherein said factor is at least five.

11. A combined photovoltaic-optical system according to claim 5, wherein said at least four primary mirrors are disposed such that a central point of all said at least four primary mirrors fall in a common plane.

12. A combined photovoltaic-optical system according to claim 1 , wherein said photovoltaic panel is disposed to receive light orthogonally from the sun.

13. A combined photovoltaic-optical system according to claim 1, wherein said secondary mirrors are adapted to convey said homogenously concentrated solar radiation towards said photovoltaic panel with a homogeneity over a receiving surface of said photovoltaic panel of at least 90%.

14. A combined photovoltaic-optical system according to claim 13, wherein said homogeneity is at least 95%.

15. A combined photovoltaic-optical system according to claim 1, wherein a utilization of said primary mirror is at least 90%.

16. A combined photovoltaic-optical system according to claim 15, wherein said utilization of said primary mirror is at least 95%.

17. A combined photovoltaic-optical system according to claim 1, wherein said secondary mirror is a selective mirror designed to pass only a part of a spectrum of said solar radiation to said photovoltaic panel.

18. A combined photovoltaic-optical system according to claim 1, wherein a distance from said primary mirror to said secondary mirror is approximately the same as a length of a side of said primary mirror.

19. A combined photovoltaic-optical system according to claim 1 , wherein said primary mirror is a section of a parabaloid.

20. A combined photovoltaic-optical system according to claim 1, wherein said primary mirror is formed from a section of a sphere.

21. A combined photovoltaic-optical system according to claim 1, wherein said primary mirror is formed as a section of a parabolic trough.

22. A combined photovoltaic-optical system according to claim 1, wherein said primary mirror is formed as a section of a cylinder.

23. A combined photovoltaic-optical system according to claim 1, wherein said secondary mirror has a surface area of less than 10% of said corresponding primary mirror.

24. A combined photovoltaic-optical system according to claim 23, wherein said secondary mirror has a surface area of less than 5% of said corresponding primary mirror.

25. A combined photovoltaic-optical system according to claim 1, wherein said primary mirror is disposed in a plane that is parallel to said photovoltaic panel.

26. A combined photovoltaic-optical system according to claim 1, wherein said primary mirror is disposed below the plane of said photovoltaic panel, thereby forming an air gap between said primary mirror and said photovoltaic panel.

27. A combined photovoltaic-optical system according to claim 1, wherein said secondary mirror is disposed above a corner of said photovoltaic panel.

28. A combined photovoltaic-optical system according to claim 1, wherein said secondary mirror is disposed above and along a side of said photovoltaic panel.

29. A method for generating electricity from solar radiation, the method comprising:

b) passing said highly concentrated solar radiation from said second reflective surface to a photovoltaic panel thereby generating homogeneous concentrated radiation; and

c) converting both:

i. said homogeneous concentrated radiation; and

ii. direct solar radiation received from the sun on said photovoltaic panel;

into electricity at said photovoltaic panel.

30. A method according to claim 29, wherein said homogeneous concentrated radiation is concentrated by at least three times the received solar radiation.

31. A method according to claim 30, wherein said homogeneous concentrated radiation is concentrated by at least five times the direct solar radiation.

32. A method according to claim 30, wherein said homogeneous concentrated radiation has a homogeneity over a receiving surface of said photovoltaic pane; of at least 90%.

33. A method according to claim 32, wherein said homogeneity is at least 95%.

34. A method for generating electricity from solar radiation, the method comprising:

a) placing a combined photovoltaic-optical system according to any of claims 1 to 28 at an outdoor location such that said primary mirror and said photovoltaic panel are disposed in a direct path from the sun to receive the solar radiation; and

b) generating electricity from said solar radiation employing said system.

35. A method according to claim 34, wherein said photovoltaic panel receives concentrated solar radiation from said at least one secondary mirror with a homogeneity of at least 90% over said photovoltaic panel.