This application is a Continuation-In-Part of U.S. patent application Ser. No. 07/898,160 filed Jun. 15, 1992, U.S. Pat. No. 5,286,305. 
    
    
     FIELD OF THE INVENTION 
     The invention describes a platform which is supported by a water layer, experiences a daily rotation about the vertical axis at the angular velocity of the sun and is rotated back during the night. To follow the solar altitude, the incident radiation is refracted towards the vertical to such an extent that all refracted sunrays strike the radiation converter. 
     PRIOR ART 
     Solar energy converters which are arranged on a body of water and track the azimuth are known. Their disadvantage is that the concentrators have to be oriented according to the particular solar altitude, which entails considerable mechanical effort. A second disadvantage is that the pivotable concentrators give rise to high drags so that wind forces cause the total installation to vibrate. In addition, all large components exposed to wind forces must be constructed using an appropriate quantity of material. A third disadvantage is that concentrators placed one behind the other viewed in the direction of the sun must be installed such a large distance apart that they do not cast shadows on one another. 
     SUMMARY OF THE INVENTION 
     The object of the invention is the realization of two axis tracking in the case of a platform whose surface presents no end surfaces to the wind and in which furthermore the entire area of the platform is used as an aperture area owing to the absence of mutual shading. 
     In order to realize this, the invention envisages a platform having a metallic base which has parallel channels whose lower surfaces extend close to the base of a water tank and which carry the photocells, so that the heat loss is passed through the base into the water and in particular into the water volume located at a higher level. The sun&#39;s rays are guided over as large a solar elevation interval as possible by a cylindrical lens system which preferably consists of several lens arrangements located one underneath the other. The invention gives preference to a system in which triangular prismatic channels whose non-transmitting boundary faces are mirrored are formed between the lens arrangements. These lens arrangements substituting mechanical tracking of the solar elevation and referred to below as tracking lenses are arranged above a concentrating Fresnel lens. An advantageous embodiment envisages the integration of a tracking lens with a Fresnel lens. The invention furthermore relates to means which ensure that the photocells are irradiated as uniformly as possible. As a first measure, the photocells float in troughs and follow the height movement of a focal line. In a more advantageous solution, secondary lenses are used for compensation of the shift in height. It proves to be advantageous if the radiation is guided in such a way that the outward-facing layer of the horizontally oriented lens arrangement has an upward-facing first, smooth boundary face and possesses, on the downward-facing side, a group of secondary boundary faces divided into steps, the boundary faces of the steps making an angle with the first boundary face such that, optionally in cooperation with further lens arrangements, a sunray which makes an angle of more than 60° with the vertical is refracted to give an emerging ray which makes an angle of more than 110° with the direction of incidence of the sunray and at the same time and, simultaneously with the vertical, makes an angle of less than 30° with a limb opposite the direction of incidence, while a sunray whose angle of incidence relative to the vertical is less than 20° is refracted to give an emerging ray which points towards the sun and whose limb points towards the sun and which makes an angle of less than 30° with the vertical, so that the emerging rays are concentrated onto a radiation converter. In the most advantageous solution, the angles of incidence have equal magnitudes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically shows a platform having an edge zone. 
     FIG. 2 shows a cut-out II from FIG. 1, in vertical section. 
     FIG. 3 shows a perspective view of a lens arrangement. 
     FIG. 4 shows the beam path in a refined version. 
     FIGS. 5a and 5b show a flat lens system with facets. 
     FIG. 6 shows a flat lens with primary and secondary steps. 
     FIG. 7 schematically shows the paths of curved steps. 
     FIG. 8a shows the beam path in a secondary lens cut vertically and at right angles to the direction of the sun. 
     FIG. 8b shows the beam path in the same secondary lens with lateral shifting of a light beam at right angles. 
     FIG. 8c shows the beam path in the same secondary lens with lateral shifting of an oblique light beam. 
     FIG. 9 shows an embodiment whose cross-section is composed of trapezoids lying one on top of the other. 
     FIG. 10 shows a secondary lens which consists of a lens which converges in the outer regions and diverges in the inner region and is supported by mirrored walls. 
     FIG. 11 schematically shows a cross-section through channels in which photocells float. 
     FIG. 12 shows a secondary lens for compensation of lateral shifting of the light beam by displacement of a component. 
     FIGS. 13a, b, c show a comparison of the deflections for different solar heights. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically shows the solar power station according to the invention, having a platform a, rotatable about the vertical axis d, and a frame b which floats in a channel c. 
     FIG. 2 shows a horizontal section of the cut-out II in FIG. 1. The energy conversion is effected on a circular platform a which is supported by a thin water layer 2 It is divided into, parallel concentrator tunnels 5. A toroidal pipe 6 surrounding the platform forms a frame for a network which keeps the concentrator tunnels an exact distance apart and which transmits the rotary movement imposed on the toroidal pipe 6 to the platform. The network is formed from sheet metal sections 7 and steel cables 8 and is kept a constant distance from metal base sheets 1 by means of thin-walled flat tubes 9. A channel 10 in which the photocells 4 are located runs along the central line of each concentrator tunnel 5. The cover consists of concentrator discs 3 which not only concentrate the sun&#39;s rays in the manner of a cylindrical Fresnel lens onto focal lines but also refract the resulting light beams downwards. The photocells 4 are located between strand-like secondary lenses 12 and the bottom of the profile channel 10. The platform is centered by the toroidal pipe 6 which rolls along the annular wall 21 on casters 13, and is rotated about the vertical axis d at the angular velocity of the sun, so that during the day the photocells, controlled by known means, always follow the azimuth of the sun. During the night, the platform is rotated back to the starting position. The secondary lens 12 guides the light beam, independently of the level of the particular focal line, to the photocell 4 which covers the underneath of the secondary lens. A light beam 15 striking vertically is shown in the concentrator tunnel 5. The gap between the toroidal pipe 6 and the wall 16 is bridged by a contacting film 17. In addition, a film 18 is clamped between the toroidal pipe 6 and the adjacent energy tunnel so that evaporation of water is prevented. The heat dissipation of the photocells 4, unless already transferred during the hours of sunlight, is passed into the water layer 2 which is separated by a film from the ground 19. The energy stored by the water is emitted 24 hours a day by convection and infrared radiation. The electricity generated by the platform is passed via flexible conductors into an earth cable leading to the center. Rainwater passes through the flat tubes 9 into the water layer 2, from which excess water can flow away. 
     FIG. 3 shows a perspective view of a lens arrangement which consists of an upper layer 30a having steps 30e and a layer 30b underneath having steps 32a and 32b, which enclose channels 31 between them. A Fresnel lens 33 whose steps 34 are at right angles to the steps 32 is located below this stepped lens 30a, 30b. The layers 30a and 30b have vertical channels 37 into which sheet metal strips 38 are inserted so that the layers cannot move towards one another. The flanks 39a and 39b are preferably mirrored. For regions with sand storms several layers 35 of an extremely thin film are fastened on the outside of the lens arrangement. If the surface of the uppermost layer 36 in each case is scratched, the said layer is removed. It has proved advantageous to bend the edge 30c downwards so that the line 30d is curved, resulting in curved roof elements according to FIG. 2. This applies correspondingly for all flat lens variants. 
     FIG. 4 shows a cross-section through a lens arrangement 30 with the rays of the sun at 20° and 80° elevation. While the rays 40a emerge as ray 40b after passing four times through boundary faces, said rays making an angle of 18° with the vertical 54, the ray 41a must be reflected at the flank 45b. For this reason, the flanks 45a and 45b are mirrored. After passing twice through boundary faces, the ray 42a passes as ray 42c through the channel 46 and emerges as ray 42b at the same angle to the vertical as the ray 40b but with opposite sign. After double refraction, the morning ray 41a with a very obtuse angle of incidence enters the channel 46 as ray and is then reflected to become ray 41d. This ray 41d is parallel to the ray 42c, so that 41b, too, emerges at an angle of +18° to the vertical 54, as in the case of ray 42b. All sunrays striking between the limits of the morning incidence of 20° and the midday incidence of 80° lead to emerging beams which lie within the interval 40b-41b of the angles of emergence. After triple refraction, the ray 43a strikes the flank 45b, undergoes total reflection there and becomes ray 43e. It then emerges as ray 43f and is reflected at the mirrored flank region 48a so that it emerges as ray 43b within the allowed angular interval. All 80° rays between the rays 44a 1  and 44a 2  would emerge outside the allowed angular interval if the flanks 45a and 45b were to merge with one another. To prevent loss of the rays, the channel 46 having the faces 46a and 46b through which rays pass is bordered by a channel 47 which has the faces 47a and 47b through which rays pass and whose walls through which rays pass guide the rays 44a 1  and 44a 2 , optionally in combination with a reflection at the upper mirrored strip 48a of the flank 48a, 48b, in such a way that they emerge in the allowed interval. The rays between the rays 45a 3  and 45a 4  undergo total reflection at the flank 48b and then emerge within the allowed angular interval. The ray 44a 5  emerges as ray 44b with a negative angle, and the subsequent rays up to 44a 6  undergo total reflection at the flank 45b and emerge with a positive angle in the allowed angular range, as does the ray 43b. All rays to the right thereof pass through the cylindrical lens with a positive angle, as in the case of the ray 42a. 
     FIG. 5a schematically shows an embodiment of the lens arrangement according to FIGS. 3 and 4, in which the downward-pointing steps 32 are combined with the perpendicular steps 34 of the Fresnel lens underneath to give faceted lenses. The flat upper surface 50 pointing upwards forms the incident surface. In the planes at right angles to the incident sunlight 53, the rectangular regions 51 make the angles x 1 , x 2 , x 3 , . . . with the vertical 54, which angles become 90° in the region of the symmetry line 52. However, the rectangular regions 51 pointing towards the photocell are simultaneously inclined at the constant angle y to the vertical 54, in planes containing the sunrays 53. This gives rise to light beams which are simultaneously refracted towards the vertical 54. The planes of the flanks 55a, 55b, etc. intersect one another close to the focal line, in order to prevent shading of the emerging radiation of the relevant subsequent step. The flanks 56 at right angles to the symmetry line 52 are all in planes which make the constant angle w according to FIG. 4 with the vertical 54. 
     In FIG. 5b, the rays 57a and 57b form the peripheral rays of the light beam produced at the elevation angle u of the sunrays 58. The peripheral rays 59a and 59b, which are assigned an elevation angle of 60°, are in a vertical plane. 
     FIG. 6a shows another embodiment of a lens according to the invention which has steps 60 at right angles to the plane of incidence, so that sunrays 61 having a very obtuse angle of incidence are refracted towards the vertical 54. The rays having a very acute angle of incidence are refracted towards the sun side. The surfaces of the steps 60 are in turn provided with steps 63 of substantially finer division. 
     FIG. 6b shows the steps in the framed region VI on a larger scale. The steps 64, 65, 66 have prism angles which point towards the symmetry line 67 and become zero at the height of the symmetry line. These steps effect the concentration. 
     FIG. 7 shows a flat lens viewed from below. Steps 71 whose prism angles assume zero value at the height of the vertex line 70 and increase on both sides with increasing distance from the vertex line 70 run symmetrically to the vertex line 70 along virtually parabolic lines. Each viewing element of the steps 71 which is a distance away from the vertex line 70 has a wedge angle which, in conjunction with the angle which the tangent to the viewing element makes with the vertex line 70, refracts a sunray passing through from the (invisible) upper side optically skew to a focal line. 
     FIG. 8a shows the cross-section of a cylindrical lens 80 which is in optical contact with the photocell 81. This contact is achieved by introducing a wetting immersion liquid or an optical cement 82 having a tailored refractive index. The geometry meets the requirement that the solar cell 81 should be illuminated with virtually identical luminous intensity for all existing light beams having the peripheral rays 87a and 87b. This is achieved by a cylindrical lens whose cross-sectional shape is trapezoidal in the lower part and corresponds to an aspherical plano-convex cylindrical lens in the upper part. In a preferred embodiment, the incident surface 83 has an elliptical sectional curve. The lateral surfaces 84 are likewise in the form of optical functional surfaces, so that total reflection takes place where required. The focal lines 85a, which would be located below the photocell 81, are produced above the photocells 81 owing to the shape of the incident surface, so that the focal line does not migrate through the cell at any solar elevation. The migration of the focal line would assume the magnitude 85b without a secondary lens. By means of the secondary lens, the focal line is positioned in such a way that, when the photocell 81 is suitably coordinated with the incident surface 83, an essentially uniform distribution of the luminous intensity over the cell width is achieved, corresponding to blurred focusing of the focal lines onto the photocells. 
     Where there is a lateral shift between the subsequent lens and the secondary lens owing to interfering forces when the power station is being operated, the incident surface 83 effects an optical compensation of the shifting of the focal line. While in the case of perpendicular light beams having the peripheral rays 87a the outer rays are caused to converge, the rays of an oblique light beam having the peripheral rays 87b experience divergent refraction. 
     FIG. 8b shows the refracted rays 88a of the vertically incident light beam having the peripheral rays 87a, the secondary lens 80 being shifted laterally relative to the light beam by a magnitude 89. 
     FIG. 8c shows the beam path for the oblique position of the light beams 122 and 126 having the peripheral rays 87c in the case of a lateral shifting by the magnitude 89, said oblique position being shown in FIGS. 13a and 13c. 
     FIG. 9 shows a further embodiment of a secondary lens which is associated with the photocell 94 and whose effect corresponds to that of the secondary lens of FIG. 8. Considerable material is saved according to the smaller cross-sectional area, and less attenuation is achieved as a result of the shorter light paths for the peripheral rays. The embodiment of the incident surface 83, described in FIG. 8, is divided into the areas 90, 91, 92 and 93. Here too, all lateral surfaces are designed so that total reflection of the rays which may be incident from inside is ensured. 
     FIG. 10 shows a secondary lens whose outer regions 103a and 103b effect convergent refraction of the outer regions of a light beam which is in a vertical plane, whereas they effect divergent refraction, through the middle region 104, of these rays of a light beam in an oblique plane. The lateral walls 101 are mirrored, with the result that even horizontally shifted rays are reflected to the photocell 102. If the lens region is produced from organic glass, the space 105 underneath, including walls 101, can be produced integrally with the lens region. 
     FIG. 11 shows a cross-section of a concentrator tunnel 115 having a channel 110 in which a pontoon 111 floats on a body 116 of water and holds the photocell 112. The height of the body 116 of water in the channel determines the distance of the photocell 112 from the flat lens 114. 
     FIG. 12 shows a secondary lens 116 which is arranged so that it can be displaced laterally relative to the photocell 112a by the magnitude 117a and which effects the shift by a maximum magnitude 117a by means of an apparatus which is not shown, as a function of the lateral shift of the light beam relative to the photocell 112a. As a result, the light beam having the peripheral rays 118a is guided towards the photocell 112a even when these peripheral rays have shifted up to the distance 117 from the symmetry line 117 into the position 118b. 
     FIG. 13a shows a light beam which is produced by the flat lenses described at the outset. The greatest refraction about axes 120 at right angles to the focal line 121 takes place in the early morning and late afternoon. The refracted light beam 122 makes an angle of about -18° with the vertical. In this oblique position of the light beam, the focal line 121 has the greatest height. 
     FIG. 13b shows that the light beam 123 is perpendicular twice a day, in each case when the solar elevation is about 60°. The theoretical focal line 124 is then below the photocell 112 which collects the total concentrated radiation. It is even below the channel 110 and thus reaches the lowest level. 
     As shown in FIG. 13c, the noon sunrays 125 are refracted about the axis 120 at right angles to the focal line towards the sun, so that the light beams 126 lie in planes which make an angle of +18° with the vertical. The focal line 127 once again reaches the same height as the focal line 121 does in the early morning and late afternoon.
In a device designed as a rotatably mounted platform (a) for recovering solar electricity, the focusing roof layer (3) further deflects the incident radiation so that the light beams (122, 123, 126) formed by the concentrating optical system are incident approximately perpendicularly on the radiation converter (112) arranged underneath.