NOVELTY - Front and back involuted shaped parts in the bottom of the reflector are of equal length and end at points on the same imaginary line extending at an angle to a horizontal plane.    DETAILED DESCRIPTION - Radiation from the sun is reflected by an internal surface in a channel-shaped reflector (1) towards an elongated absorbator (2) lying inside the channel, which is supported across its width in the vertical direction. The reflector cross-section has a part (4) with a shape corresponding to sector of a parabola according to the equation y2 + 4 px, in an orthogonal coordinate system where x is a symmetry axis for the parabola. The parabolic part forms a top section of the reflector with the parabola focal point and absorbator in the lower part of this section, and the symmetry axis for this section slopes upwards and forwards. A bottom section of the reflector is configured as two opposing involuted shapes, comprising a front involuted part (5) extending in a forwards direction from the region below the absorbator and ending at a bottom edge (8) of the reflector, and a back involuted part (6) extending in a backwards direction from the region below the absorbato r and ending at a point (9) in the parabolic part. The front and back involuted parts have the same length and end at points (8, 9) on an imaginary line sloping upwards and forwards from the cell, forming an angle of 10-40degC in relation to a horizontal plane.    ADVANTAGE -   A good compromise is achieved between minimising absorbator size, ensuring that a large amount of radiation from the sun is reflected towards the absorbator during shifting radiation angles, and keeping cell thickness to a minimum.    DESCRIPTION OF DRAWING(S) - Figure 1 shows a cross-section through the cell. (1) Channel-shaped reflector; (2) Absorbator; (3) Transparent glass or plastic panel; (4) Upper parabolic shaped reflector part; (5, 6) Front and back involuted shaped parts of reflector; (7) Imaginary line; (8) Bottom reflector edge; (9) Point on parabolic part of reflector; (10) Top reflector edge.