(US4649900-A)      The apparatus includes a reflective parabolic trough which tracks the sun the control means being such that the trough rotates about a north-south axis alternately to east and west extreme positions until the driving motor is disabled by equal irradiation of a pair of phototransistors. The phototransistors are aligned relative to the axis of the trough such that they are equally irradiated when the suns rays are concentrated on a heat transfer tube extending along the axis of the trough.    The apparatus has flexible disc drive member to mechanically couple the trough to the drive motor. The heat transfer conduit is of relatively small capacity and a two-phase flow is utilised to conserve energy. (15pp)    (US4807592-A)      The solar energy conversion appts. includes a reflective parabolic trough which tracks the sun. The trough rotates about a north-south axis alternately to east and west extreme positions until the driving motor is disabled by equal irradiation of a pair of phototransistors. The phototransistors are aligned relative to the axis of the trough so that they are equally irradiated when the sun's rays are concentrated on a heat transfer tube extending along the axis of the trough.    The appts. has a flexible disc drive member to mechanically couple the trough to the drive motor.    ADVANTAGE -   Two phone flow used to conserve energy. (14pp)p    (US4821705-A)      The solar energy conversion apparatus includes a reflective parabolic trough which tracks the sun the controller for it being such that the trough rotates about a north-south axis alternately to east and west extreme positions until the driving motor is disabled by equal irradiation of a pair of phototransistors. The phototransistors are aligned relative to the axis of the trough such that they are equally irradiated when the sun's rays are concentrated on a heat transfer tube extending along the axis of the trough.    The apparatus has a flexible disc drive member to mechanically couple the trough to the drive motor. The heat transfer conduit is of relatively small capacity and a two-phase flow in it is utilised so as to conserve energy. (14pp)