(under subclass 573) Apparatus wherein the sensing means includes a device which detects change in the volume of a solid material. [figure] [caption]A solar energy conversion device is provided with simplified means for tracking a source of solar energy. In the preferred embodiment, the tracking means is a predeterminedly shaped, heat expandable device operatively connected to an energy concentrator. The energy concentrator is movably mounted and is adapted to direct concentrated energy either towards a suitable energy conversion means in those positions in which the concentration is aligned with respect to the solar energy source, or towards the expandable device in those positions in which the concentrator is misaligned with respect to the solar energy source. Application of concentrated solar energy to the expandable device causes its expansion, which expansion is utilized to move the concentrator into alignment with the energy source. At alignment, concentrated energy is directed from the expandable device toward the energy conversion means.
