A multi-channel thermal regenerator for a heat engine comprising a coiled annulus of thin high-temperature sheet material coiled so that individual coils are in spaced substantially parallel arrangement in an annular configuration. The lower portion of the belt is adapted to be immersed in the water in a swimming pool for skimming debris floating on the surface of the water. The light in the two paths is detected by a photodetector and the output electrical signals are log amplified.