Solar eyeball: an automatic sun-tracking concentrator for use with photovoltaic generators

Abstract

The paper describes a solar-electric conversion module which uses a plastic Fresnel lens to concentrate sunlight on to a group of GaAs/GaAlAs solar cells. These cells have given 16?20% conversion efficiency and are expected to maintain this at the high intensities and temperatures involved. The necessary ability to track the sun is achieved using a novel pneumatic system powered by solar energy. When the eyeball is misaligned, the sun's image falls on a heat exchanger in one of two air reservoirs adjacent to the cells. The resultant expansion forces a magnetised piston against a fixed external magnetic field, causing the complete module to rotate until the solar cells are again in the focus. There need be no mechanical linkage with the external world apart from electrical output leads, and hence the module can be hermetically sealed and floated on water to provide a cheap, reliable, low-friction bearing. No motors or clockwork drivers are needed. Several experimental 1-axis modules have been made, either floating on water or using conventional bearings. All demonstrated the required `seek? and `hold? capabilities, finding the sun in 2?10s. Work on a damping mechanism to prevent overshoot, and tests of the slow-tracking accuracy, are presently under way. An early 2-axis model on a demountable test bed has also been demonstrated, using four air chambers. The paper reports details of the magnetic piston, the optics of the system, and the thermodynamics of the heat exchangers, and an estimate of eventual costs.