A heliostat (from Ancient Greek ἥλιος (hḗlios) 'sun' and στατός (statós) 'standing') is a device that reflects sunlight toward a target, turning to compensate for the Sun's apparent motion. The reflector is usually a plane mirror. The target may be a physical object, distant from the heliostat, or a direction in space. To do this, the reflective surface of the mirror is kept perpendicular to the bisector of the angle between the directions of the Sun and the target as seen from the mirror. In almost every case, the target is stationary relative to the heliostat, so the light is reflected in a fixed direction. According to contemporary sources the heliostata, as it was called at first, was invented by Willem 's Gravesande (1688–1742). Other contenders are Giovanni Alfonso Borelli (1608–1679) and Daniel Gabriel Fahrenheit (1686–1736). A heliostat designed by George Johnstone Storey is in the Science Museum Group collection. Currently, most heliostats are used for daylighting or for the production of concentrated solar power, usually to generate electricity. They are also sometimes used in solar cooking. A few are used experimentally to reflect motionless beams of sunlight into solar telescopes. Before the availability of lasers and other electric lights, heliostats were widely used to produce intense, stationary beams of light for scientific and other purposes. Most modern heliostats are controlled by computers. The computer is given the latitude and longitude of the heliostat's position on the Earth and the time and date. From these, using astronomical theory, it calculates the direction of the Sun as seen from the mirror, e.g. its compass bearing and angle of elevation. Then, given the direction of the target, the computer calculates the direction of the required angle-bisector, and sends control signals to motors, often stepper motors, so they turn the mirror to the correct alignment. This sequence of operations is repeated frequently to keep the mirror properly oriented. Large installations such as solar-thermal power stations include fields of heliostats comprising many mirrors. Usually, all the mirrors in such a field are controlled by a single computer. There are older types of heliostat which do not use computers, including ones that are partly or wholly operated by hand or by clockwork, or are controlled by light-sensors. These are now quite rare. Heliostats should be distinguished from solar trackers or sun-trackers that point directly at the sun in the sky. However, some older types of heliostat incorporate solar trackers, together with additional components to bisect the sun-mirror-target angle. A siderostat is a similar device which is designed to follow a fainter star, rather than the Sun.
Large-scale projects In a solar-thermal power plant, like those of The Solar Project or the PS10 plant in Spain, a wide field of heliostats focuses the Sun's power onto a single collector to heat a medium such as water or molten salt. The medium travels through a heat exchanger to heat water, produce steam, and then generate electricity through a steam turbine. A somewhat different arrangement of heliostats in a field is used at experimental solar furnaces, such as the one at Odeillo, in France. All the heliostat mirrors send accurately parallel beams of light into a large paraboloidal reflector which brings them to a precise focus. The mirrors have to be located close enough to the axis of the paraboloid to reflect sunlight into it along lines parallel to the axis, so the field of heliostats has to be narrow. A closed loop control system is used. Sensors determine if any of the heliostats is slightly misaligned. If so, they send signals to correct it. It has been proposed that the high temperatures generated could be used to split water producing hydrogen sustainably.
Small-scale projects Smaller heliostats are used for daylighting and heating. Instead of many large heliostats focusing on a single target to concentrate solar power (as in a solar power tower plant), a single heliostat usually about 1 or 2 square meters in size reflects non-concentrated sunlight through a window or skylight. A small heliostat, installed outside on the ground or on a building structure like a roof, moves on two axes (up/down and left/right) in order to compensate for the constant movement of the Sun. In this way, the reflected sunlight stays fixed on the target (e.g. window). Genzyme Center, corporate headquarters of Genzyme Corp. in Cambridge, Massachusetts, uses heliostats on the roof to direct sunlight into its12-story atrium. In a 2009 article, Bruce Rohr suggested that small heliostats could be used like a solar power tower system. Instead of occupying hundreds of acres, the system would fit in a much smaller area, like the flat rooftop of a commercial building, he said. The proposed system would use the power in sunlight to heat and cool a building or to provide input for thermal industrial processes like processing food. The cooling would be performed with an absorption chiller. Rohr proposed that the system would be "more reliable and more cost-effective per square meter of reflective area" than large solar power tower plants, in part because it would not be sacrificing 80 percent of the power collected in the process of converting it to electricity.
Design Heliostat costs represent 30-50% of the initial capital investment for solar power tower power plants depending on the energy policy and economic framework in the location country. It is of interest to design less expensive heliostats for large-scale manufacturing, so that solar power tower power plants may produce electricity at costs more competitive to conventional coal or nuclear power plants costs. Besides cost, percent solar reflectivity (i.e. albedo) and environmental durability are factors that should be considered when comparing heliostat designs.
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