A Heliostat is a device that tracks the movement of the sun. It is typically used to orient a mirror, throughout the day, to reflect sunlight in a consistent direction. When coupled together in sufficient quantities, the reflected sunlight from the heliostats can generate an enormous amount of heat if all are oriented towards the same target. It was originally developed as an instrument for use in surveying, allowing the accurate observation of a known point from a distance.

Its name comes from helios, the Greek word for sun (see also: Helios), and stat, as in stationary.

Heliostats have been used for sunlight-powered interior lighting, solar observatories, and solar power generation. Mirrors and reflective surfaces used in solar power that do not track the sun are not heliostats.

The simplest heliostat devices use a clockwork mechanism to turn the mirror in synchronisation with the rotation of the Earth. More complex devices need to compensate for the changing elevation of the Sun throughout a Solar year. Even more advanced heliostats track the sun directly by sensing its position throughout the day.

A siderostat is a similar device which is designed to track a fainter star, rather than the Sun.

Use in power production

Using the experience from Solar One and Solar Two, the Solar Tres project in Spain is building a 15MW solar power plant with a 16 hour, 600 MWh thermal power storage system based on molten sodium nitrate salt. Some have criticized the use of this salt because of its safety and environmental hazards. Others believe that it is relatively safe compared to the other chemicals used in the electrical industry, especially if handled properly.

Sandia National Laboratory also has a heliostat array. It is used for experiments in electricity production, industrial processes requiring high temperatures and waste disposal.

Solar Systems has announced a world-leading 154MW photovoltaic (PV) heliostat solar concentrator power station for north-western Victoria, in Australia. The A$420 million project will generate 270,000 MWh per year, enough for more than 45,000 homes. It will aid in reducing salinity and create jobs during manufacture, construction and operation. It will also reduce greenhouse gas emissions by approximately 400,000 tonnes per year.[1]

Use in interior lighting

The heliostat reflects the sunlight onto the transmission system. This is typically a set of mirrors that direct the reflected sunlight into the building or, alternatively, a light tube. Fiber optic cabling has also been used as a transfer mechanism. Various forms of commercial products have been designed for the point of termination (the "light bulb").

Calculating the position of the sun in an "open loop" heliostat

Since the path of the sun across the sky varies through the year, a completely automatic heliostat must be guided by continuous calculations. The National Renewable Energy Laboratory has released its Solar Position Algorithm (SPA) with complete documentation. Another resource is the libnova Celestial Mechanics and Astronomical Calculation Library, which also calculates variables such as apparent position and rise, set and transit times among many others of astronomical objects. See also the "References" section of the Seasons" article.