heliostat

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A heliostat on top of the Swedish Solar Telescope.

A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device that, in general, tracks the movement of the sun. The device typically utilizes a mirror, which can be oriented throughout the day to redirect sunlight along a fixed axis toward a stationary target or receiver.

Heliostats are used in solar telescopes, and solar power generation. Heliostats have been used in surveying in the form of a heliotrope to constantly reflect sunlight in a single fixed direction, allowing the accurate observation of a known point from a distance.

The simplest heliostat devices use an equatorial mount and a clockwork mechanism to turn the mirror in synchronisation with the rotation of the Earth. More advanced heliostats track the sun directly by sensing its position throughout the day. Others are controlled by computers. The U.S. Department of Energy funded a program for a low-cost tracking heliostat design in the late 1970s, with several major companies competing.

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

Large scale projects

One of Solar Two's heliostats in 2003. The solar power tower can be seen in the background.

In a solar power tower power plant, like the those of The Solar Project, a field of heliostats focus the sun's energy 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.

It's been proposed that the high temperatures generated could also be used to split water producing hydrogen sustainably.^[1]

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.^{[2] [3]} It is of interest to design more inexpensive 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.

One way that engineers and researchers are attempting to lower the costs of heliostats is by replacing the conventional heliostat design with one that uses fewer, lighter materials. A conventional design for the heliostat's reflective components utilizes a second surface mirror. The sandwich-like mirror structure generally consists of a steel structural support, an adhesive layer, a protective copper layer, a layer of reflective silver, and a top protective layer of thick glass. ^[2] This conventional heliostat is often referred to as a glass/metal heliostat. Alternative designs incorporate recent adhesive, composite, and thin film research to bring about materials costs and weight reduction. Some examples of alternative reflector designs are silvered polymer reflectors, glass fiber reinforced polyester sandwiches (GFRPS), and aluminized reflectors. ^[4] Problems with these more recent designs include delamination of the protective coatings, reduction in percent solar reflectivity over long periods of sun exposure, and high manufacturing costs.

References

See also

• Renewable energy
• Solar cell
• Solar power tower
• Solar thermal energy
• Solar tracker

External links

• Photos of a small-scale heliostat system in action
• Field of 63 heliostats at the Odeillo Solar Furnace
• Solar Furnaces at Odeillo
• redrok.com
• Overview of heliostat reflector materials