Heliostat
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.
Large installations such as solar-thermal power stations include fields of heliostats comprising many mirrors.
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.
However, some older types of heliostat incorporate solar trackers, together with additional components to bisect the sun-mirror-target angle.
All the heliostat mirrors send accurately parallel beams of light into a large paraboloidal reflector which brings them to a precise focus.
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.
[5][6] In a 2009 article, Bruce Rohr suggested that small heliostats could be used like a solar power tower system.
[7]: 7–12 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.
Besides cost, percent solar reflectivity (i.e. albedo) and environmental durability are factors that should be considered when comparing heliostat designs.
Alternative designs incorporate recent adhesive, composite, and thin film research to bring about materials costs and weight reduction.
[10] 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.
The movement of most modern heliostats employs a two-axis motorized system, controlled by computer as outlined at the start of this article.
There is a perpendicular secondary axis allowing occasional manual adjustment of the mirror (daily or less often as necessary) to compensate for the shift in the Sun's declination with the seasons.
In the very earliest heliostats, for example, which were used for daylighting in ancient Egypt, servants or slaves kept the mirrors aligned manually, without using any kind of mechanism.
In the 1997 film The Fifth Element an Egyptian boy holds a mirror to illuminate a wall inside a cave for a fictional archaeologist.)
Elaborate clockwork heliostats were made during the 19th Century which could reflect sunlight to a target in any direction using only a single mirror, minimizing light losses, and which automatically compensated for the Sun's seasonal movements.
