A heliostat (from helios , the Greek word for sun , and stat , as in stationary) is a device That includes a mirror, usually a flat mirror , qui turns so as to keep Reflecting sunlight Toward a Predetermined target, compensating for the sun’s apparent motions in the sky. 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 anglebetween the directions of the sun and the target of 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 Alonso Borelli (1608-1679) and Daniel Gabriel Fahrenheit (1686-1736). 
Nowadays, 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, or 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 other scientific 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, eg 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 facilities such as solar-thermal power stations include many heliostats and many mirrors. Usually, all the mirrors in such a field are controlled by a single computer.
There are more of these types of devices, which are used 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 to the sun in the sky. However, some older types of solar trackers, together with additional components to the sun-mirror-target angle.
A siderostat is a similar device which is designed to follow a fainter star , rather than the sun.
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 on 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 accurate parallel beams of light to a large paraboloidal reflector that brings them to a precise focus. The mirrors have to be located close to the axis of the paraboloid to reflect sunlight in it along lines parallel to the axis, so the field of heliostats has to be narrow. A closed loop control system is used. Sensors determined if any of the heliostats is slightly misaligned. If so, they send signals to correct it.
It has been proposed that hydrogen could be used to generate hydrogen. 
Smaller heliostats are used for daylighting and heating. This is one of the most popular solar power devices in the world, with a single target for concentrating solar power (as in a solar power tower plant), a single heliostat usually about 1 or 2 square meters in size, non-concentrated sunlight through a window or skylight. A small heliostat, one of two types of structures, one of two types of structures (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 (eg window).
Genzyme Center, corporate headquarters of Genzyme Corp. in Cambridge, Mass., uses heliostats on the roof to direct sunlight into its12-story atrium.  
In a 2009 article, Bruce Rohr suggests that small heliostats could be used as a solar power tower system.  Instead of occupying hundreds of acres, the system would have a much smaller area, like the flat rooftop of a commercial building, he said. The proposed system would be used to provide heat to the environment. The cooling would be performed with an absorption chiller . Mr. Rohr proposed that the system would be more efficient and more efficient than the rest of the world. 80% of the power collected in the process of converting it to electricity. 
Heliostat costs represent 30-50% of the initial capital investment for solar power tower power plants.   It is of interest for the design of smaller plants for large scale manufacturing, so it can be used for conventional power generation.
Besides cost, solar reflectivity (ie albedo ) and environmental sustainability are factors that should be considered when comparing heliostat designs.
One way that engineers and researchers are attempting to reduce the costs is by replacing the standard design with smaller ones. A design for the heliostat’s reflective components uses a second surface mirror. The sandwich-like mirror structure consists of a structural steel support, an adhesive layer, a protective layer, a layer of reflective silver, and a protective layer of thick glass. This heliostat is often referred to as a glass / metal heliostat. Alternative designs, recent adhesive, composite, and thin film. Some examples of alternative reflector designs are silvered reflectors, glass fiber reinforced polyester sandwiches (GFRPS), and aluminized reflectors.  Problems with these more recent designs include the delamination of the protective coatings, the reduction in the sun and the high manufacturing costs.
The movement of the most modern heliostats employed by two-axis motorized system, controlled by computer as outlined at the beginning of this article. Almost always, the primary rotation axis is vertical and the secondary horizontal, so the mirror is on an alt-azimuth mount .
One simple alternative is for the mirror to rotate around a polar aligned primary axis, driven by a mechanical, often clockwork, mechanism at 15 degrees per hour, compensating for earth’s relative rotation to the sun. The mirror is aligned to reflect sunlight along the same polar axis in the direction of one of the celestial poles . There is a perpendicular secondary axis permitting manual adjustment of the mirror (daily or less often as necessary) to compensate for the shift in the sun’s declination with the seasons. The setting of the drive can also be adjusted to compensate for changes in the equation of time. The target can be located on the same axis as the mirror’s primary rotation axis, or a second, stationary mirror can be used to reflect light from the polar axis towards the target, wherever it might be. This kind of mirror is often used with solar cookers , such as Scheffler reflectors .    For this application, the mirror can be concave , so as to concentrate sunlight onto the cooking vessel.
The alt-azimuth and polar-axis alignments are commonly used for heliostat mirrors. The third is the target-axisarrangement in which the primary axis points towards the target of the sunlight is to be reflected. The secondary axis is perpendicular to the primary one. Heliostats controlled by light-sensors have used this orientation. A small arm carries sensors that control the two axes, so it points toward the sun. (Thus this design incorporates a solar tracker.) A simple mechanical arrangement bisects the angle between the primary axis, pointing to the target, and the arm, pointing to the sun. The mirror is mounted so that its reflective surface is perpendicular to this bisector. This type of heliostat was used for daylighting , but after the initial availability of sensor control hardware.
There are heliostat designs which do not require the rotation axes to have any exact orientation. For example, there may be light-sensors to the point where they make signals to the motor vehicle. The directions of the axes need only be known, since the system is intrinsically self-correcting. However, there are many disadvantages, such as that of the mirror, when it reappears, misses the sensors, the system can not correct the orientation of the mirror. There are also geometrical problems which limit the functioning of the heliostat when the directions of the sun and the target, are seen from the mirror, are very different.
2 / 3rd motion heliostat. In general, in heliostats, the bisector angular motion of the mirror moves at a rate that is 1/2 the angular motion of the sun. There is another understanding that satisfies the definition of a heliostat yet has a mirror that is 2 / 3rd of the motion of the sun. 
Many other types of heliostat have also been used. In the very earliest heliostats, for example, which were used for the daylighting in ancient Egypt, or slaves kept the mirrors aligned manually, without using any kind of mechanism. (There are places in Egypt where this is done today, in the movie The Fifth Elementan Egyptian boy holds a mirror to illuminate a wall inside a cellar for a fictional archaeologist.) Elaborate clockwork heliostats were made during the 19th Century which could reflect a light in the light of a single mirror, minimizing light losses, and which automatically compensated for the sun’s seasonal movements. Some of these devices are still in use, but they are not used for practical purposes today. Amateurs sometimes come up with ad hoc designs which work, with some justification. An essentially limited number of such designs are possible.
- Renewable energy
- Solar cell
- Solar cooker
- Solar energy
- Solar thermal energy
- Solar tracker
- Heliograph , a similar non-tracking device, used for communication
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