Heliostats: Difference between revisions

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A heliostat is a device incorporating a mirror which moves so as to reflect sunlight in a constant direction, at a fixed target or receiver, despite the sun's motions in the sky. The normal (perpendicular) to the plane of the mirror therefore has to aim along the bisector of the angle between the directions of the sun and the target, as seen from the mirror.

Heliostats should be distinguished from solar trackers or sun-trackers, which always point at the sun in the sky. However, it is true that some types of heliostat incorporate sun-trackers, together with additional components to bisect the sun-mirror-target angle.

The principal uses of heliostats are for daylighting (bringing daylight into a space that would otherwise be poorly illuminated), and in the generation of electricity in solar-thermal power plants. They are also occasionally used, or have been used in the past, in surveying, astronomy, to improve illumination for agriculture, and to direct constant sunlight into paraboloidal reflectors for solar cooking, etc. During the 19th Century, they were used by painters and other artists in order to provide constant, bright illumination of their subjects.

WARNING: There have been occasional reports of fires and other forms of damage being caused by heliostats. If the mirror is slightly concave, as may be produced by accident during the manufacture of cheap mirrors that are supposed to be flat, it may focus (concentrate) sunlight onto a target that is some distance from it. For example, if the radius of curvature of the concavity is 100 metres, it will focus onto a target that is 50 metres away. The focal length is half the radius of curvature. Also, a heliostat reflects sunlight for prolonged periods of time onto a stationary target, possibly allowing its temperature to rise to a dangerous level. Anyone who constructs or uses a heliostat should therefore take appropriate precautions to ensure that it does not reflect sunlight onto anything that might be flammable or susceptible to damage by heat.

Types of Heliostat

The earliest known heliostats were also the simplest. They were used for daylighting in ancient Egypt, more than 4000 years ago. The interiors of Egyptian buildings were elaborately decorated, and would have been damaged by smoke from flaming torches. Instead, polished metal mirrors were used to reflect sunlight indoors. Servants or slaves moved the mirrors manually to keep reflecting sunlight in the right directions as the sun moved across the sky. (This is still done in a few places in Egypt, for the benefit of tourists.) This kind of manual operation is, of course, still practicable today, and may be the preferred method in some third-world situations. It has been suggested that animals such as monkeys might be trained to move the mirrors, but no serious effort seems to have been made to do this.

A simple type of semi-mechanical heliostat uses a mirror mounted so it can be rotated by a clockwork mechanism about an axis that is parallel with the earth's axis of rotation. The clockwork turns the mirror once every 24 hours. The mirror is oriented so it reflects sunlight along the same polar axis as its axis of rotation. At an equinox, this means that the mirror is inclined at 45 degrees to the axis. At other times of the year, this inclination angle must be changed as the sun moves north and south. Pivots are provided to allow this adjustment to be done by hand every few days. Also, the setting of the clock has to be varied seasonally to take account of the Equation of Time, a small east-west movement of the sun. This is also done manually. The beam of light that is reflected along the polar axis by the rotating mirror is intercepted by a second, stationary mirror, which reflects the light in any desired direction. This type of machine therefore runs automatically during a single day, but requires manual re-adjustment every few days to follow the sun's seasonal movements. Also, of course, the clockwork has to be wound up periodically.

More elaborate clockwork heliostats have been made that automatically follow the sun's seasonal movements as well as its daily one. They are very complex machines. Some well known ones were made by Silbermann in the 19th Century. He was a friend of several distinguished artists who used his machines to shine unmoving beams of light onto the subjects they were painting. This meant that the appearances of the subjects did not change as the sun moved across the sky. Some of Silbermann's heliostats still exist, and many replicas of them have been made. They are sometimes sold for very high prices.

If electricity is available, heliostats that use light-sensors to locate the sun in the sky are practicable. The simplest design uses a principal axis of rotation that is aligned to point at the target toward which light is to be reflected. The secondary axis is perpendicular to the first. Sensors send signals to motors that turn around both axes so that a small arm points toward the sun. (Thus this design effectively incorporates a sun-tracker.) A gear mechanism bisects the angle between the sun-pointing arm and the principal rotation axis. This gives the direction in which the perpendicular to the mirror must be pointed

Although the above types of heliostat, and others, do exist, they are not the ones that are used in the vast majority of heliostats that are now in operation. Instead, most heliostats are controlled by computers. The software they use calculates, from astronomical theory, where the sun is in the sky. Sensors are not needed, and the calculation takes account of both the daily and seasonal movements of the sun. The information that has to be available is simply the position of the heliostat on the earth's surface, as latitude and longitude, and the time and date. When the position of the sun has been calculated, it is combined with the direction in which light is to be reflected, which also has to be provided, to calculate the direction of the required angle-bisector. The computer then sends control signals to motors that rotate the mirror to the correct orientation. This whole process is repeated every few seconds, so the mirror is kept correctly aligned.

For daylighting purposes, individual mirrors controlled by their own computers are often sufficient. However, for solar-thermal power generation, "fields" of heliostat mirrors, often hundreds of them, are used to reflect large amounts of sunlight onto a boiler or other heat collector. The heat is used to make steam, which drives turbines to generate electricity. Usually, just a single computer controls all the mirrors.

Although computer-controlled heliostats sound complex, and would probably be impractical in third-world situations, they can be quite easily used where electricity and the necessary equipment are available. Several companies sell kits from which they can be built. If anyone wants to design his own hardware, he can use free, open-source, public-domain software. For example, on the website http://www.green-life-innovators.org there is a program called Sunalign which does all the necessary calculations to run a heliostat. It is available in BASIC, Perl, and C. The website also has a detailed explanation of how the code works.