Types of Heliostat
the vast majority of heliostats are designed to reflect sunlight in a fixed direction,
toward a stationary target. In the following paragraphs, it is assumed that the target is stationary.
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.
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-automatic 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 in the direction opposite to the earth's rotation.
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 occasionally to take account of the Equation of Time, a small east-west seasonal 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 can run automatically for a few days, but requires manual readjustment fairly frequently to follow the sun's seasonal movements.
Also, of course, the clockwork has to be wound up and the mirrors cleaned periodically.
A simplified version of this type of heliostat is sometimes used for solar cooking in the developing world.
There is no second mirror.
Instead, the cooking vessel is located on the polar axis around which the one mirror rotates, and the mirror is aligned so as to reflect sunlight continuously onto it.
Of course, the alignment and the setting of the clock have to be manually adjusted occasionally to compensate for the sun's seasonal movements.
Often, the mirror is concave, so as to concentrate sunlight onto the cooking vessel. This simplified design can work only if the target is on the polar axis.
More elaborate clockwork heliostats have been made that use only one mirror to reflect sunlight onto a target in any direction, and even more elaborate ones 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 the French physicist J.T. Silbermann in the 19th Century.
They were used in scientific experiments in optics, prior to the existence of electric lights.
Also, Silbermann was a friend of several distinguished artists who used his heliostats 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 considered to be works of art in themselves, and are sometimes sold for very high prices.
Heliostats Controlled by Light-Sensors
If electricity is available, heliostats that use light-sensors to locate the sun in the sky are practicable.
A simple 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, carrying the sensors, points toward the sun.
(Thus this design 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.
Another design uses light-sensors to determine the position of the reflected beam of light, rather than that of the sun.
The sensors are located close to the target and are shaded so they respond only to light reaching them from the direction of the mirror.
At the start of each day, the mirror is aligned by hand. From then on, if the reflected beam of light drifts away from the target,
the sensors detect the error and send signals to motors that turn the mirror to the correct orientation.
This is a very simple and cheap design which does not involve any determination of the sun's position in the sky, nor the bisection of any angle.
However, it has disadvantages.
For geometrical reasons, it can be used only if the target is roughly to the south of the mirror (in north-temperate latitudes).
Also, the sun must shine fairly continuously.
If it is obscured by clouds for a long time, when it reappears, the reflected beam of light misses the target and sensors, so they can not realign the mirror.
Modified versions of this design are better at surviving cloudy periods. Some have additional sensors, placed further from the target.
Others include some sort of memory so that, when the sun is obscured, the mirror is given the same alignment as it had at the same time on the previous day.
These modifications do improve the machine's performance, but they spoil its essential simplicity and cheapness.
Although the above designs of heliostat and other mechanical and sensor-controlled ones do exist, they are not 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.
Fields of heliostats are also used in solar furnaces, but in this case they are all aligned to produce beams of light that are all parallel to the axis of a large, stationary paraboloidal reflector, into which the light from the heliostats shines.
The paraboloid focuses the light accurately onto a small target, which therefore becomes very hot.
Temperatures in excess of 3500 degrees Celsius (about 6300 deg. F) have been produced this way.
At present these devices are experimental, but it is anticipated that they may be used in various industrial processes.
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.