A heliostat is a device that includes a mirror, usually a plane mirror, which 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 angle between the directions of the sun and the target as seen from the mirror. In almost every case, the target is stationary relative to the heliostat, so the light is reflected in a fixed direction.
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 stations. They are also occasionally used, or have been used in the past, in surveying, in astronomy and other sciences, to produce very high temperatures in solar furnaces, to improve illumination for agriculture, and to direct constant sunlight onto solar cookers. During the 19th Century, they were used by painters and other artists in order to provide constant, bright illumination of their subjects.
Heliostats should be distinguished from solar trackers or sun-trackers, which always point directly at the sun in the sky. However, some types of heliostat incorporate sun-trackers, together with additional components to bisect the sun-mirror-target angle.
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.
The word "heliostat" is derived from the Greek "helios" meaning "sun" and "stat" meaning "stationary". It is related to various English words that refer to the sun, and also to the name of the gas helium, which was discovered spectroscopically on the sun before it was known on the earth. It is not related to the Greek or Latin "helix" meaning "spiral", nor to English words descended from it such as "helicopter".
Types of Heliostat
Although heliostats that reflect light onto moving targets are occasionally (very rarely) used (see "Special-Purpose Machines", below), 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. 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-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. Small computers are now very cheap. Several companies sell complete heliostats, or 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 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, and a variety of other material related to the sun. To access this material, click on the following link: Link
Heliostats are sometimes attached to astronomical telescopes to allow continuous observation of the sun without the telescope having to move. For this purpose, the mirror of the heliostat has to be extremely precisely flat, and must move very exactly in the right way to reflect light into the telescope.
Slightly different machines, called "siderostats" and "coelostats" (from the Latin words for "star" and "sky"), have been used to allow other astronomical bodies, besides the sun, to be observed with stationary telescopes. Since the stars move across the sky slightly faster than the sun does, the mirrors of these machines have to move slightly faster than the mirror of a heliostat. Generally, the name "siderostat" is used for a machine with a single mirror, and "coelostat" for one with two mirrors, similar to one of the clockwork heliostats described in the preceding section.
Surveyors have used heliostats as very bright lights. For example, a heliostat might be placed on a mountain, and aligned so it reflects sunlight toward an observer far away. The observer uses a theodolite to measure the compass bearing and angle of elevation of the heliostat as seen from his location. The heliostat is then turned toward another observer, who does the same thing. The observations can later be combined to produce a survey of the area. Heliostats are so bright that they have been observed from distances of 300 kilometres or more in full sunlight, allowing large areas to be surveyed. Heliostats that are made for this purpose are called "heliotropes". They have to be rugged, easily transported, and capable of being set up and aligned easily in remote locations.
Specialized heliostats have been occasionally tested for reflecting sunlight onto moving targets, such as aircraft. The aircraft carries equipment that uses the solar energy for purposes such as running the engines, or operating a radio or television transmitter. The mirrors in these heliostats have to move so as to take into account the motions of both the sun and the target.
It should be clear that heliostats, especially computer-controlled ones, are appropriate technology for various environmentally-friendly activities in developed economies, such as generating electricity from solar energy. However, there are also good uses for them that are appropriate in poorer countries.
The most obvious among these is daylighting. Many people in the developing world live in buildings without windows. Even during daytime, they burn wood or other organic material to provide illumination. This has several unfortunate consequences. Wood is consumed, worsening deforestation. The indoor air becomes very polluted with smoke and gases produced by combustion, causing bad effects on the inhabitants' health. Accidental fires are lit. Things become dirty. And so on.
Just as the ancient Egyptians did, people nowadays can use simple heliostats to direct sunlight into their buildings. Devices similar to the Egyptian ones, with mirrors that are turned manually, are very easy and cheap to construct. Commercially-produced mirrors are not necessary. Adequately good mirrors can be made, for example, by sticking aluminum foil to flat boards or pieces of cardboard.
Schools are very suitable for daylighting by heliostats. Generally, the classrooms are indoors so there is no risk of books and other materials being damaged by rain, but the light indoors has to be quite bright, so people can read and write easily. Heliostat mirrors can be used to achieve this. Medical clinics and other buildings are also suitable. Clinics are often located in places where somewhat higher technology is available, so automatic heliostats using clockwork or electricity may be used.
Simple heliostats are also often useful for other purposes. They can improve the lighting of crops, increasing the growth of plants. They can also make life better for animals. On an iguana farm in the Dominican Republic, for example, simple heliostats keep the animals warm and healthy during the winter months.
Heliostats can be useful for solar cooking since mirrors reflect the sun's heat, as well as its light. It is usually easier to turn a heliostat mirror than to turn a whole solar cooker as the sun moves across the sky. The heliostat shines an unmoving beam of light and heat onto the cooker. Heliostats can also by used to intensify the solar heat that reaches a cooker, by having several of them directing sunlight onto the same cooker, or by adding to the heat that is being directly received from the sun.
Hazard to Eyesight
It is often claimed that the sunlight reflected by a heliostat mirror is very dangerous to the eyesight of anyone who looks at it. In fact, this is not a serious concern. If the mirror is planar (flat), even if it is a perfect reflector (which is never true) the image of the sun seen in it is no brighter than the real sun in the sky. Glancing at it is no more hazardous than glancing at the real sun. It is dangerous to stare at the sun for a prolonged period of time, and it is similarly dangerous to stare at the reflection of the sun in a plane heliostat mirror, but there is no need to take precautions to avoid brief exposures.
This section is for information that is indirectly related to heliostats and their uses, but which may be interesting to some readers.
Confused and Confusing Information in Reference Works
Heliostats are not widely known devices, even among the writers of dictionaries and encyclopedias. Many of these works contain information which is incorrect or outdated.
Some dictionaries and encyclopedias confuse heliostats with sun-trackers. This is simply wrong.
Others have definitions and descriptions which essentially date back to the early 19th Century. At that time, the ancient Egyptian use of manually moved mirrors for daylighting was unknown. It was rediscovered after Egyptian hieroglyphic writing was deciphered, following the discovery of the Rosetta Stone in 1799. (This is a stone tablet, that still exists, which carries a lengthy inscription written in both hieroglyphs and Greek. It gave scholars an invaluable insight into the structure and meaning of Egyptian hieroglyphs.) In the early 1800s, all heliostats were laboratory instruments used for optical experiments. Like the ones made by Silbermann, they were driven by clockwork. Definitions of heliostats, written then, describe them as scientific instruments with mirrors driven by clockwork to reflect sunlight at stationary targets. At that time, these definitions were adequate. However, some of them are still used in modern works of reference. They make no mention of more recent designs, nor of present-day uses of the machines, nor of the ones in ancient Egypt. Extremely few of the heliostats now in use are correctly described by these old definitions and descriptions, which should all be updated as has been done here.
The vast majority of heliostats fit into the categories described in "Types of Heliostat", above. However, machines are occasionally found that do not. Frequently, they have been constructed by amateur hobbyists.
Probably the most common are machines that are essentially the same as clockwork heliostats, but which are driven by other types of mechanism, such as electric or electronic clocks.
Devices that almost defy description also exist. For example, a few years ago the present writer met a retired carpenter who had a hobby workshop in his garage. He had a mirror outdoors to reflect daylight into it. The mirror was moved by an arrangement of electric motors, pulleys, and pieces of string. The carpenter had no relevant theoretical knowledge, did not know the word "heliostat", and was unaware that anyone else had ever done anything similar. Yet, by trial and error, he had made his device work, not well, but well enough for his purpose. Unfortunately, he was hoping to make his fame and fortune by selling his "invention".
Archimedes' Sun Weapon
The Greek scientist Archimedes, who is now best known for his work on buoyancy (Archimedes' Principle), lived in the city of Syracuse, which was then a Greek colony located on the coast of the island of Sicily. In about 215 BCE, Syracuse was attacked by Romans in ships. It is said that Archimedes helped with its defence by organizing people to hold large numbers of mirrors, using them to focus sunlight onto the ships, something like a field of heliostats focusing sunlight in a modern solar-thermal power station. The ships were set on fire by the concentrated solar heat. Modern tests of this idea have cast doubt on the story. The ships would have had to remain stationary close to the mirrors for long periods of time before igniting. Also, keeping all the mirrors accurately aligned by hand in the heat of battle would have been extremely difficult. If there is any truth behind the tale, it is more likely that the light from the mirrors served to dazzle the Romans, reducing their ability to fight.
Interestingly, Archimedes is thought to have studied in Alexandria, Egypt, which was then another Greek colony. There, he would probably have seen Egyptians using simple heliostats for daylighting their buildings. This could have interested him in different uses of mirrors.
Strange Experiences with a Heliostat
The following true story is included here since it may be instructive for anyone who is contemplating building a heliostat.
Back in the 1980s, I designed, built and programmed a computer controlled heliostat to bring more sunlight into the living room of my house. It was a simple machine, with the mirror in an alt-azimuth mount, so the principal axis of rotation was vertical and the other horizontal. The program that ran in its computer - an ancient Commodore VIC 20 - was developed from an earlier program I had written that calculated the position of the sun in the sky from astronomical theory.
Describing the heliostat in detail would be pointless. It was built from components that are now long out of date. Nowadays, duplicating it would be almost impossible. Anyone who wants to design a heliostat should start from scratch.
However, something happened when I was testing the machine that might be worth sharing...
I live in Canada, about mid-way between the Equator and the North Pole. Seen from here, the sun always moves clockwise in the sky, rising in the east, passing to the south around noon, and setting in the west. Since the heliostat mirror moves to keep reflecting sunlight in a constant direction, it was obvious to me that the azimuth drive of the machine would always turn clockwise. In fact, I contemplated using a motor for that drive which would turn only in the clockwise direction. I changed my mind only because it was convenient to use identical motors in the two drives. Since the elevation (altitude) drive changes direction as the sun rises and sets, I used motors, actually steppers, that could turn either way.
When I first set up the machine and started testing it, I encountered a couple of simple problems that I easily fixed. The next time I tried it, I was happy to see that it initialized itself and moved its mirror so as to reflect sunlight in the correct direction. It seemed to be working properly. But, as I watched it for a while, I saw to my dismay that the azimuth drive was turning anticlockwise. Obviously, that was wrong. I stopped the machine and started hunting in the hardware and software for the cause of the problem. In that, I had no success. Everything seemed to be as it should be, but the azimuth drive kept turning the wrong way.
Then I made another puzzling observation. Although the mirror was turning anticlockwise, it was continuing to reflect sunlight in the correct direction! The thing seemed to be working properly, but wrongly at the same time.
After some bemusement, I realized what was going on. It was about noon on a summer day, so the sun was high in the southern sky. The window through which I wanted the heliostat to reflect sunlight was roughly to the north of the mirror, and not much higher than it. The mirror had to be aimed in the direction that bisected the angle between the directions of the sun and the window, as seen from the mirror. At that time, the aim direction was high to the north. As the sun moved from east to west, this aim direction also moved from east to west. But since it was to the north of the zenith, this motion was anticlockwise in azimuth. My heliostat was absolutely correctly performing this anticlockwise rotation.
It was startling to realize that this machine, which I had designed and programmed, was apparently smarter than I was. It had correctly figured out which way to move, although my expectations were wrong.
Another realization was that only sheer good luck had allowed the machine to work properly. If I had used a motor in the azimuth drive that would turn only clockwise, which I had been sure would work properly, the machine would have been incapable of turning in the correct direction. I wonder how long it would have taken me to figure out what was wrong!
A few months later, as the noon-day sun sank lower in the sky as winter approached, a situation arose where the aim direction of the mirror was almost vertically upward at some time near mid-day. The angles of elevation of the sun and the window, as seen from the mirror, were equal, and their azimuths were 180 degrees apart, so the angle-bisector pointed vertically up. In that situation, a tiny movement of the bisector, as the sun moved westward, caused a large change in the bisector's azimuth. This made the azimuth drive of the heliostat turn very rapidly, about 180 degrees in just a few seconds! I hadn't expected the machine to move rapidly like that, since the sun moves very slowly in the sky. On one day, the drive spun anticlockwise, since the bisector was passing just to the north of the zenith. On the next day, the bisector passed just to the south of the zenith, and the drive spun clockwise. The reverse happened in the spring, as the sun moved northward. Of course, since the mirror was lying on its back, pointing upward, the rapid rotation about the vertical axis did not cause much change to the aim direction.
Heliostats can be very counter-intuitive machines!
DOwenWilliams 17:23, 5 July 2010 (UTC) David Williams
Some More Technical Details
For the sake of anyone who is thinking of designing and building a heliostat, here is a bit more about the VIC-controlled heliostat I built in the 1980s. (See "Strange Experience with a Heliostat", above.) The mirror was held in an alt-azimuth mount, so one of its two stepper motors turned the assembly about a vertical axis, i.e. in azimuth. The assembly included the second stepper motor and the horizontal axis about which it rotated the mirror, i.e. in elevation or altitude. There were two microswitches which closed when the two rotations reached specific positions. When I first set up the machine, I determined the compass bearing of the position where the azimuth switch closed and the angle of elevation where the altitude switch closed by experiment and measurement, and wrote these quantities into the software.
The software included an initialization routine which was executed whenever the machine was re-started, e.g. after a power outage, and also every morning at sunrise. The two drives stepped around until their respective switches closed. They then stepped slowly in the opposite directions, counting steps until the switches opened. This put the mirror into a known orientation, and also measured the backlashes in the drives. During the day, the computer sent requisite numbers of stepping instructions to the motors to turn the mirror to the required orientation. Also, when the direction of either rotation changed, additional stepping instructions were sent to take up the backlash. This meant that the machine automatically compensated for wear of its mechanical components.
At sunset each evening, i.e. when the software determined that the sun's angle of elevation became negative, the machine turned the mirror so it faced downward. This was to reduce the buildup of dust on its surface. The mirror remained in this position until sunrise, when the initialization routine was executed.
The wires leading to the elevation drive and its microswitch could have become twisted until they broke if the azimuth drive had turned many times in a single direction. To avoid this, the software automatically made the azimuth drive turn a full rotation in the opposite direction if it had previously turned more than a full rotation from its starting point, where its microswitch closed.
The VIC's internal clock was used to give the heliostat the time and date. Of course, like any other clock, it did not keep perfect time. The software therefore included a routine that allowed me to reset the clock without stopping the heliostat program. A feature of the VIC's clock was that its speed could be finely adjusted. My software made that adjustment automatically whenever I reset the clock. It kept a record of the date when each reset was done, and calculated by how much the speed should be adjusted to optimize the timekeeping. After two or three resets, the timekeeping was accurate to within a few seconds per year.
In order to make the machine capable of surviving brief interruptions of the AC power, I added a high-value capacitor essentially in parallel with the capacitor that the VIC had in its power supply. However, charging this large capacitor rapidly would have overloaded the transformer and rectifier, possibly damaging them. I therefore put a resistor and diode in parallel with each other and in series with the big capacitor. The charging current went through the resistor, which limited it to a low value. The capacitor took several minutes to charge. During a power interruption, the capacitor discharged through the diode as rapidly as required to keep the computer running. This arrangement eliminated computer crashes due to power flickers. It was still possible for the stepping motors to get slightly out of step, but this was automatically corrected the next morning when the initialization routine was executed.
I designed and built an interface unit that fed power to the stepper motors and connected them, and the microswitches, with the computer. This unit took power directly from the AC supply. It received small control signals from the computer, changed them to the necessary voltage, etc., and fed them to the motors. It was capable of producing two different voltages, a high one which was fed to the motors while they were executing steps, and a lower voltage which was used just to hold the motors stationary. Since the motors were stationary for most of the time, using the low voltage conserved power and prevented the motors from getting warm. The computer signalled which voltage should be produced. Just before steps were taken, the software made the computer select the high voltage, and return to the low voltage when stepping was temporarily completed.
None of this is of any theoretical importance, but it does show the kinds of things that must be considered when designing an operational machine.