- 1 Source and information
- 2 Heliostat vs. Solar Tracker
- 3 Heliostats are Not the same as Solar Trackers
- 4 "Solar Power Tower" Not a Widely Used Term
- 5 Strange Experience with a Heliostat
- 6 Languages
- 7 Overview of Heliostat Technologies
- 8 Video moved
- 9 Equation of Time in Tracking
- 10 Mirror turning speed
Source and information
"A computer is required for heliostats whose primary axis is aimed "the wrong way." No computer is needed if the primary axis aims directly at the target-receiver. See www.heliotrack.com" this isn't explained adequately. the commercial site referenced isn't much (or any) use as a source of information 18.104.22.168 04:23, 26 August 2007 (UTC)snaxalotl
- Commercial links are linkspam. Whole statement has been removed along with much other unverified text. 22.214.171.124 (talk) 04:19, 10 March 2008 (UTC)
I understand what he is saying and will try and explain it. "A complex controller is not needed if the mirror(s) are angled at a 45 degree angle relative to the polar rotation axis. The result is that the light shines down at the same point as the unit rotates. The unit then just has to point at the sun and can therefore use a simple "sun sensing" controller. Bill Arden (talk) 20:08, 23 March 2009 (UTC)
Snaxalotl and Bill Arden are talking about two different kinds of heliostat. Snax's machine uses light-sensors to find the sun. They are mounted on an arm that is carried in a two-axis mounting, of which the primary axis points at the receiver and the secondary axis is perpendicular to it. The sensors control motors that rotate around both axes, so that the arm points at the sun. Then a simple mechanical arrangement bisects the angle between the sun-pointing arm and the primary rotation axis, giving the direction in which the perpendicular to the mirror must be aimed.
Bill's machine can work by clockwork. A mirror is rotated once every 24 hours about a polar axis, i.e. one that is parallel with the earth's axis of rotation. The direction in which the mirror turns is opposite to the direction in which the earth rotates so, as seen from the sun, the mirror is stationary. It is oriented so it reflects sunlight along the same axis as its rotation axis. At an equinox, this means that the mirror is tilted at 45 degrees to the axis. At other times of year, the tilt is different, so pivots are provided to allow this angle to be adjusted manually every few days. The beam of light that is reflected along the rotation axis by the rotating mirror is intercepted by a second, stationary mirror, which can be adjusted to reflect the light in any desired direction.
These two machines use entirely different technologies. Neither of them uses a computer.
Heliostat vs. Solar Tracker
It appears that there are two pages for the same technology. The Solar Tracker page is not well written. Should these two merge? —Preceding unsigned comment added by 126.96.36.199 (talk) 11:07, 18 May 2008 (UTC)
- As the mergeto notice has been here for an extended time I removed it. I assume that consensus is that a heliostat is a precision device for solar telescope use while a solar tracker may be less precise and is used for power and thermal applications. Note that the articles are cross referenced. - Leonard G. (talk) 02:25, 14 October 2008 (UTC)
No. That last bit is not right. In fact, a major use for heliostats is in solar-thermal power stations, where a lot of heliostats reflect sunlight onto a boiler where steam is produced to drive turbines and make electricity.
Heliostats are Not the same as Solar Trackers
It is important to realize that heliostats and sun-trackers are different types of machine. A sun-tracker always aims at the sun. A heliostat always reflects sunlight in a constant direction. The mirror of a heliostat therefore aims along the bisector of the angle between the directions of the sun and the receiver.
Automatic sun-trackers are fairly easy to design and build. The sun appears to revolve around the earth at a speed of 15 degrees per hour. The tracking machine must therefore turn at the same speed to keep aiming at the sun. This is simple to achieve.
Automatic heliostats are more difficult. Most of the ones in practical use have computers that figure out how the mirror should be aimed. Purely mechanical designs, and ones that use light sensors to determine where the sun is in the sky, are possible, but are considerably more complicated than simple sun-trackers. There are fairly simple designs that use two mirrors, but the double reflection loses light.
In a third-world situation, the only type of heliostat that is reasonably practicable is operated manually. This was done in ancient Egypt, where servants or slaves turned mirrors so as to keep reflecting sunlight into the interiors of buildings. It is still done in a few places in Egypt, for the benefit of tourists. Sun-trackers, of course, can also be operated manually. In places where manpower is cheap and everything else is expensive, manual operation is best.
"Solar Power Tower" Not a Widely Used Term
In the article, the phrase "Solar Power Tower" is frequently used to mean a solar-thermal power station. This is not a widely accepted term. Preferably, it should be replaced, or at least explained.
Strange Experience with a Heliostat
I am posting the following true story 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 across 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!
Does anyone ever compare the versions of Wikipedia articles in different languages? I just looked at several versions of the Heliostat article, and found that they disagreed in various ways. All the ones I understand, except the English one, have incorrect definitions of a heliostat.
Incidentally, some versions mention the Frenchman J.T. Silbermann, who designed some interesting heliostats in the mid-19th Century. His given names are stated to be "Jean Thiébault". In fact, Silbermann was born in Alsace, a region of France adjacent to Germany, where many people speak a dialect of German and have German names. The names he was given were "Johann Theobald'. He may have adopted the Frenchified versions later in life, or maybe they are just assigned to him by other Frenchmen.
Overview of Heliostat Technologies
I originally posted the following material on another website. I wrote it from my own knowledge of the subject, without using or quoting any references. Maybe the editors of Wikipedia would prefer me not to put it on the main page here, for that reason. If they wish to move it onto the main page, they are welcome to do so.
The types of heliostat described below provide an overview of the ones that are in existence, or have been in the past. The list should not be regarded as exhaustive. There are heliostats, especially ones that have been built by amateur hobbyists, which do not fit into any of these categories. For example, there are machines which are similar to clockwork heliostats, but which are driven by electric or electronic clocks.
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.
More elaborate clockwork heliostats have been made that use only one mirror, 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 Frenchman 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]
Some anonymous user added:
However this requires that the sensors are able to tell the computer exactly where they are pointing which could involve more expensive sensors. So it is not completely clear that it is the best solution ?
Stepper motors are often used, which eliminate the need for direction sensors. The mirror is initialized to some known orientation, determined by fixed limit switches. From there, the computer sends the required number of stepping instructions to the motors so they turn the mirror to any other desired orientation. The motors are geared down so each step turns the mirror through a very small angle, small compared with the angular size of the sun.
Some more technology
The above few lines convince me to describe 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 couple of seconds per year.
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.
To George Plhak:
The video you put in the External Links section shows a solar tracker, rather than a heliostat. I have therefore moved it to the Solar tracker page.
Equation of Time in Tracking
It is a common error to think that variations in the Equation of Time are unimportant in adjusting a continuously-rotating mirror. In fact, at its extremes, which occur around the end of October and mid-February, the Equation of Time is equivalent to a shift in the direction of the sun of about four degees, compared with its mean position. During the 100 days or so between these dates, the direction changes by about 8 degrees, which is equivalent to the angle through which the sun moves in about half an hour. So the setting of the drive clock has to be changed by half an hour during this period.
The shift of 8 degrees in 100 days, or an average of 0.08 degrees per day, can be compared with the change in the sun's declination of about 47 degrees in the 180 days or so between the winter and summer solstices, or an average of about 0.26 degrees per day. So the rate of change of declination is about 3 times faster than the rate in change of the sun's longitude. If it is necessary to adjust the mirror every day to compensate adequately for the declination change, it is also necessary to readjust the clock a couple of times per week to compensate for the change in the Equation of Time. Of course, this is only when the Equation of Time is changing rapidly, around December and January. At other times of year, it changes much more slowly.
Mirror turning speed
Somebody anonymously left the following comment this morning:
- I note the article say the clock work driver would move the reflector at 15 degrees an hour. i believe it would be 7.5 degrees. the sun 'moves' at 15 degrees/hour the plane of the mirror on has to bisect this. logically if the sun were up 24hours a day when the sun were behind the mirror and in line with the tower you would need the mirror to flip. this is where the missing 180 degrees go.
No. In the common designs of clockwork heliostat, the mirror rotates at 15 degrees per hour.
Heliostats have been made with mirrors turning at 7.5 degrees per hour, but they work only under restricted conditions. The declination of the target has to be equal and opposite to that of the sun. Otherwise, the geometry doesn't work. Also, the mirror has to be double-sided, so it reflects with both sides. Otherwise, the mirror has to be reset every day. (I think that is what he means by saying the mirror would need to flip.)
No. We weren't talking about moving targets. However, machines have been made that reflect sunlight onto moving targets, and they are called "heliostats". The original meaning of the word has been extended to cover these newer devices. DOwenWilliams (talk) 15:15, 17 December 2011 (UTC)