الواح الهيلوستات الشمسية لتحويل الطاقة الشمسية الى طاقة حرارية فى محطات البرج المركزى لتوليد طاقة كهربية

[size=96]Heliostat[/size]

A heliostat (from helios, the Greek word for sun, and stat, as in stationary)

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.

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, e.g. 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 installations such as solar-thermal power stations include fields of heliostats comprising many mirrors.

Usually, all the mirrors in such a field are controlled by a single computer.

Heliostats should be distinguished from solar trackers or sun-trackers

that point directly at the sun in the sky.

Large-scale projects

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 onto 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.

All the heliostat mirrors send accurately parallel beams of light into a large paraboloidal reflector which brings them to a precise focus.

The mirrors have to be located close enough to the axis of the paraboloid to reflect sunlight into it along lines parallel to the axis, so the field of heliostats has to be narrow.

A closed loop control system is used.

Sensors determine if any of the heliostats is slightly misaligned.

If so, they send signals to correct it.

Design

Heliostat costs represent 30-50% of the initial capital investment for solar power tower power plants

depending on the energy policy and economic framework in the location country.

It is of interest to design less expensive heliostats for large scale manufacturing, so that solar power tower power plants may produce electricity at costs more competitive to conventional coal or nuclear power plants costs.

Besides cost, percent solar reflectivity (i.e. albedo) and environmental durability are factors that should be considered when comparing heliostat designs.

to lower the costs of heliostats is by replacing the conventional heliostat

design with one that uses fewer, lighter materials.

A conventional design for the heliostat's reflective components utilizes a second surface mirror.

The sandwich-like mirror structure generally consists of a steel structural support, an adhesive layer, a protective copper layer, a layer of reflective silver, and a top protective layer of thick glass.

This conventional heliostat is often referred to as a glass/metal heliostat.

Alternative designs incorporate recent adhesive, composite, and thin film research to bring about materials costs and weight reduction.

Some examples of alternative reflector designs are silvered polymer reflectors, glass fiber reinforced polyester sandwiches (GFRPS), and aluminized reflectors.

Problems with these more recent designs include delamination of the protective coatings, reduction in percent solar reflectivity over long periods of sun exposure, and high manufacturing costs.

Tracking alternatives

The movement of most modern heliostats employs a two-axis motorized system, controlled by computer as outlined at the start 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 the earth's rotation relative 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 allowing occasional 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 clock can also be occasionally adjusted to compensate for changes in the Equation of Time.

The target can be located on the same polar axis that is the mirror's primary rotation axis, or a second, stationary mirror can be used to reflect light from the polar axis toward the target, wherever that might be.

The alt-azimuth and polar-axis alignments are two of the three orientations for two-axis mounts that are, or have been, commonly used for heliostat mirrors.

The third is thetarget-axis arrangement in which the primary axis points toward the target at which sunlight is to be reflected.

The secondary axis is perpendicular to the primary one.

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 arrangement that satisfies the definition of a heliostat yet has a mirror motion that is 2/3rd of the motion of the sun.

What is a heliostat?

it is a large mirror (or many mirrors) attached to a rotating pole or platform (or many poles and platforms).

Unlike solar panels, heliostats don’t directly absorb the sun; instead, they use mirrors to redirect the sun’s light and aim it at stationary solar panels for absorption.

Heliostats are mostly controlled by computers.

These computers are fed certain pieces of data (the location of the heliostat,

the location of the solar panel, the time and date) and the data is crunched until the computer can calculate the sun’s position in the sky.

Once that’s done,

the computer adjusts the mirror’s angle so that the sun’s light will bounce off of it and hit the target solar panel.

The greatest benefit of the heliostat is that a multitude of them can be arranged to be aimed at a single solar receptor.

Whereas normally a solar panel might only receive some coverage of sunlight, an arrangement of heliostats can drastically amplify the amount of light being converted.

How Practical Solar's heliostat system can be used

applications include:

Water desalinization

Electricity generation

Residential heating and cooling

Contaminated soil remediation

Agricultural drying and curing

Milk pasteurization

Increasing greenhouse productivity

Melting snow and ice

Providing sunlight to dark spaces

How Practical Solar's heliostat system works

Reflecting energy:

A Practical Solar heliostat is a mirror -- not a solar panel or other complex material – that makes precise movements up/down and left/right to reflect sunlight onto a fixed spot.

As the sun marches across the sky, the heliostat adjusts its position, so the spot of reflected light remains stationary on the target.

The position of the sun in the sky is calculated using an astronomical algorithm.

If the latitude, longitude, date, and time of day are known accurately, the position of the sun can be calculated exactly.

Even small errors such as the refraction of the atmosphere near the horizon, or the tilt of the pole on which the heliostat is mounted, are compensated for in the software.

The relative spherical angular position of the thermal receiver to the heliostat is inputted to the computer.

The computer solves the spherical trigonometry problem and commands the heliostat motor drive system such that the mirror is positioned angularly exactly halfway between the sun and the thermal receiver.

Because every heliostat in an installation is in a unique position relative to the thermal receiver, each heliostat receives unique position commands.

Concentrating energy:

When multiple Practical Solar heliostats reflect sunlight onto a single thermal receiver, the concentrated heat of the sunlight can be used to produce hot water or steam.

Although such a system can generate temperatures capable of melting steel, the temperature of the water is raised to just within a degree of boiling.

The mirrors on Practical Solar heliostats have minimal reflection loss, so each heliostat reflects approximately its area in sunlight:

about 1 kilowatt of heat per square meter.

As a frame of reference, a typical electric space heater produces 1.5 kilowatts of heat.

If one hundred Practical Solar heliostats, each with 2.2 square meters of mirror area, direct sunlight onto a single thermal receiver, the sunlight will be converted into 220 kilowatts of heat.

Storing and distributing energy:

However, the sun never shines at night, and often does not shine during the day.

Energy storage is a fundamental requirement in any serious solar energy application.

The reverse is true of thermal energy.

Hot water can be stored cheaply in a thermally insulated tank.

As the volume of water and energy stored increases, the cost and losses of thermal energy storage drop rapidly.

The stored thermal energy acts as the boiler, but is larger than a conventional boiler and is charged by the heliostat array rather than fossil fuel.

Why Practical Solar's heliostat system is unique

At present, Practical Solar’s heliostats combine the highest known levels of accuracy, reliability, and affordability available in heliostat technology.

This combination of features makes it the only solar product that can compete against fossil fuels for heating and cooling commercial buildings.

The following are some of the important manufacturing advances Practical Solar has brought to its heliostat design:

Accuracy:

The mirrors on Practical Solar’s heliostats need to be positioned up/down and left/right to continuously reflect sunlight to the thermal receiver, as the sun moves through the sky.

The Practical Solar heliostat design employs two proprietary high-accuracy rotary position encoders that allow the reflected beam of sunlight to be positioned to within three inches, at a distance of one hundred feet from the receiver.

Encoders with such accuracy commonly cost hundreds of dollars each. Practical Solar has reduced the manufacturing cost to less than ten dollars per axis.

Reliability:

Practical Solar heliostats use small DC motors to drive powdered metal steel gears with a total gear ratio of many thousands to one.

Control:

The Practical Solar heliostat employs two classes of computers:

(1) embedded micro-controllers inside each heliostat that control a myriad of low level tasks

(2) a computer in the commercial building that runs Windows-based software capable of controlling the motion of thousands of individual heliostats.

Elegance:

Because each individual heliostat requires so little electrical power, the size of the wire is quite modest.

A typical array of 100 heliostats requires a single 18 AWG twisted pair cable and consumes 20 Watts of electricity, while delivering 200,000 Watts of thermal power to a central receiver.

How other solar technologies compare

Traditional photovoltaic solar panels are “passive”, meaning they have no moving parts and do not track the sun.

Solar panels convert sunlight to electricity with an average efficiency of 7-12 percent.

Practical Solar’s heliostat system concentrates the raw heat already in sunlight and distributes it through a building – in the form of heating or cooling – with an average efficiency of 85-90 percent.

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.

Manually-Operated Heliostats

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.

Clockwork Heliostats

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.

Computer-Controlled Heliostats

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.

[ltr][size=32]Heliostat Array System Power.[/size][/ltr]

[ltr]200ft² array produces about:[/ltr]

[ltr]2.2[/ltr]	[ltr]KW peak electric.[/ltr]
[ltr]9[/ltr]	[ltr]KW peak thermal.[/ltr]
[ltr]36[/ltr]	[ltr]MBtu/hour peak thermal.[/ltr]
[ltr]20[/ltr]	[ltr]KWhours/day electric.[/ltr]
[ltr].26[/ltr]	[ltr]MBtu/day thermal.[/ltr]
[ltr]81[/ltr]	[ltr]KWhours/day thermal.[/ltr]

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