Abstract

• This paper reports the design, construction and testing of a

parabolic dish solar steam generator. Using concentrating

collector, heat from the sun is concentrated on a black absorber

located at the focus point of the reflector in which water is

heated to a very high temperature to form steam. It also

describes the sun tracking system unit by manual tilting of the

lever at the base of the parabolic dish to capture solar energy.

The whole arrangement is mounted on a hinged frame

supported with a slotted lever for tilting the parabolic dish

reflector to different angles so that the sun is always directed to

the collector at different period of the day. On the average

sunny and cloud free days, the test results gave high

temperature above 200°C.

Chapter ( 1 )

1. Introduction:


• In today's climate of growing energy needs and increasing environmental
concern, alternatives to the use of non-renewable and polluting fossil fuels
have to be investigated. One such alternative is solar energy.


• Solar energy is quite simply the energy produced directly by the sun and
collected elsewhere, normally the Earth. The sun creates its energy
through a thermonuclear process that converts about 650,000,000 tons of
hydrogen to helium every second. The process creates heat and
electromagnetic radiation. The heat remains in the sun and is instrumental
in maintaining the thermonuclear reaction. The electromagnetic radiation
(including visible light, infra-red light, and ultra-violet radiation) streams out
into space in all directions.


• Only a very small fraction of the total radiation produced reaches the
Earth. The radiation that does reach the Earth is the indirect source of
nearly every type of energy used today. The exceptions are geothermal
energy, and nuclear fission and fusion. Even fossil fuels owe their origins
to the sun; they were once living plants and animals whose life was
dependent upon the sun.


• Much of the world's required energy can be supplied directly by solar
power. More still can be provided indirectly. The practicality of doing so
will be examined, as well as the benefits and drawbacks. In addition, the
uses solar energy is currently applied to will be noted.


• Due to the nature of solar energy, two components are required to have a
functional solar energy generator. These two components are a collector
and a storage unit. The collector simply collects the radiation that falls on it
and converts a fraction of it to other forms of energy (either electricity and
heat or heat alone). The storage unit is required because of the nonconstant
nature of solar energy; at certain times only a very small amount
of radiation will be received. At night or during heavy cloudcover, for
example, the amount of energy produced by the collector will be quite
small. The storage unit can hold the excess energy produced during the
periods of maximum productivity, and release it when the productivity
drops. In practice, a backup power supply is usually added, too, for the
situations when the amount of energy required is greater than both what is
being produced and what is stored in the container.


• Methods of collecting and storing solar energy vary depending on the uses
planned for the solar generator. In general, there are three types of
collectors and many forms of storage units.

·          • The three types of collectors are flat-plate collectors, focusing collectors, and passive collectors.

·        • Flat-plate collectors are the more commonly used type of collector today. They are arrays of solar panels arranged in a simple plane. They can be of nearly any size, and have an output that is directly related to a few variables including size, facing, and cleanliness. These variables all affect the amount of radiation that falls on the collector. Often these collector panels have automated machinery that keeps them facing the sun. The additional energy they take in due to the correction of facing more than compensates for the energy needed to drive the extra machinery.

·   • Focusing collectors are essentially flat-plane collectors with optical devices arranged to maximize the radiation falling on the focus of the collector. These are currently used only in a few scattered areas. Solar furnaces are examples of this type of collector. Although they can produce far greater amounts of energy at a single point than the flat-plane collectors can, they lose some of the radiation that the flat-plane panels do not. Radiation reflected off the ground will be used by flat-plane panels but usually will be ignored by focusing collectors (in snow covered regions, this reflected radiation can be significant). One other problem with focusing collectors in general is due to temperature. The fragile silicon components that absorb the incoming radiation lose efficiency at high temperatures, and if they get too hot they can even be permanently damaged. The focusing collectors by their very nature can create much higher temperatures and need more safeguards to protect their silicon components.

·       •    Passive collectors are completely different from the other two types of collectors. The passive collectors absorb radiation and convert it to heat naturally, without being designed and built to do so. All objects have this property to some extent, but only some objects (like walls) will be able to produce enough heat to make it worthwhile. Often their natural ability to convert radiation to heat is enhanced in some way or another (by being painted black, for example) and a system for transferring the heat to a different location is generally added.

·      •  People use energy for many things, but a few general tasks consume most of the energy. These tasks include transportation, heating, cooling, and the generation of electricity. Solar energy can be applied to all four of these tasks with different levels of success.

·       •   Heating is the business for which solar energy is best suited. Solar heating requires almost no energy transformation, so it has a very high efficiency. Heat energy can be stored in a liquid, such as water, or in a packed bed. A packed bed is a container filled with small objects that can hold heat (such as stones) with air space between them. Heat energy is also often stored in phase-changer or heat-of-fusion units. These devices will utilize a chemical that changes phase from solid to liquid at a temperature that can be produced by the solar collector. The energy of the collector is used to change the chemical to its liquid phase, and is as a result stored in the chemical itself. It can be tapped later by allowing the chemical to revert to its solid form. Solar energy is frequently used in residential homes to heat water. This is an easy application, as the desired end result (hot water) is the storage facility. A hot water tank is filled with hot water during the day, and drained as needed. This application is a very simple adjustment from the normal fossil fuel water heaters. 

·          •Swimming pools are often heated by solar power. Sometimes the pool itself functions as the storage unit, and sometimes a packed bed is added to store the heat. Whether or not a packed bed is used, some method of keeping the pool's heat for longer than normal periods (like a cover) is generally employed to help keep the water at a warm temperature when it is not in use.

·          •Solar energy is often used to directly heat a house or building. Heating a building requires much more energy than heating a building's water, so much larger panels are necessary. Generally a building that is heated by solar power will have its water heated by solar power as well. The type of storage facility most often used for such large solar heaters is the heat-of-fusion storage unit, but other kinds (such as the packed bed or hot water tank) can be used as well. This application of solar power is less common than the two mentioned above, because of the cost of the large panels and storage system required to make it work. Often if an entire building is heated by solar power, passive collectors are used in addition to one of the other two types. Passive collectors will generally be an integral part of the building itself, so buildings taking advantage of passive collectors must be created with solar heating in mind.

·         •These passive collectors can take a few different forms. The most basic type is the incidental heat trap. The idea behind the heat trap is fairly simple. Allow the maximum amount of light possible inside through a window (The window should be facing towards the equator for this to be achieved) and allow it to fall on a floor made of stone or another heat holding material. During the day, the area will stay cool as the floor absorbs most of the heat, and at night, the area will stay warm as the stone re-emits the heat it absorbed during the day.

·          •Another major form of passive collector is thermosyphoning walls and/or roof. With this passive collector, the heat normally absorbed and wasted in the walls and roof is re-routed into the area that needs to be heated.

·          •The last major form of passive collector is the solar pond. This is very similar to the solar heated pool described above, but the emphasis is different. With swimming pools, the desired result is a warm pool. With the solar pond, the whole purpose of the pond is to serve as an energy regulator for a building. The pond is placed either adjacent to or on the building, and it will absorb solar energy and convert it to heat during the day. This heat can be taken into the building, or if the building has more than enough heat already, heat can be dumped from the building into the pond.

·        •  Solar energy can be used for other things besides heating. It may seem strange, but one of the most common uses of solar energy today is cooling. Solar cooling is far more expensive than solar heating, so it is almost never seen in private homes. Solar energy is used to cool things by phase changing a liquid to gas through heat, and then forcing the gas into a lower pressure chamber. The temperature of a gas is related to the pressure containing it, and all other things being held equal, the same gas under a lower pressure will have a lower temperature. This cool gas will be used to absorb heat from the area of interest and then be forced into a region of higher pressure where the excess heat will be lost to the outside world. The net effect is that of a pump moving heat from one area into another, and the first is accordingly cooled.

·          •Besides being used for heating and cooling, solar energy can be directly converted to electricity. Most of our tools are designed to be driven by electricity, so if you can create electricity through solar power, you can run almost anything with solar power. The solar collectors that convert radiation into electricity can be either flat-plane collectors or focusing collectors, and the silicon components of these collectors are photovoltaic cells.

·        •  Photovoltaic cells, by their very nature, convert radiation to electricity. This phenomenon has been known for well over half a century, but until recently the amounts of electricity generated were good for little more than measuring radiation intensity. Most of the photovoltaic cells on the market today operate at an efficiency of less than 15%; that is, of all the radiation that falls upon them, less than 15% of it is converted to electricity. The maximum theoretical efficiency for a photovoltaic cell is only 32.3%, but at this efficiency, solar electricity is very economical. Most of our other forms of electricity generation are at a lower efficiency than this. Unfortunately, reality still lags behind theory and a 15% efficiency is not usually considered economical by most power companies, even if it is fine for toys and pocket calculators. Hope for bulk solar electricity should not be abandoned, however, for recent scientific advances have created a solar cell with an efficiency of 28.2% efficiency in the laboratory. This type of cell has yet to be field tested. If it maintains its efficiency in the uncontrolled environment of the outside world, and if it does not have a tendency to break down, it will be economical for power companies to build solar power facilities after all.

·        •  Of the main types of energy usage, the least suited to solar power is transportation. While large, relatively slow vehicles like ships could power themselves with large onboard solar panels, small constantly turning vehicles like cars could not. The only possible way a car could be completely solar powered would be through the use of battery that was charged by solar power at some stationary point and then later loaded into the car. Electric cars that are partially powered by solar energy are available now, but it is unlikely that solar power will provide the world's transportation costs in the near future.

·        •  Solar power has two big advantages over fossil fuels. The first is in the fact that it is renewable; it is never going to run out. The second is its effect on the environment.

·         •While the burning of fossil fuels introduces many harmful pollutants into the atmosphere and contributes to environmental problems like global warming and acid rain, solar energy is completely non-polluting. While many acres of land must be destroyed to feed a fossil fuel energy plant its required fuel, the only land that must be destroyed for a solar energy plant is the land that it stands on. Indeed, if a solar energy system were incorporated into every business and dwelling, no land would have to be destroyed in the name of energy. This ability to decentralize solar energy is something that fossil fuel burning cannot match.

·      •    As the primary element of construction of solar panels, silicon, is the second most common element on the planet, there is very little environmental disturbance caused by the creation of solar panels. In fact, solar energy only causes environmental disruption if it is centralized and produced on a gigantic scale. Solar power certainly can be produced on a gigantic scale, too.

·        •  Among the renewable resources, only in solar power do we find the potential for an energy source capable of supplying more energy than is used.

·     •     Suppose that of the 4.5x10¹⁷ kWh per annum that is used by the earth to evaporate water from the oceans we were to acquire just 0.1% or 4.5x10¹⁴ kWh per annum. Dividing by the hours in the year gives a continuous yield of 2.90x10¹⁰ kW. This would supply 2.4 kW to 12.1 billion people.

·    •      This translates to roughly the amount of energy used today by the average American available to over twelve billion people. Since this is greater than the estimated carrying capacity of the Earth, this would be enough energy to supply the entire planet regardless of the population.

·         •Unfortunately, at this scale, the production of solar energy would have some unpredictable negative environmental effects. If all the solar collectors were placed in one or just a few areas, they would probably have large effects on the local environment, and possibly have large effects on the world environment. Everything from changes in local rain conditions to another Ice Age has been predicted as a result of producing solar energy on this scale. The problem lies in the change of temperature and humidity near a solar panel; if the energy producing panels are kept non-centralized, they should not create the same local, mass temperature change that could have such bad effects on the environment.

·       •   Of all the energy sources available, solar has perhaps the most promise. Numerically, it is capable of producing the raw power required to satisfy the entire planet's energy needs. Environmentally, it is one of the least destructive of all the sources of energy. Practically, it can be adjusted to power nearly everything except transportation with very little adjustment, and even transportation with some modest modifications to the current general system of travel. Clearly, solar energy is a resource of the future.

CHAPTER ( 2 )

Section (1) : Flat Plate Collector

2.1 Flat-Plate collectors

• Flat-plate collectors are in wide use for domestic household hot-water

heating and for space heating, where the demand temperature is low.

• The construction of a flat-plate collector :

2.1.1 Full - Aperture Absorber:

• The absorber is usually a sheet of high - thermalconductivity metal with tubes or ducts either integral or attached. Its surface is painted or coated to maximize radiant energy absorption and in some cases to minimize radiant emission.

2.1.2 Transparent or translucent cover sheets

• The cover sheets, called glazing, let sunlight pass through to the

absorber but insulate the space above the absorber to prohibit cool

air from flowing into this space.

2.1.3 An insulated box

• The insulated box provides structure and sealing and reduces heat

loss from the back or sides of the collector.

2.1.1 The main element of a flat-plate collector is the absorber
plate. It covers the full aperture area of the collector and must
perform three functions

• Absorb the maximum possible amount of solar irradiance
• Conduct this heat into the working fluid at a minimum
temperature difference

• Lose a minimum amount of heat back to the surroundings.

Absorption

• Solar irradiance passing through the glazing is absorbed directly on the
absorber plate without intermediate reflection as in concentrating
collectors. Surface coatings that have a high absorptance for shortwavelength
(visible) light, are used on the absorber. Usually these
coatings appear dull or "flat," indicating that they will absorb radiation
coming from all directions equally well. Either paint or plating is used, and
the resulting black surface will typically absorb over 95 percent of the
incident solar radiation.

Fin Heat Removal

• The second function of the absorber plate is to transfer the absorbed
energy into a heat-transfer fluid at a minimum temperature difference. This
is normally done by conducting the absorbed heat to tubes or ducts that
contain the heat-transfer fluid. The heat-transfer fluid may either be a
liquid (water or water with antifreeze) or gas (air). The important design
criterion here is to provide sufficient heat transfer capability that the
difference between the temperature of the absorber surface and the
working fluid is not excessive; otherwise, the heat loss from the absorber
would be excessive. High heat-transfer rates are usually accomplished at

the expense of pumping power and absorber plate material.


• When a liquid is used as the heat-transfer fluid as is most often the case,
special problems occur in transferring the heat absorbed on the absorber
surface into the fluid. Liquid collector absorber plates often consist of a flat
sheet of metal with tubes spaced 10-25 cm (4-10 in.) apart and attached
to it in some fashion (integral, brazed or press fitted). The sheet of metal
absorbs most of the solar irradiance and acts as a fin to bring the
absorbed heat into the fluid. The following are important points in
designing a good ‘tube and sheet’ absorber:

1. The fin (absorber sheet) must he made of a material with high thermal
conductivity.

2. The fin should be thick to minimize the temperature difference required to
transfer heat to its base (tube).

3. Tubes should not be spaced too far apart; otherwise, a higher temperature
difference between the tip of the fin (midway between the tubes) and the
base will result.

4. Tubes should be thin-walled and of a high-thermal -conductivity material.

5. The tube should be brazed or welded to the absorber sheet to minimize
thermal contact resistance.

6. The tube and absorber sheet should be of similar material to prevent

galvanic corrosion between them.


• When air is the heat-transfer fluid, often the back side of the absorber
plate usually forms one surface of a duct and heat is transferred through
the absorber sheet to the air over the entire back surface of the absorber.
A thin, rather than thick, absorber sheet of high-thermal-conductivity
material will enhance this heat-transfer process. The internal air passage
must be designed to provide a sufficiently high airflow velocity past the
back of the absorber to give adequate heat transfer without producing a
high pressure drop across the collector. Low heat-transfer rates cause the
absorber plate to become significantly hotter than the heat-transfer fluid,
which increases heat loss. On the other hand, a large pressure drop
across the collector causes high pumping power consumption by the fans

supplying the air.

Emittance

• Because the temperature of the absorber surface is above ambient

temperature, the surface re-radiates some of the heat it has absorbed
back to the surroundings. This loss mechanism is a function of the
emittance of the surface for low-temperature, long-wavelength (infrared)
radiation. The dilemma is that many coatings that enhance the absorption
of sunlight (short-wavelength radiation) also enhance the long wavelength
radiation loss from the surface. This is true for most dull black paints.


• A class of coatings, mostly produced by metallic plating processes, will
produce an absorber surface that is a good absorber of short-wavelength
solar irradiance but a poor emitter of long-wavelength radiant energy. The
function of these coatings, called selective surfaces. Flat-plate absorbers
that have selective surfaces typically lose less heat when operating at high
temperature,. However, the absorptance of selective coatings is seldom
as high as for non-selective coatings, and a tradeoff must be made based
on whether the increased high-temperature performance overshadows the
reduced low-temperature performance and expense of the selective
coating.

• The absorber is usually covered with one or more transparent or
translucent cover sheets to reduce convective heat loss. In the absence of
a cover sheet, heat is lost from the absorber as a result of not only forced
convection caused by local wind, but also natural convective air currents
created because the absorber is hotter than ambient air. The cover sheet
forms a trapped air space above the absorber, thereby reducing these
losses. However, convective loss is not completely eliminated because a
convective current is set up between the absorber and the cover sheet,
transferring heat from the absorber to the cover sheet. External convection
then cools the cover sheet, producing a net heat loss from the absorber. In
addition, heat loss is reduced because of the thermal resistance of the

added air space.


2.1.2 covers


• Number of Covers The number of cover sheets on commercial flat-plate
collectors varies from none to three or more. Collectors with no cover
sheet have high efficiencies when operated at temperatures very near
ambient temperature. This is because incoming energy is not lost by
absorption or reflection by the cover sheet. When no cover sheet is used,
however, a considerable amount of the incident energy is lost during
operation at temperatures much above ambient or at low solar irradiance
levels. A typical application for an uncovered flat-plate collector is for
swimming pool heating, where temperatures less than 10ºC (18ºF) above
ambient are required.


• Increases in the number of cover-sheets increases the temperature at
which the collector can operate (or permits a given temperature to be
reached at lower solar irradiance). One or two cover sheets are common,
but triple glazed collectors have been designed for extreme climates. In
addition to the added expense, each added cover sheet increases the
collection efficiency at high temperature by reducing convection loss but
decreases the efficiency at low temperatures because of the added
absorption and reflectance of the cover.


• In regions of average mid-latitude temperatures and solar radiation,
collectors with no glazing are generally used for applications to 32ºC
(90ºF), single-glazed collectors are used for applications to 70ºC (158º F),
and double-glazing is used in applications above 70ºC (158ºF). As
discussed in Chapter 5, collector efficiency increases with increasing solar
irradiance level but decreases with increasing operating temperature. In
regions of low average solar irradiance or extremely low temperatures,
therefore, double-glazed collectors are used in applications where singleglazed
collectors should be used normally and single-glazed collectors for
unglazed applications. Also, selective absorber surfaces become more

worthwhile.


2.1.3 Materials


• Because of its superior resistance to the environment, glass is used as
the outer cover sheet on most commercial collectors. Usually the glass is
tempered, with a low iron content and 3.2-6.4 mm (0.12-0.25 in.) thick.
The surface may be either smooth, making the glass transparent, or with
a surface pattern, making it translucent. Both types have a transmittance
of around 90 per cent.


• Plastic cover sheets are sometimes used for the second cover sheet
when two sheets are required. Installation of the plastic sheet beneath the
glass protects the plastic from the environment. Glass also does not
transmit UV radiation and thus protects the plastic, which is usually
sensitive to this portion of the solar spectrum. Rigid sheets of acrylic-or
fiberglass-reinforced polymers are in use, as are stretched films of
polyvinyl fluoride. Some of these plastic cover sheets have a
transmittance approaching that of low iron glass. A major draw back of
this scheme is the potential for overheating the plastic sheet at collector

stagnation (no-flow) temperatures.

Advantages of flat plate collector:

• Flat-plate collectors will absorb energy coming from all directions above

the absorber (both beam and diffuse solar irradiance). Because of this
characteristic, flat-plate collectors do not need to track the sun. They
receive more solar energy than a similarly oriented concentrating collector,
but when not tracked, have greater cosine losses.


• Since tracking is not required, flat-plate collectors may be firmly fixed to a
mounting structure, and rigid plumbing may be used to connect the
collectors to the remainder of the system. Moving structure, motors, and
tracking control systems are eliminated, thereby reducing the complexity
of the system, however, because of the cosine effect, less total energy
falls on a fixed surface over the period of a day than on a surface that
tracks the sun about one or two axes.


• In order to increase their output, flat-plate collectors may be repositioned
at intervals or placed on a single- or two-axis tracking mechanism. Either
of these options increases the output of the collector but eliminates the

advantage of fixed piping and mounting structure.


• A flat-plate collector absorbs both the direct and the diffuse components of
solar radiation. This partially compensates for the fact that fixed surfaces
receive less energy because of the cosine effect. Although the diffuse
solar irradiance is only about 10 percent of the direct normal solar
irradiance on a clear day, on a cloudy day almost all of the available solar
irradiance is diffuse.


• Currently, flat-plate collectors cost less than concentrating collectors. Part
of reason is the lack of need for a complex tracking system. However, part
of the reason is because many more flat-plate than concentrating
collectors are being produced today. At large production rates, however, it
is still not clear whether it is less expensive to cover an aperture area with

an absorber plate or with reflective material.

Section (2) : Solar Beam Tower

2.2 Solar beam tower

2.2.1 Introduction

Solar power has the potential to solve all of humankind’s energy problems. The

amount of solar energy that reaches the surface of the Earth, per year, is

somewhere in the region of 8,000 times higher than our current power

requirements. If we were to cover just 1% (or indeed 0.01%) of the Earth’s

surface in solar panels, we would have more than enough electricity to satiate

society’s current and future needs. It isn’t that simple, of course, otherwise we

would’ve already done it.

Solar power
towers

have proven to be
a fairly efficient
way of converting
solar energy into
electricity. In a
solar power tower,
energy from a
large array of
mirrors focused
onto a tower that captures the heat in some way, and then converts that heat to electricity using a boiler and turbine. It's a great system, but building that heat-resistant tower and pumping all of those fluids up and down can be pretty expensive.
Which is why researchers at the Masdar Institute , the Tokyo Institute of
Technology and Cosmo Oil are working together on "beam down" solar. Instead
of having the heat-capturing system up on that big tower, a second set of mirrors
directs the light back down at the ground where it can be captured by a system
that doesn't have to be suspended many stories in the air.
It's certainly cheaper than a traditional solar power tower. The bad news is that
the extra set of mirrors lowers the efficiency of the system by about 20%. If that
can be made up for with reduced capital costs, however, they could be in
business. In the end, creating cheap ways of capturing solar energy is probably
going to be more important than creating efficient ways.

2.2.2 Heliostats

The tool 

A heliostat consists of a large mirror and
the mechanisms and circuitry necessary to control it, such that the heliostat tracks the sun and
reflects the light onto a given target throughout the day.
A heliostat array is a collection of these heliostats used to focus sunlight continuously on a central receptoroften called a power tower. In the context
of generating power, a heliostat array is usually defined as fifty or more mirrors with total a reflective area of at least 1076.39 square feet. The heliostat concept is by no means new, but only in recent years has the growing threat of a global energy crisis sparked enough interest in renewable energy sources to spur
heliostat development to the point of usefulness. 
Concentrated sunlight striking a non-reflective target generates a tremendous
amount of heat, which can be used directly or employed to drive a power cycle.
Current heliostat arrays use the energy of focused sunlight to heat a circulating
fluid, usually molten sodium nitrate. The fluid is cycled through a conventional
power block to generate electricity using the same boiler and steam-turbine
technologies employed in traditional fossil fuel and nuclear power plants. The
extreme heat generated in this manner may also be used for other purposes,
such as waste incineration.

The Benefits

Heliostat arrays offer a number of advantages from the clean and renewable

energy standpoint. First of all, they can transform as much as 90% of solar
energy received into heat. In contrast, conventional photovoltaic cells operate at
a maximum efficiency of about 15%. Heliostat arrays generate heat that can be
used to run boilers and drive steam-turbine power cycles. This improves the
efficiency and feasibility of the system by allowing it to take advantage of the
well-developed boiler and turbine technologies already in use by fossil fuel and
nuclear power plants around the world.
When all stages of implementation are considered, from manufacture to disposal,
heliostats are among the cleanest energy sources available. Heliostats are
primarily composed of steel and glass, both of which are readily recyclable.
Neither steel nor glass is considered particularly hazardous to human health or
the environment. In contrast, the manufacture of photovoltaic cells requires
chemicals whose lethal concentration in air is on the order of parts per billion. It is
not a problem when only a few small solar panels are being utilized to power
things such as roadside signs and electronic calculators, but manufacturing
enough solar panels to supply the world's energy demands may be hazardous to
both mankind and the environment.
Finally, heliostat arrays do not suffer from the ongoing natural degradation that
plagues selenium-based solar panels. They offer a comparatively long service
life, minimizing the frequency of manufacturing new heliostats and disposing of
old ones.
Because heliostats are not a new technology, there have been various designs
built over time, which come in many shapes and sizes and are comprised of
many different systems. As a result, it is standard to normalize costs of different
models by comparing them on the basis of cost per 10.7639 square feet of mirror
surface. This type of analysis compares component costs for annual production
figures of 5,000-50,000 units per year.

The Problem

In a conventional heliostat system, the mirror is actuated by a two-axis

servomotor drive system mounted on a concrete or steel pedestal. The motor
drive system requires large gear reduction in order to position the mirror with
sufficient accuracy and rigidity, with the consequence that this gear-based drive
system is by far the single most expensive component of the heliostat (~$5,000 -
$7,000, depending on size). Drawings of a conventional heliostat are shown in

Figure 2 and Figure 3.

Figure 2 : Front view of a conventional heliostat

Figure 3 : Rear view of a conventional heliostat; note the two-axis drive system

Second only to the drive system, installation costs are also significant. To provide

a rigid base for the gear drive system, a pedestal (steel or concrete) is sunk into

the ground. As a result, the on-site drilling and pouring of the pedestals are

significant expenses in the construction of an array of conventional heliostats.

Also, these heliostats are nearly permanent and expensive to remove and

transport.

The mirror itself, while not the most expensive part of the system, is nonetheless

a significant factor in the cost of the design. Ideally, heliostats are constructed

using mirrors with a slight parabolic curvature tuned to focus sunlight down to a

fine point at a precise distance from the heliostat. Usually the sunlight is focused

on a central target, typically referred to as a power tower, where the heat is

collected. This curvature allows for greater concentration of solar energy at the

power tower by reducing the area over which the reflected sunlight is spread.

The reduction in area also allows the light receptor to be smaller, reducing tower

costs. However, manufacturing a parabolic mirror is much more expensive than

manufacturing a flat one, and a mirror rigid enough to hold a parabolic shape and

large enough to be used for a heliostat is extremely heavy.

The Solution

In order to solve this problem, a team from the Mechanical Engineering

Department at New Mexico Tech has been designing and fabricating a new type

of heliostat that relies upon a liquid-ballast drive system. This design is much

more cost-effective and mobile than current heliostat designs. If implemented

correctly, this new design can make heliostats cost-competitive with conventional

fuel sources.

2.2.3 Solar power towers of Seville (Spain)

In a patchwork of agricultural

fields outside Seville, Spain, two giant 40-storey-high concrete towers rise. The obelisk-like structures are surrounded by an immense array of mirrors that reflect sunlight, bathing the top of the towers with a blinding white light. The rays of sunlight reflected by

hundreds of huge mirrors are so intense that they illuminate the water vapor and dust hanging in the air creating visible beams. The otherworldly spectacle is the world’s first commercially operating power station using the Sun's thermal energy to produce steam, which is used to power turbines to generate electricity.

The plant’s operator, Abengoa Solar, claims that it generates 11 Megawatts

(MW) of electricity without emitting a single puff of greenhouse gas. The solar

power plant, currently powers 60,000 homes, but when the project is completed

sometime around this year, the plant should generate enough power to service

180,000 homes. The final project, which will be able to produce over 300MW, will

include a series of towers, two more of which are being built, and standard

photovoltaic power plants, as well as a mixture of newer parabolic solar

collectors which will be installed at a later stage.

Construction of the

power plant

The power plant consist of

two towers – PS10 and
PS20. PS10 is surrounded by
(624 heliostats – huge mirrors that track the sun throughout the year,
reflecting the sun's rays to the top of the tower)

where a solar receiver and a steam turbine are located. The PS20 plant is even larger with 1,255 heliostats and will produce up to 20 megawatts when fully operational in 2013. The towers together will prevent emissions of more than 600,000 tones of carbon dioxide into the atmosphere per year over its 25-year life.