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.