Introduction: Definition of Solar Concentrators:
Solar Concentrator is a device which concentrates the solar
energy incident over a large surface onto a smaller surface. The concentration
is achieved by the use of suitable reflecting or refracting elements, which
results in a increased flux density on the absorber surface compared to that
existing on the concentrator aperture. In order to get a maximum concentration
an arrangement for tracking the sun’s virtual motion is required. An accurate focusing device is also required.
Thus a solar concentrator consists of a focusing device, a receiver system and
a tracking arrangement. Temperature as high as 3000 degrees Celsius can be got
from a solar concentrator. So they have potential applications in both thermal
and photovoltaic utilization of solar power at high temperatures.
Solar concentrating devices have been used for a long time.
In Florence as early as 1695, a diamond could be melted by solar energy.
Lavoisier carried out a number of experiments with his double-lens
concentrator. The knowledge concentrator dates back even time of Archimedes,
whose book "On Burning Mirrors" is an evidence of this fact. Many
uses of concentrators were reported in the eighteenth and nineteenth centuries,
particularly in heat engines and steam production. The advantages of
concentrator are as follows:
The advantages of solar concentrator
1. It increases the intensity by concentrating the energy
available over a large surface onto a smaller surface (absorber)
2. Due to concentration on a smaller area, the heat loss
area is reduced. Further, the thermal mass us much smaller than that of a flat
plate collector and hence transient effects are small.
3. The delivery temperatures being high, a thermodynamic
match between the temperature level the task occurs.
4. It helps in reducing the cost by replacing an expensive
large receiver by a less expensive reflecting or refracting area.
Disadvantages of Solar Concentrator
However, concentrator is a optical system and hence the
optical loss terms become significant. Further it works on beam component of
solar radiation, resulting in loss of diffuse component. Although the basic
concepts of flat plate collectors are applicable to concentrating systems, a
number of complications arise because of non-uniform flux on absorbers, wide
variations in shape, temperature and heat loss behavior of absorbers and finally
the optical considerations in the energy balance conditions. It may be noted
that higher the concentration of the collector, higher is the precision of
optics and more is the cost of the unit. In addition to the complexity of the
system, the maintenance requirements are also increased.
Classification of solar concentrator
Solar concentrators may be classified as (i) tracking type
and (ii) non-tracking type. Tracking may be continuous or intermittent and may
be one-axis or two -axes. As the sun may be followed by moving either the focusing
part or receiver or both; concentrators can be classified accordingly. Further
the system may have distributed receiver or central receiver.
The concentrators may also be classified on the basis of
optical components.
They may be
1. Reflecting or refractory type
2. Imaging or non-imaging type
3. Line focusing or point focusing type
The reflecting or refracting surface may be one piece or a
composite surface, it may be a single stage or two stage type system and may be
symmetric or asymmetric. In practice however hybrid and multistage systems,
incorporating various levels of the features, occur frequently.
Types of solar concentrators
There are a number of methods by which the flux radiation on
receivers can be increased. Some of them have been discussed here:
Tracking Concentrators classification:
Tracking Concentrators can be further classified as
Concentrators with one axis tracking
These are used to achieve
moderate concentration. A few of them have been described below.
i. Fixed Mirror Solar Concentrator (FMSC)
ii. Cylindrical Parabolic Concentrator
iii. Linear Fresnel Lens/reflector
(Follow the link of one axis tracking concentrator for getting the description of all three concentrators)
Concentrators with two-axes tracking
In order to achieve a
high concentrators for high temperatures solar processes, concentrators with
double curvatures are used. These requires two axes tracking of the sun. Some of
these have been described below:
i. Paraboloidal dish concentrators
ii. Central Tower Receivers
iii. Circular Fresnel Lens
iv. Hemispherical bowl mirror
Non-tracking concentrators classification
These are classified as follows:
i. Flat Receiver with booster mirror
ii. Tabor- Zeimer Circular Cylinder
iii. Compound Parabolic Concentrator
iv. V- tough
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