Solar Concentrators

What is Solar Concentrator?

Solar energy is a hopeful source for meeting an important proportion of the world needs in the future. Every minute sun provides energy more than the world consumes in an entire year. Beyond the earth's atmosphere sunlight intensity is about 1,350 W/m². But as it passes through the atmosphere the intensity depletes due to absorption and scattering by the various gases and dust particles in the air. So relative to the conventional sources of energy, due to its low energy density, the utilization of solar energy in technical applications is limited. There comes the importance of solar concentrators. Solar concentrators helps in attaining the solar radiation. Solar concentrators magnify the sun's intensity and this magnification translates to lower costs and greater output of electricity.

High temperature solar concentrator concepts are not new and have been used way back in the past. Olympic fire was even now lit with solar concentrators. Greeks had used concentrated light to ignite oncoming Roman fleet at Syracuse harbor in 212 BC. But it is only recently that solar concentrator technology is finding its breakthrough and commercialization.

Concentration of solar radiation is achieved using a reflecting arrangement of mirrors or a refracting arrangement of lenses. The optical system will allow the solar radiation to be directed to absorber of smaller area which is usually surrounded by a transparent cover. Because of the optical system, certain losses are introduced. These include reflection or absorption losses in the mirrors or lenses and losses due to geometrical imperfections in the optical system. The combined effect of all losses is indicated through the introduction of a term called optical efficiency. The introduction of more optical losses is compensated for by the fact that the flux incident on the absorber surface is concentrated on a smaller area. As a result the thermal loss terms do not dominate to the same extent as in a flat plate collector and the collection efficiency is usually higher.

Due to the presence of an optical system, a concentrating collector usually has to follow or track the sun so that the beam radiation is directed on to the absorber surface. As the solar concentrators mainly makes use of direct beam sun rays and is concentrated high temperatures can be achieved.  The method of tracking adopted and the precision with which it has to be done varies considerably. In collectors giving a low degree of concentration, it is often adequate to make one or two adjustments of the collector orientation every day. These can be made manually. On the other hand, with collectors giving high degree of concentration, it is necessary to make continuous adjustments of the collector orientation. The need for some form of tracking introduces a certain amount of complexity in the design. Maintenance requirements were also increased due to that. All these factors add to the cost. An added disadvantage is the fact that much of the diffuse radiation is lost because it does not get focused.

Terms used in solar concentrators

• ‘Concentrator’ is for the optical subsystem that projects solar radiation to absorber. Receiver represents a sub system that includes the absorber, its cover and accessories.
• ‘Aperture’ (W) is the opening of the concentrator through which solar radiation passes.
• ‘Acceptance angle’ (2ø_a) is the angle across which beam radiation may deviate from the normal to the aperture plane and then reach the absorber.
• ‘Concentration ratio’ (CR) is the ratio of the effective area of the aperture to the surface area of the absorber. The value of CR may change from unity to a thousand. The CR is used to classify collectors by their operating temperature range.

There are many ways to characterize or classify solar concentrators. These include:-

• Means of concentration – reflection by the use of mirrors or refraction using lenses.
• Based on focusing - Point focusing, line focusing, non focusing.
• Based on the type of tracking – One axis or two axis tracking, intermittent or continuous tracking.
• Based on shape of reflecting surfaces – parabolic, spherical or flat
• Based on the concentrator whether fixed or tracking.
• Based on the receivers whether fixed or tracking.
• Formation of image- either imaging or non imaging.
• Based on concentration ratio.
• Based on operating temperature range.

Types of concentrating collectors

Flat plate collectors with adjustable mirrors or plane reflectors

Here reflectors or mirrors to reflect radiation on to the absorber plate. It is simple in design, has a concentration ratio a little above unity and is useful for giving temperatures about 20 or 30 degree Celsius higher than those obtained with a flat plate collector alone. With a single collector, it is possible to use four reflectors all around. On the other hand, with an array of flat plate collectors, it is possible to have only two arrays of reflectors, one of which faces north and the other south. The reflectors used may reflect the radiation in a specular or diffuse manner. The concentration ratios obtained are low and normally range from one to four. Operating temperatures up to 130 degree to a 40 degree can be obtained.                              

With an array of flat plate collectors, the usual practice is to use an array of north facing reflectors only, since these are more convenient to handle and adjust than south facing reflectors. The inclination of the reflectors is usually adjusted once every few days. For the case of a north facing specular reflector array whose dimensions are equal to those of the flat plate collector array. Specular reflectors are more effective in augmenting the radiation than diffuse reflectors.

Compound parabolic concentrating collector (CPC)

This concentrator consists of curved segments which are parts of two parabolas. Like the first type this collector is also non imaging. The concentration ratio is moderate and generally ranges from 3 to 10. The main advantage of a Compound parabolic concentrating collector is that it has high acceptance angle and consequently requires only occasional tracking. In addition its concentration ratio is equal to the maximum value possible for a given acceptance angle.

Cylindrical parabolic collector

It can be also called as a parabolic trough or a linear parabolic collector.  In this type of concentrator, image is formed on the focal axis of the parabola. Many commercial versions of this type are now available. The basic elements making up a conventional collector are (i) the absorber tube located at the focal axis where heated liquid flows, (ii) the concentric transparent cover, (iii) the reflector and the support structure. Elements (iii) and (iv) constitute the concentrator and elements (i) and (ii) together constitute the receiver, while the collectors are available over a wide range of aperture areas from about 1 to 60 m² and with widths range about 1 to 6 m. The absorber is usually made up of stainless steel or copper and has a diameter of 2.5 to 5 cm. It is coated with a heat resistant black paint and is generally surrounded by a concentric glass cover with an annular gap of 1 or 2 cm. In the case of high performance collectors, the absorber tube is coated with a selective surface and the space between the tube and the glass cover is evacuated. In some small collectors, the concentric cover is replaced by a glass or plastic sheet covering the whole aperture area of the collector. Such an arrangement helps in protecting the reflecting surface from the weather. The liquid heated in the collector depends on the temperature required. Usually organic heat transfer liquids are used. Because of their low thermal conductivities, these liquids yield low transfer coefficients.                                                                           

The reflecting surface is generally curved back silvered glass. It is fixed on a light weight structure usually made of aluminium sections. The proper design of this supporting structure and of the system for its movement is important, since it influences the shape and orientation of the reflecting surface.

Collector with fixed circular concentrator and moving receiver

In this type of concentrator an array of long, narrow, flat mirror strips fixed along a cylindrical surface. The mirror strips produce a narrow line image which follows a circular path as sun moves. This path on the same circle on which the mirror strips are fixed. Thus receiver has to be moved along the circular path in order to track the sun.

Fresnel lens concentrating collector

Here Fresnel lenses are used to achieve concentration. Fresnel lenses are thin sheet, flat on one side and with fine longitudinal grooves on the other. The angles of these grooves are such that the radiation is brought to a line focus. The lens is usually made up of extruded acrylic plastic sheets. The concentration ratio ranges between 10 and 80 with temperature varying between 150 and 400 degree Celsius.

Paraboloid dish collector

Here the concentrator tracks the sun by rotating about two axes and the sun’s rays are brought to appoint focus. A fluid flowing through a receiver at the focus is heated and this heat is used to drive a prime mover. This type of collectors has concentration ratios ranging from 100 to afew thousand and has yielded temperatures upto 2000 degree Celsius. However from the point of view of the mechanical design, there are limitations to the size of the concentrator and hence, the amount of energy which can be collected by one dish. Because of the limitations on the size of the concentrator, paraboloid dish systems can be expected to generate power in kilowatts rather than megawatts. Commercial versions have been built with dish diameters up to 17 m.

Central Receiver Collector

To collect larger amounts of energy at one point, the central receiver concept has been adopted. In this case, beam radiation is reflected from a number of heliostats to receiver. Their orientation is individually controlled so that throughout the day they reflect beam radiation on the receiver.                 

In case of a central receiver system using a molten salt as the heat transfer fluid, molten salt used is a mixture of 60 percent sodium nitrate and 40 per cent potassium nitrate. Cold salt at 290 degree Celsius is pumped from a tank at ground level to the receiver at the top of a tower where it is heated by the concentrated radiation to a temperature of 565 degree Celsius. The salt flows back to another tank at ground level. In order to generate electricity, hot salt is pumped from the   hot tank through a steam generator where superheated steam is produced. The superheated steam then goes through a Rankine cycle to produce mechanical work and then electricity. The heliostat array can be sized to collect more power than is required by the electricity generation system. In that case, the excess thermal energy in the form of excess salt at 565 degree Celsius accumulates in the hot tank and serves as a thermal storage.