Construction and Working Principle of Two-axes tracking Concentrators

In order to achieve a high concentration 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: 

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Construction of Two axes tracking Concentrator 

The construction and working principle of these types of collectors are described below: 

Paraboloidal disc concentrator construction and working principle: 

A paraboloid is produced when a parabola ratates about its optic axis. When it is used to concentrate solar radiation, a high concentration ratio is achieved. 

Paraboloidal disc concentrator

Due to the compound curvature with perfect optics and a point source of light, theoretically, light is focussed at a point in a paraboloid. However, an image is produced due to the finite angular subtense of the sun. Also the surface may not be strictly parabolic so that the image will be enlarged  due to misdirection of the light rays by misaligned surface elements. A degraded image will be enlarged due to is off-axis. The rays from the edges travel a larger distance, resulting in a spread of the image. Thus a three dimensional image of the sun in the shape of an ellipsoid is formed. 
The thermal losses from paraboloid are primarily radiative and can be reduced by decreasing the aperture area. This however results in a smaller intercept factor. The optimum intercept factor is about 0.95 to 0.98. The larger the surface errors, the larger must be the absorber sixe to achieve the optimum beam intercept. 
High collection efficiency and high quality thermal energy are the features of paraboloid or parabolic dish type concentrator. The delivery temperature being very high, these devices can be used as sources for a variety of purposes. 

Illustration of formation of ellipsoid image in paraboloid

Central Tower Receiver two axes tracking concentrator 

The system consists of a central stationary receiver to which the solar radiation is reflected by heliostats. A heliostat is composed of a large erray of mirrors fixed to a supporting frame. This frame can be used to track the sun, as desired. 
The heliostats are installed in the open space and together they act like a dilute paraboloid. They focus solar radiation on a central receiver, which is stationary. Concentration ratio as high as 3000 can be achieved. The absorbed energy can be extracted from the receiver and delivered at a temperature and pressure suitable for driving turbines for power generations.  The advantage of this system s that it eliminates the need for carrying the working fluid over large distances. As a result the heat losses are reduced significantly thus eliminating the need for insulation. There are also many problems associated with this system. A majority of them are due to heliostats and receiver. A large number of heliostats are required to focus solar radiation requiring large free space. The heliostats may be provided with proper tracking arrangement amd may be arranged such that self shading is avoided; at the same time achieving desired concentration. In addition to the cost, the cleaning of mirrors to remove dust and dirt poses another problem. Further the heliostat arrangement must be strong enough to withstand extreme weather condition. The receiver must be able to interrupt the focused radiation, adsorb this heat and transfer this as energy to the working fluid with minimum heat loss. Several designs have been proposed for the receiver. Heat transfer fluids like steam, hitec and liquid metals are suggested for use. Some problems arise due to reflection and transmission losses and thermal stresses in the receiver.   

schematic view of central receiver heliostat system

Circular Fresnel lens two axes tracking concentrator

Lenses are usually not used in solar energy applications due to cost and weight. However, these are used where high temperature is required as in solar furnace, Figure shows the principle of this lens. Optically, the lens is equivalent to a thin lens approximation. It is divided into a number of zones which are spaced at a few tenths of a millimeter; the space can also be few centimeters. Within each aperture zone , the tilt of the lens surface is so adjusted that its optical behaviour resembles that a conventional spherical lens of the same focal length. The focus of the annulus need not to be curved, but is required to have the correct tilt so as to refract the light to the focus. This is because, the absrobing surface is usually much larger than the width of a Fresnel zone on the lens. Meinel (1977) has given the equation for the tilt of the facet as a function of the aperture zone and focal length. 

The circular Fresnel lens provided very good concentration. For a precise plastic lens, the brightness concentration is as high as 2000. Hence, such concentrators are usually used with silicon and gallium arsenide solar cells for high flux. In solar cell applications, the lens has to track the sun since it is required to keep small solar image centered on the receiver. It may be noted that the brightness cocentration in this case is smaller than that in case of parabolic mirrors. 
The transmitting system has an advantage over the reflecting system in that it absorbs certain wavelenght of incident beam, which may result in heating of the focus. 

schematic diagram of circular Frensel Lens

Hemispherical Bowl Mirror 

Figure shows another type of fixed mirror and movable receiver type concentrator, independently proposed by Steward (1973) and Meinel (1973). The major components of this concentrator are a fixed hemispherical mirror, a tracking linear absorber and a supporting structure. The hemisphere produces a hemispherical mirror, a tracking linear absorber and a supporting structure. The hemisphere produces a highly aberatted optical image. However, because of its symmetry all rays entering into the hemisphere after reflection cross the paraxial line at some point between the focus and mirror surface. Therefore, an absorber pivoted about the center of the curvature of the hemisphere interrupts all reflected rays. The absorber is to be moved so that its axis is always aligned with solar rays passing through the center of the sphere. This requires two axis tracking. Though this motion can be set in a number of ways, the simplest one is to adopt equatorial mount in which the absorber is driven around a polar axis, during the day, at a constant angular speed of 15degree/hour. Through a slow continuous motion or a periodic motion adjustment about an axis normal to polar axis, correct declination can be maintained. It may be noted that this type of concentrator gives a lesser concentration, owing to spherical aberration, than that in paraboloids.

Cross sectional view of hemispherical mirror concentrator

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