One-axis tracking concentrator: classifications: working Principles

Concentrators with one axis tracking are used to achieve moderate concentration. A few of them have been described below.

Please read:

• Classification, construction and working principle of two-axes tracking concentrator
• Classification and Working Principle of non tracking solar concentrators 

For knowing about the other concentrators follow the link:

• Classification of Solar Concentrators and their advantages 

One Axis Tracking Concentrator Classification 

           Fixed Mirror Solar Concentrator (FMSC) Construction and Working Principle 

          This type of  concentrator consists of a fixed mirror associated with a tracking receiver system. The fixed mirror is made of long, narrow, flat strips of mirror. These mirrors are arranged on a chosen circular cylinder of an arbitrary radius R.

          The width of the mirror strip is synchronized with the diameter of the absorber pipe. The angle of each element is set in a way that the array has a focal distance twice the radius of the reference cylinder. The array produces a narrow focal line that lies along the same circular path with the diurnal motion of the sun. The focal line can be easily tracked by the movable receiver pipe that is made to rotate about the center curvature of the reflector module. Thus the delicate part (the mirrors) of the system can be rigidly fixed.

           The image width at the absorber is ideally is same as the projected width of the mirror element. Thus the concentration ratio is exactly the same as the number of mirror elements, ignoring the solar beam spread. As the aperture is fixed and concave in shape, the mirror strips results in shading with very high or very low sun altitude angles. Also, due to strips, edge loses occur during reflection. However, mirrors can be suitably designed to have less than 10% of the total energy lost over a year time. Some (Fixed Mirror Solar Concentrator) models have shown overall efficiencies in the range of 40-50%.

Cylindrical Parabolic Concentrator Construction and Working Principle

           A cylindrical parabolic trough is a very common optical imaging instrument which is used as a solar concentrator. It is composed of a cylindrical parabolic reflector associated with a metal tube receiver at its focal plane. The outside surface of the receiver is blackened and is covered by concentrator. And it is rotated about one axis to track the sun’s diurnal motion, absorber tube carries the heat transfer fluid. And thus absorber tube transfers the heat. 

          To define the concentrator aperture diameter, rim angle, shape and the absorber size are generally used. Mild steel or copper may be used to make the absorber and is coated with a black paint which is heat resistant. Different coatings may be used for different cases for better performance. According to the need of temperature different heat transfer liquids may be used. Anodized aluminum sheet or aluminized Mylar or curved silvered glass may be used to make the reflector. Mirror strips are sometimes used in the shape of parabolic cylinder because it is difficult to curve a very large glass. A lightweight structure is used on the reflecting part . The concentration ratio for this kind of  cylinder absorber is in the range of 5 to 30.

       
          The major energy losses:
       
           The main losses are during the reflection from the reflecting part and there is also convective losses from the receiver to the surrounding environment.
           High reflecting materials are generally used for reducing convection. Twisted tapes are used to transfer heat from the absorber to the working fluid.
           East-West, North-South or polar. are the three directions in which a cylindrical parabolic trough may be oriented. The first two arrangements, although simple to assemble, have higher losses due to incidence angle cosine losses. The polar configuration gives the best performance. It intercepts more solar radiation per unit area in comparison to other models. 

           Linear Fresnel Lens/reflector Construction and Working Principle 

           A linear Fresnel lens solar concentrator is shown in figure. It consists of linear grooves on one surface of the reflecting material. The groove angles are chosen with reference to a particular wavelength of incident beam so that the lens acts as a converging one for the light which is incident normally. Although both glass and plastic can be used as refracting materials for fabricating Fresnel lenses, glass is seldom used because it is difficult to mold and has large surface tension. Plastic lenses on the other hand, are economical and the mold last for an appreciable amount of time. Plastic Fresnel lenses  with 20 grooves per mm have been molded.

           The Fresnel lens may be installed with either the groves facing the sun or the grooves facing downwards. In the first case, the ineffective facts of the grooves prevent a part of the input light from being transmitted to the focus (according to Snell’s law the refracted light is deviated away from the normal on moving from a denser medium to a rarer medium). Also dust is accumulated in these grooves results a reduced performance. In the second case, the concentrator has a higher surface reflection loss and large off axis aberrations. While reflection loss causes low efficiency, the aberrations result in a low concentration ratio. The fact that the beam is not incident normally also affects. The focal length of the lens varies rapidly with the change of the angle of incidence. So, for a better performance, the optical system needs to track the path of the sun.

           Fresnel Reflectors can be used as concentrating devices. Figure shows such a configuration, which is made up of smaller flat or curved components. It consists of a number of mirror elements mounted suitably, so that all incident parallel rays of light after reflection, are focused at a common point. Ideally, mirror elements must be parabolic in shape, but to simplify the manufacturing process and assembling problems, flat mirrors are generally used.

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