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Introduction

March 23, 2019 0 Comment

Introduction :-
Concentration of solar radiation is achieved using a reflecting arrangements of mirrors or glass and a refracting arrangements of lenses. The optical system directs the solar radiation on to an absorber of smaller area. The smaller area is surrounded by a transparent cover. Because of the optical system, some losses are introduced. It include reflection or absorption losses in the mirrors or lenses and some losses are caused due to geometrical imperfections in the optical system. The optical losses is compensated by the flux incident on a smaller area of the absorber surface by concentrating. Thermal loss terms not like a flat-plate collector and the collection efficiency is usually higher. Some of the features of a flat plate collector are simple in design and easy for maintenance but not same for concentrating collector. In the presence of the optical system concentrator usually has to track the sun so the beam radiation is concentrate on to the absorber surface. For gaining a high degree of concentration, it’s necessary to maintain continuous adjustments of the collector orientation. In tracking system there are some complexity in design so it’s maintenance requirements are also increased. In the last few years there are many significant advances have been made in the development of concentrating collectors.

Solar concentrator :-
Solar concentrator is a device that allows the collection of sunlight from a large area and focusing its on a small area. A conceptual representation of a solar concentrator used to harnessing the power from the sun to generate heat for application purpose.
Solar concentrators are made of number of flat shaped mirror, glass or reflective films which are mounted on a structural frame. They are moves to north-south axis parallel to earth’s axis to track the sun’s movement. The axis of rotation passes through the center of gravity of the reflectors and that’s how reflectors always manage its gravitational equilibrium. The frame also performs change in inclination angle , while staying in front of sun in order to obtain sharp focal point. Receiver is fixed at the focal point . receiver captures the concentrated heat and transfers to the object according to it’s application .

Classification of solar concentrator :-
Imaging concentrator:-
Imaging concentrators reflects the light from the mirror on to a receiver to produce the optical image means to concentrate. They are mainly used for achieving the highest temperature. It enable a very large aperture area into a small absorber area, effectively reducing thermal losses at high temperature. Sun rays tracing is used to evaluate concentrator during the design process.
Line focus type ( one axis tracking ) :-
Concentrator with one axis tracking are used to achieve moderate concentration.
1. Cylindrical parabolic concentrator :-
It is a very common optical imaging instrument which is used as a solar concentrator.it is rotated about one axis to track the sun’s motion. Here at focal plane metal tube receiver is used. The absdorer tube carries the heat and transfer top the fluid. Mostly mild steel or copper is used to make absorber and it is coated with the black paint. Anodised aluminium sheet, aluminized mylar or curved silvered glass are used to make the reflector.
2. Fixed mirror solar concentrator :-
This type of concentrator are having a fixed mirror which is associated with a tracking receiver system. The mirrors are made from long, narrow, flat strips of mirror. These mirrors are arranged on a circular cylinder of the radius.the focal line can be easily tracked by the movable receiver pipe which is made to rotate about the center curvature of the reflector module.
3. Linear Fresnel lens concentrator :-
Fresnel lens are similar to the conventional lens , which is refracting the rays and focusing them at on focal point. Generally there are two sections in the concentrator ; a flat upper surface and a back surface that employs canted facets. The facet is an nearly to the curvature of a lens. This concentrator is able to separate the direct and diffuse light. It is thinner than the conventional lens. Plastic and glass both are used as refracting material. For fabricating the Fresnel lens, glass is seldom used because it is difficult to mold and has large surface tension.
Point focus type ( two axis tracking ) :-
1. Hemispherical bowl mirror concentrator :-
Fixed hemispherical mirror, a tracking linear absorber and a supportive structure are the main components of this concentrator. The hemisphere produces aberrated optical image. Because of its symmetry all rays entering in to the hemisphere after reflection cross the paraxial line at some point between the focus and mirror surface. An absorber pivoted about the center of the curvature of the hemisphere. Because of spherical aberration this type of concentrator gives a lesser concentration.
2. Circular Fresnel lens concentrator :-
These type of lenses are usually not used in solar energy application due to cost and weight. These are used where high temperature is required. The circular fresenel lens provided very high concentration. Fresnel lenses are made from the glass and plastic.
3. Parabolic dish collector :-
The two dimensional design of a parabolic concentrator is equal to a parabola . it is used as a reflecting solar concentrator. A distinct property of that it can focus all the parallel rays from the sun to a single focus point . it is not necessary to use the full part of the parabola curve to construct the concentrator. It’s requires larger field of view and need a good tracking system. this concentrator provides a higher concentration.
4. Central tower receiver :-
The system consists of a central stationary receiver in 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 move as sun is move. The heliostats are installed in the open space. They focus solar radiation on a central receiver, which is fixed at some height. The receiver must be able to interrupt the focused radiation, adsorb this heat and transfer to the working fluid with minimum heat loss.

Scheffler dish :-
It is a point focus type of concentrator. The basic concept to the development of the scheffler reflector was to solar cooking as comfortable as possible. The reflector is a small lateral section of a large paraboloid. The inclined cut of the large paraboloid produces the typical elliptical shape of scheffler reflector. The axis of daily rotation is located exactly in north-south direction which is parallel to the earth axis and runs through the center of gravity of the reflector.
That way the reflector always maintains it’s gravitational equilibrium. The mechanical tracking device doesn’t require to be driven by much force to rotate it with the sun. The focus is located on the axis of rotation to prevent it from moving when the reflector is moving. The distance between focus and center of the parabolic reflector depends on the selected parabola. During the day the concentrated light will only rotate around its own center but not move in any direction. So the focus stays fixed, which is very useful, it means the cooking pot doesn’t have to moved either.
The paraboloid has to perform the change of inclination in order to stay directed at the sun. So the it’s possible to obtain a Sharp for point. It is possible by shaping the reflector after an other parabola for different seasonal inclination angle of the sun. For changing the inclination angle mirror support structure and dish stand is used. The efficiency for the cooking depends on the cleanliness of the reflector surface and the state of insulation of the cooking pot. It’s optical efficiency is up to 75%. On an average a 8m^2 reflector can bring 22 litres of cold water to boiling temperature within one hour after consuming the 700W/m^2 direct solar radiation. At the focal point backing oven, steam generator or heat storage can be installed.
Scheffler dish are build in varieties of 2.7m^2, 8m^2, 10m^2 and 12m^2. The standard size of the scheffler dish is 8m^2.

Applications :-
1. Bolier feed water preheating
2. Washing in laundries
3. Oil heating for cooking or industrial applications
4. Milk pasteurization
5. Cooking
6. Generating steam
Advantages :-
1. No any type of pollution.
2. Renewable source of energy.
3. Saves wood, electricity and other fuels.
4. Adaption to changing season.
5. Highly durable.
Disadvantages :-
1. initial cost is high.
2. Requires maintaince.
3. Replacement of parts are not easily available.
4. Not locally available.

Components :-
1. Collector:-
The collector of scheffler dish’s shape is paraboloid. All the assembly of flat shaped glass, mirrors or reflective films are arranged on a structural steel framework. The frame is mirrors support structure . it consists of bars and angles sections to form a shape of parabola. The mirrors are arranged to give paraboloidal shape and reflect the solar radiation on to focusing point or receiver.
2. Receiver :-
The receiver of scheffler dish is placed at the focusing point of the dish. it captures the solar radiation and transfer it to the thermal medium used in the system. Mostly this component is spherical dome type absorber fixed on the structural steel frame. The receiver works on thermo siphon principle. The receiver’s size is depends on the size of the focus and storage requirement. The receiver are designed according to its application of the user.
3. Dish stand :-
The basic framework of the dish stand is rectangular/triangular conical shape steel structure. The framework is made of pipes and angles. This structure is designed to withstand wind speed in operating conditions as well as rotates as sun’s movement.the rotary support, counter weight and other equipments are attached on the stand.
4. Tracking system :-
Tracking system enables the dish to focused towards the sun to capture maximum direct radiation during the day. It also tracks the sun as it change its position during the year. In tracking system many equipments are arranged like electric motor, gear box, sprockets and chains, counter weight with rope, screw shaft, solar radiation sensor, wind sensor and timer.

Working of scheffler dish system :-
The scheffler dish system works on the different principles :-
1. The reflective dish turns about North-south axis parallel to earth’s axis. It track the sun’s movement from morning (East) to evening (West) and maintaining gravitational equilibrium of the dish.
2. The parabolic reflector also performs changes in inclination angle while staying in fornt to the sun, in order to maintain a sharp focal point.
3. The Focus lies on to the axis of rotation. It remains at a fixed position, where concentrated heat is capture and it is received by the receiver.
4. The receiver pass the heat to the thermal medium of the system.