Focusing type solar collector

all about focusing type solar collectors

1. Presentation on Focusing Type Solar Collectors Session:2018-2019 Presented by: Maga Ram Patel DEPARTMENT OF RENEWABLE ENERGY ENGINEERING College of Technology and Engineering Maharana Pratap University of Agriculture and Technology Udaipur - 313001

2. Introduction: Focusing collector is a device to collect solar energy with high intensity of solar radiation on the energy absorbing surface. A focusing collector is a special form of flat collector modified by introducing a reflecting ( or refracting) surface (concentrator) between the solar radiations and the absorber. Focusing collectors can have radiation increases from low value of 1.5 to 2, high values of the order of 10,000. Reach temperature up to 450-500 ⁰C. Receivers can be convex, flat, cylindrical or concave and can be covered with glazing or uncovered.

3. Schematic of concentrating/focusing collector:

4. Solar Concentrators and Receiver Geometries: There is wide variety of means for increasing the flux of radiation on receivers. They can be classified on basis of: lenses or reflectors By the types of mounting and orienting systems By the concentration of the radiation they are able to accomplish By materials used of construction, or by orientation Concentrator is a component used to increase the intensity of energy flux on a receiver. Concentration ratio (CR): it is the ratio of the area of the concentrator aperture to the energy absorbing area of the receiver. CR = A a / A r It is determines the effectiveness of a concentrator.

5. Some possible focusing system configuration: Plane Reflector and Plane Receiver Type : that both reflector and receiver are plane. The concentration ratios are relatively low, with maximum value of four or less than four . Conical Reflector and Cylindrical Receiver Type: this system in which reflector is conical and receiver is cylindrical. Concentration ratio is little higher than that of first case, it may be of the order of 10.

6. Fresnel Reflector: This consists of a parabolic shape reflector made up of small segments. The advantage of linear Fresnel lenses is that the convenient mass production technique of extrusion of thermoplastic materials can be applied to their fabrication. A concentration ratio of about 10 is obtainable using them.

7. Parabolic system: In a system consisting of a paraboloid or a parabolic mirror and having receiver at its focal point. The concentration ratios are very high and therefore can be used where high temperatures are required. In cylindrical system , the concentration ratio is lower than paraboloid counterparts. In both the cases the receiver is placed at the focus, i.e. along the focal line in cylindrical parabolic system and at the focus point in paraboloidal system.

8. Types of Concentrating or Focusing Collectors: There are two methods by which the sun motion can be readily tracked. The first is the altazimuth method which requires the tracking device to turn in both altitude and azimuth. Paraboloidal solar collectors generally use this system. The second one is the one-axis tracking in which the collector tracks the sun in only one direction either from east to west or from north to south . Parabolic trough collectors (PTC) generally use this system. These systems require continuous and adjustment to compensate for the changes in the sun orientation.

9. Line Focus Collectors: These collectors, sometimes known as parabolic troughs, use highly reflective materials to collect and concentrate the heat energy from solar radiation. These collectors are composed of parabolically shaped reflective sections connected into a long trough. A pipe that carries water is placed in the center of this trough so that sunlight collected by the reflective material is focused onto the pipe, heating the contents. These are very high powered collectors and are thus generally used to generate steam for Solar thermal power plants.

10. Point Focus Collectors: These collectors are large parabolic dishes composed of some reflective material that focus the Sun's energy onto a single point. The heat from these collectors is generally used for driving  Stirling engines. Although very effective at collecting sunlight, they must actively track the Sun across the sky to be of any value. These dishes can work alone or be combined into an array to gather even more energy from the Sun.

11. Types of Concentrating or Focusing Collectors: There are different types of concentrating or focusing collector depending upon the concentrator and receiver geometries. There are as Follows: Cylindrical parabolic collector. Central receiver collector. Compound parabolic collector

12. Cylindrical Parabolic Collector: It is a system consisting of a parabolic reflector and having receiver at its focal point. The concentration ratios are very high and therefore can be used where high temperatures are required. In a cylindrical system, the concentration ratio is lower than paraboloid counterparts. In both the cases the receiver is placed at the focus, i.e. along the focal line in cylindrical parabolic system and at the focus point in paraboloidal system. The parabolic geometry is given by the relation Y2 = 4aX where a = semi major axis.

13. A Typical cylindrical parabolic system:

14. a) Basic geometry of a parabolic cylinder mirror. b) Basic geometry of a paraboloid

15. Central Receiver Collector: The concept of central receiver collector is simple. In order to avoid the cost and heat losses in transporting a working fluid to a central location, use of sunlight itself as the transfer medium is proposed. To implement the concept, one needs a field of mirrors provided with the means of directing reflected sunlight to a central location, or a location at one edge of the field of mirrors. In the typical central receiver, the reflector is composed of many smaller mirrors each with its own heliostat to follow the sun. The heliostat are generally located in the horizontal plane, but when the situation is favourable , can simply follow the existing terrain.

16. The basic difference between a single mirror concentrator and the heliostat system is that the heliostat system has a dilute mirror. This diluteness is generally termed as the fill factor. A central receiver with a fill factor of 40% means that 40% of the land area is covered by mirrors. Schematic arrangement of central receiver- heliostat array.

17. Compound Parabolic Collector (CPC) or Winston Collector: It is possible to concentrate solar radiation by a factor of 10 without diurnal tracking using this type of collector. A compound parabolic concentrating collector is show in Fig. ( in next slide) It consists of two parabolic reflectors which funnel the radiation from aperture to absorber. The right and left half belong to different parabolas (hence the name compound parabolic concentrator). The axis of the right branch, for instance, makes an angle Ѳ c , with the collector mid plane, and its focus is at A. At the end points C and D, the slope is parallel to the collector mid-plane.

18. The compound parabolic collector developed by Winston represents what may be called as ideal concentrator in the sense that for a given field of view it achieves the maximum possible concentration ratio given by: CR = W/b = 1/ SinѲ c Where, W = entrance aperature b = exit aperture, covered by the absorber Ѳ c = field of view (half angle)

19. A dvantage of Concentrating type Collectors: Surface required less material or provide high collection of radiation . For a concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that of the flat-plate collector. Heat loss rate is less than flat-plate collection low maintenance. Can be used for power generation We can get high concentration or radiation . Litter or no antifreeze is required to protect the absorber in concentrator systems.

20. Disadvantage of Concentrating type Collectors: Only beam radiation collected because diffuse component cannot be reflected and is thus lost. Additional requirement of maintenance particularly to retain the quality of reflecting surface against dirt, weather, oxidation etc. Non- uniform flux on the absorber whereas flux in flat-plate collector is uniform. Additional optical losses such as reflectance loss and the intercept loss, so they introduce additional factors in energy balances. These disadvantage have restricted the utility of focusing collectors and no long time practical applications of focusing collectors other than for furnaces are being made.

21. “I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait ‘till oil and coal run out before we tackle that’. Thomas Edison THANK YOU

22. Thankyou