Solar collectors nces

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Solar collectors nces

  1. 1. Solar COLLECTORS By Anu Singla Associate Professor Department of EE Chitkara University, Punjab Solar COLLECTORS By Anu Singla Associate Professor Department of EE Chitkara University, Punjab
  2. 2. Solar Energy Collectors Solar energy collector is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy thus collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment, or to a thermal energy storage tank from which can be drawn for use at night and/or cloudy days. Solar energy collector is a device which absorbs the incoming solar radiation, converts it into heat, and transfers this heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy thus collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment, or to a thermal energy storage tank from which can be drawn for use at night and/or cloudy days.
  3. 3. Types of Collectors Solar collectors can be classified according to their collecting characteristics, the way in which they are mounted and the type of transfer fluid they use. (a)Collecting characteristics: A non-concentrating or flat plate collector is one in which the absorbing surface for solar radiations is essentially flat with no means for concentrating the incoming solar radiation. A concentrating or focusing collector is one, which usually contains reflectors or employs other optical means to concentrate the energy falling on the aperture onto a heat exchanger of surface area smaller than the aperture. (b) Mounting: A collector can be mounted to remain stationary, be adjustable as to tilt angle (measured from the horizontal) to follow the change in solar declination or be designed to track the sun. Tracking is done by employing either an equatorial mounting or an azimuth mounting, for the purpose of increasing the absorption of the daily solar irradiation. (c) Types of fluid: A collector will usually use either a liquid or a gas as the transfer fluid. The most common liquids are water or a water-ethylene glycol solution. The most common gas is air. Solar collectors can be classified according to their collecting characteristics, the way in which they are mounted and the type of transfer fluid they use. (a)Collecting characteristics: A non-concentrating or flat plate collector is one in which the absorbing surface for solar radiations is essentially flat with no means for concentrating the incoming solar radiation. A concentrating or focusing collector is one, which usually contains reflectors or employs other optical means to concentrate the energy falling on the aperture onto a heat exchanger of surface area smaller than the aperture. (b) Mounting: A collector can be mounted to remain stationary, be adjustable as to tilt angle (measured from the horizontal) to follow the change in solar declination or be designed to track the sun. Tracking is done by employing either an equatorial mounting or an azimuth mounting, for the purpose of increasing the absorption of the daily solar irradiation. (c) Types of fluid: A collector will usually use either a liquid or a gas as the transfer fluid. The most common liquids are water or a water-ethylene glycol solution. The most common gas is air.
  4. 4. Solar collectors are classified basically in two different categories:  Flat plate or non-concentrating collectors  Concentrating or focusing collectors consisting of • Cylindrical parabolic collector • Central receiver collector • Compound parabolic collector Types of Collectors contd. Solar collectors are classified basically in two different categories:  Flat plate or non-concentrating collectors  Concentrating or focusing collectors consisting of • Cylindrical parabolic collector • Central receiver collector • Compound parabolic collector
  5. 5. Non concentrating collector has the same area for intercepting and for absorbing solar radiation, whereas Sun-tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux. Types of Collectors contd. Non concentrating collector has the same area for intercepting and for absorbing solar radiation, whereas Sun-tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux.
  6. 6. Flat Plate Collector (Non-Concentrating Collector) A simple flat plate collector consists of an absorber surface (usually a dark, thermally conducting surface); a trap for radiation losses from the absorber surface (such as glass which transmits shorter wavelength solar radiation, but blocks the longer wavelength radiation from the absorber, a heat transfer medium such as air, water etc. and some thermal insulation behind the absorber surface. Flat plate collectors are used typically for temperature required up to 750C although higher temperature can be obtained from high efficiency collector. A simple flat plate collector consists of an absorber surface (usually a dark, thermally conducting surface); a trap for radiation losses from the absorber surface (such as glass which transmits shorter wavelength solar radiation, but blocks the longer wavelength radiation from the absorber, a heat transfer medium such as air, water etc. and some thermal insulation behind the absorber surface. Flat plate collectors are used typically for temperature required up to 750C although higher temperature can be obtained from high efficiency collector.
  7. 7. Flat Plate Collector (Non-Concentrating Collector) contd. These flat plate collectors are further classified into two basic types, based on heat transfer fluid. Liquid type: Where heat transfer fluid may be water, mixture of water antifreeze oil etc. Air type: Where heat transfer medium is air (used mainly for drying and space heating requirements). These flat plate collectors are further classified into two basic types, based on heat transfer fluid. Liquid type: Where heat transfer fluid may be water, mixture of water antifreeze oil etc. Air type: Where heat transfer medium is air (used mainly for drying and space heating requirements).
  8. 8. Liquid-type Flat Plate Collector In general it consists of: Glazing: One or more covers of transparent material like glass, plastics, etc. Glazing may be left out for some low temperature applications. Absorber: A plate with tubes or passages attached to it for the passage of working fluid. The absorber is usually painted flat black or electroplated with a selective absorber. Header or manifolds: To facilitate the flow or heat transfer fluid. Insulation: To minimise heat loss from the back and the sides. Container: box or casing. In general it consists of: Glazing: One or more covers of transparent material like glass, plastics, etc. Glazing may be left out for some low temperature applications. Absorber: A plate with tubes or passages attached to it for the passage of working fluid. The absorber is usually painted flat black or electroplated with a selective absorber. Header or manifolds: To facilitate the flow or heat transfer fluid. Insulation: To minimise heat loss from the back and the sides. Container: box or casing.
  9. 9. Typical Liquid-type Flat Plate Collector
  10. 10. Air Type/ Space Collector Air type collectors are more commonly used for agricultural drying and space heating applications. Their basic advantages are low sensitivity to leakages and no need for an additional heat exchanger for drying and space heating applications. However, because of the low heat capacity of the air and the low convection heat transfer coefficient between the absorber and the air, a larger heat transfer area and higher flow rates are needed. Figure shows some common configurations of air heating collectors. Common absorber materials include corrugated Al or galvanised steel sheets, black metallic screens or simply any black painted surface. Air type collectors are more commonly used for agricultural drying and space heating applications. Their basic advantages are low sensitivity to leakages and no need for an additional heat exchanger for drying and space heating applications. However, because of the low heat capacity of the air and the low convection heat transfer coefficient between the absorber and the air, a larger heat transfer area and higher flow rates are needed. Figure shows some common configurations of air heating collectors. Common absorber materials include corrugated Al or galvanised steel sheets, black metallic screens or simply any black painted surface.
  11. 11. Some common configurations of air heating collectors
  12. 12. Unglazed, transpired solar air collector offers a low-cost opportunity for some applications such as preheating of ventilation air and agricultural drying and curing. Such collectors consist of perforated absorber sheets that are exposed to sun and through which air is drawn. The perforated absorber sheets are attached to the vertical walls, which are exposed to the sun. The most important components, whose properties determine the efficiency of solar thermal collectors, are glazing and absorber. Air Type/ Space Collector contd. Unglazed, transpired solar air collector offers a low-cost opportunity for some applications such as preheating of ventilation air and agricultural drying and curing. Such collectors consist of perforated absorber sheets that are exposed to sun and through which air is drawn. The perforated absorber sheets are attached to the vertical walls, which are exposed to the sun. The most important components, whose properties determine the efficiency of solar thermal collectors, are glazing and absorber.
  13. 13. Concentrating or Focusing Collectors 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 plate collector modified by introducing a reflecting (or refracting) surface (concentrator) between the solar radiation and the absorber. Focusing collectors can have radiation increase from low value of 1.5 to 2, high values of the order of 10,000. Focusing collector comprises of receiver (absorber) and concentrator. Concentrators can be reflectors or refractors, can be cylindrical or parabolic and can be continuous or segmented. Receivers can be convex, flat, cylindrical or concave and can be covered with glazing or uncovered. 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 plate collector modified by introducing a reflecting (or refracting) surface (concentrator) between the solar radiation and the absorber. Focusing collectors can have radiation increase from low value of 1.5 to 2, high values of the order of 10,000. Focusing collector comprises of receiver (absorber) and concentrator. Concentrators can be reflectors or refractors, can be cylindrical or parabolic and can be continuous or segmented. Receivers can be convex, flat, cylindrical or concave and can be covered with glazing or uncovered.
  14. 14. Schematic of concentrating/ focusing collector
  15. 15. Concentrating or Focusing Collectors contd. There are wide variety of means for increasing the flux of radiation on receivers. They can be classified on the basis of a) Lenses or reflectors. b) The types of mounting and orienting systems c) By the concentration of the radiation they are able to accomplish d) By materials 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 quantity = Aa/Ar , where Aa is the ratio of the area of the concentrator aperture and Ar , is the energy absorbing area of the receiver. It determines the effectiveness of the concentrator. There are wide variety of means for increasing the flux of radiation on receivers. They can be classified on the basis of a) Lenses or reflectors. b) The types of mounting and orienting systems c) By the concentration of the radiation they are able to accomplish d) By materials 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 quantity = Aa/Ar , where Aa is the ratio of the area of the concentrator aperture and Ar , is the energy absorbing area of the receiver. It determines the effectiveness of the concentrator.
  16. 16. Because of the apparent movement of the sun across the sky, conventional concentrating collectors must follow the sun’s daily motion. There are two methods by which the sun’s motion can be readily tracked. The first is the altazimuth method which requires the tracking device to turn in both altitude and azimuth, i.e. when performed properly, this method enables the concentrator to follow the sun exactly. 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 accurate adjustment to compensate for the changes in the sun’s orientation. Types of Concentrating or Focusing Collectors Because of the apparent movement of the sun across the sky, conventional concentrating collectors must follow the sun’s daily motion. There are two methods by which the sun’s motion can be readily tracked. The first is the altazimuth method which requires the tracking device to turn in both altitude and azimuth, i.e. when performed properly, this method enables the concentrator to follow the sun exactly. 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 accurate adjustment to compensate for the changes in the sun’s orientation.
  17. 17. Types of Concentrating or Focusing Collectors There are different types of concentrating or focusing collector depending upon the concentrator and receiver geometries. These are as follows:  Cylindrical parabolic collector.  Central receiver collector.  Compound parabolic collector There are different types of concentrating or focusing collector depending upon the concentrator and receiver geometries. These are as follows:  Cylindrical parabolic collector.  Central receiver collector.  Compound parabolic collector
  18. 18. Cylindrical Parabolic collector 1st Generation PV Crystalline Silicon  It is a system consisting of a paraboloid or 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 = 4 a X Where a = Semi Major Axis. 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost  It is a system consisting of a paraboloid or 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 = 4 a X Where a = Semi Major Axis.
  19. 19. Cylindrical Parabolic Collector Diagram 1st Generation PV Crystalline Silicon 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  20. 20. Basic geometry of paraboloid mirror 1st Generation PV Crystalline Silicon 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  21. 21. Cylindrical Parabolic Collector 1st Generation PV Crystalline Silicon 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  22. 22. Cylindrical Parabolic Collector contd. 1st Generation PV Crystalline Silicon 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  23. 23. Central Receiver Collector 1st Generation PV Crystalline Silicon 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. 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost 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.
  24. 24. Central Receiver Collector contd. 1st Generation PV Crystalline Silicon  In the typical central receiver, the reflector is composed of many smaller mirrors each with its own heliostat to follow the sun.  The heliostats are generally located in the horizontal plane, but when the situation is favourable, can simply follow the existing terrain.  The basic difference between a single mirror concentrator and the heliostat system is that the heliostat system has a dilute mirror. This means that the entire surface within the system is not covered with mirror surface.  This diluteness is generally termed as the fill factor.  A central receiver with a fill factor of about 40% means that 40% of the land area is covered by mirrors. 1st Generation PV Crystalline Silicon 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost  In the typical central receiver, the reflector is composed of many smaller mirrors each with its own heliostat to follow the sun.  The heliostats are generally located in the horizontal plane, but when the situation is favourable, can simply follow the existing terrain.  The basic difference between a single mirror concentrator and the heliostat system is that the heliostat system has a dilute mirror. This means that the entire surface within the system is not covered with mirror surface.  This diluteness is generally termed as the fill factor.  A central receiver with a fill factor of about 40% means that 40% of the land area is covered by mirrors.
  25. 25. Receiver Collector 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  26. 26. 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 collector is shown in figure (in next slide) . It consists of two parabolic reflectors which funnel the radiation from aperture to absorber. The right and the left half belong to different parabolas (hence the name compound parabolic concentrator). The axis of the right branch, for instance, makes an angle θc , 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. 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost It is possible to concentrate solar radiation by a factor of 10 without diurnal tracking using this type of collector. A compound parabolic collector is shown in figure (in next slide) . It consists of two parabolic reflectors which funnel the radiation from aperture to absorber. The right and the left half belong to different parabolas (hence the name compound parabolic concentrator). The axis of the right branch, for instance, makes an angle θc , 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.
  27. 27. Compound Parabolic collector (CPC or Winston collector) contd. 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost
  28. 28. Compound Parabolic collector (CPC or Winston collector) contd. The compound parabolic collector developed by Winston represents what may be called as the most ideal collector in the sense that, for a given field of view it achieves the maximum possible concentration ratio given by CR = W / b =1 / sin θc where, W = entrance aperture. b = exit aperture, covered by the absorber θc = field of view (half angle)2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost The compound parabolic collector developed by Winston represents what may be called as the most ideal collector in the sense that, for a given field of view it achieves the maximum possible concentration ratio given by CR = W / b =1 / sin θc where, W = entrance aperture. b = exit aperture, covered by the absorber θc = field of view (half angle)
  29. 29. Comparison of Flat Plate and Focusing Collectors Main advantages of the focusing systems over flat plate type collectors:  Reflecting surfaces requires less material and are structurally simpler than flat-plate collectors. For a concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that of the flat- plate collector.  The absorber area of a contractor system is smaller than that of a flat plate system of same solar energy collection and the insolation intensity is therefore greater.  As it is found that in case of solar energy concentrating collector the energy lost to the surrounding is less than that for flat plate collector and the insolation on the absorber is more concentrated, the working fluid can attain higher temperatures in a concentrating system than that in a flat plate collector of the same solar energy collecting surface.  Little or no antifreeze is required to protect the absorber in concentrator systems whereas the entire solar energy collection surface requires antifreeze protection in a flat-plate collector. 2nd Generation PV Thin Film 3rd Generation PV Ultra-High Efficiency Ultra-Low Cost Main advantages of the focusing systems over flat plate type collectors:  Reflecting surfaces requires less material and are structurally simpler than flat-plate collectors. For a concentrator system the cost per unit area of solar collecting surface is therefore potentially less than that of the flat- plate collector.  The absorber area of a contractor system is smaller than that of a flat plate system of same solar energy collection and the insolation intensity is therefore greater.  As it is found that in case of solar energy concentrating collector the energy lost to the surrounding is less than that for flat plate collector and the insolation on the absorber is more concentrated, the working fluid can attain higher temperatures in a concentrating system than that in a flat plate collector of the same solar energy collecting surface.  Little or no antifreeze is required to protect the absorber in concentrator systems whereas the entire solar energy collection surface requires antifreeze protection in a flat-plate collector.
  30. 30. The concentrating systems also have some disadvantages as given below: Out of the beam and diffuse solar radiation, components, only beam component is collected in case of focusing collectors 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 disadvantages have restricted the utility of focusing collectors and no long time practical applications of focusing collectors other than for furnaces are being made. Disadvantages of Concentrating type Collectors The concentrating systems also have some disadvantages as given below: Out of the beam and diffuse solar radiation, components, only beam component is collected in case of focusing collectors 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 disadvantages have restricted the utility of focusing collectors and no long time practical applications of focusing collectors other than for furnaces are being made.
  31. 31. QUERIES??

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