05-07-2012, 10:47 AM
Solar tracker
[attachment=26775]
A backyard installation of passive single–axis trackers, DC rated at 2340 watts. Seen here in winter midday position, tilted toward the south. The tall poles allow walk-under and use of the ground space underneath the panels for plantings that thrive on protection from the intense midday summer sun at this location
For solar tracking in plants, see Heliotropism. For solar telescope tracking, see Telescope.
A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices.
In flat-panel photovoltaic (PV) applications trackers are used to minimize the angle of incidence between the incoming light and a photovoltaic panel. This increases the amount of energy produced from a fixed amount of installed power generating capacity. In standard photovoltaic applications, it is estimated that trackers are used in at least 85% of commercial installations greater than 1MW from 2009 to 2012.[1][2]
In concentrated photovoltaic (CPV) and concentrated solar thermal (CSP) applications trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because such systems do not produce energy unless oriented closely toward the sun.
Contents
• 1 Types of solar collector
o 1.1 Fixed mount
o 1.2 Trackers
1.2.1 Fixed collector / moving mirror
1.2.2 Moving collector
• 2 Non-concentrating photovoltaic (PV) trackers
o 2.1 Accuracy requirements
o 2.2 Technologies supported
• 3 Concentrated photovoltaic (CPV) trackers
o 3.1 Accuracy requirements
o 3.2 Technologies supported
• 4 Single axis trackers
o 4.1 Horizontal single axis tracker (HSAT)
o 4.2 Vertical single axis tracker (VSAT)
o 4.3 Tilted single axis tracker (TSAT)
o 4.4 Polar aligned single axis trackers (PASAT)
• 5 Dual axis trackers
o 5.1 Tip–tilt dual axis tracker (TTDAT)
o 5.2 Azimuth-altitude dual axis tracker (AADAT)
• 6 Tracker type selection
• 7 Multi-mirror concentrating PV
• 8 Drive types
o 8.1 Active tracker
o 8.2 Passive tracker
o 8.3 Chronological tracker
• 9 Unusual adaptations
o 9.1 Rotating building
• 10 See also
• 11 Notes and references
Types of solar collector
Different types of solar collector and their location (latitude) require different types of tracking mechanism. Solar collectors may be:
• non-concentrating flat-panels, usually photovoltaic or hot-water,
• concentrating systems, of a variety of types.
Solar collector mounting systems may be fixed (manually aligned) or tracking. Tracking systems may be configured as:
• Fixed collector / moving mirror - i.e. Heliostat
• Moving collector
[edit] Fixed mount
Domestic and small-scale commercial photovoltaic and hot-water panels are usually fixed, often flush-mounted on an appropriately facing pitched roof. Advantages of fixed mount systems (i.e. factors tending to indicate against trackers) include the following:
• Mechanical simplicity, and hence lower installation and ongoing maintenance costs.
• Wind-loading: it is easier and cheaper to provision a sturdy mount; all mounts other than fixed flush-mounted panels must be carefully designed having regard to their wind loading due to their greater exposure.
• Indirect light: approximately 10% [3] of the incident solar radiation is diffuse light, available at any angle of misalignment with the direct sun.
• Tolerance to misalignment: effective collection area for a flat-panel is relatively insensitive to quite high levels of misalignment with the sun – see table and diagram at Accuracy Requirements section below – for example even a 25° misalignment reduces the direct solar energy collected by less than 10%.
Fixed mounts are usually used in conjunction with non-concentrating systems, however an important class of non-tracking concentrating collectors, of particular value in the 3rd world, are portable solar cookers. These utilize relatively low levels of concentration, typically around 2 to 8 Suns and are manually aligned.
Trackers
Even though a fixed flat-panel can be set to collect a high proportion of available noon-time energy, significant power is also available in the early mornings and late afternoons[4] when the misalignment with a fixed panel becomes excessive to collect a reasonable proportion of the available energy. For example, even when the Sun is only 10° above the horizon the available energy can already be around half the noon-time energy levels (or even greater depending on latitude, season, and atmospheric conditions).
Thus the primary benefit of a tracking system is to collect solar energy for the longest period of the day, and with the most accurate alignment as the Sun's position shifts with the seasons.
In addition, the greater the level of concentration employed the more important accurate tracking becomes, because the proportion of energy derived from direct radiation is higher, and the region where that concentrated energy is focused becomes smaller.
Fixed collector / moving mirror
Main article: Heliostat
Many collectors cannot be moved, for example high-temperature collectors where the energy is recovered as hot liquid or gas (e.g. steam). Other examples include direct heating and lighting of buildings and fixed in-built solar cookers, such as Scheffler reflectors[5][6][7]. In such cases it is necessary to employ a moving mirror so that, regardless of where the Sun is positioned in the sky, the Sun's rays are redirected onto the collector.
Due to the complicated motion of the Sun across the sky, and the level of precision required to correctly aim the Sun's rays onto the target, a heliostat mirror generally employs a dual axis tracking system, with at least one axis mechanized. In different applications, mirrors may be flat or concave.
Moving collector
Trackers can be grouped into classes by the number and orientation of the tracker's axes. Compared to a fixed mount, a single axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 6%.[8][9]
Photovoltaic trackers can be classified into two types: Standard Photovoltaic (PV) Trackers and Concentrated Photovoltaic (CPV) Trackers. Each of these tracker types can be further categorized by the number and orientation of their axes, their actuation architecture and drive type, their intended applications, their vertical supports and foundation type.
[edit] Non-concentrating photovoltaic (PV) trackers
Photovoltaic panels accept both direct and diffuse light from the sky. The panels on a Standard Photovoltaic Trackers always gather the available direct light. The tracking functionality in Standard Photovoltaic Trackers is used to minimize the angle of incidence between incoming light and the photovoltaic panel. This increases the amount of energy gathered from the direct component of the incoming light.
Accuracy requirements
The effective collection area of a flat-panel solar collector varies with the cosine of the misalignment of the panel with the Sun.
In non-concentrating flat-panel systems, the energy contributed by the direct beam drops off with the cosine[10] of the angle between the incoming light and the panel. In addition, the reflectance (averaged across all polarizations) is approximately constant for angles of incidence up to around 50°, beyond which reflectance degrades rapidly.[11]
For example trackers that have accuracies of ± 5° can deliver greater than 99.6% of the energy delivered by the direct beam plus 100% of the diffuse light. As a result, high accuracy tracking is not typically used in non-concentrating PV applications.
Technologies supported
The physics behind standard photovoltaic (PV) trackers works with all standard photovoltaic module technologies. These include all types of crystalline silicon panels (monocrystalline, multicrystalline, polycrystalline) and all types of thin film panels (amorphous silicon, CdTe, CIGS, microcrystalline).
Concentrated photovoltaic (CPV) trackers
The optics in CPV modules accept the direct component of the incoming light and therefore must be oriented appropriately to maximize the energy collected. In low concentration applications a portion of the diffuse light from the sky can also be captured. The tracking functionality in CPV modules is used to orient the optics such that the incoming light is focused to a photovoltaic collector.
CPV modules that concentrate in one dimension must be tracked normal to the sun in one axis. CPV modules that concentrate in two dimensions must be tracked normal to the sun in two axes.
Accuracy requirements
The physics behind CPV optics requires that tracking accuracy increase as the systems concentration ratio increases. However, for a given concentration, nonimaging optics[15][16] provide the widest possible acceptance angles, which may be used to reduce tracking accuracy.
In typical high concentration systems tracking accuracy must be in the ± 0.1° range to deliver approximately 90% of the rated power output.[17][18] In low concentration systems, tracking accuracy must be in the ± 2.0° range to deliver 90% of the rated power output. As a result, high accuracy tracking systems are typically used.
[attachment=26775]
A backyard installation of passive single–axis trackers, DC rated at 2340 watts. Seen here in winter midday position, tilted toward the south. The tall poles allow walk-under and use of the ground space underneath the panels for plantings that thrive on protection from the intense midday summer sun at this location
For solar tracking in plants, see Heliotropism. For solar telescope tracking, see Telescope.
A solar tracker is a generic term used to describe devices that orient various payloads toward the sun. Payloads can be photovoltaic panels, reflectors, lenses or other optical devices.
In flat-panel photovoltaic (PV) applications trackers are used to minimize the angle of incidence between the incoming light and a photovoltaic panel. This increases the amount of energy produced from a fixed amount of installed power generating capacity. In standard photovoltaic applications, it is estimated that trackers are used in at least 85% of commercial installations greater than 1MW from 2009 to 2012.[1][2]
In concentrated photovoltaic (CPV) and concentrated solar thermal (CSP) applications trackers are used to enable the optical components in the CPV and CSP systems. The optics in concentrated solar applications accept the direct component of sunlight light and therefore must be oriented appropriately to collect energy. Tracking systems are found in all concentrator applications because such systems do not produce energy unless oriented closely toward the sun.
Contents
• 1 Types of solar collector
o 1.1 Fixed mount
o 1.2 Trackers
1.2.1 Fixed collector / moving mirror
1.2.2 Moving collector
• 2 Non-concentrating photovoltaic (PV) trackers
o 2.1 Accuracy requirements
o 2.2 Technologies supported
• 3 Concentrated photovoltaic (CPV) trackers
o 3.1 Accuracy requirements
o 3.2 Technologies supported
• 4 Single axis trackers
o 4.1 Horizontal single axis tracker (HSAT)
o 4.2 Vertical single axis tracker (VSAT)
o 4.3 Tilted single axis tracker (TSAT)
o 4.4 Polar aligned single axis trackers (PASAT)
• 5 Dual axis trackers
o 5.1 Tip–tilt dual axis tracker (TTDAT)
o 5.2 Azimuth-altitude dual axis tracker (AADAT)
• 6 Tracker type selection
• 7 Multi-mirror concentrating PV
• 8 Drive types
o 8.1 Active tracker
o 8.2 Passive tracker
o 8.3 Chronological tracker
• 9 Unusual adaptations
o 9.1 Rotating building
• 10 See also
• 11 Notes and references
Types of solar collector
Different types of solar collector and their location (latitude) require different types of tracking mechanism. Solar collectors may be:
• non-concentrating flat-panels, usually photovoltaic or hot-water,
• concentrating systems, of a variety of types.
Solar collector mounting systems may be fixed (manually aligned) or tracking. Tracking systems may be configured as:
• Fixed collector / moving mirror - i.e. Heliostat
• Moving collector
[edit] Fixed mount
Domestic and small-scale commercial photovoltaic and hot-water panels are usually fixed, often flush-mounted on an appropriately facing pitched roof. Advantages of fixed mount systems (i.e. factors tending to indicate against trackers) include the following:
• Mechanical simplicity, and hence lower installation and ongoing maintenance costs.
• Wind-loading: it is easier and cheaper to provision a sturdy mount; all mounts other than fixed flush-mounted panels must be carefully designed having regard to their wind loading due to their greater exposure.
• Indirect light: approximately 10% [3] of the incident solar radiation is diffuse light, available at any angle of misalignment with the direct sun.
• Tolerance to misalignment: effective collection area for a flat-panel is relatively insensitive to quite high levels of misalignment with the sun – see table and diagram at Accuracy Requirements section below – for example even a 25° misalignment reduces the direct solar energy collected by less than 10%.
Fixed mounts are usually used in conjunction with non-concentrating systems, however an important class of non-tracking concentrating collectors, of particular value in the 3rd world, are portable solar cookers. These utilize relatively low levels of concentration, typically around 2 to 8 Suns and are manually aligned.
Trackers
Even though a fixed flat-panel can be set to collect a high proportion of available noon-time energy, significant power is also available in the early mornings and late afternoons[4] when the misalignment with a fixed panel becomes excessive to collect a reasonable proportion of the available energy. For example, even when the Sun is only 10° above the horizon the available energy can already be around half the noon-time energy levels (or even greater depending on latitude, season, and atmospheric conditions).
Thus the primary benefit of a tracking system is to collect solar energy for the longest period of the day, and with the most accurate alignment as the Sun's position shifts with the seasons.
In addition, the greater the level of concentration employed the more important accurate tracking becomes, because the proportion of energy derived from direct radiation is higher, and the region where that concentrated energy is focused becomes smaller.
Fixed collector / moving mirror
Main article: Heliostat
Many collectors cannot be moved, for example high-temperature collectors where the energy is recovered as hot liquid or gas (e.g. steam). Other examples include direct heating and lighting of buildings and fixed in-built solar cookers, such as Scheffler reflectors[5][6][7]. In such cases it is necessary to employ a moving mirror so that, regardless of where the Sun is positioned in the sky, the Sun's rays are redirected onto the collector.
Due to the complicated motion of the Sun across the sky, and the level of precision required to correctly aim the Sun's rays onto the target, a heliostat mirror generally employs a dual axis tracking system, with at least one axis mechanized. In different applications, mirrors may be flat or concave.
Moving collector
Trackers can be grouped into classes by the number and orientation of the tracker's axes. Compared to a fixed mount, a single axis tracker increases annual output by approximately 30%, and a dual axis tracker an additional 6%.[8][9]
Photovoltaic trackers can be classified into two types: Standard Photovoltaic (PV) Trackers and Concentrated Photovoltaic (CPV) Trackers. Each of these tracker types can be further categorized by the number and orientation of their axes, their actuation architecture and drive type, their intended applications, their vertical supports and foundation type.
[edit] Non-concentrating photovoltaic (PV) trackers
Photovoltaic panels accept both direct and diffuse light from the sky. The panels on a Standard Photovoltaic Trackers always gather the available direct light. The tracking functionality in Standard Photovoltaic Trackers is used to minimize the angle of incidence between incoming light and the photovoltaic panel. This increases the amount of energy gathered from the direct component of the incoming light.
Accuracy requirements
The effective collection area of a flat-panel solar collector varies with the cosine of the misalignment of the panel with the Sun.
In non-concentrating flat-panel systems, the energy contributed by the direct beam drops off with the cosine[10] of the angle between the incoming light and the panel. In addition, the reflectance (averaged across all polarizations) is approximately constant for angles of incidence up to around 50°, beyond which reflectance degrades rapidly.[11]
For example trackers that have accuracies of ± 5° can deliver greater than 99.6% of the energy delivered by the direct beam plus 100% of the diffuse light. As a result, high accuracy tracking is not typically used in non-concentrating PV applications.
Technologies supported
The physics behind standard photovoltaic (PV) trackers works with all standard photovoltaic module technologies. These include all types of crystalline silicon panels (monocrystalline, multicrystalline, polycrystalline) and all types of thin film panels (amorphous silicon, CdTe, CIGS, microcrystalline).
Concentrated photovoltaic (CPV) trackers
The optics in CPV modules accept the direct component of the incoming light and therefore must be oriented appropriately to maximize the energy collected. In low concentration applications a portion of the diffuse light from the sky can also be captured. The tracking functionality in CPV modules is used to orient the optics such that the incoming light is focused to a photovoltaic collector.
CPV modules that concentrate in one dimension must be tracked normal to the sun in one axis. CPV modules that concentrate in two dimensions must be tracked normal to the sun in two axes.
Accuracy requirements
The physics behind CPV optics requires that tracking accuracy increase as the systems concentration ratio increases. However, for a given concentration, nonimaging optics[15][16] provide the widest possible acceptance angles, which may be used to reduce tracking accuracy.
In typical high concentration systems tracking accuracy must be in the ± 0.1° range to deliver approximately 90% of the rated power output.[17][18] In low concentration systems, tracking accuracy must be in the ± 2.0° range to deliver 90% of the rated power output. As a result, high accuracy tracking systems are typically used.