Difference between revisions of "Shading"

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m (Text replacement - "Tilt and orientation" to "Tilt and azimuth")
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[[Category:PV source]]
 
[[Category:PV source]]
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[[File:Tiltfrontside.png|thumb|250px|'''Pole-mounted PV modules:'''<br />''(1)'' is the angle of the modules relative to the ground. Production is maximized when the sun's rays strike a module perpendicularly.]]
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The sunlight that reaches the earth is strongest when it strikes a surface perpendicularly or a 90° angle, yet the position of the sun in the sky varies through the day and through the year for every location. This means that to capture as much sunlight as possible and maximize production, a PV module must be properly positioned. There are two important ways in which a [[PV module]] can be positioned relative to the sun:
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*Direction relative to the cardinal directions (North, East, South, West).
 +
*Angle relative to the surface of the earth.
  
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The amount of energy that a PV system produces has a direct relationship to the amount of sunlight that it receives. Shade therefore has a significant effect upon production. It is ideal for [[PV module|PV modules]] to be in a location without any shading during the entire day and throughout the year, unfortunately this is often not the case. As the sun rises or falls in the sky, its angle becomes lower and the shadows that are cast become longer, which means that shade is an inevitability in most locations. If a PV module can be kept shade free from 9:00 to 15:00, its production will not be severely affected, but shading during these crucial hours when the sun is at its strongest will cause a significant drop in production. How badly production will be affected will depend upon a few different factors:
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Understanding the ideal orientation and angle requires a basic understanding of the earth's daily and annual movements.
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*The intensity of the shade (is it really weak from something in the distance or from something only a few meters away?)
 
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*How much of the module is shaded
 
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*What part of the module is shaded
 
  
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As the sun changes position in the sky throughout the day and throughout the year, it is important to consider shade during the entire year when designing and installing a PV system. Shading issues become more pronounced at higher latitudes. For more information on the position of the sun through the year, see [[Tilt and azimuth]].
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==Movements of the earth==
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The earth is at all times rotating around its own axis (completing a full rotation approximately every 24 hours) and orbiting around the sun (completing a full orbit approximately every 365 days).
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===Daily rotation===
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The earth completes a full revolution around its own axis approximately every 24 hours. This causes the strength of the sun to vary throughout the day as it arcs through the sky, rising in the East and setting in the West. The strength of the sun's rays peak when it is at the top of its arc between East and West at around 12:00.
  
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<gallery heights=250px>
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<gallery widths=400px>
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File:Barilochesummers201005.png|Shadow cast at 12:00 on December 21 in Bariloche, Argentina (41°S).
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File:Earthrotation.png|The Earth rotates around its own axis once each day. One side of the earth is always in the dark.
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File:Barilochewinter201005.png|Shadow cast at 12:00 on June 21 in Bariloche, Argentina (41°S).
 
 
</gallery>
 
</gallery>
  
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==Bypass diodes and shading==
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===Yearly orbit===
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The cells of a PV module are connected in series. If one part of a part of a series circuit has high resistance or is not performing properly, it becomes a constraint for all of the other parts of the circuit. PV modules have incorporated ''bypass diodes'' in their junction boxes which automatically bypass strings of cells that are not performing properly. The current from other modules or other cells within the module will simply bypass the poorly functioning columns. The typical PV module only has three different strings of PV cells, connected along the long axis of the module, with each one having its own bypass diode. This has important consequences for how a PV module performs when there is shade as bypass diodes respond variably depending upon the conditions. Shading along the short-axis (rows) of the module will have a greater impact than shading along the long-axis (columns) of the module
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The earth rotates around the sun in a nearly perfect circle one time every 365 days. The yearly orbit of the earth around the sun accounts for the variation in the strength of the sun in different areas between the seasons. The earth's axis is at an angle, roughly 23.5°, which creates seasonal variation as the angle of the sun's rays striking any location on earth varies depending upon the position of the earth in its orbit around the sun. This also has an effect on the number of hours of light that a location receives each day, which becomes more pronounced moving further away from the equator. June 21 in the Southern hemisphere is the shortest day of the year and December 21 is the longest day of the year. The opposite is true in the Northern hemisphere.
  
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<gallery heights=250px>
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<gallery widths=500px>
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File:Modulebypassdiodes.png|60-cell module. Two columns of PV cells are connected in series, which are then connected to a bypass diode (Triangular symbol at top).
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File:Earthorbit.png|The Earth orbits the sun in a nearly perfect circle once each year. The angle of the axis of the earth does not vary as it moves around the sun each year.<br />''1.'' December 21st. ''2.'' March 20th ''3.'' June 21st ''4.'' September 20th
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File:Horizontalshade201005.png|60-cell module. Strong shading across an entire row of cells will trigger all of the bypass diodes. 100% of module production will be lost.
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File:Earthtilt.png|''Left:'' June 21 - The Northern hemisphere is tilted towards the sun and therefore receives more direct sunlight and has more daylight each day than the Southern hemisphere.<br />''Right:'' December 21 - The Southern hemisphere is tilted towards the sun and therefore receives more direct sunlight and has more daylight each day than the Northern hemisphere.
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File:Verticalshade201005.png|60-cell module. Strong shading down an entire column of cells will only trigger one bypass diode. 33% of module production will be lost.
 
 
</gallery>
 
</gallery>
  
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==Partial shading and shade intensity==
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==Azimuth angle==
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Strong shade and shading that covers an entire PV cell is likely to trigger bypass diodes and will result in a significant loss in PV module production. Weak shading or partial shading on a PV cell may cause overall performance of the module to drop without being strong enough to trigger a bypass diode. A common example may shade resulting from a tree branch is close to the module and provides a thick dark line of shade, it will cause significant problems, but a distant branch causing a faint line of shade will not likely cause much production loss.
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The most important window for PV system production is between 9:00 and 15:00. At 9:00 the sun is rising in the East and gaining strength. At around 12:00 the sun is reaching the peak if its arc in the sky between East and West. In the Southern hemisphere, the position of the sun in the sky is due North at this point. In the Northern hemisphere it is due South. At 15:00 the sun is beginning to fall towards the West and is losing strength. To maximize the amount of sunlight that a PV module captures, it is ideal to if it is pointed with an azimuth angle of due North (0°) in the Southern hemisphere and due South (180°) in the Northern hemisphere as this enables the PV module to capture the maximum amount of sunlight as the sun arcs from East to West. Additionally, if the PV module is properly facing either of these directions it it will be able to take maximum advantage of the sun's rays when it is at its strongest.  
  
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<gallery heights=250px>
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'''Southern hemisphere:''' A PV module on a fixed mounting structure will maximize yearly production by facing directly towards the North:
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File:Weakshadeline.png|60-cell module with low intensity shading.
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<gallery widths=250px>
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File:Strongshadeline.png|60-cell module with a line of high intensity shading.
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File:Mozambiquejune219am.png|June 21, 9:00 in Quelimane, Mozambique (17°S). The sun rises in the East.<ref name="sunsmotions"> University of Nebraska-Lincoln Motions of the Sun Lab https://astro.unl.edu/naap/motion3/motion3.html </ref>
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File:Mozambiquejune213pm.png|June 21, 15:00 in Quelimane, Mozambique (17°S). The sun descends in the West.<ref name="sunsmotions"/>
 
</gallery>
 
</gallery>
  
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==Evaluating shade==
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'''Northern hemisphere:''' A PV module on a fixed mounting structure will maximize yearly production by facing directly towards the South:
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It can be difficult to evaluate the impact of shading from mountains or trees in another season when evaluating a potential project and this information can be critical when designing a system. There are various tools available that can help perform an analysis of the impact of shade on production throughout the year. The most economical and appropriate for off-grid use is the [https://www.solarpathfinder.com Solar Pathfinder]
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<gallery widths=250px>
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File:Oaxacadec219am.png|Dec 21, 9:00 in Oaxaca, Mexico (17°N). The sun rises in the East.<ref name="sunsmotions"/>
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File:Oaxacadec213pm.png|Dec 21, 15:00 in Oaxaca, Mexico (17° N). The sun descends in the West.<ref name="sunsmotions"/>
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</gallery>
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===Magnetic declination===
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A compass is necessary to determine the cardinal directions in order to point PV in the right direction. PV modules maximize production when pointed ''true'' North/South, but an unadjusted compass provides ''magnetic'' North/South which can vary significantly depending on the magnetic field in the location where the PV system is being installed. The adjustment factor between magnetic North and true north is called '''magnetic declination.''' This value can be found on some paper maps, but the most accurate and easiest source is the internet. There are many different free tools such as the [https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination NOAA Magnetic Declination Tool.]
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<gallery widths=200px>
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File:Compassdeclination.png|The needle of a compass points towards magnetic North. This compass has been adjusted for a 23° East magnetic declination (true North is 23° to the West of magnetic north) to determine true North.
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</gallery>
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===Non-standard azimuth angles===
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In some cases, it is not possible or not convenient to face a PV module true North/South. If an array is slightly off from true North/South it is unlikely to affect production greatly in any case, but the effect does become more pronounced moving further from the equator. How much production is affected varies significantly, for example:
 +
*A system with a tilt of 5° facing true East (90°) or true West (270°) at 0° latitude will produce an estimated 5% less than a system facing true North (0°).<ref name="pvwatts"> National Renewable Energy Labs PVWatts Calculator https://pvwatts.nrel.gov/index.php </ref>
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*A system with a tilt of 17° facing true East (90°) or true West (270°) at 17°S latitude will produce an estimated 7.7% less than a system facing true North (0°)<ref name="pvwatts"/>
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*A system with a tilt of 41° facing true East (90°) or true West (270°) at 41°S latitude will produce an estimated 19.8% less than a system facing true North (0°)<ref name="pvwatts"/>
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 +
==Tilt angle==
 +
The path that the sun takes through the sky changes each day as the earth moves through its orbit. The change becomes more pronounced as one moves away from the equator. A PV module is able to collect the most energy as the sun changes position in the sky throughout the year if a PV module is placed at an angle that is equivalent to the latitude of the location. At the equator it is important to still tilt the panel with at least a 5° tilt to ensure that water will run off the panel to shed dust.
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 +
<gallery widths=250px>
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File:Barilochesolstices.png|'''Pole mount system with a tilt of 41° with true North azimuth (0°).''' The path of the sun in Bariloche, Argentina 41°S at 12:00 on December 21 (summer solstice) and June 21 (winter solstice).<ref name="sunsmotions"/>
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File:Kampalasolstices.png|'''Pole mount system with a tilt of 5° with true South azimuth (180°).''' The path of the sun in Kampala, Uganda .3°N at 12:00 on December 21 and June 21.<ref name="sunsmotions"/>
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File:Gobisolstices.png|'''Pole mount system with a tilt of 41° with true South azimuth (180°).''' The path of the sun in the Gobi Desert, Mongolia 41°N at 12:00 on December 21 (winter solstice) and June 21 (summer solstice).<ref name="sunsmotions"/>
 +
</gallery>
 +
 
 +
===Non-standard tilt angles===
 +
Under some conditions it makes sense to use a tilt for a PV module that is different than latitude.
 +
*It is often the simplest, cheapest and most theft-proof option to install PV modules on the roof of a home. In this case, typically the angle of the existing roof is used as the tilt angle for the PV module. An 17° latitude, an array with a tilt angle of 7° will only produce an estimated 2.1% less than an array with a tilt angle of 17°.<ref name="pvwatts"/>
 +
*Mounting systems are frequently build that enable users to change the angle of the PV modules through the course of the year to maximize production at higher latitudes. The standard array angle adjustment is +15° at the fall solstice and -15° at the spring solstice. At 41° latitude, a system that is adjusted at the two solstices will produce an estimated 6% more than a system that is at a fixed 41° tilt. This additional energy can be crucial in the winter.<ref name="pvwatts"/>
 +
*If PV system is only going to be used seasonally then mounting the PV modules at an angle that maximizes production during that time of the year is advisable.
 +
*As in the example systems above, it is not advisable to mount PV modules at an angle of 0° near the equator as they are unable to shed water and will lose significant production as they become increasingly soiled. At least a 5° angle is recommended.
 +
 
  
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==Module orientation and shading==
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==Notes==
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If there is strong shading from an obstacle like a mountain to the East (shading in the morning) or West (shading in the afternoon), orienting the panel somewhat in the direction with the better solar resource should be considered in order to maximize production.
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<references/>
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NOAA Magnetic Declination Tool https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination

Revision as of 07:45, 1 January 2021

Pole-mounted PV modules:
(1) is the angle of the modules relative to the ground. Production is maximized when the sun's rays strike a module perpendicularly.

The sunlight that reaches the earth is strongest when it strikes a surface perpendicularly or a 90° angle, yet the position of the sun in the sky varies through the day and through the year for every location. This means that to capture as much sunlight as possible and maximize production, a PV module must be properly positioned. There are two important ways in which a PV module can be positioned relative to the sun:

  • Direction relative to the cardinal directions (North, East, South, West).
  • Angle relative to the surface of the earth.

Understanding the ideal orientation and angle requires a basic understanding of the earth's daily and annual movements.

Movements of the earth

The earth is at all times rotating around its own axis (completing a full rotation approximately every 24 hours) and orbiting around the sun (completing a full orbit approximately every 365 days).

Daily rotation

The earth completes a full revolution around its own axis approximately every 24 hours. This causes the strength of the sun to vary throughout the day as it arcs through the sky, rising in the East and setting in the West. The strength of the sun's rays peak when it is at the top of its arc between East and West at around 12:00.

  • The Earth rotates around its own axis once each day. One side of the earth is always in the dark.

Yearly orbit

The earth rotates around the sun in a nearly perfect circle one time every 365 days. The yearly orbit of the earth around the sun accounts for the variation in the strength of the sun in different areas between the seasons. The earth's axis is at an angle, roughly 23.5°, which creates seasonal variation as the angle of the sun's rays striking any location on earth varies depending upon the position of the earth in its orbit around the sun. This also has an effect on the number of hours of light that a location receives each day, which becomes more pronounced moving further away from the equator. June 21 in the Southern hemisphere is the shortest day of the year and December 21 is the longest day of the year. The opposite is true in the Northern hemisphere.

  • The Earth orbits the sun in a nearly perfect circle once each year. The angle of the axis of the earth does not vary as it moves around the sun each year.
    1. December 21st. 2. March 20th 3. June 21st 4. September 20th

  • Left: June 21 - The Northern hemisphere is tilted towards the sun and therefore receives more direct sunlight and has more daylight each day than the Southern hemisphere.
    Right: December 21 - The Southern hemisphere is tilted towards the sun and therefore receives more direct sunlight and has more daylight each day than the Northern hemisphere.

Azimuth angle

The most important window for PV system production is between 9:00 and 15:00. At 9:00 the sun is rising in the East and gaining strength. At around 12:00 the sun is reaching the peak if its arc in the sky between East and West. In the Southern hemisphere, the position of the sun in the sky is due North at this point. In the Northern hemisphere it is due South. At 15:00 the sun is beginning to fall towards the West and is losing strength. To maximize the amount of sunlight that a PV module captures, it is ideal to if it is pointed with an azimuth angle of due North (0°) in the Southern hemisphere and due South (180°) in the Northern hemisphere as this enables the PV module to capture the maximum amount of sunlight as the sun arcs from East to West. Additionally, if the PV module is properly facing either of these directions it it will be able to take maximum advantage of the sun's rays when it is at its strongest.

Southern hemisphere: A PV module on a fixed mounting structure will maximize yearly production by facing directly towards the North:

  • June 21, 9:00 in Quelimane, Mozambique (17°S). The sun rises in the East.[1]

  • June 21, 15:00 in Quelimane, Mozambique (17°S). The sun descends in the West.[1]

Northern hemisphere: A PV module on a fixed mounting structure will maximize yearly production by facing directly towards the South:

  • Dec 21, 9:00 in Oaxaca, Mexico (17°N). The sun rises in the East.[1]

  • Dec 21, 15:00 in Oaxaca, Mexico (17° N). The sun descends in the West.[1]

Magnetic declination

A compass is necessary to determine the cardinal directions in order to point PV in the right direction. PV modules maximize production when pointed true North/South, but an unadjusted compass provides magnetic North/South which can vary significantly depending on the magnetic field in the location where the PV system is being installed. The adjustment factor between magnetic North and true north is called magnetic declination. This value can be found on some paper maps, but the most accurate and easiest source is the internet. There are many different free tools such as the NOAA Magnetic Declination Tool.

  • The needle of a compass points towards magnetic North. This compass has been adjusted for a 23° East magnetic declination (true North is 23° to the West of magnetic north) to determine true North.

Non-standard azimuth angles

In some cases, it is not possible or not convenient to face a PV module true North/South. If an array is slightly off from true North/South it is unlikely to affect production greatly in any case, but the effect does become more pronounced moving further from the equator. How much production is affected varies significantly, for example:

  • A system with a tilt of 5° facing true East (90°) or true West (270°) at 0° latitude will produce an estimated 5% less than a system facing true North (0°).[2]
  • A system with a tilt of 17° facing true East (90°) or true West (270°) at 17°S latitude will produce an estimated 7.7% less than a system facing true North (0°)[2]
  • A system with a tilt of 41° facing true East (90°) or true West (270°) at 41°S latitude will produce an estimated 19.8% less than a system facing true North (0°)[2]

Tilt angle

The path that the sun takes through the sky changes each day as the earth moves through its orbit. The change becomes more pronounced as one moves away from the equator. A PV module is able to collect the most energy as the sun changes position in the sky throughout the year if a PV module is placed at an angle that is equivalent to the latitude of the location. At the equator it is important to still tilt the panel with at least a 5° tilt to ensure that water will run off the panel to shed dust.

  • Pole mount system with a tilt of 41° with true North azimuth (0°). The path of the sun in Bariloche, Argentina 41°S at 12:00 on December 21 (summer solstice) and June 21 (winter solstice).[1]

  • Pole mount system with a tilt of 5° with true South azimuth (180°). The path of the sun in Kampala, Uganda .3°N at 12:00 on December 21 and June 21.[1]

  • Pole mount system with a tilt of 41° with true South azimuth (180°). The path of the sun in the Gobi Desert, Mongolia 41°N at 12:00 on December 21 (winter solstice) and June 21 (summer solstice).[1]

Non-standard tilt angles

Under some conditions it makes sense to use a tilt for a PV module that is different than latitude.

  • It is often the simplest, cheapest and most theft-proof option to install PV modules on the roof of a home. In this case, typically the angle of the existing roof is used as the tilt angle for the PV module. An 17° latitude, an array with a tilt angle of 7° will only produce an estimated 2.1% less than an array with a tilt angle of 17°.[2]
  • Mounting systems are frequently build that enable users to change the angle of the PV modules through the course of the year to maximize production at higher latitudes. The standard array angle adjustment is +15° at the fall solstice and -15° at the spring solstice. At 41° latitude, a system that is adjusted at the two solstices will produce an estimated 6% more than a system that is at a fixed 41° tilt. This additional energy can be crucial in the winter.[2]
  • If PV system is only going to be used seasonally then mounting the PV modules at an angle that maximizes production during that time of the year is advisable.
  • As in the example systems above, it is not advisable to mount PV modules at an angle of 0° near the equator as they are unable to shed water and will lose significant production as they become increasingly soiled. At least a 5° angle is recommended.


Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 University of Nebraska-Lincoln Motions of the Sun Lab https://astro.unl.edu/naap/motion3/motion3.html
  2. ↑ 2.0 2.1 2.2 2.3 2.4 National Renewable Energy Labs PVWatts Calculator https://pvwatts.nrel.gov/index.php

NOAA Magnetic Declination Tool https://www.ngdc.noaa.gov/geomag/calculators/magcalc.shtml#declination

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