Solar constant and energy calculations

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marinacalder
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If i had a question asking me the energy incident on a solar panel at earth, would I use the area of the panel by the amount of radiation per m2 as 1397Wm-2 or by 684Wm-2 (divided by 2 since from what I understand the incoming radiation from the sun value at 1397 is that landing on the atmosphere and not on the surface of earth? divided by 2 as I took this 1370 to by on a circular disc (imaginary) at atmosphere and took into account the surface area of the earth (4pir^2 which I halved since only one side is illuminated in one time?)
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uberteknik
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(Original post by marinacalder)
If i had a question asking me the energy incident on a solar panel at earth, would I use the area of the panel by the amount of radiation per m2 as 1397Wm-2 or by 684Wm-2 (divided by 2 since from what I understand the incoming radiation from the sun value at 1397 is that landing on the atmosphere and not on the surface of earth? divided by 2 as I took this 1370 to by on a circular disc (imaginary) at atmosphere and took into account the surface area of the earth (4pir^2 which I halved since only one side is illuminated in one time?)
Hello.

This is a good question.

In practical terms for examination questions, the solar constant to be used would be stated as part of the question. The most common figure is around 1367Wm-2 based on a surface placed perpendicular to the solar radiation arriving at the outer edge of the atmosphere. (It may be stated anywhere between 1350 and 1390Wm-2 though.)

Again, this is an average estimated figure produced by taking account of many variables including facts that the earths orbit is not circular and the suns output fluctuates.

The outer edge of the atmosphere is used because anything placed at the earths surface introduces yet more variables which increases the error margin for that average received radiation at the solar panel. i.e. The earth is not a perfect sphere; atmospheric absorption and scattering varies considerably with wavelength and dependent on weather conditions; the earth's axis is tilted and therefore any given fixed point on earth will receive radiation varying with the seasons, night and day etc. etc.

In other words, unless the question asks you to estimate the solar constant for a given place on the earths surface (highly unlikely), use the 1367Wm-2 (plus or minus as given with the question) as the average radiation received by a solar panel placed perpendicular to the incident radiation at the outside edge of the earth's atmosphere.

But, as always, do read the question carefully as it will almost certainly tell you the value to use.

To give you and idea why the solar constant is always an estimate, this link provides a good insight:

http://www.itacanet.org/the-sun-as-a...arths-surface/
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marinacalder
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(Original post by uberteknik)
Hello.

This is a good question.

In practical terms for examination questions, the solar constant to be used would be stated as part of the question. The most common figure is around 1367Wm-2 based on a surface placed perpendicular to the solar radiation arriving at the outer edge of the atmosphere. (It may be stated anywhere between 1350 and 1390Wm-2 though.)

Again, this is an average estimated figure produced by taking account of many variables including facts that the earths orbit is not circular and the suns output fluctuates.

The outer edge of the atmosphere is used because anything placed at the earths surface introduces yet more variables which increases the error margin for that average received radiation at the solar panel. i.e. The earth is not a perfect sphere; atmospheric absorption and scattering varies considerably with wavelength and dependent on weather conditions; the earth's axis is tilted and therefore any given fixed point on earth will receive radiation varying with the seasons, night and day etc. etc.

In other words, unless the question asks you to estimate the solar constant for a given place on the earths surface (highly unlikely), use the 1367Wm-2 (plus or minus as given with the question) as the average radiation received by a solar panel placed perpendicular to the incident radiation at the outside edge of the earth's atmosphere.

But, as always, do read the question carefully as it will almost certainly tell you the value to use.

To give you and idea why the solar constant is always an estimate, this link provides a good insight:

http://www.itacanet.org/the-sun-as-a...arths-surface/
Fantastic, thank you!
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