Solar Lattice

Concept

Air mass

ID
concept:air-mass
IRI
https://solarlattice.org/kb/concept/air-mass
Status
active
Also known as
AM1.5, AM 1.5, AM, Absolute air mass, AMa, Air Mass 1.5 G
Category
Irradiance
Concept kind
physics

Air mass is the relative length of the path that direct sunlight takes through the atmosphere. It changes with sun position and with site altitude, which is why it is corrected using the ratio of local pressure to sea-level pressure. It is calculated as one divided by the cosine of the solar zenith angle. AM 0 means sunlight outside the atmosphere, and AM 1 means the sun is directly overhead at sea level. AM 1.5 matches a sun altitude of about 42 degrees. It is the reference spectrum used to test and rate PV cells and modules under Standard test conditions and NOCT conditions. Air mass changes both the amount of sunlight reaching the ground and its spectral content, so a module's short-circuit current moves as air mass changes through the day. For crystalline silicon, normalized Isc is typically several percent higher at high air mass than at solar noon. Over a year the spectral effect on energy production is usually small: more than 90% of the available solar energy arrives at air mass values below 3. Amorphous silicon thin-film modules respond more strongly, so models for them may add air mass as an extra parameter. When analysing monitoring data, setting a minimum sun elevation angle keeps air mass differences small, which keeps spectral mismatch low when you compare performance.

<!-- review notes

Claims

related_to

c1 drafted · lead Standard test conditions

AM 1.5 sets the spectral distribution used for STC module rating.

c2 drafted · lead Nominal operating cell temperature

NOCT conditions also specify an Air Mass 1.5 G spectrum.

c3 drafted · lead Direct normal irradiance

Air mass is the relative path length that direct solar radiation travels through the atmosphere.

review note: support check: partial - Air mass is defined as the path length of direct solar radiation, but the passages never link it to direct normal irradiance as a concept.

c5 drafted · lead Short-circuit current

Performance models use an empirical air-mass function to correct Isc for spectral effects.

c6 drafted · lead Thin-film PV

Amorphous silicon modules show a stronger spectral response, so air mass is added as a model parameter.

review note: support check: partial - Air mass is added as a model parameter for a-Si because of its spectral response, but the passages do not compare a-Si to other technologies as having a 'stronger' response, and they do not say 'thin-film' generally.; review: partial - The passage adds air mass as a model parameter because of a-Si's spectral response, but it calls this 'increased variability' rather than a 'stronger' response and covers only a-Si, not thin-film PV in general.

c7 drafted · lead Varying air mass

explains

c4 drafted · lead Spectral Effect Yield Losses

Changes in air mass shift the solar spectrum, which changes module short-circuit current.

typical_power_loss

c8 drafted · lead ≤ 3 PERCENT

Conditions: Cumulative effect of solar spectral (air mass) variation on annual energy production, as reported in the Sandia array performance model

review note: support check: partial - The passages say the cumulative spectral effect on annual energy is less than 3%, which supports an upper bound, but not a 'typical loss up to 3 percent'. Also the effect is not described as a loss.

Referenced by

Data