Estimate diffuse and direct component from global irradiance
I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting very strange results. I would expect the Direct Normal Irradiance (DNI) to be lower than the Global Horizontal Irradiance (GHI); or am I missing something? Values of GHI are not above 800 W m^2 for these days.
EDIT: As per Cliff H advice, I have limited the solar zenith to less than 85 arc degrees; the results have improved however, there are large spikes in DNI values that do not appear very reasonable, e.g. start of 07-16.
pvlib solar
asked Nov 12 at 20:23
Kievit
374
add a comment |
I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting very strange results. I would expect the Direct Normal Irradiance (DNI) to be lower than the Global Horizontal Irradiance (GHI); or am I missing something? Values of GHI are not above 800 W m^2 for these days.
EDIT: As per Cliff H advice, I have limited the solar zenith to less than 85 arc degrees; the results have improved however, there are large spikes in DNI values that do not appear very reasonable, e.g. start of 07-16.
pvlib solar
asked Nov 12 at 20:23
Kievit
374
add a comment |
I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting very strange results. I would expect the Direct Normal Irradiance (DNI) to be lower than the Global Horizontal Irradiance (GHI); or am I missing something? Values of GHI are not above 800 W m^2 for these days.
EDIT: As per Cliff H advice, I have limited the solar zenith to less than 85 arc degrees; the results have improved however, there are large spikes in DNI values that do not appear very reasonable, e.g. start of 07-16.
pvlib solar
asked Nov 12 at 20:23
Kievit
374
I am looking to separate the diffuse and direct component of global irradiance and found the Erbs model to do this in pvlib (see pvlib.irradiance.erbs) however, I am getting very strange results. I would expect the Direct Normal Irradiance (DNI) to be lower than the Global Horizontal Irradiance (GHI); or am I missing something? Values of GHI are not above 800 W m^2 for these days.
EDIT: As per Cliff H advice, I have limited the solar zenith to less than 85 arc degrees; the results have improved however, there are large spikes in DNI values that do not appear very reasonable, e.g. start of 07-16.
pvlib solar
pvlib solar
asked Nov 12 at 20:23
Kievit
374
asked Nov 12 at 20:23
Kievit
374
edited Nov 19 at 12:36
asked Nov 12 at 20:23
Kievit
374
asked Nov 12 at 20:23
Kievit
374
asked Nov 12 at 20:23
Kievit
374
374
add a comment |
add a comment |
1 Answer
1
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oldest
votes
DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.
The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.
edited Nov 19 at 16:02
Kievit
374
answered Nov 12 at 21:44
Cliff H
711
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
add a comment |
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1 Answer
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1 Answer
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active
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oldest
votes
DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.
The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.
edited Nov 19 at 16:02
Kievit
374
answered Nov 12 at 21:44
Cliff H
711
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
add a comment |
DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.
The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.
edited Nov 19 at 16:02
Kievit
374
answered Nov 12 at 21:44
Cliff H
711
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
add a comment |
DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.
The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.
edited Nov 19 at 16:02
Kievit
374
answered Nov 12 at 21:44
Cliff H
711
DNI > GHI is common at low solar elevation. GHI decreases much faster than DNI as solar elevation decreases. For example, think of a clear day with the sun right near the horizon. DNI will large because it's measured on a plane normal to the sun vector, but GHI will be near zero.
The values of DNI that are much greater than 1000 W/m2 are likely at very high zenith, since the Erbs model basically divides by cos(zenith). In practice, I limit using decomposition models like Erbs to zenith<85 degrees, to avoid the non-physical results.
edited Nov 19 at 16:02
Kievit
374
answered Nov 12 at 21:44
Cliff H
711
edited Nov 19 at 16:02
Kievit
374
edited Nov 19 at 16:02
Kievit
374
edited Nov 19 at 16:02
Kievit
374
374
answered Nov 12 at 21:44
Cliff H
711
answered Nov 12 at 21:44
Cliff H
711
answered Nov 12 at 21:44
Cliff H
711
711
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
add a comment |
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
Thanks for this. I have limited the zenith to < 85 arc degrees. This has definitely improved the results however, I still get funny artefacts.
– Kievit
Nov 12 at 22:53
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
The results appear credible to me. The Erbs model is estimating the diffuse component of GHI, and returns DNI by a closure calculation, i.e., DNI = (GHI - DHI) / cos(Z). The empirical equation that obtains DHI from GHI is a fit through a fairly broad scatter of data, so it should be regarded as one of many possible values of DHI that correspond to the observed GHI value.
– Cliff H
Nov 14 at 16:13
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
Thanks. I did not consider that DNI assumes the surface perpendicular to the rays. As this is slightly different, I have asked a new question here on how to estimate DNI and DHI for a horizontal surface from GHI.
– Kievit
Nov 19 at 12:31
add a comment |
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