Posted by *mundt* on August 2, 2008, 11:00 pm

I wish to calculate the solar flux falling on my panels (Ip) for

modeling purposes.

I have:

1. the insolation from a local meterology station (at local solar

noon) = Is

2. the solar declination (at solar noon) = Td

3. my latitude = Tl

4. my panel tilt (azimuth) =Tp

5. my panels face due south

To correct the insolation for the declination of the sun, I believe

that I need to divide the measured insolation (Is) by the cosine of

the actual sun angle (Td-Tl). Thus the solar flux on a surface NORMAL

to the sun is Is/cos(Td-Tl).

To correct for the tilt angle of my panels, I believe I need to

multiply the NORMAL flux by the cosine of the angle between the

normal surface and the actual panel surface, i.e. cos(Td-Tl-Tp).

The flux my panels then see is thus: Ip = Is *cos(Td-Tl-Tp)/cos(Td-

Tl).

Is this a correct formulation?

Thank you in advance for your comments.

Posted by *Bill Kaszeta / Photovoltaic Re* on August 4, 2008, 2:38 am

*>I wish to calculate the solar flux falling on my panels (Ip) for*

*>modeling purposes.*

*>I have:*

*>1. the insolation from a local meterology station (at local solar*

*>noon) = Is*

*>2. the solar declination (at solar noon) = Td*

*>3. my latitude = Tl*

*>4. my panel tilt (azimuth) =Tp*

*>5. my panels face due south*

*>To correct the insolation for the declination of the sun, I believe*

*>that I need to divide the measured insolation (Is) by the cosine of*

*>the actual sun angle (Td-Tl). Thus the solar flux on a surface NORMAL*

*>to the sun is Is/cos(Td-Tl).*

*>To correct for the tilt angle of my panels, I believe I need to*

*>multiply the NORMAL flux by the cosine of the angle between the*

*>normal surface and the actual panel surface, i.e. cos(Td-Tl-Tp).*

*>The flux my panels then see is thus: Ip = Is *cos(Td-Tl-Tp)/cos(Td-*

*>Tl).*

*>Is this a correct formulation?*

*>Thank you in advance for your comments.*

Not so simple.

There are two major measurements made by many observing stations,

global irradiation on a horizontal surface (using a pyranometer) and direct

normal irradiation (pyrheliometer or pyranometer with a shadow band).

Sunshine on a tilted surface has three components, direct normal (the

component that can be concentrated with a magnifying glass), diffuse

(the blue sky and/or reflections from clouds), and reflected from the

ground (albedo).

There is some good information at

http://www.nrel.gov/rredc/models_tools.html

and the related site.

Your calculations apply to the direct beam component on a clear day.

You need to make sure you understand the origin of the data you are using

before applying calculations.

There are computer programs that do these calculations, but

the old links I have no longer function. There are solar radiation

handbooks with full details.

Bill Kaszeta

Photovoltaic Resources Int'l

Tempe Arizona USA

bill@pvri-removethis.biz

Posted by *mundt* on August 4, 2008, 4:47 pm

On Aug 3, 7:38pm, wkasz...@cox.net (Bill Kaszeta / Photovoltaic

Resources) wrote:

*> >I wish to calculate the solar flux falling on my panels (Ip) for*

*> >modeling purposes.*

*> >I have:*

*> >1. the insolation from a local meterology station (at local solar*

*> >noon) = Is*

*> >2. the solar declination (at solar noon) = Td*

*> >3. my latitude = Tl*

*> >4. my panel tilt (azimuth) =Tp*

*> >5. my panels face due south*

*> >To correct the insolation for the declination of the sun, I believe*

*> >that I need to divide the measured insolation (Is) by the cosine of*

*> >the actual sun angle (Td-Tl). Thus the solar flux on a surface NORMAL*

*> >to the sun is Is/cos(Td-Tl).*

*> >To correct for the tilt angle of my panels, I believe I need to*

*> >multiply the NORMAL flux by the cosine of the angle between the*

*> >normal surface and the actual panel surface, i.e. cos(Td-Tl-Tp).*

*> >The flux my panels then see is thus: Ip = Is *cos(Td-Tl-Tp)/cos(Td-*

*> >Tl).*

*> >Is this a correct formulation?*

*> >Thank you in advance for your comments.*

*> Not so simple.*

*> There are two major measurements made by many observing stations,*

*> global irradiation on a horizontal surface (using a pyranometer) and direct*

*> normal irradiation (pyrheliometer or pyranometer with a shadow band).*

*> Sunshine on a tilted surface has three components, direct normal (the*

*> component that can be concentrated with a magnifying glass), diffuse*

*> (the blue sky and/or reflections from clouds), and reflected from the*

*> ground (albedo).*

*> There is some good information athttp://www.nrel.gov/rredc/models_tools.html *

*> and the related site.*

*> Your calculations apply to the direct beam component on a clear day.*

*> You need to make sure you understand the origin of the data you are using*

*> before applying calculations.*

*> There are computer programs that do these calculations, but*

*> the old links I have no longer function. There are solar radiation*

*> handbooks with full details.*

*> Bill Kaszeta*

*> Photovoltaic Resources Int'l*

*> Tempe Arizona USA*

*> b...@pvri-removethis.biz- Hide quoted text -*

*> - Show quoted text -*

Thank you Bill,

Your comments and pointers to more resources are appreciated.

My local insolation data appears to be from a horizontal pyranometer

(data at http://gate1.baaqmd.gov/aqmet/Met.aspx?PID=SI ).

My objective is performance modeling of my solar installation, i.e.

tracking performance changes over time. I am modeling the output of my

installation as a function of independently measured insolation,

temperature, and wind speed. I am hoping for a model accuracy

(resolution?) of less than 1% so that single cell failures could

potentially be detected.

I am selecting data only from "clear" days (by inspection of output

power vs. time graph). It looks to me like the "blue sky" component

can be ignored (for my goal) - it should effect both the pyranometer

measurement and my panel output equally (maybe some very small

spectral response related differences?).

I mainly wanted to confirm that my correction to the measured

insolation (normal direct component anyway) accurately accounted for

the geometric mounting difference between the pyranometer measurement

and my panels.

Thank you again for you comments.

Randy

>I wish to calculate the solar flux falling on my panels (Ip) for>modeling purposes.>I have:>1. the insolation from a local meterology station (at local solar>noon) = Is>2. the solar declination (at solar noon) = Td>3. my latitude = Tl>4. my panel tilt (azimuth) =Tp>5. my panels face due south>To correct the insolation for the declination of the sun, I believe>that I need to divide the measured insolation (Is) by the cosine of>the actual sun angle (Td-Tl). Thus the solar flux on a surface NORMAL>to the sun is Is/cos(Td-Tl).>To correct for the tilt angle of my panels, I believe I need to>multiply the NORMAL flux by the cosine of the angle between the>normal surface and the actual panel surface, i.e. cos(Td-Tl-Tp).>The flux my panels then see is thus: Ip = Is *cos(Td-Tl-Tp)/cos(Td->Tl).>Is this a correct formulation?>Thank you in advance for your comments.