implements the equations from Ashrae Fundamentals 2009 for clear sky in Python

• Free software: Mozilla Public License Version 2.0
• Documentation: https://pyclearsky.readthedocs.io.
• Repository: https://github.com/santoshphilip/pyclearsky

Implements the following equation of ASHRAE Fundamentals 2009, chapter 14

• Equation (4) from page 14.7
• Equation (16) from page 14.9
• Equation (17) from page 14.9
• Equation (18) from page 14.9
• Equation (19) from page 14.9
• Equation (20) from page 14.9

What does pyclearsky do ?

• pyclearsky calculates the radiation from a clear sky.
• The weather files do not always have this data, since they include cloud cover

Why would you want to calculate the radiation from a clear sky ?

• Cloud cover in the weather files do not always reflect reality.
• Sometimes you want to simulate an exteme condtion and you want to assume there is no cloud cover
• This may needed when you estimate the reflected sunlight from a water body or an adjacent building
• You are likely to want to calculate this without cloud cover
• pyclearsky will let you do that

How does pyclearsky do this calculation ?

• Chapter 14 in ASHRAE Fundamentals 2009 describes the equations that calculate the radiations from a clear sky
• the latest weather files come with three file types. They are *.epw, *.ddy and *.stat
• The raw data needed to do this calculation is in the weather file. Specifically in the *.stat file

A demonstration of pycleasky

Let us use the Phoenix AZ weather file as a way of exploring pyclearsky. And let us look into the *.stat file. The *.stat file has the following lines (at around line 37):

- Monthly Solar Irradiance Wh/m² (noon on 21st of month)
                  ib (beam)          915     937     938     920     870     827     727     750     807     833     891     907
               id (diffuse)           89     102     121     141     170     194     250     220     171     140      92      81

                         ib        = Clear Sky Noon Beam Normal Irradiance on 21st Day
                         id        = Clear Sky Noon Diffuse Horizontal Irradiance on 21st Day

This is the clear sky radiation on the 21st of every month. Let us try to use pyclearsky to calculate the same results:

from io import open # to work with python2 and python3
from pyclearsky import clearskyrad
fname = "./original_code/weatherfiles/USA_AZ_Phoenix/USA_AZ_Phoenix.722780_TMY2.stat"
fhandle = open(fname, 'r', encoding='latin1')
tau = clearskyrad.tau(fhandle)
taub, taud = tau

print taub
[0.306,
 0.317,
 0.339,
 0.366,
 0.419,
 0.465,
 0.588,
 0.547,
 0.456,
 0.393,
 0.318,
 0.298]

print taud
[0.306,
 0.317,
 0.339,
 0.366,
 0.419,
 0.465,
 0.588,
 0.547,
 0.456,
 0.393,
 0.318,
 0.298]

clearskyrad.tau(fhandle) reads the taub and taud values from the *.stat file (around line 28):

- Displaying Monthly Design Conditions "Climate Design Data 2009 ASHRAE Handbook"
- Monthly Optical Sky Depth Beam (taub) and Diffuse (taud)
                                   Jan     Feb     Mar     Apr     May     Jun     Jul     Aug     Sep     Oct     Nov     Dec
                taub (beam)        0.306   0.317   0.339   0.366   0.419   0.465   0.588   0.547   0.456   0.393   0.318   0.298
             taud (diffuse)        2.534   2.463   2.351   2.229   2.044    1.91   1.653   1.763   1.978   2.116   2.487   2.592

                       taub        = Clear Sky Optical Depth for Beam Irradiance
                       taud        = Clear Sky Optical Depth for Diffuse Irradiance

To calculate the radiation, we need the altitude of the sun. Let us find the altitude of the sun at noon on the 21st of each month. We can do this by going to the web site https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html

alts = {1:35.97,
2:45.09,
3:55.99,
4:67.74,
5:75.74,
6:78.02,
7:74.83,
8:67.34,
9:56.7,
10:45.59,
11:36.46,
12:32.8}
# month:altitude
# calculated from https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html

Now we are ready to calculate the clear sky radiation. Starting with direct normal

from datetime import datetime

for month in range(1, 13):
    print clearskyrad.directnormal(taub[month-1], taud[month-1],
        alts[month], thedate=datetime(2018, month, 21))

the direct normal results are

912.281856828
936.707585623
937.22435687
920.279543442
869.489603714
824.956794153
723.86104248
748.144302441
808.247171807
837.874397967
893.090953721
904.04138393

And for diffuse horizontal

for month in range(1, 13):
    print clearskyrad.diffusehorizontal(taub[month-1], taud[month-1],
        alts[month], thedate=datetime(2018, month, 21))

The diffuse horizontal results are

88.3239665087
102.034946163
120.595369428
140.632493558
170.230386996
193.761516975
248.413566492
219.237360391
171.24339381
140.903362551
92.1795686764
80.6806617141

Close enough to the values in the *.stat file

- Monthly Solar Irradiance Wh/m² (noon on 21st of month)
                  ib (beam)          915     937     938     920     870     827     727     750     807     833     891     907
               id (diffuse)           89     102     121     141     170     194     250     220     171     140      92      81

                         ib        = Clear Sky Noon Beam Normal Irradiance on 21st Day
                         id        = Clear Sky Noon Diffuse Horizontal Irradiance on 21st Day

If you ever need to calculate the clears sky radiation, that is how you do it.

This package was created with Cookiecutter and the audreyr/cookiecutter-pypackage project template.