Bird Clear Sky Model
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The Bird Clear Sky Model, authored by Richard Bird, is a broadband algorithm which produces estimates of clear sky direct beam, hemispherical diffuse, and total hemispherical solar radiation on a horizontal surface. The Excel^@ Spreadsheet implementation was done by Daryl Myers. The model is based on comparisons with results from rigourous radiative transfer codes. It is composed of simple algebraic expressions with 10 user provided inputs. Model results should be expected to agree within ± 10% with rigourous radiative transfer codes. The model computes hourly average solar radiation for every hour of the year, based on the 10 user input parameters; however variable atmospheric parameters such as Aerosol Optical Depth, Ozone, Water vapor are fixed for the entire year.

REFERENCES

• Bird, R. E., and R. L. Hulstrom, Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces, Technical Report No. SERI/TR-642-761, Golden, CO: Solar Energy Research Institute, 1981

ERROR NOTE: If you have downloaded the spreadsheet model before 1100 GMT 2004-10-10 there is an error in the formula for Uniform Mixed Gas Transmission (Tgases) in COLUMN M. The INCORRECT formula in cell M3 reads                 = IF(J3>0,EXP(-0.0127*J3*$A$15/1013),0) The CORRECT formula is                 = IF(J3>0,EXP(-0.0127*(J3*$A$15/1013)^0.26),0). Since Tgases is approximately 1, the effect of the error is small, ( a few watts per meter square), but we urge users to correct their workbooks. Current users should unprotect their workbook(s) [Tools>Protection>Unprotect, password BIRD], correct the formula in cell M3, copy the formula to the rest of the column, and then re-invoke protection with the password BIRD. [Tools>Protection>Protect, enter BIRD twice]. An error in the extraterrestrial irradiance and radius vector correction algorithm results in incorrect values for the ETR in column D of the previous worksheet (downloaeded before 2300 GMT 2004-10-29.) To correct the problem in a workbok already downloaded, unprotect the workbook and type in column D row 3:                 = 1367*(1.00011+0.034221*COS(6.28318*(B3-1)/365)+0.00128*SIN(6.28318*(B3-1)/365)                                   +0.000719*COS(2*(6.28318*(B3-1)/365))+0.000077*SIN(2*(6.28318*(B3-1)/365))) then copy and paste the expression into the rest of column D. The download version of the spreadsheet has been corrected. We apologize for these errors. Contact the author of the workbook, Daryl Myers, at daryl_myers@nrel.gov with questions or comments.

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