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: BEFORE 1915 GMT10-04-2011 As of 04 Oct 2011, it was determined that the formula for the computation of the Extraterrestrial (ETR) radiation value in column D of the spreadsheet model contained an error. The very last parenthetical expression should be divided by 365. Note the MAXIMUM error in the computation of ETR due to this error is +/- 0.8%. Users may unprotect the spreadsheet with password BIRD and edit the function in cell D3 which reads as =1367*(1.00011+0.034221*COS(2*PI()*(B3-1)/365)+0.00128*SIN(2*PI()*(B3-1)/365)+0.000719*COS(2*(2*PI()*(B3-1)/365))+0.000077*SIN(2*(2*PI()*(B3-1))) ) With the following CORRECT function: =1367*(1.00011+0.034221*COS(2*PI()*(B3-1)/365)+0.00128*SIN(2*PI()*(B3-1)/365)+0.000719*COS(2*(2*PI()*(B3-1)/365))+0.000077*SIN(2*(2*PI()*(B3-1)/365 ))) Then copy the cell D3 and paste in cells D4 to D8761. After completing the expansion of the spreadsheet according to the notes in the Yellow Box for cells AA to AE, we urge the user to protect the sheet using password BIRD. We regret the error, and apologize for any inconvenience. Click HERE to see all error reports.

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