A MATHEMATICAL MODEL FOR ESTIMATING THE TOTAL SOLAR IRRADIANCE FOR LAFIA, NASARAWA STATE OF NIGERIA

An empirical mathematical model for estimating the total solar irradiance on a horizontal surface for Lafia. Nasarawa State of Nigeria (Lat S.S'N and long. S.5"E) was developed using the Angstrom - page equation. The model was developed from total solar irradiance. ambient air temperature, hours of bright sunshine and cloudiness measured hourly from 0600H to 180011 Jor 3 years for the months of January, 2009 to December, 2011. The measurements were carried out at the meteorological unit of College of Agriculture, Lafia, Nigeria. The coefficients a and ‘b of the Angstrom-page linear type equation were determined by plotting the clearness index (—) against the fractional possible sunshine hours to obtain the line of best fit. The intercept of the line on the y-axis is the coefficient 'a ’ while the slope of this line is the coefficient The determined model for measuring total horizontal solar irradiance at Lafia location was ~ = 0.28 + 0.33 (^); with a coefficient of correlation of 0.50. The mean bias difference and root mean square difference used to test the performance of 'a' and ‘b' were 0.6S7o and 1.13% respectively. The developed model is therefore recommended to be used for estimating the total solar irradiance for Lafia location.