Calculates insolation on tilted surfaces, computes atmospheric transmittance and related parameters such as: Earth radius vector, declination, sunset and sunrise, daylength, equation of time, vector in the direction of the sun, vector normal to surface, and some atmospheric physics.

A new corrected version 1.1.1 is available above and in the CRAN repository.

Important!

When using a raster DEM, grid cells must be squared, with equal X and Y resolution in metres.
If they have equal resolution in degrees they are not squared: the X value will change as a function of latitude.
In this case the raster needs to be reprojected.

References

Bird, R. E. and Hulstrom, R. L. (1981a) Review, evaluation and improvements of direct irradiance models, Trans. ASME J. Solar Energy Eng. 103, 182-192.

Bird, R. E. and Hulstrom, R. L. (1981b) A simplified clear sky model for direct and diffuse insolation on horizontal surfaces,
Technical Report SERI/TR-642-761, Solar Research Institute, Golden, Colorado. pdf

Bourges, B.: 1985, Improvement in solar declination computation, Solar Energy 35(4), 367-369.

Corripio, J. G.: 2003, Vectorial algebra algorithms for calculating terrain parameters from DEMs and the position of the sun
for solar radiation modelling in mountainous terrain, International Journal of Geographical Information Science 17(1), 1-23.
pdf

Danby, J. M. Eqn. 6.16.4 in Fundamentals of Celestial Mechanics, 2nd ed. Richmond, VA: Willmann-Bell, p. 207, 1988.

Iqbal, M. (1983) An Introduction to Solar Radiation, Academic Press, Toronto.

Meeus, J. 1999. Astronomical Algorithms. Willmann-Bell, Richmond, Virginia, USA.

Reda, I. and Andreas, A. 2003. Solar Position Algorithm for Solar Radiation Applications. 55 pp.; NREL Report No. TP-560-34302, Revised January 2008.
http://www.nrel.gov/docs/fy08osti/34302.pdf