Recommendations for calculation of solar radiation on buildings
1986 Edition

The standard provides design solar radiation figures for latitudes starting from 9°N at 4° increments. These values comprise both direct and diffuse radiation for horizontal and vertical surfaces facing eight cardinal directions. Summer data corresponds to peak sun positions such as the summer solstice (June 22), while winter data aligns with the winter solstice (December 22). These values reflect near-maximum clear sky radiation conditions, covering approximately 95% of the time. Ground reflected radiation is not included in vertical surface tables but can be added using site-specific reflectivity values. Refer to Tables 3 and 4 for precise solar radiation data categorized by latitude and orientation.

Direct solar radiation on a building surface is calculated using the formula I_D = I_N × cos θ, where I_N is the direct radiation at normal incidence from Table 1, and θ is the incidence angle between the sun's rays and surface. For vertical surfaces, cos θ equals cos δ × cos φ, with δ as solar altitude angle and φ as the horizontal angle between sun direction and surface. On sloping surfaces, the formula adjusts to I_D = I_N × (cos δ × cos φ × cos o + sin δ × sin o), where o is the angle of inclination from vertical. These calculations allow precise estimation of direct radiation depending on surface orientation and solar position.

Ground reflectivity (r_g) represents the proportion of solar radiation reflected off the ground surface, impacting the reflected radiation component on vertical or tilted surfaces. It is used to compute ground-reflected solar radiation as I_GRV = r_g × I_TH, where I_TH is the total solar radiation (direct plus diffuse) on a horizontal plane. Typical reflectivity values vary by surface type, such as 0.14 for bituminous/gravel surfaces, 0.20 for brown grass or crushed rock, and up to 0.32 for new concrete. Considering ground reflectivity is vital for accurately assessing solar loads on building facades and optimizing energy performance.

IS 11907 aids HVAC design by providing design solar radiation values representing near-maximum solar exposure. By identifying building location and orientation, professionals can use the standard's tables to find appropriate solar radiation values for surfaces. The incident solar heat gain is calculated using Q = A × I × SC, where A is surface area, I is solar radiation from the standard, and SC is the shading coefficient or glass transmittance. This process ensures HVAC loads are estimated based on realistic solar heat gains, facilitating proper equipment sizing and improved indoor comfort.

Yes, the standard incorporates several illustrative examples to assist users in applying its provisions. Appendix A demonstrates the determination of total design solar radiation, Appendix B provides examples for calculating design solar radiation on building surfaces, and Appendix C details computations specific to sloping surfaces. Additionally, guidelines on numerical rounding conforming to IS 2:1960 are included to maintain consistency. These examples and instructions help engineers and designers accurately perform solar radiation assessments following the standard.