Solar Energy

The annual average energy striking the Earth from the sun is relatively constant at 430 BTU/hr/ft2.  The amount reaching the surface varies considerably based on the time of the year and local weather conditions.  In broad terms approximately 30% of the incoming solar energy is reflected and 20% is absorbed by the atmosphere.  About 50% of the incoming sunlight is available for use at the surface.

Direct sunlight arrives at the earth’s surface on a direct path from the sun.  Diffuse sunlight arrives from all directions of the sky after interacting with the atmosphere.  Reflected sunlight arrives from adjacent surfaces.

The annual radiation reaching the surface depends on location and climate conditions.  Annual average values are well known for all locations in the United States.  Most of the continental United States has a very good solar resource.  To put things into perspective Germany, which currently produces about 57% of the world’s solar electric power, has an average solar resource similar to Seattle.

The available solar energy at a given location is well documented.  The data in the figure above is for Madison, Wisconsin.  Resource Solar uses this type of data in our computer models when we design systems.  While the day-to-day variations in weather are significant, annual averages are much more consistent.  We can therefore reliably determine average annual energy gains for a solar energy system.

The solar window is a term the solar industry uses to describe the availability of sunlight for a specific building or location.  It is a window in the sky within which the sun is visible from 9 a.m. to 3 p.m. every day of the year.  Any shading from objects such as trees and adjacent buildings that obstruct parts of the solar window must be accounted for in system design.

In addition to shading, the slope and azimuth of the solar collector influence system performance.  The collector slope, also referred to as the angle of inclination, is the angle from a horizontal surface up to the collector.  For maximum annual energy gain it is set to the latitude of the site.  Winter gain is optimized by a higher slope.  The collector azimuth is the angle of the collector east or west of due south.  While due south is best, systems can be designed to function well if the collectors are positioned within 45° of south.