Solar Energy

Solar energy is a renewable energy source in the form of radiant light from the sun. The term renewable solar energy refers to a broad range of technologies and techniques for capturing and utilizing this energy. In one hour, approximately 440 exajoules (EJ) of energy from the sun reach the Earth’s outer atmosphere. This is only slightly less than the estimated world population’s energy consumption from primary energy sources, such as biomass and fossil fuels, in an entire year. The Earth’s atmosphere, land surfaces, and oceans absorb a fraction of this incoming radiation, resulting in an increase in atmospheric, oceananic, and land mass heat, in addition to water evaporation.

These heat and evaporation processes in turn drive the Earth’s water cycle and produce atmospheric phenomena, including wind and weather patterns. Plants convert solar energy into chemical energy through the process of photosynthesis. Energy from the sun is critical to processes that sustain life on Earth. Directly capturing and utilizing even a small fraction of this vast energy source would offset fossil fuel consumption, which in turn would decrease the greenhouse gas (GHG) emissions that contribute to climate change.

Solar Energy Availability

Thermonuclear reactions in the sun result in the emission of radiation or light in all directions into space. The Earth intercepts a small fraction of this emitted radiation. At the top of the Earth’s atmosphere, the amount of radiated power from the sun remains fairly constant, at 1,361 watts per square meter (W/m2), varying only slightly throughout the year by plus or minus 3.5 percent because of the eccentricity of the Earth’s orbit around the sun. As this solar energy enters the Earth’s atmosphere, some wavelengths of this light are absorbed, scattered, or reflected back into space by different molecules, such as the gases and water vapor in the atmosphere. For example, a process known as Rayleigh scattering results in the blue appearance of the sky. On clear days, with little cloud cover, the radiated solar energy reaching a flat surface on the Earth can exceed 1,000 W/m2; however, on cloudy or overcast days, this power can be reduced to less than 100 W/m2. Cloud cover, aerosols, dust, smoke, and suspended water droplets all reduce the transmittance of the atmosphere and decrease the amount of solar radiation reaching a given location.

For terrestrial solar energy applications, the amount of energy that falls on a given surface is of primary concern. The orientation of a surface relative to the incoming sun’s rays must be considered when evaluating the solar energy available for a given application. The total solar radiation available to a surface at any orientation is a combination of two components: direct beam radiation and diffuse radiation. Because the incoming solar rays are nominally parallel, geometric relations can be used to estimate the amount of beam radiation available to a given surface. The surface’s available direct beam radiation is related to its latitude, the tilt of the Earth on its axis (which depends on the day of the year), the time of day, the slope of the surface, and the orientation of the slope in the east–west direction. Maximum beam radiation on a clear day occurs when the incoming solar radiation is perpendicular to the surface of interest. Diffuse radiation refers to radiation that is scattered and reflected from the atmosphere, clouds, and the ground to a particular surface.

The estimation of diffuse radiation availability depends upon the orientation of the surface, the scattering potential of the atmosphere, and the reflectance properties of the ground. On cloudy days, and when the surface is shaded from direct beam radiation, diffuse radiation is the primary source of solar energy availability.

Because the solar energy available at a specific location depends on many factors, historical data is often used to obtain more accurate estimates of energy availability during an average day. Solar energy availability can be estimated based on data collected from pyrheliometers and pyranometers. Pyrheliometers are instruments that track the sun and measure the available direct-beam radiation. Pyranometers obtain data on the total solar radiation or the diffuse solar radiation available to a fixed surface. Using historical data from such instruments located near a proposed installation site provides a more accurate measurement of the solar energy capture potential at that location. Such data can be modified using geometric relations and empirical observations to estimate the solar energy availability on a surface at any orientation.