Solar Energy Basics
Understanding light's interaction with solar cells is the first step to begin learning learning about photovoltaic technology. The phenomenon operating at the heart of a solar cell is a study in itself, but let's take a quick look at sunlight - the energy source actually used by solar cells - and how its energy reaches earth. A brief discussion of several terms will help us better understand aspects of light's interaction with solar cells.
Air Mass
The sun is continually releasing an enormous amount of radiant energy into the solar system. The Earth receives a tiny fraction of this energy; yet, an average of 1367 watts (W) reaches each square meter (m2) of the outer edge of the Earth's atmosphere. The atmosphere absorbs and reflects some of this radiation, including most X-rays and ultraviolet rays. Still, the amount of the sun's energy that reaches the surface of the Earth every hour is greater than the total amount of energy that the world's human population uses in a year.
How much energy does light lose in traveling from the edge of the atmosphere to the surface of the Earth? This energy loss depends on the thickness of the atmosphere that the sun's energy must pass through. The radiation that reaches sea level at high noon in a clear sky is 1000 W/m2 and is described as "air mass 1" (or AM1) radiation. As the sun moves lower in the sky, the light passes through a greater thickness (or longer path) of air, losing more energy. Because the sun is overhead for only a short time, the air mass is normally greater than one—that is, the available energy is less than 1000 W/m2.
Light Energy
The energy from the sun is vital to life on Earth. It determines the Earth's surface temperature and supplies virtually all the energy that drives natural global systems and cycles. Some other stars are enormous sources of energy in the form of X-rays and radio signals, but our sun releases the majority of its energy as visible light. However, visible light represents only a fraction of the total spectrum of radiation. Specifically, infrared and ultraviolet rays are also significant parts of the solar spectrum.
The different colors of light can be arranged according to wavelength, frequency, or energy. From lowest energy to highest, the colors are red, orange, yellow, green, blue, violet, ultraviolet. That is, red has the least energy and ultraviolet has the highest energy. (A well known acronym to remember this order is ROY G BIV: red, orange, yellow, green, blue, indigo, violet.)
Sunlight contains a lot of infrared and ultraviolet as well as all other colors. Some people think heat is a different form of energy from light. Both are forms of electromagnetic radiation and both can provide energy for homes. Since most infrared and ultraviolet are blocked by Earth’s atmosphere, solar energy, whether it is used to heat water or is converted to electricity in a PV cell, comes from the visible part of sunlight.
Direct and Diffuse Light
Due to air mass, the Earth's atmosphere and cloud cover absorb, reflect, and scatter some of the solar radiation entering the atmosphere. Nonetheless, an enormous amount of the sun's energy reaches the Earth's surface and can therefore be used to produce PV electricity. Some of this radiation is direct and some is diffuse, and the distinction is important because some PV systems (flat-plate systems) can use both forms of light, but concentrator systems can only use direct light.
Flat-plate collectors, which typically contain a large number of solar cells mounted on a rigid, flat surface, can make use of both direct sunlight and the diffuse sunlight reflected from clouds, the ground, and nearby objects.
Direct light consists of radiation that comes straight from the sun, without reflecting off of clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation, referring to the portion of sunlight that comes directly from the sun and strikes the plane of a PV module at a 90-degree angle.
Diffuse light is sunlight that is reflected off of clouds, the ground, or other objects. It obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot be focused by the optics of a concentrator PV system.
Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight is composed of direct-normal and diffuse components of sunlight. Additionally, diffuse and direct-normal sunlight generally have different energy spectra or distributions of color.
Insolation
The actual amount of sunlight falling on a specific geographical location is known as insolation or "incident solar radiation." Insolation values for a specific site are sometimes difficult to obtain. Weather stations that measure solar radiation components are located far apart and may not carry specific insolation data for a given site. Furthermore, the information most generally available is the average daily total—or global—radiation on a horizontal surface.
When sunlight reaches the Earth, it is distributed unevenly in different regions. Not surprisingly, the areas near the Equator receive more solar radiation than anywhere else on the Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. For example, the amount of solar energy falling per square meter on Yuma, Arizona, in June is typically about nine times greater than that falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun's rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount of solar energy that is available to PV systems.
To learn more about solar and other resource data, visit the following Web sites:
Renewable Resource Data Center (RReDC) - The RReDC provides information on several types of renewable energy resources in the United States, in the form of publications, drata, and maps.
NASA's Surface Meteorology and Solar Energy Data - This is a renewable energy resource web site sponsored by NASA's Earth Science Enterprise Program that contains over 200 satellite-derived meteorological and solar energy parameters, monthly averaged from 10 years of data, and data tables for a particular location.
