Published on EGEE 401: Energy in a Changing World (https://www.e-education.psu.edu/egee401)

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Radiant to Electrical

Photovoltaic

a large solar array (solar panels on the roof of a building)
Figure 2.7: Solar Array.
Credit: National Renewable Energy Laboratory Photographic Information exchange [1]

In a photovoltaic process, radiant sunlight is converted to electricity.

In the 1800s, it was discovered that certain materials have the natural property of generating electricity when exposed to sunlight. But the amount of electricity was tiny. Over the years, vast improvements in materials and technology have led to the development of standard products that use this photovoltaic (PV) property to generate significant amount of electricity from sunlight.

The photovoltaic material is typically silicon, an abundant natural resource, mixed with other natural materials to enhance its electricity-generating properties. The energy from the sun invigorates the photovoltaic material at the molecular level, causing electrons to break away, creating an electrical current. To conduct the current in a controlled and orderly manner, manufacturers package thin slices of photovoltaic material between layers of carefully designed conductors to form what is called a solar cell. These cells are usually wired together in modules (or panels) that are assembled in groups called arrays. This array is the wall of shiny panels you may see on a roof or in a yard. (In the photograph above, you can see that module consists of 36 cells. This large array contains many, many modules!)

When sunlight hits an array, electricity is generated. Sunlight is the fuel (the source of energy)—the more sunlight, the more electricity. The PV array is the power plant—the larger the array, the more electricity you can generate.

Reading Assignment

Open and review the data sheet for a 240-Watt solar module [2]. Typically, many modules are installed together to form a solar array. [3]

Solar Energy

A word about the words "solar energy."

Solar energy is a broad term—it involves many technologies and applications. Photovoltaic (PV) systems generate electricity from the sun, using the photovoltaic process.

Solar energy can also be used to heat water (or other fluids) for space heating or domestic use. This is an entirely different application of solar energy and is usually called solar hot water or solar thermal. In photovoltaic applications, the photovoltaic material absorbs energy of certain wavelengths, energizing and realizing electrons. (This process actually works better at cooler temperatures.) In thermal applications, the material or fluid is simply warmed from absorbing the sun's energy.

When buildings are designed to work with the sun, absorbing heat from the sun when it is needed and rejecting it when it is not needed without the use of mechanical equipment (fans, collectors, pumps, controls), it is called passive solar. For heating, passive solar typically involves a "thermal mass" (say a stone floor) that absorbs the sun's heat which is then radiated off when ambient temperatures drop. For cooling, this means design features such as overhangs that block summer light and windows that open. In fact, deciduous trees are great passive solar devices—in the summer, they provide shade, in the winter, they allow the sunshine to come through! I have passive solar in my greenhouse, metal drums painted black and filled with water that absorb heat during the day and radiate it back overnight.

There are other ways of using solar energy (including "concentrating solar"), but the ones discussed here are the major ones in use now. The important point is, when discussing "solar energy," remember it has many possible applications.


Source URL: https://www.e-education.psu.edu/egee401/content/p2_p7.html

Links
[1] http://www.nrel.gov/data/pix/searchpix.php
[2] http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CDsQFjAA&url=http%3A%2F%2Ffiles.sharpusa.com%2FDownloads%2FSolar%2FProducts%2Fsol_dow_NUU240F2.pdf&ei=o-IOT7bSBon10gGRuNmRAw&usg=AFQjCNGi_VOn5x-U8sc9FZ-UkXpHUi3Twg&sig2=heHo4x7CCd9uzZI1l_jQlw
[3] http://www.ashrae.org/education/page/1455#1