Solar power is the ultimate renewable energy source. Here is an introduction to the basic science that makes all solar energy technologies possible.

As fossil fuels become more expensive and difficult to find, many engineers have focused on using energy from the sun to provide heat, light and electricity. What is solar energy, and how does it work? Understanding this unlimited energy source and how engineers tap into solar's power is not as difficult as it may seem.

Solar Radiation Basics

Solar technologies depend on radiation from the sun. Radiation is energy traveling through space. Thanks to Einstein, scientists understand radiation energy in two different ways. Sometimes they see energy as waves, which come in different frequencies—like the frequencies on a radio dial. The higher the frequency, the more energy.

Other times, scientists think of radiation as tiny particles of energy, called photons. These photons are streaming through space at the speed of light—186,000 miles per second. The photons carry different amounts of energy, measured in electron volts.

The Electromagnetic Spectrum

Solar radiation actually contains two parts, an electrical energy field traveling straight ahead and a magnetic field traveling in a circle around the electrical field. That's why the energy from the sun is called electromagnetic radiation.

Because electromagnetic radiation comes in a range of frequencies, scientists usually talk about it as a “spectrum.” A small part of the electromagnetic spectrum can be seen by human eyes as visible light. Other electromagnetic radiation, such as infrared, microwave, or X-ray radiation, can’t been seen by humans, but its all part of the same spectrum.

Energy from the sun comes mainly in the visible, ultraviolet and near-infrared (heat) ranges of the electromagnetic spectrum.

Using Solar Energy

Some solar technologies use the sun’s energy to directly heat buildings. This "passive" solar technology uses no moving parts. To design a passive solar building, engineers consider the direction of the sun year round. Then they specify the building's orientation, window shades, window glazing, building materials, insulation and landscaping to capture the sun's energy in winter and deflect it in summer.

Solar water heaters use collectors mounted on a roof and angled toward the sun to heat tubes of water or gas. As the sun heats the gas or liquid, it circulates through a heat exchanger to warm up the water in a swimming pool or hot water heater.

Another way to use energy from the sun is to capture and concentrate solar radiation with mirrors that focus it onto a solar collector. The collected energy heats up water to create steam. The steam is converted to electricity with a generator, just like those in traditional gas, coal-fired or nuclear power plants.

Converting Light to Electricity

Engineers can also create electricity directly from solar radiation. To do this, they use a group of natural elements called semiconductors. One of these is silicon, the main ingredient in quartz and beach sand. Silicon and other semiconductors are solid crystals.

Semiconductors have a special property called the “photoelectric effect." When photons hit the surface of a semiconductor, they travel through it, crashing into the atoms inside. The photons' energy knocks electrons in the crystal away from their atoms.

Photovoltaic Cells

By attaching metal plates to each side of the semiconductor, and running a current through a wire connecting the plates, engineers create a positive electrode on one side and a negative electrode on the other. Because electrons have a negative charge, they move toward the positive terminal, out of the crystal and into the wire itself. This flow of electrons is actually electricity.

The first photovoltaic cells (PVs) were clusters of semiconductor crystals wired together. Later, engineers improved the amount of electricity PVs make by laying thin semiconductor films over a base. Now, some engineers are developing nanotech photovoltaics, which use layers of mixed materials to maximize the amount of light converted to electricity.

For more basic introductions to engineering technologies, read An Introduction to Fiber Optics, and Electronic Waste Recycling 101.

Sources

U.S. DOE Energy Efficiency and Renewable Energy (EERE) Solar Energy Technologies Program.

"How do Photovoltaics Work?" by Gil Knier, Science@Nasa.