How does solar power work?

A. Paul Alivisatos, deputy director of Lawrence Berkeley National Laboratory and a leader of the Helios solar energy research project there, shines some light on the matter:

When the sun strikes a solar panel, the energy of the sunlight liberates electrons in the solar cells, producing an electric current that we can harness to power pocket calculators, homes, even science stations on Mars.

In a traditional crystalline silicon cell, the atoms in the silicon crystal are bound by shared electrons. When light is absorbed, some of the electrons in those bonds are excited up to a higher energy level. Those electrons can then move around the crystal more freely than when they were bound, allowing them to flow as an electric current.

Imagine that you have a ledge—a shelf on the wall—and you take a ball and throw it up onto that ledge. That is akin to promoting an electron to a higher energy level. A photon, an indivisible packet of light energy, enters the silicon crystal and bumps the electron up onto the ledge (the higher energy level), where it stays until we come and collect the energy by using the electricity.

Researchers are constantly looking for new approaches, and refining existing ones, to boost the efficiency of this process. The power efficiency of a crystalline silicon cell is in the 22 to 23 percent range, meaning the cells convert that much of the light energy striking them into electricity. The cells that you might be able to afford to put on your rooftop are less efficient than that, somewhere between 15 and 18 percent. The best-performing solar cells, such as the ones that go on satellites, approach 50 percent efficiency.

This conversion rate is one important measure, but we in the solar community are also concerned about the cost of making the cells and the scale of their production. In my opinion, the silicon technology does not scale up to mass-market size ideally because its raw materials and manufacturing processes are expensive. If researchers could produce a technology that scaled better, even one less efficient in energy conversion than crystalline silicon, we might be able to make millions of acres of the stuff to generate a great deal of energy. Many companies and universities are experimenting with a variety of materials, such as plastics and nanoparticles, to achieve this goal.

Why does my voice sound so different when it is recorded and played back?

Timothy E. Hullar, an otologist and assistant professor at the Washington University School of Medicine in St. Louis, replies:

Sound can reach the inner ear by way of two separate paths, and those paths in turn affect what we perceive. Air-conducted sound is transmitted from the surrounding environment through the external auditory canal, eardrum and middle ear to the cochlea, the fluid-filled spiral in the inner ear. Bone-conducted sound reaches the cochlea directly through the tissues of the head.

When you speak, sound energy spreads in the air around you and reaches your cochlea through your external ear by air conduction. Sound also travels from your vocal cords and other structures directly to the cochlea, but the mechanical properties of your head enhance its deeper, lower-frequency vibrations. The voice you hear when you speak is the combination of sound carried along both paths. When you listen to a recording of yourself speaking, the bone-conducted pathway that you consider part of your “normal” voice is eliminated, and you hear only the air-conducted component in unfamiliar isolation. You can experience the reverse effect by putting in earplugs so you hear only bone-conducted vibrations.

Some people have abnormalities of the inner ear that enhance their sensitivity to this component so much that the sound of their own breathing becomes overwhelming, and they may even hear their eyeballs moving in their sockets.

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