A Solar Buyer's Guide for the Home and Office by Stephen Hren

Author:Stephen Hren [Hren, Stephen]
Language: eng
Format: epub
Publisher: Chelsea Green Publishing
Published: 0101-01-01T00:00:00+00:00

Figure 5.1. Solar electricity begins with the photovoltaic (PV) cell, which creates current and voltage when exposed to photons of light. PV modules are made up of many cells wired together. PHOTO BY ROGILBERT, WIKIPEDIA.

Figure 5.2. Anatomy of a PV cell. COURTESY OF HOME POWER MAGAZINE, WWW.HOMEPOWER.COM.

There is a static region in between the two layers known as the P/N junction that acts like a one-way valve (a diode) so electrons can only flow in one direction through the circuit. Lines of a highly conductive metal such as silver (often referred to as traces) are laid onto the individual cells and gather the electrons into the circuit. The potential difference in voltage—the pull of the electrons toward the positive layer—keeps the flow going, as long as sunlight is available to provide the energy to continue to knock new electrons loose. It is the energy from these freed electrons (not the electrons themselves) in the form of an electromagnetic field (EMF) that then flows through the circuits of the PV array and out into the wires as electricity.

How They’re Manufactured

Most PV cells fall within two groups: crystalline silicon or thin-film. Crystalline modules are the classic variety, still make up over 80 percent of the market as of 2009, and are the most efficient (up to 20 percent). To make crystalline modules, you start with individual cells. There are two basic types of crystalline cells—monocrystalline and polycrystalline—which have slightly different manufacturing techniques and thus different electrical characteristics. For monocrystalline cells, a seed crystal is pulled through highly refined silicon melt doped with boron at around 2,500°F, and a boule (cylindrical crystal) about five to six inches in diameter is drawn out; this is the positive layer. After being cut into razor-thin circular wafers, it is coated with phosphorus and then heated to allow the phosphorus to seep into the silicon and adhere; this is the negative layer.

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