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A single PV cell is a thin semiconductor sandwich, with a layer of highly purified silicon. The silicon has been slightly doped with boron on one side and phosphorus on the other side. Doping produces either a surplus or a deficit of electrons, depending on which side we’re looking at. Electronics-savvy folks will recognize these as P- and N-layers, the same as transistors use. When our sandwich is bombarded by sunlight, photons knock off some of the excess electrons. This creates a voltage difference between the two sides of the wafer, as the excess electrons try to migrate to the deficit side. In silicon, this voltage difference is just under half a volt. Metallic contacts are made to both sides of the wafer. If an external circuit is attached to the contacts, the electrons find it easier to take the long way around through our metallic conductors than to struggle through the thin silicon layer. We have a complete circuit and a current flows.
The PV cell acts like an electron pump. There is no storage capacity in a PV cell; it’s simply an electron pump. Each cell makes just under half a volt regardless of size. The amount of current is determined by the number of elec- trons that the solar photons knock off. We can get more electrons by using bigger cells, or by using more efficient cells, or by exposing our cells to more intense sunlight. There are practical limits, however, to size, efficiency, and how much sunlight a cell can tolerate.
Since 0.5-volt solar panels won’t often do us much good, we usually assemble a number of PV cells for higher voltage output. A PV “module” consists of many cells wired in series to produce a higher voltage. Modules consisting of about 36 cells in series have become the industry standard for large power production. This makes a module that delivers power at 17 to 18 volts, a handy level for 12-volt battery charging. In recent years, as PV modules and systems have grown larger, 24-volt modules consisting of 72 cells have also become standardized.
The module is encapsulated with tempered glass (or some other transparent material) on the front surface and with a protective and waterproof material on the back surface. The edges are sealed for weatherproofing, and there is often an aluminum frame holding everything together in a mountable unit. A junction box, or wire leads, providing electrical connections is usually found on the module’s back. Truly weatherproof encapsulation was a problem with the early modules assembled 20 years ago. We have not seen any encapsulation problems with glass-faced modules in many years.
Many applications need more than a single PV module, so we build an “array.” A PV array consists of a number of individual PV modules that have been wired together in series and/or parallel to deliver the voltage and amperage a particular system requires. An array can be as small as a single pair of modules, or large enough to cover acres.
PV costs are down to a level that makes them the clear choice for remote and grid-intertie power applications. They are routinely used for roadside emergency phones and most temporary construction signs, where the cost and trouble of bringing in utility power outweighs the higher initial expense of PV, and where mobile generator sets present more fueling and maintenance trouble. It’s hard to find new gate opener hardware that isn’t solar powered. Solar with battery backup has proven to be a far more reliable power source, and it’s usually easier to obtain at the gate site. More than 150,000 homes in the United States, largely in rural sites, depend on PVs as a primary power source, and this figure is growing rapidly as people begin to understand how clean, reliable, and maintenance-free this power source is, and how deeply our current energy practices are borrowing from our children. Worldwide, there are currently over 1 million homes that derive their primary power from PV. Because they don’t rely on miles of exposed wires, residential PV systems are more reliable than utilities, particularly when the weather gets nasty. PV modules have no moving parts; degrade very, very slowly; and boast a lifespan that isn’t fully known yet but will be measured in multiple decades. Standard factory PV warranties are 25 years. Compare this with any other power generation technology or consumer goods. Could you find a car or truck or computer with a 25-year warranty? If you could, you’d probably buy it!