Solar Electric Systems - Performance Building Consulting August 21, 2007 1 S OLAR E LECTRIC S YSTEM B ASICS By Wade … polycrystalline cells S OLAR E LECTRIC S YSTEM C OMPONENTS (1) Solar Panel … Solar cells produce an electrical current when exposed to sunlight. A typical solar cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus- doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in …

SOLAR ELECTRIC BACKGROUND The most common method of producing electricity using solar energy is to use photovoltaic cells, often called solar cells. Solar cells produce an electrical current when exposed to sunlight. A typical solar cell is composed of a thin wafer consisting of an ultra-thin layer of phosphorus- doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon. An electrical field is created near the top surface of the cell where these two materials are in contact, called the P-N junction. When sunlight strikes the surface of a solar cell, this electrical field provides momentum and direction to light-stimulated electrons, resulting in a flow of current when the solar cell is connected to an electrical load. Diagram of a photovoltaic cell. * SOLAR CELL TYPES There are three common types of solar cells in use today. Monocrystalline Monocrystalline solar cells are made from single large crystals, cut from ingots. These cells are the most efficient, but also the most expensive. They generally perform somewhat better in low light conditions. Polycrystalline Polycrystalline solar cells are basically cast blocks of silicon which may contain many small crystals. They are sometimes called multicrystalline and are probably the most common type in use now. They are slightly less efficient than single crystal cells, but are generally considered to perform better than monocrystalline in partial shade conditions. SOLAR ELECTRIC SYSTEM COMPONENTS (1) Solar Panel - Each individual solar cell produces about 0.5 - 0.6 volts of direct current (”DC”). Multiple solar cells are grouped together to form a solar panel. The voltage rating of a solar panel is determined by the number of individual solar cells wired together in series. For example, a panel composed of 32 solar cells would have a voltage of approximately 16 - 19 volts DC. The electric power (or wattage) of a solar panel depends both on the voltage (number of cells) and the size of each cell. The larger the cell size, the greater the current (measured in amperes) the cell can produce. Wattage of the panel is determined by multiplying the panel’s voltage by its amperage. (2) Solar Array — To produce enough usable power, multiple solar panels are wired together to form a solar array. For example, ten 120-watt solar panels would form a 1,200-watt (1.2 KW) array. (3) Inverter - Inverters convert the DC electrical current produced by a solar array into alternating current (”AC”) that is utilized on the electrical grid and in most homes and business in the U.S. today. Modern inverters also perform a variety of safety and power conditioning functions and are sometimes called power conditioning units (”PCU”). Inverters designed to interface with the electrical grid will automatically and instantly shut down the solar should the grid lose power. Such inverters also match the AC frequency produced by the solar electric system with the 60 cycle (”Hz”) frequency on the grid. (4) Battery Bank - Of course, solar electric systems only produce power during sunlight conditions. Some type of electrical energy storage is required if the energy is to be used at some later period, i.e. at night or periods of low or no sun. Batteries remain the most common type of energy storage currently used in solar electric systems. (5) Charge Controller - Charge controllers control the state of charge of the battery bank. Modern charge controllers also perform a variety of other functions designed to keep batteries at maximum state of charge, prevent batteries from being discharged too deeply, and prolonging battery life. Some modern inverters have a built-in charge controller. BASIC SOLAR ELECTRIC SYSTEM TYPES There are a wide variety of solar electric systems. In net metering applications, the two most common types are line tie (with direct grid connection) and line tie with battery backup.

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