Photovoltaic (PV) literally means "light-electricity" and is the process of converting sunlight into electricity. The term "photo" comes from the Greek word for light and "volt" from Alexandro Volta (1745-1827), a pioneer in the study of electricity.
When some semi-conductor materials are exposed to sunlight, they release small amounts of electricity giving off what is known as the "photovoltaic effect." Sunlight is composed of photons, or wavelike particles of energy, that contain various amounts of energy corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell, the energy of the photon is transferred to electrons in the cell's atoms. Electrons escape from their normal position in the atom to become part of the current in an electrical circuit. When this happens, the electron creates a "hole" which is filled by another electron resulting in electrons flowing from hole to hole. The electrical field in a PV cell provides voltage that drives the current through an external load such as a light bulb or a television set.
Photovoltaic modules are the basic building blocks of solar electric power systems. PV modules can be made from several different materials which vary in cost and conversion efficiency. The most cost effective modules are made with silicon solar cells. Modules are combined to create panels. Panels are combined to form arrays. Solar electric power systems may also have batteries, charge controllers, and inverters, which convert the direct current generated by PV system into an alternating current, the type of electricity sold by utilities and required to run most appliances and electronic devices.
Solar thermal power is very different from PV power. Thermal solar is simply heat from the sun that can also be used to heat water to offset loads typically provided by natural gas and electricity. Solar water heaters, also called solar domestic hot water systems, can be a cost-effective way to generate hot water for your home. They can be used in any climate and the fuel they use, sunshine, is free. Solar water heating systems include storage tanks and solar collectors. There are two types of solar water heating systems: active, which have circulating pumps and controls, and passive, which don't. Most solar water heaters require a well-insulated storage tank. Solar storage tanks have an additional outlet and inlet connected to and from the collector. In two-tank systems, the solar water heater preheats water before it enters the conventional water heater. In one-tank systems, the back-up heater is combined with the solar storage in one tank.
Flat-plate collector - Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Unglazed flat-plate collectors, typically used for solar pool heating, have a dark absorber plate, made of metal or polymer, without a cover or enclosure.
Integral collector-storage systems - Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box. Cold water first passes through the solar collector, which preheats the water. The water then continues on to the conventional backup water heater, providing a reliable source of hot water. They should be installed only in mild-freeze climates because the outdoor pipes could freeze in severe, cold weather.
Evacuated-tube solar collectors - They feature parallel rows of transparent glass tubes. Each tube contains a glass outer tube and metal absorber tube attached to a fin. The fin's coating absorbs solar energy but inhibits radiative heat loss. These collectors are used more frequently for U.S. commercial applications.
There are two types of active solar water heating systems, direct and indirect circulating systems. Direct circulation system pumps circulate household water through the collectors and into the home. They work well in climates where it rarely freezes. Indirect circulation system pumps circulate a non-freezing, heat-transfer fluid through the collectors and a heat exchanger. This heats the water that then flows into the home. They are popular in climates prone to freezing temperatures.
Passive solar water heating systems are typically less expensive than active systems, but they're usually not as efficient. However, passive systems can be more reliable and may last longer. There are two basic types of passive systems, integral and thermosyphon. Integral collector-storage passive systems work best in areas where temperatures rarely fall below freezing. They also work well in households with significant daytime and evening hot-water needs. Thermosyphon systems water flows through the system when warm water rises as cooler water sinks. The collector must be installed below the storage tank so that warm water will rise into the tank. These systems are reliable, but contractors must pay careful attention to the roof design because of the heavy storage tank. They are usually more expensive than integral collector-storage passive systems.
Solar water heating systems almost always require a backup system for cloudy days and times of increased demand. Conventional storage water heaters usually provide backup and may already be part of the solar system package. A backup system may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. Since an integral-collector storage system already stores hot water in addition to collecting solar heat, it may be packaged with a demand (tankless or instantaneous) water heater for backup.
Solar energy can also be used for solar pool heating which is typically a low temperature application of solar thermal using unglazed polypropylene collectors.
Contact Solar Electrical Systems to find out if a solar water heating system is right for you.