Solar energy technologies use the sun’s energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. There are a variety of technologies that have been developed to take advantage of solar energy. These include: PhotovoltaicÂ (solar cell) Systems Solar cells convert sunlight directly intoÂ electricity. Solar cells are often
Solar energy technologies use the sun’s energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. There are a variety of technologies that have been developed to take advantage of solar energy.
PhotovoltaicÂ (solar cell) Systems
Solar cells convert sunlight directly intoÂ electricity. Solar cells are often used to power calculators and watches. They are made of semiconducting materials similar to those used in computer chips.Â When sunlight is absorbed by these materials, the solar energy knocks electronsÂ loose from their atoms, allowing the electrons to flow through the material toÂ produce electricity. This process of converting light (photons) to electricityÂ (voltage) is called the photovoltaic (PV) effect.
Solar cells are typically combined intoÂ modules that hold about 40 cells; a number of these modules are mounted in PVÂ arrays that can measure up to several meters on a side. These flat-plate PVÂ arrays can be mounted at a fixed angle facing south, or they can be mounted onÂ a tracking device that follows the sun, allowing them to capture the mostÂ sunlight over the course of a day. Several connected PV arrays can provideÂ enough power for a household; for large electric utility or industrialÂ applications, hundreds of arrays can be interconnected to form a single, largeÂ PV system.
Thin film solar cells use layers ofÂ semiconductor materials only a few micrometers thick. Thin film technology has
made it possible for solar cells to now double as rooftop shingles, roof tiles,Â building facades, or the glazing for skylights or atria. The solar cell versionÂ of items such as shingles offer the same protection and durability as ordinary
Some solar cells are designed to operate withÂ concentrated sunlight. These cells are built into concentrating collectors thatÂ use a lens to focus the sunlight onto the cells. This approach has bothÂ advantages and disadvantages compared with flat-plate PV arrays. The main ideaÂ is to use very little of the expensive semiconducting PV material whileÂ collecting as much sunlight as possible. But because the lenses must be pointedÂ at the sun, the use of concentrating collectors is limited to the sunniestÂ parts of the country. Some concentrating collectors are designed to be mountedÂ on simple tracking devices, but most require sophisticated tracking devices,Â which further limit their use to electric utilities, industries, and largeÂ buildings.
SolarÂ Hot Water
The shallow water of a lake is usually warmerÂ than the deep water. That’s because the sunlight can heat the lake bottom inÂ the shallow areas, which in turn, heats the water. It’s nature’s way of solarÂ water heating. The sun can be used in basically the same way to heat water usedÂ in buildings and swimming pools.
Most solar water heating systems forÂ buildings have two main parts: a solar collector and a storage tank. The most
common collector is called a flat-plateÂ collector. Mounted on the roof, it consists of a thin, flat, rectangularÂ box with a transparent cover that faces the sun. Small tubes run through theÂ box and carry the fluid â€“ either water or other fluid, such as an antifreezeÂ solution â€“ to be heated. The tubes are attached to an absorber plate, which isÂ painted black to absorb the heat. As heat builds up in the collector, it heatsÂ the fluid passing through the tubes.
The storage tank then holds the hot liquid.Â It can be just a modified water heater, but it is usually larger and very well-insulated.Â Systems that use fluids other than water usually heat the water by passing itÂ through a coil of tubing in the tank, which is full of hot fluid.
Solar water heating systems can be eitherÂ active or passive, but the most common are active systems. Active systems relyÂ on pumps to move the liquid between the collector and the storage tank, whileÂ passive systems rely on gravity and the tendency for water to naturallyÂ circulate as it is heated.
Many power plants today use fossil fuels as aÂ heat source to boil water. The steam from the boiling water rotates a largeÂ turbine, which activates a generator that produces electricity. However, a newÂ generation of power plants, with concentrating solar power systems, uses theÂ sun as a heat source. There are three main types of concentrating solar powerÂ systems: parabolic-trough, dish/engine, and power tower.
Parabolic-trough systems concentrate theÂ sun’s energy through long rectangular, curved (U-shaped) mirrors. The mirrorsÂ are tilted toward the sun, focusing sunlight on a pipe that runs down theÂ center of the trough. This heats the oil flowing through the pipe. The hot oilÂ then is used to boil water in a conventional steam generator to produce
A dish/engine system uses a mirrored dishÂ (similar to a very large satellite dish). The dish-shaped surface collects andÂ concentrates the sun’s heat onto a receiver, which absorbs the heat andÂ transfers it to fluid within the engine. The heat causes the fluid to expandÂ against a piston or turbine to produce mechanical power. The mechanical power
is then used to run a generator or alternator to produce electricity.
A power tower system uses a large field ofÂ mirrors to concentrate sunlight onto the top of a tower, where a receiver sits.Â This heats molten salt flowing through the receiver. Then, the salt’s heat isÂ used to generate electricity through a conventional steam generator. MoltenÂ salt retains heat efficiently, so it can be stored for days before beingÂ converted into electricity. That means electricity can be produced on cloudyÂ days or even several hours after sunset.
PassiveÂ Solar Heating and Daylighting
Step outside on a hot and sunny summer day,Â and you’ll feel the power of solar heat and light. Today, many buildings areÂ designed to take advantage of this natural resource through the use of passiveÂ solar heating and daylighting.
The south side of a building always receivesÂ the most sunlight. Therefore, buildings designed for passive solar heatingÂ usually have large, south-facing windows. Materials that absorb and store the sun’s heat can be built into the sunlit floors and walls. The floors and wallsÂ will then heat up during the day and slowly release heat at night, when theÂ heat is needed most. This passive solar design feature is called direct gain.
Other passive solar heating design featuresÂ include sunspaces and trombe walls. A sunspace (which isÂ much like a greenhouse) is built on the south side of a building. As sunlightÂ passes through glass or other glazing, it warms the sunspace. ProperÂ ventilation allows the heat to circulate into the building. On the other hand,Â a trombe wall is a very thick, south-facing wall, which is painted black andÂ made of a material that absorbs a lot of heat. A pane of glass or plasticÂ glazing, installed a few inches in front of the wall, helps hold in the heat.Â The wall heats up slowly during the day. Then as it cools gradually during theÂ night, it gives off its heat inside the building.
SolarÂ Process Space Heating and Cooling
Commercial and industrial buildings may useÂ the same solar technologies – photovoltaic, passive heating, day lighting, andÂ water heating – that are used for residential buildings. These nonresidentialÂ buildings can also use solar energy technologies that would be impractical forÂ a home. These technologies include ventilation air preheating, solar processÂ heating, and solar cooling.
Many large buildings need ventilated air toÂ maintain indoor air quality. In cold climates, heating this air can use largeÂ amounts of energy. A solar ventilation system can preheat the air, saving bothÂ energy and money. This type of system typically uses a transpired collector, which consists of a thin, black metalÂ panel mounted on a south-facing wall to absorb the sun’s heat. Air passesÂ through the many small holes in the panel. A space behind the perforated wallÂ allows the air streams from the holes to mix together. The heated air is thenÂ sucked out from the top of the space into the ventilation system.
Solar process heating systems are designed toÂ provide large quantities of hot water or space heating for nonresidentialÂ buildings. A typical system includes solar collectors that work along with aÂ pump, a heat exchanger, and/or one or more large storage tanks. The two mainÂ types of solar collectors used – an evacuated-tubeÂ collector and a parabolic-troughÂ collector – can operate at high temperatures with high efficiency. AnÂ evacuated-tube collector is a shallow box full of many glass, double-walledÂ tubes and reflectors to heat the fluid inside the tubes. A vacuum between theÂ two walls insulates the inner tube, holding in the heat. Parabolic troughs areÂ long, rectangular, curved (U-shaped) mirrors tilted to focus sunlight on aÂ tube, which runs down the center of the trough. This heats the fluid within theÂ tube.