Using the sun for energy can be accomplished in many different ways. However, here we will explain how two major applications work. The first type of solar power extraction discussed here will be about generating electricity from the sun. The second method discussed will be solar heating, including solar water heating.
The process of capturing and using solar energy is as old as time. Like most things in the modern world, the simplicity of capturing the sun's energy has been elevated to new technological heights. Unlike solar heating collectors that are used to warm fluids running within the collector, photovoltaic cells magically convert light from the sun directly into electricity. The magic happens simply when light hits the surface of semiconductor material, electrons are released from the outer shell of atoms in the semiconductor material. These free electrons become available for electron flow, otherwise known as electric current. All that is needed to capture this current is some kind of loading device such as an home appliance, or a storage device such as a rechargeable battery.
The photovoltaic cell used in renewable-energy systems is definitely the product of rocket science, powering communications satellites to ensure everyone on the planet can watch reruns of Friends.
The term photovoltaic is derived from the Greek word "photo", meaning light, and "voltaic", voltage which assists the flow of electricity. Friends simply call them "PV" cells for short. Bell Laboratories discovered the PV cell effect in the 1950s. It didn't take the folks at NASA very long to figure out that PV cells would be an ideal means of producing electricity in space. Many missions later, PV cells have improved in performance and have come back down to earth in price. Nowadays, the technology is used in watches, calculators, street signs, and renewable-energy systems for the urban homeowner.
For the home renewable-energy system, PV products are relatively standardized, allowing even a novice to make accurate comparisons between product lines. There are currently four major product technologies that should be seriously considered for home use: mono crystalline, polycrystalline, laminate (roof shingles) and string ribbon cell. Other cell technologies such as "thin film" are an option provided product warranty and manufacturer financial strength to honor the warranty period are acceptable. Find out more about solar panels here.
These systems are usually very large, taking up many acres, in order to make them cost effective. This technology uses mirrors to concentrate the sun's light on to a small tube fill with oil. The oil is moved through the long tube, with cool oil going in and hot oil coming out. The light is converted to heat which drives a heat engine (usually a steam turbine) connected to an electrical power generator. The heat can also be stored in some heat retentive material such as molten salts for electric generation at night.
This kind of solar energy system can be implemented in several ways.
A solar parabolic trough concentrates solar light on to a long thin pipe with oil running through it. The hot oil can then be used to boil water for steam, which can drive a turbine to generate electricity.
Solar parabolic dishes concentrate to a single focal point, converting light into heat heat, which then can be stored or changed to steam.
Performs the same function as the previous two concentrators on a larger scale. Typically, terrestrial mounted mirrors, called heliostats, surround a very tall (560 feet or 200 meters) tower. The mirrors reflect sun light into the tower to concentrate heat. Each mirror is as large as a highway bill board. The largest systems use 7,000 of these large mirrors. Each mirror is controlled by its own computer, slowly moving to maximize sun exposure. Total total field of mirrors can be 1.7 miles (2.7 km) wide. The heat is stored in a large salt tank. The hot salts can be used like natural gas to boil water, create steam to turn the electricity creating turbines. The additional feature of this heat storage system is that it can be used even at night to produce electricity because of the hot salt storage tanks. The southern California solar project, called Solar 2, used this technique before the year 2000. The site was able to produce 10 Mega Watts of power. After 8 years of cheap natural (from 2000 to 2008) and no government push for renewable generation sources, much larger scale projects using the same heat storage mechanism are being built today. These larger projects are being funded by loan guarantees from the US Department of Energy. The largest project of this type to date is due to open in 2013. It is currently being built in Tonapah, Nevada. It has awakened an old mining town that in a coma, with economic activity.
Experiments have shown that there is limit to how large the heliostat field can be and have the furthest away mirrors contribute effective heating to the tower. Power generation typically peaks at 110 MW. Beyond that, if more power is needed, its best to build more tower fields. In addition, with the cost of solar panels dropping rapidly, thanks to the Chinese manufacturing, hybrid power generation systems can be used.
A hybrid system would have both a PV field and a heating tower field. Because the tower system can store heat until night, the PV field would generate power during the day and the tower system would produce power at night. This would produce much more power than either system alone and establish a baseline of power that would be available 24 hours a day.
Another type of solar concentrating tower is very tall. Thought of by an Australian hot-air balloon enthusiast, the tower uses the chimney effect to power wind turbines at the base its base. As the ground under a large round glass or poly roof heats up from sunlight (like a greenhouse), hot air is naturally drawn to the open tower in the center. As the hot air rises up through the tower, wind turbines at the base of the tower are activated. The wind turbines generate electricity that can be fed to the grid. This idea has the least amount of moving mechanical parts, increasing its reliability and reducing cost. Once it is built, it's just absolutely free energy. This is a very large scale structure that can generate many megawatts of power. Here is a video illustrating the power.
Another benefit of this design is that as temperatures change throughout the day and night, the just above ground clear canopy condensates, making the ground under it damp. Plants start to grow because they are receiving light and water, making the land that may have once been unusable desert land into to one that could be used for farming fruits and vegetables.
This type of technology uses a solar collector (or reflector) to focus sunlight on to a small solar PV cells positioned above the reflector. The advantage of this kind of solar generation is that much smaller solar cells can generate the same amount of electricity as large panels, thereby reducing the amount of semiconductor materials and processing needed.
The technology works by concentrating light from a large surface down to a very small one. Some techniques use a large Fresnel lens, which passes light through to the small solar cell. Other techniques use a reflective bowl to send the concentrated light to a suspended solar cell. This type resembles a satellite dish.
Concentrating PV has the great potential of competing with coal fired powered stations by producing electricity at a cost less than 20 cents per watt. The high efficiency solar cells are available. The challenge is to produce the collecting dish or lens to a reasonable price using common materials that can be scaled to high volume production. Concentrating solar PV technology has the dual benefit of providing both direct electricity generation and secondary heat energy. Since the concentrated light also heats up the solar cell, the can be carried away by some liquid circulated under the solar cell. The hot liquid, usually water can be used in the same applications as other thermal solar rigs.
Currently most concentrating PV companies are developing products for utility companies rather than individual home owners. In order to maintain high efficiency levels, concentrating solar modules must be positioned for maximum sunlight. To do this complex software driven feedback systems must be employed.
Passive solar heating has two main applications: to heat water and to heat buildings.
The most popular application for solar water heating in warm climates is for swimming pools. A black array of tubes is usually placed on the roof of a home. There is an inflow spigot and an outflow spigot. Cold water comes into the bottom of the array from the pool. Hot water flows out of the array back to the swimming pool. A simple water pump keeps the liquid flowing.
Solar water heating can also be used to heat your house water, either in conjunction with your conventional gas powered water heater, or totally. Several types of solar home water heaters are available. See more detail on home solar water heaters here.
There are two main ways to heat a building with the sun's energy. One is to design the structure to capture as much winter sun as possible, given the location's latitude. This structure should also block most of the summer sun from penetrating the interior of the building to reduce summer cooling needs. The home usually has an equator facing solarium or greenhouse that captures sunlight and heats the rest of the through thermal mass. If the structure is built properly, no furnace or other carbon based heater is needed. The thermal mass of the building retains heat during the day, to be dispersed in the living quarters at night. This type of structure can be built in the coldest of climates.
The second strategy is to build a solar collector box that is mounted on the exterior of the building or roof. The insulated box is ducted into the building, delivering warm air pushed by a low speed fan. Like the solar hot water heater, the box has two ports, one to pull cool air from a low point in the building, another to pump warm air high in the building. For this externally mounted heater to be most effective, the building and the box heater must be well insulated. Any thermal leakage from the building greatly diminishes the effects of the heater. An excellent book on passive solar heaters was written by residents of a cold Colorado town, San Luis Valley. The Complete Handbook of Solar Air Heating Systems by Steve Korner with Andy Zaugg is an excellent to building one of the kind of heaters.
There are many homemade projects of this type of heater using all kinds of materials, including recycled. Here's a passive solar heater made from soda cans.
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