Solar thermal systems do not rely on advanced material properties, crystals, or specific photon energies as do solar PV systems. They simply take advantage of the heat from sunlight to warm something. Low and medium temperature systems are used to directly heat swimming pools or hot water for human use. High temperature systems usually concentrate sunlight to create steam for electric power production — or to power a heat engine.
Direct Water Heating
The sun has long been used to provide hot water for personal use and to warm swimming pools. This technology is considered one of the most cost-effective solar options. More recently, solar water heating has been configured in scaled-up versions for commercial purposes and for "district heating" to provide hot water for multiple families or neighborhoods.
日本一本道a不卡免费Solar water heaters have evolved in many variants through the years, but the operating principle is generally the same. Water is directed through panels that may be mounted on a rooftop or placed in another sunny location. After the sun heats the water, it is directed to an insulated storage tank for later use. Portable versions of solar water heaters use the sun's heat to move the water from panel to tank.
The typical solar water heater today is a flat plate collector that consists of a series of tightly coiled black metal pipes mounted above a matte black backing and covered with a glass top. The black pipes and backing provide high thermal conductivity and good heat transfer to the water. Solar water heaters are commonly paired with fueled water heaters to provide backup when the solar heat isn't sufficient or available.
日本一本道a不卡免费A variation on the common model, called an evacuated tube solar collector, uses a series of vacuum-sealed tubes that enclose thin water pipes. This system produces a significant efficiency increase for cloudy environments and moderate or hot climates, but comes with a higher cost and requires more frequent maintenance.
Concentrated Solar Power
When solar thermal energy is used to generate electricity, a second technology is needed to convert the heat into a useful energy that powers a generator. Most of these systems work best at a utility scale because they rely on traditional steam turbine and generator technology with heat exchangers. Only dish systems currently have the potential to work at much smaller scales. The main solar thermal technologies for electricity generation include dish, parabolic trough, and power tower.
日本一本道a不卡免费Dish systems use a reflective device —a parabolic dish of mirrors — to focus light onto a power conversion unit — usually a Stirling engine — that is mounted at the focal point. The focused sunlight raises the temperature of the engine's heating chamber, which then drives pistons that power a generator to produce electricity. To be most efficient, solar dish systems require dual axis tracking to maintain optimum alignment with the sun.
Due to their relatively small size and self-contained architecture, dish systems can be used for distributed generation, though current versions would provide excessive electricity for a typical home. A good example of a utility-scale dish system is located at the Maricopa Solar Project adjacent to the Agua Fria Generating Station in Peoria, Ariz.
Parabolic trough systems — also known as linear concentrator systems — use rectangular mirrors in a parabolic shape that are placed in a long linear configuration facing the sun. The mirrors focus sunlight on a tube filled with a thermal transfer fluid — usually an oil — that flows throughout the array. The focused sunlight heats the fluid, which is then used to vaporize water to power a conventional steam turbine driven generator. Tracking devices keep the mirrors focused on the sun throughout the day.
Because trough systems are fluid-based, they can provide some energy storage to produce electricity after sunset — and advantage over solar PV systems. Unlike PV systems, however, trough systems, can require considerable amounts of water to cool the thermal side of the plant, which can be a problem in desert environments like Arizona. A good example of a trough system is the Saguaro Concentrating Solar Power plant near Tucson, Ariz.
Power tower systems, also known as "solar central receivers," consist of a tall tower surrounded by a large array of sun-tracking mirrors, called heliostats. The heliostats reflect sunlight onto a receiver on top of the tower that contains a thermal transfer fluid. As with trough systems, the sunlight heats the fluid, which then boils water that drives a conventional steam turbine driven generator. Also similar to trough systems, power towers can store heat for producing electricity at night and they typically have high water use unless air cooling is used. Good examples of power towers are the Solar One and Solar Two projects designed by the U.S. Department of Energy in the 1980s and 1990s. These 10 MW generators were successfully demonstrated in the Mojave Desert near Barstow, Calif., and have since been decommissioned.
Solar power technology is a relatively new field, and rapid progress is being made to increase the efficiency of each conversion type. Several different photovoltaic (PV) technologies and substances currently exist and are described below, organized by their different "generations" of PV technology.
First Generation — Crystalline
日本一本道a不卡免费Crystalline silicon was the first PV technology to be successfully commercialized and is what most people think of when they think of solar panels. The silicon crystal on the semiconductor that converts light to electricity is a layer typically 350 micrometers thick, but it accounts for about 50 percent of the cost of the panels. Although an older solar technology than others, mono and polycrystalline technologies dominate currently installed PV systems.
日本一本道a不卡免费Two common types of crystalline solar cells:
Monocrystalline Cells日本一本道a不卡免费 contain slices of a single large silicon crystal. This gives them the highest efficiency of any single band-gap solar technology, but it also makes them the most expensive.
Polycrystalline Cells contain slices of a multi-crystal block. The crystal boundaries reduce their efficiency somewhat, so they are typically a bit less efficient than monocrystalline cells, but also less expensive.
Second Generation — Thin Film
Thin film PV technologies are characterized by their very thin active layer, as little as 0.5 microns thick. Compared to traditional crystalline technologies, this is a very small amount of material and amounts to only about 5 percent of the panel cost, thus greatly reducing the overall price tag. Thin film cells generally having a lower efficiency than crystalline, but the low cost of manufacturing and certain physical characteristics of thin films tends to make them more versatile. They are used extensively for applications ranging from hand-held calculators to rooftop solar panels.
日本一本道a不卡免费Refinements to thin film technology include the following:
Amorphous Silicon is one of the oldest solar PV technologies, consisting simply of silicon without any crystal structure. While it is very cheap and easy to produce, amorphous silicon is also very inefficient. This is the type of material that is found in some hand-held calculators.
Cadmium Telluride (CdTe) is the most commonly used of the thin film technologies. Its conversion efficiency tends to be less than that of crystalline silicon cells, but it has better resistance to heat. This is the technology most pursued to achieve the sought-after "$1 per watt" PVs that could make solar power competitive with other fuels.
Copper Indium Gallium (di)selenide (CIGS) is a more technically complex thin film than CdTe, but with similar heat resistance. Its potential advantages compared to CdTe are that it doesn't contain the toxin cadmium and it has shown potential in the laboratory to be more efficient.
Third Generation — Concentrators, Multijunction and Beyond
Researchers are tackling a variety of third generation PV technologies for two main reasons:
To find ways to surpass the theoretical maximum efficiency of current solar cells, which is only 29 percent
To make better use of PV materials that are expensive and potentially scarce.
Current third generation PV options include the following:
Solar Concentrators日本一本道a不卡免费 use mirrors or lenses to focus sunlight on high efficiency solar cells, such as multijunction cells. The intensely focused light increases the amount of electrons that can be converted to electricity.
Multijunction PV Cells essentially combine two or more solar cells, one overlaid upon the other, with each designed to catch a different bandwidth of light. This technology has the potential to capture more electrons and greatly increase efficiency in converting sunlight to electricity.
Testing and Certifications
日本一本道a不卡免费Solar panels are designed, tested and certified to be exceptionally resilient, so they can perform well over time while potentially withstanding excessive heat, cold, wind, torrential rain and the impact of hail among other hazards. Today most commercial PV panels are backed by 25 year warranties.