[WHAT IT IS] Photovoltaic (solar cell) systems convert sunlight directly into electricity. Solar water heating systems for buildings have two main parts: a solar collector and a storage tank. Typically, a flat-plate collector—a thin, flat, rectangular box with a transparent cover—is mounted on the roof, facing the sun. The sun heats an absorber plate in the collector, which, in turn, heats the fluid running through tubes within the collector. To move the heated fluid between the collector and the storage tank, a system either uses a pump or gravity, as water has a tendency to naturally circulate as it is heated. Systems that use fluids other than water in the collector's tubes usually heat the water by passing it through a coil of tubing in the tank. [USES] Many large commercial buildings can use solar collectors to provide more than just hot water. Solar process heating systems can be used to heat these buildings. A solar ventilation system can be used in cold climates to preheat air as i enters a building. And the heat from a solar collector can even be used to provide energy for cooling a building. A solar collector is not always needed when using sunlight to heat a building. Some buildings can be designed for passive solar heating. These buildings 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 walls will then heat up during the day and slowly release heat at night—a process called direct gain. Many of the passive solar heating design features also provide daylighting. Daylighting is simply the use of natural sunlight to brighten up a building's interior.
[MORE ON DAYLIGHTING] Proper building orientation, so the longest walls run from east to west, allows solar heat to enter the home in winter, while allowing in as little sun as possible during summer. Shading and overhangs also reduce excessive summer heat, while still permitting winter sun. In passive solar designs, the optimal window-to-wall area ratio is 25-35 percent. In hot and moderate climates, the strategy is to admit light while rejecting heat. Interior spaces requiring the most light, heat, and cooling are located along the south face of the building, with less used space to the north. Open floor plans allow more sun inside. The simplest passive design is the direct gain system in which the sun shines directly into a building, heating it up. The sun's heat is stored by the building's inherent thermal mass in materials such as concrete, stone floor slabs, or masonry partitions that hold and slowly release heat. With indirect gain systems, thermal mass is located between the sun and the living space. An isolated gain system is one where the system is isolated from the primary living area, such as a sunroom or solar greenhouse with convective loops into the living space. Many passive solar designs include natural ventilation for cooling. By installing casement or other operable windows for passive solar gain and adding vertical panels, called wing walls, perpendicular to the wall on the windward side of the house, you can accelerate the natural breeze in the interior. Another passive solar cooling device is the thermal chimney, which can be designed like a smoke chimney to vent hot air from the house out through the roof. Daylighting is using natural sunlight to light a building's interior. In addition to south-facing windows and skylights, clerestory windows—a row of windows near the peak of the roof—can let light into north-facing rooms and upper levels. An open floor plan allows the light to reach throughout the building. Passive solar is not just a design technique for using the sun to heat and cool a home. Passive solar heating is also a common way to heat water (see the section on Solar Hot Water), and, particularly in developing nations where the electrical grid is undeveloped, passive solar heat is sometimes captured to cook food. Solar cookers can cook just about any food a conventional oven can. A basic cooker consists of an insulated box with a glass top. Heat from concentrated sunlight gets trapped in the box and can be used to heat food enclosed in the box.
[MADE FROM] Crystalline silicon (c-Si) is the leading commercial material for photovoltaic cells, and is used in several forms: single-crystalline or monocrystalline silicon, multicrystalline or polycrystalline silicon, ribbon and sheet silicon and thin-layer silicon. [much more info on the site]
[NEGATIVE] Photovoltaics are expensive to produce because of the high cost of semiconducting materials.
Solar Hot Water and Space Heating & Cooling
Solar hot water heaters use the sun to heat either water or a heat-transfer fluid in collectors. There are passive systems and active systems. A typical system will reduce the need for conventional water heating by about two-thirds. Sometimes the plumbing from a solar heater connects to a house's existing water heater, which stays inactive as long as the water coming in is hot or hotter than the temperature setting on the indoor water heater. When it falls below this temperature, the home's water heater can kick in to make up the difference. High-temperature solar water heaters can provide energy-efficient hot water and hot water heat for large commercial and industrial facilities.
Hot Water Systems
This system uses a pump to circulate potable water from the water storage tank through one or more collectors and back into the tank. The pump is regulated by an
electronic controller, an appliance timer, or a photovoltaic panel.
In this system, a heat exchanger heats a fluid that circulates in tubes through the water storage tank, transferring the heat from the fluid to the potable water.
A thermosiphon solar water heating system has a tank mounted above the collector. As the collector heats the water, it rises to the storage tank, while heavier cold water sinks down to the collector.
In cold climates, this system prevents water from freezing in the collector by using electric valves that automatically drain the water from the collector when the temperature drops to freezing. "Drainback systems," a variation of this approach, automatically drain the collector whenever the circulating pump stops.
Swimming Pool Systems
In solar heated swimming pools, the pool's filter pump pumps water through a solar collector, and the pool itself stores the hot water.
Solar Energy Collectors
Flat Plate Collectors
The most common collector for solar hot water is the flat plate collector. It is a rectangular box with a transparent cover, installed on a building's roof. Small tubes run through the box and carry fluid-either water or other fluid, such as an antifreeze solution. The tubes attach to a black absorber plate. As heat builds up in the collector, it heats the fluid passing through the tubes. The hot water or liquid goes to a storage tank. If the fluid is not hot water, water is heated by passing it through a tube inside the storage tank full of hot fluid.
Evacuated Tube Collectors
These collectors consist of rows of parallel transparent glass tubes, each containing an absorber and covered with a selective coating. Sunlight enters the tube, strikes the absorber, and heats the liquid flowing through the absorber. These collectors are manufactured with a vacuum between the tubes, which helps them achieve extremely high temperatures (170-350 degrees F); so they are appropriate for commercial and industrial uses.
Parabolic trough-shaped reflectors concentrate sunlight onto an absorber or receiver to provide hot water and steam, usually for industrial and commercial applications.
Transpired Solar Collectors
A transpired collector is a south facing outside wall covered by a dark sheet metal collector. The collector heats outside air, which is then sucked into the building's ventilation system through perforations in the collector. They have been used for pre-heating ventilation air and crop drying. They are inexpensive to make, and commercially, have achieved efficiencies of more than 70 percent.
Batch or Breadbox Heaters
This system is also referred to as a batch heater and a breadbox. It consists of an approximately 40-gallon insulated tank, lined with glass on the inside and
painted black on the outside. It is mounted on the roof, or on the ground in the sun. Plumbing from the house supplies the box with cold water through an inlet that extends down to the bottom of the tank. The box itself acts like a collector, absorbing and trapping the sun's heat and heating the water. An outlet supplies the house with heated water from the top of the tank.
Solar Process Heat
These systems consists of several thousand square feet of ground-mounted collectors, pumps, heat exchangers, controls, and one or more large storage tanks. Typically, they provide hot water and hot water space heating for large institutions such as schools, office buildings, prisons, and military bases.
Active Solar Cooling
As water evaporates, it cools the air. Evaporative cooling systems, usually appropriate for hot dry climates, can be powered with solar technology. In humid climates, desiccant evaporative cooling systems use the same evaporative concept to cool air, but they also include a desiccant wheel to dry incoming air. Waste heat from the building, natural gas, or solar technologies can be used to regenerate the desiccant wheel. Evaporative cooling is a CFC-free and energy-efficient way to cool commercial buildings. In absorption solar cooling, an absorption device uses a heat source, such as natural gas or a large solar collector, to evaporate refrigerant.
Building-integral wind turbines can be mounted so the building channels wind to them, to increase effective windspeed at the turbine by a factor of at least 2 and possibly 4, and the building provides a high and sheltered mounting (with no need for towers and the uneven windspeed patterns they cause to spinning blades, which causes noise and blade fatigue). It also affords practical protective grills, to safeguard people from possible blade disintegration, keep birds away from the spinning blades, and protect the turbine from weather and sun damage. Doubling wind speed at the wind turbines can increase their output power by 8X. Although that increase appears easy to achieve, with no compromises in building cost or function, a building's aerodynamic properties at its intended site would need to be analyzed, in addition to wind data available for the site. Average wind speed records are available for many locations.
The energy stored in biomass (organic matter) is called bioenergy. Bioenergy can be used to provide heat, make fuels, and generate electricity. Wood, which people have used to cook and keep warm for thousands of years, continues to be the largest biomass resource. Today there are also many other types of biomass we can use to produce energy. These biomass resources include residues from the agriculture and forest industries, landfill gas, aquatic plants, and wastes produced by cities and factories.
Direct combustion involves the burning of biomass in a boiler to produce steam. The pressure of the steam then turns a turbine attached to an electrical generator, which makes electricity. Coal-fired power plants employ similar technology but use fossil fuel in their boilers. Most of today's biopower plants use a direct combustion system. Researchers are evaluating other advanced processes that are even more efficient than direct combustion.
Cofiring systems can burn up to 15 percent biomass when mixed with coal in some boilers. Cofiring biomass with coal reduces emissions and produces fewer of the chemicals that cause acid rain. Many existing coal plants could use a cofiring system with only a few modifications. Therefore, this system has a significant potential for growth in the near future. To make cofiring biomass more attractive to power companies, researchers are investigating improvements to the cofiring process and better technologies for minimizing emissions.
Engineers are developing new technologies to produce biogas from biomass. Biogas consists of methane (found in natural gas) together with hydrogen, and other gases. Researchers are learning how to produce higher quality biogas by studying coal gasification systems. Some new gasification technologies make biogas by heating wood chips or other biomass in an oxygen-starved environment.
A second method for making biogas is to let landfills do the work. As paper and other biomass decay inside a landfill, they naturally produce methane. Methane can be recovered from landfills by drilling wells into the landfill and piping the gas to a central processing facility for filtering and cleaning. Clean landfill gas is then ready to fuel a biopower plant or help heat a building. Biogas can be burned (or co-fired) in a boiler to produce steam for electricity generation. Biogas can also fuel gas turbines or combined-cycle generation systems. In a combined-cycle system, pressurized gas first turns a gas turbine to generate electricity. Then, the waste gas from the gas turbine is burned to make steam for additional power production.
Researchers are also investigating a smoky-colored, sticky liquid that forms when biomass is heated in the absence of oxygen. Called pyrolysis oil, this liquid can be burned like petroleum to generate electricity. Petroleum, however, is almost never used any more to generate electricity. There's a greater need to use petroleum as a source of gasoline, heating oil, and petrochemicals. Because pyrolysis oil can also be refined in ways similar to crude oil, it may also be more valuable as a source of biofuels and biobased products than for biopower generation. Unlike direct combustion, cofiring, and gasification, this technology is not yet in the marketplace.
Researchers are particularly interested in improving small systems sized at 5 megawatts (MW) or less. These so-called modular biopower systems can use direct
combustion, cofiring, or gasification for power generation. They are well suited for generating biopower from locally grown resources for small towns, rural industries, farms, and ranches. Modular systems may be a good choice where power lines are not available. Clusters of modular biopower systems in rural areas may eradicate the need for power companies to build larger, more expensive power plants.
[CAN BE USED FOR TRANSPORTATION TOO!]
Biomass is the only renewable source of transportation fuels. These renewable fuels, called biofuels, produce fewer emissions than petroleum fuels. Biofuels also can help us reduce our dependence on foreign sources of fossil fuels. We can open up foreign markets for U.S. products and technologies. And, we stimulate growth in industry and in rural areas, making farming and forestry more profitable.