Solar energy

1. Solar energy

2. Introduction Solar energy is energy directly from the Sun. This energy drives the climate and weather and supports virtually all life on Earth. Heat and light from the sun, along with solar-based resources such as wind and wave power, hydroelectricity and biomass, account for most of the available flow of renewable energy. Solar energy technologies harness the sun's energy for practical ends. These technologies date from the time of the early Greeks, Native Americans and Chinese, who warmed their buildings by orienting them toward the sun. Modern solar technologies provide heating, lighting, electricity and even flight.

3. Types of technologies There are many technologies for harnessing solar energy. Solar energy can be converted into other forms of energy, such as heat and electricity. Applications span through the residential, commercial, industrial, agricultural and transportation sectors. Solar energy can be used to produce food, heat, light and electricity. In the 1830’s the British astronomer John Herschel used a solar thermal collector box (a device that absorbs sunlight to collect heat) to cook food during an expedition to Africa. Today, people use the sun's energy for lots of things. The flexibility of solar energy is manifest in a wide variety of technologies such as cars, calculators,etc.

4. Convertions of the solar energy Solar energy can be converted to thermal (or heat) energy and used to: - heat water – for use in homes, buildings, or swimming pools. - heat spaces – inside greenhouses, homes, and other buildings Solar energy can be converted to electricity in two ways: - Photovoltaic (PV devices) or “solar cells” – change sunlight directly into electricity. PV systems are often used in remote locations that are not connected to the electric grid.  They are also used to power watches, calculators, and lighted road signs. - Solar Power Plants -  indirectly generate electricity when the heat from solar thermal collectors is used to heat a fluid which produces steam that is used to power generator. Out of the 15 known solar electric generating units operating in the United States at the end of 2006, 10 of these are in California, and 5 in Arizona. No statistics are being collected on solar plants that produce less than 1 megawatt of electricity, so there may be smaller solar plants in a number of other states.

5. SOLAR THERMAL HEAT Solar thermal(heat) energy is often used for heating swimming pools, heating water used in homes, and space heating of buildings. Solar space heating systems can be classified as passive or active. Passive space heating is what happens to your car on a hot summer day. In buildings, the air is circulated past a solar heat surface(s) and through the building by convection (i.e. less dense warm air tends to rise while more dense cooler air moves downward) . No mechanical equipment is needed for passive solar heating. Active heating systems require a collector to absorb and collect solar radiation.   Fans or pumps are used to circulate the heated air or heat absorbing fluid.  Active systems often include some type of energy storage system. Solar collectors can be either nonconcentrating or concentrating. 1) Nonconcentrating collectors – have a collector area (i.e. the area that intercepts the solar radiation) that is the same as the absorber area (i.e., the area absorbing the radiation). Flat-plate collectors are the most common and are used when temperatures below about 200o degrees F are sufficient, such as for space heating. 2) Concentrating collectors – where the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. 

6. SOLAR THERMAL POWER PLANTS Solar thermal power plants use the sun's rays to heat a fluid, from which heat transfer systems may be used to produce steam. The steam, in turn, is converted into mechanical energy in a turbine and into electricity from a conventional generator coupled to the turbine.   Solar thermal power generation works essentially the same as generation from fossil fuels except that instead of using steam produced from the combustion of fossil fuels, the steam is produced by the heat collected from sunlight. Solar thermal technologies use concentrator systems due to the high temperatures needed to heat the  fluid.  The three main types of solar-thermal power systems are: - Parabolic trought – the most common type of plant. - Solar dish - Solar power tower

7. The most common is parabolic troughs – long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. The liquid, at about 300 degrees Celsius, runs to a central collector, where it produces steam that drives an electric turbine. Parabolic dish concentrators are similar to trough concentrators, but focus the sunlight on a single point. Dishes can produce much higher temperatures, and so, in principle, should produce electricity more efficiently. But because they are more complicated, they have not succeeded outside of demonstration projects. A more promising variation uses a stirling engine to produce power. Unlike a car’s internal combustion engine, in which gasoline exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. These dish-stirling generators produce about 30 kilowatts of power, and can be used to replace diesel generators in remote locations. The third type of concentrator system is a central receiver . One such plant in California features a "power tower" design in which a 17-acre field of mirrors concentrates sunlight on the top of an 80-meter tower. The intense heat boils water, producing steam that drives a 10-megawatt generator at the base of the tower. The first version of this facility, Solar One, operated from 1982 to 1988 but had a number of problems. Reconfigured as Solar Two during the early to mid-1990s, the facility is successfully demonstrating the ability to collect and store solar energy efficiently. Solar Two’s success has opened the door for further development of this technology.

8. PHOTOVOLTAIC ENERGY In 1839, French scientist Edmund Becquerel discovered that certain materials would give off a spark of electricity when struck with sunlight. This photoelectric effect was used in primitive solar cells made of selenium in the late 1800s. In the 1950s, scientists at Bell Labs revisited the technology and, using silicon, produced solar cells that could convert four percent of the energy in sunlight directly to electricity. Within a few years, these photovoltaic (PV) cells were powering spaceships and satellites. The most important components of a PV cell are two layers of semiconductor material generally composed of silicon crystals. On its own, crystallized silicon is not a very good conductor of electricity, but when impurities are intentionally added—a process called doping—the stage is set for creating an electric current. The bottom layer of the PV cell is usually doped with boron, which bonds with the silicon to facilitate a positive charge (P). The top layer is doped with phosphorus, which bonds with the silicon to facilitate a negative charge (N). The surface between the resulting “p-type” and “n-type” semiconductors is called the P-N junction . Electron movement at this surface produces an electric field that only allows electrons to flow from the p-type layer to the n-type layer. When sunlight enters the cell, its energy knocks electrons loose in both layers. Because of the opposite charges of the layers, the electrons want to flow from the n-type layer to the p-type layer, but the electric field at the P-N junction prevents this from happening. The presence of an external circuit,however, provides the necessary path for electrons in the n-type layer to travel to the p-type layer. Extremely thin wires running along the top of the n-type layer provide this external circuit, and the electrons flowing through this circuit provide the cell’s owner with a supply of electricity. Most PV systems consist of individual square cells averaging about four inches on a side. Alone, each cell generates very little power (less than two watts), so they are often grouped together as modules. Modules can then be grouped into larger panels encased in glass or plastic to provide protection from the weather, and these panels, in turn, are either used as separate units or grouped into even larger arrays.

9. The clasification of the solar cells The three basic types of solar cells made from silicon are single-crystal, polycrystalline, and amorphous. Single-crystal cells are made in long cylinders and sliced into round or hexagonal wafers. While this process is energy-intensive and wasteful of materials, it produces the highest-efficiency cells—as high as 25 percent in some laboratory tests. Because these high-efficiency cells are more expensive, they are sometimes used in combination with concentrators such as mirrors or lenses. Concentrating systems can boost efficiency to almost 30 percent. Single-crystal accounts for 29 percent of the global market for PV. Polycrystalline cells are made of molten silicon cast into ingots or drawn into sheets, then sliced into squares. While production costs are lower, the efficiency of the cells is lower too—around 15 percent. Because the cells are square, they can be packed more closely together. Polycrystalline cells make up 62 percent of the global PV market. Amorphous silicon (a-Si) is a radically different approach. Silicon is essentially sprayed onto a glass or metal surface in thin films, making the whole module in one step. This approach is by far the least expensive, but it results in very low efficiencies—only about five percent. A number of exotic materials other than silicon are under development, such as gallium arsenide (Ga-As), copper-indium-diselenide (CuInSe2), and cadmium-telluride (CdTe). These materials offer higher efficiencies and other interesting properties, including the ability to manufacture amorphous cells that are sensitive to different parts of the light spectrum. By stacking cells into multiple layers, they can capture more of the available light. Although a-Si accounts for only five percent of the global market, it appears to be the most promising for future cost reductions and growth potential. Solar cell are also being used in developing countries. Solar panels can power a 17" b/w TV, a radio or a fan. Some electric lighting systems provide sufficient current for up to 10 hours of lightning each evening. Locally produced car batteries can provide up to 5 nights of energy for a 8 watt DC fluorescent light. The new Mazda 929, uses solar cells to activate a fan to ventilate the car when the car is idle and parked during a sunny hot day.

10. Advantages and disadvantages of the photovoltaic method Some advantages of photovoltaic systems are:  - Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary. - PV arrays can  be installed quickly and in any size required or allowed. - The environmental impact is minimal, requiring no water for system cooling and generating no by-products. Current drawbacks of photovoltaic cells - The use of silicon crystals in the Photovoltaic cells makes it expensive. 1) silicon crystals are currently assembled manually 2) silicon purification is difficult and a lot of silicon is wasted 3) the operation of silicon cells require a cooling system, because performance degrades at high temperatures. However, it has convinced analysts that solar cells will become a significant source of energy by the end of the century. Research is underway for new fabrication techniques, like those used for microchips. Alternative materials like cadmium sulfide and gallium arsenide are at an experimental stage. Reduction of cost will depend the economies of scale.

11. Solar Heat Collectors Besides using design features to maximize their use of the sun, some buildings have systems that actively gather and store solar energy. Solar collectors, for example, sit on the rooftops of buildings to collect solar energy for space heating, water heating, and space cooling. Most are large, flat boxes painted black on the inside and covered with glass. In the most common design, pipes in the box carry liquids that transfer the heat from the box into the building. This heated liquid—usually a water-alcohol mixture to prevent freezing—is used to heat water in a tank or is passed through radiators that heat the air. Oddly enough, solar heat can also power a cooling system. In desiccant evaporators, heat from a solar collector is used to pull moisture out of the air. When the air becomes drier, it also becomes cooler. The hot moist air is separated from the cooler air and vented to the outside. Another approach is an absorption chiller. Solar energy is used to heat a refrigerant under pressure; when the pressure is released, it expands, cooling the air around it. This is how conventional refrigerators and air conditioners work, and it’s a particularly efficient approach for home or office cooling since buildings need cooling during the hottest part of the day. These systems are currently at work in humid southeastern climates such as Florida.

12. Future of Solar Energy The success of solar power will depend on the answer to the following question: 'What do you do when the sun goes down?' The simple answer is to build an auxiliary system that will store energy when the sun is out.. However, the problem is that such storage systems are unavailable today. Simple systems, like water pipes surrounded by vacuum, do exist. It is based on the concept that provided the pipes are insulated, the water will store thermal energy. The ocean is a natural reservoir of solar power and could be used as a source for thermal energy. If we can draw warm water from the surface and cold water from the depths, an ocean thermal plant could operate 24 hours a day. George Claude tested this hypothesis as early as 1930 in Cuba. Cold water from the pipe and warm water from the surface were pumped into a plant on shore. It produced 22KW when the water temperatures were optimum and 12KW when seasonal current fluctuation reduced the efficiency. There are also the hybrid systems. Wyoming has a system that holds back water on a neighboring hydroelectric plant when the wind is blowing, which for the time being, runs the turbines. As discussed earlier, wind is an indirect form of solar energy. Thus the hybrid system is used in the fuel saver mode. Research on photovoltaic cells will continue. Compared to the other options, majority of the resources will probably flow into research for developing better and more efficient solar cells. Parallel to that, more research will be undertaken to develop rechargeable batteries that will last longer hours. Solar energy—power from the sun—is free and inexhaustible. This vast, clean energy resource represents a viable alternative to the fossil fuels that currently pollute our air and water, threaten our public health, and contribute to global warming. Failing to take advantage of such a widely available and low-impact resource would be a grave injustice to our children and all future generations.

13. Disadvantages of the solar energy The major disadvantages of solar energy are: The amount of sunlight that arrives at the earth's surface is not constant. It depends on location, time of day, time of year, and weather conditions. Because the sun doesn't deliver that much energy to any one place at any one time, a large surface area is required to collect the energy at a useful rate.

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