Photovoltaics

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Photovoltaics Photovoltaic (PV) systems convert solar energy directly into electricity using specially designed collectors known as solar cells (Figure 10-16). The photovoltaic effect was first observed by Alexander Becquerel in 1839, although it took another century before any serious attempt was made to design a practical device. In 1968, Glaser19 proposed to place a satellite solar power system in a geosynchronous orbit that collected sunlight and beamed it to collectors that then generated electricity (See box “ Solar Power Satellites”). By the mid-1970s, solar cells were primarily used in space for powering the instruments aboard space stations and in satellites. As the technology matured and the cost of production decreased, solar cells found new applications in everyday appliances, from pocket calculators and wristwatches to small remote applications such as highway call boxes, message boards, and traffic signals. Worldwide, one gigawatt of electricity is generated by photovoltaics. Japan currently leads in solar cell manufacturing and controls nearly 40% of the global solar cell market; Germany is second and the United States is third. Compared to fossil fuels, the cost of electricity generation by solar cells is expensive, but can quickly become competitive as manufacturing costs drop. Because they can be installed at any capacity, photovoltaic power is an ideal method for powering remote homes or villages too remote to connect to a grid. Figure 10-16 Photovoltaic Cells Source: National Renewable Energy Laboratory, Photographic Information Exchange (http://www.eere.energy.gov/pv). Solar Crock Pot* FYI ... Indian workers have devised a cooking system that makes no demands on the critically short supply of wood, their traditional energy source. They dug a pond about 1 m deep and 8 m in diameter and lined it with impermeable plastic. The pond was filled with water and enough salt poured in to give a saline concentration of 20 percent at the bottom. The concentration of salt gradually decreased from the bottom to the top of the pond. The salt gradient prevents convection, as the salt water is too heavy to rise. Thus the water at the bottom absorbs solar heat energy throughout the day and reaches a temperature over 80oC at the bottom. Food is placed in sealed pots and left in the pond for a whole day. This kind of cooking is equivalent to an electric “crock pot” which takes 75 W for 8 hours.

  • Excepts from a physics text currently under preparation jointly by this author and Professor Igor Glozman of Department of Physics, Highline Community College, Des Moines, Washington 98198.

19 Glaser, P. E., “Power from the Sun: Its Future,” Science, Vol. 162, pp. 957-961 (1968). Figure 10-15 The largest solar pond operated to-date is a 210,000 square-meter pond at the Dead Sea, Israel, which generates 2.5 MW of peak electric power. 232 Principle of Operation To understand the photovoltaic (or photoelectric) effect, it is best to visualize sunlight as small packets (quanta) of light called photons. The other equally valid view considers light as a wave. Photons are emitted at different frequencies and, therefore, have different wavelengths and energies. When photons strike a cell, some are absorbed, some are reflected, and some pass through. Only photons beyond a certain frequency have sufficient energy to dislodge electrons off a substrate and flow into an external circuit producing electricity. To facilitate the movement of electrons, an electric potential must be constructed. This can be done by sandwiching two layers of silicon wafers together; one layer is doped with a material that has one less electron - is positively charged (p-layer) - and the other contains a material with an extra electron (n-layer) in their valence shell. To make a positively charged semiconductor, silicon is doped with impurities such as boron, aluminum, or indium. To make a negatively charged semiconductor, silicon is doped with impurities such as arsenic, antimony, or phosphorus. Commercial silicon cells are made by pressing two thin silicon wafers together, a very thin top layer doped with a small amount of phosphorous (n-type) and a thicker bottom layer doped with a small amount of boron (p-type). When an n-type and a p-type semiconductor are pressed together, the imbalance of electrons across the p-n junction causes electrons to migrate from the n-type to the p-type material. If a wire connects two layers across a load (for example a light bulb or an electric motor), an electric field is created. As long as there is solar energy available and the circuit is closed, Solar Power Satellites (SPS) FYI ... A solar panel facing the sun in near-earth space receives about 1,350 watts of sunlight per square meter. Because of the sun’s angle and the attenuation by the atmosphere, the radiation received on the earth’s surface is considerably reduced. Furthermore, solar cells can only work during the day, so they can only receive sunlight half of the time. If solar collectors are placed in a geostationary orbit* (36,000 kilometers high), all of the extraterrestrial radiation can be collected and converted to electricity. This electricity can then be beamed as microwaves, which can pass unimpeded through clouds and rain and be received on the earth before it is converted back into electricity. This electricity is in the form of direct current, which can then be converted to the 50- or 60-cycle alternating current electricity used in homes. Because the panel always faces the sun, electricity would be generated 24 hours a day and there will be no need for storage. Such a system has indeed been proposed by the US Department of Energy and NASA. A preliminary design consisted of a 5 km x 10 km rectangular solar collector and a 1 km diameter circular transmitting antenna array. The SPS would weigh 30,000 to 50,000 tons and provide 5 billion watts of electricity, equivalent to ten 500 MW conventional coal or nuclear power plants. We are not likely to see such a huge orbiting solar collector beaming energy anytime soon. What probably makes more sense is to build a small proof-of-concept demonstration before extensive resources are dedicated toward fully-operational solar power satellites.

  • Alternatively, some have proposed to place the collectors on the surface of the moon.

Source: Glaser, P.E., “Power from the Sun: Its Future,” Science, Vol. 162, pp. 856-861, 1968. 233 Chapter 10 - Solar Energy 233 the current will continue to flow. Current is directly proportional to how much light strikes the module. To increase the current output, the top surface is coated with an anti-reflection material. Without the coating, an additional 30% of solar energy would be lost due to reflection. A typical commercially available cell, 10 cm x 10 cm, produces about 1.5 watts (0.5 volts and a current of about 3 amperes) of electric power when exposed to strong sunlight. A typical setup is shown in Figure 10-17. To achieve the proper current and voltage, cells are connected in particular configurations. Normally, modules are designed to supply electricity at a certain voltage by connecting a sufficient number of cells in series. The current can be adjusted by connecting a number of such modules in parallel. PV panels can then be grouped together to form large solar farms Figure 10-17 Schematic representation of a solar cell Sunlight Contact Semi Conductor Back Contact Anti-reflective Coating Metals, Nonmetals, and Semiconductors Digging Deeper ... Every atom is comprised of a nucleus and a number of electrons orbiting the nucleus. Electrons can only move in certain orbits. To move an electron from a lower orbital to a higher orbital, it must absorb a photon of energy proportional to the difference between the two energy levels. Similarly, an electron jumping from a higher to lower orbit emits a photon of light with a frequency proportional to the energy difference. Electrons naturally occupy the lowest available energy levels (valence bands), so upper energy levels (conduction bands) remain vacant. The energy needed to move the electrons from the highest valence band to the lowest conduction band is called gap energy. The greater the gap energy, the more difficult it is to move electrons and the less conductive (insulator) a material is considered. In non-metals, energy levels are separated by large gaps; therefore the transition from one energy level to the next is associated with frequencies that construct a discrete band. Metals, on the other hand, produce a continuous band because the electrons are free to move through the body of metal and pass through several energy levels. Metals have a continuous conduction band and are therefore good conductors of electricity. Semiconductors have intermediate gap energies. Because of its abundance and low price, silicon is the most popular semiconductor material. Atoms of silicon have four electrons in the outer valence shell and are therefore electrically neutral. Silicon is an intrinsic semiconductor, i.e. at room temperatures, electrons do not have sufficient energy to jump the gap. By raising the temperatures, some electrons gain enough energy to overcome the gap and move into the conduction band. If the source of energy is removed, the electrons fall back into their previously occupied valence band. Depending on the processing method, silicon can have different structures: Monocrystalline silicon is grown from a single atom of silicon and has a perfectly uniform molecular structure, making it ideal for the efficient transfer of electrons through the material. Single crystalline silicon is difficult to manufacture and very expensive. Polycrystalline silicon consists of, not one, but a number of crystals. It is much cheaper than single crystalline silicon to produce, but the different crystalline structures introduce boundaries that increase internal resistance and impede the flow of electrons. The greatest advantage of polycrystalline silicon is that it can be deposited monolithically as a thin film on a glass substrate. Unlike single crystalline silicon, which must be grown from a single atom (ingot), layer upon layer of polycrystalline thin-films of the required materials can be sequentially deposited to a desired thickness. Polycrystalline silicon cells are less expensive than monocrystalline cells, but their efficiencies are also lower. Amorphous silicon does not have a crystalline structure and atoms are not arranged in any particular order. Amorphous silicon contains large numbers of structural and bonding defects, where electrons and holes recombine and limit the maximum current that can flow. It is the cheapest, however, making it suitable for use in low-power consumer devices such as wristwatches and calculators. Amorphous silicon cells can be sprayed onto glass plates and onto a variety of flexible substrates like metal foils and plastic foils, eliminating much of the production cost. 234 which generate electricity that can be used locally, stored in batteries, or fed into the commercial transmission grid. The output of photovoltaic cells is in the form of DC (direct current) that can be stored in batteries or used as a source for powering various devices. If it is to be used to generate electricity, the output must be converted to AC (alternating current) and amplified by transformers to high voltages before being transmitted through commercial electrical grids. Collection Efficiency Solar cells are often characterized by their conversion efficiency – defined as the percentage of incident power that is converted into electric power. The efficiency of solar cells is limited by the number of photons of a precise frequency that can knock out the electrons from the junction. Photons of lower frequency do not have sufficient energy, whereas higher frequency photons may cause overheating of the substrate and subsequent loss of efficiency. Because of its abundance, silicon remains the most common cell material used today. However, higher efficiency cells have been constructed of gallium arsenide, cadmium telluride, cadmium sulfide, and other materials (commonly called III-V materials because they are impregnated with material from groups III and V of the periodic table). Efficiencies in the range of 5% for amorphous silicon and 17% for monocrystalline cells can be expected for commercially available silicon solar cells. Efficiencies as high as 33% may be obtained under concentrated light conditions. To increase the flux of photons, scientists have developed a spherical cavity treated with highly reflective coating and lined with multiple solar collectors. Different cells are made from different materials with different energy gaps. Each cell is covered by a filter that allows only the light with the appropriate frequency to pass through. The remaining photons are reflected back and forth within the cavity until the proper cells absorb them. Efficiencies as high as 48% have been reported.20 Another innovation, the cascade or tandem cell, is constructed by stacking several single-junction cells with different band gaps on top of each other. The top layer has the highest band gap, capturing the high-energy photons and allowing the photons with lower frequencies to reach the lower cells with smaller band-gaps. The major problem with flat panel cells is that, once they are installed with a particular orientation, they only collect maximum sunlight if the sun is directly overhead. As the sun’s incident angle changes, the light intensity drops and less power is produced. The basic idea behind the spherical cells is that a spherical receiver can collect light from all directions. This allows the capture of direct beams of light as well as light diffused from clouds and reflected from buildings, thus spherical cells would have efficiencies up to 50% greater than those of flat cells. An attempt to improve efficiency 20 Leine, J. D. et al., Proc. 15th IEEE Photovoltaic Specialist Conference, Las Vegas, IEEE, 141, 1991. 235 Chapter 10 - Solar Energy and reduce costs, a group of Japanese scientists have recently succeeded in developing spherical cells using single crystal silicon droplets (0.5-2 mm in diameter) to make the p-layer (Figure 10-18). A thin layer of n-type film is diffused and, except for a small opening for electrodes, covers the surface of the microsphere. The microspheres can be lined up along a string and connected in a series or parallel configuration with fine copper wire and are mounted on a white resin reflection plate covered with a transparent layer. The spherical solar cell module is highly flexible and can be made to match the contours of buildings or cars, thus providing integrated power sources in windows, roofing materials, canopies and other surfaces.21 Another promising technology is the multi-junction solar cell. Unlike conventional photovoltaic cells that absorb light only in the red part of the light spectrum, these cells consist of several microthin layers of light absorbing materials sandwiched together. Each layer absorbs a different color of light, increasing the overall efficiency. The technology has been implemented on the Mars Rover, but remains too expensive to be used in commercial products.22 Thermophotovoltaics Photovoltaic cells principally respond to light in the visible wavelengths. Thermophotovoltaic (TPV) cells work on the same principal, except that they are sensitive to infrared energy and use heat instead of light as their energy source.23 The source of heat could be a burning gas, a radiant furnace, waste heat, or another source in the 700-1,700°C range. TPV can also work by focusing solar radiation on to an intermediate absorber, which then re-emits it as thermal radiation. These devices are capable of converting as much as 1-10 watts of power per square centimeter, a far better power density than solar cells, which can only deliver about 100 mW of power per square centimeter. Because thermophotovoltaics are solid state devices (no moving parts), they have found applications in heating, cooling, and as sensors for temperature stabilization. Thermophotovoltaics (TPV) have been used for many years by NASA to power space crafts. The substantial drop in price of semiconductor ceramic material has prompted investigators to develop TPV systems that use waste heat from furnaces and burners to generate power, or make heat pumps that can provide cooling or heating. Bismuth-telluride materials are among the most favored because they can operate at temperatures as low as 150-200 degrees centigrade.

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