Photovoltaics

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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”).

Figure 1 Photovoltaic Cells
Figure 1 Photovoltaic Cells.

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.

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, 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.

Figure 2 Schematic representation of a solar cell.
Figure 2 Schematic representation of a solar cell.

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 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.

Contents

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 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

Figure 3 Spherical Solar Cells.
Figure 3 Spherical Solar Cells.

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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