Photovoltaic cells convert solar energy directly to electricity. No thermal engines are required to operate generators. Unlike the solar thermal technology discussed above, solar photovoltaic cells will produce energy from diffuse sunlight as well as direct. With diffuse light, the power drops off proportionally to the decrease in total light reaching the cells. As long as some light reaches the cells, there will be energy production.82
Hamakawa, Yoshihiro, "Photovoltaic Power", "Scientific American", Vol. 256, No. 4, April 1987, Page 86
NASA provides spacecraft with power by extensive use of photovoltaic cells. In this application, the lightweight of the cell, coupled with the almost continuous availability of sunlight in space, has made the photovoltaic cell a bargain almost independent of the cost. In space applications, collectors made from single crystals of silicon or gallium arsenide are used. These systems are very expensive.
Single crystal silicon cells have demonstrated efficiencies as high as 19%. These cells utilize single crystal silicon of a grade similar to that used in computer chips. As a result, they are relatively expensive. Potentially lower cost cells are being fabricated using continuously cast ribbons of silicon. Because of the lower purity and crystal structure irregularity, cast silicon ribbons cells are less efficient. They convert only 10 to 15% of the solar energy to electricity. Amorphous silicon cells have been produced at much lower costs than the crystal or ribbon silicon, but they have only demonstrated 8% efficiency.83
Gallium arsenide is a semiconductor material that can be used to manufacture photovoltaic cells. The gallium and arsenic needed to make these materials are rare compared to silicon. It is difficult, and thus expensive to produce semiconductor gallium arsenide of proper purity for photovoltaic cells. Despite the higher cost of gallium arsenide, it is of great interest because test photovoltaic cells have shown sunlight to electricity efficiencies of over 20%. They have found some use in space applications.
Several demonstration facilities use photovoltaic cells to collect solar energy. They provide electric energy to existing power grids. The Carrisa Plains solar plant near San Louis Obispo, California, consists of 756 pole mounted solar collectors. Each collector has a clock operated drive. The drive rotates the collector about the horizontal and vertical axis to keep the flat surface facing the sun. Each collector is enhanced with a set of flat mirror reflectors that approximately double the intensity of the sunlight falling on the crystalline silicon solar cells. The individual collectors are rated at 5-KWe. When the sun is shining, the plant produces 3.7 MWe of electric power that is fed into the existing power distribution grid. Figure 6.5 shows a photovoltaic collector mated to an electrolyzer. This system supplies fuel to operate an environmentally benign bus system.
Photovoltaic systems produce the maximum power when illuminated by direct sunlight. When compared to solar thermal systems they have one large advantage. They can produce modest amounts of electricity even when it is cloudy by the conversion of diffuse light to power. Like all solar collection systems, photovoltaic systems cannot produce energy at night so that at least half of the daily load must be provided by some other source or by some means of storing the solar energy.84
Solar photovoltaic collectors can be mounted almost anywhere, in any size, and will require little mechanical maintenance or supervision. Like the collectors for solar thermal power, the photovoltaic power collector will require some method of regular cleaning if its production efficiency is to be maintained. The shortcomings are continuous maintenance to keep them clean, cost and intermittent energy production. With further research, it may be possible to produce a photovoltaic system that can produce economic electric power from sunlight. 85 The lack of a good bulk storage technology for energy and the intermittent nature of sunlight currently restrict photovoltaic systems to specialized applications or to providing supplementary energy to a much larger reliable system. The system
Chalmers, Bruce, "The Photovoltaic Generation of Electricity", "Scientific American", Vol. 235, No. 4, October 1976, Page 34
84 Hubbard, H. M„ "Photovoltaics Today and Tomorrow", Science, Vol. 244, No. 4902, April 21, 1989, Page 297
85 Shah, A., Torres, P., Tschamer, R„ Wyrsch, N., and Keppner, H., "Photovoltaic Technology: The Case for Thin-Film Solar Cells", Science, Vol. 285, No. 5428, July 30, 1999, Page 692
described in "An End To Global Warming" will remedy the bulk storage challenges and make the collection of photovoltaic energy much more capable of supplying general purpose energy.
Collection of solar energy in space has been suggested. 86 Some of the problems encountered in collecting solar energy on the earth are not present in space collection. Most important is the continuous availability of the sunlight when the satellites are in the proper high orbits. Collectors do not become dirty in space. There is damage from solar wind charged particles and from impacts with space dust, but these effects take a long time to significantly reduce the capabilities of the collectors. Cost will always be a problem for systems placed in space. In addition, there are great barriers in transmitting the energy collected in space, to the ground. There is ongoing research probing these difficulties, particularly in Japan. 87 Unfortunately, tight beams of energy from the satellite, such as laser beam or microwaves, are extremely dangerous. Anyone, any animal or bird that enters the beam will be cooked as if they were in a microwave oven. The beam would be a powerful weapon and nations that did not have control of the beam would strongly object to its existence. If the beam is spread out to the point that it is not dangerous, then the collection antenna must be about the same size as the nation. To make collection of solar energy in space practical two challenges must be overcome. First, the cost of placing objects in orbit must be reduced and second, a wholly new technology for moving energy from space to the ground must be developed.
Was this article helpful?
Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.