As discussed in Appendix A, concepts for "paving" the Moon with solar power cells are being developed, because most alternative sources of power are either more expensive, more complex, or both. Fuel cells are another emerging possibility, especially for rovers that must operate during the lunar night, but they require replenishment of their constituents - converting the output water back into oxygen and hydrogen. Solar power cells are likely to be the primary method of obtaining electrical power on the Moon. Electrical power can be used directly for many operations, and can be the power source for converting the output water of fuel cells back into hydrogen and oxygen.
A solar cell (also referred to as a photovoltaic cell) is a device that converts photons from the Sun (solar radiation) into electricity. When they first became available, they were used in solar-powered calculators and other low-power applications. As their efficiency improved, they began to be used in remote areas where electrical power from transmission lines (the "grid") is unavailable. They are now becoming increasingly popular as a method of reducing our dependence on non-renewable energy sources, such as coal and petroleum products. Parking lot lights and traffic signs powered by solar cells are becoming more common. Because photovoltaic systems are modular, their electrical power output can be engineered for almost any application.
Large arrays of photovoltaic cells are also used to power spacecraft. The International Space Station, as well as the Russian Mir space station that preceded it, were both designed to use solar-generated electricity as their main source for operational power. Their solar arrays must include gimbals or other methods of rotating the solar panels so that they are in direct sunlight for as much of the time as possible, and this technology is also applicable to the devices that will be sent to the Moon in the first robotic missions.
Solar cells are a type of semiconducting device. Semiconducting devices have many applications besides photovoltaic cells, such as small semiconductor chips containing tens of millions of transistors. These chips have made possible the miniaturization of electronic devices. In addition, extremely-small devices are being made using the technique of molecular beam epitaxy.1 A laser beam is used to vaporize a substance, which is then deposited on a substrate (such as a silicon chip) in a single molecular layer.
Solar cells use the photovoltaic2 principle for converting the energy of the Sun's photons into electrical energy. The electricity is direct current and can be used that way, or it can be converted to alternating current, or it can be stored for later use. There are no moving parts, and if the device is correctly encapsulated against the environment, nothing will wear out.
Figure D.2 illustrates a common photovoltaic structure. It is a semiconductor material into which a diode,3 or p-n junction,4 has been formed. Electrical current is taken from the device through a grid structure on the front (which makes a good contact at the same time as simultaneously allowing sunlight to enter the cell) while a
1 Molecular beam epitaxy is one of several methods of thin-film deposition. Elements such as gallium and arsenic are heated until they each begin to evaporate. The evaporated elements then condense on a substrate. The process takes place in high vacuum, thus the Moon is a natural place to produce them. The term "beam" simply means that evaporated atoms do not interact with each other or any other gases until they reach the substrate, due to the large mean-free path lengths in vacuum.
2 Photovoltaic: generating an electric current when acted on by light or a similar form of radiant energy.
3 Diode: a two-terminal electronic device that has a high resistance to current in one direction but a low resistance in the other direction [from di (two) + (electr)ode].
4 p-n junction: two layers of semiconductor, one of which contains excess electrons, and the other of which lacks electrons (it is said to possess holes) and consequently has a positive charge. "p-n" refers to the positive and negative "doped" layers, which are metallurgically joined.
second contact on the back completes the circuit. An anti-reflection coating minimizes the amount of sunlight that reflects away from the device.
Silicon is the most commonly-used semiconductor at present. It is a Group IV element on the Periodic Table, which means it has four electrons in its outermost orbit. In the silicon tetrahedral crystal structure, each atom shares its four valence electrons5 with its four nearest neighbors, forming four covalent bonds.6 The valence energy band, where these electrons generally reside, has a lower energy level than the conduction band, where electrons can move about freely to carry charge. The amount of energy needed to push an electron from the valence band into the conduction band (in other words, to release a bound electron) is called the band-gap energy, which is about 1.12 eV for silicon. Even at room temperature, the amount of conductivity in pure silicon is very small: 1.6 x 1010 conductors (carriers) per cubic centimeter, as compared with 1022 carriers per cubic centimeter in a typical metal.
To modify the conductivity, small amounts of impurities are introduced into the crystal lattice. This is called "doping". If you dope silicon with a Group III element such as boron (three electrons in the outermost orbit), then that part of the crystal lattice will have a covalent bond that is deficient by one electron. In other words, it will have a "hole". Electrons and "holes" are both charge carriers. Similarly, if you dope the silicon with a Group V element, you will have an extra, loosely-bound electron that can easily jump up into the conduction band. Doping creates another
5 Valence electron: an electron in the outer shell of an atom. In a chemical reaction, the atom gains, loses, or shares these valence electrons, so that they combine with other atoms to form molecules, or ions.
6 Covalent bond: a bond formed when electrons are shared by two atoms.
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