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How Solar Cells Work by Hiroaki Okamoto Two of the most common questions about photovoltaic technology are "what is it?" and "why should I use it?" Photovoltaic (PV) technology is basically "solar electricity" that results from converting sunlight into electricity. The sun provides enough clean energy in one day to provide a year's supply of FREE FUEL for powering buildings. Why we should consider using PV is simple: it helps preserve the Earth's finite fossil-fuel resources such as coal, oil, and natural gas. It also helps reduce air and water pollution associated with these energy sources. When faced with power outages and future electricity price increases, we can also benefit from the utilization of a PV system. You've probably seen calculators that have solar cells -- calculators that never need batteries, and in some cases don't even have an off button. As long as you have enough light, they seem to work forever. You may have seen larger solar panels -- on emergency road signs or call boxes, on buoys, even in parking lots to power lights. Although these larger panels aren't as common as solar powered calculators, they're out there, and not that hard to spot if you know where to look. There are solar cell arrays on satellites, where they are used to power the electrical systems. You have probably also been hearing about the "solar revolution" for the last 20 years -- the idea that one day we will all use free electricity from the sun. This is a seductive promise: On a bright, sunny day, the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free. QuickTimeý Dz TIFFÅià• èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB In this article, we will examine solar cells to learn how they convert the sun's energy directly into electricity. In the process, you will learn why we are getting closer to using the sun's energy on a daily basis, and why we still have more research to do before the process becomes cost effective. Converting Photons to Electrons The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaics, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, photovoltaics are used more and more in less exotic ways. They could even power your house. How do these devices work? Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy 1 of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce. That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single crystal silicon cell. Silicon Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have eight electrons). To do this, it will share electrons with four of its neighbor silicon atoms. It's like every atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell. We've now described pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good conductors such as copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell. Silicon in Solar Cells A solar cell has silicon with impurities -- other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms. Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called free carriers, and can carry electrical current. There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond -their neighbors aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is. 2 Actually, only part of our solar cell is N-type. The other part is doped with boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon ("p" for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do. So where has all this gotten us? N-type Plus P-type Silicon The interesting part starts when you put N-type silicon together with P-type silicon. Remember that every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the N-type and P-type silicon are in contact. Suddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the P side, and there's a mad rush to fill them in. Before now, our silicon was all electrically neutral. Our extra electrons were balanced out by the extra protons in the phosphorous. Our missing electrons (holes) were balanced out by the missing protons in the boron. When the holes and electrons mix at the junction between N-type and P-type silicon, however, that neutrality is disrupted. Do all the free electrons fill all the free holes? No. If they did, then the whole arrangement wouldn't be very useful. Right at the junction, however, they do mix and form a barrier, making it harder and harder for electrons on the N side to cross to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides. This electric field acts as a diode, allowing (and even pushing) electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill (to the N side), but can't climb it (to the P side). So we've got an electric field acting as a diode in which electrons can only move in one direction. Let's see what happens when light hits the cell. When Light Hits the Cell When light, in the form of photons, hits our solar cell, its energy frees electron-hole pairs. Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two. 3 How much sun- light energy does our PV cell absorb? Unfortunately, the most that our simple cell could absorb is around 25 percent, and more likely is 15 percent or less. Why so little? Energy Loss Why does our solar cell absorb only about 15 percents of the sunlight's energy? Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic -- it is made up of a range of different wavelengths, and therefore energy levels. Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts (eV) and defined by our cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. We call this the band gap energy of a material. If a photon has more energy than the required amount, then the extra energy is lost (unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant). These two effects alone account for the loss of around 70 percent of the radiation energy incident on our cell. Why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, our band gap also determines the strength (voltage) of our electric field, and if it's too low, then what we make up in extra current (by absorbing more photons), we lose by having a small voltage. Remember that power is voltage times current. The optimal band gap, balancing these two effects, is around 1.4 eV for a cell made from a single material. QuickTimeý Dz TIFFÅià• èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB Single crystal silicon isn't the only material used in PV cells. Polycrystalline silicon is also used in an attempt to cut manufacturing costs, although resulting cells aren't as efficient as single crystal silicon. Amorphous silicon, which has no crystalline structure, is also used, again in an attempt to reduce production costs. Other materials used include gallium arsenide, copper indium diselenide and cadmium telluride. Since different materials have different band gaps, they seem to be "tuned" to different wavelengths, or photons of different energies. One way efficiency has been improved is to use two or more layers of different materials with different band gaps. The higher band gap material is on the surface, absorbing high-energy photons while allowing lower-energy photons to be absorbed by the lower band gap material 4 beneath. This technique can result in much higher efficiencies. Such cells, called multijunction cells, can have more than one electric field. We have other losses as well. Our electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all of our current (in some cells, transparent conductors are used on the top surface, but not in all). If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance (for an electron) to reach the contacts. Remember, silicon is a semiconductor -- it's not nearly as good as a metal for transporting current. Its internal resistance (called series resistance) is fairly high, and high resistance means high losses. To minimize these losses, our cell is covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even so, some photons are blocked by the grid, which can't be too small or else its own resistance will be too high. Finishing the Cell There are a few more steps left before we can really use our cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent. The final step is the glass cover plate that protects the cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back. Qui c k Ti m eý Ç ² T I F F Å• è i àk Ç » Ç µ Å j ê L í £ É v É ç É O É â É Ä Ç ™ Ç ± Ç Ã É s É N É ` É É Ç ¾ å © Ç È Ç ž Ç ½ Ç …• ÅÇBÕ ï K ó v Ç Ç Powering a House Now that we have our PV module, what do we do with it? What would you have to do to power your house with solar energy? Although it's not as simple as just slapping some modules on your roof, it's not extremely difficult to do, either. First of all, not every roof has the correct orientation or angle of inclination to take advantage of the sun's energy. Non-tracking PV systems in the Northern Hemisphere should 5 point toward true south (this is the orientation). They should be inclined at an angle equal to the area's latitude to absorb the maximum amount of energy year-round. A different orientation and/or inclination could be used if you want to maximize energy production for the morning or afternoon, and/or the summer or winter. Of course, the modules should never be shaded by nearby trees or buildings, no matter the time of day or the time of year. In a PV module, even if just one of its 36 cells is shaded, power production will be reduced by more than half. If you have a house with an unshaded, south-facing roof, you need to decide what size system you need. This is complicated by the facts that your electricity production depends on the weather, which is never completely predictable, and that your electricity demand will also vary. These hurdles are fairly easy to clear. Meteorological data gives average monthly sunlight levels for different geographical areas. This takes into account rainfall and cloudy days, as well as altitude, humidity, and other more subtle factors. You should design for the worst month, so that you'll have enough electricity all year. With that data, and knowing your average household demand (your utility bill conveniently lets you know how much energy you use every month),there are simple methods you can use to determine just how many PV modules you'll need. You'll also need to decide on a system voltage, which you can control by deciding how many modules to wire in series. Obstacles You may have already guessed a couple of problems that we'll have to solve. First, what do we do when the sun isn't shining? Certainly, no one would accept only having electricity during the day, and then only on clear days, if they have a choice. We need energy storage -batteries. Unfortunately, batteries add a lot of cost and maintenance to the PV system. Currently, however, it's a necessity if you want to be completely independent. One way around the problem is to connect your house to the utility grid, buying power when you need it and selling to them when you produce more than you need. This way, the utility acts as a practically infinite storage system. The utility has to agree, of course, and in most cases will buy power from you at a much lower price than their own selling price. You will also need special equipment to make sure that the power you sell to your utility is synchronous with theirs -- that it shares the same sinusoidal waveform and frequency. Safety is an issue as well. The utility has to make sure that if there's a power outage in your neighborhood, your PV system won't try to feed electricity into lines that a lineman may think is dead. This is called islanding. If you decide to use batteries, keep in mind that they will have to be maintained, and then replaced after a certain number of years. The PV modules should last 20 years or more, but batteries just don't have that kind of useful life. Batteries in PV systems can also be very dangerous because of the energy they store and the acidic electrolytes they contain, so you'll need a well-ventilated, non-metallic enclosure for them. Deep-cycle Batteries What kind of batteries are used in PV systems? Although several different kinds are commonly used, the one characteristic that they should all have in common is that they are deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle batteries can discharge more of their stored energy while still maintaining long life. Car 6 batteries discharge a large current for a very short time -- to start your car -- and are then immediately recharged as you drive. PV batteries generally have to discharge a smaller current for a longer period (such as all night), while being charged during the day. The most commonly used deep-cycle batteries are lead-acid batteries (both sealed and vented) and nickel-cadmium batteries. Nickel-cadmium batteries are more expensive, but last longer and can be discharged more completely without harm. Even deep-cycle lead-acid batteries can't be discharged 100 percent without seriously shortening battery life, and generally, PV systems are designed to discharge lead-acid batteries no more than 40 percent or 50 percent. Also, the use of batteries requires the installation of another component called a charge controller. Batteries last a lot longer if care is taken so that they aren't overcharged or drained too much. That's what a charge controller does. Once the batteries are fully charged, the charge controller doesn't let current from the PV modules continue to flow into them. Similarly, once the batteries have been drained to a certain predetermined level, controlled by measuring battery voltage, many charge controllers will not allow more current to be drained from the batteries until they have been recharged. The use of a charge controller is essential for long battery life. DC to AC The other problem is that the electricity generated by your PV modules, and extracted from your batteries if you choose to use them, is direct current, while the electricity supplied by your utility (and the kind that every appliance in your house uses) is alternating current. You will need an inverter, a device that converts DC to AC. Most large inverters will also allow you to automatically control how your system works. Some PV modules, called AC modules, actually have an inverter already built into each module, eliminating the need for a large, central inverter, and simplifying wiring issues. Throw in the mounting hardware, wiring, junction boxes, grounding equipment, overcurrent protection, DC and AC disconnects and other accessories and you have yourself a system. Electrical codes must be followed (there's a section in the National Electrical Code just for PV), and it's highly recommended that the installation be done by a licensed electrician who has experience with PV systems. Once installed, a PV system requires very little maintenance (especially if no batteries are used), and will provide electricity cleanly and quietly for 20 years or more. 7 If photovoltaics are such a wonderful source of free energy, then why doesn't the whole world run on solar power? Some people have a flawed concept of solar energy. While it's true that sunlight is free, the electricity generated by PV systems is not. As you can see from our discussion of a household PV system, quite a bit of hardware is needed. Currently, an installed PV system will cost somewhere around $9 per peak Watt. To give you an idea of how much a house system would cost, let's consider the Solar House -- a model residential home in Raleigh, North Carolina, with a PV system set up by the North Carolina Solar Center to demonstrate the technology. It's a fairly small home, and it is estimated that its 3.6-kW PV system covers about half of the total electricity needs (this system doesn't use batteries -- it's connected to the grid). Even so, at $9 per Watt, this installed system would cost you around $32,000. That's why PV is usually used in remote areas, far from a conventional source of electricity. Right now, it simply can't compete with the utilities. Costs are coming down as research is being done, however. Researchers are confident that PV will one day be cost effective in urban areas as well as remote ones. Part of the problem is that manufacturing needs to be done on a large scale to reduce costs as much as possible. That kind of demand for PV, however, won't exist until prices fall to competitive levels. It's a Catch-22 situation. Even so, demand and module efficiencies are constantly rising, prices are falling, and the world is becoming increasingly aware of environmental concerns associated with conventional power sources, making photovoltaics a technology with a bright future. Appendix I: Light and the PV Cell We've looked at how to construct a solar cell using crystalline silicon. And we've used this basic type of cell to explain the photoelectric effect, which is the phenomenon operating at the heart of a solar cell. Here, we want to take a look at sunlight, the energy source actually used by solar cells. A brief discussion of several terms will help us better understand aspects of light's interaction with solar cells. Wavelength, Frequency, and Energy The energy from the sun is vital to life on Earth. It determines the Earth's surface temperature and supplies virtually all the energy that drives natural global systems and cycles. Some other stars are enormous sources of energy in the form of X-rays QuickTimeý Dz and radio signals, but our sun releases the TIFFÅià• èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• Å B majority of its energy as visible light. However, visible light represents only a fraction of the total spectrum of radiation. Specifically, infrared and ultraviolet rays are also significant parts of the solar spectrum. The sun emits almost all of its energy in a range of wavelengths from about 2x10-7 to 4x10-6 meters. Most of this energy is in the visible light region. Each wavelength corresponds to a frequency and an energy: the shorter the wavelength, the higher the frequency and the greater the energy (which is expressed in 8 electron-volts, or eV). Red light is at the low-energy end of the visible spectrum and violet light is at the high-energy end, where it has half again as much energy as red light. In the invisible portions of the spectrum, radiation in the ultraviolet region, which causes the skin to tan, has more energy than that in the visible region. Likewise, radiation in the infrared region, which we feel as heat, has less energy than the radiation in the visible region. Solar cells respond differently to the different wavelengths, or colors, of light. For example, crystalline silicon can use the entire visible spectrum, plus some part of the infrared spectrum. But energy in part of the infrared spectrum, as well as longer-wavelength radiation, is too low to produce current flow. Higher-energy radiation can produce current flow, but much of this energy is likewise not usable. In summary, light that is too high or low in energy is not usable by a cell to produce electricity. Rather, it is transformed into heat. Air Mass The sun is continually releasing an enormous amount of radiant energy into the solar system. The Earth receives a tiny fraction of this energy; yet, an average of 1367 watts (W) reaches each square meter (m2) of the outer edge of the Earth's atmosphere. The atmosphere absorbs and reflects some of this radiation, including most X-rays and ultraviolet rays. Still, the amount of the sun's energy that reaches the surface of the Earth every hour is greater than the total amount of energy that the world's human population uses in a year. How much energy does light lose in traveling from the edge of the atmosphere to the surface of the Earth? This energy loss depends on the thickness of the atmosphere that the sun's energy must pass through. The radiation that reaches sea level at high noon in a clear sky is 1000 W/m2 and is described as "air mass 1" (or AM1) radiation. As the sun moves lower in the sky, the light passes through a greater thickness (or longer path) of air, losing more energy. Because the sun is overhead for only a short time, the air mass is normally greater than one—that is, the available energy is less than 1000 W/m2. Scientists have given a name to the standard spectrum of sunlight at the Earth's surface: AM1.5G (where G stands for "global" and includes both direct and diffuse radiation, described next) or AM1.5D (which includes direct radiation only). The number "1.5" indicates that the length of the path of light through the atmosphere is 1.5 times that of the shorter path when the sun is directly overhead. The standard spectrum outside the Earth's atmosphere is called AM0, with no light passing through the atmosphere. AM0 is typically used to predict the expected performance of PV cells in space. The intensity of AM1.5D radiation is approximated by reducing the AM0 spectrum by 28%, where 18% is absorbed and 10% is scattered. The global spectrum is 10% greater than the direct spectrum. These calculations give about 970 W/m2 for AM1.5G. 9 However, the standard AM1.5G spectrum is "normalized" to give 1000 W/m2, because of inherent variations in incident solar radiation. Direct and Diffuse Light As we have noted, the Earth's atmosphere and cloud cover absorb, reflect, and scatter some of the solar radiation entering the atmosphere. Nonetheless, an enormous amount of the sun's energy reaches the Earth's surface and can therefore be used to produce PV electricity. Some of this radiation is direct and some is diffuse, and the distinction is important because some PV systems (flat-pate systems) can use both forms of light, but concentrator systems can only use direct light. • Direct light consists of radiation that comes straight from the sun, without reflecting off of clouds, dust, the ground, or other objects. Scientists also talk about direct-normal radiation, referring to the portion of sunlight that comes directly from the sun and strikes the plane of a PV module at a 90-degree angle. • Diffuse light is sunlight that is reflected off of clouds, the ground, or other objects. It obviously takes a longer path than a direct light ray to reach a module. Diffuse light cannot be focused by the optics of a concentrator PV system. • Global radiation refers to the total radiation that strikes a horizontal surface. Global sunlight is composed of direct-normal and diffuse components of sunlight. Additionally, diffuse and direct-normal sunlight generally have different energy spectra or distributions of color. QuickTimeý Dz TIFFÅià• èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB Insolation The actual amount of sunlight falling on a specific geographical location is known as insolation—or "incident solar radiation." Insolation values for a specific site are sometimes difficult to obtain. Weather stations that measure solar radiation components are located far apart and may not carry specific insolation data for a given site. Furthermore, the information most generally available is the average daily total—or global—radiation on a horizontal surface. When sunlight reaches the Earth, it is distributed unevenly in different regions. Not surprisingly, the areas near the Equator receive more solar radiation than anywhere else on the Earth. Sunlight varies with the seasons, as the rotational axis of the Earth shifts to lengthen and shorten days with the changing seasons. For example, the amount of solar energy falling per square meter on Yuma, Arizona, in June is typically about nine times greater than that falling on Caribou, Maine, in December. The quantity of sunlight reaching any region is also affected by the time of day, the climate (especially the cloud cover, which scatters the sun's rays), and the air pollution in that region. Likewise, these climatic factors all affect the amount of solar energy that is available to PV systems. 10 Appendix II: Various PV Systems A photovoltaic (PV) or solar cell is the basic building block of a PV (or solar electric) system. An individual PV cell is usually quite small, typically producing about 1 or 2 watts of power. To boost the power output of PV cells, we connect them together to form larger units called modules. Modules, in turn, can be connected to form even larger units called arrays, which can be interconnected to produce more power, and so on. In this way, we can build PV systems able to meet almost any electric power need, whether small or large. PV systems can be classified into two general categories: flat-plate systems or concentrator systems. We will talk about the differences between these two types of systems later on. By themselves, modules or arrays do not represent an entire PV system. We also need structures to put them on that point them toward the sun, and components that take the directcurrent electricity produced by modules and "condition" that electricity, usually by converting it to alternate-current electricity. We might also want to store some electricity, usually in batteries, for later use. All these items are referred to as the "balance of system" (BOS) components. Combining modules with the BOS components creates an entire PV system. This system is usually everything we need to meet a particular energy demand, such as powering a water pump, or the appliances and lights in a home, or, if the PV system is large enough, all the electrical requirements of a whole community. PV in Simple, "Stand-Alone" Systems Stand-alone PV systems are often best in places where utility-generated power is either unavailable (because the area is so remote from power plants), undesirable (because of a possible utility power outage in an emergency), or too costly to hook up to (because of the price of extending power lines). Stand-alone systems are also excellent for uses that don't require a lot of power. The same sunny days that dry out plants, make animals thirsty, and heat up buildings and cars just happen to be very good days for generating electricity with photovoltaics. And that's exactly what stand-alone PV systems (those not connected to a utility power grid) do every sunny day, and on some cloudy days, too, all over the world. The electricity is then used to power water pumps for irrigation and drinking wells, for example, or ventilation fans for cooling. For this reason, the most simple PV systems are those that generate direct-current (dc) electricity so it can be used right away to run water pumps, fans, and many other appliances that use dc electricity. QuickTi meý Dz TIFFÅià• èkǻǵÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB 11 These basic PV systems have several advantages that make them suitable for these jobs. First, they produce energy where and when it's needed, so complex wiring, storage, and control systems aren't needed. Second, small systems that produce less than 500 watts and weigh less than 68 kilograms (150 pounds) are easy to transport and install. Most installations take only a few hours. And, although pumps and fans require regular maintenance, PV modules require only an occasional inspection and cleaning. See the case study on PV-powered water for cattle for an example of a stand-alone system in action. PV for Utility Power Production When should utilities consider PV power? Actually, large-scale photovoltaic power plants, which consist of many PV arrays working together, can be very useful to utilities in a variety of ways. For example, utilities can build PV plants much more quickly than they can build conventional fossil or nuclear power plants, because PV arrays are fairly easy to install and connect. Also, utilities can build PV power plants where they're most needed in the grid, because siting PV arrays is usually much easier than siting a conventional power plant. And, unlike conventional power plants, modular PV plants can be expanded incrementally as demand increases. Finally, PV power plants consume no fuel and produce no air or water pollution while they silently generate electricity. This makes PV power an attractive option for utilities that want or need to cut fuel costs while meeting local environmental regulations. So, why aren't more utilities building PV plants? There are actually several reasons. For example, using current utility accounting practices, PV-generated electricity still costs more than electricity generated by conventional plants in most places, and regulatory agencies require most utilities to supply the lowest-cost electricity. Furthermore, photovoltaic systems produce power only during daylight hours, and their output thus can vary with the weather. Utility planners must therefore treat a PV power plant differently than they would treat a conventional plant. Despite these obstacles, more utilities are becoming more involved in PV power. For example, DOE, the Electric Power Research Institute, and several utilities have formed a joint venture called Photovoltaics for Utility-Scale Applications (PVUSA). This project operates three pilot test stations in different parts of the country for utility-scale PV systems. The pilot projects allow utilities to experiment with newly developing PV technologies with little financial risk. In another experiment, utilities are exploring connecting PV systems to the utility grid in places where they have a higher value. For example, adding PV generation near the places where the electricity is used prevents the energy losses associated with sending an electric current long distances through conventional power lines. This means the PV system is worth more to the utility when it can be placed near the customer. PV systems could also be installed at places in utility distribution system service areas where the population is increasing rapidly. In these places, using PV systems could eliminate 12 a utility's need to increase the size of power lines as well as entire servicing areas. Installing PV systems near other utility distribution equipment, such as substations, can also prevent overloading of the equipment in the substation. For an example of a utility-scale PV application, see the case study in this section. PV Connected to the Utility Grid Using grid-connected PV power can have economic as well as environmental advantages. Where utility power is available, consumers can use a grid-connected PV system to supply some of the power they need and use utility-generated power at night and on very cloudy days. When the PV system supplies power to the grid as well as to a specific building or piece of equipment, the utility becomes a kind of storage device or battery for PV-generated power. QuickTimeý Dz TIFFÅiLZWÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB Several homeowners, considered pioneers 屋根瓦型システム in the use of renewable energy, have sizable PV systems connected to the utility grid. They like that the system reduces the amount of electricity they purchase from the utility each month. They also like the fact that PV consumes no fuel and generates no pollution. The owner of a grid-connected PV system can often sell as well as buy electricity each month! This is because electricity generated by the PV system can be used on site or fed through a meter into the utility grid. When a home or business requires more electricity than 13 the PV array is generating (for example, in the evening), the need is automatically met by utility power. When that home or business requires less electricity than the PV array is generating, the excess can often be fed (or sold) back to the utility through net metering, which is becoming more and more common throughout the nation. At the end of the month, a credit for electricity sold is deducted from charges for electricity purchased. See also the case study of an energy-efficient home for an example of this. PV Systems with Battery Storage PV systems with batteries for storage are excellent for supplying electricity when and where you need it. These systems are especially suitable in areas where utility power is unavailable or utility line extensions would be too expensive. The ability to store PV-generated electrical energy makes the PV system a reliable source of electric power both day and night, rain or shine. PV systems with battery storage are used all over the world to provide electricity for lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools! QuickTimeý Dz TIFFÅiLZWÅj êLí£ÉvÉçÉOÉâÉÄ Ç™Ç±ÇÃÉsÉNÉ`ÉÉǾå©ÇÈǞǽDžÇÕïKóvÇÇ• ÅB Speaking about the "Solar Independence" PV system, John Thornton, engineer at the National Center for Photovoltaics, says, "The objective is to raise people's awareness about the value of these technologies. And the only way to do that is to show them the technology." PV systems with batteries can be designed to power equipment that requires dc or ac electricity. People running conventional ac equipment will add a power conditioning device called an inverter between the batteries and the load. Although a small amount of energy is lost in converting dc electricity to ac, an inverter makes PV-generated electricity behave like utility power so it can operate everyday ac appliances, lights, and even computers. We operate PV/battery systems by connecting the photovoltaic modules to a battery, and the battery, in turn, to the load. During the day, the PV modules charge the battery, and then the battery supplies power to the load as needed. A simple electrical device called a charge controller keeps the batteries charged properly and helps prolong their life by protecting them from overcharging or from being completely drained. Batteries make PV systems useful in more situations, but also require some maintenance. The batteries used in PV systems are similar to car batteries, but they're built somewhat differently to allow more of their stored energy to be used each day. (They're said to be "deep cycling," like the batteries used on golf carts.) Batteries designed for PV projects pose the same risks and demand the same caution in handling and storage as automotive batteries. We need to check the fluid in unsealed batteries periodically, and protect them from extremely cold weather. 14 The amount of electricity that can be used after sunset or on cloudy days is determined by the output of the PV modules and the nature of the battery bank. Including more modules and batteries increases system costs, so energy usage needs to be studied carefully to determine the best system size for the load. A well-designed PV-battery system balances cost and convenience with meeting the user's needs, and it can be expanded if those needs change. See an example of a PV-battery system used in a modern residence for more information. Appendix III: Solar Cell Performance We measure the performance of a PV device to predict the power the cell will produce. Current-voltage (I-V) relationships that measure the electrical characteristics of PV devices are depicted by what we call "I-V curves." These I-V curves are obtained by exposing the cell to a constant level of light, while maintaining a constant cell temperature, varying the resistance of the load, and measuring the current that is produced. On an I-V plot, the vertical axis refers to current and the horizontal axis refers to voltage. The actual I-V curve typically passes through two significant points: • The short-circuit current (Isc) is the current produced when the positive and negative terminals of the cell are short-circuited, and the voltage between the terminals is zero, which corresponds to a load resistance of zero. • The open-circuit voltage (Voc) is the voltage across the positive and negative terminals under open-circuit conditions, and the current is zero, which corresponds to a load resistance of infinity. The cell may be operated over a wide range of voltages and currents. By varying the load resistance from zero (a short circuit) to infinity (an open circuit), we can determine the highest efficiency as the point where the cell delivers maximum power. Remember that power is the product of voltage times current. Therefore, on the I-V curve, the maximum-power point (Pm) occurs where the product of current times voltage is a maximum. No power is produced at the short-circuit current with no voltage, or at open-circuit voltage with no current. So we expect to find maximum power generated somewhere between these two points. Maximum power is generated at only one place on the power curve, at about the "knee" of the curve. This point represents the maximum efficiency of the solar device in converting sunlight into electricity. A parameter known as fill factor measures the "squareness" of the I-V curve and describes the degree to which the voltage at the maximum power point (Vmp) matches Voc and that the current at the maximum power point (Imp) matches Isc. The higher the fill factor's percentage or match, the "squarer" the curve. The conversion efficiency of a solar cell is the percentage of the solar energy shining on a PV device that is converted into electrical energy, or electricity. Improving this conversion efficiency is a key goal of much research and helps to make PV technologies cost competitive with more traditional sources of energy. The efficiency of solar cells is affected by a variety of factors. For an example, solar cells work best at low temperatures, as determined by their material properties. All cell materials lose efficiency as the operating temperature rises. Much of the light energy shining on cells becomes heat, so it is good to either match the cell material to the operation temperature or continually cool the cell. 15 Appendix IV: Why Solar Photovoltaics ? The world population is now rapidly increasing mainly in developing countries toward the mid 21st century. According to the prediction by the United Nations, it will reach 10 billion by the year 2050. To support this ever-increasing population, and to provide overall improvements in the living standard, the world economy will have to continue to grow at an annual rate of some 3 percents over this period. Along with this economic growth, the global energy demand around the year 2050 will increase to two or three times that in 2000 in terms of oil consumption, which is anticipated to be completely exhausted within these 100 years. An efficient utilization of alterenative & renewable energy resources including solar energy is then the most important technological evolution to resolve the energy issue as well an the environmental issue. The wide-spreaded application of solar cells is indispensable not only to accelerate the acceptance of renewable energy technologies but also to meet the worldwide demands for environmentally allowable electricity power generation. Scientists estimate that the solar radiation falling on the Earth is only 1/22-billionths of the energy that the sun emits. Yet just one hour of the energy that the Earth receives is equal to the total amount of energy consumed by humans in one year. Solar radiation can be utilized in various forms. Examples of direct utilization on a comparatively large scale include photovoltaic and solar heat power generation. Wind, hydroelectric and wave power generation can be considered examples of energy derived from solar radiation. Clean: The combustion of fossil fuels results in the production of gases such as carbon dioxide and nitrogen dioxide. These gases have a significant, negative influence on the Earth's environment. Solar energy utilization, however, produces very little exhaust gas and is extremely clean. Infinite: Many resources on Earth exist in limited amounts. For instance, it is estimated that petroleum will be exploitable for another 43 years, coal for 174 years, natural gas for 56 years, and uranium for 66 years. Solar energy, however, will exist virtually forever. To achieve practical utilization of solar energy, the following characteristics present a significant challenge to technological development and must be taken into account. Wide fluctuation: Insolation varies greatly according to climate and day-to-day weather conditions. Low density: Although the amount of energy falling on the Earth is vast, its density is 1 kW/m2, lower than would be expected. 16