ELECTRICITY FROM THE SUN
The same sunny days that dry out plants, make animals thirsty, and heat up buildings and cars are also good days for generating electricity with photovoltaics. This electricity can be used to power water pumps for irrigation and drinking wells, and ventilation fans for air cooling.
The sunlight that creates the need for water and ventilation can be harnessed using the most basic PV systems to meet those same needs. Photovoltaic modules produce the most electricity on clear, sunny days. Simple PV systems use the dc electricity as soon as it is generated to run water pumps or fans.
These basic PV systems have several advantages for the special jobs they do. The energy is produced where and when it is needed, so complex wiring, storage, and control systems are unnecessary. Small systems, under 500 watts (W), weigh less than 68 kilograms (kg) (150 pounds [lb]), making them easy to transport and install. Most installations take only a few hours. And, although pumps and fans require regular maintenance, the PV modules require only an occasional inspection and cleaning.
Water for cattle
Oliver Romey chose PV power to pump water to his drought-stricken ranch in South Dakota.
In the gumbo soil country of South Dakota there is not much water underground, so when rancher Oliver Romey s stock dams went dry in 1990 he had a hard time finding a new source of water. When he found water on his land, the well was 2.4 kilometers (km) (1.5 miles [mi]) from the power line. Extending the line to power his pump would have cost $18,000. So, like many people in the area, Romey hauled water to his cattle in tank trucks each day. After two seasons of hauling water, he read about solar-powered pumping provided by the Northwest Rural Public Power District.
The Northwest Rural Public Power District, along with many utilities around the country, offers customers the choice of running new power lines or installing a PV system. PV water pumping is particularly popular in rural areas because, although the electricity is used as it is generated, the water can be stored in tanks and reservoirs. (Storing water is much cheaper than storing electricity.)
Romey chose PV, and the utility installed 20 PV modules designed to work with the dc pump and pipeline size Romey required. He now pays the utility a monthly fee, which covers the system s installation, insurance, maintenance, and a return on the utility s investment.
The PV system pumps water through his 9 km (5.6 mi) of pipeline to four stock tanks that supply over 150 head of cattle. And with the new pipeline, he can graze cattle on two fields that he could not use before.
For the first time in years, I don t have to worry about getting water to my cattle, Romey says. I think this solar system is just as amazing as when the REA (Rural Electrification Administration) first brought electricity out to the ranch.
Water for trees
The City of Littleton, in Colorado, chose PV to irrigate 1400 newly planted trees along the South Platte River.
In 1992, Littleton launched an ambitious project to turn wasteland into parkland along the South Platte River near Denver, Colorado. Naming the project 10,000 Trees, the city planted 1400 native trees in former gravel mining areas along the river. But the city needed to water these saplings set out along a 2.7-km (1.7-mi) stretch of bike path for several years to get them established. And the setting in this wildlife refuge had to stay as natural as possible.
Before choosing PV, the city considered three energy options for pumping water. It decided against a diesel or gasoline generator because they are too noisy and demand too much attention from park department personnel. It decided against expanding the utility power line because the city did not want the expense or visual impact of power lines running over the park, and it only needed the power for a couple of years until the trees were established.
At the suggestion of the local utility, Public Service Company of Colorado, the city chose a PV-powered irrigation system. The system is quiet, requires no overhead wires, and can be moved to a new project once the trees are established. Although the PV system cost was greater than the cost of running wires to the utility line, its environmental advantages and portability made it more attractive to the city.
The PV system provides electricity during daylight hours to pump water from a small pond to water the trees. There is no need to store electricity or water, so the system is very simple a dc motor and pump float on the pond. The five PV arrays are mounted on trackers that follow the sun during the day to increase electrical output.
The townspeople are very pleased with how the solar panels look, says Littleton Mayor Susan Thornton. The PV modules quietly do their job with no muss, no fuss, and no bother. In fact, the city is so pleased it has installed several smaller PV-powered park irrigation systems in other parts of Littleton.
In the most remote and hostile environments, PV-generated electrical energy stored in batteries can power a wide variety of equipment.
Storing electrical energy makes PV systems a reliable source of electric power day and night, rain or shine. PV systems with battery storage are being used all over the world to power lights, sensors, recording equipment, switches, appliances, telephones, televisions, and even power tools.
PV systems with batteries can be designed to power dc or ac equipment. People who want to run conventional ac equipment 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 to ac, an inverter makes PV-generated electricity behave like utility power to operate everyday ac appliances, lights, and even computers.
PV systems with batteries operate by connecting the PV modules to a battery, and the battery, in turn, to the load. During daylight hours, the PV modules charge the battery. The battery supplies power to the load whenever 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 often similar to car batteries, but are built somewhat differently to allow more of their stored energy to be used each day. (They are 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. The fluid in unsealed batteries should be checked periodically, and batteries should be protected from extremely cold weather.
A solar generating system with batteries supplies electricity when it is needed. How much electricity 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 cost, so energy usage is carefully studied to determine optimum system size. A well-designed system balances cost and convenience to meet the user s needs, and can be expanded if those needs change.
Electricity for a modern home
When the Chases moved to their PV-powered home in the wilds of southwest Florida, they were able to bring along all of their electrical appliances.
When Joyce and George Chase discovered it would cost $15,000 to extend a utility line to their new home on the Peace River in southwest Florida, they decided instead to use a stand-alone PV system to power their appliances and lights. George, whose weekend cabin on the same site had been powered by a stand-alone PV system and battery, designed the house from scratch with the cooperation of the local building inspector.
Today, a visitor to this modern 185-m2 (2000-ft2 ) home would never guess there is no utility power. All fixtures, outlets, and electrical appliances are standard. Although most of the Chases appliances operate on dc, an inverter converts some of their dc power stored in batteries to ac for those that require ac power. George put the battery and control rooms in a central spot that shortened the length of wire needed, keeping costs down and increasing efficiency. Based on advice from the inspector, George installed larger-gauge wire for the dc circuits. Today, the Chases have all the conveniences of the city, from a hair dryer and a microwave oven, to a clothes washer, TV, and ceiling fans.
The PV system has operated without a problem since 1988, and is maintained with little effort. George says that he could easily forget to check on the system because there are no moving parts to squeak, grind, or use fuel. But every month he checks the water level and chemistry of the batteries. Twice a year he changes the tilt angle on the roof-mounted PV modules a nearly flat angle for summer when the sun is high, and a steeper angle for winter when the sun is lower in the sky. If prolonged cloudy weather blocks the sun, George runs a battery charger he rigged up to his tractor for a few hours and keeps the battery bank charged.
Joyce says she likes the simplicity of the system. The photovoltaic modules produce more than enough power for our needs, without gauges and switches all over the place. Our home is proof that you can use solar energy and not sacrifice modern convenience or beauty.
The Chessie System Railway chose PV as a power source for a remote signal in West Virginia because of its reliability.
Signals that control the operation of trains are crucial to the safe and efficient use of railroad tracks, and the power source for these signals must be completely reliable. Where utility power is available, signals have a bank of rechargeable batteries to draw from, in case the utility line goes down. Where it would be too costly to bring in a power line, primary batteries need to be replaced every 6 to 18 months, depending on railway traffic. By the mid-1980s, the cost to dispose of the used primary batteries was becoming greater than their original purchase price, so railroads were looking for another way to power signals far from utility lines.
In 1985, the Chessie System Railway (now CSX Transportation) needed a power source for a large new signal in a remote part of West Virginia. Because the power line was too far away, and the relatively large power load imposed by the signal would be difficult to serve with primary batteries, the train control department at Chessie decided on a PV system and rechargeable batteries.
Railway engineers sized the PV array and battery bank for a worst case weather scenario 20 days without sunlight. To aid in the system sizing, a computer program simulated the site conditions, including weather, sunlight, and signal usage. The engineers also incorporated especially rugged components to discourage vandals. The PV arrays are supported on sturdy aluminum frames designed to mount on the standard wooden poles and crossbeams used by the railroad. The modules have protective back plates and enclosed wiring connections. In addition, the batteries are housed in a concrete bunker, which protects them from vandalism and from weather.
The PV-powered signal has operated reliably since 1985. Railway personnel check out the PV installation twice a year, and, according to the installer, the rechargeable batteries are expected to last 10 years or longer.
The railroad has now installed more than 75 PV systems, and CSX Transportation routinely considers PV along with other power options for new signals. Furthermore, the company regularly upgrades old primary batteries with PV-powered, rechargeable systems to save money.
The cost of electricity from larger systems, those able, for example, to power a ;modern home, is evaluated according to the cost per kilowatt hour (kWh). The cost depends on the initial cost, interest on the loan (for paying the initial cost), the cost of system maintenance, the expected liftime of the system, and how much electricity it produces. Using typical borrowing costs and equipment life, the cost of PV-generated energy in 1993 ranged from $0.20 to $0.50/kWh.
Working together, PV and other electric generators can meet more varied demands for electricity, conveniently and for a lower cost than either can meet alone.
When power must always be available or when larger amounts of electricity than a PV system alone can supply are occasionally needed, an electric generator can work effectively with a PV system to supply the load. During the daytime, the PV modules quietly supply daytime energy needs and charge batteries. If the batteries run low, the engine-generator runs at full power its most cost- and fuel-efficient mode of operation until they are charged. And in some systems the generator makes up the difference when electrical demand exceeds the combined output of the PV modules and the batteries.
Systems using several types of electrical generation combine the advantages of each. Engine-generators can produce electricity any time. Thus, they provide an excellent backup for the PV modules which produce power only during daylight hours when power is needed at night or on cloudy days. On the other hand, PV operates quietly and inexpensively, and does not pollute. Using PV and generators together can also reduce the initial cost of the system. If no other form of generation is available, the PV array and the battery storage must be large enough to supply nighttime electrical needs. However, having an engine-generator as backup means fewer PV modules and batteries are necessary to supply power whenever it is needed.
Including generators makes designing PV systems more complex, but they are still easy to operate. In fact, modern electronic controllers allow such systems to operate automatically. Controllers can be set to automatically switch generators or to supply ac or dc loads or some of each. In addition to engine generators, electricity from wind generators, small hydro plants, and any other source of electrical energy can be added to make a larger hybrid power system.
Electricity for a telephone signal booster
At a remote signal station in the mountains of northern Nevada, Sprint Communications decided to reduce the runtime on its propane-powered electric generators by adding PV modules.
When Sprint Communications built its east-west fiber-optic cable line in the late 1980s, it needed a regenerator station every 35 km (22 mi) to boost the signal. These regenerator stations use electricity 24 hours a day, 7 days a week to power the transmission equipment and to control shelter temperatures. Utility power serves most of the stations, which have 2 hours of backup battery capacity should the power line go down.
But at Sand Pass, Nevada, 2 hours north of Reno, bringing in utility power was prohibitively expensive. When Sprint built a regenerator station there in 1986, it installed two propane-powered electric generators, or gen-sets. To maintain the gen-sets a mechanic drove out to the pass every month, and to provide fuel, a teamster hauled a heavy propane tank over rough dirt roads to the site every 3 months.
Concerned with reducing these maintenance expenses, Rich Coakley, a network engineer at Sprint Communications, read an article describing PV-powered telecommunications sites in Peru. Like Sand Pass, these sites operated unattended and the PV systems kept the battery banks charged in a hostile environment. Coakley set out to have a PV system installed at the Sand Pass station.
Installation took less than a day. The equipment arrived, preassembled and securely packed, in a refurbished metal shipping container the kind used to hold goods traveling on container ships and flatbed trucks. The container now serves as the support structure and equipment shelter at the site.
The PV array and propane gen-sets at Sand Pass complement each other. The PV array charges batteries that power the dc transmission equipment. The gen-sets run to power the ac motor in the air-conditioning unit only when the shelter thermostat calls for air conditioning. As added backup, the gen-sets can charge the batteries if necessary. Relieving the gen-sets of their everyday battery-charging duties has also reduced runtime. Thus, the PV system has significantly lowered fuel and maintenance costs for the station.
Electricity for a village power system
For 25 years the village of Xcalak, in Mexico, had struggled to keep diesel generators running. Then the village decided to provide itself with a more reliable source of electric power by connecting PV modules to the generating system.
The people of Xcalak, a remote village on the east coast of the Yucatan peninsula, wanted to replace their diesel generators because it was expensive to ship fuel to the village and because the generators kept breaking down. Located 110 km (68 mi) from the nearest utility line, this village of 350 could not convince the utility to spend $3.2 million for a line extension. But when the villagers turned to their government, a bold new solution emerged add solar electric modules and wind generators to the existing diesel system to make a large hybrid electri- cal generating system.
For Xcalak, the hybrid system offered many advantages. Construction costs would be a fraction of those required for a line extension, and fuel and maintenance expenses for the hybrid system would be lower than for running the diesels alone. The system itself would have greater generating capacity, thus providing more homes and businesses with electric power. And the hybrid system would be more reliable than diesel generators operating alone because it includes multiple backup generation and a larger battery bank. The villagers would get what they had been demanding for 25 years reliable electricity 24 hours a day.
The generation system was installed in 1992 with the support of the national government of Mexico, the local utility, and the village of Xcalak. The state of Quintana Roo and the national government of Mexico paid for the initial cost of the project. The utility updated and expanded the village wiring system to accommodate increased generating capacity. The village pays for system operation and maintenance.
Xcalak s hybrid electric system uses PV, wind turbines, and diesel to generate power. Batteries store electric energy produced by the PV and wind turbines for use 24 hours a day. An inverter converts the dc from the batteries to the ac provided to village residents. The PV arrays generate electricity during the day, and the wind turbines charge batteries whenever the wind blows. (The winds are usually best at night.) If the batteries become drained after a series of cloudy, windless days, or if the demand for electricity increases, the village can choose to run the diesel generator.
In remote villages the world over, providing reliable electric power can improve the economy and the quality of life for village residents. For Xcalak, adding renewable energy generators to their diesel system lowered operating costs and increased the reliability of electric service. When the hybrid system went on-line in 1992, it powered 80 homes, four restaurants, and one 20-room hotel. The village tourist industry centers around the hotel and restaurants, and its fishing industry benefits from reliable refrigeration all without being connected to the central utility grid.
Where utility power is available, a grid-connected PV system can <%-1>supply some of the energy needed and use the utility in place of batteries.
The owner of a grid-connected PV system buys and sells electricity each month. Electricity generated by the PV system is either used on site or fed through a meter into the utility grid. When a home or business requires more electricity than the PV array is generating, for example, in the evening, the need is automatically met by power from the utility grid. When the home or business requires less electricity than the PV array is generating, the excess is fed (or sold ) back to the utility. Used this way, the utility backs up the PV like batteries do in stand-alone systems. At the end of the month a credit for electricity sold gets deducted from charges for electricity purchased.
Utilities are required to buy power from owners of PV systems (and other independent producers of electricity) under the Public Utilities Regulatory Policy Act of 1978 (PURPA). An approved, utility-grade inverter converts the dc power from PV modules into ac power that exactly matches the voltage and frequency of the electricity flowing in the utility line, and also meets the utility s safety and power quality requirements. Safety switches in the inverter automatically disconnect the PV system from the line if utility power fails. This safety disconnect protects utility repair personnel from being shocked by electricity flowing from the PV array into what they would expect to be a dead utility line.
In addition to cooperating under PURPA, utilities are establishing rate structures that may make PV grid-connected systems more economical. (At today s prices, when the cost of installing a utility-connected PV system is divided by the amount of electricity it will produce over 30 years, PV- generated electricity is almost always more expensive than power supplied by the utility.) For example, some utilities charge higher prices at certain times of the day. In some parts of the country, the highest charges for electricity under this time-of-day pricing structure are now nearly equal to the cost of energy from PV. The better the match between the electrical output of the PV modules and the time of highest prices, the more effective the system will be in reducing utility bills.
A retired couple in southeastern Massachusetts commissioned the design of a PV-powered, all-solar home because they were concerned about the environment and wanted to be self-reliant. They did not want to maintain a battery bank at their home and utility power was available, so they specified a utility-connected photovoltaic system.
The privately financed home, completed in 1980, incorporates many energy-conserving features. Built largely of masonry, concrete, and steel with earth berms against the north, east, and west sides, it stays warm in winter and cool in summer. The south wall of the house is nearly all glass to soak up the winter sun and the overhang of the roof provides shade to the south-facing glass in the summer. A solar thermal water heater takes care of all the owners hot water needs and also helps heat the house.
The photovoltaic system generates most of the electricity the couple needs. The PV modules occupy 40 m2 (430 ft2 ) of the south-facing roof. The dc electricity from the modules passes through an inverter for conversion to utility-grade ac power. Most of this electricity is used in the home and the excess is sold to the utility.
The homeowners have what is called a net-metering arrangement with their electric utility. The home has a standard electric meter that spins forward when the house is drawing electricity from the utility and backward when the PV system is feeding excess electricity into the grid. At the end of the month, they pay for any net forward movement of the meter, or get a credit if there was net backward motion.
Ever since the system was installed in 1980, the owners have found that the home is nearly energy self-sufficient over the calendar year. During the summer months, the PV system generates more electricity than the house uses. The surplus power spins their electric meter backward as PV-generated electricity passes out into the grid. In the winter, the house uses more electricity than the PV system produces, so some power flows in from the utility grid.
The homeowners are modest in their electricity use, yet they operate a refrigerator/freezer, electric clothes dryer, and electric oven, along with lighting, power tools, TV, stereo, and various small appliances. The integration of photovoltaics with energy-conscious design allows them to live lightly on the earth in a comfortable home.
Electricity for demand-side management
In Albany, New York, Niagara Mohawk Power Corporation is exploring how a grid-connected PV system can help even out a building s electrical demand.
Electric utilities must have the capability to supply power to all their customers. During certain times such as hot afternoons when many air conditioners are running demand is particularly high. These peak demand spikes are costly to serve because the utility must fire up peaking power plants to supply demand for just a few hours a day. Peaking power plants are usually the most expensive for a utility to operate. In addition, the utility s electric distribution system has to be sized to handle these high, short-term loads.
Utilities have tried various ways to smooth out electrical demand, including demand-side management (DSM). Utility DSM tools include encouraging energy conservation, charging higher rates during peak demand times, and charging higher rates (demand charges) to those who have all their appliances turned on at once.
Installing grid-connected photovoltaic arrays on rooftops may become a good utility DSM tool. The PV-generated electricity is used directly to help supply a building s peak demand, which often nearly coincides with times when the sun is shining the brightest. For example, most summer-peaking utilities experience the highest demand on their systems because of air conditioning loads on hot, sunny days. These are the same days the output of PV systems is also high.
To evaluate PV for DSM, Niagara Mohawk Power Corporation and the Empire State Electric Energy Research Corporation installed a PV system in 1990 on an energy-efficient office building in Albany, New York. The array is an experimental system and is considerably smaller than the one that would be needed to completely offset this building s peak demand. Nevertheless, the system s output, although a small part of the building s total demand, corresponds to the time when the peaks in demand occur.
The Albany system is thus demonstrating PV s potential to help utilities reduce peak demand. It has been operating as designed, with no major problems. Because afternoon clouds were reducing the PV system s effect on peak demand somewhat, the Niagara Mohawk project team added batteries to the system. The batteries store electricity to help reduce peak demand on cloudy afternoons. The continued study of this system will shed further light on the benefits of PV for DSM for utilities and their customers.
Utilities are exploring PV to expand generation capacity and meet increasing environmental and safety concerns.
Large-scale photovoltaic power plants, consisting of many PV arrays installed together, can prove useful to utilities. Utilities can build PV plants much more quickly than they can build conventional power plants because the arrays themselves are easy to install and connect together electrically. Utilities can locate PV plants where they are most needed in the grid because siting PV arrays is much easier than siting a conventional power plant. And unlike conventional power plants, 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.
Unfortunately, PV generation plants have several characteristics that have slowed their use by utilities. Under current utility accounting, PV-generated electricity still costs considerably more than electricity generated by conventional plants, while regulatory agencies require most utilities to supply electricity for the lowest cash cost. Furthermore, photovoltaic systems produce power only during daylight hours and their output varies with the weather. Utility planners must therefore treat a PV power plant differently than a conventional plant in order to integrate PV generation into the rest of the utility s generation, transmission, and distribution systems.
On the other hand, utilities are becoming more involved with PV. For example, Photovoltaics for Utility-Scale Applications (PVUSA) a joint venture between DOE, the Electric Power Research Institute (EPRI), and several utilities operates three pilot test stations in different parts of the country for utility-scale PV systems. These 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 locations where they have a higher value. For example, adding PV generation near where the electricity is used avoids the energy losses resulting from sending current long distances through the power lines. Thus, the PV system is worth more to the utility when it is located near the customer.
PV systems could also be installed at locations in the utility distribution system that are servicing areas whose populations are growing rapidly. Placed in these locations, the PV systems could eliminate the need for the utility to increase the size of the power lines and servicing area. Installing PV systems near other utility distribution equipment such as substations can also relieve overloading of the equipment in the substation.
Electric, gas, and water utilities have been using small PV systems economically for several years. Most of these systems are less than 1 kW and use batteries for energy storage. These systems are performing many jobs for utilities, from powering aircraft warning beacons on transmission towers to monitoring air quality of fluid flows. They have demonstrated the reliability and durability of PV for utility applications and are paving the way for larger systems to be added in the future.
Electricity for a utility substation
Pacific Gas & Electric Company (PG&E) needed more capacity on hot afternoons at its Kerman substation in the San Joaquin Valley, California. PG&E installed a PV electric generating system at Kerman instead of replacing the substation.
Pacific Gas and Electric Company s Kerman substation, near Fresno, California, was becoming overloaded on hot summer days. This is the time when air conditioning and water pumping needs are at their greatest and the utility experiences its peak demand. Overloading heats up substation components, which shortens the life of expensive transformers, reduces power quality for customers, and increases line losses during transmission. Analysts in PG&E s research group argued that a PV array could meet the extra demand at Kerman, because it occurred on days when the solar cells would be producing well.
The analysts at PG&E believed using PV to support a substation during peaks in electrical demand might make economic sense for their utility, but no one knew quantitatively how much the PV grid support would be worth. Because upgrading the Kerman substation would cost several million dollars, PG&E as part of the PVUSA project decided to install a PV system as a research project to evaluate the benefits of grid support.
The PV system installed at Kerman in 1993 produces the most power on sunny summer afternoons, when PG&E experiences its peak demand and when electricity has the highest value to the utility. The PV system maintains its electrical output close to its rated capacity with the PV modules mounted on trackers that change the tilt of the modules as the sun moves across the sky during this critical period.
The Kerman substation experiment is designed to measure the value to the utility of a 500-kW generating plant that can be quickly (within 6 months) placed where extra power is needed. Monitors record electrical output throughout the day and provide detailed information on system output. The results of this experiment will be widely distributed among utilities across the country.
Each year solar electric generating systems offer people more solutions to their energy problems.
As PV technology continues to improve, it steadily moves into new and larger markets. PV systems have been the best choice for many jobs since the first commercial PV cells were developed.
For example, PV cells have been the exclusive power source for satellites orbiting the earth since the 1960s. PV systems have been used for remote stand-alone systems throughout the world since the 1970s. In the 1980s, commercial and consumer product manufacturers began incorporating PV into everything from watches and calculators to music boxes. And in the 1990s, many utilities are finding PV to be the best choice for thousands of small power needs. During the next decade, a large part of the world s population will be introduced to electricity produced by PV systems. These PV systems will make the traditional requ
Written by: National Renewable Energy Laboratory
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