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PV in Use: Getting the Job Done with Solar Electricity

http://www.eere.energy.gov/solar/pv_use.html

Modern solar electric power-generation systems such as photovoltaics (or PV) are some of the most elegant and environmentally benign energy systems ever invented. But do people actually use them? The answer is yes�and not just in space. PV systems can also be found in the most isolated spots on Earth as well as in the heart of some of our largest cities. And every place in between.

Today's PV systems are used to generate electricity to pump water, light up the night, activate switches, charge batteries, supply power to the utility grid, and much more. PV has so many uses today that it probably already touches your life in some way. You might have noticed the small PV systems attached to emergency telephones along the highways. But PV provides power in many ways we can't see�for all kinds of satellites in space, including those that keep modern communication systems "up and running."

To make PV systems even more efficient, affordable, and available, the U.S. Department of Energy (DOE) and its partners in universities and industry continue to conduct advanced research and development in this exciting, important energy technology. Here, we describe some of PV's current major uses, or applications, grouped in these categories:

• PV in Simple, "Stand-Alone" SystemsPV Systems with Battery Storage PV Systems with Backup Generator PowerPV and Hybrid Power SystemsPV Connected to the Utility GridPV Systems and Net MeteringPV for Utility Power Production

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.

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 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!

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.

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.

PV Systems with Generators

What about situations in which remote or non-grid-connected power is needed, but that power must always be available�for example, to keep vaccines cold, or a rural clinic's lights on, or communications equipment running continuously? Or the times when users know they'll occasionally need a larger amount of power than a PV system can supply alone�say, in a national park camping ground, where it's difficult to predict how much power will be needed? In those cases, PV is still a practical choice. We just need to add an electric generator that can work effectively with a PV system to supply the load.

During the day, 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 the batteries 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. See our case study on a telephone signal booster for a good example of such a remote application.

Systems that use several types of power generation have the advantages of each one. Engine generators can produce electricity any time. So, they provide an excellent backup at night or on cloudy days for the PV modules, which produce power only during daylight hours. The advantages of a PV system are that it operates quietly and does not pollute. As to the rather high initial cost, we can select a smaller PV system if we operate it with a generator.

Where no other form of power generation is available, the PV array and the battery storage have to be large enough to supply nighttime electrical needs. However, having an engine generator as a backup means fewer PV modules and batteries will be necessary to supply power whenever it's needed.

Including a generator makes designing a PV system more complex, but it's still easy to operate. In fact, modern electronic controllers allow these kinds of systems to operate automatically. Controllers can be set to automatically switch generators, to supply > loads, or to do some of each. Wind generators, small hydro plants, and any other source of electrical energy could also be added to make an even larger hybrid power system.

PV in Hybrid Power Systems

In hybrid power systems, a number of electricity production and storage elements are combined to meet the energy demand of a remote facility (such as seismic measurement equipment), a rural home, a ranch or farm, or even a whole community. In addition to PV systems, engine generators, wind generators, small hydro plants, and others source of electrical energy can be added as needed to meet the energy demand in a way that fits in with the local geography and other specifics. Hybrid systems are ideal for remote applications such as communication stations, military installations, and rural villages.

Before developing a hybrid electric system, it is essential to know the particular energy demand and the resources available at the site. Energy planners therefore must study the solar energy, wind, and other potential resources at the site, in addition to the proposed energy use. This will allow them to design the kind of hybrid system that best meets the demands of the facility, home, or community, as our case study shows.

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.

Several homeowners, considered pioneers 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 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 and Net Metering

Net metering is a policy that allows homeowners to receive the full value of the electricity that their solar energy system produces. The term net metering refers to the method of accounting for a photovoltaic (PV) system's electricity production, for example. Homeowners with PV systems can thus offset their electric bill with any excess electricity they produce. As the homeowner's PV system produces electricity, the kilowatts are used first to meet any electric requirements (e.g., appliances, lights) in the home. If more electricity is produced from the PV system than the home needs, the extra kilowatts are fed into the utility grid.

Under federal law, utilities must allow independent power producers to be interconnected with the utility grid, and utilities must purchase any excess electricity they generate. Many states have gone beyond the minimum requirements of the federal law by allowing net metering for customers with PV systems. With net metering, the customer's electric meter will run backward when the solar electric system produces more power than is needed to operate the home or business at that time. An approved, utility-grade inverter converts the dc power from the PV modules into ac power that exactly matches the voltage and frequency of the electricity flowing in the utility line; the system must also meet the utility's safety and power-quality requirements. The excess electricity is then fed into the utility grid and sold to the utility at the retail rate.

In the event of a power outage, safety switches in the inverter automatically disconnect the PV system from the line. 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.

At the end of the month, if the customer has generated more electricity than that used, the utility credits the net kilowatt-hours produced at the wholesale power rate. But if the customer uses more electricity than the PV system generates, the customer pays the difference. The billing period for net metering may be either monthly or annually. In some states, the excess generation credits at the end of each billing period are carried over to the next billing period for up to a year.

Net metering allows homeowners who are not home when their systems are producing electricity to still receive the full value of that electricity without having to install a battery storage system. Essentially, the power grid acts as the customer's battery backup, which saves the customer the added expense of purchasing and maintaining a battery system.

Generally, the preferred method of accounting for the electricity under net metering is with a single, reversible meter. An alternative is dual metering, in which customers or their utility purchase and install two non-reversing meters that measure electrical flow in each direction. This adds significant expense to a PV system, however. The current trend around the country is toward a single, reversible meter.

Some utilities are opposed to net metering because they believe it may have a negative financial impact on them. However, a number of studies have shown that net metering can benefit utilities. These benefits include reductions in meter hardware and interconnection costs, as well as in meter reading and billing costs. Grid-connected PV systems can also help utilities avoid the cost of additional power generation, increase the reliability and quality of electricity in the grid, and produce power at times of peak usage, when utility generation costs are higher and they often need the extra power.

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 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.

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