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. For this reason, the most 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 (150 pounds), 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.
PV with Battery Storage
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.
How It Works
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.
PV with Generators
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 anytime. 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 it 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 night time 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, to supply ac or dc loads, or do 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.
PV Connected to Utilities
Where utility power is available, a grid-connected PV system can supply some of the energy needed and use the utility in place of batteries.
Some homeowners, considered pioneers in the energy field, are using PV systems connected to the utility grid. They are doing so because 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 cannot only buy, but can also sell, 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 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.
How It Works
In the USA , 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.
Utility Scale Power
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, and 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 they would treat a conventional plant.
A Niche for PV
Despite the costs, utilities are becoming more involved with PV. For example, a joint venture was formed between DOE, the Electric Power Research Institute (EPRI), and several utilities. Called Photovoltaics for Utility-Scale Applications (PVUSA), the project 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 prevents the energy losses associated with sending current long distances through conventional power lines. This makes the PV system 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 prevent overloading the equipment in the substation.
Hybrid Power Systems
Hybrid systems combine a number of electricity production and storage pieces to meet the energy demand of a given facility or community. In addition to PV, engine generators, wind generators, small hydro plants, and any other source of electrical energy can be added as needed to meet energy demands and fit the local geographical and temporal characteristics. These systems are ideal for remote applications such as communications stations, military installations, and rural villages.
Essential to developing a hybrid electric system is knowing the energy demand to be met and the resources available. Energy planners therefore must study the solar energy, wind, and other potential resources at a certain location, in addition to the planned energy use. This will allow them to design a hybrid system that best meets the demands of the facility or community.