The History and the Future of Photovoltaic
The age of Solar Energy is here. Conversion of sunlight into electricity by solar photovolactic (PV) cells is an ideal source of power for an environmentally responsive home, business or public institution or agency. To put the magnitude of the sun's power into perspective, a solar cell power plant covering only 1% of the Sahara Desert would produce all the electricity consumed on this planet.
In less than 40 minutes, the United States receives more energy in the form of sunlight than it does from the fossil fuels it burns in a year!
The science of Photovoltaic began in 1838 with a pair of French physicists working on the generation of electricity through chemical reactions. The physicists noticed that exposing the apparatus to sunlight increased the output of electrical energy. By 1954, Bell Laboratories announced the developments that finally brought Photovoltaic out of the lab and into the realm of practical application, the single-crystal silicon cell.
Photovoltaic
...(or PV) modules convert light into electricity. The term "photo" is a stem from the Greek "phos," which means "light." "Volt" is named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. "Photo-voltaics," then, could literally mean "light-electricity." Most commonly known as "solar cells," PV systems are already an important part of our lives. The simplest systems power many of the small calculators and wrist watches we use every day. More complicated systems provide electricity for pumping water, powering communications equipment, and even lighting our homes and running our appliances. In a surprising number of cases, PV power is the cheapest form of electricity for performing these tasks.
Here, we describe the PV effect that allows various materials to produce electricity from sunlight; show how PV cells, modules, and arrays are made; explain why PV is the most logical power choice in many different cases; and provide real examples of how this science is improving the lives of people all over the world!
Applications and uses
Today, solar-generated electricity serves people living in the most isolated spots on earth as well as in the centre of our biggest cities. First used in the space program, PV systems are now both generating electricity to pump water, light up the night, activate switches, charge batteries, supply the electric utility grid, and more. Whether you are a homeowner, farmer, planner, architect, or just someone who pays electric utility bills, PV may already touch your life in some way.
We group PV applications into the following categories:
- Simple or "Stand Alone" PV Systems
- PV with Battery Storage
- PV Connected to the Local Utility
- Utility-Scale Power Production
- Hybrid Power Systems
- Simple PV Systems
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), with low weight, 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. One of the most simple PV/battery systems is this path light. The PV panel generates electricity during the day and stores it in the battery for use at night.
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. An 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 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 can not 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
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.
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 setting up of PV arrays is much easier than setting up of 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.
A Niche for PV
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 can 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. |