Sunpower for School Kids
Learning Objectives - After this course, you should be able to:
- Explain how photovoltaics produce electricity.
- Describe a typical photovoltaic application.
- Describe how schools are teaching photovoltaics to students.
Every day, the sun bathes the earth with enormous amounts of free energy-enough in one minute to meet the world's current energy needs for an entire year! Schools are beginning to harness this limitless natural resource as a way to meet their energy needs-and provide educational opportunities for their students in the process.
Photovoltaics are one solution. PV systems convert sunlight into electricity-one of the most elegant and environmentally benign ways to produce power. This article explores how teachers, administrators and facility managers are beginning to look for ways to produce their own power.
What are Photovoltaics?
In 1839, French scientist Edmund Becquerel observed the photoelectric effect-when a current could be measured across an electrode suspended in a solution exposed to light. And, in 1921, Albert Einstein won the Nobel Prize for his theories explaining the photoelectric effect. The photoelectric effect was not put to work until 1954, when Bell Laboratories created the silicon photovoltaic cell-the first cell able to convert enough of the sun's energy to power electrical equipment. Given a large boost by the space program, development of PV cells has proceeded uninterrupted since they were first manufactured, with conversion efficiencies (how much solar energy is converted to electrical energy) increasing from 4 percent for Bell Labs' first prototype to more than 50 percent for the specialized prototypes of today. More common for building applications are overall PV system efficiencies of 12 to 17 percent depending on the type of solar cells and system technology used.
How Does PV Work?
When sunlight strikes a PV cell, electrons are dislodged and gathered by wires attached to the cell to form an electric current. This basic action-simple, quiet, non-polluting, and requiring no moving parts-is at the core of every PV system. Cells may be more effective in areas like the Southwest, with a lot of clear, sunny days, but they will still provide substantial amounts of power in areas like the Northwest, with more overcast days.
Since most cells are relatively small, typically from 1/2-inch to 4 inches on a side. They produce very little power, and need to be electrically connected to other cells to increase energy output to levels appropriate for building applications. These collections of PV cells, referred to as "modules," are what we typically see on the roofs of buildings. They are typically 2-to-4 feet by 4-to-6 feet in size. Modules are typically connected to each other to form PV "arrays," which can range in size from one or two modules to several thousand, depending on the power output desired. School projects in the U.S. have used various sizes, from small arrays-from 1 or 2 modules for demonstration purposes-up to the array on the three buildings that make up a high school in Bayonne, New Jersey that has more than 5,000 modules and supplies a substantial portion of the school's electricity.
A functioning PV system also needs an inverter to convert the direct-current (DC) power generated by the modules into alternating current (AC) that can be used in the school. Wiring is needed to connect everything together; and some form of mounting system is necessary to attach the array to the roof, wall, or grounds of the school. The mounting system can be fixed at a set angle, or track the sun throughout the day. This can substantially increasing the energy output of the array, but at a considerable cost.