Solar 101

Quick Facts about Solar

 

HOW DOES PHOTOVOLTAIC (PV) TECHNOLOGY WORK?

PV takes advantage of the characteristics of impure silicon crystals. Pure silicon is not electrically active, because its atoms are locked into a solid crystal structure. There are no spare electrons, and no open spots for electrons. Silicon impurities create crystal with either a slight tendency to lose electrons or a slight tendency to attract them. When the two kinds of silicon are placed close together and exposed to sunlight, photons (particles of light) knock electrons loose on the unattractive side. An electrical current is created as electrons travel across the junction to the attractive side.

Sunlight is composed of particles of energy called photons. When sunlight strikes a PV material, photons will either pass through, be reflected, or be absorbed. If the photon is absorbed, its energy will be transferred to an electron in an atom of the PV material. With new energy, the electron is able to escape from its normal position in orbit around that atom. In this way, the electron can become part of, and augment, the current in an electrical circuit. This photovoltaic effect is the basic physical process through which sunlight is converted into electricity.

Light-emitting diodes (LEDs) are made of similar materials and take advantage of the same physical principles, but in reverse. Powering LEDs with a PV panel works compatibly: photons in, electrons out; electrons in, photons out.

WHAT IS A PHOTOVOLTAIC (PV) CELL?

A photovoltaic cell, or "solar cell," is the smallest semiconductor element that converts sunlight into electricity. Each cell is made of silicon or another semi-conductor material, like a computer chip. The silicon is treated so that it generates a flow of electricity when light shines on it.

A stack of thin layers of semiconductor materials exhibit the photoelectric effect, such as silicon or cadmium telluride. The layers contain small amounts of doping agents (intentional impurities), such as the element germanium. The dopants give the semiconductor the ability to produce a current when exposed to light. Cells convert about five to fifteen percent of the solar energy they receive into electricity.

Solar cells are solid-state devices in which photons collide with atoms. This process transforms the resulting energy into electrons. These electrons flow into wires connected to the cell, thus providing electric current to appliances, lighting systems or other electrical loads. A typical PV cell is a thin 3"x3", producing only a small amount of electricity.

WHAT IS A SOLAR MODULE?

Solar modules, or panels, are series of solar cells wired together into strings and enclosed in self-contained glass units for harsh weather protection. Solar cells are mounted into groups called modules that produce about 0.5 Volts of current to power lights and appliances. On the sunward side, cells are protected by a highly transparent solar glass pane. The underside takes the form of an insulating film or a second pane of glass. A connection socket picks up the generated direct current. Modules are connected together via cables, which link them to the inverter.

WHAT IS A PV ARRAY?

A PV array is an interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module. A complete set of components for converting sunlight into electricity includes a module, a support structure, wiring, an inverter, a meter and other equipment.

WHAT IS A BLOCKING DIODE?

A blocking diode connects to the cable and prevents the solar panel from discharging the battery in the absence of sunlight. For example, a car battery will not act as an impedance load on a solar panel because of reverse blocking diodes that prevent nighttime battery discharging.

WHAT IS PV CONVERSION EFFICIENCY?

PV conversion efficiency is the ratio of the electric power produced by a PV device to the power of the sunlight shining on the device. Cell efficiency defines how much energy in sunlight is actually converted into electricity. Amorphous silicon modules have lower efficiency than other PV materials. Cell efficiency degrades progressively with use.

DO SOLAR CELLS STORE ENERGY?

No. Solar cells just convert sunlight into an electric current that must be used immediately or stored in batteries for later use.

HOW WELL DO SOLAR PANELS WORK IN CLOUDY CONDITIONS?

A cloudy day provides sufficient diffuse light by which the panel will produce electricity. Optimum electrical production occurs with bright and sunny weather conditions. Under a light overcast, the modules might produce about half as much as under full sun, ranging down to as little as five to ten percent under a dark overcast day. In remote, off-grid applications, a PV system is connected to a battery storage system as a backup power source. In grid-connected applications, the PV system works in parallel with the utility power grid. So, if electrical needs exceed the solar power output, the local utility makes up for the shortfall. Conversely, when the PV system generates more energy than the building requires, the excess power is exported to the utility grid, reversing the electrical meter!

HOW WELL DO SOLAR PANELS WORK IN THE WINTER?

Solar panels work on light not heat. With shorter daylight hours in winter, solar panels produce proportionately less power. If the modules become covered with snow, they stop producing power, but snow generally melts quickly when the sun strikes the modules; if you brush the snow off, they resume operation immediately.

CAN I USE EQUIPMENT DIRECTLY FROM SOLAR PANELS?

Yes. Solar panels will directly power equipment such as fans and pumps as long as the load is accounted for correctly. Equipment load that is greater than the output of the solar panel will weaken equipment efficiency, as overcast or cloudy days reduce output. Equipment that requires a more stable voltage should pair solar power with a battery backup.

WHAT TYPE OF MAINTENANCE IS REQUIRED ON SOLAR PANELS?

Solar panels benefit from a non-abrasive cleansing agent. When reviewing battery levels, check battery connections and fuse holders to ensure that they are clean and dry. If necessary, use a silicon sealant for sealing damages around the frame.

Although the production of solar panels incorporates a high-tech manufacturing process, it's really very easy to use a photovoltaic system. Solar panels have no moving parts to wear out, they can be used alone or in combination with other energy sources, and they are silent, reliable and long lasting.

WHAT ARE THE POSSIBLE PROBLEMS WITH SOLAR PANELS AND HOW CAN I PREVENT THEM?

Solar panel failure is most often caused by water damage to the panel, sealant, or connections.

To prevent damage or failure, mount the panel carefully. Be sure not to fix it horizontally, which encourages water collection in the frame. Also, allow for a sufficient air gap beneath the panel. Keeping your panel dry and clean will ensure efficient, maximum output.

ARE SOLAR ELECTRIC SYSTEMS SAFE?

Yes. Solar cells are mostly silicon, the primary component of sand. There is no exhaust and no toxic materials to leak out of the system. The electricity coming through the inverter is just like the electricity coming from household wall sockets; you should use the same care you would with utility power. All components are approved for utility interconnection and are installed according to standard construction practices.

ARE SOLAR POWER SYSTEMS GOOD FOR THE ENVIRONMENT?

Energy created through our solar electric system produces no pollutants. Our smallest system typically cuts greenhouse gas emissions as effectively as 50 trees.

WHEN DID SOLAR CELL TECHNOLOGY DEVELOP?

Modern solar cells with practical efficiency were invented in the early 1950s, and have been used to power satellites since 1959. They became popular for terrestrial applications in the mid-1970s, mostly for remote telecommunications, navigational aids and other rugged, remote industrial uses including microwave, TV, radio and cellular repeater stations. They have been powering urban applications such as roadside emergency telephones and traffic signs since the mid-1980s. With prices dropping steadily, they are now becoming affordable for urban or remote homes and businesses.