PV Basics: What are Photovoltaics?

Photovoltaic cells convert sunlight directly into electricity. When sunlight strikes a PV cell, electrons are dislodged, creating an electrical current.

Photovoltaic cells power many of the small calculators and wrist watches in use every day. More complex systems provide electricity to pump water, power communications equipment light homes, and run appliances. Beyond the utility power line, PV is often the lowest-cost means to provide electricity, and almost always simplest and cleanest to operate.

The cost of PV has fallen by 90 percent since the early 1970s. Photovoltaics are producing electricity for critical loads from the polar ice caps to the tropics to satellites in outer space. There is a strong market today in developing countries to provide rural electrification with solar panels, which replace kerosene lamps, batteries, and wood fires at a far lower cost than the central station power plants.

Photovoltaics are also making inroads as supplementary power for utility customers already served by the grid. Currently costly compared to most conventional choices for grid power, Photovoltaics is still a very small part of the energy make-up of any country. However, more and more individuals, companies, and communities choose PV for reasons other than cost: because of a desire to develop a clean, sustainable energy source, interest in a clean back-up power source, a need for placing power generation right at the source with no fuel, noise or moving parts; and an attraction to a power technology that can be built right into building roofs, facades, canopies and windows.

 

A Primer on Solar Photovoltaics and PV Systems

A constant and universal sun, a simple device with no fuel and no moving parts, and energy as near as your backyard: this image of photovoltaics has sparked the imagination of millions of people. What this technology is and how it is packaged to produce useful electricity is the subject of this fact sheet.

The Solar Resource

The sun bathes the earth with more energy each minute than the world consumes in one year. But, except in the tropics, the sun is never directly overhead and its intensity varies by season. For example, at a latitude of 45°, solar radiation may vary from 92% (early summer) to 38% (early winter) of theoretical maximum insolation. The average intensity at this latitude is 71% (early spring and fall) of maximum.

At higher latitudes, solar radiation also follows a longer path through the earth's atmosphere. Scattering and absorption of incident and reflected radiation by gases such as CO2, methane, chlorofluorocarbons, and particulates further influence the solar resource available.

These global conditions-- plus local variations such as cloud cover, topography, and altitude-- cause solar to be a variable resource. The figure to the right presents a contour map of the U.S. which shows some of this variability. This type of map is very familiar to anyone who has investigated solar energy. One of the unfortunate aspects of such maps, however, is that they tend to suggest that the solar resource is most important in the Southwestern U.S., much less so in the far Northeast or Northwest. This perception has caused many utilities in those parts of the country to discount the technology.

But straight solar insolation values may be deceptive, especially from a utility standpoint. These maps ignore the capacity value from PV. Recent work by the National Renewable Energy Laboratory (NREL) is showing that high levels of solar insolation are not a necessary condition for finding good sites for PV. A study involving about 35 utilities that correlated system load curves with PV production found many areas of the country with lower insolation levels that received excellent capacity matches. The same study found that the utility's summer/winter peak load ratio provided a good proxy of capacity contribution, with the higher ratios showing higher contributions.

In fact, the Solar Electric Association's market evaluation work has shown that PV can make a contribution to every utility in every part of the country.