Long dismissed by some pundits as expensive and impractical, photovoltaic cells, also known as solar cells, are a popular way to generate electricity in remote areas. John Benner, a researcher at the National Renewable Energy Laboratory in Golden, Colo., said the main reason is cost: Stringing wires from existing power lines to outlying areas averages between $20,000 and $30,000 per mile. Industry experts predict that by the mid-21st century, photovoltaics will expand beyond such established niches into the energy mainstream. The cells will no longer just power satellites, calculators and emergency telephones, but will become an intimate part of the utility grid. Photovoltaic cell prices will continue to go down and efficiencies to go up, mirroring advances in the closely related semiconductor industry. (Rejected semiconductor wafers provide 80 to 90 percent of the cells' silicon.) Photovoltaic cells have come a long way since they were first popularized during the energy crisis of the '70s, just as music has progressed from eight-tracks to CDs. Large energy companies are now getting into the game. Cor Herkströter, chairman of Royal Dutch/Shell, said that Shell could be half oil and half renewables in 50 years.