The fastest-growing energy technology in the world is solar, but it’s very unlike passive solar. Photovoltaic solar collectors capture solar energy in a supremely adaptable and portable form. Photovoltaic panels and films, made mostly of silicon, convert sunlight into electricity by allowing sunlight to stimulate electrons to a higher state of energy, then converting that energy into electrical current. Basically, the photons in sunlight, hitting a collector, create free electrons that can be siphoned off as electrical current. Beginning in the early 2000s, worldwide production of photovoltaics had been doubling every two years up until 2008 when the rate of growth suddenly accelerated, more than doubling the number of photovoltaics in use in just one year. At the end of 2008 it was estimated that photovoltaics were generating about 15,000 megawatts of energy, worldwide, enough to provide about 8 million average U.S. homes with all the electricity they need, and photovoltaic expansion was accelerating.
Most photovoltaic installations are small, designed mainly to supply a single building, or even a single device, like my electric fences. But solar power stations are proliferating. A single new photovoltaic power plant, Topaz Solar Farm, proposed to cover 9.5 square miles in central California near San Luis Obispo with a 550 megawatt capacity, is scheduled to begin generating electricity in 2011. Before Topaz Solar Farm, the largest photovoltaic generator in the country, the DeSoto Energy Center in Florida, had a capacity of just 25 megawatts. The largest in the world was the 60-megawatt Olmedilla Photovoltaic Park in Spain.
The manufacturing of photovoltaics is a big business, getting bigger rapidly. Industry sources estimated the size of the industry at about $30 billion in 2009, expected to be $70 billion by 2013. Shi Zhengrong, one of the richest new billionaires in China, made his fortune manufacturing photovoltaics.
Some property owners are installing free photovoltaics thanks to “power purchase agreements,” under which an investor buys the panels and installs them, free, in exchange for a contract to buy electricity from the owner of the panel at a very low cost for a couple of decades. The City of Berkeley, California, is evaluating that program, which offers homeowners free solar installations and then takes the money saved on electricity over the next 20 years through a special property tax.  About 75 percent of commercial photovoltaics are installed on some kind of power purchase agreement.
And the technology is getting better. So-called “thin-film” photovoltaics are manufactured on a flexible surface that can be adhered directly to a metal roof. The electronics fit neatly under a cap that runs along the peak of the roof and the whole apparatus looks perfectly integrated and natural. Some power stations set up their collectors on swivels to track the sun. Others concentrate sunlight using mirrors.
The most intriguing new developments in photovoltaics are transparent collectors and concentrators. Both technologies promise to turn everyday objects into power-generating solar collectors. Japan’s National Institute of Advanced Industrial Science and Technology has developed transparent collectors that generate electricity from ultraviolet light, allowing light in the visible spectrum to pass right through – effectively a transparent solar collector. The implications are astonishing. Imagine transparent solar films that could be adhered to the roofs of cars or the windows of skyscrapers. Every outdoor surface is a potential source of electricity. Glass buildings could cool themselves. Electric vehicles, parked outside, could charge their own batteries.
A U.S. company called Covalent Solar makes glass containing subtle dyes that concentrate and deflect solar energy to photovoltaics, possibly in the frame of a window, for instance. Windows and skylights can become solar collectors, seamlessly integrated into a building or vehicle.
And because the dyes used in the glass actually concentrate the energy, less surface area is necessary to generate a given amount of electricity.
Above photo: Transparent photovoltaics on a canopy at a BP station.
Photo by the National Renewable Energy Laboratory/Gretz, Warren
 REN21: Renewable Energy Policy Network for the 21st Century. Renewables Global Status Report 2009 Update. REN21 Secretariat. Paris.
 California Public Utilities Commission. CPUC Takes Another Step Toward State’s Renewable Energy Goal with Approval of PG&E Renewable Contract. http://docs.cpuc.ca.gov/PUBLISHED/NEWS_RELEASE/96710.htm. January 29, 2009. Sourced November 5, 2009.
 DuPont Photovoltaic Solutions. DuPont Expects Continued Revenue Growth in Global Photovoltaic Market. http://www2.dupont.com/Photovoltaics/en_US/news_events/article20090317.html. Sourced November 5, 2009.
 Thomas L. Friedman. China’s Sunshine Boys. New York Times. Dec. 6, 2006.
 City of Berkeley, California, Office of Energy and Sustainable Development. Berkeley FIRST Financing Initiative for Renewable and Solar Technology. http://www.cityofberkeley.info/ContentDisplay.aspx?id=26580. Sourced Nov. 5, 2009.
 Greentech Media. Power-Purchase Agreements to Spike. http://www.greentechmedia.com/articles/read/power-purchase-agreements-to-spike-591/. Sourced November 5, 2009.
 Japan National Institute of Advanced Industrial Science and Technology (AIST). Converting ultraviolet light into electricity with transparent electronics. http://www.aist.go.jp/aist_e/museum/science/12/12.html. Sourced November 5, 2009.