Think solar power is all about photovoltaic panels? Solar thermal could power the entire country-and the technology has been tested.Solar technology is nothing new. For literal millennia, humans have been harnessing the sun's rays for energy. Over the past few decades, the dream of a solar-powered future has mostly conjured up images of rooftops covered by photovoltaic panels, turning every house into a mini power plant. But a somewhat lower-tech and much older solar solution could well prove to offer the biggest gains in carbon-free energy production. I'm talking about concentrated solar power, or as many call it, "solar thermal."Whereas photovoltaic panels directly convert sunlight into an electric current, concentrated solar uses the sun's heat energy itself to generate power. As far back as the sixth century B.C., the Chinese used mirrors to focus the sun's rays and start fires. (Much like how kids these days will torch leaves, lunch bags, and ants with a magnifying glass.) A few decades later, Greeks lit the sacred fire at Delphi using these "burning mirrors," and, supposedly, around 212 B.C., Archimedes ordered soldiers to use their shields to focus sunlight on-and ignite-Roman war ships.Today's CSP projects aim to generate power on an industrial scale, creating a baseload electricity supply that could potentially replace large, centralized fossil fuel-burning plants. Just this week, the world's largest (but not for long) CSP operation plugged in-the PS20 plant in Seville, Spain trains 1,255 mirrors on a 531-foot tall water-filled tower. The intense heat boils the water, which creates steam. The steam spins a turbine, and-voila!-electricity is generated. Under optimum conditions, the plant can churn out 20 megawatts of juice, enough to power 10,000 homes.A massive 200-megawatt project announced last week for Arizona (it's a lively time for CSP) will opt for reflective parabolic troughs, rather than a centralized tower. The principle is the same, but in this case sunlight is focused on a collector line-basically a pipe-filled with a fluid. The superheated fluid, generally oil, is pumped through an exchanger, which draws out the heat to boil water into steam. And, once again-steam spins turbine. Turbine produces power.Recently, the CSP industry has come up with something of a breakthrough in addressing solar's biggest hurdle: What happens when the sun's not shining? How can solar energy be stored for use at night? While storing electricity in batteries is prohibitively expensive (and the batteries themselves are an environmental liability), storing heat is much cheaper and easier. Molten salt, of all things, is particularly good at it. Acting as a solar heat battery of sorts, molten salt can be cooked to over 1000 degrees Fahrenheit, and the heat can be released in a controlled manner to turn water into steam. Already, current installations can retain enough heat to keep pumping out power for six hours after the sun goes down. Researchers at the U.S. National Renewable Energy Labs are hoping to soon double that time, effectively providing around-the-clock solar power on par with large-scale hydro and coal-fired plants.So just how big is the potential for concentrated solar? A recent study found that 1,000 square miles of the Mojave Desert devoted to CSP could produce enough energy to power the entire country. On a grander scale, less than one percent of the world's deserts could power the whole world, if transmission lines could accommodate the electricity.To be sure, these are huge spaces, and the issue of siting the plants is already turning contentious. Energy Secretary Steven Chu has raved about CSP's potential in the Southwest, but ultimately Ken Salazar at Interior will have more direct influence over the land use. Salazar has been an outspoken advocate of wind power (particularly offshore), but has been pretty quiet on CSP, and last month he received a surprising letter from California Senator Dianne Feinstein (who's generally pretty progressive on green issues) urging him to protect sensitive desert from disruptive energy installations. But the prospect of clean, carbon-free, renewable electricity that's tested and proven, and is far cheaper than nuclear and competitive with coal (before a likely price on carbon), is clearly appealing. I'm convinced that relatively minor and local ecological concerns won't keep this critical energy solution from seeing its moment in the sun. Related: An animation of how a CSP trough system works.