Solar Energy!
Producing electricity from solar energy was the second discovery. In 1839 a French physicist named Edmund Becquerel realized that the sun's energy could produce a "photovoltaic effect" (photo = light, voltaic = electrical potential). In the 1880s, selenium photovoltaic (PV) cells were developed that could convert light into electricity with 1-2% efficiency ("the efficiency of a solar cell is the percentage of available sunlight converted by the photovoltaic cell into electricity"), but how the conversion happened was not understood. Photovoltaic power therefore "remained a curiosity for many years, since it was very inefficient at turning sunlight into electricity." It was not until Albert Einstein proposed an explanation for the "photoelectric effect" in the early 1900s, for which he won a Nobel Prize, that people began to understand the related photovoltaic effect.
Solar energy is the cleanest, most abundant, renewable energy source available. And the U.S. has some of the richest solar resources shining across the nation. Today's technology allows us to capture this power in several ways giving the public and commercial entities flexible ways to employ both the heat and light of the sun. The greatest challenge the U.S. solar market faces is scaling up production and distribution of solar energy technology to drive the price down to be on par with traditional fossil fuel sources.
Solar energy can be produced on a distributed basis, called distributed generation, with equipment located on rooftops or on ground-mounted fixtures close to where the energy is used. Large-scale concentrating solar power systems can also produce energy at a central power plant.
There are four ways we harness solar energy: photovoltaics (converting light to electricity), heating and cooling systems (solar thermal), concentrating solar power (utility scale), and lighting. Active solar energy systems employ devices that convert the sun's heat or light to another form of energy we use. Passive solar refers to special siting, design or building materials that take advantage of the sun's position and availability to provide direct heating or lighting. Passive solar also considers the need for shading devices to protect buildings from excessive heat from the sun.