The increased awareness of global warming together with the rising costs of fossil fuels caused a surge of interest in renewable energy resources. There are a number of basic forms of renewable energy: solar, wind, water, biofuel and geothermal. Solar energy can be used in various ways, including photovoltaic cells, which convert the energy into electricity, and concentrating collectors, which focus sunlight to heat a fluid and drive a turbine that produces electricity. Another form of a solar-driven electric power plant is an energy tower that uses the form of a closed greenhouse at the base of the tower to warm air, which rises up the tower, drives a turbine, and produces electricity.