Ten years ago, America's brief love affair with renewable energy sources came to an abrupt end, the victim of declining oil prices and government indifference. But renewable energy remains the only viable long-term alternative to depletable and polluting oil, gas, and coal. Cool Energy provides the first major review of progress in the field of renewable energy technologies - solar, wind, biomass (plant matter), hydroelectric, and geothermal - since the mid-1980s. It analyzes their near-term and long-term potential to displace fossil fuels, and illuminates the role they could play in mitigating environmental problems such as air pollution, acid rain, and global warming. Energy-policy specialist Michael Brower argues that, with the right policies, renewable energy could provide as much as half of America's energy needs within forty years. He identifies the market barriers that will have to be removed and argues that if the hidden costs of fossil fuels are taken into consideration, renewables appear to be a cheaper source of new energy supply than fossil fuels: the reliability and efficiency of their equipment have improved and the cost of installing, maintaining, and running renewable systems has declined. Brower devotes a chapter to each renewable energy source, describes its current application, and discusses its costs. He also analyzes new technologies under development and assesses their positive and negative attributes. Introductory chapters set renewables in the context of current energy and environmental policy, and the last chapter outlines steps that can help speed the transition to a renewable-energy economy. Michael Brower is a physicist and holds the position of Research Director for the Union of Concerned Scientists.