The course examines the options available for providing energy from the major carbon-free energy sources: solar, wind, biomass, geothermal and hydropower, and briefly touches upon nuclear energy and sequestration of carbon from fossil fuel sources. For each carbon-free energy source covered, the physical principles, physical or biophysical limits, efficiencies, and other constraining factors are discussed, as well as examples of current applications, current and projected future costs, and possible future scenarios. The course concludes with selected studies for various regions of the world of scenarios for achieving 100% renewable energy supply systems by 2050, in line with requirements needed to (hopefully) limit global mean warming to no more than 2-3 degrees Celsius.