Abstract: Energy supply is currently undergoing a major transition from fossil fuels to more sustainable sources such as renewables and nuclear. However, in the longer term, it is unclear whether these sources will be able to meet demand increases from the developing world, increasing population, and new uses of energy. An estimate is made for the first two of these demand increases based on correlations of the population rate increase and the gap in first and third world GDP. A sustainable world may require more energy to counteract the impacts of climate change, for example, on the supply of clean water and food. Since it is uncertain whether distributed energy sources will meet this demand, new energy supplies might be considered. Some options considered already include more sustainable nuclear systems with use of thorium and fuel recycling, fusion, and large solar projects in orbit, the moon, or in deserts. Previous option analysis of these technologies are reviewed and updated based on current understanding. Potential extensions and approaches are identified for uncertainty estimation, incremental roadmapping, distinguishing serial and parallel technologies, the value of R&D information, and incentive strategies for realizing the economies of scale and learning-by-doing benefits.