When you consider that the global annual consumption of primary energy increased more than ten-fold during the 20th Century, the importance of planning future energy supply becomes clear. This study unit is just one of many that can be found on LearningSpace, part of OpenLearn, a collection of open educational resources from The Open University. Published in ePub 2.0.1 format, some feature such as audio, video and linked PDF are not supported by all ePub readers.