In his short story ‘Let There Be Light’, the science-fiction author Robert A Heinlein introduced the energy source that would power his Future History series of stories and novels. First published in Super Science Stories magazine in May 1940, it described the Douglas-Martin sunpower screens that would provide (almost) free and inexhaustible energy to fuel the future in subsequent instalments of his alternative timeline. It was simple, robust and reliable technology. ‘We can bank ’em in series to get any required voltage; we can bank in parallel to get any required current, and the power is absolutely free, except for the installation costs,’ marvelled one of the inventors as they worked out the new technology’s potential for rupturing the social order of the future.