I was recently asked about “rejected energy” by a colleague who had seen it on a website about US energy. This shows that US energy flows include a lot of rejected energy. This happens in other countries too, so it’s helpful to understand why so much energy is unloved. So what is rejected energy and why is it unloved? Rejected energy is part of the energy of a fuel – such as gas or petrol – that could be used for a purposeful activity, like making electricity or transport. However, because of the technologies that we currently use to consume fuels a lot of it gets tossed out by turning it into heat in the environment, which is totally useless. For a coal fired power station, for instance, about 2/3 of the energy released when the coal is burnt is discarded as heat in the environment. This reject energy sometimes appears as clouds of vapour coming off a power-station’s cooling towers, such as the well-known ones at Didcot in England.