A sink is also a term used for an energy resource that consumes more energy than it produces. The net energy for a 'sink' is negative, as it takes more energy to extract the resource than the energy that resource will provide.
For example, the deeper an oil deposit is beneath the ground, the more likely that the deposit will be a sink. Since economic theory has no regard for the laws of thermodynamics, the electricity hungry world will rely on sink resources to provide energy, even though they will become increasing inefficient to the point of absurdity.