In recent years, concerns on energy consumption and greenhouse pollution due to the operation of wireless devices have triggered a vast amount of research work on the so called green wireless technologies, leading to new, energy-aware proposals. Even for the case of battery powered devices, where energy conservation is a key design goal, new approaches have been proposed, based on a better understanding of the cost-performance trade-offs introduced by energy efficient operation, while increasingly focusing on emerging communication technologies, e.g., body sensor networks, MIMO or LTE. This paper presents a survey of the recent proposals for green wireless communications, with a view to understanding the most relevant sources of inefficient energy consumption and how these are tackled by current solutions. We introduce a classification of the existing mechanisms based on their operational time-scale, discuss the most important techniques employed to date from this perspective, analyze the employed evaluation methodologies and undertake a quantitative comparison of their performance gains. Following this analysis, we identify the key challenges yet to be addressed by the research community, as well as several possible future directions towards greener communications.