Thermodynamic limits for simultaneous energy harvesting from the hot sun and cold outer space

The sun and outer space are two of the most important fundamental thermodynamic resources for renewable energy harvesting. A significant amount of work has focused on understanding the fundamental limit of energy harvesting from the sun. More recently, there have been several theoretical analyses of the fundamental limit of energy harvesting from outer space. However, far less is understood about the fundamental limits of simultaneous energy harvesting from both the sun and outer space. Here, we consider and introduce various schemes that are capable of simultaneous energy harvesting and elucidate the fundamental thermodynamic limits of these schemes. We show that the theoretical limits can far exceed the previously established limit associated with utilizing only one thermodynamic resource. Our results highlight the significant potential of simultaneous energy harvesting and indicate new fundamental opportunities for improving the efficiency of energy harvesting systems. Energy harvesting: trapping the two-way flow Energy harvesting systems that combine capturing incoming solar energy with exploiting the emission of thermal energy outwards into space offer possibilities for improving our ability to generate power from the natural and freely available energy flows on Earth. Shanhui Fan and colleagues at Stanford University, USA, explore the theoretical limits of such simultaneous energy harvesting processes. While the theoretical efficiency of trapping incoming solar energy has been extensively studied, much less attention has been given to 'radiative cooling' systems, such as devices that extract energy from the dissipation of heat outwards towards the night sky. Interest in harnessing that previously neglected energy flow, however, is steadily increasing. The research team's theoretical calculations suggest that schemes exploiting energy flow in both directions should be explored to enhance our use of the energy gradients available to us.