With the world’s population fast approaching 7 billion, how do we meet the growing demand for energy in a responsible, equitable, and sustainable way? It’s a question National Geographic, in partnership with Shell, is asking in its three-year initiative, The Great Energy Challenge, which is designed to shed light on the breadth and depth of the current energy situation. Recent stories published as part of the series explore the search for green air-conditioning, including a profile of Thermax of Pune, India, which markets absorption chilling as one of its “sustainable solutions” for today’s environmental concerns. Like standard air conditioners, absorption chillers, based on a technology that has been in commercial use since the 1920s, rely on a refrigerant with a low boiling point. When the refrigerant evaporates, it removes heat from the air. Standard air conditioners then change the refrigerant gas back to liquid using an electric compressor. But absorption chillers rely on thermal compression to restart the cycle; they need only heat—no moving parts—to drive the operation.