District energy is one of the oldest concepts in all of energy, dating back at least to the ancient Romans. It simply refers to connecting multiple buildings to a common source of heating and cooling — a furnace, heat pump, geothermal well, or what have you — and distributing the heat via water or steam flowing through underground pipes. There are hundreds of district energy systems in operation, in every country in the world. (Virtually all of the buildings in Iceland, which I visited recently, are heated by district energy systems running on geothermal.)However, fossil fuel heat has been so cheap for so long that district energy has never quite become the default — it’s just been too easy to stick a natural gas furnace in every building. There hasn’t been much pressure to share heat.But with the climate crisis and the clean energy transition, that’s changing. These days, lots of people are looking for cleaner sources of heat and more efficient ways to share it, so district energy is becoming sexy again. Among other things, it’s a great way for cities to meet their carbon goals without overburdening their electrical grids.With all that in mind, I contacted Rob Thornton, the head of the International District Energy Association, to chat about the clever new sources district energy systems are drawing on (everything from sewage to deepwater lakes), the infrastructure they can integrate with, and the other services they can provide. Get full access to Volts at www.volts.wtf/subscribe