The interplay between energy and indoor environmental quality makes for a lot of really interesting problems. Think, for example, about something like natural ventilation-- harnessing wind and temperature differences to keep a space thermally comfortable and well-ventilated, without mechanical systems.

Problems in this field tend to be cross-disciplinary, ranging across controls systems, fluid dynamics, human health, and public policy. They tend to affect lots of people (people spend most of their time indoors, and buildings are a large part of total energy consumption). And a small effect, leveraged across many many installations, can become a big win.

"Hackers" in particular can contribute to this field in a number of ways. Simulation modeling, optimization algorithms, and distributed networks are big research areas. And of course the issue of data center efficiency has come to the fore in a lot of people's minds.