Some smart folks in South Korea are using the power of their minds + existing and future technology + gravity to figure out ways for sewage and drainage traveling downward through the pipes in buildings can help create energy. Think of it kind of like a hydroelectric power plant except on a tiny scale and with poop and/or soap suds mixed in.
It seems, more and more, that renewable energy production will need to take place at every level of scale, from toilets and sinks to enormous wind turbines and giant solar arrays, and everywhere in between. The situation driving this movement is terrifying, but the ideas and technology making its way into the world is exciting. The tricky part comes with making all of this: A) affordable; B) easy to install; C) accessible worldwide.