Hydroelectric power has historically been the largest renewable energy technology used. It often has some negative environmental effects associated with it, but like nuclear, is better than coal and so worth mentioning. The reason we don't go into much detail about it on this website is because it's sort of on autopilot at the moment, and that's okay. It is limited by geography and is usually installed in places where it is feasible and won't have terrible environmental effects. In short, there isn't much to be done with regards to hydroelectric except keep doing as we have been, so it didn't get it's own section. If a lot of people want to know more about it and ask for it on the suggestions page, we will consider adding it later.
Hydrogen is better described as a storage mechanism than an energy source. It takes energy to produce hydrogen, either through electrolysis or through high pressures and temperatures. This energy must come from some source, such as nuclear, solar, wind, or fossil fuels. As a result, we talk about hydrogen in the Storage and Better Batteries sections.
We talk specifically about wind and solar because they are clean technologies that are working right now and have the potential to expand to a fairly large scale. We also talk about nuclear because it is a fairly large player right now and is cleaner than coal (even if it's not renewable). In addition, we feature other technologies that some consider to be potentially large players in the future of renewable energy. These are technologies that are either limited in their implementations or are a breakthrough or two away to being major sources of power.