My Kepler Bb humanoids who actually originated on Kepler B#, a bigger planet that is also habitable, have 1 big problem. It has to do with energy. Right now they are using biomass for heat and only 1 family is using solar for electricity(The one with Robin and Lisa to be more specific). And only because humanoids on Kepler B# in the futuristic age, teleported them items that work with solar power and have batteries built into them. The other families in the whole underground city do not have any electricity. Now in a sense, this is a good thing because that means less fossil fuel use(almost 0 even). But, if a whole city is going to thrive, they will need not only domesticated animals and everybody growing their own fruits, vegetables, and grains organically(so if there are pest problems, they should find plants that deter those pests instead of using pesticides), but also renewable energy. I did my research and found 5 different sources of renewable energy. Here they are: Please Register or Log in to view the hidden image! Biomass. According to some people younger trees are better carbon sinks. Maybe per leaf but they have fewer leaves so they use less carbon dioxide, not more. Plus, using biomass destroys habitats and releases CO2 and the purpose of renewable energy is to stop releasing CO2. So this is no go. Please Register or Log in to view the hidden image! Geothermal. This is great for thermal equilibrium in an underground city because this energy source is stable. However, heating up water with geothermal energy is going to lead to a low electricity output. Plus, it would be a waste of perfectly safe water. These last 3 are the only electricity options I would consider good options. However, these are all unstable sources. Please Register or Log in to view the hidden image! Water. This is I think the third most unstable power source. Yes, normally water flow rate in a particular area stays the same but extreme weather can change that to either increase water flow rate(flash flood) or decrease water flow rate(drought). Not only that but it has a minimum. If the water is flowing too slow, no electricity will be produced. Please Register or Log in to view the hidden image! Solar. This, I think is the second most unstable power source in the daytime. Of course, it is stable at night with 0 production but otherwise, it is unstable. But there is no maximum or minimum amount of sunlight. Well, technically, there is a minimum but that minimum is 0. The only way you will get that in the daytime is if it is completely overcast. Yes, a solar eclipse will reduce production but even during totality, the sunshine of the corona is enough to produce a small amount of solar power. Please Register or Log in to view the hidden image! Wind. This, is the most unstable power source of all. Wind direction and speed can change from minute to minute. Not only that but there is a maximum useful wind speed. Any faster and you will get damage to the turbines which lowers electricity production. So I think the best renewable energy source is solar. Despite its instability, it won't cause any damage to habitats, it won't produce CO2, it is very efficient, and it has no maximum. It will work even if there hasn't been rain in weeks and it is blistering hot. Even a solar eclipse won't stop it from working. The only things that will stop solar power being produced are nighttime and totally overcast weather, assuming that the humanoids are able to somehow produce solar power, even in a deep forest.