I shall explain, and take up a good chuck of your time. First, I'm going to start with how we get energy. Now it more or less all comes from the Sun. Solar power converts the photons (light particles) from the sun into energy through use of layers of photovoltaic material that becomes electrically charged when in the presence of light. Second there is wind energy. Wind is created by the sun heating up sections of the earth more and creating convection currents which is basically warm air and cool air moving in different directions and because of gravity the air doesn't float away into space and thus moves horizontally when it collides with air of a different temperature. Hydroelectric power from from water that is pulled by gravity closer to the center of the earth. Much of that water is carried to higher elevations by rain and slow. Some of it also comes from ground water. How does ground water move uphill you ask? Well, there are probably several mechanisms but the one that sticks out in my mind is the capillary action from trees and plants and the soil itself. Things move from areas of high concentration to areas of low concentration. So the water collects at a low spot but just above that spot imagine that the ground is bone dry and there is a tree slightly uphill. Because the ground is packed so close together a wet piece of dirt beside a dry piece of dirt will create a gradient and some of the water will move to the dry piece of dirt. Now imagine the same concept but a hill side miles long. Too big of a jump you say it doesn't scale up?
Luke: "It's too big."
Yoda: "Size matters not."
How about ethanol and vegetables and other plants? Well they grow by photosynthesis which is another way of saying they use the light energy (the sun) to grow. So biofuel and ethanol and other fuels like that are basically produced from the sun. Although they have to be refined which takes a large amount of energy as well.
How about oil? Oil is thought to be created by organic material (plants and animals) that have been compressed under great pressure to form polymer chains we call oil. That means that we can create oil in a lab (ever heard of synthetic oil?) by replicating the conditions thought to produce oil in nature.
Then there is fission and fusion power. Fission is traditional nuclear reactor design and the atomic bombs they dropped on Japan. It consists of really heavy elements getting split into two smaller elements and when that occurs a lot of energy is released. Fusion is basically the bombs that are way bigger and not used in any nuclear power reactor. If we can solve fusion power and put it in a reactor that will give up so much energy. It consists of Hydrogen that is combined to form Helium and released energy when that reaction occurs. However it is so hot it will melt anything we have yet created to hold it in. This is the reaction that we think makes the sun shine.
Secondly there is another problem, besides producing a ton of energy. Containing that energy in a small place. Currently we have a bit of a problem with energy density. It's not exactly a problem because we have never known different but it limits us from flying cars, traveling to Mars in a week, and other science fiction fantasies. While lithium batteries seem so amazing with so much power imagine a battery so powerful the size of your cell phone that you put it in your car and don't have to fuel up ever again.
Now that example is a leap from where we are now but the world energy consumption is increasing. From about 5 terawatts in 1965 to 13 terawatts in 2006. Think about the energy usage in your house. Fifty years ago who had more than one television set? Twenty years ago who used the computer and internet as much as we use it today? I see no way that our energy usage can honestly be reduced. As people begin to acquire huge screens and touch capable screens and have more of them the power required is just going to go up. As the world food supply becomes incapable of handling the world population will we start farming underground or in buildings with artificial lights?
As the world progresses to easily renewable energy sources like solar and wind anther problem is how to store lots of energy for times when the sun does not shine and the wind does not blow? This goes back to energy density and also the efficiency of storing and then extracting that energy.
I feel that in the future energy is going to be a very big issue. Someone who controls energy can have clean water, electricity, heat or air conditioning, computers, internet access, transportation, food, and other communication. Basically the more energy we have the more we can do. Of course the converse of that is the more we want to do the more energy we need.
The future is going to be interesting.