In a coal or oil fired power station, the fuel is burned (converting its chemical energy into heat) and the heat used to convert water into steam at very high temperature and pressure. This then drives a steam turbine, a device which harnesses the energy in the steam (heat and pressure) to produce rotational movement (mechanical energy). The rotating shaft of the steam turbine is coupled to the armature of the alternator, so the final result is electricity.
Windmills have been around for centuries and all have harnesse d the energy of moving air (wind!) through rotating sails or fan blades. Traditionally, the mechanical energy was used directly, to turn a mill wheel. A modern wind turbine simply couples the rotating shaft to an alternator armature. The last link in the chain is always the same - electricity from mechanical rotation.
Hydro Electric Power
Here, the source energy (there always has to be https://www.youtube.com/watch?v=aWoSWNwtVKA one!) is gravitational potential energy. A mountain stream is dammed in a high place, to create an artificial lake or reservoir. Farther down the mountain, the power station is equipped with water turbines. These are simply highly efficient versions of the old fashioned water-wheel; effectively they harness the kinetic energy of a carefully channelled waterfall to produce mechanical rotation. The rest you know.
Tidal Power and Wave Power
These new technologies extract energ y from the long-term bulk movement of water in a tidal estuary and from the short-term wave motion of the surface. The principle remains the same, to harness the 'free' natural energy in moving water to drive a mechanical turbine.
Solar Power - local
In a sense. all energy on Earth is solar energy, as even fossil fuels are chemical 'memories' of ancient sunshine. But we're talking here about generating electricity from solar energy, and strangely enough, it's not very easy. The problem is that you can't easily convert sunshine into mechanical rotation to drive alternators on a commercial scale. Solar panels have no moving https://www.youtube.com/watch?v=aWoSWNwtVKA parts, and so the electricity they produce is 'DC' or direct current. This is like the electricity from a battery. It's great for local use, e.g. running a small irrigation pump, but the big problem with DC is that it is hard to distribute. (No time to explain that now - maybe another hub!)
Solar Power - commercial
Photovoltaic units, as described above, are best suited to localised applications like space or water heating. However, commercial-scale solar power plants, though still expensive to build, are becoming viable, the more so as the price of fossil fuels increases.
No single design for commercial solar power has yet won through, but all are based on the same idea - a large array of reflectors to collect the sun's rays and focus them onto a receiver which is effectively pipe-work containing a heat-absorbing fluid. Technologies are already well developed to store the collected energy as heat and to convert it to electricity using steam or gas turbines at a steady rate, night and day. The biggest problem is that the sun moves (OK, the Earth rotates!) and so ingenious tracking mechani sms are needed to make the reflectors follow the sun through the daylight hours.
Ironically, the part of the world best suited for deploying this technology is the part that least needs it - the oil rich deserts of the Middle East.
This is another underdeveloped source. If you drill down into the Earth's crust, at first the temperature drops, because the sun's warmth can't penetrate. But deeper, the temperature rises. Volcanoes are evidence of this - molten lava is pretty hot! That well of energy is there to be tapped. As always, the final conversion process http://www.indeed.com/q-electrician-jobs.html is the familiar steam turbine. And, like solar energy, it is environmentally friendly, provided you don't accidentally trigger a local volcano! But it is not as simple as it seems. The process of taking heat from a hot rock cools the rock locally. There's plenty more heat surrounding it , but can it flow quickly enough to your collectors? Again, it's another technology whose time will come, but not a panacaea.
This is the controversial one. Nuclear fission is a process in which unstable (radioactive) atomic nuclei break down, releasing energy in the form of radiation (escaping particles). By concentrating these nuclei together, a controlled chain recation is produced releasing huge amounts of energy which is used to convert water into steam. The process of generating electricity in a nuclear power plant is simply by steam turbine, exactly the same as in a fossil fuel plant. The public fear of nuclear power is twofold: the risk of meltdown - an uncontrolled nuclear reactor is not very different from an 'atomic' bomb; also the by-product, radioactive nuclear waste, is none too pleasant.
As an aside - Nuclear energy does not obey the classical Physics law of conservation of Energy. It does however obey the Modern Physics law of cons ervation of Mass-Energy which allows for interconversion of matter and energy according to Einstein's famous equation E=mc This fundamental difference means that Nuclear Energy is potentially the most fruitful source of all. It is important that proper scientific research & development into nuclear energy, and Nuclear Fusion in particular, should be allowed to continue.