What are the sources of energy?

Energy sources can be characterised in multiple ways, depending on the point of view and purpose. The first, the most straightforward
classification way, would be grouping by the physical form of the energy.

Forms of energy
The “physics” perspective distinguishes mechanical energy, chemical energy (embedded in the inter-molecular bonds), electromagnetic energy, thermal energy,  nuclear energy, gravity, electricity etc. A very good example of available mechanical energy is tidal energy generated by the Moon – the Moon, due to gravity, attracts water, and the tidal energy (wave) power plants can convert kinetic energy to useful mechanical or electricity.
The example of the source of chemical energy can be coal or natural gas, where the energy is embedded in inter-molecular bonds. Once the bond breaks out, the chemical energy is released to the environment. The chemical reaction cannot be reversed without energy delivery – so we can say it is a one-way process. Nuclear energy comes from the nuclei of atoms – for example, fission is the nuclear reaction of atom division, atomic physical bonds are breaking and excess of energy is released to the environment.  An example of nuclear fusion is the thermal fusion of hydrogen to helium,
taking place inside the Sun.
Electromagnetic energy is embedded in the electromagnetic waves. Depending on the length of the electromagnetic wave, it can be thermal radiation (relatively long length electromagnetic waves), light, ultraviolet, etc.
More reading:
Renewability perspective
Renewability means that the energy source can be practically restored. Non-renewable energy means that after the energy extraction the source has gone forever. Examples of non-renewable energy sources are coal and gas, burning converts useful resources into energy chemically inert, energyless products: water and carbon dioxide.
Burning hydrocarbon = energy + CO+ water + air pollution
From the human perspective, some energy sources are renewable or inexhaustible – we can assume that the Moon and the Sun will be here forever.  Biomass is also categorised as the renewable energy source due to its (at least theoretical) ability of renewal.
Fossil fuels are non-renewable energy resources, created in two main ways: organic and inorganic. Fossil fuels were created millions of
years ago  chiefly by living organisms. Coal was deposed at the time when nature did not develop fungi capable to digest lignin into
products edible by another organism. Because the deposition process cannot be repeated we will never be able to restore coal reservoirs.
Some of the natural gas reservoirs are created by geological processes. Sometimes renewability is artificially associated with the energy source, for example, processing communal waste in many countries enjoys the “renewability” energy financial support (tradable certificates).
Climate change perspective
From the perspective of the impact on climate change, all energy sources producing global warming gases (mainly carbon dioxide and
methane) are harmful to the composition of the protective layer of the atmosphere. The technical and economic potential of energy extraction, the use of the energy source depends on its availability and economics of processing. The civilisation recently met unprecedented dilemmas: relatively cheap, technically available energy sources like coal or natural, when burned, generate environmental hazards related mainly to gaseous emissions, harmful for the environment, humans or for the planet.
IEA, “Installed power generation capacity by source in the New Policies Scenario, 2000-2040”, IEA, Paris https://www.iea.org/data-and-statistics/charts/installed-power-generation-capacity-by-source-in-the-new-policies-scenario-2000-2040

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