Countries of the world rely heavily on petroleum, coal and natural gas for their energy sources. There are two major types of energy sources: renewable and non-renewable. Hydro-carbon or fossil fuels are non-renewable sources of energy. Reliance on them poses real big problems. First, fossil fuels such as oil, coal, gas, etc, are finite energy resources and the world eventually will run out of them. Secondly, they will become too expensive in the coming decades and too damaging for the environment. Thirdly fossil fuels have direct polluting impacts on earth's environment causing global warming, In contrast renewable energy sources such as, wind and solar energy are constantly and naturally replenished and never run out. Most renewable energy comes either directly or indirectly from the sun. Sunlight or solar energy can be used for heating and lighting homes, for generating electricity and for other commercial and industrial uses. The sun's heat drives the wind and this wind energy can be captured with wind turbines to produce electricity. Then the wind and the sun's heat cause water to evaporate. When this water vapour turns into rain or snow and flows downhill into rivers or streams. Its energy can be captured as hydroelectric energy.

Along with the rain and snow, sunlight causes plants to grow. Plants produce biomass which again can be turned into fuels such as firewood, alcohol, etc. identified as bioenergy.

Scientists have identified Hydrogen as another form of renewable energy source. It is the most abundant element in nature. But it does not exist separately as a gas. It is always combined with other elements, such as with oxygen to make water. Hydrogen, separated from another element, can be burned as a fuel to produce electricity.

Our Earth's interior contains molten lava which gives off extreme heat. This heat inside the Earth produces steam and hot water which can be tapped as geothermal energy to produce electricity for heating homes, etc. Ocean energy comes from several sources. Ocean's force of tide and wave can be used to produce energy. The surface of the ocean gets more heat from the sun than the ocean depths. This temperature difference can be used as energy source too.

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