From the perspective of ongoing sustainable use of energy resources, we have classified energy sources into two categories: renewable energy and nonrenewable energy. Renewable energy is energy derived from sources that are replenished faster than we use them. Most renewable energy sources are also clean in that they do not require combustion, which releases pollutants and greenhouse gases into the environment. Nonrenewable energy comes from sources that are used faster than they replenish (they may be finite or not). Biomass and biofuels are an interesting case: if they are used faster than their natural growth rate, or in an unsustainable way, they can also become nonrenewable.
About 90% of global energy consumption is supplied by nonrenewable sources. This is extremely problematic because nonrenewable resources will soon be exhausted, and most of these sources are also major greenhouse gas emitters. On the other hand, renewable energy sources (including biomass, biofuels, hydropower, geothermal, solar, and wind) accounted for only 11% of the global energy consumption in 2010, and are projected to account for only 15% by 2040 (from U.S. Energy Information Administration, International Energy Outlook 2013). Of these, solar, wind, geothermal, and hydropower are the cleanest energy sources as they do not require combustion and therefore have no direct greenhouse gas or air pollutant emissions, as compared to the burning of biomass and biofuels which emit greenhouse gases and particles which pollute the air and cause climate change.
Nuclear power, like fossil fuels, is a non-renewable source because there is a finite amount of uranium available in the Earth’s crust. However, nuclear power generation does not emit greenhouse gases or other air pollutants. Let’s take a more in depth look at nonrenewable and renewable energy sources.