The most sustainable energy sources are renewable bioenergy (wood, biomass, energy crops), geothermal (deep or shallow), solar energy (photovoltaic, solar thermal), hydro and wind energy. Since much more, orders of magnitudes more, solar energy hits the earth than is required for human needs, the total potential of renewable energies seems to be almost infinite. It should be noted that, with respect to our discussion about energy here, the term "potential" is not the same as in physics (see Mechanical Energy). A better term would be "availability." Also, the terms "renewable energy" and "energy sources" do not make sense physically, since in physics the energy conservation law prohibits a source or renewal of energy; only transformations are allowed. From a physical point of view, it would be better to formulate this as "availability of sustainable energies" instead of "potential of renewable energies."
The Short-term Energy Outlook (release December 2014) of the U.S. Energy Information Administration (EIA) describes the contribution from renewable energy sources to the total U.S. primary energy consumption of about 99 quadrillion Btu in the figure above. Wind and solar are increasing.
For U.S. electrical power generation, figure above, nonhydropower renewables generation surpasses hydropower on an annual basis for the first time in 2014. In 2015, total renewables consumption for electric power and heat generation increases by 4.3% as a result of similar increases in both hydropower and nonhydropower renewables. Electricity generation from wind is projected to contribute 4.7% of total electricity generation in 2015.
The U.S. Energy Information Administration's (EIA's) annual publications Annual Energy Outlook and International Energy Outlook focus on the factors that shape the energy system over the long term for the U.S. and the world, respectively. The outlooks go now until 2040 and include the expected potentials of renewable energies. The next four figures and some parts of the explanations were taken from the Annual Energy Outlook 2014.
The figure at left shows renewable electricity generating capacities in GW by energy source from 2012 to 2040. Nonhydropower renewable capacity, particularly wind and solar, nearly doubles and accounts for almost all of the growth in renewable capacity. Solar power leads the growth in renewable capacity, increasing from less than 8 GW in 2012 to more than 48 GW in 2040. Wind capacity increases from 60 GW in 2012 to 87 GW in 2040. Although geothermal capacity more than triples and biomass capacity nearly doubles in the projection, combined they account for less than 15% of renewable capacity additions. Renewable capacity grows by an average of 0.7%/year from 2020 to 2030, compared with 3.8%/year from 2010 to 2020. MSW/LFG denotes municipal solid waste plus landfill gas.
The figure at right shows the renewable electricity generation in TWh from 2000 to 2040. The total renewable electricity generation grows by 1.9%/year on average from 502 TWh in 2012 to 851 TWh in 2040. Nonhydropower renewables, averaging 3.2%/year growth, account for nearly all of the growth, with their total surpassing hydropower in 2014. Solar energy is the fastest-growing source of renewable generation, increasing by 7.5%/year from 2012 to 2040, almost exclusively as a result of increased photovoltaic capacity. Wind generation grows by an average of 2.0%/year but provides the largest absolute increase in renewable generation. Geothermal power is the second-fastest-growing source of renewable electricity generation, increasing from less than 16 TWh in 2012 to 67 TWh in 2040, a 5.4% average annual growth rate. Biomass generation also grows significantly, increasing by an average of 4.4%/year from 2012 to 2040.
The figure at left shows total electricity generation by fuel in PWh from 1990 to 2040. Natural gas is an attractive fuel for new generating capacity, and by 2035 natural gas surpasses coal as the nation’s largest source of energy for electricity generation. As natural gas prices rise and the capital costs of renewable technologies, particularly wind and solar, decrease over time, renewable generation becomes more competitive, accounting for 16% of total electricity generation in 2040. If additional existing coal-fired and nuclear generating capacity were retired, natural gas-fired generation could grow more quickly to fill the void. A strong increase of both coal prices and coal plant operating costs would lead to additional coal plant retirements. In this case, natural gas-fired generation overtakes coal-fired generation in 2019, and by 2040 the natural gas share of total generation reaches 43%. However, barring a breakthrough in electricity storage or related technologies, renewable technologies cannot fully replace the baseload generation lost as a result of coal and nuclear plant retirements.
The figure at right shows primary energy use by fuel in quadrillion Btu from 1980 to 2040. The fossil fuel share of total energy use declines from 82% in 2012 to 80% in 2040, while renewable energy use grows. The renewable share of total energy use (including biofuels) increases from 9% in 2012 to 12% in 2040. Biofuel use accounts for a small part of the increase. Natural gas consumption grows by about 0.8% / year from 2012 to 2040, led by increases in natural gas use for electricity generation and in the industrial sector. Growing production from tight shale keeps the price of natural gas to end users below 2005-08 levels through 2038. Biofuels, ibiodiesel blended into diesel, and ethanol blended into motor gasoline, account for 4% of all petroleum and other liquids consumption by energy content in 2040.Coal consumption increases by an average of 0.3% / year from 2012 to 2040, remaining between the 2011 and 2012 levels through 2040. Coal-fired capacity retirements total 51 GW between 2012 and 2040, but the remaining coal-fired plants continue to be used extensively.
In opposite, a vision for a sustainable energy strategy based on energy efficiency and 100% renewable primary and, therefore, also electrical energy was published by the German Renewable Energy Association "ForschungsVerbund Erneuerbare Energien." It claims to have the solution to achieve this and sets the share of fossil fuels and nuclear energy in energy production for 2050 at zero; see figure 2 in Energiekonzept 2050.