CHALLENGES

What is energy conservation and why is it important? What is energy efficiency and how does it differ from conservation? How do we conserve energy as individuals and as a country? How much does energy efficiency help conserve energy?

What is energy renewability and how does it fit into sustainability? Which fuels are renewable? What can we do to increase our use of renewable fuels?

Power Sources in Brief
U.S. Primary Energy Demand by Source and Sector, 2012

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A power source is a fuel that returns more energy than is invested to produce it. The sun, our star, runs on the most powerful energy source known - nuclear fusion. It's a process scientists hope to harness one day. On earth it was the carbon-based fossil fuel, coal, that powered the Industrial Revolution from the 19th century. The U.S. still depends largely on coal to generate its essential electrical energy, although natural gas and nuclear power together now almost equal coal in importance. Petroleum, the single most-consumed fuel, is the giant of transportation, but produces only 1% of our electricity.


For the 21st century, however, emergent international environmental, economic and political concerns - climate change, newly prosperous and competitive nations, dependence on other countries for critical fuel supplies - have pushed research on renewable sources of energy generation. Changing to mostly renewable power sources for a clean,affordable, sustainable future is expensive, controversial, transformative - and long way off.


 

Here's a brief overview of our fuels:


1. Fossil fuels - coal, oil, natural gas. Acid rain, smog, and smoke befouled the air of industrialized countries and large cities for more than a century. This visibly dirty air is the result of burning the carbon-based fossil fuels: oil to fuel transportation and coal for energy production. Natural gas, also important in power generation, produces cleaner results. The fossil fuels have been indicted for the over-production of carbon dioxide, the invisible "greenhouse gas" most closely associated with global warming. Yet about 84% of our overall power - in electricity, transport, industry, commerce and consumer - still comes from fossil fuels.

2. Nuclear fission. Of all our current power generators, nuclear reactors produce the greatest amount of energy per unit of fuel weight. They do this by splitting the atoms of specially processed uranium, releasing large amounts of energy. Under the right conditions the splitting process can become a self-sustaining chain reaction, allowing a continuous flow of energy and power. The process is efficient, low in cost and emits no air pollution or carbon dioxide. However, nuclear reactors are expensive to build, and the fission process results in highly radioactive waste products that must be isolated from human exposure in safe and secure storage conditions for many years. Nuclear produces 19.4% of our power, but no new plants have been built in the last 20 years due to public fears of environmental and, more recently, security dangers. Improved technology and practical energy needs may change that, but transportation and long-term storage of used fuel rods have yet to be fully addressed.

3. Renewables - hydroelectric, biomass, wind, geothermal and solar. Currently, these power sources account for about 7% of our energy supply. However, they are the most hoped-for solution to our growing need for clean energy.


Hydroelectric is the leading renewable, responsible for 6% of overall electrical generation, and 34% of renewable power. A hydroelectric station is built by damming a river and erecting turbines, which are run by falling water. The energy produced is cheap, clean and constant. But dams are costly to build, may cause flooding and can disrupt ecological plant and animal systems.


Biomass accounts for 53% of renewable power. It is now used mostly for the transportation fuels ethanol and biodiesel. However, biomass fuels, (i.e., fast-growing "energy" crops, agricultural wastes and methane from landfills) may be burned, fermented or "digested" by bacteria to provide an inexpensive, relatively clean electrical power source.


Wind is now producing 7% of renewable energy. That's just .51% of the total energy yield, but wind energy has nearly tripled in production since 2007. It works by using the wind to turn tall propeller-like turbines, whose spinning hubs connect to generators which convert mechanical to electrical energy. Wind is relatively cheap, very clean and a wind farm takes about 2 years to build. But it is intermittent as a power source, hurts birds, and neighbors say it's annoyingly noisy.


Geothermal power stations tap into reservoirs of deep underground water heated by surrounding hotbeds of rock and volcanic magma. Steam from that water is what ultimately powers geothermal generators to produce 5% of renewable energy or .35% of total electrical generation. Small geothemal systems can heat and cool buildings efficiently, though at considerable installation cost.


Solar power currently comes in two forms: concentrated solar power (CSP) and photovoltaic (PV).

  • Concentrated solar power (CSP), presently the most practical method for large-scale solar power generation, concentrates the sun's heat on an absorbant surface, which transfers the heat to water. The resulting steam drives a turbine linked to an electric generator. CSP plants employ large reflective surfaces, which tilt with the movement of the sun to focus solar rays on absorber tubes, which contain a fluid that transfers the heat to the power plant's boilers. For smaller applications like heating and cooling buildings or heating water, flat heat-absorbent panels pass heat through to tubes of fluid which transfer it to an insulated water tank. CSP plants now generate a tiny .09% of U.S. electrical power. The world's largest solar plant, built in the 1980's in the Mohave Desert of California, uses the CSP method.
  • Photovoltaic (PV) cells, when exposed to the radiant energy of the sun, produce an electrical voltage. At a photovoltaic power station, multiple panels of silicon-based PV cells covered by non-reflective glass collect electromagnetic energy directly from the sun. Electric fields within the cells force the sunlight electrons to move in a given direction, establishing direct current (DC) electricity. Photovoltaic energy must be stored in batteries or other special storage systems for use in cloudy conditions or darkness.
Go Deeper ...
EPA Interactive U.S. Map of Greenhouse Gas Emissions

An interactive map detailing major facilities and their green house gas emissions for 2010 in all 50 states and territories.