Some energy sources have advantages for specific uses or locations. For example, fuels from petroleum are well suited for transportation because they pack a lot of energy in a small space and are easily transported and stored. Solar photovoltaics are well suited for heating or electricity generation in desert climates or other sunny places with lots of flat, open space. Small hydroelectric installations are a good solution for supplying power or mechanical energy close to where it is used. Coal is widely used for power generation in many fast-developing countries—including China, India, and many others—because domestic supplies are readily available.
Efficiency is an important factor in energy costs. How efficiently can the energy be produced, delivered, and used? How much energy value is lost in that process, and how much ends up being transformed into useful work? Industries that produce or use energy continually look for ways to improve efficiency, since this is a key to making their products more competitive.
The ideal energy source—cheap, plentiful, and pollution-free—may prove unattainable in our lifetime, but that is the ultimate goal. The energy industry is continuing to improve its technologies and practices, to produce and use energy more efficiently and cleanly. A future source may be hydrogen-based.
Energy resources are often categorized as renewable or nonrenewable.
Renewable energy resources
Renewable energy resources are those that can be replenished quickly—examples are solar power, biomass, geothermal, hydroelectric, wind power, and fast-reaction nuclear power. Renewable energy sources supply about seven percent of energy needs in the United States; the other 93 percent comes from nonrenewables. The two largest categories of renewable energy now in use in the U.S. are biomass—primarily wood wastes that are used by the forest products industry to generate electricity and heat—and hydroelectricity.
In most cases, fossil energy resources are currently more affordable and easier to store and transport than renewable sources. For renewables to become more widely used, many hurdles must be overcome—most related to producing and distributing renewable energy more economically.
How far into the future will energy resources be available to supply our needs? The sustainability of any particular energy resource is an important consideration in determining where to invest in energy technology and infrastructure. All energy resources, whether renewable or nonrenewable, must be used efficiently and sustainably in order to safeguard the future for ourselves and our children.
Nonrenewable energy resources
Nonrenewable energy resources include coal, oil, natural gas, and uranium-235, which is used to fuel slow-reaction nuclear power. Projections of how long a nonrenewable energy resource will last depend on many changeable factors. These include the growth rate of consumption, and estimates of how much of the remaining resources can be economically recovered. New exploration and production technologies often increase the ability of producers to locate and recover resources. World reserves of fossil energy are projected to last for many more decades—and, in the case of coal, for centuries.
Since the beginning of the industrial revolution, significant progress has been made in the efficient use of fossil fuels. For example, new gas-powered fuel cells are 40 percent to 80 percent efficient, with no combustion or emissions involved in the energy conversion process. Likewise, the next generation of hybrid-fuel cars will improve efficiency by capturing kinetic energy from the wheels to power the battery.
Hybrid cars that use electricity from batteries together with gasoline are providing new transportation options. Automakers also are developing fuels cells that extract hydrogen from gasoline or methanol. Like batteries, fuel cells rely on chemical reactions rather than combustion.