How much renewable-energy displacement of fossil fuel is possible or feasible? Worldwide, renewable energy accounts for approximately 20% of the total primary energy supply, mainly from biomass energy and hydropower . In a highly industrialised economy such as the USA, renewable energies contribute approximately 8% to the national primary energy supply. In Austria, Australia, Sweden, Denmark, Canada, and Switzerland, renewable-energy resources, excluding hydropower, provide between 1-5% of the total primary energy. While modest in the total energy balance, these shares are significant, and the potential exists for them to be widely expanded.
Considering the modest renewable-energy contributions in most countries and the huge amount of capital invested in fossil energy infrastructure, suggestions that renewable energy could exceed the output of the conventional energy system seem irresponsible. But is it irresponsible to suggest that the world energy consumption and the economy related with this should begin to shift to renewable energy resources when they far exceed fossil energy reserves? In every country, the combined solar, biomass, geothermal, hydroelectric, and wind energy potential overshadows reserves of petroleum, natural gas, coal, and uranium.
In the U.S.A., one of the richest countries in terms of fossil energy reserves, renewable energy dominates the total energy resource base projected into the next century. Using billions of barrel of oil equivalent (BBOE) to gauge its energy resources, Table 1 shows that more than 614,000 BBOE are available in renewable energies in the USA compared with 44,000 BBOE for conventional energy resources.
Table 1: Total resource base of energy (billions of barrels of oil equivalent), U.S.A. 
Total resources expressed in Table 1 represent the sum of identified and undiscovered resources that can be economically extracted with current or future technology in a set time frame. Renewable energy resources are based on yearly output over a set period of time corresponding to the conventional resource projections established by the Federal Government that performed the resource assessment). A more realistic determination of energy resources can be derived by limiting the estimate of energy reserves to only those that are "accessible"; that is, the resource that can be delivered with current technologies or with technologies to be available in the very near term.
In Table 2, the calculation of accessible resources eliminates land areas already occupied by urban and suburban development, or for which other competing uses would preclude renewable energy conversion. Furthermore, laws that restrict energy operations in parklands are taken into account, and hydroelectric potential is severely curtailed by dam ecological impacts. These assumptions also curtail geothermal and wind energy.
Table 2: Accessible resources of energy (billions of barrels of oil equivalent), U.S.A. 
Table 2 also shows that a very large potential for renewable forms exists in the future. Accessible resources of biomass and solar energy amount to 101,153 BBOE, compared to 6,577 BBOE for coal. Furthermore, the accessible renewable energy resources are essentially infinite. Solar radiation will continue to fall uninterrupted for millions of years to come, while coal resources will ultimately be expended.
This summary of the USA energy future was used to point out that renewable energy resource dominates the available supply of fossil energy.
In addition, the use of renewable energy has benefits not accounted in the "official" economy.
Social and economic development
Production of renewable energy can provide economic development and employment opportunities. This is especially true for rural areas that otherwise, have limited opportunities for economic growth. Renewable energy can thus help reduce poverty in rural areas and reduce the pressure for urban migration.
Growing biomass for energy on degraded lands can provide the incentives and financing needed to restore lands rendered nearly useless by previous agricultural or forestry practices. Although land farmed for energy would not be restored to its original condition, the recovery of these lands for biomass plantations would support rural development, prevent erosion, and provide a better habitat for wildlife than at present.
Reduce air pollution
Renewable energy technologies, such as methanol or hydrogen for fuel-cell vehicles, produce virtually none of the emissions associated with urban air pollution and acid deposition. The need for costly additional controls can thus be avoided with the use of renewable technologies.
Abatement of global warming
Renewable energy use does not produce carbon dioxide and other greenhouse emissions that contribute to the global warming. Even the use of biomass fuels will not contribute to the global warming. The carbon dioxide released when biomass is burned equals the amount absorbed from the atmosphere by plants as they are grown for biomass fuel.
Fuel supply diversity
There will be substantial interregional energy trade in a renewable scenario, involving a diversity of energy carriers and suppliers. Energy importers would be able to choose from more producers and fuel types than they do today and thus would be less vulnerable to monopoly price manipulation or unexpected disruption of supplies. Such competition would make wide swings in energy prices less likely, leading eventually to the stabilization of the world oil price. The growth in world energy trade would also provide new opportunities for energy suppliers. Especially promising are the prospects for trade in alcohol fuels such as methanol derived from biomass, and later hydrogen.
Reducing the risks of nuclear weapons proliferation
Competitive renewable resources could reduce incentives to build a large world infrastructure in support of nuclear energy, thus avoiding major increases in the production, transportation, and storage of plutonium and other nuclear materials that could be diverted to nuclear weapons production.
The potential role for renewable technology in an electric utility system depends on the cost of producing electricity with the technology and on the value of this electricity. The most important measure of the worth of renewable power sources are the role they can play in an investment portfolio designed to minimise the cost of energy services. Also important is the extent to which their adoption would provide environmental and other non-market benefits.
In what follows, a short description of each of the renewable energy technologies will be given.