tages and can use a wider array of materials, but they are less well developed.
Concentrating solar power. The three main options are parabolic troughs, power towers, and dish-Stirling engine systems. The first two are now the lowest-cost utility-scale solar electricity technology for regions of high solar flux. Design improvements and advances in high-temperature and optical materials are the major paths to cost reduction.
Geothermal. Conventional geothermal, which relies on hydrothermal sources within 3 km of the surface to drive a heat engine, is a fairly mature technology, but it has a rather limited resource base. A study of the western United States found that 13 GW of electrical power capacity exists in identified geothermal resources in this region. Greatly expanding that base will require enhanced geothermal systems to mine heat down to a depth of 10 km. Such systems, however, face many technical challenges and are not now in operation.
Hydropower. Conventional hydropower is the least expensive source of electricity. The technology is well developed, and objectives are to increase efficiency and reduce impacts on associated water bodies, as efforts to expand are likely to be limited by environmental concerns. Hydrokinetic technologies produce electricity using currents, tides, and ocean waves; many designs and demonstration plants exist, but there are no commercial deployments.
Biopower. There are three main sources: wood/plant waste, municipal solid waste/landfill gas, and other (e.g., agriculture waste, used tires). A variety of technologies may be used to produce electricity, including current technologies based on the steam-Rankine cycle and future applications involving gasification combined-cycle plants. The use of biomass for biopower competes with its use for alternative liquid fuels.
Between now and 2020, there are no technological constraints to accelerated deployment of the major renewable resources with existing technologies. However, there are other kinds of barriers. The main ones currently include the cost-competitiveness of existing technologies relative to most other sources of electricity (with no prices assigned to carbon emissions or other externalities); markets not sufficiently shaped so as to allow the existing technologies to reach full scale