stored in deep low-permeability rock and hydrokinetic technologies that tap ocean tidal currents and wave energy, require further development before they can be considered viable entrants into the marketplace. The costs of already-developed renewable electricity technologies will likely be driven down through incremental improvements in technology, “learning curve” technology maturation, and manufacturing economies of scale. Despite short-term increases in cost over the past couple of years, in particular for wind turbines and solar photovoltaics, there have been substantial long-term decreases in the costs of these technologies, and recent cost increases due to manufacturing and materials shortages will be reduced if sustained growth in renewable sources spurs increased investment in them. In addition, support for basic and applied research is needed to drive continued technological advances and cost reductions for all renewable electricity technologies.

In contrast to fossil-based or nuclear energy, renewable energy resources are more widely distributed, and the technologies that convert these resources to useful energy must be located at the source of the energy. Further, extensive use of intermittent renewable resources such as wind and solar power to generate electricity must accommodate temporal variation in the availability of these resources. This variability requires special attention to system integration and transmission issues as the use of renewable electricity expands. Such considerations will become especially important at greater penetrations of renewable electricity in the domestic electricity generation mix. A contemporaneous, unified intelligent electronic control and communications system overlaid on the entire electricity delivery infrastructure would enhance the viability and continued expansion of renewable electricity in the period from 2020 to 2035. Such improvements in the intelligence of the transmission and distribution grid could enhance the whole electricity system’s reliability and help facilitate integration of renewable electricity into that system, while reducing the need for backup power to support the enhanced utilization of renewable electricity.

In the third time period, 2035 and beyond, further expansion of renewable electricity is possible as advanced technologies are developed, and as existing technologies achieve lower costs and higher performance with the maturing of the technology and an increasing scale of deployment. Achieving a predominant (i.e., >50 percent) penetration of intermittent renewable resources such as wind and solar into the electricity marketplace, however, will require technologies that are largely unavailable or not yet developed today, such as large-scale and distributed cost-effective energy storage and new methods for cost-effective, long-distance electricity transmission. Finally, there might be further consideration of an inte-



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