Other technologies, including enhanced geothermal systems that mine the heat stored in deep low-permeability rock, as well as hydrokinetic systems that tap the energy in ocean tidal currents and waves, require further R&D before they can be considered viable entries into the marketplace. Thus these options could potentially be available in the second timeframe (2020–2035) or third timeframe (2035–2050 and beyond) if technological and economic conditions were favorable. Meanwhile, it is possible that plastic organic solar cells, dye-sensitized solar cells, and other new photovoltaic technologies could become commercially available during the second timeframe. In any case, basic and applied research efforts are expected to drive continued technological advances and cost reductions for all renewable electricity-generation technologies.

Because some of the technologies that tap renewable resources to produce electricity must operate under temporal and spatial constraints, special consideration of systems-integration and transmission issues will be needed in order for the penetration of renewable electricity to grow. Such considerations become especially important at sizable penetrations (greater than ~20 percent) of renewables in the domestic electricity generation mix. In the second timeframe in particular, a concurrent and unified overlaying of intelligent control and communications technologies (e.g., advanced sensors, smart meters, and improved software for forecasting and operations) would be required for assuring the viability and continued expansion of renewable electricity. Such improvements in the electricity transmission and distribution grid could enhance system integration and reliability, provide significant capacity and cost advantages, and reduce the need for backup power and energy storage.

COSTS

Given the experience with renewables over the past 20–30 years, it is clear that their economics have not been favorable. The economics of renewables is about profitability, and profitability depends on three drivers: (1) the market price of electricity; (2) the costs of renewables relative to those of other resources; and (3) policies designed to promote renewables or achieve environmental goals (particularly regarding climate) that raise the costs of using fossil fuels or subsidize the costs of renewables.

In order to enjoy greater market penetration in the future, renewables need to achieve cost reductions. And they must do so at a rate that is greater than



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