• A modern T&D system can allow for greater penetration of large-scale intermittent renewable electricity sources as well as distributed generation and self-generation, thereby reducing the amount of coal that must be burned.45

  • Modern demand-response technologies (such as grid-friendly appliances that can be controlled by the utility to shift load to off-peak times) can be better accommodated, thereby reducing demand that must be met by inefficient generating equipment.

  • Battery electric vehicles (BEVs) can be better accommodated, particularly after 2030.

  • Efficiency can be improved in the T&D system as well as in end-uses, reducing the need for new generation and the siting of new transmission lines.

A modern T&D system can enable intermittent renewable electricity sources (particularly wind power) to contribute substantially to the U.S. energy supply. The electricity provided by wind power varies significantly over the course of a day and over the year because of natural variations in wind speed. As a general rule, a power-delivery system can handle the loss of 10–20 percent of the local generating capacity as long as adequate reserve capacity is available.46 Grid operators normally require generating companies to have spinning reserve (generators that can increase their output very quickly) equivalent to the largest unit on the system; if that unit fails it can be replaced without disrupting delivery of power. Because intermittent sources cannot be depended on, the spinning reserve has to include a significant fraction of the renewable capacity in addition to the largest unit of conventional power.47 Above 10–20 percent, a rapid loss of wind power could cause system instability unless the system was modernized.48 Even at lower


Self-generation is a special case of distributed generation. End users generate some portion of their own energy needs, utilizing, for example, rooftop solar panels. Under some conditions, any excess power may be sold to the utility.


Grid modernization is not needed for integrating intermittent renewable-electricity sources in relatively small percentages of the overall electricity supply. This is discussed in greater detail in Chapter 6.


Wind and solar power are the main intermittent renewable-energy sources. Other renewables, such as hydropower, geothermal, and biofuels, are not intermittent.


The changes needed to accommodate renewables are discussed in more detail in the technology section of this chapter; they involve large-scale storage as well as high-voltage long-distance transmission.

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