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Cumulative new transmission requirements in four scenarios


Rocky Mountain Institute’s four scenarios for the future U.S. electricity system ( detailed here ) all have very different requirements for an expanded transmission infrastructure.

While the Maintain (“business-as-usual”) case has a resource mix similar to today’s, rising demand will require more generation and transmission. Based on analysis conducted with the National Renewable Energy Laboratory’s (NREL) Renewable Energy Deployment System (ReEDS) expansion model, the Maintain case would require just over 25 million MW-miles of new transmission through 2050—about 60% long-distance, interregional transmission lines. The remaining 40% would be intraregional lines for moving power around within grid balancing areas and delivering power to customers. The Migrate case, relying heavily on large, centralized nuclear and gasified coal plants, will require twice as much interregional transmission as in the Maintain case.

A U.S. electricity future relying primarily on renewable energy—as RMI’s Renew and Transform cases both do—requires a much larger investment in transmission infrastructure. The Renew case, dependent on centralized, utility-scale renewables, has huge transmission requirements. Many of the most energy-rich renewable resources are located far from demand centers and off existing transmission networks. This case will need 115 million MW-miles of inter- and intra-regional transmission lines, more than four times that required in the “business-as-usual” case. RMI’s Transform case, however, with a greater emphasis on distributed renewable generation close to load centers, needs a projected 66 million MW-miles of new transmission.

The significant transmission buildout required in both renewables-heavy scenarios brings inherent risks. Transmission development is expensive, time-consuming, and often subject to permitting challenges. However, with better utility planning, a shift to a more intelligent network of microgrids, and incremental construction of small renewable plants (as compared to GW-scale nuclear or coal plants), many of these transmission risks and costs can be mitigated. Indeed, considerable transmission can probably be avoided by simply competing it against efficiency, demand response, and distributed generation—a process not yet rigorously analyzable with the tools RMI used nor with any others currently available.


RMI analysis