  Solar and wind power are cost-competitive with fossil fuels, yet their
intermittent nature presents challenges. Significant temporal and geographic
differences in land, wind, and solar resources suggest that long-distance
transmission could be particularly beneficial. Using a detailed, open-source
model, we analyze optimal transmission expansion jointly with storage,
generation, and hourly operations across the three primary interconnects in the
United States. Transmission expansion offers far more benefits in a
high-renewable system than in a system with mostly conventional generation. Yet
while an optimal nationwide plan would have more than triple current
interregional transmission, transmission decreases the cost of a 100% clean
system by only 4% compared to a plan that relies solely on current
transmission. Expanding capacity only within existing interconnects can achieve
most of these savings. Adjustments to energy storage and generation mix can
leverage the current interregional transmission infrastructure to build a clean
power system at a reasonable cost.
