What We Publish

Geophysical constraints on the reliability of solar and wind power in the United States

Matthew R. Shaner, Steven J. Davis, Nathan S. Lewis, and Ken Caldeira · Energy & Environmental Science 11, 914-925 · 2018

Key finding. Daily and seasonal variability of solar, wind, and electricity demand require substantial dispatchable power capacity in order to achieve deep decarbonization of the United States electricity system with high reliability.

Four contour panels showing the reliability of United States electricity supply, as the percentage of annual demand met, against the solar-to-wind resource mix on the vertical axis and the geographic area of resource aggregation on the horizontal axis. The left column has no storage and the right column twelve hours of storage; the top row has generation equal to demand and the bottom row one and a half times demand. Reliability rises from below 50 per cent in the darkest panel to above 99 per cent in the brightest.
Reliability improves with wind-heavy mixes, wider geographic aggregation, storage, and overbuilding — but even with twelve hours of storage and 1.5 times generation, the contours stop short of complete reliability. Figure 2 from Shaner et al. (2018), Energy & Environmental Science 11, 914-925. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Solar and wind resources in the United States are far larger than electricity demand, so the constraint on a solar- and wind-based system is not the size of the resource.

This paper asked what the constraint actually is — how the daily and seasonal timing of solar and wind availability lines up against the timing of electricity demand, and what a system would need in addition to generation in order to be reliable.

What did we find?

The analysis is built on historical weather and demand records for the United States, so the mismatch it characterizes is the one that actually occurred rather than one produced by a scenario.

Variability operates on two distinct timescales that pose different problems. Daily variability is largely a solar phenomenon and is addressable by storage measured in hours; seasonal variability is not, and storage sized for the daily problem does not solve it.

Reliability is the binding constraint rather than total energy. A system generating as much electricity over a year as is consumed over that year can still fail to meet demand in a large number of individual hours.

Closing the remaining gap requires substantial dispatchable capacity — generation that can be called on when needed — or an equivalent amount of storage, demand flexibility, or transmission.

Why does it matter?

The result reframes what "enough" means for a renewable electricity system. Sizing generation to annual demand is a necessary condition and not a sufficient one, and systems planned on annual energy alone will be unreliable in ways their design does not reveal.

It also sets up the international comparison. The same geophysical question asked across many countries shows that the severity of the problem depends strongly on a country's latitude and weather, so the United States result is not universal.

Citation

Matthew R. Shaner, Steven J. Davis, Nathan S. Lewis, and Ken Caldeira (2018). Geophysical constraints on the reliability of solar and wind power in the United States. Energy & Environmental Science 11, 914-925.

Related