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Geophysical constraints on the reliability of solar and wind power worldwide

Dan Tong, David J. Farnham, Lei Duan, Qiang Zhang, Nathan S. Lewis, Ken Caldeira, and Steven J. Davis · Nature Communications 12, 6146 · 2021

Key finding. Using 39 years of hourly weather data for 42 countries, the most reliable wind- and solar-only electricity systems — with annual generation equal to annual demand and no storage — met 72-91% of hourly electricity demand, rising to 83-94% with 12 hours of storage.

Eighteen line plots in six rows, one per country — China, Germany, the United States, South Africa, Brazil, and Australia — and three columns. The left column shows annual variability of wind, solar, and electricity demand across the months of the year for 1980 to 2018; the middle and right columns show average daily variability across the hours of the day in summer and in winter. Wind is drawn in purple, solar in yellow, and demand in red, each as a median line inside shaded bands spanning the middle 50% and the full range. Power is expressed as a multiple of its 39-year mean.
Wind, solar, and electricity demand follow very different annual and daily cycles, and the pattern differs sharply between countries. Solar output rises to a midday peak everywhere while wind stays comparatively flat through the day, and Germany's swing between abundant summer solar and winter-dominant wind is far larger than Brazil's or Australia's. Figure 1 from Tong et al. (2021), Nature Communications 12, 6146. Reproduced under CC BY 4.0. Resized for web display.

What question did this research address?

An electricity system built mainly on wind and solar generation produces power when the weather allows, not when demand arrives. This paper asked how much of a country's hourly electricity demand wind and solar resources could actually meet, given the geophysical variability of sunlight and wind.

The question was deliberately posed as a geophysical bound rather than a forecast. The analysis assumes perfect transmission within each country and annual generation equal to annual demand, so what remains is the mismatch imposed by the weather and climate themselves, before any question of cost, policy, or engineering.

What did we find?

The analysis used 39 years of hourly reanalysis data covering 1980 to 2018, varying the scale of generation, the mix between wind and solar, and the amount of energy storage, for each of 42 countries. The most reliable systems were wind-heavy, meeting 72-91% of hours of electricity demand with no storage and 83-94% with 12 hours of storage.

Reliability did not approach completeness even in the best cases. In systems that met more than 90% of demand, hundreds of hours of unmet demand still occurred in a typical year. The shortfalls are concentrated in particular periods rather than spread thinly across the year, which is what makes them difficult to cover.

The resource mismatch has a strong geographical signature. Solar output peaks near midday everywhere, while wind is comparatively flat across the day, and the seasonal swing between summer and winter availability is far larger in a country such as Germany than in Brazil or Australia.

Why does it matter?

The result reframes reliability as a geophysical constraint before it is an engineering or an economic one. The gap between what wind and solar can supply and what demand requires is set in the first instance by the atmosphere, and no amount of cost reduction in generation alone closes it.

By quantifying the residual gap, the analysis also quantifies what would be needed to close it — the power capacity, energy capacity, and utilisation rates of additional storage, demand management, or curtailment, along with the benefit obtained by aggregating supply and demand across larger regions.

Citation

Dan Tong, David J. Farnham, Lei Duan, Qiang Zhang, Nathan S. Lewis, Ken Caldeira, and Steven J. Davis (2021). Geophysical constraints on the reliability of solar and wind power worldwide. Nature Communications 12, 6146.

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