cont.
1.1 Problem statement

The major challenge in relying exclusively on “flux-limited” energy sources is to ensure that the varying energy supply can be aligned with the energy demand all the time. “Stock-limited” resources provide both storage and supply by nature and make the alignment of the supply with the demand significantly easier. Although energy flows steadily from the Sun, solar or solar-derived renewable energy sources (e.g., wind) are highly intermittent. The intermittency is due to Earth’s rotation (diurnal) and its tilted axis relative to the orbital plane around the Sun (seasonal) that is exacerbated by the chaotic behaviors of Earth’s atmosphere leading to additional stochastic variabilities. As a result, the energy supplies provided by solar and wind energy sources are highly variable and rarely align with energy demands.

To address the disconnection between energy demand and variable renewable resources, three solutions have been proposed: 1) curtailing loads (that is, modify or fail to satisfy demands), 2) providing supplemental energy sources, or 3) deploying energy storage (Clack et al., 2017). An energy system that fails to satisfy demand, forcing users to accept blackouts or adjust their demand, hardly meets the expectation of reliable services, ruling out the first option as “solution.”

Providing supplemental energy from varying resources can be achieved by installing excess capacity to meet demand all time, but such overbuilding might remain infeasible. For example, at 45° latitude, the incoming solar radiation is three to four times higher in summer than that in winter (these differences are even greater at higher latitudes). Even if winter months were as cloud free as in the summer, solar installation to provide the same power output would require three to four times more solar panels and associated infrastructure to deliver the same energy in the winter.

When the generation of the renewable fails completely, no additional capacity will be able to step in. This situation actually happens in Germany to the extent that they gave a name “Dunkelflaute” (dark lull) referring to the situation that is quite common in October and November, when the sky is covered by thick gray clouds and the air does not move for weeks. In such situation, the only solution left is the deployment of energy storage.

Alternatively, long-distance grid connectivity might allow energy transmission between regions experiencing abundance and shortfalls of renewables at different times and cope with intermittency (Jacobson et al., 2015a; Jacobson et al., 2015b). Interconnected grids can redistribute excess power generated to areas in need. This requires the grid to connect regions experiencing significantly different climate regimes.

Hydropower, while limited in its contribution to satisfying energy demands globally (Fekete et al., 2010), is less affected by intermittency, particularly when substantial water storage behind dams comprises sufficient potential energy to decouple the variation in riverine water fluxes from power generation. Hydropower is expected to partially provide energy at times when solar and wind fall short to meet the demand. Hydropower is much more flexible than solar or wind to the point that hydropower is often operated to assist power generation during peak demands. However, the magnitude that hydropower could be scaled up is poorly understood.

The “capacity factor”—which is the ratio of the power generated at any given time or averaged over a longer time period with respect to the “nameplate capacity” of the installed infrastructure—has a very different meaning for hydropower than for solar or wind. Hydropower plants are not necessarily expected to operate 24/7. They are often built with “nameplate capacity” beyond the energy available if the plant is operated 24/7. Instead, the turbines are purposely left idle and are only turned on when additional power is needed to meet peak demand. This intentionally intermittent operation leads to low “capacity factors.” In contrast, the low-load “capacity factors” of solar or wind generations are unintentional. The low-load “capacity factors” of solar and wind are representing the degree to which they failed to deliver power.

From the perspectives of the grid operator, renewables represent risk that destabilizes power delivery. Although weather forecasts are steadily improving and provide more leeway to prepare for sudden changes in the power supplies, the degree to which grid operators can turn on alternative power sources or alert customers to adjust their power demand is limited. In a truly “fossil fuel-free” energy system that relies exclusively on various renewable energy sources, the only viable means of addressing intermittency is to deploy energy storage.
1.2 Literature review

In preparation for our paper, we compiled a database of publications containing 360+ references from authors dominantly associated with the work of the National Renewable Energy Laboratory (NREL). While our list of publications is unlikely to be fully representative of the entire community researching the transitioning to renewable energy sources, some striking patterns still emerged:

1) Out of the 360+ publications, only few (Doubleday et al., 2019; Kumler et al., 2019; Denholm et al., 2021; Keskar et al., 2023) addressed the seasonal and inter-annual variability of renewables.

tbc

I'm sorry. But I can't trust anything 'renewable or environmental' that comes out of America (even China for that matter).