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| Fig. 1: Raccoon Mountain pumped hydro storage plant. (Source: Wikimedia Commons) |
There is a fundamental limitation to renewables - humans cannot control the time at which they generate energy, nor how much. This is the intermittency problem. Solar and wind do not generate power at a constant rate, which is a problem for grid operators trying to match energy supply to demand. We can call this "short-term intermittency". Moreover, there are certain stints of the year where neither wind nor solar generate power in a region. We can call this "long-term intermittency". Whether the gaps are short or long, power from elsewhere needs to fill them. This report examines whether long-duration storage can address the intermittency puzzle.
An intuitive solution is to store excess renewable energy so it can be used during intermittent periods. Currently, lithium-ion batteries dominate grid-scale storage. They have become cost-competitive over time, and are typically used for durations of 2 to 4 hours. This suggests that existing storage technologies can address short-term intermittency.
However, lithium batteries lose their economic advantage when built for greater durations of storage. This prevents them from addressing long-term intermittency, such as during cloudy or rainy seasons.
In the absence of batteries, natural gas plants are used. They are flexible, controllable, and perhaps most importantly, economically competitive.
For 12-hour durations, combined cycle natural gas has a lower levelized cost of energy (LCOE) than all other energy storage technologies, around $70-80/MWh. [1] This holds true even for future projections, where lithium-ion is expected to drop even further in price ($80-90/MWh), but still not below that of combined cycle natural gas. [1] Pumped hydro storage (see Fig. 1) is widely considered one of the cheapest forms of energy storage. But at around $120/MWh, it is still not cheaper than the gas alternative. [1] Moreover, gas has an even stronger economic advantage at 120 hours than it does at 12 hours, hovering between $50-100/MWh. [1]
At the moment, using storage to address renewable intermittency does not make economic sense. It is more expensive than natural gas for single-day, multi-day, and season-long periods of intermittency. Therefore, without some form of price manipulation or shock, it is hard to imagine storage becoming the most widely-used solution to intermittency in renewables.
Thinking deeper, there is also a land problem. Storage uses excess energy (i.e, curtailed energy) to charge. Thus, to power periods of long-term intermittency, we would need to overbuild renewable generation. Serpal et al. found that such overbuilding could require as much land as the state of Virginia, which would exceed regional limits. [2] To fully address the intermittency problem, we need an economic solution for long-term intermittency.
It could be argued that natural gas prices will fluctuate if geopolitical tensions or sourcing issues arise. However, under business-as-usual conditions, the price argument is hard to refute.
© Richard Yuan. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. A. Hunter et al., "Techno-Economic Analysis of Long-Duration Energy Storage and Flexible Power Generation Technologies to Support High-Variable Renewable Energy Grids," Joule 5, 2077 (2021).
[2] O. Serpell et al., "Feasibility of Seasonal Storage for a Fully Electrified Economy," Kleinman Center, University of Pennsylvania, April 2020.
