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| Fig. 1: Discharge time vs power rating. [4] (Image source: M. Khail) |
As the world transitions away from fossil fuels to renewable energy, there is a pressing need to develop energy storage assets that can provide power when the sun is not shining, and the wind is not blowing. The rationale behind this urgency lies in ensuring a continuous power supply, lest resorting to burning natural gas (or other fossil fuels) becomes necessary.
As shown in Table 1, wind and solar in 2021 reached a 10.2% share of electric power generation for the first time in history. [1] According to the BP estimates, the total wind and solar electric energy generated in 2021 was 2,893 TWh or ~23.1 EJ. [1]
As global renewable capacity increases, how do we make some sense of these numbers and assess the amount of storage that actually needs to be added? Firstly, it is important to describe how there are two fundamental units when describing energy storage, the amount of energy they store, which is measured in Joules (TWh or GWh can be converted into Joules) and, secondly is the rate at which they can be charged or discharged, which is measured in Watts, or the rate that Joules flow per second.
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| Table 1: Global electricity production. [1] (†Includes biomass and geothermal) | |||||||||||||||||||||||||
As an example to better understand these numbers better, consider one of the largest announced storage systems in Alamitos, Southern California. The system comprises more than 18,000 Lithium-ion batteries, and is capable of providing 100 MW of power for 4 hours, for a total of 400 MWh (or 1,440 Gigajoules) of energy, that is over two orders of magnitude lower than what is necessary to power a medium-sized city. [2] The Alamitos battery plant is dwarfed in comparison to the battery announced by the Florida Power and Light (FPL) with its 900 MWh or 3,240 GJ of energy, or California's Moss Landing which has a battery with power capacity of 400 MW and four-hour capacity, or 1,600 MWh or 5,760 GJ. [2,3]
Although lithium-ion and other electrochemical batteries are some of the most popular storage options for EVs and stationary storage alike, some of the largest are only capable of thousands of Gigajoules (1 GJ = 109 Joules) of energy, and the world needs a billion times more storage, on the scale of thousands of TWh (or equivalent EJ) which is comprises the box labelled "energy storage gap" seen in Fig. 1. [4]
Now that we have established there are some limitations on some of the largest electrochemical batteries, what role can pumped hydro, one of the oldest forms of energy storage play in terms of solving the energy storage gap? Pumped hydro energy storage (PHES) accounts for over 90 percent of the world's storage capacity, and is based on simple physics of using renewable energy to pump water above a certain height and use gravity to generate electricity when the water is released. The mechanic although simple, is still imperfect, with energy efficiency losses on the order of 25% and citing restrictions making it difficult to deploy at scale. [4,5]
PHES can still provide quite a lot of energy storage capacity and power. The worlds largest system is in China, in Fengning, and can discharge power of 3,600 MW for a little over 11 hours, for an energy storage capacity of about 40,000 MWh or 144 TJ (1012 Joules, or equivalently 0.000144 EJ). The enormous Chinese PHES plant consists of 12 reversible pumps each capable of generating close to 300 MW each. [3] This makes the project have, respectively, nine times the installed power of the Moss Landing battery system, and up to 25 times higher storage capacity. [3] Despite its size, that is still small compared to what would be needed for the demands of a megacity if required to be completely dependent on solar and wind generation, and tiny when compared to the ~23 EJ required to store all the solar and wind generated in 2021. [1-6] PHES plants take up a lot of space, especially when compared to batteries or even to fossil fuels that have a much higher energy density. Although PHES can store several hundreds of TJ worth of energy (1012 J), that is still six orders of magnitude below 1 EJ or (1018 J) of energy. If we wanted to store all the ~23 EJ of solar and wind generated in 2021, this would require over 160,000 Fengning plants!
We have to confront the apparent tradeoffs that no single energy solution fits all our energy needs right now. Every energy storage option has tradeoffs in terms of power capacity, energy density, efficiency, and scalability, and cost. [4] Lithium batteries, despite their ubiquity, have limitations in both system power rating and discharge time. They are also better for short-term storage in the 4-6 hour range, suitable for grid stabilization and load shifting. [4] In contrast, PHES, although has the majority share of storage capacity worldwide, and remarkable in their power output, suffer in terms of energy density when compared to fossil fuels, and entail significant space requirements and energy efficiency losses. [3]
Exploring these tradeoffs further, some of the newer technologies on the horizon like solid-state batteries and flow batteries can offer promises in enhancing energy density and discharge times, but need to miraculously scale to support global urban life, and overcome challenges in cost, efficiency, and commercial viability. [4-6] Understanding these tradeoffs is pivotal to selecting the most suitable energy storage system to match specific regional or sectoral requirements and fill the EJ gap.
Costs are another crucial discussion point in the energy storage discussion, and ultimately, filling the energy storage gap requires any solutions to be cost-competitive compared to the fossil fuel alternative with carbon capture and sequestration from a $/KWh perspective. In 2021, The Department of Energy cites a projected cost estimate of $263/kWh for a 100 MW, 10-hour installed system of pumped hydro energy storage. [5] Compared to battery grid storage solutions, the DOE estimates the costs for a fully installed 100 MW, 10-hour battery systems of Li-ion LFP to be roughly $356/kWh. [5] Naturally, there are steep up-front capital costs required for pumped hydro for construction of the reservoirs and powerhouse, and if we are required to deploy tens of thousands more pumped hydro storage projects, some interesting ideas to reduce the cost of their deployment is to consider repurposing old mines (ideally proximate to renewable generation) as pumped hydro energy storage batteries, as a way to reduce the up-front capital investment and increase the deployment of storage.
To decarbonize our global energy landscape and ensure a consistent supply of power from renewable sources, it is necessary that the world innovates to dramatically increase our energy storage capacity to fill the TWh/EJ energy duration storage gap. The growth of renewables, driven by an ambitious expansion in renewable capacity, is poised to reshape the energy sector. PHES remains a dominant force, but with limitations in terms of energy density, efficiency losses, costs, and citing restrictions underscore the need for more innovation. [4] Lithium batteries can be dependable for the shorter term and grid support, however, but it is apparent that other storage options are required for long-duration energy storage and to address the EJ demands of modern society. [3,6] In conclusion, meeting the world's energy storage needs involves a mix of ingenuity to create novel solutions, as well as financial commitments to fund more storage that can pave the way for our sustainable future. The future will require several orders of magnitude more reliable energy storage than we currently have, and although this road may be challenging, the pursuit of the optimal energy storage system remains a vital and evolving endeavor.
© Mark Khalil. 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] "BP Statistical Review of World Energy 2022," British Petroleum, June 2022.
[2] V. Smil, Numbers Dont Lie (Penguin, 2021).
[3] V. Smil, "Batteries for Large Scale Electricity Storage," University of Manitoba, 18 May 22.
[4] V. Smil, Energy in Nature and Society: General Energetics of Complex Systems (MIT Press, 2008).
[5] K. Mongird et al., "Energy Storage Technology and Cost Characterization Report," Pacific Northwest National Laboratory, PNNL-28866, July 2019.
[6] A. Zablocki, "Energy Storage," Environmental and Energy Study Institute, February 2019.