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| Fig. 1: Schematic for underground pumped hydro storage. (Image source: M. Khalil) |
Underground pumped hydro storage (U-PHS) has emerged as an alternative solution that can overcome some of the siting and economic challenges associated with the conventional above-ground pumped hydro. This technology utilizes subterranean caverns, mines, or underground reservoirs for storing energy, typically relying on steep height differentials of water between an upper and lower reservoir to store and generate electricity on demand.
Conceptually, U-PHS is very similar to surface level pumped hydro; the major difference is that the upper reservoir for an underground system is at ground-level while the lower reservoir is located underground, as seen in Fig. 1. [1] Underground pumped hydro leverages gravitational potential energy, using reversible pump-turbines to move water between upper and lower reservoirs situated at different elevations. The energy storage capacity of a pumped hydro storage system (E, measured in Joules) can be calculated using the formula
where ρ = 1000 kg m-3 is the mass density of water, g = 9.8 m sec-1 is the acceleration due to gravity, and h is the effective head (the vertical distance between the water surfaces of the upper and lower reservoirs in meters, and Ω is the total volume of water ready to fall downhill in m3. [2] The derivative of the energy equation with respect to time yields
| P | = | dE dt |
= | ρ g h | dΩ dt |
where P is the power delivered in Watts and dΩ/dt is the flow of water through the turbines in m3 per sec. The efficiency η, a pure number between 0 and 1, describes energy losses in the pumps and turbines. The power P' delivered to the grid can be thought of as P' = η × P.
Larger height separations h between these upper and lower water bodies (larger heads) directly correlate with increased storage capacity and power output capabilities. [2,3] By situating reservoirs in deep underground facilities, projects can leverage a thousand or more feet of head differential, allowing individual certain unique facilities to bring online several hundreds of MW to over a GW of installed capacity at a time at larger abandoned mines in some cases, with scalability through expansion of water volumes. [3-5] With the ability to go from zero power to full power dispatch in under one minute, underground pumped hydro can serve as a rapid response backbone to stabilize intermittent wind and solar infrastructure at scale. [1]
The efficiency of U-PHS is comparable to traditional pumped hydro storage, with some variation depending on the turbines used at the specific project site and its implementation details. [6] There are actually two efficiencies when considering pumped hydro systems, one for charging and another for discharging. The "round-trip efficiency" is the product of the two. The round trip efficiency rates of pumping to storage to electricity generation are on par with above-ground pumped hydro at 70-85%. [1,3] However, the relatively low energy density of PHES systems requires either a very large body of water or a large variation in height which is what makes underground systems potentially more attractive alternatives. [6,7] In addition, having the water stored underground can in some instances, reduce losses due to evaporation. [7]
Naturally, there is an extremely high variance when it comes to implementation costs, as each site has its own idiosyncratic features, which means pumped storage systems have capital costs of $600-2,000 per KW or $5-100 per KWh and 0.1-1.4 cents per KWh per cycle. [6] Underground pumped hydro, which takes advantage of natural geography and voids rather than requiring extensive dam, reservoir, and civil engineering means there is more flexibility in siting, and can often allow developers to locate projects closer to energy infrastructure and consumers. With long asset life spans of 50+ years and low maintenance requirements as system components are protected underground, projects can deliver electricity at very low costs for decades. [3]
However, the estimated costs are 1.1 to 1.3 times higher than conventional PSH plants. [4,8] Unless the market environment radically changes, returns for investors therefore seem modest but the societal benefits of lower electricity prices and better grid stability are substantial. Market incentives for large dedicated storage investments are lacking, an public-private partnerships may enable projects. [4,8] Compared to batteries and hydrogen storage, U-PHS can provide larger-scale, longer-duration storage at modest roundtrip losses, and U-PHS deserves consideration among storage options for deeply decarbonized electricity systems, given suitable geology. [4,8] Government policy and regulation changes may be needed to spur investments, as the fundamental issue is that markets lack incentives for investment in very large, capital-intensive dedicated storage. [4,8]
Other that cost issues, other important considerations for successful U-PHS implementation include understanding the drainage considerations as some sites naturally refill with water. Understanding the dynamics of water tables, or the levels where the ground will be filled with water if you go any lower is critical for the successful design and implementation of sites. [7] Despite the challenges, U-PHS could provide large-scale energy storage to facilitate greater integration of renewable energy and stabilize electricity grids. Suitable underground geology for U-PHS has been identified in coal mining areas in Germany and the Netherlands, where these risks of subsidence or seismic impacts appear low. [4,8]
In conclusion, the potential of underground pumped hydro storage heralds a transformative era in sustainable energy solutions. As an innovative approach to address the challenges of conventional above-ground pumped hydro, this unique technology leverages subterranean spaces, mines, and caverns to store and generate electricity efficiently. [4,6-8] Offering comparable efficiency rates to traditional systems and enhanced flexibility in siting are reasons underground pumped hydro emerges as a promising alternative for long-term energy storage. [4,8] However, repurposing existing underground structures, such as abandoned mines, is expensive, and despite showcasing adaptability and resourcefulness in meeting the world's growing energy demands is a capital-intensive solution that requires more granular support. As research and development continue to advance, and with successful deployments of renewables, the potential of underground pumped hydro storage stands poised to play a pivotal role in fostering a more sustainable and resilient energy landscape. [4,8]
© 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] H. Chen et al., "Progress in Electrical Energy Storage System: A Critical Review," Prog. Nat. Sci. 19, 291 (2009).
[2] T. Kousksou et al., "Energy Storage: Applications and Challenges," Sol. Energy Mater. Sol. Cells. 120 A, 59 (2014).
[3] X. Luo et al., "Overview of Current Development in Electrical Energy Storage Technologies and the Application Potential in Power System Operation," Appl. Energy 137, 511 (2015).
[4] M. Wessel, R. Madlener, and C. Hilgers, "Economic Feasibility of Semi-Underground Pumped Storage Hydropower Plants in Open-Pit Mines," Energies 13, 4178 (2020).
[5] F. Liu et al., "Pumped Storage Hydropower in an Abandoned Open-Pit Coal Mine: Slope Stability Analysis under Different Water Levels," Front. Earth Sci. 10, 941119 (2022).
[6] S. Rehman, L. M. Al-Hadhrami, and Md. M. Alam, "Pumped Hydro Energy Storage System: A Technological Review," Renew. Sustain. Energy Rev. 44, 586 (2015).
[7] X. Lyu et al., "Pumped Storage Hydropower in Abandoned Mine Shafts: Key Concerns and Research Directions," Sustainability 14, 16012 (2022).
[8] G. J. Kramer et al., "Risk Mitigation and Investability of a U-PHS Project in the Netherlands," Energies 13, 5072 (2020).