Poster Monika Ivandic
Underground large-scale energy storage: Geophysical perspectives
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A drawback of renewable energy sources such as wind or solar is strongly fluctuating electricity production.
A promising way of dealing with the intermittency from renewables is energy storage. However, long-term storage is difficult and expensive, and is a major hinder for a move towards renewables.
Current battery technology does not appear viable for long-term mass energy storage because of the vast volumes of batteries which would be required. On the other hand, energy storage technologies involving the use of geological reservoirs offer large storage capacities, as well as discharge rates, bringing all the advantages of a large-scale energy storage system while minimising environmental and social impacts, and the need for surface space.
Balancing supply and demand: geological energy storage
Short-term balancing of supply and demand (seconds, minutes, hours or days) can be easily achieved using the dispatchable hydro capacity, capacitors, batteries, and load-shifting (e.g. using Smartgrid). Long-term balancing (days, weeks, seasons, years) is more difficult and potentially massively more expensive due to longer time-scales and the large quantities of energy involved.
Swedish hydropower appears to have insufficient dispatchable capacity for future long-term balancing, unless massive new pump storage facilities are built (Bruce J., 2018). Also, the cost of batteries for long-term storage is massive: assuming (low) capital costs of 1000 kr/kWh, to store the equivalent of one mid-winter day’s consumption (about 0.5 TWh) would cost about 500 000 000 000 kr! Moreover, several times this capacity may be necessary to balance future Swedish wind and solar electricity.
Therefore, many options for long-term storage demand the use of large subsurface volumes. Some options for mass underground energy storage are, for instance, gas chemical storage – large volumes of methane (natural gas, biogas, syngas from surplus electricity) and hydrogen can be stored in natural formations, or thermal storage - high temperature heat storage for electricity production is used today, but costs are high. Low temperature storage for bulk heating is much cheaper, but demands large (underground) volumes.
Compressed air energy storage (CAES) systems represent a new technology for storing very large amounts of energy and supplying electric power to meet peak load requirements of electric systems.
Mass geological energy storage: The Gotland example and CAES
Region of Gotland has a long term goal of producing 100% of its energy from renewable sources by 2025.
The feasibility study for Compressed Air Energy Storage (CAES) on Gotland (Sopher et al., 2019) shows that the most prospective reservoir on Gotland is the Middle Cambrian Faludden sandstone (Fig. 1 and Fig. 2), which is a well sorted, medium grained sandstone deposited in a shallow marine environment. The relatively impermeable Ordovician Bentonitic limestone unit overlies the Faludden reservoir and could potentially act as a cap rock.
Excess energy is used to compress air, which is then stored at pressure within a geological formation. When required, the air is extracted and used to drive turbines to recover the stored energy (Fig. 3).
Presently, two industrial scale CAES plants exist (110 and 321 MW), which utilize artificially mined salt caverns. These plants have cycle efficiencies of 42-54%. However, a new plant design, Advanced Adiabatic CAES, currently under development is expected to deliver cycle efficiencies of about 70% (Fig. 3). It should be also noted that although no industrial scale plants exist which utilize porous reservoirs for storage, indications from modelling are positive.
CAES feasibility on Gotland
Seven structural closures where compressed air can be trapped were identified within the Faludden reservoir beneath Gotland. These structures are located at depths considered suitable for CAES.
Mean average porosity and permeability values for the Faludden reservoir of 14.9% and 559 mD, respectively, as well as estimated reservoir pressures for the structural closures are suitable for CAES .
Scoping calculations show that the energy storage capacity within one of the more promising structures identified in the Faludden reservoir is between 130 and 1300 MWh (Sopher et al., 2019).
Estimated flow rates of between 24.6 and 262.4 kg/s for the different structural closures, are of the right order of magnitude for CAES to be viable.
Cost estimates and comparison with battery storage
A scoping estimate of the costs associated with a CAES plant located on Gotland have been conducted utilizing the cost estimates quoted in Luo et al. (2015) (Sopher et al., 2019). The costs associated with CAES are lower than with battery storage, in some cases as much as an order of magnitude cheaper (Table 1).
Final remarks and Conclusions
For long-term storage, cost is dominated by the capital costs per delivered kWh. System efficiency affects necessary scale, but because long-term storage implies few storage cycles, the marginal cost per kWh of energy lost is trivial given today’s cost picture.
Therefore, long-term storage facilities need not be particularly efficient. Depending on relative capital costs of different possible solutions, even storage systems with large losses may be the most attractive.
Despite the higher system efficiency comparing to underground energy storage, current battery technology does not appear viable for long-term mass storage because of the vast volumes and costs of batteries which would be required.
Energy storage technologies, involving the use of geological reservoirs offer large storage capacities, as well as discharge rates (ENeRG), bringing all the advantages of a large-scale energy storage system while minimising environmental and social impacts, and the need for surface space. The study at Gotland shows that CAES in underground porous formations could be a viable energy storage option.
Monika Ivandic1 (firstname.lastname@example.org), Daniel Sopher1,2, Roland Roberts1, Christopher Juhlin1
1. Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden
2. Geological Survey of Sweden (SGU), Villavägen 18, 75236 Uppsala, Sweden
Bruce J., m.fl., “Flexibilitet – i en ny tid”, 2018.
Luo, X., Wang, J., Dooner, M., Clarke, J., 2015. Overview of current development in electrical energy storage technologies and the application potential in power system operations. Appl Energy (137) 511-536.
Sopher D., Juhlin C., Levendal T., da Silva Soares J.P., 2019. Evaluation of the subsurface compressed air energy storage (CAES) potential on Gotland, Sweden. Environ Earth Sci. 78(6):197.
ENeRG. The Role of the underground for massive storage of electric energy, Geo Energy, The newsletter of the ENeRG Network, 2014, Issue Nº 29, p. 4.
Succar S, Williams RH, 2008. Compressed air energy storage: theory, resources, and applications for wind power. Princeton Environmental Institute, Princeton, p 81.