Poster Monika Ivandic

Underground large-scale energy storage: Geophysical perspectives

At the bottom of this page, please find the article vesrion of this poster.

Introduction

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.

Figure 1. Map of the thickness of the Faludden reservoir beneath Gotland — Figure 1: Map of the thickness of the Faludden reservoir beneath Gotland. The inset graph in the top left hand corner shows a cross plot of the Faludden reservoir thickness and the total Cambrian thickness, for wells which reach the basement (Sopher et al., 2019).

CAES background

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.

Figure 2: Interpretation of the stratigraphy between the wells using the seismic data for an approximate north–south profile across southern Gotland.

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.

Figure 3. Schematic cross section — Figure 3: a). Schematic cross section demonstrating the concept of CAES within the porous Faludden sandstone reservoir beneath Gotland. b). A schematic of a CAES plant (figure modified from Succar and Williams 2008).

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).

Table 1. Cost estimates to build an energy storage facility — Table 1: Cost estimates to build an energy storage facility on Gotland with a capacity of 1300 MWh which can output 70MW for a total of about 18.5 hours. Costs assume that the plant will operate for 10 years. Costs are based on values from Luo et al., 2015.

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.

Authors

Monika Ivandic¹ (monika.ivandic@geo.uu.se), Daniel Sopher^1,2, 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

References

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.