As we learned in the last blog post, energy storage systems exist primarily to bridge the gap between energy production and energy consumption. The overriding benefits are peak-shifting energy related profits, reduced fossil fuel reliance, and the elimination of energy wastage during peak production; for example, excess wind energy produced on windy days is often wasted in the event of a grid overload if not stored (BINE, 2004).
Energy storage may be achieved via surface-based mechanical storage solutions, or via geological storage. Surface storage options include Pumped Hydroelectric Storage, which has the largest global capacity of all existing storage methods accounting for over 95% of installations as of 2017 (US Department of Energy, 2017). Pumped Hydroelectric Storage, however, is a net-consumer of energy, due to inefficiencies in the pumping process, and thus is showing a reduction in utilization in many European countries (Kougias et al., 2017; Kocaman et al., 2017). As a cleaner alterative, Compressed Air Energy Storage (CAES) offers a more efficient option, which could potentially eliminate fossil-fuel consumption within the system.
There are several options for geological storage of energy. These include:
- Man-made salt caverns
- Man-made rock caverns
- Depleted hydrocarbon reservoirs
- Underground mines
- Aquifers
This blog post focuses on the use of man-made salt caverns (Figure 1) which have long been recognised as promising for gas storage options, including Compressed Air Energy Storage. They are particularly favourable due to their large, open cavities (which can be exploited via solution mining; a characteristic not shared by the porous media of traditional clastic reservoirs), and the fact that they remain stable for long periods of time due to salt’s inertness to hydrocarbons, oxygen, and hydrogen. In addition, many salt structures are well explored and mapped due to their relationship with hydrocarbon plays, such that the initial exploration costs for finding salt cavern energy storage prospects is significantly lowered.
The structural stability of salt caverns can be exploited for both long term seasonal storage (supporting peak winter demands) and daily load shifting options. The caverns can withstand high rates of injection and withdrawal, and can thus be used for several storage cycles before their lifecycle is complete. These properties, shown in Figure 2, make salt caverns safe for both humans and the environment. Today, salt caverns are actively being used for brine production wells, crude oil storage and natural gas storage, as well as Compressed Air Energy Storage.
In addition to the use of salt caverns for Compressed Air Energy Storage, research is ongoing by Newcastle University, among others, for their use in hydrogen storage (Stone et al., 2009), such as at Teeside (Evans & Chadwick, 2009). Hydrogen is widely considered to be amongst the key alternative energies in the growth of renewables throughout the energy transition (Saeedmanesh et al., 2018), so it is important to understand where it can be stored safely.
Compressed Air Energy Storage systems exist in mechanical and chemical formats. Both methods of Compressed Air Energy Storage are based on compression of ambient air via excess electrical energy, such as that from a wind turbine or photovoltaic cell, to high pressures (up to 70 bar) during times of lower demand. In times of increased demand, the pressurised air is used to drive a turbine, generating electricity for the grid.
Mechanical storage is currently used worldwide to provide reserves in the minutes to hours range via compressed air in pressurizes tanks above or below ground. Chemical storage, however, is applicable to large scale, variable efficiency settings. It involves conversion of electrical energy into gas such as hydrogen or a hydrogen compound. This gas can then be stored in huge volumes in geological storage sites. The chemical ‘power-to-gas’ option thus allows for the use of existing gas infrastructure, and involves much less capital expenditure, with the technology involved in this process being based on that used for natural gas storage (Crotogino et al., 2018).
One side effect of the gas compression process is that the air temperature rises by more than 600°C, at 70 bar, which requires dissipation before geological storage. Three variations of dealing with this excess heat in chemical Compressed Air Energy Storage are:
- Diabatic
- Adiabatic
- Isothermal
Diabatic technology (Figure 3) involves dissipating the heat directly into the atmosphere, meaning this energy is lost. Later during the withdrawal process, the compressed air (which can cool below condensation point) must be heated before entering the turbine to prevent ice damage of the internal turbine components. This is often done using fossil fuels. Diabatic Compressed Air Energy Storage therefore only has a potential efficiency rate of 42- 54% (TNO, 2013).
Contrastingly, adiabatic technology (Figure 4) stores the heat generated during compression in a pressurised surface container. This provides a heat source for reheating the air during withdrawal and removes the requirement for fossil fuel use, reducing CO2 emissions up to 60%. The overall efficiency of adiabatic Compressed Air Energy Storage is estimated to be around 70%, depending on the heat storage method (Crotogino, 2002; Jakiel et al., 2007; Grazzini et al., 2012).
Finally, a more recent development is isothermal Compressed Air Energy Storage (Figure 5). In this method heat is continuously removed from the air as it is compressed, and continuously added during expansion, thereby maintaining a controlled constant temperature. This involves some complex reciprocating machinery solutions, but has a proposed efficiency of 70-80% (TNO, 2013).
Hopefully this blog has introduced you to the concepts of Compressed Air Energy Storage, and some of the possible variants of the methodology which will be applicable in different scenarios. The next and final blog in this series will explore a recent piece of research into the feasibility of employing Compressed Air Energy Storage at a large scale offshore in the Southern North Sea.
David Whitworth, Junior Geologist
References:
BINE Informationsdienst, 2006. Compressed air energy storage power plants. projektinfo.
Crotogino, F., 2002. Kali und Steinsalz, Verband der Kali und Salzindustrie e.V.
Crotogino, F., Schneider, G.-S., Evans, D.J., 2018. Renewable energy storage in geological formations. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 232, 100–114. https://doi.org/10.1177/0957650917731181
Evans, D.J., Chadwick, R.A., 2009. Underground gas storage: An introduction and UK perspective. Geological Society, London, Special Publications 313, 1–11. https://doi.org/10.1144/SP313.1
Geoscience Australia, 2022. Salt caverns and minerals across Australia unlock our nation’s hydrogen industry. Earth Sciences for Australia’s future. URL https://www.ga.gov.au/news-events/news/latest-news/salt-caverns-and-minerals-across-australia-unlock-our-nations-hydrogen-industry
Grazzini, G., Milazzo, A., 2012. A Thermodynamic Analysis of Multistage Adiabatic CAES. Proc. IEEE 100, 461–472. https://doi.org/10.1109/JPROC.2011.2163049
Jakiel, C., Zunft, S., Nowi, A., 2007. Adiabatic compressed air energy storage plants for efficient peak load power supply from wind energy: the European project AA-CAES. IJETP 5, 296. https://doi.org/10.1504/IJETP.2007.014736
Kocaman, A.S., Modi, V., 2017. Value of pumped hydro storage in a hybrid energy generation and allocation system. Applied Energy 205, 1202–1215. https://doi.org/10.1016/j.apenergy.2017.08.129
Kougias, I., Szabó, S., 2017. Pumped hydroelectric storage utilization assessment: Forerunner of renewable energy integration or Trojan horse? Energy 140, 318–329. https://doi.org/10.1016/j.energy.2017.08.106
Saeedmanesh, A., Mac Kinnon, M.A., Brouwer, J., 2018. Hydrogen is essential for sustainability. Current Opinion in Electrochemistry 12, 166–181. https://doi.org/10.1016/j.coelec.2018.11.009
Stone, H.B.J., Veldhuis, I., Richardson, R.N., 2009. Underground hydrogen storage in the UK. Geological Society, London, Special Publications 313, 217–226. https://doi.org/10.1144/SP313.13
TNO, 2013. CAES: Technology Factsheet.
US Department of Energy, 2017. Pumped-Storage Hydropower. URL https://www.energy.gov/eere/water/pumped-storage-hydropower
