Can synergies between technologies be used to fill the energy storage gap?
With renewable energy sources becoming cheaper, and a societal and political push to decarbonise our societies, the contribution from renewable sources to the total energy mix is increasing (CCC, 2019; COP, 2015). This transition from a reliable fossil fuel supply to more variable and intermittent renewable energy sources threatens the reliability and stability of our energy systems. Part of the solution is to increase the amount of energy storage in the energy system, in order to provide sufficient balancing capacity to ensure that supply and demand can be matched (Dodds and Garvey, 2016; Elliott, 2016).
This storage needs to happen over timescales ranging from seconds to years, and at power input and output spanning from Watts to hundreds of Megawatts. Currently, the inter-seasonal storage of energy is achieved primarily through hydrocarbons in the forms of stockpiles of coal and known oil and gas reserves. However, this method of achieving large-scale energy storage will have to be decarbonised in the future, particularly when the amount of variable generation from renewables exceeds approximately 80% of the power generation capacity (Cebulla et al., 2017; Elliott, 2016).
Yet at the moment the most common energy storage technologies, such as batteries and pumped storage hydroelectricity, are not scalable to achieve grid-scale inter-seasonal energy storage (Mouli-Castillo et al., 2019). If unaddressed, this situation could lead to an unreliable energy system, or one reliant on hydrocarbons by default, thus jeopardising ongoing efforts to decarbonise.
An Electric and Heated Challenge!
To make things worse, we also need to consider that the storage challenge is applicable not only to our societal electricity needs, but also to our heating (and cooling) needs. This might not be obvious today in the UK at least, as natural gas is used to both generate electricity and to heat homes. However, in a future where a significant portion of heating becomes electrified it will be necessary to be able to produce electricity rapidly and at large scale from stored energy.
Currently, many consider hydrogen as a way forward that could provide storage on a large scale to meet both electrical and heating needs. There are, however, technological limitations; an approximately 70% energy loss incurred by the conversion of electricity to hydrogen, and back to electricity, makes this option economically unattractive at present (Salameh, 2014; Samsatli and Samsatli, 2019; Shiva Kumar and Himabindu, 2019).
Compressed Air Energy Storage (CAES) provides a scalable alternative for large-scale inter-seasonal storage of electricity. In this technology, electricity is used to compress air in either underground caverns or underground porous rock reservoirs, before releasing the compressed air through a turbine to generate electricity on demand.
A suggestion is therefore to use both technologies in conjunction: hydrogen at grid scale to balance the seasonal heating needs; and CAES to balance the electricity needs.
Geology: A Key Piece of the Puzzle
The subsurface has huge potential to meet society’s needs for energy, even in a decarbonised society – in particular, by achieving reliable large-scale energy storage with a limited surface footprint. A recent study identified that CAES in porous rocks in the UK Continental Shelf could provide between 77 and 96 TWh of electricity to the grid during 60 winter days (Mouli-Castillo et al., 2019), with between 15 and 25 TWh of that energy co-located with existing and planned offshore windfarms. This juxtaposition could help in providing storage for large amounts of renewable offshore wind energy, which would otherwise have to be wasted at times of low demand. Onshore salt basins have also been studied, but in contrast to porous rocks, salt has been identified as having only between 5 and 8 TWh of storage capacity per cycle (Evans et al., 2016; Garvey, 2019).
This highlights the importance in unlocking the potential from the wider storage volumes within porous rocks. This is an expanding area of research, with hydrogen storage in porous formations currently being considered by the HyStorPor project at the University of Edinburgh (https://blogs.ed.ac.uk/hystorpor/). Hydrogen’s potential for seasonal storage has been highlighted by many other researchers (Amid et al., 2016; Sainz-Garcia et al., 2017).
Technology Collage to Maximise Value
It is important to consider other subsurface technologies, such as the ongoing hydrocarbon production and storage as well as carbon sequestration and geothermal storage. These types of technologies are, however, often seen as competing for a given geological reservoir asset. These concerns are, in some respects, valid, in that although the subsurface is vast, the portion of it which is understood sufficiently enough for the deployment of such technologies is limited.
It can be hypothesised, however, that much more value could be had if a given subsurface reservoir is considered as a valuable asset, rather than just simply looking at the technology deployed to it. Considering the synergies between different technologies could add value, since a given geological asset could be allowed to generate value from multiple technologies (Quattrocchi et al., 2013). In that sense, synergies between technologies at a single site should be considered over time. In this hypothesis, the given is the geological asset, and preserving and understanding it will allow enduring and evolving value to be generated from its use with multiple technologies.
To illustrate this point we can consider a few examples:
CO2 Enhanced Oil Recovery: This technology has been used for decades as a way to extend the production life of a subsurface asset whilst storing carbon dioxide (CO2). Although the primary focus has been the recovery of oil, rather than the storage of carbon dioxide, it still illustrates the benefits of combining technological concepts in a geological asset.
CO2 – CAES in porous reservoirs: Oldenburg and Pan (2013) proposed that CO2 could be used as a cushion gas for a site operating CAES. This stems from the need for a large portion of the gas being injected into the storage site to remain in place, whilst only a smaller portion, the working gas, is cycled in and out of the ‘store’. In addition to the apparent advantage in terms of decarbonisation, this approach also aims to capitalise on the density changes in the CO2 phase to increase the CAES efficiency.
Nitrogen – Hydrogen: A similar approach has been considered for geological hydrogen storage, where nitrogen would be used as the cushion gas (Pfeiff et et al., 2016). The aim here is to reduce the cost of developing the site, as hydrogen is expensive to produce. Air has also been considered and could provide synergies with a CAES site.
CO2 storage – geothermal: Combining CO2 storage with geothermal energy has also been recently proposed. Although this system appears more complex than others, as it requires two reservoirs with one being shallower than the other, it still provides interesting synergistic opportunities and possibly an opportunity to combine electrical and thermal storage (Buscheck et al., 2016; Fleming et al., 2018).
Aside from CO2 Enhanced Oil Recovery, these opportunities tend to be modelled and considered from site development through to operation only. Value could be added by considering the whole life-cycle of a geological asset, in particularly by learning from the implementation of technologies early on, to de-risk future usages. For example, a site used for CO2 storage could experiment (whilst still maintaining operations) with cyclic variations in injection rate to provide valuable information on the asset’s behaviour for future use as a CAES site.
Research Needed
Now is the time to embark on the research and development needed to undertake site demonstrations of these technologies and their interactions. In this way, it should be possible to lay the foundations, ready for a time when large-scale inter-seasonal storage of energy is required to maintain a reliable supply of energy to society.
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