Subsurface geology offers untapped seasonal energy storage potential to enable large-scale deployment of renewables
With the falling cost of renewable energy sources (RES) and the political and public push to decarbonise our societies, the growth of renewable power generation capacity is increasing (CCC, 2019; COP, 2015). This shift from reliable fossil fuel power generation to intermittent and variable RES does lead to some challenges. Indeed, to ensure that this transition operates in a way which ensures the energy system stability is maintained, it is necessary to adapt much more than just the means of generating power. The move towards RES requires the improvement of implementation technologies and best practice. These include, energy storage, more efficient energy use, and energy demand management. Although short-term variations in the generation capacity of RES is manageable with either of the two latter options, energy storage is the only real solution for seasonal variations. The reason being that simply expecting people to not heat or cool their homes is unfeasible. This inter-seasonal energy storage challenge, in particular with respect to the electricity system, has been largely understudied in the scientific literature. When more than 80% of the electricity generation capacity will be met by RES, inter-seasonal storage will be essential to ensure the stability of the energy system (Cebulla et al., 2017; Elliott, 2016). The next natural question is what options do we have to achieve this large transfer of energy between seasons? Indeed, in the United Kingdom the winter electricity demand is about 25% greater than the summer demand (GridWatchUK, 2018). The answer is that we do not yet have a proven and competitive technology available to fill this gap. The reservoirs from pumped hydro are not large enough to sustain seasonal production, and are subject to geographical, environmental and social limitations (Rosenberg et al., 1995; Succar and Williams, 2008). Batteries are too expensive to maintain and have a limited discharge capacity (Chen et al., 2009). Although geological storage of hydrogen might be a feasible option, its round-trip efficiency from power to gas and back to power is low (about 30%, excluding geological storage and transportation losses) (Pellow et al., 2015).