Non-linear hybrid approach for the scheduling of merchant underground pumped hydro energy storage

As a result of the increased penetration of stochastic renewable generation, power systems have a growing need of flexibility for compensating real-time mismatches between production and consumption of electricity. This flexibility can be efficiently provided by underground pumped hydro energy storage (UPHES), a new solution where end-of-life quarries or mines are rehabilitated as natural reservoirs. However, the operation of UPHES is significantly different from existing facilities, and is characterised by multiple non-linear effects with fast dynamics mainly arising from the complex geometry of the unit, and water exchanges between the porous reservoirs and their surrounding aquifers. This study aims thus at integrating these complex effects within the co-optimisation of a UPHES system in the European day-ahead energy and reserve markets. To that end, the authors leverage a hybrid iterative approach combining an optimisation tool with an advanced simulation model. The results from a real-world case study demonstrate that accurately considering these non-linear effects is a key component to fully extract the economic potential of merchant UPHES, and suggest that the proposed tool offers an effective solution for the scheduling of UPHES owners.