Preliminary feasibility analysis of a hybrid pumped-hydro energy storage system using abandoned coal mine goafs

Following the Paris climate agreement, a consensus has been made on the urgent need for increasing the use of clean energy and large-scale energy storage. This paper proposes a hybrid pumped-hydro energy storage system using goafs of abandoned coal mines. The performance of the energy storage system and the suitability potential of coal mine goafs serving as underground reservoirs were analyzed. Based on the designed conditions and meteorological data of a typical area (Inner Mongolia, China), the proposed system could have an average system efficiency of approx. 82.8% and a regulating-energy density of 1.06 kW.h/m(3). Potential analysis for goafs serving as reservoirs show that a typical coal mine with a 3 x 5 km(2) dimensions and 6 m coal thickness could have a usable capacity of 1.58 x 10(6) m(3). To ensure the smooth exchange of water in goaf reservoirs, the permeability should be above 10(-7) m(2), corresponding to the usable coefficient (>= 0.8) of goaf reservoirs. The minimum horizontal distance between two reservoirs with a typical geology has been set to 245 m so that the leakage rate could be smaller than 1%. A diameter of 1m for vertical ventilation shafts is acceptable with respect to the air pressure loss (211 Pa). Based on the reckoning of the existing coal mine goaf space in China, it has been found that developing hybrid pumped-hydro energy storage plants using abandoned coal mine goafs for daily regulation is feasible in the short term.