A grid compatible integration of fluctuating renewable energy from high capacity wind and solar energy farms to electricity systems requires large-scale energy storage methods and technologies. Currently widespread large-scale electricity storage installed is reservoir-based hydropower and Pumped Hydropower Energy Storage (PHES). Large storage volumes are necessary to deal with the relatively low volumetric storage density property of PHES plants. Investigations are made to benefit from the large volumetric storage potential of deep coal mine openings. Especially, main roadways of active and some closed hard coal mines are available and may be used as large lower reservoir volumetric storage openings for PHES plants. This paper address some basic rock engineering and stability issues associated with PHES projects in deep hard coal mines. Evaluations are made for safe and effective usage of the underground coal mine openings as PHES lower reservoir storage volumes.
The electricity power sector has been undergoing changes and an increased share of electricity from renewable sources is the stated aim of many national energy policies. However, a grid compatible integration of fluctuating renewable energy from high capacity wind and solar energy farms to electricity systems requires large-scale energy storage methods and technologies. Currently widespread large-scale electricity storage installed is reservoir-based hydropower and pumped hydropower energy storage (PHES). Alternatives are underground storage of compressed air (CAES) and hydrogen gas in suitable geological formations (Meiwes 2009, Özarslan 2013).APHES plant requires the existence of an upper and lower reservoir between which water is pumped up in off-peak periods to store hydraulic potential energy and then released down through a turbine in peak periods to produce electricity. A schematic view for a reservoir-based hydropower or PHES project is shown in Figure 1. Vertical or inclined shafts are efficiently used in pressure tunnel and shaft layouts for PHES plants. The reinforcement lining types (steel, concrete, unlined) in tunnel and shafts depends primarily on the rock conditions, depth, confinement and water pressure levels. The need for surge chambers depends on the type of turbines used, heads, length and volume of waterways, and flow velocities (Brekke & Ripley 1993).
Large storage volumes are necessary to deal with the relatively low volumetric storage density property of PHES plants. The potential for further PHES in industrialized countries in Europe is limited because of environmental considerations, lack of suitable sites and certain social acceptance issues. Investigations are made to benefit from the large volumetric storage potential of deep coal mine openings.
