How to revitalize abandoned mines in the era of the decline (hopefully fast) of the coal industry? A Chinese research team recently proposed turning these mines into hydroelectric power plants. They would become a new type of power generation plant to match the growth of solar and wind power.
According to the analysis of Chongqing University e Shaanxi Investment, in China there are currently 3.868 mines abandoned that can undergo this transformation. The pit of these coal mines (the "tower" used for the extraction of the minerals) can be further used to become an alternative reservoir for the hydroelectric power plant. How? With a system born after the war and never sufficiently exploited: hydroelectric pumping.
From mines to "tiered" hydroelectric
The hydroelectric pumping plant is divided into upper and lower levels. They work with the transfer of water between these levels, placed at different altitudes. During periods of low demand low cost energy is used, provided by the network, to pump water from the lower reservoir of the mine wells to the upper one. On the other hand, in periods of peak demand (when prices are higher) the water comes released through turbines to produce energy that is put on the market. The energy balance is negative, because more energy is expended than what is produced. But the process still pays off. Because? The pump is operated when low-cost energy (at night or, hopefully, in periods of overproduction due to wind and photovoltaics). The ideal height difference between the upper and lower reservoirs is 100 m.
The team points out that water storage capacity not only depends on the space of the mines, but must also consider the porosity and permeability of the soil and rocks. The system design developed by the team also includes a ventilation shaft 2m above the highest water level, to allow for better airflow when pumping and filling water.
Attention is needed
As indicated by the simulation model, the conversion of abandoned mines into hydro-pumped plants can guarantee average efficiency 82,8% within one year. The energy density is instead estimated at 2,82 kW · h / m3 .
The researchers believe that while the particular solution is feasible as far as technology is concerned, the mining environment needs to be studied well. The danger is that acidic water corrodes plants and soil, releasing metal ions and heavy metals that would damage the underground structure and the aquifer. Eye!
The system design is described in the study Underground Hydro-Pumped Energy Storage Using Coal Mine Goafs: System Performance Analysis and a Case Study for China, recently published on Frontiers in Earth Science .