This research from China, published in Energies in
June 2023, evaluates the use of existing coal mines in Henan Province for
potential underground pumped hydro energy storage (UPHES). The study calculates
the space available and the suitability for pumped hydro. This is a detailed
study that develops a site selection methodology that compares and high-grades
sites, mining trends, the history of underground pumped hydro, and the
utilization of mined-out space are all explored.
The paper first points out
China’s large coal consumption, which makes up about 56% of the country’s
primary energy consumption and 80% of its carbon emissions.
Figure 1. Annual coal production, consumption, and CO2
emissions from coal in China. (Data source: Our World in Data [8] and China
Statistical Yearbook 2022.
The authors note that there
are now many closed and abandoned coal mines in China with space for
underground pumped hydro energy storage (UPHES) applications. Past UPHES
studies in China have focused on technological feasibility, environmental
impact, and economic analysis. This paper focuses on site selection.
Considerations include local geology and hydrogeology. Previous studies
revealed that the permeability coefficient and horizontal distance were two
factors necessary for site evaluation. Above-ground PHES projects require
topography with significant elevation change, but UPHES can be developed
without regard to topography.
The authors developed a
two-step site selection process. The first step involves a screening assessment
that evaluates the geology and hydrogeological conditions. Step two is a
comprehensive assessment that involves an analytic hierarchy process (AHP).
The section on coal industry
trends in China notes that there are predictions that China’s coal consumption
for both power and industry will peak before 2030.
Figure 2. (a) Primary energy consumption in China by fuel
type. (b) Primary energy consumption of several countries in 2021. (Data
source: BP Statistical Review of World Energy).
Mined Underground Space Utilization
It is predicted that by 2030, there will be 15,000 closed or abandoned coal mines in China. The space
available in these mines includes deep shafts, extensive drift networks, and
goaves. The utilization of underground mine space includes four main modes:
energy storage, waste disposal, ecological restoration, and CO sequestration.
“Pumped storage is widely regarded as one of the most
reliable, cost-effective, and mature technologies for large-scale energy
storage, and it holds great promise for implementation in old coal mines. The
process of coal mining naturally forms large quantities of underground caverns,
which can serve as ready-made reservoirs with significant elevation
differences, making them ideally suited for pumped storage. Moreover, the
restored surface areas of old coal mines can be effectively repurposed as sites
for wind and solar farms, ensuring a sustainable and renewable power supply for
UPHES. This integration of renewable energy generation and energy storage
unlocks new opportunities for the coal industry. Most UPHES projects are
designed as closed-loop systems, operating independently from naturally flowing
water bodies. This design allows the direct utilization of mine water as a
supplement, mitigating the risk of water contamination and preserving water
resources.”
The Nassfeld power plant in
Austria has long utilized UPHES. The plant has a surface water body and some
excavated caverns as well as mine space. The geology of impermeable granite and
gneiss, igneous and metamorphic rocks, provides good containment for water.
Site Selection
As noted, economics, social,
and environmental factors are important for UPHES site selection, as well as
geology and hydrogeology. The presence of nearby surface wind and solar
facilities to charge the system by powering the hydro pumping is another
consideration.
The step 1 screening
assessment involves calculation and consideration of gross head, head-distance
ratio, and water source. These data are integrated with geology and
hydrogeology conditions. There are three screening indicators: geological
features, mine water disasters, and minimum installed capacity. Geological
features include the presence of karst geology, which is vulnerable to
dissolution and would make the site unsuitable for pumped hydro. Consideration
of mine water disasters includes a mine’s vulnerability to inflow greater than
600 cubic meters per hour, which indicates unacceptable hydrogeologic
conditions. Such inflow can reduce power production capacity and involve more
pumping, which makes economics more difficult. A minimum installed capacity of
20 MW was determined to be the cutoff for the screening assessment.
The Step 2 comprehensive
assessment involves determining the surface availability of wind and solar
resources and the conditions of the local power grid, where pumped hydro could
provide peak shaving services during high power demand times.
Gross head refers to the
elevation difference between the upper and lower reservoirs. Effective
reservoir volume refers to the amount of water that can be stored in both
reservoirs. If the surface water and groundwater are well connected and
interactive, then this could lead to problems, which makes such sites
unsuitable. The underground space must also be geologically stable, and the
disaster potential must be low. Permeability of the surrounding rock must be
considered. Hydraulic conductivity that is too high, combined with a high
groundwater head can result in unsuitable conditions due to the lower reservoir
filling too fast. The power regulation potential of UPHES requires the
facilities to be near urban centers where power demand peak shaving is most
needed. Being near urban centers also helps employment. Local support for
projects can also be important. Other considerations include the
cost of energy storage, the payment potential of providing peak shaving
services in the form of peak-to-valley tariff differential, maintenance and
monitoring costs (some mine water is corrosive due to water chemistry), and the
integrity of remaining equipment, including transportation and communications
equipment and substations.
The analytic hierarchy
process (AHP) includes weight calculations of many of the above factors to rank
site suitability. This is a statistical method that involves the creation of a
hierarchy, the construction of comparison matrices, and calculating priority
and consistency.
“By weight calculation, indicators that have significant
influences over site selection are identified, including the gross head (C11),
the effective reservoir volume (C12), the local peak-to-valley tariff
differential (C42), the unit cost of energy storage (C41), the stability of the
underground space (C14), and the local power demand (C21). These indicators
play crucial roles in determining the technical feasibility, safety, and
economic viability of UPHES projects in old mines.”
Next is a case study
comparing three sites in Henan Province. The first table is data for the
screening assessment, and the second table is the AHP conclusions for the
comprehensive assessment.
Henan Province has a growing
number of closed and abandoned mines as coal production from that region
continues to drop.
Estimation of Underground Space in Coal Mines
Determining available space
in mines involves estimating the volume of drifts, chambers, shafts, and
goaves. Goaves are less stable since they don’t have ventilation and supporting
structures. The authors show how a capacity coefficient was
developed in a previous study to estimate space volume. Some of the
mathematical variables in this calculation are the volume of mined coal, the
determination of volume reduction due to surface subsidence, and the determination
of volume reduction due to rock expansion after pressure relief.
UPESH Potential Estimation
Estimating the UPHES
potential of a mine site involves knowing two crucial factors: effective
reservoir volume and the elevation difference of the
upper and lower reservoirs.
“The effective reservoir volume and the elevation
difference between the upper and lower reservoirs are two crucial factors in
determining the installed capacity, power generation and economic profitability
of a pumped storage power plant.”
Effective reservoir volume
estimation must account for space that is ineffective and conditions like
reverse slopes that will affect gravity feed and reduce circulating volume if
there are cutoff stagnant zones. Groundwater inflow that is too high will result
in filling the lower reservoir prematurely. This could reduce the amount of
water available for discharge.
The elevation difference
between upper and lower reservoirs, also known as the head height, should
optimally be between 200m and 800m. Henan Province mines have head heights
between 300m and 1200m with an average just greater than 600m, which are
considered very good.
“When the head height is less than 200 m, both the
efficiency and economic benefits of the power plant significantly decrease. On
the other hand, when the head height exceeds 800 m, the current Francis turbine
is unable to meet the high-pressure requirements. Thus, multi-stage pumped
storage power plants with intermediate storage reservoirs are regarded as an
alternative.”
If only one coal seam has been mined, then the depth to the mine from the surface constitutes the head height since the upper reservoir in that case will likely be a surface water body. In that case, it is termed a semi-underground PHES. If two or more coal seams are mined, the head height can be the elevation difference between the two seams. This is termed a fully underground PHES and is the preferred configuration. It is also much rarer, as the following table for Henan Province shows. The other tables show the potential for each scenario.
The final section involves
estimating the decarbonization potential of UPESH projects, which depends on
how much curtailed or dedicated solar and wind power is provided. UPESH
projects can eliminate most curtailments, increasing emissions reduction
further than just the discharging of hydro energy production, but also
incorporating it into charging.
“In 2022, wind and solar power curtailment rates in
Henan Province were 1.8% and 0.5%, respectively. Accordingly, the abandoned
wind and solar power during the year amounted to 686.7 GWh and 102.9 GWh,
respectively. The surplus power can be consumed by UPHES power plants, with
about 631.7 GWh of energy successfully stored (assuming the plant efficiency of
80%). There is enough UPHES potential in existing old coal mine drifts to
handle this surplus power, as mentioned above.”
“…by consuming surplus renewable energy, UPHES can
reduce about 7.11 × 105 tonnes of CO2 emissions in 2025.”
The authors note that their
two-step site selection process basically adds the screening assessment to the
previous process, which is basically the comprehensive assessment. By screening
out unlikely candidates based on the three criteria of geological features,
mine water disasters, and minimum installed capacity, which effectively
characterize geological and hydrogeological conditions, effective reservoir
volume, and head height, the process is streamlined and becomes more reliable
and effective. The paper’s conclusion that the volume of goaves, areas of space
filled with caved rock, is much higher than that of drifts and shafts suggests
that if the goaf space can also be utilized, it could drastically increase the
amount of UPESH potential for a given mine.
The conclusions of the paper
are given below:
References:
A
Two-Step Site Selection Concept for Underground Pumped Hydroelectric Energy
Storage and Potential Estimation of Coal Mines in Henan Province. Qianjun Chen,
Zhengmeng Hou, Xuning Wu, Shengyou Zhang, Wei Sun, Yanli Fang, Lin Wu, Liangchao
Huang, and Tian Zhang. Energies 2023, 16 (12), 4811, June 2023. A Two-Step Site Selection
Concept for Underground Pumped Hydroelectric Energy Storage and Potential
Estimation of Coal Mines in Henan Province
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