Underground Pumped Storage Hydropower Research Articles

A reliable operation strategy on the compressed-air-regulates-pressure underground pumped storage system and its thermo-economic investigation

Reducing fossil fuel consumption and developing renewable energy sources are essential ways to reduce carbon emissions. Restricting coal consumption produces a large number of abandoned coal mines, while the rapid development of renewable energy raises the demand for energy storage in power grids. Transforming abandoned coal mines into energy storage systems not only reuses the abandoned mines, but also promotes the development of renewable energy. Traditional underground pumped storage hydropower (UPSH) systems have a limit on the range of cavern heights. The compressed-air-regulates-pressure underground pumped storage (CARPUPS) system proposed in this study improves the adaptability of the storage system to underground caverns through different operating modes, and obtains the best economic benefits under different regulated pressures. Based on a specific coal mine within China, regulation methods of compressed air are explored when the head of compressed air varies in the range of 0-350 m. The new system can help to increase space utilization by 14 % and the energy storage density by 51 % when the regulated pressure is 1.0 MPa. Based on the optimized operation mode, the system economics analysis is carried out for different market energy prices (Be). The results show that, when Be = 0.1 ¥/kWh, the levelized cost of energy (LCOE) for the system is not sensitive to the regulated pressure. However, the energy storage capacity of the CARPUPS would be 292 MWh, which is 3 times that of the conventional UPSH system, with an efficiency of 52 % when the regulated pressure is 3.5 MPa.

Hydraulic Characteristics Analysis of Double-Bend Roadway of Abandoned Mine Pumped Storage

The roadway of an abandoned mine is an ideal site for the construction of underground pumped storage hydropower, but the operation of the power station is deeply restricted by the structural characteristics of the roadway. With the common double-bend roadway of an abandoned mine as the research object, this study conducted numerical simulations based on the theory of mass conservation and momentum conservation and explored the law of the flow field characteristics and energy loss of a double-bend roadway with the roadway structure and angle. The results showed that a velocity gradient and a pressure gradient form from the outer wall to the inner wall when the fluid flows through the two bends of the roadway. The low-speed zone and maximum positive pressure appeared at the outside of the bend, while the high-speed zone and maximum negative pressure appeared at the inside of the bend. As the angle rose, the peak value of positive pressure increased correspondingly when the fluid flowed through Model A, whereas the negative pressure displayed a fluctuating trend of increasing first and then decreasing and reached its peak when β = 45°. By contrast, when the fluid flowed through Model B, the velocity gradient was symmetrically distributed at the two bends. The peak value of the positive pressure of the first bend increased, and the other positive and negative pressures displayed a trend of “first increasing and then decreasing” when the angle increased, and they reached their peak when β = 45°. When β ≥ 60°, the fluid formed a backflow zone when it flowed through each bend. With an increase in the angle, the area of the backflow zone increased correspondingly. The head loss of the two models increased with the angle. At the same angle, the head loss of Model B was greater than that of Model A. According to the requirement of abandoned mine pumped storage, the roadways with a bend angle of 15° or 30° in Model A and 15° in Model B can be used. The research results can provide some reference for the underground space exploitation and utilization of abandoned mine pumped storage.

Research on Coal Acoustic Emission Characteristics and Damage Evolution During Cyclic Loading

The acoustic emission, energy, and damage evolution of coal samples for three kinds of uniaxial cyclic loading and unloading are deeply analyzed in this study. The evolution of total absorption energy, elastic strain energy, and dissipated energy of coal samples is related to the stress path, and the increasing amplitudes cycle loading has an obvious damage effect on coal samples. During the loading stage, the acoustic emission phenomenon is most active when loading is increasing and the Felicity and post-Kaiser phenomena appear. The acoustic emission phenomenon during constant loading does not obviously change, but rather becomes active with the increase of the equivalent load. The damage to the coal sample shows nonlinear change increasing loading and unloading and shows linear change for other stress paths. Compared to waveforms with stepwise increasing amplitudes cyclic loading, the failure process of the coal sample is more closely related to the size of the external load, which indicates that reasonable hydraulic design is beneficial to the stability of the confining pressure in the chamber of an underground pumped storage hydropower station.

Study on the Influence of Moisture Dissipation on the Thermal and Humidity Environment in the Turbine Floor of Underground Pumped Storage Hydropower Plants

The turbine floor of the main plant is an important part in an underground pumped storage hydropower plants.The health of personnel and the safe operation of equipment are endangered due to high space humidity and water seepage through the walls. In this paper, numerical simulations were used to study the thermal and humidity environment of the main plant turbine floor in summer. Two working conditions with and without the dissipation of humidity are computed and compared. The psychrometric chart was also used to analyze the air treatment process in the turbine floor. Results show that the air temperature decreases by 28% and the relative humidity increases by 34% when considering the effect of the moisture dissipation source. The results of temperature and humidity analyzed by the psychrometric chart are similar to those of numerical simulation. And validition of the numerical method is conducted . The research results have a reference significance for predicting the relative humidity distribution of the turbine floor and the arrangement of dehumidification devices at design stage .

The Impact of Hydrogeological Features on the Performance of Underground Pumped-Storage Hydropower (UPSH)

Underground pumped storage hydropower (UPSH) is an attractive opportunity to manage the production of electricity from renewable energy sources in flat regions, which will contribute to the expansion of their use and, thus, to mitigating the emissions of greenhouse gasses (GHGs) in the atmosphere. A logical option to construct future UPSH plants consists of taking advantage of existing underground cavities excavated with mining purposes. However, mines are not waterproofed, and there will be an underground water exchange between the surrounding geological medium and the UPSH plants, which can impact their efficiency and the quality of nearby water bodies. Underground water exchanges depend on hydrogeological features, such as the hydrogeological properties and the groundwater characteristics and behavior. In this paper, we numerically investigated how the hydraulic conductivity (K) of the surrounding underground medium and the elevation of the piezometric head determined the underground water exchanges and their associated consequences. The results indicated that the efficiency and environmental impacts on surface water bodies became worse in transmissive geological media with a high elevation of the piezometric head. However, the expected environmental impacts on the underground medium increased as the piezometric head became deeper. This assessment complements previous ones developed in the same field and contributes to the definition of (1) screening strategies for selecting the best places to construct future UPSH plants and (2) design criteria to improve their efficiency and minimize their impacts.

An Exploratory Economic Analysis of Underground Pumped-Storage Hydro Power Plants in Abandoned Deep Coal Mines

This study researches the concept of underground pumped-storage hydro power plants in closed-down underground hard coal mines in Germany. After a review on how this could be realized technically, an economic feasibility analysis is presented, with a particular focus on the costs for the underground storage reservoir. The analysis is performed for different lower (i.e., underground) reservoir sizes and temporal arbitrage potentials (peak/off-peak electricity price spreads), and cost uncertainty is dealt with by means of a Monte Carlo simulation for two distinct head heights. The findings regarding costs and acceptability are compared with those of a classic (on-surface) pumped-storage hydro power plant in a mountainous area. Based on a techno-economic evaluation we conclude that under favorable conditions the realization of underground pumped-storage hydro power (UPSHP) plants seems both technically feasible and economically reasonable. More specifically, an extension of a tubular system seems the most promising option. A UPSHP plant in a mineshaft is probably slightly more expensive than a conventional one, an outcome that depends strongly on the feasible head height. However, the significant reduction of the adverse impacts on the landscape and on local residents, as well as a potentially large number of feasible sites in flat terrain, could make UPSHPs an interesting option for the future energy transition, not just in Germany but worldwide at sites where underground mining is being abandoned.

Empirical Analysis and Geomechanical Modelling of an Underground Water Reservoir for Hydroelectric Power Plants

The European Union policy of encouraging renewable energy sources and a sustainable and safe low-carbon economy requires flexible energy storage systems (FESSs), such as pumped-storage hydropower (PSH) systems. Energy storage systems are the key to facilitate a high penetration of the renewable energy sources in the electrical grids. Disused mining structures in closed underground coal mines in NW Spain have been selected as a case study to analyze the construction of underground pumped-storage hydropower (UPSH) plants. Mine water, depth and subsurface space in closured coal mines may be used for the construction of FESSs with reduced environmental impacts. This paper analyzes the stability of a network of tunnels used as a lower water reservoir at 450 m depth in sandstone and shale formations. Empirical methods based on rock mass classification systems are employed to preliminarily design the support systems and to determinate the rock mass properties. In addition, 3D numerical modelling has been conducted in order to verify the stability of the underground excavations. The deformations and thickness of the excavation damage zones (EDZs) around the excavations have been evaluated in the simulations without considering a support system and considering systematic grouted rock bolts and a layer of reinforced shotcrete as support system. The results obtained show that the excavation of the network of tunnels is technically feasible with the support system that has been designed.

Underground Pumped Storage Hydropower Case Studies in Belgium: Perspectives and Challenges

To avoid the geographical and topographical prerequisites of the conventional pumped hydro energy storage, the use of underground cavities as water reservoirs allows countries without steep topography, such as Belgium, to increase the potential of the energy storage capacity. Belgium abounds in disused mines and quarries convertible into water basins. In this article, two Belgian case studies are presented and discussed for their singularity. A slate quarry in Martelange is discussed in technical aspects proposing three operating scenarios. Moreover, a preliminary economic analysis of the underground pumped storage system and a greenhouse gas emission evaluation for the storage system’s lifetime are presented. The analysis for a 100 MW power plant estimates a total initial investment of over 12 million euros and two million of CO2 avoided over its lifetime. This article also proposes the use of the coal mine 500 m deep of Pérronnes-lez-Binche. The mine representation discussed here offers a high energy capacity, but the substantial head drop (from about 500 to 200 m) challenges the selection of the hydraulic turbomachinery. A 1D simulation computed in SIMSEN draws out the behaviour of the unusual hydraulic configuration of turbines in series.

Underground Pumped-Storage Hydropower (UPSH) at the Martelange Mine (Belgium): Underground Reservoir Hydraulics

The intermittent nature of most renewable energy sources requires their coupling with an energy storage system, with pumped storage hydropower (PSH) being one popular option. However, PSH cannot always be constructed due to topographic, environmental, and societal constraints, among others. Underground pumped storage hydropower (UPSH) has recently gained popularity as a viable alternative and may utilize abandoned mines for the construction of the lower reservoir in the underground. Such underground mines may have complex geometries and the injection/pumping of large volumes of water with high discharge could lead to uneven water level distribution over the underground reservoir subparts. This can temporarily influence the head difference between the upper and lower reservoirs of the UPSH, thus affecting the efficiency of the plant or inducing structural stability problems. The present study considers an abandoned slate mine in Martelange in Southeast Belgium as the lower, underground, reservoir of an UPSH plant and analyzes its hydraulic behavior. The abandoned slate mine consists of nine large chambers with a total volume of about 550,000 m3, whereas the maximum pumping and turbining discharges are 22.2 m3/s. The chambers have different size and they are interconnected with small galleries with limited discharge capacity that may hinder the flow exchange between adjacent chambers. The objective of this study is to quantify the effect of the connecting galleries cross-section and the chambers adequate aeration on the water level variations in the underground reservoir, considering a possible operation scenario build upon current electricity prices and using an original hydraulic modelling approach. The results highlight the importance of adequate ventilation of the chambers in order to reach the same equilibrium water level across all communicating chambers. For fully aerated chambers, the connecting galleries should have a total cross-sectional area of at least 15 m2 to allow water flow through them without significant restrictions and maintain similar water level at all times. Partially aerated chambers do not attain the same water level because of the entrapped air; however, the maximum water level differences between adjacent chambers remain relatively invariant when the total cross-sectional area of the connecting galleries is greater than 8 m2. The variation of hydraulic roughness of the connecting galleries affects the water exchange through small connecting galleries but is not very influential on water moving through galleries with large cross-sections.

Economic Feasibility of Underground Pumped Storage Hydropower Plants Providing Ancillary Services

The electricity generated by some renewable energy sources (RESs) is difficult to forecast; therefore, large-scale energy storage systems (ESSs) are required for balancing supply and demand. Unlike conventional pumped storage hydropower (PSH) systems, underground pumped storage hydropower (UPSH) plants are not limited by topography and produce low environmental impacts. In this paper, a deterministic model has been conducted for three UPSH plants in order to evaluate the economic feasibility when considering daily turbine cycle times at full load (DTCs) between 4 and 10 h. In the model, the day-ahead and the ancillary services markets have been compared to maximize the price spread between the electricity generated and consumed. Secondary regulation, deviation management and tertiary regulation services have been analyzed to maximize the income and minimize the cost for purchasing energy. The capital costs of an open-loop UPSH plant have been estimated for the case of using the existing infrastructure and for the case of excavating new tunnels as lower reservoirs. The net present value (NPV), internal rate of return (IRR) and payback period (PB) have been obtained in all scenarios. The results obtained show that the energy generation and the annual generation cycles decrease when the DTC increases from 4 to 10 h, while the NPV and the IRR increase due to investment costs. The investment cost of a 219 MW UPSH plant using the existing infrastructure reaches 366.96 M€, while the NPV, IRR and PB reached 185 M€, 7.10% and 15 years, respectively, participating in the ancillary services markets and considering a DTC of 8 h.

Underground Pumped-Storage Hydropower (UPSH) at the Martelange Mine (Belgium): Interactions with Groundwater Flow

Underground pumped-storage hydropower (UPSH) is a promising technology to manage the electricity production in flat regions. UPSH plants consist of an underground and surface reservoirs. The energy is stored by pumping water from the underground to the surface reservoir and is produced by discharging water from the surface to the underground reservoir. The underground reservoir can be drilled, but a more efficient alternative, considered here, consists in using an abandoned mine. Given that mines are rarely waterproofed, there are concerns about the consequences (on the efficiency and the environment) of water exchanges between the underground reservoir and the surrounding medium. This work investigates numerically such water exchanges and their consequences. Numerical models are based on a real abandoned mine located in Belgium (Martelange slate mine) that is considered as a potential site to construct an UPSH plant. The model integrates the geometrical complexity of the mine, adopts an operation scenario based on actual electricity prices, simulates the behavior of the system during one year and considers two realistic scenarios of initial conditions with the underground reservoir being either completely full or totally drained. The results show that (1) water exchanges may have important consequences in terms of efficiency and environmental impacts, (2) the influence of the initial conditions is only relevant during early times, and (3), an important factor controlling the water exchanges and their consequences may be the relative location of the natural piezometric head with respect the underground reservoir.

Transient Simulation of Underground Pumped Storage Hydropower Plants Operating in Pumping Mode

The increasing penetration of variable renewable energies (VRE) in the European electricity mix requires flexible energy storage systems (ESS), such as pumped storage hydropower (PSH). Disused mining voids from deep closed mines may be used as subsurface reservoirs of underground pumped-storage hydropower (UPSH) plants. Unlike conventional PSH plants, the air pressure in UPSH plants is variable and it differs from the atmospheric conditions. In this paper, the hydraulic transient process of an UPSH plant operating in pumping mode was investigated and a preliminary thermodynamic analysis of the closed surge tank was carried out. Analytical and CFD three-dimensional numerical simulations based on the volume of fluid (VOF) model with two-phase flow have been performed for analyzing the transient process. In the transient simulation, air and water are considered as ideal gas and compressible liquid, respectively. Different guide vanes closing schemes have been simulated. The obtained results show that the dimensioning of underground reservoir, surge tank, and air ducts is essential for ensuring the hydraulic performance and optimizing the operation of UPSH plants. The static pressure in the air duct, surge tank and lower reservoir reaches −1.6, 112.8 and −4 kPa, respectively, while a heat flux of −80 W was obtained through the surge tank walls.

Efficiency analysis of underground pumped storage hydropower plants

Large-scale energy storage systems, such as underground pumped-storage hydropower (UPSH) plants, are required in the current energy transition to variable renewable energies to balance supply and demand of electricity. In this paper, a novel method to determinate the round trip energy efficiency in pumped storage hydropower plants with underground lower reservoir is presented. Two Francis pump-turbines with a power output of 124.9 and 214.7 MW (turbine) and a power input of 114.8 and 199.7 MW (pump), respectively, have been selected to investigate the overall operation of UPSH plants. Analytical models and two-phase 3D CFD numerical simulations have been carried out to evaluate the energy generated and consumed, considering a typical water mass of 450,000 t and a maximum gross pressure of 4.41 MPa. The results obtained in both analytical and numerical models show that unlike conventional pumped-storage hydropower plants, the round trip energy efficiency depends on the pressure inside the underground reservoir. The round trip energy efficiency could be reduced from 77.3% to 73.8% when the reservoir pressure reaches -100 kPa. In terms of energy balance, the energy generation decreases down to 3,639 MWh −1 and the energy consumption increases up to 4,606 MWh year−1 compared to optimal conditions.

Comparing Subsurface Energy Storage Systems: Underground Pumped Storage Hydropower, Compressed Air Energy Storage and Suspended Weight Gravity Energy Storage

In the current energy context, intermittent and non-dispatchable renewable energy sources, such as wind and solar photovoltaic (generation does not necessarily correspond to demand), require flexible solutions to store energy. Energy storage systems (ESS) are able to balance the intermittent and volatile generation outputs of variable renewable energies (VRE). ESS provide ancillary services such as: frequency, primary and voltage control to the power grid. In order to fulfil the power system control, ESS can switch within seconds for different operation modes. Many times, ESS imply environment impacts on landscape and society. To solve this problem, disused underground spaces, such as closed mines, can be used as underground reservoir for energy storage plants. In this paper, a comparative analysis between underground pumped storage hydropower (UPSH), compressed air energy storage (CAES) and suspended weight gravity energy storage (SWGES) with suspended weights in abandoned mine shafts is carried out. Pumped storage hydropower (PSH) is the most mature concept and account for 99% of bulk storage capacity worldwide. The results obtained show that in UPSH and CAES plants, the amount of stored energy depends mainly on the underground reservoir capacity, while in SWGES plants depends on the depth of the mine shafts and the mass. The energy stored in a SWGES plant (3.81 MWh cycle-1 with 600 m of usable depth assuming 3,000 tonne suspended weight) is much lower than UPSH and CAES plants.

Numerical modelling of water subsurface reservoirs during the operation phase in underground pumped storage hydropower plants

Underground pumped storage hydropower (UPSH) plants may be an alternative to store subsurface energy with lower environmental impacts than conventional pumped storage hydropower (PSH) plants. Network of tunnels in closed mines (i.e. coal mines) could be used as water lower reservoir of UPSH plants. The amount of storable energy depends on the water mass and the net head between upper and lower reservoirs. Depending on the direction of the water flow rate, pumping or turbine modes may be used to produce or consume electrical energy. Filling and emptying processes during the operation stage in the underground reservoir are complicated due to the presence of two fluids (water and air) interacting inside the network of tunnels. This paper explores the underground reservoir during the operation stage considering a water flow rate of 55 m3s-1. Two-phase three dimensional CFD numerical models using Ansys Fluent have been developed in order to know the behaviour of the air flow on tunnels and ventilation shaft. Static pressure and air velocity have been analyzed in the simulations at the exit of the ventilation shaft as well as the junction zone between the ventilation shaft and the tunnels network. The results obtained show that a static pressure up to 8,600 Pa and air velocities up to 80 m s-1 could be reached in turbine mode considering a vent shaft with 1 m in diameter. The static pressure increases up to 258,000 Pa if a ventilation shaft of 0.5 m in diameter is considered.

Integration of renewable energies in the electricity grid from energy storage plants in disused mining structures

Energy Storage Plants (ESP) can be considered an alternative to solve the intermittency problems associated to variable renewable energy (VRE), such as wind and solar photovoltaic. This paper explores the use of closed mines as underground reservoir for Underground Pumped Storage Hydropower (UPSH) and Compressed Air Energy Storage (CAES) plants. Investment cost and stored amount of energy are compared in both technologies. Mining structures in disused coal mines in NW Spain have been selected as a case study to analyse the potential of energy storage in disused mining structures. Pumped Storage Hydropower is the most mature large-scale energy storage technology that dominates both the total installed power capacity and the energy storage capacity. An alternative to UPSH plants could be CAES plants, which are systems to store energy in the form of compressed air, and work in a similar way to conventional gas turbines. Abandoned salt cavern or closed coal underground mines are typically used as underground compressed air storage, giving new uses to the infrastructure of abandoned mines. The results obtained show that 130 MW of electrical power could be installed in UPSH and CAES plants in closed mines with an investment cost of about 225 M€.

Stability analysis of the underground infrastructure for pumped storage hydropower plants in closed coal mines

Electricity storage systems are necessary to increase the efficiency of variable renewable energies. Mine water in closed underground coal mines can be used for underground pumped-storage hydropower plants. Subsurface energy storage systems require the excavation of a powerhouse cavern and a network of tunnels as lower water reservoir. To prevent build-up of high air pressures in the network of tunnels during the water filling process it is necessary to excavate ventilation shafts. In this paper, fluid dynamics and geomechanical behavior are combined in the lower reservoir in order to know the feasibility of the underground infrastructure. Depending on the diameter of the ventilation shafts and water flow rate, air pressures up to 420 kPa can be reached. The stability of the powerhouse cavern and the effect of air pressure on the tunnels and shafts during the operation phase of the turbine are analyzed. Coal mines in Northern Spain have been selected as a case study to investigate the behavior of the underground infrastructure of a hydropower plant. Three-dimensional numerical models were developed and analyzed for different rock mass scenarios. The results obtained show that the effect of air pressure on the tunnels and shafts is moderate. The direction of air flow switches during operation time and long-term fatigue damage could be produced to the contour of excavations. The excavation of the powerhouse cavern is technically feasible in case of the application of a proper designed support system.

Pumped-storage hydropower plants with underground reservoir: Influence of air pressure on the efficiency of the Francis turbine and energy production

Closed mines can be used as lower reservoirs of Underground Pumped-Storage Hydropower (UPSH) plants. The excavation of ventilation shafts is necessary to allow the exhalation of the trapped air which is displaced during the turbine process. This paper analyzes the influence of air pressure in the lower reservoir due to the energy losses in the ventilation shafts on the overall energy production. Analytical and three-dimensional CFD numerical models (using VOF formulation for two-phase flow) have been developed in order to know the influence of the air pressure on the turbine operation for the first time. The results obtained show that depending on the usual designs of the ventilation system, the pressure inside the reservoir may vary between 0.67 and 42 mH2O. The increase in pressure implies a reduction in the available net head and therefore, the stored energy decreases. The reduction in the generation of electricity could reach up to 62 MWh/cycle (12.5% of the available energy) for an installation of a Francis turbine with a flow rate at full load of 55 m3/s and 214 MW of power with a single air duct of 0.5 m in diameter. Economically, this would imply losses of more than 1.12 M€/year.

Energy storage in underground coal mines in NW Spain: Assessment of an underground lower water reservoir and preliminary energy balance

During the last decades, the Asturian Central Coal Basin (ACCB) has been a highly exploited coal mining area by means of underground mining and its network of tunnels extend among more than 30 mines. Parts of this infrastructure will soon become available for alternative uses since most of the coal mining facilities in Spain will fade out in 2018. Increasing penetration of renewable energy sources into the electrical grid, with intermittent and fluctuating supply, leads to excessive frequency variations, so the development of energy storage technologies are required, such as Pumped Storage Hydroelectricity (PSH). Reduced environmental impacts, deep, non-flooded shafts and abundance of water from underground run-off, make coal mines in ACCB suitable for the development of Underground Pumped-Storage Hydropower projects (UPSH). The network of tunnels of a mine facility has an unusual geometry for a water storage system. Although there are numerous studies for the construction of UPSH plants, until now there have been no known projects of this type under operation. Filling and emptying processes during the operation of the turbine-pump are complex due to the presence of two fluids interacting inside the tunnels, water and air. This paper explores the viability of a network of tunnels as an underground water reservoir. Two-phase three-dimensional CFD models have been developed in order to know the flow behavior in the tunnels. The pressure and velocity results that have been obtained in the simulations confirm that the use of underground mines as a lower reservoir of a UPSH is technically possible.

Numerical study of the Martelange mine to be used as underground reservoir for constructing an Underground Pumped Storage Hydropower plant

Abstract. Underground Pumped Storage Hydropower (UPSH) using abandoned mines has been considered as a potential high capacity Energy Storage Systems. In UPSH plants, the excess of electricity is stored in the form of potential energy by pumping water from an underground reservoir (abandoned mine in this paper) to a surface reservoir, while electricity is produced (when the demand increases) discharging water from the surface into the underground reservoir. The main concerns may arise from the water exchanges occurring between the underground reservoir and the surrounding medium, which are relevant in terms of environmental impact and UPSH efficiency. Although the role of the water exchanges has been previously addressed, most studies are based on synthetic models. This work focuses on a real abandoned slate mine located in Martelange (Belgium). The effects of different rehabilitation works to prepare the mine as an underground reservoir are assessed in terms of groundwater exchanges and their associated consequences.