Underground Pumped Storage Hydropower Plants Research Articles

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.

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.

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.

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.

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€.

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.

Hydrochemical changes induced by underground pumped storage hydropower: influence of aquifer parameters in coal mine environments

Abstract. Underground pumped storage hydropower (UPSH) induces hydrochemical changes when water evolves to reach equilibrium with the atmosphere (in the surface reservoir) and with the surrounding medium (in the underground reservoir). These hydrochemical changes may impact the environment and the efficiency of the system (i.e., the UPSH plant), especially in coal mine environments where the presence of sulphide minerals is common. For this reason, it is needed to assess the variables that control the behavior of the system in order to establish criteria for the selection of abandoned mines to be used as underground reservoirs in future UPSH plants. Coupled hydro-chemical numerical models are used for investigating the influence of hydraulic parameters on the hydrochemical changes when pyrite is present in the surrounding medium. Results show the role of the hydraulic conductivity and the porosity on the system behavior, which is helpful for selecting those abandoned mines where the hydrochemical changes and their associated consequences will be less.

Hydrochemical changes induced by underground pumped storage hydropower and their associated impacts

Underground pumped storage hydropower (UPSH) using abandoned mines is an alternative system to manage electricity production in flat regions. Water from an underground reservoir is pumped to a surface reservoir to store electricity in the form of potential energy. Later, water is discharged through turbines into the underground reservoir to produce electricity when demand increases. During this operation, the water hydrochemistry continuously evolves. It varies in order to reach chemical equilibrium with the atmosphere (in the surface reservoir) and with the surrounding porous medium and groundwater (in the underground reservoir). The hydrochemical variations may lead to reactions in the reservoirs and in the surrounding porous medium, causing potentially negative consequences for the environment and the system efficiency, especially when pyrite is present in the surrounding porous medium. In this case, pyrite oxidation leads to a decrease in pH and the precipitation of goethite or schwertmannite in the surface reservoir. The decrease in pH is mitigated when calcite is present in the porous medium. However, other concerns may arise, such as slight increases in pH, the precipitation of ferrihydrite and calcite in the surface reservoir, and the oxidation of pyrite and dissolution of calcite in the surrounding porous medium. Understanding the pH variations and the precipitation/dissolution of minerals is of paramount importance in terms of the environmental impacts and system efficiency. For this reason, this work investigates the main hydrochemical changes and their associated consequences when abandoned deep mines are used for UPSH by means of numerical modelling. The main objective is to highlight the importance of considering hydrochemical aspects when designing future UPSH plants.

Water chemical evolution in Underground Pumped Storage Hydropower plants and induced consequences

Underground Pumped Storage Hydropower (UPSH) is an alternative to manage the electricity production in flat regions. UPSH plants consist of two reservoirs of which at least one is underground. For this last reservoir, abandoned mines could be considered. UPSH related activities may induce hydrochemical variations, such as the increase of the oxygen (O2) partial pressure (pO2), which may entail negative consequences in terms of environment and efficiency, especially in coal mined areas where the presence of sulfide minerals is common. This work assesses the main expected environmental impacts that UPSH using abandoned coal mines may induce.

Underground pumped storage hydropower plants using open pit mines: How do groundwater exchanges influence the efficiency?

Underground Pumped Storage Hydropower (UPSH) is a potential alternative to manage electricity production in flat regions. UPSH plants will interact with the surrounding porous medium through exchanges of groundwater. These exchanges may impact the surrounding aquifers, but they may also influence the efficiency of the pumps and turbines because affecting the head difference between the reservoirs. Despite the relevance for an accurate efficiency assessment, the influence of the groundwater exchanges has not been previously addressed.A numerical study of a synthetic case is presented to highlight the importance of considering the groundwater exchanges with the surrounding porous medium. The general methodology is designed in order to be further applied in the decision making of future UPSH plants introducing each case specific complexity. The underground reservoir of a hypothetical UPSH plant, which consists in an open pit mine, is considered and modelled together with the surrounding porous medium. Several scenarios with different characteristics are simulated and their results are compared in terms of (1) head difference between the upper and lower reservoirs and (2) efficiency by considering the theoretical performance curves of a pump and a turbine. The results show that the efficiency is improved when the groundwater exchanges increase. Thus, the highest efficiencies will be reached when (1) the underground reservoir is located in a transmissive porous medium and (2) the walls of the open pit mine do not constrain the groundwater exchanges (they are not waterproofed). However, a compromise must be found because the characteristics that increase the efficiency also increase the environmental impacts. Meaningful and reliable results are computed in relation to the characteristics of the intermittent and expected stops of UPSH plants. The frequency of pumping and injection must be considered to properly configure the pumps and turbines of future UPSH plants. If not, pumps and turbines could operate far from their best efficiency conditions.