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Variation of water pressure during inflation and deflation of the water reservoir -without overburden sand
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This paper focuses on physical modelling of a novel Underground Pumped Hydroelectric Storage (UPHS) system for storing energy. Potential energy is stored by pumping water into a shallow buried reservoir enclosed by a watertight flexible geomembrane. An updated reservoir geometry design is used to overcome previously seen issues with overstressing o...
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Context 1
... inflation and deflation of the reservoir without overburden sand serves as a reference for the subsequent inflation and deflation of the reservoir with overburden sand. In Figure 5 the variation of water pressure is plotted against stored volume of water in the reservoir as the reservoir is inflated and subsequently deflated for the first time. Series of data are shown for the case with and without overburden sand respectively and best fit linear trendlines for inflation and deflation are shown on the plot. ...
Context 2
... corresponds to 0.0024 kPa/L of water pumped into the reservoir. As can be seen from the slope of the best fit linear trendline shown on Figure 5 the observed response during inflation with no overburden is slightly lower and on average 0.0018 kPa/L. Whilst the pressure increase is seen to drop to around 0.0014 kPa/L during inflation of the reservoir with overburden. ...
Context 3
... difference corresponds to a loss of around 1.4 cm in overburden height during the first inflation if internal stresses in the sand are ignored. A rapid drop and increase in the pressure are observed in Figure 5 as a response to a reversal of the flow direction at full inflation and deflation respectively for both situations with and without overburden sand. This can be seen as a change in the dynamic pressure acting on the pressure gauge which is observed to result in almost perfectly linear relations between pressure and volume changes until a more stable flow condition is established again. ...
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Citations
... a turbine, the stored energy can be recovered later. The earliest concepts of this innovative energy storage system proposed a geomembrane-lined bag filled with water and covered with several metres of soil as a ballast [7,8]. However, to achieve a remarkable energy storage capacity with this concept, large volume fluxes are required. ...
... However, to achieve a remarkable energy storage capacity with this concept, large volume fluxes are required. Numerical investigations [6,9] in addition to large and small-scale laboratory tests [2,8] show that the resulting large deformations of the soil can cause a stability problem in the soil and can result in large energy losses and soil collapse. To overcome these drawbacks, the concept of a deep cavity was presented in [3], which is schematically shown in Figure 1. ...
Worldwide, the energy transition is reflected in the constant development of renewable energy sources such as wind power or photovoltaics. These energy sources meanwhile provide cost-effective and mass-available energy. However, renewable energy sources are subjected to natural fluctuations. This can lead to intermittent energy supply and grid instability, highlighting the urgent need for massive energy storage systems to ensure a reliable and resilient energy supply [1]. Gravity energy storage (GES) is a concept for large-scale energy storage [1, 10]. One approach is to store energy in a subsurface cavity. Through this cavity filled with water and covered by soil, potential energy can be stored through a volume and pressure increase. In a practical application, this process can be achieved by pumping water into the cavity. By discharging the water through a turbine, the stored energy can be recovered later. The earliest concepts of this innovative energy storage system proposed a geomembrane-lined bag filled with water and covered with several metres of soil as a ballast [7, 8]. However, to achieve a remarkable energy storage capacity with this concept, large volume fluxes are required. Numerical investigations [6, 9] in addition to large and small-scale laboratory tests [2, 8] show that the resulting large deformations of the soil can cause a stability problem in the soil and can result in large energy losses and soil collapse. To overcome these drawbacks, the concept of a deep cavity was presented in [3], which is schematically shown in Figure 1. The advantage of this new concept, which is the subject of the present work, is that the energy storage is carried out with significantly lower volume fluxes and, in contrast, with increased pressure. This concept leads to a significant reduction of the displacements and strains of the overlying soil, which allows investigations of the long-term stability of the overall system. Using the HCA model [5] in combination with the Hypoplasticity with intergranular strain [4], the cumulative soil behaviour can be modelled and cycles of energy storage can be simulated numerically. This corresponds to a lifetime of approx. 100 years for daily charging and discharging. Based on the investigations from [3], an extended numerical model is presented in this work and a comprehensive parameter study is carried out. In the finite element calculations, the impact of the groundwater is analysed for the first time in a quasi-static, coupled stress-pore fluid diffusion analysis both on the soil behaviour and on the energy storage. The axisymmetric numerical model and the spatial distribution of strain amplitude after cycles for saturated sand are shown in Figure 1. In addition to the energy capacity, energy efficiency and its degradation due to the cumulative soil behaviour as a result of cycles, the effects on the settlements and inclinations of structures on the ground surface are also investigated. Preliminary results are presented in Figure 1. The results of this extended numerical investigation show that the GES in the presented configurations leads to stable soil behaviour with simultaneously high energy efficiency and acceptable energy capacity. Stability problems were not detected even after storage cycles. The cumulative effects caused by the cyclic deformation are controllable. This study clearly shows that GES can contribute to energy storage and energy transition in the future.
... A pressurised and impermeable (geomembrane-enveloped) cavity that can be charged or discharged with water is used to elevate the soil mass. The stored energy corresponds in a first approximation to the potential energy of the lifted soil mass (Olsen et al., 2015;Sørensen et al., 2021). The concept of a UPHES is shown in Figure 1. ...
... The concept of a UPHES is shown in Figure 1. Sørensen (2021) In previous studies, the presented system has already been investigated by conducting model experiments (Olsen et al., 2015;Sørensen et al., 2021), field tests (Franza et al., 2022;Olsen et al., 2015), centrifuge experiments (Franza et al., 2022) and numerical calculations taking into account advanced constitutive models, such as the hypoplastic model with intergranular strain Stutz et al., 2020). ...
... The concept of a UPHES is shown in Figure 1. Sørensen (2021) In previous studies, the presented system has already been investigated by conducting model experiments (Olsen et al., 2015;Sørensen et al., 2021), field tests (Franza et al., 2022;Olsen et al., 2015), centrifuge experiments (Franza et al., 2022) and numerical calculations taking into account advanced constitutive models, such as the hypoplastic model with intergranular strain Stutz et al., 2020). ...
One of the main concerns of our days is moving away from fossil fuels and implementing renewable energy sources into the energy supply. Large-scale energy storage is one of the biggest obstacles to the energy system's change. The required energy storage systems are not yet available. This paper presents a novel concept for gravitational energy storage. The energy storage is realised in the form of potential energy in the subsurface by injecting a pressurised fluid into a previously generated cavity. High pressures can be achieved in the inflatable cavity, allowing a large amount of energy to be stored. Numerical calculations of a deep cavity are presented. The boundary condition of the charging and discharging of the cavity are realised in the FEM simulation using a fluid link. The explicit high-cycle accumulation model is applied, whereby both the soil behaviour and the energy storage capacity can be evaluated. The preliminary results are promising in the aspect of energy efficiency and storage capacity of the proposed system.
... To prevent large dynamic pressure and/or closing the inlet, a minimum volume threshold at which the flow is inverted is set to 25 m 3 ( = 0.06) starting from cycle 2. Finally, constant pressure values from the constant volume test (results are omitted) confirmed that creep and leakage was negligible, while Fig. 1d indicates a balloon-type inflation of the underground reservoir with concave shape of the overburden. The pressure-volume response in Fig. 3 agrees qualitatively with the trends observed by Sørensen et al. (2021) for a smaller laboratory reduced scale UPHS experiment at 1g. The field system had a lower rate of pressure increase per inflated volume (i.e. ...
... Importantly, − curves indicate that arching and in-locked stresses within the soil above the reservoir may develop during the first cycle, leading to: maximum pressure values greater than the self-weight of the water and soil only during inflation; stresses lower than soil self-weight only during deflation. The efficiency estimated from the input and output energies of both cycles 1 and 2 is approximately 96 %, which is higher than the 85 % values reported for lower stress tests (Sørensen et al., 2021). ...
This paper presents a physical test campaign, including both a field trial and a centrifuge test series, to characterise the geotechnical performance of Underground Pumped Hydroelectric Energy Storage (UPHS) systems in sand. This system consists of a water filled geomembrane-lined reservoir that is pressurised by overburden sand and inflated/deflated to store/recover energy. To achieve a greater storage density per unit area, a geometry involving a “reverse configuration” of the geomembrane is adopted. The paper presents the geometry of the models, ground and membrane properties, while discussing simplifying experimental assumptions. Results illustrate the cyclic response of the UPHS system (focusing on overburden surface movements and pressure-volume relationship at the reservoir) to characterise its key mechanical performance and energy efficiency.
The Underground Pumped Hydroelectric Storage (UPHS) is an energy storage system in which inflation and deflation of an underground geomembrane-lined reservoir interconnected to an open water basin enable the storing and harvesting of energy, respectively, in terms of potential energy. Axisymmetric Finite-Element Analyses (FEA), including monotonic and multi-cycle changes of the reservoir volume, are used to investigate reservoir aspects that influence the capacity and efficiency of UPHS in sand, including geomembrane stiffness and interface friction. In particular, a novel simplified approach for the modelling of UPHS reservoir is proposed. This consists of rigid piston elements lifting within a skirt incompressible material with low shear stiffness and directly tied to the overburden. The simplified approach is validated against refined models having the reservoir as a lined fluid cavity undergoing volume changes. Results indicated that the simplified modelling is suitable for preliminary estimates of overburden deformations, reservoir pressure-volume curves, and energy efficiency.
