Underwater Compressed Air Energy Storage Research Articles

Static analysis and verification of flexible riser for underwater compressed air energy storage system with different boundary conditions

ABSTRACT In an underwater compressed air energy storage system, fracture of the flexible riser often occurs. To solve this problem, we proposed a static analysis method for a flexible riser based on Cosserat theory. Bending and torsion deformation of a flexible riser were described. Using Euler parameters, static equilibrium differential equations were established. Finite element discretization, finite difference, and an improved Levenberg–Marquardt algorithm were used to perform the numerical calculations. Based on the similarity principle, experiments with three different boundary conditions were performed. Our static model and the OrcaFlex simulations were compared with experiments, and the average error of our static model was 2.9% smaller than that of the simulation. The results showed that the static model presented in this paper was more accurate than the OrcaFlex simulation calculation. This model is useful for estimating the mechanical characteristics and determining the strengths of flexible risers.

The feasibility survey of an autonomous renewable seawater reverse osmosis system with underwater compressed air energy storage

Water scarcity issue is increasingly obvious due to rapid population growth and flourished industrialization. As an energy intensive technology, the desalination has been gained more attention nowadays, which is transformed to renewable sources driven for a sustainable future. In this paper, an autonomous renewable seawater reverse osmosis system with the support of underwater compressed air energy storage is proposed. As the primary power source, the wind turbine and solar photovoltaic are applied to generate electricity. Meanwhile, the underwater compressed air energy storage system acts as an energy buffer to manage the stochastic power generation and consumption. The simulation results show that the loss of power supply probability and the loss of water supply probability in 1% maximum loss of power supply probability threshold condition are 0.9993% and 1.3667%, respectively. Meanwhile, the energy demand can be matched well by renewable supply, despite some deficient exists. In addition, the power source configuration, the volume and pressure of flexible energy bag, the reverse osmosis membrane area and the wind turbine hub height have effect on system performance. This work can provide some useful information about the potential of compressed air energy storage in seawater reverse osmosis plant for future development.

Preliminary Design and Performance Assessment of an Underwater Compressed Air Energy Storage System for Wind Power Balancing

Abstract A key approach to large renewable power management is based on implementing storage technologies, including batteries, power-to-gas, and compressed air energy storage (CAES). This work presents the preliminary design and performance assessment of an innovative type of CAES, based on underwater compressed air energy storage (UW-CAES) volumes and intended for installation in the proximity of deep-water seas or lakes. The UW-CAES works with constant hydrostatic pressure storage and variable volumes. The proposed system is adiabatic, not using any fuel to increase the air temperature before expansion; a sufficient turbine inlet temperature (TIT) is instead obtained through a thermal energy storage (TES) system which recovers the compression heat. The system includes (i) a set of turbomachines (modular multistage compressor, with partial intercooling; expansion turbine); (ii) a TES system with different temperature levels designed to recover a large fraction of the compression heat, allowing the subsequent heating of air prior to the expansion phase; (iii) an underwater modular compressed air storage, conceived as a network of rigid but open tanks lying on the seabed and allowing a variable-volume and constant pressure operation. The compressor operates at variable loads, following an oscillating renewable power input, according to strategies oriented to improve the overall system dispatchability; the expander can be designed to work either at full load, thanks to the stability of the air flowrate and of the TIT guaranteed by the thermal storage, or at variable load. This paper first discusses in detail the sizing and off-design characterization of the overall system; then it simulates a case study where the UW-CAES is coupled to a wind farm for peak shaving and dispatchability enhancement, evaluating the impact of a realistic power input on performances and plant flexibility. Although the assessment shall be considered preliminary, it is shown that round-trip efficiency (RTE) in the range of 75–80% can be obtained depending on the compressor section configuration, making the UW-CAES a promising technology compared to electrochemical and pumped-hydrostorage systems. The technology is also applied to perform peak-shaving of the electricity production from an off-shore wind farm; annual simulations, based on realistic wind data and considering part-load operation, result in global RTE around 75% with a 10–15% reduction in the average unplanned energy injection in the electric grid. The investigated case study provides an example of the potential of this system in providing power output peak shaving when coupled with an intermittent and nonpredictable energy source.

Numerical and experimental investigation of flow around a balloon-shaped bluff body

Air accumulator is a critical component in Underwater Compressed Air Energy Storage systems and Buoyancy Energy Storage systems. In order to bridge the gap between the CFD simulations and experiment verification of flow around air accumulator, in this study, the force characteristics and flow structures around a balloon-shaped air accumulator bluff body model have been investigated using a computational-experimental dual prong method for the first time. The three-dimensional Unsteady Reynolds Averaged Navier-Stokes (URANS) closure is achieved through the k-ω SST turbulence model in FLUENT. The force characteristics and flow structures are measured in a low-speed closed-loop wind tunnel to verify the simulation results. The free end and shape effects are investigated by comparing the flow structure of three different but interconnected bluff bodies at the equivalent Reynolds number of 7.0 × 104. The results show that the k-ω SST turbulence model can correctly predict the time-averaged force characteristics of the balloon-shaped bluff body and flow structures in the wake. Besides, the distinct vortex structures are induced due to the free end and tapered shape effects.

Conventional and advanced exergy analysis of a grid connected underwater compressed air energy storage facility

A data driven exergy analysis has been conducted for the first known grid connected Underwater Compressed Air Energy Storage facility, located in Toronto, Canada. Further to examining the plant through conventional exergy analysis, results were enhanced by splitting exergy destruction rates into avoidable and unavoidable, as well as endogenous and exogenous parts via advanced exergy analysis. The conventional exergy analysis showed that under real operational conditions, the exergy destruction ratio was 47.1%, while under the theoretical unavoidable operational conditions it could be reduced to 15.9%. The overall outcome of the conventional exergy analysis was confirmed by the advanced exergy analysis, the details, however, were quite different. The results of advanced exergy analysis assigned the improvement priority to heat exchanger 4, followed by the turbine and the stage 3 compressor. Conversely, the conventional exergy analysis indicated that the total exergy destruction of the turbine was higher than that for heat exchanger 3. The advanced exergy analysis also revealed that 76.4% of the exergy destruction was avoidable, highlighting the significant potential of the system for performance improvement.

Energy, exergy, and sensitivity analyses of underwater compressed air energy storage in an island energy system

Energy, exergy, and sensitivity analyses of underwater compressed air energy storage in an island energy system

Transient thermodynamic modeling of an underwater compressed air energy storage plant: Conventional versus advanced exergy analysis

Exergy analysis has been applied in various engineering fields. Most of these analyses have been conducted under steady-state conditions. This can be of limited use when the operation under consideration is highly transient. Such is the case for energy storage systems connected to a dynamic grid. In this work, a transient exergy analysis has been performed for the first known grid connected Underwater Compressed Air Energy Storage facility. Both conventional and advanced exergy analyses were applied. The conventional exergy analysis showed that at plant start up, the compressors have the highest exergy destruction ratio. Under real and unavoidable operational conditions, they alone are responsible for 40% and 35% of exergy destruction of the system. Under the steady-state condition, however, heat exchangers have the dominant effect with 29% and 31% exergy destruction ratios under real and unavoidable operating conditions, respectively. The advanced exergy analysis also revealed that at start up, about 82% of the exergy destruction is avoidable. By the beginning of the steady-state phase, this decreases to 75%, indicating the significant potential of the system for performance improvement. The transient exergetic analysis documented herein has wide applicability for design optimization of a variety of thermodynamic processes.

Comparing Electrical Energy Storage Technologies Regarding Their Material and Carbon Footprint

The need for electrical energy storage technologies (EEST) in a future energy system, based on volatile renewable energy sources is widely accepted. The still open question is which technology should be used, in particular in such applications where the implementation of different storage technologies would be possible. In this study, eight different EEST were analysed. The comparative life cycle assessment focused on the storage of electrical excess energy from a renewable energy power plant. The considered EEST were lead-acid, lithium-ion, sodium-sulphur, vanadium redox flow and stationary second-life batteries. In addition, two power-to-gas plants storing synthetic natural gas and hydrogen in the gas grid and a new underwater compressed air energy storage were analysed. The material footprint was determined by calculating the raw material input RMI and the total material requirement TMR and the carbon footprint by calculating the global warming impact GWI. All indicators were normalised per energy fed-out based on a unified energy fed-in. The results show that the second-life battery has the lowest greenhouse gas (GHG) emissions and material use, followed by the lithium-ion battery and the underwater compressed air energy storage. Therefore, these three technologies are preferred options compared to the remaining five technologies with respect to the underlying assumptions of the study. The production phase accounts for the highest share of GHG emissions and material use for nearly all EEST. The results of a sensitivity analysis show that lifetime and storage capacity have a comparable high influence on the footprints. The GHG emissions and the material use of the power-to-gas technologies, the vanadium redox flow battery as well as the underwater compressed air energy storage decline strongly with increased storage capacity.

Comparing Electrical Energy Storage Technologies Regarding Their Material and Carbon Footprint

The need for electrical energy storage technologies (EEST) in a future energy system, based on volatile renewable energy sources is widely accepted. The still open question is which technology should be used, in particular in such applications where the implementation of different storage technologies would be possible. In this study, eight different EEST were analysed. The comparative life cycle assessment focused on the storage of electrical excess energy from a renewable energy power plant. The considered EEST were lead-acid, lithium-ion, sodium-sulphur, vanadium redox flow and stationary second-life batteries. In addition, two power-to-gas plants storing synthetic natural gas and hydrogen in the gas grid and a new underwater compressed air energy storage were analysed. The material footprint was determined by calculating the raw material input RMI and the total material requirement TMR and the carbon footprint by calculating the global warming impact GWI . All indicators were normalised per energy fed-out based on a unified energy fed-in. The results show that the second-life battery has the lowest greenhouse gas (GHG) emissions and material use, followed by the lithium-ion battery and the underwater compressed air energy storage. Therefore, these three technologies are preferred options compared to the remaining five technologies with respect to the underlying assumptions of the study. The production phase accounts for the highest share of GHG emissions and material use for nearly all EEST. The results of a sensitivity analysis show that lifetime and storage capacity have a comparable high influence on the footprints. The GHG emissions and the material use of the power-to-gas technologies, the vanadium redox flow battery as well as the underwater compressed air energy storage decline strongly with increased storage capacity.

Optimal energy management of an underwater compressed air energy storage station using pumping systems

The paper is part of the development of a novel underwater isothermal Compressed Air Energy Storage (CAES) system. Compared to conventional CAES plant, the performances of this system only depend on the electrical energy required for a round-trip cycle; performances of each sub-system of the power conversion process takes part of the overall efficiency. Consequently, this work is focused on an optimal energy management of the electrical power conversion system driving the isothermal hydro-pneumatic mechanism enabling the air compression/expansion. After examining inherent characteristics of conversion components challenging the overall conversion efficiency, we propose an efficient platform layout based on the segmentation of the energy conversion multiplying power conversion systems with different power ranges. Then, we establish control laws required by the electrical multi-machines system in order to drive pumping systems closed to their best efficiency points. However, these laws subject the conversion platform to a transient and variable operating needing the design of robust controller structures. Finally, we develop a dynamic reversible modelling of the multi-physic conversion platform along with the control scheme. The layout is modelled on Matlab® Simulink® environment and the paper closes with simulation results. We evaluate the dynamic performances of the compressed air storage system in both storage and production mode. Moreover, the effectiveness of power segmentation for the grid integration of the proposed system is discussed.

A profitability investigation into the collaborative operation of wind and underwater compressed air energy storage units in the spot market

Abstract Nowadays, the integration of uncertain renewable resources has been accelerated, which encounters the operational scheduling of power system with some challenges for achieving optimal operation schedule. Hence, uncertain resources will be penalized if they create an imbalance in power generation. Both of the positive and negative imbalances will lead to a decrease in the uncertain resources' profitability. In the market environment, when the time is close to the real operation, the excess generation could not be sold. In some markets, the system operator buys the excess renewable generation by penalty factors. Furthermore, the insufficient generation must be compensated through buying from the spot market which has higher prices. The owners of uncertain resources want to have coordinated operation with storage units to mitigate the risk of commitment in the electricity market and to be protected from high prices of the spot market in case of their inaccurate forecasting. Hence, a contract model is proposed to satisfy both renewable and energy storage generating companies. Hence, a state-of-the-art storage technology which is called UWCAES is incorporated. The uncertain resources cannot be effectively profited of participation in the wholesale electricity market without collaborative operation. The results show the effectiveness of the proposed model.

Comparison of underwater and underground CAES systems for integrating floating offshore wind farms

Floating offshore wind farms (FOWF) with advanced adiabatic compressed air energy storage (AA-CAES) are thermodynamically scrutinized to evaluate if the compressed air is better stored underground or underwater. The corresponding energy efficiency is deduced based on an exergy analysis that details the exergy destruction of each system. The effects of key parameters such as storage and expansion pressures are examined parametrically. It is revealed the underwater compressed air energy storage has a higher efficiency and exergy density. These advantages tend to diminish beyond a critical storage pressure.

Tubular design for underwater compressed air energy storage

Underwater compressed air energy storage (UWCAES) in deep seas is a promising scenario for energy storage. When considered at large scales, specific difficulties arise beyond the ones present when dealing with individual energy bags. After reviewing the set of requirements that a realistic large size project should meet, we propose what we think is the optimal design, the tubular bag. A particular case study is computed numerically aiming at a 1GWh storage at 1000m depth. Depending on the material used, lengths lie in the range of 1–15km. Whereas scaling up is very natural, ballasting poses a challenge and some alternative is proposed.

Conventional and advanced exergy analyses of an underwater compressed air energy storage system

A 2MW underwater compressed air energy storage (UWCAES) system is studied using both conventional and advanced exergy analyses. The exergy efficiency of the proposed UWCAES system is found to be 53.6% under the real conditions. While the theoretical maximum under the unavoidable condition is 84.3%; showing a great potential for performance improvement. Even though there are quantitative differences between conventional and advanced results, both show that the final compressor stage has the highest potential for improvement. The advanced exergy analysis reveals the real improvement potential of the UWCAES system. Further, it is revealed that the interactions between system components are complex but not very strong. Subsequently, the total exergy efficiency may not necessarily increase by improving the performance of the components individually.

Design and thermodynamic analysis of a multi-level underwater compressed air energy storage system

Energy storage technologies are essential for the mainstream realization of renewable energy. Underwater compressed air energy storage (UWCAES) is developed from mature compressed air energy storage (CAES) technologies and retrofitted to store offshore renewable energy. Existing UWCAES technologies, however, usually operate at off-design conditions when handling fluctuating and intermittent renewable energy, which compromises the round-trip exergy efficiency. To increase efficiency, a multi-level UWCAES system is proposed. The results show that the exergy efficiency of the multi-level UWCAES system varies from 62% to 81% in different working modes. The exergy efficiency tends toward 62% when more energy is stored in the CAES subsystem and approaches 81% when more energy is stored in the design-integrated battery pack.

Evaluating the Underwater Compressed Air Energy Storage Potential in the Gulf of Maine

The United States has recognized the need to offset current and future electrical demand with clean, renewable generation. However, plans for integration of high penetration levels of often variable and uncertain renewable energy, like offshore wind, pose significant challenges to utility gird operators and system planners. The intermittent nature of renewables can result in dramatic changes in system load, indicating a need for large-scale energy storage technologies that would allow renewables to be dispatched when needed. Among different storage technologies, pumped hydro storage, batteries and fuel cells have some inherent advantages over others but only compressed air energy storage (CAES) has the capacity of pumped hydro and potentially lowest overall capital and capacity costs. Advances in system compression designs and utilization of thermal energy storage has made CAES increasingly attractive, especially as new innovations in air storage technologies are now allowing CAES to break away from site specific geological formations like salt domes by allowing the air to be stored underwater in pressure vessels. In this paper, a thermodynamic evaluation of an idealized underwater pressure-balanced CAES system is conducted and compared to other large-scale underwater storage methods. Using the Gulf of Maine as a case study area, the thermodynamic relations are integrated in ArcGIS, a geospatial analysis program, to determine the energy storage resource potential for the New England area.

Flow over submerged energy storage balloons in closely and widely spaced floral configurations

Water flow past the accumulator unit of an underwater compressed air energy storage plant was studied numerically. The accumulator unit consists of three underwater balloons arranged in a floral configuration. The numerical simulation was conducted at a Reynolds number of 2.3×105 using URANS k–ω and LES Dyna-SM turbulence models of the ANSYS-Fluent CFD software. The URANS mean values are compatible with those of LES; however, LES appeared to better predict the turbulent nature of the studied flow. The flow pattern was illustrated through iso-surfaces of the second invariant of the velocity gradient (Q criterion) and three-dimensional path lines. Several swirling tube flows were found to shed downstream of the balloons. The turbulence dynamic of the flow was illustrated through time-series snapshots of the vorticity contours on planes perpendicular to the flow direction; revealing the swinging movements of the observed swirling tube flows. The force coefficients of the hydrodynamic loading on these underwater structures were also investigated. The drag coefficient of the upstream balloon was found to be larger than the downstream one; the difference is more significant for the closely spaced configuration. It is noteworthy to mention that the total drag coefficient of the wide unit was larger than that of the closed one. A Fast Fourier transform of the time history of the force coefficients was used to find the Strouhal number. Strouhal numbers of approximately 0.52 and 0.18 were found for the widely spaced and closely spaced configurations respectively.

Structural analysis of an underwater energy storage accumulator

A full-scale three-dimensional simulation was conducted to investigate structural response of an underwater compressed air energy storage (UWCAES) accumulator to the hydrodynamic loads at Reynolds number of 2.3×105. The accumulator was assumed to be spherical, non-distensible and fixed to the bed of a water body via a cylindrical homogeneous isotropic elastic support. The simulation was carried out for three different supports with aspect ratios AR of 5, 10 and 20 where AR was defined as the ratio of the length to the diameter of the support. The effects of the aspect ratio on the frequency and amplitude of the vibrations of the solid structure induced by hydrodynamic loading were investigated. It was observed that the amplitude of the vibrations increases with the aspect ratio of the support, whereas the frequency decreases. The displacement of the spherical accumulator was illustrated on the X–Y plane for each case.

Flow past an accumulator unit of an underwater energy storage system: Three touching balloons in a floral configuration

An LES simulation of flow over an accumulator unit of an underwater compressed air energy storage facility was conducted. The accumulator unit consists of three touching underwater balloons arranged in a floral configuration. The structure of the flow was examined via three dimensional iso surfaces of the Q criterion. Vortical cores were observed on the leeward surface of the balloons. The swirling tube flows generated by these vortical cores were depicted through three dimensional path lines. The flow dynamics were visualized via time series snapshots of two dimensional vorticity contours perpendicular to the flow direction; revealing the turbulent swinging motions of the aforementioned shedding-swirling tube flows. The time history of the hydrodynamic loading was presented in terms of lift and drag coefficients. Drag coefficient of each individual balloon in the floral configuration was smaller than that of a single balloon. It was found that the total drag coefficient of the floral unit of three touching balloons, i.e. summation of the drag coefficients of the balloons, is not too much larger than that of a single balloon whereas it provides three times the storage capacity. In addition to its practical significance in designing appropriate foundation and supports, the instantaneous hydrodynamic loading was used to determine the frequency of the turbulent swirling-swinging motions of the shedding vortex tubes; the Strouhal number was found to be larger than that of a single sphere at the same Reynolds number.

Commercial grid scaling of Energy Bags for underwater compressed air energy storage

Large-scale ability to store surplus energy for use during periods of high demand is a formidable asset in reducing the energy cost, improving electric grid reliability and addressing climate change. An Energy Bag is a fabric balloon-like vessel anchored to a sea- or lakebed for the purpose of storing surplus energy in the form of compressed air. This mode of energy storage is attractive because the passive pressure force of the deep-water environment takes on the significant role of pressure vessel structure to maintain pressurization of the air stored within the Energy Bag. Upon further investigation, it becomes evident that particular attention must be given to the storage volume and pressure required to satisfy requirements economically for commercial grid-scale development of this novel technology. This paper provides an introduction to the benefits and prerequisites pertaining to commercial scale energy storage capacity as related to Energy Bag structure, volume and deployment depth.