Underwater Compressed Air Energy Storage Research Articles

Cylindrical Composite Structural Design for Underwater Compressed Air Energy Storage

Abstract The utilization of renewable energy sources is pivotal for future energy sustainability. However, the effective utilization of this energy in marine environments necessitates the implementation of energy storage systems to compensate for energy losses induced by intermittent power usage. Underwater compressed air energy storage(UWCAES) is a cost-effective and emission-free method for storing energy underwater. This technology has proven to be effective and viable, and it offers significant benefits in terms of energy efficiency and sustainability. In this paper, a cylindrical composite structure UWCAES tank is designed. At first, the materials and shapes of the different forms of air containers were evaluated, and the relationship between container diameter, length, and number of air containers was analyzed on the basis of the range of energy storage densities for different kinds of systems. Subsequently, the materials to be applied in the tanks were investigated and selected according to the actual working conditions of the system. Eventually, finite element models and prediction formula were developed and the influence of changes in the thicknesses of the steel reinforcement were discussed. The results demonstrated that the storage tank possesses adequate environmental resistance and load-bearing capacity, which can provide a reference for its practical engineering implementation.

Stress analysis of different air inlet modes for underwater compressed air energy storage considering fluid influence

Abstract Underwater compressed air energy storage has broad application prospects in the fields of marine energy development, deep-sea exploration, and energy storage away from land. The gas storage device is the core component of its energy storage, and there are two intake modes between the current gas storage device and the pipeline, the upper intake mode and the lower intake mode. There are differences in the stability of the airbag between the two modes, which affects the reliability and stability of the whole system. Therefore, it is necessary to analyze the stress of different intake modes of the gas storage device. Firstly, this paper gives the principle and structure of underwater compressed air energy storage and then establishes the model of the airbag and the corresponding parameters. Based on the finite element fluid simulation analysis system, the dynamic mesh fluid-solid coupling simulation is carried out. By comparing and analyzing the deformation of the two modes due to the impact of water flow in the flow field and the distribution of the equivalent stress at the connection between the main body of the airbag and the pipeline, it is concluded that the stress of the gas storage device in the lower air inlet mode is less than that in the upper air inlet mode, and then it is verified that its safety is better than that of the upper air inlet mode.

A novel offshore energy station with poly-generation of power, fresh water, heat, cold and ice

Energy storage system with large capacity, high efficiency, low cost and long time is major bottleneck, limiting the large-scale deployments of offshore wind power. To improve energy recovery efficiency and energy storage density, here underwater compressed air energy storage (CAES) with isobaric operation is proposed. At about 5 MPa storage pressure, energy recovery efficiency and energy storage density for traditional CAES are respectively 59.45 % and 2.94 kWh/m3, while they increase to 70.35 % and 6.59 kWh/m3 respectively for underwater CAES. The frictional pressure loss of underwater pipeline for air delivery has a remarkable effect on the performance of underwater CAES, and energy recovery efficiency and energy storage density increase from 51.92 % to 67.97 % and from 4.85 kWh/m3 to 6.28 kWh/m3 respectively when the pipeline diameter increases from 0.35 m to 0.45 m. To improve economic profit and utilize interstage compression heat and high frequency electrical energy more efficiently, an offshore energy station with poly-generation is proposed, which has the energy recovery efficiency of 58.26 % and energy storage density of 7.16 kWh/m3, and generates 358.22 MWh of electrical energy, 252.75 GJ of thermal energy, 66.10 GJ of cold energy, 123.23 tons of ice and 257.25 tons of fresh water each day, opening new ways of deploying offshore wind power and offshore energy storage and also providing strong support for developing and utilizing sea resources and conquering the oceans.

Design of Underwater Compressed Air Flexible Airbag Energy Storage Device and Experimental Study of Physical Model in Pool

Renewable energy is a prominent area of research within the energy sector, and the storage of renewable energy represents an efficient method for its utilization. There are various energy storage methods available, among which compressed air energy storage stands out due to its large capacity and cost-effective working medium. While land-based compressed air energy storage power stations have been constructed worldwide, their efficiency remains low. Underwater compressed air energy storage has the potential to significantly enhance efficiency, although no such device currently exists. This paper presents the design of an UWCA-FABESD utilizing five flexible air bags for underwater gas storage and discharge. Additionally, it introduces the working principle of the adiabatic underwater compressed air energy storage system and device. Furthermore, a small-scale physical model with similar functionality was designed and manufactured to simulate the charging process of the air bag in onshore charging and discharging tests as well as posture adjustment and lifting arrangement tests, along with underwater charging and discharging tests. These experiments validated the related functions of the designed underwater compressed air flexible bag energy storage device while proposing methods for its improvement. This research provides a new approach to underwater compressed air energy storage.

Efficiency-Driven Iterative Model for Underwater Compressed Air Energy Storage (UW-CAES)

The competitiveness of large-scale offshore wind parks is influenced by the intermittent power generation of wind turbines, which impacts network service costs such as reserve requirements, capacity credit, and system inertia. Buffer power plants smooth the peaks in power generation, distribute electric power when the wind is absent or insufficient, and improve the capacity factor of wind parks and their profitability. By substituting the variable pressure storage with an underwater variable volume air reservoir and reducing the wastage of compression heat using liquid Thermal Energy Storage (TES), which eliminates the combustor, the plant design allows overcoming the most common drawbacks of CAES plants. Underwater Compressed Air Energy Storage (UW-CAES) plants are investigated with a thermodynamic model to drive the power plant design toward efficiency maximization. Functional maps, constrained on the plant pressure ratio and the number of compressor/turbine phases with inter-refrigerated/inter-heating phases, are drawn by solving the model iteratively for the heat exchangers’ effectiveness to meet the target turbine discharge temperature, selected in advance to avoid unfeasible mathematical solutions.

Application of Underwater Constant Pressure Compressed Air Energy Storage Technology in The Direction of Hydrogen Production

With the growing global demand for renewable energy to cope with climate change and energy security issues, underwater compressed air energy storage technology has gradually attracted attention. At the same time, hydrogen production process, as an important support for green energy transition, is becoming one of the key links to achieve clean energy generation and storage. This paper aims to explore the connection between underwater compressed air energy storage technology and hydrogen production process, and put forward a device combining underwater compressed air energy storage and hydrogen production, so as to solve the problems of low efficiency and serious pollution in the traditional hydrogen production industry. In the past, scholars focused on microcosmic research in the direction of hydrogen production, and tended to the direction of catalyst. Based on this, this paper will study the case of underwater compressed air energy storage device, and conceive a new hydrogen production device to promote the collaborative application of sustainable energy and improve the integration efficiency and reliability of energy system.

Analysis of a hybrid heat and underwater compressed air energy storage system used at coastal areas

Offshore wind power has achieved a storage interest in recent years due to the advantages such as high wind speed, constant wind direction, rich wind energy resources and sufficient annual utilization hours. It has been widely recognized that the underwater compressed air energy storage is one of the most competitive technologies to be integrated into the offshore wind farms. However, the economics of the underwater gas storage device is the main obstacle when this type of energy storage is arranged in shallow-water areas. A hybrid heat and underwater compressed air energy storage system is thus suggested to be integrated with the fluctuating renewable energies. This necessitates the use of electrically heated solid thermal energy storage to provide greater flexibility. Both the system thermodynamics and economics are examined under the premise of constant or varying power input. Results demonstrate that the system behavior is strongly dependent on the water depth at 0–100 m. The combined efficiency and net present value are 44.4% and 67.41 million dollars for a 10 MW hybrid energy storage system as the water depth is 100 m, while they are 29.9% and 40.39 million dollars for the standalone system. The system integrating with the solid thermal energy storage is verified to be a much more valuable technology to manage offshore renewable powers. Furthermore, it indicates that the underwater pipe with a larger diameter is favorable to improve system performance.

2D design and characteristic analysis of an underwater airbag with mooring for underwater compressed air energy storage

Natural shapes are commonly used for balloons and can also be applied in flexible gas containers for underwater compressed air energy storage (UCAES). However, additional consideration of the mooring is required in this application, and the classic natural-shape design carries the structural risk of a significant mooring force acting at a single bottom point. To address these problems, an underwater airbag with mooring (UAM) is proposed considering the practical fixation and stress distribution. This study employs a 2D axisymmetric design method in a dimensionless form based on the natural-shape method. The weightless optimal UAM shape in the zero-pressure state is obtained when the dimensionless mooring length is 0.62. Compared with the classic design in the zero-pressure state, the optimal design decreases the structural height and significantly decreases the mooring tension. Regarding the deflated shape, the results show that the optimal design has a smoother pressure variation and smaller pressure range, which can reduce energy loss and improve the system efficiency. Moreover, a tank experiment was conducted, and the results show good agreement between the prediction and experimental results. The UAM design is closer to practical applications than the classical natural-shape design and can effectively provide preliminary design references for engineering.

Electricity, Transportation, and Water Provision of 100% Renewable Energy for Remote Areas

The integration of variable renewable energy sources in islands has become crucial in reducing their dependency on imported fossil fuels. This study aimed to assess the energy transition of an island towards a 100% renewable energy system for power generation, inland transport, and potable water provision. Linking various fossil-fuel-consuming sectors, such as transport and potable water supply systems, may strongly assist in reducing the possible mismatch between renewable energy source production and demand and contribute to fulfilling other system requirements. The use of energy storage technologies is vital and unlike traditional power systems; as the number of components in the system increases, their proper capacity needs to be accurately determined. This work employs a multi-objective optimization assessment using a modified NSGA-II algorithm to depict the energy transition for Porto Santo Island. To evaluate the solutions, we considered the main criteria of energy cost, avoided environmental impacts (CO2-equivalent emissions) of the proposed system, and loss of power supply. The Pareto front contains various solutions under different system configurations. Results indicate that full inland transport electrification (introducing 3000 EVs) can account for 18% of the avoided CO2 emissions of the island while sharing 28–40% of the up-front cost of the system, depending on the proposed system’s components. The EV’s costs incorporate subsidies and their battery replacement. Another interesting finding from the optimization process is that the solution with the highest avoided CO2 emissions involves keeping a diesel generator for supplying 4% of the island’s total demand and using an underwater compressed air energy storage with a capacity of 280 MWh. This suggests that adding more installed wind turbines or PV panels may not necessarily contribute to reducing the emissions of the entire system.

Experiment and Simulation of the Shape and Stored Gas Characteristics of the Flexible Spherical Airbag for Underwater Compressed Air Energy Storage

Underwater compressed air energy storage (UCAES) is an advanced technology used in marine energy systems. Most components, such as turbines, compressors, and thermal energy storage (TES), can be deployed on offshore platforms or on land. However, underwater gas-storage devices, which are deployed in deep water, have specific characteristics. Flexible inflatables have become a viable alternative for underwater compressed air energy storage (UCAES) as air storage devices. Few studies have been conducted on the characteristics of partially inflated structures during the inflating and deflating processes. A tank experiment of a 1 m model of an underwater spherical airbag was performed to investigate the characteristics of the deformed shape, pressure, and volume of the stored compressed air. A finite element (FE) simulation of an airbag model with the same dimensions was established in Abaqus/Explicit. The simulation under shallow testing conditions was in good agreement with the experimental results. Furthermore, studies on the performance of a 4 m airbag prototype with different water depths are presented. The results indicated that the shape variation was only related to the volume (inflation ratio). The pressure varied approximately linearly with the inflation ratio during the quasi-static process from an empty shape to a zero-pressure shape. The operating depth had a slight effect on the deformation. Because the scaling factor influences the pressure difference of the surface at the same height proportion, the larger the dimensions of the airbag, the higher the pressure difference will be.

Flow characteristics simulation of concave pipe with zero net liquid flow

Liquid accumulation often occurs during gas transmission through underwater pipeline, which affects the operation of the underwater compressed air energy storage system. To study the liquid accumulation flow characteristics in the concave pipe with zero net liquid flow, a simulation model was established in FLUENT to analyze the flow process. By analyzing the slugging distance and liquid slug frequency, the multiphase flow model, the turbulence model, and the simulation time step are determined. In the simulation, the height of the liquid accumulation is 10.4 mm and the gas velocity is 5 m/s, a series of interface waves are generated at the entrance of the pipe. However, an unstable interface causes waves to grow rapidly and merge. Also, when the wave crest of the interface wave reaches the upper part of the pipeline, it is blocked. In conclusion, the characteristics of the concave pipe with zero net liquid flow are reflected by the model.

Performance evaluation of a renewable driven standalone combined power and water supply system with cascade electricity and heat storage

The reliable supply of electricity and freshwater is linked to the living quality of local residents in remote islands. A renewable driven standalone combined power and water supply system with cascade electricity and heat storage is proposed in this paper. The hybrid wind-solar power system acts as the energy source and the humidification dehumidification (HDH) desalination is employed for freshwater production. The underwater compressed air energy storage (UW-CAES) is introduced to smooth the renewable fluctuations, whereas the heat storage system for heat addition purpose of HDH desalination is utilized to reduce dumped power during UW-CAES charging. The simulated results show that the suggested system can satisfy electricity and freshwater demand well. The lower loss of power supply probability (LPSP) or loss of water supply probability (LWSP) requires larger renewable installed capacity. At same threshold situation, the renewable configuration of LWSP is larger than that of LPSP. The performance comparison among different system structures implies that this system has lower renewable configuration and total dumped energy. Moreover, the coefficient of performance of heat pump (COP), the heat storage system capacity, the basic freshwater demand, the flexible energy bag volume and the humidifier seawater inlet temperature have obvious effect on system performance.

Underwater Compressed Gas Energy Storage (UWCGES): Current Status, Challenges, and Future Perspectives

Underwater compressed air energy storage was developed from its terrestrial counterpart. It has also evolved to underwater compressed natural gas and hydrogen energy storage in recent years. UWCGES is a promising energy storage technology for the marine environment and subsequently of recent significant interest attention. However, it is still immature. In this study, the latest progress in both academic and industrial fields is summarized. Additionally, challenges facing this emerging technology are analyzed. The pros and cons of UWCGES are provided and are differentiated from the terrestrial variant. Technical, economic, environmental, and policy challenges are examined. In particular, the critical issues for developing artificial large and ultra-large underwater gas storage accumulators and effective underwater gas transportation are comprehensively analyzed. Finally, the demand for marine energy storage technology is briefly summarized, and the potential application scenarios and application modes of underwater compressed gas energy storage technology are prospected. This study aims to highlight the current state of the UWCGES sector and provide some guidance and reference for theoretical research and industrial development.

Analysis of a Wind-Driven Air Compression System Utilising Underwater Compressed Air Energy Storage

The increasing push for renewable penetration into electricity grids will inevitably lead to an increased requirement for grid-scale energy storage at multiple time scales. It will, necessarily, lead to a higher proportion of the total energy consumed having been passed through storage. Offshore wind is a key technology for renewable penetration, and the co-location of energy storage with this wind power provides significant benefits. A novel generation-integrated energy storage system is described here in the form of a wind-driven air compressor feeding underwater compressed air energy storage. A direct drive compressor would require very high intake swept volumes. To overcome this difficulty, some prior compression is introduced. This paper discusses the constituent technologies for this concept, as well as the various configurations that it might take and the logic behind operating it. Special consideration has been given to the differences resulting from utilising a near-isothermal wind-driven compressor versus a near-adiabatic one. Multiple iterations of the system have been simulated. This has been done using a price-matching algorithm to optimise the system operation and using volumetric air flow rates to calculate exergy flow. Simulated operation has been performed for a year of real wind and electricity price data. This work has been performed in order to clarify the relationships between several key parameters in the system, including pressure and work ratios, volumetric flowrates, storage costs and profit rates. An additional objective of this paper was to determine whether the system has the potential for economic viability in some future energy grid, especially when compared with alternative wind and energy storage solutions. The results of the simulation indicated that, with proper sizing, the system might perform competitively with these alternatives. Maximum one-year return on investment values of 9.8% for the isothermal case and 13% for the adiabatic case were found. These maxima were reached with ~15–20 h of output storage. In all cases, it was found that maximising the power of the wind-driven compressor compared with the initial compressor was favourable.

Experimental and modeling investigation of critical slugging behavior in marine compressed gas energy storage systems

As a flexible-scale energy storage technology, underwater compressed gas energy storage is an emerging enabler of the transition to renewable and sustainable energy structure. Liquid accumulation often occurs in the process of underwater pipeline gas transmission, posing a challenge that must be overcome. In this study, an experimental investigation is presented to investigate the flow characteristics of liquid in a hilly-terrain tube with zero net liquid flow. By adjusting the gas velocity, pipe inclination, liquid accumulation volume, and measuring the expansion length of the liquid film in the horizontal pipe, the liquid accumulation movement is divided into three development stages, which are related to the emergence of the liquid slug. The liquid slug in the pipe is moved by absorption, consolidation, and outflow. The movement of the liquid slug is captured by a high-speed camera, and the formation mechanism of the liquid slug is divided into three categories. When the gas flow rate is low, a liquid slug is formed due to wave growth and convective extrusion. The liquid slug produced by wave merging occurs when the gas velocity is high. A theoretical model of the slugging velocity is established. The relationship between maximum growth factor, liquid film thickness, and gas velocity is analyzed. Furthermore, the critical slugging velocity is determined. The model predictions agreed with the experimental measurement results, with a maximum error of less than 10%. This work can help in furthering underwater compressed air energy storage technology.

Coupling properties of thermodynamics and economics of underwater compressed air energy storage systems with flexible heat exchanger model

Underwater compressed air energy (UW-CAES) systems own plentiful merits of high system efficiency, high energy density and stable operation. In terms of research gap of its coupling properties of thermodynamics and economics, along with research lack focusing on detailed design parameters, the comprehensive thermodynamic and economic coupling model of UW-CAES systems are developed in this paper, specifically including more details of heat exchanger structure, thermal storage medium and an improved energy density. In addition, the discharging power capacity is maintained constant in the analysis for practical design. Then, the effect of key parameters on the whole system's thermodynamic and economic performance is deeply investigated in which the coupling characteristics among parameters are focused on. Results show that the studied UW-CAES system has a good thermodynamic performance with a high system efficiency of 0.7074 and an enhanced high energy density of 26.07 MJ/ m3 considering thermal storage tanks. The system investment can be achieved as low as $3.983 million when water is thermal storage medium. Sensitive analysis shows that it is beneficial to improve the whole system performance with larger underwater pipe diameter, and the discharging pressure should not be lower than 30 bar to avoid efficiency and economic deteriorating. The growing discharging power will increase system investment, while decreasing levelized cost of electricity (LCOE) significantly. Generally, the economy of UW-CAES systems with thermal oil is much worse than counterparts with water. As the thermal storage medium is water, there exist optimum economic ranges of discharging pressure, NTU, length-width ratio and part number.

Multi-objective optimization of a renewable power supply system with underwater compressed air energy storage for seawater reverse osmosis under two different operation schemes

Water supply in rural islands or coastal areas is a basic task for people's livelihood. The all-pervading reverse osmosis (RO) technology is an energy-intensive process. To achieve sustainability goals, the development of renewable driven power supply system for RO plant is significant. Nowadays, the energy, economic and environmental indicators have rarely simultaneously investigated for the type of renewable power supply system with underwater compressed air energy storage (UW-CAES) for RO plant. In this paper, an energy-economic-environmental trade-off multi-objective optimization for such system is proposed in both grid-connected and off-grid schemes. A bi-level optimization strategy based on MOPSO algorithm and TOPSIS method is adopted to seek the optimal configuration and energy management strategy. A real-world case is implemented to demonstrate this method. The results show that the TPC/COE/EC set in optimal grid-connected scheme system is 3.716 × 106¥/0.814¥kWh−1/795.922 kg with a configuration of 511 PVs, 12 WTs and a 900 m3 bag volume in UW-CAES. Whereas, the TPC/COE/EC in optimal off-grid system is 4.076 × 106¥/2.305 kWh−1/566.254 kg with a 705/8/900 m3 configuration. In energy perspective, both schemes can match the load well. In economic perspective, the grid-connected scheme has a considerable profit, whereas the off-grid has no revenue. In environmental perspective, both schemes can reduce the carbon emission remarkably.

Assessment evaluation of a trigeneration system incorporated with an underwater compressed air energy storage

This paper is aiming at stably satisfying the diverse energy requirement of users by using intermittent renewable energies. A novel trigeneration system incorporating with an eco-friendly high temperature underwater compressed air energy storage and an ejector refrigeration cycle is thus developed and analyzed. The mathematical model for the system is established and verified by comparing with the data in open literature. A series of energy and exergy analyses are carried out to find both the qualitative and quantitative links between key properties and the system performance indicators. Results demonstrate that the unit increase of the storage pressure improves significantly the heating energy and net power (27.73 kW and 35.53 kW) while has a positive influence on cooling energy. The cooling energy and net power generation increase as the turbine inlet temperature increases. An increase in temperature has no impact on the heat produced for the end-user. The ambient temperature has positive, negative, and negligible effects on the heating, cooling energy, and the net power, in respective. Aiming to find the best performance of the system, a multi-objective optimization study is conducted to a maximal overall exergy efficiency but minimal capital cost for the novel trigeneration system. The optimized design can produce 55.85% overall exergy efficiency with $296,288 capital investment cost.

Adiabatic compressed air energy storage technology

Adiabatic compressed air energy storage technology

Use of an Under-Water Compressed Air Energy Storage (UWCAES) to Fully Power the Sicily Region (Italy) With Renewable Energy: A Case Study

The high concentration of CO2 in the atmosphere and the increase in sea and land temperatures make the use of renewable energy sources increasingly urgent. To overcome the problem of non-programmability of renewable sources, this study analyzes an energy storage system consisting of under water compressed air energy storage (UWCAES). A case study for fully power the Sicily region (Italy) with renewable energy source (wind and photovoltaic) is presented. From the real annual capacity values of the renewable plants installed in Sicily, a sizing of both the energy production and the storage system and its auxiliary services is evaluated. The optimization of the operation of the system as a whole, modeled with mathematical models already validated in previous studies, is obtained through dynamic programming. The electricity consumed annually by the region, equal to 19048.4 GWh, can be entirely satisfied by renewable energy sources. A sizing of plants powered by renewable sources for a nominal power of 15, 000 MW equally divided between photovoltaic and wind power is considered. The underwater air storage system has a maximum volume of 2.1 × 108 m3, while the compression and generation units have a total nominal power of 6, 900 and 3, 100 MW, respectively. The study finally presents a sensitivity analysis for the evaluation of the effects of the variation of the power produced by renewable energy sources and of Sicily energy consumption. The results show that carbon-free feeding is possible and that all the boundary conditions on the operation of the system can be met.