Seasonal Energy Storage Research Articles

Use of hydrogen as a seasonal energy storage system to manage renewable power deployment in Spain by 2030

Spain has presented a plan (known as PNIEC) to reduce conventional energy sources (shutting down 16 GW) and to increase the use of renewable sources (incorporating 65 GW). This generation portfolio leads to a situation in which there will be a surplus of electrical energy in spring and summer, which will be lacking in autumn and winter. The plan sets a target on batteries, but insufficient to manage more than 10 TWh over 6 months. This paper proposes the deployment of electrolysers to produce hydrogen with the surplus energy, using the already existing Spanish natural gas network to store it. The resulting gas (up to 15% blend of hydrogen in natural gas) could be used subsequently in turbines to alleviate the energy deficit. With this strategy, up to 7.27 TWh of the surplus renewable energy could be reused, and 2.54 million tons of CO2 equivalent would be avoided yearly.

Multi-objective optimisation of a seasonal solar thermal energy storage system for space heating in cold climate

Seasonal solar thermal energy storage (SSTES) system is a promising technology to minimise greenhouse gas emissions (GHGE) by harnessing solar energy for space heating applications. The SSTES system in this study includes double U-tube borehole heat exchanger, ground-coupled heat pump and evacuated tube solar collectors. The aim of this investigation is to optimise the design variables of a SSTES system for space heating in cold climate locations. Six cold climate locations were studied namely: Lukla (Nepal), Dras (India), Sivas (Turkey), Harbin (China), Ulaanbaatar (Mongolia), and Verkhoyansk (Russia). The optimisation variables considered are the total solar collector area and total borehole length. There are three separate optimisation investigations: two single-objective and one multi-objective for the SSTES system. The first investigation is to minimise total life cycle greenhouse gas emissions and the second investigation is to minimise total life cycle cost included cost of GHGE. The third investigation is to minimise both life cycle cost of SSTES system and cost of GHGE (multi objectives). The simulation model was developed using TRNSYS 17 and the Multi-Objective Building Optimisation (MOBO) tool was applied for optimisations. The optimal set of design variables and the corresponding value of the objective function were determined for each single-objective optimisation investigation. Pareto front for each location was also identified for the multi-objective optimisation investigation. The technical and financial performance were also analysed and presented.

Topology optimization for heat transfer enhancement in thermochemical heat storage

Thermochemical heat storage has a promising application prospect due to its high energy density and ability of seasonal energy storage with little heat loss. However, most of thermochemical heat storage systems suffer from very poor thermal conductivity that limit the heat transfer rate and hinder the implementation of this technology. This study presents a novel solution to the poor heat transfer problem in a multi-tube thermochemical heat storage unit through topology optimization of highly conductive fins. For the formulation of the topology optimization problem, the dehydration process of Ca(OH)2 was considered, and a density-based approach was employed. Results indicated that the topology optimized geometry had a superior heat transfer performance and a 43% improvement was achieved with respect to the normally used longitudinal design in terms of reaction time. It was found that during the dehydration process of Ca(OH)2, heat conduction was the dominant factor while convection heat transfer was only observed in a small region near the porous gas channel. Furthermore, the effects of the number of heat transfer fluid (HTF) pipes were explored. It was shown that bifurcations of intermediate branches tend to vanish with the increase of the number of HTF pipes and the charging process can be further enhanced through a more uniform HTF pipe distribution. Finally, the reconstructed model was validated, and the optimized fins were tested under the hydration process of CaO, which further confirmed the superiority of the topology design.

Simulated performance of a borehole-coupled heat pump seasonal solar thermal storage system for space heating in cold climate

The performance of a seasonal solar thermal energy storage system for space heating in cold climates was investigated. The system includes a double U-tube vertical borehole thermal energy storage integrated with a ground coupled heat pump and evacuated tube solar collectors. A validated TRNSYS model for six cold climate locations: Lukla, Dras, Sivas, Harbin, Ulaanbaatar and Verkhoyansk was applied. The hourly typical metrological year (TMY) data of each location was used. The house model for each location was based on the typical local dwelling. The annual heating loads per unit floor area (GJ m−2) was found in the range of 0.71–2.59 (1.71, 1.79, 0.71, 1.19, 1.45 and 2.59 for Lukla, Dras, Sivas, Harbin, Ulaanbaatar and Verkhoyansk respectively). The sizes of the system components such as solar collector area, borehole length, number of borehole and heat pump capacity were determined for each location. The analysis covered both heat charging and discharging. It was found that the average ground temperatures were stable in each location with solar charging. However, if the system was not charged by solar energy, the average ground temperatures were decreased over the years. The seasonal compressor heating coefficient of performance coefficient of performance were found to be more than 5.8, that shows the potential benefit of the system. The proposed system could fulfil more than 93% of the space heating demand in all selected locations. The results of the study reveal the potential benefits of utilising seasonal solar thermal energy storage system in cold climate locations.

Investigation on bi-salt chemisorption system for long term energy storage

In this study, a bi-salt sorbent was observed to exhibit better adaptability to the heating and cooling temperatures than conventional single-salt sorbents. A reactor filled with the bi-salt sorbent of CaCl2 and MnCl2 was designed, and the performance for long-term thermal energy storage, which included several months of seasonal energy storage and a few days of exhaust gas heat storage in winter, was analyzed. The thermal energy charging rate under both working conditions increased with the heating temperature; however, the COPh exhibited a downward trend. The COPh of seasonal energy storage was maintained above 0.5, and the COPh of exhaust gas heat storage was slightly higher at approximately 0.7. For both working conditions, the average heat-producing power in 50 min increased to approximately 9 kW, and the energy-storage density increased to approximately 1900 kJ∙kg−1, which was approximately 100% higher than that of existing resorption systems with single-salt sorbents.

Impact of solar field design and back-up technology on dynamic performance of a solar hybrid heating network integrated with a seasonal borehole thermal energy storage serving a small-scale residential district including plug-in electric vehicles

A solar hybrid district heating network integrated with a seasonal borehole thermal energy storage is dynamically simulated and analyzed over a 5-year period. The system is devoted to satisfying the space heating and domestic hot water requirements of a small-scale district consisting of 6 typical Italian single-family houses, located in Naples, with home charging of 6 plug-in electric vehicles.Eight different plant schemes differing in terms of solar field configuration and/or back-up system used for compensating the intermittency of solar source are investigated. Two different configurations of solar field are analyzed: the first one with solar thermal collectors only and the second one integrating solar thermal collectors and photovoltaic panels coupled with a battery storage; three different alternative auxiliary units are considered: natural gas-fired boiler, natural gas-fueled internal combustion engine-based micro-cogeneration unit and wood pellet boiler. An additional operating scheme is also proposed to better exploit the solar energy for domestic hot water production.The primary energy consumption, the carbon dioxide emissions and the operating costs of the proposed plant configurations are evaluated based on simulation results and then compared with those associated to a conventional decentralized heating system in order to assess the main energy, environmental and economic benefits/drawbacks.

Experimental investigation of underground seasonal cold energy storage using borehole heat exchangers based on laboratory scale sandbox

In order to overcome the intermittent nature of renewable energy resources, borehole thermal energy storage (BTES) systems are found to be a feasible option these days. Previous research data shows that most of the BTES projects are related to heat storage and there are very few underground cold storage applications. The proposed research work is based on the experimental strategy to determine the extracted energy storage potential in typical weather condition areas. It emphasizes on the mathematical model for soil extracted energy storage system and derives similar function relationship of soil TES system based on similarity theory. A laboratory-scale sandbox is designed with similar scale factor n = 20. Sandbox system is operated in peak winter months from October to March according to the weather data for Dunhuang, China. Approximately 69 % of the input energy is charged into the soil. Soil temperature is reduced up to 11 °C during the extraction period. The maximum heat flux value is recorded 65 W.m−2. The experimental results reveal the practicality of proposed research method. It can be used to fulfill cooling load requirements for air conditioning systems and solar thermal power plants located in desert areas with water shortage issues.

Electricity-assisted thermochemical sorption system for seasonal solar energy storage

The present paper investigated the seasonal solar thermal energy storage (SSTES) using solid-gas thermochemical sorption technology that has inherently combined function of heat pump and energy storage. The thermochemical reactions that can discharge heat at a higher temperature usually requires a relatively higher desorption temperature during charging process, which could be problematic to efficiently recover solar energy in high-latitude regions like the UK when using the most mature and economic solar thermal collector (flat-plate or evacuated tube type). The present work studied two hybrid concepts where an electric-driven compressor or an electric heater was introduced to supplement the thermochemical desorption process in terms of pressure rise and temperature lift, respectively, when the available solar heat was not sufficiently high. The SrCl2-8/1NH3 chemisorption was selected from 230 ammonia-chemisorption reactions due to its suitable adsorption/desorption temperature and large energy storage density. The performance of two hybrid systems using SrCl2-8/1NH3 chemisorption were evaluated and compared to determine the optimal solution. The results revealed that the hybrid thermochemical sorption with a compressor substantially improved the storage capacity compared to that with electric heater. With a compression ratio of 4, the SSTES system with 20 m2 solar collector under the weather condition of Newcastle upon Tyne can store 3226.8 kWh chemisorption heat in summer by charging 4465.4 kWh solar heat and 848.2 kWh electricity, indicating 60.7% storage efficiency; the corresponding energy density based on the overall system volume is 147.3 kWh/m3. Because of using the renewable solar heat and low carbon intensity electricity in summer, the proposed hybrid SSTES system has noteworthy reduction on carbon emission compared to gas boiler and conventional heat pump.

Large-scale living laboratory of seasonal borehole thermal energy storage system for urban district heating

To obtain a better understanding of the characteristics of large-scale seasonal borehole thermal energy storage (BTES), a living laboratory was developed in Chifeng, China. In the living laboratory, combined heat sources of industrial waste heat and solar energy were adopted for 500000 m3 borehole thermal energy storage. The concept and design of the system, as well as the first operation results of the system, are presented herein. First, critical considerations for developing a large-scale borehole thermal energy storage system were briefly reviewed. The living laboratory was developed to be an experimental platform to conduct long-term field tests of major system operation options while working as an actual running application simultaneously. The flexibility of the system was enhanced using changeable system integration modes and a modular design for each subsystem. According to the monitoring results of the first heat injection period, a total of 33458.6 GJ of thermal energy was injected into the storage. The average soil temperature increased from 10.0 to 35.6 °C, and the core temperature increased to approximately 40.2 °C. The increase in soil temperature 5 m outside the storage was approximately 2 °C. No obvious temperature increase was observed 10 m outside the storage. The results indicate the potential of large-scale borehole thermal energy storage to be integrated into the district heating network to improve the flexibility, robustness, and energy efficiency of the overall energy system.

DeLoop: Decomposition-based Long-term operational optimization of energy systems with time-coupling constraints

Long-term operational optimization of energy systems results in challenging, large-scale problems. These large-scale problems can be directly decomposed into smaller subproblems, in the absence of time-coupling constraints and variables. However, time-coupling is common in energy systems, e. g. due to (seasonal) energy storage and peak-power prices. To solve time-coupled long-term operational optimization problems, we propose the method DeLoop for the Decomposition-based Long-term operational optimization of energy systems with time-coupling. DeLoop calculates feasible solutions (upper bounds) by decomposing the operational optimization problem into smaller subproblems. The solutions of these subproblems are recombined to obtain a feasible solution for the original long-term problem. To evaluate the quality of the feasible solutions, DeLoop computes lower bounds by linear programming relaxation. DeLoop iteratively decreases the number of subproblems and employs the Branch-and-Cut procedure to tighten the bounds. In a case study of an energy system, DeLoop converges fast, outperforming a commercial state-of-the-art solver by a factor of 32.

Investigation on mechanical behaviors of shale cap rock for geological energy storage by linking macroscopic to mesoscopic failures

Geological energy storage using subsurface porous rock is a feasible alternative that could satisfy the needs of future large-scale seasonal energy storage, where the petrophysical and mechanical properties of cap rock is vital to ensure safe and sustainable storage of energy. Shale rock is a typical cap rock for geological storage, but its failure behaviors have not been fully understood due to its severe heterogeneity and anisotropy. Numerical simulation methods have been widely acknowledged as an accurate and effective approach to investigate the mechanical and failure behaviors of different materials. For this purpose, this paper developed a numerical model to simulate the failure process of shale specimen when subjected to triaxial compression stresses. The deformation and strength features, progressive failure processes and corresponding acoustic emission (AE) response are obtained by numerical simulation. The performance of the numerical model is validated by experimental data. Simulation results suggest that the failure events which are accompanied by forming micro-fissures are randomly distributed on the specimen at the initial fracture stage. The failure patterns of shale can be summarized into three types, including shear failure, slip failure and splitting failure. The local fractures resulting from the growth of micro-cracks are the sources of shear failure. But high confining pressure has an inhibitory effect on the extension of parts of macroscopic fractures and may induce the contact between fracture surfaces to be rebuilt. While slip failure is caused by the structural surface destruction and the failure plane follows an irregular path that jumps between the bedding plane and the matrix. The propagation of micro-cracks parallel to the bedding plane is the governing factor of tensile failure. That means numerical modeling can be able to reproduce the triaxial test results to a large extent and reflect the rock failure process in detail, and it can help us to understand the macro- and meso‑properties of cap rock for geological energy storage.

Geometric synergy of Steam/Carbon dioxide Co-electrolysis and methanation in a tubular solid oxide Electrolysis cell for direct Power-to-Methane

Power-to-methane (PtM) is a strategy for seasonal energy storage to enable wider energy share through the existing natural gas networks H2O/CO2 co-electrolysis and methanation reaction can synchronously occur in a single tubular solid oxide electrolysis cell (SOEC) reactor to facilitate direct PtM. In this study, we aim to intensify the synergy between geometrical structure and complex physical–chemical processes in a tubular SOEC reactor for higher Electricity-to-CH4 efficiency. First, we divided the reactor into two parts: Electrolysis region and Methanation reaction (MR) region, and investigated their respective contribution and interaction. According to a self-developed multi-physics tubular SOEC model, we optimized the geometry of the reactor to synergistically regulate the field temperature of the Electrolysis and MR region. The results indicate that CH4 production ratio and Electricity-to-CH4 efficiency can be enhanced through the lengthening of either the Electrolysis region or the MR region. When fed with a cathode gas of 75%H2O + 20%CO2 + 5%H2 (20 mL min−1), an tubular SOEC reactor optimized with an Electrolysis region of 0.10 m and a MR region of 0.045 m can convert 91% of CO2 to CH4 at 1.5 V with an Electricity-to-CH4 efficiency of 49%. An improved synergy between pressure, operating voltage and gas flow can significantly improve CH4 production with less electricity. A tubular SOEC reactor with the optimized geometry can convert 95% of CO2 to CH4 at 1.2 V and 16 bar with an Electricity-to-CH4 efficiency of 81%, producing 52 vol% of dry CH4 at the cathode outlet. After optimization, tubular SOEC reactor can realize a more efficient PtM, thus, promote the combination of renewable power and natural gas networks.

Global resource potential of seasonal pumped hydropower storage for energy and water storage

Seasonal mismatches between electricity supply and demand is increasing due to expanded use of wind, solar and hydropower resources, which in turn raises the interest on low-cost seasonal energy storage options. Seasonal pumped hydropower storage (SPHS) can provide long-term energy storage at a relatively low-cost and co-benefits in the form of freshwater storage capacity. We present the first estimate of the global assessment of SPHS potential, using a novel plant-siting methodology based on high-resolution topographical and hydrological data. Here we show that SPHS costs vary from 0.007 to 0.2 US$ m−1 of water stored, 1.8 to 50 US$ MWh−1 of energy stored and 370 to 600 US$ kW−1 of installed power generation. This potential is unevenly distributed with mountainous regions demonstrating significantly more potential. The estimated world energy storage capacity below a cost of 50 US$ MWh−1 is 17.3 PWh, approximately 79% of the world electricity consumption in 2017.

Numerical Analysis and Parametric Optimisation of Heat Release from a Packed Bed (MgSO4) Thermochemical Reactor

Thermochemical energy storage is one of the process which is capable of both short term and long term energy storage. Incorporating this storage method with solar energy is important when we considering seasonal or long term thermal energy storage. Thermochemical energy storage uses chemical reactions to store and release the energy. The charging or storage temperature of the thermochemical material (TCM), porosity of the reactor bed, concentration of reactants etc. are some of the important factors which affects the storage and release of the energy of a TCM. In this work we investigate the energy release from MgSO4 by modelling the hydration reaction of MgSO4 in a packed bed reactor with continuous flow of moist air through the bed. It is observed that the parameters such as porosity of the reactor bed, mass flow rate of moist air, particle diameter, concentration of water vapour etc. play an important role on the energy release from the TCM. Thaguchi method is used to optimize these parameters. The porosity of the reactor bed and the particle size of the TCM are found to be crucial in energy release.

Effective modelling of borehole solar thermal energy storage systems in high latitudes

sources of energy. The storage of solar thermal energy is a crucial aspect for implementing the solar energy for space heating in high latitudes, where solar insolation is high in summer and almost negligible in winter when the domestic heating demand is high. To use the solar heating during winter thermal energy storage is required. In this paper, equations representing the single U-tube heat exchanger are implemented in weak form edge elements in COMSOL Multiphysics to speed up the calculation process for modelling of a borehole storage layout. Multiple borehole seasonal solar thermal energy storage scenarios are successfully simulated. After 5 years of operation, the most efficient simulated borehole pattern containing 168 borehole heat exchangers recovers 69% of the stored seasonal thermal energy and provides 971 MWh of thermal energy for heating in winter.

Seasonal energy storage for zero-emissions multi-energy systems via underground hydrogen storage

The deployment of diverse energy storage technologies, with the combination of daily, weekly and seasonal storage dynamics, allows for the reduction of carbon dioxide (CO2) emissions per unit energy provided. In particular, the production, storage and re-utilization of hydrogen starting from renewable energy has proven to be one of the most promising solutions for offsetting seasonal mismatch between energy generation and consumption. A realistic possibility for large-scale hydrogen storage, suitable for long-term storage dynamics, is presented by salt caverns. In this contribution, we provide a framework for modeling underground hydrogen storage, with a focus on salt caverns, and we evaluate its potential for reducing the CO2 emissions within an integrated energy systems context. To this end, we develop a first-principle model, which accounts for the transport phenomena within the rock and describes the dynamics of the stored energy when injecting and withdrawing hydrogen. Then, we derive a linear reduced order model that can be used for mixed-integer linear program optimization while retaining an accurate description of the storage dynamics under a variety of operating conditions. Using this new framework, we determine the minimum-emissions design and operation of a multi-energy system with H2 storage. Ultimately, we assess the potential of hydrogen storage for reducing CO2 emissions when different capacities for renewable energy production and energy storage are available, mapping emissions regions on a plane defined by storage capacity and renewable generation. We extend the analysis for solar- and wind-based energy generation and for different energy demands, representing typical profiles of electrical and thermal demands, and different CO2 emissions associated with the electric grid.

The value of seasonal energy storage technologies for the integration of wind and solar power

Energy storage at all timescales, including the seasonal scale, plays a pivotal role in enabling increased penetration levels of wind and solar photovoltaic energy sources in power systems.

Hybrid solar power plant with thermochemical energy storage: A multi-objective operational optimisation

Energy storage is key to decarbonising the energy sector by reducing intermittency and increasing the integration of renewable energy. Thermochemical energy storage (TCES) integrated with concentrated solar and photovoltaic power plants, has the potential to provide dispatchable and competitive energy. Here we develop a multi-objective optimisation framework to find the best operational strategy of a hybrid solar power plant with a TCES system. The model uses a typical meteorological year to optimise one-year hourly operation. The results demonstrate that the integration of a calcium-looping process as TCES in a concentrated solar power plant provides dispatchability and, when hybridised with photovoltaic, enhances its competitiveness with current electricity prices. The low mismatch between supply and demand, even when a fixed commitment is required throughout the year, together with a high overall efficiency, indicates that the integration of calcium-looping in hybrid solar power plants is an opportunity to increase the penetration of solar energy in the power sector. Through the optimisation framework presented, a seasonal energy storage analysis can be developed, although a second optimisation stage is required to improve the sizing of the main components of the system in order to further reduce the energy costs.

Renewable electricity storage using electrolysis

Electrolysis converts electrical energy into chemical energy by storing electrons in the form of stable chemical bonds. The chemical energy can be used as a fuel or converted back to electricity when needed. Water electrolysis to hydrogen and oxygen is a well-established technology, whereas fundamental advances in CO2 electrolysis are still needed to enable short-term and seasonal energy storage in the form of liquid fuels. This paper discusses the electrolytic reactions that can potentially enable renewable energy storage, including water, CO2 and N2 electrolysis. Recent progress and major obstacles associated with electrocatalysis and mass transfer management at a system level are reviewed. We conclude that knowledge and strategies are transferable between these different electrochemical technologies, although there are also unique complications that arise from the specifics of the reactions involved.

Quantitative analysis of passive seasonal cold storage with a two-phase closed thermosyphon

Seasonal cold energy storage is an efficient and sustainable technique for energy acquisition that stores environmental cold energy in winter and offers free cooling in industries, such as food storage or data center cooling. This paper presents a visualized seasonal cold storage system utilizing a two-phase closed thermosyphon as the passive heat-transfer device. Meanwhile, a theoretical model of energy conservation for the passive cold energy storage system was established. The process of ice formation was quantitatively studied through water/ice temperature variations and icicle shape evolution. The effects of the ambient variables and the working period of a thermosyphon on the ice formation were identified. Finally, a conceptual passive cold storage warehouse based on our validated model was proposed and compared with a traditional warehouse cooled by air conditioning system in terms of energy conservation and benefit-cost ratio. This study provides a practical guidance for passive cold storage system design and performance evaluation.