A comparative analysis and optimisation of thermo-mechanical energy storage technologies

Electrical Energy Storage (EES) can decouple energy production from its consumption and is urgently needed by both conventional energy system for load leveling and renewable energy system for intermittency smoothing. Currently, Pumped Hydro Energy Storage (PHES) and Compressed Air Energy Storage (CAES) are the main technologies employed, but they both suffer from high capital cost, geographical constraints and environmental issues. Therefore, many innovative concepts of EES technologies have been proposed recently, including the Adiabatic Compressed Air Energy Storage (A-CAES), Pumped Thermal Energy Storage (PTES), Isothermal Compressed Air Energy Storage (I-CAES) and Liquid Air Energy Storage (LAES). All of these EES store electricity in the forms of thermal or mechanical energy, and are most suitable for large-scale energy storage. As a result, they have received intensive treatment from both the industry and academia, but a comparative study and optimization of these large-scale thermo-mechanical energy storage systems from a thermo-economic perspective has so far been lacking, which forms the major part of this thesis. In this thesis, a complete set of system models have been developed for each EES technology, incorporating both thermodynamic and economic factors with due consideration for the constraints and variation ranges of each parameter. Then parametric studies are carried out for each system to analyze the impact of each parameter on the system performance (e.g., efficiency and unit cost). Loss and cost distributions are given for the representative cases, and the detailed explanation and potential improvement are also provided. After that, thermoeconomic optimizations are carried out for each individual system, and the trade-off between efficiency and unit cost as well as the main factors controlling this trade-off are revealed. Finally, these optimized systems are compared with each other, and new EES systems that combined the merits of existing ones are proposed as well. The results show that A-CAES and I-CAES tend to have higher system efficiency and lower unit cost than PTES and LAES, but the PTES and LAES enjoys higher energy density and more siting freedom. More information about the component efficiency and cost is required for an accurate comparison between A-CAES and I-CAES, and between PTES and LAES

Thermodynamic analysis of a novel fossil-fuel–free energy storage system with a trans-critical carbon dioxide cycle and heat pump

This paper presents and analyzes a novel fossil-fuel–free trans-critical energy storage system that uses CO2 as the working fluid in a closed loop shuttled between two saline aquifers or caverns at different depths: one a low-pressure reservoir and the other a high-pressure reservoir. Thermal energy storage and a heat pump are adopted to eliminate the need for external natural gas for heating the CO2 entering the energy recovery turbines. We carefully analyze the energy storage and recovery processes to reveal the actual efficiency of the system. We also highlight thermodynamic and sensitivity analyses of the performance of this fossil-fuel–free trans-critical energy storage system based on a steady-state mathematical method. It is found that the fossil-fuel–free trans-critical CO2 energy storage system has good comprehensive thermodynamic performance. The exergy efficiency, round-trip efficiency, and energy storage efficiency are 67.89%, 66%, and 58.41%, and the energy generated of per unit storage volume is 2.12 kW·h/m3, and the main contribution to exergy destruction is the turbine reheater, from which we can quantify how performance can be improved. Moreover, with a higher energy storage and recovery pressure and lower pressure in the low-pressure reservoir, this novel system shows promising performance

Optimization of a Hybrid Energy System with District Heating and Cooling Considering Off-Design Characteristics of Components, an Effort on Optimal Compressed Air Energy Storage Integration

In this work, the optimal design of a hybrid energy complex, including wind turbines, an internal combustion engine, and an adiabatic compressed air energy storage system is investigated. A novel bi-level optimization strategy is proposed for optimizing the capacity and operational power of each component of the system based on techno-economic considerations. The article presents information and discussions about the impacts of the partial-load operation of the energy storage system components on the optimal rated power and working strategies. The off-design characteristics are proven to have a huge negative impact on the efficiency and economy of the hybrid system. The efficiency reduction of the compressed air energy storage system is about 21% in summer and 8.9% in winter, when the system is operating in partial-load conditions. The operation cost of the system is reduced significantly when carrying out the proposed bi-level optimization strategy

A review on compressed air energy storage - a pathway for smart grid and polygeneration

The increase in energy demand and reduction in resources for conventional energy production along with various environmental impacts, promote the use of renewable energy for electricity generation and other energy-need applications around the world. Wind power has emerged as the biggest renewable energy source in the world, whose potential, when employed properly serves to provide the best power output. In order to achieve self-sustenance in energy supply and to match the critical needs of impoverished and developing regions, wind power has proven to be the best solution. However, wind power is intermittent and unstable in nature and hence creates lot of grid integration and power fluctuation issues, which ultimately disturbs the stability of the grid. In such cases, energy storage technologies are highly essential and researchers turned their attention to find efficient ways of storing energy to achieve maximum utilization. The use of batteries to store wind energy is very expensive and not practical for wind applications. Compressed Air Energy Storage (CAES) is found to be a viable solution to store energy generated from wind and other renewable energy systems. A detailed review on various aspects of a CAES system has been made and presented in this paper which includes the thermodynamic analysis, modeling and simulation analysis, experimental investigation, various control strategies, some case studies and economic evaluation with the role of energy storage towards smart grid and poly-generation

Système intégré de stockage de l’électricité renouvelable par air comprimé

De nos jours, en raison des préoccupations liées à la protection de l'environnement et à la sécurité énergétique, l'utilisation des énergies renouvelables (ER) est en pleine croissance. L'intégration actuelle et future des ER entraîne des déséquilibres importants entre la production et la consommation d'électricité ainsi que des problèmes liés à la flexibilité et à la fiabilité de la gestion du réseau électrique. Dans ce contexte, les technologies de stockage de l'énergie électrique (SEE) s'avèrent être l'élément clé pour relever ces défis. En outre, dans les sites hors réseau qui ont recours aux moteurs diesel, les systèmes SEE sont essentiels pour accroître le taux de pénétration des énergies renouvelables et réduire la consommation d'énergie combustible. De nouvelles évolutions dans le domaine du stockage d'énergie par air comprimé CAES (compressed air energy storage) ont été effectuées en utilisant la chaleur produite durant la phase de compression et en employant des réservoirs de stockage artificiels indépendamment de la disponibilité des cavernes souterraines. Grâce à ces améliorations, le CAES se révèle être une technologie prometteuse pour des applications pratiques. Récemment, le concept de stockage d'énergie trigénérative à air comprimé T-CAES (énergie thermique, mécanique et frigorifique) a été introduit. De nombreuses études soulignent la faisabilité et les avantages de ce système pour être implanté au niveau du consommateur. Les objectifs de ce projet de recherche sont d'examiner les configurations du système T-CAES, de l'étudier par une approche couplée expériences/modélisations, ainsi que d’effectuer une optimisation technico-économique pour des systèmes à petite échelle, généralement inférieure à 500 kW. En partant d'un modèle thermodynamique simplifié, les différentes configurations du système ont été étudiées et les paramètres clés ayant une influence dominante sur l'efficacité du système ont été identifiés. Cette analyse permet de mieux comprendre les principes fondamentaux et le concept thermodynamique de notre système, ainsi que de déduire deux configurations de base du système. Ensuite, un modèle thermodynamique détaillé de ces configurations a été développé incluant les aspects technologiques existants et les interrelations entre les composants. Un banc expérimental a été utilisé pour valider le modèle des composants côté air et pour étudier l'effet des paramètres de fonctionnement sur l'efficacité du système. Les prédictions du modèle sont conformes aux mesures expérimentales pendant les phases de charge, de stockage et de décharge. De plus, il a été constaté que la chute de température à travers le régulateur de pression ne doit pas être ignorée et elle est régie par l'effet de Joule-Thomson. Par ailleurs, il a été observé que la température d'entrée du moteur pneumatique doit être étudiée pour évaluer de futures configurations. L'étude se concentre ensuite sur l'étude des effets mutuels des paramètres de conception et de leur influence sur les performances du système, la densité énergétique et l'empreinte des échangeurs de chaleur via une étude paramétrique. Il est ressorti de cette analyse que la température du stockage d'énergie thermique, le nombre d'étages de compression et l'efficacité des échangeurs de chaleur devraient être choisis comme compromis entre l'efficacité du système, l'empreinte des échangeurs de chaleur et le nombre requis d'étages de détente. Par contre, le choix de la pression maximale de stockage est un compromis à faire entre l'augmentation de la densité énergétique ou l'augmentation de l'efficacité du système. Une ligne directrice pour la conception optimale des paramètres clés mentionnés précédemment est ensuite fournie. Cette directive, la méthodologie et la procédure développée peuvent être étendues pour optimiser le système adiabatique A-CAES avec des changements mineurs. En se basant sur les technologies existantes et en utilisant une sélection optimale des paramètres, le rendement électrique de notre système à micro-échelle, généralement quelques kW, reste faible à 17%, tandis que l'efficacité du système augmente de 10.2% en ajoutant l'énergie électrique équivalente de production de froid et d'énergie thermique. Les faibles performances sont principalement liées aux pertes éxergétiques dans la vanne de détente et aux faibles rendements des machines à petites échelles. L'étude a été complétée par l'élaboration d'un modèle économique du système en fonction de son échelle de puissance et d'énergie. Les résultats montrent que le coût des réservoirs de stockage d'air représente le coût le plus élevé et que la plage technico-économique optimale de la pression maximale de stockage se situe entre 120 et 200 bars. En outre, malgré les faibles performances du système, il a été constaté qu'il pourrait être compétitif à long terme avec les batteries électrochimiques en termes de coûts d'investissement, en particulier après avoir comptabilisé les coûts de production des énergies de chauffage et de refroidissement. Les travaux futurs devraient être orientés vers l'amélioration de l'efficacité du système par l'étude du potentiel d'intégration des tubes à vortex et le développement technologique des machines de détente. De plus, les recherches futures peuvent envisager de réduire les coûts de stockage de l'air en intégrant les réservoirs sous pression en acier/béton qui sont en cours de développement.Abstract: Nowadays, as a result of environmental and energy security concerns, the use of renewable energy (RE) is growing rapidly. The actual and prospective integration of RE results in significant imbalances between electricity production and consumption as well as problems related to the flexibility and reliability of grid operations. Here, electrical energy storage (EES) technologies turn out to be the key element to address these challenges. In addition, in off-grid sites relying originally on diesel engine, EES is a critical point in order to increase the penetration rate of RE and to reduce fuel energy consumption. New advances in compressed air energy storage (CAES) have been made in the use of heat generated from compression and the use of artificial storage reservoirs independently from the availability of underground caverns. Such improvements make CAES a promising technology for practical applications. Recently, the concept of trigenerative compressed air energy storage T-CAES (heat energy, mechanical energy and cooling power) was introduced. Many studies highlight the feasibility and the benefits of this system to be placed close to the energy demand. The aims of this research project are to examine the T-CAES system configurations, to study it by a coupled experimental/modeling approach, as well as to conduct its techno-economic optimizations and economic feasibility at a small-scale, typically less than 500 kW. Starting from a simplified thermodynamic model, the different configurations of the system was investigated and the key parameters having dominant influences on the system efficiency were identified. This analysis enhances the fundamental understanding and the thermodynamic concept of our system and enabled to conclude two main basic configurations. Then, a whole detailed thermodynamic model of the system configurations was developed including the existing technological aspects and the relations between components. An experimental bench was used to validate the model of air side components and to investigate the effect of operating parameters on the system efficiency and the model accuracy. Model predictions were consistent with experimental measurements during charge, storage and discharge phases. It has been found that the temperature drop across the pressure regulator should not be ignored and is governed by the Joule-Thomson effect. Besides, it has been observed that the input temperature of the air motor must be accounted for in the assessment of future improved configurations. The study then focuses on investigating the mutual effects of the design parameters and their influences on the system performances, energy density and heat exchanger footprints via a parametric study. From this analysis, it is found that the temperature of the thermal energy storage, the number of compression stages and the effectiveness of heat exchangers should be selected as a trade-off between the system efficiencies, heat exchangers footprints and the required number of expansion stages. Meanwhile, the selection of the maximum storage pressure is a choice whether to increase the energy density or the system efficiencies. An optimal design guideline of the above key parameters is then provided. This guideline, the method and the procedure developed can be applied to the optimization of the trigenerative compressed air energy storage and could be extended for the adiabatic one with minor changes. Based on existing technologies and using an optimal set of parameters, the round-trip electrical efficiency of our system at micro-scale, typically a few of kW remains low at 17%, while the system efficiency increases by 10.2% by adding the equivalent electric energy of cooling and heating energy productions. The poor performances are mainly linked to the exergy losses in the throttling valve and the low values of the component efficiencies at a micro-scale. The study was extended by developing an economic model of the system as a function of its power and energy scale. The results show that the cost of air storage tanks accounts for the highest cost, and the optimal techno-economical range of the maximum storage pressure is [120 bars-200 bars]. Besides, regardless of the low efficiency of the system, it was found that it could be competitive with electrochemical batteries in terms of investments cost at long terms, especially when accounting for the free-cost of cooling and heating energy production. Future work should focus on the improvement of the efficiency of the system by investigating the potential of integrating of vortex tube, and on technology development of expander machineries. In addition, future research can consider reducing the air storage cost by integrating the under-development steel/concrete pressure vessels

Micro-scale trigenerative compressed air energy storage system: Modeling and parametric optimization study

International audienceIn this paper, a trigenerative compressed air energy storage system is considered giving priority to the electric energy production with the objective to apply it at a micro-scale, typically a few kW. A whole detailed thermo-dynamic model of the system is developed including the existing technological aspects and the relations between components. The study then focuses on investigating the mutual effects of the design parameters and their influences on the system performances, energy density and heat exchanger footprints via a parametric study. From this analysis, it is found that the temperature of the thermal energy storage, the number of compression stages and the effectiveness of heat exchangers should be selected as a trade-off between the system efficiencies, heat ex-changers footprints and the required number of expansion stages. Meanwhile, the selection of the maximum storage pressure is a choice whether to increase the energy density or the system efficiencies. An optimal design guideline of the above key parameters is then provided. This guideline, the method and the procedure presented in this paper can be applied to the optimization of the trigenerative compressed air energy storage and could be extended for the adiabatic one with minor changes. Based on existing technologies and using an optimal set of parameters, the round trip electrical efficiency of our system remains low at 17%, while the comprehensive efficiency reaches 27.2%. The poor performances are mainly linked to the exergy losses in the throttling valve and the low values of the component efficiencies at a micro-scale. The most optimization potentials are also addressed

Techno-economic analysis of hybrid adiabatic compressed-air and biomass gasification energy storage systems for power generation through modelling and simulation

Energy storage has gained an increasing attention as a technology to smoothen out the variations associated with renewable energy power sources and adapt them into a dispatchable product to meet variable demand loads. An energy storage system can be a hybrid or stand alone. There is a rising interest for hybrid energy storage systems cited close to local consumers which is able to exploit the amount of local renewable sources on site, to provide demand side flexibility and also help to decarbonize the heating sector. The thesis is based on modelling and simulation of overall thermodynamic performance and economic analysis of an integrated hybrid energy storage system consisting of adiabatic compressed air energy storage (A-CAES), biomass gasification system with a wood dryer coupled to a syngas-diesel fuelled electric generator for the dual production of electricity and low temperature hot water for domestic use. The first part of the research work involves the modelling of the latent heat (LH) thermal energy storage (TES) for the A-CAES component. Implicit finite difference technique was applied to discretize the energy equations of the heat transfer fluid and phase change material and the resulting equations solved using a developed Matlab computer code. The developed model of the LH TES was validated using experiment measurement from literature and its performance assessed using charging rate, energy efficiency and exergy efficiency. The second part consists of modelling of biomass gasification through a developed Matlab computer code. Kinetic free stoichiometric equilibrium modelling approach was adopted. The developed model showed good agreement with two different experimental measurements. Predictions that can be done with the model include syngas yield, temperature profiles of the pyrolysis, oxidation and reduction zones respectively including syngas yield, carbon conversion efficiency and lower calorific value of the syngas. In the third part, thermodynamic modelling of the overall novel integrated system is developed. It combines the models of different components of the integrated system earlier developed. The system designed for a maximum capacity of 1.3 MW is to utilize the high syngas temperature from the biomass gasifier and the relatively hot dual fuel engine (DFE) exhaust temperature to heat up the compressed air from the A-CAES component during the charging and discharging modes, respectively. Also, the heat contained in the DFE jacket water is recovered to produce low temperature hot water for domestic hot water use. Key output parameters to assess the performance of the hybrid systems are total system efficiency (TSE), round trip efficiency (RTE) of the A-CAES, electrical efficiency, effective electrical efficiency, and exergy efficiency for the system. Furthermore, exergy destruction modelling is done to ascertain and quantify the main sources of exergy destruction in the systems components. Finally, an economic feasibility of the overall system is presented using the electricity and heat demand data of Hull Humber region as a case study. The results of this study reveals that it is technically possible to deploy the proposed system in a distributed generation to generate dispatchable wind power and hot water for domestic use. The total energy and exergy efficiency of the system is about 37.12% and 28.54%, respectively. The electrical and effective electrical efficiency are 29.3 and 32.7 %, respectively. In addition, the round trip efficiency of the A-CAES component of the system is found to be about 88.6% which is higher than that of a standalone A-CAES system, thus demonstrating the advantage of the system to recover more stored wind electricity than in conventional A-CAES system. However, the TSE of the system is less than that of a conventional A-CAES system but comparable to similar hybrid configurations. The exergy destruction of the hybrid system components is highest in the biomass gasifier followed by the DFE and the least exergy destruction occurs in the HAD. Furthermore, economic analysis results show that the system is not profitable for commercial power generation unless a 70% of the total investment cost is waived in the form of subsidy. Expectedly, the cost of electricity (COE) of £0.19 per kWh is more than the range of the mean electricity tariff for a medium user home in the UK including taxes which is £0.15 per kWh. With a subsidy of 70%, the system becomes profitable with a positive NPV value of £137,387.2 and COE of £0.10 per kWh at the baseline real discount rate of 10%. The main contribution of the thesis is that it provides an intergraded realistic tool that can simulate the future performance (thermodynamic and economic) of a hybrid energy storage system, which can aid a potential investor to make informed decision on the profitability and financial outlays for the investmen

A comparative study of liquid, solid and hybrid adiabatic compressed air energy storage systems

The increasing penetration of renewable energy sources into the power grid has prompted the development of many energy storage systems, amongst which Adiabatic Compressed Air Energy Storage (A-CAES) is deemed one of the more promising technologies. A-CAES systems can be categorized into solid A-CAES and liquid A-CAES, both of which have received extensive treatment in the literature. In this paper, thermodynamic and economic models are built for each of these systems and their sub-components, and the appropriate materials are selected for the corresponding Thermal Energy Storage (TES). A hybrid TES system is also considered, combining solid TES for low-pressure air with liquid TES for higher pressure. Results for this are compared with the other two systems. Parametric and optimisation studies have been carried out and suggest that the hybrid system has thermodynamic and economic advantages over the other two. The trade off between efficiency and cost and the factors affecting this trade off are also investigated.Cambridge Trust and China Scholarship Council (CSC