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Renewable energy sources, such as wind and solar, can reduce environmental pollution and promote the transformation of the energy structure. However, the intermittency and uncertainty of new energy sources such as wind and photovoltaics pose challenges to large-scale grid integration. Effective utilization of intermittent energy sources requires large-scale energy storage facilities. Salt rock is recognized as an excellent medium for underground large-scale energy storage with a wide range of applications. This paper identifies the potential of salt caverns to be used for large-scale energy storage by analyzing the distribution of wind and solar energy resources in China, taking into account the grid-connected infrastructure of the Chinese power grid and the geographic location of salt mines. The overlapping areas are identified for utilization in compressed air energy storage and hydrogen storage. Through meticulous categorization and analysis of the factors influencing salt cavern underground storage site selection, the AHP-EWM method is introduced to establish an objective hierarchy model for a comprehensive evaluation system of salt cavern underground storage site selection. The comprehensive planning of China's 20 major salt mines is conducted, and the research findings hold significant reference value for facilitating the low-carbon energy transition in China. • The ways which salt mines can be used for renewable energy peaking are summarized. • Renewable energy religions combined with grid-connected infrastructure and salt mines in China were studied. • Objective hierarchical modeling of subsurface salt cavern siting was developed using a combinatorial assignment method. • Energy Storage Planning for 20 Major Salt Mines in China. [ABSTRACT FROM AUTHOR]

This paper is an Editorial for the special issue (SI) of Energy Reviews (RSER) dedicated to the 10th International Conference on Sustainable Energy and Environmental Protection SEEP, held in Bled-Slovenia. During the conference more than 300 papers were presented, around 25% of these papers were accepted for this special issue. Papers discussed different topics related to renewable energy, energy storage systems and some environmental issues related to transportation. During this conference Prof Olabi presented a keynote talk on the relation between circular economy and renewable energy, case studies about implementing this concept in Scotland were included in his presentation. This editorial will give a short description on the relation between circular economy and renewable energy, then it will be followed by addressing the accepted papers in this special issue. [ABSTRACT FROM AUTHOR]

This paper compares different strategies to transform the heating sector into a future 100% renewable energy solution. It focuses on the consequences for infrastructures in terms of grids and storage across the electricity, gas and heating sectors. The hypothesis is that these consequences are rarely taken into proper consideration, even though the costs are significant and differ substantially between the alternative pathways. While the smart grid scenarios are based on electricity as an energy carrier, the “smart energy systems” approach is based on a cross-sectoral use of all grids. Using Denmark as a case, this paper shows how the current gas and district heating grids each have twice the capacity of the electricity distribution grid. Moreover, the existing gas and thermal storage capacities are substantially higher and the additional future capacities are more affordable than within the electricity sector. The conclusion is that the “smart grid” pathway requires a 2–4 times expansion of the electricity grid and significant investments in electricity storage capacities, while the “smart energy systems” pathway can be implemented with relatively few investments in affordable minor expansions of existing grids and storage capacities. [ABSTRACT FROM AUTHOR]

Building a multi-energy complementary power generation system is a viable way to encourage the use of renewable energy and decarbonize power generation. However, the intermittent nature of renewable power generation, such as photovoltaic and wind power, has prompted concerns regarding power grid stability. To balance such fluctuations, energy storage systems or other flexible power generation technologies should be integrated. In this paper, the peak regulation ability of integrated solar combined-cycle has been enhanced via employing a gas/oil exchanger between the top and bottom cycle. When integrating high penetration intermittent renewable energy, an appropriate operational strategy towards high-quality steady power output regulation is proposed. Dynamic performance analysis of the system, coupled characteristic of heat and mass transfer between subsystems have been highlighted. The case study demonstrates that fluctuations of the multi-energy complementary system power output can be controlled below 0.3 MW without renewable energy curtailment, even though wind-PV power generation fluctuates from 0.3 MW to 26.1 MW with variations per second reaching −2.8/3.7 MW. Furthermore, the system's levelized cost of electricity is down to 0.0512 $/kWh, which is cost-competitive with conventional power generation technologies. • Combination of integrated solar combined cycle with wind and PV power plant. • Power stability of system with high penetration intermittent energy. • Novel system configuration and operation strategy. • Deviations from target power output below 0.3 MW per second. [ABSTRACT FROM AUTHOR]

In 100% renewable energy systems, the requirements for flexibility will be greater than for traditional carbon-based energy systems. New technologies and system setups are needed to provide flexibility for balancing the system. Implementing electricity storages in the energy system could provide parts of the required flexible demand and production, though most of these storage solutions have been shown to have relatively high costs. So-called Carnot batteries have been shown to have a relatively lower cost than traditional batteries, but at a reduced electric efficiency. This paper investigates to what extent large-scale integration of Carnot batteries has a role in the transition to and the operation of 100% renewable energy systems. By implementing Carnot batteries in a 100% renewable energy scenario for Denmark, the energy system effects are identified. The results indicate that the potential economic benefit could be as high as 60.5–66.2 EUR/MWh e discharged, not including costs related to investment as well as operation and maintenance of the Carnot batteries. Thus, large-scale integration of Carnot batteries must perform below this economic threshold to be economic relevant. Existing concepts for stand-alone Carnot batteries are not able to achieve these costs today, therefore solutions for cost reductions should be investigated. • Electricity storages could provide parts of future required system flexibility. • Most of these storage solutions have been shown to have relatively high costs. • Carnot batteries have a relatively lower costs but at reduced electric efficiency. • Large-scale integration of Carnot batteries is tested in a renewable energy system. • Carnot battery concepts should aim for a cost lower than 60.5–66.2 EUR/MWh e. [ABSTRACT FROM AUTHOR]

Compressed air energy storage (CAES) is one of the most promising large capacity energy storage technologies and this technology which was used only for demand side management, it has not attained the status of an economic model due to its poor round-trip efficiency. However, in the emerging renewable power scenario, this technology has received a new interest among the scientists. In this paper, a comprehensive thermodynamic analysis is performed for conventional and modified configurations that enhances the round-trip efficiency appreciably. The results reveal that when the compressed air is kept at isothermal temperature at atmospheric condition, the mass of air stored in the tank will be high which will reduce the size of the storage tank appreciably for the given capacity. Further, the possibility of generating cool energy along with power generation during the expansion of compressed air from the atmospheric temperature is also addressed. Though this configuration yields a poor round-trip power efficiency, if the heat of compression and cool energy generated during expansion are utilized for other applications, then the overall polygeneration efficiency is very high. However, if the objective is to maximize the round-trip power efficiency then the compressed air in the storage tank to be heated by the stored heat along with the external heat source. The main conclusions drawn from this work will be of great help by providing guidelines for the future development of a high-efficiency Isothermal CAES (ICAES) system for enhanced power generation/combined with the concept of polygeneration. • Isothermal CAES system helps to enhance both power and also augment the concept of polygeneration. • Possibility of extracting free energy from large water bodies/ocean to maintain isothermal condition in storage tank. • CAES system will be the future competing technology for battery storage in residential and rural electrification. [ABSTRACT FROM AUTHOR]

Abstract Energy system integration can bring several benefits to energy systems, notably to those that are in transition to high shares of renewable energy. Strategies are needed to realize the theoretical benefits of this approach in practice. Therefore, this paper proposes the organization and mathematical formulation of a multi-carrier day-ahead market in which electricity, gas and heat are traded simultaneously. This market set-up is applied to a conceptual test case to identify how - compared to a reference set-up mimicking the current practice - the multi-carrier market is able to unlock the benefits of energy system integration. It is quantitatively shown that the multi-carrier market (1) eliminates the need for forecasts of prices on subsequent markets and the consequences of the related errors, (2) allows to use the flexibility available in one carrier to facilitate the balancing in another, e.g. using the flexibility of a heat system to help balance the electricity system, and (3) enables specific market outcomes, unachievable in a sequential set-up, which increase the optimality of the market outcome. Highlights • Consistent multi energy market design solves many energy transition challenges. • Simultaneous clearing of different markets results in an increased social welfare. • Simultaneous clearing of different markets reduces the need for price forecasting. • A policy to facilitate integration of heat and electricity market is to be devised. • Measures to increase the liquidity on DA gas and heat markets should be brought. [ABSTRACT FROM AUTHOR]

Abstract The intermittency of renewable energy makes it difficult to be accommodated by the power grid. Energy storage technology can alleviate the power fluctuation and help meet peak demands. Therefore, it plays a role in assisting the renewable energy integration. In this paper, the improvement of renewable energy penetration through energy storage system in complementary and stand-alone renewable energy scenarios was investigated. As a case, two energy infrastructures of Chongming (China) in 2016 and 2040 were modeled by EnergyPLAN. The decrease of coal consumption through renewable energy integration and the enhancement of renewable energy utilization efficiency through energy storage technologies in renewable scenarios were researched by organizing the outputs of EnergyPLAN. Results showed that the coal consumption dropped by 34.62%, 38.46% and 50% when the renewable energy penetration was 100% in "wind", "photovoltaic" and "wind-photovoltaic" scenarios for Chongming, 2016 respectively. The utilization efficiency of renewable energy can be improved up to 62.94%, 55.95%, and 60.14% respectively through energy storage system in these renewable scenarios for Chongming 2040. The most critical excess electricity production and electricity import can be decreased by the heat pump for Chongming, 2040 compared with compressed air energy storage, vehicle to grid and electricity to gas. Highlights • The predicted energy structure for Chongming in 2040 is built with EnergyPLAN model. • Impact of complementary and stand-alone renewable energy sources supply on coal consumption is investigated. • Optimal scale of energy storage system in three renewable energy scenarios for regional energy system is researched. • Four energy storage technologies are compared. [ABSTRACT FROM AUTHOR]

To alleviate the issues of climate warming and fossil resources depletion, this paper proposes a wind-photovoltaic-thermal energy storage-electric heater cogeneration system, which adopts the structure of concentrated solar power plant but replaces the expensive collector field and tower with wind and photovoltaic farms and electric heater that converts the excess electricity as thermal energy source. The cogeneration system can effectively regulate the fluctuating wind and photovoltaic output to satisfy electricity load by electric heater based on resistive heating of molten salt, two-tank molten salt thermal energy storage and power block based on steam Rankine cycle, and supply heat load directly from the molten salt heat transfer fluid and heat exchanger. This paper considers minimizing levelized cost of energy, loss of power and heat supply probability, energy reduction rate and lifecycle equivalent CO 2 emissions as optimized objectives, and investigates the many-objective (having four or more objectives) capacity optimization problem by Nondominated Sorting Genetic Algorithm-Ⅲ coupled with Principal Component Analysis and Technique for order preference by similarity to ideal solution. Moreover, residual analysis is applied to consider the uncertainty of wind power prediction. The detailed optimization process and performance comparison of many-objective algorithms, the all-year energy flow and seasonal output characteristics analysis of the optimal-compromise solution are presented. Finally, the proposed cogeneration system is compared to the power-only system with the same installed capacity and equivalent load demand, and the comparison results show that the proposed cogeneration system performs better in all evaluation metrics. Therefore, the proposed renewable energy cogeneration system is feasible and cost-effective for practical applications. • A novel wind-photovoltaic-thermal energy storage cogeneration system is proposed. • A many-objective capacity optimization model with uncertainty is developed. • Economy, reliability, energy efficiency and emissions are considered as objectives. • Dimensional reduction and decision-making method are coupled with NSGA-Ⅲ. • The cogeneration system is feasible and cost-effective for practical applications. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a comparative study of two energy system analysis models both designed for the purpose of analysing electricity systems with a substantial share of fluctuating renewable energy. The first model (EnergyPLAN) has been designed for national and regional analyses. It has been used in the design of strategies for integration of wind power and other fluctuating renewable energy sources into the future energy supply. The model has been used for investigating new operation strategies and investments in flexibility in order to utilize wind power and avoid excess production. The other model (H2RES) has been designed for simulating the integration of renewable sources and hydrogen into island energy systems. The H2RES model can use wind, solar and hydro as renewable energy sources and diesel blocks as backup. The latest version of the H2RES model has an integrated grid connection with the mainland. The H2RES model was tested on the power system of Porto Santo Island, Madeira, Portugal, Corvo and Graciosa Islands, Azores Islands, Portugal and Sal Island, Cape Verde. This paper presents the results of using the two different models on the same case, the island of Mljet, Croatia. The paper compares methodologies and results with the purpose of identifying mutual benefits and improvements of both models. [Copyright &y& Elsevier]

The Norwegian Agrarian Association has the ambition to reduce greenhouse gas (GHG) emissions by 4–6 million tons of carbon dioxide equivalents (CO 2 -eq) by 2030 in the agricultural sector. As a part of this, 10–25% is expected to come from substituting fossil fuels with renewable energy sources. This paper focuses on the effects of phasing-out fossil fuel consumption in on-field tractor operations with electrification by introducing battery-electric and/or fuel cell tractors and on-site renewable energy generation and storage. The results show that electrification of on-field tractor operations can be a techno-economically feasible pathway to reduce tractor energy-use-related CO 2 emissions in the agricultural sector. Annual CO 2 emissions are observed to reduce by 69% in 2030 and 97% in 2050. However, the CO 2 reduction potential can vary significantly based on the farm type. In this regard, the analysis revealed high sensitivity to manufacturing costs of zero-emission tractors (ZETs), which in combination with a low utilization rate can render the investment to ZETs unprofitable. Moreover, electrification increases electricity consumption, especially peak electricity demand in the agricultural sector. This effect can be reduced with on-site renewable energy generation and energy storage systems. • TIMES-models with differing spatial and temporal resolutions are soft-linked. • The long-term effects of electrification in the agricultural sector in Norway are analysed. • Electrification of on-field tractor operations can reduce energy-use related CO 2 emissions. • Adverse effects of electrification can be mitigated with distributed energy generation. [ABSTRACT FROM AUTHOR]

This paper focuses on a support management system for the management and operation planning of a microgrid by the new electricity market agent, the microgrid aggregator. The aggregator performs the management of microturbines, wind and photovoltaic systems, energy storage, electric vehicles, and usage of energy aiming at having the best participation in the market. Nowadays, the electricity market participation entails making decisions aided by a support and information system, which is an important part of a microgrid support management system. The microgrid support management system developed in this paper has a formulation based on a stochastic mixed-integer linear programming problem that depends on knowledge of the stochastic processes that describe the uncertain parameters. A set of plausible scenarios computed by Kernel Density Estimation sets the characterization of the random variables. But as commonly happen, a scenario reduction is necessary to avoid the need to have significant computational requirements due to the high degree of uncertainty. The scenario reduction carried out is a two-tier procedure, following a K-means clustering technique and a fast backward scenario reduction method. The case studies reveal the performance of the microgrid and validate the methodology basis conceived for the microgrid support management system. • Microgrid support management system in the scope of electricity markets. • Stochastic programming problem to consider the uncertainty in the microgrid. • Two-tier scenario reduction capable of reducing the computation time. • Integration of electric vehicles and the consideration of demand response. • Case studies describing the performance of the microgrid. [ABSTRACT FROM AUTHOR]

The work presented in this paper deals with an assisted management of a standalone multi-source power system. The proposed structure consists of a wind turbine (WT) and a photovoltaic generator (PV) used as renewable sources, a super-capacitor (SC) as an energy storage system and a diesel generator (DG) as an emergency power supply. The aim of this work was to develop an assisted management strategy in order to control the power flow in the system operation with the highest reliability. Four limits of the SC state of charge (SOC) were defined states for the power control strategy. The main advantage of the used strategy was to protect the storage system from the deep charge and discharge and then increase its lifespan. The proposed energy management concept was tested in simulation by applying all possible cases. The obtained results confirm its efficiency and harmony between production and consumption was guaranteed. [ABSTRACT FROM AUTHOR]

The massive deployment of plug-in electric vehicles (PEVs), renewable energy resources (RES), and distributed energy storage systems (DESS) has gained significant interest under the smart grid vision. However, their special features and operational characteristics have created a paradigm shift in distribution network resource allocation studies. This paper presents a combined model formulation for the concurrent optimal resource allocation of PEVs charging stations, RES and DESS in distribution networks. The formulation employs a general objective function that optimizes the total Annual Cost of Energy (ACOE). The decision variables in the formulation are the locations and capacities of PEVs charging stations, RES, and DESS units. A Markov Chain Monte Carlo (MCMC) simulation model is utilized to account for the uncertainties of PEVs charging demand and output generation of RES units. Also, in order to enhance the accuracy of the resource allocation problem, the coordinated control of PEVs charging, RES output power, and DESS charging/discharging are incorporated in the formulated model. The formulation is decomposed into two interdependent sub-problems and solved using a combination of metaheuristic and deterministic optimization techniques. A sample case study is presented to illustrate the performance of the algorithm. [ABSTRACT FROM AUTHOR]

Assessing the applicability of an energy storage system (ESS) based on its duty cycle, i.e., its charge/discharge profile, which represents the demands (associated with a specific application) on an ESS, has attracted great attention in the field of renewable energy. The duty cycle of an ESS is determined and analyzed in this paper. The process of obtaining the duty cycle includes extracting common factors from the ESS operation data using factor analysis, clustering the original fragments using a k -means clustering analysis of the common factors, and constructing typical duty-cycle curves using representative sections of each cluster dataset. The duty cycles of the ESS in schedule output, output smoothness, and schedule and smoothness combination applications, for example, were obtained and analyzed. In a schedule output application, the power ratio of the ESS is the primary characteristic parameter, and each band (high, mid, and low) of the power grade accounts for one third of the duty cycle. The changing rate of the power is the key to the assessment and evaluation of output smoothness. The duty cycle of the schedule and smoothness combination application can be treated as a linear superposition of independent duty cycles. The results showed that a combination of factor analysis and k -means clustering methods can effectively identify and quantify the characteristic parameters of the duty cycle to determine the applicability of the ESS. [ABSTRACT FROM AUTHOR]

In the future energy system, it is likely that there is more variation in the electricity prices due to higher share of renewable energy sources in electricity production. In this paper, the effects of more variable electricity prices were analyzed in a district heating (DH) system that includes both combined heat and power (CHP) plant, fired mainly by biomass and heat only boilers. The most cost optimal dimensioning and combination of heat storages, heat pumps and solar collectors are searched in three future electricity price scenarios. When the impacts of different system components were analyzed separately, it was found that especially a larger heat storage (1% of annual DH energy) is economical. In addition, the results indicate that the most economical size for a heat pump is around 20% of peak heat demand. Yet, the most profitable solution was to include both a heat storage and a heat pump in the DH system. According to our results, solar collector was not a profitable investment in the studied DH system. [ABSTRACT FROM AUTHOR]

This paper presents an optimal planning and scheduling on energy storage systems (ESSs) for congestion management in electric power systems including renewable energy resources. The proposed problem finds optimal capacity and charging-discharging regime of ESSs. The storage units are optimally charged and discharged to tackle the uncertainty related to wind-solar units as well as relief congestion in the lines. Output power of solar and wind units is modeled by Gaussian probability distribution function (PDF) and Monte-Carlo simulation (MCS) is applied to tackle the uncertainty. Simulation results demonstrate that the proposed planning can manage congestion of the network efficiently while dealing with wind and solar resources uncertainties. [ABSTRACT FROM AUTHOR]

The aim of the paper is to simulate multi-tank storage with the thermocline moving from tank to tank and compare the results against single tank storage. No analysis of this nature has previously been performed. The results lend added impetus to developing this new type of thermal energy storage, especially as heat losses through insulation have already been reduced a minimum. Dividing a seasonal thermal energy storage tank into smaller tanks reduces the negative effect of heat transfer through the thermocline. The work is a continuation of the concept already proposed in available literature of using multiple solar energy stores, but we focus mainly on developing a dynamic model of a system of this type and presenting the results of a time and thermocline dependent simulation. The study also encompassed examination of the thickness of thermal insulation of the storage tanks, with definitions of its critical size. Our study revealed that the use of multi-tank thermal energy storage reduces heat loss through the thermocline. This is illustrated mainly by changes in water temperature in the upper part of the tank, which fluctuates across a much smaller range than for a single tank. In the single tank system, the temperature drops below 60 °C, whereas for the multi-tank system it always remains above 64 °C. In addition, in the multi-tank solution the loss caused by the thermocline occurs in only one tank, leaving 3/4 of the storage system unaffected by thermocline loss. • The concept of multi tank seasonal thermal energy storage is examined. • The examination is based on the dynamic modeling. • Results are compared with the single tank thermal energy storage systems. • The examined system is more efficient due thermocline losses limitation. [ABSTRACT FROM AUTHOR]

The optimal design of borehole thermal energy storage systems can ensure their techno-economical goals are met. Current design optimization methods either employ detailed modelling unsuitable for numerical optimization or use simplified models that do not consider operational conditions. This paper proposes an optimization-oriented model and a non-convex optimization formulation that, differently from other studies in the literature, can consider the influence of the seasonal storage size and temperature on its capacity, losses, heat transfer rate, and efficiency of connected heat pumps or chillers. This methodology was applied to a case study, considering two scenarios: storing only the rejected heat from cooling and integrating solar thermal generation. Results show that, with varying boundary conditions such as the electricity CO 2 intensity profile, cooling demand, and price of carbon emissions, not only the optimal seasonal storage size changes but also its optimal operating conditions. The potential reduction of CO 2 emissions was found, under standard boundary conditions, to be limited (up to 6.7%), but an increase in cooling demand and an enhancement of the CO 2 intensity seasonal variation led to a reduction of 27.1%. Integration of solar generation further improved it to 43.7%, with a comparably small increase in annual cost, up to 6.1%. • Energy system non-convex optimization of seasonal storage sizing and control. • Optimization varying CO 2 price, reject heat, and electricity CO 2 intensity. • Two scenarios studied, only cooling reject heat and integrating solar thermal. • Both optimal storage size and operating conditions change with boundary conditions. • CO 2 emissions can be reduced with a comparably small increase in annual cost. [ABSTRACT FROM AUTHOR]

Hydrogen is seen as the future energy that will help decarbonise the emissions of global energy use. Hydrogen-related technologies have recently attracted considerable attention due to their relatively low emissions and high energy yield. Even then, little attention was given to hydrogen's use in energy distribution networks. A renewable-based multi-energy system (RMES) considers power, cooling, heating, and hydrogen energy as utility systems for integrated urban and industrial areas to achieve urban-industrial symbiosis. This paper formulates the RMES as a multi-period mixed-integer nonlinear programming (MINLP) model to optimise the RMES, which minimises the financial implications and environmental impacts. Renewable solar energy is provided to the system using the photovoltaic solar system for electrical generation and the solar thermal collector for heat generation. Thermal, battery and hydrogen energy storages are integrated into the RMES to mitigate the energy supply and demand fluctuations and intermittency. A comparative analysis is conducted to individually identify the performance of different energy storage systems for economic and environmental objective functions. The comparison findings indicate that ESS performs better with 45% usage increases under objective environmental functions. The multi-objective optimisation using the ϵ-constraint method obtains the Pareto optimal solutions to the proposed multi-objective problem, which the 4th solution (ATC: 782,500 USD/y; ACE: 2,777.03 kg CO 2 -eq/y) appears to be the most viable. The solution maintains a high-profit level without sacrificing many opportunities for carbon emissions reduction while satisfying both objective functions simultaneously to a degree of satisfaction of 0.75. Overall, the proposed RMES is proven economical and environmentally friendly for implementation; however, the model is needed to optimise the system based on the specific situation. This study provides the optimisation model for energy recovery and technology optimisation in the multi-energy system for urban-industrial symbiosis, which minimises carbon emission and energy cost. This could lead the energy sector to achieve the Sustainable Development Goals, considering the economically and environmentally viable. • Renewable-based multi-energy system for urban-industrial symbiosis. • Heating, electricity, cooling and hydrogen energy demands are considered. • Multi-period optimisation based on economic and environmental factors. • Thermochemical, battery and hydrogen storages are compared. • Multi-objective optimisation for obtaining the non-dominating Pareto optimal. [ABSTRACT FROM AUTHOR]

This paper proposes a geothermal-solar-wind renewable energy hub framework for community multi-energy supplies. In this framework, multi-energy complementarities of geothermal-solar-wind hybrid renewable energy are fully explored based on electrolytic thermo-electrochemical effects of geothermal-to-hydrogen (GTH), which is integrated with the multi-energy conversion and storage devices for multi-energy supplies. In order to exploit the inherent multi-energy operational dispatchability and flexibility of geothermal-solar-wind renewables, a multi-energy coupling matrix is formulated for the modeling of production, conversion, storage, and consumption of hub-internal electricity, hydrogen and heating energy. A multi-energy operation scheme is further developed to dispatch the energy flows of the coupling matrix for cost-effective accommodation of community renewables. Case studies on a community microgrid under grid-connected and grid-disconnected modes are performed to verify the effectiveness and superiority of the proposed methodology. Simulations results show that the solar-wind accommodation can be improved by at most 1.59% with a significantly lower system operating cost. • An electrolytic thermodynamic network is formulated to model the thermo-electrochemical effects. • Geothermal-solar-wind 100% renewables is proposed for multi-energy supplies. • A multi-energy operation scheme is developed to dispatch the electricity, thermal, and hydrogen flows. • The costs during production, conversion, storage, and consumption process are considered. [ABSTRACT FROM AUTHOR]

The world is undergoing a transition to a more sustainable energy sector dominated by renewable energy sources. This paper proposes an innovative solution that consists of catching water from streams at high altitudes to fill storage containers and transport them down a mountain, converting the potential energy of water into electricity with the regenerative braking systems of electric trucks and storing it in the truck's battery. The energy stored in the electric truck can be sold to the grid or used by the truck to transport other goods. Results show that the levelized cost of the electricity truck hydropower is 30–100 USD/MWh, which is cheap when compared with conventional hydropower 50–200 USD/MWh. The electricity generation world potential for the technology is estimated to be 1.2 PWh per year, which is equivalent to around 4% of the global energy consumption in 2019. Apart from being a low cost and impact electricity generation technology, electric truck hydropower can operate in combination with solar and wind resources and provide energy storage services to the grid. • Innovative hydropower generation alternative. • Levelized cost of the electricity truck hydropower is 30–100 USD/MWh. • The electricity generation world potential for the technology is estimated to be 1.2 PWh per year. • Asia and South America have the highest potential for ETH. [ABSTRACT FROM AUTHOR]

Abstract: NZEB (Net zero energy building) is regarded as an integrated solution to address problems of energy-saving, environmental protection, and CO2 emission reduction in the building section. NZEB could be even possible with electricity production if enough renewable energy could be used. Moreover, various building-service systems with renewable energy sources have been widely considered for potential applications in NZEB. All of these new features extend the technical boundary of the conventional energy-efficient buildings, attach a more profound implication to the sustainable development of building technology, and therefore pose a challenge to evaluation works on NZEB performance. This paper presents a guided tour on NZEB evaluation through literature-research. An overview about definitions and energy-efficient measures of NZEB is presented so that the research object and technology boundary can be clarified for NZEB evaluation. Then, a summary of widely-used research method, tool and performance indicator in evaluation is provided for the methodology part. This part also includes a discussion on the application of LCA (life cycle assessment) in NZEB evaluation and LCA's role in promoting a well-defined NZEB. Finally, potential progress in NZEB evaluation with possible development trends is highlighted in terms of energy storage, load match and smart grid. [Copyright &y& Elsevier]

Abstract: This paper presents a pre-feasibility study of producing and using electrolytic hydrogen in Ecuador as part of a strategy towards a low carbon economy. Hydrogen could be produced using hydropower either alone or combined with other renewable energy sources. For this study, we analyzed two scenarios of energy availability based on data from the largest hydroelectric power plant in the country. The first scenario assumes that an amount of water equivalent to 30% of that spilled in 2011 could be used to generate additional electricity. Thus, an additional amount of energy equivalent to 5% of the energy produced in 2011 could be available. The second scenario doubles this amount of energy. Economic analysis showed that to obtain low-cost hydrogen (3.00 US$/kg) it is necessary to operate the electrolysis plants 24 h/day, using low-cost electricity (30 US$/MWh). A continuous supply of energy could be possible when new hydroelectric utilities start operating or by integrating hydropower with solar and wind. Three possibilities for using hydrogen are discussed: 1) production of ammonia as a raw material for nitrogenous fertilizers, 2) hydro-treating heavy oils and bio-oils in oil refineries, and 3) as an energy storage medium to offset natural instability and unpredictability of renewables. [Copyright &y& Elsevier]

In this paper, the use of bubble-driven flow on phase change material (PCM) is proposed to improve the discharge performance of a latent heat thermal energy storage system (LHTES). The upward momentum of bubbles due to its density difference can agitate liquid PCM and increase the flow velocity of liquid PCM to enhance heat transfer between the heat transfer fluid (HTF) and the PCM. To analyze the mechanism of enhanced heat transfer, visualization techniques of particle image velocimetry (PIV) and shadowgraphy were used. The result showed that the discharge time of LHTES decreased by 6%–12%. The increased flow velocity by bubble-driven flow could be observed and the expanded phase change region away from the solid/liquid interface could be identified. The average convective heat transfer coefficient is increased by maximum 1.79 times during the heat discharge period. This mechanism could accelerate the solidification process and enhance the energy discharge rate of the LHTES. Bubble-driven flow could be successfully applied to PCM to improve LHTES discharge performance. [Display omitted] • The movement and mixing of liquid PCM is increased by bubble-driven flow. • The phase change occurred over a extended region, not just at the solid/liquid interface. • Bubble-driven flow improved convective heat transfer of PCM. • The discharge time is reduced by bubble driven flow. • The discharging performance of LHTES unit is enhanced by bubble-driven flow. [ABSTRACT FROM AUTHOR]

In this paper, the use of bubble-driven flow on phase change material (PCM) is proposed to improve the discharge performance of a latent heat thermal energy storage system (LHTES). The upward momentum of bubbles due to its density difference can agitate liquid PCM and increase the flow velocity of liquid PCM to enhance heat transfer between the heat transfer fluid (HTF) and the PCM. To analyze the mechanism of enhanced heat transfer, visualization techniques of particle image velocimetry (PIV) and shadowgraphy were used. The result showed that the discharge time of LHTES decreased by 6%–12%. The increased flow velocity by bubble-driven flow could be observed and the expanded phase change region away from the solid/liquid interface could be identified. The average convective heat transfer coefficient is increased by maximum 1.79 times during the heat discharge period. This mechanism could accelerate the solidification process and enhance the energy discharge rate of the LHTES. Bubble-driven flow could be successfully applied to PCM to improve LHTES discharge performance. [Display omitted] • The movement and mixing of liquid PCM is increased by bubble-driven flow. • The phase change occurred over a extended region, not just at the solid/liquid interface. • Bubble-driven flow improved convective heat transfer of PCM. • The discharge time is reduced by bubble driven flow. • The discharging performance of LHTES unit is enhanced by bubble-driven flow. [ABSTRACT FROM AUTHOR]

Abstract: The control of the use of fossil fuels, major cause of greenhouse gas emissions and climate changes, in present days represents one of Governments' main challenges; particularly, a significant energy consumption is observed in buildings and might be significantly reduced through sustainable design, increased energy efficiency and use of renewable sources. At the moment, the widespread use of renewable energy in buildings is limited by its intrinsic discontinuity: consequently integration of plants with energy storage systems could represent an efficient solution to the problem. Within this frame, hydrogen has shown to be particularly fit in order to be used as an energetic carrier. In this aim, in the paper an energetic, economic and environmental analysis of two different configurations of a self-sufficient system for energy production from renewable sources in buildings is presented. In particular, in the first configuration energy production is carried out by means of photovoltaic systems, whereas in the second one a combination of photovoltaic panels and wind generators is used. In both configurations, hydrogen is used as an energy carrier, in order to store energy, and fuel cells guarantee its energetic reconversion. The analysis carried out shows that, although dimensioned as a stand-alone configuration, the system can today be realized only taking advantage from the incentivizing fares applied to grid-connected systems, that are likely to be suspended in the next future. In such case, it represents an interesting investment, with capital returns in about 15 years. As concerns economic sustainability, in fact, the analysis shows that the cost of the energy unit stored in hydrogen volumes, due to the not very high efficiency of the process, presently results greater than that of directly used one. Moreover, also the starting fund of the system proves to be very high, showing an additional cost with respect to systems lacking of energy storage equal to about 50%. From the above, it can be deduced that, in the aim to obtain a quick, effective penetration of hydrogen into the market, it is at the moment indispensable to enact incentivizing policies, attributing to hydrogen production fares able to cover the additional costs due to its production, storage and reconversion. [Copyright &y& Elsevier]

Abstract: A stand-alone power system is an independent system that comprises micro-turbines, diesel generators, a wind park, solar photovoltaic (PV) modules and/or energy storages, etc. This paper presents a new method for dealing with the short-term active power scheduling of a stand-alone system. The fuel cost of diesel units and CO2 emission are minimized and all operation constraints are satisfied. The maximum wind and solar PV powers with uncertainties are modeled using fuzzy sets. Adaptive chaos clonal evolutionary programming (ACCEP) is employed to solve this problem, which is formulated as an interactive multi-objective problem. Different degrees of fuzziness, preferred reference levels, and priority list for diesel generators are discussed. Simulation results show the applicability of the proposed method. [Copyright &y& Elsevier]

Abstract: The intermittent nature of wind energy poses questions on how electricity production can match the demand. The present paper analyses possible ways of achieving this goal when a large share of the energy is provided by wind. The study considers a certain share of wind energy as a target to be reached, and analyses different avenues for reaching such a target. The possible role of electricity energy storage, wind curtailment and transmission grid reinforcement is highlighted and discussed. The results show that above a certain percentage of electricity produced by wind, electricity storage becomes an economical option for integrating wind energy by reducing wind curtailments. Also, the synergy of storage and grid interconnections is highlighted. [Copyright &y& Elsevier]

Abstract: Stand-alone photovoltaic (PV) systems comprise one of the promising electrification solutions to cover the demand of remote consumers, especially when it is coupled with a storage solution that would both increase the productivity of power plants and reduce the areas dedicated to energy production. This paper presents a multi-objective design of weakly connected systems simultaneously minimizing the total levelized cost and the connection to the grid, while fulfilling a constraint of consumer satisfaction. For this task, a multi-objective code based on particle swarm optimization has been used to find the best combination of different energy devices. Both short and mid terms based on forecasts assumptions have been investigated. An application for the site of La Nouvelle in the French overseas island of La Réunion is proposed. It points up a strong cost advantage by using lead-acid (Pb-A) batteries in the short term and a mitigated solution for the mid term between Pb-A batteries and Gaseous hydrogen (GH2). These choices depend on the cost, the occupied area and the local pollution and, of course, legislation. [ABSTRACT FROM AUTHOR]

This paper introduces a mathematical formulation of energy storage systems into a generation capacity expansion framework to evaluate the role of energy storage in the decarbonization of distributed power systems. The modeling framework accounts for dynamic charging/discharging efficiencies and maximum cycling powers as well as cycle and calendar degradation of a Li-ion battery system. Results from a small-scale distributed power system indicate that incorporating the dynamic efficiencies and cycling powers of batteries in the generation planning problem does not significantly change the optimal generation portfolio, while adding substantial computational burden. In contrast, accounting for battery degradation leads to substantially different generation expansion outcomes, especially in deep decarbonization scenarios with larger energy storage capacities. Under the assumptions used in this study, it is found that battery energy storage is economically viable for 2020 only under strict carbon emission constraints. In contrast, given the projected technology advances and corresponding cost reductions, battery energy storage exhibits an attractive option to enable deep decarbonization in 2050. • Advanced battery models are incorporated into low-carbon distributed power system planning. • Battery cycle and calendar degradations have substantial impact on optimal system results. • Dynamic efficiencies and cycling powers of the battery do not have significant impact. • Battery is viable only under strict carbon emission constraints in 2020, leading to higher costs. • Battery energy storage is an attractive option toward deep decarbonization in 2050. [ABSTRACT FROM AUTHOR]

Energy storage systems are crucial for the massive deployment of renewable energy at a large scale. This paper presents a conceptual large-scale thermoelectrical energy storage system based on a transcritical CO 2 cycle. The concept is developed through the analysis of three high-efficiency systems: renewable energy storage using a thermoelectric energy storage system based on a reversible heat pump; a CO 2 storage system; and novel integration of energy storage using a reversible heat pump and geological injection of CO 2. The latter system efficiently integrates energy and CO 2 storage, taking advantage of the synergies between the operational requirements of both systems. The system uses CO 2 captured in stationary sources as a working fluid to store energy from renewables. The energy is stored and recovered in geological formation and heat/cold tanks, with energy storage based on sensible or latent heat of ice and water. A fraction of the CO 2 is expected to be permanently sequestered in the geological formation. The analysis of the time evolution of the system, under different operation profiles, shows the interest of the concept as a feasible integration for energy storage and CO 2 capture based on renewable energy, with an electric-to-electric efficiency varying between 40 and 50 %. • Reversible heat pump for electrothermal energy storage. • Transcritical CO 2 as working fluid and water for storage. • Inclusion of captured CO 2 and geological storage. • Round-trip efficiencies varying between 40 and 50 %. [ABSTRACT FROM AUTHOR]

This paper presents a unified framework for quantifying the techno-economic value of various technologies in systems with high levels of variable renewables, which is of utter importance in evaluating roadmaps towards an affordable and sustainable future for various energy industry stakeholders. The optimal planning framework is based on integrating the solution of a transmission and generation/storage expansion problem with the solution of unit commitment, with the objective to minimize the total capital and operational expenditures over a selected horizon. The formulation includes, but is not limited to, traditional thermal units, pumped-hydro storage, batteries and transmission technologies. The decision variables include new cross-zonal transmission capacity, bulk energy storage and generation capacity, which are computed over the selected horizon while accounting for the impact of wind and solar variability on hourly (or sub-hourly) basis. A reduced representative version of the European system is used to demonstrate the value of transmission and bulk energy storage for a selected set of future scenarios, including scenarios corresponding to high proliferation of distributed energy storage and e-mobility. Comparative results are presented with emphasis on the renewable energy shares, investment costs, average cost of electricity, and carbon emission targets. Sensitivity analysis is performed considering technology and fuel costs. • Techno-economic framework combines long-term planning and short-term operation. • Feasibility of intermediate steps is ensured with the multi-period formulation. • 100% renewable European power system can be achieved in a cost-efficient way. • Interconnections help to exploit resource and demand complementarity. • Threshold energy investment cost for a 10-year lifetime storage is ∼10 $/kWh. [ABSTRACT FROM AUTHOR]

Power system flexibility is the fundamental factor affecting renewable energy curtailment problem, and proposing a flexibility quantitative evaluation method becomes crucial for development of renewable energy. As the only renewable energy demonstration region in China, Zhangjiakou is facing serious renewable energy curtailment problems, and it is of great significance to explore a suitable flexibility promotion path. In this paper, the uncertainty model for each main component in generation-grid-load-energy storage is built, and combined with these models, the production simulation method based on improved universal generating function is introduced as the flexibility evaluation tool and flexibility metrics are proposed based on flexibility definition and physical mechanism. The effectiveness of the method is verified in IEEE RTS-79 and flexibility promotion path in Zhangjiakou is also studied. The results show that retrofit of coal-fire unit is the priority measure, then the flexible grid connection and implementation of demand response, and construction of energy storage is the final measure. • Multi-state probabilistic models of main components in power system are built. • A novel production simulation method is used for flexibility evaluation. • Flexibility metrics are proposed to analyze various flexibility promotion measures. • Flexibility promotion path for Zhangjiakou is put forward. [ABSTRACT FROM AUTHOR]

High levels of intermittent renewable sources will lead to large swings in demand for other generation resources, increasing the risk of overgeneration. Rooftop solar installations exacerbate the potential issues as well. Energy storage systems can mitigate these problems but need to be properly sized to reach network wide goals. This paper presents a method to estimate the necessary energy capacity and power for storage systems to align intermittent resources with network ramp-rate limitations. Case studies for the California Independent System Operator in 2017 and projecting to the renewable portfolio standard target of 60% in 2030 are presented. The effect of curtailment on storage requirements is also analyzed. Lastly, the impact of behind-the-meter solar installations in the Los Angeles area is estimated. These analyses show that significant amounts of storage and some curtailment will be necessary to reach the RPS targets given current network ramping limits unless other dispatchable renewable resources are deployed. This is the first step to develop grid-level planning tools by estimating storage needs with consideration of the demand, renewable generation characteristics, and curtailment levels. [ABSTRACT FROM AUTHOR]

This paper focuses on the Model Predictive Control (MPC) based energy scheduling of a smart microgrid equipped with non-controllable (i.e., with fixed power profile) and controllable (i.e., with flexible and programmable operation) electrical appliances, as well as photovoltaic (PV) panels, and a battery energy storage system (BESS). The proposed control strategy aims at a simultaneous optimal planning of the controllable loads, the shared resources (i.e., the storage system charge/discharge and renewable energy usage), and the energy exchange with the grid. The control scheme relies on an iterative finite horizon on-line optimization, implementing a mixed integer linear programming energy scheduling algorithm to maximize the self-supply with solar energy and/or minimize the daily cost of energy bought from the grid under time-varying energy pricing. At each time step, the resulting optimization problem is solved providing the optimal operations of controllable loads, the optimal amount of energy to be bought/sold from/to the grid, and the optimal charging/discharging profile for the BESS. The proposed energy scheduling approach is applied to the demand side management control of the marina of Ballen, Samsø (Denmark), where a smart microgrid is currently being implemented as a demonstrator in the Horizon2020 European research project SMILE. Simulations considering the marina electric consumption (340 boat sockets, a service building equipped with a sauna and a wastewater pumping station, and the harbour master's office equipped with a heat pump), PV production (60kWp), and the BESS (237 kWh capacity) based on a public real dataset are carried out on a one year time series with a 1 h resolution. Simulations indicate that the proposed approach allows 90% exploitation of the production of the PV plant. Furthermore, results are compared to a naïve control approach. The MPC based energy scheduling improves the self-supply by 1.6% compared to the naïve control. Optimization of the business economy using the MPC approach, instead, yields to 8.2% savings in the yearly energy cost with respect to the naïve approach. • A new framework for the smart energy management of a marina is proposed. • Controllable loads, shared resources, and demand-supply balance are considered. • A model predictive control approach for the optimal energy scheduling is detailed. • Energy cost and self reliance are analyzed extensively using a public real dataset. • The proposed approach effectively improves profit and sustainability. [ABSTRACT FROM AUTHOR]