Variable Renewable Energy Sources Research Articles

Development of a maintenance scheduling method for power generators considering large penetration of variable renewable energy sources and system inertia

Development of a maintenance scheduling method for power generators considering large penetration of variable renewable energy sources and system inertia

River systems under peaked stress

Abstract The change in the global energy production mix towards variable renewable energy (VRE) sources requires efficient utilization of regulated rivers to optimise hydropower operations meet the needs of changing energy market. However, the flexible operation of hydropower plants causes non-natural, sub-daily fluctuating flows in the receiving water bodies, often referred to as “hydropeaking”. Drastic changes in sub-daily flow regimes undermine attempts to improve river system health. Environmental decision makers, including permitting authorities and river basin managers facing the intense and increasing pressure on river environments, should consider ecosystem services and biodiversity issues more thoroughly. The need for research innovations in hydropeaking operation design to fulfill both the water and energy security responsibilities of hydropower is highlighted. Our paper outlines optimized hydropeaking design as a future research direction to help researchers, managers, and decision-makers prioritize actions that could enable better integration of river science and energy system planning. The goal of this is to find a balanced hydropower operation strategy.

Monitoring geomechanical pumped storage in horizontal fluid-filled lenses with surface tiltmeters

Geomechanical Pumped Storage is a long duration energy storage technology that assists with integrating variable renewable energy sources into the electricity grid. The concept involves cycling fluid in artificial subsurface lenses. These lenses act as high-pressure reservoirs, storing and releasing pressurised fluid to drive a turbine and generate power. Similar in nature to traditional pumped storage hydropower, but lower cost, modular, and without the specific topography requirements. This work describes the surface monitoring and analysis from several lenses manufactured in the Eagle Ford Shale at Quidnet Energy’s Castilleja pilot storage site in Texas. Up to 12 high-precision tiltmeters were used to verify dip and orientation of the lens as well as provide a predictive map of the lens geometry and symmetry or asymmetry of the lens growth. The surface tiltmeters used as a diagnostic tool for lens creation have been proven to be useful. It provides operators with confidence and feedback on fundamental lens behaviour and response. These improvements will help scale the technology to commercially viable modular energy storage systems.

Merit-order of dispatchable and variable renewable energy sources in Turkey's day-ahead electricity market

This study employs linear and non-linear quantile regression models to assess the merit-order effect (MOE) of integrating renewable energy sources (RESs) into the Turkish day-ahead electricity market. The analysis spans nine different quantiles from January 2019 to December 2022. Our results show that both dispatchable and variable renewable energy sources (VREs) are essential for reducing market clearing prices (MCPs) in the Turkish day-ahead market. However, they have different impacts on quantiles of MCPs and price variability. VREs enhances the price variability across all hours in linear and non-linear models, while dispatchable renewable energy reduces price variability except at night.

Optimal sizing and placement of battery energy storage system for maximum variable renewable energy penetration considering demand response flexibility: A case in Lombok power system, Indonesia

Indonesia is a tropical climate country with considerable renewable electrical energy source prospects, including photovoltaic (PV) and wind energies. Nevertheless, several variable renewable energy sources (VREs) have exhibited uncertain attributes and substantial reliance on natural conditions, leading to unstable load-related power supply risks. Hence, integrating battery energy storage systems (BESSs) with VRE generators is a dependable approach to bolster renewable energy generator applications on a large-scale grid while providing load demand flexibility. This study determined adequate sizing and placement of the BESS to achieve maximum VRE generator penetration while considering the demand response flexibility. Key indicators, including technical minimum load and system ramp capacity, were identified to achieve maximum penetration of thermal generators. This study also combined a unit commitment procedure with direct current optimal power flow (DC-OPF) as a novel approach to determine the maximum VRE penetration level. An optimization problem model concerning mixed integer linear programming (MILP) was subsequently employed in this study using the CPLEX solver in the general algebraic modeling system (GAMS). The study is based on the IEEE RTS-24 system modified and a real-life case study of the Lombok energy system in Indonesia. Results from the simulated Lombok power system highlighted that optimal sizing and placement of the BESS could lower system costs by 37.66%, 33.63%, and 22.26% compared to the current system conditions during the weekday, weekend, and the lowest day scenarios, respectively. The VRE penetrations of the generators were also higher than the current system conditions by 83%, 51%, and 39% during the weekday, weekend, and the lowest day scenarios, respectively.

A multi-criteria decision-making model to determine the optimal share of variable renewable energy sources in a wholesale electricity market

In recent years, the development of renewable energy technologies has gotten increasing attention due to the realization of cleaner production and high energy efficiency. Therefore, the proportion of these resources in electricity production has experienced a dramatic increase. However, with the features of intermittent nature and merit order effects, it is challenging to effectively optimize the integration of renewable energy sources (RES) into the electricity markets. To deal with the complex problem of selecting the proportionate share of non-dispatchable RES for participation in the market, this paper establishes a multi-criteria decision-making (MCDM) framework, which combines the Analytic Hierarchy Process (AHP) and Simple Additive Weighting (SAW). Five criteria are defined and the corresponding mathematical model is solved by the proposed AHP-SAW method. In this way, the optimal share of renewable resources for participation in the wholesale market can be determined based on the desired criteria. Also, to compare the results obtained by the AHP-SAW method, the criteria weights are determined by Shannon entropy, and the problem is solved by the TOPSIS method. To assert the effectiveness of the proposed framework, two case studies are done. According to the results, by increasing the RES penetration level, the average price of the market decreases mildly while price volatility increases significantly . Also, the amount of forward contracts is decreased when the RES share increases. Moreover, the results show that a high amount of RES is not necessarily desirable and depends on the type of renewable resources and policymakers' priorities. The results for systems 1 and 2 indicate that the maximum allowable amount of RES are 85.359 and 38.24 MW, respectively.

Role of smart charging of electric vehicles and vehicle-to-grid in integrated renewables-based energy systems on country level

The energy transition based on variable renewable energy sources raises concerns on satisfying the electricity demand at all hours of a year, which is amplified by the ongoing energy system electrification. Balancing variable electricity supply and inelastic demand requires additional sources of flexibility in the system: flexible electricity generation, energy storage, grids, flexible demand, and overall sector coupling. Electrification of transport increases the electricity demand; however, it offers the opportunity to use additional low-cost flexibility from electric vehicles batteries. The LUT Energy System Transition Model was modified to model the smart charging and vehicle-to-grid functionality of electric vehicles in integrated energy systems. The results show that, in countries with a large fleet of electric vehicles, smart charging and vehicle-to-grid allow for a substantial reduction of energy storage requirements, reducing the electricity and heat storage capacity by 35% and 25%, respectively and leading to 4% lower system cost. Flexible operation of electrolysers required for e-fuels synthesis have a similar effect and, in combination with smart charging and vehicle-to-grid, can lead to 8% lower system cost. The impact of vehicle-to-grid naturally depends on assigned operational cost; however, it contributes to system cost reduction even at high costs of 50 or 100 €/MWh.

Development of A Responsible Investment for Low Carbon Economy: Renewable Energy Responsible Investment Index and Conceptual Framework for MENA Region

Responding to climate change creates particular challenges for Middle East and North Africa (MENA) countries in terms of oil output and water scarcity, but also gives potential for new paths to sustainability. The low-carbon energy transition poses critical concerns for MENA oil and gas producers, particularly about the impact on their reliance on hydrocarbon earnings. Without adjustment, this would exacerbate macroeconomic imbalances and possibly jeopardise pre-existing social contracts. Politicians are attempting to promote a variety of renewable energy sources and implementation tactics in order to mitigate the adverse consequences of climate change. As a result, the development of an index capable of effectively measuring and benchmarking the overall sustainability and economics of a country becomes necessary. In reality, the index should be capable of reflecting a near-complete assessment of renewable energy's current and future conditions. Simultaneously, it should be capable of revealing critical ESG and economic aspects of the renewable energy business. The proposal aims to create a composite index, dubbed the Renewable Energy Responsible Investment Index (RERII). It is comprised of 19 nations and 22 distinct metrics. From 2010 to 2019, a trend in RERII is computed. The RERII suggested in this work may be utilised as a screening tool, a decision-making tool, a benchmarking tool, and a tool for guiding sustainable development.

Event-Driven Day-Ahead and Intra-Day Optimal Dispatch Strategy for Sustainable Operation of Power Systems Considering Major Weather Events

As the proportion of renewable energy installations in modern power systems increases, major weather events can easily trigger significant fluctuations in new energy generation and electricity load, presenting the system with the dual challenges of ensuring power supply and renewable energy consumption. Traditional dispatch models need more coordination and optimization of flexible resources under major weather events and risk management of system operations. This study focuses on provincial-level transmission systems, aiming to achieve the coordinated and optimized dispatch of flexible resources across multiple time scales in response to the complex and variable environments faced by the system. Firstly, by profoundly analyzing the response mechanisms of power systems during major weather events, this study innovatively proposes an event-driven day-ahead and intra-day optimal dispatch strategy for power systems. This strategy can sense and respond to major weather events in the day-ahead phase and adjust dispatch decisions in real time during the intra-day phase, thereby comprehensively enhancing the adaptability of power systems to sudden weather changes. Secondly, by considering the variability of renewable energy sources and electricity demand in the day-ahead and intra-day dispatch plans, the strategy ensures efficient and reliable power system operation under normal and major weather event scenarios. Finally, the method’s effectiveness is validated using actual data from a provincial-level power grid in China. The proposed dispatch strategy enhances the resilience and adaptability of power systems to major weather events, which are becoming increasingly frequent and severe due to climate change. The research demonstrates that an event-driven day-ahead and intra-day optimal dispatch strategy can enhance the economic efficiency and robustness of power system operations through the coordinated dispatch of flexible resources during major weather events, thereby supporting the transition toward sustainable energy systems that are resilient against the challenges of a changing climate.

Dynamic impact of hybrid wind-solar photovoltaic power injection on small signal stability of Nigerian 11kV power system using Self Organizing Map neural network

Power industry reorganization, legislation on greenhouse gas emissions and inadequate conventional generation capacity encourage the idea of incorporating Distributed Generation (DG) into the distribution networks as unconventional sources of power generation. High penetration of DG has further increased the complexity of the analysis of small signal stability because the variability of renewable energy sources affects the stability of power system. Therefore, this research work investigates the effect of wind and solar photovoltaic power injection on small signal stability of Nigerian 11 kV power network. Wind and solar photovoltaic were modelled using probabilistic methods and output power at twenty-one different locations within Nigeria was computed. Using the real and reactive power data of the network, Self-Organizing Map (SOM) neural network was trained and tested for the assessment of the small signal stability of the distribution network before and after the injection of renewable power in python virtual environment. The integration of hybrid wind-solar PV distributed generation at 30 %, 60 %, 90 % and 100 % into the studied 11 kV power distribution network improved its small signal stability by 1.4 %, 2.1 %, 2.5 % and 3.2 % respectively. Moreover, increase in the injection of power gotten from wind and solar sources reduces the oscillation frequency of the system from 1.39 Hz to 1.01 Hz and increases the damping from 4.6 % to 13.2 %. Reactive power increment in the range of 5 %-20 % improved the small signal stability of the system by the range 2.4 %-7 %. This improvement will translate to better power transfer capability of the network.

Renewable fuel production and the impact of hydrogen infrastructure — A case study of the Nordics

Hard-to-electrify sectors will require renewable fuels to facilitate the green transition in the future. Therefore, it is crucial to identify promising production locations while taking into account the local biomass resources, variable renewable energy sources, and the synergies between sectors. In this study, investments and dispatch operations are optimised of a large catalogue of renewable fuel production technologies in the open-source software SpineOpt, and this is soft-linked to the comprehensive energy system model Balmorel. We analyse future production pathways by comparing various levels of hydrogen infrastructure, including large-scale hydrogen storage, and assess system impacts. The results indicate that methanol may provide synergies in its multipurpose use as an early (2030–2040) shipping fuel and later as an aviation fuel through further refining if ammonia becomes more competitive (2050). We furthermore show that a hydrogen infrastructure increases the competitiveness of non-flexible, hydrogen-based fuel production technologies. Offshore electrolysis hubs decrease energy system impacts in scenarios with 105 TWh of Nordic hydrogen export. However, hydrogen export scenarios are much costlier compared to scenarios with no export unless a high hydrogen price is received. Finally, we find that emission taxes in the range of 250–265 €/tCO2 will be necessary for renewable fuels to become competitive.

Insights from a Comprehensive Capacity Expansion Planning Modeling on the Operation and Value of Hydropower Plants under High Renewable Penetrations

This paper presents a quantitative assessment of the value of hydroelectric power plants (HPPs) in power systems with a significant penetration of variable renewable energy sources (VRESs). Through a capacity expansion planning (CEP) model that incorporates a detailed representation of HPP operating principles, the study investigates the construction and application of HPP rule curves essential for seasonal operation. A comparative analysis is also conducted between the proposed rule curve formulation and alternative modeling techniques from the literature. The CEP model optimizes installed capacities per technology to achieve predefined VRES penetration targets, considering hourly granularity and separate rule curves for each HPP. A case study involving twelve reservoir hydropower stations and two open-loop pumped hydro stations is examined, accounting for standalone plants and cascaded hydro systems across six river basins. The study evaluates the additional generation and storage required to replace the hydropower fleet under high VRES penetration levels, assessing the resulting increases in total system cost emanating from introducing such new investments. Furthermore, the study approximates the storage capabilities of HPPs and investigates the impact of simplified HPP modeling on system operation and investment decisions. Overall, the findings underscore the importance of reevaluating hydro rule curves for future high VRES penetration conditions and highlight the significance of HPPs in the energy transition towards carbon neutrality.

Intelligent Energy Management System: Harnessing Fuzzy Logic for Charge Control

Aim: Charge controller that leverages fuzzy logic was developed to enhance the efficiency of traditional charge controllers. A model was constructed and assessed for its performance through MATLAB/Simulink. It allows for flexibility in controlling the variability of renewable energy sources. It also improves the efficiency and lifespan of energy storage systems while minimizing the impact on the grid and environment. 
 Study of the design: The PV system consists of a PV module, PWM inverter, MPPT controller and DC-DC converter which are connected using MATLAB/Simulink environment.
 Methodology: we conducted validation tests to substantiate the advantages of our fuzzy charge controller. The creation of fuzzy rules was based on the system's performance and subsequently translated into precise values with the assistance of a fuzzy inference system. This Research Project was Completed in Two Months.
 Results: Our findings clearly demonstrate that the implementation of fuzzy logic control results in superior charge controller performance. This, in turn, safeguards against battery discharging and overcharging during unpredictable weather conditions.
 Conclusion: These protective measures are made possible through the decision-making capabilities of the DC-DC buck-boost converter, which effectively regulates both the voltage and current output of the PV module.

Towards Hydrogen Sector Investments for Achieving Sustainable Electricity Generation.

Hydrogen constitutes an integral component within an expansive array of energy technologies poised to facilitate the nation's transition towards achieving a net-zero state. In additional, this endeavor involves harnessing regional resources judiciously, thereby fostering equitable and sustainable growth. The strategic development and utilization of hydrogen technologies necessitate a nuanced approach, encompassing an assessment of diverse technologies spanning various sectors especially power sector. Such a meticulous strategy aims to forge the most efficacious, cost-effective, and sustainable pathways, underpinned by the discerning adoption of these technologies in the market. The article delves into the intricate relationship between hydrogen and fuel cell technologies, shedding light on their combined impact on the evolving landscape of electricity generation. A particular focus is placed on the integration of variable renewable energy sources, elucidating how hydrogen serves as a key enabler in optimizing the utilization of these fluctuating energy resources. In addition, the article encompasses various methods of hydrogen production, exploring their technological advancements and implications for achieving sustainable electricity generation. Emphasizing the significance of technology development in the hydrogen sector, the paper delves into the potential of hydrogen production methods and their implications for advancing sustainable electricity generation. In essence, the article navigates the trajectory of the hydrogen sector's evolution within the broader context of electricity generation, offering valuable insights into the ongoing developments, challenges, and opportunities. By addressing the critical nexus between hydrogen technologies and the dynamic electricity landscape, the paper aims to contribute to the discourse on the future trajectory of investments in the hydrogen sector for enhanced electricity generation. To Conclude, the United Kingdom has committed GBP 20 billion over a span of 20 years to the development of Carbon Capture, Utilization, and Storage (CCUS) facilities. Additionally, the nation has identified and shortlisted electrolysis projects totalling 408 megawatts (MW) capacity. In Korea, Hanwha Impact has achieved a significant milestone by attaining a 60% hydrogen co-firing share in an 80 MW gas turbine, representing the largest co-firing share recorded thus far in mid-to-large gas turbines. Meanwhile, Anhui Province Energy Group in China has successfully conducted trials involving the co-firing of ammonia at a 300 MW unit. The Group has plans to further extend these trials, aiming to achieve a 50% co-firing level at a 1 GW coal unit. In the United States, notable progress has been made, with a 38% hydrogen co-firing share attained in 2023 at an operational 753 MW combined-cycle power plant.

Solar-hydro cable pooling – Utilizing the untapped potential of existing grid infrastructure

Over the years, the coupling of various variable renewable energy sources has emerged as a convenient way to partially or fully overcome their highly intermittent nature and low-capacity factors. In literature as well as in practical implementations, the coupling of solar and wind power systems is gaining special attention thanks to their advantageous complementarity in time. However, an area of yet not well-documented potential is the coupling of solar PV plants with non-dispatchable run-of-river hydropower plants. In this study, Poland, which still has significant untapped hydropower potential, is used as a case study. The objective of the work is to quantify the coupling potential with special attention paid to the unavoidable curtailment of solar generation due to the limited grid connection capacity. A simulation model was developed that uses input runoff, irradiation, and temperature hourly data covering the period 01.01.2008–31.12.2022 for 236 selected sites in Poland. Hydrological data were obtained from field measurements, while the remaining meteorological parameters were downloaded from the ERA5 reanalysis database. Hydropower generation was simulated assuming a constant head and a Kaplan turbine operating at different efficiencies depending on the flow rate through the turbine. The available flow was calculated considering environmental flow (e-flows) constraints. The design flow was determined based on the average multi-year flow in the considered river. The complementarity in time of these two renewable energy sources in each location was assessed by considering the grid connection capacity and resulting curtailment in case of too high simultaneous generation from both sources. The analysis showed that the average PV curtailment is about 10.7 % (or 13.2 % when it is solar PV capacity weighted) when the ratio of solar to hydro is equal. Still, it can be as high as 19.7 % in individual cases. To ensure that PV curtailment is no more than arbitrarily (in that case) selected threshold of 5 %, the solar-PV capacity should be between 58 % − 120 % of the designed hydropower plant capacity. The curtailment threshold can be further investigated as an expansion of a techno-economic analysis that fell out of the scope of this work. The additional analysis conducted by means of daily flow data showed a significant underestimation of solar curtailment in individual cases (i.e., up to 7 pp.) and, on average, lower by 0.24 pp. could be expected. Due to interannual flow variations, although the expected mean solar curtailment can be no more than 5 %, there are individual years when it is as low as 2 % and as high as 12 % of this year’s solar generation. The lessons learned from this study shed light on the potential maximization of local power generation through coupling with PV systems and the simultaneous reduction of environmental impacts by releasing higher quantities of e-flows offset by the integration of PV systems.

Improving vertical solar still performance for efficient Desalination: Investigating the influence of Wick, condensate plate and device dimensions

Access to sustainable energy sources and clean water remain two of the most significant challenges in the modern world. Fortunately, desalination can be achieved using a variety of renewable energy sources, with solar energy being a notable option. There has been a noticeable increase in interest in the vertical solar still (VSS) as a popular desalination technique in recent years. The performance of VSS is examined in this article in relation to a several variables, including the wick, the wettability of the condensate plate surface, and device dimensions. According to the study's findings, selecting an appropriate wick is essential for ensuring optimum water distribution on the heat absorber surface, thereby enhancing the effectiveness of the Vertical Solar Still (VSS). In comparison to gauze, cotton emerges as a superior wick, contributing to a noteworthy 5.1 % improvement in device performance. The performance of the VSS is significantly influenced by the contact angle of the condensate surface plate, a glass plate (contact angle of 5) yields 34 % more freshwater than a superhydrophobic plate (contact angle of 140). This study also shows that the dimensions of the vertical solar still (VSS) are significantly impactby the properties of the condensate plate and wick. Specifically, the most efficient VSS performance is attained when the dimensions are set at 32 cm × 30 cm.

Effects of Battery Energy Storage Systems on the Frequency Stability of Weak Grids with a High-Share of Grid-Connected Converters

To achieve an energy sector independent from fossil fuels, a significant increase in the penetration of variable renewable energy sources, such as solar and wind power, is imperative. However, these sources lack the inertia provided by conventional thermo-electric power stations, which is essential for maintaining grid frequency stability. In this study, a grid resembling Madeira Island’s power generation mix was modeled using the Matlab/Simulink platform. The model included solar, wind, hydro, and thermo-electric generation to accurately represent the energy landscape of Madeira Island. Three scenarios were examined: one reflecting the current power generation on Madeira Island, a future scenario with a substantial rise in the percentage of photovoltaic (PV) generation, and the same future scenario but incorporating a battery energy storage system (BESS). Various analyses were conducted to assess the impact on frequency stability during a ground fault and rapid load/generation changes. In the future scenario without a BESS, the thermoelectric power plant generator desynchronized, leading to system collapse in several simulations. However, with the addition of a BESS, a significant improvement in frequency stability was observed. The thermoelectric power plant generator could return to a steady state after each disturbance. Furthermore, both the maximum frequency deviation and the absolute value of the Rate of Change of Frequency (ROCOF) were reduced, indicating enhanced system performance and stability.

Continuous power imbalance assessment from multi-area economic dispatch models

To be able to efficiently maintain a continuous balance between supply and demand in power systems with high shares of variable renewable energy (VRE) sources, a variety of studies related to the topic are needed. A fundamental input parameter for such studies is an assessment of the power system’s physical needs for balancing power, in form of power imbalances. This article presents a new model for simulating physical power imbalances with a 1-minute time resolution based on multi-area economic dispatch simulations. Compared to existing models with the same purpose, the new model includes the combination of simulating power imbalances with 1-minute time resolution, simulating forecast uncertainty, simulating the continuous behaviour of all power system components and simulating the transmission for netting of power imbalances between balancing areas. By applying the model to a case study of the Nordic synchronous power system in year 2045, the impact of including these features in the model is highlighted. Case study results also show that the size and pattern of power imbalances much depends on the characteristics of a balancing area, in terms of electricity demand, available generation technologies and interconnections to other balancing areas.

Method to Model the Hourly Variability of Renewable Energy Sources in Integrated Assessment Models

Method to Model the Hourly Variability of Renewable Energy Sources in Integrated Assessment Models

Rotor–stator interaction investigations in variable speed reversible pump-turbine at higher head

Efficiency and grid stability can be improved by variable speed operation using doubly fed induction machine technology for pumped storage plants experiencing significant head variations. With the higher penetration of intermittent and variable renewable energy sources, viz., solar and wind, the grid may be stabilized by operating the reversible pump-turbines (RPTs) in off-design conditions. In a turbine mode, the RPT is more susceptible to fatigue and vibrations when operating at a higher head as a result of hydraulic instability generated by rotor–stator interaction (RSI); therefore, its performance becomes even more critical. The powerhouse structural components, including floors and columns, could experience intense vibrations because of this instability. Therefore, it is essential to investigate the RSI in the variable speed RPTs at the higher head. These investigations present the results of a numerical analysis of RSI and its associated pressure fluctuations in the variable speed RPTs at the higher head. The high-head scaled model of variable speed RPT was used, and the numerical simulations were executed by utilizing the shear stress transport k-ω turbulence model. The numerical analysis was performed at the best efficiency point and high-head operating conditions having optimized rotational speed. The results show that the main source of pressure fluctuations in the variable speed RPT at all operating conditions is RSI, where the dominant frequencies are blade passing frequency (9fn) and its harmonics. It is also found that the variable speed operation lowers the pressure fluctuations in the RPT.