Variable Renewable Energy Sources Research Articles

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.

Concept and Design of a Velocity Compounded Radial Four-Fold Re-Entry Turbine for Organic Rankine Cycle (ORC) Applications

The energy sector faces a pressing need for significant transformation to curb CO2 emissions. For instance, Czechia and Germany have taken steps to phase out fossil thermal power plants by 2038, opting instead for a greater reliance on variable renewable energy sources like wind and solar power. Nonetheless, thermal power plants will still have roles, too. While the conventional multistage axial turbine design has been predominant in large-scale power plants for the past century, it is unsuitable for small-scale decentralized projects due to complexity and cost. To address this, the study investigates less common turbine types, which were discarded as they demonstrated lower efficiency. One design is the Elektra turbine, characterized by its velocity compounded radial re-entry configuration. The Elektra turbine combines the advantages of volumetric expanders (the low rotational speed requirement) with the advantages of a turbine (no rubbing seals, no lubrication in the working fluid, wear is almost completely avoided). Thus, the research goal of the authors is the implementation of a 10 kW-class ORC turbine driving a cost-effective off-the-shelf 3000 rpm generator. The paper introduces the concept of the Elektra turbine in comparison to other turbines and proposes this approach for an ORC working fluid. In the second part, the 1D design and 3D–CFD optimization of the 7 kW Elektra turbine working with Hexamethyldisiloxane (MM) is performed. Finally, CFD efficiency characteristics of various versions of the Elektra are presented and critically discussed regarding the originally defined design approach. The unsteady CFD calculation of the final Elektra version showed 46% total-to-static isentropic efficiency.

Exploring wind curtailment effects and economic implications in the growing variable renewable energy penetration

Several countries continue to invest in variable renewable energy (VRE) sources to replace fossil fuels and achieve a nationally determined contribution. Consequently, as the share of VRE generation in each country's power grid increases, its curtailment also increases. Current studies on VRE curtailment are based on a comparative analysis of statistical data collected from different countries. However, only a few studies present detailed results based on historical curtailment data collected from individual wind farms. This study analyzes the curtailment data collected for three wind farms over four years. The duration, curtailed output, and cost per curtailment were determined to be 200 min, 38 MWh, and $3.02k, respectively. Finally, the curtailment loss to wind farms caused by the increasing share of VRE generation in the power grid was estimated. The curtailed output and curtailment cost are estimated to increase by 1.3 GWh and $107.6k, respectively, when the share of VRE in the power grid increases by 1%. These findings can aid in identifying the curtailment experienced by wind farm operators and subsequent potential economic losses. Moreover, they can form the basis for establishing an efficient curtailment cost compensation system.

Modelling flexibility requirements in deep decarbonisation scenarios: The role of conventional flexibility and sector coupling options in the European 2050 energy system

Russia's invasion of Ukraine has reaffirmed the importance of scaling up renewable energy to decarbonise Europe's economy while rapidly reducing its exposure to foreign fossil fuel suppliers. Therefore, the question of sources of flexibility to support a fully decarbonised European energy system is becoming even more critical in light of a renewable-dominated energy system. We developed and used a Pan-European energy system model to systematically assess and quantify sources of flexibility to meet deep decarbonisation targets. The electricity supply sector and electricity-based end-use technologies are crucial in achieving deep decarbonisation. Other low-carbon energy sources like biomethane, hydrogen, synthetic e-fuels, and bioenergy with carbon capture and storage will also play a role. To support a fully decarbonised European energy system by 2050, both temporal and spatial flexibility will be needed. Spatial flexibility, achieved through investments in national electricity networks and cross-border interconnections, is crucial to support the aggressive roll-out of variable renewable energy sources. Cross-border trade in electricity is expected to increase, and in deep decarbonisation scenarios, the electricity transmission capacity will be larger than that of natural gas. Hydrogen storage and green hydrogen production will play a key role in providing traditional inter-seasonal flexibility, and intraday flexibility will be provided by a combination of electrical energy storage, hydrogen-based storage solutions (e.g., liquid H2 and pressurised storage), and hybrid heat pumps. Hydrogen networks and storage will become more critical as we move towards the highest decarbonisation scenario. Still, the need for natural gas networks and storage will decrease substantially.

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

Dynamic modelling and simulation of the Graz Cycle for a renewable energy system

Critical atmospheric carbon dioxide levels and, in response, an increasing shift toward variable renewable energy sources causes the need of transient operation for thermal power plants coupled with carbon capture and storage. In this regard, dynamic process simulations are a valuable tool for predicting the cycling performance of power plants. In this study, the proposed Graz Cycle, an oxy-combustion power plant fired with natural gas, is investigated and modelled in (a) the steady-state design-point (i.e., full-load), (b) steady-state off-design (i.e., part-load), and (c) dynamic conditions. At full loads, the Graz Cycle power plant achieves a maximum net plant efficiency of 54.5%, comparable to the performance results of the competing Allam cycle found in relevant studies. In the off-design simulation, considering for the first time compressor component maps with variable inlet guide vanes, the load is reduced down to 50% achieving improved performance results compared to previous studies. The dynamic simulation boils down to a ramp rate analysis and shows that practical ramp rates of 5.55 %/min for load decrease and 18.18 %/min for load increase can be reached. The results show that the high ramp rates of the Graz Cycle enable balancing services to the electrical grid next to clean and dispatchable power generation. In this study, a systematic first-principle modelling approach has been developed with the final aim to evolve dynamic models for oxy-combustion carbon capture cycles, and a first-of-its-kind analysis of the dynamic operation of the Graz Cycle is presented.

Robust self‐adaptive fuzzy controller for load‐frequency control of islanded airport microgrids considering electric aircraft energy storage and demand response

AbstractThe rise of the renewable energy sources (RES) in microgrids has increased the impact of damping and low inertia on network stability. This gives rise to several issues in the power systems, including power fluctuations from variable RES, degradation of the frequency regulation, voltage surges, and oversupply from grid‐connected distributed generators. This paper presents a cascaded hybrid load‐frequency controller for an airport microgrid that addresses frequency fluctuations caused by load changes. It is combining a self‐adaptive fuzzy controller (SAF) and a classical two‐degree‐of‐freedom proportional integral derivative (PID) controller (2DOFPID) called CSAF‐2DOFPID. It is applied to the control section of a diesel generator, an electric aircraft energy storage system, and a demand response program. The controller parameters are optimized using a suggested hybrid differential artificial hummingbird algorithm (D‐AHA) to minimize frequency deviation. The proposed control framework is simulated on an airport microgrid comprising various components, and the impact of time delay on the controller's signals, system uncertainty is also examined. The simulation results demonstrate that the proposed self‐adaptive cascaded fuzzy controller outperforms other controllers such as PID, CF‐PID, and CF‐2DOFPID in reducing frequency fluctuations, and the D‐AHA algorithm achieves favourable results compared to other algorithms in the optimal tuning controller parameters.

Bantul's renewable energy potential: mapping the path to successful electricity transition

In the context of providing sustainable energy, it is necessary to expand access to reliable and affordable sources of energy supply. In other hand to restore confidence in the renewable energy sector, and ensure the sustainability of renewable energy projects in the future, it is very important to map the renewable energy potential. PLN, as the official institution appointed by the government to manage electricity in Indonesia, has not been able to meet the needs of the people of Bantul Regency for electricity as a whole. This study aims to provide some insights of identify various types and potential of renewable energy in Bantul Regency as an alternative to electricity generation. The research methodology was carried out quantitatively using primary data, obtained by field survey. Through the process of identification and gathering pertinent data, an evaluation is conducted to estimate the potential of electrical energy derived from a variety of renewable energy sources.

ТЕНДЕНЦИИ ЭНЕРГЕТИЧЕСКОЙ ОТРАСЛИ КИТАЯ 2020–2060 ГГ.

China consumes increasing amounts of energy to maintain high rates of economic growth. According to the IEA forecast, in 2030 the peak of consumption and, accordingly, emissions will be reached, then by 2060 consumption will gradually decrease due to increased energy efficiency and the country’s policy of substitution with renewable energy sources. Integration of higher shares of variable renewable energy sources (wind, solar) into China’s generating structure requires greater use of new ways to ensure energy system flexibility, such as energy storage, hydrogen storage to ensure energy security. Greater electrification of end-users facilitates the integration of renewable energy sources by increasing the flexibility to respond to changing demand, such as electric vehicle (EV) charging stations. Demand management, as well as hydrogen production through coal gasification or steam reforming, will allow China to reduce emissions in the long term.

Residential demand response program modelling to compliment grid composition and changes in energy efficiency

The path towards decarbonization in Canada is heavily reliant on electrification if the electricity grid is decarbonized. Besides the effectiveness in emission reduction, electrification offer opportunities to increase grid flexibility through demand response (DR) programs. Despite being widely seen in literature that DR programs are beneficial to the grid, there are limited, if any, residential DR programs available. As Canada has a diverse range of electricity grids, this study aims to determine how residential building DR programs impact grid operations on different archetypical electricity grids. The production cost model, SILVER, is used for this analysis. Additionally, various demand profiles are constructed to mimic changes in building stock efficiencies. It is found that grid composition plays a larger role in the effectiveness of a residential DR program than the efficiency of the building stock. Furthermore, increasing the efficiency of the building stock led to DR being more effective on grids where there are significant variable renewable energy sources. These results should motivate cities to consider residential DR programs as a tool to reducing emissions along with benefiting grid operations.

A Comparative Study of Electrode Parameters of Vanadium Redox Flow Batteries for Improved Design and Performance

Large-scale energy storage has become an urgent priority to integrate variable renewable energy sources into the electricity grid. Redox flow batteries offer attractive energy storage platforms due to the ease of integration of renewable energy, higher round-trip efficiency, location flexibility, and scalability [1]. Vanadium redox flow batteries (VRFBs) have already received widespread attention due to their recent commercialization for large-scale storage applications [2]. The electrode plays a significant role in determining the performance of the VRFB, which is composed of a porous microstructure characterized by fiber diameter, porosity, specific surface area, and permeability. These properties control various parameters that affect the performance of the VRFB system, such as pressure differences between the inlet and outlet of the electrode, overpotentials, cell potential, and crossover from the membrane. Various studies have reported varying electrode porosity's influence on cell charge-discharge cycles of VRFB systems [3–5]. However, these studies lack a consistent link between porous electrode microstructures and cell performance. Hence, the interrelated nature of the individual parameters and accurately predicting their effect on cell performance pose a challenge in predicting cell performance.In this work, we present a comparative study of porous electrode parameters using a 2-dimensional physics-based model of a VRFB incorporating mass, momentum, charge transport, and kinetics. We have established a consistent relationship between electrode parameters and cell performance which is incorporated into the model to verify the dependency of overpotential, pressure difference, and crossover of ions on porosity and fiber diameter by utilizing a correlation for the surface area and permeability as a function of these parameters in the porous electrode. Pressure difference at the inlet and out of the electrode is one of the driving forces for the crossover of ions across one electrode to another, decreasing with the higher value of fibre diameters during the charging of the cell (as shown in Figure 1(a)). The effect of the operating parameter, flow rate, is negligible on the ion crossover. However, the crossover is impacted by the current, which further increases by increasing the current value, as shown in Figure 1(b). We also found that the system efficiency increases with an increase in the fibre diameter and reaches a maximum at the range of 40-70 µm, but voltage efficiency decreases with the higher value of fibre diameter, as shown in Figure 1(c). This study/analysis/work will include the impact on the crossover of vanadium ions by varying the electrode and operating parameters. The system's efficiency with and without considering the pumping energy will also be presented for various parameters. Our key findings provide important insight for optimizing VRFB design. Developing a 2D model that considers multiple key parameters enables a better understanding of the complex interactions between electrode microstructure and system performance.

Climate variability on Fit for 55 European power systems.

The use of variable renewable energy sources to generate electricity introduces a dependency on meteorological factors into power systems. With the renewables share growing globally, often driven by political pressures, the reliability and efficiency of power systems are increasingly affected by this dependency. In this paper, we investigate the impact of the natural variability of meteorological parameters on the European power system in 2030. We specifically focus on (1) analysing the main European weather patterns affecting renewable energy production and (2) understanding the co-variability of this production among European countries. The identification of a set of patterns in the behaviour of key power system operation indicators allows us to analyse the relationship between large-scale weather regimes and daily power system operations in a 2030 European energy context. Regarding renewable generation, analysis of the co-variability shows that European power systems tend to form two clusters, in each of which all the regions tend to show a positive correlation among themselves and a negative correlation with the other cluster. Our analysis of the most important large-scale weather regimes shows that during cyclonic patterns, the carbon intensity of all the European power systems is lower than normal, while the opposite happens during blocking regimes.

An Open-Source Energy Arbitrage Model Involving Price Bands for Risk Hedging with Imperfect Price Signals

The increased uptake of variable renewable energy sources has increased electricity price volatility in many energy pool markets, providing an opportunity for storage systems to profit through energy arbitrage. Comparison between the cost or value of storage systems engaging in energy arbitrage should be performed on a levelised basis due to differences in system lifetime. Existing energy arbitrage models with bid/offer curves and imperfect forecasting are typically computationally expensive and are impractical for calculating lifetime levelised cost metrics. In this work, an open-source modular energy arbitrage model with bid and offer curve inputs was developed for a lithium-ion battery energy storage system (BESS) and pumped hydro system (PHS) to analyse lifetime levelised cost and revenue. The mixed integer linear program scheduling module included a new piece-wise linearised description of PHS charging behaviour for rapid optimisation. A one-at-a-time sensitivity analysis indicated that levelised cost and revenue were highly sensitive to discharging efficiency. In a case study based on Australia’s National Electricity Market, imperfect forecasting with no risk hedging was found to increase levelised costs by up to 24% and decrease levelised revenue by up to 50% relative to perfect price forecasting, despite 95% of prices being forecast to be within $35/MWh of the actual trading price. BESS levelised costs were more significantly correlated with consistent low risk bids (Kendall Tau-b of 0.75), since the undiscounted capital costs contribute to a larger proportion of the overall costs than in the PHS systems.

Enhanced demand side management for solar‐based isolated microgrid system: Load prioritisation and energy optimisation

AbstractA novel control mechanism is presented for rural microgrids, standing out in the current literature with its advanced approach to load prioritisation and energy allocation. The system's main goal is to maximise energy supply to essential loads while effectively managing available resources. Distinct from traditional methods, this mechanism dynamically classifies loads according to user‐defined priorities, adjustable based on the control system's computational power and complexity. A critical feature is the utilisation of the Particle Swarm Optimisation (PSO) algorithm to optimise demand side management (DSM). This innovative approach leverages day‐ahead load and generation forecasts to ensure optimal energy distribution across load levels, maintaining continuous power supply to high‐priority loads and reducing blackout risks due to generation and load fluctuations. Analyses under stochastic scenarios demonstrate the robustness of the control action, with percentile‐based day‐ahead forecasting allowing for adaptation to significant variations in renewable energy generation patterns. The implementation results are significant, maintaining 100% supply continuity to essential loads throughout the day, even with generation fluctuations up to ‐20%. This marks a considerable improvement in load satisfaction, increasing it from 83% to 96%. A significant advancement in microgrid control is contributed, providing an adaptive, user‐centric approach that enhances load management and energy distribution, and facilitates more resilient and efficient microgrid systems in the face of highly variable renewable energy sources (RESs).

Investigation of power quality issues in 14-bus electrical network with high penetration of renewable generation

Power quality issues arise in electrical networks when variable renewable energy (VRE) is integrated into them due to their random and intermittent nature which depends on weather conditions and other factors. The variation of solar irradiance throughout the day affects the energy produced by solar panels and the integration of solar power into electrical networks will result in changes and fluctuations in the voltage profile of buses. Reactive power compensation is required to improve the bus voltage levels of the electrical network to be within the required limits and the optimal allocation of reactive power compensation devices in the network is a complex problem to be investigated for the optimum injection of reactive power to obtain better voltage profiles for the entire network. This research investigated the penetration of variable solar energy into an electrical network in terms of voltage and reactive power flow. A variety of literature was reviewed in the scope of reactive power management in power systems and a gap in addressing the optimal allocation of compensation devices in the IEEE-14 bus was addressed based on the proposed methods followed by discussions of the results in terms of voltage profiles and reactive power flow in the buses. The objective is to produce an output power of higher quality and reliability for the loads so that intermittent sources of renewable energy can be more competent with energy sources such as fossil fuels that do not depend on weather conditions. Integration of methods using compensation optimisation (optimal allocation of capacitors) and volt-var regulation (smart inverter) to improve the voltage profile that was dropped and the fluctuations after penetration of solar power were carried out. A solar bus with variable energy generation was connected to the IEEE-14 bus to study the voltage variations. This was executed by the power flow calculation module to determine the voltages and reactive power in the buses of the network. With the optimum allocation of the capacitors, the voltage levels in all weak buses of the IEEE-14 bus were increased to be between 0.95 p.u. and 1.05 p.u. which was the voltage specifications of the Malaysian Grid Code Requirements. The voltage for every weak bus in the IEEE-14 bus showed a rise of 5.7% from 7 a.m. to 12 p.m. With that, the volt-var function was used for reactive power regulation at the point of common coupling (PCC) and a reduction of voltage deviation of 2.828 to 1.3% in the IEEE-14 bus was observed. The average voltage profile of all buses managed to attain a value of 98.99% from 95.673% (with solar power) with the optimal allocation of capacitors and volt-var regulation. The beneficiaries of this project will be the Sustainable Energy Development Authority (SEDA) which administers the Net Energy Metering (NEM) scheme and Tenaga Nasional Berhad (TNB) which is the Malaysian multinational electrical company focused primarily on the generation, transmission, and distribution of electricity in Peninsular Malaysia. The Energy Commission and Ministry of Energy and Natural Resources are also beneficiaries as they carried out a competitive bidding programme for large-scale solar (LSS) known as the LSS@MEnTARI or LSSPV4 to attain bids for the development of around 1000 MW AC of LSS power plants to be operational in Malaysia by 2022. This work will also be beneficial in future research in planning reactive power compensation devices in networks of multiple VRE sources, communication, and coordinated control of smart inverters, and incorporation of these devices for smart grid applications.

Optimal design of PV-based grid-connected hydrogen production systems

A cost-optimal design of power-to-hydrogen (PtH) systems is crucial to produce hydrogen at the lowest specific cost. New challenges arise when it comes to ensuring a reliable and cost-effective hydrogen supply in the presence of variable renewable energy sources. In this context, the aim of this analysis is to investigate the optimal design of PV-based grid-connected hydrogen production systems under different scenarios. To this end, an optimisation framework based on the mixed integer linear programming (MILP) technique is developed. Results are presented by employing a set of techno-economic and environmental indicators to provide general guidance on how to optimally size PtH systems, going beyond the analysis of a specific case study. The analysis is applied to Italy and particular attention is paid to exploring the impact of the price of grid electricity.The results indicate that the price of grid electricity strongly affects the optimal design of PtH systems. Specifically, in scenarios with high electricity prices, it is economically convenient to significantly oversize the PV plant and the electrolyser. The optimal PV ratio, representing the ratio between the PV size and the electrolyser size, increases from 1.6 to 2.7 as the electricity price rises from 50 to 300 €/MWh. Additionally, when electricity prices exceed approximately 120 €/MWh, the optimal electrolyser size (in terms of hydrogen production under rated conditions) becomes almost three times larger than the average hydrogen demand. By comparing grid-connected and off-grid scenarios, the importance of the electrical grid is also highlighted: even when poorly used, it plays a crucial role in limiting the size of the hydrogen storage. The levelised cost of hydrogen for the optimal PtH configuration falls within the range of 3.5–7 €/kg (depending on the price of grid electricity) and increases to 8.2 €/kg when the system operates off-grid. Finally, the hydrogen carbon footprint, quantified as kgCO2,e/kgH2, is also explored. Considering the current price and carbon intensity of grid electricity, the cost-optimal PtH configuration already involves the production of renewable hydrogen (<3 kgCO2,e/kgH2).

Assessment of the carbon emissions reduction potential in Thailand’s power sector with high penetration of variable renewable energy sources and electric vehicles in the year 2030

Abstract The power sector has substantial carbon emissions reduction potential that could achieve the new nationally determined contribution target in 2030 by increasing low-carbon technologies, e.g. variable renewable energy sources and electric vehicles (EVs). Therefore, two approaches were suggested in this work. In the first approach, Thailand’s power sector was modelled by using PLEXOS software to find the impact of the high penetration of variable renewable energy (RE) and EV charging load for the projected year 2030. The second approach proposed a demand–response and energy storage system solution with carbon pricing in the model to assess the targets. As a result, the electricity demand from charging EVs will cause a new peak demand at night, while the high variable RE penetration will cause curtailment in the power system due to excess supply at noon. Therefore, Thailand’s power sector has the potential for carbon emissions reduction by 45% with clean energy technologies, which could increase to 68% with carbon pricing, easily achievable by the nationally determined contribution target in 2030. However, these benefits could only be derived if various entities involved in the energy regulatory, transport and power sectors coordinate to implement the required technological and financial policies.