Intermittent Renewable Energy Resources Research Articles

Short‐term wind power prediction in microgrids using a hybrid approach integrating genetic algorithm, particle swarm optimization, and adaptive neuro‐fuzzy inference systems

This paper proposes an integrated hybrid approach combining genetic algorithm (GA), particle swarm optimization (PSO), and adaptive neuro‐fuzzy inference systems (ANFIS) for short‐term wind power generation prediction in microgrids. The increasing penetration of intermittent and uncertain renewable energy resources like wind into electric power systems poses important operational challenges. Accurate prediction of wind power generation is crucial to address the challenges and fully harness the available generation capability of wind energy generation systems. Accurate wind power generation forecasting tools play a critical role in enabling system operators to plan efficient operation of power systems and ensure reliability of supply. In this paper, a combination of GA and PSO is used to optimize an ANFIS model for short‐term wind power prediction. To demonstrate the effectiveness of the proposed method, it is tested based on practical information of weather conditions and wind power generation data of a case study microgrid system in Beijing. The performance of the proposed approach is compared with four other prediction methods. The proposed approach outperformed all the other methods, demonstrating its favorable accuracy and reliability. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Multi‐criteria optimal sizing of hybrid renewable energy systems including wind, photovoltaic, battery, and hydrogen storage with ɛ‐constraint method

Hybrid renewable energy systems (HRES) should be designed appropriately with an adequate combination of different renewable sources and various energy storage methods to overcome the problem of intermittency of renewable energy resources. A multi-criteria approach is proposed in this study to design an HRES including wind turbine, photovoltaic panels, fuel cell, electrolyser, hydrogen tank, and battery storage unit with an intermittent load. Three design criteria including loss of power supply probability, total energy loss (TEL), and the power difference between generation and storing capacity (as TELSUB) are taken into account in minimising the total cost of the system considering the interest rate and lifetime. The justifications and advantages of using these criteria are thoroughly discussed along with appropriate presentation of the results. The purpose of considering TEL and TELSUB is discussed thoroughly. The e-constraint method is used to handle practical constraints of the proposed multi-criteria problem to construct a multi-objective fitness function. Shuffled frog leaping algorithm is implemented to achieve better optimal results. The proposed approach is implemented using real wind speed and solar irradiance data for a specific location with an intermittent load demand. The results verify performance of the proposed multi-criteria design procedure.

(Invited) Electrochemical Ammonia Synthesis Using Intermediate Temperature Proton Conducting Membranes

Due to the intermittency of many renewable energy sources, their potential to significantly participate in the overall energy mix is strongly dependent on our ability to identify and develop efficient and cheap energy storage technologies1. Liquid fuels are an appealing option with energy densities typically 10-100x higher than batteries. Ammonia, in particular, has many attributes that make it the ideal energy storage compound including high energy density and an existing infrastructure for transportation. The highly developed, large-scale production of ammonia via Haber-Bosch process is not suitable for energy storage applications due to the high-temperatures and pressures used. Theory2 and experiment3 suggest that a more suitable approach is the production of ammonia from nitrogen and water via electrosynthesis whereby small-scale reactors could enable distributed ammonia production closer to the consumer and be more compatible with energy inputs from intermittent renewable energy resources - improvements that could dramatically reduce the energy and carbon intensity of ammonia production and distribution. Prior research pursuing the electrosynthesis of ammonia has focused primarily on the use of proton conducting electrolytes at high temperatures, >600˚C or low temperatures, <100˚C due to materials limitations in the intermediate temperature range. The intermediate temperature range is particularly advantageous as lower temperatures favor the thermodynamics of the forward reaction to form NH3, while higher temperatures aid in the breaking of the incredibly stable di-nitrogen bond (946 kJ/mol). In order to access this intermediate temperature range of operation, we have developed a composite SnP2O7/Nafion based electrolyte material that exhibits conductivity on the order of 20 mScm-1 at 200˚C. We have demonstrated a maximum ammonia production rate of ~1x10-8molcm-2s-1 at 200˚C using N2 and H2 as reactant gases and Pt-based electrodes. This talk will discuss the factors influencing production rates and current efficiencies of these intermediate temperature devices. References J. B. Goodenough, H. D. Abruna and M. V. Buchanan, 2007.T. H. Rod, A. Logadottir and J. K. Nørskov, The Journal of Chemical Physics 112 (12), 5343 (2000).G. Marnellos and M. Stoukides, Science 282 (1998). Acknowledgements This project is supported by ARPA-E under Award No. DE-AR0000687.

Investigation of the unsteady pressure pulsations in the prototype Francis turbines – Part 1: Steady state operating conditions

Abstract Hydropower is one of the most reliable renewable sources of electricity generation. With high efficiency and good regulating capacity, hydropower has the ability to meet rapid changes in power demand. Large investments in intermittent renewable energy resources have increased the demand for balancing power. This demand has pushed hydraulic turbines to generate electricity over the operating range from part load to full load. High-amplitude pressure pulsations are developed at off-design conditions, which cause moderate damage to the turbine components. The pressure pulsations may be either synchronous- (axial)-type, asynchronous- (rotating)-type or both. In this study, pressure measurements on low specific-speed prototype Francis turbines were performed; one of them was vertical axis and another was horizontal axis type. Four pressure sensors were mounted on the surface of the draft tube cone. Pressure measurements were performed at five operating points. The investigations showed that, in the vertical axis turbine, amplitudes of asynchronous pressure pulsations were 20 times larger than those of the synchronous component; whereas, in the horizontal axis turbine, amplitudes of asynchronous pressure pulsations were two times smaller than those of the synchronous component. For part 2 of the paper, please read Trivedi, C., Gogstad, P. J., and Dahlhaug, O. G., 2017, “Investigation of Unsteady Pressure Pulsations in the Prototype Francis Turbines during Load Variation and Startup,” Journal of Renewable Sustainable Energy, 9(6), p. 064502. https://doi.org/10.1063/1.4994884 .

Breaking Compromises in CO2 Reduction

Electrochemical reduction of CO 2 to formate is a promising approach to utilize intermittent renewable energy for high-value chemical storage and to mitigate carbon pollution. Demanding applied targets for this process require that the reaction is sustained at a high current density with near-unity product efficiency. In this issue of Joule , Zheng and De Luna et al. show that incorporating sulfur into tin catalysts greatly increases the reaction rate while maintaining high selectivity and robust stability.

Optimal sizing of a wind/solar/battery hybrid grid‐connected microgrid system

Higher cost and stochastic nature of intermittent renewable energy (RE) resources complicate their planning, integration and operation of electric power system. Therefore, it is critical to determine the appropriate sizes of RE sources and associated energy storage for efficient, economic and reliable operation of electric power system. In this study, two constraint-based iterative search algorithms are proposed for optimal sizing of the wind turbine (WT), solar photovoltaic (PV) and the battery energy storage system (BESS) in the grid-connected configuration of a microgrid. The first algorithm, named as sources sizing algorithm, determines the optimal sizes of RE sources while the second algorithm, called as battery sizing algorithm, determines the optimal capacity of BESS. These algorithms are mainly based upon two key essentials, i.e. maximum reliability and minimum cost. The proposed methodology aims to avoid over- and under-sizing by searching every possible solution in the given search space. Moreover, it considers the forced outage rates of PV, WT and utilisation factor of BESS which makes it more realistic. Simulation results depict the effectiveness of the proposed approach.

Frequency Profile Improvement of a Microgrid under Abnormal Conditions via a Demand Response Program

The frequency deviation of an electric power system from its nominal range is a significant indicator of the system’s instability. This issue is further highlighted when the system is incorporated with intermittent renewable energy resources. In this work, the frequency regulation of a microgrid is achieved by a demand response program. The proposed plan is implemented in a central controller that has two control sections. The frequency deviation is controlled by manipulating responsive loads. In the case of higher frequency deviation, the command of the first control section is proportional to the magnitude of frequency deviation and a large amount of responsive loads is manipulated to restore the frequency to its nominal range quickly. The other section of the central controller addresses minor frequency deviations. The proposed plan is implemented on a test system and outperforms the conventional methods in terms of frequency restoration. The improvement in voltage profile is also observed during abnormal conditions after the proposed plan is implemented.

Optimal Placement of Intermittent Renewable Energy Resources and Energy Storage System in Smart Power Distribution Networks

This paper presents an efficient strategy to optimally allocate renewable energy sources (RES), primarily wind and solar photovoltaic (PV), and energy storage system (ESS) in electrical distribution networks with a goal of minimizing costs and active power losses. The proposed planning strategy is used to determine the number of possible and optimal locations for the hybrid RES–ESS system. This paper further presents a control scheme to optimally dispatch the output of ESS, which increases the effective utilization of RES by reducing their day-ahead forecast errors in order to minimize the deviation between forecasted and actual values. In our proposed strategy, the location that produces the least overall power losses complying with the system constraints is considered as the optimal one. In order to evaluate the effectiveness of the proposed method, this paper considers a case study of a 16-bus test system that comprises of several loads, wind farm, PV, and ESS. The test results demonstrate that by determining the optimal bus location for the RES–ESS system, overall power losses as well as peak load can be significantly reduced.

Distributed Power-Generation Systems and Protection

Continuously expanding deployments of distributed power-generation systems (DPGSs) are transforming the conventional centralized power grid into a mixed distributed electrical network. The modern power grid requires flexible energy utilization but presents challenges in the case of a high penetration degree of renewable energy, among which wind and solar photovoltaics are typical sources. The integration level of the DPGS into the grid plays a critical role in developing sustainable and resilient power systems, especially with highly intermittent renewable energy resources. To address the challenging issues and, more importantly, to leverage the energy generation, stringent demands from both utility operators and consumers have been imposed on the DPGS. Furthermore, as the core of energy conversion, numerous power electronic converters employing advanced control techniques have been developed for the DPGS to consolidate the integration. In light of the above, this paper reviews the power-conversion and control technologies used for DPGSs. The impacts of the DPGS on the distributed grid are also examined, and more importantly, strategies for enhancing the connection and protection of the DPGS are discussed.

A case study on the effect of storage systems on a distribution network enhanced by high-capacity photovoltaic systems

Modern distribution networks with intermittent weather-dependent renewable energy resources may require energy storage to smooth out intermittency in their power generation and balance energy production and demand. Therefore, the objective of this study is to present a case study of analyzing the effect of such storage systems (SSs) on a distribution network enhanced by high-capacity photovoltaic (PV) systems. For this purpose, this study defines two metrics that quantify the effect of SSs on peak load reduction, power, and voltage such as a load factor and a change in total power generation. Then a well-known strategy that charges storage at night and discharges it to distribution networks at peak demand times is modeled in steady-state power systems analysis software, or OpenDSS. Next, as a case study, this study modifies the IEEE 37-bus test feeder enhanced by four photovoltaic systems (PVSs) with a capacity of 15 and 30 percent of total peak power, adds SSs with the same capacity as PVSs and a duration of three hours to the test feeder, and calculates steady-state power flow in 15-min intervals during a week. Finally, it presents the effect of SSs from the following point of views: (a) a reduction in peak load, (b) an increase in over-voltage (e.g., 1.05 p.u.), and (c) intermittency in voltage and total power generation.

Computational analysis on the electrode geometric parameters for the reversible solid oxide cells

Increasing global energy demands have been accelerating the research and development of reversible electrochemical systems that can realize an efficient use of the intermittent renewable energy resources. This paper thus describes a numerical investigation of reversible solid oxide cells (RSOCs), for their high energy efficiency delivered from the high operating temperatures ranging from 600 to 1000°C. Unlike the previous studies, a model-based strategy is applied for the simultaneous integration of different operating modes (namely, fuel cell and electrolysis cell modes) to enable more realistic predictions on the trade-off behavior of the effects of electrode design parameters on the cell performance. This approach was taken to investigate the effects of various geometric designs and operating parameters (electrode backing layer thickness; interconnector rib size; fuel gas composition) on the current-potential characteristic and the round-trip efficiency. The cell performance was significantly affected by the rib size, particularly when the backing layer was thin, because of the uneven distribution of the reactant species. Overall, this study provides insights into key geometric design parameters that dominate the performance of dual-mode RSOCs.

Trusted Interconnections Between a Centralized Controller and Commercial Building HVAC Systems for Reliable Demand Response

Dynamic smart grid operations require that utilities that incorporate intermittent renewable energy resources provide creative and inclusive solutions to reduce and shift electrical demand. Commercial building HVAC systems have been used as a dispatchable load, but are limited by the lack of interoperability. Increased operability can be achieved using a secure and reliable interconnection device, which can provide direct bidirectional communications between the utility and the building automation system (BAS) controllers. This paper developed and tested a building automation intrusion detection system (BAIDS) that can provide a cyber-secure connection between public and private BAS networks. The BAIDS was used in a hardware-in-the-loop experiment that connected an actual photovoltaic array with a BAS control test bed and a building zone model. The BAIDS device allowed for critical control signals to pass from the public network directly to a fan controller in a BAS private network. At the same time, the BAIDS device provided intrusion detection monitoring to identify malicious activity. The network traffic was evaluated using an adaptive resonance theory (ART) artificial neural network. The ART algorithm was able to learn normal traffic activity on the private and public networks. The algorithm was then used to detect unauthorized attempts to access the interconnection device and a malicious cyber-physical attack on the BAS.

Real time machine coordination for instantaneous load smoothing and photovoltaic intermittency mitigation

With industrial equipment causing large and variable power loads, the industrial sector has a significant impact on the power quality in the United States. Also impacting power quality is the large growth of distributed intermittent renewable energy resources such as photovoltaics and windmills. Coupling factories' high demand variability with intermittent energy resources' high generation variability creates large unpredictable swings in net demand for the utility. While we cannot control the weather driven intermittency of renewable resources, in this paper we show it may be possible to control the power demands of industrial processes in order to reduce the variability in net demand observed by the utility. This research looks at a new alternative way to alleviate some of the burden imposed by renewable energy sources on the grid by coordinating a fleet of industrial machines. By smoothing a machine fleet's power demands over time we demonstrate a significant reduction in the variability of net power draws on the power grid of photovoltaic equipped factories. We demonstrate the impact of net demand variability reduction in both a stochastic and deterministic production setting using 10 months of 1 Hz solar irradiance data. When floor shop variability is high as in the stochastic system, net demand variability can be reduced by 7% while maintaining throughput with minimal impact to total energy consumed and part time in system. When shop floor variability can be highly controlled, such as in the deterministic system presented, the net demand variability can be reduced by 35%. By reducing the net demand variability of renewable energy equipped factories, the application of instantaneous load smoothing strategies provides a new alternative solution to mitigating the impact of renewable energy source intermittency on the grid, and could provide improved power quality to facilities with integrated renewables.

Net load variability in Nordic countries with a highly or fully renewable power system

Increasing the share of intermittent renewable energy (IRE) resources such as solar, wind, wave and tidal energy in a power system poses a challenge in terms of increased net load variability. Full ...

Technical Evaluation of Superconducting Fault Current Limiters Used in a Micro-Grid by Considering the Fault Characteristics of Distributed Generation, Energy Storage and Power Loads

Concerning the development of a micro-grid integrated with multiple intermittent renewable energy resources, one of the main issues is related to the improvement of its robustness against short-circuit faults. In a sense, the superconducting fault current limiter (SFCL) can be regarded as a feasible approach to enhance the transient performance of a micro-grid under fault conditions. In this paper, the fault transient analysis of a micro-grid, including distributed generation, energy storage and power loads, is conducted, and regarding the application of one or more flux-coupling-type SFCLs in the micro-grid, an integrated technical evaluation method considering current-limiting performance, bus voltage stability and device cost is proposed. In order to assess the performance of the SFCLs and verify the effectiveness of the evaluation method, different fault cases of a 10-kV micro-grid with photovoltaic (PV), wind generator and energy storage are simulated in the MATLAB software. The results show that, the efficient use of the SFCLs for the micro-grid can contribute to reducing the fault current, improving the voltage sags and suppressing the frequency fluctuations. Moreover, there will be a compromise design to fully take advantage of the SFCL parameters, and thus, the transient performance of the micro-grid can be guaranteed.

High Temperature and Pressure Alkaline Electrochemical Reactor for Conversion of Power to Chemicals

Moving away from fossil fuels requires harvesting more and more intermittent renewable energy resources and establishing a sustainable system for the production of chemicals. This brings forward the need for efficient large scale energy storage technologies 1-3 and technologies for the conversion of renewable electricity to chemicals. Electrochemical reactors can play a crucial role in this endeavor, since they can efficiently and reversibly transform electricity to high-value chemicals, and thus serve as energy storage and recovery devices for balancing the grid, while offering a means for the sustainable production of chemicals 4-6. A novel type of alkaline electrochemical cell that can operate at elevated temperature and pressure has been developed that relies on corrosion resistant high temperature diaphragms, based on mesoporous ceramic membranes where aqueous KOH is immobilized by capillary forces. Raising the operating temperature offers a means to boost performance, as both ionic transport and reaction kinetics are exponentially activated with temperature. Indeed, we have demonstrated alkaline electrolysis cells operating at 200-250 °C and 20-50 bar at very high efficiencies and power densities. This work will provide an overview of our efforts to develop components of such high temperature alkaline electrochemical reactors for different applications. Low-cost large-scale production methods have been successfully employed for the production of ceramic diaphragms and full cells. The influence of composition and microstructure on the long-term chemical stability and mechanical durability of the mesoporous ceramic membranes has been explored. Instrumentation for electrochemical testing at elevated pressures (up to 99 bar) and temperatures (up to 300 °C) with in-line chemical analysis has been established enabling experiments with gaseous or liquids reactants/products at cell sizes of up to 25 cm2. Efforts are currently directed towards the investigation of the intrinsic activity of mixed oxides for the oxygen evolution reaction at elevated temperatures and pressures, and of the intrinsic activity, selectivity and stability of supported metal catalysts towards the electrocatalytic conversion of biomass derivatives to high-value chemicals. Finally, the use of selected electrocatalysts for the production of high performance electrodes will be reported.

Stochastic-based resource expansion planning for a grid-connected microgrid using interval linear programming

This paper represents a stochastic approach for long-term optimal resource expansion planning of a grid-connected microgrid (MG) containing different technologies as intermittent renewable energy resources, energy storage systems and thermal resources. Maximizing profit and reliability, along with minimizing investment and operation costs, are major objectives which have been considered in this model. Also, the impacts of intermittency and uncertainty in renewable energy resources were investigated. The interval linear programming (ILP) was applied for modelling inherent stochastic nature of the renewable energy resources. ILP presents some superiority in modelling of uncertainties in MG planning. The problem was formulated as a mixed-integer linear programming. It has been demonstrated previously that the benders decomposition (BD) served as an effective tool for solving such problems. BD divides the original problem into a master (investment) problem and operation and reliability subproblems. In this paper a multiperiod MG planning is presented, considering life time, maximum penetration limit of each technology, interest rate, capital recovery factor and investment fund. Real-time energy exchange with the utility is covered, with a consideration of variable tariffs at different load blocks. The presented approach can help MG planners to adopt best decision under various uncertainty levels based on their budgetary policies.

Electrochemical and Structural Investigations of Titanate Based Anodes for High Power and Long Cycle Life Li-Ion Battery

Increasing demands for utilization of intermittent renewable energy resources, cost effective, safe and long cycle life electrochemical energy storage is required. Currently, lithium-ion batteries are identified as strong contender to fulfill the future energy requirements for electric vehicle as well as stationary energy storage applications due to high voltage, energy density, and long cycle life. However, state-of-the-art lithium ion battery technology still needs improvements in terms of cost, safety, power and cycle life. For stationary application, energy density is not as critical as that for vehicle application but long cycle life and safety is required. The spinel-type lithium titanium oxide, Li4Ti5O12 is attractive as an alternative to graphite anode for advanced lithium-ion batteries, due to the reversible Li insertion and extraction property at around 1.55 V (versus Li/Li+) with so called zero strain behavior. Even though this redox potential is far higher than that of graphite or carbonaceous materials, the good cyclability of Li cells has favored its commercialization. Although the structural restriction of the spinel structure limits the theoretical capacity to 175 mAh/g, various forms of titanate compounds exists such as H2Ti6O12, H2Ti12O25 and TiO2(B) with a higher theoretical capacity up to 335 mAh/g. In our presentation, electrochemical performance of Li-ion pouch cell using NMC cathode and AlF3 coated Li4Ti5O12 anode combination will be presented. Furthermore, structural investigations and electrochemical performances of other titanate based anodes will be discussed.

Risk Optimal Pooling of Intermittent Renewable Energy Resources

Many owners of photovoltaic and wind generation capacity sign up with virtual power plants to gain access to power markets. Power generation from these sources is inherently intermittent and consequently revenue is random – inducing a risk for the owner. We investigate how pooling different technologies as well as different locations in the portfolio of a virtual power plant can reduce aggregate risk. To this end, we develop stochastic models for the factors driving the assets’ underlying market and volume risks on which we base a model for risk optimal pooling. In a case study for the German market we demonstrate that the optimal portfolios have a clearly better risk/return profile than the market portfolio. This finding holds true for the case without subsidies as well as for the case with feed-in tariffs.

Two-Stage Economic Operation of Microgrid-Like Electric Vehicle Parking Deck

This paper addresses a two-stage framework for the economic operation of a microgrid-like electric vehicle (EV) parking deck with on-site renewable energy generation (roof-top photovoltaic panel). This microgrid-like EV parking deck is a localized grouping of distributed generation (solar), energy storage (EV batteries), and load (EV charging load). Although EV parking decks can enable greater adoption of renewable energy sources by scheduling charging loads to coincide with periods of strong sun, the inherent intermittency of renewable energy resources and variable EV parking behaviors complicates the economic operation. In this paper, the proposed first stage of this framework provides the parking deck operators with a stochastic approach for dealing with the uncertainty of solar energy so as to make an optimal price decision (marginal electricity sale price and parking fee rebate) at the day-ahead time scale. The second stage introduces a model predictive control-based operation strategy of EV charging dealing with the uncertainty of parking behaviors within the real-time operation. Case studies demonstrate the better performance of the proposed framework, offering an effective day-ahead marginal electricity price for tomorrow’s operation and increasing the microgrid-like EV parking deck’s revenue during the real-time operation.