An Experimental Study of Power Smoothing Methods to Reduce Renewable Sources Fluctuations Using Supercapacitors and Lithium-Ion Batteries

The random nature of renewable sources causes power fluctuations affecting the stability in the utility grid. This problem has motivated the development of new power smoothing techniques using supercapacitors and batteries. However, experimental studies based on multiple renewable sources (photovoltaic, wind, hydrokinetic) that demonstrate the validity of power smoothing techniques under real conditions still require further study. For this reason, this article presents a feasibility study of a renewable grid-connected system, addressing various aspects based on power quality and energy management. The first of them is related to the fluctuations produced by the stochastic characteristics of renewable sources and demand. Two power smoothing algorithms are presented (ramp rate and moving average) combining photovoltaic, wind, and hydrokinetic sources with a hybrid storage system composed of supercapacitors and lithium-ion batteries. Then, the self-consumption for an industrial load is analyzed by studying the energy flows between the hybrid renewable energy sources and the grid. The main novelty of this paper is the operability of the supercapacitor. The experimental results show that when applying the power smoothing ramp rate method, the supercapacitor operates fewer cycles with respect to the moving average method. This result is maintained by varying the capacity of the renewable sources. Moreover, by increasing the capacity of photovoltaic and wind renewable sources, the hybrid storage system requires a greater capacity only of supercapacitors, while by increasing the capacity of hydrokinetic turbines, the battery requirement increases considerably. Finally, the cost of energy and self-consumption reach maximum values by increasing the capacity of the hydrokinetic turbines and batteriesPartial funding for open access charge: Universidad de Málag

The Need for Energy Storage on Renewable Energy Generator Outputs to Lessen the Geeth Effect, i.e. Short-term Variations Mainly Associated with Wind Turbine Active Power Output

Many studies investigating the short-term variations associated with the power output from wind turbine generators utilise simulated or modelled data in the analysis. This current study uses short-term empirical data downloaded directly from operational wind turbines via electrical power quality meters. The empirical data shows that the short-term variations (one-second or sub-one-second timeframe) occur continuously over most of the power output range. A novel name is proposed, the Geeth Effect, for this variability phenomenon. The Geeth Effect is measured using the coefficient of variation mathematical expression and is likely contributing to (i) lower-than-expected financial and environmental benefits associated with the vast increase in connected wind turbine capacity, (ii) significant challenges faced by the transmission system operator as they seek to deliver a stable electricity grid. Calculated coefficient of variation values include 64% (10-kW wind turbine), 46% (300-kW wind turbine), 30% (3-MW wind turbine), 1.4% (169-kW solar PV), and 3.2% (40-kW hydroelectric plant). Energy storage methods are recommended to minimise the Geeth Effect. Recommendations include the installation of (i) filters (supercapacitors) and (ii) battery energy storage systems, both systems connected to the output stage of the wind turbine generators. Supercapacitors are the preferred choice for wind turbines because of the continuous charge/discharge cycling events, which can be detrimental to battery energy systems. Low coefficient of variation values are desirable and high values undesirable

Power-electronic systems for the grid integration of renewable energy sources: a survey

The use of distributed energy resources is increasingly being pursued as a supplement and an alternative to large conventional central power stations. The specification of a powerelectronic interface is subject to requirements related not only to the renewable energy source itself but also to its effects on the power-system operation, especially where the intermittent energy source constitutes a significant part of the total system capacity. In this paper, new trends in power electronics for the integration of wind and photovoltaic (PV) power generators are presented. A review of the appropriate storage-system technology used for the integration of intermittent renewable energy sources is also introduced. Discussions about common and future trends in renewable energy systems based on reliability and maturity of each technology are presented

Reduction of power oscillations in Wave Energy Converters using prediction techniques based on Autoregressive Models

La potenza generata da molte tipologie di WECs presenta delle fluttuazioni che possono minare la stabilità della rete elettrica. L'obiettivo di questa tesi è lo sviluppo e l'integrazione di una previsione di breve termine del profilo di potenza generata nel controllo di un Power Smoothing System a base di supercondensatori, allo scopo di smorzare le oscillazioni di potenza prima dell'immissione in rete. Il funzionamento del sistema è stato testato con simulazioni e test di laboratorioope

The role of energy storage for mini-grid stabilization

48 pagesMini-grids may be designed to operate autonomously with or without connection to a central grid. While operating autonomously, they cannot rely on the central grid to provide stabilization to control the line voltage and frequency, balance supply and demand of power and manage real or reactive power. Energy storage can provide stabilization in a mini-grid as follows: when the system works autonomously, storage provides or absorbs power to balance supply and demand, to counteract the moment to moment fluctuations in customer loads and unpredictable fluctuations in generation. When grid connected, energy storage systems also can provide ancillary services to improve power quality such as voltage and frequency regulation, harmonic filtering, and fault clearing (i.e. supply of short circuit current). This is named the power use of energy storage, contrary to the usual energy use of energy storage

Direct Connection of Supercapacitor-Battery Hybrid Storage System to the Grid-tied Photovoltaic System

IEEE Penetration rate of grid-connected photovoltaic (PV) generation to the existing utility grid is rapidly increasing over the years. Since the power generated from PV systems fluctuate according to the weather condition, e.g., cloud passing, this can significantly disturb the stability of a weak utility grid. The integration of energy storage devices and its ramp-rate control technique are required to reduce the impact of PV systems output fluctuations and augment the stability of the utility grid. In this paper, ramp-rate control is applied to the direct connection of energy storage devices in PV generation system configuration. The direct connection of supercapacitors string and battery combination scheme is proposed to reduce the number of power converters so that the efficiency of the system is increased. In this work, the PV system output is controlled by directly control the energy storage system (ESS) to limit the changing rate of PV output to desired ramp-rate value. Hence, reducing the battery charge/discharge cycles and extending the expected lifetime of the ESS. The performance of the proposed direct connection scheme of ESS and its ramp-rate control strategy is verified using a 1-kW PV system prototype

Advances in Supercapacitor Technology and Applications Ⅱ

Energy storage is a key topic for research, industry, and business, which is gaining increasing interest. Any available energy-storage technology (batteries, fuel cells, flywheels, and so on) can cover a limited part of the power-energy plane and is characterized by some inherent drawback. Supercapacitors (also known as ultracapacitors, electrochemical capacitors, pseudocapacitors, or double-layer capacitors) feature exceptional capacitance values, creating new scenarios and opportunities in both research and industrial applications, partly because the related market is relatively recent. In practice, supercapacitors can offer a trade-off between the high specific energy of batteries and the high specific power of traditional capacitors. Developments in supercapacitor technology and supporting electronics, combined with reductions in costs, may revolutionize everything from large power systems to consumer electronics. The potential benefits of supercapacitors move from the progresses in the technological processes but can be effective by the availability of the proper tools for testing, modeling, diagnosis, sizing, management and technical-economic analyses. This book collects some of the latest developments in the field of supercapacitors, ranging from new materials to practical applications, such as energy storage, uninterruptible power supplies, smart grids, electrical vehicles, advanced transportation and renewable sources

Real-time testing of energy storage systems in renewable energy applications

Energy storage systems provide a promising solution for the renewable energy sector to facilitate large-scale grid integration. It is thus very important to explore means to validate their control scheme and their behaviour in the intended application before actual commissioning. This paper presents a reduced-scale hardware-in-the-loop simulation for initial testing of the performance of energy storage systems in renewable energy applications. This relieves the need of selecting and tuning a detailed model of the energy storage element. A low-power test rig emulating the storage element and the power converter is interfaced with a real time digital simulator to allow dynamic experimental tests under realistic conditions. Battery energy storage for smoothing the output power of a variable speed wind turbine is considered in this paper; however the proposed test methodology can be easily adapted for other storage elements in renewable energy, distributed generation and smart grid applications. The proposed HIL simulation is detailed and the experimental performance is shown

Review of energy system flexibility measures to enable high levels of variable renewable electricity

The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.Peer reviewe

Short-term energy recovery control for virtual inertia provision by renewable energy sources

The proliferation of Converter-Interfaced Renewable Energy Sources (CIRES), which are inertia-less, and the gradual decommissioning of synchronous generation have posed several challenges to the electric power system. This has motivated a complete a shift in the CIRES design and its corresponding control philosophy. Integrating Energy Storage Systems (ESS) within CIRES enables the implementation of different operating modes allowing them to provide ancillary services (AS) in a similar way to the synchronous generation. In order to tackle with those short-term response AS, such as virtual inertia, fast ESS (FESS) solutions with high power-to-energy ratio, particularly flywheels and supercapacitors, are preferred. In spite of several control algorithms have been proposed to provide such fast AS, very little research effort has been paid on the proper FESS energy recovery after the AS provision. This task is particularly challenging, since supercapacitors must be operated at a certain state of charge to guarantee that the required AS can be provided within its operational limits. This paper aims to fill this gap by proposing a new energy recovery control scheme for supercapacitors after the provision of short-term AS, such as virtual inertia. The proposed control is validated via simulations which clearly highlights its adequate performance.Horizonte 2020 (Unión Europea) 764090Ministerio de Economía y Competitividad (MINECO). España ENE2017-84813-R