electronics Article Investigation on Hybrid Energy Storage Systems and Their Application in Green Energy Systems Chao-Tsung Ma 1,* and Chin-Lung Hsieh 2 1 Department of Electrical Engineering, National United University, Miaoli 36063, Taiwan 2 Institute of Nuclear Energy Research (INER), Atomic Energy Council, Taoyuan 325207, Taiwan; [email protected] * Correspondence: [email protected] Received: 23 October 2020; Accepted: 11 November 2020; Published: 13 November 2020 Abstract: Power systems all over the world have been under development towards microgrids integrated with renewable energy-based distributed generation. Due to the intrinsic nature of output power fluctuations in renewable energy-based power generation, the use of proper energy storage systems and integrated real-time power and energy control schemes is an important basis of sustainable development of renewable energy-based distributed systems and microgrids. The aim of this paper is to investigate the characteristics and application features of an integrated compound energy storage system via simulation and a small-scale hardware system implementation. This paper first discusses the main components, working principles and operating modes of the proposed compound energy storage system. Then, a detailed design example composed of supercapacitors, batteries, and various controllers used in two typical application scenarios, peak demand shaving and power generation smoothing, of a grid-connected microgrid is systematically presented. Finally, an experimental setup with proper power converters and control schemes are implemented for the verification of the proposed control scheme. Both simulation and implementation results prove that the proposed scheme can effectively realize desired control objectives with the proposed coordinated control of the two energy storage devices. Keywords: microgrid (MG); renewable energy (RE); distributed generation (DG); energy storage system (ESS); compound energy storage system (CESS) 1. Introduction In recent years, with the increase of the global population and rapid economic development, human demand for energy is rapidly increasing and the global warming effects caused mostly by conventional power generation has become a major issue that countries all over the world must face together [1,2]. In addition, the use of conventional fossil fuels will soon yield problems, such as the rapid reduction in reserves and the unaffordable cost for the emission of greenhouse gases, which highlights the importance of developing alternative energy sources. Therefore, low-carbon energy technologies, such as nuclear energy and renewable energy (RE), have become the development goals of countries all over the world. Relatively mature RE power generation technologies include wind turbine (WT), solar photovoltaic (PV), hydropower, biomass energy, and geothermal energy generation, among which WT generation and solar PV generation are the mainstream of development. The projection in International Energy Outlook 2019 shows that, by 2050, the proportion of global RE power generation will increase to nearly 50%, as shown in Figure1, and the proportion of WT and solar PV together in RE power generation will increase to over 70%, as shown in Figure2[ 2]. The advantages of RE include: (1) Inexhaustible and widely distributed; and (2) no fuel and extremely environmentally friendly. However, the grid connected with a large number of such unstable generation systems will have a Electronics 2020, 9, 1907; doi:10.3390/electronics9111907 www.mdpi.com/journal/electronics Electronics 2020, 9, 1907 2 of 20 ElectronicsElectronics 20202020,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 22 ofof 2121 negative impact on the supply power quality (PQ) and reliability of existing power systems. Therefore, numbernumber ofof suchsuch unstableunstable generationgeneration systemssystems wiwillll havehave aa negativenegative impactimpact onon thethe supplysupply powerpower in order to reduce the unavoidable impact caused by the fluctuation of RE generation, such generation qualityquality (PQ)(PQ) andand reliabilityreliability ofof existingexisting powerpower systems.systems. Therefore,Therefore, inin orderorder toto reducereduce thethe unavoidableunavoidable systems are often combined with energy storage systems (ESSs), forming a hybrid generation system impactimpact causedcaused byby thethe fluctuationfluctuation ofof RERE generatigeneration,on, suchsuch generationgeneration systemssystems areare oftenoften combinedcombined in which ESSs absorb the fluctuation of power generation, achieving power smoothing and optimal withwith energyenergy storagestorage systemssystems (ESSs),(ESSs), formingforming aa hybridhybrid generationgeneration systemsystem inin whichwhich ESSsESSs absorbabsorb thethe energy management [3]. fluctuationfluctuation ofof powerpower generation,generation, achievingachieving powepowerr smoothingsmoothing andand optimaloptimal energyenergy managementmanagement [3].[3]. Figure 1. Record and projection of world electricity generation and shares by source [2]. FigureFigure 2.2. RecordRecord andand projectionprojection ofof worldworld RERE electricityelectricity generationgeneration andand sharessharesby bysource source [ 2[2].]. ThereThereThere are areare two twotwo types typestypes of ofof ESS, ESS,ESS, i.e., i.e.,i.e., power-type power-typepower-type energy enerenergygy storage storagestorage (PTES) (PTES)(PTES) units, units,units, such suchsuch asasas SC, SC,SC, flywheel, flywheel,flywheel, andandand superconducting superconductingsuperconducting magnetic magneticmagnetic energy energyenergy storage, storage,storage, and andand energy-type energy-typeenergy-type energy energyenergy storage storagestora (ETES)ge (ETES)(ETES) units, units,units, such suchsuch as variousasas variousvarious types typestypes of batteries. ofof batteries.batteries. Since SinceSince these thesethese two twotwo ESSs ESSsESSs have hahaveve their theirtheir own ownown characteristics. characteristics.characteristics. If IfIf proper properproper function functionfunction allocationallocationallocation can cancan be bebe carried carriedcarried out outout according accordingaccording to certain toto certaicertai criteriann criteriacriteria to form toto aformform compound aa compoundcompound energy storageenergyenergy systemstoragestorage (CESS),systemsystem (CESS), the(CESS), greatest thethe greatestgreatest overall overall benefitoverall benefitbenefit can be cancan obtained bebe obtainedobtained [4]. Take [4].[4]. Take theTake power-smoothing thethe power-smoothingpower-smoothing control controlcontrol in RE inin basedRERE basedbased power powerpower generation generationgeneration and energy andand energyenergy balancing balancingbalancing optimization optimizationoptimization control incontrolcontrol an MG inin as anan an MGMG example, asas anan applyingexample,example, CESSapplyingapplying can bringCESSCESS greatcancan bringbring benefits: greatgreat the bebe PTESnefits:nefits: unit thethe PTES canPTES be unitunit used cancan to smoothbebe usedused powertoto smoothsmooth fluctuations powerpower fluctuationsfluctuations with large amplitudewithwith largelarge and amplitudeamplitude high frequency andand highhigh in frequencyfrequency the active distributioninin thethe activeactive network distributiondistribution of the networknetwork MG, which ofof thethe fully MG,MG, demonstrates whichwhich fullyfully itsdemonstratesdemonstrates advantages ofitsits large advantagesadvantages power outputofof largelarge density, powerpower high outputoutput response density,density, speed, highhigh and responseresponse considerable speed,speed, numberandand considerableconsiderable of cycles; thenumbernumber ETES unitofof cycles;cycles; can be thethe used ETESETES to smooth unitunit cancan low-frequency bebe usedused toto powersmoothsmooth fluctuation low-frequencylow-frequency and perform powerpower real-timefluctuationfluctuation power andand balanceperformperform and real-timereal-time dispatch, powerpower which balancebalance fully demonstratesandand dispatch,dispatch, itswhichwhich advantages fullyfully demonstratesdemonstrates of large capacity itsits advantagesadvanta [5]. Furthermore,ges ofof largelarge thecapacitycapacity abovementioned [5].[5]. Furthermore,Furthermore, CESS the canthe abovementionedabovementioned effectively reduce CECE theSSSS number cancan effectivelyeffectively of times reducereduce of charging thethe numbernumber/discharging ofof timestimes and ofof charging/dischargingcharging/discharging andand depthdepth ofof dischargedischarge (DoD)(DoD) ofof PTESPTES toto extendextend itsits life.life. InIn addition,addition, thethe optimizedoptimized configurationconfiguration ofof CESSCESS cancan bringbring moremore economicaleconomical andand reliablereliable activeactive distributiondistribution networknetwork operation.operation. Electronics 2020, 9, 1907 3 of 20 Electronics 2020, 9, x FOR PEER REVIEW 3 of 21 depth of discharge (DoD) of PTES to extend its life. In addition, the optimized configuration of CESS At present, most of the small and medium-capacity RE power generation systems are can bring more economical and reliable active distribution network operation. equipped with various energy storage devices (ESD) and can be divided into standalone and At present, most of the small and medium-capacity RE power generation systems are equipped grid-connected architectures. Although general hybrid RE power generation systems can be easily with various energy storage devices (ESD) and can be