applied sciences Review Energy Management in Microgrids with Renewable Energy Sources: A Literature Review Yimy E. García Vera 1, Rodolfo Dufo-López 2,* and José L. Bernal-Agustín 2 1 Electronic Engineering, San Buenaventura University, Bogotá 20, Colombia; [email protected] 2 Electrical Engineering Department, University of Zaragoza, 50018 Zaragoza, Spain; [email protected] * Correspondence: [email protected] Received: 24 July 2019; Accepted: 11 September 2019; Published: 13 September 2019 Abstract: Renewable energy sources have emerged as an alternative to meet the growing demand for energy, mitigate climate change, and contribute to sustainable development. The integration of these systems is carried out in a distributed manner via microgrid systems; this provides a set of technological solutions that allows information exchange between the consumers and the distributed generation centers, which implies that they need to be managed optimally. Energy management in microgrids is defined as an information and control system that provides the necessary functionality, which ensures that both the generation and distribution systems supply energy at minimal operational costs. This paper presents a literature review of energy management in microgrid systems using renewable energies, along with a comparative analysis of the different optimization objectives, constraints, solution approaches, and simulation tools applied to both the interconnected and isolated microgrids. To manage the intermittent nature of renewable energy, energy storage technology is considered to be an attractive option due to increased technological maturity, energy density, and capability of providing grid services such as frequency response. Finally, future directions on predictive modeling mainly for energy storage systems are also proposed. Keywords: microgrids; energy management; renewable energy; optimization; photovoltaic; energy storage 1. Introduction The exponential demand for energy has led to the depletion of fossil fuels such as petroleum, oil, and carbon. This, in turn, increases the greenhouse effect gases. Energy systems have incorporated small-scale and large-scale renewable sources such as solar, wind, biomass, and tidal energy to mitigate the aforementioned problems on a global scale [1]. Global energy demand will grow by more than a quarter to 2040, when renewable sources are expected to represent 40 percent of the global energy mix. The reliability of the renewable sources is a major challenge due mainly to mismatch between energy demand and supply [2]. Renewable energy resources, distributed generation (DG), energy storage systems, and microgrids (MG) are the common concepts discussed in several papers [3]. The increase in the demand for energy and the rethinking of power systems has led to energy being generated near the places of consumption. This energy is derived from renewable sources, which are becoming increasingly competitive due to a drop in prices, especially in the case of photovoltaic solar and wind energies [4]. Due to strong dependency on climatic and meteorological conditions, in many cases the optimal system is a hybrid renewable energy system (considering one or more renewable sources) with battery storage systems (and in some cases including diesel generator) [5]. The hybrid energy systems are typically used for electricity supply for several applications such as houses or farms in rural areas without grid extension, telecommunication antennas, and equipment, and many other stand-alone Appl. Sci. 2019, 9, 3854; doi:10.3390/app9183854 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 27 Appl. Sci. 2019, 9, 3854 2 of 28 without grid extension, telecommunication antennas, and equipment, and many other stand-alone systemssystems [ 6[6,7].,7]. InIn manymany cases cases these these hybrid hybrid systems systems imply imply the the highest highest reliability reliability and and lowest lowest costs costs comparedcompared to to systems systems with with only only one one energy energy source source [8 [8,9].,9]. AA microgrid microgrid consists consists of of a a set set of of loads, loads, energy energy storage storage equipment, equipment, and and small-scale small-scale generation generation systemssystems [10 [10].]. It It can can be be defined defined in in a a broader broader sense sense as as a a medium medium or or low low distribution distribution grid, grid, which which has has distributeddistributed generation generation including including renewable renewable and and conventional conventional sources sources (hybrid (hybrid systems) systems) with with storage storage unitsunits that that supply supply electrical electrical energy energy to to the the end end users. users. The The reliability reliability of of the the microgrid microgrid is is improved improved by by thethe storage storage and and it it is is used used to to complement complement the the intermittency intermittency of of the the PV PV and and wind wind output output power power [ 11[11–13].–13]. TheseThese microgrids microgrids have have communication communication systems systems that that are are necessary necessary for for real real time time management management [ 14[14].]. MicrogridsMicrogrids can can also also operate operate either either in in isolation isolation or or when when connected connected to to a grida grid [15 [15].]. Based Based on on the the type type of of sourcesource they they manage, manage, microgrids microgrids can can be be classified classified as as direct direct current current line line (DC), (DC), alternating alternating current current line line (AC),(AC), or or hybrid hybrid (shown (shown in in Figure Figure1). 1). FigureFigure 1. 1.A A hybrid hybrid isolated isolated microgrid microgrid scheme. scheme. InIn a a microgrid, microgrid, it it is is essential essential to to maintain maintain the the power power supply-demand supply-demand balance balance for for stability stability because because thethe generation generation of theof the intermittent intermittent distributed distributed sources sources such assuch photovoltaic as photovoltaic and wind and turbines wind isturbines difficult is todifficult predict andto predict their generation and their maygeneration fluctuate may significantly fluctuate significantly depending on depending the availability on the of availability the primary of sourcesthe primary (solar sources irradiation (solar and irradiation wind). The and supply-demand wind). The supply-demand balancing problem balancing becomes problem even becomes more importanteven more when important the microgrid when isthe operating microgrid in stand-alone is operating mode in stand-alone where only limitedmode where supply only is available limited tosupply balance is the available demand to [ 16balance]. Energy the managementdemand [16]. optimization Energy management in microgrids optimization is usually in considered microgrids as is anusually offline considered optimization as probleman offline [17 optimization]. problem [17]. MicrogridsMicrogrids supported supported with with renewable renewable energies energies cancan bebe classifiedclassified asas smartgrids,smartgrids, which provide a aset set of of technological solutions to allowallow informationinformation exchangeexchange betweenbetween thethe consumersconsumers andand the the distributeddistributed generation. generation. An An energy energy management management system system (EMS) (EMS) is is defined defined as as an an information information system, system, whichwhich provides provides the the necessary necessary functionality functionality when when supported supported on on a a platform platform to to ensure ensure that that generation, generation, transmission,transmission, and and distribution distribution supply supply energy energy at at minimal minimal cost cost [18 [18].]. Energy Energy management management in in the the microgridsmicrogrids involves involves a a control control software software that that permits permits the the optimal optimal operation operation of of the the system system [19 [19].]. This This is is achievedachieved by by considering considering thethe minimal required required cost cost and and two two microgrid microgrid operation operation modes modes (isolated (isolated and andinterconnected). interconnected). The The variability variability of ofresources resources such such as as solar solar irradiation irradiation and windwind speedspeedmust must be be accountedaccounted for for when when considering considering microgrids microgrids with with renewable renewable energy energy sources sources [20 [20].]. AA review review on on the the studies studies related related toto thethe energyenergy management of microgrids microgrids can can be be found found in in [21]. [21]. A Afew few authors authors have have solved solved the problem ofof energyenergy managementmanagement usingusing didifferentfferent techniques techniques to to achieve achieve Appl. Sci. 2019, 9, 3854 3 of 28 an optimal microgrid operation. However, these techniques must incorporate better solution strategies due to the integration of distributed generation, storage elements, and electric vehicles. Other recent papers [22] have reviewed various integration methods for renewable energy systems based on storage and demand response. This covers two main areas, namely (1) the optimal usage of storage, and (2) improvement of user participation via demand response mechanisms and other collaborative methods. The authors in