Study of the use of biogas as an energy vector for microgrids

Renewable and Sustainable Energy Reviews 200 (2024) 114574 Available online 22 May 2024 1364-0321/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Study of the use of biogas as an energy vector for microgrids Valentyna Loboichenko a , b , Alfredo Iranzo a , c , Manuel Casado-Manzano a , Sergio J. Navas a , * , F.J. Pino a , Felipe Rosa a , c a Dpto. de Ingeniería Energ´ etica, Escuela T´ ecnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, Sevilla, 41092, Spain b Department of Civil Security, Lutsk National Technical University, Lvivska Street, 75, Lutsk, 43018, Ukraine c ENGREEN - Laboratory of Engineering for Energy and Environmental Sustainability, Universidad de Sevilla, Spain ARTICLE INFO Keywords: microgrid Biogas Renewable energy Power Capacity Bioreactor ABSTRACT The review presents the current state of the issue about the structural features of microgrids as potential structural elements of energy networks. By analyzing literature data, microgrids incorporating biogas were studied in more detail. It is noted that modeling methods and software computing tools are actively used to obtain optimal microgrid structures. Various options for using biomass in microgrids are shown, methods for integrating biogas plants into microgrids and features of their operation are presented. The role of individual components of a microgrids incorporating biogas has been studied, theoretical models and features of the conversion of biogas into microgrids have been discussed in detail. It is noted that microgrids with a biogas production capacity of up to 5 MWh/day are mainly considered, and the contribution of biogas in microgrids varies in the range of 1–67 % of the total production capacity. Implemented examples of such microgrids are presented and further prospects for their use are outlined. Features of the use of microgrids incorporating biogas, difficulties for their practical implementation and the need for further in-depth study of this issue are noted. It is noted that the use and implementation of microgrids incorporating biogas will contribute to the energy independence of states and their achievement of sustainable development goals. It is expected that the presented review will help specialists more clearly focus on the problematic issues of microgrids incorporating biogas, and the information presented will allow stakeholders to make informed decisions about the choice of power system structure. 1. Introduction One of the basic needs of a modern state is the provision of a sufficient amount of energy resources necessary for the smooth functioning of all sectors of the economy. The uneven distribution of fossil resources and their exhaustibility, unstable pricing policy for these resources, the development of new technologies and industries, the deterioration of the environmental situation in the world - all these factors contribute to the search for new energy sources. Significant results in the development of the use of solar, wind, water, geothermal energy, tidal energy, and other renewable energy sources have led to the next qualitatively new stage in the energy sector - their joint use [1]. The need to regulate the generation, conversion and transmission of energy in systems with several energy sources contributed to the emergence of a new structure in the operation of energy systems - the socalled “microgrids”. According to the definition of the International Electrotechnical Commission [2] “microgrid - group of interconnected loads and distributed energy resources with defined electrical boundaries forming a local electric power system at distribution voltage levels, that acts as a single controllable entity and is able to operate in either grid-connected or island mode". The modular design of microgrids, the ability to regulate energy production/energy consumption/energy storage, minimization of energy transportation costs and proximity to the end consumer made them in demand, first of all, for remote, agricultural areas [3]. At the same time, they can be a way to ensure energy autonomy for individual communities, areas and facilities [4] and eventually entire states. Under the influence of the political factor on imported energy resources, correctly functioning microgrids can act as an element of ensuring the energy, and ultimately national, security of the country. Despite the development of the concept of microgrids, there is no single understanding of the most optimal structure with the necessary quantity and composition of elements. One of the promising components is the use of biogas. The use of biogas in the microgrid concept can act as an additional source of energy and reduce the dependence of regions and * Corresponding author. E-mail address: [email protected] (S.J. Navas). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser https://doi.org/10.1016/j.rser.2024.114574 Received 8 April 2023; Received in revised form 10 May 2024; Accepted 14 May 2024 Renewable and Sustainable Energy Reviews 200 (2024) 114574 2 states on fossil energy sources. Reducing carbon emissions allows them to accelerate their progress towards achieving sustainable development. However, system research in this direction in the literature is represented very limited. This review helps fill this gap. The study is aimed at giving an idea of the structural features of microgrids using biogas, biogas conversion in them and the prospects for further development and implementation of this type of microgrids. In this review, the critical comparative analysis of the technical capabilities of different types of microgrid systems with biogas incorporation was carried out through a bibliographic examination of publications available in the Scopus database. Calculation methods were used to estimate the parameters of the systems proposed in the literature. The main contributions of this study are as follows. Energy systems in the form of microgrids allow the use of renewable energy sources and ensure the transition of states and regions to a green economy. The review points out the features of incorporating biogas into such systems with various combinations of other renewable energy sources. As novelty, it is noted that structures with biogas production capacity up to 5 MWh/day are the most popular among the considered microgrid options. At the same time, little-studied aspects are highlighted and innovative further research prospects are indicated. A unique contribution of the paper is the comparative analysis of the current state of microgrids with incorporated biogas, including a detailed comparison and benchmarking based on Key Performance Indicators (KPIs) and also including economic considerations and detail analysis of physical systems in operation. The information presented in the work will help specialists more clearly focus on problematic issues of microgrids with incorporated biogas. With the information presented in the review, scientists, politicians and entrepreneurs can make more informed decisions about choosing the structure of the energy system at the local, regional or state level. In turn, this will contribute to achieving sustainable development goals on a global scale. The review is structured as follows. Initially, in Section 2, a general description of microgrids and microgrids with included biogas was given. Further, in Section 3, a more detailed analysis of features of biogas transformation in these systems, including the individual structural elements of such microgrids, is presented, and then, in Section 4, the existing real samples of microgrids using biogas are discussed. In Section 5, further possible prospects for the development of this direction are presented. And, finally, in Section 6, conclusions are given. 2. Microgrids as a type of energy systems with various energy sources The development of the concept of microgrids contributed to the formation of their structure, hardware and software, the study of energy flow control within this system and the study of the features of its interaction with other energy systems. A separate issue is also the understanding about what these energy systems are. In particular, they are presented as a “distributed energy system” [5], μ Grid [6], hybrid renewable energy systems [7,8], multi-energy complementary system [9], integrated energy systems [10], renewable energy based power systems [11], multi-microgrid power system [12]. Further development and improvement of the concept of a microgrid with several energy sources has led to its transformation into smart grids (smart energy systems), where an indispensable element of the network is also an intelligent control system [13]. As a result, the need to study the control hierarchy, the structure of control levels and control elements in these microgrids became obvious [14]. Approaches to the classification of microgrids also vary among different authors. So, they can differ in terms of being included in a common power grid or autonomy (stand-alone system, off-grid system, islanded, isolated), by working as DC or AC distribution networks or both, by the presence of basic elements (solar batteries, wind turbines, generators, hydro turbines, storage batteries, controllers, fuel cells, etc.) [6]. Microgrids can be considered in terms of their applicability – for buildings, districts [15,16], the production of one or more types of energy [17], as well as location - stationary (land) or sea (for ships) [18]. Software tools, optimization methods used in the analysis of various microgrid configurations vary [19,20]. Thus, the most popular tools for determining the optimal size of HERS include HOMER and iHOGA (Hybrid Optimization by Genetic Algorithms for systems from few W up to 5 MW power), which allow you to simulate the storage of hydrogen energy, RETScreen (Renewable Energy and Energy Efficiency Technology Screen), which takes into account only solar, wind and battery energy, TRNSYS, which allows you to simulate the transient behavior of an integrated system, as well as a number of other commercial and free platforms [21]. A variety of predictive methods are applied, such as artificial intelligence models [8], Univariate statistical time series models [ 8] [22], linear programming and iterative methods [23], machine learning [24, 25] or considering real projects [6,26]. Another stage in the development of managed HRES is the use of hybrid optimization methods, which take advantage of several methods and provide a synergistic effect in solving optimization problems and optimization methods based on artificial intelligence [21]. As can be seen, the ongoing evolution of energy systems provides an additional understanding of the structure of the microgrid and its individual components, at the same time, despite significant efforts in this direction, a number of questions remain open. Their unsatisfactory legislative regulation can be noted even in countries actively developing alternative energy [17,27]. There are disagreements in the national legislations of countries using renewable resources [28], the need for economic incentives for green energy [26], disregard for the role of the political situation [29], the need for an individual approach when developing a microgrid structure for a particular object [30], the impact of the business model of the energy market, environmental and economic factors on the choice of microgrid structure [31,32]. Economic, political, environmental factors, among which one can note the need to solve the problem of domestic and agricultural organic waste, as well as the requirement for a stable energy supply for individual facilities and areas, have led to the search for options for integrating biogas into the operation of microgrids. Biogas can be a potential source of energy, and many landfills, livestock farms, and wastewater treatment plants currently use biogas collection systems for its further utilization. If biogas is considered as one of the sources of renewable energy, then it is important to evaluate its use in the structure of microgrids. For further development of the microgrid concept, including using systems with incorporated biogas, it is necessary to analyze both the general state of the issue and the existing gaps in this direction with a Abbreviations AC alternating current BG biogas CHP combined heat and power system CCHP combined cooling, heat and power system COE cost of energy DC direct current HOMER Hybrid Optimization of Multiple Energy Resources NPC net present cost PV photovoltaics PC production capacity SC storage capacity TRNSYS Transient System Simulation Tool VRLA svalve regulated lead–acid V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 3 further search for ways to solve them. Biogas is a product of the anaerobic decomposition of organic waste and is burned to produce heat, electricity and carbon dioxide [33]. Various types of reactors are used to produce biogas, for example, with a main tank divided into two chambers (Indian model); with a bioreactor operating on the principle of a hydraulic press (Chinese model); covered lagoon biodigesters, upflow anaerobic sludge blanket reactor [34] and etc. The resource and technical capabilities of different countries for the production and use of biogas vary greatly, despite its role in achieving sustainable development goals. This introduces additional restrictions on the potential applicability of microgrids using biogas in these countries [35,36]. Socio-economic factors [37], the existence of a regulated biowaste market [38], as well as the technical training of personnel, also affect the possibility of installation, as well as the long-term and correct operation of the equipment [39]. 2.1. Overall description of microgrids incorporating biogas A number of microgrids that use biogas in their operations have been studied in this work. Technical parameters and features of individual microgrids, their biogas production capacities, biogas storage capacities, the ratio of these capacities to general parameters, as well as the main sources of biomass are presented in Table 1. Table 1 gives a visual representation of the structure and characteristics of microgrids incorporating biogas. It presents the main energy sources for the microgrids under study and their parameters, and also indicates possible operating modes and the software used. It should be noted that most of the considered projects of such microgrids are at the stages of studying the features of the interaction of individual elements with each other and testing this interaction, developing a virtual model, or, taking into account the physical and geographical location of the object, using specialized software, conditions were simulated and the optimal configuration was searched. Given the consideration of various options for equipment that uses the sun, water, wind, biogas as a source of alternative energy, it can be noted that, for most of the options considered, the simultaneous use of all these sources is not typical. The most characteristic option is the presence of solar PV, wind turbine, battery storage and the use of biogas in the microgrid structure. Alternative energy sources such as wind, water, sun, are characterized by the instability of the energy produced, so the use of biogas is a reasonable compromise to stabilize the energy supplied to cover the needs of the serviced facility. In turn, this causes the appearance of fuel cell and battery storage in a number of cases considered. The complication of the structure of such microgrids requires converters [45,46], also it offers the possibility of developing intelligent solutions for the optimal operation of such microgrids [40,60]. As a source of biogas, the authors often consider various types of organic waste or do not mention what acts as a raw material, focusing on studying the operation of the microgrid as a whole. The applicability of biogas is also not always voiced, in most cases it is considered as a source of electrical energy, although it can also be used for heating [50,54]. The amount of biogas produced in the system can range from 72 Nm 3 /day [40] to 650.5 Nm 3 /day [41]. The resulting biogas is then used both directly for the production of electrical energy [40], and for other purposes, for example, in cooking [41]. 2.2. The role of biomass in the operation of microgrids incorporating biogas In addition to biogas itself, biomass is used in microgrids. In this case, various options for its application are assumed. Biomass is used to produce bioelectricity and biofuels by gasification, pyrolysis, direct combustion, anaerobic digestion, fermentation, and chemical reactions [76]. It is noted that wood and wood waste are predominantly burned, while animal and human waste are more suitable for producing biogas [77]. Thus, in microgrids biomass can be converted into energy using a biomass gasifier. By incompletely burning biomass in a biomass gasifier, synthesis gas is produced, which is then burned to power a generator. This equipment is cheaper than solar PV; however, there are problems with resin accumulation and keeping the feedstock dry. Spark plug and balloon coil failure and battery discharge as noted in Ref. [78] are common for these systems. At the same time, the cost of maintaining a gasifier is significantly higher compared to solar panels. Thus, its total cost is 53 % fr the gasifier/PV/battery system, given that PV produces more energy (786,705 kWh/y) compared to the gasifier (766,500 kWh/y) [79]. When operating up to 10 h/day the power capacity of the biomass gasifier can reach to 30 kW. The raw material can be forestry and agricultural waste [41].The efficiency of the gasifier is 18 % (for rice straw gasification) [79]. Different biomass components are used for different energy production methods in a microgrid. Thus, in Ref. [68], solid waste (community waste) is used in biomass-based combined heat and power. Whereas biogas obtained from animal-droppings or kitchen-wastes is supplied to the biogas turbine generator. The biomass directly used to produce biogas is goat and cattle manure [55]. Human waste, livestock waste, and rice husks can also be used, as suggested in Ref. [46], as well as a separate wet fraction of municipal solid waste [80]. Biogas is generated due to the presence of a fermenter. To determine the optimal technical characteristics of a microgrid, a biomass generator emulator can be used. Work [44] shows the performance of a system where the main components are PV, a geothermal source, a biomass generator and a storage system. It is assumed that biogas is generated in the biomass generator. Some microgrid systems involve the use of a biomass electrical generation unit, where, depending on the type of biomass, it is supposed to produce biogas (food and human waste) or syngas (agricultural and forest wastes) [58]. These gases are then used in the microgrid to generate energy. It is noted that the energy potential of syngas and biogas generators is comparable - efficiency is 26 % and 27 %, respectively. Their total estimated electricity production is 132 kW/day (biogas) and 158 kW/day (syngas). Using the example of calculating microgrid parameters for the village of Sikkim (India, 2631 people), it is shown that despite the smaller occupied area (2.93 m 2 /kW), the capital cost of the system for production of syngas is more expensive (107,895,453 $) compared to biogas (13.44 m 2 /kW and 59,796 $). If there are separate plants for converting biogas and biomass in a microgrid system, their efficiency changes somewhat. Thus, when processing the wet and dry fractions of municipal solid waste, respectively, in a biogas production plant (32 kW) and a biomass gasifier (64 kW), biogas to electricity conversion efficiency is 33.5 %, while biomass to electricity conversion efficiency is 20 % [80]. 3. Discussion and analysis of microgrids using biogas The development of any microgrid assumes the presence of a real object (objects) for which this grid plans to be applied. This leads to the active use of simulation and optimization methods, which make it possible to determine the most appropriate (economically, environmentally) technical characteristics of a microgrid, and, in some cases, to test them in practice. In this section, from the theoretical and practical sides, the contribution of biogas to the operation of the microgrid is considered. An analysis of various mathematical models describing the production, storage, conversion or utilization of biogas in the microgrid structure is presented. Various ways of practical use of biogas in microgrids are considered. The technical parameters of other possible microgrid elements using biogas were also analyzed, as were some of their economic aspects. V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 4 Table 1 Main description and parameters of microgrids using biogas [40–75]. V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 5 V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 6 V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 7 V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 8 3.1. Biogas conversion models in microgrids In a number of works presented in Table 1, mathematical models of individual elements of microgrids are considered. In particular, in Refs. [40–43,46,49,51,55,58,61,62,65,69,72,73,75] proposed different models for biogas conversion. Thus, models for biogas production are considered in Refs. [43,62,65,72]. In Ref. [43], the authors take into account thermodynamic effects when forming the biogas digestion model. A multidimensional thermal network R–C of a typical underground cylindrical digester is proposed, taking into account the structure of the digester and the influence of external factors (air, soil). A model of a biogas reactor that takes into account the effect of temperature on biogas production is discussed in Ref. [72]. The total transfer function model from feed waste input to methane production was proposed by Ref. [62]. The authors present in detail the mathematical models of the four stages of biogas production (hydrolysis, acidogenesis, acetogenesis, methanogenesis) and then describe the total transfer function model. Authors [65] calculated the requirement for the minimum amount of biogas that must be produced to supply the biogas driven generator. Electricity production by the biogas driven generator, the period of operation, the volumetric methane content of biogas, the calorific energy content of the methane gas from the biogas and the overall gas to electricity conversion efficiency of the biogas driven generator are taken into account. Several models for biogas storage have been proposed by the authors [40,42,43,46]. Thus [43], take into account the hydrodynamic interactions between reservoir pressure and biogas loading/unloading flows, and propose a hydrodynamic network model to take into account biogas pressure dynamics in storage. The papers [40,42,46] proposed options for calculating the size of the biodigester. However, most of the considered works are characterized by the study of models for biogas utilization [40–42,49,51,55,58,61,62,69,73, 75]. Modeling of biogas plant, which takes into account electrical power generated from the biogas plant, is presented in Refs. [40,60]. A model of a biogas turbine generator system, in which biogas is used to generate electricity, in the form of a transfer function, is presented in Refs. [69, 73,75]. A model of a biogas system used to generate electricity in a microgrid, based on the calculation of the biogas availability, was proposed by a team of authors in Refs. [41,55]. The biogas fired internal combustion engine model presented in Ref. [49] includes fuel consumption functions and such restrictive parameters as generator power limits, its operation and idle time, and the number of generators. A mathematical model has been proposed to determine the electrical power of a biogas power plant and the energy of biogas produced from the manure of various animals [51] or other raw materials [58]. In the work [61], the system of equations describes the stages of biogas combustion in the presence of compressed air and power generation using combustion products passing through a gas turbine. Authors [62] modeled a biogas energy production system and presented the overall transfer function of the biomass energy conversion system. 3.2. Biogas conversion in microgrids According to Ref. [40] the main equipment of a biogas plant is an insulated chamber, a vibrating screen, a coarse and fine grinding combine, a digester (bioreactor), a floating gas tank, a gas cleaning system, a pipeline, a gas generator, and a control panel. The gas formed in the bioreactor is collected in a floating gas tank and then sent to the gas cleaning system. Moisture impurities and hydrogen sulfide are cleaned in the gas cleaning system and then sent to the gas generator. The volume of the digester is 35 cubic meters with a 15 kVA generator. The resulting biogas is supposed to be used to generate electricity, although the authors note the bioenergy potential of the system with the ability to produce 220.17 kWh of thermal energy. The same plant is used to generate electricity from biogas in a biogas power plant in the work of the authors [42]. V. Loboichenko et al. Renewable and Sustainable Energy Reviews 200 (2024) 114574 9 The methane contained in biogas can act as a source of thermal energy in the microgrid and be used for heating [50]. Combustion of biogas to produce heat and electricity is proposed in Refs. [56,61,72]. Using a solid oxide fuel cell, according to the authors [53] will improve the efficiency of this process. In [45,46,48,49,51,52,60,62,63,68,75] the considered model systems microgrids involve the use of a biogas generator to generate electrical energy. Microgrid using biogas not only as a source of electrical energy, but also for cooking is discussed in Ref. [47]. Biogas in a microgrid can be used not only to generate electricity, but also to produce hydrogen [57]. Mesophilic digestion of biogas in an anaerobic 600 m 3 underground cylindrical digester is considered for biogas production in Ref. [43]. The resulting biogas is loaded into a 600 m 3 biogas storage tank, and then can be used with optimal solutions for direct cooking and heating or cogeneration with combined cooling heating and power unit [43]. It is possible to obtain biogas from animal manure and plant residues, the resulting biogas is used for cooking, and the rest is used to generate electricity [41], [55], [67]. If there is an excess biogas in the system, methane gas (CH 4 ) is split into hydrogen and carbon atoms through several processes, and the resulting hydrogen is then stored as fuel for the fuel cell. Biogas is directly produced from biodegradable animal waste and excrement and used to generate electricity using a biogas turbine generator. [69]. Analysis of the quantitative data presented in Table 1 [40,43,48] allows us to speak about the predominant role of biogas as a storage system in a microgrid, regardless of the total storage capacity of the system as shown in Fig. 1(a) and (b). The BG SC values themselves range from 15 kWh to 184 kWh, varying more than 10 times depending on the total SC of the system. As can be seen from Fig. 1, the share of energy in the microgrid produced using biogas ranges from 12.3 % to 36.06 % [40,41,51,52,58]. Although boundary cases are also acceptable, where the contribution of biogas as an energy source in the microgrid can vary from 1 % [67] to 66,5 % [61]. At the same time, the authors choose the most optimal microgrid model for the given conditions, as shown in Fig. 2. Further, individual elements of microgrids using biogas are considered in more detail. The role of biogas in the energy production system varies somewhat. With total system PC from 1 MWh/day to 1200 MWh/day [40,41,58,66, 67] BG PC can vary in a very wide range - from 380 kWh/day to 12000 kWh/day in the studied microgrids what is shown in Fig. 3(a) and (b). If BG PC in the microgrids discussed in Table 1 is conventionally divided into three subranges – up to 5 MWh/day, up to 100 MWh/day, up to 500 MWh/day as shown in Fig. 4, then it can be noted that in 67 % of these microgrids BG PC does not exceed 5 MWh/day. Since the development of a microgrid takes into account the individual characteristics of the facility, it can be noted that the most popular are microgrid structures with a biogas production capacity of up to 5 MWh/ day. 3.3. Most common structural elements of a microgrid using biogas Solar panels are a widely used energy source today. They occupy one of the key places in the microgrid structure. Their total power is determined by the design of the microgrid and can take values from 500 W [62] up to 10 MW [81], and the optimal power, as well as the number of batteries, is selected by simulation methods, taking into account the needs of the facility. Рhotovoltaic modules are combined into a solar PV system. In this case, the solar radiation affecting the surface of the solar PV system is converted into electricity [41]. In Ref. [41] solar PV system consists of several PV modules. Modeling a photovoltaic system assumes the use of a single diode model of a photovoltaic module. Its operation provides 13 % of the total energy produced by the microgrid. A set of 314.5 MW parabolic trough solar collectors is in Ref. [64] part of an integrated biomethane/biomethanol cogeneration framework using biogas as feedstock. Polycrystalline type solar panels with a maximum rated power of 62.73 W are offered as part of a 10 MW solar power plant [51]. An inverter is used to convert the DC power generated by solar panels into AC power. In Ref. [44], twelve series-connected 250 W solar panels from ET-Solar form a photovoltaic array (2.8 kW), also connected to the microgrid via an inverter. By preventing water evaporation, being more energy efficient, and improving water quality, floating solar panels are considered more environmentally friendly. However, this can increase their cost by 10–40 % compared to ground-mounted PV installations [45]. Another basic element of microgrids is the wind turbine. Their parameters are modeled taking into account the wind regime and climatic features of the area, taking into account the height of the wind generator [40]. They can have power from 1 kW [45] up to 3400 kW [67], provide up to 13 % of the total energy produced by the microgrid [67]. Although its contribution may not exceed 2.5 % in general, it is a bladed turbine connected via an inverter to a microgrid [40]. So, 1 kW HAWT (horizontal axis wind turbine) wind turbine has a rotor diameter of 2.8 m, blade number - 3, which allows it to operate at wind speeds up to 40 m/s [40]. The water potential of the area, as well as the energy needs of the facility, obviously determine the use of a hydro turbine in the microgrid. In turn, the power of the hydro turbine itself is affected by the ratio of pressure and volumetric flow of water [41]. The capacity of hydro turbines in microgrids ranges from 40 kW [41] to 300 kW [58]. In each specific case, the power of hydrokinetic turbines and their number determine the optimal microgrid configuration. Microgrids may include fuel cells. In a microgrid, they can operate in several modes - both directly in the fuel cell mode and in the electrolyzer mode. So, in Ref. [48] unified regenerative solid oxide fuel cell can be fed by a pure hydrogen and a mixture of biogas (CH 4 and CO 2 ) at different ratios, varying according to the microgrid operating conditions, with all of these gases contained in a unique tank designed to store this mixture. Inclusion of an electrolyser and a fuel cell in a microgrid [69] allows to stabilize possible fluctuations in the energy generated by the system. So, if excess energy is generated in the system, then the electrolyzer produces hydrogen and stores it. In the event of a power shortage, the stored hydrogen is used as fuel in a fuel cell. The production discussed in Ref. [64] is an improved system for the simultaneous production of biomethane and biomethanol, where biomethane is obtained by cleaning biogas, and the hydrogen produced by electrolysis is used to produce biomethanol. An important element in ensuring stable operation of microgrids is the battery. The main applications are vanadium redox flow battery, Liion battery and lead acid battery. So, in Refs. [40,42] use the 1 kW/6 kWh vanadium redox flow battery whereas 12 V VRLA battery is used in the microgrid structure in Ref. [44]. In Ref. [45] substantiates the optimal combination of a microgrid with a second-life battery energy storage system based on lithium-ion batteries. Li-ion batteries are suggested in Ref. [49] (200 kW) and in Ref. [52] (3000 kWh). In addition to the basic equipment listed above, some technical elements can also be used in a microgrid. Thus, devices for converting several types of energy can be included in the structure of a microgrid. Namely, an electric heat pump, a furnace and an absorption chiller, which are used to control the output energy in the required quantities [43]. If the use of energy from geothermal sources is expected, then a geothermal generator (2 kW) may be included in the structure of the microgrid. It can be considered as a primary source of energy provided that the geothermal water flow is constant [44]. Some authors see a converter as part of additional equipment, the power of which can be 55 kW [46], 300 kW [58], 2700 kW [45], 6834 kW [67], 20,000 kW [52]. Sometimes, in order to reduce the dependence of the system on renewable energy sources and increase its reliability, the authors separately include a diesel generator [46,55] or biodiesel generator [47,68, 69,73,75] in the design of microgrids. To convert excess energy generated in a microgrid into hydrogen, an electrolyzer can be separately integrated into its structure [69]. In some microgrids, the authors V. Loboichenko et al.