There have been a couple of significant announcements regarding green hydrogen projects in Nepal:
Sunil Prasad Lohani, Andrew Blakers, 100% renewable energy with pumped-hydro-energy storage in Nepal, Clean Energy, Volume 5, Issue 2, June 2021, Pages 243–253, https://doi /10.1093/ce/zkab011
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Rural electrification is an important measure for prompt and sustainable growth of the developing nations. Providing electricity access to extreme remote localities is a challenging task for distribution utilities. Microgrids with renewable energy based distributed generation using locally available energy resources may be one of the effective solutions. This paper presents a study on recent developments in microgrid with the Hybrid Renewable Energy System (HRES). A brief discussion and analysis of modeling, control, reliability and energy management of microgrid with HRES is presented. A brief survey and examination of the potential of power generation with HRES in South-Asian nations is also undertaken in this research.
Utilization of wind and solar energy sources are significantly popular for generating electrical energy at remote rural localities [12, 13]. However, energy from these sources is available intermittently so far, therefore continuous power supply may not be ensured [14]. To overcome this drawback electrical power generated by these sources may be integrated along with energy storage devices such as battery. A combination of two or more than two renewable power generation technologies along with energy storage may enhance performance and is termed as hybrid renewable energy system (HRES) [15]; although it is necessary to choose appropriate renewable energy (RE) technology, unit size and energy storage system (ESS) to improve reliability and performance of HRES [16].
HRES along with energy storage, power electronic (PE) controllers and connected load is also referred as microgrid [16]. PE controllers coordinate and control outputs from various generating units of the HRES [22]. Apart from PE controllers, some hardware and software interfaces are also connected for the proper coordination and control of microgrid. The control action of microgrid includes load and energy management, voltage/frequency control, active/reactive power control, security monitoring, black start, etc. Another layer of control may be added to control mode of operation of microgrid i.e. stand alone, grid connected, etc. [16, 23].
HRES include different type of RE technology to generate electrical energy. So, it is necessary to have a defined scheme for interconnection of RE sources in HRES. Wide range of interconnection schemes has been reported in the literature. These schemes may be broadly classified into three categories, i.e. alternating current (AC), direct current (DC), and Hybrid AC-DC HRES (or more widely microgrid). This paper presents a brief insight on modeling, control, optimal sizing and potential of HRES among South-Asian countries [14, 24, 25].
Subsequent part of this paper is arranged as follows: An overview of distinct combination of HRES operational around the world is presented in "An Overview of HRES". Microgrid with various HRES configuration and its modeling and control aspects are discussed in "Microgrid Configuration, Modeling and Control". ESSs and their significance in HRES based microgrid are discussed in "Energy Storage System". Scope of microgrids with HRES for rural electrification in South-Asian context is discussed in "Scope of Power Generation with HRES in South-Asia". Finally conclusions are presented in "Conclusion".
HRES is an arrangement of RE technology for generating electrical power as a unit to meet the load demand [37, 38]. Several projects on HRES have been successfully completed with a generation capacity ranging from a few kilowatts to hundreds of megawatts, as shown in Table 2. Brief discussion on the different combinations of HRES is presented below:
Wind + PV: Wind and solar energies are intermittent energy source, as solar energy is available during daytime, whereas wind energy most often at night time [28]. The average capacity factor of wind forms and PV is about 40% and 20% respectively [39]. Combining solar and wind in an interconnected system may improve overall capacity factor of the HRES [40]. However, wind-PV HRES may not be very useful in standalone microgrid operations considering the intermittent characteristics of PV and wind.
PV + Wind + BESS: Wind-PV, HRES with BESS is a predictable and dependable source of renewable electric power generation [6, 41]. This combination is suitable in standalone microgrid operation. Wind turbines are available ranging from a few watts to several megawatts [27]. Similarly, solar panels can be integrated to generate electrical power of desired rating. One of the largest wind-PV-BESS, HRES project has been commissioned by China in 2011 [42]. BESS (Lithium-ion) capacity at the time of commissioning was 20 MW, which is extended to 36 MW.
Wind + PV + PHESS: The study suggests that the RE system with PHESS is technically feasible and potentially viable for continuous power supply in remote localities [43]. It is also suggested that PHESS has lower life cycle cost as compared to BESS considering increased storage capacity and days of autonomy as controlled variable [44]. However, for microgrid applications (< 30 MW power capacity), BESS is found to be more suitable [45, 46]. In addition to this PHESS is site specific and huge areas of natural landscapes are required for the installation of reservoirs [47, 48].
Wind + PV + Wave Power: HRES with wind-PV-wave energy is another viable option for standalone microgrid operation in remote coastal areas such as island. The study suggests that wave energy is more predictable than wind and solar energy; as short term forecasting error in wave energy (5–7% approx.) is low as compared to wind (22 % approx.) and solar energy (17 % approx.) [57]. Hence, combined HRES including wind-PV-wave energy may ensure continuous electrical power supply in standalone microgrid operations. A HRES including wind-PV-wave energy has been commissioned at Dangan, island, Guangdong China [56].
Geothermal + PV: Geothermal energy has many advantages such as high thermal efficiency, higher reliability, less land requirement, etc. [34]. First geothermal power plant of 0.25 MW capacity was installed in 1913. Still a major breakthrough in power generation technologies suitable for geothermal resources is desired. Grid connected, 59 MW HRES including geothermal + solar energy has been commissioned in Nevada, USA [58].
Solar PV + FC: FC power plants are drawing attention from the electric utility due to its modularity, fast response rate, high reliability and low emissions; Although FC utilizes hydrocarbon based fuels such as natural gas, methanol, bio-gas, coal, gas, diesel fuel, etc. [36]. Combined operation of FC with any intermittent RE source such as wind, solar, etc. may be considered for the eco-friendly, standalone microgrid operations [59,60,61].
Wind + Bio-Diesel + Battery Storage: Combination of diesel engine based EPG with intermittent RE sources such as wind, PV etc. may be another alternative for standalone microgrid operation [62]. Biodiesel may be used to replace conventional fossil fuels as bio-diesel is biodegradable, nontoxic, renewable and eco-friendly [63, 64]. Biodiesel can be produced by trans-esterification of oil extracted from edible/non-edible oil sources such as mustard, soyabean, sesame, coconut, micro-algae, jatropha, karanja etc. [65].
Biomass + CSP: Availability of primary fuel for biomass power plants varies seasonally and solar irradiance is during the daytime only; hence combined CSP and biomass HRES are preferable for standalone microgrid operation [66, 67]. First biomass + CSP HRES were commissioned in 2012 by Spain. This HRES utilizes solar irradiance during daytime by means of parabolic concentrating collector tracking and biomass energy during the nighttime [68]. There are many other possible combinations such as solar PV + small hydro [69, 70], wind + FC [71] etc. HRES should be cost effective, reliable and sustainable, so that it may satisfy the economic constraints [72]. Therefore an appropriate design and control strategy for HRES should be employed [16].
Microgrids are small-scale power system that consists of HRES, energy storage, power electronic interfaces (PEIs), AC/DC buses, AC/DC loads, system monitoring and control unit and software interfaces [73]. Figure 3 shows the schematic diagram of a micro-grid with HRES. Direction of energy and communication/control signal flow is shown by the arrows. A brief discussion on various configurations, modeling and control aspects of HRES based microgrid is presented as follows:
Electrical energy generated with HRES may be distributed and utilized in the form of AC/DC [74, 75]. Therefore, microgrid may have configurations categorized as AC, DC and Hybrid AC-DC as discussed below.
In AC microgrids output power from the HRES is connected to AC bus through power electronics converters [76, 77]. Power generated with wind, CSP, biomass, small hydro, geothermal is AC whereas FC, solar PV generates DC output power. Output power from individual renewable generator is converted to AC and finally synchronized with AC bus. AC output power from renewable sources such as wind may also need to be converted with PEI (Frequency and voltage level conversion) before synchronization with the AC bus [78]. Appropriate converters and controllers need be employed for the efficient operation.
In a DC microgrid RE sources are connected with a DC bus through PE converters [79,80,81,82]. Energy sources with DC output may directly be connected to the DC bus whereas AC sources are connected through PEI. DC microgrid configuration is simple and requires less controller as no synchronization is needed to integrate the RE sources [16].
Hybrid microgrid configuration consist of both AC and DC bus connected with PEI [25, 83, 84]. In this configuration DC and AC energy sources are connected to the DC and AC bus respectively. Hybrid microgrids are capable to feed DC as well AC loads so that minimum power conversion takes place [85]. Due to this reason, hybrid AC-DC microgrids are more efficient than AC and DC microgrid configurations [86].
As shown in Fig. 1, the communication/control signals are coordinated with monitoring and control unit of microgrids. HRES, energy storage and grid are interfaced with various PEIs. Flow of energy takes place from HRES to localized load through AC/DC bus, where as in case of energy storage and grid energy flow can take place in either direction. Therefore except PEI-I all other PEIs are bidirectional power electronic converters whereas PEI-I is unidirectional power converter.
Mainly AC-DC, AC-DC-AC/AC-AC, DC-DC and DC-AC power conversion needed for the smooth integration and operation of HRES based microgrid [15, 105, 106]. Monitoring and control unit controls the switching operation of PEIs as well as it regulates the power generation (i.e. maximum power point tracking), energy management and utilization [107]. The hierarchical control operation of microgrid with HRES can be divided as primary, secondary and tertiary level of control [23, 83, 108].
Primary control schemes deals with load sharing among RE generators. There are mainly two control strategies employed for primary control i.e. grid forming and grid following. Discussion on these two control strategies has been presented in [73, 83, 109].
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