Optimal Design of a Hybrid PV Solar / Micro-Hydro / Diesel / Battery Energy System for a Remote Rural Village under Tropical Climate Conditions

: Recently, o ﬀ -grid renewable power generation systems have become good alternatives for providing reliable electricity at a low cost in remote areas. According to the International Renewable Energy Agency, more than half the population of Nigerian rural communities are outside the electricity coverage area. This research examines the potential application of hybrid solar photovoltaic (PV) / hydro / diesel / battery systems to provide o ﬀ -grid electriﬁcation to a typical Nigerian rural village. The performance of four di ﬀ erent hybrid systems was evaluated via techno-economic and environmental analysis, and the optimized solution was selected using the HOMER analysis tool. The simulation results revealed that a hybrid PV solar / hydro / diesel with battery storage was the optimized solution and most suitable with the least net present cost (NPC) of $963,431 and a cost of energy (COE) of $0.112 / kWh. The results also revealed that the optimal system prevented about 77.1% of CO 2 gas emission from being released to the surrounding air as compared with the PV / diesel system (worst case). In addition, the results also showed better performance in technical aspects, making the system appropriate and ideal for rural electriﬁcation and clean energy development. Furthermore, the e ﬀ ects of varying some variables such as interest rate, solar radiation, water discharge, capacity shortage, and battery minimum state of charge on the system cost and operational performance were discussed via the sensitivity analysis because these parameters inﬂuence the economy and technical aspect of the power system.


Introduction
Various reforms have been established in the Nigerian power sector since the beginning of the democratic ruling by the Nigerian Power Supply Industry and Nigerian Electricity Regulatory Commission. These reforms led the country to privatize its electricity production and distribution companies in 2013 [1]. To some extent, these reforms have contributed to improving the existing power infrastructure to support the ever-growing electricity needs of the masses. Rural communities are generally dispersed; geographically isolated; faced with high illiteracy levels; and lack access to good health care delivery, clean water supply, and power connection. As a result, these areas have poor standards of living as compared with urban areas where the use of electricity is common [2], and many rural communities rely on diesel generator systems to supplement the unstable grid supply or on the local kerosene-powered lighting system. According to the International Renewable Energy Agency, more than half the population of Nigerian rural communities are out of the electricity coverage [3].
In addition, the environmental benefit of the latter technologies showed a reduction in CO 2 emission of approximately 7167 kg/year. This research aims to support the development and implementation of the hydroelectric system by providing techno-economic feasibility and environmental analysis to develop and establish a standalone hybrid PV/hydro/diesel power system with continuous power generation for an isolated rural area with a tropical climate. The simulation is performed based on the hydro resources available with the level of global solar radiation, and the diesel plant and battery storage are included because of the intermittent nature of RESs to ensure a continuous supply of power. Different system configurations were simulated and analyzed, in addition to conducting a sensitivity evaluation to check the economic as well as the operational performance of the HES over variation in some parameters such as the real interest rate, RE component (solar irradiation and hydro stream flow rate), capacity shortage, and battery minimum state of charge (SOC). The RE potentiality, in addition to the proper way of implementation in this part of the country, is also discussed to serve as a guide for implementing the Renewable Energy Master Plan (REMP) that has set a target of meeting at least 10% of the country's total energy consumption by RESs by the end of 2025 [32].

Potential Renewable Energy Resources in Nigeria
The endowment of waterfalls and large rivers in Nigeria can make hydropower resources the main sources of power production and continuous supply. However, out of about 277 potential locations for small hydroelectric applications with a gross output of around 734 MW, only 32 MW were exploited, representing only 4.3% of the available hydro capacity [5]. Consequently, hydro resources have not been fully utilized in Nigeria as SHP for power generation. Moreover, hydropower application is a viable option for electricity generation in the southern region because of the perennial rainfall with the required hydraulic heads that have been favored by the topology [31]. However, considering the dams that have already been built, the implementation of hydro systems should be more cost-effective with less civil work. In this area, the hydroelectric profile of the three major rivers (Otamiri, Bumaji, and Njaba rivers) as reported in [33,34] shows a peak season discharge rate and a total head in the range of 2.65 to 9.25 m 3 /s and 3 to 40 m, respectively. In addition, Nigeria's global solar irradiation is well distributed with a yearly daily average ranging from around 3.5 kWh/m 2 (12.6 MJ/m 2 /day) in the coastal area to 7.0 kWh/m 2 (25.2 MJ/m 2 /day) in the remote part of the northern region [35]. Nigeria has a large potential for solar energy application because of its observable abundance, i.e., about 3.8 × 10 23 kW of energy is emitted from the sun, corresponding to about 1.082 million tons of oil daily at a mean solar gain of 5.535 kWh/m 2 /day [5]. Solar photovoltaic (PV) is becoming a suitable energy option for countries with average solar irradiation in the range of 3 to 6 kWh/m 2 /day [36]. The solar radiation and hydropower potential in different states of Nigeria are presented in Figure 1.
From the aforementioned data, it is evident that a considerable amount of electricity can be produced if a small hydropower/PV system can be established in this area that can adequately serve remote rural areas across the country.

Methods and Material
Selecting suitable analysis criteria is crucial to properly evaluate the operational performance of various schemes in renewable energy-based systems. The following evaluation outlines were used in this study: specifications of the selected site, the proposed HES and load profile, design specification of the HES, hybrid system components and costs, and the mathematical model.

Specifications of the Selected Location
Mboke is a village located in the township of Ihiagwa that is situated 12 km south of Owerri in Nigeria's southern region, which was considered in this research. The area is about 235 miles south of the country's capital. The village is located at 5°24.3′ N and 7°0.7′ E and is characterized by a tropical wet monsoon climate. There is a power supply from the national grid, as electricity in this region is supplied by the Affam Genco power company [39]. The electricity supply is unstable and unreliable, with many residents who have been exposed to frequent blackouts [40]. Moreover, the educational centers, as well as a marketplace such as the Nkwo Ukwu' market, are among the minimal infrastructure and economic developments in this area, and their daily operation and services could be improved through off-grid electrification [41]. According to the Nigerian Economic Summit Group and Heinrich Böll Stiftung Nigeria, the average cost of a diesel generator is around $0.3/kWh [42], which has always been the alternative option for power supply in this area. The diesel system has high operational maintenance costs and poses a serious environmental threat through the combustion of fossil fuel [43]. This community is expected to be provided with electrification using renewable sources and a conventional energy system in an off-grid mode. The Otamiri River supports the residents with a potable water supply. According to [44], about 2.28 × 10 8 m 3 volume of water is available and around 1.89 MW of power can be produced from this river. Consequently, using a hydropower system is a feasible option for producing electricity at these locations.

Proposed Hybrid Generation System and Load Profile Analysis
The proposed hybrid system is comprised of four major components that consist of a PV system, hydroelectric system, batteries, and a diesel generator (DG), as illustrated in Figure 2. The application of a hybrid system for power generation is more cost-effective with high reliability than a single source system (hydro-alone/photovoltaic-alone systems). For instance, it was shown in [12] that the hybrid system could reduce the energy cost by about $0.369/kWh as compared with a PV-alone system. Moreover, the effect of global warming could be reduced when renewable and nonrenewable

Methods and Material
Selecting suitable analysis criteria is crucial to properly evaluate the operational performance of various schemes in renewable energy-based systems. The following evaluation outlines were used in this study: specifications of the selected site, the proposed HES and load profile, design specification of the HES, hybrid system components and costs, and the mathematical model.

Specifications of the Selected Location
Mboke is a village located in the township of Ihiagwa that is situated 12 km south of Owerri in Nigeria's southern region, which was considered in this research. The area is about 235 miles south of the country's capital. The village is located at 5 • 24.3 N and 7 • 0.7 E and is characterized by a tropical wet monsoon climate. There is a power supply from the national grid, as electricity in this region is supplied by the Affam Genco power company [39]. The electricity supply is unstable and unreliable, with many residents who have been exposed to frequent blackouts [40]. Moreover, the educational centers, as well as a marketplace such as the Nkwo Ukwu' market, are among the minimal infrastructure and economic developments in this area, and their daily operation and services could be improved through off-grid electrification [41]. According to the Nigerian Economic Summit Group and Heinrich Böll Stiftung Nigeria, the average cost of a diesel generator is around $0.3/kWh [42], which has always been the alternative option for power supply in this area. The diesel system has high operational maintenance costs and poses a serious environmental threat through the combustion of fossil fuel [43]. This community is expected to be provided with electrification using renewable sources and a conventional energy system in an off-grid mode. The Otamiri River supports the residents with a potable water supply. According to [44], about 2.28 × 10 8 m 3 volume of water is available and around 1.89 MW of power can be produced from this river. Consequently, using a hydropower system is a feasible option for producing electricity at these locations.

Proposed Hybrid Generation System and Load Profile Analysis
The proposed hybrid system is comprised of four major components that consist of a PV system, hydroelectric system, batteries, and a diesel generator (DG), as illustrated in Figure 2. The application of a hybrid system for power generation is more cost-effective with high reliability than a single source system (hydro-alone/photovoltaic-alone systems). For instance, it was shown in [12] that the Electronics 2020, 9, 1491 6 of 22 hybrid system could reduce the energy cost by about $0.369/kWh as compared with a PV-alone system. Moreover, the effect of global warming could be reduced when renewable and nonrenewable energy resources were used together to serve a particular load, as demonstrated in [13]. The proposed system configuration considered the 24 h load of a total of 250 households in the village. The village load profile was estimated based on the data presented by Akinyele [30]. The Nigeria southern region energy consumption pattern was considered in that study; thus, a similar power consumption pattern was used in this paper. The loads usually consisted of refrigerators, compact fluorescent lamps, fans, televisions, and a few other appliances. The total electricity demand of the site, therefore, was computed in HOMER as the hourly load data to obtain the daily and monthly load profiles for the whole year. The village hourly and yearly load profiles are presented in Figure 3a,b for an annual average daily load demand of 3375 kWh per day with a peak demand of 357 kW. energy consumption pattern was considered in that study; thus, a similar power consumption pattern was used in this paper. The loads usually consisted of refrigerators, compact fluorescent lamps, fans, televisions, and a few other appliances. The total electricity demand of the site, therefore, was computed in HOMER as the hourly load data to obtain the daily and monthly load profiles for the whole year. The village hourly and yearly load profiles are presented in Figure 3a,b for an annual average daily load demand of 3375 kWh per day with a peak demand of 357 kW.
In general, most of the loads operated for a few hours daily, while a high percentage of the electricity demand in the remote area was attributed to the lighting and cooling devices. The lighting load was mostly operated during the night hours, with the peak time between 19:00 and 22:00, while the cooling load varied according to the community's seasonal conditions. Additionally, there was low-energy demand in the morning from 7:00 to noontime because of the presence of the sunlight and low temperature, reducing the working hours of the lighting and fan loads. However, a 10% dayto-day and time step random variability was used in HOMER for better system reliability. An optimal design assessment of PV/diesel, PV/hydro/battery, PV/hydro/diesel/battery, and PV/diesel/battery was conducted to obtain optimal system configuration along with a sensitivity analysis to examine the system's operational behavior, while varying some parameters that could affect hybrid system performance.

Hybrid System Design Specifications
The monthly average solar irradiation and clearness index obtained from NASA [45], and shown in Figure 4, were inputted in HOMER as solar global horizontal irradiance data with annual average irradiation and a clearness index of 4.71 kWh/m 2 daily and 0.474, respectively. The results showed that the months from July to September recorded low daily radiation, as well as a low clearness index because of the high rainfall experienced during these periods, revealing that the PV power production level was smaller than in other months. In addition, the ambient temperature influenced the operational performance and the electricity generation level of the solar PV system; consequently, there was a need to consider temperature variation of the selected location. The average monthly In general, most of the loads operated for a few hours daily, while a high percentage of the electricity demand in the remote area was attributed to the lighting and cooling devices. The lighting load was mostly operated during the night hours, with the peak time between 19:00 and 22:00, while the cooling load varied according to the community's seasonal conditions. Additionally, there was low-energy demand in the morning from 7:00 to noontime because of the presence of the sunlight and low temperature, reducing the working hours of the lighting and fan loads. However, a 10% day-to-day and time step random variability was used in HOMER for better system reliability. An optimal design assessment of PV/diesel, PV/hydro/battery, PV/hydro/diesel/battery, and PV/diesel/battery was conducted to obtain optimal system configuration along with a sensitivity analysis to examine the system's operational behavior, while varying some parameters that could affect hybrid system performance.

Hybrid System Design Specifications
The monthly average solar irradiation and clearness index obtained from NASA [45], and shown in Figure 4, were inputted in HOMER as solar global horizontal irradiance data with annual average irradiation and a clearness index of 4.71 kWh/m 2 daily and 0.474, respectively. The results showed that the months from July to September recorded low daily radiation, as well as a low clearness index because of the high rainfall experienced during these periods, revealing that the PV power production level was smaller than in other months. In addition, the ambient temperature influenced the operational performance and the electricity generation level of the solar PV system; consequently, there was a need to consider temperature variation of the selected location. The average monthly temperature is depicted in Figure 5. The maximum ambient temperature occurred in April, the minimum temperature occurred in August, and the annual average temperature was 24.91 • C.

Hybrid System Design Specifications
The monthly average solar irradiation and clearness index obtained from NASA [45], and shown in Figure 4, were inputted in HOMER as solar global horizontal irradiance data with annual average irradiation and a clearness index of 4.71 kWh/m 2 daily and 0.474, respectively. The results showed that the months from July to September recorded low daily radiation, as well as a low clearness index because of the high rainfall experienced during these periods, revealing that the PV power production level was smaller than in other months. In addition, the ambient temperature influenced the operational performance and the electricity generation level of the solar PV system; consequently, there was a need to consider temperature variation of the selected location. The average monthly temperature is depicted in Figure 5. The maximum ambient temperature occurred in April, the minimum temperature occurred in August, and the annual average temperature was 24.91 °C.
The hydrological data of the Otamiri River considered in this research paper were taken from [46]. The hydrological graph, shown in Figure 6, showed that the chosen site experienced a highwater flow rate for considerable power generation from the hydropower turbine. The peak water discharge was observed in October, while a low stream flow of 6.14 m 3 /s was recorded in April. An annual average discharge of 7.67 m 3 /s was found. The nominal power of 94.08 kW was determined for the water turbine through the available head and design stream flow rate.    The hydrological data of the Otamiri River considered in this research paper were taken from [46]. The hydrological graph, shown in Figure 6, showed that the chosen site experienced a high-water flow rate for considerable power generation from the hydropower turbine. The peak water discharge was observed in October, while a low stream flow of 6.14 m 3 /s was recorded in April. An annual average discharge of 7.67 m 3 /s was found. The nominal power of 94.08 kW was determined for the water turbine through the available head and design stream flow rate.

Hybrid System Components and Costs
The input parameters and pricing of the various components used in this paper are given in Table 1. Different sizes of the system components were evaluated to find the best and appropriate size suitable for the location. Two dispatch strategies (DS), i.e., load following (LF), and cycle charging (CC), were strategically implemented for the energy management systems to properly investigate the hybrid system techno-economic performance. On the one hand, in the cycle charging strategy, the diesel generator runs at its maximum rated power to serve the load in addition to feeding the surplus power for charging the storage device. On the other hand, the diesel system operates just to sufficiently satisfy the load demand without charging the battery storage in the case of a loadfollowing strategy. The operating reserve as a percentage of load and renewable output was set at 10% of the load in the current time step, and 25% of solar power output, while 3% was selected for the maximum annual capacity shortage which was within the range recommended by [47]. The project lifetime is 25 years at a diesel fuel price of $0.63/l. and a discount rate of 13.5%.

Hybrid System Components and Costs
The input parameters and pricing of the various components used in this paper are given in Table 1. Different sizes of the system components were evaluated to find the best and appropriate size suitable for the location. Two dispatch strategies (DS), i.e., load following (LF), and cycle charging (CC), were strategically implemented for the energy management systems to properly investigate the hybrid system techno-economic performance. On the one hand, in the cycle charging strategy, the diesel generator runs at its maximum rated power to serve the load in addition to feeding the surplus power for charging the storage device. On the other hand, the diesel system operates just to sufficiently satisfy the load demand without charging the battery storage in the case of a load-following strategy. The operating reserve as a percentage of load and renewable output was set at 10% of the load in the current time step, and 25% of solar power output, while 3% was selected for the maximum annual capacity shortage which was within the range recommended by [47]. The project lifetime is 25 years at a diesel fuel price of $0.63/l. and a discount rate of 13.5%.

Modeling of a Hydropower System
The principle of power production by a hydro turbine is based on the transformation of the kinetic energy of falling water into mechanical power, which is, then, converted into electrical power by a generator. In HOMER, the below equation is used to determine the electrical power generated by the hydro turbine [12]: where η hyd is the efficiency of the hydro turbine system (%), ρ water the water density (1000 kg/m 3 ), Q turbine refers to hydro turbine stream flow rate in m 3 /s, g is the gravitational acceleration, and h net is the effective head in meters, and is computed in [48] as: where h denotes available head in meters and f h , the pipe head loss.
To determine the frictional losses, the Darcy-Weisbach equation given in [12] can be utilized for a circular pipe.
where h L denotes the absolute head loss as a result of friction (in units of length), f D , the Darcy friction factor, L denotes the pipe length in meters, D the pipe diameter (m), and V is the flow speed (m/s). There are various methods in which the Darcy friction factor f D can be determined, including the Moody diagram, which is a well-known diagram, as depicted in Figure 7.

Modeling of a PV System and Temperature
PV modules consisting of solar cells are used to produce electricity for different purposes (e.g., household load, street lighting, and water pumping). Furthermore, the solar insolation level and temperature, as well as the cell material of the photovoltaic, are the main factors that influence the energy production level of a PV panel [15]. The output power of the PV panel is estimated in HOMER as follows [14]: where YPV denotes the PV power output under standard test conditions (STC) in kilo watts, ƒPV refers to the PV derating factor in %, GT represents global solar irradiation incident on the PV module in the current time step in kilo watts per square meter, GT,STC is the incident radiation under STC (1 kilo watt per square meter), αp refers to temperature coefficient of power in %/°C, TC denotes the cell temperature of the PV system (°C), and TC,STC refers to TC at STC (25 °C). PV efficiency, however, decreases as the temperature rises. The derating factor of temperature is evaluated as [23]: where Tc,NOCT is the nominal operating cell temperature (°C), Ta,NOCT refers to the ambient temperature at NOCT (20 °C), IT,NOCT represents the solar irradiation at NOCT (0.8 kW/m 2 ), Ƞmp,STC denotes the efficiency of the PV panel under maximum power at STC (%), and αp represents the power temperature coefficient (%/°C).

Modeling of Economic Parameters
The net present cost (NPC) comprising the cost of replacement, initial cost, fuel cost, salvage cost, and the operational maintenance cost is computed using the following equation [43]:

Modeling of a PV System and Temperature
PV modules consisting of solar cells are used to produce electricity for different purposes (e.g., household load, street lighting, and water pumping). Furthermore, the solar insolation level and temperature, as well as the cell material of the photovoltaic, are the main factors that influence the energy production level of a PV panel [15]. The output power of the PV panel is estimated in HOMER as follows [14]: where Y PV denotes the PV power output under standard test conditions (STC) in kilo watts, f PV refers to the PV derating factor in %, G T represents global solar irradiation incident on the PV module in the current time step in kilo watts per square meter, G T,STC is the incident radiation under STC (1 kilo watt per square meter), α p refers to temperature coefficient of power in %/ • C, T C denotes the cell temperature of the PV system ( • C), and T C,STC refers to T C at STC (25 • C). PV efficiency, however, decreases as the temperature rises. The derating factor of temperature is evaluated as [23]: where T c,NOCT is the nominal operating cell temperature ( • C), T a,NOCT refers to the ambient temperature at NOCT (20 • C), I T,NOCT represents the solar irradiation at NOCT (0.8 kW/m 2 ), η mp,STC denotes the efficiency of the PV panel under maximum power at STC (%), and α p represents the power temperature coefficient (%/ • C).

Modeling of Economic Parameters
The net present cost (NPC) comprising the cost of replacement, initial cost, fuel cost, salvage cost, and the operational maintenance cost is computed using the following equation [43]: where TAC is the total annualized cost in $ per year; n and i refer to the project lifetime in years and the annual interest rate in percent, respectively; and the capital recovery factor (CRF) is given in [25] as a function of n and i: The levelized cost of energy (COE) is the average cost per kilowatt hour of effective power generated by the system and is given in [49] as: where E anloadserved refers to the gross annual load (kWh) served.

Results and Discussion
In this analysis, the HOMER simulation tool was used to determine the best system configuration through a techno-economic and environmental evaluation corresponding to the selected remote location in Nigeria. A sensitivity evaluation was performed to determine system performance, while varying variables that directly affect the operational behavior of hybrid systems. The procedure for the economic evaluation to obtain the optimal configuration is illustrated in Figure 8. The maximum capacity shortage of 3% was set for 25 years of project lifespan and a diesel fuel price of $0.63/L [50].
Electronics 2020, 9, x FOR PEER REVIEW 11 of 22 where TAC is the total annualized cost in $ per year; n and i refer to the project lifetime in years and the annual interest rate in percent, respectively; and the capital recovery factor (CRF) is given in [25] as a function of n and i: The levelized cost of energy (COE) is the average cost per kilowatt hour of effective power generated by the system and is given in [49] as: where Eanloadserved refers to the gross annual load (kWh) served.

Results and Discussion
In this analysis, the HOMER simulation tool was used to determine the best system configuration through a techno-economic and environmental evaluation corresponding to the selected remote location in Nigeria. A sensitivity evaluation was performed to determine system performance, while varying variables that directly affect the operational behavior of hybrid systems. The procedure for the economic evaluation to obtain the optimal configuration is illustrated in Figure  8. The maximum capacity shortage of 3% was set for 25 years of project lifespan and a diesel fuel price of $0.63/L [50].

Optimization Results
In this section, we discuss the optimization results of the power system produced at a fuel price of $0.63/L and a discount rate of 13.5%. The hybrid system configuration obtained from the simulation analysis is shown in Table 2. The configuration includes PV/diesel, PV/hydro/battery, PV/hydro/diesel/battery, and PV/diesel with battery system configurations. The table reveals that PV/diesel/micro-hydro/battery configuration is a more economically optimal solution than other models and that the PV/diesel configuration has the worst economic prospect. The optimal system consists of a 50 kW PV, 94.1 kW hydro turbine with 111 kWh nominal battery capacity, 100 kW DG capacity, and a 50 kW converter. The operating cost for this optimized configuration is $83,667/year with a COE of $0.112/kWh and an NPC of $963,431 and an initial capital cost of $369,820. The results also indicate that the optimal configuration reduces the operating cost, COE, and NPC by 76%, 64%, and 64.3% as compared with the PV/diesel system (worst case), respectively. The renewable fraction representing the contribution of RESs in the total power generated by the entire system was found to be 77.2%. This result shows that the system produces more energy from renewable power sources with the additional energy supplied by the diesel system. The outcome also shows that dispatch strategy significantly affected the renewable penetration rate. For example, when the simulation was conducted for the optimal system using CC and LF strategy, the renewable fraction was found to be 77.2% and 81.6%, respectively, for the same load demand. The optimal dispatch strategy for the proposed system was found to be cycle charging, as shown in Table 2. The optimized system produces an annual total of 1,518,895 kWh of energy, whereas the hydro turbine system contributes about 1,178,600 kWh/year which represents around 77.6% of the total production. The diesel plant system supplies a share of 18.3% (277,222 kWh/year), with the extra electricity provided by the PV system at a share of 4.15%. Figure 9 shows the monthly power generation of each component.   The cash flow summary, presented in Figure 11, reveals that both capital and fuel costs contribute to the highest share of the total system cost. The high capital cost was because of the amount of money spent on the installation of the hydro and PV system, whereas the fuel cost was attributed to the generator operating conditions. The maintenance of the moving part of the diesel Furthermore, the PV/hydro/battery configuration shows 100% renewable penetration with no fuel consumed because of the absence of a diesel plant. This system is superior in terms of environmental aspects because no pollutant emissions are reported. However, it has economic negatives because the NPC and COE are 125% and 127% more than that of the optimal configuration. Moreover, the optimized system had a small unmet electric load of 15,412 kWh/year, which was only 1.25% with a capacity shortage of 35,816 kWh/year (2.91%) of total capacity. However, the capacity shortage was considered to be acceptable, showing that the system had the maximum uptime and was able to satisfy most of its electrical load.
Concerning the performance of the diesel plants, the outcome revealed that the optimal system had the lowest annual consumption of 76,492 L of fuel, at 8.73 L/h, as shown in Figure 10, when operated for about 4,705 h per year. The diesel fuel consumption of this system was 77.1% less than that of the PV/diesel configuration (worst case). The generator operational life was found to be 3.19 years because it was considered to have a lifespan of 15,000 h [51].  The cash flow summary, presented in Figure 11, reveals that both capital and fuel costs contribute to the highest share of the total system cost. The high capital cost was because of the amount of money spent on the installation of the hydro and PV system, whereas the fuel cost was attributed to the generator operating conditions. The maintenance of the moving part of the diesel plant also led to high operation and maintenance (O&M) costs of around $174,075. The replacement cost, however, was reported to be a low value of $78,515, because the renewable power systems incurred no cost from replacement, since their lifetime was the same as that of the project's lifetime.

Environmental Analysis
The production of greenhouse gases (GHGs) and pollutant emissions in hybrid power systems are attributed to the fuel consumption level of the DG system. In this paper, CO2 and other GHGs emissions were used to determine the environmental effect of the HESs. The concentration of pollutants and GHGs produced is directly related to global warming, and a trillion tons of carbon  The cash flow summary, presented in Figure 11, reveals that both capital and fuel costs contribute to the highest share of the total system cost. The high capital cost was because of the amount of money spent on the installation of the hydro and PV system, whereas the fuel cost was attributed to the generator operating conditions. The maintenance of the moving part of the diesel plant also led to high operation and maintenance (O&M) costs of around $174,075. The replacement cost, however, was reported to be a low value of $78,515, because the renewable power systems incurred no cost from replacement, since their lifetime was the same as that of the project's lifetime.  The cash flow summary, presented in Figure 11, reveals that both capital and fuel costs contribute to the highest share of the total system cost. The high capital cost was because of the amount of money spent on the installation of the hydro and PV system, whereas the fuel cost was attributed to the generator operating conditions. The maintenance of the moving part of the diesel plant also led to high operation and maintenance (O&M) costs of around $174,075. The replacement cost, however, was reported to be a low value of $78,515, because the renewable power systems incurred no cost from replacement, since their lifetime was the same as that of the project's lifetime.

Environmental Analysis
The production of greenhouse gases (GHGs) and pollutant emissions in hybrid power systems are attributed to the fuel consumption level of the DG system. In this paper, CO2 and other GHGs emissions were used to determine the environmental effect of the HESs. The concentration of pollutants and GHGs produced is directly related to global warming, and a trillion tons of carbon Cash flow summary

Environmental Analysis
The production of greenhouse gases (GHGs) and pollutant emissions in hybrid power systems are attributed to the fuel consumption level of the DG system. In this paper, CO 2 and other GHGs emissions were used to determine the environmental effect of the HESs. The concentration of pollutants and GHGs produced is directly related to global warming, and a trillion tons of carbon emitted can cause a peak warming of around 2 • C [8]. Ngan and Tan [52] recommended maintaining the amount of carbon release below a trillion tons to limit the effect of global warming. The type and amount of harmful pollutant emissions (CO 2 , CO, SO 2 , NO x , unburned hydrocarbons, and particulate matter) produced by the optimized system and PV/diesel configuration (worst case) are shown in Table 3. The optimum configuration was considered to be environmentally friendly because it emitted the least pollutant gas as compared with the PV/diesel system and other system designs analyzed. The results also revealed that the optimal system prevented about 77.1% of CO 2 gas emission from being released to the surrounding air as compared with the PV/diesel system (worst case). The table shows that the prime contributor to unhealthy atmospheric air in this location is carbon dioxide (CO 2 ), followed by carbon monoxide, while particulate matter contributes the least to total pollutant emissions [51].

Sensitivity Assessment
Sensitivity analysis was performed on the optimized configuration by varying some variables that could directly affect the cost and its overall performance. In this section, the following parameters were analyzed: real interest rate, RE component (solar irradiation and hydro stream flow rate), capacity shortage, and battery minimum SOC.

Real Interest Rate
The interest rate (RIR) is a function of both the nominal discount and expected inflation rates that are utilized in HOMER to compute from NPC, the discount factors, and annualized costs. The effects of varying this parameter from 12% to 18% on the operating cost, NPC, and COE are shown in Figure 12. The figure reveals that this variable is linearly related to the COE, as given in Equation (8), because the interest rate increases with an increase in total annualized cost (TAC), which is directly proportional to COE. However, the total NPC and operating cost decreases linearly as the interest rate increases. The COE increases from $0.108/kWh for a 12% interest rate to $0.124/kWh as the interest rate rises to 18%. The NPC decreases by approximately 20%, while the operating cost is affected slightly with about a 0.69% reduction, as the interest rate increases from 12% to 17%. Again, an increase in the interest rate by 50% is found to increase the COE by about 14.8%.

Renewable Energy Component Parameters
The level of global solar irradiation (GSI) and availability of water stream flow (WSF) rate in a place determines the periods it will take for the PV and hydro system to meet the load requirement. High solar insolation and water discharge reduce the working hours of DG, as well as the fuel consumption rate, and vice-versa. Furthermore, the amount of electricity contributed by RE systems (PV and hydro) depends on the level of streamflow and solar radiation and intensity; thus, varying these parameters can affect the system's operational performance and cost. In this case, the annual average global solar irradiation was increased from 4.71 to 9.71 kWh/m 2 daily at an interval of 1 kWh/m 2 /day, whereas the average annual streamflow was varied in the range of 7.67 to 8.67 m 3 /s with an increasing step of 0.2 m 3 /s. The effects of varying solar radiation on the COE, NPC, and operating costs are illustrated in Figure 13, and Figure 14 presents a graph of NPC, operating cost, and COE as a function of water discharge variation. As indicated in Figure 13, the NPC and COE decrease by only 0.76% and 0.89%, respectively, whereas the operating cost reduces by about 4.2% when the level of global solar radiation increased from 4.71 to 9.71 kWh/m 2 /day in the HES. Again, similar to solar radiation, Figure  14 shows that the NPC decreased linearly from $963,431 to $920,009 (4.5% reduction) along with the COE that reduced from $0.112/kWh to $0.107/kWh (4.46% decrease), and the operating cost reported the highest reduction of 8.8%, as the water discharge rate increased from 7.67 m 3 /s to 8.67 m 3 /s. The amount of decrease in NPC and COE in both cases could be justified by the reduction in diesel consumption, as shown in Figures 15 and 16, as well as the cost related to diesel. Moreover, the renewable penetration rate increased as the GSI and WSF increased. The results also show, in the case of GSI, that the COE remains constant for solar irradiation above 5.51 kWh/m 2 /day. The results clearly show the impact of these parameters on the overall system cost as well as the electricity cost.

Renewable Energy Component Parameters
The level of global solar irradiation (GSI) and availability of water stream flow (WSF) rate in a place determines the periods it will take for the PV and hydro system to meet the load requirement. High solar insolation and water discharge reduce the working hours of DG, as well as the fuel consumption rate, and vice-versa. Furthermore, the amount of electricity contributed by RE systems (PV and hydro) depends on the level of streamflow and solar radiation and intensity; thus, varying these parameters can affect the system's operational performance and cost. In this case, the annual average global solar irradiation was increased from 4.71 to 9.71 kWh/m 2 daily at an interval of 1 kWh/m 2 /day, whereas the average annual streamflow was varied in the range of 7.67 to 8.67 m 3 /s with an increasing step of 0.2 m 3 /s.
The effects of varying solar radiation on the COE, NPC, and operating costs are illustrated in Figure 13, and Figure 14 presents a graph of NPC, operating cost, and COE as a function of water discharge variation. As indicated in Figure 13, the NPC and COE decrease by only 0.76% and 0.89%, respectively, whereas the operating cost reduces by about 4.2% when the level of global solar radiation increased from 4.71 to 9.71 kWh/m 2 /day in the HES. Again, similar to solar radiation, Figure 14 shows that the NPC decreased linearly from $963,431 to $920,009 (4.5% reduction) along with the COE that reduced from $0.112/kWh to $0.107/kWh (4.46% decrease), and the operating cost reported the highest reduction of 8.8%, as the water discharge rate increased from 7.67 m 3 /s to 8.67 m 3 /s. The amount of decrease in NPC and COE in both cases could be justified by the reduction in diesel consumption, as shown in Figures 15 and 16, as well as the cost related to diesel. Moreover, the renewable penetration rate increased as the GSI and WSF increased. The results also show, in the case of GSI, that the COE remains constant for solar irradiation above 5.51 kWh/m 2 /day. The results clearly show the impact of these parameters on the overall system cost as well as the electricity cost.

Maximum Annual Capacity Shortage (MACS)
The maximum annual capacity shortage (MACS) shortage is the maximum allowable amount of the total deficit that occurs between the required capacity and the actual capacity the system can supply divided by the total load, whereas the capacity shortage fraction is a function of total capacity shortage and the total electrical load [48], and any system having a higher amount of this fraction is considered to be unacceptable. The results can be altered in some cases if a certain amount of capacity shortage is allowed, such as the occurrence of a very high peak demand lasting for a very short duration. However, selecting zero for the maximum annual capacity shortage means that the system must serve the electric load at all times including the very high peak demand.
Givler and Lilienthal [47] recommended varying this parameter within a range of 0.5% and 5% to determine the performance and cost-effectiveness of a RE system. In this part, the maximum annual capacity shortage was allowed to vary between 0% (in which the system must serve 100% of the load at all times) and 5% (in which the system must serve 95% of the load at all times) to check the operational and economic behavior of the HES. Figures 17 and 18

Renewable fraction
Fuel consumption

Maximum Annual Capacity Shortage (MACS)
The maximum annual capacity shortage (MACS) shortage is the maximum allowable amount of the total deficit that occurs between the required capacity and the actual capacity the system can supply divided by the total load, whereas the capacity shortage fraction is a function of total capacity shortage and the total electrical load [48], and any system having a higher amount of this fraction is considered to be unacceptable. The results can be altered in some cases if a certain amount of capacity shortage is allowed, such as the occurrence of a very high peak demand lasting for a very short duration. However, selecting zero for the maximum annual capacity shortage means that the system must serve the electric load at all times including the very high peak demand.
Givler and Lilienthal [47] recommended varying this parameter within a range of 0.5% and 5% to determine the performance and cost-effectiveness of a RE system. In this part, the maximum annual capacity shortage was allowed to vary between 0% (in which the system must serve 100% of the load at all times) and 5% (in which the system must serve 95% of the load at all times) to check the operational and economic behavior of the HES. Figures 17 and 18

Renewable fraction
Fuel consumption Figure 16. Effect of WSF on the renewable fraction and annual fuel consumption.

Maximum Annual Capacity Shortage (MACS)
The maximum annual capacity shortage (MACS) shortage is the maximum allowable amount of the total deficit that occurs between the required capacity and the actual capacity the system can supply divided by the total load, whereas the capacity shortage fraction is a function of total capacity shortage and the total electrical load [48], and any system having a higher amount of this fraction is considered to be unacceptable. The results can be altered in some cases if a certain amount of capacity shortage is allowed, such as the occurrence of a very high peak demand lasting for a very short duration. However, selecting zero for the maximum annual capacity shortage means that the system must serve the electric load at all times including the very high peak demand.
Givler and Lilienthal [47] recommended varying this parameter within a range of 0.5% and 5% to determine the performance and cost-effectiveness of a RE system. In this part, the maximum annual capacity shortage was allowed to vary between 0% (in which the system must serve 100% of the load at all times) and 5% (in which the system must serve 95% of the load at all times) to check the operational and economic behavior of the HES. Figures 17 and 18 show that allowing a certain percentage of capacity shortage can significantly affect the system's total NPC and unserved annual load, as well as the energy cost and operating cost. The NPC and COE of the system reduce from $1,069,184 to $915,297 and from $0.122/kWh to $0.108/kWh, respectively, when the MACS rises from 0% to 5%. In addition, this increase causes a 17.6% reduction in operating cost, while the unfulfilled load increases from 0.018% to 2.68%.
Electronics 2020, 9, x FOR PEER REVIEW 18 of 22 load, as well as the energy cost and operating cost. The NPC and COE of the system reduce from $1,069,184 to $915,297 and from $0.122/kWh to $0.108/kWh, respectively, when the MACS rises from 0% to 5%. In addition, this increase causes a 17.6% reduction in operating cost, while the unfulfilled load increases from 0.018% to 2.68%.  The battery system minimum state of charge (SOCmin) is the relative charge state below which the storage system is never drawn. Most storage batteries that are rechargeable are expected not to be completely discharged, otherwise, such action can permanently damage the batteries. The percentage range from 30 to 50 is normally selected for the battery SOC to avoid an excessive discharge that could damage the battery bank and to increase the battery's lifetime [49]. In this analysis, the effect of varying the SOCmin in the range of 20% to 50% in 5% increments on the hybrid system operational performance and the total cost was evaluated. The effect of increasing the SOCmin on the total NPC, COE, operating cost, and fuel consumption rate is illustrated in Figures 19 and 20     The battery system minimum state of charge (SOCmin) is the relative charge state below which the storage system is never drawn. Most storage batteries that are rechargeable are expected not to be completely discharged, otherwise, such action can permanently damage the batteries. The percentage range from 30 to 50 is normally selected for the battery SOC to avoid an excessive discharge that could damage the battery bank and to increase the battery's lifetime [49]. In this analysis, the effect of varying the SOCmin in the range of 20% to 50% in 5% increments on the hybrid system operational performance and the total cost was evaluated. The effect of increasing the SOCmin on the total NPC, COE, operating cost, and fuel consumption rate is illustrated in Figures 19 and 20  The battery system minimum state of charge (SOC min ) is the relative charge state below which the storage system is never drawn. Most storage batteries that are rechargeable are expected not to be completely discharged, otherwise, such action can permanently damage the batteries. The percentage range from 30 to 50 is normally selected for the battery SOC to avoid an excessive discharge that could damage the battery bank and to increase the battery's lifetime [49]. In this analysis, the effect of varying the SOC min in the range of 20% to 50% in 5% increments on the hybrid system operational performance and the total cost was evaluated. The effect of increasing the SOC min on the total NPC, COE, operating cost, and fuel consumption rate is illustrated in Figures 19 and 20 The results show that a rise in the SOC min from 20% to 50% causes the NPC and COE to increase by 1.59% and 1.8%, while the operating cost and annual consumption increase from $79,093/yr. to $83,658/yr. and from 62,812 L to 76,479 L of fuel. The increase in the annual liters of fuel consumed shows that the system relies more on the DG in meeting the load demand whenever the battery minimum SOC increases. In addition, the results show that increasing the SOC min poses a significant environmental challenge, as more carbon emission would be produced because of its direct relationship with the amount of fuel in-take by the diesel plant.
Electronics 2020, 9, x FOR PEER REVIEW 19 of 22 The results show that a rise in the SOCmin from 20% to 50% causes the NPC and COE to increase by 1.59% and 1.8%, while the operating cost and annual consumption increase from $79,093/yr. to $83,658/yr. and from 62,812 L to 76,479 L of fuel. The increase in the annual liters of fuel consumed shows that the system relies more on the DG in meeting the load demand whenever the battery minimum SOC increases. In addition, the results show that increasing the SOCmin poses a significant environmental challenge, as more carbon emission would be produced because of its direct relationship with the amount of fuel in-take by the diesel plant.

Conclusions
In this article, we present an optimal design and environmental evaluation of an off-grid hybrid energy system meant to power a rural village in Nigeria. The HOMER analysis tool was employed for the economic evaluation of the proposed energy supply system along with the technical feasibility based on the energy sources available in the southern part of the country. Four different system configurations from a combination of hydro, DG, battery storage, and PV generation system were Operating cost Fuel consumption Figure 19. Impact of the battery minimum state of charge (SOC min ) on the system total NPC and COE.
Electronics 2020, 9, x FOR PEER REVIEW 19 of 22 The results show that a rise in the SOCmin from 20% to 50% causes the NPC and COE to increase by 1.59% and 1.8%, while the operating cost and annual consumption increase from $79,093/yr. to $83,658/yr. and from 62,812 L to 76,479 L of fuel. The increase in the annual liters of fuel consumed shows that the system relies more on the DG in meeting the load demand whenever the battery minimum SOC increases. In addition, the results show that increasing the SOCmin poses a significant environmental challenge, as more carbon emission would be produced because of its direct relationship with the amount of fuel in-take by the diesel plant.

Conclusions
In this article, we present an optimal design and environmental evaluation of an off-grid hybrid energy system meant to power a rural village in Nigeria. The HOMER analysis tool was employed for the economic evaluation of the proposed energy supply system along with the technical feasibility based on the energy sources available in the southern part of the country. Four different system configurations from a combination of hydro, DG, battery storage, and PV generation system were analyzed during the simulation process to obtain the most suitable and least expensive option, while

Conclusions
In this article, we present an optimal design and environmental evaluation of an off-grid hybrid energy system meant to power a rural village in Nigeria. The HOMER analysis tool was employed for the economic evaluation of the proposed energy supply system along with the technical feasibility based on the energy sources available in the southern part of the country. Four different system configurations from a combination of hydro, DG, battery storage, and PV generation system were analyzed during the simulation process to obtain the most suitable and least expensive option, while considering the environmental effect of each system design. The simulation results reveal that the hybrid system with 50 kW PV panels, 94.1 kW hydro system with 111 kWh nominal battery capacity, 100 kW DG capacity, and a 50 kW power converter is the most economically feasible option for this location as compared with the other models, with the lowest NPC and COE being $963,431 and $0.112/kWh, respectively.
In addition, the area shows a high potential for RE projects, as the RE penetration stands at a high rate of 77.2% that has reduced the operating hours and consumption of the diesel plants to 4,705 h/yr. and 76,492 L of fuel; thus, this system is appropriate for the environment because it helps to maintain a clean and safe atmosphere because it has low pollutants and GHGs.
The sensitivity assessment conducted on the optimum configuration shows that parameters such as the real interest rate, RE component (solar irradiation and hydro stream flow rate), capacity shortage, and battery minimum SOC have significant effects on the costs and operational performance of the system. The obtained results show that the total NPC and electricity cost varies whenever these parameters change at different rates, showing the sensitivity of the HES parameters toward variations in these variables. Lastly, the results of this work should assist in the implementation of rural electricity framework plans and increase the RE share in the total energy mix, supporting the limitation of the global warming effect. Further research is required involving the feasibility study of solar, wind, and hydro resources considering the six geopolitical zones of Nigeria.
Author Contributions: All authors contributed equally. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.