1. Off-Grid Hybrid Energy System optimization results

In this design, the standalone system comprises a diesel generator (DG), wind turbine, PV arrays, batteries, and a converter. These components are interconnected together to meet the energy consumption requirement of the system. The simulation of four different scenarios with various integrations of power sources was performed by HOMER; namely: PV/wind/battery (System 1), PV/DG (System 2), PV/DG/battery (System 3), and DG-only (System 4). Based on the dispatch strategy employed, the RESs are the main sources of the HPS. The diesel generator is operated as a back-up generation source whenever the RESs and/or the storage system are incapable of meeting the energy demand of the load. Several component combinations are selected by HOMER as the optimum system regarding NPC and COE, as reported in Table 1. The result indicates that the optimum configuration (System 3) consists of a 21-kW power converter, 54  kW PV, 70 battery group (484 kWh nominal capacity battery bank), and a 12-kW diesel generator. The NPC, COE, Operating Cost, and Renewable Fraction (RF) are $259,354–385,555, 0.218– 0.323$/kWh, 2292–22,968$/, and 0%–100% respectively, when the capacities of the wind turbine system, diesel generator, battery storage, and PV panel are 0–1 kW, 0–20 kW, 0–761 kWh, and 0–76 kW, respectively. The result also shows that the optimum system reduces CO₂ emissions by 51,123 kg/year in comparison to the diesel-only system (System4) used in the current situation as shown in Table 2.

To meet the load requirement, the generator system runs for 1301 h/year and contributes to 8.51% (8,152 kWh/year) of the overall production, which leads to fuel consumption of 2278 L/year with a rate of 0.279 L per kWh. The annual generated energy of the PV is 87,661 kWh with a COE of $0.0623/kWh.  

The fact that the surplus energy produced by the off-grid system cannot be sold to the national utility grid poses a serious challenge for off-grid HPS. In this case, a small percentage of excess electricity is generated. It constitutes about 6% (5741 kWh/year) of the overall electricity production. However, the system can be connected to the national grid to sell back excess energy; in this way, all the excess electricity can be consumed, in addition to increasing the system’s earnings.

Table 1. Summarized optimization results of multiple optimum configurations.

Table 4. Pollutant emissions produced by the optimum hybrid system.

1.1 Details of the NPC of the optimized PV/battery/diesel hybrid system by cost type are illustrated below. A cost analysis of different cost types as a function of each component is given as follows:

• The initial capital cost of the PV is around 46% of the overall capital cost of the system, closely followed by battery storage, which constitutes around 44% of the overall capital cost. The converter and diesel generator contributed about 6.44% and 3.43%, respectively. The total capital cost of the system is calculated to be $174,792, with the cost of the PV system accounting for most of the cost in this scenario.
• The total replacement cost of the system is $69,138. The PV system has no replacement cost since the lifetime of this component is 20 years, which corresponds to the project’s lifetime. The replacement cost is, however, the sum of the converter, batteries, and diesel generator cost of replacement, where the storage system contributes to about 84% of the total cost due to its short- term operational lifetime compared to the overall system lifetime. The battery bank expected life and the diesel generator operational life are 14.3 years and 11.5 years respectively.
• The overall operating, salvage and fuel costs of the proposed system are estimated to be $28,280, $36,322, and $23,465, respectively.

1.2 Battery storage:

The inclusion of battery storage in any off-grid system can increase the renewable penetration rate and reduce excess energy at the expense of increasing the COE and capital cost. Furthermore, batteries are used to support load requirements during times when the output power of RESs is insufficient, thereby improving system reliability as well as mitigating the variability associated with the RE system. Battery autonomy refers to the periods of time (in hours) during which the battery can adequately supply the load without needing to recharge. In this paper, one string of battery storage forms 2.49 h of autonomy with a nominal capacity of 34.6 kWh. In the charging and discharging process of the battery, the use of a traditional control method leaves some parameters out of the user’s control. This results in the aging of the batteries in addition to decreasing the battery life cycle thereby causing irreversible battery damage. However, the introduction of other control methods such as model predictive and fuzzy logic controls in battery charging management for renewable energy systems can reduce the charging time, avoid deep discharging through the maintenance of the state of charge (SOC) above 50%, as well as protect the battery from wear and tear [51] The monthly average SOC of the battery shows that the months of July through to September recorded the lowest charging cycles due to the low solar irradiation levels reported in these months.

This reduces the energy penetration of solar; thus, additional energy is expected to be supplied by the battery to meet the load demand. Conversely, the highest charging cycle was reported in the months of January through to May. This indicates the system mostly relies on other components to serve the load.