Optimization of Mantsonyane and Semonkong mini hydro power stations by hybridization

Abstract

This paper shows the optimized design and performance of a hybrid energy system for the following study sites, Mantsonyane and Semonkong in Lesotho. The main objective is to design a hybrid system with Low Cost of Energy (LCOE), high Renewable Fraction (RF) and reduced carbon emissions from a diesel generator at Semonkong hydro-diesel hybrid system. This study employs HOMER Pro simulation software to demonstrate the performance of the Mantsonyane and Semonkong hybrid renewable energy systems. The proposed Semonkong system design is made up of 360 kW of solar PV array, 100 kW wind turbine, 500 kW inverter, 1 MWh battery storage, 180 kW mini hydro and 410 kW diesel generator with a Load Following (LF) dispatch strategy. The simulation results indicate the environmentally friendly system with a renewable fraction of 97.3% which also reflects the high utilization of renewable energy in the system and the remaining small portion is diesel generator. The total energy produced is 1,978,099 kWh per year out of which 25.2% is contributed by wind turbines, hydro power generation contributed 41.8% while solar PV supplied 31.6% and diesel generator injected only 1.42%. The LCOE for this system is $0.129/kWh and these results are taken at the solar irradiation of 5.44 kWh/m2, wind speed of 9.71 m/s, average flow rate of 1,595 L/s and the diesel price of $1.00/L. The project would demand an initial capital contribution of roughly $1.25M with a total NPC of $2.65M. The proposed Mantsonyane system design comprises of twenty-five wind turbines with a total capacity of 7.5 MW, one unit of 1MWh battery storage, 2 MW mini hydro turbine and a 2 MW converter. The simulation results show that the renewable fraction for the most cost effective system configuration is 100% with the LCOE of $0.149/kWh. The results were taken at the solar irradiation of 5.44 kWh/m2, wind speed of 9.71 m/s, and average flow rate of 1,731 L/s. This project would demand an initial capital contribution of roughly $22.8M with a total NPC of $45.2M. Sensitivity analysis is used to investigate the impact of variation in wind speed, solar radiation and river flow rates at Mantsonyane. The analysis indicates that a high COE is experienced when the plant is operating at a very low load with comparatively low O&M costs. It is also found that good river flow rates and high wind speeds result in a more affordable unit price. As for Semonkong site, the optimal solution shows a minimal impact from the instability of diesel price, river flow rate and solar radiation. The LCOE drops with the increasing wind speed and river flow rates. However, the diesel generator will remain part of the system in order to boost generation during dry season from July to September.