Optimization of the choice of solar minigrid architecture and management in Lesotho

Abstract

Installation and maintenance of the solar photovoltaic systems for power generation is highly discouraged by the high costs of storage units resulting from the traditional approach of sizing the systems. In order to reduce these costs, Solar PV systems sizing using a time-step approach is used in this study as opposed to traditional approach. Comparison of the traditional and timestep approaches used for sizing solar PV systems was performed and showed that time-step approach is the most cost-effective way of sizing the PV systems. The time-step approach is very important in this study since it addresses the country’s lack of progress in mini-grid establishment regarding appropriate mini-grids architectural combinations versus costs best for Lesotho. The primary aim of this research work was to develop a comprehensive computer-based model to be used for performance and optimization of mini-grid systems in order to reduce the system costs, operation costs as well as enhancing the systems reliability. This involved developing an approach to modelling hourly load profile in the absence of historical consumption data and finally determine the best mini-grid system architectural combination which should be used in Lesotho, based on considerations of reliability and cost of energy. The current work successfully developed a simple computer-based program for optimally sizing, performance prediction and economic analysis of mini-grids systems. It shows how optimally sized solar mini-grid systems are determined by the model. The only data required to differentiate between mini-grid systems is the daily energy load as well as its hourly distribution and the desired supply reliability. The current work uses Sehong-hong mini-grid among sites identified by Sustainable Energy for All (SE4ALL) in Lesotho’s mountainous districts and the objective function used for determining the cost effective solar mini-grid architectural combination best for Lesotho is the Levelized Cost ofEnergy (LCOE). The study also explores several diesel dispatch strategies on system performance and energy cost. The study presents an optimised design and performance of solar mini-grid architectural configurations comprising solar PV array, solar inverter, battery bank, battery chargers as well as diesel generator. In this study, system component sizing is defined in terms of daily-energyload related dimensionless variables, 𝑃𝑃 for PV array size, 𝐵���𝐿�� for battery size and 𝑄��𝐿 for diesel generator size. This allows generalization of the design for similar locations and similar hourly load profiles. Results of simulations using the study method show that the ii most cost-effective configuration for mini-grid systems in Lesotho comprises a PV array, a battery and a diesel generator, and should operate at a high solar fraction. For 100% supply reliability, the optimum system comprises solar PV array size (𝑃𝑃 = 11.2), battery bank size (𝐵���𝐿�� = 1.8) and diesel generator size (𝑄��𝐿 = 2.2), operating at 83 % solar fraction and at LCOE of 0.62 USD/kWh. For 99% supply reliability, the optimum system has 𝑃𝑃 = 3.9, 𝐵���𝐿�� = 0.292 and (𝑄��𝐿 = 2.2), operating at 85% solar fraction and at LCOE of 0.30 USD/kWh. It is opined to go for 99% reliability ahead of 100% reliability as only a 1% increase in reliability results in 54% cost increase. The used dispatch strategy in this study for the diesel generator is charge cycling strategy.