Cascading wind and solar pumping systems to recirculate water for pumped hydropower storage system at Letsa-La-Letsie dam
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Date
2024
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National University of Lesotho
Abstract
This dissertation investigates the use of cascading wind and solar power pumping systems for recirculating water for pumped hydropower generation at Lets'a-la-Letsie in the Quthing district, Lesotho. The study's main goals are to identify optimal wind and solar resources for positioning wind turbines and solar panels; to design a solar/wind pumping system for recirculating water for pumped hydropower storage; and to design a hybrid system that considers the LCOE (least cost of energy) and system performance.
The digital elevation, solar energy and wind energy potential maps, in conjunction with QGIS, were used to analyse the heads of the study regions, and TURBNPRO was used to estimate the power outputs and specifications of the selected Pelton turbine. The net head for Lets'a-la-Letsie was 157 metres. The design flow rate for Lets'a-la-Letsie was 0.06 m3/s. To provide water to the top reservoir, 19 similar pumps are required. Each pump has a power control of 37 kW (kilowatts). The top reservoir volume was calculated to be 2700 m3.
This study uses the HOMER Pro simulation software to illustrate the performance of the Lets'a-LaLetsie hybrid renewable energy system. The system's total load includes the school load, clinic load, residential load, pumping load, and commercial load. The proposed hybrid system design consists of a 5379 kW solar PV array, a 1500 kW wind turbine, a 1985 kW converter, and a battery model for a pumped storage with a maximum capacity of 4331 Ampere hours (Ah). The optimal system has an LCOE of $0.417/kWh (Maluti 7.61/kWh) . The average solar irradiation and wind speed are determined to be 5.26 kWh/m2/day and 4.77 m/s, respectively. The project would require an initial capital commitment of about $27.9 million (509 million Maluti), with a total NPC (Net present Value) of $39.6 million (720 million Maluti).
The findings show that Lets'a-La-Letsie has enough sun resources as well as the topography for the construction of pumped hydropower storage facilities. However, the country lacks a legislative and commercial framework to support the development of pumped-storage power facilities.