Monont'sa pumped-storage power project, Lesotho

This paper describes the pre-feasibility design of a high-head pumped-storage scheme in Lesotho. The underground powerhouse accommodates four 300 MW Francis pump turbines of regulated double-stage type achieving net heads of 723 m and 696 m in pumping and generating modes, respectively. Power drawn from the grid at off-peak periods will be regenerated at peak periods. The generation capacity is 1200 MW for 21 h. Roller-compacted concrete and concrete-faced rock-fill dams were selected for the particular reservoir sites. The headrace and tailrace tunnels, pressure shaft and manifold were concrete lined and sited where available minimum in situ rock stresses can safely contain internal pressures. Penstocks were steel lined. A twin-cavern arrangement was selected for the powerhouse set 800 m below ground. Features include downward demountable turbine runners in the machine cavern and rapid-acting bonneted draft tube gates accessed from the transformer cavern. A system of non-waterway tunnels provides access, separate emergency escape, ventilation and power cable routing to the surface switchyard. This paper addresses the energy characteristics and cycle efficiency of the project, and the considerations involved in the selection of the hydro-mechanical plant, electro-mechanical plant and civil works.