GENERATING HYDROPOWER within municipal water networks
Drakenstein Municipality’s Leliefontein Pump-AsTurbine (PAT) station is believed to be a first-time application in South Africa. Originally intended as a booster station, the switch to a PAT approach provides the added benefit of small-scale hydropower generation, contributing to lower municipal operating costs and optimising mechanical efficiencies.
D
rakenstein’s PAT station provides a vital service for the town of Wellington, ensuring sustained water supply. This is especially important during the one- to twoweek period annually when the Wemmershoek Water Treatment Works (WTW) is temporarily offline for scheduled maintenance. The Wemmershoek WTW, which is owned and operated by the City of Cape Town, supplies water to the Leliefontein Bulk Reservoir Complex (LBRC) within the Drakenstein municipal footprint via a 19 km gravity pipeline, and to Wellington’s Con Marine and Newton zone reservoirs via a further 11 km pipeline at a maximum capacity of 19 Mℓ/day.
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During scheduled or unscheduled downtime at the Wemmershoek WTW, water from the LBRC can still gravitate towards Wellington at a maximum capacity of 11.2 Mℓ/day. This is fine for the winter months, but not during summer when demand is at its highest. “Drakenstein’s original approach was to install a booster pump station at Leliefontein to cater for intermittent supply risks in Wellington. However, the downside to this scenario was that the booster pump station’s mechanical equipment would remain dormant most of the time, potentially leading to premature equipment failure,” explains Holiday Kadada, electrical engineer at Zutari, and part of the multidisciplinary PAT design and commissioning team. “Going the PAT route therefore made better business sense in terms of pump efficiency and hydropower potential,” she continues. “Leliefontein uses the same set of pumps to pump water and generate electricity, which ensures that the PATs are active for most of the year, solving the problem of underutilisation.”
“Another important consideration was the cost. For low-scale hydropower to be viable, it has to be affordable to implement, as is the case with the PAT solution for Drakenstein in terms of return on investment,” says Kadada.
Optimising the setup During the design stages, a solution had to be found to allow the PAT to operate at a rotational speed of 920 rpm (generation speed) when in turbine mode and at
The estimated annual generation for the Leliefontein PAT station is 320 MWh, depending on water demand and loadshedding, translating into 44 days of free pumping for Drakenstein Municipality.”
PAT explained A PAT is essentially a centrifugal pump that can be used as a turbine to generate electricity. The most efficient way for a PAT to operate is by reversing the direction of water flow through the volute. In addition to reversing the direction of water flow, the rotational direction of the pump shaft is also reversed. Therefore, a PAT’s shaft rotates in the opposite direction to that of a similar pump. To confirm the feasibility of utilising PAT technology, the quantum of the available hydropower potential at Leliefontein was calculated. The projected average inflow rate to the LBRC over a 30-year period was estimated at 31 Mℓ/day, while the residual head within the Wemmershoek supply pipeline at LBRC was calculated to be 19 m. This equates to a total hydropower potential of 46.8 kW, at an assumed generator efficiency of 70%.
Leliefontein PAT Motor Control Centre
