Water, Sewage & Effluent May-June 2017 - Page 36

Boston A Dam Boston A spillway and spillway channel Botterkloof Dam spillway Figure 1: Illustration of the location of the Stortemelk Hydropower Station, as per cover image. excavations in ‘weak’ material requiring strong supports to the vertical slopes through a series of anchoring and shotcrete. The power station had to be built in the narrow space between the two existing dams, and construction work had to continue around fully operational infrastructure: the Botterkloof Dam (owned by the DWS) and the adjacent privately owned Boston A Dam. Permission had to be obtained from the respective owners and all regulatory permits approved before the project could be submitted to the REIPPP. Another significant challenge in the construction itself included the need for deep excavations through the left embankment of the Botterkloof Dam and adjacent to the spillway stilling basin, while such construction needed to be done without affecting operations and stability of either of the two dams. This was aggravated by the high water table, being in-between two dams that had for effect seepage during construction and an impact on the ‘buoyancy’ design (Rochecouste Collet et al., 2016). The power station design solution was a shallow intake, followed by a cut and cover concrete penstock, leading to a compact hydropower station housing a single 4.4MW vertical Compact Axial Turbine Kaplan turbine ending in the tailrace, which was rotated at 90 degrees at the draft tube. Two turbines were considered further, namely the saxo and the conventional arrangement with a concrete spiral. The saxo type was eventually selected, as it offered a smaller power station footprint, reducing civil works, while offering easy erection and easy access to the runner and guide vanes for inspections and maintenance purposes. The Andritz Compact Axial Turbine (CAT) also offers the benefit that the turbine guide vane is closed by means of gravity (counterweight). Nevertheless, an emergency gate connected to the turbine controller is also provided, as guide vanes jamming remains a risk owing to high vegetation, although coarse and fine trash racks are provided at the intake. The generator is cooled by means of a two-circle closed loop water and air system, with heat exchangers located in the penstock (Rochecouste Collet et al., 2016). The successful implementation of the Stortemelk project has made a noteworthy contribution of renewable energy to the South African power grid and boosted the local economy. The Aurecon team is proud of the fact that they have now 34 Water Sewage & Effluent May/June 2017 been part of three successful hydropower stations in the eastern Free State. This has brought them to the attention of private sector developers and lenders, and the Aurecon team is now involved in several hydropower projects across Africa. In fact, Werner Comrie, unit manager: Water at Aurecon Tshwane, stated that the hydropower service line is now one of the fastest growing service lines in this multinational consultancy company. The project was also a successful collaboration between various stakeholders, for which Aurecon and the project developer would like to thank TCTA, DWS, Eskom, LHWC, and the landowner, J Farrell, for their enthusiastic support. u References Rochecouste Collet, B.J., Blersch, P.C., and Olivier, A.L. (2016). Design Considerations for the Stortemelk Hydropower Station, South Africa. Paper presented at ANCOLD 2016. Van Vuuren, S.J. (2010). A High Level Scoping Investigation into the Potential of Energy Saving and Production/Generation in the Supply of Water Through Pressurized Conduits. WRC Report no. KV238/10. The successful implementation of the Stortemelk project has made a noteworthy contribution of renewable energy to the South African power grid and boosted the local economy. Acknowledgements The author wants to thank Werner Comrie and Bertrand Rochecouste Collet at Aurecon for the valuable information and photos supplied, and their willingness to be interviewed for this article.