Conduit-hydropower potential in the City of Tshwane water distribution system : a discussion of potential applications, financial and other benefits

Abstract

In water distribution networks, water is often fed under gravity from a higher reservoir to another reservoir at a lower level. The residual pressure head at the receiving reservoir is then dissipated through control valves (mechanically or hydraulically actuated), sometimes augmented by orifice plates where there is a propensity for cavitation. There are possibilities to add turbines in parallel and generate hydroelectricity at these locations using the flow and head available. The benefit of this hydropower generating application is that minimal civil works need to be done, as the control valves are normally inside a control room/valve chamber. No negative environmental or social effects require mitigation, and the anticipated lead times should be short. From a topographical perspective the City of Tshwane has a lower elevation than the bulk service reservoirs of Rand Water, which is the main water supply. Water is distributed through a large water system that includes 160 reservoirs, 42 water towers, 10 677 km of pipes and more than 260 pressure reducing stations (PRS) that operate at pressures of up to 250 m. The top ten hydropower potential sites in the City of Tshwane water distribution network have a total energy generating capacity of approximately 10 000 MWh/a. A number of potential conduithydropower sites have shown promise of short payback periods. The identifying and development of these sites in Tshwane to convert water pressure to electricity is ongoing and exploited further. Various challenges currently exist with reservoir communication in isolated areas due to vandalism and theft of necessary infrastructure, including electricity cables and solar panels. Because conduit-hydropower systems can be housed completely inside chambers, vandalism and theft can be mitigated. Therefore, one of the major benefits of hydropower turbines at these sites is that the hydroelectric potential could be exploited to power telemetry, pressure management, flow control and monitoring/security systems. Alternatively or additionally, other local demand and/or (depending upon the quantum of energy available) off-site energy demand clusters, or even a municipal or national grid, could also be serviced by these power stations. The capacity of hydroelectric installations can vary to suit the application for the amount of power needed or to be generated. Short payback periods, especially when using pumps as turbines, also make conduit-hydropower systems attractive.