“Leliefontein is a true marriage between the civil, mechanical and electrical engineering disciplines and serves as an example of how municipalities can use low cost, off the shelf equipment like centrifugal pumps and induction motors to generate clean power using potential energy in their existing infrastructure,” – Jacobus Kriegler, Aurecon Professional Civil Engineer.
In 2015, Aurecon was appointed by the Drakenstein Municipality to augment the supply of potable water to Wellington through the implementation of the Leliefontein Pump Station project. The Pump Station needed to increase the conveyance capacity to Wellington to 30 Mℓ/d in the interim, and to 60 Mℓ/d in the future.
The location for the Leliefontein Pump Station has hydropower potential, which led Aurecon’s design team to create a station that would have the ability to pump water and generate electricity using the same pumps. The Pump-As-Turbine (PAT) Station is considered a first of its kind in South Africa. While using pumps as turbines is not a new technology, Leliefontein uniquely uses the same set of pumps to pump water and generate electricity by reversing flow through the pumps.
The challenge that the project team faced was that the pump station was only required for two weeks out of the year. Installation would be underutilised with the risk of premature failure of mechanical equipment. This led the team to solve the underutilisation challenge by using the same pumps to pump water and generate electricity. As the main purpose of the installation was to pump water, the challenge faced by the team was building an efficient pump station with a minimal cost of conversion to generation. To overcome this challenge, the team designed the pump station before making cost-effective alterations to the design for generation purposes. A further challenge was that the pump-as-turbines (sized for efficient pumping) were unable to generate power with available hydropower potential. The solution was to reduce the speed of the pump-as-turbines via the electrical control system.
The resulting Pump-As-Turbine Station was achieved through the innovative use of active front-end variable speed drives to lower the speed of the PATs to generate electricity at the available hydropower potential, a series of actuated valves and some creative pipework. The power generated at the station is fed back into the municipal grid, offsetting the power consumed during pumping. The PAT conversion cost only R3 M extra to a contract value of R30 M. The estimated annual generation is 320 MWh, which translates to 44 days of free pumping.
“The power generated at Leliefontein can be offset against the power that the Drakenstein Municipality would have had to purchase from the electricity public utility. Not only does it reduce the Municipality’s environmental impact through the consumption of renewable energy, but it also has a social impact because the client can invest the money saved by utilising the renewable free energy, back into the community through the delivery of services. We commend the client on their future-ready Pump-As-Turbine Station,” says Kriegler.
The project won ‘The Most Outstanding Project in Water Engineering’ award at the South African Institution of Civil Engineering (SAICE) National Awards in 2018.