Power and water: Leliefontein pump-as-turbine station

August 21st, 2019, Published in Articles: EE Publishers, Articles: Energize

Originally intended to be a booster station, Leliefontein’s location proved ideal for the establishment of a mini hydro-electric power station. The project, a marriage between the civil, mechanical and electrical engineering disciplines, shows how municipalities can use low cost, off the shelf equipment to generate clean power in their existing infrastructure.

Wellington is home to a growing industrial and agricultural sector which is highly dependent on reliable water supply. 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 had hydro-electric power potential, which led the consulting company’s design team to create a station which would have the ability to pump water and generate electricity using the same pumps.

Fig. 1: Aerial view of Leliefontein PAT station.

This 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. It is also different from standard pumped-storage installations, in that it generates electricity from water received via a gravity pipeline, supplying the Leliefontein reservoir; while in pump mode, it pumps water from the Leliefontein reservoir through a different pipeline to Wellington.

The PAT station made use of active front-end variable speed drives to lower the speed of the PATs to generate electricity at the available hydropower potential. The power generated at the station is fed back into the municipal grid, offsetting the energy consumed during pumping. The PAT conversion cost R3-million extra on a contract with a value of R30-million. The estimated annual generation is 320 MWh, which translates to 44 days of free pumping.

Project evaluation criteria

Quality of engineering design

The project team needed to consider various factors when determining the best location for the pump station. This included considerations around security and the impact the pump station operation would have on direct-offtake users along the pipeline. The evaluation concluded that the pump station could be located at the Leliefontein bulk reservoir complex (LBRC).

The guaranteed duty point of the pump station for the initial capacity of 30 Mℓ/d was 347 ℓ/s at a differential head of 70 m, pumping into the 11 km long pipeline towards the Con Marine and Newton reservoirs. Since the final design duty of 694 ℓ/s for the capacity of 60 Mℓ/d, at a differential head of 27m, pumping into the existing 11 km pipeline and a new parallel pipeline, the same pumps were used for the initial and future pump duty, removing the need to make changes to the pump station pipework or oversize the civil structure to accommodate future equipment.

Fig. 2: The motor control centre at the Leliefontein PAT station.

Variable speed drives (VSD) were selected to adjust the pump’s performance curves to meet the different duties. Two duty pumps were installed for the initial duty point, with one standby to meet the criteria for the pumping operation of the station. For the future duty point, another pump will be added. The conversion of the pump station to a PAT station required the following alterations to the original pump station design:

  • An additional high-pressure turbine supply pipeline from the Wemmershoek-Leliefontein pipeline up to the pump delivery pipeline valve chamber.
  • An additional turbine return pipeline to the Leliefontein reservoir’s inlet valve chamber.
  • Converting the pumps to PATs, which required locking screws on the pump impellers to allow reverse rotation.
  • Converting the single quadrant variable speed drives to active front-end drives to allow the converting of the 30 Hz generated power to 50 Hz grid power.
  • Converting the discharge non-return valves to actuated ball valves, to allow flow in two directions.
  • An additional power meter to measure the power generated and supplied to the grid.

Ingenuity, originality and innovation

This was the first project of its kind to be undertaken by Aurecon. The project team considered multiple options to solve issues with underutilisation, one of which was having an automated maintenance procedure to periodically run the pumps using the programmable logic controller. This option would have solved the problem, however, the LBRC site suggested the possibility of a much more innovative, economical solution. During the investigation to find a suitable site for the pump station, it was determined that the LBRC location had hydropower potential. The LBRC receives about 73% of the total volume of water conveyed through the Wemmershoek pipeline, with the balance carried to the Wellington reservoirs.

With the Leliefontein reservoirs located closer to Wemmershoek Water Treatment Works, the flow rate into the reservoirs is controlled through two electrically actuated sleeve valves. It was calculated that at the average inflow rate to LBRC over 30 years, estimated at 32 Mℓ/d, the residual head at Leliefontein was 19 m. This equated to a total hydropower potential of 48,9 kW, at an assumed generator efficiency of 70%.

Innovative PAT solution

A PAT is essentially a centrifugal pump, which can be used as a turbine by reversing the direction of water flow through the pump and reversing the rotation of the pump shaft. Not only would the PAT solution solve the problem of underutilised pumps, but it would also provide the client with the added benefit of generating clean energy into their electrical network, a first for the municipality, which would reduce their electricity bill.

Two types of reaction water turbines were considered for the Leliefontein installation: a Francis turbine and a PAT installation. The reason for considering only reaction turbines, was that these turbines can operate with a flooded draft tube, which means the turbine can be installed below the tail race water level. Due to the topography of the Leliefontein reservoir site, the pumps would have to be installed below ground to ensure sufficient suction head. The outcome of the turbine options analysis determined that the PAT was the preferred option, as it would involve:

  • Lower capital cost.
  • No increase in footprint of the pump station.
  • Minimal alterations to pump station design and size.
  • Ease of maintenance by using off-the-shelf equipment with which the client was familiar.

Management of planning and technical design

Effective and ongoing client engagement proved extremely beneficial to the quality of the project’s outcome. The client’s expertise, combined with Aurecon’s experience and innovative design thinking, allowed the team to determine the best workable solutions and overcome the challenges inherent in ground-breaking engineering projects. With extensive experience in pump station and pipeline design, as well as electrical and control and instrumentation design, the team had a clear understanding of the complexity of such a project and the possible risks, which they addressed in the options analysis, which allowed for efficient planning, risk mitigation and programme management.

Fig. 3: The pump house at Leliefontein PAT station.

Aesthetics of engineering elements

The building was designed to blend seamlessly with the surroundings. The pump station building was constructed from facebrick and concrete to reduce maintenance requirements.

Budgetary compliance

Ongoing collaboration between the project team during all phases of the project resulted in the project being delivered on time and within budget despite its technical complexity. Regular meetings between the client and the full project team ensured that everyone was informed of progress and required actions and timelines.

Influence of consulting engineer on conceptual design

The team worked closely with the contractors and the client to make ongoing adjustments and identify issues to mitigate any risks before or during construction. The issue of an under-used pump station was foremost in the consultant’s mind, as this was a significant operational risk to the client. The team harvested the energy available, thus managing risk, while creating additional benefit to the client.

Complexity and sophistication

The design approach and objective were to, as far as possible, use the same equipment, instrumentation and pipes as the pump station design with minimal necessary alterations. To meet the municipality’s condition of designing an efficient pump station, the pumps were sized to be efficient pumps rather than efficient generators. However, to push an induction machine into generation, the rotor of the motor must rotate at speeds higher than the synchronous speed of the machine, which in this case is just over 1500 rpm for a four-pole motor at a frequency of 50 Hz. It was found that at the 50 Hz speed, the PAT, sized for efficient pumping, required significantly more residual head than was available from the Wemmershoek supply to generate electricity. This was a significant stumbling block in converting the pump station to a PAT.

The project team concluded that a turbine follows similar principles as pumps when the rotational speed of the turbine is reduced, i.e. its required residual head reduces. Therefore, slowing down the rotational speed of the PAT would allow the system to generate electrical power at the available residual pressure. The speed of the PAT had to be reduced to 30 Hz or approximately 900 rpm to achieve this. With a solution at hand, the team had to figure out a way of reducing the speed of the PAT when in generation mode.It was decided to change the single quadrant variable speed drives required for pumping to active front end variable speed drives (AFE VSDs). The AFE drives allowed for four-quadrant operation, meaning they could change the speed of the PATs in pump and generation modes as well as enable discharge of power into the grid at the required power quality. This solved the problem of having to slow down the PAT for them to generate electricity at the available residual pressure.

Responsibility carried by the consulting engineer (risk)

Apart from the inherent risk in undertaking any innovative, large-scale engineering project, the project team took full responsibility for managing risks throughout the project with thorough planning, investigation and transparency with the client at all stages. The project was designed to the highest standard, with a strong focus on creating infrastructure that is functional, environmentally-conscious and future proof.

Sustainability and social impact

The PAT station generates clean hydro-electric power, which offsets the municipality’s use of electricity generated from coal-fired power stations. The generated power is estimated to result in a reduction in carbon emissions of 346 t. Furthermore, the majority of the equipment used was manufactured locally, with minimal imported equipment required also reducing the carbon footprint. The use of stainless steel where metal pipes were required means there is no necessity to protect the pipes against corrosion. By generating a portion of its power through the Leliefontein PAT station, the Drakenstein municipality’s power purchase expenditure has reduced significantly, making additional funding available for other infrastructure projects to the benefit of the municipality’s residents and local job creation.

Responsiveness to the needs of the client

Although the initial brief was for a booster station, the PAT solution was able to be implemented with minimal cost implications and added efficiency of 77% and a 60 kW power generation since commissioning. The PAT only needs to generate at peak generating capacity for six hours to offset the energy consumed during one hour of operating as a pump station, pumping at 30 Mℓ/d. The power generated at Leliefontein results in a saving of R198 000 in power purchase costs for the unicipality per annum and the estimated maximum monthly power generated (2018 to 2041) is 42 400 kWh (enough to energise 63 households), with an estimated total power generation (2018 to 2041) of 9,9 GWh.
Through the innovative use of a simple off the shelf pump, the project team was able to solve the client’s capacity problem, generate clean electricity using water flowing in the existing municipal infrastructure, reduce the municipality’s electricity bill and keep mechanical equipment active throughout the year without adding any complexity to the municipality’s maintenance procedures.

Meeting the client’s deadlines for readiness

Close collaboration between the client, contractors and the project team, along with efficient project and programme management, proved vital in meeting the client’s objectives and completing the project on time and within budget. The PAT station has been in operation since June 2018 and has generated 105 MWh of renewable energy to date.

Contact Adrienne Brookbanks, Aurecon, Tel 082 468-4566, adrienne.brookbanks@aurecongroup.com

 

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