A small hydropower station between Bethlehem and Clarens in the Free State is helping renewable energy gain momentum in the region.

The Stortemelk plant adds 4.4MW to the national electricity grid. This “run-of-river” plant turns the kinetic energy of flowing water into clean, renewable electricity.

Not only has it gained international acclaim, it is already producing more electricity than expected after just a year in operation. Initially expected to generate 27GWh/annum, due to higher than expected efficiencies and lower losses it has produced 30GWh in its first 12 months. It has also already caught the attention of those in the engineering field, gaining a commendation at the 2017 CESA Aon Engineering Excellence Awards for projects with a value between R50million and R250million.

Stortemelk employs the age-old strategy of generating electricity using the kinetic energy of moving water. Its vertical turbine spins a generator to produce electricity, which is then connected to Eskom’s grid and consumed within the Dihlabeng region.

Stortemelk is the third in a series of small hydropower plants developed by Renewable Energy Holdings (REH), which has also been involved in the Merino and Sol Plaatje hydropower stations developed under the Bethlehem Hydro Project.

Stortemelk runs entirely on the green energy it produces and saves about 28tonnes of CO2-equivalent emissions a year during its operation.

In recent years, very few new hydro projects have been developed locally. Although hydropower potential is constrained in South Africa given its status as a water-scarce country, there are still many opportunities for plants like these to contribute meaningfully to renewable energy targets, says Anton-Louis Olivier, managing director of REH.

Opportunities also exist to retrofit existing dams and reservoirs, to generate clean and efficient energy from water. “Unlike big hydro plants, these small-scale plants can be built in three to four years from inception,” Olivier points out.

Going with the flow

Stortemelk was awarded preferred-bidder status in terms of the South African Renewable Energy Independent Power Producer Programme (REIPPP) in 2012 and reached financial close in July 2014, when a power purchase agreement was signed with Eskom and the Department of Energy. Construction on the project started in September 2014, and it has been contributing energy to the grid since 28 July 2016.

Stortemelk harnesses kinetic energy from water in the Ash River, which is plentiful all year round. This is because the river is fed by the nearby Lesotho Highlands Water Project that conveys water from the Katse and Mohale dams through the transfer tunnel, via the Muela hydropower station and Muela Dam and into the river. From here, its journey continues to the Liebensbergvlei River, the Wilge River and into the Vaal Dam from where water is pumped to Gauteng for domestic and industrial use.

Several structures were constructed in 2000 to mitigate the erosion caused along the river channel by the increased flow. The Botterkloof Dam is one of these structures, located approximately 1.5km from the tunnel outfall. The plant was retrofitted on the left bank of this existing dam and basically bypasses it.

Stortemelk sits on a narrow strip of land between the Botterkloof Dam and a nearby farm dam (the Boston A Dam). This meant it had to be carefully designed and constructed to fit in the limited space between these two dams.

The design addressed environmental concerns and ensures low visual impact. This is because the site is situated next to the river and in a privately-owned conservancy.

The nuts and bolts

The project team integrated operations and maintenance requirements in the design process.

“We wanted to create a project that would last, that worked really well and could do so for a long time,” Olivier recalls.

The plant is designed to allow for autonomous operation. This is a relatively new approach in small hydro in Southern Africa, where most plants still require full-time operators on site. Operations and maintenance are contracted to REH Operations and Maintenance, which also operates two other hydropower plants on the Ash River.

The plant building is very compact, yet designed to ease operation and maintenance work. It comprises a simple, reinforced concrete structure, cast against the excavated rock. The design of the building (above ground) had to be compatible with the natural beauty of Clarens. “The idea was to create a non-traditional power plant that blended with its environment and caused minimum visual impact,” says André Eksteen from Earthworld Architects. The team used Corten (rusted steel) cladding attached to a galvanised steel structure that mimics the surrounding ridges.

Inside the plant, the sustainable focus continues

The type of turbine selected offered a smaller power station footprint, hence less civil works.

All the water abstracted for power generation is returned to the river and any potential seepage and leaks inside the plant are pumped out through an oil separator unit to ensure that no contaminated water ends up in the river. The turbine also uses a water-lubricated bearing to prevent any oil from ending up in the water.

The generator itself is completely sealed and cooled by a double-circle closed-loop water and air system. It exchanges heat internally and reduces cooling requirements.

The type of power transformer is a highly efficient dry type (without oil) and is located indoors to reduce its visual impact. The plant also has a small water filtration facility for cooling and sealing water as well as other water needs.

According to Olivier, the smooth start-up and successful operations of the Stortemelk plant can be traced back to the initial specification and design of the project – and in particular the feedback that REH was able to get from the operations of its other hydropower plants. “We are fortunate in that we were not starting this project as a first-time hydro developer, but had some experience to fall back on.

Future contribution

The project contributed to the local economy through local procurement worth over R10.5million during the construction process. Almost half of the total staff complement working on the project (including the design and architectural team) came from the surrounding communities.

Stortemelk’s unique ownership structure will ensure the project continues to benefit disadvantaged communities in the surrounding area, Olivier believes.

During its 20-year power purchase agreement term, the project will contribute 1% of its gross revenue to support socio-economic development activities, including the support of the Combined Churches in Action (CCIA), a local charity focused on supporting orphans in the disadvantaged communities around the town of Clarens.

“Stortemelk is an example of how multi-disciplinary engineering and design thinking can create future-proofed solutions,” Olivier concludes.

By Jorisna Bonthuys

See earthworks magazine issue 40 October-November 2017 for the full feature.