Cape crusader26 October 2004
The dual role of South Africa’s award winning Palmiet pumped storage scheme makes best use of scarce water resources, writes Jenny Holthuysen
The Palmiet pumped storage scheme is situated on the Palmiet river in the Western Cape, South Africa. It is unusual in that it not only generates peaking power for the country’s national grid, but its reversible pump/turbines also act as a link in an intercatchment water transfer.
In addition to its dual role, extraordinary measures were taken during construction of the project to safeguard the environment as it is located in the heart of the Cape Floral Kingdom; the smallest, but most diverse, plant kingdom in the world.
For decades, the Steenbras river and its reservoirs have supplied water to Cape Town, but population and industrial growth create an ongoing need for more water in the Western Cape. The South African Government Department of Water Affairs and Forestry (DWAF) identified the Palmiet river as a potential source of water for the city, provided its waters could be transferred across the range of mountains separating it from the Steenbras catchment.
The Palmiet scheme was carried out as a joint venture between the electricity utility Eskom Holdings Ltd and DWAF.
Given South Africa’s scarce water resources, a conventional hydro power station was not considered feasible. The solution, it was decided, lay in the construction of a pumped storage scheme, in which water stored in an upper reservoir is released to drive reversible pump/turbines during periods of high electricity demand before being pumped back when demand is less.
DWAF and Eskom had previously carried out another joint venture at the Drakensberg pumped storage scheme in the mountains of Kwa-Zulu Natal. This facility transfers water from the Tugela river to the Vaal dam in South Africa’s industrial heartland and provides 1000MW of capacity to the national grid.
Engineers began work on the Palmiet scheme in 1983. Construction of the two reservoirs was undertaken by DWAF, while Eskom oversaw the building of the power station with its reversible pump/turbines and associated waterways. Water from the Palmiet river is pumped to the high lying reservoir in the mountains from where it gravitates into the Steenbras system for Cape Town. The volume of water stored permanently within the two reservoirs provides Eskom with the generating capacity it needs for peak electricity demands.
Eskom commissioned the two 200MW units in 1988. Both organisations contributed to the capital costs of the project and the partnership continues with Eskom operating the power station and DWAF the reservoirs.
At the conclusion of construction work in 1987, the Palmiet scheme picked up the South African Institution of Civil Engineering award for the Most Outstanding Civil Engineering Achievement.
The upper reservoir (Rockview dam) has no natural catchment as it is situated on the watershed between the two rivers. Its basin is formed by a rockfill and an earthfill wall, known respectively as the main and northern embankments. A waterway leads from the main embankment to the power station. A separate canal and pipeline from the northern embankment connects the reservoir to the Steenbras system. The reservoir has a gross storage capacity of 20.8Mm3.
The lower Kogelberg reservoir is located on the Palmiet river and comprises a gravity arch concrete wall and an earth embankment. River outlets are provided to allow compensating water into the river downstream. The gross storage capacity is 19.3Mm3.
A 2km-long conduit from the main embankment in the upper reservoir conveys water to the reversible pump/turbines in the 400MW power station.
In the generating mode, up to 185m3/sec of water is admitted at the intake in the upper reservoir, while water is pumped back at an average rate of 126m3/sec. The overall efficiency of the generating/pumping cycle is 77.6%.
The two pump/turbine generator/motor sets and their auxiliary equipment are located approximately 60m below ground level at the base of two 23m diameter concrete-lined machine shafts. The free-standing concrete surge tank, which prevents excessive pressure fluctuations in the waterways, is 61m high and 21m in diameter – one of the largest in the world at the time of construction.
Production plant features
The generator/motors are directly coupled to the pump/turbines and, in addition to supplying peak power, are also used for system frequency control, system voltage control and emergency standby. The machines have been designed to cope with high-fatigue stresses resulting from frequent mode changes, which can total up to 600 per month. They produce at 16.5kV, which is stepped up to the 400kV system voltage.
An unusual feature is the magnetic thrust bearing, which decreases the load by means of a magnetic field, thus reducing bearing friction losses. The mechanical bearings are self-lubricating except during start-up. The elimination of the need for oil pumps and the associated piping reduces the cost of unit auxiliaries and increases plant efficiency.
When the pump/turbine operates as a pump the generator/motor assumes a motoring condition. A static frequency convertor (SFC) is used to achieve a ‘soft’ start of the motor. Converting the 50Hz mains frequency to lower frequencies, it increases the generator/motor speed gradually to avoid the voltage dips caused by sudden excessive current demand. Before starting rotation, air is blown into the pump/turbine area to reduce the run-up current to a minimum.
The generator/motors are phase-reversed and connected to the generator transformer through 16.5kV isolators and a 16.5kV load switch.
Low-level tubular busbars are used in the high-voltage yard instead of high-voltage connecting cables, thus minimising the visual impact.
The Cape Floral Kingdom is the smallest but most diverse of the world’s plant kingdoms. Some 70% of the ‘fynbos’ (from the Dutch ‘fijn bosch’, which means fine-leaved bushes) plant species in the Cape Floral Kingdom are endemic and not found naturally anywhere else on earth. The Kogelberg is regarded as a centre of this biodiversity and is justifiably known as the heart of the fynbos plant kingdom. In 1998, UNESCO registered the area as a biosphere reserve; the first in South Africa.
The multi-disciplinary Palmiet Committee was established in the early stages of planning for the scheme. It comprised representatives from many state bodies, educational institutions and societies, all with specialist knowledge. An Environmental Impact Control Plan, considered a forerunner of its time, was developed and implemented. The overall approach was to implement controls from the very outset and to rigorously follow them throughout the construction, with particular attention paid to the protection of the unique fynbos flora. This proved to be an effective and economically viable approach.
The instream flow required to satisfy ecological needs was established, and operating rules ensure that releases of water mimic natural seasonal cycles and that extractions do not compromise the instream flow requirements. Transfers from the Palmiet river increase the annual yield of the Western Cape System by an average of 22.5Mm3.
At Hydro 2003 in Cavtat, Croatia the Palmiet pumped storage scheme received the Blue Planet Prize from the International Hydro Power Association. The prize recognises good practice and sound management in the development and operation of a hydro power scheme, on the basis of technical, economic, social and environmental criteria.
Eskom Holdings provides 95% of the electricity in the Republic of South Africa and approximately 60% of the electricity generated on the African continent. With a total nominal capacity of 42,011MW it is the eleventh largest producer of electricity in the world. South Africa is rich in coal with 90% of Eskom’s electricity generated by coal-fired thermal power stations.
Steady growth in the demand for electricity in South Africa calls for a corresponding increase in power generating capacity. Eskom’s current supply and demand plan identifies a number of different energy sources, as well as demand side management, in order to meet this growth. These include nuclear energy (pebble bed modular reactor); renewables such as wind and solar power; gas; additional coal-fired capacity; and imported hydro power.
With regard to hydro capacity, South Africa does not have the water resources for large conventional hydro units but another pumped storage scheme is on the cards to provide much needed peak demand capacity.
Jenny Holthuysen is with Eskom. For more information, email her at: [email protected]