Eskom is a large consumer of freshwater in South Africa, accounting for approximately 1.5% of the countryís total water consumption annually. Eskom power stations run constantly, supplying in excess of 95% of South Africaís electrical energy and more than half of the electricity used on the African continent. Without water, this output would not be possible.
Eskom uses raw water, which is put through extensive purification and treatment before entering the production processes. The salinity of the raw water dictates the volume of effluents that are produced during the treatment process. During 2005 (April 2005 to March 2006) Eskom used approximately 292 million cubic metres of water for electricity generation, mainly at its coal-fired power stations.
The water catchment areas in which many of Eskomís power stations were built are relatively water scarce, necessitating the need for inter basin transfers. Over the years, various water supply schemes have been constructed to supply the necessary water to the power stations. These schemes consist of dams, pipelines, pumping stations and reservoirs and are inter-linked and operated as a system.
It is expected that Eskomís water consumption will increase over the next 10 years, due to increased demand for electricity. However, decades ago Eskom recognised that the organisation would have to find ways of limiting increases in water consumption and contribute to sustainable water use in South Africa. Eskom is thus committed and determined to support the drive to improve the management of South Africaís scarce water resources.
Water use targets in terms of litres of water used per unit of electricity sent out are set for each power station every year. The water targets are linked to the Eskom Sustainability Index contained in performance compacts, which are in turn linked to business unit and individual performance bonuses. The targets are benchmarked against historical as well as theoretical water consumptions for each particular type of plant.
Management strategies are formalised in the companyís Environmental Management Policy and more specifically its Water Management Policy. These policies are cascaded down into all levels of the organisation. Where relevant, specific business units have their own water policies, standards and guidelines.
Eskom has over the last two decades introduced a number of innovative technologies to save water. These include dry cooling, desalination of polluted mine water for use at the power stations, and technical improvements on treatment regimes to maximize the beneficial use of water. In so doing, more than two hundred million litres of water are saved every day.
a) Dry Cooling Technology
A conventional wet cooled power station uses a re-circulating system in which cooling takes place via evaporation in an open cooling tower. Approximately 85% of the total quantity of water supplied to a power station evaporates through these open cooling towers. In contrast, dry-cooling technology does not rely on open evaporative cooling for the functioning of the main systems. As a result, overall power station water use is approximately 15 times lower than a conventional wet-cooled power station. For this reason, Eskom has implemented dry-cooling technology on power stations wherever feasible, despite the fact that dry cooled stations comparatively are less efficient than wet-cooled stations and there is higher capital and operating costs associated with the technology. This water conservation effort results in an estimated combined saving of over 200 Ml/day, or in excess of 70 million m3/annum.
Matimba Power Station near Lephalale in the Limpopo Province is the largest direct-dry-cooled station in the world, with an installed capacity greater than 4000 Megawatts (MW). It makes use of closed-circuit cooling technology similar to the radiator and fan system used in motor vehicles. Water consumption is in the order of 0.1 litres per kWh of electricity sent out, compared with about 1.9 litres on average for the wet-cooled stations. The choice of dry-cooled technology for Matimba was largely influenced by the scarcity of water in the area.
Fig 1: (a) Matimba Power Station during construction, (b) and (c) Finned tube condensers above forced draught fans
Kendal Power Station near Witbank in the Mpumalanga Province, is the
largest indirect dry-cooled power station in the world with a installed
capacity of greater than 4100 Megawatts, with a water consumption in the
order of 0.08 litres per kWh of electricity sent out. Indirect
dry-cooling entails the cooling of the water through indirect contact
with air in a cooling tower, a process during which virtually no water
is lost in the transfer of the waste heat.
Fig 2: (a) Kendal Power Station, (b) and (c) Tube bundle heat exchangers within the cooling tower.
Where power station design permits, Eskom has endorsed a policy of zero liquid effluent discharge (ZLED) at its wet cooled stations. In terms of the ZLED policy, water is cascaded from good to poor quality uses until all pollutants are finally captured in the ash dams. The objective is to dispose of the maximum mass of salts with the smallest possible volume of water without compromising the ability of the ash to encapsulate the salt load imposed.
The effective use of this practice has seen the company introduce the use of desalination plants at two of its power stations, namely Lethabo and Tutuka. These treatment processes allow the company to introduce polluted mine-water from the tied collieries for re-use at the power stations. This assists with the prevention of negative environmental impacts on the surface and ground waters of the country.
Tutuka Power Station, near Standerton in Mpumalanga, is a
wet-cooled station using a dry ashing system, where moistened ash is
conveyed to the ash dump on overland conveyors. Blowdown water from the
concentrated cooling water (CCW) system is used for ash conditioning.
When the power station operates at a low load factor the amount of ash
generated is normally insufficient to contain all the blowdown water.
Also, the salinity of the Vaal River water used at the station, in
combination with the dry ashing system, limits the stationís ability to
contain water in the ashing system. A desalination plant was therefore
required to reduce the volume of water that is disposed of at the ash
dumps, to counteract the effect of the poor water quality. For this
reason, the power stationís desalination plant was built in 1985. The
product of the desalination plant is mainly fed into the CCW system with
first stage permeate being used for the production of demineralised
water, whilst the brine is used for ash conditioning. Initially, the
plant was intended to be used only for the treatment of the CCW
blowdown. In an effort to explore existing and possible synergies
between the power station and its mine, New Denmark Colliery, it was
realised that greater benefits will be derived from treating the mineís
water in addition to the CCW blowdown.
Fig 3: The desalination plant and high pressure pump
c) Water Infrastructure
Eskom has over the past 40 years contributed to the development of an extensive network of pipelines and dams with the South African Department of Water Affairs and Forestry (DWAF), especially on the Mpumalanga Highveld. This has been done through partnering with DWAF in either directly contributing to the infrastructure development financially or by joint involvement in projects. These projects, primarily aimed at providing a secure water supply to the power stations and their associated collieries, has had a significant impact on the viability of supplying water to both industries in the area and water for domestic use. Currently, Eskom is the major contributor to a pipeline linking Vaal Dam to the water supply system on the Mpumalanga Highveld which would provide water security to the area for the next 20 years.
d) Water Metering and Monitoring
The DWAF measures the water they supply to the power stations at the boundary of the power station terrace. Eskom and DWAF has adopted a metering procedure which has seen the implementation of revenue class meters that measure to a level of accuracy of 0.5%. This is an improvement on the previously accepted 5% level of accuracy.
On the power station terraces ongoing meter verification and upgrades takes place. Both on-terrace and third party usersí water meter readings are taken at least once per month. Water balances and salt balances are carried out at least once a month using the readings to verify performance and identify potential problems. At the power stations, inspections are carried out during every shift and any leaks are recorded and reported for repair as part of their formal operating reporting systems and maintenance procedures. On the raw water supply pipelines, major leaks are rapidly indicated by the remote supervisory control system that senses any reduced water levels in the terminal raw water reservoirs at the power stations. Regular pipeline inspections are carried out to identify minor leaks and all leaks are recorded in mandatory inspection schedules. Water balances are also carried out monthly on the water supply systems.
e) Demand Side Management
Electricity demand side management as practiced in Eskom endeavours to integrate the demand and supply side options to find the lowest cost options for the provision of electricity. Demand side management in electricity may be defined as the practice of a supplier of electricity influencing the manner in which its customers use electricity to increase the beneficial use of the commodity. Although water conservation has not been the primary motive for these initiatives, there have been water savings spin-offs. For every kilowatt hour of electricity that is saved, approximately 1.32 litres of water is also saved on average.
f) International Co-operation
Eskom is a participant in the Southern African Power Pool, which allows for the trading of electricity between countries in Southern Africa. This means that Eskom can import electricity from neighbouring countries to meet increased demands. Since the electricity generated in the countries to the north of South Africa is mainly from hydro power stations e.g. Cahora Bassa in Mozambique, the importation of this electricity effectively reduces the utilisation of South Africaís water resources for electricity generation.
Over the years Eskom has decreased its water use from 2.85 l/kWh in 1980 to approximately 1.32 l/kWh in 2006. Based in the performance over the last 12 years, the quantity of energy produced between 1994 and 2006 has increased by 38% in comparison to an increase in water consumption of 41% (Figure 4). This marginal increase has been due to the significantly higher usage of wet cooled units versus dry cooled units over the last three years. Water consumption has been increasing faster than electricity production since 2004 due to the commissioning of wet cooled units at the Return to Service sites and the use of Tutuka and Majuba power stations.
Note: 2005 is indicated as a 15 month year due to a financial year change
Figure 4: Water Use compared to Energy produced
Access to water and water availability remains a key factor in ensuring the sustainability of development in southern Africa. The efforts by Eskom to use this precious resource more efficiently are an integral part of the companyís commitment to sustainable development.
The Sustainability Index (SI) was introduced in 1996 to ensure long-term sustainability of Eskomís business in the technical, financial, social and environmental arenas. The specific water use indicator (l/kWh) forms part of the SI, and is included in the performance contracts of all those who have an impact on the organizations water use. The SI alarm value for water has been 1.30 l/kWhSO and the standard has been set at 1.40 l/kWhSO. In 2005 the alarm was revised to 1.35 l.kWhSO to accommodate the re-commissioning of the older wet cooled stations like Camden, Grootvlei and Komati. The standard has remained at 1.40 l/KWhSO.
From Fig 5, the specific water consumption performance has been
such that the organisation has performed below the alarm level
throughout the period. This performance is expected to be maintained
into the long term despite the short term increase of water use due to
greater usage of wet cooled technologies until new power generation
capacity is implemented.
Figure 5. Specific Water Use for Eskom