Drought forces Eskom to seek dry cooling technology
JOBURG - With water scarcity prevalent in many parts of the country, Eskom is heavily investing in world-class dry cooling technology and is actively driving research and development in this field.
The public electricity entity’s spokesperson, Khulu Phasiwe said that having pioneered developments in the field of dry cooling technology, Eskom is currently operating both the largest direct dry-cooled and the largest indirect dry-cooled power stations in the world.
Phasiwe said, “Matimba and Kendal [power stations] both… consume about 0.1 litres of water per kilowatt per hour (kWh) of electricity produced. In comparison, a traditional wet-cooled plant needs close to 2 litres per kWh.”
He said that Eskom has embarked on direct and indirect cooling systems in its endeavour to conserve the country’s limited water resources. However, the expense involved in the construction and operation of a dry cooling system is greater than a conventional wet cooling system.
“Limited water resources may override economic considerations in determining the choice between the two technologies,” Phasiwe said.
Dry cooling technology keeps the cooling water in a separate, closed circuit which is cooled by heat transfer rather than evaporation. Thus, the amount of water needed to cool the plant is significantly reduced. As a result, the water usage for cooling of a dry-cooled plant is on average more than 90 percent lower than that of a wet-cooled plant.
The indirect, dry cooling system adopts a principle similar to that used in a car radiator where heat is conducted from the water to the metal of the radiator, and from there to the air passage. The air remains dry as it does not come into contact with the hot water. The water runs in a closed circuit and evaporation is thus minimised.
However, in the direct system, steam from the low-pressure turbine is channelled directly into the radiator-type heat exchanger. The heat is conducted from the steam to the metal of the exchanger. The air passing through the exchanger removes the heat, thus condensing the steam into water to be pumped back to the boiler.
Phasiwe concluded, “Cooling in both the direct and indirect systems can be achieved by natural draughts in cooling towers, or by forced draughts using fans. Cooling in the Matimba direct dry cooling system is by forced draught, whereas an indirect natural draught system is applied at the Kendal Power Station.”
