The cooling surface

Used steam arrives through the green tubes.
Cooling towers must be used because of thermodynamics: a Carnot-cycle is the most efficient if the difference between the working temperature and the temperature of the working agent at the biginning is the highest. The turbines are fed by 500 °C steam, and the lower the temperature of the supply water of the furnace is the more efficient the power plant will be. As the plant isn’t on fresh water, recirculated water must be cooled down in cooling towers. Power plants using only fresh water and depositing used coolant to a river don’t have cooling towers.