Can a cooling tower be too big
Condensation increases the efficiency - which is what cooling towers are used for
Wet cooling is the most common cooling method. This picture shows the "cup" at the bottom of a natural draft wet cooling tower, which catches the trickled water so that it can be used again as cooling water.
When the steam has been processed as much as possible and leaves the turbine, it could be released into the open at overpressure - that is, at temperatures of over 100 degrees - where it would then condense into white clouds, as it used to be with the steam locomotives of the Case was. In the case of steam locomotives, however, the processed steam was only allowed to escape because there was no other choice. It makes much more sense to cool it in a closed "condenser" to such an extent that precisely that amount of heat is extracted from it that is required for the change from the gaseous to the liquid state of aggregation. The steam then condenses into water of the same temperature. The 30 to 40 degrees warm water can be used again as feed water for the steam cycle.
Due to the condensation of the steam, the working medium water suddenly takes up a much smaller volume. This creates a strong negative pressure in the condenser, which is far below the air pressure. Correspondingly, the temperature gradient (or "enthalpy gradient") between the inlet and outlet of the machine increases the thermal efficiency. For the efficiency of a steam power plant, this effect of condensation is even more important than the possibility of reusing the condensate as preheated feed water.
The Brokdorf nuclear power plant on the Elbe manages with fresh water once-through cooling.
Different cooling methods
A lot of cooling water is required to operate the condenser. This is the reason why the steam power plants are mostly located on rivers (and why the condensation had to be dispensed with in steam locomotives). The easiest way is to take the water you need from the river or sea and return it to the water after it has been heated in the condenser. The Brokdorf nuclear power plant on the Elbe, for example, manages with such fresh water cooling. However, if the water supply is insufficient or if there is a risk of impermissible warming of the water due to the reintroduction of the cooling water, a cooling tower must be erected. In this cooling tower, the cooling water is trickled out so that it can give off its heat to the air which flows into the cooling tower from below and rises upwards. At the bottom of the cooling tower there is a container which takes up the trickled water again after cooling, so that it can again serve as cooling water for the condenser. A small part of the water (approx. 1.5 to 2 percent) evaporates during the trickling and is carried away by the rising air. When this vapor condenses on cooler layers of air, the typical white cooling tower swaths are created. The water loss caused by this must be replaced by adding fresh water to the cooling circuit of the condenser.
Dry cooling towers must be built much larger than wet cooling towers with the same cooling capacity.
Hybrid cooling towers are a combination of wet and dry cooling towers.
Fan cooling towers are lower than natural draft cooling towers, but they need electrical energy for the fans.
In contrast to the wet cooling towers described here, there are also cooling towers over which no plumes can be seen. In such dry cooling towers, the water is not trickled out, but rather circulates through pipes through which the rising air sweeps past and thus provides for cooling. They have to be built much larger than wet cooling towers with the same cooling capacity. There are also combinations of wet and dry cooling towers, which are known as hybrid cooling towers.
The most economical and technically advantageous solution is fresh water cooling, which, however, requires large amounts of cooling water. Wet cooling towers only need a little fresh water to supplement their cooling water cycle. However, they are more expensive and reduce the efficiency of the steam power plant by around one percentage point compared to fresh water cooling. Dry cooling towers manage completely without additional cooling water and can therefore be erected independently of water. But they are even more expensive and reduce the efficiency compared to fresh water cooling by around two percentage points.
Natural draft and fan cooling towers
The different types of cooling towers can be designed in two variants: as natural draft cooling towers and as fan cooling towers. The natural draft cooling towers require a fairly large construction height so that the "draft" for the rising air comes about. The fan cooling towers manage with lower overall heights. To do this, they need additional electrical energy for the fans that help the draft.
Discharge of flue gases via the cooling tower
In some newer power plants with fossil firing, the cleaned flue gases are also discharged via the cooling towers. They are transported to relatively great heights with the warm cooling air. This eliminates the need to build a chimney and energy-absorbing reheating of the flue gases that have cooled down after cleaning.
Steam power plants have very complex systems for flue gas cleaning. Almost one hundred percent of the dust is filtered out. In the case of sulfur dioxide, the degree of separation is over 90 percent and in the case of nitrogen oxides around 80 percent. Over 90 percent of the residual materials (gypsum, ash) are recycled.
Thermal load plan limits warming of rivers
Cooling towers have generally been provided for new steam power plants since the mid-1970s. If the cooling water is taken from a river, a "heat load plan" prescribes the maximum extent to which the water is allowed to heat up. If this temperature is exceeded, the power plant must, if necessary, restrict its operation or even stop it temporarily. This can be the case in warm summers, when the water flow is low and the water temperature is very high due to the sun's rays. For example, the Brokdorf nuclear power plant had to limit its electricity generation in August 1995, taking into account the temperature of the Elbe, and in August 1994 even had to interrupt it for three days.
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