Cooling water is a significant source of water in thermal power plants (TPPs), accounting for around 80 per cent of the total water consumption. Other processes such as steam cycle, ash handling and flue gas desulphurisation account for the remaining water consumption in a plant. TPPs always release excess heat, that is, heat that is not converted into electricity. The hot steam/ hot water is released into the atmosphere/nearby freshwater stream after being cooled. Cooling water is also required for condensing steam in a surface condenser and for auxiliary cooling in heat exchangers, air condensing and the ventilating system, etc.
Therefore, the quantum of water consumed for cooling purposes is proportional to the amount of residual heat emitted by the TPP, which is dependent on the size, efficiency and fuel type of the plant. The consumption of cooling water is also dependent on the technology used for cooling water.
Design and types of cooling systems
The cooling water system cools the hot water ejected from different heat generation operations in a TPP and puts it back into the system. It helps in reducing the overall water consumption.
Broadly, cooling systems can be classified as wet and dry. In a dry cooling tower, the water passes through a heat exchanger where it is cooled by air. In a wet cooling tower, which is most familiar to process industries, some of the water is evaporated, thus cooling the entire stream. The wet cooling systems are subdivided into once-through wet systems and closed loop systems.
Once-through wet cooling systems are the most water intensive, withdrawing anywhere from 10 to 100 times more water than other types of cooling systems. In once-through systems, the water directly absorbs the heat as it flows through a condenser. A large volume of water is essential for keeping the water temperature to a manageable level to protect the aquatic organisms. However, very little of this withdrawal is consumed and more than 99 per cent is returned to the source. The temperature at which the water is returned is higher than the temperature at which it is withdrawn. Government regulations have been established to ensure that the discharge temperature does not harm the wildlife.
Closed-loop systems rely on a cooling tower or a pond to recirculate the water and remove excess heat. Cooling towers or ponds allow a portion of the water to evaporate, cooling the remaining water, which is recirculated back through the system. While these systems require significantly less water to be withdrawn, they consume more water than once-through systems. Closed-loop systems typically withdraw around 2 per cent less water than once-through systems but consume around 2.5 times more water than them.
Wet cooling towers utilise the evaporation process for cooling. Hot water comes from the condenser into the distribution system of the cooling tower, which sprays it over a set of horizontal slats or bars called packing or fill. The water droplets mix with the ambient air moving through the packing as the water splashes down from one level to another. The temperature difference between the hot water and cold ambient air is large enough, so that water droplets partly evaporate and therefore transfer heat to the dry air. Cold water is collected in a basin at the bottom of the tower and then pumped back to the condenser, while hot and moist air leaves the tower from the top.
Dry cooling towers conduct heat transfer through air-cooled heat exchangers that separate the working fluid from the cooling air. Because there is no direct contact between the working fluid and the ambient air, there is no water evaporation or water loss in this system. However, dry cooling towers require more area and consume more auxiliary power, making them less cost-attractive than wet cooling towers. For sites where adequate quantity of water is not available, dry cooling systems offer a possible solution for power plant installation, with much reduced water requirement.
Limiting water use
Thermal power stations can benefit from the move from once-through cooling technology to cooling tower-based systems. For extreme water stress scenarios, dry cooling could also become a reality. The move from once-through technology to cooling towers, and further to dry cooling technology entails additional expenses. This additional expense could be argued as the shadow price of scarce water. There is a trade-off between the cost of water technology and the water that could be saved, and policymakers have to balance these trade-offs.
Further, water consumption in Indian TPPs can be reduced by implementing measures such as increasing the number of cycles made by circulating cooling water also known as cycles of concentration. A TPP can reduce the water consumption by 17,520-157,680 tonnes per annum (tpa) by increasing the cycles of concentration from two to eight in a 250 MW unit. Similarly, the plant can reduce its water consumption by 8,760-227,760 tpa by increasing the cycles of concentration to eight, up from two cycles.
However, as the levels of dissolved minerals elevate with higher cycles of concentration, the scaling and corrosion potential also increase. All dissolved minerals have a saturation limit, and if this is exceeded, it will lead to scale formation. In addition, high levels of dissolved minerals (high cycles of concentration) increase the tendency of water to be corrosive. Chemical and mechanical treatment programmes allow the thresholds of scaling tendencies and corrosion to be pushed. There is also a need for management of dissolved mineral (conductivity) levels through elimination of high mineral content water through blowdown.
Traditional water treatment programmes are designed and implemented to account for such system concerns. This ensures optimal tower system operation and helps meet the cooling requirement. These programmes consist of chemical additives including corrosion inhibitors, dispersants, scale inhibitors and biocides that function to protect the cooling system and keep heat exchange surfaces clean and free of deposits or biofilms. When this is accomplished, maximum cycles of concentration can be achieved, and the cooling system can be operated at peak efficiency, in terms of both water use and energy use.
Automation systems can provide a broad range of capacities to control single or multiple parameters in the cooling system such as conductivity and blowdown control, pH control, and real-time chemical monitoring and dosing. Blowdown controllers offer a range of control points from simple conductivity/blowdown control, to timed or meter relay chemical dosing. Many of them incorporate water meter inputs and alarm relays if threshold measurements are exceeded.
Conclusion
India is a water-scarce country with declining water reserves and increasing demand. A large part of coal-based power generation comes from the states of Jharkhand, Madhya Pradesh, Chhattisgarh and Uttar Pradesh, which are also projected to face major water crisis in India. Around 36-40 per cent of India’s TPPs are situated in water-stressed areas. In the case of freshwater insufficiency, several plants will be forced to shut down until water is available.
Hence, decision-makers need to develop policies that to incentivise generators to reduce water consumption as water stress is expected to get worse in the coming years. This will ensure that these TPPs can operate smoothly regardless of the decline in water availability. Stakeholders also need to ensure that the upcoming TPPs are situated in areas with water availability.