One of the most important uses of water in a thermal power plant is for cooling purposes. It is one of the most water consuming processes in a power plant. To transfer heat from the core, water is circulated continuously in a closed loop steam cycle. It is then turned into steam in the process, in order to drive the turbine to work for making electricity, to be condensed and then returned under pressure to the heat source in a closed system. The amount of cooling required by any steam cycle power plant is determined by its thermal efficiency. Notably, plants that have flue gas desulphurisation systems installed need additional water to go through with the process, raising a plant’s water requirement further.
Given these factors, it is necessary to focus on cooling technologies that align with water conservation goals as well.
Direct or “once-through” cooling is usually used at plants that are located next to the sea, a big river, or large inland waterbody. The procedure requires running a large amount of water through condensers in a single pass and discharging it back into the source water body. The discharged water is a few degrees warmer and is nearly the same as the amount withdrawn. Although this process is water efficient, high temperatures of the discharge water are harmful to the aquatic life. Another drawback of the process is that it can only be used in plants located near waterbodies.
Recirculating or indirect cooling is used for power plants that do not have access to abundant water. In this process, cooling is done by passing the steam through the condenser and then using a cooling tower. In the tower, an updraught of air via water droplets cools the water. Many plants create an on-site reservoir or canal to store the cooling water. The cooling is driven by evaporation, which occurs with simple heat transfer to the air. The cooling tower evaporates up to 5 per cent of the flow before the cooled water is returned to the condenser.
Dry cooling involves cooling simply by air without evaporation. Cooling towers with a closed circuit are used alongside high forced draught air flow, which passes through a finned assembly. The technology, however, is three to four times more expensive than a recirculating wet cooling system. It also has significantly higher capital and operating costs as it is more suited to smaller units than large units in which the process may have safety implications.
Many newer technologies have evolved over the years that are cost effective and water efficient at the same time. Some of these have also been actively deployed in other industries and can be successfully used in this sector as well.
One of the best ways to improve the efficiency of dry cooling towers is to add Adiabatic cooling pads, which are made moist with water such that the moisture is evenly distributed over the top of the evaporative pads. As the air passes through the pads, water is evaporated, humidifying the air that cools it down. The pre-cooler is constructed in stainless steel, with water distribution piping and a gutter system to drain the water. Debris and minerals stay behind on the pad after evaporation. This technology can also be used as a once-through system. This way, the cooler does not require any water treatment and minimises the risk of microbiological contamination. This technology is flexible, economical and safe. It uses less energy and water, with an evaporation loss of just 0.001 per cent.
Cooling towers use copious amounts of water. When water evaporates during heat transfer, residual ions collect in the equipment and increase the need for makeup water back into the tower. The more the ions in the water, the more frequent the blowdowns requiring a new cycle of makeup water. A water treatment system can maximise the efficiency of the water being used inside the tower without scaling or corroding the system, which increases the asset life and reduces capex. One simple intervention can decrease the concentration of ions in the makeup water, allowing it to operate at higher cycles of concentration.
Hybrid towers that can accommodate both wet and dry technologies, coupled with water treatment plants can tremendously reduce the water requirement.
Best practices adopted by Indian gencos
Among many efforts to make its plants more environment friendly, NTPC ensures water management in all its plants, especially in cooling systems. It engages in practices such as increasing cycles of concentration in cooling water systems. In its recent statement, NTPC has explained that cooling technologies comprise 70 per cent of the water requirement and it has been able to reduce its water requirement by 16 per cent by reducing blowdown quantity just by increasing the cycles of concentration in circulating cooling water systems. In many of its plants, NTPC implemented dry cooling systems to reduce water use. It also engages in rainwater harvesting for water efficiency in its Ratnagiri Gas and Power Private Limited plant in Maharashtra and 139 per cent of its sweet water requirements internally.
Apart from maximisation of the cycles of concentration, there are many more measures that can help reduce water requirement for cooling systems as given below.
Plants need to aim to reduce blowdown through careful monitoring and measures such as minimising scaling and biological growth, increasing blowdown water, which causes water loss. This may lead to increased corrosion due to low pH, hence, the need for careful monitoring.
Plants can install a conductivity controller to automatically control blowdown. In cooling water, it indicates the amount of dissolved minerals in the water. A conductivity meter or controller can continuously measure the conductivity and discharge water only when the conductivity set point is exceeded.
Plants can install flow meters on makeup and blowdown lines. Plants should routinely check the ratio of makeup flow to blowdown flow and then proceed to check the ratio of conductivity of blowdown water and the makeup water. Both ratios should match the target cycles of concentration and check for leaks or other unauthorised draw-off in case they do not.
Monitoring water levels by switching away from ballcock-style fill valves that are prone to leakage, to more reliable fill valves can be beneficial to prevent water loss.
Keeping air handler coils well maintained by regularly checking for dirt has proven beneficial when the load increases on the chilled water system. The additional load increases the use of electricity and is taxing for the evaporative cooling process as it uses more water.
Apart from the aforementioned measures, gencos can also consider auxiliary support systems like leak detection and monitoring system, which are AI based. Such systems can provide the visibility and analytics needed to detect leaks and monitor water usage. They can track cooling tower performance and save time by automating the manual process. Tracking of makeup water, makeup to blowdown ratio, cost, cycle of concentration and number of alert days needs to be performed and results across several cooling towers have to be compared with stored data to eventually work out systems to improve efficiency.
NTPC worked on creating a water system for stations, which is monitored on a real-time basis with a water dashboard. The dashboard stores inputs from flowmeters installed in the water system right from drawal point to consumption end. Owing to the real-time monitoring, the utility is able to take corrective actions quickly in case of mishaps and deviations.
Choosing the right cooling system is critical to plant and environmental health. The right cooling technology and auxiliary systems can save water and improve the plant’s overall performance in the long run.