Thermal power plants (TPPs) utilise significant amount of fresh water during power generation for condenser cooling, removal of plant heat from plant auxiliaries and cycle make-up. Currently, TPPs in India account for less than 5 per cent of the total water consumption; however, by 2050, power generation is expected to account for 9-12 per cent of the total consumption. Furthermore, coal power plants are responsible for about 70 per cent of the total freshwater withdrawal by all industries in India. It is estimated that most of India’s TPPs today are located in water-stressed areas and water shortages have led to electricity generation disruptions. Thus, limiting the water consumption by TPPs has been a key action area for policymakers and the industry.
Water consumption by TPPs
The water requirement of TPPs is governed by a number of factors such as the quality of raw water, type of condenser cooling system, quality of coal, ash utilisation, type of ash disposal system and efficient wastewater management. Major water consumption areas in TPPs include cooling water make-up (in the case of wet cooling systems), ash handling system make-up (in the case of wet ash systems), power cycle make-up, condensate polishing unit regeneration, coal handling plant dust suppression, potable water use, service water use and fire water. Approximately 80 per cent of the water consumption in TPPs is related to cooling directly or indirectly.
Cooling towers and their types
A cooling water system cools the hot water ejected from the different heat generation operations in a TPP and sends it back to the system. It helps in reducing water consumption in general. Cooling water is required for condensing steam in surface condensers and also for auxiliary cooling such as heat exchanger, air condensing and ventilating system cooling.
A make-up stream of fresh water is needed to replace what is lost through evaporation and the small amount that is blown down. Therefore, evaporation loss is a major component determining efficiency of cooling water tower as it determines the extent of heat energy rejected by cooling towers.
Broadly speaking, cooling towers can be classified as either wet or dry. In a dry cooling tower, water passes through a heat exchanger where it is cooled by air. In a wet a cooling tower, the type that is most familiar to the process industries, some of the water is evaporated, thus cooling the entire stream.
Wet cooling tower: Wet cooling towers utilise an 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 hot water and cold ambient air is large enough, so that water droplets partly evaporate and, therefore, transfer the heat to 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.
Wet cooling water systems are of two types, that is, once-through type or closed-cycle type. In a once-through system, additional evaporation from the surface of the waterbody to dissipate the imposed heat load by the power station amounts to about 1 per cent of the circulating water flow rate. In a closed-cycle system, that is, recirculating system, the use of a cooling tower results in the evaporation of 1.5-1.7 per cent of cooling water flow for heat removal from the circulating water.
Most of the TPPs in India utilise wet cooling towers and recirculate water for their operations, given that they are more techno-economically superior to dry cooling towers as well as once-through systems. According to the International Energy Agency, two-thirds of the total cooling systems in Indian TPPs utilise wet recirculating systems, while wet cooling systems of once-through type account for another third. The vast majority of water withdrawn for power generation is passed through a once-through cooling system and then returned to the original source. In this case, water is rapidly used by the TPP for the cooling process and later discharged back to the source. Nonetheless, once-through cooling may not be an environmentally friendly option for modern power plants due to the large intake of water and thermal discharge to the aquatic ecosystem.
Dry cooling towers: 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 an adequate quantity of water is not available, dry cooling systems offer a possible solution for power plant installation, with much reduced water requirement.
In 2015, the Ministry of Environment, Forest and Climate Change (MoEFCC), for the first time, took cognisance of the extent of water consumption by TPPs and introduced water consumption norms. These norms specified that all TPPs commissioned after 2017 will have to restrict water consumption to 2.5 cubic metres per MWh. It also required all TPPs installed before 2017 to limit their water consumption to 3.5 cubic metres per MWh, in addition to mandating the installation of cooling water tower.
However, the MoEFCC extended the deadline related to these norms, which were revised in 2018, in view of the limited compliance by TPPs by 2017. These revised norms also raised water consumption for TPPs installed from 2017 onwards to limit it to 3 cubic metres per MWh, instead of 2.5 cubic metres per MWh, as water consumption as per the earlier clause was found to be extremely stringent. Additionally, TPPs installed from 2017 onwards were obliged to adopt zero-liquid discharge technology in order to augment their efficiency even further.
In June-July 2019, the Central Pollution Control Board (CPCB) issued directions to freshwater-based once-through plants (about 17 GW capacity), requiring them to install cooling towers and comply with the standards by June 30, 2022. TPPs operating on once-through cooling systems are noted to cause problems for the aquatic ecosystem as there is a thermal difference in the discharged water, hence, it is important to urgently oblige them to improve their efficiency.
Issues and challenges
According to a study conducted by the Centre for Science and Environment (CSE) in 2021, around 41 per cent of TPPs in India were non-compliant with the MoEFCC notification (for cooling tower and once-through cooling systems).
Additionally, in 2019 the CPCB issued directions to freshwater-based once-through plants (about 17 GW capacity) to install cooling towers and comply with the standards by June 30, 2022. These TPPs were also required to submit six-monthly progress report on actions taken in order to comply with the above-mentioned direction. As per the CSE’s analysis and survey, the majority of them have still not installed cooling towers and continue to flout the norms.
Further, many TPPs in India are financially stressed and are unwilling to invest in reducing water consumption of their units. Transitioning from a once-through cooling system to a closed-loop wet cooling system by installing a cooling tower costs Rs 2 million-Rs 2.5 million per MW. It takes three years for a TPP to install a cooling tower. Additionally, it takes 10 years for a TPP to recover the amount invested in this transition, hence, it is unviable for plants with less than 10 years of the remaining life.
The way forward
The total water consumption by all Indian TPPs stands at around 1,100 million cubic metres, which is higher than that consumed by Chinese TPPs. Hence, it is indisputable that India has an enormous potential to pare down water consumption. Water resources continue to become scarce, while water demand continues to rise unabated, hence, it is necessary for the government to incentivise industries to improve the efficiency of their water consumption and to reduce emissions. On both counts, the government has already introduced dedicated policies and mandates with time-bound targets. However, their effective implementation and enforcement will be crucial for meeting the targets.