The IEA has assessed the future water needs of rapid energy transitions as envisaged by its Sustainable Development Scenario — an integrated approach focused on tackling climate change, delivering energy for all and reducing the impacts of air pollution. In such a scenario water withdrawals are 20% lower than today. The increased deployment of solar PV and wind, a shift away from coal-fired power generation and a greater focus on energy efficiency all contribute to this decline. However, the energy sector’s water consumption moves in the opposite direction in this scenario — increasing by 50%. Withdrawal refers to the amount of water used that can be returned to its source while consumptive water use reduces the amount of water that can be used for other purposes because it is lost to evaporation or incorporated into byproducts or waste streams. The change in water withdrawals and consumption under the IEA Sustainable Development Scenario is captured in image 1 below.
Figure 1: IEA. Global water use in the energy sector by fuel and power generation type in the Sustainable Development Scenario
Thermal power plants – which include coal, nuclear, oil, biomass and natural gas fueled generation – currently account for 80 percent of global electricity production. Most of these power plants utilize steam-electric technology, in which water is used to produce steam which spins turbines to produce electricity. This steam is passed through a condenser and cooled before being used again. Cooling is accomplished through one of three main methods:
Although power plants use water for various processes including pollutant scrubbing to control air emissions, sanitary systems, plant cleaning and fuel processing, the vast majority of water use is for cooling. Lifecycle analysis of water use from extraction through operation found that water for cooling purposes dominates water use in natural gas-fueled power plants.
The amount of cooling required by a steam-electric power plant correlates with its efficiency, irrespective of the fuel used. More efficient power plants have less heat loss and therefore lower cooling needs. Reviews of water consumption rates at power plants have shown that while a nuclear power plant with cooling towers will consume about 2500 liters/MWh, a combined cycle gas turbine power plant (CCGT) with a recirculating system will consume approximately 780 liters/MWh. In comparison, a Wärtsilä combustion engine power plant operating in simple cycle on natural gas will consume mere 3 liters/MWh. This is due to the high efficiency and low cooling needs of Wärtsilä engines.
In combined cycle plants, the output from the steam portion of the plant affects water consumption. About half of the output in a CCGT power plant is generated from through steam cycle, and one-quarter of the energy is lost through evaporation. In a Wärtsilä Flexicycle power plant the steam cycle only contributes 10% of the load. Thus, because of lower steam cycle temperatures, a Flexicycle plant with cooling towers uses about 50% less water than a comparably-sized CCGT with cooling towers. In combined cycle, a Flexicycle plant with a cooling tower will consume only 409 liters/MWh. Water use at a Wärtsilä Flexicycle plant is compared with other technologies using cooling towers in Figure 2.