Combustion Engine vs Gas Turbine: Water consumption
Electric power represents one of the largest uses of water globally. In 2010, water use for energy production accounted for 583 billion cubic meters – 15% of the world’s water withdrawals. In some countries the energy sector accounts for even high percentage of water withdrawals. In the U.S. for example, over 40% of freshwater withdrawals are for thermoelectric power. Water is used during the extraction and processing of fossil fuels, to power hydroelectric generation and for thermal power plant cooling and emissions control systems.
Water scarcity is already a challenge for the electric power sector in many countries. Drought and diminishing reservoir levels have reduced hydropower output in Brazil and the western United States.
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
Power plant cooling needs dominate water use
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:
- Once-through cooling systems require very high water withdrawals from adjacent waterbodies such as rivers, lakes or oceans. In these cooling systems, a large volume of water is passed through the condenser, and a portion of the water is returned to the source at a higher temperature. Although once-through systems are highly effective, large water intake structures and excess heat from discharged water can be detrimental to aquatic organisms. Environmental regulations have sought to limit such impacts and as a result many new power plants employ recirculating cooling systems.
- Recirculating cooling systems cool water by exposure to ambient air in either cooling towers or (less frequently) cooling ponds. Heat transfer with air occurs primarily through evaporation. As water evaporates, minerals and other impurities in the remaining coolant water become increasingly concentrated and must be removed by periodic “blowdown” cycles. Recirculating systems only withdraw make up water to replace evaporative losses and maintain water quality. Recirculating cooling systems withdraw 20 – 80 times less water than once-through systems, but the percentage of water consumed is much greater. Once-through cooling systems consume about 4% of the water withdrawn, while recirculating systems consume 80% of the water withdrawn. There are several different designs for cooling towers to facilitate air to water contact, depending on the ambient conditions and heat load from the condenser.
- Dry cooling systems use mechanical forced air systems to condense steam and have no water requirements. While well-suited to arid climates, dry cooling systems are less efficient, particularly at high ambient temperatures. Dry cooling is not suitable for power plants that have significant steam production and thus large cooling needs such as coal and nuclear units.
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.
Water consumption comparison
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.
Flexicycle plants typically utilize a water-cooled condenser and induced draft cooling tower. In water-stressed regions, Wärtsilä’s Dry Flexicycle™ plants utilize air-cooled condensers (dry cooling) to reduce water use to near zero. The cooling system uses a radiator closed-loop circuit and fans to help dissipate heat. Dry cooling is seldom used at CCGT plants as it imposes increased costs and reduction in plant efficiency. Analysis of derating due to dry cooling compared with cooling towers found that on hot days, CCGT output would degrade by 3% to 9%. Water consumption at a Wärtsilä 12x18V50SG power plant (gas engines) in simple cycle and Flexicycle operation is compared with a CCGT plant in Figure 3. All plants are nominally sized at 220 MW. Dry Flexicycle plants use 96% less water than a CCGT with cooling towers. Wärtsilä power plants offer efficient electric generation with the lowest water use of any thermoelectric technology.