This is a summary of a Union of Concerned Scientists (UCS) report released in July 2013 − 'Water-Smart Power: Strengthening the U.S. Electricity System in a Warming World'. The report is posted at www.ucsusa.org or use this shortcut: http://tinyurl.com/ucs-water
The power sector is built for a water-rich world. Conventional fossil-fuel and nuclear power plants require water to cool the steam they generate to make electricity. At some power plants, a lot of the water they withdraw gets evaporated in the cooling process; at others, much of the water is discharged back to its source (albeit hotter). The bottom line: Most power plants need a huge, steady supply of water to operate, and in hot dry summers, that water can become hard to secure.
As climate change brings extreme heat and longer, more severe droughts that dry up − and heat up − freshwater supplies, the US electricity system faces a real threat. Shifting to less water-intensive power can reduce the risk of power failures and take pressure off our lakes, rivers, and aquifers.
The phrase "energy-water collision" refers to the range of issues that can crop up where our water resources and the power sector interact. The UCS report provides some recent examples of each type of collision:
- Not enough water: Heat and drought in Texas in 2011 caused water levels in Martin Creek Lake to drop so low that Martin Creek Power Plant had to import water from the Sabine River to cool its coal-fired plant and keep it operating.
- Incoming water too warm: During a 2006 heat wave, incoming Mississippi River water became too hot to cool the two-unit Prairie Island nuclear plant in Minnesota, forcing the plant to reduce output by more than 50%. In the first such case in northern New England, the Vermont Yankee nuclear plant was forced to reduce its power production by as much as 17% over the course of a week in the Summer of 2012 due to high water temperatures and low flow in the Connecticut River. One of the two reactors at the Millstone nuclear plant, Connecticut, was shut down for 11 days in mid-July 2012 as its water source, Long Island Sound, got too warm − this was the first open-water collision on record and signals that even plants on large bodies of water are at risk as temperatures increase.
- Outgoing water too warm: To prevent hot water from doing harm to fish and other wildlife, power plants typically aren't allowed to discharge cooling water above a certain temperature. When power plants bump up against those limits, they can be forced to dial back power production or shut down. Alabama's Browns Ferry nuclear plant, on the Tennessee River, has done that on several occasions in recent years − cutting its output during three of the past five summers, for example, and for five consecutive weeks in one of those years (2010). In the Summer of 2012, four coal plants and four nuclear plants in Illinois each sought and received "thermal variances" from the state to let them discharge hotter water than their permits allow, even amidst extensive heat-related fish kills and tens of millions of dollars in fisheries-related losses.
Nuclear power cycle
The nuclear power cycle uses water in three major ways: extracting and processing uranium fuel, producing electricity, and controlling wastes and risks. Reactors in the US fall into two main categories: boiling water reactors (BWRs) and pressurised water reactors (PWRs). Both systems boil water to make steam (BWRs within the reactor and PWRs outside the reactor); in both cases, this steam must be cooled after it runs through a turbine to produce electricity.
Like other thermoelectric power plants, nuclear reactors use once-through and/or recirculating cooling systems. Once-through systems withdraw enormous amounts of water, use it once, and return it to the source. Recirculating (or closed-loop) systems circulate water between the power plant and a cooling tower. About 40% of nuclear reactors in the US use recirculating cooling systems; 46% use once through cooling. Recirculating cooling systems withdraw much less water than once through systems but they consume much of what they do withdraw, typically operate less fuel-efficiently, and cost more to install. Dry (air) cooling is not currently used in nuclear power generation due to high costs (although World Nuclear News reported on 17 April 2013 that an air cooling system is to be constructed for Loviisa's two pressurised water reactors in Finland.)
Boiling water reactors and pressurised water reactors use comparable amounts of water to produce a unit of electricity. Nuclear plants as a whole withdraw and consume more water per unit of electricity produced than coal plants using similar cooling technologies because nuclear plants operate at a lower temperature and lower turbine efficiency, and do not lose heat via smokestacks.
In addition to cooling the steam, nuclear power plants also use water in a way that no other plant does: to keep the reactor core and used fuel rods cool. To avoid potentially catastrophic failure, these systems need to be kept running at all times, even when the plant is closed for refueling.
During an accident, 10,000 to 30,000 gallons (38,000−114,000 litres) of water per minute may be required for emergency cooling.
Low-carbon power is not necessarily water-smart. Electricity mixes that emphasise carbon capture and storage for coal plants, nuclear energy, or even water-cooled renewables such as some geothermal, biomass, or concentrating solar could worsen rather than lessen the sector's effects on water. That said, renewables and energy efficiency can be a winning combination. This scenario would be most effective in reducing carbon emissions, pressure on water resources, and electricity bills. Energy efficiency efforts could more than meet growth in demand for electricity in the US, and renewable energy could supply 80% of the remaining demand.
Further reading:
- Synapse Energy Economics, paper prepared for the Civil Society Institute, Sept 2013, 'Water Constraints on Energy Production: Altering our Current Collision Course', www.synapse-energy.com/Downloads/SynapseReport.2013-06.CSI.Water-Constra...
- Benjamin Sovacool, January 2009, 'Running On Empty: The Electricity-Water Nexus and the U.S. Electric Utility Sector', Energy Law Journal, Vol.30:11, pp.11-51.
- Benjamin Sovacool and Kelly Sovacool, 2009, 'Identifying future electricity–water tradeoffs in the United States', Energy Policy, 37, pp.2763–2773.