LONDON (Reuters) - Water and energy are inextricably linked.
Power plants are the largest users of water in the United States, while substantial amounts of energy are needed to supply fresh water to homes, farms and factories and treat waste water prior to safe disposal.
Rising water consumption for hydraulic fracturing and production of biofuels, coupled with severe droughts in Texas in 2011 and across more than 60 percent of the continental United States in 2012, have propelled that link up the policymakers' agenda.
The threat to hydroelectric generation is obvious. But in 2007-2009, drought put the water supplies of 24 of the nation's 104 reactors at nuclear plants at risk. In 2011, more than 3,000 megawatts of thermal generating capacity in Texas also was considered at risk of having to shut down if the drought persisted as reservoir levels plunged.
Texans were asked to conserve water to keep the lights on. The state was only spared blackouts because of high output from wind farms.
On April 25, the Energy and Natural Resources Committee of the U.S. Senate held a hearing to explore the effect of drought on the energy sector and water management, reflecting lawmakers' fears about the instability created by the tightening links between water and energy supplies. (link.reuters.com/cak77t)
The United States withdrew 410 billion gallons of water from aquifers, rivers and the ocean every day in 2005, of which 350 billion gallons were fresh water and 60 billion gallons were saline or brackish, according to a U.S. Geological Survey (USGS) report on "Estimated use of water in the United States" published in 2009.
Cooling systems for nuclear plants and power plants that burned coal, gas and oil accounted for 41 percent of fresh water withdrawals and 49 percent of all water withdrawals. That put them ahead of irrigation (31 percent of total withdrawals) and public supply to homes and offices (11 percent). The remaining uses including industry, mining, livestock and aquaculture accounted for less than 10 percent combined.
In addition to power plants, water used in growing crops for biofuels as well as for drilling and fracking oil and gas wells accounts for a rapidly increasing amount of total consumption.
The broader energy sector has been the fastest growing water consumer in the United States in recent years and is projected to account for 85 percent of the growth in domestic water consumption between 2005 and 2030, according to the Congressional Research Service (CRS).
Environmentalists and community groups cite water scarcity as one reason to ban or restrict fracking. In 2008, Evergreen Underground Water Conservation District, which has jurisdiction over several counties in the Eagle Ford shale area of Texas, introduced restrictions on the amount of water that could be used in fracking operations.
Prompted by the 2011/12 drought, several other water conservation districts in Texas have said they will consider introducing restrictions if reservoirs and the water table drop too low, according to a recent study in the St Mary's Law Journal. ("The south Texas drought and the future of groundwater use for hydraulic fracturing in the Eagle Ford shale" Jan 2013)
Most of the water employed in thermoelectric power stations is in the cooling system. In once-through cooling (OTC) systems, water is withdrawn from a source, normally a river or coastal location, circulated through heat exchangers, then returned to the surface water body. OTC systems withdraw large amounts of water but use comparatively little, returning most to the source.
By contrast, in a closed or recirculating system, water is withdrawn from the source, circulated through heat exchangers, and then cooled using ponds or towers, before being reused. Small amounts of extra water are withdrawn from the source to top up the system.
Most thermal plants in the United States employ OTC systems. Net water consumption is therefore only 3 percent of the total, compared with gross withdrawals of almost 50 percent.
The distinction between withdrawals and consumption is crucial. Water that is consumed cannot readily be used for another purpose.
The water actually consumed in energy production, which includes thermoelectric power, biofuels and oil and gas extraction, amounts to 11 percent of the total, second only to irrigation at 74 percent, according to the Government Accountability Office (GAO). ("Coordinated federal approach needed to better manage energy and water trade-offs" Sept 2012)
Power plants need sufficient water at a low enough temperature to operate efficiently. Most states impose restrictions on the temperature at which water can be discharged back into rivers and the sea to prevent the animals and plants in the waterway from being cooked.
If there is insufficient water or it is too hot, power plants may be forced to close or cut output, which explains the near shutdowns of many power plants in 2007-2009 and again in 2011-2012.
In August 2012, the Illinois Environmental Protection Agency waived the normal environmental restrictions and allowed four coal-fired and four nuclear power stations to release hundreds of millions of gallons of hot water at nearly 100 degrees Fahrenheit into state lakes and rivers to keep the lights on.
On other occasions, low water levels forced power plants to turn down. During the 2003 heat wave in France, which was responsible for more than 10,000 deaths, nuclear plants had to reduce their output, worsening the crisis. The rising temperature of river water meant they could not achieve sufficient cooling and still observe discharge limits, Michael Webber of the University of Texas Energy Institute told senators.
Nuclear and coal-fired power plants with OTC systems are especially vulnerable to droughts and heat waves because they rely on by far the largest volume of water withdrawals. Combined-cycle gas plants are much more efficient. And gas turbines, solar and wind generators use negligible quantities, Webber testified.
Options for reducing the power sector's vulnerability include switching the type of fuel from nuclear and coal to gas, solar or wind; switching to recirculating or dry and hybrid wet-dry cooling systems; or switching the water source from fresh water to saline or waste water.
The drawback is the capital cost and reduced efficiency of the plant. Cleaning waste water so it can be used in the cooling system or using a dry cooling mechanism imposes an "energy penalty" on the plant's efficiency.
Oil and gas extraction uses prodigious quantities of fresh water and produces large amounts of brackish waste water, which is normally reinjected far below the drinking water table.
Drilling a conventional well uses relatively small quantities of water for drilling mud. Some water is also used for secondary recovery by waterflooding and pressure maintenance in declining conventional fields.
Hydraulic fracturing, however, uses vast amounts. A typical well drilled and fracked in the Eagle Ford uses 4.3 million gallons, according to one estimate.
On a life-cycle basis, unconventional gas actually has relatively little impact on total water use. Extra water used in fracking is offset by reduced water used when it is burned in power plants rather than coal. Unconventional oil production has a far greater impact.
Pressure to reduce the amount of fresh water used in fracturing operations has led to interest in switching to saline or waste water, recycling more water from each fracturing operation, or fracking with diesel or hydrocarbon gels, though all these systems are less efficient and remain experimental.
Biofuels present far more of a problem. Huge amounts of water are being used to grow crops to produce ethanol. Pressure on water supplies is especially acute in the Northern Plains, which rely on irrigation and are already water constrained, according to GAO.
"Because biofuels need so much water for their growth, they are particularly vulnerable to droughts. Just as traditional agricultural crops are hindered in times of drought, so are energy crops," Webber warned the senators, forging another dangerous link between water and energy supplies.
(John Kemp is a Reuters market analyst. The views expressed are his own)
editing by Jane Baird