From a global perspective, agriculture is the largest user of water accounting for approximately 70%.
Water is a core issue of our time. The United Nations estimates that 2/3 of the global population will be living with water scarcity by 2025. Urbanization, changes in climate and failing states are exacerbating the crises around the globe. In many parts of the world, demand for water is outstripping its availability. According to the Food and Agriculture Organization, and other supporting reports (USGS water use report), on a global average, human use of water is dominated by agriculture (~70%). The United States uses more water to make electricity (~38%) than it does to irrigate crops (~32%). Further, the vast majority (99%) of the water used for thermal electric power generation is withdrawn from rivers and lakes.
This is why the anticipated changes in energy generation resulting from the Clean Power Plan (CPP) are of great importance to the surface fresh waters of the United States. The CPP is a powerful US policy that will shift energy production from coal to natural gas, wind and solar. It is worth exploring how these shifts will influence water by fuel type and cooling technology, and local geography.
(1) FUEL TYPE
With respect to fuel type, the CPP will likely result in a shift away from coal, with the predominant replacement fuel being natural gas, renewables, and improved energy efficiency. As a result, we will see a change in the supply chain of energy, most significantly in the use of fresh water.
Nuclear power, which has a very high demand for water cooling (1,100 gallons of water per MWh ) is the largest user of water per MWH in the energy sector (UCS report). Coal is a near second (varies, but roughly 500-1,000 gallons of water per MWh), with natural gas typically ranging between 250 and 500 gallons of water per MWh. Renewable energy, however, has virtually no water use in its generation of electricity. Figure 2 shows the relative differences in average water withdrawal per MWh when using once-through cooling technology. A shift in fuel type from nuclear or coal to natural gas and renewables can have a dramatic impact on freshwater withdrawals, and thus on our rivers, lakes and groundwater.
(2) COOLING TECHNOLOGY
The second major factor that will drive a reduction in water withdrawals is the cooling technology used by the power plant. Generally, there are two widely used technologies: once-through and recirculating. Once-through technology transfers waste heat to water and directs the water back to the source (typically a river or lake). This technology relies on large volumes of water, in a continuous flow paradigm, little water is removed from the watershed, but water quality is impaired with thermal pollution. Recirculating cooling systems transfer waste heat to the water and then evaporates the water (and associated heat) into the atmosphere. Recirculating systems withdraw far less water than once-through systems, and they avoid the thermal pollution of the waterways, but recirculating systems “consume” water from the watershed through evaporation.
As Figure 3, from the Energy Information Administration (EIA) Annual Electric Generator Report shows, the national trend for power generation cooling systems is a clear decline of once-through cooling since the 1960s, while the recirculating systems have maintained a relative strength and we see the emergence of dry cooled systems which use air in lieu of water, and is a strong option for integrated energy/water planning.
Figure 3: Trend in US powerplant cooling technology
(3) Geographical Differences
In the west, where physical water scarcity has driven policy and technology, power plants are largely utilizing the recirculating cooling technologies. This may seem counter intuitive (consuming water where water is physically scarce) but the consumed volume is far less of an impact than caused by thermal pollution. In the Eastern United States, rivers generally run larger and colder than the west and policy has traditionally promoted the use of once-through cooling to leverage the dissipation of heat in the waterway as opposed to evaporating it into the atmosphere. But that appears to be changing.
Map Source: USGS: http://water.usgs.gov/edu/wupt.html
Although much is to be determined, it is clear that changes in energy generation have the potential to impact the water that flows through our rivers and lakes. But the degree of impact will vary by the selected fuel type, cooling technology and region. The CPP can lead to an unparalleled opportunity to integrate the planning of our energy and water resources. It is up to us to make the connection, and bridge the divide between Water and Energy leaders. Great work has been done by federal and nonprofit agencies who are concerned that the opportunity is slipping away; we as practitioners have the responsibility to understand the research and make holistic decisions.