This is the fourth in a series of blog posts discussing the water-energy nexus.
Hydraulic fracturing, the technique used to extract natural gas from shale and coalbed reservoirs, typically uses between 2 and 4 million gallons of water per well over the course of a few days – as much as the city of Washington, D.C. consumes in an hour. With such “unconventional” gas contributing a growing share of U.S. natural gas supplies, this water-intensive process has raised concerns that a large-scale shift in the power sector from coal to natural gas could strain the nation’s water supplies. In a new Worldwatch briefing paper, my coauthor Emily Grubert of University of Texas – Austin and I compare the water needs of electricity generated from coal and natural gas, from the point of fuel extraction to the point of power generation. Our analysis shows that generating a kilowatt-hour of electricity from a combined cycle power plant, even if unconventional gas is used, can consume less than half the water that generating the same amount of electricity from a coal steam turbine plant does.
The take-home message here is not that hydraulic fracturing uses less water than one might expect – on the contrary, millions of gallons per well aggregated over hundreds or thousands of wells represents a very significant volume to the areas where hydraulic fracturing takes place. Gas developers must generally obtain permits to withdraw water from local streams, rivers, or other freshwater bodies, or even, as in the case of many operations in Texas’s Barnett Shale region, purchase it from municipal water utilities – water good enough to drink – and truck it to their drilling sites, adding substantially to vehicle traffic, fuel consumption, and costs. In fact, because freshwater supplies do represent a constraint to natural gas drilling in many areas, numerous companies are exploring ways to use less water, mainly by filtering and reusing fracturing water for multiple wells. Recycling fracturing water is also critical to reducing the overall environmental impacts of unconventional gas development – the more wastewater that operators reuse, the less they have to dispose of, risking leaks and spills of fluids that contain chemical additives and natural contaminants dissolved from ancient rock formations.
Rather, our analysis highlights the really enormous water requirements of power generation, which is responsible for over 80 percent of the water consumed to produce a unit of electricity from coal or natural gas. In fact, U.S. power plants withdraw some 143 billion gallons of freshwater every day – 41 percent of all freshwater withdrawals! (Withdrawal, in the language of water usage, refers to the removal of water from a natural source that may either be returned to the source or consumed.)
Steam turbines, a key generating technology in most coal, nuclear, solar thermal, biomass, and natural gas plants, use a fuel to heat water to steam within a circulatory system of tubes. This steam converts much of its heat energy to mechanical energy by expanding through a turbine, which turns a generator that produces electricity. The steam passes to a heat exchanger or “condenser,” where it is cooled and condensed to a liquid so it can go through the cycle again.
Power plants generally use one of three main types of cooling systems: open loop and closed-loop cooling (both of which use water), and dry cooling (which uses air). The type of cooling system a plant employs dictates its water needs: open-loop systems withdraw large quantities of water from a lake, river, ocean, or other body of water, run it past the condenser, and then return it back to the source, losing relatively little to evaporation. Closed-loop cooling systems, the most common type of system in power plants less than 40 years old, store water in a cooling tower or ponds, where water that has passed through the plant is allowed to cool before being used again. While closed-loop cooling systems withdraw much less water than their open-loop counterparts, they can lose more than half of what they withdraw to evaporation, leading their water consumption to be higher. Dry cooling systems use much less water than open-loop or closed-loop wet cooling, making them popular in water-scarce areas. However, they reduce the power plant’s efficiency, forcing it to consume more fuel and create more emissions for each unit of electricity it produces.
Natural gas combined cycle power plants link a gas turbine, which needs no water for cooling, with a steam turbine component, making them 50 – 70 percent more water-efficient than plants that use only steam turbines, which represent almost all those that burn coal in the United States. Put another way, replacing electricity from a steam with electricity from a combined cycle plant (the dominant type of natural gas plant in recent decades) could save around half a gallon of water for every kilowatt-hour of electricity produced. Aside from plants fitted with dry cooling systems, only wind turbines and solar photovoltaic cells can generate electricity with less water.
Using natural gas from shale rather than conventional natural gas in a combined cycle power plant requires an additional 0.01 gallons of water per kilowatt-hour – the water used for hydraulic fracturing. In an illustrative example, hydraulic fracturing of gas shales in the Marcellus accounts for just one to five percent of the total water consumption per kilowatt-hour over the gas’s full fuel cycle.
For its part, coal mining, processing, and transport also consume water, though generally less than unconventional gas per unit of energy. Most coal from Appalachia, however, must be washed to remove impurities, a process which adds 1-2 gallons of water demand per million British thermal units (Btu) of coal.
Water and energy are valuable resources whose fates are closely linked. As the United States – and the world – enters a 21st century marked by carbon, energy, and water constraints, decision makers must increasingly take into account the impact that energy choices have on both quantity and quality of water supplies. While wind, solar PV, and other water-efficient technologies are still scaling up, natural gas may offer the largest water savings over coal.
How Energy Choices Affect Fresh Water Supplies: A Comparison of U.S. Coal and Natural Gas is the second in a series of briefing papers by Worldwatch’s Natural Gas and Sustainable Energy Initiative (NGSEI), which studies the environmental questions surrounding the future role of natural gas.