This is the first in a series of blog posts discussing the water-energy nexus, a current focus of the Worldwatch Institute’s Climate and Energy team. A Worldwatch study that investigates water consumption over the lifetime of a natural gas power plant will be available soon.
In a world with rapidly diminishing natural resources, the interdependence of water and energy is becoming increasingly apparent. According to the Institute of Electronic and Electrical Engineers, an estimated 500 billion liters of freshwater travels through power plants in the United States each day—more than twice the daily flow of the Nile River. Water and energy demands are coming into competition throughout the country, creating water-energy choke points. These competing demands were discussed at a recent event titled “Choke Point US: Understanding the Tightening Conflict between Energy and Water in the Era of Climate Change,” held September 22 at the Woodrow Wilson International Center for Scholars in Washington, D.C.
For the last four months, the communications network Circle of Blue has been investigating the potential impacts of rising U.S. energy demand on the nation’s water resources. At the Wilson Center event, Keith Schneider and J. Carl Ganter from Circle of Blue talked about the possible effects of the Department of Energy’s projected 40 percent increase in U.S. energy consumption by 2050. Schneider warned that if we are not cautious, this new energy development will come at the expense of “the nation’s water, land, and quality of life.”
Will the shift to a clean energy economy exacerbate water scarcity in the United States? Schneider, Ganter, and other Circle of Blue reporters agreed that it will, unless we are extremely careful. Out of today’s market-ready, low-carbon technologies, only solar photovoltaic (PV) and wind energy require little-to-no water over their lifecycles. In fact, some renewable energy technologies have higher water demands than conventional energy sources.
For example, there is a growing buzz about building concentrating solar power (CSP) plants in the U.S. Southwest; however, based on Department of Energy figures, CSP plants with conventional cooling systems use two or three times more water than coal-fired power plants. A conventional cooling system in a CSP plant uses the wet cooling technique, where the generator’s steam exhaust is either condensed for reuse or cooled by evaporation—much like a coal-fired power plant. In reality, a CSP plant utilizes the same Rankine cycle steam turbine-generator as a typical coal plant, so the cooling systems are nearly identical.
The debate over solar power development has been playing out in the United States’ federal agencies. Last November, the U.S. Department of the Interior announced that it had “identified 23 million acres of public lands in six southwestern states as prime locations for new solar electrical generating plants.” The National Renewable Energy Laboratory (NREL) projects that CSP facilities with wet cooling will generate 55 gigawatts of electricity in the Southwest by 2050. However, according to the Congressional Research Service, these plants would also require 164 billion gallons of water annually—as much as is used by one million Southwestern families.
Dry cooling may be the most practical solution for curtailing CSP water demands. Also known as convective cooling, dry cooling circulates ambient air through a closed-loop system. There are two main types of dry cooling systems: a direct air cooling system, typically known as an air-cooled condenser (ACC), and an indirect air cooling system, also called a Heller system. An ACC takes steam exhaust from the turbine and pumps it through the top of a long A-frame structure. The steam is then blown using large mechanical fans, which condenses it into water and sends it through the rest of the system.
A study by NREL found that dry cooling using an ACC, “offers highly viable alternatives [to wet cooling] that could reduce the total water usage of steam-generating CSP plants by 80 to 90 percent at a penalty in electricity cost in the neighborhood of 2 to 10 percent, depending on plant location and on assumptions.” Generally, an ACC would also reduce energy generation by around 5 percent compared to a wet cooling system.
A Heller system, in the right circumstances, may be even better than an ACC. The Heller system sprays water in the steam exhaust flow, which condenses the steam into a large volume of warm water and directs it to the natural-draft hyperbolic cooling tower. The warm water circulates around the tower’s base, creating a strong updraft that pushes cool air through the rest of the system, and then returns to the sprayer. These dry cooling systems are fairly common in conventional power plants but much less common in CSP plants.
Heller systems have not been employed in the United States yet. However, in a recent presentation to NREL, a German government agency, Deutsches Zentrum fur Luft- und Raumfahrt e.V. (DLR), showed that a Heller system would reduce water consumption in a CSP plant by over 95 percent, with a minimal loss of efficiency. If these findings are true, then this is a technology worth developing in the United States. Even with dry cooling’s cost and generation trade-offs, it seems likely that water scarcity will lead to the rise of CSP plants with dry cooling systems in the U.S. Southwest. After all, we need to make sure that water does not limit the expansion of clean, sustainable solar energy.