Clean, Green Solar Power Falls Short in Achieving Water Efficiency

By Joe Gelt

Semi-arid Arizona is a mother lode of the raw material needed for solar energy - sunlight.

Despite the impressive advantages of solar power - it converts sunlight into energy and is non-polluting and sustainable - the same concern is being raised about solar facilities that has been raised about coal or nuclear energy plants: water use.

One of the sunniest states in the country, Arizona is poised to become the North American capital of solar power. But does the semi-arid state have adequate water resources for large-scale solar energy production? Further, would producing solar energy largely for export out of state be a wise use of those water resources? (See News Briefs, page 5 for information about state water export through power generation.)

University of Arizona water researcher Gary Woodard appreciates the value of solar energy, but believes more attention should be devoted to the amount of water used to produce solar power. He says, "The water issue is not being squarely addressed." He believes this is an important but overlooked issue saying,"Solar thermal tends to be the least efficient of all thermal types of power generation."

Consider water-use information from the 2006 report Energy Demands on Water Resources, Report to Congress on the Interdependency of Energy and Water prepared by the U.S. Department of Energy: a coal fired plant uses 110 to 300 gallons per megawatt hour; a nuclear plant uses between 500 and 1100 gallons/MWh; and a solar parabolic trough plant uses 760 -920 gallons/MWh.

Considering the large number of solar plants being proposed for Arizona and the West the question of the amount of water needed to produce solar energy is an important one. The Bureau of Land Management has received 130 applications for large-scale photovoltaic and concentrated-solar projects on 1 million acres of land, most of which are in the West. If all currently proposed plants became actual projects, about 70 billion watts of power could be added to the nation?s electrical grid, capable of meeting the electric needs of 20 million homes.

Included in the above are plants proposed for Arizona. Eight solar companies or investment groups have submitted applications to BLM to build 27 solar projects in the state, with the proposed projects having the potential to generate more than 12 billion watts of power.

Concerns overlooked
The high expectations for solar energy tend to overshadow some concerns about this much acclaimed clean, green alternative energy option. Solar facilities require electrical transmission lines, are often located on land providing habitat for plants and animals and, at issue here, consume large quantities of water.

In response to these concerns BLM decided to take a cautious route when approving applications for solar projects. After receiving the above-mentioned 130 applications, the federal agency announced it would accept no other applications for large-scale solar projects on its western lands pending the completion of an Environmental Impact Statement.

According to a press release, the process will ?assess the environmental, social, and economic impacts associated with solar energy development on BLM-managed public land in six western States: Arizona, California, Colorado, Nevada, New Mexico, and Utah.? Water would be one of the issue addressed by the EIS along with visual impact, effects on species and competing uses of land.

The delay raised the ire of members of Congress and the solar industry; BLM reversed its decision. Many of the proposals already submitted, however, may be approved through individual National Environmental Policy Act reviews before EIS guidelines are written.

Solar energy and water use
What all thermal-based energy production has in common, whether involving solar concentrators, coal, nuclear or, to some extent, natural gas, is that they all use heat to boil water and produce super heated steam. The steam then rotates a large turbine activating a generator that produces electricity. The steam is then condensed back into water in cooling towers, giving off waste heat in the process. This waste heat is dissipated by evaporating water from another source.

Those who know solar energy only from the photovoltaic systems installed on roofs and that provide electricity for domestic use might wonder about the concern about water use. Photovoltaic systems convert sunlight directly to electricity using the semiconductor materials in solar panels, with no water needed.

The big water users are solar facilities relying on concentrated solar power or CSP projects. As its name suggests CPS uses long parabolic mirrors or Fresnel lenses to concentrate the sun?s energy on black tubes carrying molten sodium or high-temperature oil. These fluids are used in turn to boil water, with the steam turning a conventional turbine to produce electricity. CPS systems require a water source and cover large areas of land, up to several square miles, to produce sufficient electricity for export or local use.

All but one of the solar projects proposed for Arizona are CPS projects use parabolic-trough technology. The lone exception is a project proposed on BLM land just south of Eloy that would use photovoltaic panels.

Efforts to increase water efficiency in solar energy operations involve modifying the conventional cooling tower. For example, dry desert air could be used instead of water to cool the operation. This, however, would greatly increase building costs because enormous cooling towers would need to be constructed. Also relying on air to cool would not cool the water circulating through the plant to a low enough temperature for peak performance, decreasing the efficiency of the plant.

Woodard says hybrid cooling towers have been designed and built that are both dry and wet. In winter or in the middle of the night, with temperatures in the cool range, dry cooling is effective. When temperatures rise to a certain degree, water is then used, with devices Woodard describes as "outdoor misting systems on steroids" activated in the cooling towers.

Woodard says, ?They can be operated either as dry cooling or wet cooling towers. You get a compromise. You still use some water but not nearly as much if you were always using water, and you still have to build somewhat larger and more expensive cooling towers.?

Joseph Simmons, co-director of the Arizona Research Institute for Solar Energy at the University of Arizona, says that solar processes are being developed that will not use any water. One process will use hydrogen heating fluid and another will rely on hot acid. Simmons expects the former will likely be available in about a year.

In the meantime, while these innovative solar processes are being developed, Simmons says, " The work horses of the utilities are solar thermal systems using parabolic-trough technology ... They use quite a bit of water."

Reallocating water for solar power
The Sierra Club has varied concerns about the proposed solar power plants including water use. Its primary concern, however, has more to do with location, whether, for example, a plant proposed for a particular area may pose a threat to environmental values. Sandy Bahr, spokesperson for the Sierra Club, says, "We support solar power, but it is an industrial activity and putting it next to a wilderness area just is not a good idea.

Bahr says, "Water use is one of the things to consider. If they are proposing to place [solar power plants] on undisturbed desert lands and pump groundwater where currently no groundwater pumping is occurring, we will give it more of a critical eye."

The issue that concerns the Sierra Club is not just the amount of water a proposed plant would use but also the source of that water, whether new wells would be drilled or an existing water source reallocated to the new use. The one solar plant in Arizona the Sierra Club is supporting is the Solana plant, a 280-megawatts plant proposed for the Gila Bend area. Bahr says, "It is on private land, not public land, and is currently agricultural land. Depending on how you calculate it [the power plant] will use 75 to 85 percent less water than the current agricultural use. It is still a fair amount of water but it is much less than it takes to grow alfalfa."