The falling price of photovoltaic panels and public concerns about aquifers and rivers in the western United States are boosting solar energy technologies that save water.

In December, the U.S. Department of the Interior (DOI) approved a 300-megawatt (MW) solar energy project on public land in southwestern Arizona on condition that the developer changes the plant’s design from concentrated solar thermal collectors — which use mirrors and fluids to generate steam to run a turbine — to photovoltaic (PV) panels.

The PV technology will cut down the plant’s water consumption by almost 99 percent. Instead of pumping 3.7 million cubic meters (977 million gallons) per year from the nearby Rainbow Valley aquifer, the facility will use a mere 40,705 cubic meters (10.7 million gallons), primarily for washing the panels.

Each drop conserved is important in the arid Southwest, where the water from most river basins is fully allocated and where climate models predict an even drier future.

The Sonoran Solar Energy Project, as the facility in southwestern Arizona is known, is among a growing number of solar energy projects across the deserts of the American West to make design changes that use less water. As the federal government pushes solar power production, developers both on public and private lands are increasingly switching from wet cooling to dry cooling — like the proposed Quartzsite Solar Energy Project in Arizona — or swapping mirrors for PV panels, which are getting cheaper each year.

A Cooling Breeze Dry cooling, a technology that uses ambient air instead of water, reduces water consumption by about 90 percent, according to the National Renewable Energy Laboratory . But it also increases the cost of electricity between 3 and 8 percent.

Government Targets, Public Pressure, Market Forces

The solar boom in the United States was kick-started by a provision in the Energy Policy Act of 2005, which required the DOI to approve 10,000 MW of non-hydroelectric renewable energy projects by 2015. Then in 2009, the Obama administration called for rapid development of solar and wind power on public lands, and set a target of getting 25 percent of the nation’s electricity from renewable sources by 2025.

Yet in the midst of this development, concerned citizens have pushed regulators to consider solar technologies that save water. During the permitting process for the Sonoran project, the DOI requested less water-intensive PV panels after several residents in the area worried that water withdrawals for solar power generation would draw down the shallow aquifer that the community is using for drinking water.

“Given feedback from stakeholders, the decision is one that we support,” said Steve Stengel, a press officer for NextEra Energy Resources, the parent company of the Sonoran project developer.

Though the DOI generally does not have jurisdiction over water rights, it does assess the total environmental consequences of resource use. As part of the evaluation process, the department can require an alternate technology if a project is estimated to use too much water.

“There haven’t been [federal] regulatory changes,” said Arreola. “But we have seen changes in the developers.”

Those changes, Arreola told Circle of Blue, are being driven by industry economics. The falling price of PV panels has in recent years prompted more and more developers to choose this technology instead of the more established solar thermal systems. According to GTM Research, the price of silicon panels fell by half in 2011, to $0.90 per watt, and will drop even more.

The downward price pressure caused Solar Trust of America, which is developing the world’s largest solar project — a 1,000 MW facility in the Mojave Desert near Blythe, Calif. — to alter its plans.

In August, Solar Trust decided to switch the Blythe plant from a solar thermal design with parabolic troughs to PV panels. The company is “deploying the right technology at the right time,” Uwe Schmidt, the chairman and chief executive, said in a statement.

The trend is evident across the U.S. solar industry. According to data from the Solar Energy Industries Association, a trade group, some 82 percent of projects under development plan to use PV technology. By contrast, just 61 percent of projects already in operation do so.

The fall in solar panel prices comes as a group of U.S. manufacturers have accused the Chinese government of unfairly subsidizing the Chinese solar industry and flooding the American market with artificially cheap goods. The U.S. Commerce Department is investigating whether tariffs should be imposed on solar panel imports from China.

Meanwhile, the U.S. Department of the Interior is conducting a broad environmental review of solar development on public lands in six western states. The review aims to amend the department’s resource management plans so that solar projects are directed to the most suitable parcels of land.

Water concerns, among other issues, have already prompted the DOI to cut the number of proposed “solar energy zones” from 24 to 17. Officials from Lincoln County, Nev., for instance, have requested that only PV technology be allowed in a neighboring zone because of worries about groundwater withdrawals. The DOI eventually withdrew the land from consideration.

Until the environmental review is complete, all public lands administered by the Bureau of Land Management (BLM) are open for solar development. In the last two years, no less than 16 solar projects have been approved for BLM land.

Solar development on private lands can also result in a net gain in water if it displaces irrigated agriculture, as has happened in many western states.

Interactive Map: Solar on BLM Land Click on the points below to learn more about the 16 solar projects approved for BLM-managed land.