Desalinatori a energia solare. Pratici, smontabili e mobili
Solar power turns the sun’s energy into electricity. Desalination removes unwanted minerals from saltwater so it can be used for drinking or agriculture.
These two technologies have typically been employed separately in the effort to live more sustainably and limit dependence on finite resources. Now in California, a company has found a way to merge the two with the aim of providing long-term relief to farmers suffering the impacts of the state’s devastating four-year drought. The implications are far-reaching, as agriculture accounts for 80percent of water use in California and roughly 70 percent of water use globally. In California alone, there is an estimated one million acre-feet of irrigation drainage that could be treated and reused if solar desalination catches on.
“Conserving or recycling even a small share of this water can make a big difference,” Sandra Postel, director of the Global Water Policy Project and a Freshwater Fellow of the National Geographic Society, told ThinkProgress.
WaterFX, a San Francisco-based water producer for agricultural and commercial users, recently announced that its California subsidiary, HydroRevolution, plans to build the state’s first commercial solar desalination plant. To be located in the agriculture-intensive Central Valley, the plant will ultimately generate up to 5,000 acre-feet, or 1.6 billion gallons, of clean water per year — enough water for 10,000 homes or 2,000 acres of cropland. It will be built on 35 acres of land currently used to grow salt-tolerant crops, and will recycle unusable irrigation water from a 7,000-acre drainage area into a new and much-needed source of freshwater for nearby water districts by removing unwanted mineral and salts.
Using something called Aqua4 technology, the desalination process creates zero excess discharge and produces only freshwater and solid salt as co-products. This differs from traditional desalination where up to half the discharge ends up as brine back in the ocean.
This is not the only way solar desalination differs from traditional reverse osmosis desalination projects, where sea water is the main input. There are currently several of these large-scale projects in use or under construction along the California coastline. Conventional desalination plants force salt and other minerals through a membrane; they are energy-intensive and can also harm marine life and disturb coastal ecosystems. The solar desalination plants developed by WaterFX use solar thermal energy to avoid the use of fossil fuel-powered electricity.
“The energy intensity of conventional reverse osmosis plants has dropped considerably over the last two decades, but they still have a relatively high energy price tag compared to other water supply and demand management strategies,” said Postel. “I see no elegance in a technology aimed at ensuring there’s enough drinking water during droughts if it employs a process that will hasten climate change, which in turn will worsen droughts.”
Postel said WaterFX’s technology has several advantages, including “not contributing to climate change,” cleaning up local salty, toxic irrigation drainage, and being more cost-effective. She said while she hasn’t done an independent cost comparison, she’s read that solar desalination produces clean water at rough one-fourth the cost of conventional desalination.
“Lastly, it’s super exciting to me because it opens up the possibility of farmers and irrigation districts leasing some water back to the environment,” said Postel. “It could be a win-win for farmers and the environment” if they could lease some water “to safeguard habitats for fish and wildlife.”
The solar desalination technology is also “modular and movable,” Ivy Wisner with the WaterFX communications team told ThinkProgress.
“The equipment is delivered in modules and mounted on skids so installation is easy and equipment can be moved depending on water treatment needs,” she said.
According to WaterFX, the HydroRevolution system is the most efficient of its type available. It uses heat generated from parabolic solar panels to evaporate clean water out of the original source water. The condensate is then recovered as pure water at over 90 percent efficiency. When the sun isn’t shining, thermal heat storage used to hold excess heat allows the process to continue.
“WaterFX hopes this project is merely the first step in revolutionizing the way California uses water,” said Wisner.
WaterFX Co-founder and Chairman Aaron Mandell, who studied groundwater engineering, told ThinkProgress that in order to confront climate change, any solution to the water crisis must be long-term.
“Droughts come and go, but the water problem in California is driven by climate change,” he said. “While lack of rain is temporary, elevated temperatures due to a warming climate is permanent and as a result will have a long-lasting impact on the amount of available water.”
WaterFX’s mission is to expand the availability and reliability of freshwater generation — very few places need this more than right now than California’s dried-out Central Valley. In some places in the Central Valley, groundwater tables have dropped 50 feet or more in just a few years, and many shallower wells have run dry.
While the HydroRevolution plant is currently in pre-production and seeking investors, WaterFX installed a demonstration plant in the Panoche Water and Drainage District in the Central Valley,where the federal Bureau of Reclamation has cut back water deliveries from dams and canals by up to 80 percent. Central Valley water districts are also under pressure to limit polluted irrigation runoff from their fields into the San Joaquin River. The solar desalination plants could also provide a fix to this issue.
“The technology is being piloted in the perfect place for it,” said Postel. “The drainage water from irrigation in this western side of the San Joaquin River in the Central Valley has an unusually high load of salts, selenium, and other contaminants.”
As the Central Valley publication Ag Alert recently reported, the plant “will help the district clean up salts, selenium, boron and other minerals in tile-drain water coming from irrigated fields and reach its goal of zero agricultural water being discharged into the San Joaquin River by 2019, which is required by an agreement with federal agencies.”
Part of HydroRevolution’s innovative approach to the solar desalination project is to pursue a crowdsourcing effort, or capital-raising campaign, that will be available to California residents only. It is still in the preparatory stages.
Postel considers WaterFX’s approach one way of confronting the bigger challenge of repairing the overall water cycle.
“Our approach to water management has been very disruptive of the natural water cycle and all the benefits that cycle provides,” she said.
Other avenues that can aid in this process include better storm water management, green infrastructure that helps rainwater infiltrate back into the earth, and wastewater recycling and reuse.
“We still have a long way to go with water conservation and efficiency improvements,” she said. “Both indoors and outdoors.”