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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Can Solar-Powered Desalination Aid California?

By Catherine A. Cardno, Ph.D.

A successful proof-of-concept prototype that separates freshwater from mineral salts has concluded in California's Central Valley. Construction of a full-sized facility begins next year.

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Next year a 2 million gal/day solar-powered desalination plant will be constructed in California’s Central Valley. At the facility, large solar thermal parabolic collectors, A, will maximize the collection of solar energy. Excess heat will be stored in thermal storage tanks, B, filled with a solid, highly conductive material. The desalination process will use the collected heat in a distillation system, C, where the mineral salts will be separated from the freshwater. Salts will be stored in a tank, D, before being sold as a useful byproduct, and freshwater will be stored in a tank, E, before being discharged into a canal, F. Courtesy of HydroRevolution

October 27, 2015—It is not breaking news that the state of California is suffering from a debilitating drought that led Governor Edmund G. Brown, Jr., to issue the first-ever state-wide mandatory water restrictions earlier this year (read "Engineering for Every Drop" by Robert L. Reid in the forthcoming November issue of Civil Engineering ). These restrictions have resulted in residents letting their lawns brown, or go "California Gold," as it's been called; at least one city required for a period of time that restaurants serve food with disposable plates and utensils to conserve water; and farmers are letting what would otherwise be lucrative fields lie fallow. The state's snow pack is almost nonexistent in the Sierra Mountains, reportedly hitting a 500-year low, and the entire state is collectively holding its breath for the promised wetter-than-usual winter that is predicted by some to arrive with this year's El Niño.

But what if a cost-efficient desalination system was already in the works, with the potential to help Californians with their water supply regardless of drought conditions? A successful 25,000 gal/day prototype built in the state's Central Valley—long known for its fertile fields and successful crops—has proven that a grid-independent, solar-powered desalination plant can operate successfully. With its success come plans to begin construction of a full-size, 2 million gal/day facility next year in the rural community of Panoche. Design of the facility is currently under way.

The Central Valley is one of the best places for solar thermal energy, according to Brian Fojtasek, a chemical engineer, a senior process engineer, and the president of Amherst, New York-based ATSI, the full-service engineering and consulting firm that helped develop the solar-powered desalination technology for commercial use. ATSI, which has also been involved in early commercialization of such renewable technologies as waste-to-energy systems, partnered with the Cambridge, Massachusetts, and Healdsburg, California-based water production company WaterFX to develop the solar desalination technology. The team created the new facility, dubbed HydroRevolution, to be both its own company and a test project, based in Firebaugh, California.

The system uses agricultural runoff that is not absorbed by plants or soils during irrigation. In the Central Valley, this agricultural runoff typically includes concentrations of 1.5 to 3 percent of salts, and those salts contain boron and selenium, which can contaminate any bodies of water into which they are discharged. "The project was initially intended to treat the agricultural runoff water to prevent contamination of the nearby rivers," Fojtasek explains. 

Despite the drought that has afflicted the state, "this runoff treatment remains one of the primary goals for the Panoche site [of the] HydroRevolution project," Fojtasek notes. "In fact, whenever 'normal' rains return to California, there will be even more runoff water to treat, making the project continually viable regardless of whether drought conditions exist."

With the process that has been developed to treat the water in Panoche, the salty runoff is collected and sent through the desalination process so that the mineral salts can be separated from the freshwater. The freshwater is then directed to a canal so that it can be reused in the fields, while the mineral salts are collected as solids that can be sold as a useable by-product to either consumers or businesses. "Specific minerals such as boron [as borate] can be extracted for use in agriculture fertilization, or salt compounds such as gypsum [calcium sulfate] can be used for industrial materials," explains Fojtasek.

What makes this system so unusual, however, is its high efficiency compared to existing systems, which enables it to operate in remote locations and to be powered by renewable energy. "A solar still is a device for capturing solar energy and using that energy to evaporate and condense pure freshwater," Fojtasek explains. Unlike typical stills, however, the HydroRevolution technology—which is called Aqua4—dramatically increases the efficiency of the process. "While stills have been around for a very long time, the basic solar still will only produce 9 cubic meters of water per day per acre of solar collection area," Fojtasek says. "Aqua4 incorporates state-of-the-art technology for using solar energy as efficiently as possible to produce over 275 cubic meters of freshwater per acre, every day. This is a thirtyfold increase in the productivity of solar distillation and results in greater water production in a smaller footprint."

The system provides a 100 percent recovery rate of freshwater from the source water, according to Fojtasek. 

At the Panoche facility, large solar thermal parabolic collectors will maximize the collection of solar energy. The array's mirrors will direct the sun's energy to a mineral oil-based heat-transfer fluid located in glass-insulated piping, heating it to greater than 400 degrees F. The fluid will then carry the heat to an absorption heat pump that optimizes the system's efficiency and heat utilization, according to Fojtasek. Excess heat will be stored in thermal storage tanks filled with a solid, highly conductive material that can be drawn upon to reheat the transfer fluid for 24-hour-a-day operation and during cloudy spells. A natural gas backup will also be built on-site in Panoche, in case it proves necessary for full operations to continue in the typically cloudy periods of January and February. 

While the parabolic array is being designed to minimize its structural mass, its overall 115 m by 6 m size will nevertheless result in a significant potential for impacts from wind-based forces, according to Fojtasek. So the array's collectors will require robust foundations, and these will need to address the poor soil conditions on-site. Designs for micropilings, screw pilings, spread footings, and other foundations are currently being evaluated so that the most economical and suitable foundation can be selected, Fojtasek says.

In addition, wind fences will be installed on two sides of the array to protect it from prevailing winds, and the winds will be continually monitored so that the system can be put into a "storage" mode during unusually strong wind events. 

The desalination process, which is separate from the solar energy-collection system, works by feeding the collected heat into a distillation system. "The steam generated by the thermal energy drives a multieffect distillation process that 'multiplies' the energy by reducing pressure in multiple distillation stages," Fojtasek explains. "That means that the energy used to generate steam from one stage, or 'effect,' is used in subsequent effects at lower pressure to generate more steam.

"Instead of condensing the last effect-generated steam and effectively throwing away that energy, WaterFX's technology recycles and recovers that steam in a proprietary heat-pump system," he continues. "That recycled energy is then applied to the first effect, thereby reducing the amount of outside energy required for distillation." 

In all, the amount of energy required to operate the system will thus be reduced by between 40 and 60 percent versus typical thermal desalination systems, according to Fojtasek. This reduction—which also equates to significant cost savings—has been documented in a peer-reviewed paper published in the journal Desalination(Elsevier, 2015), he notes. 

The lower energy requirements make the system a good candidate for the use of renewable energy production. "By demonstrating the ability to use renewable thermal energy, it makes the project and technology viable for energy-stranded regions," Fojtasek notes, such as island nations or developing countries in which extensive energy infrastructure is lacking. "Any thermal source is sufficient for running the unit, but solar is 100 percent renewable and readily available in the Valley," he points out. Such sources as geothermal, biomass, or waste heat from other industrial sites could also be used to run the system.

"One of the most valuable innovations of the WaterFX technology is the ability to recycle thermal energy in our process," Fojtasek says. And this is a system that can be implemented in traditional coastal desalination systems that draw on seawater, as well—lowering their energy consumption by similar percentages, he notes. By retrofitting existing thermal systems, this processing capability reduces the direct cost and carbon footprint of existing desalination facilities because it can upgrade the systems without adding suction and discharge lines to the ocean, according to Fojtasek. 

In addition, "the WaterFX technology allows for the efficient and cost-effective 100 percent recovery of the seawater, thereby eliminating the need for brine reject to the ocean and doubling the recovery of water from the intake," Fojtasek says. "The resulting sea-salt is a marketable product for commercial and industrial markets." 

The technology solves numerous water problems, according to Fojtasek: it generates a new water supply, because it creates freshwater from previously unusable water; it reduces farmers' reliance on federal water supplies by recycling agricultural runoff; and it draws little to no energy from the grid, reducing the environmental impact of the plant. The technology can also be used to treat the contaminated water that is a by-product of the oil extraction. 

And what does the future hold? According to Fojtasek, the idea is to develop fully renewable, municipal-scale projects that can convert municipal solid waste—trash—into low-cost electrical and thermal energy that, in turn, could be used to create low-cost freshwater from currently unusable water sources. "ATSI has helped develop these technologies with different partners, and these combined-renewable-product projects serve communities' mandates of reducing landfill [use], generating low-cost renewable power, and providing a low-cost, stable source of 'new' water," he says.

 

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