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| A solar collector system |
This year, farmers in California's Central Valley won't receive any water through the federal irrigation program, a network of reservoirs, rivers, and canals that is normally replenished yearly by ice melt from the Sierra mountains. Crippling water shortages have made desalination technology more attractive. A solar collector system uses the sun to produce heat. The heat separates salt and water through evaporation. This has fewer environmental repercussions than traditional methods of desalination that rely on fossil fuels to generate electricity.
In a solar collector system, a solar trough that looks like a jumbo-sized curved mirror, collects energy from the sun's rays and transfers that heat to a pipe filled with mineral oil. The mineral oil feeds the heat into a system that evaporates the salty water being treated. Steam is produced, which condenses into pure liquid water. The remaining salt solidifies and can be removed. That salts can be used in other industries as building materials, metals, or fertilizers. In order to operate continuously, the solar trough is very large so that it collects extra heat during the day. The energy is stored and used to run the system at night when the sun isn't shining.
By using sun as the fuel source, a solar collector system uses roughly one-fifth of the electricity consumed by traditional desalination plants. Less electricity means lower operating costs. With conventional desalination, electricity makes up 50-60% of the water costs. A typical desalination plant in San Diego operates at about $900 per acre-foot, while it costs around $450 to produce an acre-foot of water with a solar collector system. (An acre-foot is 325,000 gallons, or the amount of water it takes to cover an acre at a depth of one foot). A solar collector system also helps solve an issue that has long plagued irrigated land. Soils in the arid west of San Joaquin Valley naturally contain a lot of salt as well as high concentrations of metals, like selenium, which can be toxic to humans and wildlife. When the soil is irrigated, the salt, selenium, and other elements become concentrated in the drainage water that collects under the crops. In the past, drainage water have been discharged into rivers, wetlands, and aquifers in the San Joaquin Valley. Now, that otherwise unusable water can be diverted to a solar collector system and turned into irrigation water again.
During a drought-free year, the federally run Central Valley Project provides enough water to irrigate 3 million acres of agricultural land. Last year, farmers only received 20% of their allotment. The lack of water is not just worrying for growers. It affects all people who eat food. One third of the nation's produce is grown in the Central Valley — composed of Sacramento Valley in the north and San Joaquin Valley in the south — and the deep water cuts mean that more than half a million acres of crop land will be left unplanted. Some scientists predict California's drought could last as long as a century. Going forward, the state is going to need a substantial water supply that doesn't rely on the aqueduct system. However, in order to counter California's drought, the push must be toward renewable desalination plants rather than fossil-fuel dependent facilities that further contribute to climate change.
Many desalination facilities, including the $1 billion Carlsbad plant set to open in 2016, use a process known as reverse osmosis that forces seawater through billions of tiny holes that filter out salt and other impurities. This method can produce fresh water on a large scale, but has economic and environmental drawbacks. It uses an immense amount of electricity and only about half of the seawater that goes into the system comes out as clean water. The remaining half is dumped back into the ocean as salty brine where it can be harmful to marine plants and animals. By contrast, a solar collector system has a 93% recovery rate, meaning that for every 100 gallons of water that goes in, 93 gallons of usable water are spit out.
The Panoche Water District in Central Valley is home to the first demonstration plant, a 6,500-square foot system that is capable of producing around 10 gallons of freshwater a minute, or roughly 14,000 of freshwater each day. When the demonstration plant is operating in commercial mode, running 24 hours a day, it can put out 25 to 30 gallons of freshwater a minute.
The pilot project, funded by the California Department of Water Resources, will hopefully prove that a solar collector system system is more reliable and affordable than other freshwater sources.
The water that's being treated by the pilot plant streams in from a canal that collects salty drainage water from around 200 farms in the area and brings it to a single location. In the pilot phase, the clean water that's produced is blended back in with the drainage water, but a commercial plant would send the water back to farmers through a series of canals that are already in place.
Additionally, small-scale systems could be used by individual farmers on site to recycle their own drainage water. The Sahara Forest project in Qatar and an Australian company called Sundrop Farms are using the technology to grow food in greenhouses. The goal is to eventually be able to treat salty groundwater in addition to drainage water.
