The sun is a vastly underused resource in the U.S. — though the federal Department of Energy estimates it could provide more than six times the country’s energy needs, it accounts for less than 1 percent of its power.
But researchers at the University of California, Merced have developed technology they say makes harnessing the power of the sun cheaper, easier and more efficient. Not by capturing light and turning it into electricity — by capturing the heat it gives off. The External Compound Parabolic Concentrator (XCPC), a product of the UC Solar program, can channel heat into a variety of applications. It can power an air conditioning system, provide heating or possibly help with dairy pasteurization.
Lately, UC Merced researchers have been working on using the technology to help farmers in California’s Central Valley. Many farmers, including those in the Merced area, have pipes running underground beneath their crops that are designed to collect water. Then they use what's collected to water their crops again.
“It’s agricultural drainage water, which is already reused multiple times,” said Ronald Durbin, executive director of UC Solar. “But eventually you reuse it so much that it gets too high a salt content to reuse again. And those salts come from the ground itself, from fertilizer, from selenium, lots of stuff.”
It’s an important system because California has been battling a drought for years and farmers, like everybody else in the state, are under an executive order to use as little water as possible. But when salt builds up in the water, farmers have to desalinate it, and all the salty brine left over has to go somewhere. In coastal regions, it just goes back into the ocean where there’s so much water the salt can be sufficiently diluted.
But in places distant from the shore, like California’s fertile Central Valley, it’s a tougher problem. In the 1980s, desalination of agricultural water wrought havoc on animals at the Kesterson National Wildlife Refuge west of Merced. Wildlife officials eventually closed the refuge because of the problem.
That’s where UC Solar’s XCPC comes in. Researchers at UC Merced won a grant from the California Department of Water Resources (DWR) in August to hook the solar thermal capturing technology up to an evaporator and run the brine from agricultural desalination projects through it.
“Think of us as brine disposal,” Durbin said.
The system removes the last remaining water from the brine, evaporating it and releasing water vapor into the atmosphere. The result is a slurry that's pretty much just a bunch of salty stuff.
It might sound like an exacerbation of the problem, but it’s a step toward what the researchers call a “zero-liquid discharge system.” The idea is that someday that super-salty slurry could be used for other purposes.
“Think of it as a mine, and it contains things that have some value,” Durbin said. “So the question is, ‘How do we get those things out of the water and then segregate it so that we can then extract the value?’”
That’s not part of the DWR grant project, but it’s a step toward that system. In the future, Durbin said researchers at the university want to solve that last step of the process, which would make desalination a much more attractive prospect for California’s massive agricultural industry. That would bolster water conservation efforts.
The reason Durbin and the UC Merced researchers think XCPC is the way to go is that the system is cheap and efficient compared to others. It captures heat from both direct and indirect light, allowing it to function even when it’s cloudy out. The system is also “non-tracking,” meaning it doesn’t follow the sun as it moves across the sky. That means fewer parts to add to the cost and easier installation.
“We like to say fewer moving parts, and fewer engines and fewer motors,” Durbin said. “They just sit there.”
There’s another possibility for the system that isn’t a part of the grant project but could become part of evaporation systems in the future — one that would make the systems even more efficient and allow farmers to reuse even more water.
“We can capture that water vapor and use it again, that is an option,” Durbin said. “So in that type of a system, it would be close to a 100 percent water recovery system.”