Research Spotlight: Measuring the Energy Potential of Saltwater-Freshwater Exchange

The Great Salt Lake Photo: Charles Uibel, GSLcouncil.utah.gov

The Great Salt Lake
Photo: Charles Uibel, GSLcouncil.utah.gov

By Hilary Smith, Sustainability Resource Center

The Great Salt Lake, a remnant of the prehistoric Lake Bonneville, is the fourth-largest terminal lake in the world. Covering some 1,700 square miles, it hosts a rich and surprisingly diverse ecosystem, comprised of unique critters like the brine shrimp and brine fly, as well as three major types of algae. Some 257 species of birds make use of its shores, including massive populations of eared grebes, pelicans, and phalaropes.

Birds in flight at the Bear River Migratory Bird Refuge on the Great Salt Lake.  Photo: Bear River Migratory Bird Refuge, fws.gov

Birds in flight at the Bear River Migratory Bird Refuge on the Great Salt Lake.
Photo: Bear River Migratory Bird Refuge, fws.gov

It’s also incredibly, notoriously salty. In some spots the lake’s salinity reaches 25 percent or higher—three to five times saltier than the ocean, on average.

The Great Salt Lake's salinity allows for some stellar, seemingly impossible floating.

The Great Salt Lake’s salinity allows for some stellar, seemingly impossible floating. Photo: Summitpost.org

Recently, researchers in the U of U Mechanical Engineering Department have been experimenting with a process that aims to put that saltiness to good use. The technology they are exploring, called pressure-retarded osmosis, or PRO, captures pressure from the osmotic movement of fresh water (e.g. inflow from rivers) into salt water (e.g. seawater, or water from salty lakes), and uses that pressure to generate green, renewable energy.

Thomas Tran (L) and Joseph Melville (R), engineering students at the U, with the model pressure retarded osmosis system they designed and constructed.

Thomas Tran (L) and Joseph Melville (R), engineering students at the U, with the model pressure retarded osmosis system they designed and constructed.

PRO was first theorized in 1975, at Israel’s Ben-Gurion University of the Negev, not far from the Dead Sea—a body of water that’s even saltier than the Great Salt Lake.

Based on background research performed last year by mechanical engineering undergrad David Elwell, the U of U research team—comprised of PhD students Thomas Tran and Carlo Bianchi, and undergraduate Joseph Melville, advised by assistant professors Amanda Smith and Kay Park—has constructed an experimental PRO system inside a lab in the Merrill Engineering Building. They’ve been running trials on the system since January, tweaking various components and monitoring the results. Their goal, says Tran, is to collect enough solid baseline data to understand whether PRO might be scalable for commercial use locally: for example, where the Jordan and Weber rivers flow into the Great Salt Lake.

PRO carries the benefit of being carbon-free. Its main byproduct is brackish (mix of salty and fresh) water. However, as National Geographic reporter Dean Clark notes, some questions remain as to how the pre-process redirection of both salt and fresh water might impact local marine life.

A system that generates energy from PRO might work something like this:

  1. Distilled freshwater is pumped across the length of a semi-permeable membrane, contained within a (usually metal) casing.
  2. Saltwater is pumped across the other side of the membrane.
  3. A certain amount of the freshwater moves through the membrane toward the saltwater side, seeking to equalize the salinity of both solutions, via the natural process of osmosis.
  4. The displaced freshwater increases the total volume of saltwater, which generates a buildup of pressure in the saltwater tank.
  5. That pressure can be harnessed and used to turn a turbine, which can power a generator.

It might also be possible to couple a PRO system with a desalination process, says Tran, creating a system that could produce energy and potable water at the same time.

The U’s experimental system stops short of actually producing power. Instead, it aims solely to quantify the amount of energy that could be generated by PRO, under a variety of conditions. This it does by measuring the amount of freshwater that crosses through the membrane to the saltwater side. Algorithms already exist to determine how much power a given amount of osmotic movement can generate, Tran explains.

Student researchers' working model of a PRO, or pressure retarded osmosis system.

Student researchers’ working model of a PRO, or pressure retarded osmosis system.

Only one other full-scale, experimental PRO model has ever been constructed. In 2009, a Norwegian company called Statkraft developed a prototype PRO system in the town of Tofte, on the Oslo Fjord. The prototype system was only capable of generating 2 to 4 kilowatts—about enough to power an electric hot water heater. The company had stated a goal of building a full-scale commercial PRO plant by 2015, but they pulled the plug on the project early last year, citing their skepticism as to whether they could make the energy marketable any time in the near future. Tran says the U of U team contacted Statkraft but was unable to obtain any data from their experiments.

The University of Utah’s current experimental set-up offers researchers a way to recreate the results that theoretical PRO models have predicted, and to experiment with various components of the system, says Tran.

Tran, Bianchi, and Melville constructed their experimental set-up in-house, using a mix of commercially available pieces, like the membrane—a papery film composite, often used in desalination processes—and hand-made parts, like the metal casing, which was designed and built by Tran.

So far, the team has experimented with the shape of the plastic mesh that holds the membrane in place, varied the pressure at which the saltwater and freshwater are pumped across the membrane, and adjusted the salinity gradient by varying the saltiness of the saltwater solution. Melville says they plan to start experimenting with water temperature soon, as well.

What they’ve loved about the work so far, the researchers say, has been observing the results of each of their creative tweaks to the system. They recognize that sometimes even the smallest discoveries can yield significant clean energy solutions.

“It’s something new,” says Tran. “Not a lot of people have done this before.”

Hilary Smith is a graduate assistant in the Sustainability Resource Center. She is a graduate student in Environmental Humanities.

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