The idea of getting energy from salinity variant, where freshwater meets seawater, was first considered in the 1970s to be largely forgotten. But over the last few years interest has been resurrected and now a team from Penn State has developed a new hybrid technology to create power using the electrochemical charge created by the meeting of different salt concentrations.
The team at Penn State aims to harness this charge using a custom built technology made of two cells, one containing saltwater and one containing freshwater. Between the two is an anion-exchange membrane, and there is a nickel hexacyanoferrate electrode in each cell. As the saltwater passes through the cells it creates a charge that can be collected using graphite foil and discharged as electricity.
Researchers predict that the technology could meet up to 40% of the world’s energy needs, if it can be successfully rolled out. But a host of challenges lie ahead before it can be produced on a large scale.
Uniting salinity gradient technologies
This hybrid combines elements of two older methods used in the field: capacitive mixing (CapMix) and reverse electrodialysis (RED). Both have proved inefficient for a number of reasons and this hybrid seeks to adopt the best elements from both.
“One approach is to use what's called reverse electrodialysis, in which potentials develop across ion exchange membranes,” says Penn State assistant professor in environmental engineering Christopher Gorski. “So, as saltwater and freshwater are flying through an electrochemical cell this potential develops and then generates a voltage in the cell which can be used to discharge electricity.”
“The other approach is often referred to as capacitive mixing, in which potentials develop at electrode surfaces,” says Gorski. “An electrode develops a potential based on the amount of salt in the solution and when the two electrodes are exposed to different salt concentrations, they develop a voltage between the two electrodes of a potential difference. Then that can be discharged to generate electricity also.” While this has proven to be a relatively successful technique, as with RED, it simply doesn’t produce enough power to be viable.
However, Gorski and his team believe they may have found a way to yield a far greater amount of energy from salient differences by combining aspects of RED and CapMix. “In the cell we had electrodes that developed potentials based on the salt concentration, as well as the membrane that developed the potential across it,” says Gorski. “By combining those two and reconfiguring the cell a little bit, we were able to generate far more electricity than either of those two technologies could do independently.”
The potential to create more energy
In a recent test, 12.6W/m2 of membrane power was generated. “In principle, the amount of power that we could generate was competitive with the best technologies that are out there [currently] that are mainly based on osmotic pressure,” says Gorski.
The team claims that this technology could meet as much as 40% of the world’s energy supply, a bold claim based on the study ‘Membrane-Based Production of Salinity-Gradient Power’ by Guy Z Ramon, Benjamin J Feinberg and Eric M V Hoek, published in the journal Energy & Environmental Science. The study examined points where river water meets seawater, taking into account both geographical and practical constraints, to estimate how much energy each location could generate.
“What goes into that are several assumptions that are probably somewhat impractical, like you'd have 100% efficiency,” expands Gorski. “If you just look at solar energy, for example, and said that you would assume 100% efficiency, you'd easily provide all of the global demand for energy too, so there's that consideration.”
While the previous study gives an indication of the technology’s potential, there are further variables affecting the accuracy of the calculations. “There are other factors that need to be taken into account, like sea level rise,” says Gorski. “As you have tides and high tide and low tide, that influences where freshwater and seawater mix and how much pumping energy you need to put into it, so that will probably also reduce that value.”
“I think in a very practical sense, where this technology would be used would be more where you have large cities on coasts, and you have a discharge of wastewater,” he says.
“We did some back of the envelope calculations and in San Francisco they discharge about 20 billion gallons of wastewater per year to the San Francisco bay,” explains Gorski. “So, if you just mix that treated wastewater with the seawater, you'd generate about 20% of the electricity that San Francisco needs. So there you have a very controlled water stream that you're mixing with, and that seems feasible to me.”
Along with wastewater plants, it’s hoped that the hybrid could be incorporated into the infrastructure at desalination plants. As water is desalinated, a salty substance is left over which is usually dumped straight back into the ocean which could instead be used with wastewater to generate power.
More time for testing
Tests of the hybrid technology were very promising but it’s still got a long way to go before it could be rolled out. “One of the main things is longevity,” says Gorski, explaining that “We did these experiments using pretty pure solutions and relatively short timescales. But if you wanted to implement this on a pilot-scale plant, you'd want to run it for years, ideally.”
The brief time period of testing the hybrid leaves a degree of uncertainty over its durability. Furthermore, the lab conditions raise concerns over the membrane’s ability to perform in real-life applications. “It's not really clear how well the membranes hold up over time, or the electrodes hold up over time when they are exposed to all the random things that are in seawater and freshwater, like small particles, bacteria, strange metals that might get into the water,” says Gorski. “We don’t know how that will affect their performance over a time. So that's something that we're looking at now.”
As tests continue, the team is experimenting with various materials for the membrane to ensure the hybrid is as efficient as possible. “We used nickel and iron, but there's other metals that you can put into it that will change its properties,” says Gorski, and it seems that there is a long way to go from here as, “lots of people have studied how those metals influence it and, sort of, the theory behind that, but haven't explored their use for these sorts of materials, these purposes”.