Author Biography

Seth Beddes, a freshman at Utah State University, is passionate about a sustainable future. With a love for SCUBA diving and surfing, he plans to major in electrical engineering and research sustainable energy production. Combining his passion for the ocean with his academic pursuits, Seth aims to ensure that future generations can enjoy all the ocean offers by working towards a more sustainable future.

This essay was composed in April 2023 and uses MLA documentation.

Many, if not most, people are not concerned with how the world meets its energy needs. However, with the threat of irreversible climate damage on the horizon, it is an element of our future that needs to be discussed. The current methods of producing sustainable energy could be better at best. Nuclear energy is wildly misunderstood and perceived as “dangerous” due to this misinformation. Many believe that research and funding in nuclear energy is a waste of tax dollars and resources. Solar and wind, while viable sources, are only sometimes reliable and are subject to constant change and yearly weather patterns. With hydropower, water is continuous. Well, almost. Our typical method of generating power with water uses hydroelectric dams that spin a turbine with collected runoff and lake water. Even this, however, is only sometimes reliable. This is where the idea of salt gradient energy is derived from. The sea has always been considered an unstoppable source; even our rivers flow outwards, resulting in the water being destined to meet the ocean. Around the 1950s, scientists first conceived of creating electricity from saltwater. Since then, scientists have made remarkable discoveries, yet this technology has yet to be spoken about and is rarely used worldwide. This technology has tremendous potential; all it needs is its time in the spotlight. Salt gradient energy could change the world and significantly reduce our dependency on fossil fuels, which is why people need to grasp its fundamental concepts and advocate for a shift towards cleaner, more sustainable energy.

Salt gradient energy, SGE, is the means of producing energy, primarily electricity, by exploiting salt water’s chemical and biological properties. We can produce electricity from salt water in various ways, and some that we can do at home as a little science experiment. While those methods work, they aren’t practical or effective and often produce minuscule voltage levels. Primarily, electricity is produced in two general ways: through pressure-retarded osmosis (PRO) and reverse electrodialysis (RED). Both involve the use of special membranes and two different saline concentrations. Like any new up-and-coming technology, there is always room for improvement, and SGE is no exception. While this technology has been around for many years, lobbying and conflicts of interest for politicians caused it never to get the attention it deserves. It’s relatively cheap and easy to implement. That being said, the technology still has a great deal of room for growth, and as our understanding of electrophysics widens, so does our ability to manipulate pre-existing methods of energy generation, like SGE.

Understanding this technology’s effectiveness is a key element to a firm grasp of the subject. Many experts in the field who have done experiments and modeling with SGE have estimated that it could account for a large portion of the world’s energy supply as it stands, meaning that if implemented as it is today, we could reduce the amount of energy supplied by fossil fuels (Nano). When discussing SGE and its different methods, the question of how we can accomplish this feat often arrives, and the answer to that question is simpler than one would imagine. Since PRO and RED require two different saline concentrations, we can build these plants from estuaries, water treatment facilities, and desalination plants. By using pre-existing infrastructure, it makes the cost of building these plants much cheaper.

The two main methods of producing electrical energy from saltwater, pressure-retarded osmosis (PRO) and reverse electrodialysis (RED), often need clarification about how these processes function. For PRO, let’s dissect the name itself. Pressure-retarded, meaning to go from an area of low pressure to high pressure, seems counterintuitive to basic physics. This is because most natural processes go from an area of high pressure to low pressure, like bodies of air and water. Most people are familiar with the term osmosis and that it allows water molecules to flow from an area of low-water potential to an area of high-water potential through a semi-permeable membrane to reach equilibrium. PRO manipulates this biological concept to create a pressure differential or a rise in water pressure on one side of a membrane-separated tank. This allows freshwater (low-water potential) to flow continuously into the saline solution (high-water potential) despite the saline side having higher water pressure. In a peer-reviewed study conducted by Xia Zhou, they found that “[t]he theoretical energy produced by PRO is decided by the membrane and operating conditions” (Zhou). Since osmosis is a naturally occurring process, there isn’t one single membrane that can be used. Several naturally occurring and unnatural membranes can be produced to make osmosis more efficient under certain operating conditions. However, cellulose acetate membranes have been considered common practice due to their high durability and ability to work well under varying conditions (Zhou). After the freshwater has flowed through the membrane and into the saline solution, it raises the water pressure and can be used in one of two ways. First, the created pressure is converted from mechanical to electrical energy via a turbine generator. Secondly, as freshwater moves into the saline solution, it raises the water level and creates an overspill that converts mechanical energy into electrical energy via a turbine generator.

PRO was the first conceived method of SGE; however, reverse-electrodialysis (RED) is seen as the standard of SGE. To best explain this process, it is best to dissect its name once more. Electrodialysis is the process of using electricity to separate particles with a membrane, essentially functioning almost the same way that reverse osmosis functions, which is the process of desalinating/purifying water by providing an electric current to separate the ions and creating a clean water source. The reverse, or “R,” part of its name means that it works backward, and instead of providing an electrical current, we receive a current. RED alternates solutions of varying salinity levels with ionic exchange membranes. These membranes allow certain ions of either positive or negative charge to flow through them. This constant flow of ions creates electrical energy. Much like PRO, the flow of ions will eventually reach an equilibrium, and the brine can be safely discarded or potentially reused as the high saline concentrate.

While this process has been recognized as a more efficient form of SGE, it’s important to recognize that it can be improved, and the good news is that we already know how. We can effectively double its power output by using saltwater as the low saline concentration and brine as the high saline concentration (Sarp and Nidal). Another way to increase its power output is to match the ratio of salinity and the number of membranes in the device. By finding the “golden ratio” – as it’s called – we can maximize our output and efficiency (Tamburini). It’s easy to see that as our understanding of chemical and engineering principles improves, there is much more we could do with this technology. Only time will tell whether we make the move to a more sustainable future or not. Combining the most efficient models of RED with PRO could yield very impressive results; however, one may ask how far we can take this technology.

Just as every coin has two sides, it’s easy to see that this technology needs to be fixed. We must ask ourselves how we plan to produce and dispose of these membranes needed for PRO and RED. Most membrane production techniques involve numerous chemicals and are typically very energy-intensive (Elsaid). This raises the question of whether or not we can achieve a positive net gain of energy with SGE, and to answer this briefly, we can. Simply put, these membranes do wear out slowly over time. Fortunately, the degradation process is slow, typically one to three years of constant use, according to several models (Elsaid). Secondly, we have the fact that some methods of SGE, typically PRO, can produce brine, highly concentrated saltwater. If released into the sea, Brine could cause ecological damage to the surrounding marine environment and even make the area a dead zone. While this might be enough to cause some skepticism, it’s important to note that by collecting the brine produced, it can be recycled into other materials and even reused with RED. Another important factor to consider is that, on average, solar panels require maintenance two to four times per year compared to a membrane replacement every one to three years.

The capabilities of any new emerging technology seem endless. If we compare computers from just twenty years ago to today, it is easy to see how far they’ve come. This idea of constant technological and scientific advancement sets the stage for an individual to wonder about the scientific and technological advancements that can be made with salt gradient energy. The primary area of inefficiency with SGE, particularly with RED, is that the longer the ions are allowed to flow between membranes, the lower the output becomes. This is due to the different salinity levels becoming solute. This can be fixed. One method that scientists and engineers have begun to explore is the idea of an integrated closed system. This takes the process of RED, utilizes the chemical properties of salt water, and recycles the wastewater back into the system via ammonium bicarbonate (a solution that increases salinity levels in water). Andrea Cipollina, a professor and researcher at the University of Palermo in Italy, found that “This concept uses a RED stack with a closed loop for the energy generation and a thermally driven regeneration step for restoring the initial salinity gradient of the feed solutions.” (Cipollina pg. 82). By recycling the brackish water, mixing it with the ammonium bicarbonate solution, and adding heat, we can reactivate the salt gradient and reuse it. This is just one example of the possibilities of SGE, but there’s more. We can also look at hybrid systems that combine multiple methods of energy production.

A singular RED device produces around a few watts; it is common practice to have several RED devices run in series – meaning their power output is added onto each other – to create a meaningful power output. This is why researchers at Penn State are beginning to reimagine how they look at reverse electrodialysis. They designed a cell called CapMix-RED. Capacitive mixing is a method of fluid movement that involves placing porous electrodes and applying a small electrical current, which forces the water to flow in a specific direction. The researchers found that utilizing CapMix could significantly increase the power output of RED. The Penn State research team found, “At 12.6 watts per square meter, this technology leads to peak power densities that are unprecedentedly high compared to previously reported RED (2.9 watts per square meter).” (Choi). This study used a scaled-down model over twelve watts, which is very impressive; by adding relatively simple modifications, we can significantly increase the power generated by RED. This study highlights the significance of the research and shows us just how much potential there is within this technology. Knowing where we can take this technology is very important as it brings in investors and future research, but knowing and understanding how effective these methods are in the present is equally important.

Most people don’t buy cars with horrible gas mileage, and many do not want to invest in worthwhile energy. Salt gradient energy (SGE) includes pressure-retarded osmosis (PRO) and reverse electrodialysis (RED) technologies, which have attracted significant attention due to their potential for generating clean energy. Their effectiveness is measured by their energy efficiency, power density, and cost-effectiveness. PRO generates electricity from the osmotic pressure difference between two solutions. In this process, a concentrated salt solution and a freshwater solution are separated by a semi-permeable membrane, which allows water molecules to move from the freshwater to the saltwater side, generating a pressure that can be used to produce electricity. According to a study published in the Journal of Membrane Science, PRO has reported efficiencies ranging from 15% to 35% and power densities of up to 5 W/m² in laboratory and pilot-scale experiments (Zhou).

On the other hand, RED generates electricity from the difference in ion concentration between two solutions. This process involves alternating ion-exchange membranes and spacers with a concentrated salt solution and a freshwater solution on opposite sides. As ions move through the membranes, a potential difference is created that can be used to generate electricity. According to a study published in Energy & Environmental Science, RED has reported efficiencies ranging from 20% to 50% and power densities of up to 4 W/m² in laboratory and pilot-scale experiments (Valladares). These numbers may seem small and insignificant, but when scaled up to their actual size, the power output will be much greater, even enough to power hundreds of homes with clean energy.

Salt gradient energy technologies such as PRO and RED offer a promising avenue for generating clean energy from salinity gradients. While laboratory and pilot-scale studies have demonstrated their effectiveness, scaling up these technologies to commercial applications will require overcoming challenges such as membrane fouling and high initial production costs. As research continues to address these challenges, salt gradient energy has the potential to provide a significant source of renewable energy that could help reduce our reliance on fossil fuels. Fabio la Mantia, a professor and researcher of electrochemistry, found that “the renewable energy production could potentially reach 2 TW or ∼13% of the current world energy consumption.” (La Mantia). Increasing the amount of power generated with clean energy reduces the global need for fossil fuels. Thirteen percent – to most people – does not seem like a lot; however, even the slightest changes can have drastic impacts. However, none of this means anything unless we implement this technology in our society.

Unfortunately, good things don’t come quickly, and often, it takes a lot of work and time to change people’s views, even for the better. SGE holds enormous potential and is proven efficient; however, one question remains: How can we implement this in our society? Pressure-retarded osmosis requires a feed of both freshwater and saltwater. This means that a suitable location would have access to both of these elements and luckily, the infrastructure already exists. Many coastal water treatment facilities are built on or near estuaries – where rivers drain into the ocean – providing near–perfect conditions for PRO. In a review of PRO by Bassel Abdelkader, a professor and researcher at the University of Guelph, he found that implementing PRO into pre-existing facilities could reduce the initial start-up cost by 40%. However, he also found that buildings that could be used for PRO often require maintenance and construction to become suitable (Abdelkader). Retrofitting old infrastructure for PRO would greatly decrease the cost of development. However, work still needs to be done to ensure the building’s safety and that the PRO system can be integrated within the building.

Likewise, since RED also needs two different salinity concentrations, we can implement them in our pre-existing infrastructure. We have two choices since RED often takes up significantly less space than PRO. First, we can add onto the buildings to ensure they have the necessary elements and room. Secondly, we have the option to add RED devices where they fit. The first option is more cost-effective as there are often issues with the building that need to be addressed before the location is suitable (Abdelkader). Now that we know where to use this, it all comes down to who can do it. The answer to this question is more complex. Projects of this scale often require large amounts of funding and oversight. We need our elected officials to see that this technology is worthwhile and either have them take the first step or incentivize outsiders to start this blue revolution.

Salt gradient energy technologies like PRO and RED offer a promising solution for generating clean energy from salinity gradients. These technologies have the potential to provide a significant source of renewable energy that could help reduce our reliance on fossil fuels. However, scaling up these technologies to commercial applications will require overcoming challenges such as membrane fouling and high initial production costs. We need to continue our research to address these challenges and incentivize funding and support from our elected officials to implement this technology in our society. By increasing the amount of power generated with clean energy, we can reduce the global need for fossil fuels and positively impact our planet. Retrofitting pre-existing infrastructure for PRO and RED would greatly decrease the development cost, making it more accessible for implementation. We must take action to support and promote the development of salt gradient energy technologies as a viable solution for clean energy. We are responsible for working towards a sustainable future, and embracing this technology is a step towards achieving that goal. Let us advocate for its implementation and consciously reduce our carbon footprint by utilizing renewable energy sources like salt gradient energy.

Works Cited

Abdelkader, Bassel A., and Mostafa H. Sharqawy. “Challenges Facing Pressure Retarded Osmosis Commercialization: A Short Review.” Energies, vol. 15, no. 19, 5 Oct. 2022, p. 7325., doi:10.3390/en15197325.

Altıok, Esra, et al. “Salinity Gradient Energy Conversion by Custom-Made Interpolymer Ion Exchange Membranes Utilized in Reverse Electrodialysis System.” Journal of Environmental Chemical Engineering, vol. 11, no. 2, 1 Apr. 2023, p. 109386., Doi: 10.1016/j.jece.2023.109386.

Choi, Jongmoon, et al. “Membrane Capacitive Deionization-Reverse Electrodialysis Hybrid System for Improving Energy Efficiency of Reverse Osmosis Seawater Desalination.” Desalination, vol. 462, 14 May 2019, pp. 19–28., Doi: 10.1016/j.desal.2019.04.003.

Cipollina, Andrea, and Giorgio Micale. Sustainable Energy from Salinity Gradients. Woodhead Publishing, 2016.

Elsaid, Khaled. “Environmental Impact of Desalination Technologies: A Review.” Science of The Total Environment, Elsevier, 8 Aug. 2020, www.sciencedirect.com/science/article/abs/pii/S0048969720350579.

La Mantia, Fabio, et al. “Batteries for Efficient Energy Extraction from a Water Salinity Difference.” Nano Letters, vol. 11, no. 4, 17 Mar. 2011, pp. 1810–1813., doi:10.1021/nl200500s.

Sarp, Sarper, and Nidal Hilal. Membrane-Based Salinity Gradient Processes for Water Treatment and Power Generation. Elsevier, 2018.

Shadravan, Arvin, et al. “Feasibility of Thin Film Nanocomposite Membranes for Clean Energy Using Pressure Retarded Osmosis (PRO) and Reverse Electrodialysis (Red).” SSRN Electronic Journal, 12 Nov. 2021, doi:10.2139/ssrn.4126120.

Valladares Linares, R., et al. “Impact of Spacer Thickness on Biofouling in Forward Osmosis.” Water Research, vol. 57, 27 Mar. 2014, pp. 223–233., Doi: 10.1016/j.watres.2014.03.046.

Ye, Meng, et al. “Charge-Free Mixing Entropy Battery Enabled by Low-Cost Electrode Materials.” ACS Omega, vol. 4, no. 7, 8 July 2019, pp. 11785–11790., doi:10.1021/acsomega.9b00863.

Zhou, Xia, et al. “Principles and Materials of Mixing Entropy Battery and Capacitor for Future Harvesting Salinity Gradient Energy.” ACS Applied Energy Materials, vol. 5, no. 4, 11 Apr. 2022, pp. 3979–4001., doi:10.1021/acsaem.1c03528.