”It is a huge potential — it could be a clean energy source for millions of people”

Osmotic power is also known as blue energy or salinity gradient power and is the energy that is released during the mixing of two solutions with different levels of salinity, such as seawater and river water. In 2009, Tavajohi tells, a pilot plant in Tofte, a village located by the Oslo fjord in Norway, with a generating capacity of 10 kW was started up. Five years later, another one was installed at Afsluitdijk, the Netherlands. That same year, the Dutch king Willem-Alexander led the construction of the world’s first blue energy power plant in Leeuwarden producing electrical energy by mixing freshwater with seawater.

— Theoretically, we can calculate the amount of energy by using thermodynamic calculation (the study of the relations between heat, work, temperature, and energy, Ed’s note). For instance, we know when 1 m3 of freshwater flows into the sea, 0.8 kW/h energy can be obtained. The principle is simple but the potential is huge if we consider the number of rivers and seas in the world. The theoretical investigation estimates the total salinity gradient power (blue energy) in the world is around 2.6 TW which is the second-largest marine-based renewable energy source next to ocean waves. However, this number for Salinity Gradient Power (SGP) can be increased significantly by considering the other source of SGP. For instance, the SGP potential from wastewater discharge into an ocean is estimated to be 18 GW. It is a completely clean and sustainable energy source with no toxic gas emissions. Unlike intermittent wind and solar energy sources, blue energy can be exploited continuously 24h per day and 365 days a year. It can be obtained from natural or artificial resources. Naturally, SGP can be harvested in river mouths, where two solutions with different salinity meet, from saline groundwater, saltworks, salt lakes, and brine of natural. The river has a low salinity in comparison with the sea. The river mouth where the river is entering the sea is one natural spot. Any two water streams with different salinity gradients that exist in nature could potentially be a good spot for harvesting blue energy. An artificial source of SGP includes, but is not limited to, oil-gas fields, and brine from desalination units. Any spot in the industry which generates high salt content water could be a good spot for producing it. For instance, we have a desalination plant. We take water from sea and separate the water to be used as drinkable water. Meanwhile, we are producing the concentrated salty water which needs to be discharged. The produced concentrated salt has a high salinity in comparison with seawater and can be used to produce blue energy. We are producing concentrated brine in different industries. Therefore, we can harvest blue energy in different industries, says Tavajohi. He continues:

— Harvesting blue energy was not practically feasible on large scale due to the low performance of the technologies. Currently, there are different technologies for harvesting the salinity gradient energy such as Reverse Electrodialysis (RED), Pressure Retarded Osmosis (PRO), vapour pressure difference utilization, mixing entropy batteries, and capacitive mixing. Among them, RED and PRO are the most promising technologies for harvesting blue energy on large scale. Yet, they have challenges in the cost, performance, and stability of the semi-permeable membranes that are being used in RED or PRO systems. In our new project, we are aiming to assess the commercial membranes based on Swedish water resources.

Tavajohi received his PhD in Energy Engineering from Hanyang University in Korea and in Chemical Engineering from the University of Calabria in Italy in 2015. He moved to Sweden in October 2018 and one year later, he built his laboratory at the chemistry department at Umeå University, where he now works as an assistant professor. For the project he mentioned, he, together with researchers at Lund University, has received €470.000 from the Swedish Energy Agency.

— The first and most important aspect is to assess the potential of blue energy in Sweden considering the artificial and natural resources, which we do not yet have any information about. This could be very interesting for us and for future researchers who may come up with other novel ideas and for SMEs that are active in the energy sector. The second point is to identify our standing point based on Swedish water resources and commercial existing membranes. As mentioned, the semi-permeable membrane plays a key role in RED and PRO systems. The third aspect of the project is to develop the membrane.

It’s said that the potential equals the capacity of 2,000 nuclear reactors. How come it’s so unexamined?

— To be honest, it was surprising for me as well — I checked and re-checked the Science report. It is a huge potential. It could be a clean energy source for millions of people. Then I thought: why don’t we take any action to assess this source of energy in Sweden? Indeed, it was the starting point of this project. As I mentioned the capacity is huge. Yet, the technology for harvesting has not been developed. The heart of the system for harvesting is a semi-permeable membrane. The membranes suitable for harvesting blue energy are the main obstacle. As I said, we are working on the membrane as well in this project.

In Scandinavia, hydropower is fundamental for the energy system, however not as sustainable as it may seem when releasing a large amount of CO2. What’s the situation for blue energy?

— To my best knowledge blue energy does not have a negative impact on the environment. Yet, we need to assess the life cycle of this process to identify any possible negative impact on humans or the environment, which we’re planning to do in this project.

Can the technology also be beneficial in other ways? Can it help to turn undrinkable salt water into drinking water?

— Desalination plants are one of our main targets for harvesting blue energy in this project. The concentrated brine leaving the Reverse Osmosis process (a technology that is used to remove a large majority of contaminants from water, Ed’s note) is an ideal solution for generating electricity. Furthermore, there are some novel applications of harvesting blue energy with a different system. For instance energy recovery in the industry, such as low-grade waste heat. It is worth noting that salinity gradient power can collaborate with seawater desalination, brine production, and other renewable energy generation, where, for instance, NASA has investigated a technology that can purify water and create energy at the same time.

What’s next for you now?

— My main goal in this step is to prepare a salinity gradient power map for Sweden based on existing Swedish natural and artificial resources. This map provides valuable information for the SMEs, researchers, and regulator officers in the coming years. Following that we are aiming to tailor the existing technology based on Swedish water resources to maximize extractable energy. I believe advances in blue energy will contribute to sustaining Sweden’s leadership in the renewable energy sector, Tavajohi concludes.