For centuries, the idea of harnessing freshwater from rivers before it reaches the sea has been prevalent. Even today, despite our greater environmental awareness, we understand that there is no waste, as everything has its purpose in conserving natural cycles, maintaining environmental balance, and preserving ecosystems.
Blue energy represents a milestone in humanity’s long-standing aspiration to harness river water, as well as a new, very promising renewable energy source. However, it is not as well-known as river hydroelectric energy or marine-based energies such as tidal energy, which we have already discussed in a publication on its relevance in Spain.
What is Blue Energy?
Both tidal energy, which we just mentioned, and offshore wind and wave energy (energy production from sea waves) are also often classified as blue energy. However, in strict terms, blue energy is specifically the energy generated from the contact between freshwater and saltwater due to their salinity difference, through a phenomenon known as osmosis. In osmosis, liquids with lower saline concentration tend to flow toward those with a higher level, generating a pressure that can be converted into energy, capable of powering, for example, a turbine.
This process occurs naturally at the mouths of rivers and can be induced to generate renewable energy in three different ways:
- Pressure Retarded Osmosis (PRO): a membrane is used to balance the salt concentration of both types of water, which increases the pressure and thus pushes water toward the turbine.
- Reverse Electrodialysis (RED): in this case, special membranes are used to separate both types of water, specifically designed to allow ions of a certain charge to move in one direction, generating a voltage of electrical energy.
- Capacitive Mixing (CapMix): this is the most experimental method, using similar electrodes in both saltwater and freshwater to produce electricity from the ions present in both types of water.
The first research on osmotic energy dates back to 1954, when the viability of extracting an energy source from the chemical processes resulting from mixing fresh and saltwater was studied. But it wasn’t until the 1970s that the membrane-balancing method was successfully tested to harness the osmosis process through pressure retarded osmosis. A few years later, in 1977, blue energy was produced with reverse electrodialysis using a thermal motor.
Regarding membranes, while types capable of restricting certain substances and allowing others have existed for centuries, those designed to operate with ions or electrodes are the result of technological developments from the 1990s. They continue to be a major area of focus today, as work continues to enable cells to generate higher voltage and reduce their size, which is very important, as we will see a little further on.
The Great Advantage of Blue Energy Compared to Other Renewables
Blue energy is highly promising, even though it is still in an embryonic phase compared to other fully operational renewable energies. This is because, due to its characteristics, it overcomes the challenges of continuous use faced by solar, wind, or some marine energy sources, simply because the sun doesn’t always shine, the wind doesn’t always blow, or there aren’t always waves in the sea…
In contrast, river flows into the sea are constant and occur in all countries with coastlines (more than 43% of the global population lives in coastal areas), although not all have rivers with high flow rates.
Thus, when we talk about blue energy, we refer to a renewable that, like solar and wind, does not release CO2 during its generation. However, unlike these sources, blue energy is not climate-dependent, which means it offers continuous availability.
Additionally, since all the world’s rivers flow into the sea, the potential amount of energy that could be generated is immense. In fact, it has been estimated that this energy source could meet 40% of global needs, based on a study by Pennsylvania State University, which is currently the global reference center for blue energy research.
At this university’s laboratories, membranes capable of producing 12.6 watts per m² have been developed by combining the methods of reverse electrodialysis and capacitive mixing. This has been a major advancement for the efficiency prospects of blue energy, as according to these figures, a small power plant could be sufficient to supply a population of 30,000 people.
However, these results have been achieved in laboratory conditions, and we must wait for the outcomes of projects in real environments, where issues such as membrane fouling from wastewater and the passage of organic microcontaminants through ion exchange cells will occur.
Ongoing Blue Energy Projects
The first successful blue energy projects were developed in the past decade in Japan, the Netherlands, and Norway, where the world’s first osmotic power plant was established in the city of Tofte.
Currently, the most ambitious project in Europe is being developed by Compagnie Nationale du Rhône in collaboration with Sweetch Energy, aiming to build an osmotic plant at the mouth of the Rhône River capable of generating large amounts of energy. It is expected that once the plant becomes operational in 2030, it will be able to produce 4 TWh of electricity per year.
To put this amount in perspective, considering that the average household electricity consumption in Spain is approximately 3,000 kWh (kilowatt-hours) per year, the blue energy generated by the plant could meet the needs of 1.33 million households, which is more than the households in a city the size of Barcelona with its entire metropolitan area.
Additionally, other major projects are underway in Europe to explore the potential of blue energy resources at the mouth of the Danube River in the Black Sea.
In Spain, although there is a Roadmap for the Development of Offshore Wind and Marine Energy promoted by the Ministry for Ecological Transition, it does not specifically include actions for blue energy projects, focusing instead on tidal energy (energy generated from tides) and wave energy, as well as offshore wind, an area in which the country is a global leader in terms of generation.
There is a strong Spanish company, Sacyr, leading a major European renewable blue energy project. This project, called Life Hyreward, is focused on generating renewable energy from the brine remaining in the desalination process, using a combination of reverse osmosis and reverse electrodialysis (RED).
The first experimental phase of the project began at the Alicante desalination plant at the end of 2021 and is expected to conclude in the first half of 2025.
Challenges of Blue Energy
Blue or osmotic energy generation still faces several significant challenges to be resolved before it can be viable on a large scale, despite its evident potential and the advances made in its research and development.
Firstly, a mature technology has yet to be developed to avoid the high production cost; while solar and wind energy production only require solar panels and wind turbines, blue energy currently requires the construction of a salinity plant.
As a result, in the most optimistic estimates, the cost per megawatt is double that of fossil fuels like gas and oil.
The key lies in the effective reduction of membrane size necessary for osmosis processes. Currently, research is underway to reduce the pores of the membranes using nanotechnology so that ions as small as atoms can pass through them. This would enable the feasibility of installations comparable in size and cost to solar panels or at least wind turbines, thus avoiding the need to build large industrial plants in river estuaries, which entail substantial investments in infrastructure, resources, personnel, and maintenance, driving up the production cost per megawatt to the point of being very difficult to make profitable.
In short, blue energy offers many potential benefits due to its non-climate-dependent nature, unlike other dominant renewables like solar and wind energy. However, there is still a significant journey of technological development ahead to scale up production and make it a viable, efficient, and profitable energy source.