Osmotic power: a huge and untapped resource

Two energy workers beside a turbine at Statkraft's osmotic power plant, Norway
Two energy workers beside a turbine at Statkraft's osmotic power plant, Norway

The energy released when waters with different salt concentrations are mixed constitutes a form of renewable energy, and it’s receiving a lot of attention around the world.

We’re talking about salinity gradient power or osmotic power – otherwise known as ‘blue energy’.

The amount of energy that can be obtained, as for example, when a river meets the sea, is huge. According to a study carried out in 2012 by a team from Yale University led by Professor Menachem Elimelech, about 0.75kWh (2.7MJ) is dissipated when one cubic metre of river water mixes with one cubic metre of seawater. Theoretically, 1MW of power can be obtained when one cubic metre of fresh water mixes with one cubic metre of seawater.

The International Energy Agency (IEA) estimates the global potential of salinity gradient energy at about 2000TWh/a. According to Wetsus – European Centre for Excellence for Sustainable Water Technology – by mixing seawater and river water, it is theoretically possible to obtain more than 2TW of power, which is close to the present worldwide electricity demand.

Salinity gradient energy also entails the advantages that it is a renewable energy alternative that has a minimal environmental impact, does not release CO2 and entails no fuel costs, and constitutes a base-load power source, available all the time.

Several meetings of the Integrated Network for Energy from Salinity Gradient Power (INES) – a project designed to foster the development of salinity gradient power – have been held since the inaugural meeting in April 2010 in Brussels. Follow up meetings were held in Oslo, Barcelona, Singapore, and Brussels. Most recently a meeting took place in Singapore on July 11.

INES is part of the Institute for Infrastructure, Environment and Innovation, a private non-profit organisation based in Brussels.

Energy from salinity gradients is a consequence of a fundamental law of nature, expressed as the second law of thermodynamics, which says in an isolated system the entropy – the measure of molecular disorder within a macroscopic system – never decreases. In other words, the system tends toward increased disorder. That is, the system tends toward the state of highest probability. Therefore, solutions with different concentrations, such as waters with different salt concentrations, tend to mix.

Professor Charles Lemckert and doctor Fernanda Helfer from the School of Engineering, at Griffith University in Queensland, have carried out a study of Australia’s potential for osmotic power production. They concluded Australia has a great potential for osmotic power production with many favourable factors. The country has various sources of saline solutions that could be used, including salt lakes, brine from desalination plants and saline groundwater.

Australia’s largest urban centres are located near the ocean and close to river mouths, ideal conditions for osmotic power plants.

Osmotic power plants can also be used to reduce the energy required to operate desalination plants. Professor Lemckert et al have also studied ways of reducing the consumption of energy in seawater desalination based on the utilisation of the energy harnessed from the mixture of brine (the main seawater desalination by-product) and seawater.

Several technologies have been developed to obtain energy from salinity gradients, but two of them have been most extensively studied and have been tested in pilot scale plants: pressure retarded osmosis (PRO) and reverse electrodialysis (RED).

PRO employs a semi-permeable membrane separating two solutions with different salt concentrations. The membrane allows water through but does not allow salt or ions through. Water flows from the less saline solution (feed solution) to the saltier solution (draw solution) to equalise the salt concentrations on the two sides. This increases the volume (level) of the saltier solution and therefore, its pressure. A system of valves allows the saltier solution through a hydro turbine, which produces electricity in the usual way.

A PRO plant has been in operation since 2009 by the Norwegian utility Statkraft at Tofte, a village in the municipality of Hurum, in Norway’s southern coast. The facility, located on the premises of the Sodra Cell cellulose factory at Tofte, has also been used for testing components and membranes. The plant combined 10 litres of fresh water and 20 litres of salt water per second. It was equipped with 2000sq m of membranes and was only capable of a modest power output of 2kW to 4kW. Statkraft planned to develop a plant capable of producing 25MW. According to the utility, up to 2.85GW would be available from the PRO process in Norway.

However, in 2013 Statkraft decided to halt investing in osmotic power because it decided that the technology would not be competitive with other renewable energy technologies within the foreseeable future. Statkraft department manager Stein Skilhagen said: “We have proven that the technology works and we achieved substantial improvements.”

“We are now leaving the process of maturing the technology to others, as several independent enterprises around the world are looking into this already.”

RED operates very differently from PRO. RED is basically a salt battery. It consists of an array of alternating cation and anion exchange membranes. Each membrane has fresh water on one side and salt water on the other side. The anion exchange membranes allow only negatively charged ions (Cl- ions) to pass and cation exchange membranes, which allow only positively charged ions (Na+) to pass. Thus the ions – positive and negative – migrate in opposite directions, from salt water into fresh water. All the positively charged ions will only be able to diffuse through the cation-exchange membranes and all the negatively charged ions will only be able to diffuse through the anion-exchanged membranes. Thus, the bulk transport of positive ions will be in one direction and that of the negative ions will be in the opposite direction. This creates positive and negative poles, as in a battery. As a result, voltage is generated over each membrane and the total voltage of the system is the sum of the voltages over all membranes. Connecting the two poles
by a conductor causes an electrical current to flow.

Two plants have been operating with the RED process: one in Trapani, on the west coast of Sicily, and one in the Netherlands.

The Trapani plant ceased operating about six months ago, but the Dutch plant opened in November 2014 and is now the only running pilot plant utilising the RED technology. It is located on the closure dam Afsluitdijk, and produces electricity directly from the difference in salt concentration in the surface water on each side of the dam.

According to Professor David Vermaas of the Technical University of Delft, who had a part in developing the technology, the plant is running well. The plant will be fed with 220 cubic meters per hour of seawater and fresh water, and will have a maximum capacity of 50kW.

Professor Vermaas said the results from that pilot plant will help overcome issues such as membrane fouling and lead to a more economically competitive plant design. The plant is operated by Redstack BV, the company that developed the technology based on RED using specially designed membranes. The project to develop the pilot plant, called Blue Energy, is a joint initiative of Redstack, Fujifilm who supplied the ion exchange membranes and the water technology knowledge institute Wetsus.

While at the University of Twente, in the Netherlands, Professor Vermaas carried out research on strategies to prevent fouling of ion exchange membranes, with a group that included Professor Kitty Nijmeijer. Professor Vermaas et al have founded a company, AquaBattery, to develop a rechargeable salinity gradient battery – not for energy generation, but only for energy storage.

The Trapani project was an accomplishment of the REAPower project, carried out by a European consortium, which included staff from the University of Palermo. A RED unit equipped with almost 50sq m of ion exchange membranes (125 cell pairs, 44x44sq cm) was tested with feed solutions corresponding to brackish water and saturated brine. About 40W (1.6W/sq m of cell pair) was obtained using natural solutions, while an increase of 60 per cent was observed when testing with artificial solutions, reaching about 65W (2.7W/sq m of cell pair). The plant performance was monitored over five months and no significant performance losses were observed.

Membrane power density (power generation per unit area of membrane) is an important factor in osmotic power, since it determines the amount of membrane required in the plant. The higher the membrane power density the less membrane area is required, and the less will be the costs of installation and maintenance. For a large-scale osmotic power plant operating with river water and seawater, the minimum power density required for commercial viability is 5W/sq m.

According to Michael Papapetrou, one of the members of the REAPower team that developed the system, conditions at Trapani were particularly favourable for the RED process. The difference in saline concentrations at Trapani was far greater than the saline difference between seawater and fresh water. The use of brackish water rather than river water, as the low salinity stream was a major factor in Trapani’s high levels of electrical output per square metre of membrane, because it increased the overall conductivity of the system.

According to Professor Lemckert and doctor Helfer, with the currently available membrane technology, the capital costs associated with an osmotic power plant remain very high, making osmotic energy uncompetitive when compared with other sources of renewable energy. They believe osmotic power still remains several years away from commercial viability. However, there is an increasing interest in osmotic power due to its great potential and the technology behind osmotic power has been advancing rapidly.

By Paul Grad, engineering writer