When you want to disgorge a cucumber, you sprinkle it with salt. In this way, we create a very salty solution on its surface that causes water to move from the less salty part, the inside of the cucumber, to the more salty part, the outside of the cucumber. This is an example of the phenomenon of osmosis.
The key point is that the water moves under the action of a driving force, called the osmotic pressure, caused by the concentration difference of salt. Thus, the phenomenon of osmosis converts chemical energy into mechanical energy through a fluid flow that goes from the least salty to the most salty area.
Osmotic forces are found in the biological world in a considerable variety of phenomena: to store energy, induce mechanical motion, or control ejection and absorption of compounds, for instance.
One may even assess that plant life depends on osmotic forces: indeed to produce motion or growth, plants cannot rely on muscles but instead on an underlying hydraulic machinery driven by osmotic or humidity gradients .
Humanity faces a major challenge for the near future: if we want to minimise climate change caused by human activity, it is of the utmost importance to change our energy consumption habits and turn to viable, renewable and non-polluting energy sources.
In this context, nanomaterials play a role that we believe will become essential in the future, providing solutions to technical problems and often opening up new perspectives as we will see in this article on specific examples.
In our search for new energy solutions, it might be useful to draw inspiration from living organisms. For example, research mimicking photosynthesis is being conducted to find new solutions for energy storage.
Also, we are seeking to use osmotic pressure as a new source of energy in the same way as wind, hydraulic force or the sun. More precisely, we can ask ourselves whether it would be possible to use osmotic force to efficiently extract energy from sea water, the so-called blue energy.
If we want to use osmotic force, the first question to ask is the intensity of that force: is it large enough to be reasonably used to extract energy?
To get an idea of this, let’s imagine two reservoirs, one filled with fresh water and the other with sea water, separated by a membrane that allows only the solvent to pass while retaining salt, as a biological membrane can do.
The fluid will undergo a driving force that will push it towards the reservoir with the highest salt concentration. Reversely, in order to prevent the fluid from passing through the membrane, a pressure has to be applied to the fluid to counteract the flow: the applied pressure is then equal to the osmotic pressure.
This is a very powerful phenomenon: considering seawater and fresh water, we obtain an osmotic pressure of 30 atmospheres which corresponds to the pressure felt under a 300m water column!
Blue energy: power and challenges
Where can we find natural areas where waters with different concentrations of salts mix? In fact, there are a lot since it happens in every estuary.
Blue energy is the energy that could be harvested from differences in salinity, by mixing sea water and fresh river water. An estimate could be done by counting estuaries over the Earth, amounting to 8,500TWh , as compared to energy production from other resources: in 2015, hydraulic energy production was about 4,000 TWh, nuclear energy around 2,600 TWh and wind and solar a combined 1,100 TWh.
According to this estimate, blue energy is so plentiful that it could feed all our needs if we can just find an effective way to tap it!
How can we manage this fantastic driving force to harvest electricity? Among several approaches that are being developed, the most promising are Pressure-Retarded Osmosis (PRO) and Reverse Electro-Dialysis (RED).
- PRO uses the flow of water through the membranes, producing pressurised water that generates electricity using mechanical turbines. It has been tested for large-scale energy production by a pilot plant developed by Statkraft in Norway and by the Mega-ton Water System project in Japan.
- RED, on the other hand, uses membranes for ion transport, not water, and the electrical current generated is captured directly from the flow of ions. This technique is currently explored by REDStack on a large scale with a prototype plant in the Netherlands.
Despite encouraging results, none of the current attempts has reached yet a sufficient power density to be considered as economically profitable. Given the very high osmosis pressure, how is it possible to get such poor results?
Both strategies rely on separation of water from ions or ions from water. Yet, the main source of difficulties of these systems is the membrane. In order to be completely impermeable to the solute molecules, the membrane may only be slightly permeable to the solvent molecules, or vice versa.
Indeed, for the salt not to pass through we need a membrane that has nanometric to sub-nanometric pores. With such small pores, the permeability of the membrane is extremely low and therefore, the water flow is not very intense.
So, even if the pressure difference is gigantic, the power that can be recovered from these systems is measured in a few W/m2, below or sometimes slightly above the threshold of economic viability that is estimated to be 5 W/m2 .
Membranes with new nanomaterials
For blue energy to become a large-scale exploitable and competitive energy source in the future with respect to other renewable energy resources, it is necessary to make progress not only in the design of membranes, but also in our understanding of the physico-chemical mechanisms at work within these membranes.
Major efforts have been made in recent years to this end through multidisciplinary research combining chemistry, materials science and nanoscale fluid dynamics. A new class of materials has recently appeared, improving the conversion efficiency by several orders of magnitude, thus offering new and encouraging perspectives.
In 2013, membranes made of boron nitride (BN) nanotubes were investigated . These are materials structurally similar to carbon nanotubes except that carbon atoms are substituted by nitrogen and boron atoms.
Nanotubes with a radius of tens of nanometers showed an osmotic energy conversion density of several kilowatts per square meter, three orders of magnitude larger than the energy conversion density obtained with PRO or RED processes.
In 2016, membranes made of an atomically thin molybdenum disulfide (MoS2) layer with nanopores of few nanometers in diameter led to an outstanding power density of 106 W.m-2 !
The crystal structure of MoS2 takes the form of a hexagonal plane of sulfur atoms on either side of a hexagonal plane of molybdenum atoms. This is the second most investigated 2-dimensional material for potential device applications after graphene.
These remarkable performances are in part attributed to the extremely narrow widths of the membranes. Indeed, the flow through the membrane is expected to be inversely proportional to its width.
Thus, one year later atomically thin membranes based on hexagonal BN and graphene were investigated showing power densities in the range of 700 W.m-2 . All these recent results of extremely high osmotic power densities for devices using new materials as membrane supports have undoubtedly revived the field.
It is now necessary to further explore the class of materials that matches the conditions for a scalable osmotic power production.
Osmosis phenomenon at nanoscale
Lastly, it is puzzling to note that these new materials are all totally permeable to ions and water, unlike those used in membranes so far. What is then at the origin of the high osmotic flow, if even molecular separation does not seem to be a prerequisite as it has been assumed until now? To answer this question, it is necessary to consider, in detail, the mechanisms at work within the membrane at the nanoscale.
The important point is that surface charge of all BN, MoS2 and graphene membranes are very large compared to membranes used so far: surface charge up to 1 C.m-2 has been measured, which is two orders of magnitude larger than that of other conventional materials such as silica.
Surface charges act on the solution, creating a gradient of salt at the direct vicinity of the membrane. This causes an intense electric current which explains the exceptional performance of this new class of materials.
These examples stress the point that at the nanoscale, new phenomena may exist that may pave the way to novel strategies to develop devices.
Let’s observe nature more carefully
An ideal membrane must be both highly selective and highly permeable. How could this be possible with such seemingly contradictory prerequisites? Well, living organisms have already achieved this objective!
Indeed, transport channels at the nanometric scale are already known in biology as aquaporin, or water channels. They are membrane proteins present in many living organisms, animals and plants, which have unparalleled performances in terms of selectivity and permeability. These performances are attributed both to the particular structural shapes of these membranes and to electrical phenomena.
With our recent discoveries we have certainly made great progress on the road to ideal artificial membranes and, in the meantime, have advanced our knowledge, but there is still a long way to go. Perhaps we should listen more carefully to what nature has to tell us about how to effectively harness blue energy.
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