ReviewOsmotic power with Pressure Retarded Osmosis: Theory, performance and trends – A review
Introduction
Global energy supply for human activities is dominated by fossil fuel combustion [1], which due to high emissions of greenhouse gases, is accelerating changes in our climate towards dangerous long-term effects [2], [3]. It is estimated that only 13% of our energy is sourced by renewable resources, mainly shared between biomass and waste (75%), hydro (17%) and solar and wind (6%) [1]. Geothermal, wave and tidal energies account for the rest of the share (2%). To reduce the reliance on fossil fuels while also satisfying growing energy requirements, new alternative sources have to be explored and embraced, particularly renewable sources due to the smaller impact on our environment.
A type of renewable and gas emission-free energy that has just recently been given credibility is salinity-gradient energy, which is based on the release of free energy upon mixing of waters with different salt concentrations, as between rivers and oceans. When appropriately harnessed, this energy can be used to produce power [4].
In the context of this review, the process of harnessing salinity-gradient energy is best explained in terms of osmotic pressure. Osmosis occurs when two solutions of different concentrations (for example, different salinities) are separated by a membrane which will selectively allow some substances through it but not others. If these two solutions are fresh water and sea water, for example, and they are kept separated by a semipermeable membrane that is only permeable to water, then water from the less concentrated solution side (fresh water) will flow to the more concentrated solution side (sea water). This flow will continue until the concentrations on both sides of the membrane are equalized or the pressure on the concentrated solution side is high enough to stop further flow. Under no flow conditions, this pressure will be equal to the osmotic pressure of the solution. Osmotic pressure of a given solution is therefore not a pressure that the solution itself exerts, but a pressure that must be applied to the solution (but not the solvent) from outside in order to just prevent osmotic flow.
Pressure Retarded Osmosis (PRO) is the process through which osmotic energy can be harnessed and power generated [5]. Putting it simply, in PRO, a water flow is diverted at low pressure into a module wherein a semipermeable membrane keeps it separated from a pressurized and saltier water flow. The saltier water flow draws the less concentrated water through the semipermeable membrane due to its higher osmotic pressure, increasing the volume of the flow. A turbine is coupled to the pipe containing the increased pressure flow to generate power. Power generated via PRO is referred to as ‘osmotic power’.
The most known and studied application of PRO technology for power generation is the pairing of river water (less concentrated solution or feed solution) and sea water (more concentrated solution or draw solution), as schematized in Fig. 1. Under this arrangement, incoming river water and seawater are both diverted into adjacent chambers of a membrane module. The two flows are separated by a semipermeable membrane with the active layer facing the seawater side, allowing only river water to flow through it. This process increases the volume of water on the seawater side. The resultant high-pressure, brackish water is then split into two paths: part of the flow is used to drive a turbine, and generate power, and the other part returns to the pressure exchanger. The pressure exchanger is designed to transfer pressure energy from the pressurized brackish water to the incoming sea water. Similarly, sea water could also be used as feed solution, paired with a more concentrated solution, such as brine from seawater desalination plants [6], [7], [8], or hypersaline water from salt lakes or salt domes [9], [10].
PRO was invented by Prof. Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel, with his first publication released in 1975 [5]. The method has been improving over the years, particularly after the opening of the first osmotic power plant prototype by the Norwegian state-owned power company, Statkraft, in 2009 [12]. This prototype has been designed to develop and test new PRO technologies, particularly novel semipermeable membranes, and is projected to become the first large-scale osmotic power production facility in the world by 2015 [13]. The plant operates using river water and sea water, as shown in Fig. 1.
This article analyses technical, economical, environmental and other aspects of PRO. It combines the findings of the latest research, outlining the advancements achieved in the last few years and the hurdles that need to be overcome for the effectuation of osmotic power production on a commercial scale. This article also discusses some combinations of water solutions under which osmotic power could be produced, beyond the traditional pairing of river water and sea water. It is also an objective of this paper to provide an informative document that encourages governments, research institutions and private investors to combine efforts to accelerate the development of PRO technology and its availability as a renewable energy source.
Section snippets
World's potential for osmotic power
Salinity-gradient energy is the energy released when waters with different salt concentrations are mixed together. Presumably, this energy can be easily encountered at the interface between waters of differing salt concentrations, for instance where rivers meet the ocean. Approximately 0.70–0.75 kWh (2.5–2.7 MJ) is dissipated when 1 m3 of fresh water flows into the sea [14], [15], meaning that 1 m3 s−1 of fresh water can potentially generate 2.5–2.7 MW). Table 1 summarizes the maximum energy that
Osmotic processes
The energy released through the mixing of fresh water and salt water can be more easily explained using the osmosis effect, hence the name ‘osmotic energy’. Osmosis is the transport of water across a semipermeable membrane from a solution of higher chemical potential (i.e., lower osmotic pressure or lower salt concentration) – typically referred to as the ‘feed solution’ – to a solution of lower chemical potential (i.e., higher osmotic pressure or higher salt concentration) – referred to as the
Osmotic power with PRO
The concept of harvesting the energy generated from mixing waters of different salinities was first reported by Pattle [4], and then re-investigated in the mid 1970s, when the world's energy crisis prompted further research into energy supply alternatives. The discussions on PRO expanded rapidly after 1970 particularly due to the theoretical and experimental publications of Loeb [5], [9], showing the feasibility of PRO. Loeb [9] was the first to report that osmotic energy could indeed be
Membrane performance
Membrane performance in PRO is usually measured in terms of power output per unit area of membrane – referred to as membrane power density. The power density of the membranes is particularly important as it will directly affect the costs of osmotic power. The higher the power output per unit area of membrane, the cheaper the costs with installation, maintenance and plant operation. It should be noted, however, that the power generation capacity of an osmotic power plant is not limited by the
The existing PRO pilot power plant
Until 2009, PRO studies had only been conducted in laboratory scale and no one had tested the feasibility of the technology in real scale. In 2009, the first osmotic power plant prototype based on the PRO technology was finally opened in Tofte, Norway. The plant prototype belongs to Statkraft and was built driven by the encouraging results demonstrated by ‘The Osmotic Power Project’ (2001–2004), funded by the European Union and conducted by a joint effort of Statkraft, ICTPOL of Portugal,
Environmental impacts of osmotic power
Overall, osmotic power with PRO is claimed to have very limited to non-existent environmental impacts in comparison to current power production methods e.g., [26], [29], [56], [69]. This is mainly attributable to emission-free energy production and to the fact that the brackish water discharged from the plant would mimic the natural discharge of a river into the ocean [78]. The water from the river diverted into the plant would not be consumed, but only cycled through the plant [58]. However,
The economics of PRO
As with some other ocean energy technologies, it is difficult to estimate the cost of osmotic power due to the absence of large-scale plants to validate cost assumptions. The main advantage of PRO in relation to other renewable energy sources lies in its reliable baseload power, which can make the annual energy costs comparable and competitive with other renewables. Under a constant supply of feed and draw solutions, it is anticipated that an osmotic power plant can be designed to operate
Final considerations
The world should reduce its dependence on fossil fuel combustion by increasing the production of renewable energy. Continued reliance on fossil fuel to meet our growing energy demands is unsustainable due to its finite availability [40] and the fact that it is accelerating climate change towards long-term, dangerous effects [2], [3]. The harnessing of the salinity-gradient energy originated at the interface between waters of different salt concentrations through the PRO technology could make an
Acknowledgments
Funding for this project has been provided by the Griffith Climate Change Response Program and by the Centre for Infrastructure Engineering and Management, Griffith University, Australia.
References (106)
Production of energy from concentrated brines by pressure-retarded osmosis: I. Preliminary technical and economic correlations
J. Membr. Sci.
(1976)- et al.
Production of energy from concentrated brines by pressure-retarded osmosis: II. Experimental results and projected energy costs
J. Membr. Sci.
(1976) - et al.
The potential for power production from salinity gradients by pressure retarded osmosis
J. Membr. Sci.
(2009) Power from salinity gradients
Energy
(1978)- et al.
Forward osmosis: principles, applications, and recent developments
J. Membr. Sci.
(2006) - et al.
Osmotic power–power production based on the osmotic pressure difference between waters with varying salt gradients
Desalination
(2008) - et al.
Membrane processes in energy supply for an osmotic power plant
Desalination
(2008) - et al.
Comparative mechanical efficiency of several plant configurations using a pressure-retarded osmosis energy converter
J. Membr. Sci.
(1990) Large-scale power production by pressure-retarded osmosis, using river water and sea water passing through spiral modules
Desalination
(2002)- et al.
A two-coefficient water transport equation for pressure-retarded osmosis
J. Membr. Sci.
(1978)