A Literature Review On Pressure Retarded Osmosis
In this review the use of pressure retarded osmosis to generate power will be investigated. It’s potential for generating power and its viability will be analysed by examining the findings of numerous research papers that discuss the use of different salinity gradients and the amount of power that can potentially be produced from said gradients. The issues that are preventing its success will also be analysed and methods of overcoming these issues will also be explored. Existing PRO plants and their performance will also be analysed.
Introduction
“The global economic system has a strong dependence on fossil fuels” (Kim, et al., 2012), and as the global energy demands continue to increase the search for new renewable energy resources has become increasingly more important. Salinity Gradient Power is generated through the controlled mixing of different salt solutions and is seen to be a promising sustainable energy. In a nutshell, the mechanism works as follows; a low concentration feed solution and a high concentration draw solution are separated by a semipermeable membrane. Water molecules are forced through the membrane from the feed to the draw solution due to the chemical potential difference between the different solutions while the solutes are retained. To increase the hydraulic pressure in the draw tank , the volume expansion in the draw solution is then constrained, and the pressurized flow that results from this is forced through a turbine to generate power. Despite it’s increasing popularity, it’s viability and its potential as a source of renewable energy is still under scrutiny.
Power Generation
It has been estimated that global energy consumption will increase by 56% and the total energy usage will be approximately 240 kTWh by the year 2040 (Han, et al., 2015), and currently only 13% of the world’s energy comes from renewable sources, mainly energy from biomass, and some from hydro, solar, and wind. To put into context how much energy can be produced from the mixing of water of different salinity, it is said that more than one terawatt of energy could be produced and obtained when the annual global river discharge, which is approximately 37,300 km3, comes into contact with the sea, or in smaller terms, approximately 0.70 to 0.75 kWh of energy is released when 1m3 of water enters the sea (Helfer, et al., 2013), while others say that up to 2.6 TW of osmotic energy could potentially be produced globally every year. Statistics such as these would have anyone convinced that producing energy by mixing differing salt solutions would make a great contribution to the world’s energy supply.
Many experts in this area have defined the “Gibbs energy of mixing as the theoretical upper limit of extractable energy” (Straub, et al., 2016) which is useful when conducting experiments regarding PRO performance as the results can be compared to this theoretical upper limit to determine the efficiency of the system. Claims have been made by various articles that typical PRO plants can produce anything from 2 to 10W/m2 of membrane, in some cases higher when systems are run at higher pressures e.g., 20 bar.
Different Sources of Salinity Gradients
There are different sources of salinity gradients that can be used to generate power using pressure retarded osmosis. Sea water and river water are probably the most widely used, other sources include brine from desalination plants and effluent streams from wastewater treatment plants. The feasibility of these sources depends on a number of factors; the amount of power they can generate, their availability, their likelihood of causing problems such as fouling, and their potential effect on the environment.
The vast majority of research papers focus on the mixing of sea water and river water. The most popular saline sources include the Great Salt Lake and the Dead Sea. Hypersaline lakes are very suitable due to the fact that they exist in many locations across the globe making it possible for this energy to be generated almost everywhere, most likely in costal cities at river mouths. Another reason that these saline sources are popular is because water sources such as the Dead Sea are only hospitable to bacteria, which means that using this water poses little threat to the environment, as it does not interfere with the living conditions of marine life. This is a very interesting point considering that PRO will be used as a renewable energy source; it can be used to help the environment, without harming it in the process, making it an extremely ‘green’ form of energy. It is also worth mentioning here that PRO is considered to be extremely environmentally friendly due to the fact that no chemical or greenhouse gases are released during the process of generating osmotic power (Han, et al., 2015). Unfortunately, some argue that an appropriate osmotic pressure differential across the membrane needed to produce high power density cannot be achieved using sea water and river water. Theoretical studies have shown that PRO is not capable of extracting net positive energy using these sources due to the problem just mentioned, and the high cost of operation (Straub, et al., 2016). However, the first pilot plant that was built for PRO power generation was capable of generating approximately 10kW of energy using sea water and fresh water, despite the fact that a power in the order of 5 W/m2 of membrane is considered to be a realistic value when the different losses are taken into account, and is also considered the requirement for economic viability of these water sources. It is estimated that there will be a power loss of approximately 1 W/m2 of membrane in a large, optimised plant. Other arguments related to these salinity sources are that being able to meet the condition needed for solutions such as those coming from the Dead Sea with an osmotic pressure of 507 bar is nearly impossible, and commercial membranes are not advanced enough for this particular process because of their low water permeability and the suboptimal building of their support layer.
The Mega-ton water project in Japan built a pilot plant with a PRO system that could recover energy from the mixing of brine from the Tua Spring desalination plant and wastewater effluent from the NEWater plant using a “state-of-the-art TFC-PES hollow fibre membrane”. It is thought that wastewater effluent could be used as an alternative to river water seeing as a power density of over 10W/m2 of membrane can be achieved using these solutions, however the problem with this is the need for pre-treatment of the wastewater effluent; the effluent would need to be treated prior to entering the PRO system to ensure that membrane fouling would not be an issue.
Brine and river water produce very high osmotic pressure differentials meaning that this could be a possible system however the major problem with this system is that it is limited to only certain places because the majority of desalination plants are normally located in arid regions, and in these regions a sufficient amount of river water would not be available to warrant building a PRO plant. So, while it may work well, the amount of energy that this system can produce is quite limited, only minimally contributing to the world’s energy supply.
Brackish water is water that has more salinity than fresh water but does not have a salt concentration as high as sea water. The use of it with sea water has been discussed in various research papers. Brackish groundwater is usually of high-quality meaning that it would not require extensive pre-treatment. Again, availability is the problem with this solution; there is just simply not enough available to supply the demands of a PRO plant.
It is also expected that brines of high salt concentrations will be paired with sea water to produce energy using PRO in the future once high-performance membranes are developed. The complexity of membrane design would mean that sufficient membranes may not be developed any time soon.
Issues preventing the success of Pressure Retarded Osmosis
There are a couple of issues that are hindering the success of PRO as a source of renewable energy. Many research papers have highlighted some practical limitations and other potential problems.
Despite the fact that PRO plants can product approximately 10 kW of power per m2 of membrane, ensuring that the net amount of energy produced is enough to be considered viable is a challenge. Currently, it is estimated that PRO plants consume approximately 2kW of power per metre cubed of water, significantly reducing the amount of extractable energy. Until plant power consumption can be reduced, the amount of energy produced by PRO is not feasible.
Operating pressure is another problem regarding total extractable energy. To obtain the maximum amount of extractable energy from the mixing of saline solutions, the system has to be run at very high pressures, up to 20 bar. The constraint of constant-pressure operation can lead to a decrease of 20 to 30% in the specific extractable energy.
Other factor that affects the maximum amount of extractable energy that can be obtained from a PRO process is membrane size. Very large membranes are required to extract the maximum amount of energy, meaning a balance will need to be obtained between these different factors. By increasing the power density, it will enable a high-power output system coupled with a low membrane area. This is an important observation as the membrane is likely to be one of the biggest capital costs for the plant. Fabricating membranes that can withstand the high operating pressures required for PRO while satisfying all other requirements has also proven to be quite difficult.
PRO is also affected by non-ideal phenomena including concentration polarisation and membrane fouling. There are two types of concentration polarisation; external concentration polarisation, and internal concentration polarization “Internal concentration polarisation is considered as the most severe constraint in the osmotic membrane processes, as it significantly depresses the water permeation across the membrane” (Chou, et al., 2011). ICP takes place on the feed side of the membrane due to the fact that the support layer limits mixing of the feed solution. Unfortunately, increasing the feed flowrate or causing turbulence across the membrane will not reduce ICP due to the fact that it occurs within the membrane. ICP was found to be largely dependent on factors such as membrane morphology, water permeation flux, and the characteristics of the solute. The most common methods of combatting ICP are decreasing the thickness of the membrane and increasing porosity and lowering tortuosity. However, the issue with these actions is that there is a risk of creating membranes with bad mechanical properties which could break when put subjected to high pressures. Also, enhancing hydrodynamic conditions within the system in an attempt to reduce ICP will cause an increase in the demand of pump energy which will again affect the net power produced. ECP is considered to be a phenomenon where concentration boundary layers form outside of the membrane.
Membrane fouling is another major issue that needs to be addressed. Fouling is often found to be a problem when wastewater effluent is used in the system. Membrane fouling decreases pure water permeability and reduce water flux which will reduce the osmotic driving force (Wan & Chung, 2014). In an attempt to prevent membrane fouling the wastewater effluent would can be treated using both ultrafiltration and nanofiltration, adding extra costs and extra time to the overall process.
Reverse solute diffusion occurs when salt diffuses back into the membrane through the selective layer due to the concentration gradient. This event is definitely not desirable as it can encourage membrane fouling, reduce the osmotic driving force, as well as worsen the effects of internal concentration polarisation.
One other worrying issue is that the amount of studies that have been conducted on the problems discussed above are still quite limited. Creating predictive models would mean that they could be used to help in the determination of membrane characteristics (Achilli, et al., 2008).
Plant location also poses it’s own challenges; building a plant in an estuary is difficult and the plant needs to be somewhere where it is guaranteed a suitable supply of both water sources due to the fact that the reliability of the process is highly dependent on intake.
Existing Pilot Plants
Before the year 2009, PRO systems could only be found at laboratory scale. But on the 24th of November during this year the first osmotic power prototype plant was built by Statkraft, a leading company in hydropower generation, in Tofte, Norway. The build was based on ‘The Osmotic Power Project’ that took place between the years 2001 and 2004 which was funded by the EU and designed to produce 10kW of energy. Statkraft made a deal with Nitto Denko/Hydranautics who agreed to develop and supply membranes for the plant, these membranes needed to be capable of producing 5 w/m2 of membrane to ensure a positive net energy. The company also had a plant to construct an even larger pilot plant in Sunndalsøra, Norway. However, on the 20th of December 2012 Statkraft announced that they were ‘pulling the plug’ on osmotic power. While it was possible for Nitto Denko to produce appropriate membranes, membranes capable of producing 10 W/m2 of membrane can actually be formed, the price of these membranes are much larger than standard RO membranes which became the biggest problem for Statkraft, so they decided to discontinue their work in the development of osmotic power technology. When looking at the current market outlook, Statkraft has realised that the technology will simply not develop enough to become competitive in the near future and the company believes that there “are more competitive and relevant investments for us in the future” (Statkraft, 2013). The company’s decision to discontinue its investments into osmotic power has made researchers and other potential PRO commercial producers question the viability of this system and wonder if it will ever become widespread industrial practice.
In the year 2012, Japan also entered the osmotic power generation game when they built a plant in Fukuoka City. The research was carried out by Kyowakiden Industry Co who partnered with the Tokyo Institute of Technology and Nagasaki University (Helfer, et al., 2013). Very limited results have been released from this research as of now.
