This paper presents a study on the potential of osmotic energy for power production. The study includes both pilot plant testing and theoretical modelling as well as cost estimation. A projected cost of £30/MWh of clean electricity could be achieved by using a Hydro-Osmotic Power (HOP) plant if a suitable membrane is used and the osmotic potential difference between the two solutions is greater than 25 bar; a condition that can be readily found in many sites around the world. Results have shown that the membrane system accounts for 50%–80% of the HOP plant cost depending on the salinity difference level. Thus, further development in membrane technology and identifying suitable membranes would have a significant impact on the feasibility of the process and the route to market. As the membrane permeability determines the HOP process feasibility, this paper also describes the effect of the interaction between the fluid and the membrane on the system permeability. It has been shown that both the fluid physical properties as well as the membrane micro-structural parameters need to be considered if further development of the HOP process is to be achieved.
A two-step forward osmosis (FO) desalination process combining both FO and reverse osmosis (RO) systems has been developed by the Centre for Osmosis Research and Applications at the University of Surrey and commercialised by Modern Water plc. In the FO + RO process seawater was used as feed water (FW) and a concentrated aqueous solution was used as a draw solution (DS). By taking advantage of natural osmosis, pure water is transferred from the FW to the DS and then recovered from the DS by the RO process utilising low resistance membranes, and hence lower specific energy consumption (SEC). This paper presents results of FO experiments conducted on flat sheet membrane using a bench-scale rig. The osmotic agent investigated in this study was magnesium sulphate, which is non-toxic, and highly soluble in water. Furthermore experiments were carried out on the RO pilot in order to regenerate the DS for reuse in the FO process and produce clean water. This paper also presents some pilot plant results and data from commercial plants in Oman and Gibraltar. The data demonstrates the efficiency of the FO + RO compared with the conventional RO process in terms of SEC and membrane fouling performance.
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The availability of clean water for agriculture has become a growing concern because of the increasing global water scarcity. Agriculture sector already accounts for around 70% of the total water withdrawals in the world whereas domestic and industrial water use is about 10% and 21% respectively. Desalinated water has high quality and irrigating with desalinated water would result in 24% increase in the crop yield and assist a 45% reduction in the current water irrigation volume simultaneously. However desalination is an energy intensive process and an expensive option. The current membrane and thermal desalination systems have many limitations, including high energy consumption and capital cost, especially for thermal methods, coupled with negative environmental impacts due to the discharge of brines and chemicals. Forward Osmosis (FO) promises to overcome most of the practical difficulties in conventional RO desalination process such as fouling, scaling, chemical treatment and high power consumption. In addition, FO process gives higher throughput with minimal environmental impact including minimal chemical additives and rejection of waste stream. Forward Osmosis (FO) process has the potential to increase the availability of freshwater both in coastal areas with limited resources and in areas where seawater, salinized groundwater and municipal wastewater are available. The novelty of FO process lies in using natural osmosis as a driving force for water to move across a semi-pearmeable membrane from a solution of low osmotic pressure (seawater) to a solution of high osmotic pressure (DS). The choice of draw solution (DS) has a large impact on the performance and viability of the FO process. The draw solutes should be able to generate high osmotic pressures and be completely regenerated using simple and energy efficient techniques.This study presents a novel FO process for producing irrigation water using thermolytic draw solution. The main energy intensive stage in FO process is the separation of draw solute from the freshwater. In this research, the concept of employing liquefied gas compounds as a draw agent has been investigated among 137 gaseous compounds by determining their high solubility in water. In this process a liquefied gas as DS with high solubility in water resulting high osmotic pressure has been used. The DS could be separated from water by changing the operating temperature and or pressure allowing for an efficient and complete removal of the DS. The modified FO process operates at low hydraulic pressure
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