Abstract
The development of renewable and sustainable energy sources has been acknowledged as the most straightforward strategy for global energy security to deal with growing environmental concerns. Out of the available renewable resources, oceans represent a huge untapped potential candidate to provide clean energy while addressing water shortage issue. The utilization of salinity gradients between salt- and freshwater or two salt solutions with different concentrations is a relatively new concept simultaneously to generate electricity and desaline seawater. Salinity gradient energy (SGE) is a zero-emission and sustainable technology that can be practically applied worldwide. When the two mediums are mixed, the SGE can be generated based on the Gibbs free energy. Additionally, SGE can also be harvested from industrial processes where more concentrated salt solutions are produced. In fact, the location where many industries such as sewage treatment plants have discharged substantial amount of fresh- or saline water into the ocean can be a candidate to implement a salinity gradient system to capture the energy. Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are the two most commonly known SGE methods that are based on membrane technology. Norway and the Netherlands have been especially active in the research and development of osmotic power based on PRO and RED, respectively. Both technologies are dependent on the semipermeable and ion-specific membrane that is selective in its permeability. The performance of both PRO and RED are strongly dependent on the salinity, volume, and cleanliness of the feed water supply. In general, the higher the salinity gradient between the two mixing mediums, the greater the energy that can be harvested in the system. The quality of feed water is also influential on the performance of the membranes as it is known that membranes are generally prone to various forms of foulants present in the water sources. This phenomenon is deemed to be detrimental to the membrane performance and causes a severe reduction in the power output. In terms of application, PRO is known to be more feasible for power generation using concentrated saline brines while RED is more promising to be applied at the locations when seawater and river water meet. From a technical point of view, RED seems to be more attractive experimentally due to its relatively simple set up that does not require high-pressure setups, pressure exchangers, and turbines. It has been commonly agreed that the membrane is one of the most important components to determine the performance of PRO. Hence the membrane characteristics have been carefully tailored to meet the desired requirement to optimize the osmotic power. Meanwhile for the development of RED, the optimization of the ion-exchange membranes (IEMs) is also crucial to ensure reliable performance. Additionally, system characteristics such as internal resistance are also the keys of practical RED performance.