Blue energy which is also called salt gradient power (SGP) has been established in 1954 when Pattle introduced the theoretical production of electric power after the mixture of fresh and salt water streams. This theory was not proven to be feasible, however, until the team led by Sidney Loeb proposed and demonstrated the first known applications of pressure retarded osmosis (PRO). To this date, only two technologies have even reached pilot scale demonstration, and we will briefly introduce these two technologies. These are PRO and reverse electrodialysis (RED).
PRO utilizes a semi-permeable membrane placed between the two streams of different salinity, wherein water flows from the less concentrated stream towards the more concentrated one, in order to equilibriate the concentration gradient between the two solutions. RED, on the other hand, utilizes ion-exchange membranes (either anion-exchange or cation-exchange membranes), which can selectively allow negatively-charged or positively-charged ions through. This technology, using the membranes, has allowed charge separation which then leads to the formation of an electrochemical potential. Using a stack of alternately anion-exchange or cation-exchange membranes facing two streams of different salinity, electrical potential can be built up and utilized to generate electricity.
Future research on PRO & RED are expected to continue to focus on the optimization of the process and development of suitable high-performance membranes to improve the larger-scale application of this processes. Once the challenges for both PRO & RED have been addressed, implementation of PRO & RED on a larger scale may lead to an efficient utilization of blue energy and generation of salt gradient energy. Furthermore, new technologies such as capacitive systems and salt batteries are currently in development. These new technologies will only drive the research on blue energy forward, allowing us to further implement blue energy utilization.