Closed loop PRO and RED methods are developing which would be capable of extracting much higher power density from salinity gradient. Not only that, closed loop systems would reduce bad environmental impact of PRO and RED. A closed-loop system, which is termed as a heat engine , converts thermal energy to mechanical work or electricity. In this method PRO or RED method is used to generate electricity from mixing of sea water and river water and then the resultant mixture is separated by thermal energy to high-concentration and lowconcentration solution to recycle back. To make this process economically viable, in general, waste heat, like geothermal energy from shallow wells and low-grade heat of power plants, is used for thermal separation. Advantage of this process is that we may use any concentrated electrolyte instead of sea water and river water as recycling is possible. It is obvious that the use of high concentrated solution would produce higher energy density extraction. For example, an ammonia-carbon-dioxide solution of 5M concentrations is capable of generating power densities of 250W m -2 . Thus, closed loop heat engine is also promising, but also have the same limitations as PRO and RED.