The paper is to report the work on a preliminary feasibility study of energy storage by concentrating/desalinating water. First, a novel concentrated water energy storage (CWES) is proposed which aims to use off-peak electricity to build the osmotic potential between water bodies with different concentrations, namely brine and freshwater. During peak time, the osmotic potential energy is released to generate electricity. Two scenarios of CWES are specified including a CWES system using reverse osmosis (RO) and pressure retarded osmosis (PRO), and a CWES system co-storing/generating energy and freshwater using "osmo tic-equivalent" wastewater. A comprehensive case study is carried out with focusing on the configuration of CWES using RO and PRO. It is found that the limiting cycle efficiency of the CWES using RO and PRO is inversely proportional to the RO water recovery and independent of the initial salinity. Therefore, to balance the energy density and cycle efficiency of CWES, it is recommended to operate a system at lower RO water recovery with higher concentration of the initial solution. Detailed energy analysis of detrimental effects in mass transfer, e.g. concentration polarization and salt leakage, and energy losses of pressurisation and expansion of pressurized water, are studied. Finally, a preliminary cost analysis of CWES is given. (C) 2016 Elsevier Ltd. All rights reserved.