Reliable, economic and efficient energy storage is needed to help shift from primarily fossil fuel generated electricity to clean energy. This report develops a thermodynamics based model for an osmotic energy storage (OES) system. This system uses reverse osmosis (RO) to store energy and pressure retarded osmosis (PRO) to produce power during the discharge cycle. Bottom up RO and PRO models were created in MATLAB for hollow fiber membrane modules. These models were then used in an overall OES system model. Preliminary results were produced, with a maximum round trip efficiency of 8.97%. Note that operating conditions were not optimized, and higher efficiencies can be achieved using the model developed here. Salt leakage plays a large role in limiting system efficiency.