Operating Conditions Optimization And Energy Efficiency Evaluation Of The Osmotic Power Generation Process Based On The Pressure Retarded Osmosis Technology

Pressure retarded osmosis?PRO?is a new membrane separation technology mainly used in the osmotic power generation process,the bottlenecks of the process are low power density and low energy generation efficiency.In the PRO process,the power density was influenced by the solutions system and the operating conditions.To maximize the power density of the PRO process,a theoretical model considering the non-ideal effects of concentration polarizations,reverse salt flux and pressure losses in draw side through the membrane module was developed to derive the water flux and the power density of the PRO process.The influences of the operating conditions,including the applied hydraulic pressure difference??P?,the ratio of feed to draw flow rate?V_F/V_D?and the ratio of draw flow rate to membrane area?V_D/S_m?,on the power density of seawater-river and RO brine-river systems in the PRO process were studied.A commercial forward osmosis membrane was used to verify the two systems'theoretical results of the water flux and power density in the maximum pressure range of the membrane.When the operating conditions of seawater-river system are?P of 17 bar,V_F/V_D of 0.8 and V_D/S_m of 47Lmin^-1m^-2,the theoretical power density is 5.13 Wm^-2,higher than the benchmark of5.00 Wm^-2.When the operating conditions of RO brine-river system are?P of 29 bar,V_F/V_D of 1.0 and V_D/S_m of 47 Lmin^-1m^-2,the theoretical power density is 13.61 Wm^-2,higher than the power density of seawater-river system.The error between the experimental and theoretical results is less than 5%,which demonstrates the theoretical model is reliable.This study is important for the selection of operation conditions in actual PRO process.In the PRO process,the energy generation efficiency was influenced by the solutions system.Therefore,the models of water flux and mix Gibbs free energy recovery efficiency along the unit length of the membrane module in the draw solution flow direction were constructed.The influences of the four different solutions systems,including SW-RW,RO-RW,DS-WW and DS-SW systems,on the water flux and mix Gibbs free energy recovery efficiency along the membrane module were studied.The results show that when the length of the membrane module is one meter,the water flux gradually decreases along the membrane module,and the water flux's declines of the four systems are 2.12%,12.01%,20.07%and 48.24%,respectively.The total mix Gibbs free energy recovery efficiencies of the four systems gradually increase along the membrane module,which are 10%,15%,32%and 42%,respectively.But the mix Gibbs free energy recovery efficiency in the unit membrane length is gradually reduced,due to the decrement of the water flux along the membrane module.Study on the change law of Gibbs free energy recovery efficiency using different solution systems in the membrane module by theoretical model provides a basis for the energy recovery effiency of actual PRO process.