Study Of The "Thermal Energy-electricity" Cycle Characteristics Driven By Solar Thermal Energy

Salinity gradient energy is available whenever two solutions with different salinity levels are mixed together,as occurs in nature when a river discharges into the sea.An open circle is suitable for using salinity gradient energy in nature.But impurity contained in sea water and river water has a negative effect on the conversion of salinity gradient energy.There are two methods to convert salinity gradient energy into electricity based on membrane technology: Pressure Retarded Osmosis(PRO)and Reverse Electro-Dialysis(RED).Currently,the domestic and foreign research on RED is not sufficient,and the power generation and energy conversion efficiency of RED technology have a large room to improve.A novel thermal energy-electricity conversion method was proposed.This method contains the low-temperature multi-effect distillation desalination technology driven by solar energy and RED technology.By this method,dilute and concentrated solutions can cycle in a closed loop,which avoid the impurity issue existed in an open circle.As the simulation of low-temperature multi-effect distillation desalination technology driven by solar energy has been studied by our research group.This paper focus on the optimization of RED.Because of the EMF losses existed in RED stack,increasing the number of system-contained RED stack can effectively improve power generation and energy conversion efficiency.Therefore,there are three aspects of this paper:Firstly,models for RED stacks were established.Effect of total flow passageway length of RED,solution flow velocity and the number of RED stacks on main output parameters of RED stack and system energy conversion efficiency of two circuits in co-flow and counter-flow system was discussed.Secondly,effect of output current on system output voltage and energy utilization of RED stack series circuit was studied.Thirdly,combined with the former research of low-temperature multi-effect distillation desalination driven by solar energy,efficiency of “thermal energy-electricity” circulation was calculated.Simulation results show that under the given operation parameters,as the total flow passageway length increases and solution flow velocity decreases,output voltage and power density of RED stacks in multiple stack circuits decreases,and output current of RED stacks increases.Along the dilute solution flow direction,output voltage and power density of each RED stack in multi-stack system decrease.Affected by the interrelationship between solution resistance and EMF of IEMs,the change of output current in different working conditions is complex.Reducing solution flow velocity and increasing total flow passageway length and RED stack numbers can improve power generation efficiency of the system.Power generation efficiency in a counter-flow RED system is higher than that in a co-flow system.The increase of series RED stack numbers not only enhances the output voltage and power density of the system,but also improves the internal resistance,which leads to a significant influence of system output current on output voltage and power density.Energy conversion efficiency-current curve has a parabolic trend.When at the maximum energy conversion efficiency,increasing current leads to lower energy conversion efficiency and significantly higher ohmic losses.The calculated “thermal energy-electricity” conversion efficiency is low.In the future,the system conversion efficiency can be improved by increasing the number of effects in LT-MED system,selecting the working fluid with high electrical conductivity and low latent heat of vaporization.