Thermodynamic Analysis And Optimization Of Water And Power Co-generation System Based On Multi-phase Flow Evaporation

Currently, the shortage of water resources has become a bottleneck slowing down the development of our society. Seawater desalination technology is an effective method to solve the problem. Thermal desalination has become one of the dominating technologies of seawater desalination. The emergence of water and power co-generation technology, which takes the low pressure steam turbine extractions as the heating source of thermal desalination, could reduce the cost of desalination significantly. But scaling is one of the main problems in thermal desalination technology. The multi-phase flow technology, which adds the solid particles into the evaporator, not only can strengthen heat transfer as well as anti-scaling, but can also improve the gained output ratio(GOR). In this paper, the multi-phase flow evaporator is introduced in the water and power co-generation technology, which could provide fresh water for coastal power plants. The obtained high concentration brine could be used as raw materials of salt production, which provides an effective way to dispose the high concentrated seawater.In this paper, the water and power cogeneration system is composed by 300 MW condenser unit and multi-phase flow evaporation desalination plant. The equivalent enthalpy drop matrix model, the mathematical model of steam turbine under off-design operation, and the mathematical model of multiple phase flow evaporator(including the evaporator, flash tank, condenser and steam ejector) are established. The temperature losses caused by the heat and pressure loss, as well as the variation of brine concentration are taken into consideration. Moreover, the optimization model is also established which is aims at minimizing unit product cost. All these models are solved by using the tool of MATLAB.Thermodynamic analysis of water and power co-generation system is based on four effects forward feed multi-phase flow evaporation. The equivalent enthalpy drop method is utilized to analyze the energy cost of water production for the condenser. Different thermal vapor compression(TVC) extraction location, the effect of solid particle volume fraction, the liquid velocity at the inlet of heating pipe, and concentration ratio on the thermodynamic properties of the system under different conditions are analyzed. The results show that, at the same condition for the same pumping steam outlet,the power consumption for water production, the heat transfer area of each effect, and the unit product cost increase gradually with the TVC extraction location moving backward, while the performance ratio decreases. Through increasing the solid particle volume fraction, liquid velocity at the inlet of heating pipe, and concentration ratio can improve the performance ratio. Meantime, the electricity consumption for water production, the heat transfer area, and the unit product cost can be reduced.Exergy balance for the multi-phase flow evaporation system is analyzed using the black box model method. Exergy destruction for the evaporation system under different solid particle volume fraction, liquid velocity at the inlet of heating pipe, and concentration ratio is investigated. The results show that, all the parameters mentioned above can reduce the exergy destruction of the evaporation system. The largest exergy destruction occurs in the condenser, and the second largest exergy destruction occurs in the TVC.