Analysis Of Seawater Desalination Energy Economy In Water And Electricity Cogeneration System

Water and electricity cogeneration is one of the important ways of thermal seawater desalination,which can effectively use the low-pressure extraction steam of power plants to produce high-quality fresh water,so as to reduce the cost of seawater desalination.Seawater desalination is an energy-intensive process.In the process of developing sea water desalination and water and electricity cogeneration system,how to evaluate the energy consumption of water production is an academic issue worth further discussion.For thermal power plants,seawater desalination is equivalent to a relatively stable heat user.A large number of studies have been carried out on energy consumption and share of heat and power of cogeneration.At present,seawater desalination in China is basically built on the basis of existing power plants,and the evaluation of cogeneration is often compared with the separate generation of heat and power.Given the different construction purpose and time,the paper’s seawater desalination energy consumption evaluation method in the water and electricity co-generation system is different from that in heat and power of cogeneration system,consider only power generation runtime(or thermal power plant is pure power or heating runtime),in the case of excess heating capacity,the coal consumption of the same generating capacity(heating)condition without extraction steam of the unit is used as the reference,the net increment of coal consumption after adding seawater desalination unit is taken as the index to analyze the coal consumption of water generation,and for the seawater desalination unit,the utilization degree of power steam heat and available energy is discussed without considering the influence on power generation.Based on the principle of energy conservation,conservation of mass and variable working condition,this paper adopts MATLAB software to model the thermal calculation of thermoelectric unit under variable working conditions.Coupled with the thermal process calculation of seawater desalination,the water production under different operating conditions of water and electricity cogeneration is determined,and then the variation rule of water-making energy consumption is determined.Based on the analysis of the calculation results,the following conclusions are obtained:(1)For different extraction schemes,under the condition of the same power generation load and extraction steam volume,the water-making coal consumption made by hot re-extraction is the highest,and the water-making coal consumption made by exhaust steam extraction from medium pressure cylinder is the lowest.For exhaust extraction steam working condition of medium pressure cylinder,when the power generation load is constant,the net coal consumption per unit of water generation decreases gradually with the increase of extraction steam.When the power generation load changes,there is a steam extraction flow change curve as the "power generation load-steam extraction volume" reference curve of the water and electricity cogeneration system.On the right side of the curve,given the steam extraction volume,the lower the power generation load,the lower the net coal consumption per unit water-making net coal consumption.For hot re-extraction steam and cold re-extraction steam conditions,water-making net coal consumption increases gradually with the decrease of power generation load.However,when the steam extraction volume is low,water-making net coal consumption is slightly lower under 60% of power generation load.(2)When the influence of power generation is not taken into account,with the decrease of the quality of power steam,the heat consumption per unit of water generation of the seawater desalination device increases gradually,and the exergy consumption per unit of water generation decreases gradually.(3)The increase of valve throttling loss improves the water-making ratio under the condition of the same power generation load and steam extraction volume,and reduces the heat consumption per unit of water generation,but the net coal consumption and exergy consumption per unit of water generation increases.According to the above conclusions,the use of low temperature and low pressure power steam in seawater desalination plant can make full use of heat,reduce the consumption of power steam available energy and the net coal consumption of water generation for water and electricity cogeneration system.For the operation of the unit,on the premise of satisfying the safe and thermoelectric load supply,reducing the pressure loss of regulating butterfly valve is more conducive to reducing the net coal consumption of water generation.