Research On Simulation And Optimization Of The Combined Turbine And Heat Pump Cycle Heating System In Condensing Power Plant

Cogeneration has attracted extensive attention as it reduces the primary energy consumption and environmental pollution.Temperature and pressure reducers can be used to transform condensing power plants into cogeneration systems.Unfortunately,this transformation may bring two problems to the cogeneration process,that is,on the one hand,high grade energy is wasted in the steam passing through the temperature and pressure reducer,the other is that large amounts of waste heat in circulating cooling water is also released to the ambient without recycling.This paper presents a combined turbine and heat pump(CTHP)system and establishes the correlations of the system characteristic parameters according to scientific problems in this system.Based on the optimization design of each component,the dynamic model of the whole system and the objective function of the optimization are established to achieve the optimal operation of the system.The results of a case study show that the system is feasible and can bring excellent economic and social benefits.Based on the characteristics of the CTHP system,the matching relation between the steam turbine and the heat pump is determined.The correlation of steam turbine efficiency and power is regressed by field test.By investigating the interaction effects between the steam turbine and the heat pump,circulating technical parameters including the maximum efficiency coefficient,the maximum quantity of steam extraction,dynamic load ratio and circulation ratio,are determined,which provides necessary theoretical basis for the optimization of the CTHP system.Adopting the distributed parameter method in the flooded evaporator and routine method in other Heat transfer components,the optimization design model and solution method of each component are established using the minimum entransy dissipation as the objective function,and the optimal structure parameters of each component,including the diameter of the heat exchanging tube,the tube number,the tube length,the inner diameter of the shell and the pitch of tubes,are obtained.The results show that the heat transfer efficiency of the heat exchanger is increased by 7.79% and the power consumption is reduced by 19.61% with an optimized design.The dynamic simulation model of the CTHP system and the coupling relationship between the devices are constructed treating the system high grade thermal parameters and the state parameters of the circulating water as main variables.From an overall point of view,the dynamic changes and the interaction of the devices in the system are analyzed.The effect of the cooling water temperature on the discharging pressure of the steam turbine is greater than that on the discharging temperature.The heating capacity of the system linearly decreases with the increase of the outlet water temperature of the condenser,and the outlet water temperature of the heat pump results in a sharp increase of the steam consumption.There is an optimal steam flow ratio under the same performance of the system.For the two regulating methods,the variation margins of the system parameters are greater in the constant-variable flow control method.Results of case studies show that,during the whole heating period,the average value of the heat pump COP is 2.18,and the total consumption of steam is 223430 t.Compared with operation status of the original condensing steam turbine,the consumption of steam is reduced by 38197.6t,and the energy utilization rate is improved from 0.938 to 1.064.Employing the maximal energy efficiency coefficient of the CTHP as objective function,the operation process is optimized and analyzed on the premise that the optimized operation model and the constraint conditions of all the components are established.The optimal parameters of operating condition in full load and part load conditions are obtained,and the influences of the cooling water temperature and heating load on the system's optimal operating parameters are analyzed.As the inlet water temperature of the cooling water increases,the optimal outlet temperature and the discharging pressure of the turbine firstly increase fast and then remain stable.The heating load has the biggest influence on the cooling water temperature and flow rate.The discharging pressure of the turbine reduces as the heating load increases,but the drop amplitude declines,while the drop amplitude of discharging pressure improves gradually.When system is operating in 60 percent load condition,the optimal temperature of cooling water is 14.7 centigrade,the optimal temperature and discharge pressure are 189.6 centigrade and 0.295 MPa respectively,the optimal outlet water temperature of heat pump is 79.6,and the optimal steam flow ratio is 0.457.Taking the application of CTHP system in Huaneng Dalian power plant as example,the optimal parameters of CTHP system are obtained through optimized design and operation simulation.For CTHP system,the optimal steam flow ratio lies between 0.428 and 0.48,and the energy utilization efficiency ranges from 1.01 to 1.085.Comparing with the original system,the amount of exhausted steam in optimal system,decreases from 24088.4t to 199341.6t.The technology economic benefit of the CTHP system is calculated and compared with the original system by means of annual cost,net present value and dynamic payout period.Results verify that the CTHP system show better economy and society benefit,as the initial cost of CTHP system is 1.73 times as that of original system,the annual cost of CTHP system is 90 percent of the original system,and the dynamic payout period of CTHP system is only 2.3 year,which less than 3.13 year in the original system.The reconstruction of cogeneration system in power plants is still in the initial stage in our country.Thus the CTHP system proposed in this thesis and the research results on it can provide theoretical basis and technical support for the energy saving transformation in power plants.