Performance Study On Ammonia-Water Power Cycles For Power-Heating Cogeneration In Winter And Dual-Pressure Evaporation

This dissertation mainly presents the investigation and analysis on some specific applications of Kalina cycle processes and their performances,including an ammonia-water Kalina-Rankine cycle combination system for heating/power cogeneration in winter and high efficient power generation in other seasons,and two schemes of dual pressure evaporation Kalina cycle to further improve the efficiency of the Kalina cycle driven by medium temperature waste heat source.Working medium property calculation is the basis of thermodynamic cycle calculation.Because the working parameter range for power generation is different from the application range of original refrigeration cycle,the calculation method of the properties of working medium for the Kalina cycle needs be discussed and verified according to the working conditions.The Schulz equation and PR equation were used to calculate the thermal status parameters of ammonia water mixture,and the results were obtained with high precision.The thermodynamic calculation models were established for the corresponding ammonia power cycles analyzed in this dissertation.The power recovery efficiency and exergy efficiency as the evaluation indexes of the system were used respectively from the view points of the first law and second law of thermodynamics to complete the thermodynamic analysis and the optimization of the cycle.The application situation discussed first in this dissertation is the demand for supplying central heating in winter in the northern regions.The applied principles are that the variable concentrations of the Kalina cycle with the characteristics of the larger temperature difference in evaporation in the boiler and smaller temperature difference in the absorptive condenser is conducive to the efficient power generation from the waste heat source,while the fixed concentration of the ammonia-water medium in the Rankine cycle with larger temperature differences in both evaporation and condensation enables to supply heating capacity by raising the temperature of the cooling water in a counter-flow condenser.By combining the Kalina cycle with the ammonia-water Rankine cycle,the integrated system of the ammonia-water Kalina-Rankine cycle(AWKRC)was formed.The flow circuit of the AWKRC system is based on the Kalina cycle by adding 3 pairs of three-way valves(or 3 sets of four-way valves)to make certain equipment out of operation service and to be converted to the Rankine cycle.During the winter,the demand for central heating in the northern region is realized by the Rankine cycle operation,and in the non-heating seasons,the Kalina cycle is operated in a more efficient way to produce power only.The calculation model of the AWKRC system was established including the mass and the energy balance equations of system and equipment.The equations of the evaluation criteria for the cycle were presented such as thermal efficiency,waste heat recovery ratio and power recovery efficiency,and the calculation software of AWKRC system were programed for the theoretical calculation.Under the initial setting conditions the influences of the work concentration and basic concentration on the performances of the cycle were analyzed,and the optimal matching relationship of the basic concentration and the work concentration was obtained for the Kalina cycle.The external conditions were selected as: the inlet temperature of the heat source is 300℃,the cooling water inlet temperature is 25 for Kalina cycle or 15 for Rankine cycle,℃ ℃and the temperatures of the heating water and returning water are 90 ℃and 40 ℃respectively.The main internal conditions were set as: the work concentration is 0.5 and the corresponding optimal basic concentration is 0.314 for Kalina cycle,and the working medium concentration for Rankine cycle is 0.5,the same as the work concentration of Kalina cycle.The state point parameters and the cycle performances were respectively calculated for the two cycles under the given conditions.The influence of evaporation dew point temperature in the evaporator on the cycle performances was discussed.The analysis results show that under the conditions of different temperatures of heat source and cooling water,optimal value of evaporation dew point temperature of working medium in the evaporator exists corresponding to a given work concentration,to achieve the optimal cycle performance.And the optimal values of the evaporation dew point temperature and cycle performance are slightly different at different work concentrations.Furthermore,the curves of the optimal values of the cycle performances under different working conditions are presented.The results show that,under the above calculation conditions,the power recovery efficiencies of the Kalina cycle and the ammonia-water Rankine cycle are 16.05% and 12.95% respectively for power generation.However,combined with the equivalent electrical efficiency of the heating capacity converted by COP of a heat pump,the comprehensive power recovery efficiency of the ammonia-water Rankine cycle can reach 23.72%.At the same time,the exergy analysis was performed for the AWKRC system under the condition of optimal work and basic concentration matching.The relative calculation and analysis of losses and efficiencies of exergy of the system and equipment in the circulation process were performed.Under the above given external conditions and internal parameter optimization,the exergy efficiency of the Kalina cycle is 41.9%;while exergy efficiency for power generation of the ammonia-water Rankine cycle is only 33.1%,however,the comprehensive exergy efficiency including the heating part is 46.5%.The second case studied by this dissertation is for the power recovery from waste heat source with temperature between 350~400℃,and a cascade utilization of the heat source with the dual-pressure evaporation Kalina cycle(DPV-KC)was proposed.On the basis of the Kalina cycle,a second evaporator is added after the first evaporator,and the second evaporator take advantage of the heat source from the first evaporator exit to further increase the net power output of the system.By calculation,the best matching relationship between work concentration and basic concentration is also existed for the dual-pressure evaporation Kalina cycle that for the work concentrations of 0.35,0.4 and 0.45,the corresponding optimal basic concentrations are 0.214,0.243 and 0.277,respectively.Under the inlet temperatures of the heat source and the cooling water are 400 and 25 respectively,the influence variation ℃ ℃trends of the evaporation of dew point temperature on the performances of DPV-KC system with different work concentrations were obtained and the optimal evaporation dew point temperatures corresponding to the best performances were presented.Under the conditions of the work concentration and basic concentration took the 0.45 and 0.277 respectively and the corresponding optimal first evaporation dew point temperature of 300℃,the power recovery efficiency of the dual-pressure evaporation the Kalina cycle reached 25.43%,13.2% higher than that of the single evaporation pressure Kalina cycle.Considering the temperature difference between heat source and working medium in the first evaporator subcooling section of the DPV-KC system is still great with larger irreversible losses,the parallel-cascade dual pressure evaporation Kalina cycle(DPV-KC2)was further discussed,that the second evaporator and the part of liquid heating section(economizer)of the first evaporator are arranged in parallel.The main factors influencing the performances of the DPV-KC2 system were discussed and optimized,including the dew point temperature of first evaporation,the subcool degree of the work solution at inlet of the first evaporator and the superheat degree of the work solution at outlet of the second evaporator.The results show that the power recovery efficiency of the DPV-KC2 is up to 26.61%,which is 18.5% higher than that of the single-pressure evaporation Kalina cycle and 4.64% higher than that of the DPV-KC.The modified Kalina cycles discussed in this dissertation provide new ways to provide seasonal power/heating cogeneration and to improve further the energy conversion efficiency in the power recovery system of mid-temperature waste heat source.