Thermodynamic Optimization And Experimental Study On Dual-absorption Kalina Cycle System For Low-and Medium-grade Geothermal Energy

With the shortage of the fossil fuel, the power technology using renewableenergy is developed. The Organic Rankine Cycle and Kalina Cycle are therepresentative bottom cycles in the field of power technology. For the low-andmedium temperature geothermal energy, the various evaporating temperature ofammonia-water mixture is superior, and the advantage makes the performance ofKalina cycle is better than that of ORC cycle. In the low-and medium temperaturepower generation system, the temperature of tail geothermal water of the Kalina cyclesystem is always relative high (above80℃), and the thermal energy has not been wellused. The absorption heat transformer can be introduced to recover the thermal energyof tail water. The temperature of tail water can be decreased and the powe output ofthe power cycle system can be increased.Based on the low-and medium temperature geothermal power technology, thefirst and second laws of thermodynamics are adopted. For Kalina cycle (KCS11andKCS34) and ORC cycle, the power output, thermal efficiency and exergetic efficiencyare determined to be objective functions, in order to make full use of geothermalresources. The optimization of cycle parameters were performed on the different cyclemodes. Besides, the optimal cycle and its corresponding parameters were obtained, inorder to improve the performance.Besides, the concerning point is to expand the scope of application of low-andmedium temperature geothermal energy, and to improve the cycle mode of powergeneration system, so the dual-absorption Kalina cycle is proposed in the paper. Thespecific net work (the net power output of nuit mass flow of brine) was established asthe objective performance, and the methods of sensitivity analysis and parameteranalysis were developed to analyze the influences of the heat soure temperature, thecold source temperature, the ammonia mass fraction, the mass flow, the circulatingratio to the performance of heat transformer. The numerical results were comparedwith the experimental results to validate the feasibility of the dual-absorption Kalinacycle system, and were used as the foundation. The results were shown as follows:(1) There is a best match relationship between the ammonia mass fraction andpressure of turbine inlet under the constant heat and cold source temperatures. The netpower output is proportional to heat source temperature and inversely proportional to cold source temperature. With the increase of condensation temperature, the pressureof turbine outlet increases exponentially. When the cooling water is relatively high,the effect of the low-temperature recuperator is little.(2) For the ORC system, with the increase of heat source temperature, theoptimal evaporating temperature of working fluid and the corresponding evaporatingpressure increase. When the temperature of heat source is above100℃, theperformance of KCS34system is better than that of ORC system, but the evaporatingpressure of KCS34system is3.5times higher than that of ORC system.(3) The net power output of the dual-absorption Kalina cycle is more8%thanthat of Kalina cycle under the same conditions. The optimum values of thermalefficiency and net power output are existed with the increase of evaporating pressure.When the heat source temperature and cold source temperature are122℃and25℃,respectively, the optimum ammonia mass fraction and pressure of turbine inlet are0.7and46bar, respectively. The performance of heat transformer device has been lessaffected by the evaporating pressure of10~20bar.(4) The impacts of cold soure temperature, heat source temperature andammonia mass fraction on the heat transformer device are10%,9.8%and6%,respectively. When the flow state of circulating water in the absorber is turbulent state,the performance of heat transformer device is better. Based on the experimentalresults, the exergetic loss of condenser is the most, and the evaporator and absorberare less in turn. The exergetic loss of generator is least.(5) The heat transfer coefficient of absorber is lower than that of numericalresults, which is ranged from260to340W/m2·K. The trend of numerical results andexperimental results are similar, but the temperature difference is about6℃. Thepressure difference of heat transformer device is lower3bar than that of numericalresults of dual-absorption Kalina cycle.