Investigations On Internal Generator Cooling For Hermetic Expanders In Organic Rankine Cycles

Organic Rankine Cycle(ORC)technology is an effective way to realize power generation from low and medium grade energy.The expander is a core component in ORC system to realize energy conversion.The machine can be divided into three different construction types: open,semi-hermetic and hermetic construction types.Hermetic expander can avoid all kinds of problems caused by the shaft seal.Therefore,it is called one of the most important development directions of ORC technology.Due to shaft sealing,the heat produced by the generator cannot be dissipated in time,and this would lead to temperature rise of the generator,affecting the safe operation of the unit.This thesis specially studies the internal generator cooling for hermetic expanders in organic Rankine cycles.The main contents and conclusions are as follows.Using the organic working fluid as the cooling medium of the internal generator,three schemes of exhaust cooling,spray cooling and exhaust-spray cooling are introduced.The structures,characteristics and applicability of different cooling schemes were discussed.The structure of the exhaust cooling scheme is the simplest,while it was necessary to add pipelines and install atomizing nozzles for the spray cooling and exhaust-spray cooling schemes,but they are all non-moving parts and will not cause leakage of the working medium.The organic Rankine cycle unit with exhaust cooling and spray cooling can also use regenerative heat to further improve the thermodynamic efficiency.At the same time,a thermodynamic model was established to study the effect of different cooling schemes on the performance of ORC system.The results show that the maximum thermodynamic efficiency drops by 0.21%,0.40%,and0.77% when R245 fa was used as the working fluid for spray cooling,exhaust cooling,and exhaust-spray cooling,respectively.In the exhaust-spray cooling scheme,the thermodynamic efficiency drop caused by exhaust resistance loss accounts for 52%.When the exhaust resistance loss is greater than 2 k Pa,the decrease of thermodynamic efficiency caused by the spray cooling scheme was the smallest.The detailed structures of the hermetic expander are discussed.The convective heat transfer coefficient of generator cooled by gas working medium is analyzed by means of experimental correlation formula.The generator structural is equivalent to cylinder structure and the heat transfer model between the generator structure was established by using the "T" thermal resistance model.Based on the equivalent thermal circuit method,a mathematical model of the internal generator cooling was constructed,including convection heat transfer,natural heat conduction on shell surface and forced convection cooling on organic working fluid.In addition,the electromagnetic calculation of the generator is performed to obtain the loss of each component.A complete equivalent thermal circuit model equation system is thusly developed.By solving the generator loss model,it is found that the stator core loss is basically2.6% of the rated power with the change of ORC.However,the losses of stator and rotor winding loss have obvious changes.Using R245 fa as the working fluid,when the evaporation temperature was 115°C,the stator winding losses rotor winding losses and stator core losses account for 42.9%,35.2% and 21.9% of the total losses,respectively.In this thesis,two different schemes of exhaust cooling and exhaust-spray cooling are selected.By using the above mathematical model,the variations of the generator temperature field under different working conditions are studied.The results show that with the increase of the evaporation temperature of the ORC system,the stator winding temperature increases rapidly due to the double influence of the increase of the output power and exhaust temperature of the expander.The condensation temperature has little influence on the generator temperature.Compared with the exhaust cooling scheme,the exhaust spray cooling scheme can effectively improve the cooling conditions of the generator.Using R245 fa as the working fluid of the ORC unit,when the evaporation temperature is 115°C,the maximum temperature of the stator winding for the exhaust cooling scheme can reach 142°C,and for the exhaust-spray cooling scheme,the value is reduced to 125°C.The temperature of the stator winding can be reduced by increasing the equivalent thermal conductivity of the insulation layer of the groove.However,changing the central Angle of stator outer flow channel has little effect on winding temperature.