Theoretical And Experimental Investigation On The Small Scale Gravity Driven Low-temperature Organic Rankine Cycle

In recent years, basic researches and applied researches aiming at the efficient utilization of medium and low temperature heat source have gradually become a hot research field all over the world. In the field of low temperature heat recycling technology, organic Rankine cycle(ORC) technology is the fastest growing heat utilization technology in recent years and it is also the focus of domestic and foreign researchers.Focusing on the actual operating performance and the efficiency of the working fluid pump in the small scale low-temperature ORC system whose evaporation temperature is below 100℃ and the net power output is less than 10 k W, an experiment prototype is installed and the related investigations are carried out. Based on the experimental investigation results, in order to reduce the working fluid pump energy consumption and to increase the system’s net power output and net electricity output, a small scale low-temperature ORC system pressurized and driven by gravity is introduced. Performance investigations on this new cycle are conducted with different working fluids, different system structures and different working conditions. Finally, a reaction turbine used in the gravity driven ORC system and its basic design are proposed. Beyond that, the reaction turbine operating performances with different geometric parameters and working fluids conditions are also investigated and discussed.In order to confirm the working fluid pump’s actual operating characteristics and their effects on the system performance, an experiment prototype is built according to the former theoretical analysis. A positive displacement working fluid pump is installed in the prototype while the popular organic fluid R245 fa is employed as the working fluid. The experimental data indicate that, within the low temperature range, the R245 fa working fluid pump’s highest isentropic efficiency is about 70%, while its mechanical efficiency would be lower than 30% when the electrical motor efficiency is integrated. Combined the isentropic, the mechanical and the motor efficiency, the overall efficiency of the working fluid pump is lower than 20%, which is much lower than the estimated value used by the former researchers.Continuous dynamic experimental research shows the pump efficiency is highly related to the flow rates. The mechanical efficiency reaches its maximum value at the maximum volumetric flow rate zone. Larger pressure difference between the pump inlet and outlet will always lead to a higher mechanical efficiency. Low efficiency also comes from the mismatch between the pump and the electrical motor. The pump efficiency would be improved obviously if a special developed motor with low rotating speed and large torque is employed. Experiments on the system performance indicate that, the pump efficiency affects the system Rankine efficiency directly. The latter will be improved obviously when the pump isentropic efficiency is promoted. Limited by the pump efficiency, there will be a capacity floor when the ORC system is operating at part load condition. Extremely low part load operation will lead to a negative net electricity output. With the same working fluid flow rate, small scale ORC system net electricity output will increase following the pump efficiency’s promotion while it will decrease in pace with the pump efficiency’s depression. For an ORC system whose pump is driven by electrical motor, system performance analysis and evaluation should be carried out from the system net electricity output rather than other criterions, otherwise, there would be a large error or even an absolutely opposite conclusion. The ORC system net electricity output and operation performance will be improved greatly when the pump efficiency is promoted by technology means or a more efficient pump is employed.In order to investigate the performance of the small scale gravity driven low-temperature ORC system, eight different organic working fluids including R113, R123, R134 a, R152 a, R227 ea, R236 fa, R245 fa and RE347mcc(HFE-7000) are chosen as the research subjects. Comparing to the traditional pump driven ORC, the gravity driven ORC possesses a higher net power output and higher system Rankine cycle efficiency. The required pressurized height increases when the evaporating temperature rises or when the condensing temperature declines, but the evaporating temperature is the main decision factor. For the gravity driven ORC whose net power is lower than 10 k W, the overall pressure drop during its operation is relatively small. Main pipeline pressure loss occurs inside the uplink steam pipe, and it will increase rapidly when the system capacity increases. Dry fluids are more suitable for the gravity driven ORC. Among the object working fluids, R113, R123, R245 fa and RE347 mcc show better cycle performances, considering the required pressurized height and the Rankine cycle efficiency. With the evaporating temperature of 100℃ and the condensing temperature of 35℃, R113 possesses the minimum required pressurized height which is just 24.6m while its Rankine cycle efficiency is still above 10% at the same time. Different from the traditional pump driven systems, the gravity system performance is not obviously improved when a recuperator is introduced to the system.Zeotropic isentropic mixture’s performance is quite different from the pure isentropic fluid when they are used in the gravity driven ORC. With the same set working conditions, zeotropic isentropic mixture’s efficiency is not the highest, but it possesses a quite lower pressure ratio. With the same cycle efficiency and pressure ratio, multiple mass ratios are available. By changing the mass ratio of the zeotropic mixture component, different application height requirements could be satisfied. With the same cycle performance, zeotropic mixtures could replace the pure ones and could be the optimized working fluids.By using the “length to diameter ratio” as the variable, the highest efficiency curve equation and the corresponding torque, rotating speed and mass flow rate equations of the reaction turbine for the small scale gravity driven low-temperature ORC system are gained. The design equations of the reaction turbine which use dimensionless torque as variable are developed when the vapor stagnation condition equations are introduced. The reaction turbine operating performance research shows that, with the set working conditions, the power output and the efficiency of the reaction turbine share the same variation trend. With the revolving speed increasing all the time, the power output and the efficiency increase before reaching the peak point and after that they both decrease. For the same revolving speed conditions, different working fluids lead to different power output and efficiency values. At the low revolving speed zone, the efficiency differences between the objective fluids are small while the differences become significantly at the high revolving speed zone.