Research On Thermosyphon Trilateral Cycle Power System

The scarcity of fossil fuels necessitates both the development of renewable energy sources and increased energy utilization.The effective recovery and utilization of lowgrade heat sources can play a significant role in conserving energy.Therefore,the development of low-temperature thermoelectric conversion technology has been a focus of attention for researchers worldwide.Many researchers consider the trilateral cycle(TLC)to be a promising technology for low-grade heat conversion.However,the low efficiency and immaturity of twophase expanders have limited the practical application of TLC systems.In this article,a novel TTLC system based on thermosyphon is proposed,which uses a hydraulic turbine instead of a two-phase expander to avoid the problems of traditional TLC systems.A two-phase flow model of the riser was established based on the conservation of energy,mass,and momentum laws,and a thermodynamic model of the new system was established using the conservation of energy law.The riser model was used to select the working fluid for the system,and the performance of the TTLC system was analyzed based on the first and second laws of thermodynamics,and compared with traditional TLC and organic Rankine cycle(ORC)systems.Based on the thermodynamic and riser models,the dynamic models of other components of the TTLC system were established using the three conservation laws,and further research was conducted on the dynamic behavior of the new system.The critical value of the working fluid charge for the system was studied using a condenser outlet subcooling of zero as the threshold,and nondesign conditions of the TTLC system were studied based on the critical charge quantity.Research on the riser model has shown that gravity pressure drop is an important factor affecting the efficiency of the riser.The height of the riser is influenced by two physical properties of the working fluid: the ratio of the two-phase density and the specific heat.Working fluids with smaller ratios of two-phase density and specific heat are more suitable for the TTLC system,such as R502,R218,R125,and R115.Research on the first law of thermodynamics of the TTLC system revealed that increasing the inlet temperature of the heat source is more beneficial for improving system output power performance than decreasing the inlet temperature of the cold source.When the inlet temperature of the heat source is below 50℃,the output power of the TTLC system is slightly higher than that of the TLC system.When the inlet temperature of the heat source is in the range of 50-70℃,the output power of the TLC system is only about 10% higher than that of the TTLC system.The ORC system has the lowest system performance due to poor matching with the heat source temperature.The volumetric flow rate at the outlet of the hydraulic turbine in the TTLC system is only 2～17% of that in the TLC system,indicating that the hydraulic turbine can be made smaller in size.Research on the second law of thermodynamics showed that with increasing heat source temperature,the system exergy efficiency varies between 15%and 30%,and increasing the heat source temperature is beneficial for increasing system exergy efficiency.With decreasing cold source temperature,the system exergy efficiency varies between 23% and 27%,and there exists an optimal cold source temperature.The temperature difference at the hot end of the heater can also be optimized,with a value of 4℃ being a reasonable choice.The pressure drop in the riser is the main factor affecting system exergy efficiency,especially when there is a large temperature difference in the system.Reducing the pinch point temperature difference is beneficial for reducing the exergy losses in the heater and heat rejection,but it will increase other exergy losses.Overall,reducing the pinch point temperature difference is beneficial for increasing system exergy efficiency.Taking a dynamic perspective,research on the critical charge amount of the working fluid has found that an increase in the hot source inlet temperature and flow rate leads to a decrease in the critical charge amount of the system to varying degrees.In contrast,the effects of cold source inlet flow rate and temperature are opposite: the greater the flow rate,the smaller the critical charge amount,and the higher the cold source inlet temperature,the greater the critical charge amount of the system.Increasing the heat transfer area of the evaporator or condenser can both increase the critical charge amount of the system to a certain extent.Additionally,raising the height of the system can lead to an increase in the critical charge amount of the working fluid.Research on the non-design operating conditions of the TTLC system,considering the chosen working fluid,structural parameters,and filling quantity,reveals that the mean density difference of the working fluid between the riser and downcomer is the primary factor influencing system performance.The power output and inlet temperature of the heat source are negatively correlated,with higher inlet temperatures leading to lower system performance.Increasing the heat source flow rate results in a lower power output for the TTLC system.As the inlet temperature of the cold source rises,the power output continually decreases.An increase in the cold source flow rate leads to a reduction in the performance of the hydraulic turbine power output.