Research On Optimization,dynamic Modeling And Control Strategy Of Low-grade Heat Power Generation System

Energy shortage and climate change have been enormous threats to human life.Thus,to deal with the challenges brought by climate deterioration,governments worldwide designed guidelines of reducing the CO2 to zero.In the guidelines,improving the utilization rate of fossil energy and develop renewable energies are the most effective route.Low-grade heat energy power generation technology(LGHEPGT)is one of the key technologies.More than 78%of fossil energy is wasted in the form of low-grade heat energy,and a large amount of renewable energy exists in the form of low-temperature thermal energy,such as ocean thermal energy,geothermal energy,low-grade heat solar energy,etc.The LGHEPGT,based on Organic Rankine Cycle(ORC),is considered the most perspective technology.However,the current LGHEPGT based on ORC technology had high cost,and low thermal efficiency.Besides,the system had complex nonlinear phase change process,strong model coupling,multiple system characteristics,control difficulties and other problems need to be solved.Therefore,this dissertation conducted an in-depth study of the critical issues in the design,modeling,and control of ORC-based Low-grade heat energy power generation systems(LGHEPGS),and the specific research contents and innovations are as follows.1)This dissertation presented a comparative analysis of the thermodynamic performance and economics of the single heat source multi-loop power generation system with three configurations:series,parallel and series parallel.Thermodynamic and economic models of these three configurations are developed.The multi-objective optimization algorithm adopted NSGA II to provide Pareto frontiers,the optimal solution on the frontiers were solved by TOPSIS,and all the optimal solutions are evaluated by GRA.The evaluated results showed that series dual-loop ORC has the highest GRA score.2)A series heat exchangers ORC(SHEORC)is proposed to recover multiple heat sources.The thermodynamic and pinch point analysis models of the SHEORC were established,and multi-objective optimization were carried out.The analysis results showed that adding intermediate temperature heat sources among two heat sources with large temperature difference could improve matching rate.The optimization comparison results revealed that the proposed configuration could significantly increase the power output and reduce the initial investment cost compared to the individually designed ORC power generation system.3)The thermal-mechanical-electrical-magnetic multi-timescale ORC power generation system modeling method is proposed to address the problem of long computation time for the dynamic model of ORC power generation system built by the current single-timescale ORC power generation system modeling method.The response time of each component and the speed of parameter changes was analyzed when the ORC power generation system under disturbance and the whole system was divided into multiple time scales for solving separately.The dynamic model of the basic ORC power generation system is established using the proposed method,and the simulation results show that the error between the proposed method and the actual experiment is small,and the simulation time is reduced significantly.4)By analyzing the optimal pressure and optimal superheat temperature of the ORC system under different heat and cold source temperatures,an optimal evaporative pressure and superheat temperature MPPT strategy was proposed to obtain the maximum power of the ORC system.The optimal values were used as reference values under different heat and cold sources.The results showed that the electric energy obtained by the proposed MPPT strategy is increased by 14.15%compared to the optimal evaporative pressure strategy.5)The grid-connected/stand-alone control strategies of the grid-side converter switching were designed for the basic and dual-loop ORC generation systems.For the basic ORC system,the Voltage Oriented Control(VOC)strategy was used on the grid side when connected to the grid.When off-grid,the V/f strategy was applied on the grid side.For multi-loop ORC generation systems,the VOC strategy was used for both loops during grid connection.However,for off-grid operation,the V/f strategy was used for the first loop and the VOC strategy is used for the second loop.The simulation results verify the correctness of the designed control strategy and that the harmonics are within the required range.6)The low temperature thermal energy generation system based on ORC power generation technology was applied to a ship as an example.The waste heat distribution on the ship was analyzed,and exhaust gas,cylinder liner water and intercooler cooling water were selected as the heat sources.According to the temperature range distribution of heat sources,a series heat exchanger type configuration was selected for the optimized design of the multi-source ORC power generation system.The optimization results showed that the designed ORC waste heat power generation system could achieve a net output of 5485.31 kW with an efficiency of 20.17%when Cyclopentane is used as the work fluid.The investment cost per kW is 1404.74 $/kW,the payback time is 2.61 years,and 24,000 tons of CO2 emissions could be reduced annually.This dissertation provided references for the design,modeling and control of low-grade heat power generation systems based on ORC technology and accelerated the development and application of ORC power generation systems.