Thermodynamic Analysis And Optimization For Energy Systems Based On Stirling Cycle

With the development of science,technology,and ecological civilization,people have come to realize the importance of environment protection and sustainable energy development.As the combustion of fossil fuels has caused serious harm to the environment,people are constantly exploring clean energy and effective energy utilization methods.As a heat-power conversion device with high efficiency,cleanness,and stability,the Stirling engine has significant potential for utilization of clean energy.The theoretical research and optimization design of the Stirling engine have important guiding significance for the production and use of the Stirling engine.Regarding the theoretical research of the regenerator,a significant component of the Stirling engine,the irreversibility caused by temperature differences is considered to model the regeneration using the finite time thermodynamics method.The regenerator is modeled and discussed in detail with different assumptions of temperature distribution.For the regenerator with an even distribution temperature assumption,the regenerative efficiency and its limitations are obtained.Results demonstrate that the efficiency of regenerator with an even distribution cannot be higher than 50%.For an uneven distribution temperature assumption,the regenerator is modeled by dividing it into n sub-regenerators.Two cases,for either constant or varying temperatures of the sub-regenerators,are discussed in detail,and the same regenerative effectiveness is obtained for the limit n ??.Furthermore,the thermal efficiency and output power of the Stirling engine are obtained,and the effects of the parameters regarding the performance of the Stirling engine are investigated.Regarding the theoretical research of Stirling engine cycle,the effects of certain irreversibilities are considered,and three different thermal models are developed using finite time thermodynamics.First,finite time thermodynamics is employed to construct a refined model that considers the isothermal processes in expansion and compression strokes,and isochoric processes in regeneration.Second,as the actual expansion and compression stroke are more likely to be polytropic processes,a polytropic model of a Stirling engine is developed and its polytropic exponents are obtained via finite time thermodynamics.Third,different from previous isothermal,adiabatic,or polytropic models,a finite time thermodynamic model based on the movements of cylinder pistons is proposed and investigated.As a result,the relationship between pressure and volume of working fluid is formulated as normalized ordinary differential equations and subsequently are solved numerically.In addition,to evaluate the performance of the new model,the effects of design parameters on output power and thermal efficiency are studied and certain conclusions are obtained.Regarding the optimization of the Stirling engine,intelligent optimization algorithms are adopted in multi-objective optimization of the Stirling engine models.Owing to the nature of conflicts among different optimization objectives,traditional single objective optimization methods always result in inferior values of other objectives.Therefore,multiobjective optimizations are considerably instructive in engineering.First,the non-dominated sorting genetic algorithm II is employed to multi-objective optimization of the proposed Stirling engine models.Second,a particle swarm optimization algorithm based on crowding distance is employed to the multi-objective optimization of the Stirling engine models for the first time.The output power,thermal efficiency,and ecological coefficient of performance are considered as the objectives.From the comparison of results obtained by multi-objective and single-objective optimization methods,the multi-objective optimization method is more practical in engineering applications for its superiority in coordinating different objectives of Stirling engines.For the research regarding cascade utilization of energy systems,three types of hybrid energy systems based on the Stirling engine are proposed and studied.First,a solar refrigerator consisting of a dish solar collector,a Stirling engine,and a Stirling refrigerator is proposed and studied.Solar energy is captured by the dish solar collector and converted into thermal energy.Using the Stirling engine,the thermal energy is converted to work,which is used to drive the Stirling refrigerator.Effects of direct solar flux intensity,collector optical efficiency,and convective heat transfer coefficients on the performance of the system are investigated.Second,a hybrid system combined power and distillation(CPD)is introduced and analyzed.In the system,a Stirling engine is adopted to produce power and a direct contact membrane distillation device is used to harvest the waste heat of the Stirling engine for distillation.Effects of design parameters on the performance of the CPD system are studied to aid the design and optimization.Some conclusions are obtained to guide the design and optimization of the CPD system.