presentation thesis thermodynamic analysis of stirling engine systems
DESCRIPTION
Presentation of a PhD thesis in Energy TechnologyTRANSCRIPT
Slide 1
Thermodynamic analysis of Stirling engine systemsAdhemar Araoz, PhD studentSupervisors : Prof. Torsten Fransson Marianne Salomon, PhD.
KTH ROYAL INSTITUTEOF TECHNOLOGY1Outline2Background-MotivationResearch ObjectivesMethodologyTechnology OverviewThermodynamic Analysis of Stirling enginesThermodynamic analysis of SE-CHP systemsConclusionsFuture work2Motivation3The need for the development of different energy technologies
Different technologies Particular research challengesSource: International Energy Agency3
Stirling systems4
Definition : A Stirling engine is a thermal machine that transforms heat into work to produce electricity. The engine operates on a thermodynamic cycle, with compression and expansion of the working fluid at different temperature levels
Attractive Stirling PropertiesStirling engine Technological Needs1. Multi-fuel capability2. Waste heat recovery3. High Theoretical efficiency4. Modular systems suitable to access isolated regions.5. Lower noise levelsThere are needs to:Reduce the mechanical and thermal losses in prototypes. Increase the power to heat ratio.Improve the engine efficiency.Increment the reliability of the system(long life, long service intervals).Reduce the manufacturing costs.Source : D.G. Thombare, S.K. Verma, Technological development in the Stirling cycle engines, Renewable and Sustainable Energy ReviewsResearch challenge: Improve the design of Stirling engine systems4Research Objectives5Main objective
Assess the design of Stirling engine systems with emphasis on applications for small scale CHP systems
Specific objectives
Assess the design of Stirling engine prototypes
Develop an adequate design tool for Stirling engines.Determine the effect that different design and operational parameters have on the engine performance
Propose design guidelines to increase the thermodynamic performance of CHP-SE systems
Evaluate the integration of the engine into Combined Heat and Power SystemsDetermine the main parameters that affect the system performance
5Methodology6Modelling approachModel Development(Articles 1,2)Model Validation(Articles 1,2)Parametric Analysis for a Prototype(Article 3)Integration of SE into CHP systems(Article 4)Literature Review67Technology Overview
7Technology overviewStirling engine components8The pistons, the cylinder volumes, the heat exchangers and the crank mechanism. These are arranged in different configurations
Piston-Displacer ConfigurationsAlpha BetaGamma
8Technology OverviewHeat exchangers in Stirling engines9
Heater heads
Cooler
RegeneratorThe main heat exchangers are the heater, cooler and regenerator.
HeaterCoolerInternal regenerator
9Thermodynamic Analysis of Stirling engines(Articles 1, 2,3)1010
Ideal Stirling cycle11The Ideal Stirling cycle consists on four thermodynamic processes
1-2: Isothermal compression. 2-3 :Constant Volume heating.3-4:Isothermal expansion. 4-1 :Constant volume cooling.
Real cycle largely differs from the ideal description.
11Thermodynamic analysis12Second order analysis were chosen, considering a compromise between accuracy and computational requirements
12Proposed second order model (Article 1) 13
Engineering thermodynamic approach
Coupling thermodynamic and heat transfer analysis
13
Ideal adiabatic model 14AssumptionsThe engine is divided into 5 characteristic control volumes The expansion and compression spaces are adiabatic Sinusoidal volume variations Ideal gas inside the engine No heat, no mechanical losses Ideal Regenerator Source: Urieli, I.; 1977, A computer simulation of Stirling engine machines Ph. D Thesis, University of the Witwatersrand, Johannnesburg.
Mass balanceEnergy balanceEquation of stateVolume variationsGoverning equations
14Heat transfer model15
External combustion systemDependent on appropriate correlations15Energy losses module 16Pressure drop through the regeneratorSource: B. Thomas, D. Pittman, Update on the evaluation of different correlations for the flow friction factor and heat transfer of Stirling engine regenerator, American Inst. Aeronaut. & Astronautics, 2000: pp. 7684.sPressure drop through heater and cooler
f : Estimated considering one dimensional flow and cyclic steady state conditions Shuttle conduction
Oscillating flow of the displacer across a temperature gradientInternal conduction losses
Qlk
16Numerical solution17Set of algebraic differential equations for the engine and the heat transfer modulesIterative initial value numerical method until cyclic state conditions are reachedFourth order Runge Kutta scheme for the time discretization
17The GPU-3 engine is a single cylinder displacer engine, with rhombic and sliding rod seals
Capable to produce approximately 7.5 kW with hydrogen working fluid at 6.9 MPa and 3600 rpm rotational speed
The engine was studied by NASA-Lewis Research Centre (LeRC) and is well documented
In addition, the NASA-Lewis Research Centre developed a computational model which was also compared with the model proposed
Simulation of GPU-3 Stirling Engine18
Brake Power Th=704 C
Brake Power at Mean Pressure=2.76 MPa - Good capability of the model for the prediction of the brake power at different conditions - The results reflect the effect that different pressure levels and temperatures on the brake power
P=1.38 MPaP=2.76 MPa P=4.14 MPaFrequencyBrake Power kWFrequencyBrake Power kWT=583 CT=704 C18Simulation of Genoa Stirling engine19
The experimental measurements presented very low power output (55 W). But the thermal performance corresponded with the calculated with the model. For this reason the evaluation of the mechanical efficiency was included
Heat ExchangersCooler
Regenerator
Balancing flywheel
CrankcaseCrankshaftGenerator19
Mechanical efficiency of the system20 The mechanical efficiency is evaluated with Senft efficiency theorem.
W- is the forced work in the systemHypothesis: The model attributed the main losses to the forced work
20Parametric Analysis21ParameterOperational parametersCharge pressure (Pch)Temperature ratio (Tr=Twater/Tad )Design parametersCrankmechanismPhase angle Mechanism effectiveness HeaterTubes internal diameterLength of tubesCoolerTubes internal diameter LengthRegeneratorHousing internal diameterRegenerator length Porosity ()Wire diameterTypeMaterial
21
ResultsInfluence of the charging pressure and temperature ratio
22Improve brake power by increasing the pressure but considering the limits determined by the temperature ratioCritical pointCritical point
Improvements on the brake power may be reached with increments on the charge pressure and reductions on the temperature ratio (high temperatures). The brake power increases until a critical pressure is reached, and after this point the brake power decreases drastically
At higher temperature ratios(low temperature operation) the break point is found at lower pressures
Therefore, in order to improve the engine brake power, both the flame temperature, which affects the temperature ratio and the charged pressure, must be increased.
2223 ResultsInfluence of the crank mechanism on the engine performanceA crank mechanism with low effectiveness will drastically reduce the brake power and thus the efficiency of the enginePhase angles closer to 70 degrees represent the optimum design values
Brake power
23Results24 Heat exchangers sizingThe curves reflect a balance between the positive effect of the increased heat transfer area and the negative influence of the dead volume and pressure drop
Brake powerBrake EfficiencyHeater
24Regenerator analysis (Housing diameter)25Regenerator capacity vs negative increment of dead volume and pressure dropHousing diameter influence on the engine brake powerBrake power start to decrease, around dihous=0.095 m
25Thermodynamic analysis for small scale Stirling engine CHP systemsArticle 42626
Description and Modelling of the system27
Generic CHP systemCombustion system (conventional fuels, renewable fuels)Stirling engine (Alpha, beta,gamma,free piston)Boiler heat exchanger
27Simulation Results28Energy Balance CHPEnergy Balance Stirling engineThe power output of the SE corresponds only to 1.2% of the heat input and the main output is the heated waterThe main losses correspond to the heat lost to the surroundings through the connections and the interface box
28Combustion analysisDifferent Biomass fuels-Overall EfficiencyAir Excess Ratio (wood pellets as fuel )Fuel humidity (wood pellets)Large and negative effectHigher chemical losses(Ash +UHC) Maximum thermal power around 1.9
29
Efficiency Analysis30Overall efficiencyExergy efficiency3 Parameters: -Heat transfer to surroundings-Heat transfer to the engine-Heat transfer to the boiler hexLarge sensitivityHeat to surroundingsHeat to the engineHeat to the boilerIncrement electrical out Small reductionHeat t recoveredVery low exergyIncreasing the size of the engineIncreasing the engine size would increment the exergy efficiency
30Conclusions 31Objective: Design assessment of Stirling engine prototypesA mathematical model that included the integration of thermodynamic, heat transfer and mechanical efficiency was developed and probed adequate for the design assessment of Stirling engine systemsFrom the different curves:
Pressure and temperatures Energy Balance
Design crank mechanism Reduce the forced work.
Design of heat exchangers Dead volumes vs heat transfer area.
- Design of regenerator Pressure drop vs heat transfer
31Conclusions 32Objective: Propose design guidelines to increase the thermodynamic performance of CHP-SE
.
Identify and reduce Energy lossesCombustion parameters Operating parameters Engine designHeat Transfer SEHeat Transfer Boiler-HEXThe study allowed to propose and evaluate design improvements by using the thermodynamic analysis of the system32Future work33Model DevelopmentPropose low cost engine prototypesExtend the analysis to Free piston enginesOptimization routines33Future work34Analysis of the SE-CHP system
34
Thanks
KTH ROYAL INSTITUTEOF TECHNOLOGY35Team TitleCompany NameCompany NameDepartment NameMethod of characteristics
Stirling engine analysis
0th order
1st order
2nd order
3rd order
Empirical correlations (Beale equation)
Isothermic modelsSchmidth analysis
Decoupled approach
Coupled approach
Nodal models
4th order
CFD models
Adiabatic models
Modified adiabatic models
Ideal Adiabatic Module
Internal Heat Transfer Module
External Heat Transfer Module
Energy Losses Module
Design variables
Outputs
External Heat Transfer
External Cooling
Outputs
Qh
Qk
Inputs: Design variables
Regenerator Space
Hot Source (Tflame)
External Wall of the Heater(Twoh)
Internal Wall of the heater(Twih)
Radiation + Convection
Conduction
Working FluidHeater (Th)
Convection
Qh
Qh
Qh
Convection
Convection
Working Fluid inside the cooler(Tk)
Internal Wall of The cooler (Twik)
External wall of the cooler (Twok)
Conduction
Cooling Fluid Temperature(Tek)
Heater
Qk
Cooler
Qk
Qk
Ideal Adiabatic Module
Internal Heat Transfer Module
External Heat Transfer Module
Energy Losses Module
Model outputs
Mechanical Efficiency
Lh
Cooling passages
Lk
Combustion
Fuel