Theoretical simulation, manufacture and experimental evaluation of a free piston stirling engine electric generator.

De La Bat, Jean Gerard (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: A Stirling engine is a suitable candidate for heat and electrical power co-generation when integrated with a parabolic-dish solar concentrator. Such solar systems are envisaged as being a viable option for micro off-grid co-generation using smart energy control strategies in developing countries rich in solar irradiance. This dissertation presents the theoretical simulation, fabrication and experimental evaluation of a novel prototype free-piston Stirling engine electric generator. The theoretical model was developed from first principles, by discretising the working fluid into a network of one-dimensional control volumes. By applying the conservation of mass, momentum and energy and applicable property functions to each control volume, a system of non-linear time-dependent partial differential equations was generated. These equations were solved sequentially using a fully-explicit numerical method with linear upwind-differencing and by employing a backwardsstaggered velocity scheme. A transient electromagnetic finite element analysis of the linear generator was performed and is represented in the theoretical simulation model through generator characteristic functions. A novel experimental engine prototype was manufactured that includes a linear motor attached to the displacer shaft. Finite element analyses were used to design the strength and safety-critical mechanical components of the engine. The electromechanical, flexure spring stiffness and displacer and piston mechanical friction characterising equations of the theoretical model were matched to the as-built engine by direct experimentation. Seven selected experimental test runs were used to establish whether the theoretical model is capable of emulating the behaviour of the as-manufactured engine, with hot-end temperatures varied between 300 and 600 C and working fluid charge pressures between 1.0 and 2.0MPa. Self-sustained, closed-circuit operation of the engine was achieved at a hot-end temperature of 470 C and charge pressure of 1.7MPa absolute, with a peak electrical output power of approximately 73W. A good correlation between experimental and simulation results was demonstrated for several operating conditions, thereby validating the theoretical simulation model. It is concluded that it is critical to ensure proper alignment of the moving parts so that internal sliding friction may be minimised. To achieve continuous and sustained operation without piston-casing collisions, it is recommended that an electronic feedback control system be integrated to the existing engine.

AFRIKAANSE OPSOMMING: ‘n Stirling enjin is ’n geskikte kandidaat vir gekombineerde warmte en elektriese kragopwekking tesame met die gebruik van ‘n paraboliese sonkrag konsentreer skottel. Só ‘n stelsel word beoog as ‘n lewensvatbare oplossing vir mikro kragopwekking in ontwikkelende lande waar sonkrag vollop beskikbaar is. Hierdie proefskrif dokumenteer die teoretiese simulasie, vervaardiging en eksperimentele evaluering van ‘n nuwe prototipe vry-suier Stirling enjin elektriese kragopwekker. Die teoretiese simulasiemodel is vanuit eerste beginsels afgelei deur die werkvloeistof in ‘n netwerk van eendimensionele kontrolevolumes te verdeel. Die beginsels van behoud van massa, momentum en energie, asook toepaslike eienskapsvergelykings is dan aan elke kontrolevolume toegeken om sodoende ’n stelsel van nie-lineêre, tydafhanklike parsiële differensiaalvergelykings af te lei. Hierdie tydsafhanklike vergelykings was opgelos deur gebruik te maak van ’n volledig-eksplisiete numeriese metode met windopse-verskilvorming en ‘n terugwaards-gestapelde snelheidskema. ‘n Ongestadigde elektromagnetiese eindige element analise was uitgevoer om die werking van die lineêre elektriese kragopwekker te simuleer. Die kragopwekker word in die teoretiese model verteenwoordig deur gebruik te maak van analitiese kragopwekkerkenmerkende funksies. ‘n Nuwe eksperimentele enjin prototipe was gemasjineer. Eindige element analises was gebruik om die sterkte en veiligheidskritieke meganiese komponente van die enjin te ontwerp. Die elektromeganiese, veërbuigstyfheid en verplaser- en krag-suier meganiese wrywing eienskappe was vasgestel deur eksperimentele ondersoeke van die soos-vervaardigde enjin. Sewe uitgekiesde eksperimentele toetslope was gebruik om vas te stel of die teoretiese model in staat is om die gedrag van die vervaardigde enjin na te boots. In hierdie toetse is die verwarmer temperatuur gewissel tussen 300 en 600 C en die werkvloeistof lugdruk gewissel tussen 1.0 en 2.0MPa. Selfonderhoudbare, geslote-stroom werking van die enjin was behaal met ‘n temperatuur van 470 C en ‘n lugdruk van 1.7MPa. By hierdie toestande is ‘n piek elektriese krag uitset van 73W behaal. ‘n Goeie korrelasie tussen eksperimentele en simulasie resultate was getoon vir ‘n reeks werkstoestande, wat die teoretiese model valideer. Daar is tot die gevolgtrekking gekom dat dit noodsaaklik is om die belyning en passing van die bewegende suiers te verbeter om ten einde interne glywrywing te minimaliseer. Dit word aangebeveel om vir die bestaande prototipe ‘n elektroniese terugvoerbeheerstelsel te ontwikkel sodat volhoubare enjine werking sonder suier botsings kan verseker word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107164
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