Numerical Investigation of a novel blade for use in vertical axis wind turbines.

Erfort, Gareth (2019-12)

Thesis (PhD)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Renewable energy prospects in South Africa have been growing thanks to the government’s commitment to alternative energy sources. The country has committed to 36 projects ranging in size from 52 MW to 140 MW. South Africa’s sole energy distributor has been implementing rolling black-outs due to unscheduled maintenance on their plants. This has resulted in average two hour periods of no power for citizens and companies alike. These entities have turned to storage and small scale renewable generation to tide them over during a black out. Within the country wind power is therefore being utilized on both a commercial and private scale. Vertical axis wind turbines have been identified for their applicability in large scale off shore wind farms and ability to operate in urban environments, as a future power generation technology. At this time the vertical axis wind turbine is however not a common sight for power generation. Studies have indicated that a few inherent traits of this turbine design have hindered deployment due to increased manufacturing cost associated with their mitigation. The variation in torque generated during the course of rotation is an example. It can result in drive train stresses, and negatively affect the fatigue life of drive components. This dissertation is aimed at reducing the variation in torque experienced by a straight bladed Darrieus turbine during operation. The study proposed a novel blade that would allow for adjustments to the forces experienced by the turbine during rotation. A virtual laboratory was used to analyse the effect of the blade. An analytical model for a two-bladed H-rotor was implemented in Python and validated against published data. The model was based on the double multiple streamtube (DMST) method as it provided fast accurate solutions. The blade is designed to have an adaptive distortion on the upper surface. The distortion is able to change height and thereby control the tripping of the boundary layer from laminar to turbulent flow. Lift and drag coefficients for the blade were obtained through computational fluid dynamic (CFD) simulations in the open source software OpenFOAM. A transitional turbulence model based on momentum thickness and intermittency was implemented and adjusted to increase efficiency. A random forest surrogate model was used in optimization to determine the exact nature of the proposed distortion. The blade design proved effective in reducing the torque ripple. Placement of the distortion was predominantly on the leading edge (LE) of the blade, where a small change in shape had the largest effect on the boundary layer. The optimized solution reduced the maximum possible torque that the turbine could achieve by synchronously increasing the drag experienced by the blade with the torque fluctuations. The reduction in ripple resulted in an increased life span of the drive train shaft by an estimated 36%. An equation relating the reduction in torque ripple to the reduction in coefficient of performance was identified.

AFRIKAANSE OPSOMMING: Hernubare energievooruitsigte in Suid-Afrika het verbeter danksy die regering se verbintenis tot alternatiewe energiebronne. Die land is verbind tot 36 projekte wat wissel van 52 MW tot 140 MW. Suid-Afrika se enigste energieverspreider is tans besig om kragonderbrekings te implementeer weens ongeskeduleerde instandhouding van aanlegte. Dit het burgers sowel as maatskappye blootgestel aan twee-uur lange kragonderbrekings. Hulle het hulle dus gewend na die stoor van energie en na kleinskaalse hernubare kragopwekking om hulle te help tydens beurtkrag. In die land word windkrag tans kommersiëel en privaat gebruik. Weens die toepaslikheid van die vertikale-as windturbine in grootskaalse aflandige windplase, en die vermoë om in stedelike omgewings te funksioneer, word dit erken as ’n toekomstige kragopwekkingstegnologie. Tans is die vertikale-as windturbine egter nie in algemene gebruik vir kragopwekking nie. Studies het aangedui dat ’n paar inherente eienskappe van hierdie turbine-ontwerp die implementering verhinder het as gevolg van verhoogde geassosieerde vervaardigingskoste. Die variasie in opgewekte wringkrag tydens die rotasie is ’n voorbeeld. Dit kan lei tot spanning in aandryfstelsels, en die vermoeidheidslewe van aandryfonderdele negatief beïnvloed. Hierdie proefskrif is daarop gemik om die variasie in wringkrag wat ’n reguit-lem Darrieus-turbine ondervind te verminder. Dit studie stel ’n nuwe lem voor wat toelaat dat die kragte wat die turbine tydens rotasie ervaar, aangepas kan word. ’n Virtuele laboratorium is gebruik om die effek van die lem te analiseer. ’n Analitiese model vir ’n twee-lem H-rotor is geïmplementeer in Python, en bevestig teen gepubliseerde data. Die model is gebaseer op die tweevoudige stroomstrook (DMST) metode, aangesien dit vinnige en akkurate oplossings verskaf. Die lem is ontwerp om ’n aanpasbare vervorming op die boonste oppervlak te hê. Die vervorming kan van hoogte verander en sodoende die oorgang van die grenslaag van laminêre tot turbulente vloei beheer. Hef- en sleurkoëffisiënte vir die lem is verkry deur middel van ’n oorgangsturbulensiemodel gebaseer op momentumdikte en onderbrokenheid, en is geïmplementeer en aangepas om doeltreffendheid te verhoog. ’n "Random forest“ surrogaatmodel is gebruik in optimering om die presiese aard van die voorgestelde vervorming te bepaal. Die lemontwerp was effektief in die vermindering van die wringkrag rimpeling. Plasing van die vervorming was hoofsaaklik aan die voorpunt van die lem, waar ’n klein verandering in vorm die grootste effek op die grenslaag gehad het. Die geoptimeerde oplossing verminder die maksimum moontlike wringkrag wat die turbine kan behaal, deur die sleur wat die lem ervaar met die wringkragskommelinge sinchronies te verhoog. Die vermindering in die rimpeling het gelei tot ’n verhoogde lewensduur van die aandryfas met ’n geskatte 36%. ’n Vergelyking wat die vermindering in wringkrag rimpeling met betrekking tot die vermindering in koëffisiënt van prestasie weergee, is geïdentifiseer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107231
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