Development of a micro-gas turbine for central receiver concentrating solar power systems

Ssebabi, Brian (2020-03)

Thesis (PhD)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: There is limited access to electricity in much of Southern Africa, yet this region receives some of the highest solar irradiation worldwide. Consequently, there is a huge potential for the deployment of concentrating solar power (CSP) distributed electricity generation systems. The application of gas turbines in central receiver CSP systems combines high concentration ratios of central receiver technology and the ability to scale in size, with inherent gas turbine advantages. This makes these systems ideal for distributed electricity generation applications. However, there are currently no commercial solar-hybrid gas turbine systems readily available off-the-shelf. Through both a theoretical and experimental approach, this study therefore aims to develop a micro gas turbine (MGT) system for central receiver CSP distributed electricity generation applications. The theoretical work involved modeling the performance of a MGT system under solar-hybrid operation, in order to predict the possible operating range and develop suitable operation and control strategies for the MGT system. The experimental work involved designing, building, testing and characterising the performance of an experimental MGT system and then using the obtained test data to validate the predicted performance, as well as assess the technical feasibility of adapting such a system to solar-hybrid operation. The component matching showed a shift of the equilibrium running point on the compressor characteristic, to counter the additional system pressure losses and ensure a useful work output, albeit with a reduced surge margin. Solar-hybrid operation was only possible for a solar share of at least 20 %, while the work output and cycle thermal efficiency drop below standard operation levels beyond certain solar share. In contrast to standard operation, a higher nominal work output of 20 kW, at a lower specific fuel consumption of 0.0004 kg/kWh and a higher cycle thermal efficiency of 8 % was predicted, the latter potentially increasing to 20 % with recuperation. Based on the proposed control strategy of operating the MGT system at the determined solar-hybrid equilibrium running point, sudden changes in solar irradiation were corrected by altering the fuel flow. From the validation of the predicted performance, the predicted MGT system equilibrium running point matched the optimum experimental equilibrium running points, and a similar shift on the compressor characteristic was predicted for varying levels of system pressure losses. The use of turbocharger technology should allow for easy coupling of the individual MGT system components, some of which could be specially designed for solar-hybrid operation. The twin-shaft configuration allows for flexibility in operation, with the added advantage of ease of starting. The large swallowing capacity of the much bigger power turbine should further ensure that the MGT system is insensitive to load variation. The findings from this study should guide operation and control strategies for the proposed, and future solar-hybrid MGT systems, which should in turn contribute to their development and commercialization.

AFRIKAANSE OPSOMMING: Alhoewel Suider Afrika van die hoogste sonbestralingsvlakke ter wêreld het, is daar beperkte toegang tot elektrisiteit in die streek. Gevolglik is daar ’n beduidende potensiaal vir die ontplooiing van gekonsentreerde sonkrag (CSP) verspreide elektrisiteitsopwekking stelsels. Die gebruik van gasturbines in sentrale ontvanger CSP stelsels kombineer die hoë konsentrasievlakke van sentrale ontvanger tegnologie en die vermoë om te skaleer, met die inherente voordele van gasturbines. Dit maak hierdie stelsels ideaal vir verspreide elektrisiteitsopwekking toepassings. Daar is egter tans geen kommersiële van-die-rak son-hibriede gasturbine stelsels beskikbaar nie. Die studie poog om deur middel van beide ’n teoretiese en eksperimentele benadering ’n mikrogasturbine (MGT) stelsel vir sentrale ontvanger CSP verspreide elektrisiteitsopwekkings toepassings te ontwikkel. Die teoretiese werk het die modellering van ’n MGT stelsel onder son-hibriede werking behels, ten einde die moontlike bedryfsgebied te voorspel en om gepaste bedryfs- en beheer strategieë vir die MGT te ontwikkel. Die eksperimentele werk het die ontwerp, bou, toets en karakterisering van ’n eksperimentele MGT stelsel behels. Die toetsdata is gebruik om die voorspelde werksverrigting van die stelsel te valideer, sowel as om die tegniese haalbaarheid van die aanpassing van so ’n stelsel vir son-hibriede werking te bepaal. Die stelselbalansering vir son-hibriede toepassing het gewys dat die ekwilibrium werkspunt op die kompressor karakteristiek skuif, ten einde die addisionele stelsel verliese teen te werk, asook om steeds ’n bruikbare drywingsuitset te verseker, alhoewel teen ’n vermindering in staakwydte. Son-hibriede werking was net moontlik vir ’n sonaandeel van ten minste 20 %, terwyl die werksuitset en siklus termiese benuttingsgraad geval het tot onder standaard bedryfsvlakke bokant ’n sekere sonaandeel. In vergelyking met standaard werking van die MGT, is ’n hoër nominale drywingsuitset van 20 kW, teen ’n laer spesifieke brandstofverbruik van 0.0004 kg/kWh en ’n hoër termiese benuttingsgraad van 8 % voorspel. Laasgenoemde kan potensieel tot 20 % verhoog word met tussenverhitting (recuperation). Gebaseer op die voorgestelde beheer strategie om die MGT stelsel by die spesifieke son-hibriede ekwilibrium werkspunt te bedryf, is daar vir skielike verandering in die sonbestraling gekorrigeer deur die brandstofvloei aan te pas. Uit die validasie van die voorspelde werksverrigting is gevind dat die voorspelde MGT werkspunt ooreenstem met die eksperimentele optimum eksperimentele ekwilibrium werkspunt en dat ’n soortgelyke skuif in die kompressor karakteristiek voorspel is vir wisselende vlakke van stelsel drukverlies. Die gebruik van bestaande turbo-aanjaer tegnologie behoort voorsiening te maak vir die vereenvoudigde koppeling van die MGT stelsel komponente, waarvan sommige doelontwerp kan word vir sonhibriede werking. Die dubbel-as konfigurasie lei tot buigbare stelselwerking, asook die bykomende voordeel dat die stelsel maklik in werking gestel kan word. Die groot vloeikapasiteit van die veel groter drywingsturbine behoort ook te verseker dat die MGT stelsel onsensitief is vir lasvariasies. Die bevindinge van hierdie studie sal as gids dien vir die opstel van bedryfs- en beheer strategieë vir die voorgestelde en toekomstige son-hibriede MGT stelsels, wat weer ’n bydrae sal maak tot die ontwikkeling en kommersialisering van hierdie stelsels.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108262
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