Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Concentrating solar power as a sustainable energy technology is considered favourable
in South Africa due to the high solar resource and the integration possibilities with
the current electricity grid. Considering the various concentrating solar power
technologies, the central receiver system is thought to be one of the most promising
due to its high capacity factor and cost-efficient thermal storage capabilities.
These thermal systems are able to reach high temperatures (more than 1000°C
have been shown), and to obtain such temperatures, a high solar flux is required.
This is achievable by aiming all of the heliostats at the centre of the target. High flux
gradients over the receiver surface area and between the outer and inner surfaces
of the receiver material can exist. These thermal gradients account for differences
in temperatures on the receiver which result in thermal stresses leading to elastic
and potentially plastic deformation of the material.
To eradicate the thermal stresses, the aim points of the heliostats can be managed
such that the flux density distribution over the receiver aperture is decreased and
homogenised. The primary objective of this thesis is to develop an aiming strategy
specifically for small experimental heliostat fields. To reach this objective, a simulation
platform was developed to model any heliostat field and receiver, and an
analysis was conducted to compare analytical flux prediction methods to ray tracing.
At high incidence angles of between 30° and 60°, the standard deviations of the
circular Gaussian flux approximation methods were found to differ between approximately
10 % and 30 % from the ray traced results. A novel method, the Gaussian
mixture model, was suggested and deviated less than 4 % from the ray traced
results when considering the standard deviation of the flux profile.
Two basic aiming strategies were developed using the Tabu search and Genetic algorithm optimisation methods. These two strategies make use of approximate
method of flux prediction. Experiments were conducted to investigate static aiming
strategies on physical systems and to identify factors that could have an effect
on the experimental results. Finally a method of implementing the aiming strategy
on a dynamic system was proposed.
By implementing both optimisation methods to complement each other, a new
aiming strategy was developed that proved to provide better homogenisation of
the flux distribution than either of the two methods alone. The accuracy of the final
predicted flux distribution was improved by using the Gaussian mixture model as
the flux distribution approximation method. / AFRIKAANSE OPSOMMING: Gekonsentreerde sonenergie as ’n hernubare energie bron, word in Suid-Afrika as
uitvoerbaar beskou weens die hoë stralingsdigtheid asook die moontlikheid om
hierdie tegnologie te koppel aan die huidige energienetwerk. As die verskeie gekonsentreerde
sonenergie tegnologieë in ag geneem word, word sentrale ontvangerstelsels
beskou as die mees belowende sonenergie konsep as gevolg van die hoë
energie kapasiteit en die vermoë om termiese energie te stoor.
Hierdie termiese stelsels besit die vermoë om hoë temperature te bereik (meer as
1000°C is al opgemerk), en om hierdie temperature te verkry word ’n hoë termiese
vloed benodig. Dit is bereikbaar deur al die heliostate op die middel van die ontvanger
te rig. Hoë termiese vloedgradiënte oor die oppervlakte van die opvanger
en tussen die buite- en binne laag van die opvanger materiaal kan ontstaan. Termiese
vloedgradiënte kan verskille in temperatuur op die ontvanger veroorsaak
wat termiese spanning tot gevolg het kan lei tot elastiese en potensieel plastiese
vervorming van die materiaal.
Omvan die termiese spanning ontslae te raak kan die heliostate deur beheerstelsels
na ander dele van die ontvanger gerig word om sodoende ’n laer en meer egalige
termiese vloed oor die oppervlak van die ontvanger te verkry. Die hoofdoel van
hierdie tesis is die ontwikkeling van ’n mikstrategie, hoofsaaklik bedoel vir klein
eksperimentele heliostaatvelde. Om hierdie hoofdoel te bereik, was ’n simulasieplatform
ontwikkel waardeur heliostaatvelde en opvangerstelsels gemodelleer kan
word, en ’n analise is gedoen om die verskeidenheid benaderende analitiese metodes
te vergelyk met die straalopsporingsmetode.
By hoë invalshoeke tussen 30° en 60° is daar gevind dat die benaderende modelle
wat die termiese vloed as ’n normale Gaussiese verspreiding beskou, ’n afwyking van die straalopsporingsresultate toon van ongeveer 10 % tot 30 %. ’n Nuwe metode,
die Gaussiese mengsel model, was voorgestel en het minder as 4 % van die
straalopsporings resultate afgewyk, met die standaard afwyking van die termiese
vloed profiel in ag geneem.
Twee basiese mikstrategieë is ontwikkel deur gebruik te maak van die Tabu soek en
Genetiese algoritme optimeringsmetodes. Hierdie strategieë maak gebruik van die
benaderende metodes om termiese vloed te voorspel. Eksperimente is uitgevoer
om die implementering van statiese mikstrategieë op ’n fisiese stelsel te beskou en
die faktore wat ’n invloed op die eksperimentele resultate sal hê te identifiseer. Ten
laaste word ’n metode voorgestel vir die implementering van die mikstrategieë op
’n dinamiese stelsel.
Deur beide optimeringsmetodes te implementeer sodat hul mekaar komplimenteer,
word ’n nuwe mikstrategie ontwikkel wat beter homogenisering van die termiese
vloed verspreiding bewys het as enige van die alleen staande metodes. Die
akkuraatheid van die finale voorgestelde termiese vloed verspreiding was verbeter
deur gebruik te maak van die Gaussiese mengsel model as die benaderende
metode van die termiese vloed verspreiding.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97051 |
| Date | 03 1900 |
| Creators | Grobler, Annemarie |
| Contributors | Gauche, Paul, Smit, Willie, Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 117 pages : illustrations |
| Rights | Stellenbosch University |
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