Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The ability of concentrating solar power (CSP) to efficiently store large amounts of
energy sets it apart from other renewable energy technologies. However, cost
reduction and improved efficiency is required for it to become more economically
viable. Significant cost reduction opportunities exist, especially for central receiver
system (CRS) technology where the heliostat field makes up 40 to 50 per cent of the
total capital expenditure.
CRS plants use heliostats to reflect sunlight onto a central receiver. Heliostats with
high tracking accuracy are required to realize high solar concentration ratios. This
enables high working temperatures for efficient energy conversion. Tracking errors
occur mainly due to heliostat manufacturing-, installation- and alignment tolerances,
but high tolerance requirements generally increase cost. A way is therefore needed to
improve tracking accuracy without increasing tolerance requirements.
The primary objective of this project is to develop a heliostat field control system
within the context of a 5MWe CRS pilot plant. The control system has to govern the
tracking movement of all heliostats in the field and minimize errors over time. A
geometric model was developed to characterize four deterministic sources of heliostat
tracking errors.
A prototype system comprising 18 heliostats was constructed to function as a scaled
down subsection of the pilot plant heliostat field and to validate the chosen control
method and system architecture. Periodic measurements of individual heliostats’
tracking offsets were obtained using a camera and optical calibration target combined
with image processing techniques. Mathematical optimization was used to estimate
model coefficients to best fit the measured error offsets. Real-time tracking error
corrections were performed by each heliostat’s local controller unit to compensate for
a combination of error sources. Experimental tracking measurements were performed using the prototype system.
Daily open-loop RMS tracking errors below one milliradian were obtained, thereby
satisfying the project’s primary objective. The thesis concludes that high tracking
accuracy can be achieved using the control method proposed here. This could
potentially lead to a reduction in heliostat cost, thereby lowering the levelised cost of
electricity for CRS plants. / AFRIKAANSE OPSOMMING: Gekonsentreerde sonkrag se vermoë om groot hoeveelhede energie effektief te stoor
onderskei dit van ander hernubare energie tegnologieë. Kostebesparing en hoër
effektiwiteit word egter vereis om dit ekonomies meer lewensvatbaar te maak.
Beduidende kostebesparingsgeleenthede bestaan wel, spesifiek vir tegnologieë vir
sentraal-ontvangerstelsels (central receiver system (CRS)) waar die heliostaatveld 40
tot 50 persent van die totale kapitaalbestedings uitmaak.
CRS aanlegte gebruik heliostate om sonlig op ’n sentrale ontvanger te reflekteer.
Heliostate met ’n hoë volgingsakkuraatheid word vereis om hoë sonkragkonsentrasieverhoudings
te laat realiseer. Dit maak hoë werkstemperature moontlik vir effektiewe
energie-omsetting. Volgingsfoute kom hoofsaaklik voor a.g.v. die heliostaat se
vervaardigings-, installasie- en instellingstoleransies, maar hoë toleransie-vereistes
verhoog gewoonlik die koste. Daar is dus ’n manier nodig om volgingsakkuraatheid
te verbeter sonder om die toleransie-vereistes te verhoog.
Die primêre doel van hierdie projek is om ’n heliostaat aanleg kontrole-stelsel te
ontwikkel binne die konteks van ’n 5 MWe CRS toetsaanleg. Die kontrole-stelsel
moet die volgingsbeweging van al die heliostate in die aanleg bestuur en ook met
verloop van tyd volgingsfoute verminder. ’n Geometriese model is ontwikkel om die
vier bepalende bronne van heliostaat volgingsfoute te karakteriseer.
’n Prototipe stelsel met 18 heliostate is gebou om as ’n funksionele skaalmodel van
die toetsaanleg heliostaatveld te dien en om die gekose kontrole-metode en stelselargitektuur
geldig te verklaar. Periodieke metings van die individuele heliostate se
volgingsafwykings is verkry deur ’n kamera en optiese kalibrasie teiken te kombineer
met beeldprosesseringstegnieke. Wiskundige optimering is gebruik om die model se
koëffisiënte te skat om die beste passing te bepaal vir die gemete foutafwykings.
Intydse volgingsfoutregstellings is deur elke heliostaat se plaaslike beheereenheid
gedoen om te vergoed vir ’n kombinasie van foutbronne. Eksperimentele volgingsmetings is uitgevoer met die prototipestelsel. Daaglikse ooplus
RMS volgingsfoute onder een milliradiaan is verkry, en sodoende is die projek se
primêre doel behaal. Die tesis maak die gevolgtrekking dat hoë volgingsakkuraatheid
behaal kan word deur die gebruik van die beheer-metode soos hier voorgestel. Dit
kan potensieel bydra tot kostebesparing in die heliostaatveld van CRS aanlegte om
sodoende die geykte koste van elektrisiteit te verminder.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86674 |
| Date | 04 1900 |
| Creators | Malan, Karel Johan |
| Contributors | Gauche, P., Treurnicht, J., 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 | xiv, 114 p. : ill. |
| Rights | Stellenbosch University |
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