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.

This thesis presents the thermodynamic and economic assessment of gas power cycles for 100 MW solar thermal power generation systems. A gas power cycle for solar power generation is a totally different technology from the current state of the art solar power generation systems. As a result, this thesis provides an assessment of the solar power generation systems with gas power cycles and provides guidance in the selection of design and operating parameters for gas power cycle based solar power generation system. The gas power cycle based power generation systems are assessed by means of thermodynamic and economic models developed and simulated using commercial thermodynamic analysis software. The gas cycle based power generation systems considered in this study are Cold Gas Turbine, High Temperature Solar Gas Turbine and Lorentz Cycle Gas Turbine. The system models are assessed for their thermodynamic performance using theory based turbo-machinery models with practical performance and loss data. In addition, extensive cost models have been developed for assessing the economic performance of the system models to determine their practical feasibility. The results from this study indicate that the most economical power generation system is the HTSGT system for a high peak cycle temperature utilizing the central receiver power tower solar collector system. The LCGT system also has a comparable performance at the same operating temperature. The CGT system assessed for operating with parabolic trough solar collector system at a lower peak cycle temperature had an inferior performance compared to the current state of the art technology for the power generation using parabolic troughs.

Solar power tower technology can achieve higher temperatures than the most common commercial technology using parabolic troughs. In order to take advantage of higher temperatures, new power cycles are needed for generating power at higher efficiencies. Supercritical carbon dioxide (S-CO2) power cycle is one of the alternatives that have been proposed for the future concentrated solar power (CSP) plants due to its high efficiency. On the other hand, carbon dioxide can also be a replacement for current heat transfer fluids (HTFs), i.e. oil, molten salt, and steam. The main disadvantages of the current HTFs are maximum operating temperature limit, required freeze protection units, and complex control systems. However, the main challenge about utilizing s-CO2 as the HTF is to design a receiver that can operate at high operating pressure (about 20 MPa) while maintaining excellent thermal performance. The existing tubular and windowed receivers are not suitable for this application; therefore, an innovative design is required to provide appropriate performance as well as mechanical strength. This research investigates the application of s-CO2 in solar power tower plants. First, a computationally efficient method is developed for designing the heliostat field in a solar power tower plant. Then, an innovative numerical approach is introduced to distribute the heat flux uniformly on the receiver surface. Next, different power cycles utilizing s-CO2 as the working fluid are analyzed. It is shown that including an appropriate bottoming cycle can further increase the power cycle efficiency. In the next step, a thermal receiver is designed based on compact heat exchanger (CHE) technology utilizing s-CO2 as the HTF. Finally, a 3MWth cavity receiver is designed using the CHE receivers as individual panels receiving solar flux from the heliostat field. Convective and radiative heat transfer models are employed to calculate bulk fluid and surface temperatures. The receiver efficiency is obtained as 80%, which can be further improved by optimizing the geometry of the cavity.

Concern over the altering climate due to the release of anthropogenic greenhouse gases has caused a major shift in the developments of ways to minimise human impact on the climate. Solar energy is seen as one of the most promising sources to transform the energy market for low-carbon energy generation. Currently, solar power is generated via photovoltaic (PV) and concentrating solar power (CSP) technologies. The advantage of CSPs to scale up renewable energy to utility level, as well as to store thermal energy for electrical power generation when the sun is not available (after sunset or during cloudy periods) makes this technology an attractive option for sustainable clean energy. CSP development, however, is still in its infancy, and for it to be a competitive form of energy-generation technology, techno-economic developments in this field need to improve the efficiency and decrease the costs of this technology. A policy report by the European Academies’ Science Advisory Council (EASAC) (2011) indicated that central receiver (solar tower) CSP systems show the greatest margin for technological improvements (40% to 65% is estimated), and that an improvement in receiver technology could make the greatest contribution to increase efficiency. This study therefore focused on analysing the optical and thermal performance of a new proposed solar cavity molten salt receiver design for a central receiver CSP system using a numerical approach. In this study, the receiver’s performance was analysed by first selecting an existing heliostat field, Planta Solar 10 (PS-10). For the numerical analysis to reflect conditions that are as realistic as possible, numerical models for different aspects were selected and validated. For modelling the sun, the solar tracking numerical model proposed by Iqbal (1983) was selected and implemented after literature and comparison showed adequate results. The direct normal irradiation (DNI) was modelled by applying a clear sky model, with the parameterisation model C proposed by Iqbal (1983) as the chosen model. The variables in this model that were subject to temperature, and humidity values were more accurately presented by adding numerical approximations of the region’s actual weather data. The DNI model reflected realistic fluctuations. For the thermal modelling, a validation study was conducted on impingement flow heat transfer to select an appropriate Reynolds-averaged Navier-Stokes (RANS) model that would provide accurate results when conducting the thermal performance test on the receiver. The study concluded that the transitional Shear Stress Transport (SST) turbulence model performed the best. A new method was also developed and validated that allows one to not only simulate complex geometries within the Monte Carlo ray tracing environment SolTrace, but also to apply the results obtained by simulating this model as a heat source within the computational fluid dynamics (CFD) environment ANSYS Fluent. This allows SolTrace modelling to be more accurate, since models do not need to be approximated to simple geometries. It also provides an alternative for solar modelling in ANSYS Fluent. The optical analysis was conducted by first performing an analysis on the receiver aperture and studying its sensitivity on the captured flux. This was followed by analysing the optics of the proposed receiver, the flux distributions on a simplified absorber surface area, and how these distributions are altered by changing some parameters. An in-depth analysis was finally done on the absorber area by applying the aforementioned model to simulate complex geometries within SolTrace, with the results illustrating the difference of the detailed geometry on optical modelling. An alternative receiver design with improved optical features was proposed, with an initial study providing promising results. The thermal analysis was done within the CFD environment, with only a section of the absorber surface area considered, and by applying the solar flux simulated during the optical analysis as heat source within the geometry model. This allowed the model to simulate the effects of re-radiation at the surface of the absorber while simulating the heat transfer at the fluid molten salt side simultaneously. The results showed that, for the current design and requirements, the absorber surface temperature reaches impractical temperatures. Altering the design or being more lenient on the requirements has, however, shown dramatic improvements in terms of thermal performance. Sensitivity studies for both the optical and thermal analyses have shown that changes in design can dramatically improve the performance of the design, making it a possible feasible receiver design for central receiver systems. / Dissertation (MEng)--University of Pretoria, 2019. / National Research Foundation (NRF) / Mechanical and Aeronautical Engineering / MEng / Unrestricted

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.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: There are two methods generally used for heliostat field layout optimization: the field growth method and the pattern method. A third, less commonly used method also exists: the free variable method. Each of these three methods offers its own set of advantages and disadvantages. The purpose of this study is to explore the subject of heliostat field layout optimization and to examine the free variable method. In order to achieve this objective, optimization was conducted using the free variable method. To perform optimization, a technical model of the heliostat field was constructed using approximating functions from literature and geometric analysis. Results of this study indicated that the free variable method is possible using a gradient-based optimization algorithm. The free variable method was applied to redesign the field of a commercial plant, PS10, located in Spain. The original plant was improved by 1.2% in annual intercepted energy. The study has demonstrated some of the characteristics, advantages and pitfalls of the free variable method and has proved useful in contributing to the understanding of heliostat field layout optimization. The free variable method requires more research and development before it may be used commercially. / AFRIKAANSE OPSOMMING: Daar word oor die algemeen twee metodes gebruik om 'n heliostaatveld se uitleg te optimeer: die terreinvermeerderingsmetode (field growth) en die patroonmetode (pattern). 'n Derde, minder algemeen gebruikte metode bestaan ook: die vrye veranderlike metode (free variable). Elkeen van hierdie drie metodes bied sy eie voordele en nadele. Die doel van hierdie studie is om die onderwerp van heliostaat veld uitleg optimalisering te verken en die vrye veranderlike metode te ondersoek. Ten einde te hierdie oogmerk te bereik, is die optimalisering uitgevoer met gebruik van die vrye veranderlike metode. Om optimalisering uit te voer, is 'n tegniese model van die heliostaat veld gebou met behulp van benader funksies van letterkunde en geometriese analise. Resultate van hierdie studie het aangedui dat die vrye veranderlike metode moontlik is deur gebruik te maak van 'n gradientbebaseerde optimeeringslagoritme. Die vrye veranderlike metode is ook aangewend om die terrien van 'n kommersiele aanleg, PS10 in Spanje, te herontwerp. Die oorspronklike aanleg is in jaarlikse onderskep energie verbeter met 1.2%. Die studie het sommige van die eienskappe, voordele en slaggate van die vrye veranderlike metode getoon en het nuttige bewys om by te dra tot die begrip van heliostaat veld uitleg optimalisering. Die vrye veranderlike metode vereis meer navorsing en ontwikkeling voordat dit kommersieel gebruik kan word.

Thesis (MScEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Concentrating solar power technology is a modern power generation technology in which central receiver systems play a significant role. For this technology a field of heliostats is used to reflect solar irradiation to the receiver located on top of the tower. An extensive review has shown that contemporary receiver designs face geometric complexities, lack of thermal efficiency as well as issues with durability and cost. The purpose of this study is to develop a new receiver concept that can potentially reduce these issues. A parametric analysis was used to identify potential means of improvement based on an energy balance approach including sensitivities involved with convection and radiation heat transfer. Design criteria such as the use of headers to minimize pressure drop was also investigated. Based on these findings the hybrid pressurized air receiver was developed which is a combination of tubular and volumetric receiver technologies. The fundamental idea of the receiver was investigated by simulating the ray-tracing and coupled natural convection and radiation heat transfer. The ray-tracing results have shown that the use of quartz glass is a prospective solution to higher allowable flux densities, but with reflection losses in the order of 7 %. The coupled natural convection heat transfer simulation further revealed that the receiver concept effectively eliminates the escape of buoyant plumes and radiative heat losses are minimized. Empirical data was gathered from a medium flux concentrator and good agreement with the numerical results was obtained. The thesis therefore concludes that the research outcomes were met. Ongoing research aims to optimise the receiver concept for a 5MW pilot plant. / AFRIKAANSE OPSOMMING: Gekonsentreerde sonkrag tegnologie is ’n moderne kragopwekkingstegnologie waar sentrale ontvangersisteme ’n beduidende rol speel. Vir hierdie tegnologie word ’n veld heliostate gebruik om sonstraling na die ontvanger wat aan die bopunt van die toring geleë is te reflekteer. ’n Omvattende hersiening het daarop gewys dat kontemporêre ontwerpe van die ontvangers ’n aantal geometriese kompleksiteite, ’n tekort aan termiese doeltreffendheid sowel as probleme in terme van duursaamheid en koste in die gesig staar. Die doel van die studie is om ’n nuwe ontvangerskonsep te ontwikkel wat moontlik hierdie probleme kan verminder. ’n Parametriese analise is gebruik om potensiële maniere van verbetering aan te dui wat gebaseer is op ’n energiebalans benadering; insluitend sensitiwiteite betrokke by konvektiewe en stralingswarmteoordrag. Ontwerpkriteria soos die gebruik van spruitstukke om drukverliese te minimaliseer is ook ondersoek. Gebaseer op hierdie bevindinge is die hibriede saamgepersde-lug ontvanger ontwikkel. Laasgenoemde is ’n kombinasie van buis- en volumetriese ontvangertegnologie. Die fundamentele idee van die ontvanger is ondersoek deur straalberekening asook die gelyktydige natuurlike konveksie en stralingswarmteoordrag te simuleer. Die straalberekeningsresultate het getoon dat die gebruik van kwarts glas ’n moontlike oplossing is om hoër stralingsintensiteit te bereik, maar met refleksieverliese in die orde van 7 %. Die gelyktydige natuurlike konveksie en stralingswarmteoordrag simulasie het verder aan die lig gebring dat die ontvangerkonsep die ontsnapping

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Concentrating solar power plants are a promising means of generating electricity. However, they are dependent on the sun as a source of energy, and require thermal storage to supply power on demand. At present thermal storage – usually molten salt – although functional, is expensive, and a cheaper solution is desired. It is proposed that sensible heat storage in a packed bed of rock, with air as heat transfer medium, is suitable at temperatures of 500 – 600 °C. To determine if this concept is technically feasible and economically competitive with existing storage, rock properties, packed bed pressure drop and thermal characteristics must be understood. This work addresses these topics. No previously published data is available on thermal cycling resistance of South African rock, and there is limited data from other countries in the proposed temperature range for long-term thermal cycling, so samples were thermally cycled. There is rock which is suitable for thermal storage applications at temperatures of 500 – 600 °C. New maps of South Africa showing where potentially suitable rock is available were produced. Dolerite, found extensively in the Karoo, is particularly suitable. Friction factors were measured for beds of different particles to determine the importance of roughness, shape, and packing arrangement. Five sets of rock were also tested, giving a combined dataset broader than published in any previous study. Limitations of existing correlations are shown. The friction factor is highly dependent on particle shape and, in the case of asymmetric particles, packing method. The friction factor varied by up to 70 % for crushed rock depending on the direction in which it was poured into the test section, probably caused by the orientation of the asymmetric rock relative to the air flow direction. This has not been reported before for rock beds. New isothermal correlations using the volume equivalent particle diameter are given: they are within 15 % of the measurements. This work will allow a techno-economic evaluation of crushed rock beds using more accurate predictions of pumping power than could previously be made. Thermal tests below 80 °C show that bed heat transfer is insensitive to particle shape or type. A heat transfer correlation for air in terms of the volume equivalent diameter was formulated and combined with the E-NTU method. The predicted bed outlet temperatures are within 5 °C of the measurements for tests at 530 °C, showing that the influence of thermal conduction and radiation can be reasonably negligible for a single charge/discharge cycle at mass fluxes around 0.2 kg/m2s. A novel method for finding the optimum particle size and bed length is given: The Biot number is fixed, and the net income (income less bed cost) from a steam cycle supplied by heat from the bed is calculated. A simplified calculation using the method shows that the optimum particle size is approximately 20 mm for bed lengths of 6 – 7 m. Depending on the containment design and cost, the capital cost could be an order of magnitude lower than a nitrate salt system. / AFRIKAANSE OPSOMMING: Gekonsentreerde son-energie kragstasies is n belowende manier om elektrisiteit op te wek, maar hulle is afhanklik van die son as n bron van energie. Om drywing op aanvraag te voorsien moet hulle energie stoor. Tans is termiese stoor – gewoonlik gesmelte sout – hoewel funksioneel, duur, en n goedkoper oplossing word gesoek. Daar word voorgestel dat stoor van voelbare warmte-energie in n gepakte rotsbed met lug as warmteoordrag medium geskik is by temperature van 500 – 600 °C. Om te bepaal of dié konsep tegnies gangbaar en ekonomies mededingend met bestaande stoorstelsels is, moet rotseienskappe, gepakte bed drukval en hitteoordrag verstaan word. Hierdie werk spreek hierdie aspekte aan. Geen voorheen gepubliseerde data is beskikbaar oor die termiese siklus weerstand van Suid-Afrikaanse rots nie, en daar is beperkte data van ander lande in die voorgestelde temperatuurbereik, dus is monsters onderwerp aan termiese siklusse. Daar bestaan rots wat geskik is vir termiese stoor toepassings by temperature van 500 – 600 °C. Nuwe kaarte van Suid-Afrika is opgestel om te wys waar potensieel geskikte rots beskikbaar is. Doleriet, wat wyd in die Karoo voor kom, blyk om veral geskik te wees. Wrywingsfaktore is gemeet vir beddens van verskillende partikels om die belangrikheid van grofheid, vorm en pak-rangskikking te bepaal. Vyf rotsstelle is ook getoets, wat n saamgestelde datastel gee wyer as in enige gepubliseerde studie. Beperkings van bestaande korrelasies word aangetoon. Die wrywingsfaktor is hoogs sensitief vir partikelvorm en, in die geval van asimmetriese partikels, pakkings metode. Die wrywingsfaktor het met tot 70 % gevarieer vir gebreekte rots, afhanklik van die rigting waarin dit in die toetsseksie neergelê is. Dit is waarskynlik veroorsaak deur die oriëntasie van die asimmetriese rots relatief tot die lugvloei rigting, en is nie voorheen vir rotsbeddens gerapporteer nie. Nuwe isotermiese korrelasies wat gebruik maak van die volume-ekwivalente partikel deursnee word gegee: hulle voorspel binne 15 % van die gemete waardes. Hierdie werk sal n tegno-ekonomiese studie van rotsbeddens toelaat wat meer akkurate voorspellings van pompdrywing gebruik as voorheen moontlik was. Termiese toetse onder 80 °C wys dat die warmteoordrag nie baie sensitief is vir partikelvorm en -tipe nie. n Warmte-oordragskorrelasie vir lug in terme van die volume-ekwivalente deursnee is ontwikkel en met die E-NTU-metode gekombineer. Die voorspelde lug uitlaat temperatuur is binne 5 °C van die meting vir toetse by 530 °C. Dit wys dat termiese geleiding en straling redelikerwys buite rekening gelaat kan word vir n enkele laai/ontlaai siklus by massa vloeitempos van omtrent 0.2 kg/m2s. n Oorspronklike metode vir die bepaling van die optimum partikelgrootte en bedlengte word gegee: Die Biot-getal is vas, en die netto inkomste (die inkomste minus die bed omkoste) van n stoomsiklus voorsien met warmte van die bed word bereken. n Vereenvoudigde berekening wat die metode gebruik wys dat die optimum grootte en lengte ongeveer 20 mm en 6-7 m is. Afhangende van die behoueringsontwerp en koste, kan die kapitale koste n orde kleiner wees as dié van n gesmelte nitraatsout stelsel

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Central receiver power plants generate renewable electricity by exploiting the energy provided by the sun. The conditions experienced in the Northern Cape region of South Africa provide the ideal conditions for the development of these plants. Without a storage medium these plants have capacity factors in the range of 25-30%. The inclusion of a thermal energy storage medium provides the ability to increase the capacity factors of these plants. Although storage increases the costs, it results in better utilisation of the power block and a decrease in the levelised electricity cost (LEC). Eskom intends building a 100MWe central receiver dry cooled power plant in the Upington region. This research identifies the appropriate storage medium and ideal storage capacity to achieve the lowest LEC. A literature survey was performed to identify the different methods of storage that are available. The different storage methods were evaluated and the best storage medium for a central receiver power plant based on the developments of the various storage technologies was identified. To determine the costs associated with a central receiver power plant, data published by NREL was used. Different plant parameters were required to evaluate the costs. A power plant model based on efficiencies and energy balances was created to determine the required plant parameters. It provided the ability to determine the effect of changing different plant parameters on the LEC and estimate the plant output. The power block parameters were initially varied to determine the most efficient power block configuration. Once the most efficient power block configuration was identified the solar field and storage parameters were varied to determine the plant configuration which resulted in the lowest LEC. The most efficient power block configuration of 0.4206 was found for a system comprising of six feedwater heaters with the feedwater temperature of 230°C, main steam pressure 140 bar and an exit steam generator salt temperature of 290°C. A solar multiple of 3.0 with 16 hours of storage resulted in a LEC of R1.41/kWh with no system constraints. A capacity factor constraint of 60% resulted in a solar multiple of 1.8 with 8 hours of storage and a LEC of R1.78/kWh. / AFRIKAANSE OPSOMMING: Sonkragaanlegte met sentrale ontvangers wek hernubare elektrisiteit op deur sonenergie te ontgin. Die klimaat in die Noord Kaap-streek van Suid-Afrika is ideaal vir die oprigting van hierdie aanlegte. Sonder ’n bergingsmedium is die kapasiteitsfaktore van sulke aanlegte ongeveer 25-30%. Met die insluiting van ’n bergingsmedium vir termiese energie kan die kapasiteitsfaktore egter verhoog word. Hoewel berging aanlegkoste verhoog, lei dit terselfdertyd tot beter aanwending van die kragblok en ’n afname in die konstante eenheidskoste van elektrisiteit (LEC). Eskom beplan om ’n droogverkoelde kragaanleg van 100 MW met ’n sentrale ontvanger in die Upington-streek op te rig. Hierdie navorsing was dus daarop toegespits om die mees geskikte bergingsmedium en ideale bergingskapasiteit te bepaal om die laagste moontlike LEC uit die aanleg te verkry. ’n Literatuurstudie is onderneem om die verskeie beskikbare bergingsmetodes te bestudeer. Die verskillende metodes is beoordeel, waarna die beste bergingsmedium vir ’n kragaanleg met ’n sentrale ontvanger op grond van die ontwikkelings in die verskillende bergingstegnologieë bepaal is. Om die koste van ’n kragaanleg met ’n sentrale ontvanger te bepaal, is gepubliseerde data van die Amerikaanse Nasionale Laboratorium vir Hernubare Energie (NREL) gebruik. Verskillende aanlegparameters was egter nodig om die koste te beoordeel. Dié parameters is gevolglik bepaal deur ’n kragaanlegmodel op grond van doeltreffendheidsfaktore en energiebalanse te skep. Sodoende kon vasgestel word watter uitwerking veranderinge in die verskillende parameters op die LEC sou hê, en kon die aanleguitset geraam word. Die kragblokparameters is aanvanklik afgewissel om die doeltreffendste kragbloksamestel te bepaal. Nadat dít bepaal is, is die sonenergieveld en bergingsparameters weer afgewissel om vas te stel watter aanlegsamestel die laagste LEC tot gevolg sou hê. Die beste termiese benuttingsgraad is behaal vir ʼn stoom siklus met ses water verhitters en ʼn water temperatuur van 230 °C by die ketel se inlaat, ʼn stoom druk van 140 bar, en sout uitlaat temperatuur van 290 °C. ʼn Vermenigvuldigingsfaktor van drie vir die heliostaat veld, en 16 uur termiese energie storing gee ʼn opwekkingskoste van R 1.41/kW/h indien daar geen beperkings op die grootte of koste van die stelsel geplaas word nie. Indien die kapitaal uitgawe ʼn perk van 60 % op die kapasitiet van die stelsel plaas, verander die optimale ontwerpspunt na ʼn vermenigvuldigingsfaktor van 1.8, en die termiese stoorkapasitiet verlaag na 8 uur. In hierdie geval is die opwekkingskoste R 1.78/kWh.

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