Rozprawy doktorskie na temat „Solar aided thermal power plants”

This thesis analyses novel technology for renewable electricity generation: the solar thermal chimney (STC) power plant and the suspended chimney (SC) as a plant component. The STC consists of a solar collector, a tall chimney located at the centre of the collector, and turbines and generators at the base of the chimney. Air heated in the collector rises up the chimney under buoyancy and generates power in the turbines. STCs have the potential to generate large amounts of power, but research is required to improve their economic viability. A state-of-the-art STC model was developed, focussing on accurate simulation of collector thermodynamics, and providing data on flow characteristics and plant performance. It was used to explore power generation for matched component dimensions, where for given chimney heights, a range of chimney and collector radii were investigated. Matched dimensions are driven by the collector thermal components approaching thermal equilibrium. This analysis was complemented with a simple cost model to identify the most cost-effective STC configurations. The collector canopy is an exceptionally large structure. Many of the designs proposed in the literature are either complex to manufacture or limit performance. This thesis presents and analyses a series of novel canopy profiles which are easier to manufacture and can be incorporated with little loss in performance. STC chimneys are exceptionally tall slender structures and supporting their self-weight is difficult. This thesis proposes to re-design the chimney as a fabric structure, held aloft with lighter-than-air gas. The performance of initial, small scale suspended chimney prototypes under lateral loading was investigated experimentally to assess the response to wind loads. A novel method of stiffening is proposed and design of larger prototypes developed. The economic viability of a commercial-scale suspended chimney was investigated, yielding cost reductions compared to conventional chimney designs.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 43-44).
Carbon Dioxide emissions from power production are believed to have significant contributions to the greenhouse effect and global warming. Alternative energy resources, such as solar radiation, may help abate emissions but suffer from high costs of power production and temporal variations. On the other hand, Carbon Capture and Sequestration allows the continued use of fossil fuels without the CO2 emissions but it comes at an energetic penalty. The Advanced Zero Emissions Plant (AZEP) minimizes this energy loss by making use of Ion Transport Membrane (ITM)-based oxy-combustion to reduce the cost of carbon dioxide separation. This work seeks to assess if there are any thermodynamic gains from hybridizing solar-thermal energy with AZEP. The particular focus is hybridizing of the bottoming cycle with supplemental solar heating. A simple model of parabolic solar trough was used to hybridize a model of the AZEP cycle in ASPEN Plus*. Two cycle configurations are studied: the first uses solar parabolic troughs to indirectly vaporize high pressure steam through Therminol and the second uses parabolic troughs to directly preheat the high pressure water stream prior to vaporization. Simulations of the solar vaporizer hybrid by varying the total area of collectors (holding fuel input constant) show an increase of net electric output from 439MW for the non-hybridized AZEP to 533MW with an input solar share of 38.8%. The incremental solar efficiency is found to be around 16% for solar shares of input ranging from 5% to 38.8%. Moreover, simulations of variable solar insolation for collector area of 550,000 m2 , show that incremental solar efficiency increased with solar insolation reaching a plateau around 17%. Simulations of the direct solar preheater, show a net electric output of 501.3 MW for a solar share of 35%, (an incremental solar efficiency of 13.73%). The power generation and hence incremental efficiency is lower than in hybridization with steam vaporization with the same input solar share. Synergy analysis for the steam vaporization hybrid indicates no thermodynamic gains from hybridization.
by Ragheb Mohamad Fawaz El Khaja.
S.B.

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 (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: Solar energy is by far the greatest energy resource available to generate power. One of the difficulties of using solar energy is that it is not available 24 hours per day - some form of storage is required if electricity generation at night or during cloudy periods is necessary. If a combined cycle power plant is used to obtain higher efficiencies, and reduce the cost of electricity, storage will allow the secondary cycle to operate independently of the primary cycle. This study focuses on the use of packed beds of rock or slag, with air as a heat transfer medium, to store thermal energy in a solar thermal power plant at temperatures sufficiently high for a Rankine steam cycle. Experimental tests were done in a packed bed test section to determine the validity of existing equations and models for predicting the pressure drop and fluid temperatures during charging and discharging. Three different sets of rocks were tested, and the average size, specific heat capacity and density of each set were measured. Rock and slag samples were also thermally cycled between average temperatures of 30 ºC and 510 ºC in an oven. The classical pressure drop equation significantly under-predicts the pressure drop at particle Reynolds numbers lower than 3500. It appears that the pressure drop through a packed bed is proportional to the 1.8th power of the air flow speed at particle Reynolds numbers above about 500. The Effectiveness-NTU model combined with a variety of heat transfer correlations is able to predict the air temperature trend over the bed within 15 % of the measured temperature drop over the packed bed. Dolerite and granite rocks were also thermally cycled 125 times in an oven without breaking apart, and may be suitable for use as thermal storage media at temperatures of approximately 500 ºC. The required volume of a packed bed of 0.1 m particles to store the thermal energy from the exhaust of a 100 MWe gas turbine operating for 8 hours is predicted to be 24 × 103 m3, which should be sufficient to run a 25-30 MWe steam cycle for over 10 hours. This storage volume is of a similar magnitude to existing molten salt thermal storage.
AFRIKAANSE OPSOMMING: Sonenergie is die grootste energiebron wat gebruik kan word vir krag opwekking. ‘n Probleem met die gebruik van sonenergie is dat die son nie 24 uur per dag skyn nie. Dit is dus nodig om die energie te stoor indien dit nodig sal wees om elektrisiteit te genereer wanneer die son nie skyn nie. ‘n Gekombineerde kringloop kan gebruik word om ‘n hoër benuttingsgraad te bereik en elektrisiteit goedkoper te maak. Dit sal dan moontlik wees om die termiese energie uit die primêre kringloop te stoor, wat die sekondêre kringloop onafhanklik van die primêre kringloop sal maak. Dié gevalle studie ondersoek die gebruik van ‘n slakof- klipbed met lug as hitteoordragmedium, om te bepaal of dit moontlik is om hitte te stoor teen ‘n temperatuur wat hoog genoeg is om ‘n Rankine stoom kringloop te bedryf. Eksperimentele toetse is in ‘n toets-bed gedoen en die drukverandering oor die bed en die lug temperatuur is gemeet en vergelyk met voorspelde waardes van vergelykings en modelle in die literatuur. Drie soorte klippe was getoets. Die gemiddelde grootte, spesifieke hitte-kapasiteit en digtheid van elke soort klip is gemeet. Klip en slak monsters is ook siklies tussen temperature van 30 ºC en 510 ºC verkoel en verhit. Die klassieke drukverlies vergelyking gee laer waardes as wat gemeet is vir Reynolds nommers minder as 3500. Dit blyk dat die drukverlies deur ‘n klipbed afhanklik is van die lug vloeispoed tot die mag 1.8 as die Reynolds nommer groter as omtrent 500 is. Die ‘Effectiveness-NTU’ model gekombineerd met ‘n verskeidenheid van hitteoordragskoeffisiënte voorspel temperature binne 15 % van die gemete temperatuur verskil oor die bed. Doloriet en graniet klippe het 125 sikliese toetse ondergaan sonder om te breek, en is miskien gepas vir gebruik in ‘n klipbed by temperature van sowat 500 ºC Die voorspelde volume van ‘n klipbed wat uit 0.1 m klippe bestaan wat die termiese energie vir 8 ure uit die uitlaat van ‘n 100 MWe gasturbiene kan stoor, is 24 × 103 m3. Dit behoort genoeg te wees om ‘n 25 – 30 MWe stoom kringloop vir ten minste 10 ure te bedryf. Die volume is min of meer gelyk aan dié van gesmelte sout store wat alreeds gebou is.

The current energy scenario in the developing nations with abundant sun resource (e.g. southern Mediterranean countries of Europe, Middle-East & North Africa) relies mainly on fossil fuels to supply the increasing energy demand. Although this long adopted pattern ensures electricity availability on demand at all times through the least cost proven technology, it is highly unsustainable due to its drastic impacts on depletion of resources, environmental emissions and electricity prices. Solar thermal Hybrid power plants among all other renewable energy technologies have the potential of replacing the central utility model of conventional power plants, the understood integration of solar thermal technologies into existing conventional power plants shows the opportunity of combining low cost reliable power and Carbon emission reduction. A literature review on the current concentrating solar power (CSP) technologies and their suitability for integration into conventional power cycles was concluded, the best option was found be in the so called Integrated solar combined cycle systems (ISCCS); the plant is built and operated like a normal combined cycle, with a solar circuit consisting of central tower receiver and heliostat field adding heat to the bottoming Rankine cycle. A complete model of the cycle was developed in TRNSYS simulation software and Matlab environment, yearly satellite solar insolation data was used to study the effect of integrating solar power to the cycle throw-out the year. A multi objective thermo economic optimization analysis was conducted in order to identify a set of optimum design options. The optimization has shown that the efficiency of the combined cycle can be increased resulting in a Levelized electricity cost in the range of 10 -14 USDcts /Kwhe. The limit of annual solar share realized was found to be around 7 % The results of the study indicate that ISCCS offers advantages of higher efficiency, low cost reliable power and on the same time sends a green message by reducing the environmental impacts in our existing power plant systems.

Concentrating solar power (CSP) plant is a promising technology that exploits direct normal irradiation (DNI) from the sun to be converted into thermal energy in the solar field. One of the advantages of CSP technology is the possibility to store thermal energy in thermal energy storage (TES) for later production of electricity. The integration of thermal storage allows the CSP plant to be a dispatchable system which is defined as having a capability to schedule its operation using an innovative dispatch planning tool. Considering weather forecast and electricity price profile in the market, dispatch planning tool uses an optimization algorithm. It aims to shift the schedule of electricity delivery to the hours with high electricity price. These hours are usually reflected by the high demand periods. The implementation of dispatch optimizer can benefit the CSP plants economically from the received financial revenues. This study proposes an optimization of dispatch planning strategies for the parabolic trough CSP plant under two dispatch approaches: solar driven and storage driven. The performed simulation improves the generation of electricity which reflects to the increase of financial revenue from the electricity sale in both solar and storage driven approaches. Moreover, the optimization also proves to reduce the amount of dumped thermal energy from the solar field.

This thesis examined solar thermal collectors for use in alternative hybrid solar-biomass power plant applications in Gujarat, India. Following a preliminary review, the cost-effective selection and design of the solar thermal field were identified as critical factors underlying the success of hybrid plants. Consequently, the existing solar thermal technologies were reviewed and ranked for use in India by means of a multi-criteria decision-making method, the Analytical Hierarchy Process (AHP). Informed by the outcome of the AHP, the thesis went on to pursue the Linear Fresnel Reflector (LFR), the design of which was optimised with the help of ray-tracing. To further enhance collector performance, LFR concepts incorporating novel mirror spacing and drive mechanisms were evaluated. Subsequently, a new variant, termed the Elevation Linear Fresnel Reflector (ELFR) was designed, constructed and tested at Aston University, UK, therefore allowing theoretical models for the performance of a solar thermal field to be verified. Based on the resulting characteristics of the LFR, and data gathered for the other hybrid system components, models of hybrid LFR- and ELFR-biomass power plants were developed and analysed in TRNSYS®. The techno-economic and environmental consequences of varying the size of the solar field in relation to the total plant capacity were modelled for a series of case studies to evaluate different applications: tri-generation (electricity, ice and heat), electricity-only generation, and process heat. The case studies also encompassed varying site locations, capacities, operational conditions and financial situations. In the case of a hybrid tri-generation plant in Gujarat, it was recommended to use an LFR solar thermal field of 14,000 m2 aperture with a 3 tonne biomass boiler, generating 815 MWh per annum of electricity for nearby villages and 12,450 tonnes of ice per annum for local fisheries and food industries. However, at the expense of a 0.3 ¢/kWh increase in levelised energy costs, the ELFR increased saving of biomass (100 t/a) and land (9 ha/a). For solar thermal applications in areas with high land cost, the ELFR reduced levelised energy costs. It was determined that off-grid hybrid plants for tri-generation were the most feasible application in India. Whereas biomass-only plants were found to be more economically viable, it was concluded that hybrid systems will soon become cost competitive and can considerably improve current energy security and biomass supply chain issues in India.

The provision of a sustainable energy supply is one of the most importantissues facing humanity at the current time, and solar thermal power hasestablished itself as one of the more viable sources of renewable energy. Thedispatchable nature of this technology makes it ideally suited to forming thebackbone of a future low-carbon electricity system.However, the cost of electricity from contemporary solar thermal power plantsremains high, despite several decades of development, and a step-change intechnology is needed to drive down costs. Solar gas-turbine power plants are apromising new alternative, allowing increased conversion efficiencies and asignificant reduction in water consumption. Hybrid operation is a furtherattractive feature of solar gas-turbine technology, facilitating control andensuring the power plant is available to meet demand whenever it occurs.Construction of the first generation of commercial hybrid solar gas-turbinepower plants is complicated by the lack of an established, standardised, powerplant configuration, which presents the designer with a large number ofchoices. To assist decision making, thermoeconomic studies have beenperformed on a variety of different power plant configurations, includingsimple- and combined-cycles as well as the addition of thermal energy storage.Multi-objective optimisation has been used to identify Pareto-optimal designsand highlight trade-offs between costs and emissions.Analysis of the simple-cycle hybrid solar gas-turbines revealed that, whileelectricity costs were kept low, the achievable reduction in carbon dioxideemissions is relatively small. Furthermore, an inherent trade-off between thedesign of high efficiency and high solar share hybrid power plants wasidentified. Even with the use of new optimised designs, the degree of solarintegration into the gas-turbine did not exceed 63% on an annual basis.In order to overcome the limitations of the simple-cycle power plants, twoimprovements were suggested: the integration of thermal energy storage, andthe use of combined-cycle configurations. Thermal energy storage allowed thedegree of solar operation to be extended, significantly decreasing carbondioxide emissions, and the addition of a bottoming-cycle reduced the electricitycosts. A combination of these two improvements provided the bestperformance, allowing a reduction in carbon dioxide emissions of up to 34%and a reduction in electricity costs of up to 22% compared to a combination ofconventional power generation technologies.
Hållbar energiförsörjning är för närvarande en av de viktigaste frågorna förmänskligheten. Koncentrerad solenergi är nu etablerad som en tillförlitlig källaav förnybar energi. Den reglerbara karaktären hos tekniken gör den specielltintressant för uppbyggnaden av ett framtida koldioxidsnålt elsystem.Kostnaden för elektricitet från nuvarande termiska solkraftverk är hög trottsflera decennier av utveckling. Ett genombrått på tekniknivå krävs för att drivaned kostnaderna. Sol-gasturbiner är ett av de mest lovande alternativen, somger en ökad verkningsgrad samtidigt som vattenkonsumtionen reducerasdrastiskt. Sol-gasturbintekniken gör det möjligt att blandköra solenergi ochandra bränslen för att möta efterfrågan vid alla tidpunkter, en attraktiv aspekt iförhållande till alternativa lösningar.Uppbyggnaden av första generationens kommersiella hybrida solgasturbinkraftverkförsvåras dock av bristen på etablerade och standardiseradekraftverkskonfigurationer. Dessa ger planeraren ett stort antal valmöjlighetersom underlag för beslutsfattande. Termoekonomiska studier har genomförtsför ett flertal olika kraftverkskonfigurationer, däribland kraftverk med enkelcykel, kombikraftverk samt möjligheten att utnyttja termisk energilagring.Pareto-optimala konfigurationer har identifierats med hjälp av multiobjektsoptimeringför att belysa balansen mellan kostnader och utsläpp.Analysen av det enkla hybrida sol-gasturbinkraftverket visade attelektricitetskostnaden hållits på en låg nivå, men att den möjliga minskningen avkoldioxidutsläpp är relativt liten. Dessutom identifierades en inre balans mellanatt bibehålla en hög verkningsgrad hos konfigurationen och en hög andelsolenergi i produktionen. Andelen av solenergi i gasturbinen överskred aldrig63% på årlig bas, även med optimerade kraftverkskonfigurationer.Två förbättringar föreslås för att övervinna begränsningarna hos kraftverk medenkel cykel: integration av termisk energilagring samt nyttjande avkombikraftverkskonfigurationer. Termisk energilagring tillåter en ökad andelsolenergi i driften och reducerar koldioxidutsläppen drastiskt, medan denytterligare cykeln hos kombikraftverket reducerar elektricitetskostnaden.Kombinationen av dessa förbättringar ger den bästa prestandan, med enreduktion av koldioxidutsläppen på upp till 34% och reducerade elektricitetskostnaderpå upp till 22% i jämförelse med andra kombinationer avkonventionella kraftverkskonfigurationer.

QC 20130503

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

The percentage of renewable energy within the global electric power generation portfolio is expected to increase rapidly over the next few decades due to increasing concerns about climate change, fossil fuel costs, and energy security. Solar thermal energy, also known as concentrating solar power (CSP), is emerging as an important solution to new demands for clean, renewable electricity generation. Dish-Stirling (DS) technology, a form of CSP, is a relatively new player in the renewable energy market, although research in the technology has been ongoing now for nearly thirty years. The first large plant utilizing DS technology, rated at 1.5 MW, came online in January 2010 in Peoria, AZ, and plants rated for several hundred MW are in the planning stages. Increasing capacity of this technology within the utility grid requires extensive dynamic simulation studies to ensure that the power system maintains its safety and reliability in spite of the technological challenges that DS technology presents, particularly related to the intermittency of the energy source and its use of a non-conventional asynchronous generator. The research presented in this thesis attempts to fill in the gaps between the well established research on Stirling engines in the world of thermodynamics and the use of DS systems in electric power system applications, a topic which has received scant attention in publications since the emergence of this technology. DS technology uses a paraboloidal shaped dish of mirrors to concentrate sunlight to a single point. The high temperatures achieved at the focal point of the mirrors is used as a heat source for the Stirling engine, which is a closed-cycle, external heat engine. Invented by the Scottish clergyman Robert Stirling in 1816, the Stirling engine is capable of high efficiency and releases no emissions, making it highly compatible with concentrated solar energy. The Stirling engine turns a squirrel-cage induction generator, where electricity is delivered through underground cables from thousands of independent, autonomous 10-25 kW rated DS units in a large solar farm. A dynamic model of the DS system is presented in this thesis, including models of the Stirling engine working gas and mechanical dynamics. Custom FORTRAN code is written to model the Stirling engine dynamics within PSCAD/EMTDC. The Stirling engine and various other components of the DS system are incorporated into an electrical network, including first a single-machine, infinite bus network, and then a larger 12-bus network including conventional generators, loads, and transmission lines. An analysis of the DS control systems is presented, and simulation results are provided to demonstrate the system's steady state and dynamic behavior within these electric power networks. Potential grid interconnection requirements are discussed, including issues with power factor correction and low voltage ride-through, and simulation results are provided to illustrate the dish-Stirling system's capability for meeting such requirements.

El protagonisme creixent de la tecnologia solar termoelèctrica entre el ventall de les energies renovables es centra en la seva capacitat d’adaptar la seva producció a la demanda energètica exigida. La gestionabilitat d’aquest tipus de centrals s’ha aconseguit amb la integració de sistemes d’emmagatzematge tèrmic en les mateixes. La major part dels sistemes d’emmagatzematge tèrmic, ja sigui els que s’utilitzen a nivell comercial com aquells que es troben en fase de desenvolupament proposen l’ús de sals inorgàniques foses com a medi d’emmagatzematge. Aquestes sals presenten l’inconvenient de la seva alta corrosivitat a altes temperatures. Per un costat, s’han analitzat els fenòmens de corrosió associats a les sals solars utilitzades a la planta pilot TES-PS10 mitjançant la instal·lació de racks de testimonis de corrosió als tancs de sals. A més, al finalitzar l’operació de la instal·lació pilot s’ha dut a terme un estudi post-mortem dels seus. Finalment, amb l’objectiu d’abaratir el cost de l’inventari de sals, s’ha analitzat a nivell de laboratori la corrosivitat de diferents mescles de nitrats de baixa puresa. El segon bloc de la tesi es centra en els sistemes d’emmagatzematge tèrmic en calor latent. Concretament, s’analitza la corrosió associada a la mescla peritèctica 46% LiOH-54% KOH proposta com a material de canvi de fase en un mòdul d’evaporació d’instal·lacions termoelèctriques de generació directa de vapor. D’aquesta forma, s’han dut a terme una sèrie d’assajos a nivell de laboratori amb l’objectiu d’avaluar el comportament envers la corrosió de diferents materials en contacte amb aquests hidròxids.
El creciente protagonismo de la tecnología solar se centra en su capacidad para adaptar su producción a la demanda energética exigida. La gestionabilidad de este tipo de centrales se ha conseguido mediante la integración de sistemas de almacenamiento térmico en sales fundidas. El uso de sales fundidas en sistemas de almacenamiento térmico presenta el hándicap de su corrosividad a alta temperatura. El primer bloque de la Tesis analiza los fenómenos de corrosión asociados a las sales solares en la planta piloto TES-PS10 mediante la instalación de racks de corrosión en los tanques de sales. Además, se ha llevado a cabo un estudio post-mortem de componentes de la instalación. Finalmente, se ha analizado a nivel de laboratorio la corrosividad de distintas mezclas de nitrato de baja pureza. El segundo bloque de la tesis se centra en los sistemas de almacenamiento en calor latente. En concreto, se analiza la corrosión asociada a la mezcla peritéctica 46% LiOH-54% KOH propuesta como material de cambio de fase en el módulo de evaporación en plantas de generación directa de vapor. De este modo, se han llevado a cabo ensayos de corrosión a nivel de laboratorio para evaluar el comportamiento a corrosión de distintos materiales en contacto con los hidróxidos.
The growing of concentrated solar power (CSP) within the different renewable energies is due to its ability to adapt the production to the required energy demand. The dispatchability of this type of plants has been achieved through the integration of molten salts thermal storage systems (TES). Molten salts have a handicap associated to their corrosiveness at high temperature. First block of this Thesis analyzes the corrosion phenomena associated with solar salts used in TES-PS10 pilot plant by installing corrosion racks in the salt tanks. Moreover, a postmortem study of different components was performed after facility shut down. Finally, in order to reduce the cost of the salt inventory in TES systems, the corrosivity of different low purity nitrates mixtures has been analyzed at laboratory scale. The second block of the Thesis focuses on latent heat storage systems. Specifically, it has been analyzed the corrosion associated with the proposed 46% LiOH-54% KOH peritectic mixture as a phase change material in the evaporation module of direct steam generation (DSG) CSP plants. Thus, corrosion tests have been performed at laboratory level to evaluate the corrosion performance of several materials in contact with such hydroxides.

Thesis (MEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2007.
ENGLISH: The design, construction and testing of a passive thermal system, a roof solar chimney, for a loft is considered. Unlike conventional solar chimneys the solar collector is constructed from corrugated iron roof sheets with the aim that it can be integrated into existing buildings at a lower cost or used in low cost housing developments. The main objective of the study was to determine the feasibility of such low-cost design to regulate thermal conditions in a loft, that is heating the loft during winter and enhancing natural ventilation during summer, by carrying out an experimental and analytical study. The results obtained from the experimental study showed that for winter the solar chimney, having a channel width, depth and length of 0.7 m, 0.1 m and 1.8 m respectively and with a peal solar radiation of 850 W/m², heated the room air 5°C higher than the ambient temperature during the hottest periods of the day, which is only marginally better than a loft with conventional roof insulation. At night, it was found that reverse airflow occurred through the chimney, cooling the loft down to ambient temperature, due to radiation heat loss from the roof collector to the night sky. For summer operation, the experimental data showed that the chimney was able to maintain the loft at ambient temperature and the analytical study found that the chimney was able to enhance natural ventilation effectively, reaching air exchange rate of 6.6 per hour for the 4.6 m³ volume space. It was also found that the chimney’s performance dropped rapidly and significantly during periods of low solar radiation and at night. A sensitivity analysis illustrated that for both summer and winter operation, the size, tilt angle and absorptivity of the roof collector greatly effected the efficiency and mass flow rates of the system, agreeing well with other literature. These results prove that this low cost solar chimney cooling design was feasible to enhance natural ventilation mainly during hot summer conditions with high solar radiation. Compared to a loft with only conventional roof insulation, the chimney did not perform effectively during the winter to heat the loft up, meaning that winter operation for this specific design is not feasible. Possible improvements to the design include using construction materials with higher thermal capacities to retain heat energy and ensure continued operation during periods of low solar radiation, as well as using selective absorber coatings on the collector surface. It is recommended that further work on the project include the integration of these improvements into the present design and to use the findings obtained from the sensitivity analysis to improve system efficiencies. CFD analysis of the test-rig will be insightful as an additional means to validate and compare with the analytical and experimental data obtained in this report. With the continuation of these studies, this low-cost solar chimney design can be optimised, validated on a commercial scale and built into existing and new housing developments. Incorporating such a passive thermal device will aid homeowners in air regulation and thermal comfort of their living space as well as saving on energy requirements.
Sponsored by the Centre for Renewable and Sustainable Energy Studies, Stellenbosch University

Concentrating solar power plants can integrate cost-effective thermal energy storage systems and thereby supply controllable power on demand, an advantage against other renewable technologies. Storage integration allows a solar thermal power plant to increase its load factor and to shift production to periods of peak demand. It also enables output firmness, providing stability to the power block and to the grid. Thus, despite the additional investment, storage can enhance the performance and economic viability of the plants. However, the levelized cost of electricity of these plants yet remains higher than for other technologies, so projects today are only viable through the provision of incentives or technology-specific competitive bid tenders. It is the variability of the solar resource, the myriad roles that storage can assume, and the complexity of enhancing the synergies between the solar field, the storage and the power block, what makes the development of adequate policy instruments, design and operation of these plants a challenging process. In this thesis a comprehensive methodology for the pre-design and analysis of concentrating solar power plants is presented. The methodology is based on a techno-economic modeling approach that allows identifying optimum trade-off curves between technical, environmental, and financial performance indicators. A number of contemporary plant layouts and novel storage and hybridization concepts are assessed to identify optimum plant configurations, in terms of component size and storage dispatch strategies. Conclusions highlight the relevance between the sizing of key plant components, the operation strategy and the boundaries set by the location. The interrelation between critical performance indicators, and their use as decisive parameters, is also discussed. Results are used as a basis to provide recommendations aimed to support the decision making process of key actors along the project development value chain of the plants. This research work and conclusions are primarily meant to set a stepping stone in the research of concentrating solar power plant design and optimization, but also to support the research towards understanding the value of storage in concentrating solar power plants and in the grid.
Koncentrerad solkraft erbjuder möjligheten att integrera kostnadseffektiv termisk energilagring och därmed behovsstyrd kraftkontroll. Detta är en viktig fördel jämfört med andra förnybara energiteknologier. Lagringsintegration tillåter solkraftsanläggningar att öka sin lastfaktor och skifta produktion till tider med största efterfrågan. Vidare möjliggör lagring fast elproduktion vilket leder till förbättrad nät- och kraftturbinstabilitet. Därför kan termisk lagring öka anläggningsprestanda och ekonomiskt värde trots ökande initiala kapitalkostnader. I termer av specifik elproduktionskostnad (LCOE) ligger koncentrerade solkraftsanläggningar med lagring fortfarande högre än andra kraftteknologier och anläggningsprojekt blir endast lönsamma genom subventionsmodeller eller teknologispecifika konkurrensutsatta anbudsförfaranden. Att hitta adekvata policylösningar och optimala design och operationsstrategier är en utmanande process eftersom det gäller att hitta rätt balans mellan variabel solinstrålning, lagring av energi och tid för produktion genom optimal design och operation av solmottagarfält, kraftblock och lagringskapacitet. I denna avhandling presenteras en omfattande metodik för pre-design och analys av koncentrerande solkraftverk. Metodiken baseras på en tekno-ekonomisk modelleringsansats som möjliggör identifiering av optimala avvägningssamband för tekniska, ekonomiska och miljöprestanda indikatorer. Metodiken tillämpas på ett antal moderna anläggningslayouter  och lagrings- och hybridiseringskoncept för att identifiera optimal kraftanläggningsdesign i termer av komponentprestanda och lagringsanvändningsstrategier. I slutsatsen poängteras relevansen av att hitta rätt storlek på nyckelkomponenter i relation till lagringsstrategi och randvillkoren som ges av konstruktionsläget för optimal ekonomisk och miljömässig prestanda. Resultaten används för att formulera rekommendationer till nyckelaktörer i beslutsprocessen genom hela kraftanläggningens värdekedja från politisk beslutsfattare till anläggningsingenjör. Forskningen och slutsatserna i detta arbete skall i första hand ta ett steg framåt för optimering och design av solkraftsanläggningar men även tillhandahålla en metodik för utvärdering av lagringslösningar och dess specifika värde för solkraftsanläggningar och elnätet.

QC 20160829

This thesis analyzes binary cycle power plants (Organic Rankine Cycles) for electricity generation from low enthalpy geothermal resources. The objective is the maximization of the net power output by means of the proper selection of the working fluid and cycle parameters. A critical review of many studies on ORCs in the scientific literature is carried out to provide a basis for an optimization study having the exergy recovery efficiency as objective function. Two working fluids are analyzed taking into account both supercritical and subcritical pressures and different temperatures of the geothermal fluid. The application of advanced techniques derived from Pinch Analysis (HEATSEP method) allowed finding also sub-optimal solutions, corresponding to small deviations of the cycle parameters from the optimal design values. These solutions, although sub-optimal from a thermodynamic point of view, may be selected when different aspects related to the technology, economics, flexibility or safety of the system are considered. The costs of the optimal thermodynamic solutions are estimated using the module costing technique that relates all capital and operating costs to the purchased cost of equipment evaluated for some base conditions. The economic results show the impact of the geothermal fluid temperature and working fluid selection on the economics of the system. The results of this study are applied to the Stillwater real binary cycle power plant that started operating in 2009 in Nevada (USA). The power plant operates at subcritical pressures with isobutane as working fluid and uses a dry cooling system as heat rejection system. Due to the limited geothermal resource the plant net power output is much lower than expected. A detailed off-design model of the power plant is developed using the software Aspen. The model is tested and adjusted against the plant data collected during the first year of operation. After validation, the model is run to evaluate the operating parameters that maximize the annual energy production. A study is then performed to increase the performance of Stillwater geothermal binary power plant with the addition of the solar source. The combination of the high exergy solar resource with the low exergy geothermal resource could provide many benefits such as the improvement of the thermal efficiency and the increase of the power output during the day and especially during the warm season, a time when the energy production of air-cooled geothermal power plants is markedly reduced. The addition of the solar heat in the Stillwater geothermal plant restores operating conditions close to design point also in presence of reduced geothermal flow rate and temperature. The detailed off-design model of Stillwater power plant is used to carry out this hybridization study. Cycle parameters are optimized for different values of the ambient temperature and solar irradiation in order to maximize the annual energy production. Two different designs of hybrid geo-solar plants, with and without storage, are compared, and the levelized cost of electricity (LCOE) of the incremental generation from solar energy is calculated. As expected, this LCOE is quite high due to the high costs of the solar collectors and could be competitive only in presence of appropriate incentives.
La tesi analizza gli impianti a ciclo binario (Organic Rankine Cycles) per generazione di elettricità da risorse geotermiche a bassa entalpia. L'obiettivo è la massimizzazione della potenza netta tramite una selezione appropriata del fluido operativo e dei parametri di ciclo. La valutazione critica di molti studi sui cicli di Rankine organici ha fornito le basi per formulare uno studio di ottimizzazione avente come funzione obiettivo il rendimento di recupero exergetico. Sono analizzati due fluidi operativi considerando sia pressioni supercritiche che subcritiche e diverse temperature del fluido geotermico. L'applicazione di tecniche avanzate derivate dalla Pinch Analysis (metodo HEATSEP) ha consentito di trovare anche soluzioni sub-ottimali, corrispondenti a piccoli scostamenti dei parametri di ciclo dai valori di progetto ottimali. Queste soluzioni, sebbene sub-ottimali da un punto di vista termodinamico, potrebbero essere scelte se venissero considerati anche aspetti legati alla tecnologia, economia, flessibilità o sicurezza del sistema. I costi delle soluzioni termodinamicamente ottimali sono quindi valutati mostrando l'impatto della temperatura del fluido geotermico e della scelta del fluido operativo sull'economia del sistema. I risultati di questo studio sono applicati all'impianto reale a ciclo binario di Stillwater che iniziò l'operazione nel 2009 in Nevada (USA). L'impianto opera a pressioni subcritiche con isobutano come fluido operativo e usa un sistema di raffreddamento ad aria. A causa della limitata risorsa geotermica la potenza netta prodotta dall'impianto è molto più bassa di quella attesa. E' stato sviluppato un modello dettagliato di fuori progetto dell'impianto usando il software Aspen. Il modello è stato validato con i dati dell'impianto raccolti nel primo anno di attività. Dopo validazione il modello è utilizzato per ottenere i parametri operativi che massimizzano la produzione energetica annuale. E' stato quindi effettuato uno studio per aumentare le prestazioni dell'impianto geotermico di Stillwater con l'aggiunta della risorsa solare. I parametri di ciclo sono ottimizzati per valori differenti della temperatura ambiente e della radiazione solare per massimizzare la produzione energetica annuale. Sono confrontate due configurazioni di ciclo ibrido, con e senza accumulo, ed è calcolato il costo dell'energia elettrica (LCOE). Come atteso, questo LCOE è abbastanza elevato a causa dei costi elevati dei collettori solari e potrebbe essere competitivo solo in presenza di incentivi appropriati.

Doutoramento em Engenharia Mecânica
The PhD project addresses the potential of using concentrating solar power (CSP) plants as a viable alternative energy producing system in Libya. Exergetic, energetic, economic and environmental analyses are carried out for a particular type of CSP plants. The study, although it aims a particular type of CSP plant – 50 MW parabolic trough-CSP plant, it is sufficiently general to be applied to other configurations. The novelty of the study, in addition to modeling and analyzing the selected configuration, lies in the use of a state-of-the-art exergetic analysis combined with the Life Cycle Assessment (LCA). The modeling and simulation of the plant is carried out in chapter three and they are conducted into two parts, namely: power cycle and solar field. The computer model developed for the analysis of the plant is based on algebraic equations describing the power cycle and the solar field. The model was solved using the Engineering Equation Solver (EES) software; and is designed to define the properties at each state point of the plant and then, sequentially, to determine energy, efficiency and irreversibility for each component. The developed model has the potential of using in the preliminary design of CSPs and, in particular, for the configuration of the solar field based on existing commercial plants. Moreover, it has the ability of analyzing the energetic, economic and environmental feasibility of using CSPs in different regions of the world, which is illustrated for the Libyan region in this study. The overall feasibility scenario is completed through an hourly analysis on an annual basis in chapter Four. This analysis allows the comparison of different systems and, eventually, a particular selection, and it includes both the economic and energetic components using the “greenius” software. The analysis also examined the impact of project financing and incentives on the cost of energy. The main technological finding of this analysis is higher performance and lower levelized cost of electricity (LCE) for Libya as compared to Southern Europe (Spain). Therefore, Libya has the potential of becoming attractive for the establishment of CSPs in its territory and, in this way, to facilitate the target of several European initiatives that aim to import electricity generated by renewable sources from North African and Middle East countries. The analysis is presented a brief review of the current cost of energy and the potential of reducing the cost from parabolic trough- CSP plant. Exergetic and environmental life cycle assessment analyses are conducted for the selected plant in chapter Five; the objectives are 1) to assess the environmental impact and cost, in terms of exergy of the life cycle of the plant; 2) to find out the points of weakness in terms of irreversibility of the process; and 3) to verify whether solar power plants can reduce environmental impact and the cost of electricity generation by comparing them with fossil fuel plants, in particular, Natural Gas Combined Cycle (NGCC) plant and oil thermal power plant. The analysis also targets a thermoeconomic analysis using the specific exergy costing (SPECO) method to evaluate the level of the cost caused by exergy destruction. The main technological findings are that the most important contribution impact lies with the solar field, which reports a value of 79%; and the materials with the vi highest impact are: steel (47%), molten salt (25%) and synthetic oil (21%). The “Human Health” damage category presents the highest impact (69%) followed by the “Resource” damage category (24%). In addition, the highest exergy demand is linked to the steel (47%); and there is a considerable exergetic demand related to the molten salt and synthetic oil with values of 25% and 19%, respectively. Finally, in the comparison with fossil fuel power plants (NGCC and Oil), the CSP plant presents the lowest environmental impact, while the worst environmental performance is reported to the oil power plant followed by NGCC plant. The solar field presents the largest value of cost rate, where the boiler is a component with the highest cost rate among the power cycle components. The thermal storage allows the CSP plants to overcome solar irradiation transients, to respond to electricity demand independent of weather conditions, and to extend electricity production beyond the availability of daylight. Numerical analysis of the thermal transient response of a thermocline storage tank is carried out for the charging phase. The system of equations describing the numerical model is solved by using time-implicit and space-backward finite differences and which encoded within the Matlab environment. The analysis presented the following findings: the predictions agree well with the experiments for the time evolution of the thermocline region, particularly for the regions away from the top-inlet. The deviations observed in the near-region of the inlet are most likely due to the high-level of turbulence in this region due to the localized level of mixing resulting; a simple analytical model to take into consideration this increased turbulence level was developed and it leads to some improvement of the predictions; this approach requires practically no additional computational effort and it relates the effective thermal diffusivity to the mean effective velocity of the fluid at each particular height of the system. Altogether the study indicates that the selected parabolic trough-CSP plant has the edge over alternative competing technologies for locations where DNI is high and where land usage is not an issue, such as the shoreline of Libya.
O projeto de Doutoramento aborda o potencial de usar centrais de energia solar concentrada (CSP) como um sistema de produção de energia alternativa disponível na Líbia. Uma análise nas vertentes exergética, energética, económica e ambiental foi realizada para um tipo particular destas centrais – um sistema de 50 MW com receção parabólica, porém ela é suficientemente geral para ser aplicada a outras configurações. A originalidade do estudo, para além da modelação e análise da configuração selecionada encontra-se na utilização do estado da arte em termos da análise exergética combinada com a avaliação do ciclo de vida (LCA). A modelação e simulação da central CSP selecionada são efetuadas no terceiro capítulo tendo em consideração as duas componentes: ciclo de potência e campo de coletores solar. O modelo computacional para a análise do sistema foi desenvolvido com base em equações algébricas que descrevem o sistema, e que são resolvidas usando o software EES. Deste modo, são definidas as propriedades em cada ponto de interesse para os diferentes elementos do sistema, o que assim permite determinar as energias, eficiências e irreversibilidades desses elementos. O modelo desenvolvido tem o potencial de se tornar uma ferramenta de grande utilidade para o projeto preliminar de engenharia de centrais CSP, e também para a avaliação da eventual reconfiguração de centrais elétricas solares comerciais em operação. Além disso, o modelo pode ser utilizado no estudo de viabilidade da operação de centrais CSP, através da análise energética, económica e ambiental, para regiões diferentes da que foi escolhida no presente estudo -Trípoli (Líbia). O cenário total da viabilidade da operação da central CSP é completado através da análise horária com base anual apresentada no quarto capítulo. Esta análise permite a comparação de diferentes sistemas e, eventualmente permite fazer a seleção com base nas componentes económicas e energéticas, que são determinadas dentro do contexto do software greenius. A análise também toma em conta o impacto de financiamento e incentivos dados aos projetos no custo da produção de energia. O principal resultado desta análise é a verificação que o desempenho é mais elevado, com o consequente menor custo nivelado da eletricidade, para a Líbia em comparação com o Sul da Europa (Espanha). Assim a Líbia tem o potencial de se tornar um candidato atrativo para o estabelecimento de centrais CSP com o objetivo, como foi considerado em várias iniciativas europeias, de exportar eletricidade gerada através de fontes de energia renováveis de países do Norte de África e Médio Oriente para a Europa. A análise apresenta uma breve revisão do custo corrente da eletricidade e o potencial para reduzir o custo da energia a partir da tecnologia de receção parabólica de centrais CSP. A avaliação do ciclo de vida com base exergética (ELCA) e a avaliação do ciclo de vida convencional são realizadas para a centrais CSP específicas no quinto capítulo. Os objetivos são 1) avaliar o impacto ambiental e custo, em termos de do ciclo iv de vida exergético do sistema; 2) identificar pontos fracos em termos da irreversibilidade dos processos; e 3) verificar se as centrais CSP podem reduzir o impacto ambiental e o custo de geração de eletricidade em comparação com centrais que consomem combustível fóssil. O capítulo ainda apresenta uma análise termoeconómica com base na metodologia do custo específico da exergia (SPECO), que avalia o custo relacionado com a destruição de exergia. A análise verificou que o impacto mais importante é a contribuição apresentada pelo campo solar (79%), e os materiais com maior impacto são: aço (47%), sal fundido (25%) e óleo sintético (21%). A análise ELCA mostra que a maior demanda de exergia é devida ao aço (47%); a análise existe uma considerável demanda de exergia relacionada com o sal fundido e ainda o óleo sintético. Em comparação com as centrais que consomem combustível fóssil (NGCC e óleo) a central sistema CSP apresenta menor impacto ambiental, enquanto o pior desempenho ambiental é o da central com queima de óleo seguida pela central a gás natural (NGCC). Na central CSP, o campo solar apresenta o custo mais elevado, enquanto o gerador de vapor, entre os componentes do ciclo de potência, apresenta o maior custo. O armazenamento de energia térmica permite que as centrais CSP superem a intermitência de radiação solar para responder à procura de energia elétrica independentemente das condições climáticas, e também possam estender a produção de eletricidade para além da disponibilidade da radiação solar diária. A análise numérica do transiente térmico de um sistema de armazenamento de gradiente térmico é realizada durante a fase de carregamento. O sistema de equações que descreve o modelo numérico é resolvido através da utilização de diferenças finitas implícitas no tempo usando o software Matlab. Os resultados da análise indicam que as previsões estão em boa concordância com os dados experimentais para a evolução no tempo da região de gradiente térmico, em particular para regiões mais afastadas da entrada. Nesta região os desvios observados são provavelmente causados pelo alto nível de turbulência devido à penetração do jato no seio do tanque de armazenamento. O modelo analítico simples para simular a turbulência que foi desenvolvido melhora os resultados. Esta abordagem não requer esforço computacional adicional e determina a difusidade térmica efetiva ao longo do tanque.

Renewable energy for energy production, like Solar, is turning out to be very pertinent in today's world [1]. It is very clear that Solar Energy is going to emerge as one of the key sources of energy in future. Moreover, the storage option is going to play an essential role to the future deployment of solar power plants. Concentrated solar power plants with thermal storage, photovoltaic plants integrated with battery energy storage, and hybrid plants are attractive solutions to obtain a stable and dispatchable energy production. Investors or policymakers usually find it challenging to come up with the most feasible solar storage technology because they need to consider techno-economic feasibility, and at the same time, from a market or administrative perspective as well. So, this thesis study will address the key problem which is aimed at investors or policymakers since there is a need to choose the best solar storage technology at a utility level in future based on so many attributes. The thesis project was carried out in two phases which includes forecast modelling & estimations and techno-economic assessment of virtual plants. These two phases helped to address various questions in relation to the problem statement of this study. The entire thesis study broadly covered seven countries spanning across four major regions around the world. The first phase of the thesis, forecast modelling estimations shows how the seven countries will look in future (2020 – 2050) with respect to installed capacity and costs for PV, CSP, and BESS technologies. Some major results from phase 1 include, in low-cost estimates, China will remain to be the market leader in PV & CSP by 2050. In U.S.A and India, the installed costs of PV are projected to decline by 70% by 2050. By 2050, the installed costs of Solar Tower technology are estimated to drop by about 65% in China and Spain. In U.S.A, the prices of BESS technology are likely to fall by around 58 – 60 % by 2050. In the second phase of thesis study, a techno-economic evaluation of virtual plants addressed the aspects which are to be considered for a solar project if it is deployed in future across seven specific countries. Results from this analysis helps investors or policymakers to choose the cheapest solar storage technology at a utility level across seven specific countries in future (2020 – 2050). Key results from this analysis show that, in the U.S.A, by 2050, PV+BESS will be the cheapest storage technology for 4 – 10 storage hours. Addition of another renewable technology will add up more viability to the comparison. In China, Hybrid will be the cheapest storage technology for 4 – 8 hrs by 2050. There is huge potential for deployment of CSP & hybrid plants in future than PV. In South Africa, CSP will be the cheapest storage technology by 2050 for 4 – 10 hours of storage. It is assumed that deployment of BESS projects at utility level starts from 2025 in South Africa. Beyond this, market forces analysis was carried out which offers insights especially for the policymakers of how various drivers and constraints are influencing each solar technology across the specific countries in future. Overall, the entire thesis study provides guidelines/insights to investors or policy makers for choosing the best solar storage technology in future at a utility scale for a particular country.
Förnybar energi för energiproduktion, liksom Solar, visar sig vara mycket relevant i dagens värld [1]. Det är mycket tydligt att solenergi kommer att framstå som en av de viktigaste energikällorna i framtiden. Dessutom kommer lagringsalternativet att spela en väsentlig roll för den framtida distributionen av solkraftverk. Koncentrerade solkraftverk med värmelagring, solcellsanläggningar integrerade med batterilagring och hybridanläggningar är attraktiva lösningar för att få en stabil och skickbar energiproduktion. Investerare eller beslutsfattare brukar tycka att det är utmanande att komma på den mest genomförbara solcellstekniken eftersom de måste överväga teknikekonomisk genomförbarhet, och samtidigt, ur ett marknads- eller administrativt perspektiv också. Så denna avhandlingsstudie kommer att ta itu med nyckelproblemet som riktar sig till investerare eller beslutsfattare eftersom det finns ett behov av att välja den bästa solenergilagringstekniken på en användningsnivå i framtiden baserat på så många attribut. Avhandlingsprojektet genomfördes i två faser som inkluderar prognosmodellering och uppskattningar och teknikekonomisk bedömning av virtuella anläggningar. Dessa två faser hjälpte till att ta itu med olika frågor i samband med problemstudien i denna studie. Hela avhandlingsstudien omfattade i stort sju länder som sträcker sig över fyra stora regioner runt om i världen. Den första fasen i avhandlingen, prognosmodelleringsuppskattningar visar hur de sju länderna kommer att se ut i framtiden (2020 - 2050) med avseende på installerad kapacitet och kostnader för PV-, CSP- och BESS -teknik. Några viktiga resultat från fas 1 inkluderar, i lågkostnadsuppskattningar, att Kina kommer att vara marknadsledande inom PV och CSP år 2050. I USA och Indien beräknas de installerade kostnaderna för PV minska med 70% år 2050. Av 2050 beräknas de installerade kostnaderna för Solar Tower -teknik sjunka med cirka 65% i Kina och Spanien. I USA kommer priserna på BESS -teknik sannolikt att sjunka med cirka 58 - 60 % år 2050. I den andra fasen av avhandlingsstudien behandlade en teknikekonomisk utvärdering av virtuella anläggningar de aspekter som ska övervägas för ett solprojekt om det används i framtiden i sju specifika länder. Resultaten från denna analys hjälper investerare eller beslutsfattare att välja den billigaste solenergilagringstekniken på en användningsnivå i sju specifika länder i framtiden (2020 - 2050). Viktiga resultat från denna analys visar att i USA, år 2050, kommer PV+BESS att vara den billigaste lagringstekniken på 4 - 10 lagringstimmar. Tillägg av en annan förnybar teknik kommer att öka jämförbarheten. I Kina kommer Hybrid att vara den billigaste lagringstekniken i 4-8 timmar fram till 2050. Det finns en enorm potential för distribution av CSP & hybridanläggningar i framtiden än PV. I Sydafrika kommer CSP att vara den billigaste lagringstekniken år 2050 för 4 - 10 timmars lagring. Det antas att distributionen av BESS -projekt på verktygsnivå börjar från 2025 i Sydafrika. Utöver detta genomfördes marknadskravsanalys som ger insikter speciellt för beslutsfattarna om hur olika drivkrafter och begränsningar påverkar varje solteknik i de specifika länderna i framtiden. Sammantaget ger hela avhandlingsstudien riktlinjer/insikter till investerare eller beslutsfattare för att välja den bästa solenergitekniken i framtiden i en nyttoskala för ett visst land.

Thesis (MEng)--Stellenbosch University, 2014.
ENGLISH ABSTRACT: Most rural African villages enjoy high levels of sunlight, but rolling out solar power generation technology to tap into this renewable energy resource at remote rural sites in Africa pose a number of design challenges. To meet these challenges, a project has been initiated to design, build and test/evaluate a knock down 3 kW peak electrical stand-alone self-tracking dual-axis concentrating solar power system. This study focusses on the mechatronic engineering aspects in the design and development of a dynamic mechatronic platform and digital electronic control system for the stand-alone concentrating solar power system. Design specifications require an accurate automatic positioner and control system for a motorized parabolic solar reflector with an optical solar harnessing capacity of 12 kWt at solar noon. It must be suitable for stand-alone rural power generation. This study presents a conceptual design and engineering prototype of a balanced cantilever tilt-and-swing dual-axis slew drive actuation means as mechatronic solar tracking mobility platform for a ∼12 m2 lightweight parabolic solar concentrator. Digital automation of the concentrated solar platform is implemented using an industrial Siemens S7-1200 programmable logic controller (PLC) with digital remote control interfacing, pulse width modulated direct current driving, and electronic open loop/closed loop solar tracking control. The design and prototype incorporates off-the-shelf components to support local manufacturing at reduced cost and generally meets the goal of delivering a dynamic mechatronic platform for a concentrating solar power system that is easy to transport, assemble and install at remote rural sites in Africa. Real-time experiments, conducted in the summer of South Africa, validated and established the accuracy of the engineering prototype positioning system. It shows that the as-designed and -built continuous solar tracking performs to an optical accuracy of better than 1.0◦ on both the azimuth and elevation tracking axes; and which is also in compliance with the pre-defined design specifications. Structural aspects of the prototype parabolic dish are evaluated and optimized by other researchers while the Stirling and power handling units are under development in parallel projects. Ultimately, these joint research projects aim to produce a locally manufactured knock down do-it-yourself concentrated solar power generation kit, suitable for deployment into Africa.
AFRIKAANSE OPSOMMING: Landelike gebiede in Afrika geniet hoë vlakke van sonskyn, maar die ontwerp van betroubare sonkrag tegnologie vir die benutting van hierdie hernubare energie hulpbron by afgeleë gebiede in Afrika bied verskeie uitdagings. Om hierdie uitdagings te oorkom, is ’n projek van stapel gestuur om ’n afbreekbare 3 kW piek elektriese alleenstaande selfaangedrewe dubbel-as son-konsentreeder te ontwerp, bou en te toets. Hierdie studies fokus op die megatroniese ingenieurs-aspekte in die ontwerp en ontwikkeling van ’n dinamiese megatroniese platform en ’n digitale elektroniese beheerstelsel vir die alleenstaande gekonsentreerde sonkrag stelsel. Ontwerp spesifikasies vereis ’n akkurate outomatiese posisionering en beheer stelsel vir ’n motor aangedrewe paraboliese son reflekteerder met ’n optiesekollekteer- kapasiteit van 12 kWt by maksimum sonhoogte, en veral geskik wees vir afgeleë sonkrag opwekking. Hierdie studie lewer ’n konsepsuele ontwerp en ingenieurs-prototipe van ’n gebalanseerde dubbelas swaai-en-kantel swenkrat aandrywingsmeganisme as megatroniese sonvolg platform vir ’n ∼12 m2 liggewig paraboliese son konsentreerder. Digitale outomatisering van die son konsentreerder platform is geimplementeer op ’n industriële Siemens S7-1200 programmeerbare logiese beheerder (PLB) met ’n digitale afstandbeheer koppelvlak, puls-wydte-gemoduleerde gelykstroom aandrywing en elektroniese ooplus en geslote-lus sonvolg beheer. Die ontwerp en prototipe maak gebruik van beskikbare komponente om lae-koste plaaslike vervaardiging te ondersteun en slaag in die algemeen in die doel om ’n dinamiese megatroniese platform vir ’n gekonsentreerde sonkrag stelsel te lewer wat maklik vervoer, gebou en opgerig kan word op afgeleë persele in Afrika. Intydse eksperimente is gedurende die somer uitgevoer om die akkuraatheid van die prototipe posisionering sisteem te evalueer. Dit toon dat die sisteem die son deurlopend volg met ’n akkuraatheid beter as 1.0◦ op beide die azimut en elevasie sonvolg asse, wat voldoen aan die ontwerp spesifikasies. Strukturele aspekte van die prototipe paraboliese skottel word deur ander navorsers geëvalueer en verbeter terwyl die Stirling-eenheid en elektriese sisteme in parallelle projekte ontwikkel word. Die uiteindelike doel met hierdie groepnavorsing is om ’n plaaslik vervaardigde doen-dit-self sonkrag eenheid te ontwikkel wat in Afrika ontplooi kan word.

L'énergie solaire est connue pour sa nature intermittente par rapport aux ressources d’énergie fossile. Cette observation souligne la nécessité d'utilisation d’un système de stockage d'énergie thermique. Le système de stockage thermocline est considéré comme un système de stockage rentable. La présente thèse vise à étudier le potentiel des roches basaltiques et siliceuses comme des candidates matériaux de stockage pour les centrales solaires concentrées. Les études expérimentales des propriétés thermo-physiques et thermomécaniques de ces roches à des températures allant jusqu'à 1000°C montrent que ces roches offrent de bonnes propriétés thermiques par rapport aux matériaux classiques de stockage. L'analyse du système de stockage thermocline sur un lit de roches à air direct est réalisée par une approche numérique. En outre, cette recherche vise également à évaluer l’impact environnementale de ce type de système de stockage en effectuant une analyse comparative de son cycle de vie. Enfin, une étude complémentaire réalisée dans le but de produire une carte d'indice de pertinence a permis d’identifier les zones les plus appropriées pour la construction des centrales solaires en Egypte. L'originalité de cette approche alternative pour le stockage d'énergie thermique est qu’elle combine la performance et la disponibilité des matériaux de stockage tout en réduisant leurs impacts environnementaux et financiers
Compare to fossil fuel energy resources, solar energy is known for its intermittent nature. This observation highlights the need for the use of a thermal energy storage system. The thermocline storage system is considered as a cost-effective storage system. This thesis aims to study the potential of basalt and silex rocks as candidate storage materials for concentrated solar power plants. Experimental studies of the thermo-physical and thermo-mechanical properties of these rocks at temperatures up to 1000°C show that these rocks offer good thermal properties compared with conventional storage materials. The analysis of the thermocline storage system of air rock-packed bed is carried out using a numerical approach. This research also aims to assess the environmental impact of this type of storage system by conducting a comparative analysis of its life cycle. Finally, a complementary study carried out with the aim of producing a relevance index map made it possible to identify the most suitable areas for the construction of solar power plants in Egypt. The originality of this alternative approach for thermal energy storage is that it combines the performance and availability of storage materials while reducing their environmental and financial impacts

Though economically favorable when compared to other renewable energy storage technologies, thermal energy storage systems for concentrating solar thermal power (CSP) plants require additional cost reduction measures to help transition CSP plants to the point of grid-parity. Thermocline packed bed storage is regarded as one potential low cost solution due to the single tank requirement and low cost storage media. Thus sensible heat storage (SHS) and latent heat storage (LHS) packed bed systems, which are two thermocline varieties, are frequently investigated. LHS systems can be further classified as single phase change material (PCM) systems or cascaded systems wherein multiple PCMs are employed. This study compared the performance of SHS, single PCM, and cascaded PCM direct storage systems under the conditions that may be encountered in utility-scale molten salt CSP plants operating between 565°C and 288°C. A small-scale prototype SHS packed bed system was constructed and operated for use in validating a numerical model. The drawbacks of the latent heat storage process were discussed, and cascaded systems were investigated for their potential in mitigating the issues associated with adopting a single PCM. Several cascaded PCM configurations were evaluated. The study finds that the volume fraction of each PCM and the arrangement of latent heat in a 2-PCM and a 3-PCM system influences the output of the system, both in terms of quality and quantity of energy. In addition to studying systems of hypothetical PCMs, real salt PCM systems were examined and their selection process was discussed. A preliminary economic assessment was conducted to compare the cost of SHS, single-PCM LHS, cascaded LHS, and state-of-the-art 2-tank systems. To the author's knowledge, this is the first study that compares the cost of all three thermocline packed bed systems with the 2-tank design. The SHS system is significantly lower in cost than the remaining systems, however the LHS system does show some economic benefit over the 2-tank design. If LHS systems are to be viable in the future, low cost storage media and encapsulation techniques are necessary.

Renewable energies have a major role in today’s energy systems development, energy security and climate change fight. Thermal Concentrating Solar Power (CSP) has the potential to get up to 11.3% of world’s electricity production with the adequate support. This type of renewable energy has proved to be price competitive and to have the advantage of integrating Thermal Energy Storage (TES). This adds the generation flexibility that other renewable energies, like wind or photovoltaics, does not have integrated. In order to continue developing this technology, solid particle CSP has been proposed. This design uses granular solid materials as Heat Transfer Fluid (HTF) and TES material in solar towers in order to be able to achieve higher operation temperatures, than current commercial CSP. Higher temperature means more efficiency in heat-to-electricity conversion, due to the use of better power generation cycles. The main objective of this thesis is to enhance relevance and provide theoretical and experimental background for solid particles to be used as TES material and HTF for CSP tower power plants, from the materials perspective, by using existent or new methodologies. During this dissertation, current scientific output and relevance were studied in two separate contributions, one for CSP and the other for TES, both by using bibliometric methods. For the CSP study, additional analyses were carried out according to the harvesting technologies (parabolic trough, solar tower, Stirling dish and linear Fresnel). For the TES study, the additional analyses were performed according to the different ways to store thermal energy (sensible, latent and thermochemical). For both analyses, most productive countries, regions, authors, journals and research communities were identified. Moreover, funding impact and cooperation between countries and authors were analyzed. For developing these bibliometric analyses, a specific methodology was implemented following Bibliometrics principles. For these purposes, two existing software programs were used for a part of the analysis, while for performing the rest of the analysis a special software was developed ad-hoc for this study. For providing background, two state-of-the-art analyses were performed in order to get current development status of solid particle CSP. The first one was oriented to the plant design itself. Several solar receivers were analyzed, as well as TES, Heat Exchanger (HEX) and conveyance systems. During the second state-of-the-art, a material driven study was carried out in order to understand the behavior expected by the particle media and to identify some of the materials proposed by the most relevant researchers in this field. Next step of this dissertation was focused on establishing the design criteria for solid particle CSP technology, from the materials science perspective. This was achieved by finding the most relevant objectives that a power plant of this kind must comply, as well as the influence of the particle media properties and parameters. Last part of this dissertation is related with two studies regarding the durability of some of the most promising solid particle materials from high temperature exposure effect perspective. The first study was focused on analyzing the effect of long term high temperature (900 °C) in the optical, mechanical, thermal and chemical properties and parameters of the solid particle material. The second study was focused in the effect of long term thermal cycling, in which is considered that the materials should resist several thousand charge-discharge cycles remaining with acceptable operational conditions. For achieving an accelerated thermal cycling test with realistic thermal conditions, a novel device was developed to perform the thousands of thermal cycles required. Electronic, software and hardware design was developed and implemented. Current device has performed more than 20 thousand cycles for different kind of materials, analyzing the same properties and parameters as the first study.
Para continuar desarrollando la energía solar de concentración (CSP), se ha propuesto el uso de sólidos particulados. En este nuevo tipo de planta CSP de torre, se utilizan materiales sólidos granulados como Fluido de Transferencia de Calor (HTF) y material para el almacenamiento de energía térmica (TES). El objetivo principal de esta tesis es establecer la relevancia y proporcionar antecedentes, tanto teóricos como experimentales, sobre el uso de sólidos particulados para esta nueva tecnología. Durante este trabajo, la producción científica actual y la relevancia científica fueron estudiados mediante dos estudios bibliométricos, una para CSP y otra para TES. Para ambos análisis, se identificaron los países, regiones, autores, revistas, comunidades de investigación más productivos, el impacto del financiamiento y la cooperación entre países y autores. Para este fin, se utilizaron tres programas informáticos, de los cuales uno tuvo que se desarrollado a la medida. Se realizaron dos análisis del estado del arte para obtener el estado actual de desarrollo de las plantas CSP con uso de sólidos particulados. En el primero, se analizaron varios receptores solares, así como sistemas TES, intercambiadores de calor (HEX) y sistemas de transporte del material granulado. En el segundo, se llevó a cabo un estudio basado en los sólidos particulados, comprendiendo el comportamiento de estos materiales, así como recopilar los materiales potenciales. Se establecieron los criterios de diseño desde la perspectiva de los materiales, logrando encontrar los objetivos relevantes, y la influencia de las propiedades y parámetros de estos materiales. Se realizaron dos estudios sobre la durabilidad en cuanto a la exposición a altas temperaturas. El primero, se centró en analizar el efecto de la temperatura a largo plazo y su efecto en las propiedades ópticas, mecánicas, térmicas y químicas. El segundo, se centró en el efecto de los miles de ciclos térmicos esperados. Se desarrolló un nuevo dispositivo capaz de realizar los ciclos térmicos requeridos en las condiciones requeridas. Se desarrolló e implementó el diseño electrónico, de software y de hardware. Se caracterizaron las mismas propiedades analizadas en el primer estudio.

Thesis (PhD) -- Stellenbosch University, 2014.
ENGLISH ABSTRACT: Currently the reduction of the levelised cost of electricity (LCOE) is the main goal of concentrating solar power (CSP) research. Central to a cost reduction strategy proposed by the American Department of Energy is the use of advanced power cycles like supercritical steam Rankine cycles to increase the efficiency of the CSP plant. A supercritical steam cycle requires source temperatures in excess of 620°C, which is above the maximum storage temperature of the current two-tank molten nitrate salt storage, which stores thermal energy at 565°C. Metallic phase change materials (PCM) can store thermal energy at higher temperatures, and do not have the drawbacks of salt based PCMs. A thermal energy storage (TES) concept is developed that uses both metallic PCMs and liquid metal heat transfer fluids (HTF). The concept was proposed in two iterations, one where steam is generated directly from the PCM – direct steam generation (DSG), and another where a separate liquid metal/water heat exchanger is used – indirect steam generation, (ISG). Eutectic aluminium-silicon alloy (AlSi12) was selected as the ideal metallic PCM for research, and eutectic sodium-potassium alloy (NaK) as the most suitable heat transfer fluid. Thermal energy storage in PCMs results in moving boundary heat transfer problems, which has design implications. The heat transfer analysis of the heat transfer surfaces is significantly simplified if quasi-steady state heat transfer analysis can be assumed, and this is true if the Stefan condition is met. To validate the simplifying assumptions and to prove the concept, a prototype heat storage unit was built. During testing, it was shown that the simplifying assumptions are valid, and that the prototype worked, validating the concept. Unfortunately unexpected corrosion issues limited the experimental work, but highlighted an important aspect of metallic PCM TES. Liquid aluminium based alloys are highly corrosive to most materials and this is a topic for future investigation. To demonstrate the practicality of the concept and to come to terms with the control strategy of both proposed concepts, a storage unit was designed for a 100 MW power plant with 15 hours of thermal storage. Only AlSi12 was used in the design, limiting the power cycle to a subcritical power block. This demonstrated some practicalities about the concept and shed some light on control issues regarding the DSG concept. A techno-economic evaluation of metallic PCM storage concluded that metallic PCMs can be used in conjunction with liquid metal heat transfer fluids to achieve high temperature storage and it should be economically viable if the corrosion issues of aluminium alloys can be resolved. The use of advanced power cycles, metallic PCM storage and liquid metal heat transfer is only merited if significant reduction in LCOE in the whole plant is achieved and only forms part of the solution. Cascading of multiple PCMs across a range of temperatures is required to minimize entropy generation. Two-tank molten salt storage can also be used in conjunction with cascaded metallic PCM storage to minimize cost, but this also needs further investigation.
AFRIKAANSE OPSOMMING: Tans is die minimering van die gemiddelde leeftydkoste van elektrisiteit (GLVE) die hoofdoel van gekonsentreerde son-energie navorsing. In die kosteverminderingsplan wat voorgestel is deur die Amerikaanse Departement van Energie, word die gebruik van gevorderde kragsiklusse aanbeveel. 'n Superkritiese stoom-siklus vereis bron temperature hoër as 620 °C, wat bo die 565 °C maksimum stoor temperatuur van die huidige twee-tenk gesmelte nitraatsout termiese energiestoor (TES) is. Metaal fase veranderingsmateriale (FVMe) kan termiese energie stoor by hoër temperature, en het nie die nadele van soutgebaseerde FVMe nie. ʼn TES konsep word ontwikkel wat gebruik maak van metaal FVM en vloeibare metaal warmteoordrag vloeistof. Die konsep is voorgestel in twee iterasies; een waar stoom direk gegenereer word uit die FVM (direkte stoomopwekking (DSO)), en 'n ander waar 'n afsonderlike vloeibare metaal/water warmteruiler gebruik word (indirekte stoomopwekking (ISO)). Eutektiese aluminium-silikon allooi (AlSi12) is gekies as die mees geskikte metaal FVM vir navorsingsdoeleindes, en eutektiese natrium – kalium allooi (NaK) as die mees geskikte warmteoordrag vloeistof. Termiese energie stoor in FVMe lei tot bewegende grens warmteoordrag berekeninge, wat ontwerps-implikasies het. Die warmteoordrag ontleding van die warmteruilers word aansienlik vereenvoudig indien kwasi-bestendige toestand warmteoordrag ontledings gebruik kan word en dit is geldig indien daar aan die Stefan toestand voldoen word. Om vereenvoudigende aannames te bevestig en om die konsep te bewys is 'n prototipe warmte stoor eenheid gebou. Gedurende toetse is daar bewys dat die vereenvoudigende aannames geldig is, dat die prototipe werk en dien as ʼn bevestiging van die konsep. Ongelukkig het onverwagte korrosie die eksperimentele werk kortgeknip, maar dit het klem op 'n belangrike aspek van metaal FVM TES geplaas. Vloeibare aluminium allooie is hoogs korrosief en dit is 'n onderwerp vir toekomstige navorsing. Om die praktiese uitvoerbaarheid van die konsep te demonstreer en om die beheerstrategie van beide voorgestelde konsepte te bevestig is 'n stoor-eenheid ontwerp vir 'n 100 MW kragstasie met 15 uur van 'n TES. Slegs AlSi12 is gebruik in die ontwerp, wat die kragsiklus beperk het tot 'n subkritiese stoomsiklus. Dit het praktiese aspekte van die konsep onderteken, en beheerkwessies rakende die DSO konsep in die kollig geplaas. In 'n tegno-ekonomiese analise van metaal FVM TES word die gevolgtrekking gemaak dat metaal FVMe gebruik kan word in samewerking met 'n vloeibare metaal warmteoordrag vloeistof om hoë temperatuur stoor moontlik te maak en dat dit ekonomies lewensvatbaar is indien die korrosie kwessies van aluminium allooi opgelos kan word. Die gebruik van gevorderde kragsiklusse, metaal FVM stoor en vloeibare metaal warmteoordrag word net geregverdig indien beduidende vermindering in GLVE van die hele kragsentrale bereik is, en dit vorm slegs 'n deel van die oplossing. ʼn Kaskade van verskeie FVMe oor 'n reeks van temperature word vereis om entropie generasie te minimeer. Twee-tenk gesmelte soutstoor kan ook gebruik word in samewerking met kaskade metaal FVM stoor om koste te verminder, maar dit moet ook verder ondersoek word.

Modele de fonctionnement capable de simuler des modes de marche, de demarrage et d'arret base sur un systeme de pilotage utilisant les methodes effectivement appliquees a themis. Ce modele est limite a la clarine principale de conversion de l'energie. Presentation de trois exemples d'utilisation qui illustrent les possibilites d'ameliorer la conduite de la centrale: une loi de commande du debit du fluide caloporteur en tout ou rien; de conseils d'exploitation des journees mal ensoleillees; une recommandation de maintien a chaud des installations pour de courtes durees d'arret

Comme les autres technologies liées aux énergies renouvelables, le solaire à concentration souffre des problèmes liés à l’intermittence de la ressource. La technologie thermocline est une solution prometteuse qui réduirait le coût du stockage thermique dans les centrales solaires de ce type. Cependant, aucune étude n’a jusqu’ici porté sur l’impact de la variation de la température en sortie du réservoir de stockage de type thermocline sur les autres composants de la centrale. Ce travail de thèse a pour but d’améliorer les connaissances sur ce sujet, grâce à l’utilisation d’une mini boucle solaire cylindro parabolique intégrant un stockage thermocline.En premier lieu, la compatibilité entre le fluide de transfert de la centrale (huile synthétique) et les potentiels matériaux de garnissage de la cuve de stockage (Cofalit, briques de cendres volantes, alumine) est vérifiée. Puis les performances de chacun des composants de la centrale (cuve de stockage, collecteurs solaires, générateur de vapeur) sont analysées expérimentalement et numériquement. Enfin, le comportement du système global est étudié, avec un accent porté sur l’impact de la variation de la température de sortie de la cuve thermocline sur les autres composants.Il a été montré qu’avec un dimensionnement et une stratégie de contrôle appropriés, la technologie thermocline diminue très peu les performances de la centrale solaire par rapport à la technologie conventionnelle à deux cuves (maximum 3 4 % de diminution de la production électrique)
Like other renewable energy technologies, concentrated solar power (CSP) suffers from resource intermittence. Thermocline technology is a promising solution to decrease cost of thermal energy storage in CSP plants. Thermocline behavior has thoroughly been studied in the past years and its behavior is considered well known. However no study treated of thermocline tanks integrated in CSP plants. Thus, the impact of the varying outlet temperature of the thermocline storage has not been assessed yet. This work aims to fill this lack of knowledge by studying a mini parabolic trough power plant integrating a thermocline tank as storage.First, the compatibility between the heat transfer fluid of the plant (synthetic oil) and various potential filler materials (Cofalit, coal fly ash bricks, alumina) of the storage tank is verified. Then, some performance studies are performed on the three main components of the power plant (energy storage tank, solar collectors, steam generator). Finally, the behavior of the whole system is assessed, with a focus on the impact of the varying fluid temperature at the outlet of the thermocline tank on the other components.It has been shown that, with a proper sizing and an appropriate control strategy, thermocline technology induces very low decrease of the solar power plant performance in comparison to the conventional two tank technology (maximum 3-4% of electrical power production difference)

Ce travail traite la modélisation et l'optimisation des centrales solaires thermodynamiques à concentration produisant de l'électricité pour l'électrification des zones rurales isolées et mal raccordées au réseau électrique. D’abord, un modèle optique et thermique détaillé des concentrateurs solaires cylindro-paraboliques est présenté permettant l'identification de capteurs existants et la création de corrélations qui peuvent être injectées dans un modèle plus global. Dans un second temps, un modèle original d'un stock de chaleur stratifié de type « lit de roche » est développé. Le nouveau modèle proposé permet de déterminer analytiquement le profil de température dans le stock à n'importe quel instant dans le cas d'une température d'entrée régulée. Ensuite, deux alternatives de bloc moteur sont modélisées : le moteur Stirling et le Cycle Organique de Rankine (ORC acronyme anglais pour Organic Rankine Cycle). Concernant le moteur Stirling, une revue critique des modèles existants a été effectuée. Certains de ces modèles ont été implémentés et complétés par des modèles originaux des pertes par fuite de matière et par effet navette. Le cycle organique de Rankine, lui, est modélisé par un modèle orientée vers l'optimisation. Enfin, une optimisation mono et multicritère d’une centrale solaire est effectuée. La configuration étudiée est équipée d’un stock de chaleur et d’une chaudière d’appoint. Elle est optimisée selon trois critères : le coût moyen actualisé de l'électricité (LCOE acronyme anglais pour Levelized Cost Of Electricity), le rendement énergétique de la centrale et la quantité de CO2 émise par Kilowatt heure d'électricité produite
This work deals with the modelling and the optimization of thermodynamic solar power plants intended to supply electricity to isolated locations. Firstly, a state of the art of solar collectors is achieved and a model for parabolic trough collectors is proposed. This model is used for actual collectors identification. It is used also to propose correlations to be introduced in the whole system model. In a second time, a state of the art of energy storage technologies is conducted and an original model of a packed bed storage tank is proposed. This model gives an explicit solution of the temperature inside the tank without using a time step based numerical resolution. Two alternatives for the power block are given: Stirling engines and Organic Rankine Cycles. For Stirling engines, a critical review of existing models is performed. Some losses occurring in Stirling engines are not well documented in the literature as leakage losses at the power piston and displacer gap losses. Therefore, original models are proposed to estimate these losses. When compared to former models in the literature, the new model of the displacer gap losses demonstrates clearly that it is very important to use decoupled models with caution. Concerning the ORC, an optimization-oriented model is proposed. Finally, a mono and multi-objective optimization of a solar power plant is performed. The optimized system is composed of a solar field, a packed bed heat storage, a power block and an auxiliary fired heater. Objective functions used in this study are: the Levelized Cost of Electricity (LCOE), the energetic efficiency of the power plant and CO2 emission per kilowatt hour of electricity

The United Nations (UN) project "Sustainable energy for all" sets three ambitious objectives to favor a sustainable development and to limit climate change: - Universal access to modern energy services. Electricity is currently not available for 1.3 billion people and the global energy demand is expected to grow of about 35% within 2040, due to the increasing world population and the expanding economies - Double the global rate of improvement in energy efficiency - Double the share of renewable energy sources (RESs) in the global energy mix In addition, according to the climate scenario assessed in the fifth assessment report (AR5) of the International Panel on Climate Change (IPCC), the prevention of undesirable climate effects requires a 40-70% reduction of greenhouse gas (GHG) emissions, compared with 2010 levels, by mid-century, and to near-zero by the end of this century (IPCC, 2014). The achievement of such objectives requires and encourages the spread of RESs in the global energy mix, gradually replacing depleting and polluting energy sources based on fossil fuels, which still have the main incidence on the energy sector. RESs already play a major role in several countries, due to the technological development and the increasing market competitiveness, and the world renewable power capacity reached 22.1% in 2013, showing an increasing trend in 2014 (REN, 2014). However, supporting policies, robust investments from the private sector and efforts from the scientific community are still crucial to demonstrate the technical and economic sustainability and effectiveness of RESs, helping their large-scale diffusion. Starting from such a background, this Ph.D dissertation focuses on the study, design and development of methods and tools for the optimization and enhancement of renewable energy technologies and their effective integration with energy storage solutions and traditional energy sources powered by fossil fuels (hybrid energy systems). The analysis of the major literature and the different scenarios and perspectives of RESs in the national and international contexts have shown that their economic sustainability, and then their diffusion, is closely connected to a number of technical, economic/financial and geographical parameters. Such parameters are the input of the analytic models developed for the techno-economic design of photovoltaic (PV) plants and small wind turbines (SWTs) and applied to the economic feasibility study, through multi-scenario analysis, of such systems in some of the main European Union (EU) Countries. Among the obtained results, the self-consumption of the produced energy plays a crucial role in the economic viability of SWTs and PV plants and, particularly, after the partial or total cut of incentives and uncertainties related to supporting policies within the EU context. The study of the energy demand profile of a specific user and the adoption of battery energy storage (BES) systems have been identified as effective strategies to increase the energy self-consumption contribution. Such aspects have led to the development of an analytic model for the techno-economic design of a grid connected hybrid energy system (HES), integrating a PV plant and a BES system (grid connected PV-BES HES). The economic profitability of the grid connected PV-BES HES, evaluated for a real case study, is comparable with PV plants without storage in case of a significant gap between the cost of energy purchased from the grid and the price of energy sold to the grid, but high BES system costs due to the initial investment and the maintenance activities and the eventual presence of incentives for the energy sold to the grid can make the investment not particularly attractive. Thus, the focus has shifted to the techno-economic analysis of off-grid HES to meet the energy demand of users in remote areas. In this context, BES systems have a significant role in the operation and management of the system, in addition to the storage of exceeding energy produced by the intermittent and variable RESs. The analysis has also been strengthened by an industrial application with the aim to configure, test and install two off-grid HESs to meet the energy demand of a remote village and a telecommunication system. In parallel, two experimental activities in the context of solar concentrating technology, a promising and not fully developed technology, have been carried out. The former activity deals with the design, development and field test of a Fresnel lens pilot-scale solar concentrating prototype for the PV energy distributed generation, through multi-junction solar cells, and the parallel low temperature heat recovery (micro-cogeneration CPV/T system). The latter activity deals with the development of a low cost thermal energy (TES) storage prototype for concentrating solar power (CSP) plants. TES systems show a great potential in the CSP plants profitability since they can overcome the intermittent nature of sunlight and increase the capacity factor of the solar thermal power plant. Concluding, the present Ph.D dissertation describes effective methods and tools for the optimization and enhancement of RESs. The obtained results, showing their critical issues and potential, aim to contribute to their diffusion and favor a sustainable development
Il progetto delle Nazioni Unite "Sustainable energy for all" ha fissato tre obiettivi ambiziosi per favorire uno sviluppo sostenibile e limitare l'impatto del cambiamento climatico: - Accesso universale a moderni servizi elettrici. Tali servizi sono attualmente indisponibili per circa 1.3 miliardi di persone ed è previsto un aumento del 40% della domanda globale di energia elettrica entro il 2040, a causa dell'incremento della popolazione mondiale e delle economie in crescita nei paesi in via di sviluppo - Raddoppio del tasso globale di miglioramento dell'efficienza energetica - Raddoppio del contributo di fonti di tipo rinnovabile nel mix energetico globale Inoltre, lo scenario climatico proposto nel "fifth assessment report (AR5)" redatto da "International Panel on Climate Change (IPCC)" stabilisce la necessità di ridurre l'emissione di gas ad effetto serra del 40-70%, rispetto ai valori registrati nel 2010, entro il 2050 ed eliminarli in modo quasi definitivo entro la fine del secolo con lo scopo di evitare effetti climatici indesiderati. Il raggiungimento di tali obiettivi richiede e incoraggia la diffusione di fonti energetiche rinnovabili (FER) all'interno del mix energetico globale, rimpiazzando gradualmente le fonti di energia convenzionali basate su combustibili fossili, inquinanti e in via di esaurimento, che hanno ancora l'incidenza principale nel settore energetico. A seguito nel loro sviluppo tecnologico e la crescente competitività nel mercato, le FER rivestono già un ruolo fondamentale nel mix energetico di numerose Nazioni ricoprendo il 22.1% del fabbisogno globale di energia nel 2013 e mostrando un andamento in rialzo nel 2014 (REN, 2014). Tuttavia, sono ancora cruciali politiche di supporto, ingenti investimenti privati e contributi della comunità scientifica per dimostrare l'efficacia e la sostenibilità tecnica ed economica delle FER e favorire, quindi, una loro diffusione in larga scala. In questo contesto, la seguente tesi di dottorato è rivolta allo studio, progettazione e sviluppo di metodi e strumenti per l'ottimizzazione e la valorizzazione di tecnologie energetiche rinnovabili e la loro integrazione efficace con fonti di produzione di energia convenzionali alimentate da combustibili fossili e sistemi di accumulo di energia (Sistemi energetici di tipo ibrido). I contributi scientifici disponibili in letteratura e l'analisi dei diversi scenari e delle prospettive delle FER nei vari contesti nazionali ed internazionali hanno dimostrato che la loro sostenibilità economica, e quindi la loro diffusione, è strettamente legata ad una serie di parametri tecnici, economico / finanziari e geografici. Tali parametri sono stati impiegati come input in due modelli analitici sviluppati per la progettazione tecnico-economica di impianti fotovoltaici (FV) e micro turbine eoliche e applicati per lo studio della loro fattibilità economica, attraverso analisi multi-scenario, in alcuni dei maggiori Paesi Europei. I risultati ottenuti hanno mostrato come l'autoconsumo dell'energia prodotta rivesta un ruolo fondamentale nella redditività economica dei citati impianti ed, in particolare, a seguito del taglio parziale o totale dei sistemi di incentivazione e l'incertezza attorno alle politiche di supporto all'interno del panorama Europeo. Lo studio specifico del profilo di domanda elettrica delle utenze e l'impiego di sistemi di accumulo di energia sono stati identificati come strategie efficaci al fine di incrementare la quota di autoconsumo. Tali considerazioni hanno portato allo sviluppo di un modello analitico utile alla progettazione tecnico-economica un sistema energetico ibrido connesso alla rete Nazionale integrante un impianto FV e un sistema di accumulo a batterie. La redditività del sistema, valutata su un caso reale, risulta comparabile a un impianto fotovoltaico privo di batterie in caso di un gap significativo tra il costo dell'energia elettrica acquistata dalla rete e il prezzo di vendita dell'energia elettrica ceduta in rete. Tuttavia, gli elevati costi dovuti all'acquisto iniziale e alle attività di manutenzione, e l'eventuale incentivazione sulla vendita dell'energia in rete, non rendono l'investimento particolarmente attrattivo per impianti connessi alla rete. L'attenzione si è quindi rivolta all'analisi tecnico-economica di sistemi energetici ibridi non connessi alla rete, comunemente definiti in isola o off-grid, per soddisfare il fabbisogno energetico di utenti in area remote e quindi prive di allaccio a una rete elettrica. In tali sistemi, i sistemi di accumulo a batterie, oltre alla capacità di accumulo dell'energia prodotta in eccesso variabili e intermittenti FER, hanno funzioni fondamentali nella gestione del sistema stesso. L'attività è stata anche rafforzata da un'applicazione industriale per la configurazione, test e installazione di due sistemi energetici ibridi in isola impiegati per soddisfare il fabbisogno energetico di un villaggio e di un sistema di telecomunicazione situati in aree remote. In parallelo, sono state svolte due attività sperimentali applicate alla promettente, ma non ancora completamente sviluppata a livello industriale, tecnologia solare a concentrazione. La prima attività riguarda la progettazione, sviluppo e test sperimentali di un prototipo in scala ridotta di concentratore solare a lenti di Fresnel per la produzione distribuita di energia elettrica, mediante l'uso di celle fotovoltaiche multi giunzione, ed energia termica a bassa temperatura, tramite un sistema di recupero termico. La seconda attività concerne lo sviluppo e test sperimentali di un prototipo di sistema di accumulo termico per impianti termodinamici alimentati da sistemi a concentrazione solare. Il sistema di accumulo consente di compensare la natura intermittente e variabile della fonte solare incrementando le ore di funzionamento dell'impianto termodinamico con i conseguenti benefici economici. Concludendo, la presente tesi di dottorato include la descrizione di metodi e strumenti per l'ottimizzazione e valorizzazione delle FER. I risultati evidenziano le criticità e potenzialità dei sistemi studiati con lo scopo di contribuire a una loro diffusione e favorire uno sviluppo sostenibile

abstract: Climate change is affecting power generation globally. Increase in the ambient air temperature due to the emission of greenhouse gases, caused mainly by burning of fossil fuels, is the most prominent reason for this effect. This increase in the temperature along with the changing precipitation levels has led to the melting of the snow packs and increase in the evaporation levels, thus affecting hydropower. The hydropower in the United States might increase by 8%-60% due to Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 scenarios respectively by 2050. Wind power generation is mainly affected by the change in the wind speed and solar power generation is mainly affected by the increase in the ambient air temperature, changes in precipitation and solar radiation. Solar power output reduces by approximately a total of 2.5 billion kilowatt- hour (kWh) by 2050 for an increase in ambient air temperature of 1 degree Celsius. Increase in the ambient air and water temperature mainly affect the thermal power generation. An increase in the temperature as per the RCP 4.5 and RCP 8.5 climate change scenarios could decrease the total thermal power generation in the United States by an average of 26 billion kWh and a possible income loss of around 1.5 billion dollars. This thesis discusses the various effects of climate change on each of these four power plant types.
Dissertation/Thesis
Masters Thesis Electrical Engineering 2020

The focus of this research was to create and characterize high temperature alumina and nitrate salt eutectic nanofluids for use in thermal energy storage (TES) systems. The nitrate eutectic was originally used in the TES system demonstrated as part of the Solar Two power tower and is currently employed as the TES material at Andasol 1 in Spain. Concentrations of alumina nanoparticles between 0.1% and 10% by weight were introduced into the base material in an effort to create nanofluids which would exhibit improved specific heat capacity to reduce the $/kWht thermal energy storage system costs. The composite materials were created using an aqueous mixing method in which both the nanoparticles and nitrate eutectic were placed into solution using acidic water. This solution was then sonicated in an ultrasonic bath in an effort to reduce nanoparticle agglomeration and to improve homogeneity. After boiling off the excess water, the nanoparticle-nitrate eutectic composite was recovered for characterization. The thermal properties of both the composite and base materials were characterized using the differential scanning calorimetry techniques outlined in ASTM E 1269. The created nanofluids were not stable and did not offer a cost-effective alternative to the current nitrate eutectic TES material. Despite these setbacks, a positive correlation between alumina concentration and nanofluid specific heat was demonstrated. Additionally, the specific heat capacities of the created nanofluids exceeded that predicted by the current theoretical models. These findings suggest that further work in the field of high temperature nanofluids for use in TES systems is warranted.