Rozprawy doktorskie na temat „Solar Parabolic Trough Collector”

Concentrated Solar Power (CSP) technologies are gaining increasing interest in electricity generation due to the good potential for scaling up renewable energy at the utility level. Parabolic trough solar collector (PTC) is economically the most proven and advanced of the various CSP technologies. The modelling of these devices is a key aspect in the improvement of their design and performances which can represent a considerable increase of the overall efficiency of solar power plants. In the subject of modelling and improving the performances of PTCs and their heat collector elements (HCEs), the thermal, optical and aerodynamic study of the fluid flow and heat transfer is a powerful tool for optimising the solar field output and increase the solar plant performance. This thesis is focused on the implementation of a general methodology able to simulate the thermal, optical and aerodynamic behaviour of PTCs. The methodology followed for the thermal modelling of a PTC, taking into account the realistic non-uniform solar heat flux in the azimuthal direction is presented. Although ab initio, the finite volume method (FVM) for solving the radiative transfer equation was considered, it has been later discarded among other reasons due to its high computational cost and the unsuitability of the method for treating the finite angular size of the Sun. To overcome these issues, a new optical model has been proposed. The new model, which is based on both the FVMand ray tracing techniques, uses a numerical-geometrical approach for considering the optic cone. The effect of different factors, such as: incident angle, geometric concentration and rim angle, on the solar heat flux distribution is addressed. The accuracy of the new model is verified and better results than the Monte Carlo Ray Tracing (MCRT) model for the conditions under study are shown. Furthermore, the thermal behaviour of the PTC taking into account the nonuniform distribution of solar flux in the azimuthal direction is analysed. A general performance model based on an energy balance about the HCE is developed. Heat losses and thermal performances are determined and validated with Sandia Laboratories tests. The similarity between the temperature profile of both absorber and glass envelope and the solar flux distribution is also shown. In addition, the convection heat losses to the ambient and the effect of wind flow on the aerodynamic forces acting on the PTC structure are considered. To do this, detailed numerical simulations based on Large Eddy simulations (LES) are carried out. Simulations are performed at two Reynolds numbers of ReW1 = 3.6 × 105 and ReW2 = 1 × 106. These values corresponds to working conditions similar to those encountered in solar power plants for an Eurotrough PTC. The study has also considered different pitch angles mimicking the actual conditions of the PTC tracking mechanism along the day. Aerodynamic loads, i.e. drag and lift coefficients, are calculated and validatedwith measurements performed in wind tunnels. The indepen-dence of the aerodynamic coefficients with Reynolds numbers in the studied range is shown. Regarding the convection heat transfer taking place around the receiver, averaged local Nusselt number for the different pitch angles and Reynolds numbers have been computed and the influence of the parabola in the heat losses has been analysed. Last but not the least, the detailed analysis of the unsteady forces acting on the PTC structure has been conducted by means of the power spectra of several probes. The analysis has led to detect an increase of instabilities when moving the PTC to intermediate pitch angles. At these positions, the shear-layers formed at the sharp corners of the parabola interact shedding vortices with a high level of coherence. The coherent turbulence produces vibrations and stresses on the PTC structure which increase with the Reynolds number and eventually, might lead to structural failure under certain conditions.

The demand for modern energy services is increasing rapidly. Solar energy has the potential to meet a significant share of the world’s energy request. Solar energy is one of the cleanest renewable forms with little or no effect on the environment. The concentrating solar power is one of the methods to harvest sun’s energy. Concentrating solar power has the advantage of easier energy storage compared to photovoltaic systems. However, the cost of energy generated by those systems is higher than conventional energy sources. It is necessary to improve the performance of concentrating solar power to make them cost competitive. Moreover, few countries such as Saudi Arabia are moving from energy based on fossil fuel to renewable energy, therefore, improving the performance of concentrating solar systems and reducing their cost is considered to emulate photovoltaic systems. This research aims to develop an innovative design of parabolic trough solar collector that uses magnetic nanofluids as a heat transfer fluid to enhance the thermal efficiency compared to conventional parabolic trough. Based on past researches, new parabolic trough design is then proposed and investigated. Ferromagnetic nanoparticles dispersed in common heat transfer fluids (ferrofluids) exhibit better thermos-physical properties compared to the base fluids. By applying the right magnetic intensity and magnetic field direction, the thermal conductivity of the fluid increased higher than typical nanofluids. Moreover, the ferrofluids exhibit excellent optical properties. The external magnetic source is installed to alter the thermo-physical properties of the fluid. This thesis is comprised of four studies including two experimental studies, one heat transfer analysis, and one economic and environmental study. A small scale parabolic trough collector was manufactured and assembled at the laboratory based on the British Standards. A steady-state method was used to measure the performance of the parabolic trough collector in corresponding studies. The performance of the ferrofluids as a heat transfer fluid was compared to the base fluid. The two experimental studies differ in the absorber used. The two absorbers used were a conventional non-direct absorber and a direct absorber without a selective surface that allows ferrofluids to absorb the incoming solar irradiation directly. The effects of nanoparticle concentration, anti-foaming, external magnetic field intensity were investigated. The volume fraction of nanoparticles was 0.05%, 0.25%, and 0.75%. Three different magnetic field intensities were investigated, 3.14 mT, 6.28 mT, and 10.47 mT. Using ferrofluids to enhance the heat transfer performance the efficiency of the ferrofluids solar collector was compared to the based fluid (water). The results show that the parabolic trough solar collector in the experiment has similar performance of flat-plate solar collectors. The efficiency of the collector improved when ferrofluids water used compared to water. Ferrofluids with low concentration improved the performance of the solar collector. The ferrofluids showed much better performance at higher reduced temperature with lower overall heat loss coefficient. Due to the non-Newtonian behaviour of the fluid, increasing the volume fraction of particles will suppress the enhancement. The pH of ferrofluids influences the behaviour of the fluid. pH values higher than 5 showed a Newtonian behaviour of the fluid. In the presence of magnetic field, the performance of the solar collector enhanced further. By increasing the magnetic field intensity, the absorbed energy parameter increased, and at higher magnetic field intensity, the rate of enhancement decreases due to the magnetic saturation of ferrofluids. In this study, the performance of non-direct absorption receiver was better than the direct absorption receiver. However, the performance of the collector with a direct absorption receiver and using ferrofluids in the presence of the external magnetic field in some cases was higher than the performance of non-direct receiver with water as heat transfer medium. The performance of ferrofluids based parabolic trough collector was theoretically investigated. The correlation, equations, and specifications used in the model were discussed in detail. The model was used to study two different parabolic trough designs. First, the parabolic trough was validated with the experimental results of AZTRAK platform. The results of the model show a good agreement with the experimental data. Thereafter, nanoparticles were added to the heat transfer fluid, and the performance of the collector with and without the presence of external magnetic field was determined. The performance of the collector did not change a lot unless the external magnetic field was present. Moreover, the effect of the glass envelope on the performance was observed. A glass cover with vacuum in the annulus has higher performance and less thermal loss. Second, the model was used to study the performance of the test rig ferrofluids based parabolic trough. The performance of the parabolic trough was first considered as concentrating collector and then as a non-concentrating collector. With the lack of an external magnetic field, the efficiency changed slightly, wherein the presence of the external magnetic field the performances of the collector enhanced and showed higher performances. In General, the presence of the magnetic field showed promising enhancement. Economic and environmental effects of using ferrofluids based solar collector compared to a flat-plate collector for household water heating systems. Results show that the ferrofluids based parabolic trough has lower payback period and higher economic saving at its useful life end than a flat-plate solar collector. The ferrofluids based collector has higher embodied energy and pollution offsets tan flat-plate collector. Moreover, if 50% insertion of ferrofluids based parabolic trough for domestic hot water could be achieved in Tabuk over 83,750 metric Ton of CO2 could be eliminated.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 26).
This thesis documents my personal contribution to the development of a hydraulic-based actuation system for a solar trough collector. The goal of this project was to design the actuation system using hydraulic actuators for a four meter solar collector prototype in Pittsfield, New Hampshire. After considering several hydraulic system architectures and conducting in-depth analysis into two of them, the idler pulley scheme was chosen. This mechanism uses a double rod end hydraulic actuator connected to wire rope wrapped around a capstan drum and an idler pulley. The model was optimized for mechanical performance, and it is expected to be a more cost effective option than the existing actuation system in New Hampshire once the controls equipment required to actuate the hydraulic cylinders for the new design is specified.
by Juan Felipe Carrillo.
S.B.

A novel type of parabolic trough collector was characterised using a very basic theoretical model. This model looked at an ideal case and provided a basic expectation that was compared to actual measurements. The model showed that greater improvements can be achieved if heat losses to the environment are limited or omitted. This can be achieved by using a glass shield to insulate the receiver in a vacuum to limit the effect wind has and therefore limit convective losses. The experimental characterisation of the PTC consisted of taking six different temperature measurements to better understand the energy balances taking place. Four different configurations were tested, using two different types of concentrator and in each case a receiver that was either unpainted or painted with a semi matte black paint. The different types of concentrator were either stainless steel sheet metal or discretised glass mirror strips, similar to a linear Fresnel collector. Experimental runs were conducted on cloudless days for an hour and 15 minutes. This allowed for three runs to be performed on a single day. Using the theoretical model and comparing it to the experimental data, an efficiency was calculated. This efficiency averaged 14 % when the receiver was unpainted and 13 % when the receiver was painted for the metal sheets. The glass mirror strips had average efficiencies of 54 % and 45 % for an unpainted and painted receiver respectively. The model is very basic and can be improved upon if more variables are taken into consideration, such as convective heat losses. It was also recommended that wind measurements are taken in future tests. A property looked at to evaluate the effectiveness of each type of configuration was the average energy supplied to the thermal heating fluid over the course of an experimental run. For this the averaged values over all the experimental runs conducted for stainless steel sheet metal were 258 W and 332 W for an unpainted and painted pipe respectively. When using the glass mirrors an average energy value of 1049 W was supplied when the pipe was unpainted and an average of 1181 W was gained in the runs conducted after the pipe had been painted. Painting the receiver had little to no effect. The surface temperature of the receiver after painting the pipe was not higher and a slight increase in the energy gained by water was observed. This was explained by inaccuracies during testing as scattered light may have caused an interference on some of the measurements. There were also human inaccuracies in testing which should be omitted in future tests by implementing, for one, a functional tracking system. Future tests should be designed in such a way to completely omit irradiance affecting the thermocouple taking the measurement. Glass mirrors fared far better than the stainless steel sheet metal counterpart. It was recommended that they are used as the concentrator of choice. Higher efficiencies were achieved and in some cases almost four times the energy was supplied to the water in the pipe. This was attributed to a much lower concentrator temperature, on average 11 °C lower than the temperature of the metal sheets, as well as a much better ability to concentrate sunlight onto a single focal point. However, the glass mirror strips were proven to be very fragile and as such, require protection from the elements. While the strips were lighter and caused less of a load during windy conditions, they were susceptible to oscillations from gusty wind. This led to a number of strips breaking and needed to be replaced. By discretising the strips into individual pieces, they had the benefit of only needing to replace the strips that were damaged. This is also true for all future runs. It is still recommended that a tarp be used to protect the glass mirrors. Using glass mirror strips as a concentrator combined LFC technology with PTC technology and a novel PTC design was achieved. The design still required the installation area of a PTC. The novel design was compared to Industrial Solar’s industrial LFC module, LF-11, as it shares many similarities to LFC technology. The peak thermal output of the rig was significantly lower at 346 W/m2 compared to the industrial value of 562 W/m2. However, the noteworthy differences in design and optimisation between the two modules meant the results achieved were comparable. It is expected that better and more comparable results can be realised once the inherent flaws in the design, such as tracking the sun, aperture size and adding a vacuum absorber, are addressed. It is recommended that more research and emphasis is put into this field as an alternative energy power plant for South Africa.
Dissertation (MEng)--University of Pretoria, 2017.
Chemical Engineering
MEng
Unrestricted

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 193-200).
Parabolic trough collectors are a low cost implementation of concentrated solar power technology that focuses incident sunlight onto a tube filled with a heat transfer fluid. The efficiency and cost of the parabolic trough collector designs is influenced by structural stiffness, choice of materials, assembly tolerances, mirror cleanliness and wear. Current performance estimates of solar trough optical field efficiencies are 54.2% [1]. The goal of this research is to identify general methods and specific design concepts for achieving increased collector efficiency. This thesis has investigated improvements in the design of a parabolic trough module by looking first at the overall structural concept of the collector to reduce complexity while maintaining structural stability under wind loading conditions. In the process of evaluating the feasibility of one such concept, a monolithic reflector panel with a mirror film front surface, details related to the mirror surface efficiency were investigated. At the panel-structure to mirror interface, surface roughness of the underlying structural backing was studied to understand performance effects on the mirror film surface that would make one backing material potentially more suitable than another would. In this case it was found that three materials tested: gel-coated fiberglass, rolled aluminum, and rolled steel were all similarly effective when compared to a more expensive mirrored aluminum backing material. When looking at the integration of the larger structural changes with the factors that affect surface reflectivity of parabolic mirrors, it became apparent that contamination of the surfaces and cleaning were major factors in reduced module effectiveness. Given that the conceptual development of the structure is ongoing, research into contamination factors and potential cleaning solutions were considered in such a way that panel cleaning solutions could be integrated into the trough module design from the start. A vortex generator cleaning concept, which uses V-shaped extruded forms to create vortices over a mirror panel in the presence of flow over the surface, was tested as a passive cleaning solution.
by Stacy L. Figueredo.
Ph.D.

La radiazione solare alla sua origine è una fonte di energetica ad alta exergia: il sole ha un’irradianza pari a 63 MW/m2. Ma all’arrivo sulla superficie terrestre questo flusso diminuisce drasticamente. Per questa ragione, quando si necessita di elevate temperature o elevate exergie si adottano sistemi solari a concentrazione. Fra tutte le possibili geometrie i concentratori solari parabolici assiali sono di gran lunga la tecnologia più adottata. Un campo di utilizzo dei PTC (parabolic trough collectors) è quello del calore destinato ai processi industriali: questa applicazione ha un elevatissimo potenziale anche alle latitudini dell’Europa centro-­‐meridionale. Nella presente tesi sono discussi i risultati di un progetto di ricerca (PTC.project) per lo studio dei PTC applicati alla domanda di calore dei processi industriali o di altre utenze nell’intervallo di temperatura fra 80 e 250 °C. Sono descritti la progettazione e la realizzazione di due prototipi di PTC, con informazioni complete riguardo alle caratteristiche geometriche, ai materiali e ai processi produttivi. Successivamente sono illustrati i risultati di test preliminari sui prototipi, assieme alle caratteristiche di un banco per il test di apparati solari a temperature comprese fra 10 e 150 °C. E’ poi esposto il modello matematico sviluppato per descrivere l’efficienza ottica e termica dei concentratori, completo delle routine per il calcolo della posizione del sole. Infine è esposto un ambiente per la simulazione dell’esercizio annuale di un campo di concentratori accoppiato ad uno specifico profilo di domanda termica. I risultati suggeriscono lo sviluppo di questa tecnologia nel panorama delle fonti di energia rinnovabile che dovranno essere adottate per raggiungere gli obiettivi energetici ed ambientali fissati in vari contesti internazionali. Ma saranno necessari forti investimenti se si vorrà imprimere un’accelerazione allo sviluppo dei PTC e delle tecnologie solari termiche in genere.
Solar radiation at its origin is a high-exergy energy source: the Sun has an irradiance of about 63 MW/m2. But on the Earth’s surface solar energy flow dramatically decreases. For this reason, when high temperatures or high-exergy need to be reestablished, concentrated solar systems are adopted. Among all possible geometries, parabolic trough collectors are by far the most widespread technology. A field of usage of PTCs is in industrial process heat: this application has a dramatic potential and can be adopted at latitudes like those of central and southern europe. In this thesis the results of research project (PTC.project) for the study of PTCs in IPH and other heat demands in the temperature range from 80 to 250 °C are exposed. The design and manufacture of two prototypes are described in detail, giving complete information on geometrical characteristics, materials and manufacturing processes. Then the results of preliminary tests on the mentioned prototypes are produced, together with the characteristics of a test bench designed to determine PTCs performances with water and heat transfer oil as working fluids in a temperature range from 10 to 150 °C. Then a mathematical model, able to determine the performance of any PTC is described: the model accounts for optical and thermal losses of the collector, and also contains a routine code to calculate the solar position. In the end a simulation environment for annual analysis of the performance of a PTC applied to a specific process heat demand load is presented and the results obtained on a realistic heat demand yearly profile are described. The energetic results suggest that there could be space for this technology in the variety of renewable energies that will be needed to meet international goals in terms of energy and environment in the nearest future. But the experience acquired also suggests that investments are needed if an acceleration on the spreading of PTCs and other CSP technologies is to be realized

Absolicon Solar Collector AB in Härnösand, Sweden, develops concentrating solar collectors with a parabolic trough. In the solar collector trough, there is thermal loss due to convection. A convection suppressor was made and used as a method to reduce thermal loss due to convection in the trough. The objective of the project was to evaluate the convection suppressor for solar collectors with a parabolic trough and its impact on the performance (thermal loss characteristics) in two different orientations of the trough, horizontal and inclined. The performance of the solar collector was first measured without the convection suppressor; these results were compared to two previous quasi-dynamical tests of the solar collector performance made by two different institutes, Research Institute of Sweden and SPF Institut für Solartechnik (Switzerland). The comparison was made to validate the test results from the tests without the convection suppressor, which matched. Secondly, when the convection suppressor was made and tested in the two different orientations, the results of the performance with and without the convection suppressor was evaluated as well as the convection suppressor itself. The results showed a significant improvement of the solar collector performance in the aspect of reduced thermal loss when the convection suppressor was used, hence higher efficiency.
Absolicon Solar Collector AB I Härnösand, Sverige, utvecklar koncentrerande solfångare med ett paraboliskt tråg. I solfångarens tråg uppstår termiska förluster som en följd av konvektion. En konvektionsreducerare tillverkades och användes som metod för att minska de termiska förlusterna i tråget. Målet med projektet var att testa och utvärdera konvektionsreduceraren för koncentrerande solfångare med ett paraboliskt tråg samt dess inverkan på verkningsgraden i två olika positioner för tråget, horisontell och lutande. För att kunna mäta konvektionsreducerarens inverkan på solfångaren mättes först solfångarens prestanda utan konvektionsreduceraren i de två olika positionerna, detta resultat användes som referens efter validering. Valideringen gjordes genom att resultatet jämfördes sedan med två andra prestandamätningar (quasi-dynamical test) av solfångaren gjorda av två olika institut, Research Institute of Sweden och SPF Institut für Solartechnik (Schweiz). Därefter, när konvektionsreduceraren var tillverkat och testad i de olika positionerna på samma sätt som mätningarna utan konvektionsreducerare, jämfördes resultaten med och utan konvektionsreducerareet samt att en utvärdering gjordes av dess inverkan. Resultatet visade en signifikant förbättring av solfångarens prestanda i form av minskade termiska förluster när konvektionsreduceraren användes och därav ökad verkningsgrad.

This work outlines the conceptualization, modelling and design of a novel electrical energy storage (EES) receiver for use in solar parabolic trough collector (PTC) power plants. A hybridization of sodium sulphur (NaS) battery and parabolic trough collector (PTC) Technologies, the EES receiver concept could one day enable PTC power plants to operate 24 hrs using solar energy only, while simultaneously providing them significant ancillary power benefits. Modelling of the EES receiver operation is achieved using of a system of ten steady state (algebraic) equations and two transient (partial differential) temperature dependent equations. The method of solving the system consisted of precedence ordering and back substituting of the steady state equations to develop a single complex and highly non-linear algebraic equation, in terms of the main process heat flux ݍ′̇ ௖௢௡ௗ,௔௧,. This equation was solved with the assistance of the Microsoft Excel goalseek tool. For the partial differential equations, a one dimensional finite difference approximation, consisting of a forward difference predictor, and a modified central difference corrector was used in discretization. Visual Basic code was then written to solve the system at each increment, each time utilizing the solution obtained for the complex non-linear algebraic equation in ݍ′̇ ௖௢௡ௗ,௔௧. This allowed investigation of the initial heat-up and charge/discharge function of the conceptual solar field. Results of simulations indicate the concept is both promising and implementable and that the slightly higher heat losses in the order of 400 – 600 W/m (a direct result of the unavoidably larger size of the conceptual receiver), are seen to be insignificant when compared to the possible energy storage and power support benefits. Though NaS batteries are currently expensive, this condition is thought to be ephemeral, since cells are made from low cost and widely available materials. Thus falling battery prices (with future mass production) could make this novel energy storage concept worthy of evaluation in a prototype PTC power plant.

This thesis aims at improving the understanding of the effects of the wind on parabolic trough Concentrating Solar Power technology. Parabolic trough power plants are often located in areas that are subjected to high wind speeds, as an open terrain without any obstructions is beneficial for the plant performance. The wind impacts both the structural requirements and the performance of the plant. The aerodynamic loads from the wind impose strong requirements on the support structure of the reflectors, and they also impact the tracking accuracy. On a thermal level the airflow around the glass envelope of the receiver tube cools its outer surface through forced convection, thereby contributing to the total heat loss of the system. The influence of the shape and design of the trough is studied with the aim to minimise wind loads and thermal losses and, thus, contribute to making parabolic trough technology more efficient and hence reduce the cost of the generated electricity. Starting with an investigation on the level of a single row of collectors, the influence of different trough depths on the wind effects — the aerodynamic loads, as well as the thermal effects — is analysed via numerical simulations that are validated against experimental data from wind tunnel tests. While a deeper trough geometry leads to higher forces than a shallow one, it also significantly reduces the wind speed around the receiver and hence the thermal loss on its outer surface. Based on these results alterations to the standard trough design of a continuous parabolic shape are undertaken, analysed in numerical simulations, and validated in wind tunnel experiments. A staggered reflector layout with different focal lengths in different sections of the trough is found to be able to reduce the wind loads by up to 24%,while some designs also retain the sheltering effect on the receiver. Various numerical simulation approaches for an adequate representation of the wind effects on individual rows of collectors, as well as in a solar field are investigated and compared. For the simulation of a solar field, time-averaged simulations of a large domain with several collector rows are compared with a transient simulation with stream-wise periodic boundary conditions. At the level of an individual collector row, the performance and results of a transient scale resolving simulation are compared with those of a simulation using synthetic turbulence generation at the inlet boundary.

Les sources d’énergies utilisées pour le chauffage de l’eau dans les bâtiments commerciaux et résidentielles sont multiples. Ces ressources sont essentiellement électriques dans les milieux urbains et utilisent le bois dans les milieux ruraux. Le pourcentage de l’énergie solaire utilisé reste assez faible. Les méthodes les utilisées pour produire l’eau chaude sont pour basés pour l’essentielle sur l’utilisation des résistances électrique ou des capteurs solaire plat. Le travail présenté dans cette thèse est basé sur l’utilisation des concentrateurs solaires pour chauffer des collecteurs d’énergie. Le rendement est augmenté par le développement de nouveau matériaux pour le stockage.La structure pour le support du collecteur a été conçue et analysée utilisant le logiciel Solidworks®. Les forces agissant sur les éléments de la structure sont simulées pour assurer la fiabilité du support lors des différentes conditions de fonctionnement. L’analyse par la méthode des éléments finis a permis la vérification de la structure utilisée pour le réflecteur et son support.Les performances énergétiques ont été simulées pour cinq ans d’opération utilisant le logiciel Matlab Simulink®. Cette simulation a été basée sur l’utilisation de trois données différentes. La première est une base de données météorologique de cinq ans en Afrique du Sud dans la Ville de Tshwane. La deuxième est un profil d’utilisation pour un foyer type. La troisième est le coût de complément de chauffage en électricité dépendant de l’heure de l’utilisation. Cette simulation a permis la validation des choix de dimensions de différents éléments du système de chauffage.Cette étude a permis le développement d’une approche pour la conception d’un système de chauffage solaire en optimisant les dimensions des différents éléments pour un foyer type et une région spécifique.De plus, nous avons conçu un autre réservoir d’eau chaude. Nous avons démontré que l'utilisation de matériaux polymères et d'autres matériaux comme le polyuréthane, le sel et l'aluminium est possible pour le développement d'un réservoir de stockage d'eau chaude en fonction de leurs propriétés inhérentes.L'extension des résultats de cette thèse améliorera encore les conceptions des technologies de concentrateurs solaires et des systèmes de chauffage solaire de l'eau. Par conséquent, certaines recommandations et suggestions sont mises en évidence afin d'améliorer la conception, l'analyse et les performances globales du système
In recent years, various energy sources and methods have been used to heat water in domestic and commercial buildings. The known sources for water heating include electrical energy and solar radiation energy in the urban regions or burning of firewood in the rural areas. Several water heating methods may be used such as electrical heating elements, solar concentrators, flat plate collectors and evacuated tube collectors. This thesis focuses on ways to further improve the system’s performance for water heating through the combined use of solar energy and solar concentrator technique. Furthermore, the study proposed an alternative design method for the hot water storage tank.The solar collector-supporting frame was designed and analysed using Solidworks®. The forces acting on the structural members were simulated to determine the capacity of the frame to sustain the load, and the possible regions on the supporting frame, which could potentially fail while in operation.Energy performance was simulated for five years of operation using Matlab Simulink® software. This simulation was based on the use of three different data. The first is a five-year weather database of the City of Tshwane in South Africa. The second is a hot water consumption profile for a typical household. The third is the cost of additional heating with electricity depending on the time of use. This simulation allowed the validation of the choices of the different elements of the heating system.This study allowed the development of an approach for the design of a solar heating system by optimising the dimensions of the different elements for a typical household and a specific region.In addition, the use of polymeric materials and other materials like polyurethane, salt and aluminium is possible for the development of a hot water storage tank based on their inherent properties.Extending the findings in this thesis will further improve the designs for solar concentrator technologies and solar water heating systems. Therefore, some recommendations and suggestions are highlighted in order to improve the overall system design, analysis and performance

This study focused on a numerical second law analysis and optimisation of a receiver tube op- erating in a parabolic trough solar collector for small-scale application. The receiver functioned in a Rankine cycle. The focus was on entropy generation minimisation in the receiver due to the high quality exergy losses in this component. Water functioned as the working uid and was heated from ambient conditions (liquid) to a superheated state (vapour), consequently, the receiver tube was subject to both single phase as well as two-phase ow. Entropy generation in the receiver tube was mainly due to nite temperature di erences as well as uid friction. The contribution of each of these components was investigated. Geometrical as well as operating conditions were investigated to obtain good guidelines for receiver tube and plant design. An operating pressure in the range of 1 MPa (Tsat = 180 C) to 10 MPa (Tsat = 311 C) was considered. Furthermore a mass ow range of 0:15 kg=s to 0:4 kg=s was investigated. Results showed that beyond a diameter of 20 mm, the main contributor to the entropy generation was the nite temperature di erences for most conditions. Generally, operating pressures below 3 MPa showed bad performance since the uid friction component was too large for small operating pressures. This phenomenon was due to long two-phase lengths and high pressure drops in this region. The nite temperature di erence component increased linearly when the tube diameter was increased (due to the increase in exposed area) if the focused heat ux was kept constant. However, the uid friction component increased quadratically when the diameter was reduced. In general when the concentration ratio was increased, the entropy generation was decreased. This was due to more focused heat on each section of the receiver pipe and, in general, resulted in shorter receiver lengths. Unfortunately, there is a limit to the highest concentration ratio that can be achieved and in this study, it was assumed to be 45 for two-dimensional trough technology. A Simulated Annealing (SA) optimisation algorithm was implemented to obtain certain optimum parameters. The optimisation showed that increasing the diameter could result in a decrease in entropy generation, provided that the concentration ratio is kept constant. However, beyond a certain point gains in minimising the entropy generation became negligible. Optimal operating pressure would generally increase if the mass ow rate was increased. Finally, it was seen that the highest operating pressure under consideration (10 MPa) showed the best performance when considering the minimisation of entropy in conjunction with the maximisation of the thermodynamic work output.
Dissertation (MEng)--University of Pretoria, 2014.
tm2015
Mechanical and Aeronautical Engineering
MEng
Unrestricted

Numerical analysis on a three-dimensional PTSC receiver’s tube model equipped with conical/wavy turbulators was conducted with various types of nanofluids/hybrid nanofluids. The Navier-Stokes equations were solved using FVM coupled with the MCRT method. The flow, thermal and entropy characteristics of the PTSC’s receiver tube were investigated. This research revealed that the coupling of conical/wavy turbulators and hybrid nanofluids effectively augmented the thermal and exergetic efficiencies and reduced the entropy generation rate compared with pure base-fluid.

In this study, the theoretical performance of a concentrating solar thermal electric system (CSTES) using a field of parabolic trough collectors (PTC) is investigated. The commercial software TRNSYS and the Solar Thermal Electric Components (STEC) library are used to model the overall system design and for simulations. The model was constructed using data from the literature for an existing 30-MW solar electric generating system (SEGS VI) using PTC&rsquo
s in Kramer Junction, California. The CSTES consists of a PTC loop that drives a Rankine cycle with superheat and reheat, 2-stage high and 5-stage low pressure turbines, 5-feedwater heaters and a dearator. As a first approximation, the model did not include significant storage or back-up heating. The model&rsquo
s predictions were benchmarked against published data for the system in California for a summer day. Good agreement between the model&rsquo
s predictions and published data were found, with errors usually less than 10%. Annual simulations were run using weather data for both California and Antalya, Turkey. The monthly outputs for the system in California and Antalya are compared both in terms of absolute monthly outputs and in terms of ratios of minimum to maximum monthly outputs. The system in Antalya is found to produce30 % less energy annually than the system in California. The ratio of the minimum (December) to maximum (July) monthly energy produced in Antalya is 0.04.

The industry sector accounts for a high share of the final energy consumption, with industries in EU-28accounting for a quarter of the final energy demand. Studies also show that 45 % of the industrial heatdemand in EU-27 is in a temperature range that can be supplied with present day solar collectors. Despitethis large potential, solar heat faces obstacles hindering its growth in the industrial sector. The mostsignificant obstacle is the low insight of the industrial system designs and energy demands. Those arecrucial factors for the feasibility and dimensioning of solar heating systems. Three case studies aretherefore conducted in dairy and pharmaceutical plants in order to review the most promising integrationpoints for parabolic trough solar collectors in terms of annual heat demand, temperature level andintegration effort. Two case studies are performed in dairy plants and one in a pharmaceutical plant, alllocated in Sweden. The analyses comprised reviewing energy mappings, process and instrumentationdiagrams of processes and boiler systems, and hourly energy demand data. Simulations have beencarried out with Polysun for the processes with hourly energy data available.Four integration points have been determined to be high priority solar heat integration points in dairyplants, when considering annual thermal energy demand, temperature levels and integration effort.Those are the low pressure steam line, heating of feedwater, clean in place systems and pasteurizers.Solar heat integration concepts have been presented for all the aforementioned heat sinks andsimulations have been conducted for the low pressure steam line and heating of feedwater. A significantamount of excess heat is produced as a result of fluctuating heat demands and peak solar heat productionhours. Further investigation should be carried out, in order to review the potential of supplying excessheat to other heat sinks. Despite the reviewed potential of the clean in place systems and pasteurizers,lack of the hourly energy demand has hindered further analyses of those systems. It is thereforerecommended to conduct energy measurements before taking further measures.Two integration points have been identified in the pharmaceutical plant, namely autoclaves andmultiple-effect distillers. Solar steam generation concepts have been presented for both processes. Theautoclaves are provided with 4,5 bar steam intermittently, as they work with batches and can have ondutyand off-duty intervals ranging from 3-30 minutes. The multiple-effect distillers are providedwith 7 bar steam, which is of rather high pressure for the solar collectors model on which thesimulations are based. The heat demand of the distillers is more or less constant.It was generally easier to acquire data for the integration points at the supply level. For instance, all heatsinks at the supply level had energy demand data available, contrary to the process level. This inclinesadditional focus on integration to the supply level, if the extent of the feasibility study is to be kept to aminimum.

L'utilizzo di energia solare termica deve essere sostenuto per ridurre il consumo di fonti fossili climalteranti. Nel presente studio si sono progettati e realizzati due sistemi solari a concentrazione: un collettore parabolico assiale (PTC) ed un forno solare a scatola. Il PTC ha un angolo di bordo di 90° ed un rapporto di concentrazione di 19,89. Sul concentratore, realizzato in sandwich composito, sono state applicate pellicole in alluminio ad elevata riflettanza. Il ricevitore è un tubo di acciaio rivestito da una vernice selettiva. Il sistema di inseguimento è governato da un algoritmo solare. I test sperimentali sono stati condotti con acqua ad una temperatura massima di 85 °C. Il PTC è stato caratterizzato ottenendo curve di efficienza termica, modificatore dell'angolo di incidenza e costante di tempo. I risultati mostrano che l'equazione dell'efficienza termica è confrontabile con quella di collettori simili. I dati sperimentali sono stati utilizzati per validare un ambiente di simulazione della resa annuale di PTC. Si è determinata la convenienza nell'adozione di nanofluidi a base di metalli rispetto al fluido di base (acqua). Sono state analizzate 5 temperature del fluido in ingresso e 3 portate in massa. I risultati mostrano che solo le nanoparticelle di Au, TiO2, ZnO e Al2O3 alle più basse concentrazioni presentano ridotti miglioramenti. Il forno solare a scatola ha un rapporto di concentrazione di 11,57, ed è costituito da una camera di cottura, un coperchio superiore vetrato e una doppia fila di specchi riflettenti. Il prototipo consente un allineamento solare manuale sia azimutale che zenitale. La temperatura massima del forno è stata determinata attraverso prove a vuoto. Sono state inoltre svolte prove a carico inserendo nel forno una o due pentole di alluminio, verniciate o meno in nero, riempite con acqua o olio di arachidi. In quest’ultimo caso, si è giunti a temperature superiori a 200 °C e a risultati confrontabili con quelli in letteratura.
Use of solar thermal energy has to be sustained to reduce consumption of climate-changing fossil fuels. Thus, in this study two concentrating solar prototypes were designed and manufactured: a parabolic trough collector (PTC) and a solar box cooker. The PTC has a 90° rim angle and a concentration ratio of 19.89. The concentrator is a sandwich composite structure with high-reflectance aluminum foils applied on it. The receiver is a steel pipe painted with a selective coating. The tracking system is based on a solar-position computer program. Experimental tests were carried out with water and temperatures up to 85 °C. Thermal efficiency, incident angle modifier, and time constant curves were found. Results show that the thermal efficiency equation is comparable with that of other PTCs in literature. Experimental data were utilized to validate a simulation environment able to determine the yearly yield of PTCs. The simulation was carried out to evaluate the convenience in adopting metal-based nanofluids respect to the base fluid (water). Five inlet fluid temperatures and three mass flow rates were analyzed. Results show that only Au, TiO2, ZnO, and Al2O3 nanoparticles, at the lowest concentrations, present reduced improvements respect to water. The solar box cooker is a high concentration ratio prototype (11.57). The cooker has a cooking chamber with a glass cover on the top and is composed by two rows of booster mirrors. The prototype allows both an azimuth and a zenith manual orientation. Tests without load were carried out to evaluate the maximum cooker temperature. Tests with load, conduced using aluminum vessels containing a certain amount of water, were accomplished both with non-painted vessels and black-coated ones, and with one or two vessels. Additional tests were carried out with peanut oil. Using this fluid, temperatures higher than the water ones were achieved (> 200 °C) and results exhibited values comparable to those in literature.

In this thesis report for Dalarna University in Borlange and Absolicon company the study of a possibility to add an array of concentrating solar collectors to a Torsång district heating system was done. The whole idea of this work was to make a simulation of this kind of system, trying to get 15-20% of solar fraction, and make an economical evaluation. At the same time, another goal was to make two comparisons: between concentrating and flat-plate collector in the same system, and between two tools for collector analysis – Polysun and Absolicon tool, based on TRNSYS, which was designed to estimate the output of the collector for a certain temperature, without any load. During the study, the analysis of the simulating tools was made and the combination of those two tools was used. Using long iteration cycles, involving changing the field layout, number of collectors and distance between collector rows in flat-plate collector case, both types of collectors were analyzed. The method of the analysis was to get an equal output of the field and see the differences, which appear while using different collector types.

A new method for the evaluation of the efficiency of parabolic trough collectors, called Rapid Test Method, is investigated at the Solar Institut Jülich. The basic concept is to carry out measurements under stagnation conditions. This allows a fast and inexpensive process due to the fact that no working fluid is required. With this approach, the temperature reached by the inner wall of the receiver is assumed to be the stagnation temperature and hence the average temperature inside the collector. This leads to a systematic error which can be rectified through the introduction of a correction factor. A model of the collector is simulated with COMSOL Multipyisics to study the size of the correction factor depending on collector geometry and working conditions. The resulting values are compared with experimental data obtained at a test rig at the Solar Institut Jülich. These results do not match with the simulated ones. Consequentially, it was not pos-sible to verify the model. The reliability of both the model with COMSOL Multiphysics and of the measurements are analysed. The influence of the correction factor on the rapid test method is also studied, as well as the possibility of neglecting it by measuring the receiver’s inner wall temperature where it receives the least amount of solar rays. The last two chapters analyse the specific heat capacity as a function of pressure and tem-perature and present some considerations about the uncertainties on the efficiency curve obtained with the Rapid Test Method.

A predominant amount of energy needed in the industrial sector is in the form of heat. A significant number of industries in the world still relies on fossil fuels for meeting their heat requirements. A transition to renewable energy for heating needs is at a snail's pace due to fossil fuel lock-in, cost superiority of conventional fuels, and less government support for renewable technology for thermal requirements. The dairy industry is one of the sectors that need heat energy for its production process. This study deals with a techno-economic analysis on the integration of parabolic trough collectors in the dairy industry. The thesis finds the barriers for solar-thermal collectors to evolve in the dairy sector and the viewpoint of the dairy industry towards the acceptance of solar thermal for meeting their thermal needs. From a literature review, it is observed that the need for dairy product will increase in the coming year. To meet the demand, the production process has to be increased. For sustainable production, companies have to rely on environment-friendly energy sources to meet the thermal demand. In the thesis work, it was also found that for several solar fractions, the LevelizedCost of Heat (LCoH) of solar-assisted heating system is less than the LCoH of the fossil-fueled conventional boiler. Therefore, it is economically viable to integrate solar thermal collectors in the dairy industry. The project also compares the LCoHof solar-assisted heating system when solar integration is done at a) feed water heating, b) direct steam generation, and c) process integration. The effect of integration point on the solar fraction, LCoH, and carbon mitigation potential is presented for a real case dairy unit in Dubai. The simulations are performed using a dynamic simulation tool. Results show that minimum LCoH and solar fraction are achieved for process integration. The process integration results in up to 90 % of the solar fraction. Through process integration, the LCoH of the conventional boiler can be reduced by 60%.

Transitioning the industrial sector’s energy system to renewable sources is crucial to reduce climate change. There is no exception for China, currently having the highest absolute levels of greenhouse gas (GHG) emissions in the world. The industrial sector accounts for about two thirds of the national energy consumption and coal is the country’s most important energy source. The integration of alternative energy sources such as solar can help transitioning the country’s energy system. By presenting a techno-economic analysis, this thesis gives an indication for profitability and in what extent there is a potential to cover the steam demand with a decentralized solar heat technology for two industries, fish and textile, in the Zhejiang province in eastern China. The used solar technology is a system with parabolic trough collectors (PTCs) with an integrated gas fired boiler. The PTC-system is compared with a coal-fired centralized supplier. The analyzed factors were roof area, solar irradiation, solar fraction, cost for steam from the centralized suppliers and cost of coal and natural gas. The maximum CO2 reduction is found to be dependent on the potential installation area. The greater area installed, the larger is the capacity and thus also the CO2 savings. The share of total steam demand covered by solar is directly proportional to the demand in relation to the installed solar capacity. The fish industry, having the lowest steam demand in relation to the roof area, is found to be able to save the largest relative proportion of CO2 emissions. Different scenarios are presented, modifying the fuel cost and fuel type for the PTC-system’s boiler, adjusting the steam cost from the centralized suppliers and using two different solar fractions of 35% and 50%. The CO2 savings depends on what fuel is being used and the solar fraction. Larger CO2 reductions are possible with a gas fired boiler compared to a coal fired one. But using a coal fired boiler makes it more economically profitable, matching the low coal price used for the centralized supplier. The scenario with most CO2 reductions is attained when using a high solar fraction of 50% and a natural gas fired boiler. The annual CO2 savings is then ranging from 15 tons per year for the company having the lowest steam capacity, up to 2090 tons/year for the company with one of the highest. Another significant factor is the companies’ seasonal activity. For the company having the least amount of active days per year (84 out of 365 days), the PTC-system is unprofitable whichever scenario. However, fuel costs for the boiler is found to be one of the most significant factors for the outcome determining if the investigated PTC-system is profitable or not. For all of the companies, there was only one that could provide all its steam demand with the PTC-system. This indicates that other energy sources need to be integrated to provide the steam demand of the enterprises with a renewable energy system.
Att omvandla industrins energisystem med förnyelsebara energikällor är väsentligt för att bromsa klimatförändringarna. Det är inget undantag för Kina, som nuvarande har de största absoluta nivåerna av utsläpp av växthusgaser i världen. Industrin står för ca två tredjedelar av den nationella energiförbrukningen och kol är landets största energikälla. Integrerandet av andra alternativ såsom solenergi kan dock hjälpa till i landets energiomvandling. Denna rapport syftar till att presentera en tekno-ekonomisk analys av en decentraliserad solfångare och ge indikation på lönsamhet samt i vilken grad tekniken kan förse behovet av ånga för två industrier, textil och fiske, i Zhejiang provinsen i östra Kina. Den solfångarteknik som används är ett system med Parabolic trough collectors (PTCs) med integrerad gaseldad ångpanna. Systemet jämförs med kraftvärmeverk som drivs med kol. De analyserade faktorerna är takytan, solar fraction, solinstrålning, kostnad för ånga samt ångpannans bränslekostnader. Besparingarna för CO2-utsläpp beror på den potentiella installationsytan. Ju större installationsyta, desto högre kapacitet och därmed högre CO2-besparingar. Andelen av behovet ånga som kan förses med solfångare är i direkt proportion till takytan och det totala behovet. Fiskeindustrin, som har lägre ångbehov i relation till takyta, visar sig kunna spara den största relativa mängden CO2-utsläpp. Olika scenarier presenteras, där bränslekostnaden och typ av bränsle för PTC-systemets ångpanna modifieras, kostnaden för ånga från de centraliserade leverantörerna justeras och solar fraction ändras mellan 35% och 50%. Besparingarna i CO2-utsläpp beror på vilket bränsle som används samt solar fraction. Större CO2-reduktion är möjlig med en gaseldad panna jämfört med en koleldad. Dock är en koleldad panna lönsammare när den matchar det låga priset på kol som används för de centraliserade leverantörerna. Scenariot med de största CO2-besparingarna uppnås med en hög solar fraction på 50% och en naturgaseldad panna. De årliga CO2-besparingarna varierar från 15 ton per år för det företag som har den lägsta ångkapacitet, upp till 2090 ton per år för ett företag med en av den högsta kapaciteten ånga. En annan viktig faktor är företagens aktivitet per år. Företaget som har minst aktiva dagar per år (84 av 365 dagar), är ej lönsamt i något av de testade scenarierna. Bränslekostnaderna för pannan har emellertid visat sig vara en av de viktigaste faktorerna för resultatet som avgör om det undersökta PTC-systemet är lönsamt eller inte. Bland alla företagen fanns det bara ett som kunde förse hela sitt ångbehov, med PTC-systemet. Detta indikerar att andra energikällor måste införas för att förse företagens ångbehov med ett förnybart energisystem.

The performance of parabolic trough based solar power plants over the last 25 years has proven that this technology is an excellent alternative for the commercial power industry. Compared to conventional power plants, parabolic trough solar power plants produce significantly lower levels of carbon dioxide, although additional research is required to bring the cost of concentrator solar plants to a competitive level. The cost reduction is focused on three areas: thermodynamic efficiency improvements by research and development, scaling up of the unit size, and mass production of the equipment. The optimum design, performance simulation and cost analysis of the parabolic trough solar plants are essential for the successful implementation of this technology. A detailed solar power plant simulation and analysis of its components is needed for the design of parabolic trough solar systems which is the subject of this research. Preliminary analysis was carried out by complex models of the solar field components. These components were then integrated into the system whose performance is simulated to emulate real operating conditions. Sensitivity analysis was conducted to get the optimum conditions and minimum levelized cost of electricity (LCOE). A simplified methodology was then developed based on correlations obtained from the detailed component simulations. A comprehensive numerical simulation of a parabolic trough solar power plant was developed, focusing primarily on obtaining a preliminary optimum design through the simplified methodology developed in this research. The proposed methodology is used to obtain optimum parameters and conditions such as: solar field size, operating conditions, parasitic losses, initial investment and LCOE. The methodology is also used to evaluate different scenarios and conditions of operation. The new methodology was implemented for a 50 MWe parabolic trough solar power plant for two cities: Tampa and Daggett. The results obtained for the proposed methodology were compared to another physical model (System Advisor Model, SAM) and a good agreement was achieved, thus showing that this methodology is suitable for any location.

A prototype solar desalination system (SODESAL) with a parabolic-trough solar concentrator (PTSC) and evacuated tube was designed and analyzed to determine the solar thermal capabilities for small-scale distillation and energy generation. A proof-of-concept study verified that distillation is possible with the system as designed, however a rupture occurred in the copper heat-pipe heat exchanger due to overheating. The internal temperatures of an aluminum heat transfer fin were measured inside an evacuated tube typically used in solar water heater systems to understand the lateral heat distribution and identify possible causes of the rupture. Solar radiation was measured for both the summer and winter solstices to understand the relationship between incident solar radiation and the potential freshwater yield of the system. The lateral heat distribution of the AHTF is dependent upon the PTSC’s solar incident angle. A consistent lateral heat distribution occurred across the AHTF approximately 40 mins after solar noon. The temperature difference between each end of the AHTF can exceed over 225 °C leading up to and following solar noon when the PTSC was set at a static slope. The SODESAL system’s future applications, system improvements and additional research are also discussed along with the capability of small-scale CSP systems.

The majority of the power generated today is produced using fossil fuels,emitting carbon dioxide and other pollutants every second. Also, fossil fuels will eventually run out. For the increasing worldwide energy demand, the use f reliable and environmentally beneficial natural energy sources is one of the biggest challenges. Alongside wind and water, the solar energy which is clean, CO2-neutral and limitless, is our most valuable resource. Concentrated solar power (CSP) is becoming one of the excellent alternative sources for the power industry. The successful implementation of this technology requires the efficient design of tracking and operation system of the CSP solar plants. A detailed analysis of components needed for the design of cost-effective and optimum tracker for CSP solar systems is required for the power plant modeling, which is the primary subject of this thesis. A comprehensive tracking and operating system of a parabolic trough solar power plant was developed focusing primarily on obtaining optimum and cost effective design through the simplified methodology of this work. This new model was implemented for a 50 kWe parabolic trough solar power plant at University of South Florida, Tampa.

This thesis is interested in improving the operation of a parabolic trough technology based solar thermal power plant by means of automatic control. One of the challenging issues in a solar thermal power plant, from the control point of view, is to maintain the thermal process variables close to their desired levels. In contrast to a conventional power plant where fuel is used as the manipulated variable, in a solar thermal power plant, solar radiation cannot be manipulated and in fact it ironically acts as a disturbance due to its change on a daily and seasonal basis. The research facility ACUREX is used as a test bed in this thesis. ACUREX is a typical parabolic trough technology based solar thermal power plant and belongs to the largest research centre in Europe for concentrating solar technologies, namely the Plataforma Solar de Almería (PSA) in south-east Spain. The plant exhibits nonlinearities as well as resonance characteristics that lie well within the desired control bandwidth. Failure to adequately capture the resonance characteristics of the plant results in an undesired oscillatory control performance. Moreover, measured disturbances are an integral part of the plant and while some of the measured disturbances do not have a significant impact on the operation of the plant, others do. Hence, with the aim of handling the plant nonlinearities and capturing the plant resonance characteristics, while taking explicit account of the measured disturbances, in this thesis a gain scheduling feedforward predictive control strategy is proposed. The control strategy is based upon a family of local linear time-invariant state space models that are estimated around a number of operating points. The locally estimated linear time-invariant state space models have the key novelty of being able to capture the resonance characteristics of the plant with the minimal number of states and hence, simple analysis and control design. Moreover, while simple classical, series and parallel, feedforward configurations have been proposed and used extensively in the literature to mitigate the impact of the measured disturbances of the ACUREX plant, the proposed control strategy incorporates a feedforward systematically by including the effects of the measured disturbances of the ACUREX plant into the predictions of future outputs. In addition, a target (set point) for a control strategy is normally set at the ACUREX plant by the plant operator. However, in this thesis it is argued that, in parallel, the operator must choose between potentially ambitious and perhaps unreachable targets and safer targets. Ambitious targets can lead to actuator saturation and safer targets imply electricity production losses. Hence, in this thesis a novel two-layer hierarchical control structure is proposed with the gain scheduling feedforward predictive control strategy being deployed in a lower layer and an adequate reachable reference temperature being generated from an upper layer. The generated reference temperature drives the plant near optimal operating conditions, while satisfying the plant safety constraints, without any help from the plant operator and without adding cost. The proposed two-layer hierarchical control strategy has the potential benefits of: (i) maximising electricity production; (ii) reducing the risk of actuator saturation; (iii) extending the life span of various elements of the plant (e.g. synthetic oil, pump and valves) and (iv) limiting the role of the plant operator. The efficacy of the proposed two-layer hierarchical control strategy is evaluated using a nonlinear simulation model that approximates the dynamic behaviour of the ACUREX plant. The nonlinear simulation model is constructed in this thesis and validated in the time and frequency domain.

Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2016.
Cataloged from PDF version of thesis.
Concentrating solar power (CSP) stands as a promising renewable energy technology with the ability to contribute towards global reduction of carbon emissions. A major obstacle to increased adoption of CSP plants has to do with their high initial investment cost; consequently, there is a powerful desire to find improvements that decrease the initial capital investment for a CSP plant. One such improvement involves connecting modularized parabolic trough segments, each with the same dimensions, decreasing the overall amount of actuators required along with greatly simplifying system control architecture. This thesis is concerned with the extent to which parabolic solar trough modules can be connected together while still being able to operate to desired accuracy under expected load. Accuracy requirements are calculated, along with expected loads resulting in frictional torque on the trough. These expected loads are combined with a model for the effect of connecting multiple trough modules to generate a relationship between number of chained modules and required torsional stiffness. To verify said model, an experimental setup was designed and constructed to simulate loads due to both trough weight and wind loads.
by Kabir Abiose.
S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 47).
This thesis documents my personal contribution to the engineering and design of an actuation system with the purpose of rotating a parabolic solar trough to track the sun throughout the day. The primary focus of the design was to create a robust system with minimal cost while meeting a number of functional requirements. After considering a number of possible designs and conducting an in-depth analysis into two of them, the final design chosen was a slider crank mechanism that rotates the trough about its focus. This mechanism uses a lead screw to drive the base of two passively extensible arms in a lateral direction and translates that force into a rotational motion. Whichever arm is in compression actuates the trough. One of these bases is driven by the lead screw while the other is fixed a distance away using a rigid connection. The model for this system was optimized for cost and design simplicity resulting in the selection and purchase of parts for a full scale prototype at a site in New Hampshire using one 4ft lead screw and a 0.16 horsepower motor to drive each end of the trough.
by Conor R. O'Rourke.
S.B.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.
Page 47 blank. Cataloged from PDF version of thesis.
Includes bibliographical references (p. 42).
This thesis presents the error analysis pertaining to the design of an innovative solar trough for use in solar thermal energy generation fields. The research was a collaborative effort between Stacy Figueredo from Prof. Alexander Slocum's Precision Engineering Research Group at MIT and a team of students from the 2.752 Mechanical Engineering course and resulted in a final design that uses two linear actuators to rotate a monolithic parabolic trough over a 2600 range. A measure of efficiency, based on the geometric tracing of incident and reflected rays under different parabola deformations, was developed and used to determine the impact of several key parameters on the accuracy of the system. The resulting error analysis demonstrated the substantial influence of the crank arm length and actuator precision on the overall system efficiency and set an upper bound of 1 degree in permissible angular error in order to maintain 80% efficiency under sustained wind loading.
by Cyril Koniski.
S.B.

Includes abstract.
Includes bibliographical references.
This thesis reports on the technical and economic analysis of wet and dry cooling technologies of parabolic trough CSTP plant. This was done through modelling and simulation of a standalone and grid connected parabolic trough using the System Advisor Model (SAM).

The change in climate as a consequence of anthropogenic activities is a subject ofmajor concerns. In order to reduce the amount of greenhouse gas emissions inthe atmosphere, the utilization of renewable, fossil-free power generationapplications becomes inevitable. Geothermal and solar energy play a major rolein covering the increased demand for renewable energy sources of today’s andfuture’s society. A special focus hereby lies on the Concentrating Solar Powertechnologies and different geothermal concepts. The costs for producingelectricity through Concentrating Solar Power and therefore Parabolic Trough Collectorsas well as geothermal conversion technologies are still comparatively high. Inorder to minimize these expenses and maximize the cycle’s efficiency, thepossible synergies of a hybridization of these two technologies becomeapparent. This thesis therefore investigates the thermodynamic and economicbenefits and drawbacks of this combination from a global perspective. For that,a Parabolic Trough Collector system is combined with the geothermal conversionconcepts of Direct Steam, Single and Double Flash, Organic Rankine as well asKalina Cycles. The solar integrations under investigation are Superheat,Preheat and Superheat & Reheat of the geothermal fluid. The thermodynamicanalysis focuses on the thermal and utilization efficiencies, as well as therequired Parabolic Trough Collector area. The results indicate that in the caseof the Superheat and Superheat & Reheat setup, the thermal efficiency canbe improved for all geothermal concepts in comparison to their correspondinggeothermal stand-alone case. The Preheat cases, with the major contributionfrom solar energy, are not able to improve the cycle’s thermal efficiencyrelative to the reference setup. From an exergy perspective the findings varysignificantly depending on the applied boundary conditions. Still, almost allcases were able to improve the cycle’s performance compared to the referencecase. For the economic evaluation, the capital investment costs and thelevelized costs of electricity are studied. The capital costs increasesignificantly when adding solar energy to the geothermal cycle. The levelizedelectricity costs could not be lowered for any hybridization case compared tothe reference only-geothermal configurations. The prices vary between20.04 €/MWh and 373.42 €/MWh. When conducting a sensitivity analysison the solar system price and the annual mean irradiance, the Kalina Superheatand Superheat & Reheat, as well as the Organic Rankine Preheathybridizations become cost competitive relative to the reference cases.Concluding, it is important to remark, that even if the hybridization of the ParabolicTrough and the different geothermal concepts makes sense from a thermodynamicperspective, the decisive levelized costs of electricity could not be improved.It is, however, possible that these costs can be further reduced under speciallocal conditions, making the addition of Parabolic Trough solar heat tospecific geothermal concepts favorable.

Thesis (S.B.)--Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (page 34).
This thesis documents Katarina Popovic's contribution to the design of hydraulic cylinder actuation system for day to day solar trough sun tracking, a semester long project within 2.752 Development of Mechanical Products class. The primary goal of this project was to design a robust mechanical system while reducing the complexity and thus overall cost of the solar trough assembly for the production line. The mechanism suggested in this thesis actuates both sides of the solar trough simultaneously, as well as exploit hydraulic cylinder's full range stroke in order to deliver +/-110° rotational requirement. The rotational motion is achieved through a pulley and a wire rope system, actuated by a single, double action double rod cylinder. As this project funding was received from our sole sponsor, an Italian multinational energy company, during the design process the ultimate goal of eventual production line was kept in mind. However slight design modifications have been made in order to install and test the actuation system on the already existing 4m solar trough prototype on site in Pittsfield, New Hampshire.
by Katarina Popović.
S.B.

L’objectif de cette thèse est d’étudier expérimentalement les performances énergétiques d'une installation de micro-cogénération solaire. Le prototype réalisé est constitué d'un concentrateur cylindro-parabolique associé à un moteur à vapeur fonctionnant suivant un cycle de Hirn (Rankine avec surchauffe). Les originalités de ce projet sont l’utilisation de l’énergie solaire, renouvelable et inépuisable mais intermittente, la génération directe de vapeur au sein d'un concentrateur de taille réduite (46,5 m²), le système de suivi solaire sur deux axes et le couplage à un moteur à piston non lubrifié. La première partie de l'étude porte sur le concentrateur seul. Son fonctionnement est étudié sur deux journées types (ensoleillée et nuageuse) et son rendement thermique est évalué. La dynamique du système est également abordée notamment par l'étude de sa réponse à des perturbations. Une régulation de type boucle ouverte a été mise en place et validée. La seconde partie concerne la caractérisation du moteur seul. Des essais ont été menés avec une puissance de source chaude stable puis variable. À partir des résultats obtenus, un modèle empirique est développé, puis exploité dans le cadre d'une étude paramétrique du moteur. Cette étude montre l'influence importante du ratio de pression et de la vitesse de rotation sur le rendement. Dans la dernière partie, les performances globales (rendement, puissances électrique et thermique produites) du micro-cogénérateur sont évaluées. Des essais à pression et à vitesse régulées sont présentés. A partir de cartographies de fonctionnement réalisées à l’aide d’un modèle empirique, une régulation basée sur l'utilisation d'un by-pass est alors mise en place, puis testée
The objective of this thesis is the experimental study of the energy performances of a micro combined solar heat and power (micro-CHP) unit. The prototype is composed of a solar parabolic trough collector coupled to a Hirn (superheated Rankine) cycle engine. The originalities of this project are the use of solar energy which is renewable and inexhaustible but intermittent, the direct steam generation with a reduced size parabolic trough collector (46.5 m²), the two axis tracking system and the coupling with an oil-free reciprocating steam engine. The first part of this study is focussed on the solar collector. Thermal performances under sunny and cloudy conditions are presented and the thermal efficiency is evaluated. The system dynamic is also investigated through the characterization of the inertia as well as a study of its response to perturbations. Then a control strategy is set up and validated. The second part deals with the characterization of the engine. Tests have been performed with a stable and variable heat source power. From these tests, an empirical model has been developed and used in a parametrical study. This study shows the significant influence of the pressure ratio and of the rotational speed on the efficiency of the engine. In the last part, global performances (efficiency, output thermal and electrical powers) of the entire micro-CHP unit are evaluated. Tests with controlled pressure and speed are presented. From operating maps established from an empirical model, a control strategy based on the use of a by-pass is set up and tested

In recent years power generation from renewable energy has grown substantially both in South Africa and around the world. This growth is set to continue as there is more pressure to reduce the burning of fossil fuels. However, renewable energy power generation suffers from unpredictability, which causes problems when it comes to managing power grids. Concentrated Solar Power (CSP) plants offer a practical solution to store power in the form of thermal energy storage (TES). Thus, the plant can run when there is no solar energy available, leading to a more stable power supply. Unfortunately, CSP plants cost more than other renewables such as photovoltaic and wind power. Thus, there is a need for research into how to bring down the cost of CSP plants. One of the most proven types of CSP is the parabolic trough plant. The most recent innovation is to try and use molten salt as the heat transfer fluid which would reduce the cost of the plant. However, this new technology has not been implemented on a full scale CSP plant and little testing has been done to prove the technology. The HPS2 is a test facility aimed at testing the use of molten salt as a heat transfer fluid (HTF). This test facility, located in Evora Portugal, is being developed by an international consortium led by the German DLR institute of Solar Research. It is one of the first test facilities of its kind where experiments will be conducted to demonstrate the validity of using molten salt as a HTF and a storage medium in a parabolic trough CSP plant. The HPS2 test facility is not yet operational and there is a need for a dynamic thermofluid process model to better understand and predict both its steady state and transient operational behaviour. This dissertation reports on the development of such a dynamic thermofluid process model and the results obtained from it. The process model developed primarily focuses on the steam cycle with the TES incorporated into the model. The physical geometry of each of the components are employed to construct discretized elements for which the conservation of mass, energy, and momentum are applied in a one-dimensional network approach. The economizer and evaporator combined has a helical coil geometry and uses molten salt as a heat transfer fluid, which is unique. Thus, correlations had to be adjusted for the flow characteristics found in the economizer/evaporator. Results from the steady state simulations of the steam cycle show that the molten salt mass flowrate through the steam generation system will have to be reduced from the initially expected value to meet operational requirements. Results of the dynamic simulations show that the test facility will be able to produce a constant power supply despite transient solar conditions and highlights key dynamic responses for operators to be aware of.

Concentrated solar power (CSP) has the potential of fulfilling the world’s electricity needs. Parabolic-trough system using synthetic oil as the HTF with operating temperature between 300 and 400o C, is the most matured CSP technology. A thermal storage system is required for the stable and cost effective operation of CSP plants. The current storage technology is the indirect two-tank system which is expensive and has high energy consumption due to the need to prevent the storage material from freezing. Latent heat storage (LHS) systems offer higher storage density translating into smaller storage size and higher performance but suitable phase change materials (PCMs) have low thermal conductivity, thus hindering the realization of their potential. The low thermal conductivity can be solved by heat transfer enhancement in the PCM. There is also lack of suitable commercially-available PCMs to cover the operating temperature range. In this study, a hybrid cascaded storage system (HCSS) consisting of a cascaded finned LHS and a high temperature sensible or concrete tube register (CTR) stages was proposed and analysed via modelling and simulation. Fluent CFD code and the Dymola simulation environment were employed. A validated CFD phase change model was used in determining the heat transfer characteristics during charging and discharging of a finned and unfinned LHS shell-and-tube storage element. The effects of various fin configurations were investigated and heat transfer coefficients that can be used for predicting the performance of the system were obtained. A model of the HCSS was then developed in the Dymola simulation environment. Simulations were conducted considering the required boundary conditions of the system to develop the best design of a system having a capacity of 875 MWhth, equivalent to 6 hours of full load operation of a 50 MWe power plant. The cascaded finned LHS section provided ~46% of the entire HCSS capacity. The HCSS and cascaded finned LHS section have volumetric specific capacities 9.3% and 54% greater than that of the two-tank system, respectively. It has been estimated that the capital cost of the system is ~12% greater than that of the two-tank system. Considering that the passive HCSS has lower operational and maintenance costs it will be more cost effective than the twotank system considering the life cycle of the system. There is no requirement of keeping the storage material above its melting temperature always. The HCSS has also the potential of even lower capital cost at higher capacities (>6 hours of full load operation).

The integration of concentrated solar thermal technology with PCC-retrofitted power plants has the potential to recover the power plant penalty while capturing carbon through the PCC plant. The main contribution of this thesis is showcasing such synergy between solar and PCC-retrofitted power plants through development of a model investigating the solar plant dynamic behaviour and response to external disturbances including climatic changes for the Australian context. Firstly, the potential of solar thermal and/or power energy to compensate for both PCC penalties is investigated. The mode of integrated solar energy with the PCC-retrofitted power plant and the integration point in the superstructure, result in a number of possible configurations among which, three are shown to have a higher potential to meet the operational objective, maintaining the original output rate while PCC is operating. The basic sizing data, a preliminary size of each configuration, operational capability and complexity level involved in the integration of each configuration are established. A limitation was faced in assessing the performance of each option using levelised cost of energy due to the lack of a dynamic profile of high-pressure side feed water preheaters of the PCC-retrofitted power plant. Additionally, a dynamic study on the superstructure was required using the annual dynamic profile of PCC load requirements, carbon price and power demand. Despite these, this work has shown near optimal values for solar multiple and number of full load hours of thermal storage for each configuration. These results are used to select one option for a dynamic model. A comprehensive dynamic study is performed on Option 2 using TRNSYS modelling package. The main operating and control schemes are identified and details of the modelling are provided followed by an analysis of performance results and conclusion and a note for future directions.

The  aim  of  this  master  thesis  is  an  investigation  of  the  thermal  performance  of  a  thermal compound parabolic concentrating (CPC) collector from Solarus. The collector consists of two troughs with absorbers which are coated with different types of paint with  unknown  properties.  The  lower  and  upper  trough  of  the  collector  have  been  tested individually. In  order  to  accomplish  the  performance  of  the  two  collectors,  a  thorough  literature  study  in  the  fields  of  CPC  technology,  various  test  methods,  test  standards  for  solar thermal  collectors  as  well  as  the  latest  articles  relating  on  the  subject  were  carried  out. In addition, the set‐up of the thermal test rig was part of the thesis as well. The thermal  performance  was  tested  according  to  the  steady  state  test  method  as  described in the European standard 12975‐2. Furthermore, the thermal performance of  a  conventional  flat  plate  collector  was  carried  out  for  verification  of  the  test  method. The  CPC‐Thermal  collector  from  Solarus  was  tested  in  2013  and  the  results  showed  four  times  higher  values  of  the  heat  loss  coefficient  UL (8.4  W/m²K)  than  what  has been reported for a commercial collector from Solarus. This value was assumed to be too large and it was assumed that the large value was a result of the test method used that time. Therefore, another aim was the comparison of the results achieved in this work with the results from the tests performed in 2013. The results of the thermal performance showed that the optical efficiency of the lower trough of the CPC‐T collector is 77±5% and the corresponding heat loss coefficient UL 4.84±0.20  W/m²K.  The  upper  trough  achieved  an  optical  efficiency  of  75±6  %  and  a  heat loss coefficient UL of 6.45±0.27 W/m²K. The results of the heat loss coefficients  are  valid  for  temperature  intervals  between  20°C  and  80°C.  The  different  absorber paintings have a significant impact on the results, the lower trough performs overall better.  The  results  achieved  in  this  thesis  show  lower  heat  loss  coefficients UL and higher optical efficiencies compared to the results from 2013.

The increasing availability on the market of different types of solar reflectors such as: polymeric film mirrors, aluminum mirrors and thin glass mirrors, together with: the lack of available norms in this area, and a valid methodology to compare the performances of the candidate reflectors; highlights the necessity to conduct a more detailed analysis on these new technologies. The objective of the present work is to suggest a valuable method to compare the reflectance performance of mirrors, evaluating also their performances in order to assess: - the most durable to ageing and weathering effects; - the different reflectance behavior with the variation of the solar incident angle. .For these reasons the work here proposed was carried out with an experimental apparatus composed by: - An Agilent Cary 5000 UV/Vis/NIR spectrophotometer to test the different performance of the mirrors at different characterization steps; - An integrating sphere of 150 mm in diameter (DRA ¡V Diffuse Reflectance Accessory); - A VASRA (Variable Angle Specular Reflection Accessory); - A UV chamber to accelerate the ageing process; - A £gScan SMS Scatterometer for RMS Roughness and BDSF measurement; - An outdoor bench The work was completed with two modeling tools: - An engineering equation solver (Mathcad) to dynamically evaluate the behavior; - A ray tracing software (Soltrace) to evaluate the system¡¦s optical efficiency. The analysis indicates that the candidate reflectors can be accurately characterized with five fundamental parameters: a) £lSWH, the solar-weighted hemispherical reflectance; b) £lSWS, the solar-weighted specular reflectance; c) £lSWS( á), the solar weighted specular reflectance function of the variable angle of incidence; d) BDSF, Bi Directional Scattering Function; e) RMS Roughness This evaluation will provide a valuable tool, for the companies who want to invest in concentrating solar power technology, to decide whether or not using a candidate reflectors to realize new plants, assessing their performances, their costs, and their durability.

Os processos oxidativos avançados (POAs) têm sido apontados como alternativa eficiente para a degradação de poluentes recalcitrantes. Entre os POAs, a fotocatálise utilizando luz solar vem sendo muito estudada tendo em vista sua aplicação no tratamento de efluentes aquosos contendo pesticidas. No presente trabalho, estudou-se a degradação do herbicida amicarbazona (AMZ) por meio do processo TiO2/UV em um reator com coletores parabólicos compostos irradiados por luz solar. Os experimentos foram realizados segundo uma matriz Doehlert para o estudo da influência da concentração inicial de AMZ (20-100 mg L-1), da concentração de catalisador (0,1-1 g L-1) e do número de tubos expostos à luz solar (1-9). Amostras retiradas ao longo do tempo foram analisadas quanto às concentrações de AMZ e de carbono orgânico total (TOC). As medidas radiométricas realizadas indicaram que a radiação UVB-UVA correspondeu em média a ca. 4% da radiação solar total incidente entre 310-2800 nm; a actinometria de ferrioxalato indicou fluxo fotônico médio de 3,58×10-5 mol fótons m-2 s-1 para dias ensolarados típicos. O processo TiO2/UV mostrou-se eficiente para degradação do pesticida, que foi totalmente removido antes de 45 minutos de tratamento, para as seguintes condições: [AMZ]0=21,3 mg L-1; [TiO2]=0,5 g L-1; e 7 tubos. Contudo, nesse caso houve apenas ca. 24% de mineralização e na grande maioria dos casos os valores de TOC permaneceram praticamente constantes, o que indica a formação de sub-produtos recalcitrantes, cuja toxicidade e biodegradabilidade devem ser caracterizadas. A análise estatística dos resultados confirma os efeitos importantes da concentração inicial do pesticida e do número de tubos expostos (volume irradiado), cujo aumento permite compensar a menor incidência de radiação solar. Em alguns experimentos os resultados sugeriram que a degradação da AMZ foi favorecida pela maior concentração de TiO2. Na grande maioria dos casos os valores de ACM foram inferiores a 50 m2 kg-1, o que torna este parâmetro interessante para aumento de escala de processos fotocatalíticos irradiados por luz solar empregados no tratamento de efluentes aquosos contendo amicarbazona.
Advanced oxidative processes (AOP) have been considered as an efficient alternative for the degradation of recalcitrant pollutants. Photocatalysis using solar radiation has been studied for the treatment of wastewaters containing pesticides. In this work, the degradation of the herbicide amicarbazone (AMZ) by the TiO2/UV process was studied in a reactor equipped with compound parabolic collectors irradiated by solar light. The experiments were carried out according to a Doehlert matrix to study the effects AMZ initial concentration (20-100 mg L-1), catalyst concentration (0.1-1 g L-1), and number of tubes exposed to solar light (1-9). Samples were analyzed for AMZ and total organic carbon (TOC) concentrations. Radiometric measurements indicated that UVB-UVA radiation corresponded in average to about 4% of the solar radiation between 310-2800 nm; ferrioxalate actinometry resulted in an average photonic flux of 3.58×10-5 mol fótons m-2 s-1 for typical sunny days. The TiO2/UV process showed to be efficient for the degradation of the pesticide, which was completely removed before 45 minutes of treatment, for the following conditions: [AMZ]0=21.3 mg L-1; [TiO2]=0.5 g L-1; and 7 tubes. However, in this case only ca. 24% of mineralization was achieved, and in most cases TOC values remained practically constant, indicating the formation of recalcitrant by-products whose toxicity and biodegradility should be characterized. Statistical analysis of the results confirmed important effects of pesticide initial concentration and number of tubes exposed (irradiated volume), whose increase enables to compensate the lower incidence of solar radiation. Some experimental results suggested that AMZ degradation was favored by higher TiO2 concentrations. In most cases the values of ACM were lower than 50 m2 kg-1, in such a way that this parameter is interesting for scale-up of solar irradiated photocalytic processes used in the treatment of amicarbazone-containing wastewaters.

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)

The thesis deals with the design, the realization and the experimental test of an innovative small size parabolic trough collector (mPTC) suitable to produce heat at medium temperature (100−250°C) with higher efficiency than the solar thermal collectors. Starting from the study of the state of the art, the design phase for the small parabolic trough collector has dealt with the numerical modelization of the parabolic collector from the optical, thermal and structural point of view. A termofluidodynamic FEM model of the receiver tube has been developed and a parametric analysis has been carried out to optimize the components of the receiver tube. Furthermore, the numerical model has allowed to obtain useful details for the realization of the prototype and for the design of the test rig such as the optimal mass flow and the rise in temperature along the collector. A structural finite element model has been realized in order to compute the thermal stress on the absorber tube. Following the indications of the numerical models the prototype of an innovative parabolic trough collector has been realized. The m-PTC collector is characterized by extremely small size since it has been designed to be suitable for the integration on the roofs of industrial environments where the space for installation of solar collectors is in general limited. An indoor test rig has been realized to test the thermal performances and to verify the good quality of the receiver tube. The test rig allows the measurement of the heat losses of receiver heating up the absorber tube through the Joule effect. In order to fully characterize the collector, a test rig for outdoor test has been designed. The test bench has been carefully projected in order to satisfy the requirements imposed by the standard test on solar concentrating collectors. The measurement instrumentation has been properly selected in order to minimize the uncertainty on the final variables to be obtained, the useful thermal power and the efficiency of the collector. Tests have been run for different inlet temperatures of the fluid and different conditions of irradiance. An accurate analysis of the measurement uncertainties has been conducted. The data have been fitted through a multiple linear regression based on weighted least squares obtaining the efficiency curve of the collector. The peak optical efficiency of the m-PTC has been estimated to be 69%. The predicted thermal efficiency at fluid temperature of 180°C, is 63% ±4%. The experimental results have been compared with the numerical outcomes of the termofluidodynamic FEM model that has been validated. The yearly performances of the m-PTC have been evaluated through dynamical simulations with TRNSYS. It has been compared the m-PTC with an evacuated collector and a linear Fresnel collector for four different locations and different levels of inlet temperature.

Il mercato del solare termodinamico offre varie soluzioni tecnologiche e impiantistiche in funzione dei livelli di temperatura che si vogliono ottenere. Le esigenze energetiche nei vari settori industriale, residenziale e commerciale, però, spingono il mercato verso i collettori solari capaci di operare con rendimenti maggiori del 50% a temperature superiori a 100 °C, fino anche a 250 °C (nel range cosiddetto “a media temperatura”'). In questo ambito, la tecnologia che dimostra di essere più matura per la penetrazione del mercato risulta essere quella dei collettori parabolici lineari (PTC), e in particolar modo quelli di piccola taglia. L'assorbitore solare riveste un ruolo di estrema importanza per il buon funzionamento dell'intero PTC. In particolare la scelta del coating superficiale per il tubo rappresenta un punto focale per lo sviluppo e l'ottimizzazione del sistema in termini tecnici ed economici. Per il raggiungimento degli obiettivi è necessario orientarsi verso soluzioni tecnologiche che abbiano proprietà chimiche, fisiche e ottiche tali da garantire elevate prestazioni in termini di efficienza energetica e stabilità nel tempo alle temperature di esercizio desiderate. I coatings a base di cromo nero presentano ottime caratteristiche ottiche (α≈0.90-0.92; εT≈0.10-0.15) e risultano essere stabili anche fino a 300 °C. Il più grande problema legato alla realizzazione di rivestimenti cromati è legato all'inquinamento conseguente all'utilizzo nel bagno elettrolitico di ossidi di cromo esavalente. Agli inizi del nuovo secolo, con l'avvento di nuove soluzioni chimiche meno inquinanti per la produzione di oggetti cromati e con il crescente interesse verso i collettori solari piani, le tecniche galvaniche hanno trovato largo uso nella produzione di impianti solari termodinamici. L'assorbitore che è stato studiato è un assorbitore a base di cromo nero, e questo rientra nella categoria dei “tandem-absorber”: lo studio è cominciato quindi dal substrato. Lo studio del substrato ha portato alla comprensione delle caratteristiche che questo deve possedere e quali sono le condizioni operative per ottenerle. Il substrato per il cromo nero deve possedere appropriate caratteristiche ottiche, ovvero bassa emittanza, ma deve anche favorire la deposizione e l'adesione dell'assorbitore. I materiali candidati a questo scopo sono stati il nichel, ottenuto con due diversi processi di deposizione e il rame. Poiché questi tre materiali favoriscono egualmente deposizione ed adesione del cromo nero, è da preferire il materiale che garantisce la minore emittanza, ovvero il nichel ottenuto con il processo di deposizione di Watts (ε300 °C≈ 0.4). Si è mostrato inoltre, facendo chiarezza rispetto a quanto riportato in letteratura, come gli spessori dei substrati non influenzino le caratteristiche ottiche. Quindi al fine di contenere i costi di produzione è da preferire il minore spessore che garantisca una buona adesione del substrato e questo è stato individuato in uno spessore di 2 µm. Si sono poi trattate le caratteristiche ottiche dell'assorbitore, ponendo particolare attenzione al contesto in cui questo verrà utilizzato: infatti il parametro di selettività, comunemente utilizzato in letteratura per il confronto degli assorbitori solari selettivi, non fornisce indicazioni valide sul comportamento dell'assorbitore nell'impianto solare. Si è quindi introdotto il parametro di efficienza η che tiene conto delle condizioni in cui verrà impiegato l'assorbitore. Ipotizzando un plausibile caso di lavoro con temperatura di esercizio 300 °C e rapporto di concentrazione di 40, si è mostrato come sia necessario cercare di massimizzare l'assorbanza del materiale assorbitore al fine di ottimizzare l'efficienza, piuttosto che limitarne l'emittanza. L'analisi dei parametri di deposizione che ha portato a determinare l'insieme di condizioni da cui deriva la migliore efficienza ha mostrato la fondamentale importanza della composizione chimica del bagno galvanico: infatti, oltre alla presenza del costituente principale, ovvero il Cr+3, si è verificato il contributo determinante apportato dai ``catalizzatori''. Questi facilitano il trasporto dello ione principale in soluzione e la sua deposizione al catodo, limitando al contempo le reazioni collaterali. In questo modo si riesce ad ottenere il cromo nero con un miglioramento di η del 5-8 % ed a densità di corrente molto inferiori rispetto al caso in cui i catalizzatori non sono presenti. Densità di corrente e temperatura del bagno galvanico sono i parametri principali su cui operare. I migliori risultati sono stati ottenuti a 20 °C con una densità di corrente di 60 A dm-2. Il tempo della deposizione è molto importante: infatti, dagli studi condotti, il tempo ottimale di deposizione è 1 minuto, poiché sia per tempi minori che maggiori si ha un peggioramento dell'efficienza. Dall'analisi della superficie si è visto che il cromo nero è uno strato di materia soffice, non compatta, con aspetto estremamente frastagliato e composta da globuli di piccole dimensioni costituiti da un nucleo di cromo metallico circondato da uno strato di ossidi e idrossidi di cromo. L'aspetto della superficie influenza le caratteristiche ottiche del materiale: infatti esiste una correlazione tra la rugosità superficiale e α/εT secondo cui all'aumentare di Rz si ha una perdita di selettività. Numerose e importanti informazioni si sono ottenute dalla valutazione degli effetti provocati dai trattamenti termici: infatti per un assorbitore solare è di fondamentale importanza conoscere le caratteristiche ottiche alla temperatura di funzionamento. Il cromo nero analizzato è caratterizzato da un miglioramento della selettività dopo essere stato esposto alle alte temperature (300 °C e 400 °C) soprattutto nei casi in cui il substrato sia Ni Watts. Si è potuto valutare anche che il rame non è un buon substrato per applicazioni che possano trovarsi a temperature superiori ai 300 °C a causa della sua facile interdiffusione con altri metalli. L'esposizione dell'assorbitore alla temperatura di esercizio si comporta inoltre come una sorta di livellante nei confronti di η il cui valore medio si attesta a circa 0.8. Infatti il trattamento termico a 300 °C provoca un miglioramento delle efficienze degli assorbitori che inizialmente possedevano delle η abbastanza basse e un cambiamento esattamente opposto per gli assorbitori che appena deposti presentavano le migliori efficienze. La presenza dello ione fluoruro nella composizione del bagno galvanico comporta invece una minore resistenza dello strato assorbitore nei confronti della temperatura. Alla luce di queste considerazioni la composizione ottimale del bagno galvanico individuata è costituita da CrCl3·6H2O 266 g l-1, H2SiF6 10 g l-1, NaH2PO4 4 g l-1 e CoCl2 ·6H2O 15 g l-1 . Infine, nonostante in precedenza si sia individuato il miglior substrato in base alle sue caratteristiche ottiche, si è visto sperimentalmente come questo parametro non si rifletta in maniera determinante sull'efficienza finale. Infatti, come già detto, è importante massimizzare l'assorbimento piuttosto che limitare l'emissione dell'assorbitore. Per questo motivo e considerate le prove effettuate, si può affermare che i substrati considerati sono tra loro equivalenti. Alla luce di ciò il substrato più adatto è il nichel ottenuto con il metodo di Wood, poiché è quello che necessita di minor lavorazione e non presenta le limitazioni riguardo alle temperature di utilizzo viste per il rame. Il miglior campione ottenuto, che rispetta le condizioni appena elencate, presenta una efficienza di conversione energetica η=0.88: questo valore non è molto lontano, e talvolta migliore, delle efficienze dei ben più costosi CERMET (η=0.85-0.93), oltre ad essere migliore delle efficienze dei campioni ottenuti da cromo esavalente. ***************** The use of a low-intensity source like sunlight, for energy generation requires an efficient system to concentrate and capture radiation and to transfer the energy to the exchange fluid. Sunlight is abundant, renewable and free of charge. Therefore the development and diffusion of solar energy exploitation is a key issue for the future. However, at present solar energy technologies are generally affected by a not high enough efficiency and a high cost, making them not fully competitive yet over conventional fossil fuels. Thus, it is clear that both increasing the efficiency and reducing the cost is mandatory to promote solar energy exploitation. Systems operating at mid-temperatures (i.e. using fluids at about 200-300 °C) and in particular parabolic trough collectors (PTCs) offer several advantages in comparison with conventional flat plates thanks to their higher efficiency and reduced receiver surface. In these systems the incident solar radiation is converted into heat either by sunlight absorption by blackened or specially developed absorbing surfaces that collect the solar energy and transfer it to the fluid. Required characteristics for the absorber surface are chemical and physical stability at the operating temperatures, as well as good performances in terms of energy efficiency. Moreover a production process characterized by a low cost and a high repeatability should promote a large scale diffusion. Several direct industrial applications, like Direct Steam Generation (DSC) and Solar Heating and Cooling (SHC), could exploit mid-temperature solar energy as energy source. This interest drives the research of novel technologies focused on this market sector where the technologies developed for systems operating at higher temperatures (e.g. CSP plants) cannot be used. Electrodeposition techniques are a promising route to obtain surfaces with tailored optical characteristics. Black chrome coatings have excellent optical properties, as they are strongly absorbing in the sunlight spectral region, with a high absorbance α ≈ 0.90-0.92 and a low thermal emittance ε ≈ 0.10-0.15. Moreover they remain stable up to 300 °C. However, a relevant drawback correlated to chrome electrodeposition is represented by pollution derived from Cr6+ ions. Because of that, the technological development of these processes underwent a sharp slowdown since '90. Only with the advent of new studies about Cr3+ baths, since the beginning of 2000's, the electrodeposition processes have found new interest in mass production of components for thermal solar plants. To obtain a good coating by black chrome, a preliminary deposition of a nickel layer on the substrate is needed to ensure a better chrome adherence to the surface and an improved wear and corrosion resistance. Moreover this creates an ``absorber/reflector tandem'' having both the high solar absorbance of the black exterior deposit and the low thermal emittance of the metallic inner coating. The first step of this study was the investigation of structural features and optical properties of the nickel and copper surfaces, correlating them to coating thickness and deposition process, in the perspective to assess optimal conditions for solar absorber applications. The second step of this study was the investigation of structural features and optical properties of the black chrome absorber taking into account several bath's operational parameters. This black chrome was obtained by a solution of Cr+3. In order to compare the performance obtained by the materials in a working configuration has been paid attention to a parameter that can provide some information: this parameter is the efficiency η that take into account the working temperature and the concentration ratio. Moreover has been done several thermal aging cycle on the materials in order to predict the effect of the aging on the optical properties. The optimal set-up that has been found is: for a galvanic bath composition CrCl3·6H2O 266 g l-1, H2SiF6 10 g l-1, NaH2PO4 4 g l-1 and CoCl2 ·6H2O 15 g l-1; for the operational parameters 20 °C and current density of 60 A~dm-2. With this set-up the best result is a sample with η=0.88: this value is rather similar to the efficiency of the more expensive CERMET (η=0.85-0.93).

One of the most widely used solar collectors for process heating and large scale electricity generation is the parabolic trough collector (PTC), in which a tube placed at the focus of the parabola receives concentrated solar radiation. In this work, a novel solar receiver design is proposed that bridges the gap in efficiency between the evacuated and non-evacuated receivers which are presently in use. In the standard or commercial non-evacuated receivers, the absorbing surface loses the heat to the surrounding ambient and to the heat transfer fluid (useful heat). A novel receiver has been proposed here, in which the absorbing surfaces (metal inserts) are immersed in the heat transfer fluid which is flowing through the inner tube of the receiver. The proposed design reduces the heat loss to the surrounding ambient. Experiments were conducted using water and air as heat transfer fluid (HTF), to compare the performance of the novel receiver with the standard receiver. Single pass experiments using water as HTF did not produce high fluid temperatures. In order to achieve higher fluid temperatures, experiments with recirculation of water were performed. The difference in the thermal performance of the novel receiver and the standard receiver became conspicuous as the losses became predominant. Also, it was observed that the thermal performance of the novel receiver over the standard receiver improved with an increase in the outlet temperature. Experiments using air as heat transfer fluid showed that the novel receiver outperformed the standard receiver in thermal performance. Also, the time response to changes in solar radiation was much lower for novel receiver as compared to standard receiver. Numerical simulations were performed using a one dimensional steady state heat transfer model for both these receivers. These results also indicate that the thermal performance of the novel receiver is superior to the standard receiver. Some interesting observations with regard to the influence of the heat transfer coefficient and incoming solar radiation on energy gain and loss have been noted and will be presented.