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DESIGN OF METHOD FOR CONTROLLING TEMPERATUREIN A CENTRAL SOLAR THERMAL RECEIVERClaudia CammarataBachelor of Engineering in Mechanical EngineeringDepartment of Mechanical EngineeringMacquarie UniversitySeptember 4, 2017Supervisor: Senior Lecturer Dr. Yijiao JiangACKNOWLEDGMENTSI would like to acknowledge my academic supervisor Senior Lecturer Dr. YijiaoJiang for her guidance and sharing of insightful background knowledge relatedto this thesis project. I also acknowledge my co-supervisor Research Fellow Dr.Sicong Tian for his encouragement and willingness to exchange ideas.STATEMENT OF CANDIDATEI, Claudia Cammarata, declare that this report, submitted as part of the requirement for the award of Bachelor of Engineering in the Department of MechanicalEngineering, Macquarie University, is entirely my own work unless otherwise referenced or acknowledged. This document has not been submitted for qualificationor assessment at any other academic institution.Student’s Name: Claudia CammarataStudent’s Signature:Date: 04/09/2017ABSTRACTA method for controlling the temperature in a central cavity solar receiver isdescribed in this report. Reasons why temperature control is important are discussed, along with previous solutions in a summary of available literature. Basedon a variable aperture size method, a unique solution is proposed which utilisesthe circle-like properties of a squircle in order to simplify the size adjustmentmechanism whilst maintaining an aperture profile suitable in terms of opticsand heat transfer. The solution is described and illustrated through the use of 3-dimensional computer models and prototypes. Issues with the preliminary designare discussed and further work to be carried out is outlined.ContentsAcknowledgments iiAbstract ivTable of Contents vList of Figures vii1 Introduction 11.1 Statement of Intent & Purpose . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Projected Outcomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Background 32.1 CST Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1.2 Central Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2 Summary of Methods . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Variable Aperture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.1 Reasons for Selection . . . . . . . . . . . . . . . . . . . . . . . . . . 82.3.2 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 The Design 113.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 Squircle Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.1 What is a squircle? . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2.2 Why is it relevant? . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3 Motion Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 The Build 154.1 CAD Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Prototyping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19vCONTENTS vi5 Conclusions and Future Work 205.1 Preliminary Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 Abbreviations 22Bibliography 22List of Figures2.1 Parabolic trough collector with linear tube receiver [6] . . . . . . . . . . . . 42.2 Parabolic dish collectors with central receivers [1] . . . . . . . . . . . . . . 42.3 External (left) and cavity (right) solar receiver diagrams [8] . . . . . . . . . 52.4 Simple cavity receiver schematic with aperture [9] . . . . . . . . . . . . . . 52.5 Ideal solar-to-fuel energy conversion efficiency to operating temperature TH(K) for a blackbody cavity receiver converting concentrated solar energyinto chemical energy. Toptimum is shown for maximum ηsolar-to-fuel;ideal [10] 62.6 Variable aperture mechanism using 2 sliding blades [9] . . . . . . . . . . . 92.7 Mechanism as the aperture is opened [9] . . . . . . . . . . . . . . . . . . . 92.8 Iris mechanisms with 8 blades: straight (left) and curved (right) blades [18] 102.9 8-blade iris mechanism shape design [18] . . . . . . . . . . . . . . . . . . . 102.10 Aperture area shape change as it closes [18] . . . . . . . . . . . . . . . . . 102.11 Replacing bar linkage with an SMA spring aperture mechanism [20] . . . . 103.1 Transition between a square and circle by varying s [23] . . . . . . . . . . . 123.2 Superimposed squircle profiles [24] . . . . . . . . . . . . . . . . . . . . . . . 123.3 Diffraction patterns of squircle s = 0.6 (left) and circle s = 0 (right) [22] . 133.4 Sketch of the cam used in the present work . . . . . . . . . . . . . . . . . . 143.5 Diagram of cam, follower and spring assembly [27] . . . . . . . . . . . . . . 143.6 CAD model of a needle roller bearing based on a video tutorial [28] . . . . 144.1 Fully assembled CAD models of Prototypes 1 and 2 . . . . . . . . . . . . . 154.2 Inner rings of Prototypes 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . 164.3 Prototypes 1 rings and rollers assembly . . . . . . . . . . . . . . . . . . . . 164.4 Cam and follower parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.5 Fully assembled without the top cap and guides . . . . . . . . . . . . . . . 174.6 Top cap with grooves for joining to the inner ring and guides for the followers 174.7 Section view of top cap with guides for followers . . . . . . . . . . . . . . . 174.8 Aperture mechanism with the top off in the fully open and fully closedpositions and with top on . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.9 3D printed prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.1 Schematic of planetary gear with four planets [29] . . . . . . . . . . . . . . 21viiChapter 1Introduction1.1 Statement of Intent & PurposeThe aim of undertaking this thesis topic is to develop a working design and prototypefor a method of controlling the temperature of solar receivers heated using concentratedsolar thermal (CST) technology. This is specifically as it pertains to solar thermochemicalconversion (STC), using concentrated solar radiation to drive endothermic chemical reactions. CST works by concentrating energy from the sun using mirrors or lenses focusedon either a central or linear receiver to heat up a heat transfer fluid (HTF), transferringits heat to where it can be used [1]. This renewable energy source can be used in manydifferent industries and for numerous applications. The focus of this project is on drivingimportant chemical reactions, for example H2O splitting to get H2 and O2 [2].In order to narrow the scope of the project, the relevant CST technology will be smallscale central solar receivers (having all solar energy directed to one receiver) using oneparabolic dish. The target of the work in the long term is local energy harvesting to drivechemical reactions in villages or small communities, rather than to generate electricity.It has been decided that the chosen method of temperature control is the creation of avariable aperture mechanism, altering the size of the window through which the solarradiation can enter the receiver.This work is valuable as the energy harvested through the use of CST technology canbe used more effectively through the implementation of temperature control. This is dueto improvements in efficiency of energy conversion and the ability to facilitate chemicalreactions that rely on specific temperature ranges to work [3]. In the solar fuels domain,resources such as natural gas and CO2 that are not considered green” can be used toform fuels. With CO2 conversion, not only is CO2 not being produced through burningfossil fuels, it is also being taken out of the atmosphere, a double benefit [2].In summary, a temperature control mechanism designed through undertaking thisproject would be addressing numerous limitations of current CST technology.11.2 Projected Outcomes 21.2 Projected OutcomesAs of the time of submission of this progress report, 2 preliminary prototypes have beendesigned and created through the use of Computer-Aided Design (CAD), specifically CreoParametric 3.0, and 3D printing using the OMNI3D Factory 2.0 Production System atMacquarie University.The prototypes are non-functional but promising with further development and designchanges going forward. It is projected that by the final thesis submission, a workingdesign and prototype that is more manufacturable and practical for the application willbe developed.Chapter 2Background2.1 CST Technology2.1.1 OverviewCST technology utilises energy from the sun in the form of heat, hence thermal” being inthe name. This is accomplished through the concentration of the sun’s rays by mirrors orlenses (collectors) onto either a linear or point target (receiver) [4]. The receiver, whetherit be a tube as with parabolic trough technology (see figure 2.1) or a single unit as withparabolic dish types (figure 2.2), is where the heat is directed and carried by the HTF.The heat can then be used to generate electricity, drive chemical reactions etc.The focus of this thesis is on designing a temperature control mechanism for use withcentral receivers (point target) such as those used in parabolic dish or central tower CSTtypes. These can typically reach higher temperatures than linear receiver technologiesdue to a large concentration of sunlight being directed onto a small receiver [5].2.1.2 Central ReceiverIn order to understand how temperature in the receiver can be controlled, one must firstunderstand the basics of solar receiver design. Central refers to the position of the receiverrelative to the concentrators, having the solar flux directed to a point focus. The two maincategories of central receivers are cavity and external [7]. The difference between them isthe location of the heat absorbing material. As the names suggest, an external receiverhas the absorbing material on the exterior of the receiver, whilst it is within a cavity fora cavity receiver.Large surface area is a major cause of heat loss for external receivers, making reducingthe surface area desirable [7]. However, absorbers have maximum operating temperaturesand therefore the HTF must be able to carry away the heat faster (higher heat capacity)if the surface area is reduced. Cavity receivers do not suffer from as much convective and32.1 CST Technology 4Figure 2.1: Parabolic trough collector with linear tube receiver [6]Figure 2.2: Parabolic dish collectors with central receivers [1]radiative heat loss since the absorber lies within a cavity. However, its main drawback isthat the solar radiation in contact with the absorber is limited by the acceptance angleof the cavity aperture (60-120◦ [8]), whereas external absorbers can be targeted from anyangle. Since this work is based on a small-scale unit with a single collector, a cavity typewith one aperture is the receiver on which the temperature control method is based.There are multiple types of cavity receivers, for example; tube, volumetric and solidparticle. However, describing them is beyond the scope of this report since the internalworkings of the receiver do not affect the design of the external aperture mechanism.2.2 Temperature Control 5Figure 2.3: External (left) and cavity (right) solar receiver diagrams [8]Figure 2.4: Simple cavity receiver schematic with aperture [9]2.2 Temperature Control2.2.1 PurposeIn the context of STC, controlling the temperature in a solar receiver could lead to higherefficiency in converting solar to chemical energy. Applying basic thermodynamic principles, one might think having the highest maximum temperature (TH) is best to promoteheat transfer to the absorber due to increased temperature difference. However, a higherTH also leads to increased heat loss due to re-radiation [2]. There is a Toptimum at whichthese conflicting factors balance to achieve the highest energy transfer efficiency, whichvaries based on conditions. Being able to change the temperature to consistently achievethe highest possible efficiency would be a great asset to any solar thermochemical process.If a constant temperature in the receiver is desired, the transient nature of solar energyavailability due to time of day, weather and so forth is a barrier to achieving this [7]. Thusconstant temperature despite external factors can only be achieved through some means2.2 Temperature Control 6Figure 2.5: Ideal solar-to-fuel energy conversion efficiency to operating temperature TH(K) for a blackbody cavity receiver converting concentrated solar energy into chemicalenergy. Toptimum is shown for maximum ηsolar-to-fuel;ideal [10]of bypassing these conditions. One main reason why this would be desired is that manychemical reactions can only occur effectively under specific temperatures. Further, a singleprocess can have multiple steps that require different temperatures, for example splittingCO2 into CO and O2 [11]. This means that effective control of temperature can makesuch reactions run more effectively and allow for more complex processes.2.2.2 Summary of MethodsIn order to demonstrate how the decision to base the temperature control method on avariable aperture mechanism came about, a brief summary of the main tested techniquesis provided here.Flow RateThe most widely used method of compensating for solar fluctuations, allowing for temperature to be controlled, is the adjustment of feed gas flow rate [12]. Feed gas is the gascarrying the unreacted chemicals (together or separated) before any conversion processes2.2 Temperature Control 7have occurred. In a direct catalytic absorption receiver (DCAR), which has been used inCO2 reforming of methane (CH4), receiver temperature and energy loss are dependent onavailable solar radiation as well as feed gas flow rate [13]. Since the feed gas carries heat,by increasing its flow rate, more heat is brought to the next stage of the reaction processover the same time period.In the Catalytically Enhanced Solar Absorption Receiver (CAESAR) project, it wasproposed that tailoring of the distribution of feed gas flow may have led to the observedincrease in absorber performance [13]. Flow controllers have also been used with heatpipe receivers as well [14]. The feed flow rate could be kept constant or controlled automatically depending on solar flux variations to control the temperature [14].Collector PositionAnother method is manipulating the position of the collectors. With solar tower CSTplants, a single tall tower holding a receiver is surrounded by a field of heliostats (mirrorswith two axis tracking) that concentrate solar energy onto the receiver [1]. Since they collect the solar energy, they have the greatest controllable impact on the heat at the receiver.By changing the position of a heliostat, one can alter where the solar energy it collects is being directed, or have it not collect any energy at all (facing downwards). Inthis way, altering the positions of the heliostats can control the temperature at the receiver by compensating for any discrepancies between a chosen reference and a measuredtemperature [3], by directing more or less light in its direction.Aperture SizeAs incident solar radiation into the receiver increases, so does the receiver’s temperature.The aperture is the window through which solar radiation enters the receiver. Thus, ithas been reported that both aperture size manipulation and covering have been used astemperature control methods [12,14,15] by allowing more or less light to enter. Aperturediameter has an optimum size which depends on the balance between solar exposure andre-radiation losses [15]. Thus being in control of the opening size can assist in achievingthis balance by adapting to the conditions. It would also bring about the benefits oftemperature control as stated in 2.2.1.A reduction in temperature can be achieved by blocking the light entering the aperturewith some sort of shutter or door of an external housing [14, 15]. Aperture size can alsobe more directly manipulated through a variable aperture mechanism [12] which variesthe size based on the difference between temperature readings and a desired temperature.This was developed in order to reduce the impact of solar radiation fluctuations and allowed for more stable receiver operation than with an uncontrolled system [12].2.3 Variable Aperture 82.3 Variable Aperture2.3.1 Reasons for SelectionFrom the options outlined in section 2.2.2, aperture size variation was chosen as themethod to be utilised in the design of a temperature control method for a solar receiver,as per the aims of this thesis. The reasons for the selection will be discussed here.Multiple factors were considered during the selection process. These include appropriateness of the solution to the defined problem, efficacy of the technique and room forfurther development. Starting with the first consideration, the solution is to be based onthe following assumptions:• The solar receiver is a central cavity type• The system is small-scale, having one collector (parabolic dish)• Precise temperature control is desired in the receiverThese assumptions immediately eliminate the collector position option. This is because it is based on the control of the position of heliostats in a solar field, having onereceiver atop a tower surrounded by many solar collectors. Though one could argue that asingle collector could be controlled in the same way, it would be difficult to achieve precisetemperature control as the single aperture is dependent on the single collector moving itsfocus in minuscule increments. The dish would have to track the sun and have some sortof offset from the ideal position if the temperature was required to be lower.Next, considering efficacy in achieving precise temperature control, flow rate manipulation and aperture size adjustment were compared. As stated in section 2.2.2, feed gasflow rate manipulation has been the most widely used technique. It has been demonstrated that this method can help achieve greater stability in receiver temperature thanthat of an uncontrolled system [13,14,16].However, in studies directly comparing aperture size adjustment to feed gas flow ratemanipulation for regulating temperature in a solar receiver, aperture control has beenfound to be the superior method [12, 17]. This is due to more precision in the controland less overshoot of the system (overcompensating for discrepancies between desired andmeasured temperatures). In the case of a 50% increase in solar radiation, the system controlled via feed gas flow rate manipulation was found to have 3 times greater overshootthan that controlled by aperture size adjustment [12]. Additionally, varying flow rates candisturb the flow dynamics in a receiver, causing problems for processes where constantflow patterns are required [18,19].The fact that aperture size adjustment has not been as widely used as a temperaturecontrol method as changing feed gas flow rate also made it more likely that the technique2.3 Variable Aperture 9could be developed further or a new mechanism could be designed. Finally, an exterioraperture mechanism does not affect the interior design of a receiver the way a flow variation method likely would, making it easier to isolate the problem from more complexconsiderations.2.3.2 Previous WorkEarly work involving adjusting the amount light able to enter the receiver through theaperture involved indirect control. One example is the input solar energy being varied viamechanically opening and closing the doors of the building that housed the concentrating mirror [14]. In another similar case, the energy in was controlled using Venetian-likeshutters [15]. Both of these studies involved receivers with a fixed aperture size so aredissimilar to the work of this thesis, but paved the way for the development of more directaperture-based temperature control solutions.More recently, the focus has been on being able to manipulate the amount of solarenergy able to enter the receiver by changing the size of the aperture itself. A 2-bladedmechanism was proposed, but due to the non-circular nature of the shape as the aperturewas closed, the mechanism sacrificed a lot of available solar energy [9]. 8-blade mechanismshave also been proposed since having more blades creates a more circular shape. However,the profile still becomes more sharp and octagonal as the aperture closes rather thanbeing a perfect circle [18, 20]. Novel solutions have tried to incorporate shape memoryalloy (SMA) springs instead of typical bar-linkages [20]. See images below for clarificationand visualisation.Figure 2.6: Variable aperture mechanism using 2 sliding blades [9]Figure 2.7: Mechanism as the aperture is opened [9]2.3 Variable Aperture 10Figure 2.8: Iris mechanisms with 8 blades: straight (left) and curved (right) blades [18]Figure 2.9: 8-blade iris mechanism shape design [18]Figure 2.10: Aperture area shape change as it closes [18]Figure 2.11: Replacing bar linkage with an SMA spring aperture mechanism [20]Chapter 3The Design3.1 IntroductionThe design process that led to the development of the prototype as discussed in Chapter4 is described here. The design is based on an aperture with a circle/squircle profile and amotion mechanism to open and close the aperture using an adapted roller bearing, camsand followers.3.2 Squircle Profile3.2.1 What is a squircle?As the name implies, a squircle is a shape between a square and circle. It differs froma square with rounded corners in that there are no straight edges. Transitioning from asquare to a circle, the corners become 4 mirrored parabolas. To clarify, the shape of eachcorner” is governed by a varying parabolic equation, rather than a circular arc. Theseparabolas together form the squircle. See figure 3.2 for a visual representation of thistransition between square and circle. A squircle’s shape is governed by the equation: x2 + y2 –k2 x2y2 = k2(3.1) s2where x and y are the coordinates along the horizontal and vertical axes on the Cartesianplane respectively, s is the squareness parameter and k is the radius or half length of thecorresponding shape [21,22]. For s = 0, the shape is a circle of radius k. For s = 1, theshape becomes a square of length 2k. Any value of s between 0 and 1 yields a squircle.3.2.2 Why is it relevant?While attempts have been made to have an increasingly circular aperture for reasonsmentioned in section 2.3.2 by adding more blades to an iris mechanism, there is littlein the literature about considering another shape entirely. In order to come up with a113.2 Squircle Profile 12Figure 3.1: Transition between a square and circle by varying s [23]Figure 3.2: Superimposed squircle profiles [24]unique solution, the idea of trying to improve the multi-blade camera-like aperture wasabandoned. Instead, research was conducted to find a shape that would behave like acircle in terms of the behaviour of light as it passes through the aperture.Reiterating from section 2.3.2, a circular aperture is desired as it allows more radiationinto the receiver than other shapes of the same area due to optical and heat transfer factors [9,18]. In photography, if an aperture is non-circular and has edges, diffraction willoccur which causes spikes” of light radiating outwards from the centre of the image andoverall blurring [25]. This just means that the light is being spread out. In the case of a solar receiver, this would cause some of the available light and thus heat, to not be captured.Where the squircle comes in, is that it has been documented that moving from a squareto a circle (s = 1 to s = 0), there is a point at which the diffraction pattern through anaperture becomes insensitive to changes in s [22]. Through both numerical [22] and experimental [26] means, this occurs somewhere between s = 0.8 and s = 0.6. It has beenshown that small departures from a circular shape have little effect on diffraction throughan aperture and that a squircle can mimic the properties of a circle very closely [22,26].See figure 3.3 for a comparison between the diffraction patterns of a squircle of s = 0.6and a circle.Instead of getting sharp edges as the aperture closes, with a squircle profile, one wouldhave a squircle of varying radius and s across the transition from open to closed for mostof the cycle (see Chapter 4 for a demonstration of this).3.3 Motion Mechanism 13Figure 3.3: Diffraction patterns of squircle s = 0.6 (left) and circle s = 0 (right) [22]3.3 Motion MechanismUsing a squircle as the aperture shape, a mechanism is needed in order to make the aperture size variable. It was decided that 4 pieces (followers) each with one quarter of an s =0.8 squircle profile would be used (see Chapter 4). They would be translated in 4 differentdirections, along 2 axes. Controlling the motion of these followers is achieved throughthe use of cams. As can be seen in figure 3.4 that since the axis of rotation is not in thecentre, as the cam is rotated, the follower is translated along its axis by a varying amount.Figure 3.5 demonstrates how a cam changes the position of a follower as it rotates, witha guide being used to keep the motion along one axis.The proposed solution involved the cams being fixed to rollers in a roller bearing sothat as the outer ring was rotated, the rollers would also rotate with the cams, pushingthe followers in an out using springs. See 3.6 for the basic bearing before any adjustmentswere made and Chapter 4 for the final design.3.4 SummaryIn summary, though the squircle has been studied in relation to its similarities to a circlein terms of diffraction, nothing has been found in the literature linking the squircle to avariable aperture mechanism or solar technology. Therefore this thesis project is exploringa new concept entirely for controlling temperature in a solar receiver.3.4 Summary 14Figure 3.4: Sketch of the cam used in the present workFigure 3.5: Diagram of cam, follower and spring assembly [27]Figure 3.6: CAD model of a needle roller bearing based on a video tutorial [28]Chapter 4The Build4.1 CAD ModellingCAD models of the designed solutions (Prototypes 1 and 2) are shown here. Where onlyone Prototype is shown, it is Prototype 2 as this was more successful (removed unnecessaryfeatures and made more sturdy).(a) Prototype 1 (b) Prototype 2Figure 4.1: Fully assembled CAD models of Prototypes 1 and 2154.1 CAD Modelling 16(a) Prototype 1 (b) Prototype 2Figure 4.2: Inner rings of Prototypes 1 and 2(a) Inner ring withrollers(b) Inner ring, outerring and rollers assembledFigure 4.3: Prototypes 1 rings and rollers assembly(a) Cam(b) FollowerFigure 4.4: Cam and follower parts4.1 CAD Modelling 17Figure 4.5: Fully assembled without the top cap and guides(a) Top cap(b) Top cap with followers inguidesFigure 4.6: Top cap with grooves for joining to the inner ring and guides for the followersFigure 4.7: Section view of top cap with guides for followers4.1 CAD Modelling 18(a) Fully open (b) Fully closed (c) Fully closed with topFigure 4.8: Aperture mechanism with the top off in the fully open and fully closedpositions and with top on4.2 Prototyping 194.2 PrototypingPrototypes based on the above CAD models were made by 3D printing using the OMNI3DFactory 2.0 Production System. For Prototype 1, it is not fully assembled due to printererrors causing the top cap part to fail and rollers not being able to fit on the rods of theinner ring. Prototype 2 is fully assembled but is non-functional due to rolling slip (rodsslide along outer ring rather than rotate).(a) Prototype 1(b) Prototype 2Figure 4.9: 3D printed prototypesChapter 5Conclusions and Future Work5.1 Preliminary ConclusionsBased on the results of prototyping the designed variable aperture mechanism, it appearsthat the use of a squircle as the profile was successful. It allowed for a smooth transitionbetween open and closed without edges that would cause unwanted diffraction of enteringlight, within certain boundaries.The motion mechanism proposed was unsuccessful in this preliminary prototype. Therollers did not rotate as the outer ring of the bearing was rotated, meaning that thefollowers were not translated as they should be. This could be due to 3D printing error, incorrect tolerances between parts, inappropriate material or a solution that needsrethinking.5.2 Future WorkIt is proposed that to overcome the issue of non-rotating cams, the roller bearing conceptshould be abandoned. There is the potential for slip and this would create an inaccurateadjustment system. In future work, a solution based on a planetary gear system with thecams fixed to the 4 planets (see figure 5.1) is to be designed. Due to the meshing of theteeth, precise control of the rotation of the cams and thus the translation of the followers,could be achieved.Attempts to have a more easily manufacturable design will also be made so that themechanism is more practical for moving into real use.205.2 Future Work 21Figure 5.1: Schematic of planetary gear with four planets [29]Chapter 6Abbreviations heyCADCAESARbrahComputer-Aided DesignCatalytically Enhanced Solar Absorption ReceiverCSTConcentrated Solar ThermalDCARDirect Catalytic Absorption ReceiverHTFHeat Transfer FluidSMAShape Memory AlloySTCSolar Thermochemical Conversion 22Bibliography[1] Australian Solar Institute. Realising the potential of Concentrating Solar Power inAustralia, 2012.[2] M. Romero and A. Steinfeld. Concentrating solar thermal power and thermochemicalfuels. Energy Environ. Sci., 5:9234{9245, 2012.[3] B. A. Costa and J. M. Lemos. Temperature control of a solar tower receiver basedon the lyapunov method. 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