A concrete solar collector – From design to assembly in ... · A concrete solar collector – From design to assembly VGB PowerTech 9 l 2018 A concrete solar collector – From

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A concrete solar collector – From design to assembly VGB PowerTech 9 l 2018

A concrete solar collector – From design to assembly

Authors

Kurzfassung

Entwurf und Errichtung eines Parabolrinnenkollektors aus Beton im Originalmaßstab

Erneuerbare Energieformen werden infolge des Rückgangs fossiler Brennmaterialien und der Abschaltung von Kernkraftwerken immer wich-tiger. Eine etablierte Technologie solarthermi-scher Kraftwerke sind Parabolrinnenkraftwer-ke, welche die solare Strahlung linienartig fo-kussieren. Diese Rinnen bestehen zumeist aus filigranen Stahlfachwerken mit punktweise aufgelagerten, gekrümmten Spiegelelementen. Im Projekt ConSol („Concrete Solar Collector“) wurde ein Solarkollektor als Schale aus hoch-festem Beton entwickelt und errichtet, der diese Trennung zwischen Trag- und Reflektorstruk-tur aufhebt. Dieser besteht aus einer dünnen Betonschale und einer sichelförmigen Auflager-konstruktion mit neuartigen Betonzahnrädern für die Nachführung mithilfe eines speziell ent-wickelten Antriebssystems mit Kettenverbin-dungen. Vom Entwurf unter Berücksichtigung auslenkungsabhängiger Beanspruchungen bis hin zur Montage des Kollektors bestehend aus verschiedenen Betonfertigteilen, Einbauteilen und einem verbesserten PVD-beschichteten Spiegelmaterials auf elektrochemisch polierten Aluminiumbech wurde der so ganzheitlich opti-mierte Betonkollektor entwickelt. Mithilfe von digitaler Nahbereichsphotogrammmetrie wur-de der Kollektor optisch bewertet und die Leis-tungsfähigkeit sowie Kosten wurden mit etab-lierten Kollektoren verglichen. l

Dr.-Ing. Patrick FormanProf. Dr.-Ing. habil. Peter MarkRuhr University Bochum Bochum, GermanyDipl.-Ing. Sebastian PenkertProf. Dr.-Ing. Jürgen SchnellTechnische Universität Kaiserslautern Kaiserslautern, GermanyDipl.-Ing. Dirk KrügerDipl.-Ing. Klaus HenneckeGerman Aerospace Center Cologne, GermanyDr. Joachim KrügerSolarlite CSP Technology GmbH Duckwitz, GermanyDr. Reinhard DasbachALMECO GmbH Bernburg, Germany

A concrete solar collector – From design to assembly in full scalePatrick Forman, Sebastian Penkert, Dirk Krüger, Peter Mark, Jürgen Schnell, Klaus Hennecke, Joachim Krüger and Reinhard Dasbach

Introduction

Concentrating solar power (CSP) genera-tion makes use of direct solar radiation by focusing it via mirrors on a focal line (par-abolic trough, Fresnel) or a focal point (so-lar towers), heating a heat transfer medi-um which is used in a power block to gen-erate electricity. In most of the power plants, especially recently constructed ones, a heat storage is included so that electricity can be supplied for several hours after sunset. The thermal storage is charged by surplus heat from the solar field during sunny hours of the day and is later discharged to operate a turbine at the desired time adapted to the demand. Ad-ditional fuel back-up firing can further ex-tend the flexibility of such power plants.Today the operational capacity of CSP is around 5 GW (F i g u r e 1 ) worldwide. Thus it is a technology still at the beginning of the learning curve compared to other technologies as e.g. PV with 304 GW. Para-bolic trough collectors have widely been used in CSP plants and nowadays their effi-ciency is quite far developed leaving few room to optimize their thermal output. Therefore the main focus is now on invest-ment cost reduction and on lowering non-technical costs.A recent example of significant cost reduc-tion was presented in Sep. 2017 in Dubai, where a consortium was the preferred bid-der for the 700 MW combined tower and trough station DEWA IV, at a PPA price of USD 0.073 per kWh (AED 0.27 per kWh)

[1, 2]. This includes a thermal storage al-lowing turbine operation for 10 hours. Due to a long PPA duration of 35 years, very good financing conditions, production guarantee conditions, it is found that the technology costs are low [3].Clearly the reduction of investment costs is one important way to lower the Levelized Costs of Electricity (LCOE). The ConSol project therefore aimed at reducing the in-vestment costs with the idea of replacing steel structures by concrete components.

Conceptual design of a concrete collector

The conceptual design of the concrete col-lector is leaned on the geometrical charac-teristics of the already existing steel collec-tor EuroTrough [4, 5] with an aperture width of 5.77 m and a length of 12.00 m. The structural system and bearing condi-tions are adopted from a first small-scale concrete prototype (F i g u r e 2 ) with an aperture width of 2.205 m and a length of 3.20 m, which has been built up within a cooperative project between the TU Kai-serslautern and the Ruhr University Bo-chum [6, 7]. It shows the general feasibil-ity of line-like concentrating solar collec-tors made from concrete merging the supporting and reflecting (so directly sup-porting the mirrors) structure to a para-bolic shell [8, 9]. It exhibits a mean shell thickness of 2.5 cm. The design significant-ly depends on the concrete’s high tensile strength, as cracking would cause soften-

Fig. 1. Map of CSP plants worldwide (source: https://www.solarpaces.org/csp-technologies/csp-projects-around-the-world/).


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VGB PowerTech 9 l 2018 A concrete solar collector – From design to assembly

ing. The non-cracked shell allows only small deformations to fulfil the demands on accuracy for the solar concentration. Furthermore, the prototype is character-ized by a novel bearing concept which serves as the bearing structure and simul-taneously tracks the sun, while the centre of gravity stays on a horizontal line [10]. This means, that almost no mechanical work – just overcoming friction and geo-metric uncertainties – with respect to self-weight is needed to move the collector during the course of the day. In the longi-tudinal direction the system of the module can be interpreted as a single span girder with two cantilever arms.In the framework of the 6th Energy Re-search Programme of the Federal Govern-ment funded by the Federal Ministry for Economic Affairs and Energy, seven part-ners comprised of scientific research insti-tutions and industrial partners (Ta b l e 1 ) under the leadership of the German Aero-space Center (DLR) developed within the interdisciplinary project ConSol (“Con-crete Solar Collector”) a holistically opti-mized collector made from concrete in full scale. The collector concludes an optimized design of the shell, the supporting struc-ture, the driving system and mirroring. It scales up the prototype by a magnitude of

ial rocker bearings, two upper gears, two lower gears with a running surface and two middle sleeve foundations. Both modules share a drive bearing and two edge sleeve foundations. F i g u r e 3 illustrates the two shells with substructures in an exploded drawing.Due to manufacturing demands of the of the prototype that should be produced in simple formworks, a one-walled shell with a constant cross-section is chosen. Thus, the torsional stiffness compared to ordi-nary steel framework modules that com-prise an additional torque tube is pro-nounced smaller. Therefore, the driving system of the collector is designed to move only two modules, one per shell side, to minimize torsion by the drive. Considering longer collector lengths, several driving systems have to be installed and even more important synchronized to a simultaneous motion. The driving system is installed on a special drive bearing between two mod-ules. It is attached to sickles with a lower circular shape that are connected to the shell and enable the movement of the col-lector. Hence, the collector consists of two shells, which both can be interpreted as a one span girder of 8 m with a cantilever arm of 4 m in longitudinal direction. Al-most every structural part of the collector is built of precast elements connected to each other. Additionally, mounting parts with respect to the connections of precast elements and for external installations, e.g. the absorber tube’s support and the driving system’s pins, were necessary. Therefore, a 3-dimensional BIM (Building Information Modelling) model [11] (cf. F i g u r e 3 ) has

been developed to ensure an integrated de-sign, to tackle collision queries between the elements and to include all geometrical as well as material data and other mechan-ical properties.The components are made of two different types of concrete. For the sleeve founda-tions and rocker bearings serves a standard concrete C30/37. For elements with high demands on accuracy and/or bearing ca-pacity, a high-performance concrete based on the binder pre-mixture NANODUR [12] was necessary. The concrete exhibits a very dense microstructure and a high com-pressive strength as well as a high ten-sile strength. Furthermore, the concrete has a good workability due to high fluidi-ty and a self-compacting behaviour. The principal material properties are given in Ta b l e 2 .The design of parabolic trough collectors strongly depends on the specific inherent and environmental actions on the mod-ules. They mainly result from self-weight and wind loads that both differ with re-spect to the collector’s position during the course of the day. Thus, a lot of varying load situations occur due to the sun track-ing. Additionally and only for the here pre-sented prototype, a snow load was addi-tionally assumed, since the collector is as-sembled in Borchen, Germany. In typical locations of solar collectors, high tempera-tures and severe solar irradiation domi-nate making snow load assumptions un-necessary. The self-weight is applied pro-portional to the shell thickness and snow according to standard code regulations. The specific wind loads have been derived

Fig. 2. Small-scale prototype at TU Kaiserslautern.

Tab. 1. Project partners of ConSol.

German Aerospace Center (DLR), Cologne

Solarlite CSP Technology GmbH, Duckwitz

Pfeifer Seil- und Hebetechnik GmbH, Memmingen

Stanecker Betonfertigteilwerk GmbH, Borchen

ALMECO GmbH, Bernburg

Technical University of Kaiserslautern, Kaiserslautern

Ruhr University Bochum, Bochum

4. Moreover, it is fully equipped and work-able to be included in a power unit.

Structural and material design restrictionsThe ConSol prototype collector was built up of different precast concrete compo-nents to ensure a fast assembly on site. It is composed of two modules which share an engine in between. Each of the two mod-ules consists of a shell, two sickles, two ax-

Shell

Upper gear

Lower gear +Running surface

Drive bearingSickle

Edge sleeve foundation

Middle sleeve foundation

Axial rockerbearing

4.00 m 8.00 m 8.00 m 4.00 m

Fig. 3. 3D BIM model of the collector.

Tab. 2. Material properties of the used NANODUR concrete.

Description Value Unit

Young’s modulus Ecm 52,700 N/mm²

Compressive strength fcm 139.4 N/mm²

Bending tensile strength (average and characteristic values)

fctm,flfct,fl

18.215.5

N/mm²N/mm²

Tensile strength (average and characteristic values) fctmfct

9.38.0

N/mm²N/mm²

Density ρc 2,440 kg/m³


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from particular wind tunnel experiments [13]. Three different states with respect to the wind speed have to be considered:

– Operational state, where the collector tracks the sun under low wind speed conditions (v ≤ 10 m/s)

– Transition state, where the collector is moved to a safety position under upcom-ing moderate wind speed conditions (v ≤ 15 m/s)

– Survival state, where the collector holds in a constant survival position under strong wind speed (v ≤ 33 m/s)

In state (1) and (2) the collector’s orienta-tion is variable. The collector’s position of state (3) corresponds to the deflection with the lowest aerodynamic wind load coefficients being the zenith position with the opening facing vertically upwards. In operational state (1) the serviceability of the shell is ensured by means of accuracy demands to ensure full solar concentration in all positions. Thereby, the deformations of the shell are limited by means of the re-sulting waviness of the surface. The transi-tion (2) and the survival state (3) are the main states for the design in ultimate limit state (ULS), where its robustness is guar-anteed due to reinforcement. In addition, serviceability is provided by limiting the first principle stress to a reduced share of the characteristic tensile stress fct of the concrete to achieve a (computationally) non-cracked state.

Shell designThe design of the shell can be divided into an outer and inner form finding process aiming for a robust, low-weight structure but high stiffness. The outer form finding ensures a full solar concentration by di-mensioning the cross section of the shell restricted by accuracy and material de-mands. The inner form finding ensures ro-bustness by means of the reinforcement design. Therefore, the shell with outer sick-les was numerically built up as a parame-terized Finite-Element model consisting of 4-noded shell elements with 6 degrees of freedom per node. Due to the central sym-metry, only one module was modelled (F i g u r e 4 ). The outer sickles have a ra-dius of 2.10 m. The focal length – the dis-tance between the parabola vertex and the focal point where the receivers are ar-ranged – corresponds to 1.71 m defining the curvature of the parabola.The thickness of the shell was designed to only 4.5 cm in average. A maximum wavi-ness criterion of the surface serves as a re-striction for the design. This holds true, as a surface deviation causes twice as much solar ray deviations. The criterion is de-rived from empirical values of the Euro-Trough collector [14] and it is defined by the root mean square of the slope devia-tions SDx,rms – being the deviation between the slope values of the undeformed parab-

ola and the load induced distorted parabo-la – over the surface of the shell. It is limit-ed to 2.0 mrad. It should be noted, that it has to be fulfiled only in transverse direc-tion, since deviations in longitudinal direc-tion deflect the solar rays just along the absorber tube. This does not prevent rays to meet the tube. Therefore, the deforma-tion of the surface in operational state (F i g u r e 5 , top) and the resulting slope deviations SDx (F i g u r e 5 , bottom) for the dominating load situation were deter-mined. A rotation of 45 ° is assumed. F i g -u r e 5 shows the results of the accuracy analysis for the developed optimized de-sign with a decreasing shell thickness of 5.5 cm at the vertex to 3.5 cm at the edge of the cross section. It results into a root mean square of 2.08 mrad that seems acceptable for a first full scale prototype. Additionally, the tensile strength criterion is continu-ously maintained over the shell.For the reinforcement a single-layered, standard steel mesh is chosen for economic reasons. It was designed from a superposi-tion analysis of maximum and minimum sectional forces resulting from the load situations in transition and operational state. A mat Q257 is chosen that provides 2.57 cm2/m reinforcement area crosswise. A concrete cover of only 1 cm suffices

against environmental impacts due to the dense structure of the high-performance concrete. Additional reinforcement is re-quired at locally high stressed areas. These areas lie at the borders of the shell, espe-cially next to the middle bearing due to bending in longitudinal direction and at the end of the sickles. No shear reinforce-ment is required.The sickles below the shells enable the movement of the superstructure. They are connected to the shell by a two component adhesive and threaded rods. Therefore, special mounting parts of steel are installed into the shell. To avoid slipping or tipping while tracking the sun, a novel gear is in-stalled parallel to the sickle and vice versa on the axial rocker bearings. It is shaped with teeth and is made from high-perfor-mance concrete (cf. F i g u r e 9 ).

Substructure with rolling up conceptEvery shell is placed on two bearings that are held by sleeve foundations. Threaded rods and mortar grouting ensure a solid connection. Fluting between the single parts strengthen the composite. Two differ-ent types of foundations are fabricated. Two foundations of a first type are installed at the interconnection of the two shells in-cluding the drive bearing. The other type corresponds to the middle sleeve founda-tion which is indented from the edge of the shell (cf. F i g u r e 3 ). All sleeve founda-tions rest on block foundations with threaded rods to keep a gap of about 5 cm. This allows adjusting the substructure to a constant horizontal level, necessary for a precise solar tracking.The movement of the collector is described by an unrolling of the shell along the rocker bearings. Doing so, the centre of gravity of the superstructure remains on a constant horizontal level. Therefore, sickle and rocker bearing need corresponding geo-metrical shapes. The sickle exhibits a circu-lar shape, the rocker bearing the one of a shortened cycloid.F i g u r e 6 shows the kinematics of track-ing. The centre of gravity (red point) fol-lows a horizontal line. Moreover, it remains in a pure vertical offset to the contact point

Aperture width

Sickle

Receiver

ShellCenter ofgravity M

odule

leng

th

Shell

Sickles

5.77 m

4.0

m

8

.0 m

R =

2.10

m

1.71

m

Fig. 4. Finite-Element model of the parabolic trough‘s cross section (left) and discretised system (right) of one module of the collector

Aper

ture

wid

thin

m

Module length in m

Aper

ture

wid

thin

m

Module length in m

Deformation in mm

Slope deviation SDx in mrad

210-1-2

210-1-2

0 2 4 6 8 10 12

0 2 4 6 8 10 12

0

-2

-4

-6

-8

4

2

0

-2

Fig. 5. Numerically estimated deformations and corresponding slope deviations of the collector module’s surface.


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to the rocker bearing. Consequently, no tor-sional effects occur from self-weight and very economic engines can move the shells, despite its quite large dead load.

Power unit and motionsThe power unit consists of an electric en-gine of 180 W, chains, several rollers and interconnections to the drive bearing. A longitudinal shaft harmonises the tracking from shell to shell and avoids offsets. F i g -u r e 7 shows the components in an ex-ploded drawing. It should be noted that very robust components are chosen and the energy demand – compared to the pro-nounced self-weight of the two shells of about 20 tons – remains very limited.The tracking in operational slow speed mode works automatically with a high pre-cision of <0.1 ° [15] and a precise drive speed in the range of 10 °/h to 15 °/h. An angle sensor attached to one sickle meas-ures the actual deflection and in case of a deviation between the correct and actual position of more than 0.05 ° the system re-positions the shell. Ta b l e 3 summarizes the relevant technical data of the drive sys-tem.

Detailing and realisation of a large-scale prototype

The prototype collector composed of two modules was built in Borchen near Pader-born (Germany). It comprises eight differ-ent precast concrete components with vari-able quantities and two different types of concrete (Ta b l e 4 ). Each of the two mod-ules consists of a shell, two sickles, two ax-ial rocker bearings, two upper gears, two gears with a running surface and two mid-dle sleeve foundations and share a drive bearing as well as two edge sleeve founda-tions (cf. F i g u r e 3 ). The whole collector weights about 40 t, whereby the unrolling superstructure of one module has a weight of almost 10 t.The manufacturing process can be divided into two parts, namely the manufacturing of the substructure, mainly the bearings and foundations, and the fabrication of the shell superstructure.

Rocker bearings and foundationThe substructure consists of the block foundations, the rocker bearings with inte-grated concrete gears and the running sur-face as well as the bearing of the drive sys-tem. In order to manufacture the six sleeve

foundations, three wooden formworks (two for the middle foundations, one for the edge foundation) were needed (F i g -u r e 8 , (1)). All parts have a width of 1.40 m and lengths of 1.00 m at the middle foundations and 1.10 m at the edge founda-

tions. They are designed for a usual soil pressure of sands or gravel. The edge sleeve foundations exhibit three gaps of 17 cm for the rocker and the drive bearings, while just one gap for the axial rocker bearing suffices for the middle sleeve foundations. All gaps were lined with a riffle sheet to in-crease the roughness. F i g u r e 8 (2) shows the completed foundations with ver-

Shell

Contactpoint

Rocker bearing

SickleDeflectionα = 90o

α = 10o α = 45oα = 135o

α = 170oCenter of gravity

level

Fig. 6. Horizontal movement of the centre of gravity with an orthogonal contact point.

Shaft

Roller chain drive

Sickle

Drive bearing

Electricmotor

Fig. 7. 3D model of the drive engine.

Tab. 3. Technical data of the drive.


Driveway length 4.40 m

Total angular movement +80 zenith -80 °

Rotation speed (slow speed mode) 10 to 15 °/h

Rotation speed (fast speed mode) 7 °/min

Nominal driving force 20 kN

Maximum driving force 40 kN

Holding force 90 kN

Power consumption 0.6 W/max

Protection class IP65 -

Tab. 4. Overview of the produced prototype precast elements.

No. Name Quantity Concrete material Mass per element

[kg]

Total mass [t]

1 Edge sleeve foundation 2 C35/45 1,400 2.80

2 Middle sleeve foundation 4 C35/45 1,550 6.20

3 Lower gear with running surface 4 Nanodur® 750 3.00

4 Sickle 4 Nanodur® 560 2.24

5 Upper gear 4 Nanodur® 290 1.16

6 Axial rocker bearing 4 C35/45 1,480 5.92

7 Drive bearing 1 C35/45 2,050 2.05

8 Shell 2 Nanodur® 8,640 17.28

Sum 25 40.65

tical and horizontal cladding tubes of 60 mm in diameter for assembly. The axial rocker bearings were made of two different types of concrete (C35/45 and NANO-DUR) motivated by an economic use of the materials. The high-performance con-


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crete was only used for the high-loaded parts with specific accuracy demands such as the running surface and the gear of the axial rocker bearing. Here, increased ten-sile and compressive strengths as well as accurate surfaces and shapes are required. F i g u r e 8 (3) shows the formwork for the running surface and the gear. Due to the mainly curved surfaces of this element, a formwork of extruded polystyrene was used. It was cut by a 3D heating wire steered by a CAD model to achieve high precisions (<1 mm). For the connection to the rocker bearing made from normal con-crete, the running surface additionally pro-vides stirrup starter bars. Upon completion of the running surface and the gear, the component was used as a formwork ele-ment for the axial rocker bearing (F i g u r e 8 , (4)). Further parts of the formwork were made of wood and extruded polysty-rene (for the notches). Within the starter bars, a monolithic composite construction between the already concreted running surface and the axial rocker bearing could be realized. The completed rocker bearings have a length of 5.34 m, a height of 2.04 m and a thickness of 15 cm (F i g u r e 8 , (5)).The sickles and the upper gears were made from high-performance concrete solely. Sickle and gear have a width of 3.45 m and a thickness of 15 cm for the sickle and 8 cm for the gear, respectively. Sickle and upper gear were manufactured as single elements and connected afterwards. So, both parts had to be produced geometrically accu-rate at the connection faces. Therefore, special steel parts were manufactured and encased in the concrete to ensure a necessary tolerance of 1 mm. The sickle in-cluding running surface and the upper gear were built using extruded polystyrene formwork (F i g u r e 9 , (1)) The upper gear (F i g u r e 9 , (2)) was installed on top of the sickle and connected by thread-ed rods with a certain gap to avoid colli-sion queries between the gear of the rock-er bearing and the sickle and vice versa (F i g u r e 9 , (3)). A stainless steel sheet with a thickness of 1 mm was installed along the surface of the sickle to protect the concrete from abrasion and wear. Fi-

nally, the drive bearing was build. Except for two notches to save material costs, no polystyrene elements were needed and the formwork was completely made of wood (F i g u r e 9 , (4)). The component has a width of 4.77 m, a thickness of 12 cm and a maximum height of 2.76 m (F i g u r e 9 , (5)). Several transport anchors were placed for the subsequent installation of the drive system.

Shell superstructureThe superstructure consists of the shell and the outer sickles with connected con-crete gear segments. Sickles and gear seg-ments were manufactured along the sub-structure components. The primary de-mand on the shell was a precise surface, so the shell was cast with its inner surface downwards. Therefore, the concreting could be done from the backside of the shell, where no accuracy demands with re-spect to solar concentration have to be ful-filled. The formwork was made from ply-wood, while the downside part was cov-ered with steel sheets to ensure the precise surface (F i g u r e 10 , (1, 2)). Onto the

(1)

(2) (3)

(4)

(5)

Fig. 8. Formwork of the sleeve foundation (1), sleeve foundations (2), formwork of the running surface (3), formwork of the axial rocker bearing (4) and axial rocker bearing (5).

(1) (2)

(3)

(4)

(5)

Fig. 9. Formwork of the sickle (1), gear (2), sickle and gear (3), formwork of the drive bearing (4) and drive bearing (5).

(1) (2) (3)

(4) (5)

Fig. 10. Plywood formwork with steel sheet (1, 2), bending reinforcement (3), lifting of the shell (4) and finished shells with mounted sickles and concrete gear segments (5).

sheets, the reinforcement with spacers of 1 cm (F i g u r e 10 , (3)) and the additional mounting parts were placed. The shell was cast along the apex at the top of the form-work. After concreting and hardening, the counter formwork was removed and the shell lifted out of the bottom formwork. This was done over 8 mounting anchors, 4 in each axis of the sickles, by two indoor cranes (F i g u r e 10 , (4)). Then, the outer


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sickles with already attached concrete gears were connected to the shell by threat-ed bars and an additional layer of two-com-ponent adhesive to provide a full connec-tion between sickles and shell (F i g u r e 10 , (5)).

Assembly on site and performance evaluation of the collector

For assembly, all elements and structural parts have to be installed with enhanced accuracy, since uncertainties of single parts would sum up. This could lead to losses in solar concentration or even avoid opera-tion due to different heights of bearings. Therefore, the block foundations were ex-actly aligned on the prepared and levelled working panel (F i g u r e 11 , (1)). After-wards, the six sleeve foundations were placed on the block foundations that con-tain the necessary threaded rods fitting into the cladding tubes. A steel template cut by waterjet was set in the block founda-tions to provide the exact position of the threaded rods. Then, the axial rocker bear-ings and the drive bearing were installed (F i g u r e 11 , (2)). They were placed in the gaps of the corresponding sleeve foun-dations and fixed by horizontal threaded rods. After levelling the substructure, the gaps between the bearings and sleeve foundations were filled with mortar. Be-fore placing (F i g u r e 11 , (3)), both shells had to be turned around, since the concret-ing was done upside down. Therefore, the outer sickles with already attached gears were mounted onto the shells (F i g -u r e 11 , (4)) and auxiliary steel frames were attached (F i g u r e 11 , (5)). It should be noted that this process only occurs for the prototype. In serial production, alter-natives have to be considered, like a form-work with integrated tilting table to auto-matically flip the shell during manufactur-ing. Here, the shells are finally placed on top of the bearings and the drive engine is installed at both modules to ensure stabili-ty as well as the ability of suntracking. Ta -b l e 5 summarizes the basic technical data of the collector.

The receiver tubes, three per module, with their brackets were subsequently attached to the shell. In the framework of the pro-ject, a new mirror material made from electrochemical polished aluminium strip with a thickness of 0.4 mm with a silver based multilayer PVD coating and an ad-ditional sol-gel protective coating with a solar weighted specular reflectivity of 92.3 %. has been developed by the Almeco company and glued to the concrete surface by adhesive tape (F i g u r e 11 , (6)). Un-like conventional parabolic troughs, where a multitude of modules is arranged to a solar field, the here presented prototype is not utilized for standard power genera-tion, since the overall aperture is not suf-ficient for this. Nevertheless, the collector

is connected to a water circulation to dem-onstrate its effectiveness by heating up wa-ter that is pumped through the receiver. F i g u r e 1 2 shows the complete collector.

To specify the optical efficiency, the collec-tor was measured by digital close range photogrammetry, which is a common tool to evaluate surface qualities of parabolic

(1) (2) (3)

(4) (5) (6)

Fig. 11. Working panel (1), sleeve foundations with bearings (2), shell mounting on bearings (3), mounting of the sickle (4), turning of the sickle (5) and prototype with mirrors (6).

Tab. 5. Geometrical and technical data of the collector.


Aperture width 5.77 m

Aperture length 24 (12 per module) m

Aperture area 138.5 m²

Focal length 1.71 m

Shell thickness 3.5 to 5.5. cm

Weight of superstructure 141 kg/m²

Engine power 180 W

Mirror solar weighted specular reflectivity 92.3 %

Intercept factor 58.6 (85.6*) %

Costs 22 €ct/kWh

* with assumed focal length of 1.78 m

Fig. 12. Built-up prototype.


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troughs. By means of target points ar-ranged on the concrete shell the actual co-ordinates of the surface have been gath-ered in zenith (90 °) and horizon position (19.45 °). Subsequently, these data points are compared to an ideal parabolic shape and the calculated deformation shapes. As a result, the deformation in zenith position due to self-weight appears as a vertical mis-alignment of the shell’s edges which is al-most constant over the length of the collec-tor. Furthermore, the maximum deforma-tions amount to about 30 mm, larger than numerically expected. Since deformation occurs almost constant over the length, a systematic error is supposed. Most likely, it is caused by one-sided shrinkage that pro-vokes dishing and typically effects slender slab-like concrete elements. In addition, high deviations occur in the areas of the formwork clamps, what causes local peaks of misalignment. Initial displacements and superposed deformations from self-weight result in slope deviations in the range of +/- 25 mrad. This corresponds to a root mean square of 13.63 mrad for the zenith position and 10.12 mrad in horizon. Only 58.6 % of the solar rays will than meet the receiver tubes (intercept factor). Adjusting the absorber to a focal length of 1.78 m can almost overcome the shrinkage effect to yield 85.6 % of intercept. In further investi-gations a main focus should lie on prevent-ing shrinkage deformations, e.g. by a tem-perature treatment [12, 16]. Moreover, the discontinuity due to clamping can be elimi-nated by costly, but more efficient steel formworks. Those formworks are a matter of course in the case of large-scale produc-tion.The costs for the prototype have been esti-mated and compared to the benchmark of the solar power plant ANDASOL [4]. Doing so, field conditions are assumed. The holis-tic calculations result in a cost output of 21.9 €ct/kWh what lies slightly above the reference of ANDASOL with 19 €ct/kWh. Performance parameters, cost for materials, formworks, field fabrication, staff etc. accumulated for a solar field with 510,000 m2 aperture area have been taken into account. Material costs domi-nate. Cost reductions, mainly effecting material costs, have been identified. A shell with stiffeners rather than constant cross sec-tion and minimisations of material efforts in the substructure can reduce concrete volumes up to about 50 %. Numerical sim-ulations prove that the stiffness demands are still met. Further cost reductions are expected in the mirroring, the drive engine and the mounting parts. Though, the pre-sented prototype holds reasonable poten-tial for improvements.

Conclusions

The interdisciplinary research project has shown the general feasibility of a fully equipped and workable concrete trough collector with an overall aperture area of 138.5 m2, which was developed and assem-bled within 2.5 years. The parabolic shell made from high-performance concrete was designed with a mean thickness of 4.5 cm and mesh reinforcement by means of outer and inner form finding. The supporting structure consists of rocker bearings for suntracking. Concrete gears with specifi-cally designed teeth avoid tipping. The drive system of the collector consists of a power engine with 180 W only to drive two modules with a total weight of about 20 t. Additional collectors with similar driving systems can easily be integrated by means of a shaft synchronizing the modules. The collector was covered with a newly devel-oped mirror material made from electro-chemical polished aluminium strip with a silver based multilayer PVD coating and an additional sol-gel protective coating with a solar weighted specular reflectivity of 92.3 %. By means of digital close range photogrammetry the shell was measured and an intercept factor of up to 85.6 % was gained, assuming a corrected receiver posi-tion.Costs for electricity output with respect to a reference solar power plant result to 21.9 €ct/kWh which is – for this single pro-totype – not yet fully competitive. Minimi-sation of material efforts, a heat treatment to prevent dishing by shrinkage and elabo-rations in the driving and mirroring system are improvement examples. Concrete proved to be an appropriate material for sustainable solar structures.

Acknowledgment

The authors thank the German Federal Ministry for Economic Affairs and Energy (BMWi) for the financial support of the project “ConSol – Concrete Solar Collector” in the framework of the 6th Energy Re-search Programme on the basis of a deci-sion by the German Bundestag.

References[1] ACWA, Dubai to Get Solar Power Day and

Night Without Subsidy at Lower Cost Than Gas-fired Electricity, ACWA, 2017 http://www.acwapower.com/en/newsroom/press-releases/latest-news/dubai-to-get-solar-power-day-and-night-without-subsi-dy-at-lower-cost-than-gas-fired-electrici-ty/ (accessed 31.08.2018)

[2] DEWA, DEWA Awards AED14.2 Billion Largest CSP Project in the World, Dubai Electricity and Water Authority (DEWA), 2017 https://www.dewa.gov.ae/en/about-dewa/news-and-media/press-and-news/latest-news/2017/09/dewa-awards-aed142-billion-largest-csp (ac-cessed 31.08.18).

[3] Lilliestam J.; Pitz-Paal, R.: Concentrating solar power for less than USD 0.07 per kWh: finally the breakthrough? Renewable En-ergy Focus, Volume 26, Number 00, Sep-tember 2018.

[4] Solar Millennium: The parabolic through power plants Andasol 1 to 3. self-published Solar Millennium, 2008.

[5] Geyer, M.; Lüpfert, E.; Osuna, R.; Este-ban, A.; Schiel, W.; Schweitzer, A.; Zarza, E.; Nava, P.; Langenkamp, J.; Mandelberg, E.: EuroTrough – Parabolic Trough Collector Developed for Cost Efficient Solar Power Generation. In: Proceedings of the 11the SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, 4.-6. September, Zürich, Switzerland, 2002.

[6] Müller, S.; Forman, P.; Schnell, J.; Mark, P.: Leichte Schalen aus hochfestem Beton als Parabolrinnen solarthermischer Kraft-werke. In: Beton- und Stahlbetonbau 108(11), 2013, S. 752-762.

[7] Forman, P.; Müller, S.; Ahrens, M.A.; Schnell, J.; Mark, P.; Höffer, R.; Hennecke, K.; Krüger, J.: Light concrete shells for para-bolic trough collectors – Conceptual design, prototype and proof of accuracy. In: Solar Energy 111 (2015), S. 364-377.

[8] Kämper, Ch.; Forman, P.; Stallmann, T.; Ahrens, M.A.; Mark, P.; Schnell, J.: Opti-mised High-Performance Concrete Shells for Parabolic Trough Collectors. In: Journal of the International Association for Shell and Spatial Structures (J. IASS), Vol. 58 (2017), No. 1 March n. 191, pp. 105-119.

[9] Forman, P.; Kämper, C.; Stallmann, T.; Schnell, J.; Mark, P.: Parabolschalen aus Hochleistungsbeton als Solarkollektoren. In: Beton- und Stahlbetonbau 111(12), 2016, S. 851-861.

[10] Weissbach, R.: Die abrollende Parabol-spiegelrinne. Schutzrecht DE102011011805 A1. Industrial property right (23.08.2012).

[11] Bocklenberg, L.; Winkler, K.; Mark, P.: Digitale Planung und geometrische Präzi-sionsfertigung für Durchstanzversuche. Bautechnik 95 (2018), Heft 6, S. 432-438.

[12] Sagmeister, B.: Maschinenteile aus zement-gebundenem Beton. Berlin, Wien, Zürich: Beuth Verlag, 2017.

[13] Oya, J.; Kalender-Wevers, C.; Winkel-mann, U.; Höffer, R.: Experimental and numerical investigation of the dust trans-port on the reflector panels of a parabolic trough power plant. In: Proceedings of the European-African Conference on Wind Engineering ACWE 2013, Cambridge, 2013.

[14] Pottler, K.; Ulmer, S.; Lüpfert, E.; Land-mann, M.; Röger, M.; Prahl, C.: Ensuring performance by geometric quality control and specification for parabolic trough solar fields. Energy Procedia 49 (2014), pp. 2170-2179.

[15] Ullah, F.; Min, K.: Performance Evaluation of Dual-axis Tracking System of Parabolic Trough Solar Collector. In: IOP Conf. Se-ries: Materials Science and Engineering 301, 2018.

[16] Tkocz, J.; Mark, P.: Industrielle Präzisions-fertigung von Hohlstäben aus hochfestem Feinkornbeton. In: W. Breit et al. (Hrsg.): Beiträge zur 5. DAfStb-Jahrestagung mit 58. Forschungskolloquium, Bd. I, Kaisers-lautern, 2017, S. 174-185. l

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