recent experiences on reflectant module components for innovative heliostats

Solar Energy Materials 24 (1991) 320-332 North-Holland

Solar Energy Materials

Recent experiences on reflectant module components for innovative heliostats

Manuel Romero a, Eduardo Conejero and Marcelino Sanchez " CIEMAT-IER, Acda Comphttense, 22, E-28040 Madrid, Spain

C I E M A T is promoting experiences on heliostats for replacement of damaged facets on CESA-I field, design of new mod-les for Phoebus-like plants and innovative high-risk options for the future. Glass-metal facets, stretched tmembranes, glass fiber and holographic concentrators are being considered in a number of developments.

1. Introduction

CIEMAT is promoting and perfor}iiing some experiences on cortventional facets and innovative refiectant units. Glass-metal mirrors are com;idered for near-term and realistic options in plants like Phoebus where uncertainties and risks should be avoided. Most of changes are concerned with reduction of costs during manufacturing, elimination of bonding procedures and construction of lighter facets and structures. More innovative materials like stretched membranes or glass fiber reinforced polyester sandwiches are tested but some uncertainties in manufacturing and economical aspects should be overcome before their utilization in commercial plants. Holographic concentrators and high-UV reflectant surfaces arc being assessed for photochemical applications where reflectivity in the UV portion of the solar spectrum needs to be optimized.

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M. Romero et a L / Recem experiences on reflectant module components 321

2. R e p l a c e m e n t o f C E S A - I face t s

CESA-1 damaged facets are being replaced with a glass /metal facet designed and fabricated by CIEMAT. This facet follows a former concept created by MBB in the G A S T project, where glass sheets are bonded to ceramic dishes with a silicone adhesive and conformed with 26 metallic rods to the metallic frame. During 1989 and 1990, 100 heliostats are being repaired with two versions of 3.3 m 2 facets. The main advantages of such a concept were: - Bonding procedure is performed in a separated flat plate, and only facet

conformation is made in a vacuum plate, therefore 1 facet each 15 minutes can be easily constructed.

- Reduct ion of 15% in product ion cost can be obtained from the Asinel-Gast facet (285 D M / m 2 against 337 DM/m2) .

- The number of focusing rods has been optimized down to 26. - Focusing is reversible and can be corrected just re-adjusting the fixing clamps. - Mirror backside should be white painted in order to avoid high temperature

gradients and mirror damages. - Tes ted heliostats showed optical performance similar to second generation ones.

A beam dispersion error of 2.6 mrad has been measured.

3. S t r e t c h e d s t r u c t u r e p r o t o t y p e

A light stretched structure has been used for a new prototype with two glass facets and 36 m ~ of reflectant surface. Each facet has 12 mirrors of 1.5 m 2 manufac tured by Flachglas. No bonding is necessary for the mirrors. Stretching and focusing is performed with cables and rods over the aluminium frames, and complete assembling and focusing is carried out on field.

Fig. 2. Rear view of new facet for CESA-1 heliostat field replacement. Tw_~ mirrors are bonded with a silicone adhesive to the metallic frame by using 26 ceramic dishes.

322 M. Romero et aL / Recent experiences on reflectant module components

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Fig. 3. Flux contours of a replaced heliostat spot (in %). Day: 5-Jul-90 Time: 13:50 h. Direct insolation: 822 W / m z. Slant range: 224.3 m Diameter (95%): 4.54 m. Beam dispersion error: 2.6 tared.

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Fig. 4. Schematic of the 80 m-" stretched structure prototype suggested for Phoebus.

M. Romero et al. / Recent experiences on reflectant module components 323

Fig. 5. Rear view of a 18 m 2 module assembled in a CESA-I heliostat.

Fig. 6. Exploded view of ~;¢straim rods and focusing rods and cables.

324 M. Romero et al. / Recent experiences on reflectant module components

A central restraint in each mirror is necessary to avoid gravity influence over the mirror contour and appears to be enough for focusing. Fig. 4 shows a 80 m ~- prototype suggested for Phoebus plant.

Glass/metal heliostats with stretched structure represent an excellent option for current CRS projects: - A number of 6 tensioning cables provide stiffness. Light facets with 40 m 2 can

be manufactured. - Signif;~"cnt improvement of production cost. - No b~. ~ 'ng. Reversible mirror fixing. Easy replacement. - On site manufacturing. - Significant weight reduction (19 K g / m 2 of reflectant surface including struc-

tural trusses where ! 1 K g / m 2 come from the mirror). - Production cost estimated (Phoebus plant):

Reflectant module (79 m 2)

Trusses, cables and bolts . . . . . . . . . . . . . . . . . 3174.0 DM (40.2 D M / m 2) Aluminium frames . . . . . . . . . . . . . . . . . . . . . . . 793.6 DM (10.0 D M / m 2) Glass mirror . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5542.0 DM (70.2 D M / m 2)

9509.6 DM (120.4 D M / m 2)

Pedestal and foundation . . . . . . . . . . . . . . . . . 2692.0 DM (34.0 D M / m 2) Drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3946.0 DM (50.0 D M / m 2) Torque . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800.0 DM (10.1 D M / m 2)

Assembling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 635.0 DM (8.0 D M / m 2)

Tot# direct costs . . . . . . . . . . . . . . . . . . . . . . . . 17582.6 DM (222.5 D M / m 7)

- Optical performance: Even though no quantitative measurement has been made yet, an excellent spot with less than 3 m has been obtained at 250 m slant range.

4 . S t r e t c h e d m e m b r a n e s

Stretched membranes have been analyzed in small modules of 7 m 2 [1]. As a result of previous tests on that experimental modules a 120 m 2 prototype is being developed [2]. The final goal is to get a dual-facet unit with the following characteristics:

Membrane tension is supplied by a number of clamps that provide a simple and reversible tensioning device. With such a procedure, tension can be supplied at the end of the assembling chain, on field. By a simple correlation, torque applied over the regulating-nut is directly connected to tension on the membrane. Focus control of the active vacuum focusing system is carried out by a set of rectilinear potentiometer and speed variator connected to an axial fan.

That dual-facet concept is going to be tested in two steps: first a single 60 m 2 round facet is to be tested on a conventional drive and afterwards the whole dual-facet structure on a low-cost drive.


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Fig. 7. Dual facet s t re tched m e m b r a n e prototype (120 m2). Diamete r round facet = 9 m. Reflective area = 120 m 2. Reflective film = ECP-305 (3M). Membrane : galvanized steel; thick. = 0.!5 mm; preload = 7500 N / m ; membranes a t t achmen t = Sn welding; m e m b r a n e / m e m b r a n e overlap = 3 ram. Tension- ing torpid: AISI-304; No. c lamps = 24 per membrane ; thick. = 3 mm; diamete.r = 40 mm. Ring: ST44; t h i c k . - 4 ram; cross sect. = 200× 100 ram; Nod of ring supports = 5; r i n g / m e m b r a n e at tach. = Sn

welding. Focus control: Rect i l inear po ten t iome te r + Speed variator + vacuum fan.

For a plant like Phoebus, spot diameter predicted for our 120 m 2 commercial prototype oscillates from 1.5 m to 8 m and peak flux from 85 k W / m 2 until only 5 k W / m 2 for slam ranges between 100 m and 500 m.

Structural analysis was done with Ansys. As "~hown in fig. 12, for the maximum wind load (80 k m / h ) tensions are still at 50% of yield strength, and maximum displacement is 85 mm. Initial tension (To) was cho-;en as 7500 N / m as a compromise b~:tween optical response and load over the ring, though structural design allows i0r an increase up to 10000 N/ re .

Ring design has contemplated different rectangulal sections, in order to opti- mize the relationship weight-stiffness by static analysis and buckling analysis of i he ring. As depicted in fig. 7, for a wind speed of 80 k m / h we have a maximum ring deflection of 2.155 mm (max. tensien of 8.71 Kp/mm2). In fig. 13, for the first buckling mode, out of plane, we got a security factor T c r / T = 4.10Z It should be pointed out that for membranes tensioning up to 7500 N / m we got a factor of 6.29 for the first buckling mode (in plane) and 6.51 for the second (out of plane).

Concerning the tensioning device, we selected a toroidal tube with D = 40 mm and e = 3 mm, and 24 clamps per membrane. With such a system we obtain a maximum deviation from nominal qo of 4%, between supports. A moment of 25 N.m must be applied on the clamp nut for a T O -- 7500 N / m .

5. Glass fiber facets

Substitution of glass mirrors and metallic frames by light composite ~aterials is the final goal of this development. Advantages and achievements collected u:~ti! now are:

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Fig. 9. Peak flux versus slant range for a 120 m 2 stretched membrane hc!iostat.

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Fig. 10. Diameter containing 90% beam power versus slant range for a i20 m z dual facet stretched membrane heliostat.

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328 M. Romero et al. / Recent erperiences on reflectant module c,~mponents

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Fig. 12. (Left) Displacements of front membrane for T,, = 7500 N / m and v = 80 km/h (Ansys). (Right) Radial tension (SX) and circumferential tension (SY), both in Kp /mm 2 versus membrane radius in mm

(Ansys).

- Substantial weight reduction (10 K g / m 2 versus 21 K g / m 2 in second generation glass facets).

- Mirror and support manufactured in a single piece. -- Size can be increased up to |!':. m z per facet. - Print through effect avoided with high smoothness achieved on front sheet.

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Fig. 13. Deformed ring simulation (T o = 7500 N / m and 5 supports)

(Up) Static load corresponding to v = 80 km/h. (Dw) Critical buckling stress.


Fig. 14, Front view of a 1.4x2,3 m glass tibet facet laminated with ECP-300.

-- GFRP sandwiches provide a stiff stcucture with only 3 or 4 plies in upper and lower laminates. Nevertheless, manufacturing problems concerning the spherical contour remain

still unsolved. Six units with a size of 1.4 x 2.30 m and structures of ( 9 0 / 0 / 9 0 / 0 / Z c = 13.5 ram) were constructed as shown in fig. 14 and ECF-300 film was laminated in front. All tests showed a trend of focal length reduction after resin curing. Even though conforming plate was constructed at 240 m focal length, all facets got focal lengths around I00 m Up to now, resin and curing temperature changes did not reduce the focal length gap.

6. Holographic concentrators

Photochemical reactions and solar detoxification claim for new kind of reflec- tors able to cut the soler ~pectrum, reflecting onto the chemical receiver only the active wavelengths. This is the case of water detoxification by photocatalysis since only,, a narrow bandwidth around 380 nm is useful. Therefore reflectivity should be optimized in the UV and the rest of the spectrum must be rejected.

For such applicatio~ls holographic concentrators with either point focus or line focus could be used as a selective filter.

The University of Alicante is cooperating with CIEMAT for development of holograms in photochemical applications.

Main effort was focused on:

330 M. Romero et al. / Recent experiences on ref!.ectant mod,al.e components

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M Romero et al. / Recent experiences on reflectant module components 331

- Increase of single holograms size up to 20 × 20 cm This size is limited by current Ar laser power around 4 W.

- Reflectivity up to 80% in the UV wavelengths, which means 7.7-10% of the whole sr~ectrum reflected between 370-470 nm.

- Increase of bandwidth from 30 to 100 nm, controlling gelatine swelling and hardening, and making double layer holograms.

- Wavelength has been moved towards 380 nm by heating up to 140 °C during gelatine curing, without bandwidth reduction. Above 130°C, transmittance losses are significant. As final goal of this phase, a 1 m 2 point focus concentrator has been assembled

with 24 holograms, 20 × 20 cm each. Total weight including the supporting glass

Fig. 16. Point focus holographic concentrator made from 24 double-layer holograms. Holograms size is 20 × 20 cm.

332 M. Romero et al. / Recent experiences on reflectant module components

was 11.7 kg. Each hologram provided a spot with 2.1 W of UV power. With an appropriate hologram canting we got 2000 × , a 1.75 c m 2 spot and 200 °C as peak temperature.

The concentrator has been created canting holograms made, all of them, from ti~c same spherical model, therefore holograms located in the edges had non- negligible aberration.

Our present design has a wide spectrum (100 nm) and each A is focused at different focal length, therefore if bandwidth is reduced, concentration ratio can be improved.

Next steps in our development will look for: - Displacement towards lower ,~ for potential link with T iP 2 active bandwidth - Bigger size in single holograms with a moving laser scanning (1 m2). - Single unit concentrator to avoid aberrations in the edges. - Development of a linear focus concentrator for water detoxification processes

and low quantum concentrations. Since most of glass mirrors and current solar polymers have low efficiencies in

UV part of the solar spectrum, CIEMAT is also making some lab-zca!e experiences concerning metaUization and adhesivation of UV transparent polymers and sol-gel coating of metallic membranes for direct metaUization of reflector surfaces.

R e f e r e n c e s - -

[1] M. Romero and M. Silva, Proc. Fourth Symp. on R + D Appl. of Solar Thermal Tech., Santa Fe, June 13-17, 1988, Ed. Hemisphere Pub. New York, 1990, pp. 153-162.

[2] M. Romero, E. Conejero and J.M. Figarola, Proc. ISES Solar World Congress-1989, Kobe, Sept. 4-8, Japan, Pergamon Press, Forthcoming.

recent experiences on reflectant module components for innovative heliostats

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