Fiber Optic2002

8/4/2019 Fiber Optic2002

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Solar Energy Vol. 72, No. 6, pp. 459472, 20022002 Elsevier Science Ltd

Pergamon P I I : S 0 0 3 8 09 2 X ( 02 )0 0 0 2 5 7 All rights reserved. Printed in Great Britain0038-092X/02 /$ - see front matter

www.elsevier.com/locate/solener

SOLAR FIBER-OPTIC MINI-DISH CONCENTRATORS: FIRSTEXPERIMENTAL RESULTS AND FIELD EXPERIENCE

,* ** *DANIEL FEUERMANN , JEFFREY M. GORDON and MAHMOUD HULEIHIL*Department of Solar Energy and Environmental Physics, Jacob Blaustein Institute for Desert Research,

Ben-Gurion University of the Negev, Sede Boqer Campus 84990, Israel**The Pearlstone Center for Aeronautical Engineering Studies, Department of Mechanical Engineering,

Ben-Gurion University of the Negev, Beersheva 84105, Israel

Received 12 March 2002; accepted 19 April 2002

Communicated by JOACHIM LUTHER

AbstractThe experimental realization and field experience of a recently proposed solar fiber-optic mini-dishconcentrator are reported. The prototype is 200 mm in diameter. We have repeatably transported concentratedsunlight in a one-millimeter-diameter optical fiber and measured flux levels of 1112 kilosuns at a remotetarget (up to 20 m away). The prototypeassembled from off-the-shelf parts and customized items that relysolely upon existing commercial technologiesproved impervious to dust penetration and condensation. Forthe particular application of solar surgery, dielectric second-stage concentrators were designed and fiber tipswere sculpted to boost flux concentration by a factor of 2 to 4, for light extraction into air and tissue,respectively. Our findings strengthen the feasibility of the efficient and complete de-coupling of the collectionand remote delivery of highly concentrated solar radiation. 2002 Elsevier Science Ltd. All rights reserved.

1. INTRODUCTION physical and engineering concepts into a func-

tional prototype (Feuermann and Gordon, 2001b).Certain solar applications uniquely require very

Fig. 1 is a schematic of the mini-dish unit. Fig.

high photon densities, i.e., high flux concentra- 2 includes photographs in the field of our firsttion. Recent innovations include solar surgeryoperating prototype (200 mm in diameter, with a

(Feuermann and Gordon, 1998), solar-pumpedfocal length of 120 mm), including the remote

lasers (Arashi and Kaneda, 1993), the solar-drivenextraction (delivery) of sunlight at flux levels

synthesis of carbon nanomaterials (Pitts et al.,surpassing 11 kilosuns (i.e., 11 000 suns) from an

1993; Flamant et al., 1999) and electricity gene-optical fiber 1.0 mm in diameter, at a distance up

ration with advanced semiconductor materialsto 20 m from the mini-dish collector.

(Feuermann and Gordon, 2001a; Reddy et al.,The sections that follow provide a detailed

2001).component-by-component description of the de-

An essential element in the practical realizationsign, assembly, testing and field measurements.

of these applications is the total separation of theWith this itemized accounting, we then proceed to

collection and delivery of highly concentrated

report on the performance and analysis of oursunlight, including high efficiencies for collectionoutdoor solar experiments on the assembled

and optical transport. Toward solving this prob-prototype. This is followed by our observations

lem in a pragmatic and modular fashion, the useregarding future generations of improved viable

of solar fiber-optic mini-dishes was proposedsolar fiber-optic mini-dishes.

(Cariou et al., 1982; Feuermann and Gordon,

1998, 1999, 2001a; Gordon, 2001). This article

constitutes a detailed report on the experimental2. PROTOTYPE CONCENTRATOR DESIGNresults and field experience in which these

miniaturized concentrators were translated from A paraboloidal dish is specified completely by

its diameter D and rim half-angle f. The f-

number of the dish,

f5 (focal length)/(diameter)5F/D (1)Author to whom correspondence should be addressed. Tel.:1972-7-659-6923; fax: 1972-7-659-6921; e-mail:

[email protected] is related to f by

459


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460 D. Feuermann et al.

flux that reaches the focal plane (Baum and

Strong, 1958).

The focal spot diameter d that acceptsmaxessentially all reflected rays is (Rabl, 1976)

2 2Du 11 16fs d

s

]]]]

d 5 . (5)max 28f 16f 21s d

The corresponding averaged flux concentration is

2 2sin (f) cos (f)av g]]]]]C 5 . (6)flux 2

us

For comparison, the thermodynamic limit for

flux concentration (in air) is (Rabl, 1976; Welford

and Winston, 1989)

1ma x]

C5

. (7)flux 2us

In Section 7, we shall explore how overall flux

concentration can be heightened to the thermo-

dynamic limit, at high collection efficiency, with

dielectric secondary concentrators sculpted from

the distal tips of optical fibers.

When designing for maximum flux concen-

tration from the dish, one selects the fiber coreFig. 1. Schematic of a solar fiber-optic mini-dish. A small flatmirror below the focal plane redirects solar rays reflected from diameter d to be d ; while for maximumfiber minthe dish downward to facilitate practical coupling into an efficiency designs d will be closer to d .fiber maxoptical fiber. Extreme rays are shown traced into the fiber,

Furthermore, achieving a reasonably low numberwhich is enclosed in a protective sleeve. Concentrated sunlight of fibers per collection area demands systems ofis transported in the optical fiber to a remote applicationrelatively low f-number. Fiber core diameters canillustrated here as solar surgery (Feuermann and Gordon,

1998)with the option of a significant boost in power density of course be chosen between d and d . Amin maxwith a nonimaging secondary concentrator at the distal end comprehensive study of the associated efficiency(Drawing courtesy of New Scientist (Marks, 1999)).

concentration tradeoff for paraboloidal concen-

trators covering a wide range of f-numbers was1 /f5 4 tan(f / 2). (2)presented in Feuermann et al. (1999). The optimal

compromise between efficiency and concentration

is case-specific, with Feuermann et al. (1999)The diameter d of the approximately uni-mi nproviding the quantitative tools for such determi-form-flux core region of the focal spot isnations.

d 52Dfu (3)min s We selected a mini-dish diameter D5200 mm,a focal length F5 120 mm (hence f5 458), andwhere u is the effective solar half-angle (a

s

an optical fiber core diameter d 51.0 mm as afiberconvolution of the actual size of the solar diskconsequence of several considerations.with optical errors in the mini-dish contour,1. The dish size should be sufficiently small thattracker accuracy and fiber alignment), which is

eventual mass production of the dishes, andtaken throughout to be sufficiently small thatmodularity, would be feasible.sin(u )u . As justified in Section 6, we adopt a

s s

2. The dish should be small enough to permitrealistic value for u of 0.005 radian. The con-s

affordable production of prototypes by a num-centration in the core region is the highest localber of companies.flux a paraboloidal dish generates (Rabl, 1976) 2

3. The number of optical fibers per m of mini-2

dishes should be as small as possible forsin (f)core

]]

C 5 (4)flux 2 eventual large-scale viability. Hence high con-us

centration is required, which in turn demands4

and encompasses a fraction cos (f/2) of the total high rim-angle dishes (Eq. (4)).


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Solar fiber-optic mini-dish concentrators: first experimental results and field experience 461

Fig. 2. Photographs of our first operational prototype in the field. (a) Photograph of the assembled fiber-optic mini-dish, dual-axis

tracker, tracker sensors and scope for determining tracker accuracy (foreground). (b) Front view, with a 20-meter long optical

fiber, where the fiber output has approximately 1/4 the brightness of the sun. (c) Side view of Fig. 2b. The solar fiber-optic

mini-dish is at the far left and the technician holding the distal fiber end is at the far right. (d) Projection of the output from the

20-meter fiber onto a white diffuse screen.


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4. High rim-angle dishes enjoin optical fibers of plastic buffer, and sit snugly inside the fiber

high numerical aperture (NA). NA5sin(u ), holder. Thus there is virtually no dust penetration.ai rwith 2u denoting the full angular range of Moreover, we did not observe any condensationairrays incident from air, that nominally is sup- inside the dish at any time.

ported by the optical fiber, i.e., for which total3.2. Achieving adequate mirror contourinternal reflection is respected. Therefore weaccuracyconstrained ourselves to commercially avail-

able fibers of high NA and large core diame- We did not find commercial off-the-shelf high-

ters. quality parabolic mirrors for the dimensions re-

Our aim was to achieve the highest flux con- quired. The only practical fabrication technique

centration at high radiative efficiency for practical for the first handful of prototypes was diamond

configurations. The flux concentration achievable turning of an aluminum substrate followed by2

in the mini-dish alone is proportional to (NA) . first-surface silver coating and a protective thin

(Equivalently, the number of fibers, per square layer of silica. Diamond turning can provide a

meter of solar collecting area, is proportional to dish contour accuracy of several microns, which2

1 /(NA) .) At u 5 0.005 radian, a dish with the translates into a mirror slope error of order 0.0001s

dimensions noted above can be used between the radian. Several manufacturers have demonstrated

extremes of (a) maximum concentration with comparable accuracy with plastic (coated) mirrorsd 5 d 51.2 mm; and (b) maximum ef- produced by mold injection (Avimo, 2001;fiber minficiency with d 5 d 52.0 mm. Syntec, 2002; G-S, 2002). Improper diamondfiber max

Realizable flux concentration is also propor- turning can result in deleterious diffraction losses

tional to the fibers transmittance, so fiber core that stem from excessive groove width. If the

materials with near-negligible attenuation inte- groove size is kept well below the wavelength

grated over the full solar spectrum are essential range of the solar spectrum (which is readily

(such as pure quartz). Furthermore, the number of achievable with commercial equipment), then

(expensive) optical fibers, per square meter of diffraction losses can be rendered negligible.2

solar collecting area, is proportional to 1/(d ) .fiber3.3. Accurate alignment of all elements andTherefore we aimed to use commercially avail-

positioning of the small flat mirror

able fibers with the highest NA, lowest attenua-tion, and largest core diameter. Alignment of the elements was achieved by

producing the machined encasement on an accur-

ate lathe. The positioning of the mirrors and3. PRACTICAL CONCERNS

glazing relative to the encasement was checked

As we embarked upon the experimental pro- with a laser in our indoor laboratory. There was a

gram reported here, our principal practical con- tolerance of60.1 mm in positioning the fiber tip

cerns were: because the fibers core diameter (d 5 1.0 mm)fiberwas less than the diameter of the uniform flux

3.1. Dust penetration and accumulation on the core region in the focal plane of the dish (d 5mi nmirror or fiber tip, as well as condensation in 1.2 mm). A small frame with adjustment screwsthe space between the glazing and the mirror that attached to the back of the mini-dish main-

These were not expected to pose a major tained the fiber at its proper position and permit-problem, based on the success of related tech- ted fine-tuning of fiber tip location in the x, y and

nologies for totally different applications. For z directions to within about 60.05 mm. Element

example, the reflectors in car headlights (of alignment in future mass-produced units should

dimensions comparable to our mini-dish units) be achievable with common manufacturing meth-

remain clean for years in demanding environ- ods, such as those used for automatic pencils and

ments. We produced a tight encasement of the plastic lenses.

mirror, proximate fiber tip and small flat mirror Our design introduces a small flat mirror to

with two O-rings: one between the glazing and re-direct rays reflected from the dish downward,

the encasement, and the other between the mirror so that a rigidly-positioned optical fiber can point

and the encasement. The opening in the center of upward and thereby obviate the practical prob-

the parabolic mirror (for insertion of the fiber lems associated with the conventional downward-holder) was sealed by a small flange machined facing absorber in solar dishes. Furthermore,

into the sleeve in a metal-to-metal seal. The cleaning the protective glazing would be proble-

optical fibers are manufactured with a protective matic were the fibers to extend outwards and be


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required to bend backwards behind the dish. This specular reflectivities of both the small flat mirror

is particularly important for modules comprising and the parabolic dish, transmission of the cover

tens to hundreds of mini-dishes. In eventual large- glazing, as well as attenuation and angular trans-

volume production, we recommend that the small mission of optical fibers and sculpted fiber tips. A

mirror be deposited directly on the protective source of collimated white light and a HeNe

glazing. But for the first prototypes, toward laser together with a calibrated integrating sphere

granting maximum operational flexibility, we had were used to test optical properties of componentsthe small mirror deposited on a pedestal that in the visible and near infrared. The measurement

extends downward from the glazing. procedures for the angular transmission and at-

tenuation of optical fibers were documented in3.4. Tracking precision Feuermann et al. (2002).

We identified several commercially-available Fig. 2a is a photograph of the assembled

small precision dual-axis trackers, of which two prototype installed outdoors. The installation

certified a tracking accuracy of better than 60.002 comprised: (1) a battery-driven dual-axis solar

radian. Tracker details and test results are reported tracker and stand, (2) a telescope-like device used

in Section 6. in determining tracker accuracy (Section 6), (3)

accommodation of the full fiber-optic mini-dish

3.5. Commercial availability of suitable low- assembly, and (4) a pyrometric power meter toattenuation optical fibers that can withstand measure radiative output at the fibers distal end.high solar flux

The attenuation of commercial quartz-core

optical fibers, integrated over the solar spectrum, 5. EVALUATION OF OPTICAL ELEMENTS

is sufficiently low that flux levels of order 105.1. Mini-dishkilosuns are easily accommodated (Polymicro,

2001; Feuermann et al., 2002). Whereas the best- We ordered several diamond-turned parabolic

suited optical fibers should have relatively large mirrors from two companies (referred to as A and

diameters and high NA, practical problems arose B), and developed in-house procedures for asses-

because: (a) fiber stiffness is a strongly increasing sing contour accuracy, reflectivity and specularity.

function of fiber diameter; and (b) we did not Lacking sophisticated profiling equipment, wediscover commercial fibers with core diameters determined the contour accuracy by comparing

much above 1.0 mm that are suitable for concen- the spot size produced in the dish focal plane by a

trated sunlight. While fiber manufacturers con- distant (approximately point) light source against

firmed that there is no major material or tech- the theoretical result. Reflectivity was measured

nological factor that militates against high-NA with a laser and integrating sphere. Specularity

fibers, the highest NA fibers that we found was ascertained by comparing the observed

produced commercially have a nominal NA of spread of a laser beam reflected from several

0.66, and could be purchased from only one points on the mirror to the calculated value for a

company (Polymicro, 2001). We purchased sam- parabola.

ples of all the commercially-available so-called Satisfactory accord between our in-house mea-

high-NA fibers for comparative testing and surements, and those from an independent testing

evaluation. For the highest-NA fibers, the core laboratory to which one of the mini-dishes wasdiameter is 1.0 mm. Some of the fibers with a sent, established confidence in our procedures for

nominal NA of about 0.4 are available with core the precision required for our solar concentrator.

diameters up to 2.0 mm. Test results are summa- Company A also independently supplied the

rized in Section 5.2. mirrors measured spectral reflectivity from a

witness piece, which confirmed our measurements

(a) with the laser and (b) integrated over the4. LABORATORY EQUIPMENT

lamps (white-light) spectrum.

Outdoor measurements on the assembled Our measurements revealed that only Company

prototype are of limited value unless the per- As mirrors satisfied our specifications, to wit, a

formance of each component is first characterized specular reflectivity (integrated over the solar

separately under controlled conditions. Conse- spectrum) of 0.96, with a contour error com-quently, individual components were tested in the mensurate with u not exceeding 0.005 radian.

s

laboratory prior to assembly. These measurements Fig. 3 highlights the distinction between the

included the mini-dish contour accuracy, mirror image produced with a surface that does not


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Fig. 3. Illustration of diffraction losses. Photographs of a far-field target irradiated by laser light or white light that has been

reflected from mini-dishes A and B. (a) Dish A, laser light. (b) Dish A, white light. (c) Dish B, laser light. (d) Dish B, white light.

appear to suffer from diffractive losses (Company per air-quartz interface) and attenuation in the

A), and one that does (Company B). The reflected core (no more than 1% for fiber lengths of up to

spot should be a single disk of light. With 10 m) were predictable from known material

Company Bs dish, optical interference results in properties and the attenuation spectrum provideda sizable fraction of the reflected light being by the manufacturers. They were confirmed ex-

distributed at distances far beyond the diameter of perimentally.

the central spot. Based on manufacturers claims, the residual

losses were expected to be negligible. However,5.2. Optical fibers we discovered significant transmission losses at

The optical characterization of the fibers was off-normal incidence that increased dramatically

reported in Feuermann et al. (2002). In particular, with incidence angle within the fibers NA. Sam-

we measured the transmission of the fibers, as a ple measurements are plotted in Fig. 4. Integrated

function of incidence angle, for a broad-spectrum over the fibers nominal NA, these residual losses

collimated light source. reached 20% in some instances. At angles near

We divided optical losses into: (a) Fresnel (but less than) the fibers NA, losses could exceedreflections off the air-quartz interfaces; (b) at- 50%. Transmission losses of this magnitude have

tenuation in the fibers core; and (c) residual also been documented in other laboratories (Irvin

losses. The Fresnel reflective losses (around 4% and Nakamura, 1991; Liang et al., 1997), al-


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Fig. 4. Measured angular response of commercial low-attenuation optical fibers of nominal NA50.66, intended for high-flux

solar applications. L/d5ratio of fiber length to core diameter.

though a satisfactory model to account for the both sides, produced by WZW Ltd (Switzerland).losses and predict how they depend on fiber The theoretical prediction for Fresnel reflective

properties had not been forthcoming. losses was 1% at each airglass interface, plus a

We identified these losses as light leakage from 1% absorption loss in the glazing. The small flat

the core to the cladding of the fiber, and de- mirror was specified to have a specular reflectivity

veloped a theory to model the phenomenon of 97% integrated over the solar spectrum. We

(Feuermann et al., 2002). Fig. 5 is a photograph measured the transmission of the glazings to be

that highlights this sort of light leakage, with red 0.9760.005, and the specular reflectivity of the

laser light injected into one of the NA50.66 small flat mirrors to be 0.9660.005.

optical fibers at an angle within the fibers nomi-

nal NA.6. SOLAR TRACKER

With our theoretical model, we can forecast

how light leakage depends on: (1) the properties We aimed for a tracking accuracy better thanof the fiber cladding, and (2) fiber length. The 60.18. Attainable flux concentration is propor-

2highest degree of light leakage occurred in the tional to 1/ (u ) . u comprises the natural size of

s s

highest-NA (teflon-clad) fibers, which mitigated the solar disk (0.0047 radian) enlarged by contri-

our reaching the originally predicted maximum butions from: mirror contour errors, imperfect

power densities. The shortfall is around 20% for specularity of the mirrors, diffraction from the

fiber lengths of a few meters. Our model can mini-dish, tracking inaccuracy, and alignment

predict the improvements related to the use of errors in the mini-dish and fiber placement.

superior existing cladding materials that can The errors are viewed as statistically indepen-2

produce the same NA yet result in negligible dent. So the effective (u ) is the sum of thes

leakage. squares of these individual errors. For example,

when a tracker accuracy of 0.058 (0.00087 rad)5.3. Protective glazing and small flat mirror and a mini-dish contour and diffraction error of

We used BK7 Schott glass windows for good around 0.0015 radian are convolved with the finite

transmissivity, with anti-reflective coatings on size of the solar disk (Rabl, 1985), the effective us


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Fig. 5. Photograph to illustrate light leakage from the fibers core through its cladding and buffer. Laser light was injected into

the fiber at an angle within the fibers nominal NA. In this black-and-white reproduction, the fiber appears white where light

leakage occurs. In the absence of leakage, the light should only be visible at the entrance and exit of the fiber (and not along its

length). The light exiting the fiber is projected onto a small screen.

increases to 0.005 radian. This is the value as- tracker, however, moved in sharp distinct steps of

sumed in the analyses that follow. The agreement 0.18. Namely, the tracker remains stationary until

reported in Section 9 between experimental mea- a tracking movement is required, and then abrupt-surements of flux concentration and predictions ly advances by 0.18. Given the optical tolerances

based on u 50.005 radian supports this estimate. needed to reach power densities of orders

22Standard accurate dual-axis solar trackers are 10 W mm , we proceeded with the SPS tracker.

produced for the mechanical loads commonly

encountered in large solar dishes and heliostats7. SECOND-STAGE CONCENTRATORS2

(each unit being tens to hundreds of m in area).

We required only a small tracker that could The applications being considered hereespe-

accommodate at most a few prototypes. Two US cially solar surgerydemand ultra-high flux con-

companies, Small Power Systems Inc (SPS) and centration. The concentration that can be achieved

Enhancement Electronics Inc (EEI), claimed they in the paraboloidal mini-dish at reasonably high

could satisfy our specifications with a robust collection efficiency is less than half the thermo-

device for continuous outdoor use. One tracker dynamic limit (Eqs. (4) and (7)). Realizablewas purchased from each company. concentration from the mini-dish alone is further

To measure tracker accuracy, we built a tubular reduced by the restricted NA in commercially-

pinhole camera, and compared the movement of available off-the-shelf optical fibers of admissibly

the suns image at the tubes exit against a high transmission over the solar spectrum. Ac-

reference position. The tube has a high aspect cordingly, from the start, our system designs

ratio and is internally black (see Fig. 2a). It incorporated second-stage nonimaging concen-

effectively absorbs all solar radiation outside the trators, with the intention of boosting flux con-

solar disk. centration to the maximum extent possible.

Tracker accuracy measurements were per- A second-stage concentration enhancement can

formed under clear-sky conditions during the in principle be performed either at the proximate

central daytime hours, with a sample run for the fiber end in the dish, or in the distal fiber tip at theSPS tracker presented in Fig. 6. The SPS tracker remote absorber. Introducing a maximum-concen-

exhibited an average accuracy of 60.058 with tration second stage in the dish necessitates fibers

essentially continuous tracking motion. The EEI with a NA near 1.0 (Feuermann and Gordon,


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Fig. 6. Absolute value of the measured tracking error of the SPS tracker, and the normal beam irradiation, for a typical clear day

(7 May 2001).

1999). Such fibers could in principle be produced, (Eq. (9) being a generalization of Eq. (7)). Formax

but would suffer from severe fragility and are not optical fibers with NA50.66, C 52.3 and 4.12commercially available. Accordingly, we de- for light extraction into air and water, respectively

signed second-stage concentrators at the remote (n51.331.34, which is also approximately that

delivery point. of many biological tissues).

We chose a one-fiber-one-secondary-concen- Our secondary concentrators are based on edge-

trator strategy with sculpted fiber tips: the ability ray and V-cone designs (Welford and Winston,

to generate immense flux concentration from 1989), for an input NA of 0.66 and an exit NA of

individual units without the need for active cool- 1.00, i.e., an exit half-angle of 908. Concentrator

ing. We designed dielectric nonimaging concen- cross-sections are drawn in Fig. 7. In thesetrators that are sculpted directly from the distal designs, the concentrator profiles are tailored to

tips of the optical fibers. Our strategy was to

design devices that, barring material imperfec-

tions, would produce flux levels approaching the

thermodynamic limit. The actual measured per-

formance of the manufactured concentrators

would fall short of maximum concentration only

due to flaws in the materials and production

processes.

The thermodynamic limit for the secondary fluxma x

concentration boost C from an optical fiber2with numerical aperture NA is (Welford andWinston, 1989)

2nma x]C 5S D (8)2 NA

where n denotes the refractive index of the

medium into which the light is extracted. This

secondary boost augments the maximum con-

centration produced by the mini-dish (Eq. (4)) to

an overall concentration that in principle reaches

the thermodynamic limit when light is extractedFig. 7. Cross-sections of our dielectric nonimaging sculpted

into a medium of refractive index n (Rabl, 1976) fiber tip secondary concentrators. Each concentrator is anatural extension of the optical fiber used to transport sunlight

2nma x concentrated in the mini-dish to a remote target (i.e., the]C 5 (9)S Dflux u concentrator tip is not cut separately and then attached).s


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meridional rays only. Namely, it is a two-dimen- attempt to produce the sculpted fiber tips drawn in

sional procedure applied to an axisymmetric 3D Fig. 7. Fig. 8 offers photographs of two samples,

device. We performed 3D raytrace simulations, with and without light transmission (8a and 8b,

and confirmed that skew-ray rejection is at the respectively). Fig. 8b reveals a less-than-perfect

12% level for all the devices we designed. shape: light leakage from the sculpted tips due to

An exit NA of 1.00 creates a subtle problem for imperfect manufacturing (mostly, above and

radiation measurement with commercial integra- beyond contour errors). As even visual inspectionting spheres and power meters which typically are confirmed, light leakage remained at such high

certified for light injection angles up to 6458. The levels that these second-stage concentrators were

exit-angle constraint posed no problem for the deemed unsuitable for the planned solar experi-

measurements depicted in the preceding sections, ments. Reports from other laboratories indicate

which were inherently restricted to NA values that sculpted tips of these dimensions, with the

within around 0.7. We could not, however, obtain required accuracy and negligible defects, can be

accurate radiative power measurements from the produced, at least in-house (Liang et al., 1997).

sculpted fiber tips where a substantial fraction of Currently we are investigating alternative routes

the power emerges beyond 458. Presently we are for the commercial preparation of our sculpted

exploring viable measurement techniques with fiber tip secondary concentrators. For a properly

readily available equipment. produced tip, the expected boost in flux con-Polymicro Inc (Polymicro, 2001) agreed to centration is more than a factor of 2, to around

Fig. 8. Photographs of some sculpted tip secondary concentrators. (a) Without light transmission through the fibers (magnified

view, untapered fiber diameter51.00 mm). (b) With laser light transmission through the fiber, to illustrate light leakage that stems

from imperfect manufacturing. Note the light leakage at the entrance to the sculpted tip and along its contour.


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25 kilosuns, for the extraction of sunlight into air, clear days distributed throughout the year. The

and a factor of 4, to around 45 kilosuns, for the power output at the fibers distal end was mea-

extraction of light into water or tissue. sured with an Ophir Ltd pyrometer-detector

power meter, and the normal solar beam ir-

radiance was measured with an Eppley normal-8. OPERATING PROTOTYPE AND

incidence pyrheliometer.EXPERIMENTAL RESULTS There was no measurable degradation of the

Photographs of a completed prototype are optical or mechanical elements due to outdoor

presented in Fig. 2. exposure during our experiments. A future task is

We positioned the optical fiber in the mini-dish establishing whether the dearth of degradation

with a two-pronged strategy. First, we calculated persists with long-term continuous operation.

the precise position for the fibers proximate tip The pyrheliometer has a full acceptance angle

and, guided by visual inspection, sited the fiber of 0.100 radian. Our concentrator, however, is

accordingly. Then we exposed the mini-dish designed for the far smaller angular extent of the

assembly to the sun (with dual-axis tracking in solar disk. We had no direct measurement of

operation), and fine-tuned the fibers position by circumsolar radiation. For solar climates similar to

simultaneously measuring the radiative output at that in Sede Boqer, clear-day circumsolar frac-

the fibers distal end and adjusting the position of tions tend to be in the range of 16% (Rabl,the fibers proximate tip. The final position was 1985; Gueymard, 2001). We conservatively

chosen so as to maximize power output. adopted a nominal uniform correction from mea-

Experiments were performed at Ben-Gurion sured solar beam to collectible radiation of 0.98.

Universitys Sede Boqer Campus, under clear-sky Typical measured values are: (1) a remote

conditions, and covered periods within 64 h of power delivery of 8 W, (2) at a collectible solar22

solar noon for durations of up to several hours on beam irradiance of 900 W m . With a fiber core

Table 1. Representative and repeatable experimental results for our solar fiber-optic mini-dish prototype, for nominal fibera

lengths of 1, 7 and 20 m

Component Measured optical Improved optical throughput

throughput attainable with existing materialsor production techniques

Protective glazing with anti-re-flective coating 0.97 0.97Paraboloidal mini-dish mirror 0.96 0.97Small flat secondary mirror 0.96 0.97Airquartz interface at the fibertips (per tip) 0.96 0.99Light leakage from fiber core tocladding: 0.99

1 meter fiber 0.877 meter fiber 0.82

20 meter fiber 0.79Attenuation in the fiber core 0.99 0.99Net prototype throughput with: 0.88

1 meter fiber 0.717 meter fiber 0.66

20 meter fiber 0.64

2Maximum possible flux concentration in a loss-less system5(NA/u ) 517.4 kilosuns

s

Maximum flux concentration with real materials and improved optical throughput515.3 kilosunsRepresentative experimental results for:Optical fiber core diameter51.0060.03 mm, fiber nominal NA50.66Effective solar angular radius (incorporating all optical errors) u 5 0.005 radian

s

Fiber length Measured Measured Power density Corresponding Predicted fluxpower output normal beam attained flux concentrationfrom distal irradiance experimentally concentration including losses

22 22fiber tip (W) (W m ) (W m ) (kilosuns) (kilosuns)

1.1 m 7.8560.3 838620 10.060.6 11.960.8 12.460.87.2 m 8.160.3 921620 10.360.6 11.560.8 11.560.8

20.0 m 8.160.3 931620 10.360.6 11.160.8 11.260.8a

We also include detailed accounting of optical performance on a component-by-component basis. The performance of eachelement individually is based on our indoor laboratory measurements.


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diameter of 1.0 mm, this corresponds to a flux They might enhance cost-effectiveness by reduc-

concentration of more than 11 kilosuns. Repre- ing the number of fibers per unit of collection

sentative measurements are summarized in Table area.

1 for three different fiber lengths, along with the Our mini-dishes were diamond turned. This

predictable optical losses associated solely with was the only feasible method we identified for

material properties. producing a handful of precision concentrators.

The predicted flux levels are consistent with Diamond-turned devices are inherently expensive,our measured values, to within the experimental and could not be deemed economically feasible

uncertainty. The agreement between the measured for large mini-dish systems. A workable alter-

outdoor performance of the assembled prototype native is preparing a die and mold-injecting mini-

and the projections would appear to indicate that dishes in mass production (with subsequent mirror

all key effects have been accounted for. Also deposition). With a precision die, the price for

included are the potential improvements attainable creating and coating a mini-dish would be

with existing materials and technologies, i.e., dramatically less than the diamond-turning pro-

without the need for future advances in optical cedure: about several US$ per mirrored mini-dish

surfaces or production techniques. at very high volume production. But the precision

die typically costs of the order of tens of

thousands of US$ (G-S, 2002; Syntec, 2002). Afuture mini-dish system of sufficient extent could

9. OBSERVATIONS AND QUALIFICATIONSbenefit from this economy of scale, and could

Our mini-dish prototype was purposely slightly thereby advance the demonstration of economic

oversized in order to accommodate fibers of and technical feasibility.

varying diameter and NA. That means that the NA Finally, a recent outgrowth of our experimental

of the mini-dish (0.707) is larger than that of the program is high-flux photovoltaic power gene-

optical fiber (0.66), and that both the dish focal ration with advanced multi-junction semiconduct-

length and diameter could have been smaller. A ors. The concept was originally delineated in

mini-dish with the uniform-flux core region of its Feuermann and Gordon (2001a), and subsequently

focal spot equal to the fiber core diameter, d 5 commissioned for experimental realizationfiber

1.00 mm, would have F5

100 mm and D5

(Reddy et al., 2001).151 mm. Alternatively expressed, whereas our

actual mini-dish typically intercepted 28 watts of22

collectible solar beam irradiation (of 900 W m ),10. SUMMARY

a mini-dish exactly matched to the fibers would

intercept 16 watts. In the first phase of our solar fiber-optic mini-

Given known material properties and our abili- dish program, the primary milestones included:

ty to account for all system losses (Table 1), we 1. The experimental realization of our proposed

can assess that the power density produced in the solar fiber-optic mini-dish, including the de-

core region of the focal spot of the mini-dish was sign, construction, and testing of an operational

approximately 15.4 kilosuns. The attainable flux prototype. We have demonstrated the collec-

concentration can be noticeably improved exclu- tion, concentration and transmission of sun-

sively through the use of superior existing materi- light to remote targets. Discussions withals and technologies, i.e., without the need for producers of molds and injection-molded opti-

future advances in material or production tech- cal elements indicate that the miniaturized

nologies. Hence we have included in Table 1 what units (200 mm in diameter, with a focal length

we view as tenable projections of the performance of 120 mm) are well suited to mass production.

of solar fiber-optic mini-dishes of the type we 2. The measurement of flux concentration ex-

have designed, built and tested. ceeding 11 kilosuns at a remote receiver (20 m

Optical fibers with core diameters up to 2.0 mm removed), solely from the fiber-optic mini-dish

are commercially available, but only for noticeab- (primary) concentrator. Flux levels in the

ly lower NA. The highest nominal NA value, for encased mini-dish at the proximate fiber tip

which we found optical fibers with core diameters exceed 15 kilosuns, for a system with a nu-

of 1.5 and 2.0 mm at admissibly low attenuation merical aperture of 0.66. There appear to be noover the solar spectrum, was 0.4. These large- unaccounted-for loss mechanisms, i.e., we can

diameter fibers have enough mechanical flexibility confidently project optical performance from

to remain robust in solar concentrator systems. known material properties. It should be feas-


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f rim half-angle of the paraboloidal dishible eventually to attain up to 30% higher fluxu half-angle of acceptance of the optical fiber

airconcentration, based wholly on available ma-u effective solar angular radius

sterials and existing production technologies.

3. Testing the tracking accuracy of small dual-

axis trackers suitable for miniaturized high- AcknowledgementsThis research was supported by grantsfrom the Israel Ministry of National Infrastructures (Jerusalem)concentration units, and establishing a toler-and the Rita Altura Foundation ( Los Angeles, CA). We areance of60.058.indebted to Michael Altura for his encouragement and spon-

4. An experimental study of the measurement of sorship. We thank Jonathan Molcho of Ben-Gurion Universi-tys Department of Electrical and Computer Engineering forthe angular transmission characteristics of opti-generously granting the use of facilities and space in hiscal fibers suitable for high-flux solar uses,laboratory, as well as his valuable recommendations in the

including the discovery of deleterious light selection of measurement equipment. We are also grateful toDavid Faiman of Ben-Gurion Universitys Blaustein Instituteleakage (Feuermann et al., 2002). A theoreticalfor Desert Research for the use of facilities and technicalmodel that can account for these observationsassistance at the Ben-Gurion National Solar Energy Center.

permits distinguishing between inherent limita-

tions versus technological problems for the

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