1. A SUBDEGREE PRECISION SUN TRACKER
FOR lOOOX MICROCONCENTRATOR MODULES
lgnacio Luque-Heredia', Carlos Martin', hlaria T. Madanes', Jose M. Moreno', Josd L. Auger', Vincent Bodin',
Jeslis Alonso', Vicente Diaz', and Gabriel Sala'
1. INSPIRA, SL C/Chile, IO. 28290 Las Matas, Madrid - Spain
PhiFax: +34 91630 45 34/4087,e-mail: iluque@inspira.es
2. ISOFOTON, SA, Pol. Ind. Sta. Crur, C/La Gitanilla, 26,29006 Malaga- Spain
3. Instituto de Energia Solar, E.T.S.I. Telecomunicaci6n, Ciudad Universitaria, 28040 Madrid - Spain
ABSTRACT to come, which in many cases could profit from sunlight
concentration means when secking, at least in its first
In this paper we present a subdegree precision stages, for cost effcctiveness.
pedestal sun tracker developed for IOOOX Oriented towards one of the referred very high
microconcentrator modules. Setting a cost target of concentration attempts already on its pre-industrial stage
lOO€/m', a lightweight structure has been designed that, [7], we present here thc first prototypes of a pedestal
fully made out of commercially availablc parts, stays subdegree precision sun trackcr. This novel concentration
operative for wind speeds which account for at least 95% concept is to he rcleased with the appearance and
ofthe available direct solar radiation. dimensions of a convcntional flat PV panel, but housing
Thc electronic tracking control unit relies on a instead a parquet of IOOOX and i1.5"acceptance angle
strategy which resorts to high accuracy solar ephemeris, non-imaging lenses, which focuses on an array of Imm'
parameter fitting for a model of installation and Ill-V cells with expected module cfficicncies in the 23%
manufacturing crrors, an adaptive algorithm for absorbing to 26% range.
unforeseen or time varying errors, and automatic At the onset stagc we reckoned thc importance, in
calibration of cheap sun pointing sensors against array order to fulfil the posed cost targets, of constraining our
power output. tracker design, both drives and structure, within the
possibilities of commercially available parts, both in the
construction and automation industrics, thus avoiding
1. INTRODUCTION premiums imposed by specially manufactured
components. In the other hand bcing the manufacturing
Since in the late sevcnties, concentration systems cost of the structure mostly driven by its overall weight,
first captivated the interest of the solar community, when emphasis was placcd in lightening and additionally
it went on stage as a clear track towards strong PV cost devising efficient wind managemcnt strategies.
reduction, a reasonable amount of field cxperience has Regarding the tracking control we set off from our
been acquired which clearly keeps on pointing out its previous experience in the design and production of the
underlying tracking systems as onc of its most error pronc EPS-TENERIFETh' tracking control units for BP Solar's
components [1][2], and probably one of the overloads EUCLIDESTM concentration technology [8], based on an
which is delaying this technology's definitive industrial hybrid strategy powered by high accuracy solar ephemeris
take ofE cornplemcnted by an adaptive alignment routine.
But thc low reliability of tracking systems is not at
all unjustificd; what past PV concentration episodes have
shown is that it is really no small Sent to design tracking 2. COST ANALYSIS
structures and drives highly precise in aiming heavy loads
~ and even more with recent rising trends in the A" initial cost targct of io~€/rn'of aperture was set
concentration factors - and stiff enough to withstand for the trackcr, target which includes the tracking structure
additional wind loads. Neither is it an easy task to and drives and the tracking control unit, hut lcaves aside
develop reliable sun tracking control systems immune the cost of land, land preparation, foundation, in-field
enough to factors such as overcast and hazy skies, assembly and DC wiring.
softwarc failures, extreme weather conditions, or low cost Under this assumption and in order to perform an
low qualification installation crews. All of which is to he electricity cost comparison between present commercial
attained bound by strong cost constraints and with little flat plate technology and microconcentrator modules. we
option, in principle, to learning curvc discounts. follow the method proposed by Foetzberger and Stahl 191
Being this the situation, serious R&D effort in for the cost of cnergy estimation for tracking and non-
concentration tracking systems, will be required in order tracking PV systems, now updated with cost figures
to breed a safe mount not only for the promising very foresecn by Luque and Yamaguchi [IO] for future IOOOX
high concentration concepts now under progress in microconcentrator modules, and convcntional PV cost
different countries [3][4][5][6], hut also to ease the estimates used in recent concentration cost assessments
introduction and implementation of those aery high [Ill,(see Table I). The estimation has been made for a
efficiency 3". generation leapfrogging PV dcvelapments short term scenario. in which microconccntrator
Poster 857

2. 3rd World Confireme on Photovolruic Enrrgv Converriun Moy 11-18,2003 030ko. Japon
technology cnters the market based on GaAs one junction Being the weight of the structure highly influenced
cells, with a 265 Urn2 module, cost which is almost by the desired capability to withstand wind loads, this has
present prototype cost estimate, and a 19% module to be reasonably well set. The highest the Wind Speed
efficiency, while 550 € h z , 15% modules represent present Operative Threshold (WSOT) - the wind speed below
flat plate PV. Note that in the long term the use of W O which the tracker, whichever its position, is able to
junction cells and consideration of learning cost reduction maintain accurate tracking and over which stowing is
rates could drive to a microconcentrator module cost in the ordered the higher the direct solar radiation intake but
~
100 Wm2range and 37% module efficiencies. also the sturdier and therefore more expensive the
tracking structure results. The optimum which brings into
Table I. Data used for the cost of electricity comparison line these two opposing trends, will be the WSOT which
between IOOOX microconcentrator module technology and results in the cheapest cost of solar energy due to
conventional flat plate PV. tracking, i.e. the cumulated direct solar energy input
allowed divided by the tracker's overall cost.
Concentrarion cells ( ~ r n ~ ) . 134.000 Afler the basic structural design concept of the
Concentrarionfactor 1.000 pedestal tracker design was accomplished, a simplified
Optics, heat sink & assemblv (€/mz) 131 model was analysed to determine how the increase in its
Microconcentrotor module efficiency (%) 19 WSOT affectcd the overall tracking cost due to its
Tracking srructure, driws & control (€/mz) 100 progressive stiffening. WSOT was here defined as the
Cunventionai module (€/m') 550 threshold over which 0.1" or higher tracking error is
Conventional module efficiency P )A 15 causcd by wind load bending of the tracking structure,
Land, land preparation, foundation, in-field both pedestal and aperture. Three different aperture
assembly and DC wirinp 20 surfaces (9, 16, 25rn') were analyzcd (see Fig. 2), where
growing reinforcement was attained by escalating
Thus, "1st. generation" microconcentrator panels, structural components in a discrete manner imposed by
could result between 35% and 68% cheaper, when the constraint of only using standard commercial sizes.
compared to present flat panel photovoltaics, for which Costs estimation was performed for a preliminary low to
polar mounting is assumed, for all latitudes 60" above or medium volume industrial scenario. Linear intelpolation
below equator, and direct radiation ratios ranging from of the cost curves offer aperture cost vs. WSOT increase
50% to 80% (see Fig. I), being highest the cost premium - ratios of 0.78, 0.95 and 1.2 €/m' per kmm, respectively
the concentration to flat panel cost ratio - the highest the for 25, 16 and 9m2apertures, showing also that within this
latitudc and the sky's direct radiation ratio. range the biggest the mounted aperture the lowest the
impact of WSOT increase.
0.70 -
4 80%
- -G 70% 250 3
&SO% ~
9 m2
16 m2
-25 m2
200 4
IR = 1.20
0.35 . ~ . .- . .-. .. .
~ . ~ ~ ~
O L -
0 20 40 60 80 100
0 IO 20 30 40 50 60 WSOT (kmlb)
Latitude (")
Fig. 2 Overall tracking cost as a function of its WSOT,
Fig. 1 Cost Premium of microconcentrator technology vs. for three different aperture sizes. Linear intclpolation is
conventional flat panel as a function of latitude, for also shown giving the value of the Aperture Cost vs.
different direct solar radiation ratios. WSOT increase ratio IR.
These curves permit the deduction of the tracker's
optimum WSOT for a certain location, provided
3. WIND LOAD ASPECTS O F T H E DESIGN
cumulative direct solar radiation vs. wind speed plots can
be obtained. This type of analysis could be of use when
Steel is the cost driving factor of tracking structures,
tracker design is tailored to the specific climate of a
in such a way that when ordered in volume cost is mainly
determined by weight. In order to converge towards the certain location, which could be justified in case of
medium to big size plants.
referred cost target, weight optimization was sought for
In this direction a case study was performed for
the tracking structure.
Granada (Sp.) in order to determine the optimum WSOT
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3. 3rd World Conference on Phoruvoltnic Energv Conversion May /I-18, 2003 Osaka. Jnpon
for a 9m2 aperture model from its cost point of view. One aperture, have been built up to know in which other
year ofcontinuous hourly direct radiation, and wind speed driving and structural variations have been tested (see Fig.
and direction measurements were used for this analysis 3).
which revealed that almost 95% of the yearly direct solar Load simulation, both due to extreme winds and
energy could be captured with a WSOT of 15 kmih and concentrating modules, in the form of distributed sand
that the cheapest WSOT was 25 kmih which would allow bags have been applied to these prototypes in order to
capture of up to 99% of the yearly direct radiation. validate design targets and calibrate CAD Finite Element
Taking a less location sensitive and more general tools. Monitoring of the tracking stmchre elastic
approach, a 43 kmPn WSOT was chosen for our design, behaviour under real wind conditions, is at present being
which permits a worst case 95% direct radiation performed in a LabviewT" environment gathering data
availability, within the big climatic variability inside the from an on-site meteorological station, the systems wind
US limits, as can be derived from the data tapes generated sensors, and high precision inclinometers placed on the
by the US National Climatic Center SOLMET stations shucture.
[121. The extra-costs due to its subdegree accuracy
A system of low cost wind speed sensors have been requirements were analysed, to get a cost comparison to
developed in order to stow the tracker whenever wind conventional flat pancl sun trackers. The analysis was
speed grows over the tracker's WSOT. Reliability is done for the three referred aperture sizes, defining the 43
granted to the system through redundancy in the k d h WSOT for three different conditions: 0.1". I" and
measurements. This system is also capable of breakage bending. This analysis shows that 0.1" stiffness
simultaneously providing global radiation measurements means an extra cost of 20-30 €lm2 over I" or breakage
in the aperture plane, which are used by the tracking conditions which are scarcely I Urn2 apart wherever
control strategy. breakage overcomes in the 3" range.
4. TRACKING STRUCTURE AND DRIVES 5. THE TRACKING CONTROL UNIT
A pedestal configuration was chosen due to its In order to avoid the detected flaws of outright open
relative low cost single point foundation, whcn compared or closed loop strategies employed in the past 1131, a
to other options such as tilt-roll. The pedestal option also hybrid improved method has been developed which resorts
permits reductions of on-site assembly work, which to an assortment of algorithms of proven efficiency.
usually dominate installation costs.
Fig. 3 9m' aperture prototype of the pedestal tracker
Dummy panels with centred hanging sand bags simulate
its real operative load.
Fig. 4 Two examples of the error transform applied to a
Standard I-beams, channels, and structural tubing,
commercially available in the construction industry rectangular grid in the azimuth-elcvation plane
determined by the four angle vector (upper figure
compose the structure, avoiding the cost surcharge
lower graph (0'. 15°,00,Z50))
(O~So,0~,Oo),
required by specifically manufactured parts.
Non back-driving drives arc chosen for both axes; a
Taxonomy of errors for ephemeris based open loop
screw jack is used for the elevation axis, while the azimuth
movement is provided by a worm and worm gear set sun tracking was first done. An specific error model which
which makes the tube holding up the aperture, to turn accounts for all characterization errors, bath stcmming
around an inner concentric post to which it is kept coupled from manufacturing and in-field installation, but leaving
by rolling contact hearings. Three prototypes, 9 m' in aside those which derive from the drive & positioner
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4. 3rd World Con/rrence on Fhotovolraic EnergV Conversion Mu.v 11-18,2003 Osaka Jamn
which can he partly suppressed with primary axis position 6. ACKNOLEDGEMENTS
feedback, is derived based on four angles, whose
transforms over the azimuth elevation-plane have been This work has partly been funded by the Direccion
studied (see Fig. 4). General de Investigacion of the Consejeria de Educacibn
AAer a tracking error measurement session, the four de la Comunidad de Madrid under contract no
parameters of the error model are fitted using a nonlinear 09/0105/2000.
least-squares routine. Two error measurement strategies
can be selected depending on the feedback signal used:
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860 Poster