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Solar Energy Vol. 68, No. 3, pp. 263283, 20002000 Elsevier Science Ltd

Pergamon P I I : S 0 0 3 8 09 2 X ( 99 )0 0 0 6 8 7 All rights reserved. Printed in Great Britain0038-092X/00 /$ - see front matter

www.elsevier.com/locate/solener

COMPACT LINEAR FRESNEL REFLECTOR SOLAR THERMALPOWERPLANTS

,DAVID R. MILLS* and GRAHAM L. MORRISON***School of Physics, University of Sydney, Sydney 2006, Australia

**School of Mechanical and Manufacturing Engineering, University of New South Wales, New South

Wales 2052, Australia

Received 25 November 1998; revised version accepted 30 August 1999

Communicated by LORIN VANT-HULL

AbstractThis paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scalesolar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linearreceivers elevated on tower structures that are close enough for individual mirror rows to have the option ofdirecting reflected solar radiation to two alternative linear receivers on separate towers. This additional variablein reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirrorinclination can be set up such that shading and blocking are almost eliminated while ground coverage ismaximised. Preferred designs would also use secondary optics which will reduce tower height requirements.The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining thecost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steamline cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system,namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes asthe receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorberorientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector fieldconfigurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was thedevelopment of specific raytrace and thermal models to simulate the new concepts. 2000 Elsevier ScienceLtd. All rights reserved.

1. INTRODUCTION This paper describes a new design approach

that came from a realisation that trough technolo-The majority of direct solar electricity worldwide

gy was near its design limits and that fundamentalis generated in nine large solar thermal electric

changes to the absorbing surface and collectorplants in California built by Luz International

configuration were needed for large scale im-Limited (LIL), with design and major solar com-

plementation of solar thermal power. New highponents produced by Luz Industries Israel (LII).

temperature selective surfaces with very lowThese solar fields have achieved better than 97%

emissivity for evacuated tube solar absorbers haveavailability over more than 10 years of operation

been developed (Mills, 1991; Zhang and Mills,and have successfully demonstrated the technolo-

1992). Mills and Keepin (1993) suggested that

gy, but in a financial environment requiring these surfaces could be used in new low con-substantial government subsidy. Advanced ver-

centration designs to reduce system costs, andsions of the Luz LS3 collector technology may

increase performance as lowering the absorbingsoon reduce unsubsidised electricity generation

surface emissivity allows greater flexibility incost to about ECU0.085 per kWh (EC, 1998).

geometric concentration as a design variable.However, these costs are still too high for many

Mills and Keepin used polar axis trough collec-markets. Even in markets where there is renew-

tors as the example of a low concentrationable energy obligation legislation, significant cost

collector, but there are several new design con-reductions in solar thermal power systems are still

figurations that could use advanced evacuatedneeded to compete against other renewable energy

tubes. In this paper, we advocate the use of angrid-electricity systems using waste biomass fuel

advanced form of linear Fresnel reflector as aor wind energy.

more cost-effective alternative to parabolic troughor parabolic dish systems. The peak solar energy

radiant flux concentration of such systems canAuthor to whom correspondence should be addressed. Fax:161-2-9351-3577; e-mail: [email protected] range between 50 and 150% of that used by LS2

263

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264 D.R. Mills and G.L. Morrison

and LS3 systems (23:1 and 26:1, respectively). and secondary concentrators, both of which allevi-

Higher concentrations are being investigated ate the requirement for a small primary image size

using advanced secondary reflector systems and and very high optical concentration.

will be reported in future publications. A more recent effort to produce a tracking

Linear Fresnel Reflector was made by the Israeli

Paz company in the early 1990s (Feuermann and2. LINEAR FRESNEL REFLECTOR Gordon, 1991; Feuermann, 1993). Although in-

TECHNOLOGYtended for 1508C operation, this technology is the

Geometrically, the ideal reflectors to use with closest in the literature to that proposed here. This

single receivers of solar energy are continuous array exhibited, among others, aberration difficul-

reflectors, usually parabolic for linear axis sys- ties caused by the movement of reflectors around

tems, or paraboloidal for two axis systems. Large an axis parallel to but displaced from the reflector

continuous reflectors or lenses can be simulated optical axis. The analysis approach taken by the

by small elements distributed over a plane thus Paz system developers was to use an optical ray

avoiding the problems associated with very large trace program for a system with finite reflector

reflectors. Baum et al. (1957) discussed large sizes. This is also the method used in the course

two-axis solar tracking systems of this type, but of the current study.

the first to apply this principle in a reasonably One fundamental difficulty with the Linearlarge linear system for solar collection was Fran- Fresnel Reflector (LFR) technology is the avoid-

cia (1968), who developed both linear and two- ance of shading of incoming solar radiation and

axis tracking Fresnel reflector systems. This work blocking of reflected solar radiation by adjacent

showed that elevated temperatures could be reflectors. Shading and blocking can be reduced

reached using such systems. Following this, Riaz by using higher absorber towers, which increases

(1976) developed theory associated with two-axis cost, or by increasing absorber size, which allows

systems, which was soon accompanied by addi- increased spacing between reflectors remote from

tional work by Vant-Hull and Hildebrandt (1976), the absorber. The latter leads to increased ground

Abdel-Monem et al. (1976), Lipps and Vant-Hull usage relative to collector area and also increases

(1977), Lipps and Vant-Hull (1978), Igel and both thermal losses and shading by the absorber.

Hughes (1979) and Dudley and Workhoven(1978, 1979). The work by Riaz can be adapted to

linear systems, and he discusses shadowing ef- 3. COMPACT LINEAR FRESNEL REFLECTOR

fects in a general way. Wei (1980, 1981) discusses (CLFR)

simplified calculations for two-axis systems.3.1. Basic conceptMuch of this work was associated with early

modelling of the US Central Receiver programme Compact Linear Fresnel Reflector (CLFR) tech-

which culminated in Solar One, a 10 MW(e) nology is, in effect, a second type of solution for

two-axis tracking solar power plant constructed in the Fresnel reflector field problem which has been

the early 1980s. However, Di Canio et al. (1979) overlooked until now. The classical linear Fresnel

of the FMC Corporation produced a detailed system has only one linear absorber on a single

project design study for a linear plant of between linear tower, and therefore there is no choice

10 MW(e) and 100 MW(e), with a mirror field on about the direction of orientation of a givenone side of a 1.68-km linear cavity absorber reflector. However, if one assumes that the size of

mounted on 61 m towers. Vant-Hull (1991) sug- the field will be large, as it must be in technology

gests that the increased image size and lowered supplying electricity in the multi-MW class, it is

concentration for ray incident angles not perpen- reasonable to assume that there will be many

dicular to the linear axis would permit no advan- linear absorbers in the system. If they are close

tages over the Central Receiver plants. However, enough, then individual reflectors will have the

the FMC report itself acknowledges these optical option of directing reflected solar radiation to at

shortcomings but says these are compensated by least two absorbers. This additional variable in

lower costs of manufacture and maintenance; the reflector orientation provides the means for much

authors were aware of Central Receiver develop- more densely packed arrays, because patterns of

ment going on in parallel at the time and proposed alternating reflector inclination can be set up suchuse of the same generating system. Since this that closely packed reflectors can be positioned

report was made, substantial advances have been without shading and blocking. The interleaving of

made in the areas of spectrally selective absorbers mirrors between two linear absorber lines is

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Compact Linear Fresnel Reflector solar thermal powerplants 265

Fig. 1. Schematic diagram showing interleaving of mirrors without shading between mirrors.

shown in Fig. 1. This arrangement minimises tube. The absorber of the glass evacuated tube

beam blocking between adjacent reflectors and is connected to the central steel pressure tube

allows higher reflector densities and lower ab- by a heat transfer fin. The evacuated tubes

sorber tower heights to be used. Land or roof area exhibit very low radiative losses and arecost is in many cases not a serious issue, but inexpensive, the current cost of a 1.2-m long,

available area can be restricted in industrial or 45-mm diameter evacuated tube is |US$15.

urban situations. Avoidance of large reflector Low array maintenance costs due to ease of

spacing and high towers is an important cost issue access for cleaning, and the capability to

when one considers the cost of ground prepara- remove the single ended evacuated tubes with-

tion, array substructure, tower structure, steam out breaking the heat transfer fluid circuit.

line thermal losses, and steam line cost for This paper investigates alternative versions of

installation next to an existing fossil fuel generat- the CLFR concept to determine which are worthy

ing plant where the objective is the retrofit of a of further development. Areas of study include

low pollution steam source. field orientation, absorber orientation, absorber

The CLFR power plant concept proposed in structure, usage of auxiliary reflectors adjacent tothis paper is intended to reduce costs in all the absorbers, reflector packing density, and tower

elements of the solar array. The following features height.

enhance the cost effectiveness of this system3.2. Horizontal tracking axis arrayscompared with trough technology.

Flat or elastically curved glass reflectors CLFR arrangements can include analogues of

mounted close to the ground are used to horizontal EastWest axis, NorthSouth axis and

minimise structural costs. Costly sagged glass polar axis parabolic troughs. In the latter, the

reflectors are avoided. plane of the CLFR array is inclined toward the

The heat transfer loop is separated from the equator at the latitude angle, and would require an

reflector field and fixed in space thus avoiding inclined support structure or favourable ground

the high cost of flexible high pressure lines or inclination. A scaled layout of a CLFR system

high pressure rotating joints required in trough with a 50-m tower spacing, 10-m high absorberand dish systems. and 48 mirrors, each 1 m wide, is shown in Fig. 2.

The heat transfer fluid is water, and passive The high-density arrangement of reflectors shown

direct boiling heat transfer is proposed to in Fig. 2 is such that the reflectors are separate but

minimise parasitic pumping losses and the would be close to touching if all were tilted to the

need for flow controllers. Steam supply may horizontal. Lower densities of reflectors may be

either be directly into the power plant steam more cost-effective in some cases. The scale of

drum or via a heat exchanger. Steam can also the moving elements is relatively small, even

be supplied in a similar manner for power though the unit scale of the overall system is very

plant preheating cycles. The steam delivery large compared to other linear concentrator con-

conditions considered in this study are 3508C figurations; the pictured array is equivalent to a

and 16 MPa wet steam. parabolic trough of focal length of around 10 m. An absorber composed of a pressure tube In this project most of the investigation was

containing the high pressure water, mounted done on a horizontal EastWest axis array using a

inside an advanced all-glass evacuated Dewar vertical absorber system consisting of a vertical

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Fig. 2. Layout to scale of a CLFR array with 48 mirror rows, 50-m absorber spacing and absorbers 10 m above the primary

reflector field. Scale marks are in metres. Mirrors near each tower are trained on it alone because close packing can be achieved

without blocking, mirrors in the middle of the two absorber rows have alternating directions.

wall of all-glass evacuated tubes illuminated from inclination required. The stationary reflector redi-

both sides, or a horizontal absorber tube array rects rays that have not yet hit the finite length

illuminated from one side. primary mirror but would be intercepted by an

infinite length primary mirror and also collects

3.3. Inclined NorthSouth and polar axis rays that have been reflected from the primaryarrays mirror but have struck the reflecting wall before

A CLFR array can also be inclined toward the striking the finite length absorber. Inclination2

equator to increase winter and annual collection. markedly improves collected energy per m of

A NorthSouth axis array inclined at the latitude reflector for locations outside tropical latitudes.

angle (a polar axis tracking array) will yield close Inclining the array necessitates spacing between

to the optimal annual performance. Performance the inclined reflector arrays to avoid winter shad-2

simulations for different tracking axis orientation ing, and decreases output per m of ground area

and inclination are given in this study. An in- occupied compared to a horizontal NorthSouth

clined array would be similar to that shown in array. Inclined NorthSouth arrays have a flatter

Fig. 3, in which a short NorthSouth array is seasonal output profile compared to horizontal

tilted toward the equator. A stationary reflector at arrays, however, they require a more expensivethe back end of each array is used to reduce the substructure than horizontal arrays. The lower

Fig. 3. Inclined CLFR field with an inverted receiver. The stationary vertical reflector wall improves winter collection.

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input in the early morning and late afternoon due evacuated tubes, since rays reflected from the

to the raised artificial horizon for an inclined array glass cover tubes or absorber elements could find

is accounted for in the analysis that follows. their way to other tubes. Primary, secondary and

tertiary reflections were tracked in the absorber. In

this study reflector slope errors were not consid-

4. SOLAR COLLECTOR OPTICAL ered.

MODELLING

4.2. CLFR receiver optical modelling4.1. Raytrace modelling

Two primary receiver types are proposed forA raytrace model was used to generate optical this technology. The first uses all-glass evacuated

collection maps in terms of transverse and longi- absorber tubes in a vertical rack illuminated fromtudinal incidence angles. The concentration maps both sides. The second uses tubes in a single-and beam radiation data were used as inputs to sided horizontal receiver facing downward.thermal modelling routines. A two-dimensional

model was used to generate concentration data for 4.2.1. Optical modelling of vertical evacuated

a series of slices through the array. The two- tube receiver rack. The first configuration consid-

dimensional solutions were then assembled to ered was a receiver with a vertical absorber

generate three-dimensional maps of the optics of protected by evacuated absorber tubes that werethe systems. Absorber angular response and inter- illuminated from both sides as shown in Fig. 4.

actions with the curved surfaces of the glass All-glass evacuated tubes having an outer cover

Dewar tubes was accurately modelled. The three tube diameter of 45 mm and a cover tube thick-

dimensional map of optical concentration and ness of 1.5 mm were evaluated. The absorber

absorption as a function of two orthogonal inci- surface on the outside of the inner glass tube is

dence angles was used in a radiation and thermal 1200 mm long, with a diameter of 37 mm. The

model developed in TRNSYS (Klein et al., 1996). University of Sydney has licensed the basic

The ray tracing model incorporated a branch- selective coating technology to a manufacturer in

ing ray concept for modelling reflections in an China (Turbosun, 1998), and large volume low

array of evacuated absorber tubes. A complex cost tubes having such dimensions are available.

incremental raytrace model was used to establish Due to the use of single ended absorber tubes thethe optimal orientation of each mirror row for a array can only operate as a boiler and thus cannot

given solar incidence angle. A model that in- generate super heated steam.

cluded tracking of a ganged field of mirror rows A potential difficulty with this arrangement is

was also used. The branching ray model was that evacuated spaces between the inner and outer

necessary to gauge the optical absorption ef- glass tubes allow radiation to pass through the

ficiency of arrays of closely spaced all glass absorber rack. Such losses could be significant.

Fig. 4. Vertical Dewar-type absorber tube banks illuminated from both sides. Fluid flow occurs entirely in fixed tubing.

Feedwater is introduced from the header pipe into the branch pipes enclosed by the evacuated glass tube. Boiling occurs in the

branch pipe and we saturated steam leaves via the header.

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absorber surface because much of the available

absorber surface faces other absorber tubes.

The absorptance of such an arrangement varies

with the size of the gap between the tubes. A

raytrace program was developed which follows

primary ray paths through the tube assembly,

together with ray reflections from the glass tubesurfaces, as many reflections are collected on the

second bounce. It was found that the absorptance

of the tube array with 25-mm spacing could be

approximated quite well by a flat plate receiver

covered by a flat glass sheet (Fig. 6). The best

configurations were about 2.5% better than a flat

plate although the flat plate was the best perform-Fig. 5. Double row tube arrangements of branch tubes en-

er at normal incidence. A tube spacing of 25 mmveloped by all-glass evacuated tube absorbers. (a) Close

was chosen for analysis (equivalent to a singlepacked zig-zag absorber array, (b) absorber array with 2.5-cmline spacing of 0.64 diameters), as the hemispheri-gap between tubes.

cal absorptance for this tube spacing is almostidentical to a flat plate absorber (Fig. 7). This

However, because the tubes are inexpensive it choice also allowed use of a flat plate receiver in

was possible to consider a staggered double row subsequent modelling as a convenient approxi-

tube configuration (Fig. 5) with high tube density mation to the evacuated tube receiver. More

and very high optical interception. In a single row detailed analysis may be needed to determine tube

of touching evacuated tubes the gap between the spacing sensitivity, but a spacing of 0.64 diame-

inner and outer tubes amounts to 18% of the face ters is expected to be close to the optimum. In a

area of the absorber. For vertical tube racks a double row vertical absorber system the tube

solution to the gap loss problem is to use a pitch, or interval between tube centres in each

zig-zag double row of evacuated tubes (Fig. 5b). row, is 45 mm1 25 mm5 70 mm. The number of

Such a receiver traps light passing through gaps tubes per lineal metre of receiver using a 25-mmbetween the inner and outer tubes of the vacuum spacing and two rows is 28.6 / m.

tube fitted over the branch tubes. The absorber A second approach to the vertical evacuated

surface consists of both sides of the tube rack, as tube gap loss problem is to use secondary reflec-

in Fig. 4. The absorptance as a function of tube tors behind the tubes. This requires two separated

spacing shown in Fig. 6 is dictated primarily by rows of tubes with individual non-imaging reflec-

the plane aperture (face area) of the receiver, not tors for each tube between the two rows. This will

the circumferential area of the evacuated tube allow an increased tube spacing and a decreased

tube related cost. However, the optical efficiency

will always be less than that of the quasi-vacuum

flat plate receiver due to both increased ray

spillage and absorption in the reflector. The

benefit of increased tube spacing with secondaryreflectors depends upon component costs. Costs

are not presented in this paper, but our estimate is

that the increased optical losses and reflector

absorption would require an increase in plant size

which will be in excess of the evacuated tube and

pressure pipe related cost savings. Similarly, the

40% increase in tube numbers needed for a

smaller tube spacing of 5 mm would cost more

than the 3% performance increase gained. There-

fore, for modelling of vertical receivers, a 25-mm

zig-zag spacing was assumed.Fig. 6. Angular absorptance of zig-zag tube racks withdifferent tube spacing compared to a flat absorber covered by a

4.2.2. Optical modelling of horizontalglass sheet. The acceptance of the zig-zag rack averaged overall angles is almost identical to a flat plate absorber. evacuated tube receiver rack. In horizontal receiv-

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Fig. 7. Hemispherical absorptance of double row absorber relative to a flat plate absorber with a glass cover.

2ers, the absorber face area is 1.2 m per lineal A secondary reflector can be used underneath

metre because it is single-sided (facing down- the horizontal absorber to enhance optical collec-

ward). One option is to use the same zig-zag tube tion and increase concentration. A schematic

arrangement as the vertical absorber. This would diagram of one such reflector design is shown in

achieve the same high absorptance but would be a Fig. 8 without an upper backing reflector. This

very expensive arrangement because only one uses a horizontal array of tubes with a backing

side of the rack (the underside) is receiving solar reflector above the tubes to collect rays passingradiation. Alternatively a backing reflector behind between the absorbers. The bifurcated secondary

a single row of tubes could be used. A system reflector system is designed such that most rays

with a tube pitch of 49 mm (to avoid tubes from the primary reflector strike the absorber

touching) requires only 20.4 tubes per lineal directly and do not incur an absorption loss on

metre of absorber and has negligible penalty in this secondary reflector. Only rays on the

optical efficiency. Approximately 1 / 4 of the rays periphery of the reflected beam use the secondary

find their way through the gaps in the worst case reflector.

of normal incidence, but these can be reflected4.3. Field raytraceback with high efficiency, and rays from lower

angles mostly strike the tubes directly. This Optimisation of the mirror field requires con-

absorber configuration would be less costly than sideration of the two possible positions of each

the zig-zag arrangement, but optical performance mirror row for each solar radiation input angle,and heat losses would be similar. corresponding to two absorber targets. The model-

Use of a non-imaging CPC backing reflector ling process involved beginning raytracing with

would also reduce the number of tubes and tube an arbitrary starting configuration for the linear

related costs. However, as this is a more open elements in the mirror field, then flipping the first

pitch tube arrangement with less direct tube mirror in the field and raytracing the whole field

absorption and increased reflector absorption the again. Having chosen the best position for the first

overall gain in cost-effectiveness will be minimal reflector, the second reflector was flipped and the

because the entire array must be enlarged in order best position chosen for it. This process was

to achieve the same output. In this paper we have repeated for the entire field, and for all incidence

evaluated the performance of the horizontal ab- angles. To account for the finite size of the solar

sorber systems using an inverted vacuum insu- source a secondary raytrace between 60.758 waslated flat plate receiver model which would be carried out, which increased the number of rays

similar in performance to the high density, tube by a factor of five. The resulting raytrace compu-

configuration. tations were very large. For a 48-mirror row

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Fig. 8. Secondary reflector for a horizontal tube rack. Rays from the outer edge of the primary Fresnel reflector use the secondary

reflector more because of beam spread.

array, the model included 48 mirrors, two posi- configuration was 84% of the total beam radiationtions, 1000 rays, 19 angles and five beam spread arriving between the linear towers. The 2D inter-

divisions amounting to 9.12 million rays for each ception factor increases with incidence angle

simulation of the field optical characteristics. because gaps between the reflectors are covered

The optical specifications used were:

normal incidence absorptance of the evacuated

tube absorber surface5 0.94;

refractive index of transparent coating on the

mirror5 1.47;

reflectance of mirror surfaces5 0.95;

mirror segment width5 1 m.

In operating systems, reflectivity will be lostdue to dirt deposited on the mirrors between

cleaning operations. The raytrace in this study

was performed on the basis of clean mirrors.

Optimum performance in a CLFR is obtained

by directing each tracking mirror strip to the best

receiver for the time of day. This implies that

each mirror row must have independent tracking.

However, the simplest and cheapest mechanical

arrangement is to have many mirror rows me-

chanically attached to each other and run from a

single motor. Thus it is important to know the

performance penalty associated with mirror rowganging, and this is modelled for some of the

configurations. Having mirrors of identical curva-

ture could lower the array cost, and this was also

investigated.

Fig. 9a and b shows optical concentration maps

for two field configurations. A wide spacing (Fig.

9b) allows each mirror to gather more energy

without blocking or shading, and the output is

flatter throughout the day. A dense mirror con-

figuration (Fig. 9a) approaches the 2D cosine

collection characteristic of a horizontal flat plate Fig. 9. Ray trace map of CLFR solar radiation flux con-collector. The peak interception of beam radiation

centration, (a) absorber 15 m above a 48-mirror array, (b)for solar radiation arriving from a direction per- absorber 10 m above a 24-mirror array with mirror segmentspendicular to the CLFR array plane in the dense spaced by one mirror width.

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by adjacent reflectors for low angles of incidence. designed to accept an incidence angle modifier

Peak flux concentration of the configurations map with up to 50 incidence angles in both the

considered without including optical losses (com- longitudinal and transverse planes. The optical

parable to peak geometrical concentration for a map data was generated using the ray tracing

trough) is 35:1 for the 48-mirror array. The routine described in Section 4.1.

geometrical concentration of the Luz LS2 collec- The radiation processor in TRNSYS was used

tor was 23:1 (aperture to absorber tube circumfer- to compute beam radiation from hourly globalence). Higher concentrations can be achieved with radiation records. For analysis in locations where

the Fresnel system, but the concentration is only hourly solar radiation was available the

limited by the heat flux capacity of the Dewar Reindl et al. (1990) radiation model was used to

tubes. compute beam radiation in terms of the clearness

index and the solar elevation. Radiation on the

tracking surface was calculated using the Hay and5. THERMAL MODELLING OF CFLR

Davies (1980) model which accounts for both

Solar radiation and thermal simulation models circumsolar and non-isotropic diffuse radiation

of the collectors were developed in the TRNSYS using an anisotropy index to quantify the portion

modelling environment (Klein et al., 1996). For of diffuse radiation considered as isotropic. Radia-

this project a series of extensions were developed tion data for various sites modelled is shown inwithin TRNSYS to simulate the linear Fresnel Fig. 10. For Dubbo, 1-min time step beam

concentrating collector. Due to the modular nature radiation measurements were available. The mea-

of TRNSYS these new routines were integrated sured data were converted to normal incidence

with the existing data handling and solar radiation beam radiation for the horizontal or inclined

analysis routines that are built into TRNSYS. planes of the Fresnel mirror field being consid-

The primary routines that were used to simulate ered. The annual solar radiation at each of the

concentrating solar collector performance were as design sites is shown in Table 1.

follows.

Radiation processor, nonisotropic radiation 5.1. Heat loss from an array of evacuated tubesdistribution model (TRNSYS TYPE16).

The absorber of the proposed linear Fresnel

Extended optical map-based solar collector collector consists of a rack of evacuated tubemodel for Fresnel concentrator (Morrison,absorbers mounted in two rows so that the

1997).absorber is equivalent to an evacuated flat plate

Nonlinear heat loss solar collector model forabsorber. The heat loss from the evacuated tube

evacuated tube absorber (Morrison, 1997).rack was determined from the measured charac-

The collector thermal mass was modelled interistics of single evacuated tubes (Harding et al.,

TRNSYS using an instantaneous collector ef-1985). The heat loss modes are as follows.

ficiency model coupled to a zero heat-loss Conduction through the insulated header.

storage-tank (TYPE4 tank). This procedure used Conduction through the glass envelope at the

the proven TRNSYS tank routine and solver toopen end of the tube.

include the effect of thermal capacitance, rather Conduction through the metal retainer near the

than developing a complex collector model with

closed end of the absorber tube.built in capacitance. This model follows the start Radiation from the absorber.

up and shut down transients at the beginning andThe heat loss (Q ) from a single tube can be

lend of each day and transient temperature effectsexpressed as

during cloudy periods. The transient effects, due

to thermal capacitance within the absorber, were Q 5 k (T 2T )1 k (T 2T )l 1 m a 2 s a

found to reduce the annual output of the collector4 4

1 k (T 2T ) W/ tube. (1)3 s aarray by 3 to 6% depending on the array con-

centration.For a single tube of the Sydney UniversityTo model the CLFR performance the TRNSYS

design (1.4 m long), Harding et al. (1985) havecollector routine was modified to include a spe-2

shown that k 50.26 W/(K m ), k 50.039 W/(Kcification of optical concentration through a biaxi- 1 229 4

al incidence angle modifier map (see Fig. 9). This tube) and k 58.5310 W/(K tube) where k is3 1was implemented via an extension of the optical the header heat loss factor, k is the conduction2mode 4 option in the extended TRNSYS TYPE1 heat loss factor for conduction heat loss from the

solar collector model. The new routine was absorbing surface to the top of the tube and

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Fig. 10. Beam radiation at Australian design sites.

9through the retainer clip, k is the radiation heat The header heat loss coefficient k for a close3 1loss factor, is the absorber emissivity 5 a1 packed tube rack is

bT where a and b are coefficients determineds 2pkfrom measurement of surface properties (Fig. 11),]]

]9k 51 d2T is the absorber surface temperature, and T iss m]ln S Ddthe mean fluid temperature in the evacuated tube. 1

The tube array proposed for the absorber of the5 0.343 W/ (K m run of header) (2)

CLFR presents a continuous outer face to the

surroundings, thus the radiation heat loss is where k is the header insulation conductivityequivalent to that of a flat plate evacuated collec- (0.06 W/m K), d , d are the inner and outer1 2tor. The radiation heat loss per unit face area of diameters of the insulation (100-mm steam tubethe close packed array will be the same as the with 100 mm insulation).radiation heat loss per unit circumferential area of The heat loss due to conduction around the topa single evacuated tube as measured by Harding of the tube and through the retainer clip is givenet al. (1985). The header heat loss per tube will bybe reduced due to a closer tube spacing than

considered by Harding. Q 5 k (T 2T ) W/ tube. (3)C 2 s a

Table 1. Annual solar irradiation data for Australian design sites2

Location Latitude Climate Annual irradiation MJ /(m day)

Global Diffuse Total at Beamhorizontal latitude angle

Longreach 238 S Dry desert 21.8 7.6 23.1 22.2Dubbo (1994) 328 S Dry inland 19.4 6.5 22.6 24.6

Sydney 348 S Temperate coastal 16.9 7.0 18.6 16.3Wagga 358 S Temperate 17.7 7.1 19.3 17.2

inland

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Fig. 11. Emissivity of selective surfaces.

The conduction heat loss per unit face area of double sided zig-zag evacuated tube rack (28.6

the rack is tubes / m) is

2 Q5 0.143 T 2T 1 0.93 T 2T 1 6.44s d s dQ 5Nk (T 2T ) /L W/ m (4) m a s aC 2 s a28 4 4 2

310

T2

T W/ m . (6)s ds awhere N is the number of tubes per metre lengthof rack (28.6/m for the vertical tube configuration

The selective surfaces investigated in this pro-and 20.6 for horizontal tube configuration in this

ject are two formulations of a stainless steel/study), L is the length of tubes51.2 m active

aluminium nitride cermet with a copper reflectorlength. The radiation from a single tube is given

layer (SS/Cu) and two formulations of a stainless4 4by k T 2T W/tube.s d3 s a steel/aluminium nitride cermet with a2

The surface area of a single tube is 0.132 mmolybdenum reflector (SS/Mo). The temperature

hence the radiation heat loss per unit surface areadependence of the emittance of the four selective

of a single tube or per unit face area of a tubesurfaces being considered for this application is

rack isshown in Fig. 11. The trade off between high

4 4 absorptance and low emittance is considered inQr5 k T 2T /0.132s d3 s a the analysis.

28 4 4 25 6.443 10 T 2T W/ m (5)s ds a The heat loss per unit face area from the

evacuated tube rack absorber for the CLFR sys-

The overall heat loss per unit face area of a tem is shown in Table 2 for a range of absorber

Table 2. Heat loss per unit face area of evacuated tube rack using stainless steelcopper selective surface (a50.93)2

Absorber surface Heat loss W/ mtemperature Header Tube conduction Tube radiation Total

8C

100 10 64 32 105200 22 143 146 312

300 34 223 423 681400 47 303 988 1337500 59 383 2020 2462

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Table 3. Heat loss per unit mirror area for alternative CLFRsystems

Absorber surface Heat loss per unit area of mirror2

temperature W/ m8C CLFR 24 CLFR 36 CLFR 48

100 10.5 7.0 5.3200 31.2 20.8 15.6300 68.1 45.4 34.0400 133 89.2 66.9500 246 164.1 123

temperatures. The heat loss from the absorber perFig. 13. Cross section through Dewar-type evacuated tube andunit mirror area for three CLFR reflector packingpressure pipe, dimensions in mm.

densities is shown in Table 3. CLFR24 refers to a

CLFR system using 24 mirror rows in the 50-m

space between two towers, CLFR36, 36 mirrors in Schmid et al. (1990) have shown that a large

the same space, etc. temperature difference will occur between the

absorber surface and the fluid in the pressure tube

unless a fin system is used in the clearance space6. ABSORBER CONFIGURATIONSbetween the evacuated tube and the pressure tube.

The absorbers in the CLFR systems are single-6.1. Heat transfer in absorberended evacuated absorber tubes mounted horizon-

tally (Figs. 12 and 13) or vertically (Fig. 4). The The CLFR systems considered in this study

steel pressure tube inside the evacuated tube can have concentrations (mirror area / face area of the2

be either a single-ended tube or a flow through absorber surface) up to 20:1. For 900 W/m beam

U-tube system. A single ended system will be intensity the incident radiation flux on the ab-2

better at the radiation flux levels typical of this sorber will be 18 kW/m . The tube density

absorber system because a U-tube of sufficient adopted for the CLFR system is 28.6 tubes / m

diameter to provide adequate feedwater flow with each tube having 1.2 m exposed length hence

could not be easily fitted inside the inner glass the maximum heat transfer will be 755 W/tube.tube. The absorber surface of the Dewar-type This maximum heat transfer will occur only when

evacuated tube is on the vacuum side of the inner the sun is directly overhead of the mirror array.

glass tube. The absorbed energy must be con- To minimise heat loss the temperature drop

ducted through the 1.5-mm wall of the inner between the absorber surface and the inner pres-

borosilicate glass tube, then across the clearance sure tube must be minimised.

gap between the glass tube and the pressure tube Thermal resistance between the absorber sur-

and then through the pressure tube wall, Fig. 13. face and the water / steam working fluid is due to

The pressure tube is supplied with feed water conduction through the absorber glass wall;

from the header and returns steam through the heat transfer across the gap between the ab-

same opening. To obtain separation between the sorber tube and the pressure tube (via an

liquid and gas streams a slight inclination from internal fin);

the vertical or horizontal is required. The flow conduction through the pressure tube wall;configuration has been successfully demonstrated convection into the boiling water in the pres-

in a number of prototype systems (Mills, 1991). sure tubes.

Fig. 12. Transverse cross section through absorber rack, the evacuated absorber tube can be vertical or near horizontal and can be

easily slipped off the boiler tubes.

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The temperature drop across the absorber wall temperature drop across the pressure tube wall is

DT is given by DT 51.3 K.1 3The convective heat transfer coefficient inside

dQ o the pressure tube during pool boiling will be very]] ]DT 5 ln (7)S D1 2pk L d high and the temperature drop will be small.g i

Fouling of the inner surface of pressure tube

where Q is the heat transfer 755 W/tube maxi- should not significantly add to the overall thermalmum; d is the outer diameter of inner tube ofo resistance between the absorber surface and thevacuum envelope537 mm; d is the inner diam-i steam.eter of inner tube of vacuum envelope534 mm; L The fin system will have additional thermalis the length of vacuum tube51.2 m; k is theg resistance due to the contact resistance betweenborosilicate glass conductivity51.1 W/m K. The the glass tube and the fin, and between themaximum temperature drop across the glass ab- pressure tube and the fin. The outer contactsorber tube is DT 57.7 K.1 between the glass and the fin has not been a

Heat transfer between the glass absorber tube problem in low temperature tubes using a similarand the pressure tube wall is via conduction type of fin. The contact resistance with thethrough the fin system and convection and radia- pressure tube could be minimised by using a fin

tion through the gap. The fin system could consist system consisting of a web sandwich so that thereof a circumferential plate and radial elements as is full circumferential contact over both the glassshown in Fig. 13. The fin system is a combination absorber and the pressure tube. This fin systemof two split cylindrical sections 0.5 mm thick with would be inserted into the evacuated tube andradial fins 0.2 mm thick and 4.5 mm long between twisted as it is slipped over the pressure tube. Thethe two cylinders. A finite element analysis of contact resistance can also be reduced by increas-conduction in the outer ring and through the radial ing the number of fins beyond the 16 fins atfin has indicated that it is equivalent to a simple 11.258 pitch considered in this analysis. At the

20.2-mm-thick fin that is 7.5 mm long. If the radial maximum beam intensity of 900 W/m when thefins were formed from 0.2-mm copper on an beam is normal to the array the temperature drop11.258 pitch the temperature drop (DT ) across the2 across the absorber tube is 13.2 K1the contactfin system (assuming all heat transfer is through

temperature drop.the fins as radiation and convection through the In the following analysis an overall temperaturegap will be minimal) is given by drop of 20 K has been assumed for beam radia-

2tion input of 900 W/m . The development of the

Qabsorber tube will require assessment of a number]]DT 5 l (8)2 NLt kf f of configurations to determine the most effective

finning and contact arrangement.where Q is the heat transfer per tube5755 W; tfis the fin thickness50.2 mm; k is the finfconductivity5350 W/m K (copper at 3508C); N 7. COLLECTOR PERFORMANCE ESTIMATESis the number of fins around the circumference5

In this section, array performance simulation is16; l is the equivalent length of fins between theused to select optimum CLFR configurations. The

glass wall and the pressure tube5

7.5 mm; L is basic selection calculations were carried out forthe width of fins5length of vacuum tube51.2 m.Sydney, Australia, but it was found that per-For a copper fin system the temperature drop isformance relativities are maintained at all sites.

DT 54.2 K.2This is because all of the sites in this study haveThe temperature drop across the pressure tubelow circumsolar radiation and therefore plants inwall is given bythese sites will operate similarly to a plant in

Sydney. Because of the low circumsolar radiationdQ o]] ]DT 5 ln (9)S D3 in Australia, it is sensible to optimise performance2pk L dw i

on the basis of a direct beam capture half angle of

0.758. Diffuse and circumsolar radiation outsidewhere Q is the heat transfer per tube5755 W; dothis range is ignored in the simulation, but there isis the outer diameter of pressure tube525.4 mm;

little energy in this angular range in Australia. Ind is the inner diameter of pressure tube519 mm;ipractice there will be beam spread due to mirrork is the conductivity of pressure tube520 W/mwirregularities and mirror tilt error and slightK (chrome steel at 3508C); L is the length ofdifferences in diffuse collection between differentpressure tube in the evacuated tube51.2 m. The

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absorber configurations but these factors have between the improved optical collection and

been ignored in this initial investigation. In addi- increased thermal losses for a larger receiver

tion, beam spread for rays with a significant surface. The receiver size that delivered maximum

directional component parallel to the array linear energy collection was evaluated for a 50-m wide

axis has not been included, but is not serious for a array using 36 mirror rows. Annual delivered

concentrator of this receiver aperture with mini- energy for different absorber tube lengths is

mal aiming error. However, in some climates, shown in Fig. 14 for both horizontal and verticalnotably tropical regions and parts of the Northern absorbers. The operating temperature in each case

Hemisphere, beam radiation is more forward was 3208C. In neither case does the energy

scattered than in Australia and the amount of collection vary significantly with absorber size.

circumsolar radiation is greater. For such loca- The size of the horizontal absorber was found to

tions the acceptance angle of the collector must be optimal at 1.2 m tube length, and the vertical

therefore be larger to collect both the direct and absorber at 1.0 m tube length. The available

near circumsolar components. It is possible to evacuated tubes are 1.2 m long but an absorber

alter the design to enlarge the absorber for this 1.0 m long can be constructed by angling the

purpose. This is not done in this paper, as it leads tubes in the rack.

to a relatively greater capital cost and increased

7.1.2. Secondary reflector. Secondary reflec-thermal loss. tors can be used near the receiver to capture

reflected solar radiation that would otherwise have7.1. CLFR configuration assumptionsmissed the receiver. The ends of the secondary

A 50-m wide array is assumed with mirror rowreflector reposition the edge of the receiver aper-

densities of 24, 36 and 48 rows per absorber line.ture to better face the primary reflector array. One

Each mirror was taken as 1 m wide with 0.95 mconfiguration of a horizontal absorber (1.2 m

reflecting width and 25 mm edge structure. Eachwide, 12.5 m absorber height, 36 mirror rows)

configuration was evaluated for absorber heightswas chosen to test optimal secondary reflector

of 10 m, 12.5 m, and 15 m. An equal spacing oflength. After the ray trace, an annual performance

mirror rows is assumed and a space 1 m wide iscalculation was performed for each secondary

left clear on either side of the array centre linereflector length all using the same receiver con-

under the absorber for access and maintenance. figuration. The optimal secondary reflector isBecause the effective aperture does not change

similar to that shown in Fig. 8, and is quite short.as a simple cosine function as in a parabolic

The gap between the secondary reflector andtrough (because gaps between reflectors are

absorber allows rays to strike the absorber direct-blocked by adjacent mirrors at high angles of

ly, decreasing secondary reflector absorption loss-incidence), a nominal peak effective aperture was

es. A similar calculation was carried out for aselected. The peak effective apertures of the

vertical absorber with a short circular (orarrays were calculated on the basis of the maxi-

parabolic) section secondary reflector as shown inmum energy that could be collected by an ab-

Fig. 15. The optimal secondary reflector lengthssorber which picks up all beam ray spillage, and

depend on absorber height. However, for sub-is 87.5% of the curved mirror surface area: for 48

m of reflector, this is 42 m of nominal peakaperture in a space of 50 m. This presentation is

analogous to a parabolic trough, where the peak

aperture is used as a parameter for collector

attributes such as cost and energy collection.

There are several options for mirror construc-

tion and mounting. For the purposes of this study

the mirrors are assumed to be a self-supporting

laminated structure using silvered microsheet

glass as the reflector. The auxiliary reflectors2

adjacent to the absorber are assumed to be 2 m

per lineal metre of the absorber.

7.1.1. Optimal absorber length. One important

issue is how large to make the absorber relative to Fig. 14. Useful delivered thermal energy as a function ofthe array field dimensions. There exists a trade off absorber length for a 50-m-wide array.

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Fig. 15. Vertical evacuated tube receiver using secondary reflector above the absorber rack.

top secondary reflector effectively angles thesequent analysis the secondary reflector wasreceiver aperture toward the array, reducing rayscaled with absorber size.spillage at the image boundary. Mirror rows close

7.2. Performance optimisation for Sydney to the absorber line deliver a smaller image and

would be aimed at the receiver itself, avoidingHaving sized the secondary reflectors and thesecondary reflector losses. Mirror rows under-absorber length, it is now possible to determineneath the absorber would use the secondarythe optimum absorber and mirror field configura-reflector.tions. NorthSouth axis and polar axis versions

Table 4 shows the annual thermal energywere used as the basic configurations. The per-

delivery. As tower height increases, performanceformance of EastWest systems was also investi-also increases due to reduced mirror row shading.gated as a variation on the basic design. TheHaving fewer field mirror rows allows moreprimary cases calculated are described in Sectionscollection by each row, but reduces energy col-7.2.17.2.4 using Sydney solar radiation data. Thelected by the fixed size absorber array. This is theselective surface of the tubes was taken as stain-original configuration proposed, but it is out-less/copper with normal incidence absorptance of

0.93. performed both by horizontal primary reflector

fields having horizontally oriented absorbers as7.2.1. Vertical evacuated tube absorber with shown in Fig. 8, and polar inclined fields, Fig. 3.

overhead secondary reflector and horizontal

NorthSouth primary reflector field. This configu- 7.2.2. Vertical evacuated tube absorber with

ration was found to perform better than a vertical overhead secondary reflector and polar axis

absorber without a secondary reflector, Fig. 4. The tracking NorthSouth primary reflector field. Thissecondary reflector is circular (or parabolic, they is a version of the vertical absorber array placed

are almost indistinguishable) in curvature as upon a NorthSouth structure inclined at the

shown in Fig. 15 and is sized according to results latitude angle to improve winter performance

of a large number of annual simulation runs (Figs. 3 and 16). If an end reflector is used at the

determining optimal solar radiation capture. The upper end of each segment of the array the system

Table 4. Performance of vertical evacuated tube absorber with overhead secondary reflector and horizontal NorthSouth primaryreflector field, output per unit area of mirror

Number of mirror rows Thermal energy delivery2

in the primary array MJ /(m day)

10 m tower 12.5 m tower 15 m tower

24 5.54 5.61 5.5736 5.31 5.32 5.3248 4.64 4.80 4.91

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Fig. 16. Polar axis CLFR array.

would closely approximate the performance of a This configuration delivers the highest collection

segment of an infinite array. Table 5 shows that per unit aperture area (Table 7) of all the CLFR

the annual thermal energy delivery of this system configurations considered and is equivalent to

is substantially better than for the horizontal about 70% of a two-axis tracking paraboloidal

reflector configuration (Section 7.2.1). dish.

7.2.3. Horizontal evacuated tube absorber with7.3. Design variationsunderneath secondary reflector and horizontal

NorthSouth primary reflector field. This arrange- There are a number of design variations thatment yields substantially improved performance have the capability of improving performance or(Table 6) compared to the horizontal mirror field lowering cost. In the following, each variation iswith a vertical absorber, since fewer rays are evaluated for Sydney.missed due to a larger total receiver aperture. The

tubes would be tilted slightly off horizontal to 7.3.1. Use of more sophisticated secondary

ensure natural circulation of feedwater into the reflector design to decrease absorber size. The

tubes. configuration shown in Fig. 17 uses a short

horizontal absorber and an upper secondary semi-7.2.4. Horizontal evacuated tube absorber with circular reflector to provide a horizontal aperture

underneath secondary reflector and polar axis for a bifurcated lower secondary reflector. In this

tracking NorthSouth primary reflector field. This case, the absorber rack width is reduced to half

is a polar version of the NS horizontal evacuated the size of that for the previously describedtube absorber array placed upon a structure in- horizontal case. Losses are similar, because loss

clined at the latitude angle facing the equator. can take place from both sides of the receiver, but

Table 5. Vertical evacuated tube absorber with overhead secondary reflector and polar axis primary reflector field


in primary array MJ /(m day)


24 7.07 7.16 7.1136 6.82 6.83 6.8848 5.99 6.22 6.40

Table 6. Performance of horizontal evacuated tube absorber with secondary reflector and horizontal NorthSouth primaryreflector field


in primary array MJ /(m day)


24 6.56 7.06 7.3536 6.15 6.65 6.9648 5.22 5.86 6.23

Table 7. Performance of horizontal evacuated tube absorber with secondary reflector and polar axis tracking primary reflectorfield


in primary array MJ /(m day)10 m tower 12.5 m tower 15 m tower

24 8.26 8.82 9.1636 7.76 8.36 8.7148 6.59 7.41 7.88

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from the absorber and it is these mirror elements

that produce most of the beam spread at the

absorber. The envelope of rays diverge from the

mirror because of the finite angular size of the

solar disk. If the mirror is moved closer to the

absorber, fewer rays are spilled even if the focal

length of the mirror is incorrect. This means that acurvature correct for the outer mirrors should also

work for mirrors closer to the absorber and

explains why the results for beam collection for

the constant curvature are almost the same as for

variable curvature.

The question arises as to whether flat mirrorsFig. 17. Schematic of an upper and lower secondary reflectorarrangement for a horizontal tube rack. can be used for the field to further simplify

production and cleaning. Because the mirror has a

the absorber cost is halved. However, optical physical width of 0.95 m, the half image of the

sun at the absorber will fall outside 0.95 by 0.738,efficiency is reduced because of increased optical

or by about 0.32 m for a mirror at the centre oflosses in the dual secondary reflectors, which nowthe field. This means the total image extent is 0.95intercepts a greater fraction of incoming rays. Them1(230.32 m)51.59 m, about 0.39 m greatermain disadvantages of this approach are the verythan the absorber aperture size. The situation islarge unit size of the collector field 100-mworse for mirrors far from the receiver. Therefore,wide fields would be required for a 1.2-m longin spite of the small curvature, it is necessary totube and the high solar radiation flux on eachcurve the mirrors to capture all of the beamtube. There are several methods for using reflec-radiation, or else increase the absorber size by thetors in this way, however, at this stage, noorder of 50% if flat mirrors are to be used.definitive avenue for improving cost-effectiveness

The performance of horizontal and polar mirrorhas been identified.fields using optimised fixed curvature and flat

7.3.2. Use of a larger receiver to increasemirrors are compared to those of variable curva-acceptance angle. A simulation was performed on ture in Table 8. The optimised constant curvature

the standard array with the absorber doubled in performs only 0.50.6% lower than the variablesize. The net result was a 3% decrease in de- curvature, but the flat mirror is 13% lower. The

livered energy due to increased thermal losses net result is that constant curvature elastically

formed mirrors will be used because of highfrom the larger absorber. However, increasing theperformance and simplicity in production. Largeabsorber size also increases cost. Hence it isconcentrating systems typically require mirrorimportant to find the smallest receiver assemblyelements with different curvature in differentsize suitable for the radiation conditions applyingsections of the mirror field. In the proposedat the location of interest. Adoption of increasedconstant curvature system, all reflector elementsabsorber size may allow this technology to beare identical, and moulding or sagging of glass isused in climates with a high circumsolar fraction,not required. This represents a very low cost andas it increases the acceptance angle at a minorpractical option. Due to the very slight curvaturecost in shading and reflector absorption. Note thatrequired (|30 m radius) the mirrors will be asin place of varying the receiver size, one mayeasy to clean as flat mirrors.simply vary the field size for a given absorber

width for different locations.7.3.4. Ganging of mirror rows. The mirror

rows in the primary array can be constrained since7.3.3. Use of a standard mirror curvature. The

curvatures required in the field mirrors are small

but important in their effect on performance. AnTable 8. Average daily delivered energy for different mirror

optimisation calculation was carried out using the shapes15-m tower and 48 mirror row configuration Mirror shape Delivered energy

2

modified for use with constant curvature mirrors MJ/(m day)in all sections of the array. Table shows results for Horizontal array Polar arraya constant focal length of 30 m for a 50-m wide Variable curvature 6.23 7.88

Constant curvature 6.20 7.84array. A 30-m focal length was selected since theFlat 5.40 6.85outer field mirrors are approximately this distance

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Table 9. Annual energy delivery of alternative tracking sys- Table 10. Annual delivered energy for different evacuatedtems (vertical absorber) tube selective surfaces, 24 mirror CLFR system in Sydney

Tracking mode Delivered energy Selective surface Annual energy delivery2 2

MJ /(m day) MJ /(m day)

Horizontal array Polar array SS / Cu a50.93 7.35SS/Cuha50.95 7.30

Row tracked 6.20 7.84SS/Mo a50.93 7.04

Optimal ganged 6.18 7.83SS/Mo a50.95 7.07

they all move together through the same tracking treatments is shown in Table 10 for Sydney

angle, even though the absolute angle of each conditions. Annual performance is shown for the

mirror row is different from the others. The four new selective surfaces. The new high tem-

advantage of this is that the cost of the tracking perature selective surfaces developed by Sydney

system may be reduced. The disadvantage is that University all have significantly better perform-

performance is also reduced due to shading ance than the cermet surface used in the Luz LS3

between some of the fixed mirror lines, because SEGS plant (earlier SEGS systems used chrome

the optimal allocation of mirror rows to the black selective surfaces). The stainless steel and

alternative towers changes throughout the year. copper surface with a slightly lower absorptance

Thus, there is a cost / performance trade off that delivered more energy than the surface with thehas to be assessed. higher absorptance. This shows that a small

A ganged mirror field configuration was opti- decrease in absorptance can be traded off for a

mised for annual collection and compared against significant decrease in emissivity. For the stainless

the standard configuration with mirrors switching steel molybdenum surface the loss of performance

between absorbers to minimise shading. The due to a lower absorptance was not compensated

annual energy delivery of the ganged field is only by a matching heat loss reduction due to lower

0.2% less than the independently row-tracked case emissivity, but the performance of the two formu-

(Table 9). The mirror arrangement of the ganged lations is very close. For a higher concentration

configuration used was approximately the optimal system with 36 or 48 mirrors per tower, higher

arrangement of the unganged configuration at absorptance will be preferred.

equinox (in unganged fields some mirrors change 7.4. Performance for different climaticabsorbers to optimise performance on a seasonalconditionsbasis). The mirror arrangement was fine-tuned by

trial and error with each change being evaluated The performance of the horizontal absorber

by an annual performance calculation. Although a with a 15-m tower and 24- or 48-mirror array in

ganged field might lead to lower capital cost, a both horizontal field and polar field configurations

non-ganged configuration has practical advan- was evaluated for a range of climatic conditions

tages, since focusing can be finely tuned, and all in Australia (Table 11). The sites ranged from

mirrors can be aligned vertically in hailstorms, or cloudy coastal conditions (Sydney), dry and clear

horizontally in high winds. Independently tracked inland sites ( Dubbo) and latitudes ranging from

mirror lines can also be aligned or inverted for the dry tropics to 358 S. Surprisingly good

cleaning. During absorber maintenance, arbitrary performance was obtained for Dubbo, however it

sections of the mirror array can be realigned to must be noted that the weather data for Dubbo areother absorbers, maintaining output, and individ- based on one particular year (1994) of measured

ual rows can be aligned vertically to provide 1-min data. The performance for other locations is

walk-through paths. A single control system could based on 1-h time step typical meteorological year

control many hundred drive motors of this slow- weather data, which gives a good estimate of the

moving tracking system. The issue of ganging of long term performance. Thus, Table 11 shows the

mirrors must therefore rest as a minor issue that performance during 1994 in Dubbo and the long

needs to be resolved during detailed equipment term average performance for the other locations.

and operation design. In the remainder of this The variation of annual output in Dubbo as a

study it is assumed that the unganged system is function of array slope for a NorthSouth align-

used. ment is shown in Fig. 18. The effect of the

shortening of the solar day on an inclined receiver7.3.5. Effect of selective surface properties. was accounted for in the system analysis. A polar

The performance of the 24-mirror CLFR system axis array has 22% higher output than a horizontal

with different evacuated tube selective surface array. It can be seen that the performance is not

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Table 11. Annual average delivered thermal energy for different locations

Location Latitude Annual average delivered energy2

MJ/(m day)

Horizontal Polar Horizontal Polar24 mirrors 24 mirrors 48 mirrors 48 mirrors

Longreach 238 S 11.3 12.6 9.49 10.7Dubbo 328 S 11.4 14.1 9.49 11.9Sydney 348 S 7.35 9.16 6.23 7.88Wagga 358 S 7.95 9.49 6.70 8.12

therefore conceivable that both array types may

find geographical niches.

8. CONCLUSIONS

This paper has evaluated Compact Linear Fres-

nel Reflector (CLFR) concepts suitable for large

scale solar thermal electricity generation plants,and recommends the concepts most suitable to

pursue. In the CLFR, it is assumed that there will

be many parallel linear receivers that are closeFig. 18. Annual energy delivery as a function of array slope to enough for individual mirror rows to have thethe North, for Dubbo, latitude 328 S.

option of directing reflected solar radiation to two

linear receivers on separate towers. This addition-

al degree of freedom in mirror orientation can

allow closely packed mirror rows to be positionedsensitive to inclination within 5 degrees of theso that shading and blocking are almost elimi-latitude.nated.

7.5. Seasonal variation of CLFR performance

The avoidance of large mirror spacings andtower heights is an important issue in determiningPolar and horizontal arrays can have markedlythe cost of ground preparation, array substructuredifferent seasonal performance variation, with thecost, tower structure cost, steam line thermalpolar type having much better winter performancelosses, and steam line cost. The improved abilityat high latitudes. The improved winter perform-to use the Fresnel approach still delivers theance of polar arrays at high latitudes (Figs. 19 andnormal benefits of such a system, namely small20) may ensure its adoption, but for dry semi-reflector size, low structural cost, fixed receivertropical locations such as Longreach (Fig. 21), theposition without moving joints, and non-cylindri-winter performance of the horizontal array iscal receiver geometry. The modelled array usesreasonable as the sun is higher in winter. It is

Fig. 19. Seasonal performance of 48 mirror arrays in Sydney, latitude 348 S.

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Fig. 20. Seasonal performance of 48 mirror arrays in Dubbo, latitude 328 S.

advanced all glass evacuated tubular absorbers for vertical and horizontal configurations was

with low emittance selective coatings. determined.

The CLFR concepts evaluated in this study (b) For optimum receiver performance a small

included absorber orientation, absorber structure, secondary reflector is required to reorient the

the use of secondary reflectors adjacent to the receiver aperture to a more favourable angle to

absorbers, mirror field configurations, mirror receive rays from the outer edge of the mirror

packing densities, and tower heights. A necessary field, and also to provide a slight degree of

requirement in this activity was the development concentration. The size of the secondary reflec-

of specific raytrace and thermal models to simu- tor is limited when the increased reflection and

late the new concepts. The primary results of the shadowing losses outweigh additional collec-

evaluations together with discussion of implica- tion.

tions are as follows. (c) A NorthSouth polar field option was also

(a) A new absorber tube configuration has been evaluated. This configuration has a high ef-developed using a purpose designed multi- ficiency and good unit aperture collection but

branched raytrace model. It was important to lower ground usage efficiency than the

have high absorber efficiency because rays lost horizontal mirror field collectors, because

between tubes necessitate enlargement of the spaces must be left between rows to avoid

entire array to compensate. The best orienta- shading. Quite large structures will be required

tion of the planar absorber (a rack of evacuated (at least 20 m along the slope) and mainte-

Dewar-type absorber tubes) was found to be nance will be more difficult than with the

horizontal rather than vertical as originally horizontal configuration. However, seasonal

proposed. The optimum size of the absorber performance is more uniform than with the

Fig. 21. Seasonal performance of 48 mirror arrays in Longreach, latitude 238 S.

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Feuermann D. (1993). Analysis and Evaluation of the Solarhorizontal North South array for locationsThermal System at the Ben-Gurion Sede Boqer Test Center

outside of the tropics. for Solar Electricity Generating Technologies, Israel Minis-try of Energy and Infrastructure, Jerusalem, Final Report,(d) Optimised ganged mirror row versions ofContract No. 88169101; July.the CLFR were found to perform almost as

Feuermann D. and Gordon J. M. (1991) Analysis of a two-well as the row tracked version that can flip stage linear Fresnel reflector solar concentrator. J. Solar

Energy Eng. ASME 113, 272279.between targets.Harding G. L., Yin Z. and McKay D. W. (1985) Heat(e) It was found that a standard mirror curva- extraction efficiency of a concentric glass tubular collector.

ture could be used on all primary array reflec- Solar Energy 35, 7179.Hay J. E. and Davies J. A. (1980) Calculation of the solartors with negligible effect on performance, this

radiation incident on an inclined surface. Canadian Solargreatly simplifies manufacture.

Radiation Workshop, 5972.Igel E. A. and Hughes R. L. (1979) Optical analysis of solar

facility heliostats. Solar Energy 22, 283295.AcknowledgementsThis project was carried out with finan- Klein S. A. et al. (1996). TRNSYS. A Transient Systemcial support of the NSW Department of Energy through the Simulation, University of Wisconsin, Madison.State Energy Research and Development Fund, and the Lipps F. W. and Vant-Hull L. O. ( 1978) A cellwise method forfinancial assistance of his Royal Highness Prince Nawaf Bin the optimisation of large central receiver systems. SolarAbdul Aziz of the Kingdom of Saudi Arabia through the Energy 20, 505516.Science foundation of the University of Sydney. Mr. Wesley Morrison G. L. (1997). TRNSYS extensions (TRNAUS), Uni-Stein of Pacific Power was involved with the initiation of the versity of New South Wales, STEL 1/97.

project and provided technical input. Mr. Tony Monger Mills D. R. (1991) High temperature solar evacuated tube for(Sydney University) supplied data on circumsolar characteris- applications above 3008C. In Energy Research and De-tics for Australian climates. velopment Corporation Final Grant Report[1368, Depart-

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