CLFR Solar Thermal Power Plants

Pergamon

PII:

Solar Energy Vol. 68, No. 3, pp. 263–283, 2000
© 2000 Elsevier Science Ltd
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COMPACT LINEAR FRESNEL REFLECTOR SOLAR THERMAL
POWERPLANTS
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

Abstract—This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale
solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear
receivers elevated on tower structures that are close enough for individual mirror rows to have the option of
directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable
in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror
inclination can be set up such that shading and blocking are almost eliminated while ground coverage is
maximised. 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 the
cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam
line 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 noncylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as
the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber
orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field
configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the
development of specific raytrace and thermal models to simulate the new concepts. © 2000 Elsevier Science
Ltd. All rights reserved.

This paper describes a new design approach
that came from a realisation that trough technology was near its design limits and that fundamental
changes to the absorbing surface and collector
configuration were needed for large scale implementation of solar thermal power. New high
temperature selective surfaces with very low
emissivity for evacuated tube solar absorbers have
been developed (Mills, 1991; Zhang and Mills,
1992). Mills and Keepin (1993) suggested that
these surfaces could be used in new low concentration designs to reduce system costs, and
increase performance as lowering the absorbing
surface emissivity allows greater flexibility in
geometric concentration as a design variable.
Mills and Keepin used polar axis trough collectors as the example of a low concentration
collector, but there are several new design configurations that could use advanced evacuated
tubes. In this paper, we advocate the use of an
advanced form of linear Fresnel reflector as a
more cost-effective alternative to parabolic trough
or parabolic dish systems. The peak solar energy
radiant flux concentration of such systems can
range between 50 and 150% of that used by LS2

1. INTRODUCTION

The majority of direct solar electricity worldwide
is generated in nine large solar thermal electric
plants in California built by Luz International
Limited (LIL), with design and major solar components produced by Luz Industries Israel (LII).
These solar fields have achieved better than 97%
availability over more than 10 years of operation
and have successfully demonstrated the technology, but in a financial environment requiring
substantial government subsidy. Advanced versions of the Luz LS3 collector technology may
soon reduce unsubsidised electricity generation
cost to about ECU0.085 per kWh (EC, 1998).
However, these costs are still too high for many
markets. Even in markets where there is renewable energy obligation legislation, significant cost
reductions in solar thermal power systems are still
needed to compete against other renewable energy
grid-electricity systems using waste biomass fuel
or wind energy.
†

Author to whom correspondence should be addressed. Fax:
161-2-9351-3577; e-mail: d.mills@physics.usyd.edu.au
263

264

D.R. Mills and G.L. Morrison

and LS3 systems (23:1 and 26:1, respectively).
Higher concentrations are being investigated
using advanced secondary reflector systems and
will be reported in future publications.
2. LINEAR FRESNEL REFLECTOR
TECHNOLOGY

Geometrically, the ideal reflectors to use with
single receivers of solar energy are continuous
reflectors, usually parabolic for linear axis systems, or paraboloidal for two axis systems. Large
continuous reflectors or lenses can be simulated
by small elements distributed over a plane thus
avoiding the problems associated with very large
reflectors. Baum et al. (1957) discussed large
two-axis solar tracking systems of this type, but
the first to apply this principle in a reasonably
large linear system for solar collection was Francia (1968), who developed both linear and twoaxis tracking Fresnel reflector systems. This work
showed that elevated temperatures could be
reached using such systems. Following this, Riaz
(1976) developed theory associated with two-axis
systems, which was soon accompanied by additional work by Vant-Hull and Hildebrandt (1976),
Abdel-Monem et al. (1976), Lipps and Vant-Hull
(1977), Lipps and Vant-Hull (1978), Igel and
Hughes (1979) and Dudley and Workhoven
(1978, 1979). The work by Riaz can be adapted to
linear systems, and he discusses shadowing effects in a general way. Wei (1980, 1981) discusses
simplified calculations for two-axis systems.
Much of this work was associated with early
modelling of the US Central Receiver programme
which culminated in Solar One, a 10 MW(e)
two-axis tracking solar power plant constructed in
the early 1980s. However, Di Canio et al. (1979)
of the FMC Corporation produced a detailed
project design study for a linear plant of between
10 MW(e) and 100 MW(e), with a mirror field on
one side of a 1.68-km linear cavity absorber
mounted on 61 m towers. Vant-Hull (1991) suggests that the increased image size and lowered
concentration for ray incident angles not perpendicular to the linear axis would permit no advantages over the Central Receiver plants. However,
the FMC report itself acknowledges these optical
shortcomings but says these are compensated by
lower costs of manufacture and maintenance; the
authors were aware of Central Receiver development going on in parallel at the time and proposed
use of the same generating system. Since this
report was made, substantial advances have been
made in the areas of spectrally selective absorbers

and secondary concentrators, both of which alleviate the requirement for a small primary image size
and very high optical concentration.
A more recent effort to produce a tracking
Linear Fresnel Reflector was made by the Israeli
Paz company in the early 1990s (Feuermann and
Gordon, 1991; Feuermann, 1993). Although intended for 1508C operation, this technology is the
closest in the literature to that proposed here. This
array exhibited, among others, aberration difficulties caused by the movement of reflectors around
an axis parallel to but displaced from the reflector
optical axis. The analysis approach taken by the
Paz system developers was to use an optical ray
trace program for a system with finite reflector
sizes. This is also the method used in the course
of the current study.
One fundamental difficulty with the Linear
Fresnel Reflector (LFR) technology is the avoidance of shading of incoming solar radiation and
blocking of reflected solar radiation by adjacent
reflectors. Shading and blocking can be reduced
by using higher absorber towers, which increases
cost, or by increasing absorber size, which allows
increased spacing between reflectors remote from
the absorber. The latter leads to increased ground
usage relative to collector area and also increases
both thermal losses and shading by the absorber.

3. COMPACT LINEAR FRESNEL REFLECTOR
(CLFR)

3.1. Basic concept
Compact Linear Fresnel Reflector (CLFR) technology is, in effect, a second type of solution for
the Fresnel reflector field problem which has been
overlooked until now. The classical linear Fresnel
system has only one linear absorber on a single
linear tower, and therefore there is no choice
about the direction of orientation of a given
reflector. However, if one assumes that the size of
the field will be large, as it must be in technology
supplying electricity in the multi-MW class, it is
reasonable to assume that there will be many
linear absorbers in the system. If they are close
enough, then individual reflectors will have the
option of directing reflected solar radiation to at
least two absorbers. This additional variable in
reflector orientation provides the means for much
more densely packed arrays, because patterns of
alternating reflector inclination can be set up such
that closely packed reflectors can be positioned
without shading and blocking. The interleaving of
mirrors between two linear absorber lines is

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
beam blocking between adjacent reflectors and
allows higher reflector densities and lower absorber tower heights to be used. Land or roof area
cost is in many cases not a serious issue, but
available area can be restricted in industrial or
urban situations. Avoidance of large reflector
spacing and high towers is an important cost issue
when one considers the cost of ground preparation, array substructure, tower structure, steam
line thermal losses, and steam line cost for
installation next to an existing fossil fuel generating plant where the objective is the retrofit of a
low pollution steam source.
The CLFR power plant concept proposed in
this paper is intended to reduce costs in all
elements of the solar array. The following features
enhance the cost effectiveness of this system
compared with trough technology.
• Flat or elastically curved glass reflectors
mounted close to the ground are used to
minimise structural costs. Costly sagged glass
reflectors are avoided.
• The heat transfer loop is separated from the
reflector field and fixed in space thus avoiding
the high cost of flexible high pressure lines or
high pressure rotating joints required in trough
and dish systems.
• The heat transfer fluid is water, and passive
direct boiling heat transfer is proposed to
minimise parasitic pumping losses and the
need for flow controllers. Steam supply may
either be directly into the power plant steam
drum or via a heat exchanger. Steam can also
be supplied in a similar manner for power
plant preheating cycles. The steam delivery
conditions considered in this study are 3508C
and 16 MPa wet steam.
• An absorber composed of a pressure tube
containing the high pressure water, mounted
inside an advanced all-glass evacuated Dewar

tube. The absorber of the glass evacuated tube
is connected to the central steel pressure tube
by a heat transfer fin. The evacuated tubes
exhibit very low radiative losses and are
inexpensive, the current cost of a 1.2-m long,
45-mm diameter evacuated tube is | US$15.
• Low array maintenance costs due to ease of
access for cleaning, and the capability to
remove the single ended evacuated tubes without breaking the heat transfer fluid circuit.
This paper investigates alternative versions of
the CLFR concept to determine which are worthy
of further development. Areas of study include
field orientation, absorber orientation, absorber
structure, usage of auxiliary reflectors adjacent to
the absorbers, reflector packing density, and tower
height.

3.2. Horizontal tracking axis arrays
CLFR arrangements can include analogues of
horizontal East–West axis, North–South axis and
polar axis parabolic troughs. In the latter, the
plane of the CLFR array is inclined toward the
equator at the latitude angle, and would require an
inclined support structure or favourable ground
inclination. A scaled layout of a CLFR system
with a 50-m tower spacing, 10-m high absorber
and 48 mirrors, each 1 m wide, is shown in Fig. 2.
The high-density arrangement of reflectors shown
in Fig. 2 is such that the reflectors are separate but
would be close to touching if all were tilted to the
horizontal. Lower densities of reflectors may be
more cost-effective in some cases. The scale of
the moving elements is relatively small, even
though the unit scale of the overall system is very
large compared to other linear concentrator configurations; the pictured array is equivalent to a
parabolic trough of focal length of around 10 m.
In this project most of the investigation was
done on a horizontal East–West axis array using a
vertical absorber system consisting of a vertical

266


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
both sides, or a horizontal absorber tube array
illuminated from one side.

3.3. Inclined North–South and polar axis
arrays
A CLFR array can also be inclined toward the
equator to increase winter and annual collection.
A North–South axis array inclined at the latitude
angle (a polar axis tracking array) will yield close
to the optimal annual performance. Performance
simulations for different tracking axis orientation
and inclination are given in this study. An inclined array would be similar to that shown in
Fig. 3, in which a short North–South array is
tilted toward the equator. A stationary reflector at
the back end of each array is used to reduce the

inclination required. The stationary reflector redirects rays that have not yet hit the finite length
primary mirror but would be intercepted by an
infinite length primary mirror and also collects
rays that have been reflected from the primary
mirror but have struck the reflecting wall before
striking the finite length absorber. Inclination
markedly improves collected energy per m 2 of
reflector for locations outside tropical latitudes.
Inclining the array necessitates spacing between
the inclined reflector arrays to avoid winter shading, and decreases output per m 2 of ground area
occupied compared to a horizontal North–South
array. Inclined North–South arrays have a flatter
seasonal output profile compared to horizontal
arrays, however, they require a more expensive
substructure than horizontal arrays. The lower

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


input in the early morning and late afternoon due
to the raised artificial horizon for an inclined array
is accounted for in the analysis that follows.

4. SOLAR COLLECTOR OPTICAL
MODELLING

4.1. Raytrace modelling
A raytrace model was used to generate optical
collection maps in terms of transverse and longitudinal incidence angles. The concentration maps
and beam radiation data were used as inputs to
thermal modelling routines. A two-dimensional
model was used to generate concentration data for
a series of slices through the array. The twodimensional solutions were then assembled to
generate three-dimensional maps of the optics of
the systems. Absorber angular response and interactions with the curved surfaces of the glass
Dewar tubes was accurately modelled. The three
dimensional map of optical concentration and
absorption as a function of two orthogonal incidence angles was used in a radiation and thermal
model developed in TRNSYS (Klein et al., 1996).
The ray tracing model incorporated a ‘branching ray’ concept for modelling reflections in an
array of evacuated absorber tubes. A complex
incremental raytrace model was used to establish
the optimal orientation of each mirror row for a
given solar incidence angle. A model that included tracking of a ganged field of mirror rows
was also used. The branching ray model was
necessary to gauge the optical absorption efficiency of arrays of closely spaced all glass

267

evacuated tubes, since rays reflected from the
glass cover tubes or absorber elements could find
their way to other tubes. Primary, secondary and
tertiary reflections were tracked in the absorber. In
this study reflector slope errors were not considered.

4.2. CLFR receiver optical modelling
Two primary receiver types are proposed for
this technology. The first uses all-glass evacuated
absorber tubes in a vertical rack illuminated from
both sides. The second uses tubes in a singlesided horizontal receiver facing downward.

4.2.1. Optical modelling of vertical evacuated
tube receiver rack. The first configuration considered was a receiver with a vertical absorber
protected by evacuated absorber tubes that were
illuminated from both sides as shown in Fig. 4.
All-glass evacuated tubes having an outer cover
tube diameter of 45 mm and a cover tube thickness of 1.5 mm were evaluated. The absorber
surface on the outside of the inner glass tube is
1200 mm long, with a diameter of 37 mm. The
University of Sydney has licensed the basic
selective coating technology to a manufacturer in
China (Turbosun, 1998), and large volume low
cost tubes having such dimensions are available.
Due to the use of single ended absorber tubes the
array can only operate as a boiler and thus cannot
generate super heated steam.
A potential difficulty with this arrangement is
that evacuated spaces between the inner and outer
glass tubes allow radiation to pass through the
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.

268


Fig. 5. Double row tube arrangements of branch tubes enveloped by all-glass evacuated tube absorbers. (a) Close
packed zig-zag absorber array, (b) absorber array with 2.5-cm
gap between tubes.

However, because the tubes are inexpensive it
was possible to consider a staggered double row
tube configuration (Fig. 5) with high tube density
and very high optical interception. In a single row
of touching evacuated tubes the gap between the
inner and outer tubes amounts to 18% of the face
area of the absorber. For vertical tube racks a
solution to the gap loss problem is to use a
‘zig-zag’ double row of evacuated tubes (Fig. 5b).
Such a receiver traps light passing through gaps
between the inner and outer tubes of the vacuum
tube fitted over the branch tubes. The absorber
surface consists of both sides of the tube rack, as
in Fig. 4. The absorptance as a function of tube
spacing shown in Fig. 6 is dictated primarily by
the plane aperture (face area) of the receiver, not
the circumferential area of the evacuated tube

Fig. 6. Angular absorptance of ‘zig-zag’ tube racks with
different tube spacing compared to a flat absorber covered by a
glass sheet. The acceptance of the ‘zig-zag’ rack averaged over
all angles is almost identical to a flat plate absorber.

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 tube
surfaces, 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 performer at normal incidence. A tube spacing of 25 mm
was chosen for analysis (equivalent to a single
line spacing of 0.64 diameters), as the hemispherical absorptance for this tube spacing is almost
identical to a flat plate absorber (Fig. 7). This
choice also allowed use of a flat plate receiver in
subsequent modelling as a convenient approximation to the evacuated tube receiver. More
detailed analysis may be needed to determine tube
spacing sensitivity, but a spacing of 0.64 diameters is expected to be close to the optimum. In a
double row vertical absorber system the ‘tube
pitch’, or interval between tube centres in each
row, is 45 mm 1 25 mm 5 70 mm. The number of
tubes per lineal metre of receiver using a 25-mm
spacing and two rows is 28.6 / m.
A second approach to the vertical evacuated
tube gap loss problem is to use secondary reflectors behind the tubes. This requires two separated
rows of tubes with individual non-imaging reflectors for each tube between the two rows. This will
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 secondary
reflectors 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. Therefore, for modelling of vertical receivers, a 25-mm
‘zig-zag’ spacing was assumed.

4.2.2. Optical
modelling
of
horizontal
evacuated tube receiver rack. In horizontal receiv-


269

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

ers, the absorber face area is 1.2 m 2 per lineal
metre because it is single-sided (facing downward). One option is to use the same zig-zag tube
arrangement as the vertical absorber. This would
achieve the same high absorptance but would be a
very expensive arrangement because only one
side of the rack (the underside) is receiving solar
radiation. Alternatively a backing reflector behind
a single row of tubes could be used. A system
with a tube pitch of 49 mm (to avoid tubes
touching) requires only 20.4 tubes per lineal
metre of absorber and has negligible penalty in
optical efficiency. Approximately 1 / 4 of the rays
find their way through the gaps in the worst case
of normal incidence, but these can be reflected
back with high efficiency, and rays from lower
angles mostly strike the tubes directly. This
absorber configuration would be less costly than
the zig-zag arrangement, but optical performance
and heat losses would be similar.
Use of a non-imaging CPC backing reflector
would also reduce the number of tubes and tube
related costs. However, as this is a more open
pitch tube arrangement with less direct tube
absorption and increased reflector absorption the
overall gain in cost-effectiveness will be minimal
because the entire array must be enlarged in order
to achieve the same output. In this paper we have
evaluated the performance of the horizontal absorber systems using an inverted vacuum insulated flat plate receiver model which would be
similar in performance to the high density, tube
configuration.

A secondary reflector can be used underneath
the horizontal absorber to enhance optical collection and increase concentration. A schematic
diagram of one such reflector design is shown in
Fig. 8 without an upper backing reflector. This
uses a horizontal array of tubes with a backing
reflector above the tubes to collect rays passing
between the absorbers. The bifurcated secondary
reflector system is designed such that most rays
from the primary reflector strike the absorber
directly and do not incur an absorption loss on
this secondary reflector. Only rays on the
periphery of the reflected beam use the secondary
reflector.

4.3. Field raytrace
Optimisation of the mirror field requires consideration of the two possible positions of each
mirror row for each solar radiation input angle,
corresponding to two absorber targets. The modelling process involved beginning raytracing with
an arbitrary starting configuration for the linear
elements in the mirror field, then flipping the first
mirror in the field and raytracing the whole field
again. Having chosen the best position for the first
reflector, the second reflector was flipped and the
best position chosen for it. This process was
repeated for the entire field, and for all incidence
angles. To account for the finite size of the solar
source a secondary raytrace between 60.758 was
carried out, which increased the number of rays
by a factor of five. The resulting raytrace computations were very large. For a 48-mirror row

270


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 positions, 1000 rays, 19 angles and five beam spread
divisions amounting to 9.12 million rays for each
simulation of the field optical characteristics.
The optical specifications used were:
• normal incidence absorptance of the evacuated
tube absorber surface 5 0.94;
• refractive index of transparent coating on the
mirror 5 1.47;
• reflectance of mirror surfaces 5 0.95;
• mirror segment width 5 1 m.
In operating systems, reflectivity will be lost
due 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 mechanically attached to each other and run from a
single motor. Thus it is important to know the
performance penalty associated with mirror row
ganging, and this is modelled for some of the
configurations. Having mirrors of identical curvature 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 configuration (Fig. 9a) approaches the 2D cosine
collection characteristic of a horizontal flat plate
collector. The peak interception of beam radiation
for solar radiation arriving from a direction perpendicular to the CLFR array plane in the dense

configuration was 84% of the total beam radiation
arriving between the linear towers. The 2D interception factor increases with incidence angle
because gaps between the reflectors are covered

Fig. 9. Ray trace map of CLFR solar radiation flux concentration, (a) absorber 15 m above a 48-mirror array, (b)
absorber 10 m above a 24-mirror array with mirror segments
spaced by one mirror width.


by adjacent reflectors for low angles of incidence.
Peak flux concentration of the configurations
considered without including optical losses (comparable to peak geometrical concentration for a
trough) is 35:1 for the 48-mirror array. The
geometrical concentration of the Luz LS2 collector was 23:1 (aperture to absorber tube circumference). Higher concentrations can be achieved with
the Fresnel system, but the concentration is
limited by the heat flux capacity of the Dewar
tubes.
5. THERMAL MODELLING OF CFLR

Solar radiation and thermal simulation models
of the collectors were developed in the TRNSYS
modelling environment (Klein et al., 1996). For
this project a series of extensions were developed
within TRNSYS to simulate the linear Fresnel
concentrating collector. Due to the modular nature
of TRNSYS these new routines were integrated
with the existing data handling and solar radiation
analysis routines that are built into TRNSYS.
The primary routines that were used to simulate
concentrating solar collector performance were as
follows.
• Radiation processor, nonisotropic radiation
distribution model (TRNSYS TYPE16).
• Extended optical map-based solar collector
model for Fresnel concentrator (Morrison,
1997).
• Nonlinear heat loss solar collector model for
evacuated tube absorber (Morrison, 1997).
The collector thermal mass was modelled in
TRNSYS using an instantaneous collector efficiency model coupled to a zero heat-loss
storage-tank (TYPE4 tank). This procedure used
the proven TRNSYS tank routine and solver to
include the effect of thermal capacitance, rather
than developing a complex collector model with
built in capacitance. This model follows the start
up and shut down transients at the beginning and
end of each day and transient temperature effects
during cloudy periods. The transient effects, due
to thermal capacitance within the absorber, were
found to reduce the annual output of the collector
array by 3 to 6% depending on the array concentration.
To model the CLFR performance the TRNSYS
collector routine was modified to include a specification of optical concentration through a biaxial incidence angle modifier map (see Fig. 9). This
was implemented via an extension of the optical
mode 4 option in the extended TRNSYS TYPE1
solar collector model. The new routine was

271

designed to accept an incidence angle modifier
map with up to 50 incidence angles in both the
longitudinal and transverse planes. The optical
map data was generated using the ray tracing
routine described in Section 4.1.
The radiation processor in TRNSYS was used
to compute beam radiation from hourly global
radiation records. For analysis in locations where
only hourly solar radiation was available the
Reindl et al. (1990) radiation model was used to
compute beam radiation in terms of the clearness
index and the solar elevation. Radiation on the
tracking surface was calculated using the Hay and
Davies (1980) model which accounts for both
circumsolar and non-isotropic diffuse radiation
using an anisotropy index to quantify the portion
of diffuse radiation considered as isotropic. Radiation data for various sites modelled is shown in
Fig. 10. For Dubbo, 1-min time step beam
radiation measurements were available. The measured data were converted to normal incidence
beam radiation for the horizontal or inclined
planes of the Fresnel mirror field being considered. The annual solar radiation at each of the
design sites is shown in Table 1.

5.1. Heat loss from an array of evacuated tubes
The absorber of the proposed linear Fresnel
collector consists of a rack of evacuated tube
absorbers mounted in two rows so that the
absorber is equivalent to an evacuated flat plate
absorber. The heat loss from the evacuated tube
rack was determined from the measured characteristics of single evacuated tubes (Harding et al.,
1985). The heat loss modes are as follows.
• Conduction through the insulated header.
• Conduction through the glass envelope at the
open end of the tube.
• Conduction through the metal retainer near the
closed end of the absorber tube.
• Radiation from the absorber.
The heat loss (Q l ) from a single tube can be
expressed as
Q l 5 k 1 (T m 2 T a ) 1 k 2 (T s 2 T a )
1 k 3 ´(T 4 2 T 4 ) W/ tube.
s
a

(1)

For a single tube of the Sydney University
design (1.4 m long), Harding et al. (1985) have
shown that k 1 50.26 W/(K m 2 ), k 2 50.039 W/(K
tube) and k 3 58.5310 29 W/(K 4 tube) where k 1 is
the header heat loss factor, k 2 is the conduction
heat loss factor for conduction heat loss from the
absorbing surface to the top of the tube and

272


Fig. 10. Beam radiation at Australian design sites.

through the retainer clip, k 3 is the radiation heat
loss factor, ´ is the absorber emissivity ´ 5 a 1
bT s where a and b are coefficients determined
from measurement of surface properties (Fig. 11),
T s is the absorber surface temperature, and T m is
the mean fluid temperature in the evacuated tube.
The tube array proposed for the absorber of the
CLFR presents a continuous outer face to the
surroundings, thus the radiation heat loss is
equivalent to that of a flat plate evacuated collector. The radiation heat loss per unit face area of
the close packed array will be the same as the
radiation heat loss per unit circumferential area of
a single evacuated tube as measured by Harding
et al. (1985). The header heat loss per tube will
be reduced due to a closer tube spacing than
considered by Harding.

The header heat loss coefficient k 9 for a close
1
packed tube rack is
2p k
k 9 5 ]]]
1
d2
ln ]
d1

S D

5 0.343 W/(K m run of header)

(2)

where k is the header insulation conductivity
(0.06 W/ m K), d 1 , d 2 are the inner and outer
diameters of the insulation (100-mm steam tube
with 100 mm insulation).
The heat loss due to conduction around the top
of the tube and through the retainer clip is given
by
Q C 5 k 2 (T s 2 T a ) W/ tube.

(3)

Table 1. Annual solar irradiation data for Australian design sites
Location

Latitude

Annual irradiation MJ /(m 2 day)

Climate
Global

Longreach
Dubbo (1994)
Sydney
Wagga

238
328
348
358

S
S
S
S

Dry desert
Dry inland
Temperate coastal
Temperate
inland

Diffuse
horizontal

Total at
latitude angle

Beam

21.8
19.4
16.9
17.7

7.6
6.5
7.0
7.1

23.1
22.6
18.6
19.3

22.2
24.6
16.3
17.2


273

Fig. 11. Emissivity of selective surfaces.

The conduction heat loss per unit face area of
the rack is

double sided zig-zag evacuated tube rack (28.6
tubes / m) is

Q C 5 Nk 2 (T s 2 T a ) /L W/ m 2

Q 5 0.143sT m 2 T ad 1 0.93sT s 2 T ad 1 6.44

(4)

where N is the number of tubes per metre length
of rack (28.6 / m for the vertical tube configuration
and 20.6 for horizontal tube configuration in this
study), L is the length of tubes51.2 m active
length. The radiation from a single tube is given
by ´k 3sT 4 2 T 4d W/ tube.
s
a
The surface area of a single tube is 0.132 m 2
hence the radiation heat loss per unit surface area
of a single tube or per unit face area of a tube
rack is
Qr 5 ´k 3sT 4 2 T 4d / 0.132
s
a
4
4
5 6.44 3 10 28 ´sT s 2 T a d W/ m 2

(5)

The overall heat loss per unit face area of a

4
4
3 10 28 ´sT s 2 T a d W/ m 2 .

(6)

The selective surfaces investigated in this project are two formulations of a stainless steel /
aluminium nitride cermet with a copper reflector
layer (SS / Cu) and two formulations of a stainless
steel / aluminium
nitride
cermet
with
a
molybdenum reflector (SS / Mo). The temperature
dependence of the emittance of the four selective
surfaces being considered for this application is
shown in Fig. 11. The trade off between high
absorptance and low emittance is considered in
the analysis.
The heat loss per unit face area from the
evacuated tube rack absorber for the CLFR system is shown in Table 2 for a range of absorber

Table 2. Heat loss per unit face area of evacuated tube rack using stainless steel–copper selective surface (a 50.93)
Absorber surface
temperature
8C

Header

100
200
300
400
500

10
22
34
47
59

Heat loss W/ m 2
Tube conduction
64
143
223
303
383

Tube radiation

Total

32
146
423
988
2020

105
312
681
1337
2462

274


Table 3. Heat loss per unit mirror area for alternative CLFR
systems
Absorber surface
temperature
8C
100
200
300
400
500

Heat loss per unit area of mirror
W/ m 2
CLFR 24
CLFR 36
CLFR 48
10.5
31.2
68.1
133
246

7.0
20.8
45.4
89.2
164.1

5.3
15.6
34.0
66.9
123

temperatures. The heat loss from the absorber per
unit mirror area for three CLFR reflector packing
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
the same space, etc.
6. ABSORBER CONFIGURATIONS

The absorbers in the CLFR systems are singleended evacuated absorber tubes mounted horizontally (Figs. 12 and 13) or vertically (Fig. 4). The
steel pressure tube inside the evacuated tube can
be either a single-ended tube or a flow through
U-tube system. A single ended system will be
better at the radiation flux levels typical of this
absorber system because a U-tube of sufficient
diameter to provide adequate feedwater flow
could not be easily fitted inside the inner glass
tube. The absorber surface of the Dewar-type
evacuated tube is on the vacuum side of the inner
glass tube. The absorbed energy must be conducted through the 1.5-mm wall of the inner
borosilicate glass tube, then across the clearance
gap between the glass tube and the pressure tube
and then through the pressure tube wall, Fig. 13.
The pressure tube is supplied with feed water
from the header and returns steam through the
same opening. To obtain separation between the
liquid and gas streams a slight inclination from
the vertical or horizontal is required. The flow
configuration has been successfully demonstrated
in a number of prototype systems (Mills, 1991).

Fig. 13. Cross section through Dewar-type evacuated tube and
pressure pipe, dimensions in mm.

Schmid et al. (1990) have shown that a large
temperature difference will occur between the
absorber surface and the fluid in the pressure tube
unless a fin system is used in the clearance space
between the evacuated tube and the pressure tube.

6.1. Heat transfer in absorber
The CLFR systems considered in this study
have concentrations (mirror area / face area of the
absorber surface) up to 20:1. For 900 W/ m 2 beam
intensity the incident radiation flux on the absorber will be 18 kW/ m 2 . The tube density
adopted for the CLFR system is 28.6 tubes / m
with each tube having 1.2 m exposed length hence
the maximum heat transfer will be 755 W/ tube.
This maximum heat transfer will occur only when
the sun is directly overhead of the mirror array.
To minimise heat loss the temperature drop
between the absorber surface and the inner pressure tube must be minimised.
Thermal resistance between the absorber surface and the water / steam working fluid is due to
• conduction through the absorber glass wall;
• heat transfer across the gap between the absorber tube and the pressure tube (via an
internal fin);
• conduction through the pressure tube wall;
• convection into the boiling water in the pressure 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.


The temperature drop across the absorber wall
DT 1 is given by

S D

do
Q
DT 1 5 ]] ln ]
2p k g L
di

(7)

where Q is the heat transfer 755 W/ tube maximum; d o is the outer diameter of inner tube of
vacuum envelope537 mm; d i is the inner diameter of inner tube of vacuum envelope534 mm; L
is the length of vacuum tube51.2 m; k g is the
borosilicate glass conductivity51.1 W/ m K. The
maximum temperature drop across the glass absorber tube is DT 1 57.7 K.
Heat transfer between the glass absorber tube
and the pressure tube wall is via conduction
through the fin system and convection and radiation through the gap. The fin system could consist
of a circumferential plate and radial elements as
shown in Fig. 13. The fin system is a combination
of two split cylindrical sections 0.5 mm thick with
radial fins 0.2 mm thick and 4.5 mm long between
the two cylinders. A finite element analysis of
conduction in the outer ring and through the radial
fin has indicated that it is equivalent to a simple
0.2-mm-thick fin that is 7.5 mm long. If the radial
fins were formed from 0.2-mm copper on an
11.258 pitch the temperature drop (DT 2 ) across the
fin system (assuming all heat transfer is through
the fins as radiation and convection through the
gap will be minimal) is given by
Q
DT 2 5 ]]l
NLt f k f

(8)

where Q is the heat transfer per tube5755 W; t f
is the fin thickness50.2 mm; k f is the fin
conductivity5350 W/ m K (copper at 3508C); N
is the number of fins around the circumference5
16; l is the equivalent length of fins between the
glass wall and the pressure tube57.5 mm; L is
the width of fins5length of vacuum tube51.2 m.
For a copper fin system the temperature drop is
DT 2 54.2 K.
The temperature drop across the pressure tube
wall is given by

S D

do
Q
DT 3 5 ]] ln ]
2p k w L
di

(9)

where Q is the heat transfer per tube5755 W; d o
is the outer diameter of pressure tube525.4 mm;
d i is the inner diameter of pressure tube519 mm;
k w is the conductivity of pressure tube520 W/ m
K (chrome steel at 3508C); L is the length of
pressure tube in the evacuated tube51.2 m. The

275

temperature drop across the pressure tube wall is
DT 3 51.3 K.
The convective heat transfer coefficient inside
the pressure tube during pool boiling will be very
high and the temperature drop will be small.
Fouling of the inner surface of pressure tube
should not significantly add to the overall thermal
resistance between the absorber surface and the
steam.
The fin system will have additional thermal
resistance due to the contact resistance between
the glass tube and the fin, and between the
pressure tube and the fin. The outer contact
between the glass and the fin has not been a
problem in low temperature tubes using a similar
type of fin. The contact resistance with the
pressure tube could be minimised by using a fin
system consisting of a web sandwich so that there
is full circumferential contact over both the glass
absorber and the pressure tube. This fin system
would be inserted into the evacuated tube and
twisted as it is slipped over the pressure tube. The
contact resistance can also be reduced by increasing the number of fins beyond the 16 fins at
11.258 pitch considered in this analysis. At the
maximum beam intensity of 900 W/ m 2 when the
beam is normal to the array the temperature drop
across the absorber tube is 13.2 K1the contact
temperature drop.
In the following analysis an overall temperature
drop of 20 K has been assumed for beam radiation input of 900 W/ m 2 . The development of the
absorber tube will require assessment of a number
of configurations to determine the most effective
finning and contact arrangement.
7. COLLECTOR PERFORMANCE ESTIMATES

In this section, array performance simulation is
used to select optimum CLFR configurations. The
basic selection calculations were carried out for
Sydney, Australia, but it was found that performance relativities are maintained at all sites.
This is because all of the sites in this study have
low circumsolar radiation and therefore plants in
these sites will operate similarly to a plant in
Sydney. Because of the low circumsolar radiation
in Australia, it is sensible to optimise performance
on the basis of a direct beam capture half angle of
0.758. Diffuse and circumsolar radiation outside
this range is ignored in the simulation, but there is
little energy in this angular range in Australia. In
practice there will be beam spread due to mirror
irregularities and mirror tilt error and slight
differences in diffuse collection between different

276


absorber configurations but these factors have
been ignored in this initial investigation. In addition, beam spread for rays with a significant
directional component parallel to the array linear
axis has not been included, but is not serious for a
concentrator of this receiver aperture with minimal aiming error. However, in some climates,
notably tropical regions and parts of the Northern
Hemisphere, beam radiation is more forward
scattered than in Australia and the amount of
circumsolar radiation is greater. For such locations the acceptance angle of the collector must
therefore be larger to collect both the direct and
near circumsolar components. It is possible to
alter the design to enlarge the absorber for this
purpose. This is not done in this paper, as it leads
to a relatively greater capital cost and increased
thermal loss.

7.1. CLFR configuration assumptions
A 50-m wide array is assumed with mirror row
densities of 24, 36 and 48 rows per absorber line.
Each mirror was taken as 1 m wide with 0.95 m
reflecting width and 25 mm edge structure. Each
configuration was evaluated for absorber heights
of 10 m, 12.5 m, and 15 m. An equal spacing of
mirror rows is assumed and a space 1 m wide is
left clear on either side of the array centre line
under the absorber for access and maintenance.
Because the effective aperture does not change
as a simple cosine function as in a parabolic
trough (because gaps between reflectors are
blocked by adjacent mirrors at high angles of
incidence), a nominal peak effective aperture was
selected. The peak effective apertures of the
arrays were calculated on the basis of the maximum energy that could be collected by an absorber which picks up all beam ray spillage, and
is 87.5% of the curved mirror surface area: for 48
m of reflector, this is 42 m of nominal peak
aperture 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 construction 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 reflectors
adjacent to the absorber are assumed to be 2 m 2
per lineal metre of the absorber.

7.1.1. Optimal absorber length. One important
issue is how large to make the absorber relative to
the array field dimensions. There exists a trade off

between the improved optical collection and
increased thermal losses for a larger receiver
surface. The receiver size that delivered maximum
energy collection was evaluated for a 50-m wide
array using 36 mirror rows. Annual delivered
energy for different absorber tube lengths is
shown in Fig. 14 for both horizontal and vertical
absorbers. The operating temperature in each case
was 3208C. In neither case does the energy
collection vary significantly with absorber size.
The size of the horizontal absorber was found to
be optimal at 1.2 m tube length, and the vertical
absorber at 1.0 m tube length. The available
evacuated tubes are 1.2 m long but an absorber
1.0 m long can be constructed by angling the
tubes in the rack.

7.1.2. Secondary reflector. Secondary reflectors can be used near the receiver to capture
reflected solar radiation that would otherwise have
missed the receiver. The ends of the secondary
reflector reposition the edge of the receiver aperture to better face the primary reflector array. One
configuration of a horizontal absorber (1.2 m
wide, 12.5 m absorber height, 36 mirror rows)
was chosen to test optimal secondary reflector
length. After the ray trace, an annual performance
calculation was performed for each secondary
reflector length all using the same receiver configuration. The optimal secondary reflector is
similar to that shown in Fig. 8, and is quite short.
The gap between the secondary reflector and
absorber allows rays to strike the absorber directly, decreasing secondary reflector absorption losses. A similar calculation was carried out for a
vertical absorber with a short circular (or
parabolic) section secondary reflector as shown in
Fig. 15. The optimal secondary reflector lengths
depend on absorber height. However, for sub-

Fig. 14. Useful delivered thermal energy as a function of
absorber length for a 50-m-wide array.


277

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

sequent analysis the secondary reflector was
scaled with absorber size.

7.2. Performance optimisation for Sydney
Having sized the secondary reflectors and the
absorber length, it is now possible to determine
the optimum absorber and mirror field configurations. North–South axis and polar axis versions
were used as the basic configurations. The performance of East–West systems was also investigated as a variation on the basic design. The
primary cases calculated are described in Sections
7.2.1–7.2.4 using Sydney solar radiation data. The
selective surface of the tubes was taken as stainless / copper with normal incidence absorptance of
0.93.

7.2.1. Vertical evacuated tube absorber with
overhead secondary reflector and horizontal
North–South primary reflector field. This configuration was found to perform better than a vertical
absorber without a secondary reflector, Fig. 4. The
secondary reflector is circular (or parabolic, they
are almost indistinguishable) in curvature as
shown in Fig. 15 and is sized according to results
of a large number of annual simulation runs
determining optimal solar radiation capture. The

top secondary reflector effectively angles the
receiver aperture toward the array, reducing ray
spillage at the image boundary. Mirror rows close
to the absorber line deliver a smaller image and
would be aimed at the receiver itself, avoiding
secondary reflector losses. Mirror rows underneath the absorber would use the secondary
reflector.
Table 4 shows the annual thermal energy
delivery. As tower height increases, performance
also increases due to reduced mirror row shading.
Having fewer field mirror rows allows more
collection by each row, but reduces energy collected by the fixed size absorber array. This is the
original configuration proposed, but it is outperformed both by horizontal primary reflector
fields having horizontally oriented absorbers as
shown in Fig. 8, and polar inclined fields, Fig. 3.

7.2.2. Vertical evacuated tube absorber with
overhead secondary reflector and polar axis
tracking North–South primary reflector field. This
is a version of the vertical absorber array placed
upon a North–South structure inclined at the
latitude angle to improve winter performance
(Figs. 3 and 16). If an end reflector is used at 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 North–South primary
reflector field, output per unit area of mirror
Number of mirror rows
in the primary array

Thermal energy delivery
MJ /(m 2 day)
10 m tower

24
36
48

12.5 m tower

15 m tower

5.54
5.31
4.64

5.61
5.32
4.80

5.57
5.32
4.91

278


Fig. 16. Polar axis CLFR array.

would closely approximate the performance of a
segment of an infinite array. Table 5 shows that
the annual thermal energy delivery of this system
is substantially better than for the horizontal
reflector configuration (Section 7.2.1).

7.2.3. Horizontal evacuated tube absorber with
underneath secondary reflector and horizontal
North–South primary reflector field. This arrangement yields substantially improved performance
(Table 6) compared to the horizontal mirror field
with a vertical absorber, since fewer rays are
missed due to a larger total receiver aperture. The
tubes would be tilted slightly off horizontal to
ensure natural circulation of feedwater into the
tubes.
7.2.4. Horizontal evacuated tube absorber with
underneath secondary reflector and polar axis
tracking North–South primary reflector field. This
is a polar version of the NS horizontal evacuated
tube absorber array placed upon a structure inclined at the latitude angle facing the equator.

This configuration delivers the highest collection
per unit aperture area (Table 7) of all the CLFR
configurations considered and is equivalent to
about 70% of a two-axis tracking paraboloidal
dish.

7.3. Design variations
There are a number of design variations that
have the capability of improving performance or
lowering cost. In the following, each variation is
evaluated for Sydney.

7.3.1. Use of more sophisticated secondary
reflector design to decrease absorber size. The
configuration shown in Fig. 17 uses a short
horizontal absorber and an upper secondary semicircular reflector to provide a horizontal aperture
for a bifurcated lower secondary reflector. In this
case, the absorber rack width is reduced to half
the size of that for the previously described
horizontal case. Losses are similar, because loss
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 2 day)
10 m tower

24
36
48

12.5 m tower

15 m tower

7.07
6.82
5.99

7.16
6.83
6.22

7.11
6.88
6.40

Table 6. Performance of horizontal evacuated tube absorber with secondary reflector and horizontal North–South primary
reflector field
in primary array

MJ /(m 2 day)
10 m tower

24
36
48

12.5 m tower

15 m tower

6.56
6.15
5.22

7.06
6.65
5.86

7.35
6.96
6.23

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

MJ /(m 2 day)
10 m tower

24
36
48

12.5 m tower

15 m tower

8.26
7.76
6.59

8.82
8.36
7.41

9.16
8.71
7.88


Fig. 17. Schematic of an upper and lower secondary reflector
arrangement for a horizontal tube rack.

the absorber cost is halved. However, optical
efficiency is reduced because of increased optical
losses in the dual secondary reflectors, which now
intercepts a greater fraction of incoming rays. The
main disadvantages of this approach are the very
large unit size of the collector field — 100-m
wide fields would be required for a 1.2-m long
tube — and the high solar radiation flux on each
tube. There are several methods for using reflectors in this way, however, at this stage, no
definitive avenue for improving cost-effectiveness
has been identified.

7.3.2. Use of a larger receiver to increase
acceptance angle. A simulation was performed on
the standard array with the absorber doubled in
size. The net result was a 3% decrease in delivered energy due to increased thermal losses
from the larger absorber. However, increasing the
absorber size also increases cost. Hence it is
important to find the smallest receiver assembly
size suitable for the radiation conditions applying
at the location of interest. Adoption of increased
absorber size may allow this technology to be
used in climates with a high circumsolar fraction,
as it increases the acceptance angle at a minor
cost in shading and reflector absorption. Note that
in place of varying the receiver size, one may
simply vary the field size for a given absorber
width for different locations.
7.3.3. Use of a standard mirror curvature. The
curvatures required in the field mirrors are small
but important in their effect on performance. An
optimisation calculation was carried out using the
15-m tower and 48 mirror row configuration
modified for use with constant curvature mirrors
in all sections of the array. Table shows results for
a constant focal length of 30 m for a 50-m wide
array. A 30-m focal length was selected since the
outer field mirrors are approximately this distance

279

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 a
curvature 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 mirrors
can be used for the field to further simplify
production and cleaning. Because the mirror has a
physical width of 0.95 m, the half image of the
sun at the absorber will fall outside 0.95 by 0.738,
or by about 0.32 m for a mirror at the centre of
the field. This means the total image extent is 0.95
m1(230.32 m)51.59 m, about 0.39 m greater
than the absorber aperture size. The situation is
worse for mirrors far from the receiver. Therefore,
in spite of the small curvature, it is necessary to
curve the mirrors to capture all of the beam
radiation, or else increase the absorber size by the
order of 50% if flat mirrors are to be used.
The performance of horizontal and polar mirror
fields using optimised fixed curvature and flat
mirrors are compared to those of variable curvature in Table 8. The optimised constant curvature
performs only 0.5–0.6% lower than the variable
curvature, but the flat mirror is 13% lower. The
net result is that constant curvature elastically
formed mirrors will be used because of high
performance and simplicity in production. Large
concentrating systems typically require mirror
elements with different curvature in different
sections of the mirror field. In the proposed
constant curvature system, all reflector elements
are identical, and moulding or sagging of glass is
not required. This represents a very low cost and
practical option. Due to the very slight curvature
required (|30 m radius) the mirrors will be as
easy to clean as flat mirrors.

7.3.4. Ganging of mirror rows. The mirror
rows in the primary array can be constrained since
Table 8. Average daily delivered energy for different mirror
shapes
Mirror shape

Delivered energy
MJ /(m 2 day)
Horizontal array

Variable curvature
Constant curvature
Flat

Polar array

6.23
6.20
5.40

7.88
7.84
6.85

280


Table 9. Annual energy delivery of alternative tracking systems (vertical absorber)

Table 10. Annual delivered energy for different evacuated
tube selective surfaces, 24 mirror CLFR system in Sydney

Tracking mode

Selective surface

Annual energy delivery
MJ /(m 2 day)

SS / Cu a 50.93
SS / Cu ha 50.95
SS / Mo a 50.93
SS / Mo a 50.95

7.35
7.30
7.04
7.07

Delivered energy
MJ /(m 2 day)
Horizontal array

Row tracked
Optimal ganged

Polar array

6.20
6.18

7.84
7.83

they all move together through the same tracking
angle, even though the absolute angle of each
mirror row is different from the others. The
advantage of this is that the cost of the tracking
system may be reduced. The disadvantage is that
performance is also reduced due to shading
between some of the fixed mirror lines, because
the optimal allocation of mirror rows to the
alternative towers changes throughout the year.
Thus, there is a cost / performance trade off that
has to be assessed.
A ganged mirror field configuration was optimised for annual collection and compared against
the standard configuration with mirrors switching
between absorbers to minimise shading. The
annual energy delivery of the ganged field is only
0.2% less than the independently row-tracked case
(Table 9). The mirror arrangement of the ganged
configuration used was approximately the optimal
arrangement of the unganged configuration at
equinox (in unganged fields some mirrors change
absorbers to optimise performance on a seasonal
basis). The mirror arrangement was fine-tuned by
trial and error with each change being evaluated
by an annual performance calculation. Although a
ganged field might lead to lower capital cost, a
non-ganged configuration has practical advantages, since focusing can be finely tuned, and all
mirrors can be aligned vertically in hailstorms, or
horizontally in high winds. Independently tracked
mirror lines can also be aligned or inverted for
cleaning. During absorber maintenance, arbitrary
sections of the mirror array can be realigned to
other absorbers, maintaining output, and individual rows can be aligned vertically to provide
walk-through paths. A single control system could
control many hundred drive motors of this slowmoving tracking system. The issue of ganging of
mirrors must therefore rest as a minor issue that
needs to be resolved during detailed equipment
and operation design. In the remainder of this
study it is assumed that the unganged system is
used.

7.3.5. Effect of selective surface properties.
The performance of the 24-mirror CLFR system
with different evacuated tube selective surface

treatments is shown in Table 10 for Sydney
conditions. Annual performance is shown for the
four new selective surfaces. The new high temperature selective surfaces developed by Sydney
University all have significantly better performance than the cermet surface used in the Luz LS3
SEGS plant (earlier SEGS systems used chrome
black selective surfaces). The stainless steel and
copper surface with a slightly lower absorptance
delivered more energy than the surface with the
higher absorptance. This shows that a small
decrease in absorptance can be traded off for a
significant decrease in emissivity. For the stainless
steel molybdenum surface the loss of performance
due to a lower absorptance was not compensated
by a matching heat loss reduction due to lower
emissivity, but the performance of the two formulations is very close. For a higher concentration
system with 36 or 48 mirrors per tower, higher
absorptance will be preferred.

7.4. Performance for different climatic
conditions
The performance of the horizontal absorber
with a 15-m tower and 24- or 48-mirror array in
both horizontal field and polar field configurations
was evaluated for a range of climatic conditions
in Australia (Table 11). The sites ranged from
cloudy coastal conditions (Sydney), dry and clear
inland sites (Dubbo) and latitudes ranging from
the dry tropics to 358 S. Surprisingly good
performance was obtained for Dubbo, however it
must be noted that the weather data for Dubbo are
based on one particular year (1994) of measured
1-min data. The performance for other locations is
based on 1-h time step typical meteorological year
weather data, which gives a good estimate of the
long term performance. Thus, Table 11 shows the
performance during 1994 in Dubbo and the long
term average performance for the other locations.
The variation of annual output in Dubbo as a
function of array slope for a North–South alignment is shown in Fig. 18. The effect of the
shortening of the solar day on an inclined receiver
was accounted for in the system analysis. A polar
axis array has 22% higher output than a horizontal
array. It can be seen that the performance is not


281

Table 11. Annual average delivered thermal energy for different locations
Location

Latitude

Annual average delivered energy
MJ /(m 2 day)
Horizontal
24 mirrors

Longreach
Dubbo
Sydney
Wagga

238
328
348
358

S
S
S
S

Polar
24 mirrors

Horizontal
48 mirrors

Polar
48 mirrors

11.3
11.4
7.35
7.95

12.6
14.1
9.16
9.49

9.49
9.49
6.23
6.70

10.7
11.9
7.88
8.12

therefore conceivable that both array types may
find geographical niches.
8. CONCLUSIONS

Fig. 18. Annual energy delivery as a function of array slope to
the North, for Dubbo, latitude 328 S.

sensitive to inclination within 5 degrees of the
latitude.

7.5. Seasonal variation of CLFR performance
Polar and horizontal arrays can have markedly
different seasonal performance variation, with the
polar type having much better winter performance
at high latitudes. The improved winter performance of polar arrays at high latitudes (Figs. 19 and
20) may ensure its adoption, but for dry semitropical locations such as Longreach (Fig. 21), the
winter performance of the horizontal array is
reasonable as the sun is higher in winter. It is

This paper has evaluated Compact Linear Fresnel 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 close
enough for individual mirror rows to have the
option of directing reflected solar radiation to two
linear receivers on separate towers. This additional degree of freedom in mirror orientation can
allow closely packed mirror rows to be positioned
so that shading and blocking are almost eliminated.
The avoidance of large mirror spacings and
tower heights is an important issue in determining
the cost of ground preparation, array substructure
cost, tower structure cost, steam line thermal
losses, and steam line cost. The improved ability
to use the Fresnel approach still delivers the
normal 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 uses

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

282


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

advanced all glass evacuated tubular absorbers
with low emittance selective coatings.
The CLFR concepts evaluated in this study
included absorber orientation, absorber structure,
the use of secondary reflectors adjacent to the
absorbers, mirror field configurations, mirror
packing densities, and tower heights. A necessary
requirement in this activity was the development
of specific raytrace and thermal models to simulate the new concepts. The primary results of the
evaluations together with discussion of implications are as follows.
(a) A new absorber tube configuration has been
developed using a purpose designed multibranched raytrace model. It was important to
have high absorber efficiency because rays lost
between tubes necessitate enlargement of the
entire array to compensate. The best orientation of the planar absorber (a rack of evacuated
Dewar-type absorber tubes) was found to be
horizontal rather than vertical as originally
proposed. The optimum size of the absorber

for vertical and horizontal configurations was
determined.
(b) For optimum receiver performance a small
secondary reflector is required to reorient the
receiver aperture to a more favourable angle to
receive rays from the outer edge of the mirror
field, and also to provide a slight degree of
concentration. The size of the secondary reflector is limited when the increased reflection and
shadowing losses outweigh additional collection.
(c) A North–South polar field option was also
evaluated. This configuration has a high efficiency and good unit aperture collection but
lower ground usage efficiency than the
horizontal mirror field collectors, because
spaces must be left between rows to avoid
shading. Quite large structures will be required
(at least 20 m along the slope) and maintenance will be more difficult than with the
horizontal configuration. However, seasonal
performance is more uniform than with the

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


horizontal North South array for locations
outside of the tropics.
(d) Optimised ganged mirror row versions of
the CLFR were found to perform almost as
well as the row tracked version that can flip
between targets.
(e) It was found that a standard mirror curvature could be used on all primary array reflectors with negligible effect on performance, this
greatly simplifies manufacture.
Acknowledgements—This project was carried out with financial support of the NSW Department of Energy through the
State Energy Research and Development Fund, and the
financial assistance of his Royal Highness Prince Nawaf Bin
Abdul Aziz of the Kingdom of Saudi Arabia through the
Science foundation of the University of Sydney. Mr. Wesley
Stein of Pacific Power was involved with the initiation of the
project and provided technical input. Mr. Tony Monger
(Sydney University) supplied data on circumsolar characteristics for Australian climates.

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