MIDDLE EAST TECHNICAL UNIVERSITY

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MIDDLE EAST TECHNICAL UNIVERSITY Phys. 471 project
HELIOSTAT FIELD
PRESENTED BY Ertug ÖZYIGIT Bahtiyar
RUZIBAYEV
INSTRUCTOR Prof. Dr. Ahmet ECEVIT
2004-1
2
OUTLINE

• Page
  
• Central Receiver System (CRS)
  ...3
• Components of CRS...8
  
• 1. Solar Concentrators (Heliostats)
  ....10
• 1.1 How Heliostats move..
  13
• 1.2 Ideal Heliostat.
  .15
• 1.3 Heliostat field types.
  ....17
• 1.4 Heliostat errors
  ....19
• 1.5 Cosine Effect
  ....23
• 1.6 Shadowing and Blocking
  ....27
• 2. Receiver...29
  
• 2.1 Types of receiver
  31
• 3. Tower design.3
  6
• 4. Beam characterization targets
  .....40
• 5. Heat transfer fluids
  .....41
• 6. Storage system...
  47
• 7. Power generator..
  ...49
• 8. Multi Tower Solar Array (MTSA)
  .....51
• References56

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Central Receiver System (CRS)

• The central receiver concept for solar energy
  concentration and collection is based on a field
  of individually sun-tracking mirrors (heliostats)
  that reflect the incident sunshine to a receiver
  (boiler) at the top of a centrally located tower.
  Typically 80 to 95 percent of the reflected
  energy is absorbed into the working fluid which
  is pumped up the tower and into the receiver. The
  heated fluid (or steam) returns down the tower
  and then to a thermal demand such as a thermal
  electrical power plant or an industrial process
  requiring heat 1. In figure 1 you can see a CRS.

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Figure 1. CESA-1 CRS 2.
5

• The basic difference between the central receiver
  concept of collecting solar energy and the trough
  or dish collectors is that in this case, all of
  the solar energy to be collected in the entire
  field, is transmitted optically to a small
  central collection region rather than being piped
  around a field as hot fluid. Because of this
  characteristic, central receiver systems are
  characterized by large power levels (1 to 500 MW)
  and high temperatures (540 to 840C) 1.

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• Central receiver technology for generating
  electricity has been demonstrated in the Solar
  One pilot power plant at Barstow, California.
  This system consists of 1818 heliostats, each
  with a reflective area of 39.9 m2 covering
  291,000 m2 of land. The receiver is located at
  the top of a 90.8 m high tower and produces steam
  at 516C at a maximum rate of 42 MW. In figure 2
  you can see Solar One power plant 1.

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Figure 2. Solar One Power Plant 1.
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Components of CRS

• Central receiver consists of,
• Solar concentrator (heliostat field)
• Receiver
• Storage system
• Power generator
• Figure 3 shows the schematic diagram of CRS.

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Figure 3. Schematic Diagram of CRS 3.
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1. Solar concentrators (Heliostats)

• The heliostats are mirrors with solar tracking on
  two axes and capable of concentrating the
  reflected solar radiation on a focal point
  located at the top of a tower in which the
  receiver element is placed 4. See the figure 4.

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Figure 4. Heliostats 3.
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• Heliostats sizes varies according to the the
  receiver used on the tower.
• Heliostats are generally made from iron glass
• Heliostats made from low iron float glass have a
  reflectivity 0.903. However, dirt reduces
  reflectivity to 0.82 1.

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1.1 How heliostats move

• The mirrors are mounted on individual frames that
  are tipped up and down and rotated east to west
  by small motors much like those used in electric
  clocks.
• The motors are controlled by a computer which
  determines how to position each heliostat so that
  its reflection hits the receiver at any time of
  the day and any day of the year 4.
• In figure 5 you can see an example of sun
  tracking heliostat design.

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Figure 5. Erik Rossens Heliostat Design 3.
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1.2 Ideal Heliostat

• Low cost
• Maximum reflection
• No absorbtion transmission
• In table 1 you can see the reflectivity and
  emissivity of some surfaces.

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E
    Average
Surface Reflectivity Emissivity
Aluminum foil, bright 92 - 97 0.05
Reflective Mylar Film 90 - 93 0.05
Aluminum sheet 80 - 95 0.12
Plate glass mirrors coated with aluminum on back 85  
Aluminum-coated paper, polished 75 - 84 0.20
Steel, galvanized, bright 70 - 80 0.25
Aluminum paint 30 - 70 0.50
Building materials wood, paper, glass, masonry, nonmetallic paints   5 - 15 0.90
Table 1. Reflectivity and Emissivity for
Different Surfaces 1.
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1.3 Heliostat Field Types

• Surrounding the tower
• On one side of the tower
• You can see these types in figure 6.

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Figure 6. One Side and Surrounding Type 3.
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1.4 Heliostat Errors

• A perfectly flat heliostat would produce an image
  on the receiver the size of the heliostat
  (projected normal to the heliostat-receiver
  direction) increased by the approximately 0.5
  degree of sun spread. For most applications, each
  mirror segment is concaved slightly and each
  mirror segment on a heliostat is canted toward a
  focal point. This produces a higher flux density
  at the aim point 1.

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• A number of factors tend to increase the image
  size from a particular heliostat. Mirror surface
  waviness is an important factor for heliostats as
  it is with parabolic collector surfaces. In
  addition, the gross curvature error of each
  mirror segment and the errors associated with
  accurate canting of each mirror segment on the
  heliostat frame further increase the image error.
  This last source of error can be amplified by the
  effects of differential thermal growth and
  gravity (heliostat position) on the heliostat
  frame. The important heliostat performance
  parameter is the size of the isoflux contour
  containing 90 percent of the total reflected
  power 1.

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• In addition to producing a high flux density, the
  ability of the heliostat tracking system to
  position the centroid of the flux profile at the
  center of the receiver (aim point) is critical.
  Positioning errors may be caused by vertical and
  horizontal errors in the heliostat positioning or
  feedback mechanisms. In addition, wind can
  produce structural deflections, causing position
  errors 1.

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• Most of the heliostat errors discussed become
  more significant (in terms of the flux spilled
  from the receiver), the farther the heliostat is
  located from the receiver. However, the flux
  contour and positioning errors are also critical
  for heliostats close to the tower because the
  projected area of the receiver is very small at
  that location 1.

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1.5 Cosine Effect

• The major factor determining an optimum heliostat
  field layout is the cosine efficiency of the
  heliostat. This efficiency depends on both the
  suns position and the location of the individual
  heliostat relative to the receiver. The heliostat
  is positioned by the tracking mechanism so that
  its surface normal bisects the angle between the
  suns rays and a line from the heliostat to the
  tower. The effective reflection area of the
  heliostat is reduced by the cosine of one-half of
  this angle as seen in figure 7.

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Figure 7. The cosine effect for two heliostats in
opposite directions from the tower.  For the
noontime sun condition shown, heliostat A in the
north field has a much greater cosine efficiency
than does heliostat B 1.
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• Field cosine efficiency, calculated by using
  equation 1.

Equation 1. Field Cosine Efficiency 1
where a and A are the suns altitude and azimuth
angles, respectively, and z, e, and n are the
orthogonal coordinates from a point on the tower
at the height of the heliostat mirrors as
depicted in figure 8.
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Figure 8. Coordinates defining the reflection of
the suns rays by a heliostat to a single aim
point. Vector H is normal to the heliostat
reflecting surface 5.
27
1.6 Shadowing and Blocking

• Shadowing occurs at low sun angles when a
  heliostat casts its shadow on a heliostat located
  behind it. Therefore, not all the incident solar
  flux is reaching the reflector. Blocking occurs
  when a heliostat in front of another heliostat
  blocks the reflected flux on its way to the
  receiver. Blocking can be observed in a heliostat
  field by noting reflected light on the backs of
  heliostats. Both processes are illustrated in
  figure 9 1.

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Figure 9. Shadowing and Blocking Effect 1.
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2. Receiver

• The receivers normally consist of a large number
  of metal tubes that contain a flowing fluid. The
  outer surface of the tubes are black to assure
  that the light is absorbed and converted to heat.
  The metals used for the tubes are the same as
  those used in other high-temperature, nonsolar
  processes. Central receivers are usually very
  large and have a capacity to generate 100 MW of
  useful power or more 1.

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• The primary limitation on receiver design is the
  heat flux that can be absorbed through the
  receiver surface and into the heat transfer
  fluid, without overheating the receiver walls or
  the heat transfer fluid within them 1.

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2.1 Types of Receivers

• External Type
• Cavity type

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• External type These normally consist of panels
  of many small (20-56 mm) vertical tubes welded
  side by side to approximate a cylinder.  The
  bottoms and tops of the vertical tubes are
  connected to headers that supply heat transfer
  fluid to the bottom of each tube and collect the
  heated fluid from the top of the tubes 1.

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• Cavity type In an attempt to reduce heat loss
  from the receiver, some designs propose to place
  the flux absorbing surface inside of an insulated
  cavity, thereby reducing the convective heat
  losses from the absorber. The flux from the
  heliostat field is reflected through an aperture
  onto absorbing surfaces forming the walls of the
  cavity. Typical designs have an aperture area of
  about one-third to one-half of the internal
  absorbing surface area. Cavity receivers are
  limited to an acceptance angle of 60 to 120
  degrees (Battleson, 198l). Therefore, either
  multiple cavities are placed adjacent to each
  other, or the heliostat field is limited to the
  view of the cavity aperture 1.

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• The aperture size is minimized to reduce
  convection and radiation losses without blocking
  out too much of the solar flux arriving at the
  receiver. The aperture is typically sized to
  about the same dimensions as the suns reflected
  image from the farthest heliostat, giving a
  spillage of 1 to 4 percent. For a 380 MW plant
  design, the aperture width for the largest of the
  four cavities (the north-facing cavity) is 16 m,
  and the flux at the aperture plane is four times
  that reaching the absorbing surface inside. In
  figure 10 you can see the two different types of
  receivers 1.

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Figure 10. External and Cavity Type Receivers
3.
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3. Tower design

• The height of the tower is limited by its cost.
  The weight and wind age area of the receiver are
  the two most important factors in the design of
  the tower. Seismic considerations are also
  important in some locations. Figure 11 shows a
  solar tower 1.

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Figure 11. Solar Tower 3.
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• The weight and size of a receiver are affected by
  the fluid choice as discussed previously. Typical
  weights for a 380 MW receiver range from 250,000
  kg for an external receiver using liquid sodium
  to 2,500,000 kg for a cavity air receiver. These
  would be placed at the top of a 140 to 170 m
  tower if a surrounding heliostat field is used
  1.

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• Proposed tower designs are of either steel frame
  construction, using oil derrick design
  techniques, or concrete, using smokestack design
  techniques. Cost analyses indicate that steel
  frame towers are less expensive at heights of
  less than about 120 m and that concrete towers
  are less expensive for higher towers 1.

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4. Beam characterization targets

• Prominent on any photograph or drawing of a
  central receiver tower are the white targets
  located just below the receiver. These are beam
  characterization system (BCS) targets used to aid
  in periodic calibration and alignment of
  individual heliostats. They are coated with a
  diffusely reflecting white paint, and are not
  designed to receive the flux of more than one or
  two heliostats. Instrumentation within the target
  area is used to determine the centroid and flux
  density distribution of the beam from a selected
  heliostat. If the centroid of the beam is not
  located where the field tracking program predicts
  it to be, tracking program coefficients are
  modified appropriately 1.

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5. Heat transfer fluids

• The choice of the heat transfer fluid to be
  pumped through the receiver is determined by the
  application.  The primary choice criterion is the
  maximum operating temperature of the system
  followed closely by the cost-effectiveness of the
  system and safety considerations.  Five heat
  transfer fluids have been studied in detail for
  application to central receiver systems 1.

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• The heat transfer fluids with the lowest
  operating temperature capabilities are heat
  transfer oils. Both hydrocarbon and
  synthetic-based oils may be used, but their
  maximum temperature is around 425C. However,
  their vapor pressure is low at these
  temperatures, thus allowing their use for thermal
  energy storage. Below temperatures of about
  -10C, heat must be supplied to make most of
  these oils flow. Oils have the major drawback of
  flammable and thus require special safety systems
  when used at high temperatures. Heat transfer
  oils cost about 0.77/kg 1.

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• Steam has been studied for many central receives
  applications and is the heat transfer fluid used
  in the Solar One power plant. Maximum temperature
  applications are around 540C where the pressure
  must be about 10 MPa to produce a high boiling
  temperature. Freeze protection must be provided
  for ambient temperatures less than 0C. The water
  used in the receiver must be highly deionized in
  order to prevent scale buildup on the inner walls
  of the receiver heat transfer surfaces. However,
  its cost is lower than that of other heat
  transfer fluids. Use of water as a
  high-temperature storage medium is difficult
  because of the high pressures involved 1.

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• Nitrate salt mixtures can be used as both a heat
  transfer fluid and a storage medium at
  temperatures of up to 565C. However, most
  mixtures currently being considered freeze at
  temperatures around 140 to 220C and thus must be
  heated when the system is shutdown. They have a
  good storage potential because of their high
  volumetric heat capacity. The cost of nitrate
  salt mixtures is around 0.33/kg, making them an
  attractive heat transfer fluid candidate 1.

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• Liquid sodium can also be used as both a heat
  transfer fluid and storage medium, with a maximum
  operating temperature of 600C. Because sodium is
  liquid at this temperature, its vapor pressure is
  low. However, it solidifies at 98C, thereby
  requiring heating on shutdown. The cost of
  sodium-based systems is higher than the nitrate
  salt systems since sodium costs about 0.88/ kg
  1.

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• For high-temperature applications such as Brayton
  cycles, it is proposed to use air or helium as
  the heat transfer fluid.  Operating temperatures
  of around 850C (1560F) at 12 atm pressure are
  being proposed.  Although the cost of these gases
  would be low, they cannot be used for storage and
  require very large diameter piping to transport
  them through the system 1.

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6. Storage System

• A storage system makes it possible to run the
  steam turbine under constant conditions even
  during periods of varying insolation (clouds) or
  after sunset. It consists of two main parts which
  are hot and cold storage tanks. In figure 12, you
  can see these tanks 3.

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Figure 12. Storage Tanks 3.
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7. Power Generator
An electric generator is a device that converts
mechanical energy to electric energy. See figure
13.
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Figure 13. Power Generator 3.
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8. Multi Tower Solar Array (MTSA)

• The Multi Tower Solar Array (MTSA) is a new
  concept of a point focusing two-axis tracking
  concentrating solar power plant (Fig.14). The
  MTSA consists of several tower-mounted receivers
  which stand so close to each other that the
  heliostat fields of the towers partly overlap.
  Therefore, in some regions of the total heliostat
  field the heliostats are alternately directed to
  different aiming points on different towers. Thus
  the MTSA uses radiation which would usually
  remain unused by a conventional solar tower
  system due to mutual blocking of the heliostats
  6.

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Figure 14. MTSA 6.
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• In an urban environment small MTSAs can be
  installed on the flat roofs of big buildings such
  as industrial halls or shopping complexes or over
  open areas like parking sites. Even in central
  Europe, parking sites do have the problem that
  the cars can overheat on hot and sunny days.
  Therefore an MTSA reflector field, serving as a
  sun protecting roof at a height of 3 to 6 m,
  could be advantageous. The solar radiation would
  be utilized and additionally the cars would be
  protected from overheating. In figure 15 and 16
  you can see visualizations 6.

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Figure 15. A visualization of an MTSA field over
a parking site at the Munich Trade Centre 6.
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Figure 16. Impression of conditions in a parking
lot topped by an MTSA solar array 6.
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References

1. Web page http//www.powerfromthesun.net/Chapter10
   /Chapter10new.htm
2. Web page http//www.ciemat.es/eng/instalacion/psa
   -cesa-1.html
3. Sengul Topcu, phys471 project. 20/04/2004.
4. Web page http//www.eia.doe.gov/kids/energyfacts/
   sources/renewable/solar.html
5. Web Page http//www.powerfromthesun.net/Chapter8/
   Chapter8new.htm
6. Web Page http//www.physics.usyd.edu.au/app/resea
   rch/solar/mtsa.html