Transcript Slide 1

MIDDLE EAST TECHNICAL UNIVERSITY
Phys. 471 project
HELIOSTAT FIELD
PRESENTED BY : Ertuğ ÖZYİĞİT
Bahtiyar RUZIBAYEV
INSTRUCTOR : Prof. Dr. Ahmet ECEVİT
2004-1
1
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……………………………………………………………………………….36
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
References………………………………………………………………………………………56
2
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.
3
Figure 1. CESA-1 CRS [2].
4
• 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 840°C) [1].
5
• 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 516°C
at a maximum rate of 42 MW. In figure 2
you can see Solar One power plant [1].
6
Figure 2. Solar One Power Plant [1].
7
Components of CRS
•
•
•
•
Central receiver consists of,
Solar concentrator (heliostat field)
Receiver
Storage system
Power generator
Figure 3 shows the schematic diagram of
CRS.
8
Figure 3. Schematic Diagram of CRS [3].
9
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.
10
Figure 4. Heliostats [3].
11
• 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].
12
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.
13
Figure 5. Erik Rossen’s Heliostat Design [3].
14
1.2 Ideal Heliostat
• Low cost
• Maximum reflection
• No absorbtion & transmission
In table 1 you can see the reflectivity and
emissivity of some surfaces.
15
%
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
5 - 15
0.90
Building materials: wood, paper, glass, masonry, nonmetallic
paints
Table 1. Reflectivity and Emissivity for Different Surfaces [1].
16
1.3 Heliostat Field Types
• Surrounding the tower
• On one side of the tower
You can see these types in figure 6.
17
Figure 6. One Side and Surrounding Type [3].
18
1.4 Heliostat Errors
• A perfectly flat heliostat would produce an
image on the receiver the size of the
heliostat (projected normal to the heliostatreceiver 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].
19
• 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].
20
• 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].
21
• 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].
22
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 sun’s 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 sun’s 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.
23
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].
24
• Field cosine efficiency, calculated by using
equation 1.
Equation 1. Field Cosine Efficiency [1]
where α and A are the sun’s 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.
25
Figure 8. Coordinates defining the reflection of the sun’s rays by a heliostat to
a single aim point. Vector H is normal to the heliostat reflecting surface [5].
26
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].
27
Figure 9. Shadowing and Blocking Effect [1].
28
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].
29
• 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].
30
2.1 Types of Receivers
• External Type
• Cavity type
31
• 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].
32
• 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].
33
• 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 sun’s 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].
34
Figure 10. External and Cavity Type Receivers [3].
35
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].
36
Figure 11. Solar Tower [3].
37
• 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].
38
• 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].
39
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].
40
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].
41
• 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 425°C. However, their vapor pressure is
low at these temperatures, thus allowing their
use for thermal energy storage. Below
temperatures of about -10°C, 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].
42
• 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
540°C where the pressure must be about 10
MPa to produce a high boiling temperature.
Freeze protection must be provided for ambient
temperatures less than 0°C. 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].
43
• Nitrate salt mixtures can be used as both a
heat transfer fluid and a storage medium
at temperatures of up to 565°C. However,
most mixtures currently being considered
freeze at temperatures around 140 to
220°C 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].
44
• Liquid sodium can also be used as both a
heat transfer fluid and storage medium,
with a maximum operating temperature of
600°C. Because sodium is liquid at this
temperature, its vapor pressure is low.
However, it solidifies at 98°C, 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].
45
• 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
850°C (1560°F) 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].
46
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].
47
Figure 12. Storage Tanks [3].
48
7. Power Generator
An electric generator is a device that converts
mechanical energy to electric energy. See
figure 13.
49
Figure 13. Power Generator [3].
50
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].
51
Figure 14. MTSA [6].
52
• 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].
53
Figure 15. A visualization of an MTSA field over a parking site at
the Munich Trade Centre [6].
54
Figure 16. Impression of conditions in a parking lot topped by
an MTSA solar array [6].
55
References
1.
2.
3.
4.
5.
6.
Web page:
http://www.powerfromthesun.net/Chapter10/Chapter10new.htm
Web page: http://www.ciemat.es/eng/instalacion/psa-cesa-1.html
Şengul Topcu, phys471 project. 20/04/2004.
Web page:
http://www.eia.doe.gov/kids/energyfacts/sources/renewable/solar.
html
Web Page:
http://www.powerfromthesun.net/Chapter8/Chapter8new.htm
Web Page:
http://www.physics.usyd.edu.au/app/research/solar/mtsa.html
56