A User's Manual for DELSOL3 - prod.sandia.gov - Sandia National ...
A User's Manual for DELSOL3 - prod.sandia.gov - Sandia National ...
A User's Manual for DELSOL3 - prod.sandia.gov - Sandia National ...
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The most common choice <strong>for</strong> the curvature of a rigid heliostat is a symmet-<br />
rical “on-axis” focusing or cant. In this case, the heliostat is perfectly focused or<br />
canted at a point along the heliostat optical axis, A, when the sun is positioned<br />
along A, (i.e., A = s^ in Figure 11-1). When the sun is in other positions (A # 8),<br />
there will be off-axis aberrations.<br />
A second possibility is an asymmetric curvature such that the heliostat is per-<br />
fectly focused or canted <strong>for</strong> a specific sun position (defined by the HCANT hour<br />
past noon on the DCANT day of the year). In this case the relative positions of<br />
the sun, heliostat, and receiver are important because, in general, the focal point<br />
will not lie along A. This “off-axisn cant or focus results in off-axis aberrations<br />
when the sun position differs from that specified by (HCANT, DCANT).<br />
On a yearly average, off-axis aberrations are less with an on-axis than off-axis<br />
cant or focus. Moreover, the symmetry of the on-axis cant or focus allows <strong>for</strong> a<br />
simpler manufacturing procedure. However, at the time specified by (HCANT,<br />
DCANT), an off-axis cant or focus <strong>prod</strong>uces a smaller image and higher flux den-<br />
sity. This may be useful in test facilities where the number of heliostats is small,<br />
and periodic readjustment of their canting or focusing is possible. DELSOL al-<br />
lows both on-axis and off-axis canting but only on-axis focusing.<br />
For canting, several options are available according to the choice of the parameters<br />
ICANT, NCANTX, and NCANTY. There are NCANTX x NCANTY<br />
equally sized submirrors per heliostat with NCANTX (NCANTY) submirrors<br />
along the 2, (jn) edge of the heliostat, as shown in Figure 11-11(A). The larger<br />
NCANTX and NCANTY, the more closely a canted heliostat approximates a focused<br />
heliostat. Uncanted heliostats are specified by ICANT=O and have only<br />
one sub-mirror equal to the heliostat itself; there<strong>for</strong>e, NCANTX=NCANTY=l.<br />
Fields in which each heliostat is individually canted off-axis on the HCANT hour<br />
past noon and the DCANT day of the year (default is solar noon, equinox, March<br />
21) are specified by ICANT=3. A heliostat field in which every heliostat is focused<br />
”on-axis” with a focal length equal to its slant range is specified by the<br />
parameter ICANT=-1. Finally, fields in which heliostats have “on-axis” canting<br />
with user defined focal lengths (= RCANT(K) tower heights) are specified by<br />
ICANT=l. (This option can be used to <strong>prod</strong>uce a single cant <strong>for</strong> the whole field.)<br />
Independent of canting, the mirror or submirrors can be focused in 0 to 2 di-<br />
mensions. If the mirror is curved along the 2, (;,) direction then the parameter<br />
XFOCUS=1.0 (YFOCUS=l.O). Thus, no focusing is specified by XFOCUS=O.O,<br />
YFOCUS=O.O. One dimensional focusing is specified by XFOCUS=1.0, YFO-<br />
CUS=O.O, or XFOCUS=O.O, YFOCUS=l.O; 2-d focusing is specified by XFO-<br />
CUS=l.O, YFOCUS=1.0. The parameter IFOCUS determines the specification of<br />
the focal lengths. DELSOL automatically sets the focal length equal to the slant<br />
range when IFOCUS=O. User defined focal lengths, XFOCAL and YFOCAL, can<br />
be specified when IFOCUS=l.<br />
11.0-9. Default Hefiostat-The default heliostat is a high reflectivity square<br />
advanced design with a single cant and focus, as defined below:<br />
49
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