A User's Manual for DELSOL3 - prod.sandia.gov - Sandia National ...

Given that heliostats will be mass produced and assembled, it is assumed in
DELSOL that the error sources are essentially the same for each heliostat re-
gardless of field location. Hence, the user supplies as input the magnitude of the
sources of error (e.g., motor inaccuracies, surface distortions), and the code cal-
culates the time and field dependent effects of these sources. (This is identical to
the approach in MIRVAL and HELIOS (References 1, 2, 5).)
The sources of heliostat errors can be grouped into three types according to
the variable used to describe the error distribution. Each type produces different
qualitative and quantitative effects on the position and profile of the heliostat im-
age. The code assumes that the errors have normal probability distributions char-
acterized by standard deviations in two perpendicular directions. The three error
groups, typical sources, and the coordinate systems in which they are defined are
listed in Table 11-2. The default values are also included, and are consistent with
a fairly accurate, high reflectivity mirror such as a Second Generation Heliostat
(Reference 13).
1I.D-2. Focusing and Cunting-The finite size of the sun, the heliostat per-
formance errors, and the size of the heliostat all determine the size of the image
produced by a heliostat on the receiver, as illustrated in Figure II-lZ(A). Reduc-
ing the contribution of the heliostat size can lead to a smaller image size, and in
turn, lower spillage, smaller receivers, and lower receiver radiation and convection
losses. DELSOL simulates the two methods, focusing and canting, employed to
reduce image size by decreasing the contribution from heliostat size.
In focusing, the mirror panels are concave in a manner so that rays from the
center of the sun reflected from point on the mirror panel hit the same point
on the receiver, as shown in Figure II-l2(B). A canted heliostat is divided into a
number of submirrors. Each submirror is displaced relative to the others so that
rays from the center of the sun reflected from analogous points of the submirrors
all converge to the same point on the receiver, as indicated in Figure II-lZ(C).
Thus, perfect focusing results in the minimum size image by eliminating the con-
tribution of the heliostat size to the reflected image. Perfect canting approximates
perfect focusing by reducing the total heliostat size to that of a single submir-
ror. The greater the number of canted submirrors for a given size heliostat, the
smaller the contribution of heliostat size to the image. In other words, canting is
a Fresnel approximation to focusing. (Note also that the submirrors of a canted
heliostat can each be focused in 0 to 2 dimensions.)
The curvature or displacement required for focusing or canting depends on
the angles between the heliostat, sun, and receiver, and is therefore time depen-
dent. For most heliostat designs the curvature cannot be varied, and the heliostat
will be perfectly focused or canted for only the one or two times of the year when
the sun is in the correct position. At all other times, the heliostat will produce
“off-axis aberration” of the image; Le., distortions of the ideal image due to oper-
ation when the sun is not in the correct position for the heliostat to be perfectly
focused or canted.
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