Environmental Engineering Reference
In-Depth Information
parabolic first-surface mirror was selected, despite its high initial cost and
sole-source limitation. The rationale was that molded plastic parabolic mir-
rors could eventually be produced in quantity. To keep the mirror pointed at
the sun, a tracking systemhad to be identified or developed. Early experiments
with sun-pointing sensors had suggested that they might not be adequate for
maintaining the required pointing accuracy. Ultimately, a dual-axis open-
loop tracking system was selected from a third-party source. This system,
when accurately programmed with the latitude, longitude, date and time was
capable of maintaining 0.1 pointing accuracy without any position sensor
feedback. The drawback to the open-loop tracking systemwas that it required
careful calibration to account for any alignment errors in the system. A
specialized camera-based sun pointer was developed for use in the calibration
procedure [11].
The choice of an appropriate optical fiber was another essential element.
Glass and plastic bundles, large core plastic waveguides, and even a liquid
core specialized waveguide were all considered. Ultimately, large core
(12-mm) waveguides constructed from fully cured polymethacrylate (PMA)
were selected. These waveguides eliminated the optical losses due to the
packing factor in fiber bundles, by having only one core. In addition, the
material had higher optical transmission and was significantly softer than
polymethylmethacrylate (PMMA). The combination of optical and mechan-
ical properties made the PMA waveguides attractive for this application.
Although the basic PMA product was offered by several vendors, the optical
properties varied significantly [12]. Eventually a product was selected that
had good transmission and negligible color shift.
Directing the light from the primary mirror required a secondary mirror
assembly. A segmented secondary mirror assembly was designed that
divided the concentrated light from the primary into eight individual foci.
The secondary mirror segments were designed with a dichroic coating that
reflected only the visible wavelengths, allowing the IR to pass through the
mirrors. The optical waveguide mounting assemblies were designed with
alignment features that allowed each of eight waveguides to be aligned to
one of the segmented foci. A specialized alignment system and procedure
was developed to ensure that each of the fibers was aligned to the optical
axis of the parabolic primary mirror [13]. Due to the fact that there were no
alignment features on the primary mirror that could reliably be used to
identify the optical axis, a portion of the initial assembly procedure
required measuring the parabolic mirror with a portable coordinate mea-
surement instrument, as shown in Fig. 5.9. Using these measurements,
the optical axis of the mirror was identified, and the mirror was cemented
into a specially designed mounting cradle. The fully assembled prototype
concentrator-tracker assembly with transmission fibers is shown in
Fig. 5.10. The sun pointer used for the calibration of the system is mounted
on the segmented secondary mirror assembly.
