A performance evaluation was conducted on a commercially available point-focus solar concentrator, the Heliodyne model HTC-25s tracker/concentrator. Thermal power test results indicate that slightly more than six kilowatts of thermal energy is available from a system of this configuration using a 10 cm aperture

Patzold, J. D.

English

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N80 24777
OMNIUM-G CONCENTRATOR TEST RESULTS*
J. D. Patzold
.Tot Propulsion Laboratory
Pasadena, California	 ORIGINAL PACK IS
OF POUR QUALITY
ABSTRACT
The Jet Propulsion Laboratory Solar Thermal Power Systems Project!
Module Developments conducted a performance evaluation on a commercially
available point-focus solar concentrator manufactured by the Omnium-G
Company. Thermal power test results indicate that slightly more than six
kilowatts of thermal energy is available from a system of this config-
uration using a 10 em ,aperture under the conditiono outlined in this paper.
INTRODUCTION
Preliminary results of evaluation Nesting conducted to determine the
thermal performance of a HeliodyneTM model I1TC-25e tracker/concentrator
manufactured by the the Omnium-G Company of Anaheim, California are summarized
in. this paper. The concentrator is pant of the OG-7500 module purchased by
VAT 	 ^.... 10788
This system connists of a two-axis, sun-tracking parabolic dish
concentrator six meters in diameter with a four meter focal length. The dish
structure has 15 pie-shaped elementso or petals, surfaced with anodized
aluminum (Alzak R ).
All tests were conducted with petals that were new, clean, and in an "as
received" condition. Sun-tracking was done in the manual override mode, i.e.,
automatic sun-tracking was not employed. More detailed results of these tests
will appear in an external report.
TESTING
The chronological order of the tests performed on the system is shown in
Table 1. These thermal power tests are part of a more extended series.
A concentrator retrofit is indicated in Table 1. Within that time
interval the Omnium-G Company recovered the original "A"-mold petals and
replaced them with "C"-mold petals; only results for the latter are presented
here.
The system checkout testing was conducted to determine an operational
ready-state and to establish remote control capability of the system on the
Parabolic Dish Test Site (PDTS) at the JPL Edwards Test Station.
'The research described in this paper was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, and was sponsored by the
U.S. Department of Energy through an agreement with NASA.
125
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Table 1.	 Chronological Order of Significant Tests
Performed on the Omnium-G System
12-21-78 System Checkout 04-12-79 Convertor (Cold)
01-27-79 Reflectivity Tests 04-20-79 Fluxmapper
02-06-79 Alignment 04-23-79 Concentrator Retrofit
02-06-79 Calorimeter thru
02-22-79 Alignment 05-10-79
02-27-79 Calorimeter 05-15-79 Alignment
03-12-79 Moonlight Focal. Plane Evaluation 05­25-79 Fluxmapper
Plane Evaluation 06-06-79 Calorimeter
03-1:3-79 "Step Testing" Memo 06
-22-79 Convertor (Cold)
08-20-79 Convertor (Hot)
The concentrator is positioned in the azimuth and elevation axes by a null-
seeking sun-tracker system consisting of a sun-sensor photocell box and tracker
electronics module. The sun-sensor photocell determines the error and the
tracker electronics module controls the tracker DC drive motors. The diagram
shown in Figure 1 illustrates the concentrator with the convertor (receiver)
mounted at the fecal point. A hemispherical reflectivity test series was
performed before the first set of concentrator petals was installed. In this
test a light source was used to illuminate a Small area of a concentrator
petal and the refler..ted light intensity was measured. Twenty-two concentrator
petals were measured at nine locations and the average reflectivity was
eighty-four percent.
The concentrates petals were aligned at night to assure that all reflected
images were superimposed at the focal plane. Three different alignment methods
were used. Figure 2 illustrates the method used for the "C"-type petal con-
centrator installation. After positioning the concentrator toward the light
source, an observer views the reflecting surface through the wire hoop
eyepiece. Technicians then adjusted each individual petal manually until full
illumination of the reflective surface was observed. The manufacturer sel-
ected the mirror elements for installation prior to delivery, and no further
provision was made to refocus the petals after delivery to the test. site.
Boresighting, the final alignment operation, requires the manual
repositioning of the sun-sensor photocell box relative to the concentrator
support structure. Repositioning assures that the aggregate group of solar
images is centered on the receiver aperture. This operation, or its
verification, must take place following each changeout of an experiment to
compensate for mass changes at the focal-point end of the support structure.
EXPERIMENTAL APPARATUS AND METHODS
The thermal performance output of the concentrator was determined by three
different tests. The flat-plate cold-water calorimeter, the fluxmapper, and
the Omnium-G receiver were the devices used in testing. The flat-plate calori-
meter and the fluxmapper yield data as a function of aperture diameter. There
was special interest in the results of the 10 cm (4 inch) dimension because it
was the cavity entrance diameter for the receiver supplied by Omnium-G.
126
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Figure I. Omnium-G Heliodyne tracking
	
Figure 2 Nighttime alignment
concentrator/collector	 technique
The flat-plate calorimeter was fabricated from two 36 cm by 36 cm square
copper plates. The plates were furnace-brazed together after cooling water
passages were machined into one of them. These parallel passages were
connected to inlet and outlet passages forming manifolds. The calorimeter was
designed to permit a low water temperature rise and a hot-side surface
temperature of less than 3800. The front surface of the calorimeter was
painted with 3M Velvet Black spray paint providing a 0.97 solar absorptivity.
The water temperature rise across the calorimeter- was controlled in the
range 30C to 60C by adjusting flow rate. If the temperature rise is too
low there will be a high degree of uncertainty in the measurement results and
if the temperature rise is too high it will cause an excessive calorimeter
operating temperature.
The calorimeter water flow rate was measured using a turbine- type flow
meter. The absolute inlet temperature of the feed water was measured with a
thermocouple probe near the flow meter. The rise in water temperature was
measured with thermocouples arranged in series (a thermopile) so that a larger
output voltage for the small temperature difference could be measured.
Nominal water flow rate was 38 liter/mina
The calorimeter was mounted approximately 10 cm behind the focal plane
location supplied by the Omnium-G Company. Separate aperture plates (with
various size apertures) were mounted at the focal plane. These plates were
fabricated from 1.9 cm-thick sheets of transite (asbestos), and had a lifetime
of about two hours, sufficient to acquire test information. Aperture size
(diameter) was varied in the range of approximately 8 to 18 cm.
Initial calorimetric data indicated that a problem existed in the original
concentrator petals. As a result of this data, the concentrator system was
realigned so that thermal performance could be improved. Some improvement was
noted but further information was required to evaluate the optical image near
the focal plane.
127
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6Visual evaluation of focal length and focusing quality of each concen-
trator metal was accomplished by using the !soon as a light source and viewing
the reflected lunar image on a moveable target located at, and near, the focal
plane. Representatives of Omnium-G Company were present for these tests a.ad
they determined that the concentrator petals were not properly focused indi-
viduall y or collectivel y . The recommendation of the manufacturer was to
terminate the evaluation on the original "A" type petals and replace them with
the "C" type. After the concentrator petals were replaced, the final alignment
was performed by
 Omnium-G personnel.
The flu;rmanper wa g then installed and the first thermal performance data
obtained for the "C"-type concentrator assembly. The fluxmapper is a device
that utilizes a water cooled probe that moves in an X-Y plane perpendicular to
the focal axis, or Z-axis. Motion on the 7.-axis, ahead and behind of the focal
plane, facilitates a three-dimensional mapping of the vicinity of the focal
plane. (See paper by W. Owen in these Proceedings.) Figure 3 illustrates the
mounting of the open ring support structure and electronics package.
Figure 3. Fluxmapper mounted at the nominal focal plane
128
All controls for the fluxmapper 'were programmed into a microprocessor that
was also used to gather, store, and process the data acquired by the probe.
Output from the processor was displayed on an X-X plotter or line printer for
real-time evaluation of concentrator focal plane shape, and flux intensity.
The third device used to determine thermal performance of the concentrator
was the convertor (receiver) manufactured by the Omnium-G Company„, This
receiver, an early design, incorporated a 10 cm diameter aperture and a steam
coil buried in an aluminum block. Originally this aluminum was to have been
heated to a molten state, but present design limits the aluminum mass
temperature to 638 0C (11800F).
The receiver was tested at two temperature levels: 930C (2000F), called
a "cold” test, and at 2040C (4000F), called a "hot" test. In the first test
series facility tap water was used and no steam was produced. The second series
utilizes: the arrangement shown in Figure 4; sufficient back pressure was
applied to maintain a saturated water condition at the receiver exit. Steam
formed across the expansion valve was condensed and returned to the feed-
water reservoir. During this test, the receiver outlet water temperature was
maintained for an hour to allow the receiver to achieve thermal equilibrium.
TESTING CONDITIONS
The flatplate calorimeter data and the .fluxmapper data were obtained with
new; clean petals. Some of the receiver data were obtained with petals that
were dirty. In all cases, a manual override tracking mode was employed to
assure a continuous on-sun condition, and automatic tracking was not utilized
because of occasional tracker drift.
Data was recovered continuously for each test run but only data taken at
insolation levels greater than 800 W/M2 was analyzed. All data was norm-
alized to a solar insolation value of 1000 W/M 2 . Solar insolation data was
acquired by using three Kendell Mark III, and one Eppley, Pyroheliometers.
Wind speed and direction also were recorded.
Omnium-G personnel were invited and encouraged to observe all tests at the
PDTS. In addition, JPL furnished the Omnium-G Company with various test data
and other results.
THERMAL PERFORMANCE TEST RESULTS
A typical flux contour plot obtained for the "C"-type petals is shown in
Figure 5. Superimposed is a hypothetical aperture diameter of 10 cm (4
inches). The peak flux near the center of 700 W/in 2 corresponds approx-
imately to 110 W/cm2 . Fluxmapper data corresponding to.various aperture
sizes as well as flatplate, coldwater calorimeter data is shown in Figure 6.
Flatplate and coldwater calorimeter data had an uncertainty of +400 watts,
which is indicated by bound-bars in Figure 6. Agreement between fluxmapper
data and calorimetric data is excellent.
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500
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10 V 1000 W/n2
Figure 4. Receiver Thermal Performance
	 Figure 5. Typical flux map at
Test Schematic Diagram	 the focal plane
"Cold" and "hot" receiver test data are shown in Figure 7 relative to the
previous data, which is represented in Figure 7 by the cross-hatched region.
Of course, the receiver test results apply only for a 10 cm aperture. The
cold receiver test data agrees well with the previous data.
For the cold receiver tests, the receiver outlet temperature was maintained
at less than 93 0C (2000F) to prevent two-phase flow. A comparison series
of tests was performed during the cold water receiver series to determine the
effect of dirt and dust accumulation on the concentrator. A 16% improvement
in thermal performance was obtained following a mirror cleaning operation
recommended by Omnium-G.
The data plotted for the hot receiver test series was obtained while the
petals were dirty, but assuming the same 16% improvement; could be achieved,
thermal performance very close to the calorimetric, fluxmapper, and cold con-
vertor data might be expected. However, the difference in the cold and hot
S
2	
FLATPLAIE CALORIMETER
• FLU%MAPPER
	 ~
0—
Y9 >
r 4>`Y
s	 e'
6.
Y
•
4
	 ....^^
~	 ALL DATA NORMALIZED
2	 ti TO INSOLATION OF
I, ODD W/ M2
0
0	 2	 4	 6	 8	 10	 12	 14	 16	 16
RECEIVER APERTURE DIAMETER, cm
Figure 6. Preliminary thermal power
test results
l-
12	 • COLD RECEIVER TEST ICIEANI
• COLD RECEIVER TEST IDIRTYI
•
10	
HOT RECEIVER TEST (CLEAN)
8
6	 •
4	 •
ALL DATA NORMALIZED
TO INSOIAJION OF
2	 7,000 WIM
0
0	 2	 4	 6	 8	 10	 12	 14	 16	 18	 20
RECEIVER APERTURE DIAMETER, Cm
Figure 7. Receiver (converter)
test results
y
130
4
receiver data (Figure 7, dirty petals) cannot be explained solely on the basis
of differences in thermal reradiation.
Uncertainty in data, including the statistical variation of calorimetric
data of +400 watts (approximately 6% of the energy through a 10 cm diameter
aperture'T,
 has been estimated. Thermal reradation received by the flatplate
calorimeter from the heated aperture plates was estimated to be about 420
watts for an aperture plate temperature of 325 0C. This effect, now being
investigated experimentally, is expected to decrease with increasing aperture
size. Estimates of thermal losses from forced convection )
 due to winds of the
magnitude seen during test data acquisition ( 20 mph), indicate about 1 to 2%
wind effect.
CONCLUSIONS
Thermal power test results on an Omnium-G tracking concentrator, purchased
by JPL in the fall of 1978, have been presented. This includes coldwater
calorimeter and fluxmapper data, and some preliminary data using the early
Omnium-G receiver design. The measured thermal power was in the range 6 to 7
thermal kilowatts for new, clean petals, and a 10 cm aperture. Design changes
to increase the receiver aperture diameter to 20 cm have been completed by the
Omnium-G Company in accordance with their continuing development program. A
new receiver design has been delivered to JPL and plans are to test it by the
summer of 1980.
ACKNOWLEDGEMENTS
The assistance and cooperation of 11. Carley, S. Holian, W. Owen, J.
Roschke, and D. Ross of JPL, as well as PDTS personnel, was greatly
appreciated. Valuable exchange of information was afforded with .} 9ohn of
the Solar Energy Research Institute (SERI) of Golden, Colovado. k- o
acknowledged is the support and encouragement of J. Lucas and V. Truscello of
JPL, and G. Braun of DOE. Finally, the timely cooperation of Omnium-G Company
personnel was appreciated.
13-


OMNIUM-G concentrator test results

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