To insure that the insolation values accurately represent the input power to a power conversion unit the field of view (FOV) of the concentrator aperture and the insolation radiometer must be the same. The calculations, implementation, and results of this approach are covered. Three instruments were used to measure the insolation: an Eppley Normal Incidence Radiometer (NIP) and two versions of the kendall cavity radiometer. The shrouds used to limit the FOV of the radiometers were designed to simulate the FOV of the PDC-1 concentrater with the cold water cavity calorimeter. This technique of matching the FOV of an insolation radiometer to the FOV of a specific concentrater and receiver aperture appears to be both practical and effective. The efficiency of a power conversion unit will be too low if the insolation is measured with a radiometer which has a FOV which is larger than the FOV of the concentrator

Dennison, E. W.

English

NASA Technical Reports Server

Special Pyrheliometer Shroud Development
E. Dennison
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91109
Introduction The need for simple methods to compensate for circumsolar
radiation became very important during the spring of 1982 when large
amounts of dust appeared in the upper terrestrial atmosphere as a result
of a volcanic eruption in Mexico. Prior to this time definitive
calorimeter measurements at the JPL Parabolic Dish Site (PDTS) at the
Edwards Test Station were limited to times when the insolation was
greater than 950 W/sq.M, i.e., times when the amount of circumsolar
radiation was negligible. This limitation was used because of the
inconsistent data which was found at low elevation angles and on days
of high circumsolar radiation.
After the appearance of the volcanic dust the maximum insolation was
less than 900 W/sq.M and the amount of circumsolar radiation was
significant. In addition, there was a very substantial increase in the
presence of high thin cirrus clouds which added significant errors to
the calorimeter measurements. The presence of circumsolar radiation is
clearly shown in Figure 1. In this photograph of the PDC-l concentrator
the image of the sun is covered by the calorimeter and the bright halo
around the calorimeter is the circumsolar radiation.
Insolation measurements are used to analyze atmospheric conditions,
evaluate potential thermal power sites, and to determine the amount of
input power to a power conversion unit. For the latter, the insolation
measurements are used as an interpolation parameter between calorimeter
measurements.
To insure that the insolation values accurately represent the input
power to a power conversion unit it is important that the Field Of View
(FOV) of the concentrator aperture and the insolation radiometer are
*The work described in this paper was carried out or coordinated by the
Jet Propulsion Laboratory, California Institute of Technology, and was
sponsored by the U.S. Department of Energy through an agreement with
the National Aeronautics and Space Administration.
215
https://ntrs.nasa.gov/search.jsp?R=19840020186 2020-03-22T10:10:52+00:00Z
the same. The word "radiometer" will be used to refer to both radiometers
and pyrheliometers. If the calorimeter and the power conversion unit
have the same aperture, the radiometer can be used to normalize all
power measurements to a standard insolation value x (1000 W/sq.M for the
JPL data).
This report covers the calculations, implementation, and results of the
JPL use of this approach. Three instruments were used to measure the
insolation: an Eppley Normal Incidence Radiometer (NIP) and two versions
of the cavity radiometer developed by J. M. Kendall, Sr. at JPL. One
of the Kendall radiometers was of the Mark VI windowless design used
for calibration of radiometers and the other was the Mark III quartz
window design used for routine field measurements. The shrouds used to
limit the FOV of the radiometers were designed to simulate the FOV of
~he PDC-1 concentrator with the Cold Water Cavity Calorimeter (Figure 2).
Field of View Calculations The quantitative description of the FOV of
a concentrator or radiometer will be referred to as the Angular Acceptance
Function (AAF). The AAF is the fraction of the radiation coming from a
point source at an infinite distance as a function of the angular
distance of the point source from the optic axis of the concentrator or
radiometer. The AAF does not depend on the angular size of sun but
only on the geometric parameters of the concentrator or radiometer.
The AAF will be 1.0 for a source on the optic axis and will decrease
continuously from an inner limiting angle to zero at the outer limiting
angle. The form of a concentrator AAF is different from the AAF of a
radiometer because of the respective optical geometries. As a result,
the radiometer AAF cannot be matched exactly to the concentrator AAF,
but the differences between these functions can be made acceptably small.
Concentrator AAF One practical method for calculating the AAF of a
solar concentrator was developed by Prof. G. F. Trentelman of Northern
Michigan University using vector analysis. It has been found that in
practice it is convenient to represent the AAF as a function of the
ratio of the tangent of the source angle divided by the tangent of the
receiver radius angle (receiver radius divided by the concentrator focal
length). This parameter will be referred to as the "radius number."
In this form the AAF changes substantially with the concentrator f-number
(focal length divided by diameter) and insignificantly with receiver
aperture. When the AAF is expressed in terms of these dimensionless
parameters it can be easily applied to any paraboloidal concentrator
with any receiver aperture (Figure 3). Slope errors cause some change
in the AAF but in most cases these effects appear to be small.
Radiometer AAF The AAF of a radiometer can be calculated directly from
the aperture and detector radii, the aperture/detector separation, and
the angular distance of the point source from the optic axis. This
calculation is based on the assumption that the detector aperture is
uniformly sensitive. With the ratio of the front aperture to the
216
detector aperture and the tangent of the receiver aperture times the
aperture/detector separation as free parameters the radiometer AAF can
be calculated as a function of the radius number.
Matching Radiometer AAF to Concentrator AAF One practical method for
finding the radiometer parameters which make the radiometer AAF fit the
concentrator AAF is to use an interactive computer program. By a careful
choice of parameters it is possible to make the radiometer AAF rapidly
converge on the concentrator AAF. Because the concentrator AAF is
determined from a finite number of rays, it can be fitted to a polynomial
to give a smooth representation of the data. In practice this does not
significantly change the radiometer parameters.
Experimental Results To verify the expected advantages of using a FOV
limiting shroud on an insolation radiometer, a series of calorimeter
measurements were made using the PDC-1 concentrator and the shrouded
radiometers. One of the radiometers was an Eppley NIP mounted on the
concentrator. The other radiometers were of the Kendall type and were
attached to an equatorial mount with a clock drive. The boresight
alignment images were checked frequently during the measurement period
to insure that no erroneous data resulted from tracking errors.
The thermal power measured by the calorimeter and the insolation measured
bv the radiometers was plotted for each measurement period. To test the
validity of this technique, the calorimeter values were divided by
each of the radiometer values and the results were also plotted. These
ratios gave the net power output of the concentrator normalized to
1000 W/sq.M under sky conditions which varied from light haze to thin
cirrus clouds. No completely clear days occurred during the time these
tests were made. There is no reason to believe that the normalized
power values would differ from these values for completely clear skies.
During the passage of the cirrus clouds the normalized power values
showed a substantial variation over short periods of time as a result of
long time constant of the calorimeter relative to the time constants of
the radiometers.
Figures 4a and 5a show the radiometer data plots for two different days
and Figures 4b and 5b show the corresponding plots of the direct and
normalized power measurements. Figure 5b demonstrates that the normalized
power is relatively constant under a wide range of sky conditions. The
value of the normalized power in this figure was too high because of a
faulty flow meter. This problem was corrected and the normalized power
values shown in Figure 4b more accurately represent the performance of
the PDC-1 concentrator.
The insolation values measured with these modified radiometers were
lower than the values which would have been obtained with standard
radiometers. However, the purpose of these measurements was to determine
the relationship between the radiometers and the net power throughput
217
of the concentrator with a specific aperture. This calibration would
have been used to determine the operating efficiency of the power conversion
unit which was to have been used with this concentrator.
During this limited test program it was not possible to make a direct
comparison between these shrouded radiometers and standard radiometers.
However, this test program did demonstrate that the normalized power
output of the PDC-l concentrator was constant under a wide range of sky
conditions.
Recommendations Because this program to evaluate the effectiveness of
approach to insolation measurements was limited, it is recommended that
these experiments be repeated with other concentrators and radiometers.
These tests should be made with a wide range of sky conditions and
solar elevation angles using both a standard and a shrouded NIP. These
data would be very useful for determining the accuracy of existing NIP
records. Finally, it is recommended that future site surveys be made
with both standard and FOV limited NIP instruments to insure that
proposed solar power systems are suitable for the proposed sites. This
is most important when the proposed site has a substantial number of
days with strong haze or thin clouds.
Conclusions This technique of matching the FOV of an insolation
radiometer to the FOV of a specific concentrator and receiver aperture
appears to be both practical and effective. It would appear that the
efficiency of a power conversion unit will be too low if the insolation
is measured with a radiometer which has a FOV which is larger than the
FOV of the concentrator.
Author's Note An expanded version of this report with the AAF algorithms
will be published at a later date.
218
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Special Pyrheliometer Shroud Development

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