NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 100 of 144
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IMPROVED ACCURACY FOR LOW-COST SOLAR IRRADIANCE SENSORS
David L.King, WilliamE.Boyson, andBarryR Hansen
Sandia National Laboratories
Albuquerque, New Mexico, USA
ABSTRACT: Accurate measurements of broadband (full spectrum) solar irradiance are fundamental to the successful
implementation of solar power systems, both photovoltaic and solar thermal. istorically, acceptable measurement
accuracy has been achieved using expensive thermopile-based pyranometers and pyrheliometers. The measurement
limitations and sensitivities of these expensive radiometers are a topic that has been addressed elsewhere. This paper
demonstrates how to achieve acceptable accuracy ( 3%) in irradiance measurements using sensors costing less than
one-tenth that of typical thermopile devices. The low-cost devices use either silicoiphotodiodes or photovoltaic cells as
sensors, and in addition to low-cost, have several operational advantages.
Keywords: Pyranometer-1: PVModule-2: Reference Cell-3
achieved by applying these corrections.
Thousands of photovoltaic systems, large and small,
are now being installed worldwide. As a result, there is a
growing demand for inexpensive devices for accurately
monitoring the solar irradiance. Most often, the total
(global) solar irradiance is the desired measurement
Occasionally, the direct normal (beam) irradiance is
required. For most system applications, reasonable
accuracy (i5%) at low cost (-200 SUS) is usually
preferable to high accuracy ( 2%) at high cost (~2000
$US). As a result, silicon photodiode pyranometers
manufactured by companies such as LI-COR Incorporated
 and Kipp & Zonen  are now commonly used for
solar resource measurements and photovoltaic system
monitoring. One manufacturer alone (LI-COR) has sold
over 31,000 of their low-cost silicon photodiode-based
pyranometers. Commercial solar cells have also been
packaged in a variety of ways to serve as solar irradiance
sensors. Traditional photovoltaic reference cells  have
been used for many years in test laboratories, occasionally
for field applications. Irradiance sensors designed for
easy temperature compensation have been produced using
two solar cells and standard module lamination
procedures by the European Solar Test Installation (ESTI)
, and by module manufacturers such as AstroPower
Incorporated . Small commercial photovoltaic modules
have also frequently been used for measuring the solar
These photovoltaic-based devices have typically
provided a reasonable method for measuring the
integrated daily solar irradiance (radiation). However,
when used to measure the instantaneous broadband solar
irradiance, their accuracy has been less than desired.
Their inaccuracy has been due to errors introduced by
systematic, time-of-day dependent, variations in the solar
spectrum, solar angle-of-incidence, and operating
temperature. A method was described in our previous
work for obtaining empirical relationships that
compensate for these systematic errors [6, 7]. The
purpose of this paper is to demonstrate the improvement
Sandia is a multiprogram laboratory operated by Sandia
Corporation, aLockheedMartin Company, for the U. S.
Department ofEnergy under contractDE-ACO4-94AL85000.
The corrections result in measurement accuracy
comparable to more expensive instruments, for both
global and direct normal solar irradiance.
2. LOW-COST IRRADIANCE SENSORS
Fig. 1 illustrates a few of the low-cost sensors
evaluated in our work. The low-cost devices illustrated
include a LI-COR LI-200SA silicon photodiode
pyranometer, a LI-COR L-200SA fitted with a baffled
plastic (PVC) collimator, an ESTI Sensor using two
crystalline silicon cells, a two-cell mini-module fabricated
by AstroPower using their Silicon-FilmTM cell technology,
and a common silicon reference cell. For comparison, an
Eppley PSP pyranometer  is also shown in Fig. 1. For
photovoltaic-based devices, empirical "corrections" were
developed to compensate for the systematic influences
mentioned. Controlled tests were then conducted to
compare irradiance measurements, with and without the
corrections, to the measurements obtained using carefully
calibrated Eppley thermopile-based instruments.
3. SOLAR SPECTRAL INFLUENCE
Compensation for the influence of the time-of-day
dependent solar spectrum was achieved by using an
empirically determined function . This empirical
function, fi(AKd), related solar spectral variations to the
absolute air mass (AM,). "Air mass" is the term used to
describe the path length that sunlight traverses through
the atmosphere before reaching the ground. When
adjustment is made for the altitude of the site, it is called
the "absolute" air mass. AMa is readily calculated
knowing the zenith angle of the sun and the site altitude
. At sea level, AM=l with the sun directly overhead,
AMa=1.5 when the sun's zenith angle is 48 degrees, and
AM, of about 10 at sunrise and sunset As AM
increases, the spectrum of the sun shifts to longer
wavelengths, becoming more "red."
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/100/: accessed December 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.