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An LED solar simulator for student labs
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2017 Phys. Educ. 52 035002
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Paper
Phys. Educ. 52 (2017) 035002 (5pp)
iopscience.org/ped
An LED solar simulator for
student labs
Manuel I González
Departamento de Física, Universidad de Burgos, Avda. Cantabria E-90006, Burgos, Spain
E-mail: miglez@ubu.es
Abstract
Measuring voltage–current and voltage–power curves of a photovoltaic
module is a nice experiment for high school and undergraduate students.
In labs where real sunlight is not available this experiment requires a
solar simulator. A prototype of a simulator using LED lamps has been
manufactured and tested, and a comparison with classical halogen simulators
has been performed. It is found that LED light offers lower levels of
irradiance, but much better performance in terms of module output for a given
irradiance.
1. Introduction
requisites concerning irradiance, spectral matching to natural sunlight, spatial uniformity and
temporal stability. The requisites are detailed in
documents such as [3, 4].
For a student lab, of course, the requisites
are much looser, and a common practice is to use
incandescent halogen lamps as the source [5, 6].
Tungsten-based incandescent lamps are widespread and cheap, but lack spectral quality. The
color temperature of sunlight is about 6000 K,
whereas the color temperature of a tungsten filament does not usually exceed 2800 K [7]; otherwise the filament lifetime is drastically shortened.
According to Wien’s law the emission peak at
2800 K lies at about 1030 nm, i.e. at the near
infrared zone of the electromagnetic spectrum.
By Planck’s law, this implies that most photons emitted by the filament simply do not have
enough energy to create electron-hole pairs in an
ordinary silicon PV module. Therefore one may
expect a very low conversion efficiency of the
module when lit with an incandescent source, as
will be seen below.
The recent spread of light emitting diode
(LED) systems offers a new approach to solar
simulation. LED light is not governed by the
Energy sources and energy conversion have
always been a major concern for physics teachers at all levels. Recent development of renewable
sources makes this interest even greater especially (but not only) for laboratory instructors.
Nowadays, catalogues of laboratory equipment
dealers [1, 2] include a variety of experiments
about solar thermal, solar photovoltaic, wind
energy and other renewable sources.
As for photovoltaic (PV) energy, a simple
but enlightening experiment consists of obtaining the characteristic curves of a PV module. This
experiment, which will be explained in detail in
section 4, explores the conversion of solar radiation into electricity and involves concepts such as
irradiance, power output, conversion efficiency
and others.
To carry out this experiment in labs where
the supply of sunlight is not ensured, a solar simulator is required. A solar simulator is an artificial
light source whose physical characteristics are
similar to those of sunlight. In fact, solar simulators are routinely used to perform standard tests
on PV modules in certification facilities. These
‘serious’ simulators must fulfil a number of
1361-6552/17/035002+5$33.00
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© 2017 IOP Publishing Ltd
M I González
laws of thermal emission and LED lamps can be
found in various equivalent color temperatures
ranging from 2500 K (warm white) to 6000 K
(daylight). This work deals with the manufacture
of a prototype for a solar simulator using commercially available LED lamps. Specifically, the
relationship between irradiance and distance to
the simulator will be measured and an estimate of
spatial uniformity in irradiance will be obtained.
The characteristic curves of a student PV module
under three sources (sunlight, halogen and LED)
will be measured and discussed. Finally the pros
and cons of the prototype will be examined.
2. The prototype
Figure 1. Prototype of LED solar simulator.
An LED solar simulator suitable for student labs
should satisfy the following requirements:
The arrangement occupied an approximate
area of 37 × 30 cm2, which was considered sufficient to light a PV module having 24 × 18 cm2.
The lamps were all connected in parallel by means
of standard wires located in the rear of the panel.
Figure 1 shows a front view of the prototype.
Figure 2 shows the light beams of a conventional halogen simulator and the proposed unit.
The photo is strongly underexposed in order to
reveal the different color temperatures, although
the difference is even more remarkable under
direct vision.
• Color temperature of the lamps should be
substantially higher than that of halogen
lamps. Otherwise the spectral matching to
sunlight will be poor.
• The number of lamps and the directional
characteristics of their light beams should
provide an irradiance close to 1000 W m−2,
which is typical of natural sunlight under
good conditions [8]. This is also the irradiance under which standard tests are carried
out on PV modules.
• The irradiance on the plane of the PV module
should have an acceptable degree of spatial
uniformity.
• The whole simulator should not be prohibitively expensive.
3. Irradiance and spatial uniformity
The first comparison performed on both simulators referred to the irradiance on the PV module
and its spatial uniformity. The test module was
centered in front of each simulator, and oriented
to normal incidence. The distance between each
simulator and the PV module was then varied. For
each distance the irradiance was measured at nine
points of the module: the center, the four corners
and the midpoint of each edge. Irradiances were
measured with a student lab pyranometer. Average
irradiances for each distance were computed as
weighted averages, instead of simple ones. The
assigned weights to the aforementioned locations
were 1, 0.25 and 0.5, respectively. Figure 3 shows
the results. A simple estimate of spatial uniformity was computed as the standard deviation of
the nine measurements, and displayed in figure 3
as error bars.
Inspection of figure 3 shows that for a given
distance, the halogen simulator provides higher
According to these criteria the selected lamps
had 5 W power consumption, 220 V (the European
AC Standard), 345 lm luminous flux (similar to the
luminous output of a 40 W incandescent lamp),
4000 K color temperature, GU10 bulb socket and
a built-in reflector cup providing 36° beam aperture. The unit cost was approximately €5.
42 lamps were mounted on a wooden panel,
in a hexagonal arrangement with 5.5 cm distance
between adjacent lamps. The packing density
was very high, since the diameter of the reflecting cups was only slightly lower, namely 5 cm. In
this way, the density of electrical power consumption was about 1900 W m−2 although, as will be
shown below, the irradiance on the PV module
was much lower.
May 2017
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Phys. Educ. 52 (2017) 035002
Average irradiance / W m–2
An LED solar simulator for student labs
Figure 2. ‘Classic’ halogen (left) and LED (right)
simulators.
Ihal = 256 000·d –1.581
1000
500
0
ILED = 588·e–0.019·d
0
40
Distance/cm
80
Figure 3. Irradiance versus distance. Upper data set:
halogen simulator; lower data set: LED simulator.
Dashed lines: least squares fits.
irradiance. This is simply due to its higher power
consumption (500 W against 210 W). The degree
of spatial uniformity was reasonably good for the
LED simulator at all distances and for the halogen
simulator at large distances. At relatively shorter
distances (40 cm or less) the halogen simulator
exhibits poor uniformity as a logical consequence
of the quasi point-like character of its filament.
The trend lines in figure 3 do not have any
particular physical meaning. In fact, the best fits
have been achieved with different types of functions. They must be interpreted as simple tools for
irradiance versus distance calibration.
setup (except for the pyranometer) during the test
of the LED simulator.
In order to compare the performances of a
given PV module under different light sources
three trials were conducted. The first trial was carried out under sunlight at 1010 W m−2, which is
very close to the irradiance level in standard tests.
The second and third trials used our LED and halogen simulators, respectively, at 500 W m−2, which
is accessible to both simulators. The calibration
curves shown in figure 3 were used to select the
distance between module and simulators.
Figure 5 depicts the results of these trials.
To compare the experiment performed at 1010
W m−2 to those performed at 500 W m−2, the
latter were corrected by using the well-known
property [9] that the output current of a PV module is approximately proportional to the irradiance, whereas the output voltage does not depend
significantly on irradiance. Therefore the main
comparison should involve the continuous lines
in figure 5. Once the corrections are made short
circuit currents were nearly equal for sunlight and
LED (325 and 315 mA, respectively) and much
lower for halogen light (170 mA), whereas open
circuit voltages were similar: 18.8 V (sun), 18.2 V
(LED) and 17.9 V (halogen).
A similar conclusion can be drawn if one
computes the power output by multiplying volt­
age and current and plots the P–V (power output
versus voltage) diagram, as depicted in figure 6.
Under sunlight the maximum power was 4.2 W,
4. Characteristic curves and discussion
The classical test of a PV module consists of measuring its I–V (current versus voltage) curve. In a
student lab one can ignore the subtleties required
in a certification facility, and the experiment can
be carried out with very little equipment: the module itself, a rheostat, an ammeter and a voltmeter.
A pyranometer is also required if one wants an
accurate measurement of irradiance. The electrical circuit is shown in the upper right inset of figure 4. Different pairs (I, V ) can be obtained by
varying the load on the rheostat, and the I–V curve
is plotted by joining them. Intersections of the
I–V curve with both axes define the open circuit
voltage Voc and the short circuit current Isc (shown
in the upper central inset of figure 4). These are
readily measured by connecting the voltmeter and
the ammeter, respectively, to the output ports of
the PV module. Figure 4 shows the experimental
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Phys. Educ. 52 (2017) 035002
M I González
Figure 4. Experimental setup for measuring the characteristic curves of a PV module. The upper right inset shows
the electric connections, whereas the upper central inset depicts the typical aspect of an I–V curve, showing the
open circuit voltage and short circuit current.
close to the 4.5 W rated power of the module.
After correction, LED light provided a similar
maximum output, 3.9 W, whereas halogen light
delivered at most 2.0 W.
For a PV module it is customary to define
the conversion efficiency as the quotient between
the maximum electrical power output and the
radiant power on the collection plane. According
to this definition the efficiencies at 1000 W m−2
were 10.4% (rated), 9.6% (sunlight test), 9.0%
(LED simulator test) and 4.6% (halogen simulator test).
In any case, figures 5 and 6 and the efficiencies tell us the same: for a given irradiance level
PV module output under LED light is similar to
that under sunlight and twice as much as under
halogen light. The poor performance of halogen
light is clearly due to its richness in high wavelength, low energy photons.
Sun, 1010 W m–2
300
Current/mA
LED, corrected to 1000 W m–2
Halogen, corrected to 1000 W m–2
LED, 500 W m–2
100
Halogen, 500 W m–2
0
0
10
Voltage/ V
20
Figure 5. I–V curve for a PV module under different
light sources. Black line: sunlight; blue: LED simulator;
red: halogen simulator. Dashed lines: tests performed
at 500 W m−2; continuous lines: tests performed or
corrected to 1000 W m−2. The minute correction of
sunlight curve from 1010 to 1000 W m−2 has been
omitted.
lamps, mounts and cabling amounted to some
€300 ($330 at the time of writing). Moreover
the manufacture required a few hours of
do-it-yourself.
• Low power consumption of common LED
lamps makes it a challenge to reach the
standard 1000 W m−2 irradiance. Our prototype required a very dense lamp packing to
deliver 500 W m−2.
• Similar levels of spatial uniformity can be
expected from LED and halogen simulators.
5. Conclusions
Data presented in section 4 have been obtained by
lighting a particular PV module with a particular
LED simulator and a particular halogen simulator, but a number of useful and more general conclusions can be drawn:
• A halogen simulator is expected to be
cheaper than an LED one. The cost of a high
power halogen lamp and its mount is about
€50, whereas our LED prototype, including
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Phys. Educ. 52 (2017) 035002
An LED solar simulator for student labs
Power output/ mW
4000
–2
,
un
S
2000
10
10
W
c
rre
–2
m
0W
m
00
o1
t
ted
[3] IEC 60904-9:2007 document, International
Electrotechnical Commission Webstore
(https://webstore.iec.ch/publication/3880)
(Accessed: 13 March 2017)
[4] ASTM E927—10(2015) document, Standard
Specification for Solar Simulation for
Photovoltaic Testing (www.astm.org/Standards/
E927.htm) (Accessed: 13 March 2017)
[5] FREDERIKSEN—manufacturer of Physics
Equipment for teaching (http://int.frederiksen.
eu/shop/product/halogen-lamp--350-400-w)
(Accessed: 13 March 2017)
[6] PHYWE SYSTEME web page (www.phywe.
com/en/catalog/product/view/id/11064/s/
filament-lamp-220v-120w-with-reflector/
category/16696/) (Accessed: 13 March 2017)
[7] Pritchard D C 1998 Lighting 5th edn (Longman:
Singapore)
[8] Duffie J A and Beckman W A 2006 Solar
Engineering of Thermal Processes 3rd edn
(Hoboken, NJ: Wiley)
[9] Messenger R and Ventre J 2000 Photovoltaic
Systems Engineering (Boca Raton, FL: CRC
Press)
W
000
–2
m
co
to 1
D, ected
–2
m
LE , corr
0W
en
0
g
5
o
,
Hal
LED
m–2
n, 500 W
Haloge
0
0
10
Voltage / V
20
Figure 6. P–V curve for a PV module under different
light sources.
• As for spectral content, LED lamps with
high color temperature are far superior to
halogen lamps. Under an LED at 4000 K, the
response of a PV module in terms of short
circuit current, maximum power output and
efficiency is close to that under sunlight.
Received 16 December 2016, in final form 24 January 2017
Accepted for publication 9 February 2017
https://doi.org/10.1088/1361-6552/aa5f86
Manuel I González teaches physics
and renewable energy to engineering
students at the University of Burgos.
His main research areas are solar
cooling and new technologies applied
to teaching physics.
References
[1] PHYWE webpage (www.phywe.com/en/physics/
age-16-19/energy/) (Accessed: 13 March 2017)
[2] PASCO SCIENTIFIC webpage (www.pasco.com/
filters/topics/index.cfm?topics=02-065-000)
(Accessed: 13 March 2017)
May 2017
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Phys. Educ. 52 (2017) 035002