FIELD OF THE INVENTION
The present invention relates to performance monitoring of solar
photovoltaic systems installed at different geographic locations. More
particularly, the invention relates to a portable solarimeter device for
direct measurement of incident solar radiation intensity.
BACKGROUND OF THE INVENTION
For measurement of incident solar radiation intensity, the commonly
used instruments are Pyrheliometers and Pyranometers.
Pyrheliometers follow the sun as it traverses across the sky and
measure direct radiation only. Pyranometers measure global (direct
and diffused) radiation incident on a surface. These instruments work
on the thermoelectric effect, and in which, the sensing element
constitutes a thermopile consisting of a number of thermo-junctions.
The voltage output of the thermopile is measured to obtain the
radiation level. The sensitivity obtained is rather low, about 10mV per
W/m2, and requires an accurate instrument to measure the voltage.
Also the sensor used with these measuring devices is very expensive.
It is known that crystalline silicon solar cells can be used as a source
of DC electricity generation. Crystalline silicon solar cells can also be

used as a sensor in solar cell modules for measurement of incidental
solar radiation intensity. A solar array can be formed with the solar
modules which is used in a solar power plant. The output of the
crystalline silicon solar sensor and the modules are compared to
check the performance of the modules.
Some known solarimeter use photo sensor as the sensor for the
measurement of solar irradiance and are installed for the tracking of
solar arrays having Solar cell module and Fresnel Concentrator lens.
A handy and high cost solarimeter with photo sensor is known in the
art which measures the solar irradiance in watt/m2.
Solar photovoltaic systems such as streetlights, installed at various
locations, are required to be monitored regularly for their
performance. The measurement of incident solar radiation intensity
at the place of installation is an important task. This needs to be
done every time the electrical performance of the system is
monitored. This requires an insolation measurement tool having the
following attributes:
1. identical spectral response as the SPV modules under study.
2. light weight & portable
3. inexpensive, reliable and rugged sensor such as solar cell

4. easy to operate e.g. with a push button
5. calibrated and easy to read display directly measuring the
intensity
6. inexpensive (the complete tool)
Such an insolation measurement tool is not available in the art.
Therefore, a need exists to propose a low cost, portable and easy to
use solarimeter device which enables direct measurement of
insolation with the help of a solar cell, acting as a sensor. Such an
innovative and novel structure shall enable monitoring the electrical
performance of SPV systems in relation with the insolation level.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a solarimeter
device for on-site measurement of insolation, which is low-cost and
easy to operate.
Another object of the invention is to propose a solarimeter device for
on-site measurement of insolation, which enables to carry out
performance monitoring of SPV systems such as street lights or
stand-alone solar power plants.

A further object of the invention is to propose a solarimeter device
for on-site measurement of insolation, which adapts a sensor with
spectral response identical to that of the solar photovoltaic modules
generating the solar energy.
A still further object of the invention is to propose a solarimeter
device for on-site measurement of insolation, which allows direct
measurement of the insolation.
SUMMARY OF THE INVENTION
Accordingly, there is provided a portable solarimeter device which is
low-cost and easy to use. It essentially consists of a square piece of
crystalline silicon solar cell cut from a larger solar cell and laminated
with ethylene vinyl acetate (EVA) between two thin glass plates. A
current from the solar cell is allowed to pass through a standard
shunt resistance connected between the cell terminals and the
voltage developed is measured by a digital voltmeter having an LCD
display. The reading on the display directly exhibits the solar intensity
in mW/sq. cm. The solarimeter is configured in such a way that
when placed on a horizontal plain, the solar cell always faces the sun
at the latitude angle, while the display faces the user. The reading on

the display is exhibited when a push button placed next to the
display is kept pressed. The various components of the device are so
selected that the display directly indicates the solar intensity in
mW/sq. cm.
Therefore, the solarimeter device formed of crystalline silicon solar
cell as a sensor according to the invention is able to measure
incidental solar radiation directly and instantaneously. The
solarimeter device is portable, handy, and constitutes a low cost
measurement tool compared to the existing devices. According to the
invention, a crystalline silicon solar cell is employed as a sensor for
measurement of incidental solar radiation intensity, and, has the
advantage that it generates area-dependent electric current, which is
directly proportional to the incident solar radiation. Also, the spectral
response of the sensor (solar cell) is similar to that of the solar
modules, which are to be regularly monitored for their performance.
The device is housed in a triangular block made out of nylon sheets
that allows the solar cell to face the sun, the device being placed on
its horizontal base. The solar cell used as a sensor is so selected that
it produces approximately 1000 mA of short-circuit current at 1.0 Sun
intensity. The current generated by the solar cell passes through a

precision shunt resistance of 100 m Ohm (+/- 5%) and the voltage
generated is measured with a digital voltmeter having an LCD
display. The current generated and therefore the voltage produced
across the shunt is directly proportional to the incident solar radiation
intensity. The measurement of mV across the shunt resistance is the
direct measure of incidental radiation intensity in mW per sq. cm.
The invention is used for the measurement of incident solar radiation
intensity and in turn, the data is useful for evaluating the
performance of solar photovoltaic systems installed at different
locations.
BRIEF DESCPRIPTION OF THE ACCOMPANYING DRAWINGS :
FIGURE 1 - Shows the constituent components of a solarimeter
device in disassembled condition according to the invention.
FIGURE 2 - Graphically shows I-V results of a laminated solar cell
according to the invention.
FIGURE 3 - Shows an isometric view of the solarimeter device of the
invention.

DETAILED DESCRIPTION OF INVENTION
As shown in figure 1, a solar cell sensor (1) used for the
measurement of incidental solar radiation is a crystalline silicon solar
cell. The selection of a crystalline silicon solar cell as a sensor (1) is
made on the basis that
• It is a stable device
• The electric current generation from the solar cell is directly
proportional to the incident solar radiation and
• The spectral response of the device is similar to that of the
modules installed on site.
The crystalline silicon solar cell is cut from a larger cell and the cell
area is so selected that after lamination between 2 thin (~1 mm)
glass sheets, it has a short-circuit current of about 1A or 1000 mA at
one sun. Prior to lamination, the I-V characteristics of the solar cell
(1) is measured and the cell is exposed to outdoor sun for a period of
three months. This stabilizes the performance of the solar cell (1). An
l-V curve of the laminated solar cell (1) under indoor light intensity of
one sun is shown in Figure 2.

Precision shunt resistance: (2)
For determination of insolation, the current generated by the solar
cell (1) in short-circuit condition is allowed to flow through a precision
shunt resistance (2). The precision shunt resistance (2) used in the
inventive device is a wire-wound type of shunt, the wire material
being nickel-chromium. The value of the shunt (2) is accurately
determined with a Quad Tech LR 2000 m Ohmmeter and is designed
to have a value of 100 m Ohm (+/-5%).
Digital Voltmeter (3)
A digital voltmeter (3) with LCD display is used to measure and
display the voltage produced. The voltmeter (3) is based on a single
micro controller based single chip having Programmable logic on the
Chip. This configuration essentially avoids noise pickup from the
surroundings, minimizes voltage drop arising out of connecting wires
and also reduces number of interconnections due to the absence of
any amplifier, ADC or differential amplifier.

Integration of components:
The above components (1, 2, 3) are housed in a triangular piece of
nylon block with a horizontal base. These are placed in such a way
that the solar cell faces the sun almost at the latitude angle i.e. 28
degrees and the LCD panel of the voltmeter (3) faces the user. This
arrangement allows easy visibility of the LCD display to the user. The
shunt (2) is connected with the voltmeter (3) through a high quality,
low resistance push-button switch (4). The fully integrated
solarimeter, which is handheld during use, is a compact tool that is
easy to handle. Isometric view of the solarimeter is shown in Figure
3.
Method for determination of solar intensity :
For measurement of insolation, the solarimeter is to be placed with
its base almost horizontal. Under this condition, the solar cell
automatically faces the sun almost at latitude angle and a reading
corresponding to the insolation level appears on the LCD screen at
the press of a push button switch(4). Due to design considerations
(100 mV generated across the shunt resistance at the intensity level

of 100 mW/sq. cm when the solar cell produces 1A short-circuit
current), this reading directly indicates the insolation level in mW/sq.
cm. However, the solarimeter is rotated along both the vertical and
horizontal axes so that the reading on the panel is maximized. This
then becomes the true measurement of the insolation level.
Calibration :
That the display on the panel is a true indication of the insolation is
borne out by a calibration curve plotted with the help of a reference
solar cell calibrated and certified by the National Renewable Energy
Laboratory (NREL), USA.

WE CLAIM :
1. A portable solarimeter device for direct measurement of
incident solar radiation intensity, comprising :
- a shaped piece of crystalline silicon solar cell (1) disposed
between two thin glass sheets laminated with ethylene vinyl
acetate;
- a shunt resistance (2) connected between the cell terminals,
with the short circuit current generated by the solar cell (1)
allowed to pass through the shunt resistance (2);
- a digital voltmeter (3) with an integrated LCD display and on
activation of a push-switch (4) enabled to read the generated
voltage across the shunt resistance (2), and exhibit the
insolation value directly on the LCD display,
- designed in a manner that the crystalline silicon solar cell is
configured to produce after lamination a short-circuit current of
1 ampere at an intensity level of 1-sun which is directly
proportional to the insolation, and in that the shut-resistance is

a wire wound resistance with wire material of nickel-chromium
having a predetermined value of 100m Ohm.
2. The device as claimed in claim 1, wherein the voltmeter (3)
comprises a single micro controller based single chip
incorporated with a logic rule.
3. The device as claimed in claim 1, wherein a triangular
shaped nylon block with a horizontal base accommodates
the constituent components such as the solar cell (1) and
the shunt resistance (2).
4. The device as claimed in any of the preceding claims,
wherein the crystalline silicon solar cell (1) is square shaped.
5. A portable solarimeter device for direct measurement of
incident solar radiation intensity as substantially described
and illustrated herein with reference to the accompanying
drawings.

The invention relates to a portable solarimeter device for direct
measurement of incident solar radiation intensity, comprising a
shaped piece of crystalline silicon solar cell (1) disposed between two
thin glass sheets laminated with ethylene vinyl acetate; a shunt
resistance (2) connected between the cell terminals, the short circuit
current generated by the solar cell (1) allowed to pass through the
shunt resistance (2); a digital voltmeter (3) with an integrated LCD
display, and on activation of a push-switch (4) enabled to read the
generated voltage across the shunt resistance (2), and exhibit the
insolation value directly on the LCD display, designed in a manner
that the crystalline silicon solar cell is configured to produce after
lamination a short-circuit current of 1 ampere at an intensity level of
1-sun which is directly proportional to the insolation, and in that the
shut-resistance is a wire wound resistance with wire material of
nickel-chromium having a predetermined value of 100m Ohm.