A Novel Optical Sensor of Light Source Directions

A Novel Optical Sensor of Light Source Directions
Chia-Yen Lee 2 , Po-Cheng Chou 1 , Wen-Jen Hwang 2
1 Department of Interior Design, Shu-Te University, Kaohisung County, Taiwan
2 Department of Mechanical and Automation Engineering, Da-Yeh University, Chunghua, Taiwan
ABSTRACT
This paper presents a novel technique for the measurement systems of solar orientation based
on solar cells. A methodology for the calculation of the solar orientation is developed which uses
solar cells as solar sensors. The time and latitude angles of the sun are proposed to be a function of
the output voltages of sloped solar cells. The solar cells are located at different angles of elevation
and azimuth for the comparative output voltages between the back-to-back solar cells. A variation in
the time and latitude angles of the sun causes a change of the output voltages of the eastern-western
(E-W) and southern-northern (S-N) solar cells, which changes the relative measured voltages
between the solar cells, respectively. The current experimental data show that the optimized
assembly of solar cells and the detailed calibration of time and latitude angles of the sun yield a
high degree of sensitivity. The relationship between the measured relative voltages of solar cells is
fully explored and documented. The proposed study indicates not only simplified solar orientation
measurement systems but also convenient and accurate correlations of the comparative output solar
cell voltages and the sun angles. As such, the proposed measurement systems make a valuable
contribution to the development of tracking systems in solar energy technologies.
Keywords: Solar cell, Solar collector, Solar orientation
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1. Introduction
In the past decades, emerging solar energy systems have been popular in the renewable energy
technology. As a result of these systems, solar collectors has now been developed which is capable
of collecting incident solar radiation and converting it into electrical power, thermal energy …etc.
Importantly, the absorbed solar energy within the collectors can be increased through their
integration with solar tracking systems that compute the direction of the solar vector on location and
time [Blanco-Muriel et al., 2001]. Therefore, the effectiveness of the solar collectors can also be
increased if it is always aimed at the sun [Berenguel et al., 2004; Hj Mohd Yakup et al., 2001;
Algifri et al., 2001].
Solar orientation measurement is essential in such solar energy fields. Many previous studies
have addressed the application of solar collectors to the development of solar energy technologies
[Kowalski, 1997; Surman, 1996; Raasakka, 1997; Bari, 2000; Tesfamichael et al., 2000]. Automatic
regulation systems of solar collectors integrated with sun sensors were proposed to improve the
solar radiation absorption. A review of the related literature reveals many forms of sun sensors
[Berenguel et al., 2004; Popat Pradeep, 1998; Wen et al., 2002; Falbet et al., 2002], including
brightness sensors [Popat Pradeep, 1998], artificial vision techniques and CCD devices [Berenguel
et al., 2004]. Measured solar energy values could be used to compute the absorbed solar energy as
function of time of day [Wen et al., 2002]. Of particular interest is the two axis analog device,
which measures the sun’s location relative to its optical axis based on the relative signal obtained in
a quadrant silicon detector [Falbet et al., 2002]. In the four detector quadrants, relative radiant
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powers are applied to estimate the angle that the sun line makes with the sensor’s optical axis. This
device is attractive since it is capable of providing highly precise solar orientation measurement
despite its complicated structure. Therefore, the intention of this present study is to present a novel
solar orientation measurement system which incorporates four elevated solar cells fixed on a wedge,
and which includes a methodology to calculate the time and latitude angles of the sun.
Generic algorithms with climate data were used to find out the optimum installation angle of
the solar collector for different locations in Taiwan [Chen et al., 2001]. The best monthly and annual
installation angles were obtained by computer simulations. In spite sufficient installation
information for solar collectors were provided, the solar orientation measurement was not
instantaneous and connectable with a tracking system. The current study develops a methodology
for the solar orientation based on solar cells and geometry models of direct solar irradiation into
different sun sensor configurations, namely single cell, double cells and quadrantal cells. By
correlating the output voltage signals of two back-to-back solar cells (eastern and western / southern
and northern), the time angle and the latitude angle of the sun can be estimated, respectively. The
characteristics of the three types of sun sensors were investigated and the correlations were
calibrated in this study. Experimental results show high coincidence between the calibrated angles
and the sun angles.
2. Sensor Design and Methodology
In this study, solar cells are adopted as the sun sensors for different solar time and latitude
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angles. The sun sensors are connected with a personal computer to record and analyze data. In solar
cells, the photo-induced current, or called the generation current is proportional to the number of
photons that can be collected on the surface area of the solar cells. Since the objective of the solar
cells is to sense the change of solar incident ray and to convert it to the analyzer, the irradiation
absorbency differs at different incident angles by time in a day-time. As a result, the output voltage
of the solar cells varies at different solar time and latitude angle. In this study, the time angle θ is
defined as the angle between the incident ray and the horizontal plane and the latitude angle φ is the
angle between the incident ray and the eastern direction. Many studies presented the geometry of
solar irradiation into the solar devices [Blanco-Muriel et al., 2001; Hj Mohd Yakup et al., 2001;
Wen et al., 2002; Chen et al., 2001; Duffie et al., 1991; Lorenz, 1998]. The present study uses the
rectangular coordinate-system to model the geometrical relationships between the sun and the sun
sensors (Fig. 1). To simplify the problem, the plane which equipped with sun sensors is assumed to
be horizontal. The latitude plane is bounded by the latitude of the sun location as season changes.
As day-hours go, the time plane sweeps the hemisphere from the east to the west. The intersection
line of the two planes (the latitude and the time planes) is the trajectory of the incident ray from the
sun. The latitude and time angles of the instantaneous locations of the sun can be established by the
geometrical analysis.
In order to compare the performance of different designs of solar orientation measurement
systems based on solar cells, three types of measurement systems were developed in the study, i.e.
(a) single cell, (b) double cells and (c) quadrantal cells. Incandescent lamps (SC5848, HOMES,
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Taiwan) were used to simulate the sun in a dark room. The wavelength of the incandescent lamp is
400-780 nm, which resemble the wavelength of the solar light (400-800 nm). A solar cell was lain
on a horizontal plane and connected with a voltage meter to record the voltage changes as the
elevation angle of the light source changed in the single cell type of measurement system (Fig. 2).
In the double cell type of system, two sloped solar cells leaned against each other to measure the
comparative output voltages of the two cells at different solar orientation (Fig. 3). Two sets of
double cell type of sensors were integrated into a quadrantal types of measurement system (Fig. 4).
Not only the time plane can be defined by sensors E and W, but also the latitude plane can be found
by sensors S and N. The output voltages were measured by a voltage meter and operated in a
personal computer to define the time and latitude angles of the incident ray from the sun. The
detailed experimental results were described in section 3.
3. Experimental Results
A systematic investigation of the performance of the three types of solar orientation systems
was conducted. The characterization of the sun sensors was carried out in a dark room (L: W: H =
3.5 m: 3.5 m: 2.5 m) using a voltage meter (3136A, Escort, Taiwan), which was connected to the
solar cells in the dark room to record the signal response to changes of light source locations. The
distance between the light source and the sun sensors was kept constant at 140 cm because the
distance change between the sun and any area on earth can be ignored as compared with the actual
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distance. All the measured data was recorded and operated in a personal computer.
3.1 Single cell type of measurement system
The output voltage of the single solar cell increases with the increase of the elevation angle
from -45 o to 90 o (Fig. 2). The output voltage increases abruptly as the elevation angle of the light
source is more than -5 o , and it increases almost linearly when the elevation angle is more than 0 o
(Fig. 5). To find the relationship of the output voltage and the negative elevation angle, the output
voltage was measure from the elevation angle = 45 o . As the power of the light source increases, the
output voltage increases. As the light source is 250 W of incandescent lamp, the linearity of the
measured curve is better than the other two. In spite the output voltage is linear both at the ranges of
-45 o - -5 o and 0 o – 90 o at 250 W, the measured signal may decrease when it is cloudy or partly
cloudy. To compensate the deviation due to the environmental effects, a two cell type of system was
developed.
3.2 Double cell type of measurement system
For the optimization of the slope angle of the double cell type of measurement system, two
sloped solar cells leaned against each other to measure the comparative output voltages of the two
cells at different sloped angles δ (Fig. 3). The solar orientation measurement system was
characterized with different solar elevation angles θ in the range of 0 o to 90 o at a constant distance
from the light source of 140 cm. A voltage ratio R of the two solar cells was calculated to normalize
the measurement results. In Figure6, the results showed that the ratio drops as the elevation angle,
which is the time angle in the actual case, increases and the ratio approaches 1 when the time angle
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is more than the same value of the sloped angle δ. As the time angle is less than δ value, the fitted
curve equations for the voltage ratio R of different sloped angles are expressed by:
R = -0.0059θ 2 + 0.905 θ + 9.9888 for δ = 45 o (1)
R = -0.0099θ 2 + 0.055 θ + 8.9372 for δ = 30 o
R = -0.0223θ 2 – 0.0315 θ + 6.0736 for δ = 15 o
As the time angle is more than δ value, the fitted curve equations for the voltage ratio R of different
sloped angles are linear and can be expressed by:
R = -0.0053θ + 1.4587 for δ = 45 o (2)
R = -0.0027θ + 1.2385
for δ = 30 o
R = -0.0010θ + 1.0922 for δ = 15 o
where R is the voltage ratio of the two solar cells and θ is the time (elevation) angle ( o ). Eqs. (1) and
(2) are useful for the estimation of the solar time angle at low elevation angle (δ).
4. Conclusions
This study has successfully demonstrated a new solar orientation measurement system with
integrated solar cells. A new assembly method and a geometrical model have been developed for the
precise measurement of the time and latitude angles of the sun at day-time and four seasons. It has
been shown that the change of the output voltages of two back-to-back sun sensors caused by their
sloped installation angle gives rise to a measurable change in the voltage ratio between the two
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sensors. In addition to its precise solar orientation measurement and its simplified geometrical
model, the device also exhibits a high degree of integrable with other solar energy devices.
Acknowledgement
The authors would like to thank the financial support provided by the National Science Council in
Taiwan (NSC 93-2218-E-212-011).
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latitude plane
W
incident
ray
A
N
O
B
time plane
E
S
Figure 1 Geometry of the irradiation of direct sunlight to sun sensors.
N
sun sensor
E
N : normal Vector
personal
computer
singnal lines
voltage meter
RS232 Cable
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Figure 2 A schematic representation of single cell type of measurement systems.
2 1
N 2 N 1
sun sensor 2
sun sensor 1
E
personal
computer
N : normal Vector
singnal lines
voltage meter
RS232 Cable
Figure 3 A schematic representation of double cell type of measurement systems.
N
personal
computer
E
voltage meter
sensor N
sensor W
sensor E
sensor S
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Figure 4 A schematic representation of quadrantal type of measurement systems.
250 W
◊ 100 W
∆ 40 W
Figure 5 Output voltages at different elevation angles for single cell type of measurement
systems.
48

voltage ratio
11
9
7
5
3
1
45 o
30 o
15 o
0 10 20 30 40 50 60 70 80 90
time angle (degree)
voltage ratio
11
9
7
5
3
(a) 0 o – 90º
45 o
30 o
15 o
1
0 5 10 15 20 25 30 35 40 45
time angle (degree)
voltage ratio
1.25
1.2
1.15
1.1
1.05
(b) 0 o – 45º
45 o
30 o
15 o
1
45 50 55 60 65 70 75 80 85 90
time angle (degree)
(c) 45º-90º.
Figure 6 Output voltages at different time angles for double cell type of measurement systems. (a)
0 o – 90º, (b) 0 o – 45º and (c) 45º-90º.
49