A Novel Optical Sensor of Light Source Directions
A Novel Optical Sensor of Light Source Directions
A Novel Optical Sensor of Light Source Directions
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A <strong>Novel</strong> <strong>Optical</strong> <strong>Sensor</strong> <strong>of</strong> <strong>Light</strong> <strong>Source</strong> <strong>Directions</strong><br />
Chia-Yen Lee 2 , Po-Cheng Chou 1 , Wen-Jen Hwang 2<br />
1 Department <strong>of</strong> Interior Design, Shu-Te University, Kaohisung County, Taiwan<br />
2 Department <strong>of</strong> Mechanical and Automation Engineering, Da-Yeh University, Chunghua, Taiwan<br />
ABSTRACT<br />
This paper presents a novel technique for the measurement systems <strong>of</strong> solar orientation based<br />
on solar cells. A methodology for the calculation <strong>of</strong> the solar orientation is developed which uses<br />
solar cells as solar sensors. The time and latitude angles <strong>of</strong> the sun are proposed to be a function <strong>of</strong><br />
the output voltages <strong>of</strong> sloped solar cells. The solar cells are located at different angles <strong>of</strong> elevation<br />
and azimuth for the comparative output voltages between the back-to-back solar cells. A variation in<br />
the time and latitude angles <strong>of</strong> the sun causes a change <strong>of</strong> the output voltages <strong>of</strong> the eastern-western<br />
(E-W) and southern-northern (S-N) solar cells, which changes the relative measured voltages<br />
between the solar cells, respectively. The current experimental data show that the optimized<br />
assembly <strong>of</strong> solar cells and the detailed calibration <strong>of</strong> time and latitude angles <strong>of</strong> the sun yield a<br />
high degree <strong>of</strong> sensitivity. The relationship between the measured relative voltages <strong>of</strong> solar cells is<br />
fully explored and documented. The proposed study indicates not only simplified solar orientation<br />
measurement systems but also convenient and accurate correlations <strong>of</strong> the comparative output solar<br />
cell voltages and the sun angles. As such, the proposed measurement systems make a valuable<br />
contribution to the development <strong>of</strong> tracking systems in solar energy technologies.<br />
Keywords: Solar cell, Solar collector, Solar orientation<br />
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1. Introduction<br />
In the past decades, emerging solar energy systems have been popular in the renewable energy<br />
technology. As a result <strong>of</strong> these systems, solar collectors has now been developed which is capable<br />
<strong>of</strong> collecting incident solar radiation and converting it into electrical power, thermal energy …etc.<br />
Importantly, the absorbed solar energy within the collectors can be increased through their<br />
integration with solar tracking systems that compute the direction <strong>of</strong> the solar vector on location and<br />
time [Blanco-Muriel et al., 2001]. Therefore, the effectiveness <strong>of</strong> the solar collectors can also be<br />
increased if it is always aimed at the sun [Berenguel et al., 2004; Hj Mohd Yakup et al., 2001;<br />
Algifri et al., 2001].<br />
Solar orientation measurement is essential in such solar energy fields. Many previous studies<br />
have addressed the application <strong>of</strong> solar collectors to the development <strong>of</strong> solar energy technologies<br />
[Kowalski, 1997; Surman, 1996; Raasakka, 1997; Bari, 2000; Tesfamichael et al., 2000]. Automatic<br />
regulation systems <strong>of</strong> solar collectors integrated with sun sensors were proposed to improve the<br />
solar radiation absorption. A review <strong>of</strong> the related literature reveals many forms <strong>of</strong> sun sensors<br />
[Berenguel et al., 2004; Popat Pradeep, 1998; Wen et al., 2002; Falbet et al., 2002], including<br />
brightness sensors [Popat Pradeep, 1998], artificial vision techniques and CCD devices [Berenguel<br />
et al., 2004]. Measured solar energy values could be used to compute the absorbed solar energy as<br />
function <strong>of</strong> time <strong>of</strong> day [Wen et al., 2002]. Of particular interest is the two axis analog device,<br />
which measures the sun’s location relative to its optical axis based on the relative signal obtained in<br />
a quadrant silicon detector [Falbet et al., 2002]. In the four detector quadrants, relative radiant<br />
38
powers are applied to estimate the angle that the sun line makes with the sensor’s optical axis. This<br />
device is attractive since it is capable <strong>of</strong> providing highly precise solar orientation measurement<br />
despite its complicated structure. Therefore, the intention <strong>of</strong> this present study is to present a novel<br />
solar orientation measurement system which incorporates four elevated solar cells fixed on a wedge,<br />
and which includes a methodology to calculate the time and latitude angles <strong>of</strong> the sun.<br />
Generic algorithms with climate data were used to find out the optimum installation angle <strong>of</strong><br />
the solar collector for different locations in Taiwan [Chen et al., 2001]. The best monthly and annual<br />
installation angles were obtained by computer simulations. In spite sufficient installation<br />
information for solar collectors were provided, the solar orientation measurement was not<br />
instantaneous and connectable with a tracking system. The current study develops a methodology<br />
for the solar orientation based on solar cells and geometry models <strong>of</strong> direct solar irradiation into<br />
different sun sensor configurations, namely single cell, double cells and quadrantal cells. By<br />
correlating the output voltage signals <strong>of</strong> two back-to-back solar cells (eastern and western / southern<br />
and northern), the time angle and the latitude angle <strong>of</strong> the sun can be estimated, respectively. The<br />
characteristics <strong>of</strong> the three types <strong>of</strong> sun sensors were investigated and the correlations were<br />
calibrated in this study. Experimental results show high coincidence between the calibrated angles<br />
and the sun angles.<br />
2. <strong>Sensor</strong> Design and Methodology<br />
In this study, solar cells are adopted as the sun sensors for different solar time and latitude<br />
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angles. The sun sensors are connected with a personal computer to record and analyze data. In solar<br />
cells, the photo-induced current, or called the generation current is proportional to the number <strong>of</strong><br />
photons that can be collected on the surface area <strong>of</strong> the solar cells. Since the objective <strong>of</strong> the solar<br />
cells is to sense the change <strong>of</strong> solar incident ray and to convert it to the analyzer, the irradiation<br />
absorbency differs at different incident angles by time in a day-time. As a result, the output voltage<br />
<strong>of</strong> the solar cells varies at different solar time and latitude angle. In this study, the time angle θ is<br />
defined as the angle between the incident ray and the horizontal plane and the latitude angle φ is the<br />
angle between the incident ray and the eastern direction. Many studies presented the geometry <strong>of</strong><br />
solar irradiation into the solar devices [Blanco-Muriel et al., 2001; Hj Mohd Yakup et al., 2001;<br />
Wen et al., 2002; Chen et al., 2001; Duffie et al., 1991; Lorenz, 1998]. The present study uses the<br />
rectangular coordinate-system to model the geometrical relationships between the sun and the sun<br />
sensors (Fig. 1). To simplify the problem, the plane which equipped with sun sensors is assumed to<br />
be horizontal. The latitude plane is bounded by the latitude <strong>of</strong> the sun location as season changes.<br />
As day-hours go, the time plane sweeps the hemisphere from the east to the west. The intersection<br />
line <strong>of</strong> the two planes (the latitude and the time planes) is the trajectory <strong>of</strong> the incident ray from the<br />
sun. The latitude and time angles <strong>of</strong> the instantaneous locations <strong>of</strong> the sun can be established by the<br />
geometrical analysis.<br />
In order to compare the performance <strong>of</strong> different designs <strong>of</strong> solar orientation measurement<br />
systems based on solar cells, three types <strong>of</strong> measurement systems were developed in the study, i.e.<br />
(a) single cell, (b) double cells and (c) quadrantal cells. Incandescent lamps (SC5848, HOMES,<br />
40
Taiwan) were used to simulate the sun in a dark room. The wavelength <strong>of</strong> the incandescent lamp is<br />
400-780 nm, which resemble the wavelength <strong>of</strong> the solar light (400-800 nm). A solar cell was lain<br />
on a horizontal plane and connected with a voltage meter to record the voltage changes as the<br />
elevation angle <strong>of</strong> the light source changed in the single cell type <strong>of</strong> measurement system (Fig. 2).<br />
In the double cell type <strong>of</strong> system, two sloped solar cells leaned against each other to measure the<br />
comparative output voltages <strong>of</strong> the two cells at different solar orientation (Fig. 3). Two sets <strong>of</strong><br />
double cell type <strong>of</strong> sensors were integrated into a quadrantal types <strong>of</strong> measurement system (Fig. 4).<br />
Not only the time plane can be defined by sensors E and W, but also the latitude plane can be found<br />
by sensors S and N. The output voltages were measured by a voltage meter and operated in a<br />
personal computer to define the time and latitude angles <strong>of</strong> the incident ray from the sun. The<br />
detailed experimental results were described in section 3.<br />
3. Experimental Results<br />
A systematic investigation <strong>of</strong> the performance <strong>of</strong> the three types <strong>of</strong> solar orientation systems<br />
was conducted. The characterization <strong>of</strong> the sun sensors was carried out in a dark room (L: W: H =<br />
3.5 m: 3.5 m: 2.5 m) using a voltage meter (3136A, Escort, Taiwan), which was connected to the<br />
solar cells in the dark room to record the signal response to changes <strong>of</strong> light source locations. The<br />
distance between the light source and the sun sensors was kept constant at 140 cm because the<br />
distance change between the sun and any area on earth can be ignored as compared with the actual<br />
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distance. All the measured data was recorded and operated in a personal computer.<br />
3.1 Single cell type <strong>of</strong> measurement system<br />
The output voltage <strong>of</strong> the single solar cell increases with the increase <strong>of</strong> the elevation angle<br />
from -45 o to 90 o (Fig. 2). The output voltage increases abruptly as the elevation angle <strong>of</strong> the light<br />
source is more than -5 o , and it increases almost linearly when the elevation angle is more than 0 o<br />
(Fig. 5). To find the relationship <strong>of</strong> the output voltage and the negative elevation angle, the output<br />
voltage was measure from the elevation angle = 45 o . As the power <strong>of</strong> the light source increases, the<br />
output voltage increases. As the light source is 250 W <strong>of</strong> incandescent lamp, the linearity <strong>of</strong> the<br />
measured curve is better than the other two. In spite the output voltage is linear both at the ranges <strong>of</strong><br />
-45 o - -5 o and 0 o – 90 o at 250 W, the measured signal may decrease when it is cloudy or partly<br />
cloudy. To compensate the deviation due to the environmental effects, a two cell type <strong>of</strong> system was<br />
developed.<br />
3.2 Double cell type <strong>of</strong> measurement system<br />
For the optimization <strong>of</strong> the slope angle <strong>of</strong> the double cell type <strong>of</strong> measurement system, two<br />
sloped solar cells leaned against each other to measure the comparative output voltages <strong>of</strong> the two<br />
cells at different sloped angles δ (Fig. 3). The solar orientation measurement system was<br />
characterized with different solar elevation angles θ in the range <strong>of</strong> 0 o to 90 o at a constant distance<br />
from the light source <strong>of</strong> 140 cm. A voltage ratio R <strong>of</strong> the two solar cells was calculated to normalize<br />
the measurement results. In Figure6, the results showed that the ratio drops as the elevation angle,<br />
which is the time angle in the actual case, increases and the ratio approaches 1 when the time angle<br />
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is more than the same value <strong>of</strong> the sloped angle δ. As the time angle is less than δ value, the fitted<br />
curve equations for the voltage ratio R <strong>of</strong> different sloped angles are expressed by:<br />
R = -0.0059θ 2 + 0.905 θ + 9.9888 for δ = 45 o (1)<br />
R = -0.0099θ 2 + 0.055 θ + 8.9372 for δ = 30 o<br />
R = -0.0223θ 2 – 0.0315 θ + 6.0736 for δ = 15 o<br />
As the time angle is more than δ value, the fitted curve equations for the voltage ratio R <strong>of</strong> different<br />
sloped angles are linear and can be expressed by:<br />
R = -0.0053θ + 1.4587 for δ = 45 o (2)<br />
R = -0.0027θ + 1.2385<br />
for δ = 30 o<br />
R = -0.0010θ + 1.0922 for δ = 15 o<br />
where R is the voltage ratio <strong>of</strong> the two solar cells and θ is the time (elevation) angle ( o ). Eqs. (1) and<br />
(2) are useful for the estimation <strong>of</strong> the solar time angle at low elevation angle (δ).<br />
4. Conclusions<br />
This study has successfully demonstrated a new solar orientation measurement system with<br />
integrated solar cells. A new assembly method and a geometrical model have been developed for the<br />
precise measurement <strong>of</strong> the time and latitude angles <strong>of</strong> the sun at day-time and four seasons. It has<br />
been shown that the change <strong>of</strong> the output voltages <strong>of</strong> two back-to-back sun sensors caused by their<br />
sloped installation angle gives rise to a measurable change in the voltage ratio between the two<br />
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sensors. In addition to its precise solar orientation measurement and its simplified geometrical<br />
model, the device also exhibits a high degree <strong>of</strong> integrable with other solar energy devices.<br />
Acknowledgement<br />
The authors would like to thank the financial support provided by the National Science Council in<br />
Taiwan (NSC 93-2218-E-212-011).<br />
References<br />
Algifri, A. H., Al-towaie, H. A., 2001. Efficient Orientation Impacts <strong>of</strong> Box-type Solar Cooker on<br />
the Cooker Performance, Solar Energy 70, 165-170.<br />
Bari, S., 2000. Optimum Slope Angle and Orientation <strong>of</strong> Solar Collectors for Different Periods <strong>of</strong><br />
Possible Utilization, Energy Conversion & Management 41, 855-860.<br />
Berenguel, M., Rubio, F.R., Valverde, A., Lara, P. J., Arahal, M. R., Camacho, E. F., López, M.,<br />
2004. An artificial vision-based control system for automatic heliostat positioning <strong>of</strong>fset correction<br />
in a central receiver solar power plant, Solar Energy 76, 563-575.<br />
Blanco-Muriel, M., Alarcón-Padilla, D.C., López-Moratalla, T., Lara-Coira, M., 2001. Computing<br />
the Solar Vector, Solar Energy 70 , 431-441.<br />
Chen, Y. M., Wu, H. C., 2001. Determination <strong>of</strong> the Solar Cell Panel Installation Angle, Power<br />
Electronics and Drive Systems Proceedings 2, 549-554.<br />
Duffie, J., Beckman, W., 1991. Solar Engineering <strong>of</strong> Thermal Processes. Wiley Interscience, New<br />
York.<br />
44
Falbel, G., Puig-Suari, J., Peczalski, A., 2002. Sun Oriented and Powered, 3 Axis and Spin<br />
Stabilized Cubesats, IEEE Aerospace Conference Proceedings 1, 9-16.<br />
Hj Mohd Yakup, M. A., Malik, A. Q., 2001. Optimum tilt angle and orientation for solar collector in<br />
Brunei Darussalam, Renewable Energy 24, 223-234.<br />
Kawalski, S., 1997. Solar Powered <strong>Light</strong> Fixture, Renewable Energy 11, 399.<br />
Lorenz, W., 1998. Design Guidelines for a Glazing with a Seasonally Dependent Solar<br />
Transmitttance, Solar Energy 63, 79-96.<br />
Popat Pradeep, P., 1998. Autonomous, Low-cost, Automatic Window Covering System for<br />
Daylighting Applications, Renewable Energy 13, 146.<br />
Raasakka, B., 1997. Solar Skylight Apparatus, Renewable Energy 12, 117.<br />
Surman, R. L., 1996. Solar Powered Illuminated Address Number Device and Mailbox Structure,<br />
Solar Energy 57, VIII.<br />
Tesfamichael, T., Wäckelgård E., 2000. Angular Solar Absorptance and Incident Angle Modifier <strong>of</strong><br />
Selective Absorbers for Solar Thermal Collectors, Solar Energy 68, 335-341.<br />
Wen, J., Smith, T. F., 2002. Absorption <strong>of</strong> Solar Energy in a Room, Solar Energy 72, 283-297.<br />
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latitude plane<br />
W<br />
incident<br />
ray<br />
<br />
A<br />
N<br />
O<br />
<br />
B<br />
time plane<br />
E<br />
S<br />
Figure 1 Geometry <strong>of</strong> the irradiation <strong>of</strong> direct sunlight to sun sensors.<br />
N<br />
<br />
sun sensor<br />
E<br />
N : normal Vector<br />
personal<br />
computer<br />
singnal lines<br />
voltage meter<br />
RS232 Cable<br />
46
Figure 2 A schematic representation <strong>of</strong> single cell type <strong>of</strong> measurement systems.<br />
2 1<br />
<br />
N 2 N 1<br />
sun sensor 2<br />
<br />
sun sensor 1<br />
E<br />
personal<br />
computer<br />
N : normal Vector<br />
singnal lines<br />
voltage meter<br />
RS232 Cable<br />
Figure 3 A schematic representation <strong>of</strong> double cell type <strong>of</strong> measurement systems.<br />
N<br />
personal<br />
computer<br />
E<br />
voltage meter<br />
sensor N<br />
sensor W<br />
sensor E<br />
sensor S<br />
<br />
<br />
47
Figure 4 A schematic representation <strong>of</strong> quadrantal type <strong>of</strong> measurement systems.<br />
<br />
<br />
<br />
<br />
<br />
250 W<br />
◊ 100 W<br />
∆ 40 W<br />
<br />
<br />
Figure 5 Output voltages at different elevation angles for single cell type <strong>of</strong> measurement<br />
systems.<br />
48
voltage ratio<br />
11<br />
9<br />
7<br />
5<br />
3<br />
1<br />
45 o<br />
30 o<br />
15 o<br />
0 10 20 30 40 50 60 70 80 90<br />
time angle (degree)<br />
voltage ratio<br />
11<br />
9<br />
7<br />
5<br />
3<br />
(a) 0 o – 90º<br />
45 o<br />
30 o<br />
15 o<br />
1<br />
0 5 10 15 20 25 30 35 40 45<br />
time angle (degree)<br />
voltage ratio<br />
1.25<br />
1.2<br />
1.15<br />
1.1<br />
1.05<br />
(b) 0 o – 45º<br />
45 o<br />
30 o<br />
15 o<br />
1<br />
45 50 55 60 65 70 75 80 85 90<br />
time angle (degree)<br />
(c) 45º-90º.<br />
Figure 6 Output voltages at different time angles for double cell type <strong>of</strong> measurement systems. (a)<br />
0 o – 90º, (b) 0 o – 45º and (c) 45º-90º.<br />
49