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Solar Panels as a Source of Noise-Free Power A. Dutta 1 , J. Martin 2 1 College of Chemistry, University of California, Berkeley 2 Medart Elementary School, Crawfordville, Florida Work done in the Quantum Spin Dynamics group at NHMFL, under supervision of Dr. I. Chiorescu ABSTRACT AND MOTIVATION Solar energy is acknowledged to be one of the main sources of non-pollutant energy of the future. Furthermore, many research disciplines that are dependent on sensitive research instrumentation would benefit from it, since solar power precludes the 60Hz (and multiples) noise of grid power. We present a study that implements solar D.C. power to eliminate such background disturbance. We measured the electrical characteristics of a set of three solar panels (SHARP: NU-U235FI) as a function of load resistance, sun intensity, and varying weather conditions. -10 10 30 50 70 90 110 130 150 170 0 2 4 6 8 10 12 0 5 10 15 20 25 30 35 Power (W) Current (A) Voltage (V) Electrical characteristics of Solar Panels Current vs. Voltage 615 W/m 2 837 W/m 2 981 W/m 2 433 W/m 2 Power vs. Voltage BACKGROUND In contrast to the Photoelectric (PE) effect, which expels the electron from the material, the Photovoltaic (PV) effect retains the electron while promoting it from the Valence Band (VB) to the Conduction Band (CB) [Figure 1]. However, the efficiency of the PV cell is limited by the decay of electron from the CB back to the VB. To analyze characteristics such as voltage, power, and efficiency, we use Ohm’s law. The current (I) through a conductor between two points is directly proportional to the potential difference (V) across the same two points where R is conductor resistance. Other relevant equations are: EXPERIMENTAL SETUP One of the three solar panels is connected to a circuit (see green box in Fig 1) consisting of a switch, a variable resistor, and an ammeter. A light meter (EXTECH EA33) is mounted outdoors to measure the Sun intensity at an angle identical to that of the solar panels. This light meter, however, is limited by its spectral sensitivity. Therefore, to get an accurate measurement of the Sun intensity that is radiated on the solar panels, we performed a calibration analysis that treats the Sun radiation as a blackbody emission with a T=5777 K [2]. Our analysis indicates that the intensity measured by the light meter has to be multiplied by a factor of ~7.2 to estimate the proper sunlight intensity. DATA AND ANALYSIS 0 5 10 15 20 25 30 0 5 10 15 20 25 30 35 Efficiency (%) Voltage (V) Efficiency vs. Voltage 172 257 339 446 855 880 943 433 615 837 981 Using series-parallel combinations of resistors, we create circuits that had resistances varying from 0 to 24Ω. We then simultaneously measure the Sun intensity outdoors and the current through the created circuit. To account for the decrease in efficiency due to elevated temperature of the solar cells, we measure the temperature using an Infrared thermometer (EXTECH 42540). I at max power STC* 433 W/m 2 615 W/m 2 837 W/m 2 981 W/m 2 Maximum Power 235W 65.9W 100W 128W 152W Open Circuit Voltage 37.0V 31.1V 31.2V 32.2V 30.3V Short Circuit Current 8.60A 2.58A 4.33A 4.75A 7.36A Maximum Power Voltage 30.0V 28.1V 24.5V 22.6V 24.6V Maximum Power Current 7.84A 2.34A 4.09A 5.65A 6.16A Module Efficiency (corrected for temperature) 14.4% 11.0% 12.4% 10.7% 12.6% Temperature (⁰C) 25 40.5 51.1 34.3 62 The loss of efficiency could be accounted for by the resistance in connecting wires and diodes. ACKNOWLEDGEMENTS REFERENCES The irregular curve shown in Figure 4 at 837 W/m 2 was a result of data collected under inconsistent conditions (cloudy weather) while the others were a result of data collected over a more constant range. Looking at the more constant data, the following trends are observed: (1) The maximum power, the short circuit current, and the maximum power current increase with sun intensity. (2) The open circuit voltage and maximum power voltage decrease with increasing sun intensity. (3) The module efficiency (corrected for temperature effects) increases with increasing sun intensity. NOISE- FREE EXPERIMENTAL SIGNALS These solar panels will be used in future projects to continuously recharge batteries that power the amplification and data acquisition stages since a significant decrease in noise is observed when batteries are used. Figure 1. Sketch of the electrical processes ongoing in a solar panel, and of the testing circuit. Photons promote electrons from the VB to the CB; the efficiency is limited by decay. The obtained current through a known resistor is measured by an ammeter, to evaluate the electrical power and efficiency. Figure 2: Monthly variation of solar energy per m 2 per day, for different panel tilt angles (from [1]). For a tilt of 45 degrees the variance of energy output is minimum, allowing research experiments all year long. Table : Electrical characteristics of the solar panel, detailed for four solar intensities at maximum power, compared with the *STC specifications (STC= standard test conditions, 1kW/m2, 25`C, solar spectrum of air mass 1.5)[3] On the left, we show current and power as a function of voltage recorded for four solar intensities. Solar intensities, measured for maximum power, are given above each curve. Light meter 3000 3500 4000 4500 5000 5500 6000 6500 7000 1 2 3 4 5 6 7 8 9 10 11 12 Solar Radiation (Wh/m 2 /day) Months Optimization of Solar Panel Orientation 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 [1] “PVWATTS: Florida- Tallahassee” <http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/Florida/ Tallahassee.html> [2] C.A. Gueymard, D.Myers, and K.Emery, Solar Energy, Vol 37, Issue 6, Dec 2002, 443-467 [3] “SHARP-solar electricity”< http://www.solarelectricsupply.com/Solar_Panels/Sharp/NU-U235F1.html> On the right, we show a graph of efficiency as a function of voltage at eleven solar intensities that range over various weather conditions. The values in the legend correspond to sun intensities at maximum power in W/m 2 . The figure shows real time signals (upper inset, amplified 8,000 times) detected by the research equipments in two situations: powered from the grid (blue) or from batteries to be recharged by the solar panels (red). A Fourier analysis (main figure) shows the frequencies composing the real- time signals (same colors). One can observe a strong noise of 60 Hz and multiples, which almost disappears when using batteries. We thank graduate students Lei Chen and Nick Groll, and the NHMFL teams of Richard Brooks and Andy Powell, for help and support during the experimental implementation of the project. We acknowledge the financial support from the NSF programs Res. Exp. for Undergraduates and Res. Exp. for Teachers hosted by the Center for Integrating Research and Learning (Dir. P. Dixon) at NHMFL. Work funded by the NSF Cooperative Agreement DMR-0654118, NSF DMR-0645408, Florida State University and the Sloan Foundation. Figure 3 Figure 4 I max (W/m 2 ) The orientation of the solar panels could be determined using two types of optimizations: (1) Smallest variance in maximum output and (2) Largest amount of total solar radiation input over a year. Using optimization (1) [Figure 2], we determined the preferred angle of the solar panels to be 45 o . Above, we show the setup of our study that corresponds to the sketch shown in Fig 1. Ammeter Resistor (024Ω) Solar Panels Circuit Breaker
