The effect of light intensity on solar cell efficiency
Leighann VanCleef - Aaron Baker
Introduction
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Background
The idea of harnessing the energy of the sun is not a new one. In fact, way back in 7th century BC our ancestors were using crude magnifying glasses to focus the sun’s energy and light fires. Then, fast forward to 1767, when Swiss scientist Horace de Saussure built the world's first solar collector, later used by Sir John Herschel to cook food during his South African expedition in the 1830s. However, it would not be until 1839 that the basis of most of our more modern solar power energy would be discovered by French scientist Edmond Becquerel, who named his discovery the photovoltaic effect. Even then, technology was not quite ready to explore this new discovery, so it would take more than a century to truly understand this new process.
PV power is one of the most environmentally safe forms of energy that we have in use. This, plus the fact that solar power is a renewable energy source, makes PV power very attractive for the future of our energy production. Current energy production methods such as fossil fuels are non-renewable and greatly harmful to our environment. If we want to continue to lead healthy, long lives, we need to start looking to the future, and that is solar power.
The photovoltaic effect is one by which PV cells (there are many different materials which PV cells can be made of) convert light energy into electrical energy at the atomic level. Light, however, may be reflected, absorbed, or pass right through the PV cell, and it is only the absorbed light that generates electricity. Determining how the increase in the intensity of this light affects the output of a solar cell is the focal point of our study, and is critical to determining when (or more specifically, where) building a new solar cell will result in a net gain of electricity.
Research
Preliminary research indicates that while solar cell voltage output in an ideal cell is directly proportional to the light intensity it is exposed to, numerous inefficiencies and inaccuracies in the mechanism throw off the linear variation. The study “Series Resistance Effects on Solar Cell Measurements” indicates that series resistance within the structure of the solar cell itself affects the voltage output proportionately more at higher light intensities, causing the IV curve to become non-ohmic (Wolf and Rauschenbach, 1963). This would mean that voltage output and power production drops off for non-ideal solar cells. Temperature increase also hurts the efficiency of solar cells, which would naturally be higher at higher light intensity levels (see Katz et al, 2001). Finally, a study of the “Dependence of the Photocurrent Conversion Efficiency of Dye-Sensitized Solar Cells on the Incident Light Intensity” indicates that at extreme light values (very high or low light), the incident photon to current conversion efficiency (IPCE) decreases significantly. At high light values, this is as a result of “diffusion limitation of the electrolyte within... the electrode” (Trupke, Würfel and Uhlendorf, 2000).
Hypothesis
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For non-ideal solar cells, voltage output related to varying light intensity exhibits diminishing returns: that is, as light intensity increases, the increase in voltage output drops off.
Setup (Table of Contents)
Materials:
Lamp with 60W lightbulb
Variable transformer, 120V
Cardboard box
Solar cell panel (9V, 50mA)
Electronic voltmeter with 0-20V range
Electronic Light probe
Computer with light probe software (LoggerPro™)
Procedure: (Table of Contents)
1. Set up your materials as shown in picture. The lamp, solar cell panel and light probe should all be inside the cardboard box, to avoid light pollution by ambient light. The light probe should be propped up at such an angle that it is directly facing the light that the lamp is providing. Connect the lamp to the transformer, and connect the transformer to your power source. The solar cell panel should be connected to the voltmeter, and the light probe connected to the computer or other device to interpret the readings of the light probe.
2. Start by taking your baseline readings. Record the amount of light intensity in lux and amount of voltage that is output by the solar cell when there is no power to the lamp.
3. Next, turn the transformer to 100% of its power source (to avoid artificially deflated lux values due to hysteresis) and record the power output by the solar cell. Also, to record the light intensity, set the light probe to get data points for five seconds, with five probes per second. This way you can record the fluctuating light intensity and determine the average light intensity for that power output.
4. Repeat step 3 at 90% of the transformer's power, then continue to reduce the power at 10% intervals until reaching 0% again.
5. Take off the cardboard box for about five minutes, as the temperature inside the box builds up and might skew results.
6. Repeats steps 3 through 5 two more times, to provide more data for accurate results.
Results (Table of Contents)
Conclusion (Table of Contents)
Our hypothesis is completely justified, given the results of our experiment. As the light intensity increases, the increase in voltage production drops off. It took nearly 3 times as much light intensity to create a 10 volt output as it did to produce 7.5 volts of output: a one-third increase in productiveness for a 3x increase in solar “raw materials.”
The strengths of our approach are many. First, the use of the electronic light probe enabled us to take 25 samples for each trial for three trials, resulting in 75 measures of light intensity. This helps reduce the margin of error for the independent variable. Second, our setup shielded the solar cell as much as possible from ambient light, and used a non-reflective surface and container (black butcher paper and cardboard box) which ensures that the cell and the probe are exposed to the same proportion of the available light. Additionally, we left a five-minute cool-down period between each trial, which limits interference from raised temperatures. The efficiency-hindering effects of high-temperatures on solar cells are well-documented (Katz et al, 2001)
Most of the limitations of our approach are related to the poor quality of the equipment used, specifically the solar cell itself. Series resistance in the structure of the cell is a part of what creates the “diminishing returns” effect, and the low quality of our cell exaggerates this tendency beyond what may be considered reasonable in one of industrial or research quality. Additionally, because we did not stop between each different lux value but instead between different trials, accumulated radiant heat would deflate the voltage output at later values within each trial.
Bibliography
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Katz, E. A., Faiman, D., Tuladhar, S. M., Kroon, J. M., Wienk, M. M., Fromherz, T., Padinger, F., Brabeck, C. J, & Sariciftci, N.S. (2001).
Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions. Journal of Applied
Physics. 104, 5343-5350.
Trupke, T., Würfel, P., & Uhlendorf, I. (2000). Dependence of the photocurrent conversion efficiency of dye-sensitized solar cells on the incident
light intensity. Journal of Physical Chemistry B. 104, 11484-11488.
Wolf, M., & Rauschenbach, H. (1963). Series resistance effects on solar cell measurements. Advanced
Energy Conversion. 3, 455-479.
Links (Table of Contents)
Solar Energy Technologies Program
A
Department of Energy-maintained website. It is very comprehensive and covers not
just the how photovoltaics work but also how they are used and their social and
economic importance.
An easy-to-read explanation of how exactly a solar cell generates power, and other important issues like local power production and cost.
A non-technical website with lots of information about the history and development of solar cells, their uses, and social implications.
A (very extensive) list of science fair projects for students wanting to investigate the properties of solar cells on their own.
Solar Panel Simulation and User's Guide
A neat applet-type program which allows the user to simulate a number of factors in solar power generation, like angle of incident light, light intensity, series resistance, efficiency, etc.