Solar Cell Experiment

Solar Cell Experiment
Aim: To see how individual factors affect the output of a solar cell. Factors affecting the output of a solar cell: This experiment is going to be performed in the confines of a school laboratory, and so the complexity and cost of the experiment(s) should reflect this. However, to see how different factors affect the solar cell output, I will need to perform at least two experiments. The question is, which ones? Â? Distance from the light source will affect the solar cell output, because intensity of light on the solar cell will decrease, the further away from the light the cell is. This is because many waves, including light, will travel away from the filament in a circular motion, not straight lines directly towards the solar cell, so the further away it is, the less rays will hit it. Â? A changing power to the light source, and therefore, to the solar cell, will affect the solar cell output. Theoretically, an increase in power at the light source should result in more energy being transferred to the solar panel, per second, resulting in a greater solar cell output.
The different colours of light would also affect the solar cell output, as each colour of light has a different frequency. We can tell this from Einstein?s theory of photons being directly proportional to frequency: Energy = Planck?s constant x speed / wavelength Different colours of light have the same speed, but different wavelengths, which, in the above equation, changes the value for energy, which is the solar cell output. And, because the wavelength is the denominator for this equation, we can make the statement, that ?the larger the wavelength, the lower the energy?. A colour of light with a larger wavelength will result in a lower solar cell energy output. The two factors that I am going to study in this experiment are: 1. Changing power at the light source 2. Distance of solar cell, from the light source I have chosen these factors purely for ease of measuring the values of the energies involved, and for the (comparatively) low cost of the equipment required. Studying the colours of light would have required lasers, specialised equipment and a very strictly controlled environment, which we can?t really get in school. Prediction: I predict that, the greater the power at the light source, the greater the solar cell output. I also predict that, the further away the solar cell from the light source, the lower the solar cell output. Hypothesis: There must be a greater output at the solar cell if there is greater power at the light source, if the cell is the same distance away, because the intensity will be greater. We know this through the equation intensity = power / area. The power is a variable, but the area will stay the same because we are using the same solar cell throughout the experiment, and because power is the nominator in the above equation, we can say that ?the greater the value for power, the greater the value for intensity? of light upon the surface of the solar cell, and therefore, the greater the solar cell output. Text Box: Because this one changing variable is the only changing value and therefore the only one that influences the intensity, the power and solar cell output (dictated by the intensity of light upon the cell) must be proportional, giving the graph in fig.1. Text Box: Fig. 1The graph doesn?t go through the origin (0,0) because, at a very low voltage, the intensity of the light on the solar cell won?t be great enough for a reading to noticeably register on the cell?s milliammeter. The further away from the filament the solar cell finds itself, the lower the solar cell output. The reason for this is not that the individual light rays become greatly weaker than they were at the source because they have had to travel an extra few centimetres through the air, but that the light rays, collectively, become weaker at the surface of the cell because less are hitting the cell. This simply means that the light is less intense on the solar cell, when it is further away from the filament - the light attenuates over distance. Why is this? For the solar cell output to be the same, no matter what distance it was away from the light source, light rays would have to travel in the exact direction from the source to the cell. As this obviously isn?t the case, we can deduce that light rays do not travel in any single, fixed direction. Text Box: Fig. 2Text Box: In fact, the light rays move steadily outwards from the filament, equally in all directions, so that if you were to draw a line around them at any given point, your line would meet to make an exact circle (see fig. 2), and because the rays go upwards as well as outwards, this makes a sphere. This is a problem for the intensity of light that the solar cell receives, for the rays of light that hit its edges at, for example, 5cm, don?t hit it at all at 10cm (see fig.3), leaving less and less rays, and therefore less and less photons, to hit the surface of the solar cell. This reduces the solar cell?s output as the distance grows. Text Box: Text Box: Fig. 3 There is a distinct relationship between distance of the solar cell from the lamp, and the area of the ?sphere? of light. For every time the ?radius? is doubled, the area of the sphere is quadrupled. We can work this out by applying the change in radius to the formula for the area: Text Box: Original equation : 4Ï?r2 Double the radius : 4Ï?(2r)2 Multiply out the bracket : 4Ï?4r2 Factorise : 4(4Ï?r2) = 4 times the original equation To prove this works, here is a theoretical example: Let?s say that r = 5. When the solar cell is r distance from the lamp, area of the sphere = 4Ï?r2 = 314 When the solar cell is 2r distance from the lamp, area of the sphere = 4Ï?(2r)2 = 1256 1256 / 314 = 4 (area for r is quadrupled). This relationship comes under the ?inverse square law?, for we can see, through re-arranging the area of a sphere formula, that: r = (4Ï?r2) Text Box: (see fig. 4 for visual representation) 2r = 4(4Ï?r2) 3r = 9(4Ï?r2) 4r = 16(4Ï?r2) and so onâ?? There is a proportion between the area and distance squared ? area Î? distance2. Text Box: In the above diagram: S = source strength I = intensity r = radius of sphere A= area Text Box: Fig. 4 Text Box: Due to the proportion that exists, and therefore the Inverse Square Law, the results I will obtain can be best analysed on a graph of ?solar cell output vs. 1 / distance2.? If it looks something like fig. 5, then I will be able to state that the correlation I have explained in the hypothesis does exist, or if the graph is different, that it doesn?t exist, or the results are too bad to tell. Text Box: Fig. 5Text Box: Method, diagram & apparatus: Â? First of all, we set up the following circuit: [image] [image] [image] [image] [image] Â? The apparatus consists of: 15V power pack / filament lamp with bulb / solar cell / voltmeter / ammeter / milliammeter / ruler. Â? Power experiment: I kept the solar cell a constant 6 cm away from the light bulb, so that it would not be so close as for some of the bulb to be on the non-photovoltaic side of it, but we be close enough for a strong, clear reading. The solar cell was leaning against a heavy paperweight, so that the chances of it moving were very slim. I took the readings as close to every voltage from 1V to 12V as possible, measuring the ammeter and milliammeter readings at the same time. I didn?t have time to take two readings for every value in the experiment, but I took two whenever I felt that a variable we had not been testing had changed, and the result would therefore not be as accurate as it should be. Â? Distance experiment: From the hypothesis, I could see that, due to the ?area of a sphere? effect, the distance I could move the solar cell over and still get a sizeable reading on the milliammeter would probably be fairly small. For this reason, I decided to take the readings every centimetre, from 1cm to 15cm away from the bulb. The ruler on the table, against which the solar cell was placed, was held in place by a clamp so that it wouldn?t change position and disrupt my results. At all times, the voltage and current were at constant 8.6V and 1.51I respectively. Fair test: Â? Background light was always going to be a key factor in this experiment, and, as we couldn?t get rid of it completely, we made sure that the same conditions were maintained throughout the experiment. That meant no lights were turned on or off in the laboratory and no blinds were taken up or put down, after the practical had been started. Also, during the experiment, I made sure that the group with whom I was performing the experiment were quite still, so that they would not move in the way of one of the background lights or the lamplight itself. Â? Variables that we weren?t testing at a particular time were constantly monitored so that, if they did change while we were taking a result, we could repeat that part of the experiment. Â? I made sure that I turned the power off when I was not taking a result, so that the bulb and solar cells had the maximum amount of time to cool down, so that their resistances would not be affected more than was strictly necessary, given the time constraints. Safety: Â? The only safety worries in this experiment were those of the electrical appliances overheating, and when they inevitably did, not to touch them with your bare hands. Â? Also, the milliammeter, an extremely delicate, sensitive device, was only allowed to be connected to the low-power solar cell circuit, as the higher power lamp circuit would have immediately blown it up. Results: [image] [image] The only graph to plot for the first prediction -?the greater the power at the light source, the greater the solar cell output? ? is solar cell output vs. power of the lamp. This graph, below, shows a nearly straight correlation for the points where the power is greater, but the line overall is a curve of equation y = x1.5, or something close to that. This is because the lower power values don?t follow the same straight-line correlation (as predicted in my hypothesis), as the greater values. However, I think I can say that the correlation on this graph, between power of lamp and solar cell output, is very strong. Text Box: The second prediction was ?the further away the solar cell from the light source, the lower the solar cell output?. The values that need to be compared and seen to be correlated, for the above statement to be true, are solar cell output and distance from the lamp. The graph on the next page shows a steady curve of equation y = 1/x or y = 1/x2, or somewhere in between. For the lack of a straight line, we cannot say that there is any correlation. Text Box: As the above graph might have had an equation of y = 1/x, I decided to try it out, making the new graph ?solar cell output vs. 1/distance?. Text Box: The resulting graph also followed a curve, whose equation was approximately y = x1.7. There was still no correlation directly linking the two values. There was only one logical step to take after thisâ?? Being backed by the Inverse Square Law that I had researched in my hypothesis, I tried the graph I had mentioned in that section, solar cell output vs. 1/distance2. This time, the line of best fit was a straight line, although it missed quite a few of the points where the value for 1/distance2 was at its lowest, but more about that in the evaluation. Text Box: This showed correlation between the output and distance away from the source ? it told me that the solar cell output was indirectly proportional to the square of the distance Analysis: Â? The graph showing a variation in power versus solar cell output corresponds with my first prediction, that solar cell output increases as the power in the lamp increases, if the distance between them stays the same. However, the hypothesis I made, although correct (I think), was defeated in practice by the presence of background light, which didn?t let the milliammeter drop beyond a certain point of 2mA or so. The higher values of power, which weren?t as greatly affected by the small presence of background light, produced a straight line. From this, I could draw the conclusion that ?solar cell output increases in direct proportion to power, at a fixed distance?. Â? The trend for the second prediction was detailed in the hypothesis, and was completely supported by the findings from my experiment - solar cell output has a 1/x2 relationship with distance; there is concrete proof of the inverse square law. The reason for this relationship must be that light does in fact expand in an ever-expanding spherical shape, and the surface area of the sphere increases in proportion to the square of the distance between the cell and the bulb (the radius of the sphere). The graph of 1/radius2 dependence is produced, and from both it and the above proportion, we can deduce that ?energy twice as far from the source is spread over 4 times the area, and therefore the intensity is ¼ of what it was?. This is the inverse square law. The fact that a consistent proportion exists will explain why the graph of solar cell output vs. distance has such a steep initial curve - the intensity (and therefore the solar cell output) decreases by greater amounts, the greater the energy value it is dealing with. What is the science behind this theory? When photons of light hit solar cells, they give an electron(s) the energy to break away from its atom, so that it becomes a free electron and can conduct electricity. Obviously, the more photons that hit the solar cell, the more electrons that will be freed and the more electricity that will conducted (a greater the solar cell output). Text Box: Text Box: Fig. 3As is shown in fig. 3, the further away from the light the cell is, the less photons reach its surface due to the? ever-expanding sphere theory? of the area the light covers. It is only the inverse square law photons that hit the solar cell, meaning there are a high intensity of photons at the cell close to the lamp, but a lower intensity at the cells further away. Evaluation: In an experiment involving many measurements by the human eye, there were always lots of things that could go wrong, and many of them did, and they unfortunately affected our results: Â? The voltmeter readings throughout the experiment are unlikely to be completely correct, as the reading constantly fluctuated, so that I to guess an average of the numbers that were flickering before my eyes. This could be the reason why some of the points on the graph of solar cell output vs. power look slightly out of place in the curve. Â? Reading the milliammeter was an error-strewn process as well, because the needle was a few millimetres off the scale, and cast a deceptive shadow across a point about 4 milliamps off the actual reading. This made reading the instrument quite time-consuming and didn?t contribute positively to the accuracy of my results. I would have preferred a milliammeter whose needle was closer to the surface of the scale, or that simply had a digital display. Â? In the varying distances experiment, the milliammeter reading would not go lower than 2mA because that was the level of background light. This was the reason why the graphs for this experiment have flat bottoms and make the straight-line graphs look like curves. This problem also made the curves have a straighter line than they would have, otherwise! To remedy the negative effects of background light, I would perform the practical in a photography lab or a room with a similar, dark environment. Â? The inefficiency of both the filament lamp and the solar cell (they were 20% and 4% efficient, respectively) served to give us a very small range of results to work with. At least their efficiencies were fairly constant over the course of the experiment. Â? Another factor that altered the results was the resistance that built up in the circuit, as we couldn?t afford to give the instruments time to cool down properly in between readings. A solution to this would have been to have more time to do the experiment in. This factor was, in my opinion, at east partly to blame for the last point on the graph of solar cell output vs. 1/distance2. Â? Finally, to make the readings more accurate overall, and negate the effect of any anomalies that might have been taken (luckily, my experiment didn?t have any serious anomalies), if we had had more time, the whole practical could have been performed twice, and an average of the results taken. Improvements: Â? Have more time for the experiment. Â? Perform it in a darkened room or fume cupboard etcâ?? Â? Study the effect of colour of light, maybe using basic colour filters instead of the complicated equipment mentioned at the beginning of the coursework.

Solar Cell Experiment 7.6 of 10 on the basis of 2186 Review.