Naked Science Forum
Non Life Sciences => Technology => Topic started by: Edwina Lee on 18/12/2015 09:29:48
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Question 1: Do solid state solar cells convert multi-directional light less efficiently than uni-directional light?
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Question 1: Do solid state solar cells convert multi-directional light less efficiently than uni-directional light?
it isnt the unidirectional aspect but the angle of incidence. For example a beam of light coming perpendicularly onto the surface will have less light reflected off the surface than one which comes in at an angle. You can see this effect by looking directly at a piece of glass an then tipping it at an angle.
Tests on solar cells show that most of the losses occur at the first air/glass interface and this holds true for all types of cell.
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multi-directional light
1. The most severe cause of multi-directional light is an overcast day, where most of the light is reflected back into space, and the remaining light comes through as diffuse illumination.
Our eyes adapt to the reduced light levels by opening the iris, so we don't really notice the change in illumination so much. But it really impacts the output of solar cells, because there is less light to convert into electricity.
2. Another cause happens on every clear day, as the Sun passes across the sky. The solar cell produces maximum power when the solar cell is directly facing the Sun, as this presents the maximum area to the sunlight, and intercepts the maximum amount of sunlight. In the morning and evening, the Sun is at an extreme angle to the solar cell, and the solar cell presents a very small area to the Sun (for the mathematically inclined, the area presented to the Sun is a Sine function of the angle to the Sun). This factor is in addition to the reflection effects mentioned by Colin2B.
A heliostat (http://en.wikipedia.org/wiki/Heliostat#Large-scale_projects) keeps the solar cell pointed straight at the Sun, all day. But these are large and expensive, so they are only cost-effective on large solar farms.
Another recently-published solution (based on a fossilized insect eye) is to apply a nanoscale texture to the surface of the solar cell, which allows it to capture more of the sunlight from a greater range of angles, with less reflection.
3. For very compact and portable solar cells, a lens or mirror is used to concentrate light from the Sun onto the solar cell. These can only pick up light from a very narrow view of the sky (unidirectional light), so they need to be kept pointing at the Sun. These have high efficiency on clear days, but on a cloudy day, a flat solar cell is more efficient at collecting multi-directional light.
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First of all waves of a perpendicular polarization component should more easily transmit in any medium whereas the parallel waves reflect. Waves that reflect off the ground become horizontal.
Some kinds of cells transduce more anisotropicly than others:
https://en.wikipedia.org/wiki/Thin-film_solar_cell#Amorphous_silicon
https://en.wikipedia.org/wiki/Anomalous_photovoltaic_effect
https://en.wikipedia.org/wiki/Perovskite_solar_cell#Hysteretic_current-voltage_behavior
But all cells should do this as they rely on a stack of elements. If a shallow wave makes it inside it should be easily trapped in the n layer where the elèctròn's greater mobility over the hole's in the p layer yields less resistance. (Which suggests the best shape for a panel isn't flat but a concave cone.) The worst thing you could do is point or install a panel backwards. I can't personally verify this but maybe someone on here has a meter and cell ready.