Theory/How Solar Works 1: Sunlight

Sunlight to electricity - How it works

Solar panels convert energy in the form of light into energy in the form of electricity. But how does this really happen inside the device? - This is the first lesson out of a little online solar photovoltaics physics course, explained in the most simple way. Most of the information is from websites like Wikipedia and I will also include some Youtube videos that help visualize the concept. If you're interested in learning more, you can click the reference links!

Now, lets start at the beginning: Sunlight.

Light can be thought of as tiny bundles of energy, moving at 186,000 miles per second. It takes about 8 minutes for light to travel from the sun, to earth (yes, the Sun is as far away as it is awesome). Now, take a moment to imagine how much the average power consumption of all humanity on earth is.. That is, all the fuel we burn in our cars, power plants, stoves, etc etc. It's a lot, right?

Well, the sun produces about 25,000,000,000,000 times that amount [2]. Even though the surface of the Earth only receives a fraction of that power, it's still approximately 8,000 times our energy consumption [3]. All this energy comes in tiny bundles of energy, called: photons.

Photons

Ever wondered how light consists of different colors and what the difference is between blue and red light? Photons carry energy, but some photons carry more energy than others and this is how we experience different colors. Blue light photons carry more energy than red light photons. You might also have heard of 'invisible light' like Infra-Red (IR) and Ultra-Violet (UV). UV gives us that nice tan in the summer, while IR is simply 'heat'. I will now explain a little bit more about photon energy, because like our eyes, solar cells also react different to photons with high and low energy!

Photons have characteristics of both particles, as well as waves. Back in the 17th century, Christiaan Huygens (a smart Dutch guy) and Isaac Newton (click here for his portrait!) argued about whether light was a wave (Huygens) or a particle (Newton). Quantum mechanics later described that photons can not be described as only waves or particles. That discussion goes far beyond the goal of this lesson, but it is important to know that you can characterize light in terms of 'wavelength' (similar to sound waves) and 'electronvolts' (tiny amounts of energy). These two can be directly converted from one to another using a simple formula:

Converting wavelength to energy

Red light has a wavelength of about 600 nano-metres (nm). Using the equation above, you find: 1240/600 = 2.07 eV. Don't worry about these weird units if you haven't seen them before, just remember that 'nm' stands for the length of the lightwave and 'eV' stands for the energy of the photon (or: light bundle).

The sun produces a whole spectrum of different colors: below is a graph that shows the intensity of light from the sun, for each wavelength (nm). Do you see that the visible part of the spectrum (colors we can observe) forms exactly around the peak of the curve? This is not a coincidence! You can understand from this that our eyes have evolved over time to become best at observing the light that is most present: sunlight!

Sunlight Spectrum

Another interesting thing you can see are the dips in the curve. It is not that the sun doesn't emit light at those wavelenghts, but the earth's atmosphere has particles in it that absorb light of certain wavelengths. Most of the dips you see in the curve, are because of water (H2O) in the air. At even higher wavelengths (not shown in this graph), you can actually see the effect of greenhouse gases. That's where the black sheep called: 'CO2' does his work, see here.

Next: Bandgaps

Now that you understand that light actually consists of photons carrying different amounts of energy, you know the basics of the energy that is served to solar panels by the Sun. Next week, we will go into more detail on how the solar cell device uses these photons to generate a current of electrons. You will also learn what a 'bandgap' is and how it decides what photons are used to generate electricity and which ones are not. To warm you guys up, here's a one-minute youtube video that you should see.

Sun Voyage!

Rob

 


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