Solar energy

It sounds like magic: capturing energy from the sun and converting it into electricity to run your toaster. It's science, not magic, though: Solar panels use a curious bit of science called the photovoltaic effect to convert one form of energy into another. Here's how solar panels harvest light, converting it into electricity.

The sun is an intense powerhouse, outputting in the region of 10 decillion joules (that's a 1 followed by 34 zeroes) of energy per year. That's a lot. The whole earth is bathed in this wash of energy, and it is one of the things that makes this such a nice place to live. It drives the weather, heating the atmosphere and making life possible. So, wouldn't it be nice to be able to convert some of this energy into a form that we could use? That's what plants do through photosynthesis, using light to combine carbon from the atmosphere into sugars that they metabolize to grow. And the solar panels that you see on an increasing number of roofs do something similar, converting light into electricity. They do this because of something called the photovoltaic effect, which converts solar energy into electrical energy.

It works like this: sometimes, when light hits an atom, it is absorbed by one of the electrons around the atom, boosting the energy of the electron. On some materials (such as some metals and silicon), this extra energy is enough to kick the electron loose of the atom, letting it move within the crystal structure of the material. If you create two crystal layers, you can exploit this. One layer is contaminated (scientists call this doping) with a chemical that means there are lots of electrons in it, so it has electrons to spare (called an N-type material). The other layer is contaminated with another material that makes it want to soak up more electrons, called a P-type material. These electrons can't easily jump over the junction of these two materials, (called an NP junction) so there is a voltage difference between the two layers. If you then connect a circuit to each side of this panel, that voltage can be used to power an appliance, or to charge a battery.

Each solar cell generates only a small voltage, typically about 0.5V. The amount of current that is created depends on the size of the cell. If you connect multiple cells together, this voltage can be increased. So, if you cover your roof with solar cells wired together, you can harvest enough electricity to power most of the house.

Solar panels also generate DC (direct current) rather than the AC (alternating current) that your utility company delivers. This means that the energy from a solar panel has to be converted before it can drive the toaster. It also often has to be stored: you may want toast when the sun isn't up, and the solar panels don't generate power in the dark.

Of course, this depends on how much light there is to convert, and there are other limitations to the process. The amount of energy they capture is dependent on the properties of the material absorbing the light, and the junction between the two materials. This combination will only absorb specific frequencies of light. Some modern solar panels get around this by including multiple materials and junctions between them (called multijunction cells) that can absorb different frequencies of light to capture more of the available energy.

And there is the cost. Solar cells are not cheap to make, as they require precise growth of large crystals with very exact chemical compositions. The other parts of the system (such as the controller) are also expensive, as they need to be able to handle large amounts of energy. When I put the details of my home near Boston into the Google site Project Sunroof (which calculates the cost and benefits of putting solar panels on your roof), it estimated that I would be able to generate about 7 kilowatts of energy by putting up just under 490 square feet of solar panels on my roof. That, it estimated, could save me over $900 a year, or $18k over the 20-year life of the system. However, the cost would be high: that system would cost over $30k to install. You don't have to buy a solar system outright, though, as there are companies that lease these systems, so you pay over time. The Feds also kick in to help, as installing a solar system can get you a big tax credit of up to 30 percent of the cost.

This price is constantly falling, though, as new ways to make solar cells are being developed and new factories are opening up to make solar cells. Although solar power isn't likely to replace other ways of generating power, we are going to see solar power as an increasingly important source of electricity. And that's all because of that strange quirk of some materials that converts light into electricity.