Understanding light

Light is all around us; most of us it we can't even see. Light is composed of individual photons, each with a wavelength and an energy, and all travelling at the same velocity.

The photon's wavelength determines the color, in the portion of the spectrum that we can actually see (much of the light spectrum is invisible to the human eye). The photon's frequency determines the energy of a photon. The higher the frequency, the higher the energy. You can visualize this by picturing two snakes, one large and one small, crossing a hot road. The large snake wriggles across the road with much longer strides (wavelength) than the small snake. To wriggle fast enough to keep up with the large snake, the small snake has to expend much more energy.

As mentioned previously, only a small portion of the spectrum is visible. At the low end is blue light, with a lot of energy (high frequency), and at the upper end is red light, which is lower in energy. Above the red zone is infrared, which is basically heat radiation. This is relevant because solar collectors are selective in which wavelengths they work with. An efficient collector will convert all the wavelengths in the spectrum into useable energy, while an inefficient collector will favor infrared, for instance. Solar water heaters convert much more of the spectrum to useable energy than PV cells, although technical advances in PV cells have enabled a much broader range of selectivity, making them more efficient.

On the passive front, windows are available which selectively filter both ultraviolet light and infrared light. Ultraviolet is damaging to fabrics and carpets, while infrared light is responsible for a lot of heat. Neither of these spectral regions are visible, and so these high-tech windows do not alter the view to the outside.

A brief history of solar power

Since time immemorial, humans have been using the sun for warmth, and devising inventive schemes to heat water and make electricity. In 1860, Auguste Mouchout, a French mathematician, invented the first solar-powered steam engine. It didn't offer much power, but it could get the job done, albeit slowly (unfortunately for Mouchout's wallet, humans don't like slow). In 1870, American John Ericsson devised a solar water trough that focused concentrated radiation onto a liquid — water or oil — and the steam pressure was used to spin turbines. As with Mouchout's invention, the job could be done, but only slowly, and only when the sun was shining. Solar engines inevitably gave way to fossil fuel power plants.

Most of the early advances in solar power were geared toward thermal applications, where the heat of the sun converted liquids into steam, which was used in steam engines. Some of the more outrageous inventions included huge mirrors, which focused a lot of energy onto a small spot size. These inventions did not fare well in windy conditions, as one might imagine. And engines like this are fixed into place, so they were never used in transport.

It was not until 1905 that Albert Einstein theorized the PV (photovoltaic) effect, which exploits the dual nature of light (photons act as both radiation and matter — a rather counterintuitive fact, but fact it is). From Einstein's work evolved the modern PV cell, which converts light energy into electrical energy, a revolutionary technological advancement that underpins the entire PV solar industry today. Einstein established some of the basics of nuclear power as well; his influence is inestimable.

Of all the solar energy that reaches the planet earth, here's where it goes:

✓ 35 percent is reflected away from the earth, back into space

✓ 43 percent is absorbed as heat radiation, both ground based and atmospheric

✓ 22 percent evaporates water, creating rain and water distribution

✓ 0.2 percent creates wind energy, or kinetic energy of the atmosphere

✓ 0.02 percent is used up in photosynthesis by plants

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