Solar Energy Safety Messages

    Solar energy is largely light energy. What we already know about light can be useful in gathering and concentrating solar energy.

REFLECTION  - Light bounces. We can reflect light. We use this all the time in mirrors. But by reflecting light we are reflecting energy. It is possible to concentrate light energy by using several mirrors to reflect light to the same spot. Or by using a curved mirrored surface.



Concentrating Solar Power
"Concentrating solar power plants produce electric power by converting the sun's energy into high-temperature heat using various mirror configurations."

Arba Minch Solar Cooking Initiative
"...experiences on promoting the use of a simple parabolic solar
cooker to save firewood in Ethiopia."


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REFRACTION - Light bends. Another way to concentrate light energy is to bend the rays of light so that they gather together (focus) at one particular spot. We use this most often in lenses.


    An interesting lense design is the fresnel lense. August Fresnel, a French mathematician and physicist of the early 19th century, duplicated the effect of a huge convex lense using concentric prisms. In this age of plastic, a relatively flat plate of thin circular prismatic lines can be stamped out on a plastic sheet. Fresnel lenses are now sold in office supply stores in 8" x 11" sheets as page magnifiers, for approximately $2.00.

Fresnel Lense

    Lenses can be used not only to concentrate solar energy for heating purposes, but also to boost the amount of energy falling on solar cells.

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ABSORBTION...AND RE-RADIATION  - Light enters in. We know from color studies that what we perceive as colors are actually mixtures of reflected light photons. When light strikes an object, some of the wavelengths of light energy are absorbed, some are reflected.

    We perceive leaves as green because they are reflecting radiant energy of a wavelength that our eyes and brains "see" as the colour green. The other light energy wavelengths are being absorbed by the leaves.

    At the extremes of colour reflection are the "colours" white (all colours combined) and black (no colours at all). A white object is reflecting nearly all of the light energy falling on it. A black object is absorbing nearly all of the light energy falling on it. Which colour of object will get warmer in sunlight?

    Black collector plates and black cooking pots are most often used in solar heat collectors or cookers.

    Light energy absorbed is re-radiated as lower wavelength heat radiation. An object of any temperature will radiate energy as electromagnetic radiation. The hotter an object is, the shorter the wavelengths of electromagnetic radiation that it emits (and the higher the energy per photon). The Sun radiates energy from its heated outer gases at about 5900 degrees Kelvin, and so emits energy peaking in the short wavelengths of our visible light range.
    When an object is heated up on the Earth's surface, by absorbing solar energy, it is never as hot as the sun. The electromagnetic radiation it emits will be of lower energy and longer wavelengths.

See also, as mentioned before :
Electromagnetic Radiation, Temperature, Colour, Light
[This is a great educational site, but the interactive applets take a while to download. Well worth the visit.]

   We, who are existing at about 300 degrees Kelvin* (heated by the plant-absorbed solar energy released by digestion), radiate energy in the far-infra-red.

    [*A degree Kelvin is the same size as a Celsius degree, but the Kelvin scale starts at Absolute Zero, rather than the Celcius scale which has zero as the freezing point of plain water...which is at 273 degrees Kelvin.]


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TRANSMISSIVITY - Light goes through. Some materials allow light pass to through them. We call these transparent. Good examples are window glass, and clear plastic.

    But the heat stays back. There is a feature of glass that allows us to trap solar energy. Glass is somewhat like a one-way valve to solar radiation. It allows the wavelengths of light and near-infra-red from the Sun to pass through it, but it is not as transparent to the longer wavelengths of farther-infra-red radiant energy emitted by solar-heated objects behind the glass. Glass reflects back most of these longer wavelengths of infra-red radiation.

    What this means is that energy can build up in a glass solar energy trap. While energy is coming in all the time, it can't re-radiate back out directly through the glass. Most of us have experienced this effect in a car in summer-time, or in front of a house window through which the sun is shining. It's the working principle of greenhouses, and solar box cookers.
    Selective films allow us to control to some extent which wavelengths enter, which are reflected. Different glasses can allow different wavelengths of radiation to enter.

Principles of Solar Cooker Design
[This is the best I've found for explaining solar energy principles clearly. There is simply nothing like building a solar cooker to get hands-on experience with solar energy, building techniques...and cooking!]