RESOURCES FOR TEACHERS: The Sun
The sun is the source of all life on earth, powering our giant weather machine and fueling the temperature differences that drive the winds. All plant life, and hence all animal life, is dependent upon its beneficial light and heat. Because of the tilt our earth makes with the sun, we have seasonal changes, and variations in our light, temperature, and weather.
All of our fossil fuels and the current biomass of the earth are also solar derived, having utilized the radiant power of the sun to drive plant growth processes, both now and in the past. Fossil fuels are nothing more than “frozen” sunshine in photosynthetic chemical form, a bank account of past eon’s of the sun’s energy that has been captured for later use—stored solar energy.
The amount of incoming energy from the sun is quite enormous by earthly standards, but rather puny by astronomical measures. Our sun is basically a mediocre star, but even this common class star is capable of converting 4 million tons of hydrogen to helium every second. The sun is a thermonuclear furnace. Like all other stars, it is nuclear powered. We the creatures of earth have learned to adapt to living in the energy shine of a nuclear furnace.
About 1% of the sun’s radiant energy actually intercepts the earth. In scientific terms, this energy input to our world can be expressed as
5,400,000 quads of energy. You may not be familiar with the unit of energy known as a “quad”. It is 1,000,000,000,000,000 Btu; and is often used as a measure to express the entire energy use of nations. For instance, the U.S. uses about 100 quads of energy per year; the world…over 400 quads.
Our current energy consumption appears quite small compared to what our world is capable of intercepting from the sun; but you must keep in mind that this enormous amount of solar energy is spread over Earth’s large surface area, so it is relatively diffuse and must be collected. Solar energy does not come fully ready to use. We must meet the challenges of building systems that can economically collect and store this free energy resource.
This large amount of incoming solar energy gets apportioned in the following manner:
- 27% is reflected back into space by the atmosphere and the clouds
- 10% is absorbed by the clouds
- 9% is reflected from the earth’s surface
- 9% is absorbed in the atmosphere
- 45% is absorbed by the earth itself
Of the energy penetrating our atmosphere, 45% is visible to us as light; another 45% we perceive as heat; and the remaining 10% is high energy radiation that we cannot detect with our senses, representing ultra-violet and other high energy forms.
The visible energy is further broken into two major components: direct sunlight or beam radiation; and, diffuse or scattered radiation. Diffuse or scattered radiation is that component of sunlight reflected off clouds and other natural objects. Diffuse radiation is what we see on hazy, overcast days -the sun’s direct rays being obscured by the clouds.
The sun does not shine with equal intensity over the face of the planet. Some places are gifted with greater amounts and more cloudless days, which make it more economic to use the sun than those places where cloud cover is more common. For instance, in the United States, the southwestern part of the country receives about 1.5-1.7 times as much solar energy as the northeast where cloud cover is prevalent about 50% of the daytime period.
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