Solar energy

Posted Wednesday, March 20, 2013 in Sustainable Maine

Solar energy

Solar flare, courtesy NASA

by Paul Kando

The Sun is a mass, about 330,000 times that of Earth, of hot plasma and magnetic fields. Plasma is matter in an ionized state, distinct from the gas, liquid and solid states.  About three quarters of the plasma mass is hydrogen, the rest is mostly helium, with smaller amounts of heavier elements. The Sun came into being through the gravitational collapse of part of a large molecular cloud 4.6 billion years ago when most of its matter concentrated in the center and the rest evolved into today’s planets. Under the immense pressure of gravitational forces the central Sun-mass became sufficiently dense and hot (27,000,000ºF in the core, 11,000ºF at the surface) to sustain the thermonuclear fusion of some 620 million metric tons of hydrogen per second, releasing enough energy to power the U.S. for 9 million years.

Because of Earth’s elliptical orbit around the Sun, the Sun-Earth distance varies through the year. At the mean distance of 93 million miles it takes 8 minutes and 19 seconds for sunlight to reach Earth, packing 1,360 Watts of energy per m2 of Earth surface (126.4 W/ft2). Some of this energy warms the atmosphere. Only 1,000 Watts per m2 (92.9 W/ft2) reaches the ground, more than enough potentially useful energy per hour to meet annual world demand. We can harvest it as heat: objects warm in the sun because electromagnetic radiation, (sunlight), increases molecular vibration, commonly called “temperature”.  We can also convert sunlight directly to electricity through photovoltaics, or to mechanical energy by heat engines or wind and water turbines. In turn, mechanical energy can be converted to electricity. The potential of wind alone is several times the world’s total energy demand.  A sunny Midcoast Maine roof receives an average 4.51 kWh/m2 (0.42 kWh/ft2) of solar energy per day – on par with Georgia and the Carolinas and more than either Pennsylvania or Germany.

Solar energy supports almost all life on Earth, powering the photosynthesis plants use to make sugars from ground-water and air-borne carbon dioxide. The sugars form the bodies of plants and provide food for animals that eat the plants. The byproduct is oxygen animals breathe. Life on Earth hangs in the delicate balance between solar heat and an atmosphere that moderates it. The key is just the right amount of carbon in the atmosphere. Comparing Earth and two fellow planets illustrates the point: The atmosphere of Venus is 96% carbon dioxide, and its global average temperature is 887ºF. Mars, with virtually no carbon in its atmosphere (it is sequestered underground), has an average temperature of minus 80ºF. Venus is too hot, Mars too cold for life as we know it.  Earth’s comfortable 59ºF average temperature results from its atmospheric CO2 content of just 0.003%.

By powering the water cycle, solar energy drives Earth's climate and weather. Warming the air it creates wind: as warm air rises cool air rushes to replace it. It heats water and evaporates it. Warm air absorbs the vapor, only to release it when it cools -- thus relative humidity and precipitation.  As water vaporizes from the warming ocean surface, lots of energy is absorbed by the vapor, which, in turn, is absorbed by the warm air. Earth revolving on its axis adds the familiar circular pattern of storms. The absorbed energy, released when the vapor condenses into rain, provides a hurricane’s awesome power. The heat of evaporation of water is 970 Btus per pound. Imagine the number of pounds of water in Hurricane Sandy’s downpours!

Scientific understanding of the Sun and its climate impact came about slowly -- 19th century scientists still had little knowledge of the Sun's composition and source of energy. But the enormous power the Sun exerts over the Earth has been recognized since prehistoric times. Many cultures worshiped a Sun-god. Others hung on to other myths. As recently as the 17th century, Galileo Galilei was prosecuted as a heretic for documenting that Earth orbits around the Sun, not the other way around. Is it so surprising that some among us – including elected politicians – still have a hard time acknowledging the fact that global warming threatens climate change, and our release of 20 billion extra tons of CO2 per year into the atmosphere by burning fossil fuels is enough to upset the planet’s life-sustaining balance? It takes mental capacity and effort to reason and know, not much to heave to some belief.

We know how to reduce our energy consumption by 75-90%, in houses, transportation and industry. We know that, courtesy of the Sun, there is plenty of renewable energy available to replace all the fossil fuel we use, and we know how to harness it. We see others creating jobs acting on this knowledge.  So, let’s not waste time on those paid to deny facts and let’s forgive those who lack the brains to observe them.  Let’s act. The hour is late. 

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