Sunshine and Energy

Energy is the key to the development of humankind. With the taming of fire, early humans learned to cook their food and use fire for warmth and security from animal attack. Wood from trees and bushes became a commodity. This commodity was either gathered or used within a local tribe or, when early humans settled down with agriculture, some group supplied the wood. Wood was burned directly (as it is now in many parts of the world), or it was turned into various products: bowls, utensils, etc. Later, wood was used to make boats and other tools necessary for existence. These tools included throwing sticks (atlatl), spears, and later bows and arrows. Fire from wood also allowed metalworking and the creation of harder and more useful tools made from bronze, copper and other alloys allowing humankind to advance out of the Stone Age.

Of course wood was used for housing and many other uses. Inevitably, as the population grew, there wasn’t enough wood to supply everyone and the land was virtually denuded. England’s forests were mostly lost, and when explorers encountered the new world, one exclaimed that “a squirrel could run from the Atlantic Ocean to the farthest of the great lakes without ever touching the ground”! What’s even worse, as this limited natural resource was used up, conflicts arose. For example, on Easter Island, it is believed that wars broke out between the various tribes until only a few people were left as the wood ran out. That is only one of many stories that are going on even today. But wood was essential to human survival, so why did it run out? That’s simple; trees were being used up faster than they could grow. Trees grow as their leaves turn sunlight into wood via the process called “photosynthesis”. A leaf converts less than 1% of the sunlight that falls on it into cellulose to make the tree grow. An average pine tree, for example, takes 10 years to grow to harvestable size.

How did humans handle that insoluble problem? They discovered coal and oil. At first, some wood was turned into charcoal (a process that’s still in use today) by burning it in a semi-sealed furnace where there wasn’t enough oxygen from the air to completely burn up the wood. Charcoal was easier to carry than bundles of wood, so that was a step forward. They also discovered that peat would burn and that became a commodity fuel. Finally, deposits of coal were discovered. The coal came ready for burning, all one had to do was light it. In other places, a gooey substance was found oozing out of the ground in places called “seeps” was discovered and called oil or pitch. This material has been used for at least 5,000 to 6,000 years. The earliest users were the Babylonians, Assyrians and Sumerians. American Indians used oil that they skimmed off the surface of streams or lakes. As far as oil is concerned, its future was set with the discovery of pools of oil under the ground in Titusville, Pennsylvania in 1859.

But let’s step back a little – say about 300 million years ago to the Carboniferous Period in the Paleozoic Era. This period was before the dinosaurs and ran from 360 to 286 million years ago. The land was swampy and filled with plants and trees and the oxygen content of the atmosphere was high, encouraging this growth. Of course, the trees and plants died and sank to the bottom of the swamps. There they apparently formed layers of what we now call peat. Over many thousands of years, the peat was covered by sand and clay that turned into sedimentary rocks. As time continued, more of these materials were deposited and the pressure on the peat increased, squeezing all the moisture out of it. Over millions of years, it turned into coal. Some turned into oil or asphalt and natural gas as well. Some coal deposits existed at the time of the dinosaurs, 65 million year ago. But the fact remains, it took around 200 million years to make all of the earth’s coal.

What’s the story on oil? There the tale is a little different from that of coal. Oil is believed to have been created by small creatures called plankton that lived in the earth’s oceans during the Jurassic Period about 180 million years ago. Algae and diatoms may also have been involved. However, these creature which are smaller than the head of a pin, capture solar energy and use it to convert carbon dioxide and water into oxygen and carbohydrates through photosynthesis in a way similar to trees. When they died, they fell to the bottom of the oceans and were gradually covered over by sediments. Similar to trees turning into coal, as more sediment was deposited, the pressure got higher and the carbon in the plankton bodies was turned into oil. Because this occurred deep within the ocean, the process took place with less oxygen, and like making charcoal from wood, oil resulted rather than coal. Some of these sediments were porous and, as time went on harder rocks were deposited on top of them. The oil that was in the porous rock began seeping up into pools that were trapped under the hard, or cap, rock. That’s why oil occurs in large pools. Scientists are still seeking to understand this process, but the fact is that it took about 200 million years for this to happen.

What’s happening now? Well, it’s pretty obvious, we’re using up oil at a prodigious rate and the same is true for coal. The good news is that there is quite a bit of coal that remains. The bad news is that the same isn’t true for oil. The oil-rich companies know that. In fact, it appears that the production of oil in the entire world has peaked a decade or two ago and production is now declining. That means that we’ll be running out of oil in a relatively short while. That’s a really scary thought: it took the earth 200 million years to create oil and we will use it up in only a couple of hundred years! The same is true for coal. These hydrocarbon fuels are not being created now, so they represent a finite resource. When it’s gone, it’s gone.

That’s a really scary thought, but what are our options. After all, energy is essential to life and to economic well-being. The one resource that we will have at hand is solar energy. It created and sustains the earth. It was responsible for creating all the hydrocarbon fuels we have now. It provides energy for the crops we grow and, most importantly, won’t run out until the sun uses up its nuclear fuels a few billion years from now! When that occurs, people won’t be around anyway because the sun will first turn into a red giant star and incinerate all the planets! Thus we don’t need to worry about sunlight running out on us!

How can we use sunlight for energy? There are several ways: use it to heat water. We can focus the sunlight an use it to boil water or other fluid and turn a turbine to generate electricity. We can use this high temperature heat to make chemicals, including fuels. Or we can use a material called a semiconductor and turn it directly into electricity with no moving parts. There are other ways we can use sunlight to our advantage, but let’s talk about the photovoltaic effect in semiconductors. Photovoltaic is a word that means “light creating a voltage”. The photovoltaic effect was first observed by Alexandre-Edmond Becquerel in 1839. This effect amounted to no more than a curiosity until, in the 1930’s and 1940’s, the effect was observed in the semiconductor element called silicon at the Bell Laboratories by R.S. Ohl. The modern silicon solar cell was invented by Chapin, Fuller and Pearson, also at Bell Laboratories in 1954. These early devices converted sunlight directly into electricity with an efficiency of about 5%! A small number, but it is a gigantic increase over the photosynthesis process and the conversion of sunlight into wood, or ultimately coal and oil. The efficiency of those processes cannot be determined but in infinitesimally small. After all, the solar cell produces electricity the instant sunlight strikes it. With a tree, it has to grow for 10 years to be useful, not to mention the time it took to create oil or coal.

Let’s do some simple comparisons to put the advantage of photovoltaic (PV) cells over energy created by crops or trees. First though, we need to define some terms and get some background numbers. For example, each square meter of ground in the U.S. receives about 5 kilowatt-hours of solar energy each day. A square meter is about 10.7 square feet. The peak intensity of sunlight is about 1 kilowatt/square meter (kW/m2), but it is only that bright near noon time. It’s dim at sunrise and sunset and clouds and rain over the year reduce the amount of sunlight. The easy way to think about it is that the average length of peak sunlight over a full year averages only five hours. Thus we get the average annual energy deposited by sunlight to be 1 kW/m2 times 5 hours duration or 5 kW-h/m2/day. In any case, that’s how the averages work out, but we can use this number to do some interesting comparisons with trees and crops.