2006-03-31 / Columnists

Drawing On Science


Alternative Sources Of Energy, Part II
Alternative Sources Of Energy, Part II

by Stephen Yaeger

Last month, we wrote about Ethanol driven cars and hydrogen cells. (Confusing, wasn't it?) Now let's see how Solar power, wind power and geothermal energy are and can be used as alternative (renewable) energy sources.

The sun's energy has been harnessed by green plants, as part of the process of photosynthesis (see column of 9/17/ 2004,) since they appeared on Earth. Now, we have a chance to use the same energy as an alternative energy source. This can be accomplished through the use of photovoltaic cells or modules (=a group of cells packaged as a single unit). If you have a calculator that is powered by light, it is the photovoltaic cell that makes it possible. The photovoltaic cell uses a semiconductor , which loses electrons by absorbing the sun's (light) energy. The most common semiconductor is silicon (Si), element number 14 on the Periodic Table of Elements. Silicon is a metalloid or semimetal, which makes it, along with other semimetals, a good semiconductor. It has three electron energy levels or shells. The first two are completely filled, but the last energy level has only four electrons. Now Si would "prefer" to have eight electrons in this outermost energy level (Why? Remember your high school chemistry?), so it shares electrons with four other Si atoms forming a crystalline structure.

Unfortunately in order for Si to act as a conductor of electricity, it must be able to release its electrons. But unlike copper (Cu), for instance, none of its electrons are free to move about-they are locked up in the crystalline structure. Light energy can knock some of the Si electrons out of place and become, for a moment, free to move around producing an electric current. There are too few of them and the current that they produce is quite small. They soon fall back in place. So, what to do? Simple, just introduce other, non-silicon atoms to the crystalline structure. These are known as impurities and the process is called doping . Phosphorus (P), element number 15, makes for a good impurity as it has five electrons in its outermost energy level. If one of these P atoms is added to, say every million or so Si atoms, it can bond with three other Si atoms. And here's the good part: it still has one electron, which is an "orphan." Get it? Since the orphan electron is not bonded to anything (except it is held in place by a positive proton in the P atom) it takes less energy to knock it loose from the atom. So there are many of these electrons now able to roam freely. When Si is doped with P it is known as n-type (=negative) Si because of the free orphan electrons. Are you thoroughly confused? No? OK, let's continue.

Now, what happens if we add boron (B) to Si? Three Si atoms, then, bond with an atom of B which has only three electrons in its outermost shell. Now there is an opening or hole, which can be filled by a free electron. This is known as p-type (=positive) Si. When we put the n-type and p-type silicons together an electric field is formed. This allows for the free electrons in the n-type Si to rush over the p-type Si to fill in the openings. The n-type zone becomes positively charged and the p-type zone becomes negatively charged. By adding metal contacts to the two zones a diode is created.

If light is absorbed by the cell photons (=packages of light energy) come in contact with electrons close to the electric field, electrons will be set free to move to the n side. If an external circuit is provided then the electrons will flow back to the p side to fill in the holes that were created by the loss of the electrons. In the process a current is produced to do work. That's why your solar powered calculator works without batteries.

Wind is a form of solar energy. It is the sun that warms up the air causing it to circulate, but we must consider land irregularities (such as mountains and valleys) and the rotation of the earth also, which results in the Coriolis Effect (=global wind patterns). Wind is commonly used for sailing and drying off things, but it is also used to generate electricity. A wind turbine is basically a large fan in reverse. Instead of producing wind it captures wind, which is used to convert wind energy into mechanical energy by turning a shaft. The shaft is connected to a generator, which converts the mechanical energy into electrical energy. The electricity is then distributed to homes, businesses, etc.

Finally geothermal energy is harnessed by bringing the heat energy up near the surface. This is accomplished by thermal conduction (=using a heat conducting device). Hydrothermal resources , which is ground water heated by molten rock deep within the earth, may be used most economically to generate electricity. This is done by well-drilling and energy-conversion technology. The hot water may also be used directly for heating homes, etc.

If alternate sources of energy technology is developed to the extent that it can become commonplace, we would not be dependent on non-renewable (and politically controlled) fossil fuels. Another bonus would be the fact that our environment would not suffer continuous pollution and increased global warming.

Questions/comments?

E-mail Steve: Drawingonscien ce@aol.com

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