Friday, June 11, 2010

It's Electric

The funny thing about “energy” is that it is neither created nor destroyed; it only changes form. So when coal is burned at a power plant the coal’s chemical energy is ultimately converted into electricity. This process is typically only 35% efficient ; wasting approximately 65% of coal’s energy when converting its chemical energy into electricity. Basically the heat from the fire is used to boil water, which is used to turn a turbine, which turns a magnet inside a coil of wires to produce electricity. So heat from burning (considered chemical energy) is converted into mechanical energy by turning the turbine which is then converted into electrical energy simply by moving a magnet through a coil, losing energy at each step along the way.

As the world becomes more energy conscious, we are seeing new technologies that convert mechanical energy into electricity. It’s pretty simple to convert mechanical energy into electricity so I encourage you to use your imaginations and invent stuff. Wind farms already exists (many on the Tug Hill Plateau west of here) that convert steady wind into electricity as do hybrid vehicles that use the rolling wheels while breaking to produce electricity. You can sort of think of a coal-fired power plant as a fancy way to create a steady wind that will turn a turbine to produce electricity.

Solar energy conversion does not rely on mechanically moving a magnet through a coil of wires as with many other electricity producing methodologies. It relies on the sun to induce electron motion inside a semiconductor device. Solar energy conversion is very attractive since it is passive and does not rely on burning carbon. However, it’s relatively expensive and requires sunlight. I’m still amazed at the magic of solar power. Think about what we are asking a solar cell to accomplish. We want to create electrical energy from a piece of semiconductor that has no moving parts and we want it to last more than 20 years being blasted by the sun daily and we want it to be cheap and reliable. I would not last 20 years if you put me in the sun everyday and asked me to reliably produce and I think I’m a pretty sophisticated machine, as far as humans go.

How does a solar cell do it? There are two key components to solar cells, the semiconductor material from which it is made and the special junction between two semiconductor materials which acts as an electrical one-way sign. I will leave detailed physics and electrical engineering explanations of solar technology to other more eloquent sources. Instead I will opt to explain it through interpretative dance…. of course.

Imagine a dark stage with dancers on the floor in fetal positions; this is our un-illuminated solar cell. As you watch the motionless dancers in dark silence you notice a line that divides the stage in half and the line has a large one-way symbol pointing to the right. As the stage lights come up, our dancers awaken and begin to randomly walk around the stage. This represents the motion of electrons in a solar cell once the sun is shining. You marvel at the dancers’ movements under the bright lights of the stage and notice something curious. Although each dancer (a.k.a. electron) has a random motion, they are all beginning to congregate on the right hand side of the stage because after they cross the center line, the one-way (P/N junction) does not let them randomly walk stage left. So the stage is loaded with flying dancers on the right while only a few remain on the left. This represents a charge separation and will produce electricity if “released”. So, the last step in our geek-fest interpretive dance of the solar cell is to picture a tube (representing an electrically conducting wire) placed on the stage that allows the dancers on the right to crawl through and get back to the left where there is more space. Many of them crawl through the tube and as they do, you realize that they represent electricity flowing through a wire and are awestruck by the simple elegance that is a solar cell. As the curtain closes you clap, cheer and exhale greenhouse gases to show your appreciation.

Hope this helps to explain how things work. I figure there is never a wrong time to invoke an interpretive dance analogy when explaining complex physics processes.

Wednesday, June 2, 2010

Oil and Water

You may have heard that oil has been leaking in the Gulf of Mexico for weeks as a result of a rig accident and is causing an environmental nightmare. News about global warming and the effects of burning fossil fuels are all over the news. We are bombarded by dire predictions that we are running out of energy and that we need to reduce our dependence on carbon-based fuels. Why is it so difficult to get energy from other sources? To understand why this is tough we need to first understand energy and all its forms.

Energy is an illusive thing. From a physics standpoint it is the ability to do work on a system. Translated into English this means we can use energy to do stuff like propel our cars, heat our homes, power the refrigerator, send a baseball flying over the left field fence, etc. Carbon happens to be a very nice way to store energy. When you burn carbon, it produces heat, a form of energy (as well as water and carbon dioxide a.k.a. a “green-house” gas). Even cavemen knew that fire was good because it can be used to warm the cave. What they may not have realized is that energy is easily stored in the form of wood, oil or other chemical forms. They just knew: “fire good”. Long after the cavemen, electricity was discovered and quickly became popular because it is easy to transport from a central location and can be used to power up many devices that make life easier like toasters, fans, lights, etc. I’ll speak more about electricity later.

Getting back to the Gulf oil leak, the reason oil is so nice is because it can be refined into gasoline to power our cars. Like most chemical sources of energy, this is a very convenient form to store energy in for use at a later time. Have you ever run out of gas and tried to push a car down the road? It requires a lot of energy to move a car. Gasoline packs a lot of energy in a relatively cheap, easy-to-use, liquid form that stores well. Simply stated, it requires energy to move an object like a car from point A to point B. And while gasoline is the most convenient energy source for this task, there are many environmental drawbacks.

It has been difficult to find an alternative power source for automobiles despite all the scientific research. One alternative is Ethanol from corn, sugar cane and other sources. Although there is no chance of a catastrophic oil-leak-type disaster from a corn field, it has other drawbacks and still produces “green house” gasses when burned to power your car. Battery powered cars seem pretty cool. A battery is a nice way to convert chemical energy into electricity without any moving parts but they don’t last very long and currently are really heavy. Don’t be fooled into thinking that batteries don’t contribute to green house gases either as they are typically charged by plugging them into the wall. The electrical energy from the wall socket might come from coal-fired power plants. So I say, just bike it! Remember however that when you bike, you are burning (in the biological sense) calories and producing greenhouse gases each time you exhale.

In summary, chemical energy in the form of fossil fuels are particularly easy to store, use and extract from the ground. The typical way to release the energy from a chemical source is by burning them but remember this produces greenhouse gasses. I’ll consider other energy sources like electrical, nuclear, wind, etc later. Right now though, I have to go chop some wood so I can use it to heat my ‘man cave’ next winter.