A couple of my friends joined me for breakfast the other day to discuss various forms of Green Energy. The term “Green Energy” is no more self-descriptive than “good energy”, thus perhaps it is best first to sort out some of the related terminology.
The term “Sustainable Energy” tries to capture energy producing processes that do not deplete some non-renewable resource such as oil, natural gas, coal, or even wood. I suppose one should add nuclear fuels such as uranium, as the earth only has a finite amount of it as well, but perhaps it’s not politically correct to include it due to the problem of radioactive waste. Instead, sustainable energy comes from producing energy from renewable resources such as sunlight, geothermal heat, wind, tides, river flow, waves and tidal movement. I suppose we could argue that these aren’t really renewable, but we’re really talking about thousands of years and certainly not millions or billions of years. Coming back to nuclear, today’s nuclear power generators use fission, i.e. the breakdown of large atoms into smaller ones to generate energy. There is a process, yet to be made practical on earth, called fusion. Fusion generates energy by combining atoms, e.g. combining forms of heavy hydrogen to get helium, some neutrons, and a lot of energy. Fusion occurs in the stars, including our sun. In theory, fusion is sustainable.
Breeder Reactors have a nuclear process to produce energy and fissionable fuel. Some breeder reactors use thorium and produce uranium. This is interesting, since thorium is more abundant on earth than uranium. If the amount of fuel consumed by weight is N grams and the amount produced is n grams, then the ratio n/N is called the “breed-up ratio”, usually expressed as a percentage. Technology improvements have taken this ratio from around 30% to about 70%. Sadly, breeder technology coupled with reprocessing technology can be used to produce material for nuclear bombs, and thus not only is nuclear waste a tough political problem; it is coupled with tricky nuclear (weapons) containment policies.
There is another class of energy producing processes that aren’t really sustainable, but yet are often considered “Green”. These relate to converting waste to energy. Movie buffs might remember Mad Max, Beyond Thunderdome, with Mel Gibson and Tina Turner. In that movie methane gas from human and animal waste was captured and burned to create energy. This process is quite practical, and the methane production can be increased by putting the waste into anaerobic “digesters”. Many waste treatment plants do this on a huge scale, and then power generators by burning the resulting methane. Such digesters need not be huge. In fact they can be small enough to serve a single household, and these are mildly popular in rural areas. Applying energy to burn the remaining waste can still result in a net energy production. These basic ideas give rise to a spectrum of energy producing processes from harvesting pure methane, to impure methane, to just burning the remaining waste. I’m reminded by another movie, Back to the Future, Part 2 where Doc returns to the present with his time traveling DeLorean, but this time the DeLorean is powered by waste taken out of household trash cans. This technology has a ways to go for that.
I’m not sure how to classify the processing of corn and sugar cane into ethanol. This is somewhat renewable, since the corn and sugar cane plants can be replanted after harvesting. The political upside with this is that we mix this ethanol with our gasoline, and thus lower the price of such a mixture by reducing the amount of gasoline we burn. The major political downside of this is that it raised the world-wide cost of this food. Since some of this food is used to feed farm animals, it indirectly raises the cost of these animal byproducts. I guess people can argue as to whether the good outweighs the bad so that this energy production can be called Green.
A final form of Green Energy isn’t really the production of energy, but rather it is the production of hot water. Such mechanisms driven directly by sunlight are very popular in desert areas, such as Israel, and they are becoming increasingly popular in the US. Heat can also be obtained as a byproduct of other energy producing processes such as solar and nuclear, and this heat can sometimes be captured into usable hot water.
“We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature’s inexhaustible sources of energy–sun, wind and tide. I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that.” Other wonderful quotes of Thomas Edison with attributions can be found in http://en.wikiquote.org/wiki/Thomas_Edison .