One of my students at UNT asked me the following question: "What is the difference between alternative energy, renewable energy, sustainable energy, and green energy?"

To me, these terms represent a spectrum of energy technologies ranging from the least environmentally friendly to the most. My gut reaction went something like this:

"Alternative" refers to any non-fossil-fuel energy source, but it does not necessarily have to be renewable. Nuclear energy is one example, since the available fissionable material on the planet is finite. Geothermal energy, which can be exhausted if seismic events disrupt heat sources, is another alternative energy example.

"Renewable" energy arises from sources that are consumed at the same rate they are produced. However, these sources are not necessarily sustainable because they cause pollution. Combustion of organically-derived products falls into this category; for example, burning bio-diesel for transportation. The fuel is derived from sugar, corn, and other plants, which can be re-grown. However, the products of combustion beyond carbon dioxide and water (i.e., unburned hydrocarbons and NOx) are not absorbed by plants as they grow. So, these pollutants can build up in the atmosphere over time.

"Sustainable" refers to energy sources that create no pollution in the conventional sense but may have other undesirable environmental consequences and hence are not green. Wind power is one example, given environmentalists' concerns about migrating birds being struck and killed by rotating turbine blades. Hydroelectric energy is another example. Damning rivers provides a continuous flow of energy but destroys natural habitats and dramatically upsets the surrounding environment.

"Green" energy is the most environmentally benign. It creates no pollution and has no detrimental environmental impacts. Photovoltaic and solar-thermal energy closely match this definition. However, even these technologies do induce some environmental impact. The fabrication of PV panels consumes energy from non-renewable sources and produces pollution. Solar installations block sunlight that would otherwise reach the ground. Therefore, these installations (especially large ones) can dramatically impact the surrounding environment.

Of course, these definitions merely represent my opinions on the meanings of alternative, renewable, sustainable, and green. I am eager to hear other opinions from blog readers on what these terms mean to them.