Alternative energy refers to any source of usable energy intended to replace fuel sources without the undesired consequences of the replaced fuels. Typically, official uses of the term, exclude fossil fuels and nuclear energy.

Over the years, the nature of what was regarded alternative energy sources has changed considerably.

The term "alternative" presupposes a set of undesirable energy technologies against which "alternative energies" are opposed. As such, the list of energy technologies excluded is an indicator of what problems that the alternative technologies are intending to address. Controversies regarding dominant forms of energy and their alternatives have a long history.

It is interesting to note that coal was an alternative source of heat, to wood.  Europeans had become so skilled at deforestation that by 1500 AD they were running short of wood for heating and cooking. By 1500 Europe was on the edge of a fuel disaster. This changed in the sixteenth century only by the burning of soft coal.

Whale oil was the dominant form of lubrication and fuel for lamps in the early 19th century, but by mid century and the depletion of the whale stocks, whale oil prices were skyrocketing and could not compete with the newly discovered source of cheap petroleum from Pennsylvania in 1859. 

In 1917, Alexander Graham Bell advocated ethanol alcohol from corn and other foodstuffs as an alternative to coal and oil, stating that the world was in measurable distance of depleting these fuels.

In the 1970s, President Jimmy Carter's administration advocated coal gasification as an alternative to expensive imported oil. The program was scrapped when petroleum prices plummeted in the 1980s.

Today, alternative energy commonly refers to renewable energy. Renewable energy is energy generated from natural resources, such as sunlight, wind, rain, tides and geothermal heat. They are renewable, because they are naturally replenished. 

When comparing the processes for producing energy, there remain several fundamental differences between renewable energy and fossil fuels. The process of producing oil, coal, or natural gas fuel is a difficult and demanding process that requires a great deal of complex equipment, physical and chemical processes. On the other hand, alternative energy can be widely produced with basic equipment and naturally basic processes.  Wood, the most renewable and available so called "alternative" energy, burns the same amount of carbon it would emit if it degraded naturally.

Relatively new concepts for alternative energy include floating wind farms, which can float in water, that is nearly 2,300 feet deep, as opposed to the offshore wind farms which are placed in water only 131 feet deep.  The advantages to this, there are no obstructions such as hills, mountains, trees or buildings. And, winds from the open ocean can reach double the speeds of winds in coastal areas.

A couple other new concepts include, biogas digestion and helioculture.  Biogas digestion deals with harnessing the methane gas that is released when waste breaks down. This is usually taken from garbage and sewage system. The other, helioculture, is a newly developed process which is claimed to be able to produce 20,000 gallons of fuel per acre per year.  This concept involves removing the carbon dioxide from the air, and converting it into energy

The world of Alternative Energy, is an ever changing one. It evolves with improvements in technology, as it improves upon technology. The laws of physics are constantly changing, even if just slightly, as our comprehension of this universe expands.