We can use the traditional `elements' to categorize the types of modern `alternative sources of energy'. I find this a useful aid to memory.
Most of the energy sources that come under the rubric of the traditional four elements depend on low density transformations. This means they are expensive:
Aristotle's fifth element can represent a `high density' transformation, nuclear energy:
Like the other alternative sources of energy, nuclear energy is expensive, but for different reasons. All kinds of alternative source are examples of high initial cost development; they will only become widespread with low incremental costs.
Let's look at each element in turn:
Earth is for biological, self-replicating sources, such as wood or sawdust from trees, oils used in cooking and then for diesel fuel, and ethanol from biomass.
In addition, Earth is for geothermal energy sources, both those which occur in certain places, such as Iceland, and those which might be tapped by drilling deep enough. (Geothermal energy may be concentrated: a `high density' source, but readily accessible geothermal energy is rare.)
The European Union could pay for growing biomass in its new members. This would enable the Union to sidestep the Common Agricultural Policy, which is no longer useful, enable richer countries to transfer money to poorer ones, and help reduce net carbon emissions.
This growth cannot provide for more than a small fraction of Europe's energy needs, but it could help.
Poland, 1 for example, currently consumes a bit less than 2 Exajoules per year, 1.85x10^18 joules. It has about 14 million hectares (about 35 million acres) of arable land.
Thus, in theory, if it used all its land for fuel, and none for food, it might well be able to provide for its current energy use. As a practical matter, it could supply only a portion of its energy from biomass.
For comparison, the United states has about 180 million hectares (440 million acres) of arable land. In theory, this land could supply 20 – 40 Exajoules per year, but the US uses approximately 100 Exajoules per year.
Air is for energy from the winds.
As far as I know, the cost of electricity generated by wind turbines is coming close to the cost of electricity generated from coal or oil and in some places is less. The problem with wind turbines is that birds and bats fly into them and die; their noise bothers people (and probably animals); and in the numbers needed, wind turbines look unsightly. Also, none can depend on the wind, and we lack acceptable, large batteries to take up the load during a lull.
Fire is for solar heating and electricity generation.
In many places, solar heating is both practical and cost efficient, so long as the capability is designed into the building at the beginning. Retrofitting is different.
Solar hot water heating is cost effective in some places, but not in others. As a rule of thumb, you can figure that one square meter of collector produces each year the same heat as burning about 40 liters of oil (one square foot is roughly equivalent to one gallon of oil).
At the moment, solar electricity generation is cost efficient only for sites which lack power generated in other ways.
Water is for energy from waves, tides, ocean currents, and ocean temperature differences.
For centuries, people have dammed rivers and extracted energy from them. These are not `alternative sources of energy.' On the contrary. Over the past century, hydroelectric generation has become very important. Many rivers have already been dammed. One of the great questions is whether, over the next few centuries, this method of gaining energy will have been any good? Will the reservoirs behind the dams fill up? Put another way, is hydroelectric power be sustainable? Moreover, is the loss to people, plants, and animals from being displaced and the loss of beauty that is submerged less than the gains, or more?
Leaving aside river dams, researchers are developing devices to make use of the energy in waves and ocean currents. These are lower energy density sources than rivers. It is correspondingly harder to extract energy from them in a cost effective manner.
One suggestion is to put wings in a deep current of water. The device sits below ships. To produce energy the wings generate lift and move up, and then tip to generate lift downwards and move down. Since I own an airplane and depend on wings, I find this method intriguing . I do not know how expensive it will be.
Unfortunately, oceans are rough on equipment. For example, ocean temperature difference machines tend to clog with barnacles.
Aristotle's quintessence is for uranium fueled nuclear power plants and for future hydrogen or hydrogen-boron nuclear fusion plants.
As a practical matter, radioactive waste is a byproduct of both uranium fission and regular hydrogen fusion (the latter because of the neutrons released in tritium reactions). Only hydrogen-boron nuclear fusion is clean. (I leave out helium-3 reactions because helium-3 is so expensive. The closest big source is the moon; the closest good source is the planet Uranus. Helium-3 reactions are clean.)
We know that hydrogen fusion succeeds, since the sun depends on it and humans have built and exploded hydrogen bombs. But controlled hydrogen or hydrogen-boron fusion is another matter. No self-sustaining, inexpensive devices have been made. Perhaps the task is impossible. Or perhaps someone will invent a superconducting magnet that can tolerate even higher fields and build a good reactor. Because sustainable and inexpensive fusion promises so much, I hope it becomes possible. In the meantime, we can consider possible side effects.
One outcome is straightforward: if the price of electricity falls far enough, we will be able to replace fossil fuels. Perhaps we will be able to synthesize ethanol and methane. If so, we will be able to cut back on the amount of carbon dioxide we humans release. This way, we will be able to reduce the effects of extra carbon dioxide on weather as well as compensate for natural changes.
Countries that depend on exporting their oil and natural gas, such as Saudi Arabia, Russia, and Nigeria, will lose to countries that import oil and natural gas, such as the United States, Japan, and China.
Since regular hydrogen fusion plants will give off neutrons, another issue will be how to prevent their being diverted or copied to create material suitable for weapons, such as `dirty bombs'.
Moreover, as a practical matter, support for those forms of energy production that produce strong, long lived radioactive waste must include support for organizations such as The Long Now Foundation, which thinks in terms of a time period longer than human history.
I myself like the idea of hydrogen-boron nuclear fusion. But it is more difficult than regular hydrogen fusion (although I wonder whether boron nuclei will be as unstable in a plasma as the lighter protons). Moreover, I do not know whether hydrogen-boron plants could readily be converted to plain hydrogen fusion plants; if so, they will be dangerous.
The calculations for land area and energy use are based on data from the CIA Fact book's page on Poland. The calculations for biomass energy come from Oak Ridge National Laboratory, which suggests that a bioenergy crop might yield 11.2 tonnes/hectare (5 US tons/acre) and that the energy content of the crop, depending on moisture content, would range from 10 – 17 Gigajoules per ton (4,300-7,300 Btu/lb) or 110 – 200 GJ/hectare.
The conclusion is that Poland could obtain 1.5 – 2.8 Exajoules of energy per year from agriculture, but only if it grew all for fuel and none for food. It currently consumes 2 Exajoules per year.
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