Science Forums 2

[CraigD]

Originally Posted by Not half, but whole!

What truly are the risks of tinkering with these systems?

As best I can figure, the risk to the gross mechanical energy of these systems is insignificant, the smaller-scale ecological risks significant, but, with research and planning, manageable.

It’s always a good – nay, essential – idea to determine as accurately as possible the consequence of any act of engineering. That said, the impact of most of the alternative energy technologies NHBW mentions – wind, geothermal, and tidal (which I think should be consider more broadly as ocean currents in general) – appears to be low, because the maximum human power requirements are very small compared to the available power and its natural variation, and because these supplied of energy are effectively inexhaustible.

At first order approximations, a major ocean current system such as the gulf stream has about J of kinetic energy, while the atmosphere has about J. The Earth’s total power (energy/time) from the Sun is about W. All artificial energy consumption by humans – transportation, electric, etc. - is about W. (sources: wikipedia articles “Earth’s Energy budget”, “Orders of magnitude (power)”)

So, human currently consume only about 1/10000th of the available non-exhaustible energy on Earth. If the Sun were to stop supply the Earth with power, and all of the world’s energy were obtained from a natural source which were somehow freed of natural sourced of drag (an unrealistic scenario, but useful for this illustration), the Gulf stream would last about 2 minute, wind about 2.5 hours, illustrating that these systems have a considerable amount of “inerta”.

Intuitively, one should consider natural features that currently have an effect indistinguishable from artificial ones that could provide human-usable power. For example, the aerodynamic effect of a large tree is about equivalent to that of a 10 m, 10000 W wind turbine. If all human energy needs were met with wind turbines (another unrealistic, but illustratively useful scenario), about 150 million such turbines would be required. I’m unable to find an estimate of the number of large trees in the world, but extrapolating from my neighborhood, where there seems to be one at least every 10000 m^2, there should be on the order of 10,000 to 100,000 million worldwide, or about 100 times as many as the number of number of equal-effect wind turbines required to supply all human energy needs. (source: wikipedia article “Wind turbine”)

The main difficulties with these technologies, IMHO, are economic – with rare exceptions, they are more costly than fossil fuel burning, so until fossil fuel becomes much more expensive, can’t be afforded in our current market-driven economy.

Adverse ecological impacts, although insignificant in terms of effecting the large scale energy dynamics of natural systems, can be considerable in terms of impact on local fauna and flora – eg: killing fish in water turbines, killing birds in wind turbines. Tidal power systems, attractive because they permit water current power generation near large coastal power consumers (eg: cities), can upset local ecologies by changing the normal tidal patterns. Geothermal power generally is a closed heat engine with nearly no ecological impact other than waste heat, but is a small resource compared to these others.

In summary, on a gross mechanical level, we humans should be able to increase our power use by about a factor of 10,000 without exceeding the amount of effectively inexhaustible energy on Earth.