
Originally Posted by
genebujold
5. All the solar, wind, tide, and geothermal sources combined won't amount to but a small fraction of our current needs, much less our future needs.
The sun is good for another 4-5 billion years or so, after which there won't be a planet left for bacteria to inhabit

. The US could supply all it's current electricity needs with a square solar photovoltaic array about 300 miles on a side ... and that's with panels that are only 10-11% efficient at converting sunlight to electricity. Experimental cells have demonstrated efficiencies in the 30% range so it's not unreasonable to think that improvements can be made in this area.
On the other hand, all the solar panel ever made would only form an array about 10 miles on a side so there is a long way to go. The bigger challenge with solar PV is building storage systems so that you still have power at night. This can be done on the scale of a single family residence but it's not clear how big storage systems can be made. As a result wind turbines will be an important component of our energy future ... the wind is always blowing somewhere. In addition, if you start putting solar arrays in space and beaming the power back to earth then the amount of power that you can potentially collect gets much larger.
They key, IMHO, is realising that a social and economic system based on continuous and therefore unlimited growth is not conducive to long term survival. Here's an example:
It is possible to calculate an absolute upper limit to the amount of crude oil the earth could contain. We simply assert that the volume of petroleum in the earth cannot be larger than the volume of the earth. The volume of the earth is 6.81 x 1021 barrels, which would last for 4.1 x 1011 yr if the 1970 rate of consumption of oil held constant with no growth. The use of Eq. (6) shows that if the rate of consumption of petroleum continued on the growth curve of 7.04 % / yr of Fig. 2, this earth full of oil will last only 342 yr!
http://www.npg.org/specialreports/bartlett_section3.htm
By a similar analogy, you can show that if we had fusion and we increased the amount of fusion generated by 2-3% per year it would not be long (a few hundred years) before the amount of power generated became a sizable fraction of the total amount of energy received from the Sun.
At some point, humanity has to figure out how to start living a "steady-state" existence. Physical systems that don't tend to a steady-state from some set of initial conditions either destroy themselves or oscillate. Why should humanity be different?