Hi All

Raoul's discussion of tidal power coincides with some calculations I did on the Earth-moon system's evolution. Currently the Earth dissipates about 3 TW of its rotational energy as friction against the tidal bulges raised by the Moon in the shallower parts of the oceans. The human race currently uses about 15 TW-years per annum, but could use a lot more - about 75 TW-years would bring everyone up to US usage levels. If the population doubles before it hits equilibrium then we'll need 150 TW-years. Imagine if we derived the bulk from the tides - how long would it take to brake the Earth's rotation relative to the Moon?

At current natural loss rates the Moon will retreat to a stationary orbit of 50.7 days at ~ 581,000 km in about 15 billion years. Earth's day will then match the month in length. But if humans tapped the Earth's spin at a constant ~ 150 TW, then the Earth will brake to a relative halt in ~ 45 million years. Wild idea.

What if power demand kept rising at its current 2.6% pa? The Earth's total rotational energy is 6.75 billion TW-years. Hence the Earth will brake to a relative halt in a mere ~ 882 years. Of course Earth will be glowing like the Sun at ~ 7700 K, but hey that's exponential growth for you...

Adam

BTW Demand at ~ 2.6 % growth will surpass the Sun's terrestrial input of 174,000 TW by the year 2365 AD. We will surpass the biosphere's usage of solar energy ~ 0.5% by 2160. The doubling time is ~ 27 years, and we'll be using 100% of the Sun's power by 3205 AD at that pace... reductio ad absurdum