Renewable Energy or a Perpetual Motion Machine?

Axel Kleidon of the Max Planck Institute has concluded that it is a mistake to assume that energy sources like wind and waves are truly renewable.
His analysis says that if you build enough wind farms to replace fossil fuels, we could seriously deplete the energy available in the atmosphere, with consequences as dire as severe climate change.

It appears that the idea that we can harvest unlimited amounts of renewable energy from our environment is as much of a fantasy as a perpetual motion machine.

Humans currently use energy at the rate of 47 terawatts. Axel calculates that this is roughly 5 to 10 per cent of the “free” energy available. About 17 TW comes from burning fossil fuels. So to replace this, we would need to build enough sustainable energy installations to generate at least 17 TW, but because no technology can ever be perfectly efficient, some of the free energy harnessed by wind and wave generators will be lost as heat.

Axel says "Large-scale exploitation of wind energy will inevitably leave an imprint in the atmosphere." "Because we use so much free energy, and more every year, we'll deplete the reservoir of energy." He says this would probably show up first in wind farms themselves, where the gains expected from massive facilities just won't pan out as the energy of the Earth system is depleted.

Axel says that we are going to need to think about fundamental principles much more clearly than we have in the past. He says "We have a hard time convincing engineers working on wind power that the ultimate limitation isn't how efficient an engine or wind farm is, but how much useful energy nature can generate."

The answer may lie in direct solar energy, but to be sustainable, it will probably have to be based on cheaper and more readily available materials such as zinc and copper. Unfortunately many of the most efficient of today's thin-film solar cells require rare elements such as indium and tellurium, whose global supplies could be depleted within decades. The trouble is that current systems convert only a small fraction of the light that hits them to power, and absorb most of the rest, passing heat into the environment.

New Scientist - Wind and wave farms could affect Earth's energy balance

As with everything there must be a balance somewhere. But this type of article does make one stop and think.