Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena, Germany, says that efforts to satisfy a large proportion of our energy needs from the wind and waves will sap a significant proportion of the usable energy available from the sun. In effect, he says, we will be depleting green energy sources. His logic rests on the laws of thermodynamics, which point inescapably to the fact that only a fraction of the solar energy reaching Earth can be exploited to generate energy we can use.At some level, this is just "Science from the Duh! Files". Solar energy captured to generate electricity is not available to warm the earth (at least in that location), or to grow plants. A wind generator extracts energy from wind, and in the process, slows the wind. A wave, or tide generator does the same. There is a finite amount of energy that can be extracted from any such sun driven process, and long before that ultimate amount of energy is achieved, the extraction will begin to have significant consequences. Slowing the wind significantly would have dire consequence for the climate, since winds (and their oceanic equivalents, currents) are the mechanism for distributing heat around the globe, cooling the tropics, and taking it to the poles where it is lost to space as radiation. Waves are simply a consequence of winds, and suffer a similar limitation. Tides are also dependent on a steady input of gravitational energy from sun/moon/earth interactions, and using much of that energy would substantially change how tides act, which is globally interconnected. How likely are we to get close to these extreme levels of energy extraction?
When energy from the sun reaches our atmosphere, some of it drives the winds and ocean currents, and evaporates water from the ground, raising it high into the air. Much of the rest is dissipated as heat, which we cannot harness.
Humans currently use energy at the rate of 47 terawatts (TW) or trillions of watts, mostly by burning fossil fuels and harvesting farmed plants, Kleidon calculates in a paper to be published in Philosophical Transactions of the Royal Society. This corresponds to roughly 5 to 10 per cent of the free energy generated by the global system.So if we were to switch to solar, wind, tidal and completely at our current energy use we would be using 5-10% of the available free energy of the system? Sounds like enough to start having significant consequences.
"It's hard to put a precise number on the fraction," he says, "but we certainly use more of the free energy than [is used by] all geological processes." In other words, we have a greater effect on Earth's energy balance than all the earthquakes, volcanoes and tectonic plate movements put together.