A very interesting paper by Axel Kleidon from the Max Planck Institute which calculates the 'free' energy (thermodynamically speaking) in the Earth system. This number is compared to the free energy consumption by humans and the needs we have to convert free energy equivalent to the energy provided by fossil fuels in order to replace them. It turns out that human requirements for free energy - renewables- to replace the fossil fuels are substantial in comparison to the free energy total available.
Thus, unlike what we have commonly believed about wind, solar and tidal, the amount of 'free' energy generated by the energy being delivered to the Earth by the sun is not in practical terms actually unlimited and we need to keep in mind that
if we use the free energy we need to convert our global system to all renewables using today's technologies we will have a substantial negative impact on the global environmental systems.In other words we dare not do it with the technology we currently possess. The author suggests that to obtain the free energy we require "safely" would require that we resort to geoengineering schemes like 'greening the desert' mentioned in one of our earlier topics. Cautionary results and scary options.
A very interesting read.
From news link at New Scientist
Wind and wave farms could affect Earth's energy balance
WITNESS a howling gale or an ocean storm, and it's hard to believe that humans could make a dent in the awesome natural forces that created them. Yet that is the provocative suggestion of one physicist who has done the sums.
He concludes that it is a mistake to assume that energy sources like wind and waves are truly renewable. Build enough wind farms to replace fossil fuels, he says, and we could seriously deplete the energy available in the atmosphere, with consequences as dire as severe climate change.
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 present, humans use only about 1 part in 10,000 of the total energy that comes to Earth from the sun. But this ratio is misleading, Kleidon says. Instead, we should be looking at how much useful energy - called "free" energy in the parlance of thermodynamics - is available from the global system, and our impact on that.
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.
"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...
Of the 47 TW of energy that we use, 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. And because no technology can ever be perfectly efficient, some of the free energy harnessed by wind and wave generators will be lost as heat. So by setting up wind and wave farms, we convert part of the sun's useful energy into unusable heat.
"Large-scale exploitation of wind energy will inevitably leave an imprint in the atmosphere," says Kleidon. "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.
Using a model of global circulation, Kleidon found that the amount of energy which we can expect to harness from the wind is reduced by a factor of 100 if you take into account the depletion of free energy by wind farms. It remains theoretically possible to extract up to 70 TW globally, but doing so would have serious consequences.
Although the winds will not die, sucking that much energy out of the atmosphere in Kleidon's model changed precipitation, turbulence and the amount of solar radiation reaching the Earth's surface. The magnitude of the changes was comparable to the changes to the climate caused by doubling atmospheric concentrations of carbon dioxide (Earth System Dynamics, DOI: 10.5194/esd-2-1-2011).
From the actual paper.
How does the Earth system generate and maintain thermodynamic disequilibrium and what does it imply for the future of the planet?
...
6. How does human activity change planetary free energy generation?
The free energy used for human activities are, of course, drawn out of the Earth system and thereby affect its state. At present, much of the free energy needs for industrial use are met by depleting a stock of geological free energy (in form of fossil fuels) and this results in global climatic change due to higher concentrations of carbon dioxide in the Earth’s atmosphere. If this depletion is going to be replaced by renewable sources of free energy – as commonly suggested to avoid emissions of carbon dioxide –, then this is going to leave an impact on the free energy balance of the planet. Other impacts of human activity, such as the emission of methane, nitrous oxide, or soot also relate, directly or indirectly, to the combustion of fuels or to food production, and should also relate to the Earth’s free energy balance. Hence, it would seem appropriate to relate human activity as well as its impacts on the Earth system to its basic driver, the need for free energy. This need for free energy would seem to be the most important metric to measure the impact of humans on the planet and would seem to serve to be a highly useful metric to evaluate potential future impacts. As we have already seen in the last section, human activity already consumes a consid- erable share of the free energy in relation to how much is generated within the Earth system. When we think about the future state of the planet, it would seem almost inevitable that hu- man activity will increase further, in terms of population size and standard of living, among others. Both of these will require more free energy to sustain. Then, the central question is going to be whether this increase in human activity is going to be met by degrading the ability of the Earth system to generate free energy or whether these demands will be met by enhancing the ability of the Earth system to generate free energy. ...
....Both examples of meeting the human demands for free energy suggest that human activity will result in detrimental effects in terms of the ability of the Earth system to generate free energy. We can, however, also imagine another scenario. If human activity is directed to have impacts of the sort that these would act to enhance free energy generation within the Earth system, as shown in Fig. 5b, then this could have beneficial effects on the overall system in that the ability to generate free energy within the Earth system is enhanced. ....
1st link is to the paper via a pdf link on the right side of the page. 21 pages
http://arxiv.org/abs/1103.2014http://www.newscientist.com/article/mg21028063.300-wind-and-wave-farms-could-affect-earths-energy-balance.html?full=true#bx280633B1
