A few further thoughts. The driving force for atmospheric motions is the equator-pole temperature gradient‹energy continues to come in and decrease entropy (that is, enhancing the gradient), and then the motions tend to increase the entropy (trying to smooth out the gradient).
I guess what wind power really does is create an alternate form of dissipation to small eddies moving vegetation and just dissipating heat. So, instead of local heat dissipation by the vegetation (entropy increase), the energy is drained by windmills and then the electricity is dissipated as heat, so it is an alternative pathway. And I gather what you find is that the atmosphere above does not really care if the dissipation is locally friction heat loss or goes through the wind power bypass, doing some useful work for humanity. I guess that is just sort of saying that having fully efficient wind power machines would be no different than having higher orography, as far as the atmospheric circulation is concerned. So, then the question is what fraction of the surface friction that one can replace with wind power machines. While you did the calculation for land areas, there is really no reason that one could not have floating windmills out over the ocean, diverting energy away from the wind driven currents and evaporation. If one could develop the optimal machine, one could potentially extract energy at the rate that it is created by the differences in energy being supplied and radiated away from low and high latitudes, not wasting any energy in wind, etc.--so there is a theoretical upper limit‹the supply is not infinite. But, the amount available could be pretty large using large windmills, tethered wind turbines in the jet stream, etc.--for an interim period, this would likely cause an increase in the gradient, but ultimately the limiting rate of removal of energy is the pole-equator difference (times some fractional potential efficiency). As for the Jacobson calculation, the US is a pretty small area and so I would think creating a stronger drag over it would tend to attract energy in from elsewhere as Nature likes to push down gradients. This can take time, but there surely will be some infilling, especially given large scale motions due to the atmospheric circulation. Interesting. Mike On 7/12/11 9:36 PM, "David Keith" <[email protected]> wrote: > Mike & Bala > > A few answers: > > First there is almost no link to geo here so we should probably take this off > this list. The only (weak link) is weather control, see: > http://www.atmos-chem-phys.org/10/769/2010/acp-10-769-2010.html > > 1. Bala said ³Generation of wind energy would increase the KE dissipation rate > but this is not an external forcing to the climate system.² And ³The current > KE dissipation rate is about 2 watts/m^2. Over land, this translates to about > 300 TW. Suppose wind farms extract 150 TW (which may be impractical), the > dissipation rate over land will increase to 3 Wm^2. Don't you think the KE (or > available PE) generation rate in the atmosphere would correspondingly > increase? Of course these would be large regional climate changes.² > > Answer: As the surface drag is increased the total dissipation does not change > much. That is, as you increase the KE sink in some locations with wind > turbines the dissipation decreases elsewhere keeping total about constant. See > Figure 2 of our 2004 PNAS where we tried this. This is what one would expect > because dissipation of KE must balance its creation from APE (see pexoto and > ort or my encyclopedia article cited below for an overview of atmo > energetics). Going a bit deeper one might think that with more to ³push > against² the APE generation rate would go up and the atmo heat engine get more > efficient, Kerry Emanuel have suggested to me that this should not be true > because of a maximum entropy principle that I do not fully understand. > > Bottom line: very likely Bala¹s assumption is wrong. > > 2. Bala said: ³I agree there would be local and regional climate changes but > there should be no global mean warming. Right?² > > Answer: mostly. One can see either warming or cooling depending on where the > wind drag is applied. The point is that (a) climate changes due to drag are > non-local, and (b) they can be large. > > 3. Mike asked about the Jacobsen paper that says no effect. > > Answer: I think this paper is just wrong. If it were true I could violate the > first law by extracting power without altering KE and then using that power to > increase APE generating infinite power with no input. Nice trick. There are > now about 5 studies that confirm the broad results in our 2004 paper. The > Jacobsen paper is an outlier. I expect a convincing critique will be published > in the next few years. > > Yours, > David > > > > > > > From: [email protected] [mailto:[email protected]] > On Behalf Of Mike MacCracken > Sent: Tuesday, July 12, 2011 6:22 AM > To: Govindasamy Bala; David Keith; Ken Caldeira > Cc: Geoengineering > Subject: Re: [geo] Wind and wave energies are not renewable after all > > Dear David--I was going to ask a similar question to Bala¹s‹as this has > actually been an ongoing argument in some circles of the energy community, > with a scientific study by a Royal Society lead physicist in their energy > analysis talking about a limit based on extracting a share of the existing > atmospheric KE and Mark Jacobson at Stanford saying there is plenty of KE as > it will be restored. > > It seems to me that the KE pulled out will be replaced‹if not, the atmosphere > would eventually not be moving and so a huge equator-pole temperature gradient > would build up. With solar energy concentrated at the low latitudes and IR > loss in excess at high latitudes, the atmosphere will be seeking balance; take > some energy out and the atmosphere will try to restore it, rather like what > happens when one puts a rock in a stream, maybe with a bit different flow, but > I would not think significantly less KE. Right? > > Mike > > > > On 7/12/11 7:25 AM, "Govindasamy Bala" <[email protected]> wrote: > Hi David, > > Couple of questions. > Generation of wind energy would increase the KE dissipation rate but this is > not an external forcing to the climate system. I agree there would be local > and regional climate changes but there should be no global mean warming. > Right? > > The current KE dissipation rate is about 2 watts/m^2. Over land, this > translates to about 300 TW. Suppose wind farms extract 150 TW (which may be > impractical), the dissipation rate over land will increase to 3 Wm^2. Don't > you think the KE (or available PE) generation rate in the atmosphere would > correspondingly increase? Of course these would be large regional climate > changes. > > Bala > > On Tue, Jul 12, 2011 at 8:37 AM, David Keith <[email protected]> wrote: > Responding to a VERY old thread on wind power: > > The only link to geoengineering here is that there is a possibility of > manipulating wind turbine drag for weather control, see: > > At 10¹s TW scale extraction of wind does begin to be constrained by the > generation of kinetic energy. I led the a joint NCAR-GFDL group that published > the first paper on this topic see: > David W. Keith et al, The influence of large-scale wind-power on global > climate. Proceedings of the National Academy of Sciences, 101, p. 16115-16120. > http://people.ucalgary.ca/~keith/papers/66.Keith.2004.WindAndClimate.e.pdf > <http://people.ucalgary.ca/%7Ekeith/papers/66.Keith.2004.WindAndClimate.e.pdf> > > See > http://people.ucalgary.ca/~keith/papers/94.Kirk-Davidoff.SurfaceRoughnessJAS.p > .pdf > <http://people.ucalgary.ca/%7Ekeith/papers/94.Kirk-Davidoff.SurfaceRoughnessJA > S.p.pdf> for a paper that says a bit about why it happens. > > The following web page gives and overview but it¹s now out of date: > http://people.ucalgary.ca/~keith/wind.html > <http://people.ucalgary.ca/%7Ekeith/wind.html> > > Alvia¹s comment that about ³kinetic energy, i.e. the motion of molecules², > confuses the physics. Kinetic energy is macroscopic velocity, random motion of > molecules is just heat. It is true that large scale production and dissipation > of kinetic energy must balance, have a look at Peixoto and Oort¹s the Physics > of Climate or a short encyclopedia article I one wrote on atmospheric > energetics: > http://people.ucalgary.ca/~keith/papers/15.Keith.1996.Energetics.s.pdf > <http://people.ucalgary.ca/%7Ekeith/papers/15.Keith.1996.Energetics.s.pdf> > > Bottom lines: > > 1. Commonly cited estimates for global wind power potential are too large. On > cannot get to 100 TW in any practical scheme I know about. > > 2. At even a few TW large scale climate effects will begin to be important. > But, this does not say we should not make a few TW of wind, just that--like > any energy technology‹there are tradeoffs. > > David > > From: [email protected] [mailto:[email protected]] > On Behalf Of Nando > Sent: Saturday, April 02, 2011 8:25 AM > To: [email protected] > Cc: [email protected]; geoengineering > Subject: Re: [geo] Wind and wave energies are not renewable after all > > My reading of the article suggested that the authors of the study were > principally claiming that wind has an impact on climate, so it is already > being "used". What wasn't clear from the article was what type of impact > reducing the energy level of winds all over the globe through the prolific use > of wind turbines might have. In a warming world, I understand we should expect > stronger winds. On a simplistic generalized level that might not be relevant > to local climate, slowing those stronger winds down might have an ameliorating > effect on climate change. Hence the claim that "The magnitude of the changes > was comparable to the changes to the climate caused by doubling atmospheric > concentrations of carbon dioxide" might not be as bad as it is made to seem. > > > > As usually, I'm grasping at straws, but as a layman, that's what stood out for > me. > > > > Nando > > On Sat, Apr 2, 2011 at 3:15 PM, Alvia Gaskill <[email protected]> wrote: > > Wind and wave energy are the result of the conversion of solar energy into > kinetic energy, i.e. the motion of molecules. Once converted into kinetic > energy it's a use it or lose it proposition. Extracting kinetic energy from > the atmosphere or the ocean doesn't mean it won't be replaced by more energy > from sunlight. Planting more trees will also intercept winds, albeit without > the electricity generation. Who funded this research? The same people who > want to prevent contact with alien civilizations? I note that the Royal > Society was also a party to that one too. Note to Royal Society. When you > actually find something under the bed I should be afraid of, wake me up. > ----- Original Message ----- > > From: Andrew Lockley <mailto:[email protected]> > > To: geoengineering <mailto:[email protected]> > > Sent: Friday, April 01, 2011 8:10 > > Subject: [geo] Wind and wave energies are not renewable after all > > > Wind and wave energies are not renewable after all > · 30 March 2011 by Mark Buchanan > <http://www.newscientist.com/search?rbauthors=Mark+Buchanan> > · Magazine issue 2806 <http://www.newscientist.com/issue/2806> . > Subscribe and save <http://www.newscientist.com/subscribe?promcode=nsarttop> > · For similar stories, visit the Energy and Fuels > <http://www.newscientist.com/topic/energy-fuels> and Climate Change > <http://www.newscientist.com/topic/climate-change> Topic Guides > > Editorial: "The sun is our only truly renewable energy source > <http://www.newscientist.com/article/mg21028062.500-the-sun-is-our-only-truly- > renewable-energy-source.html> " > Build enough wind farms to replace fossil fuels and we could do as much damage > to the climate as greenhouse global warming > 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. > 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. > 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 <http://arxiv.org/abs/1103.2014> . 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. > Radical as his thesis sounds, it is being taken seriously. "Kleidon is at the > forefront of a new wave of research, and the potential prize is huge," says > Peter Cox, who studies climate system dynamics at the University of Exeter, > UK. "A theory of the thermodynamics of the Earth system could help us > understand the constraints on humankind's sustainable use of resources." > Indeed, Kleidon's calculations have profound implications for attempts to > transform our energy supply. > 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 <http://dx.doi.org/10.5194/esd-2-1-2011> ). > "This is an intriguing point of view and potentially very important," says > meteorologist Maarten Ambaum of the University of Reading, UK. "Human > consumption of energy is substantial when compared to free energy production > in the Earth system. If we don't think in terms of free energy, we may be a > bit misled by the potential for using natural energy resources." > This by no means spells the end for renewable energy, however. Photosynthesis > also generates free energy, but without producing waste heat. Increasing the > fraction of the Earth covered by light-harvesting vegetation - for example, > through projects aimed at "greening the deserts" - would mean more free energy > would get stored. Photovoltaic solar cells can also increase the amount of > free energy gathered from incoming radiation, though there are still major > obstacles to doing this sustainably (see "Is solar electricity the answer?") > <http://www.newscientist.com/article/mg21028063.300-wind-and-wave-energies-are > -not-renewable-after-all.html?full=true#bx280633B1> . > In any event, says Kleidon, we are going to need to think about these > fundamental principles much more clearly than we have in the past. "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." As Kleidon sees it, 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. > Is solar electricity the answer? > A solar energy industry large enough to make a real impact will require cheap > and efficient solar cells. 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 > <http://www.newscientist.com/article/dn16550-why-sustainable-power-is-unsustai > nable.html> . > For photovoltaic technology to be sustainable, it will have to be based on > cheaper and more readily available materials such as zinc and copper, says > Kasturi Chopra of the Indian Institute of Technology, New Delhi. > Researchers at IBM showed last year that they could produce solar cells from > these elements > <http://onlinelibrary.wiley.com/doi/10.1002/adma.200904155/abstract;jsessionid > =A766B41341BD4059B74B2F28AE9B8A80.d03t03?systemMessage=Wiley+Online+Library+wi > ll+be+disrupted+2nd+Apr+from+10-12+BST+for+monthly+maintenance> along with > tin, sulphur and the relatively rare element selenium. These "kesterite" cells > already have an efficiency comparable with commercially competitive cells, and > it may one day be possible to do without the selenium. > Even if solar cells like this are eventually built and put to work, they will > still contribute to global warming. That is because they convert only a small > fraction of the light that hits them, and absorb most of the rest, converting > it to heat that spills into the environment > <http://www.newscientist.com/article/mg20026845.200-heat-we-emit-could-warm-th > e-earth.html> . Sustainable solar energy may therefore require cells that > reflect the light they cannot use. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
