Di seguito gli interventi pubblicati in questa sezione, in ordine cronologico.
Modeling crowd behavior can help engineers design buildings and other public spaces so as to prevent deaths and injuries during emergencies. But it is hard to design virtual crowds that realistically mimic real ones.
European researchers have now shown that a simple model based on one cognitive factor—vision—can predict pedestrian behavior in various types of crowds. It represents significant progress in a field that has been trying to move away from purely physics-based models.
"There's no clear way to describe the cognitive processes of each individual, but with this vision-based approach, it's actually very simple," says Dirk Helbing, of the Swiss Federal Institute of Technology in Zurich, who carried out the work with Mehdi Moussaïd and Guy Theraulaz, of Université Paul Sabatier in Toulouse, France.
The study, which appears in this week's issue of Proceedings of the National Academy of Sciences, was inspired by previous research that used eye-tracking data to determine how people predict the trajectory of an airborne ball in order to catch it. Numerous other studies have suggested that walking, like catching a ball, is primarily governed by vision. So the researchers hypothesized that using visual factors, mainly line of sight and visibility, would allow them to better model crowd behavior.
The researchers gave virtual crowd members the ability to "see" their surroundings and navigate accordingly. They found that their vision-based model predicted pedestrian behavior surprisingly well for both small and large crowds as long as the physical influence of the crowd as a whole was also considered. They suggest that the model could help avert such crowd disasters as the Love Parade incident that killed 19 concertgoers in Germany last summer, by providing designers with new information about how pedestrians will attempt to move quickly through a specific space.
The model primarily indicates how vision affects pedestrians' direction and speed—two forces that often compete when a person is navigating pedestrian traffic. The researchers predicted pedestrian trajectories using the model and then compared their predictions with data from real-life pedestrian scenarios. They found the trajectories matched up almost exactly.
To model crowd disasters, though, they had to consider involuntary as well as voluntary behaviors. What the pedestrian can see remains important, but sometimes the push and pull of the crowd can be even more so. "When the crowd becomes high-density, the simple model isn't enough," says Theraulaz. "You have to take into account the rules of physical contact."
Adding a physical-force component to the vision-based model allowed the study authors to predict pedestrian behavior in different types of overcrowding situations, such as a bottleneck around a blocked exit or a pileup that forms behind a fallen pedestrian.
When the study authors applied their modified model to a real-world bottleneck disaster, they were able to predict the location of the highest-risk areas and map out how pedestrian collisions would spread once the situation became critical. "This is the most dangerous type of case," says Helbing. "You can do video analysis afterward, but even then it's hard to see exactly what's going on, because people are hardly moving."
One of the biggest advantages of the vision-based model is its versatility, says Michael Batty, an urban planning researcher at University College London, who studies crowd modeling. "It's relevant to a whole range of pedestrian situations, and that's what makes it more testable," he says. The study authors suggest that the model could also be used to analyze crowd disasters in low-visibility cases, such as fires, and could help improve the design of crowd-navigating robots.
Source: Technology Review
Their work will help engineers develop a new generation of high-performance, energy-efficient lighting that could replace incandescent and fluorescent bulbs.
"Identifying the root cause of the problem is an indispensable first step toward devising solutions," says Chris Van de Walle, a professor in the Materials Department at UC Santa Barbara who heads the research group that carried out the work.
Van de Walle and his colleagues are working to improve the performance of nitride-based LEDs, which are efficient, non-toxic and long-lasting light sources. They investigated a phenomenon referred to as "droop"?the drop in efficiency that occurs in these LEDs when they're operating at the high powers required to illuminate a room. The cause of this decline has been the subject of considerable debate, but the UC Santa Barbara researchers say they've figured out the mechanism responsible for the effect by performing quantum-mechanical calculations.
LED droop, they conclude, can be attributed to Auger recombination, a process that occurs in semiconductors, in which three charge-carriers interact without giving off light. The researchers also discovered that indirect Auger effects, which involve a scattering mechanism, are significant?a finding that accounts for the discrepancy between the observed degree of droop and that predicted by other theoretical studies, which only accounted for direct Auger processes.
In nitride LEDs, "These indirect processes form the dominant contribution to the Auger recombination rate," says Emmanouil Kioupakis, a postdoctoral researcher at UC Santa Barbara and lead author of a paper published online April 19 in Applied Physics Letters. The other authors are Van de Walle, Patrick Rinke, now with the Fritz Haber Institute in Germany, and Kris Delaney, a project scientist at UC Santa Barbara.
LED droop can't be eliminated because Auger effects are intrinsic, but it could be minimized, the researchers say, by using thicker quantum wells in LEDs or growing devices along non-polar or semi-polar growth directions in order to keep carrier density low.
"With Auger recombination now established as the culprit, we can focus on creative approaches to suppress or circumvent this loss mechanism," Van de Walle says.
Several of Japan's nuclear power plants, especially the Fukushima Naiishi plant in northeastern Japan, are experiencing serious problems in the wake of earthquake and tsunami. If you've been following the news, you've seen some pretty alarming stuff going on at this plant--terms like "explosion," "partial meltdown," "evacuation," and "radiation exposure." With details sparse from the chaotic scene, here's what you need to know to understand and make sense of the news unfolding in Japan.
Fukushima Dai-ichi Nuclear Plant, March 14, 2011 - DigitalGlobe via Getty Images
What Is a Nuclear Reaction?
A nuclear reaction is at its most basic nothing more than a reaction process that occurs in an atomic nucleus. They typically take place when a nucleus of an atom gets smacked by either a subatomic particle (usually a "free neutron," a short-lived neutron not bound to an existing nucleus) or another nucleus. That reaction produces atomic and subatomic products different from either of the original two particles. To make the kind of nuclear reaction we want, a fission reaction (in which the nucleus splits apart), those two original particles have to be of a certain type: One has to be a very heavy elemental isotope, typically some form of uranium or plutonium, and the other has to be a very light "free neutron." The uranium or plutonium isotopes are referred to as "fissile," which means we can use them to induce fission by bombarding them with free neutrons.
In a fission reaction, the light particle (the free neutron) collides with the heavy particle (the uranium or plutonium isotope) which splits into two or three pieces. That fission produces a ton of energy in the form of both kinetic energy and electromagnetic radiation. Those new pieces include two new nuclei (byproducts), some photons (gamma rays), but also some more free neutrons, which is the key that makes nuclear fission a good candidate to generate energy. Those newly produced free neutrons zoom around and smack into more uranium or plutonium isotopes, which in turn produces more energy and more free neutrons, and the whole thing keeps going that way--a nuclear fission chain reaction.
Nuclear fission produces insane amounts of energy, largely in the form of heat--we're talking several million times more energy than you'd get from a similar mass of a more everyday fuel like gasoline.
Getting Usable Energy From Fission
There are several types of nuclear fission reactors in Japan, but we're going to focus on the Fukushima Naiishi plant, the most hard-hit facility in the country. Fukushima, run by the Tokyo Electric Power Company (TEPCO), has six separate reactor units, although numbers 4, 5, and 6 were shut down for maintenance at the time of the earthquake (and more importantly, the subsequent tsunami). Numbers 1, 2, and 3 are all "boiling water reactors," made by General Electric in the early- to mid-1970s. A boiling water reactor, or BWR, is the second-most-common reactor type in the world.
A BWR contains thousands of thin, straw-like tubes 12 feet in length, known as fuel rods, that in the case of Fukushima are made of a zirconium alloy. Inside those fuel rods is sealed the actual fuel, little ceramic pellets of uranium oxide. The fuel rods are bundled together in the core of the reactor. During a nuclear fission chain reaction, the tubes heat up to extremely high temperatures, and the way to keep them safe turns out to also be the way to extract useful energy from them. The rods are kept submerged in demineralized water, which serves as a coolant. The water is kept in a pressurized containment vessel, so it has a boiling point of around 550 °F. Even at such a high boiling point, the burning hot fuel rods produce large amounts of steam, which is actually what we want from this whole complicated arrangement—the high-pressure steam is used to turn the turbines on dynamos, producing electricity.
Boiling Water Reactor Schematic: 1. Reactor pressure vessel (RPV) 2. Nuclear fuel element 3. Control rods 4. Circulation pumps 5. Engine control rods 6. Steam 7. Feedwater 8. High pressure turbine (HPT) 9. Low pressure turbine 10. Generator 11. Exciter 12. Condenser 13. Coolant 14. Pre-heater 15. Feedwater pump 16. Cold water pump 17. Concrete enclosure 18. Mains connection Nicolas Lardot - Wikimedia Commons
Since lots of heat is being produced, as well as the production and use of lots of pretty nasty radioactive materials, nuclear plants employ a variety several safety efforts beyond simply the use of the cooling water (which itself is backed up by redundant diesel generators--more on that later). The plant's core, the fuel rods and the water, is encased in a steel reactor vessel. That reactor vessel is in turn encased in a giant reinforced concrete shell, which is designed to prevent any radioactive gases from escaping.
Isn't There an "Off" Switch?
Sure! But needless to say, safely shutting down and controlling a nuclear reactor is not at all as simple as unplugging a rogue kitchen appliance. This is due to the extreme heat still present well after fission has subsided--mostly due to chemical reactions inherent in the fission reaction.
A functioning fission plant employs a system of control rods, essentially structures that limit the rate of fission inside the fuel rods by absorbing roaming free neutrons. The rate of fission can be controlled--even stopped--by inserting and removing the control rods in the reactor. At the time of the quake, the Fukushima reactors' control rods functioned normally, shutting down the fission reaction. But even with the fission reaction stopped, the fuel rods remain at extremely high temperatures and require constant cooling.
Which isn't typically a problem, so long as the cooling system (and, failing that, its diesel-powered backup) is still intact. But after losing main power in the quake, the subsequent tsunami wave also destroyed Fukushima's diesel backup generators. Which is a serious problem; even though the fission had stopped, coolant is still very much required to keep the plant safe.
That's due to the heat that remains in the nuclear core, both from the recently-disabled but still-hot fuel rods and from the various byproducts of the fission process. Those byproducts include radioactive iodine and caesium, both of which produce what's called "decay heat"--residual heat that is very slow to dissipate. If the core isn't continuously cooled, there's still more than enough heat to cause a meltdown long after it's been "turned off."
In the case of the Fukushima plant, with both the main and backup coolant systems down for the count, TEPCO was forced to rig a method to flood the core with seawater laced with boric acid (the boric acid to stave off another fission reaction if one were to restart due to a meltdown--more on that below). That's a bad sign--it's a last-ditch effort to prevent catastrophe, as the salt in the seawater will corrode the machinery. It's also a temporary fix: TEPCO will need to pump thousands of gallons of seawater into the core every day, until they can get the coolant system back online. Without it, the seawater method might have to go on for weeks, even up to a year, as the decay heat slowly subsides.
The Dreaded Meltdown
First of all, a "meltdown" is not a precisely defined term, which makes it fairly useless as an indicator of what's going on. Even the terms "full meltdown" and "partial meltdown" are pretty unhelpful, which is partly why we've written this guide--you'll be able to understand what's actually happening without relying on spurious terms that the experts themselves are often loathe to use.
Anyway, let's start at some of the less severe (though still unsettling) things that can happen when the coolant liquid is no longer present in the core. When the fuel rods are left uncovered by water, they'll get far too hot--we're talking thousands of degrees Celsius here--and begin to oxidize, or rust. That oxidation will react with the water that's left, producing highly explosive hydrogen gas. This has already happened in reactor No. 1 at Fukushima (see the video below). The hydrogen gas can be vented in smallish doses into the containment building, but if they can't vent it fast enough, it'll explode, which is exactly what happened at reactor No. 1. Keep in mind, this is not a nuclear reaction, but a simple chemical explosion that often (as in this case) results in little or no radioactive material being leaked into the outside world.
TEPCO has announced that after the explosion, radiation levels in the area around the plant were still within "normal" parameters. This is an important distinction--not to say that a hydrogen explosion at a nuclear plant is particularly fun news, but it is not nearly as panic-inducing as a meltdown.
What people mean when they say "meltdown" can refer to several different things, all likely coming after a hydrogen explosion. A "full meltdown" has a more generally accepted definition than, say, a "partial meltdown." A full meltdown is a worst-case scenario: The zirconium alloy fuel rods and the fuel itself, along with whatever machinery is left in the nuclear core, will melt into a lava-like material known as corium. Corium is deeply nasty stuff, capable of burning right through the concrete containment vessel thanks to its prodigious heat and chemical force, and when all that supercharged nuclear matter gets together, it can actually restart the fission process, except at a totally uncontrollable rate. A breach of the containment vessel could lead to the release of all the awful radioactive junk the containment vessel was built to contain in the first place, which could lead to your basic Chernobyl-style destruction.
The problem with a full meltdown is that it's usually the end result of a whole boatload of other chaos--explosions, fires, general destruction. Even at Chernobyl, which (unbelievably, in retrospect) had no containment building at all, the damage was caused mostly by the destruction of the plant by explosion and a graphite fire which allowed the corium to escape to the outside world, not the physical melting of the nuclear core.
Over the weekend, Chief Cabinet Secretary Yukio Edano somewhat hesitatingly confirmed a "partial" meltdown. What does that mean? Nobody knows! The New York Times notes that a "partial" meltdown doesn't actually need to have any melting involved to qualify it as such--it could simply mean the fuel rods have been un-cooled long enough to corrode and crack, which given the hydrogen explosion, we know has already happened. But we'd advise against putting too much stock in any term relating to "meltdown"--it'll be much more informative to find out what's actually going on, rather than relying on a vague blanket term.
As TEPCO grapples with the damage the earthquake and tsunami did to the nuclear system, there's going to be lots of news--there could be more explosions, mass evacuations, and more "meltdowns" of one kind or another. All we can do is learn about what's going on, think calmly about the situation, and hope that TEPCO can eventually regain control of the plants
But we're fighting back, and winning! We played a key role in stopping Murdoch's grab for media control in the UK. Now we're taking our red-hot UK campaign global, to roll back the Murdoch menace everywhere with campaigns, investigations and legal action.
Hacking murdered children's phones, paying off police, destroying evidence of crimes, threatening politicians -- UK leaders say Rupert Murdoch's empire has "entered the criminal underworld". For decades, Murdoch has ruled with impunity -- making and breaking governments with his vast media holdings and scaring opponents into silence, but we're fighting back, and winning!
Murdoch at the World Economic Forum Annual Meeting in 2009.
Through almost 1 million actions, 7 campaigns, 30,000 phone calls, investigations and countless stunts and legal tactics, we've played a lead role and stopped Murdoch from buying over 50% of UK commercial media! Now we're taking our red-hot UK campaign global, to roll back the Murdoch menace everywhere.
Here's the plan: together we can a) hire investigators to expose Murdoch's corrupt tactics beyond the UK b) organize prominent voices to break the cycle of fear and speak out on this issue and c) mobilise people in key countries behind new laws and legal actions that stop Murdoch and clean up our media for good.
Avaaz members live in every country where Murdoch works, making our movement the only one that can truly take a campaign against his global empire and win. The time is now -- If just 20,000 of us donate a small amount each, we can seize this once-in-a-generation chance. Click below to chip in:
For weeks, nearly daily revelations have uncovered the extent of Murdoch media's corruption in the UK. His operatives hacked the phones of thousands of people, including grieving widows and soldiers who died in Iraq, stole a Prime Minister's bank information and harassed him for 10 years, paid huge sums to police officers, and Rupert's son, James Murdoch, himself authorized hush money to victims.
But this is the tip of the iceberg -- Murdoch is a global problem. He's famous for dictating editorial positions to his papers. He corrupts and controls democracies by pushing politicians to back his extremist ideas on war, torture and a host of other planetary ills, and destroying the careers of politicians with smear campaigns unless they do his bidding. In the US, he helped elect George W. Bush and has most of the Republican presidential candidates actually on his payroll (see sources below). His Fox News Network spread lies to promote the war in Iraq, pushed resentment of Muslims and immigrants and spawned the right-wing tea party. Maybe worst of all, he has helped block critical global action on climate change.
Murdoch's reign of fear is breaking down, and many are on the edge of speaking out against his tactics. The dam is about to break in the US, Australia and elsewhere, but we need to give it an urgent push by investigating Murdoch further, organising high profile opposition, and making sure that our politicians pass laws that will clean up our media for good. Let's make it happen together:
Our community kept campaigning on this issue when almost everyone else in the UK gave up hope. Because we're people-powered, we don't have the same fear of Murdoch that almost everyone else does. It's part of the promise that people power has for change in the world. Today, hope is breaking out in the UK -- let's take it global.
Ricken, Emma, Maria Paz, Giulia, Luis, Alice, Brianna and the rest of the Avaaz team
Decision on BSkyB takeover could take weeks after surge in online campaigning (Huffington Post)
BSkyB bid final clearance unlikely to be given before September (The Guardian)
Culture Secretary Jeremy Hunt will take 'several weeks' to review 100,000-plus submissions on News Corp/BSkyB takeover
Murdoch maimed by social media (The Scotsman)
Who is Rupert Murdoch? (Center for American Progess)
The global reach of Murdoch's News Corp (BBC)
Rebekah Brooks must go over Milly 'hacking' - Miliband (BBC)
Latest Updates on British Phone Hacking Scandal (New York Times)
Fox News 2012? Nearly All Potential GOP Presidential Candidates On FNC Payroll (Huffington Post)
Hydrogen gas is not only explosive but also very space-consuming. Storage in the form of very dense solid metal hydrides is a particularly safe alternative that accommodates the gas in a manageable volume. As the storage tank should also not be too heavy and expensive, solid-state chemists worldwide focus on hydrides containing light and abundant metals like magnesium.
Sjoerd Harder and his co-workers at the Universities of Groningen (Netherlands) and Duisburg-Essen (Germany) now take the molecular approach. As the researchers report in the journal Angewandte Chemie, extremely small clusters of molecular magnesium hydride could be a useful model substance for more precise studies about the processes involved in hydrogen storage.
Magnesium hydride (MgH2) can release hydrogen when needed and the resulting magnesium metal reacts back again to form the hydride by pressurizing with hydrogen at a "gas station". Unfortunately, this is an idealized picture. Not only is the speed of hydrogen release/uptake excessively slow (kinetics) but it also only operates at higher temperatures. The hydrides, the negatively charged hydrogen atoms (H-), are bound so strongly in the crystal lattice of magnesium cations (Mg2+) that temperatures of more than 300 ?C are needed to release the hydrogen gas.
Particularly intensive milling has made it possible to obtain nanocrystalline materials, which, on account of its larger surface, rapidly release or take up hydrogen. However, the high stability of the magnesium hydride still translates to rather high release temperatures. According to recent computer calculations, magnesium hydride clusters of only a few atoms possibly could generate hydrogen at temperatures far below 300 °C. Clusters with less than 20 Mg2+ions are smaller than one nanometer and behave differently from the bulk material. Their hydride ions have fewer Mg2+ neighbors and are more weakly bound. However, it is extremely difficult to obtain such tiny clusters by milling. In Harder's "bottom-up" approach, magnesium hydride clusters are made by starting from molecules. The challenge is to prevent such clusters from forming very stable bulk material. Using a special ligand system, they could trap a cluster that resembles a paddle wheel made of eight Mg2+ and ten H- ions. For the first time it was shown that molecular clusters indeed release hydrogen already at the temperature of 200 °C.
This largest magnesium hydride cluster reported to date is not practical for efficient hydrogen storage but shines new light on a current problem. It is easily studied by molecular methods and as a model system could provide detailed insights in hydrogen storage.
Click here for a compendium of important, interesting or historical coverage of the extradition trial of Julian Assange in other publications, twitter, video, etc. For all WL Central coverage of the Julian Assange extradition trials see this thread or in list form here. A summary of the arguments under appeal is here.
*Image from emnnews.com
This is a "WikiLeaks News Update," constantly updated throughout each day. The blog tracks stories that are obviously related to WikiLeaks but also follows stories related to freedom of information, transparency, cybersecurity, and freedom of expression. All the times are GMT.
09:40 AM Julian Assange has arrived, wearing glasses. (image via @Robert_Booth)
09:00 AM A crowd of supporters gathers outside Royal Courts of Justice where Julian Assange's extradition appeal hearing is to take place in approximately 1h30 (images via @m_cetera).
07:45 AM The first short film by Revolution Truth is out. Tangerine Bolen and Michael Moore can be seen reading an open letter in support of Wikileaks and government accountability (4:37) that you can sign here.
Description : The US government is going out of its way to muzzle Julian Assange and shut down WikiLeaks. We are a group of people from around the world who defend WikiLeaks, whistleblowers, and legitimate democracies. We demand truth and justice in our systems. Michael Moore has endorsed this campaign and organization and appears in these short films.
07:30 AM Collateral Experiments, art installation in "response to Wikileaks’ Collateral Murder".
Assange has been under house arrest in England for over six months, following a ruling in February at a London district court that the extradition of Assange to Sweden was valid and would not breach any of his human rights.
Assange’s lawyers argued unsuccessfully that he would face an unfair trial in Sweden since the press and the public are excluded from parts of sexual assault trials.
Assange voluntarily turned himself in to the police in the UK after Sweden filed a European Arrest Warrant for him.
A widely held view amongst WikiLeaks’ supporters is that the extradition to Sweden is a preface to an eventual trial in the US; and that extradition for the charges against Assange would not be pursued if it was any ordinary citizen.
Assange and his colleagues at the whistle-blowing website WikiLeaks have subjected the US government to extreme duress and embarrassment due to its publication of many thousands of US diplomatic cables including: the July 12 Baghdad Collateral Murder Video and other Iraq war documents; material on extrajudicial killings in Kenya, and the Guantanamo Bay files, to name a few.
Julian Assange is an Australian citizen and deserves all the support our government can offer him. Instead he was thrown to the wolves. Australian Prime Minister Julia Gillard failed to support Assange and called the leaks “an illegal act” according to an article in The Australian in 2009.
Gillard came under widespread condemnation for failing to give support to Assange. Hundreds of prominent lawyers, journalists, editors, and academics signed a letter to the Gillard government calling for her to support Assange but the government has maintained its hardline stance from the outset.
Assange’s legal team will include prominent human rights lawyers Gareth Peirce and Ben Emmerson, a change from his representitives at the original hearing.
Wikileaks continues its release of over 250,000 US diplomatic cables with 16,068 of the 251,287 cables published so far; and the cables will continue to come out, day by day.
A University of Missouri researcher says people who overuse credit have very different beliefs about products than people who spend within their means. Following a new study, Marsha Richins, Myron Watkins Distinguished Professor of Marketing in the Trulaske College of Business, says many people buy products thinking that the items will make them happier and transform their lives.
"There is nothing wrong with wanting to buy products," Richins said. "It becomes a problem when people expect unreasonable degrees of change in their lives from their purchases. Some people tend to ascribe almost magical properties to goods -- that buying things will make them happier, cause them to have more fun, improve their relationships -- in short, transform their lives. These beliefs are fallacious for the most part, but nonetheless can be powerful motivators for people to spend."
Richins identified four types of changes that materialistic people expect when making purchases. Previous research has shown that often these expectations are not fulfilled. The four types of transformations expected are:
-> Transformation of the self is the belief that a purchase will change who you are and how people perceive you. This is commonly held by young people and people in new roles. For example, a woman interviewed for the study wanted to have cosmetic dental surgery because she thought it would improve her appearance and self-confidence.
-> Transformation of relationships is the expectation that a purchase will give someone more or better relationships with others. For example, a woman interviewed for the study wanted to buy a new home because she thought it would enable her to entertain more often and make more friends.
-> Hedonic transformation implies that a purchase will make life more fun. For example, a man in the study wanted a mountain bike because he thought it would give him more incentive to get out and go on "an adventure."
-> Efficacy transformation is the expectation that purchases will make people more effective in their lives. For example, some study participants wanted to buy a vehicle because they thought it would make them more independent and self-reliant.
People who have strong and unrealistic transformational beliefs are more likely than others to overuse credit and take on excessive debt. According to Richins, this finding highlights a limitation of financial literacy and credit counseling programs. She recommends that financial literacy and credit counseling programs be revised to help people better understand their motivations for purchasing goods and to help them recognize that products are not a quick fix for improving their lives.
"Many financial literacy programs seek to prevent people from getting into financial problems by presenting the facts about interests rate and loans," Richins said. "However, few programs seek to directly influence behavior or focus on why people purchase things they cannot afford and go into debt."
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. 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).
"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?").
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.
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 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. Sustainable solar energy may therefore require cells that reflect the light they cannot use.
Con il decollo avvenuto poco tempo fa della navetta Endeavour, l’era degli Shuttle volge al termine e con la missione STS 135, la cui partenza è prevista per l’inizio di luglio, il Programma Shuttle della NASA sarà definitivamente concluso.
(© NASA/Dimitri Gerondidakis)
Quali sono i motivi che hanno decretato il pensionamento dei gloriosi velivoli spaziali? Da cosa saranno sostituiti? Che ne sarà della Stazione Spaziale Internazionale e dei suoi occupanti? E quale sarà il futuro dell’esplorazione del cosmo? Ve lo spieghiamo in 7 domande e risposte.
1) Perchè gli Shuttle vengono dismessi?
Le navicelle Shuttle hanno costi di missione e manutenzione che crescono con il passare degli anni. Sono in servizio dal 1981 e il loro compito, cioè la costruzione della ISS, è praticamente terminato. La NASA vuole quindi utilizzare le risorse assorbite dal Programma Shuttle in qualcosa di nuovo.
L'idea di pensionare gli Shuttle non è comunque nuova: già nel 2003 la commissione d’inchiesta sull’incidente del Columbia concludeva il suo rapporto con la frase "È nell’interesse del paese rimpiazzare gli Shuttle il prima possibile"
2) Chi ha deciso di fermare gli Shuttle?
La decisione fu presa dal Presidente degli Stati Uniti George Bush nel 2004. Il Presidente chiese alla NASA di sviluppare un nuovo programma spaziale che portasse nuovamente l’uomo sulla Luna e magari anche su Marte.
Per raggiungere questi obiettivi l’agenzia spaziale aveva però bisogno di ingenti fondi e sospendendo il programma Shuttle si potevano liberare oltre 4 miliardi di dollari l’anno.
Lo scorso anno il Presidente Obama ha però annullato tutti i progetti lunari per concentrarsi sulla costruzione di un grande razzo in grado di portare l’uomo sugli asteroidi e su Marte.
Il trasporto di materiali e uomini da e verso la Stazione Spaziale Internazionale dovrebbe essere affidato ai vettori russi e alle future compagnie private.
3) Quando terminerà il programma Shuttle?
L’ultimo volo degli Shuttle sarà compiuto dall’Atlantis ed è programmato per luglio: porterà sulla ISS componenti elettronici e pezzi di ricambio.
4) Quali erano gli obiettivi del programma Shuttle?
Questi cargo cosmici avrebbero dovuto permettere voli spaziali sicuri ed economici, e la possibilità di lanci frequenti, addirittura settimanali.
Così non è stato: le navette si sono rivelate costose e insicure. In trent' anni di servizio, i due gravi incidenti che hanno coinvolto le navette Columbia e Challenger hanno causato la morte di 14 astronauti.
Ma gli Shuttle, a metà strada tra un tir spaziale e una gru, si sono rivelati il mezzo più idoneo per la messa in orbita e la manutenzione di grandi apparecchiature come il telescopio spaziale Hubble.
5) Che fine faranno le navicelle?
Andranno a vari musei degli States: l’Endeavour verrà esposto al California Space Center di Los Angeles, l’Atlantis rimarrà a disposizione dei visitatori del Kennedy Space Center e il Discovery andrà in un hangar della Smithsonian Institution presso l’aeroporto Dulles di Washington.
L’Enterprise, un prototipo utilizzato per alcuni voli di prova fin dalla fine degli anni ‘70, fa già bella mostra di sè nei locali del New York City's Intrepid Sea, Air and Space Museum
6) E la Stazione Spaziale Internazionale? Che ne sarà?
La ISS continuerà a funzionare almeno fino al 2020. Ora è abbastanza grande da poter ospitare 6 persone e tutti gli esperimenti e le ricerche propedeutici alla conquista di Marte.
Gli astronuauti raggiungeranno la Stazione a bordo delle capsule russe Soyouz affittate dalla NASA per questo scopo, in attesa che gli americani dispongano nuovamente di vettori spaziali propri.
7) Quanto costa mandare un uomo nello spazio?
Con le capsule Soyouz il costo del traporto di un astronauta da e per la Stazione Spaziale Internazionale si aggirerà sui 50 milioni di euro. Una cifra mostruosa, ma comunque inferiore a quella di un volo dello Shuttle. Nei prossimi 2 - 3 anni saranno pronti anche i primi vettori spaziali privati e i costi dei biglietti per la ISS potrebbero scendere.
La NASA infatti, all’inizio del 2011, ha affidato a Space X, Boeing, Blu Origin e Sierra Nevada, una commessa da oltre 200 milioni di euro per la progettazione di un missile e una capsula in grado di portare uomini sulla ISS.