Limits to nuclear power

[dieseltaylor]

Kind of interesting to see what the limitations are and realise that other sources HAVE to be used:

PhysOrg.com) -- The 440 commercial nuclear reactors in use worldwide are currently helping to minimize our consumption of fossil fuels, but how much bigger can nuclear power get? In an analysis to be published in a future issue of the Proceedings of the IEEE, Derek Abbott, Professor of Electrical and Electronic Engineering at the University of Adelaide in Australia, has concluded that nuclear power cannot be globally scaled to supply the world’s energy needs for numerous reasons. The results suggest that we’re likely better off investing in other energy solutions that are truly scalable. As Abbott notes in his study, global power consumption today is about 15 terawatts (TW). Currently, the global nuclear power supply capacity is only 375 gigawatts (GW). In order to examine the large-scale limits of nuclear power, Abbott estimates that to supply 15 TW with nuclear only, we would need about 15,000 nuclear reactors. In his analysis, Abbott explores the consequences of building, operating, and decommissioning 15,000 reactors on the Earth, looking at factors such as the amount of land required, radioactive waste, accident rate, risk of proliferation into weapons, uranium abundance and extraction, and the exotic metals used to build the reactors themselves.

“A nuclear power station is resource-hungry and, apart from the fuel, uses many rare metals in its construction,” Abbott told PhysOrg.com. “The dream of a utopia where the world is powered off fission or fusion reactors is simply unattainable. Even a supply of as little as 1 TW stretches resources considerably.”

His findings, some of which are based on the results of previous studies, are summarized below.

Land and location: One nuclear reactor plant requires about 20.5 km2 (7.9 mi2) of land to accommodate the nuclear power station itself, its exclusion zone, its enrichment plant, ore processing, and supporting infrastructure. Secondly, nuclear reactors need to be located near a massive body of coolant water, but away from dense population zones and natural disaster zones. Simply finding 15,000 locations on Earth that fulfill these requirements is extremely challenging.

Lifetime: Every nuclear power station needs to be decommissioned after 40-60 years of operation due to neutron embrittlement - cracks that develop on the metal surfaces due to radiation. If nuclear stations need to be replaced every 50 years on average, then with 15,000 nuclear power stations, one station would need to be built and another decommissioned somewhere in the world every day. Currently, it takes 6-12 years to build a nuclear station, and up to 20 years to decommission one, making this rate of replacement unrealistic.

Nuclear waste: Although nuclear technology has been around for 60 years, there is still no universally agreed mode of disposal. It’s uncertain whether burying the spent fuel and the spent reactor vessels (which are also highly radioactive) may cause radioactive leakage into groundwater or the environment via geological movement.

Accident rate: To date, there have been 11 nuclear accidents at the level of a full or partial core-melt. These accidents are not the minor accidents that can be avoided with improved safety technology; they are rare events that are not even possible to model in a system as complex as a nuclear station, and arise from unforeseen pathways and unpredictable circumstances (such as the Fukushima accident). Considering that these 11 accidents occurred during a cumulated total of 14,000 reactor-years of nuclear operations, scaling up to 15,000 reactors would mean we would have a major accident somewhere in the world every month.

Proliferation: The more nuclear power stations, the greater the likelihood that materials and expertise for making nuclear weapons may proliferate. Although reactors have proliferation resistance measures, maintaining accountability for 15,000 reactor sites worldwide would be nearly impossible.

Uranium abundance: At the current rate of uranium consumption with conventional reactors, the world supply of viable uranium, which is the most common nuclear fuel, will last for 80 years. Scaling consumption up to 15 TW, the viable uranium supply will last for less than 5 years. (Viable uranium is the uranium that exists in a high enough ore concentration so that extracting the ore is economically justified.)

Uranium extraction from seawater: Uranium is most often mined from the Earth’s crust, but it can also be extracted from seawater, which contains large quantities of uranium (3.3 ppb, or 4.6 trillion kg). Theoretically, that amount would last for 5,700 years using conventional reactors to supply 15 TW of power. (In fast breeder reactors, which extend the use of uranium by a factor of 60, the uranium could last for 300,000 years. However, Abbott argues that these reactors’ complexity and cost makes them uncompetitive.) Moreover, as uranium is extracted, the uranium concentration of seawater decreases, so that greater and greater quantities of water are needed to be processed in order to extract the same amount of uranium. Abbott calculates that the volume of seawater that would need to be processed would become economically impractical in much less than 30 years.

Exotic metals: The nuclear containment vessel is made of a variety of exotic rare metals that control and contain the nuclear reaction: hafnium as a neutron absorber, beryllium as a neutron reflector, zirconium for cladding, and niobium to alloy steel and make it last 40-60 years against neutron embrittlement. Extracting these metals raises issues involving cost, sustainability, and environmental impact. In addition, these metals have many competing industrial uses; for example, hafnium is used in microchips and beryllium by the semiconductor industry. If a nuclear reactor is built every day, the global supply of these exotic metals needed to build nuclear containment vessels would quickly run down and create a mineral resource crisis. This is a new argument that Abbott puts on the table, which places resource limits on all future-generation nuclear reactors, whether they are fueled by thorium or uranium. As Abbott notes, many of these same problems would plague fusion reactors in addition to fission reactors, even though commercial fusion is still likely a long way off.

Of course, not many nuclear advocates are calling for a complete nuclear utopia, in which nuclear power supplies the entire world’s energy needs. But many nuclear advocates suggest that we should produce 1 TW of power from nuclear energy, which may be feasible, at least in the short term. However, if one divides Abbott’s figures by 15, one still finds that 1 TW is barely feasible. Therefore, Abbott argues that, if this technology cannot be fundamentally scaled further than 1 TW, perhaps the same investment would be better spent on a fully scalable technology.

“Due to the cost, complexity, resource requirements, and tremendous problems that hang over nuclear power, our investment dollars would be more wisely placed elsewhere,” Abbott said. “Every dollar that goes into nuclear power is dollar that has been diverted from assisting the rapid uptake of a safe and scalable solution such as solar thermal.”

Solar thermal devices harness the Sun’s energy to produce heat that creates steam that turns a turbine to generate electricity. Solar thermal technology avoids many of the scalability problems facing nuclear technology. For instance, although a solar thermal farm requires a little more land area than the equivalent nuclear power infrastructure, it can be located in unused desert areas. It also uses safer, more abundant materials. Most importantly, solar thermal can be scaled to produce not just 15 TW, but hundreds of TW if it would ever be required.

However, the biggest problem with solar thermal technology is cloudy days and nighttime. Abbott plans to investigate a number of storage solutions for this intermittency problem, which also plagues other renewable energy solutions such as wind power, in a future study. In the transition period, he suggests that the dual-use of natural gas with solar thermal farms is the pathway to building our future energy infrastructure.

More information: Derek Abbott. “Is nuclear power globally scalable?” Proceedings of the IEEE. To be published.

© 2010 PhysOrg.com

[Magpie_Oz]

Check out Thorium reactors, saw them in New Scientist they look promising.

[Other Means]

d/t, do you even read these things before you post them?

Location: He's assuming a certain MW o/p - why?

Lifetime: Another assumption based on generation 1 stations.

Nuclear waste: The only reason there's so much waste produced now is because the G1 reactors were built to create material for weapons. Current designs create little to no waste. Besides, if it's such a problem dig holes in front of tectonic subduction zones - the material will return to the core soon enough.

“Accident rate: To date, there have been 11 nuclear accidents at the level of a full or partial core-melt.” So what? Core melt != THE END OF THE WORLD. In Three Mile Island the molten mass froze when it hit the colder reactor vessel, and stopped its downward journey at five-eights of an inch through the five-inch thick vessel wall. I've no data on Fukushima yet. Chernobyl had no containment vessel – total deaths, 52.

Proliferation: Yes, true. But also true of a lot of chemical processes etc.

Uranium abundance: Not true. Funny this guy’s an Aussie, that’s where most of the available uranium is. The world could survive on the *currently mined* uranium for 5 years without breaking apart any bombs – and uranium isn’t the only fuel.

Exotic metals: this isn’t even funny. He does know it takes material to build anything, doesn’t he?

“More information: Derek Abbott. “Is nuclear power globally scalable?” Proceedings of the IEEE. To be published.”

Yeah I'll wait for that thanks.

[Magpie_Oz]

“Accident rate:....

New Scientist again quotes in order of descending number of deaths Coal, Hydro, LNG, nuclear

Seems about 13,200 people die each year in the US alone from Coal related circumstances.

[Other Means]

Deaths per TWH by energy source.

This doesn't even take into account deaths from poor air quality etc, which are hard to quantify.

Coal plants also put out far more radioactivity/year than nuclear.

[dieseltaylor]

Tsk tsk, Of course I read it. And as I get the New Scientist each week its not like I have no clue.

OM - if you read it then you will see that the total chosen was simply based on current usage and explored the situation from there. If you prefer the author might have said I will assume 33% of current usage and extrapolated from there - but then no doubt some bright spark is going to start talking of why restrict nuclear power to 33%!! : ) I am sure we can argue parts of the general thesis, and then I will ask what do YOU think the worlds power demand will be in 10 years time.

I agree some of the assumptions made seem to be based on current technologies and we all know how quickly science will move us on to newer less waste producing forms.

I cannot argue the specialist mineral aspects as I do not know enough.

Thorium reactors are fine in theory unfortunately the details have not yet been ironed out. AFAIR the first thorium reactor lasted 4 years before being mothballed because the highly specialist steels used were not up to the corrosion of the process.

But thorium is very much back in favour and perhaps this year India will have the first commercial one operating. Given though the time-lags in some countries I would imagine it is going to be 20-30 years before this design makes serious in-roads to the power generation spectrum.

In fact, an MSR was chosen as the base design for the 1960s DoD Atomic Plane largely because of its great safety advantages,

SO 51 years and counting : )

The point that Abbot highlights on rare minerals may be a problem however advances in nano-technology - and things like the glass like steel - may actually solve that particular niggle . However there is plenty time likely needed to resolve that. There is also how often new facilities would be required to be built to replace old reactors, and the area required.

Capture of solar energy has a lot to be said for it still as it can be captured pretty nearly everywhere during the day. WIndows can capture energy, roof tiles can etc with current technology - perhaps not commercially yet but then these things take time

So overall it does make sense not to ignore the simpler solutions whilst waiting to see if thorium reactors will be successful in a commercial way.

[Other Means]

The thing about solar, wind or wave power is the collecting methods take up so much land. They are unreliable, meaning there's got to be some sort of backup to them meaning huge redundancy, and they're distributed which means collecting and transmitting the energy is hugely expensive.

The unreliability means that even if we blanket the country with windmills and solar collectors we'll STILL need nuclear for calm, overcast days.

Have a look at this site:

http://www.withouthotair.com/

from David MacKay, who in 2008 was appointed Chief Scientific Advisor to the Department of Energy and Climate Change. Here he actually does the maths on what renewables can actually do for us – even if we turn the country into a giant, passive energy collecting machine.

FWIW Over the last 20 years I've seen New Scientist go from a highly respected, trustworthy magazine into "Hey kids - this is science!". I quit my subscription in disgust a few years ago.

[Stalins Organ]

I wonder if anyone has done a similar study on wind, totalling the number of turbines required to generate 15 TW at 1MW/turbine (that would be 15 million at how much area, dead birds & neighbour each?), plus they take some materials to build to, dunno what their "life" is, but assuming 50 years like nuke plants that means building/replacing 820 or so every day.......

Disposing of the composite materials in old blades might be a problem - are they biodegradable? safe for the environment? etc., etc.

why is it pepole take these things to extremes?

Nuclear power is never going to supply 100% of power in the first place -home roof-top hot water systems see to that if nothing else:rolleyes:

[dieseltaylor]

India, aiming to increase atomic-energy generation capacity 13-fold in the next two decades, is in talks with Kazakhstan, Niger and Namibia to acquire uranium mines, the head of the nation’s nuclear program said

Todays news. What is interesting in this is that India has one of the largest stocks of thorium and is also about to run a thorium reactor. Curious !?

[dieseltaylor]

If we get away from the "little" England approach to considering power production :

How much wind power could the U.S. produce?

According to the U.S. Department of Energy, all U.S. electrical energy needs could be met by the wind in Texas and the Dakotas alone.

I understand other countries have superior sunshine hours and more land than the UK so in general perhaps it is a matter of choosing what makes the most sense for each country. As a fundamental rule though I believe in having some redundancy in the system so that areas can exist without total reliance on National grids.

Generating hydrogen as a fuel would make renewable electricity more functional than purely electrical generation and the need to balance load.

[Other Means]

And when the wind doesn’t blow? You need something else. So if you DO invest all that money in power you can't count on, what aren't you building? Like clean fossil fuel stations with carbon capture or - what I think is needed - properly built nuclear plants.

[dieseltaylor]

I am working my way through

http://www.inference.phy.cam.ac.uk/withouthotair/c24/page_171.shtml

I am enjoying it. Good stuff. Particularly interesting was

The safety of nuclear operations in Britain remains a concern. The THORP

reprocessing facility at Sellafield, built in 1994 at a cost of £1.8 billion, had

a growing leak from a broken pipe from August 2004 to April 2005. Over

eight months, the leak let 85 000 litres of uranium-rich fluid flow into a

sump which was equipped with safety systems that were designed to detect

immediately any leak of as little as 15 litres. But the leak went undetected

because the operators hadn’t completed the checks that ensured

the safety systems were working; and the operators were in the habit of

ignoring safety alarms anyway.

The safety system came with belt and braces. Independent of the failed

safety alarms, routine safety-measurements of fluids in the sump should

have detected the abnormal presence of uranium within one month of the

start of the leak; but the operators often didn’t bother taking these routine

measurements, because they felt too busy; and when they did take mea-

surements that detected the abnormal presence of uranium in the sump

(on 28 August 2004, 26 November 2004, and 24 February 2005), no action

was taken.

By April 2005, 22 tons of uranium had leaked, but still none of the

leak-detection systems detected the leak. The leak was finally detected by

accountancy, when the bean-counters noticed that they were getting 10%

less uranium out than their clients claimed they’d put in! Thank goodness

this private company had a profit motive, hey? The criticism from the Chief Inspector of Nuclear Installations was withering: “The Plant was

operated in a culture that seemed to allow instruments to operate in alarm

mode rather than questioning the alarm and rectifying the relevant fault.”

If we let private companies build new reactors, how can we ensure that

higher safety standards are adhered to? I don’t know.

Regarding alternative power - well one of the solutions might be storing the hydrogen gas when there is oversupply to burn in generators if sufficient is derived from the wind. However I think the US has figures for days of sufficient wind when calculating their figures. It is a matter of looking at base load and what can be stored from fluctuating sources. Though sunshine has a pretty good availability record which could be harnessed.

And anyone familiar with Spain will wonder how much energy they are saving by solar heating water from their roofs. Compare this to the US where I suspect the idea is very much less common even in similar latitudes. Less power used less needs generating.

[Other Means]

Good for you d/t

I've not had time to read it all - tell me how you get on.

[gunnergoz]

d/t, do you even read these things before you post them?

Chernobyl had no containment vessel – total deaths, 52.

52 deaths? By whose reckoning? These folks disagree with your source: http://www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf

And the death toll was only the start of the horror:

http://mediastorm.com/publication/chernobyl-legacy

Mind you, I'm all for any rational, relatively safe and sane power source, including fusion if and when we ever get there. But to deliberately downplay an existing disaster to justify potentially reckless activity in the future, is just plain crazy.

And yes, every nuclear reactor plan begins in the premise that safety is the first concern, but once you get politics, corporations and lobbies involved in the process, the actual safety issues usually take a back seat to profits and election sound bites.

[LemoN]

The main problem of nuclear accidents compared to, let's say, coal related accidents (except for underground coal-fires) is that with a nuclear accident you can get tens of thousands of km² poisoned landscape that can't be inhabited for thousands of years. Now that isn't such a huge problem areas with a low population density but in mainland Europe for instance it would be devastating.

Heck, even now, 25 years after Chernobyl, up to 5% of all wild boars shot in the Bavarian Black forest have to be burned because the amount of radioactivity present in the meat is exceeding the maximum allowance by up to 40 times. And we still shouldn't eat mushrooms from that region either.

I'm aware that there's a load of hype, scaremongering and propaganda involved on all sides, but saying that the total death-toll of Chernobyl was 52 is laughable at best.

[Other Means]

You're both right, my mistake. 56. From here.

Total deaths from Fukishima - none.

And the death toll was only the start of the horror:

The risk of radiation-induced mutations in sperm and eggs, resulting in heritable disease "is sufficiently small that it has not been detected in humans, even in thoroughly studied irradiated populations such as those of Hiroshima and Nagasaki".

Source - hard green George Monbiot, in The Guardian.

Heck, even now, 25 years after Chernobyl, up to 50% of all wild boars shot in the Bavarian Black forest have to be burned because the amount of radioactivity present in the meat is exceeding the maximum allowance by up to 4000 times. And we still shouldn't eat mushrooms from that region either.

A) Source.

4000 times what?

BTW the IAEA is just a collection of green parties from across Europe - they're not any kind of scientific body.

[LemoN]

A) Source.

4000 times what?

First of all, I don't know how I managed it but sorry for the typos, I meant 5, not 50% and 40, not 4000. Well, I wrote that post in a hurry. O.o

Maximum allowance in the EU is 600Bq/kg, Wild boars often are around 12,000Bq/kg with some reported cases being more than 40 times above the 600Bq, that is more than 24,000Bq/kg. The reason for this is that the Wild Boars eat a lot of deep-soil truffles and the C-137 particles have accumulated in these depths. Normal mushrooms in that area at at ~2800Bq while the truffles the boars especially eat are at ~26,000Bq.

It's just a few small areas in the Bavarian forests btw, so it's not like the whole landscape is poisoned or something.

BTW the IAEA is just a collection of green parties from across Europe - they're not any kind of scientific body.

And "The Guardian" is?

Also, you could maybe revert back to a bit of an less aggressive tone, you come across quite rude.

[gunnergoz]

BTW the IAEA is just a collection of green parties from across Europe - they're not any kind of scientific body.

Is that "fact" from the "Guardian" too?

This is from their official site: http://www.iaea.org/

The IAEA Secretariat - the international body of staff tasked with running the Agency - is made up of a team of 2300 multi-disciplinary professional and support staff from more than 100 countries. They come from scientific, technical, managerial, and professional disciplines.

Most of these men and women work at Agency headquarters in Vienna, Austria. Others work at IAEA regional offices in Toronto and Tokyo, liaison offices in New York and Geneva, and research laboratories in Seibersdorf, Austria, and in Monaco. Six major IAEA departments - management, nuclear sciences and applications, nuclear energy, nuclear safety and security, technical cooperation, and safeguards and verification - set the organizational framework.

The work they do is as diverse as the landscape of peaceful nuclear technologies. Safeguards inspectors and analysts check and verify the whereabouts of sensitive nuclear material. Technical officers run projects that help countries bring fresh water to cities and richer harvests to farmers' fields. Others help scientists to better understand and protect the environment and medical doctors to prevent and treat diseases. Nuclear experts, radiation specialists, and engineers help countries to meet safety standards at nuclear plants, or to more safely manage and transport radioactive material.

Doesn't sound like a bunch of non-scientific tree-huggers to me.

[Speedy]

As soon as I read that he was a lecturer at an Australian university I stopped reading.

Was it a good article?

[Magpie_Oz]

As soon as I read that he was a lecturer at an Australian university I stopped reading.

And what exactly does that mean ?

[costard]

You're both right, my mistake. 56. From here.

Total deaths from Fukishima - none.

Actually, one carked it today. Older bloke, probably past his use-by date. (Press statement said no indication of radiation poisoning.)

[Bigduke6]

I think I can comment with some confidence on Chernobyl.

About 30 people died immediately from radiation poisoning. These were plant workers in coveralls sent to do stuff in super-polluted areas, or firemen wearing waterproofs sent to fight the graphite fire. The exposure took place in the first 12 hours of the accident.

After that the Soviets were putting their people in more proper gear, although whether it was fully adequate is still debated. The guys clearing debris from the reactor roof, for instance, typically wore Red Army rubberized biowar suits with zinc and/or lead armor hung over their torsos and groins, and they handled the debris with shovels, mostly by tossing it into containers. There was a max accumulated radiation established, sometimes it was exceeded and sometimes it wasn't, and once that level was hit the worker got sent home. Sometimes it took a day, sometimes weeks, it depended on the job.

There were about 50,000 of these workers that really got close to the hot materials, most of whom at the time were men of middle school education or less living say 300 km. or less from Chernobyl. Of these maybe 10-12 thousand are alive today. People are still arguing about how much that death rate is due to radiation exposure, however, most agree that working at the clean up almost always lowered the life span.

There was a team of ueber-scientists that got the job of going inside the reactor, taking pictures, looking for bodies, poking about in radioactive sludge, looking for mutants, etc. This was called "The Chernobyl Expedition." These guys got full hazmat suits and the Soviets approached it pretty much like a landing on hostile planet. For practical purposes, despite the fact they were at some points being bombarded by lethal radiation for minutes or if you believe the stories hours at a time, none of these workers suffered excess exposure.

I point this out because when one looks at Chernobyl and tries to use it as a measuring stick on how bad is a bad nuclear power accident, one must keep in mind the casualties suffered (and indeed the accident's taking place in the first place) were directly affected by Soviet attitudes towards safety and the relative value of personnel. It is not reasonable to look at Chernobyl and say another Chernobyl could cause the same damage elsewhere. Only if the operator were Soviet in his approach towards engineering and protection of human life, can one say that one might expect a Chernobyl-type accident from that operator.

Beyond direct radiation exposure it appears to have entered the population via diary products produced by cattle eating grass that either had irradiated dust on it, or less often, drew water from an irradiated water table. This caused a substantial bump in leukemia in the mostly rural children that drank it - once again, poor people with limited education living outside the big city were the ones most screwed.

As to irradiated terrain, the Chernobyl accident did not pollute evenly. Lower areas are more polluted as radioactive dust washes downhill. There are people still living in the Chernobyl Exclusion Zone today, and if you geiger counter their homes and yards radiation levels are normal. The problem is in a swamp or lower area a few hundred meters away, radiation levels can be 5-10 times normal, which won't kill you unless you live with it day in and day out. It is useful to remember these radioactive rays lose effect in a couple of meters from the emitter.

The Ukrainians and the Belarusians are now saying wood cut in areas with limited radiation, i.e mostly clean with spots 5-10 above normal, is safe to use. This sounds nuts but again, you put a dosimeter on the lumber, and the radiation level is normal.

To me, the lesson taught by Chernobyl is that if you do commit to nuclear power, the absolutely top priority must be staff that know what they are doing and who are deadly serious about safety, and who have the authority to spend money whenever they choose - usually by shutting the reactor down - when they see fit, and the accountants won't second guess them. Provided you have that, I would say nuclear power is certainly safe enough to consider as an energy source, you just need to factor in the costs of extra levels of safety nuclear power requires.

I would argue that from a safety POV Fukushima has done brilliantly. Considering what else people dump in the ocean and do to the land, pollution around the plant or some radioactive water into the sea is, comparatively speaking, no big deal. Radiation is far from the only poison mankind generates, nor is it the worst. (My vote would be dioxins or maybe nerve agents or gold mining if you're talking toxicity, or fossil fuels if you're talking overall harm.)

The question is, will building the next nuclear plant be worth it, given the alternatives? Sure the wind doesn't blow all the time, but if it blows most of the time and you put up enough windmills and build enough storage capacity, then you have a choice between which you dislike more, fields of windmills messing up the view or always worrying some goofball at the nuke plant is going to do something stupid.

52 deaths? By whose reckoning? These folks disagree with your source: http://www.iaea.org/Publications/Booklets/Chernobyl/chernobyl.pdf

And the death toll was only the start of the horror:

http://mediastorm.com/publication/chernobyl-legacy

Mind you, I'm all for any rational, relatively safe and sane power source, including fusion if and when we ever get there. But to deliberately downplay an existing disaster to justify potentially reckless activity in the future, is just plain crazy.

And yes, every nuclear reactor plan begins in the premise that safety is the first concern, but once you get politics, corporations and lobbies involved in the process, the actual safety issues usually take a back seat to profits and election sound bites.

[chris talpas]

I wonder if anyone has done a similar study on wind, totalling the number of turbines required to generate 15 TW at 1MW/turbine (that would be 15 million at how much area, dead birds & neighbour each?), plus they take some materials to build to, dunno what their "life" is, but assuming 50 years like nuke plants that means building/replacing 820 or so every day.......

Disposing of the composite materials in old blades might be a problem - are they biodegradable? safe for the environment? etc., etc.

why is it pepole take these things to extremes?

Nuclear power is never going to supply 100% of power in the first place -home roof-top hot water systems see to that if nothing else:rolleyes:

It is a few years old now, but there was a good seminar by Saul Griffith at the Long Now Foundation that discussed various alternatives to fossil fuels and the sheer numbers required.

http://longnow.org/seminars/02009/jan/16/climate-change-recalculated/

Chris