Stabilization Wedges (Part 2)

Okay. We’re going through this paper, which you can read yourself:

• Stephen Pacala and Robert Socolow, Stabilization wedges: solving the climate problem for the next 50 years using current technologies, Science 305 (2004), 968-972.

The paper lists 15 ‘wedges’, each of which could ramp up to reducing carbon emissions by 1 gigaton/year by 2054. We’re going through all these wedges and discussing them. And the Azimuth Project is lucky to have a new member on board — Frederik De Roo — who is summarizing our discussion here:

• Azimuth Project, Stabilization wedges.

So, let’s get going!

Last time we covered four wedges related to energy conservation and increased efficiency. Wedge 5 is in a category of its own:

5. Shifting from coal to natural gas. Natural gas puts out half as much CO₂ as coal does when you burn them to make a given amount of electricity. After all, it’s mainly methane, which is made from hydrogen as well as carbon. Suppose by 2054 we have coal power plants working at 90% of capacity with an efficiency of 50%. 700 gigawatts worth of coal plants like this emit 1 gigaton of carbon per year. So, we can reduce carbon emissions by one ‘wedge’ if we replace 1400 gigawatts of such plants with gas-burning plants. That’s four times the 2004 worldwide total of gas-burning plants.

Wedges 6-8 involve carbon capture and storage:

6. Capturing CO₂ at power plants. Carbon capture and storage at power plants can stop about 90% of the carbon from reaching the atmosphere, so we can get a wedge by doing this for 800 GW of coal-burning power plants or 1600 GW of gas-burning power plants by 2054. One way to do carbon capture and storage is to make hydrogen and CO₂ from fossil fuels, burn the hydrogen in a power plant, and inject the CO₂ into the ground. So, from one viewpoint, building a wedge’s worth of carbon capture and storage would resemble a tenfold expansion of the plants that were manufacturing hydrogen in 2004. But it would also require multiplying by 100 the amount of CO₂ injected into the ground.

7. Capturing CO₂ at plants that make hydrogen for fuel. You’ve probably heard people dream of a hydrogen economy. But it takes energy to make hydrogen. One way is to copy wedge 6, but then ship the hydrogen off for use as fuel instead of burning it to make electricity at power plants. To capture a wedge’s worth of carbon this way, we’d have to make 250 megatons of hydrogen per year from coal, or 500 megatons per year from natural gas. This would require a substantial scale-up from the 2004 total of 40 megatons of hydrogen manufactured by all methods. There would also be the task of building the infrastructure for a hydrogen economy. The challenge of injecting CO₂ into the ground would be the same as in wedge 6.

8. Capturing CO₂ at plants that turn coal into synthetic fuels. As the world starts running out of oil, people may start turning coal into synfuels, via a process called coal liquefaction. Of course burning these synfuels will release carbon. But suppose only half of the carbon entering a synfuels plant leaves as fuel, while the other half can be captured as CO₂ and injected underground. Then we can capture a wedge’s worth of CO₂ from coal synfuels plants that produce 1.8 teraliters of synfuels per year. For comparison, total yearly world oil production in 2004 was 4.7 teraliters.

Now: What are the pros and cons of these four wedges? What is the biggest thing that Pacala and Socolow overlooked?

I’m puzzled about the last wedge. Pacala and Socolow say 1 gigaton carbon/year is the flow of carbon in 24 million barrels/day, or 1.4 teraliters/year. They assume the same value for synfuels and allow for imperfect capture, which leads them to conclude that carbon capture at synfuels plants producing 1.8 teraliters/year of synfuel can catch 1 GtC/year. But this calculation doesn’t make sense to me. If we’re catching just half the carbon, and 1 GtC/year = 1.4 teraliters oil/year, don’t we need to generate at least twice that — 2.8 teraliters synfuel/year — to catch wedge’s worth of carbon?

I’m also unclear what percentage of the carbon you can actually capture while turning coal into synfuels. Can you really capture half of it?

There’s also another funny feature of this last wedge. If we assume people are already committed to making synfuels from coal, then I guess it’s true, they’ll emit less carbon if they use carbon capture and storage as part of the manufacturing process. But compared to making electricity or hydrogen as in wedges 6 and 7, turning coal into synfuels seems bound to emit more carbon, even with the help of carbon capture and storage.

In general, it only makes sense to talk about how much carbon emission some action prevents when we compare it to some alternative action. That’s pretty obvious, but it gets a bit confusing when some of Pacala and Socolow’s wedges look like plausible alternatives to other ones.

Another question: how does carbon capture and storage work, actually? Summarizing Pacala and Socolow, I wrote:

One way to do carbon capture and storage is to make hydrogen and CO₂ from fossil fuels, burn the hydrogen in a power plant, and inject the CO₂ into the ground.

But I’d like to learn the details!

Fossil Fuel Subsidies

Today my friend Bruce Smith pointed out this:

• How to save $300 billion, The Economist Online, 12 November 2010.

Here’s an executive summary for you busy folks:

This year’s World Energy Outlook, put out by the International Energy Agency, says that governments spent $312 billion on subsidies for fossil fuels in 2009.

According to the IEA, eliminating these subsidies would reduce the world’s energy demand by 5%: the current energy consumption of Japan, Korea and New Zealand combined. It would also reduce carbon emissions by about 0.4-0.5 gigatons by 2020.

(I think they mean annual carbon emissions. They also say “this is more than a third of the difference between business-as-usual emissions and the level needed to stand something like a 50:50 chance of limiting global warming to two degrees centigrade”, but that seems overly optimistic to me, given the figures I’ve been reading.)

Of the $312 billion in subsidies, more than a fifth comes from one country: Iran. To keep fuel prices as low as ten American cents a liter for gasoline — two cents for diesel — the Iranian government spent $66 billion in 2009. That’s $895 per person, or 20% of their GDP.

Saudi Arabia’s subsidy is even higher per capita, though lower overall and under 10% of GDP. (Guess what percent of their GDP comes from oil.)

Uzbekistan’s fossil fuel subsidy is even more absurd: a whopping 32% of GDP.

What is it for the USA, and the other countries the readers of this blog live in? What can we do to reduce or end these obscene subsidies?

Stabilization Wedges (Part 1)

Okay, let’s look at some plans for combating global warming! And let’s start with this paper:

• Stephen Pacala and Robert Socolow, Stabilization wedges: solving the climate problem for the next 50 years using current technologies, Science 305 (2004), 968-972.

I won’t try to summarize it all today, just a bit.

Stephen Pacala and Robert Socolow wrote this now-famous paper in 2004. Back then we were emitting about 6.2 gigatons of carbon per year, there were 375 ppm of carbon dioxide in the atmosphere, and many proposals to limit global warming urged that we keep the concentration below 500 ppm. Their paper outlined some strategies for keeping it below 500 ppm.

They estimated that to do this, it would be enough to hold emissions flat at 7 gigatons of carbon per year for 50 years, and then lower it to nothing. On the other hand, in a “business as usual” scenario, they estimate the emissions would ramp up to 14 gigatons per year by 2054. That’s 7 too many.

So, to keep emissions flat it would be enough to find 7 ways to reduce carbon emissions, each one of which ramps up linearly to the point of reducing carbon emissions by 1 gigaton/year in 2054. They called these stabilization wedges, because if you graph them, they look like wedges:

Their paper listed 15 possible stabilization wedges, each one with the potential to reduce carbon emissions by 1 gigaton/year by 2054. This is a nice way to start thinking about a very big problem, so many people have adopted it and modified it and criticized it in various ways, which I hope to discuss later. Right now I’ll only tell you about the original paper.

But before I even list any of their stabilization wedges, I should emphasize: stabilizing emissions at 7 gigatons is not enough to stay below 500 ppm forever! Carbon dioxide stays in the atmosphere a very long time. So, as Pacala and Socolow note:

Stabilization at any level requires that net emissions do not simply remain constant, but eventually drop to zero. For example, in one simple model that begins with the stabilization triangle but looks beyond 2054, 500-ppm stabilization is achieved by 50 years of flat emissions, followed by a linear decline of about two-thirds in the following 50 years, and a very slow decline thereafter that matches the declining ocean sink. To develop the revolutionary technologies required for such large emissions reductions in the second half of the century, enhanced research and development would have to begin immediately.

What’s the “declining ocean sink”? Right now the ocean is absorbing a lot of CO₂, temporarily saving us from the full brunt of our carbon emissions — while coral reefs, shellfish and certain forms of plankton suffer from increased acidity. But this won’t go on forever; the ocean has limited capacity.

Pacala and Socolow consider several categories of climate wedges:

• efficiency and conservation
• shifting from coal to gas
• carbon capture and storage
• nuclear fission
• renewable energy sources
• forests and agriculture

Today let me just go through the first category. Here they list four wedges:

1. Efficient vehicles: increase the fuel economy for 2 billion cars from 30 to 60 miles per gallon. Or, for those of you who don’t have the incredible good luck of living in the USA: increasing it from 13 to 26 kilometers per liter. When they wrote their paper, there were 500 million cars on the planet. They expected that by 2054 this number would quadruple. When they wrote their paper, average fuel efficiency was 13 kilometers/liter. To achieve this wedge, we’d need that to double.

2. Reduced use of vehicles: decrease car travel for 2 billion 30-mpg cars from 10,000 to 5000 miles per year. In other words: decreasing the average travel from 16,000 to 8000 kilometers per year. (Clearly this wedge and the previous one are not additive: if we do them both, we don’t save 2 gigatons of carbon per year.)

3. Efficient buildings: cut carbon emissions by one-fourth in buildings and appliances. This could be done by following “known and established approaches” to energy efficient space heating and cooling, water heating, lighting, and refrigeration. Half the potential savings are in the buildings in developing countries.

4. Efficient coal plants: raise the efficiency of coal power plants to 60%. In 2004, when they wrote their paper, “coal plants, operating on average at 32% efficiency, produced about one fourth of all carbon emissions: 1.7 GtC/year out of 6.2 GtC/year.” They expected coal power plants to double their output by 2054. To achieve this wedge, we’d need their average efficiency to reach 60%.

There are lot of questions to ask! Which do you think are the most easily achieved of these wedges? What are the biggest problems with their reasoning so far? And so on…

I would love any interesting information you have on: 1) ways to make vehicles more efficient, 2) ways to coax people to drive less, 3) ways to make buildings more efficient, and 4) ways to make coal power plants more efficient. Please post it here, with references if you can!

I’ll conclude for now with a couple of tiny points. First, they seem to vacillate a bit between saying there were 6.2 and 7 gigatons of carbon emitted in 2004, which is a bit odd, but perhaps just a way of giving the world a bit of slack before levelling off emissions at 7 GtC/year. I guess it’s not really a big deal.

Second, they aren’t idiots: despite the above graph, they don’t really think carbon emissions will increase linearly in a business-as-usual scenario. This is just a deliberate simplification on their part. They also show this supposedly more accurate graph:

They say the top curve is “a representative business as usual emissions path” for global carbon emissions in the form of CO₂ from fossil fuel combustion and cement manufacture, assuming 1.5% per year growth starting from 7.0 GtC/year in 2004. Note this ignores carbon emissions from deforestation, other greenhouse gases, etc. This curve is growing exponentially, not linearly.

Similarly, the bottom curve isn’t flat: it slopes down. They say the bottom curve is a “CO₂ emissions path consistent with atmospheric CO₂ stabilization at 500 ppm by 2125 akin to the Wigley, Richels, and Edmonds (WRE) family of stabilization curves described in [11], modified as described in Section 1 of the SOM text.”

Here reference [11] is:

• T. M. L. Wigley, in The Carbon Cycle, eds. T. M. L. Wigley and D. S. Schimel, Cambridge U. Press, Cambridge, 2000, pp. 258–276.

and the “SOM text” is the supporting online material for their paper, which unfortunately doesn’t seem to be available for free.

Our Future

I want to start talking about plans for cutting back carbon emissions, and some scenarios for what may happen, depending on what we do. We’ve got to figure this stuff out!

You’ve probably heard of 350.org, the grassroots organization that’s trying to cut CO₂ levels from their current level of about 390 parts per million back down to 350. That’s a noble goal. However, even stabilizing at some much higher level will require a massive effort, given how long CO₂ stays in the atmosphere:

In a famous 2004 paper, Pacala and Socolow estimated that in a “business-as-usual” scenario, carbon emissions would rise to 14 gigatons per year by 2054… while to keep CO₂ below 500 ppm, they’d need to be held to 7 gigatons/year.

Alas, we’ve already gone up to 8 gigatons of carbon per year! How can we possibly keep things from getting much worse? Pacala and Socolow listed 15 measures, each of which could cut 1 gigaton of carbon per year:

(Click for a bigger image.)

Each one of these measures is big. For example, if you like nuclear power: build 700 gigawatts of nuclear power plants, doubling what we have now. But if you prefer wind: build turbines with 2000 gigawatts of peak capacity, multiplying by 50 what we have now. Or: build photovoltaic solar power plants with 2000 gigawatts of peak capacity, multiplying by 700 what we have now!

Now imagine doing lots of these things…

What if we do nothing? Some MIT scientists estimate that in a business-as-usual scenario, by 2095 there will be about 890 parts per million of CO₂ in the atmosphere, and a 90% chance of a temperature increase between 3.5 and 7.3 degrees Celsius. Pick your scenario! The Stern Review on the Economics of Climate Change has a chart of the choices:

(Again, click for a bigger image.)

Of course the Stern Review has its detractors. I’m not claiming any of these issues are settled: I’m just trying to get the discussion started here. In the weeks to come, I want to go through plans and assessments in more detail, to compare them and try to find the truth.

Here are some assessments and projections I want us to discuss:

• International Panel on Climate Change Fourth Assessment Report, Climate Change 2007.

• The Dutch Government, Assessing an IPCC Assessment.

• The Copenhagen Diagnosis. Summary on the Azimuth Project.

• National Research Council, Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia. Summary on the Azimuth Project

• K. Anderson and A. Bows, Reframing the climate change challenge in light of post-2000 emission trends. Summary on the Azimuth Project.

• William D. Norhaus, A Question of Balance: Weighing the Options on Global Warming Policies.

• The Stern Review on the Economics of Climate Change.

And here are some “plans of action”:

• The Kyoto Protocol.

• World Nuclear Association, Nuclear Century Outlook. Summary and critique on the Azimuth Project.

• Mark Z. Jacobson and Mark A. Delucchi, A path to sustainable energy: how to get all energy from wind, water and solar power by 2030. Summary and critique on the Azimuth Project.

• Joe Romm, How the world can (and will) stabilize at 350 to 450 ppm: The full global warming solution. Summary on the Azimuth Project.

• Robert Pacala and Stephen Socolow, Stabilization wedges: solving the climate problem for the next 50 years with current technologies. Summary on the Azimuth Project

• New Economics Foundation, The Great Transition: A Tale of How it Turned Out Right. Summary on the Azimuth Project.

• The Union of Concerned Scientists, Climate 2030: A National Blueprint for a Clean Energy Economy.

• The Scottish Government, Renewables Action Plan.

• Bjorn Lømborg and the Copenhagen Business School, Smart Solutions to Climate Change.

As you can see, there’s already a bit about some of these on the Azimuth Project. I want more.

What are the most important things I’m missing on this list? I want broad assessments and projections of the world-wide situation on carbon emissions and energy, and even better, global plans of action. I want us to go through these, compare them, and try to understand where we stand.

2010 Singapore Energy Lecture

Yesterday morning Lisa and I took a bus downtown to see the Prime Minister of Singapore, Lee Hsien Loong, give the Singapore Energy Lecture. It was part of a big annual event, the Singapore International Energy Week. We met our friend Walter Blackstock, had a last cup of coffee, and filed into a banquet room, along with about 800 businessmen, to see what the Prime Minister had to say.

His lecture was clear and precise — very different than the rhetoric-filled talk I’m used to from American politicians when it comes to energy. He began by discussing the twin problems of peak oil and global warming.

‘Peak oil’ refers to the idea that oil production, having risen, is bound to eventually fall; of course this idea gains teeth when one believes it will fall rather soon. Mr Lee gave some evidence that oil production will fall in the next few decades, but then pointed out that similar predictions had been made before, and had turned out to be wrong. He concluded in an agnostic vein, and added that there are still huge supplies of coal, which become more useful as gasification technologies improve. What interested me was his use of the term ‘peak oil’, which I’ve never heard from the lips of an American president. But of course, I’ve never seen one speak at an energy conference.

He then noted that even if there are plenty of fossil fuels, burning them leads to global warming. He mentioned the 2010 United Nations Climate Change Conference, which will be held in Cancún, Mexico, from 29 November to 10 December. He said that if an agreement is reached, Singapore would abide by it and impose a price on carbon. He said:

“If there’s a global regime to curb carbon emissions, that means that Singapore will have to reduce our own emissions more sharply than we are doing now, in order to comply with international obligations, and we would have to make the carbon price explicit to send the right price signals.”

Increases in efficiency would not be enough, he noted, because of the ‘rebound effect’: more efficient energy usage just makes people use more energy. He also said:

“At present, we don’t have a carbon tax, but we calculate a shadow price for carbon in our cost-benefit analysis so that Government policies and decision making can be better-informed and rational.”

On the other hand, he said, if an agreement is not reached, uncertainty will continue to prevail about when the problem of global warming will finally be addressed. Given this, he said, Singapore supports the goal of reaching an agreement. He explicitly noted that this was a ‘commons’ problem: every individual nation stands to benefit by being the only one who continues to burn lots of carbon, but if every nation does this, the world will be harmed.

He was not optimistic about an agreement being reached in Cancún; he mentioned how Obama had begun his term in office strongly supporting climate change legislation, but was unable to stick with this intention. Nonetheless, he seemed to indicate that a price on carbon was inevitable in the long term.

He discussed three main ways to reduce carbon usage:

1) switching to sustainable forms of energy production like solar, wind and hydropower,

2) technologies such as carbon sequestration and nuclear power, and

3) conservation.

He said that Singapore is “an alternative energy-disadvantaged country”, so option 1) is limited. He explicitly mentioned that most sustainable forms of energy have a low ‘power density’, and again his correct use of jargon pleased me. He said that even if every building in Singapore was covered with solar cells, that would only generate 10% of the necessary power.

He said that the use of nuclear power was an option one cannot afford to dismiss:

“There is often strong resistance in countries – from the green movement, from populations who have witnessed accidents like Chernobyl, and are fearful and anxious about their safety. But if we look at this rationally, without nuclear energy, the world cannot make sufficient progress in dealing with global warming”.

He pointed out that America is beginning to build more nuclear plants, and that Angela Merkel, despite great pressure from the Greens, had put off the closure of such plants in Germany. He said that more plants would eventually be built in Germany, even though it’s “unspeakable” now.

He argued that it is important to start moving forward on this issue, even in a small state like Singapore where any nuclear plant would necessarily be close to densely populated areas. The crucial first step, he said, is to develop the technical know-how and the necessary “culture of safety”. When the moderator asked him whether nuclear power would be introduced during his time in office, he replied:

“I would say possibly during my lifetime.”

He also spent a lot of time discussing option 3), energy conservation. He said Singapore has a pilot program for a “smart grid” that lets households see how they’re using electrical power, and if this turns out to increase their efficiency, this would be adopted more widely.

All in all, an interesting talk.

Energy Return on Energy Invested

The Azimuth Project wiki has been up and running for exactly one month!

We’ve built up a nice bunch of articles sketching some of the biggest environmental problems we face today — and some ideas for dealing with them. I invite you to look these over and improve them! It’s very easy to do.

I also invite you to join us at the Azimuth Forum, where we are deciding the fate of humanity (or something like that). We need your help!

In the weeks to come I want to tell you what we’ve learned so far. I especially want to talk about various plans of action that people have formulated to tackle global warming. Even sitting here in the comfort of this cozy blog, you can help us compare and criticize these plans.

But I also want to tell you about some interesting concepts. And the first is EROEI, or “Energy Return On Energy Invested”. The Azimuth Project entry on this concept was largely written by David Tweed. Three cheers for David Tweed!

It also had help from Eric Forgy, Graham Jones and David Pollard, and a major contribution from Anonymous Coward. I’ll shorten it and amp it up for the purposes of this blog. I know you’re here to be entertained.

The Idea

You’ve probably heard the saying “it takes money to make money”. Similarly, it takes energy to make energy. More precisely, it takes useful energy to make useful energy.

Energy Returned On Energy Invested or EROEI captures this idea: it’s simply the ratio of “useful energy acquired” to “useful energy expended”. Note that money does not enter into this concept. The difficult and often heated debate arises when we try to decide which inputs and outputs count as “useful”.

There are other names for this concept and closely related concepts. “Energy profit ratio”, “surplus energy”, “energy gain”, and “EROI”, and EROEI all describe virtually the same idea: how much energy we receive per energy put in. See:

• Nate Hagen, A Net Energy Parable: Why is ERoEI Important?, The Oil Drum, 2006.

The concept of “energy yield ratio” is also very similar, but tends to be used in slightly different ways. See the Azimuth Project article for more.

Details

The definition of EROEI for a process of “extracting energy”
is the useful acquired energy divided by the useful energy expended. The “useful” tag denotes energy which is usable by human beings now. For example: a supernova wastes a lot of energy in the process of making uranium and blasting it out into space. But that was done long before we came along, so it makes no sense to include it in the EROEI inputs.

In practice, people include inputs and outputs that aren’t strictly “energies”, but rather “substances from which energy can be extracted”. For example: one could look at the EROEI of growing trees for fuel, where the wood produced is counted as an output according to the energy extractable by burning.

In general, having a high EROEI value counts as “good”. Indeed, when the EROEI drops below 1 more energy is being used in the extraction process than is being output at the end! But because it only considers energy issues (and not resource scarcity, scalability, pollution, etc.), EROEI should only be one input into our process of deciding on technologies and actions.

When it comes to computing EROEI, the hard part is deciding which inputs and outputs should be included in the ratio — particularly since this involves considering which other competing processes are genuinely viable.

Another complication is that while various forms of energy can generally be converted to each other, this will incur losses due to conversion inefficiencies. So, you can’t look at two schemes with the same useful energy inputs that produce different kinds of energy — e.g., electricity and heat — and declare the one with the higher EROEI as more suitable.

Examples

To see some of the difficulties in calculating an EROEI, let’s imagine growing a crop of grass and then fermenting it to produce a liquid fuel. The most obvious inputs and outputs are:

“Energy” outputs:

1. The liquid fuel itself. This is unarguably useful output “energy”.

2. There may be excess heat produced by the fermentation process. Whether this is useful is debatable since the energy is of high entropy and produced at plants located away from energy consumers.

3. The remaining biomass may be suitable for burning. Again the usefulness is debatable, since the biomass may be better used for fertilising the fields used to grow the crop. Even if this isn’t the case, the biomass may require yet more energy to collect into a dry, burnable state.

“Energy” inputs:

1. Sunlight. Except for exceptional circumstances, there is no other use for sunlight falling on fields so this does not count as a useful input.

2. Artificial fertilizer. This requires energy to produce and could be used for growing food or other crops, so it definitely counts as a useful energy input.

3. Energy used by motorized vehicles, both during farming and transportation to the biomass plant. For the same reasons as fertilizer, this counts as a useful energy input.

4. Mechanical energy used to extract liquid fuel after fermentation and clear waste products from the apparatus. Again a useful energy input.

Thus one computation of EROEI would count outputs 1 and inputs 2, 3 and 4.

However, suppose that the grass crop is genuinely being grown for other reasons — e.g., as part of a crop rotation scheme — and the plant is sufficiently small that the excess heat can be used fully by the plant for staff heating. Then you could argue that the EROEI should count outputs 1 and 2 and count inputs 3 and 4. So, to determine the EROEI you need to decide which alternative uses are genuinely viable.

Note also that this EROEI calculation is purely about energy! It does not reflect issues such as whether the land usage is sustainable, possible soil depletion/erosion, scarcity of mineral inputs for artificial fertilizer, etc.

Comparison

Okay, but enough of these nuances and caveats. Important as they are, I know what you really want: a list of different forms of energy and their EROEI’s!

Natural gas: 10:1
Coal: 50:1
Oil (Ghawar supergiant field): 100:1
Oil (global average): 19:1
Tar sands: 5.2:1 to 5.8:1
Oil shale: 1.5:1 to 4:1

Wind: 18:1
Hydro: 11:1 to 267:1
Waves: 15:1
Tides: ~ 6:1
Geothermal power: 2:1 to 13:1
Solar photovoltaic power: 3.75:1 to 10:1
Solar thermal: 1.6:1

Nuclear power: 1.1:1 to 15:1

Biodiesel: 1.9:1 to 9:1
Ethanol: 0.5:1 to 8:1

This list comes from:

• Richard Heinberg, Searching for a Miracle: ‘Net Energy’ Limits & the Fate of Industrial Society.

You can read this report for more details on how he computed these numbers. If you’re like me, you’ll take a perverse interest in forms of energy production with the lowest EROEIs. For example, what idiot would make ethanol in a way that yields only half as much useful energy as it takes to make the stuff?

The US government, that’s who: the powerful corn lobby has been getting subsidies for some highly inefficient forms of biofuel! But things vary a lot from place to place: corn grows better in the heart of the corn belt (like Iowa) than near the edges (like Texas). So, the production of a bushel of corn in Iowa costs 43 megajoules of energy on average, while in Texas it costs 71 megajoules.

Similar, ethanol from sugar cane in Brazil has an EROEI of 8:1 to 10:1, but when made from Louisiana sugar cane in the United States, the EROEI is closer to 1:1.

“Solar thermal” also comes out looking bad in the table above, with an EROEI of just 1.6:1. But what’s “solar thermal”? Heinberg has a section on “active” or “concentrating” solar thermal power, where you focus sunlight to heat a liquid to drive a turbine. He also has one on “passive” solar, where you heat your house, or water, by sun falling on it. But he doesn’t give EROEI’s in either of these sections — unlike the sections on other forms of energy. So I can’t see where this figure of 1.6 is coming from.

Anyway, there’s a lot to think about here. Each one of the numbers listed above could serve as the starting-point for a fascinating discussion! Let’s start…

Power Density

Today I’ve been thinking about “power density”, and I’ve got some questions for you.

But let’s start at the beginning!

In his 2009 talk at the Long Now Foundation, the engineer Saul Griffith made some claims that fill me with intense dread. Stewart Brand summarized the talk as follows:

The world currently runs on about 16 terawatts (trillion watts) of energy, most of it burning fossil fuels. To level off at 450 ppm of carbon dioxide, we will have to reduce the fossil fuel burning to 3 terawatts and produce all the rest with renewable energy, and we have to do it in 25 years or it’s too late. Currently about half a terrawatt comes from clean hydropower and one terrawatt from clean nuclear. That leaves 11.5 terawatts to generate from new clean sources.

That would mean the following. (Here I’m drawing on notes and extrapolations I’ve written up previously from discussion with Griffith):

“Two terawatts of photovoltaic would require installing 100 square meters of 15-percent-efficient solar cells every second, second after second, for the next 25 years. (That’s about 1,200 square miles of solar cells a year, times 25 equals 30,000 square miles of photovoltaic cells.) Two terawatts of solar thermal? If it’s 30 percent efficient all told, we’ll need 50 square meters of highly reflective mirrors every second. (Some 600 square miles a year, times 25.) Half a terawatt of biofuels? Something like one Olympic swimming pools of genetically engineered algae, installed every second. (About 15,250 square miles a year, times 25.) Two terawatts of wind? That’s a 300-foot-diameter wind turbine every 5 minutes. (Install 105,000 turbines a year in good wind locations, times 25.) Two terawatts of geothermal? Build 3 100-megawatt steam turbines every day — 1,095 a year, times 25. Three terawatts of new nuclear? That’s a 3-reactor, 3-gigawatt plant every week — 52 a year, times 25”.

In other words, the land area dedicated to renewable energy (“Renewistan”) would occupy a space about the size of Australia to keep the carbon dioxide level at 450 ppm. To get to Hansen’s goal of 350 ppm of carbon dioxide, fossil fuel burning would have to be cut to ZERO, which means another 3 terawatts would have to come from renewables, expanding the size of Renewistan further by 26 percent.

The main scary part is the astounding magnitude of this project, and how far we are from doing anything remotely close. Griffith describes it as not like the Manhattan Project, but like World War II — only with everyone on the same side.

But another scary part is the amount of land that needs to get devoted to “Renewistan” in this scheme. This is where power density comes in.

The term power density is used in various ways, but in the work of Vaclav Smil it means the number of usable watts that can be produced per square meter of land (or water) by a given technology, and that’s how I’ll use it here.

Smil’s main point is that renewable forms of energy generally have a much lower power density than fossil fuels. As Griffith points out, this could have massive effects. Or consider the plan for England, Scotland and Wales on page 215 of David MacKay‘s book Without the Hot Air:

That’s a lot of land devoted to energy production!

Smil wrote an interesting paper about power density:

• Vaclav Smil, Power density primer: understanding the spatial dimension of the unfolding transition to renewable electricity generation

In it, he writes:

Energy density is easy – power density is confusing.

One look at energy densities of common fuels is enough to understand while we prefer coal over wood and oil over coal: air-dried wood is, at best, 17 MJ/kg, good-quality bituminous coal is 22-25 MJ/kg, and refined oil products are around 42 MJ/kg. And a comparison of volumetric energy densities makes it clear why shipping non-compressed, non-liquefied natural gas would never work while shipping crude oil is cheap: natural gas rates around 35 MJ/m³, crude oil has around 35 GJ/m³ and hence its volumetric energy density is a thousand times (three orders of magnitude) higher. An obvious consequence: without liquefied (or at least compressed) natural gas there can be no intercontinental shipments of that clean fuel.

Power density is a much more complicated variable. Engineers have used power densities as revealing measures of performance for decades – but several specialties have defined them in their own particular ways….

For the past 25 years I have favored a different, and a much broader, measure of power density as perhaps the most universal measure of energy flux: W/m² of horizontal area of land or water surface rather than per unit of the working surface of a converter.

Here are some of his results:

• No other mode of large-scale electricity generation occupies as little space as gas turbines: besides their compactness they do not need fly ash disposal or flue gas desulfurization. Mobile gas turbines generate electricity with power densities higher than 15,000 W/m² and large (>100 MW) stationary set-ups can easily deliver 4,000-5,000 W/m². (What about the area needed for mining?)

• Most large modern coal-fired power plants generate electricity with power densities ranging from 100 to 1,000 W/m², including the area of the mine, the power plant, etcetera.

• Concentrating solar power (CSP) projects use tracking parabolic mirrors in order to reflect and concentrate solar radiation on a central receiver placed in a high tower, for the purposes of powering a steam engine. All facilities included, these deliver at most 10 W/m².

• Photovoltaic panels are fixed in an optimal tilted south-facing position and hence receive more radiation than a unit of horizontal surface, but the average power densities of solar parks are low. Additional land is needed for spacing the panels for servicing, access roads, inverter and transformer facilities and service structures — and only 85% of a panel’s DC rating is transmitted from the park to the grid as AC power. All told, they deliver 4-9 W/m².

• Wind turbines have fairly high power densities when the rate measures the flux of wind’s kinetic energy moving through the working surface: the area swept by blades. This power density is commonly above 400 W/m² — but power density expressed as electricity generated per land area is much less! At best we can expect a peak power of 6.6 W/m² and even a relatively high average capacity factor of 30% would bring that down to only about 2 W/m².

• The energy density of dry wood (18-21 GJ/ton) is close to that of sub-bituminous coal. But if we were to supply a significant share of a nation’s electricity from wood we would have to establish extensive tree plantations. We could not expect harvests surpassing 20 tons/hectare, with 10 tons/hectare being more typical. Harvesting all above-ground tree mass and feeding it into chippers would allow for 95% recovery of the total field production, but even if the fuel’s average energy density were 19 GJ/ton, the plantation would yield no more than 190 GJ/hectare, resulting in harvest power density of 0.6 W/m².

Of course, power density is of limited value in making decisions regarding power generation, because:

1. The price of a square meter of land or water varies vastly depending on its location.

2. Using land for one purpose does not always prevent its use for others: e.g. solar panels on roofs, crops or solar panels between wind turbines.

Nonetheless, Smil’s basic point, that most forms of renewable forms of energy will require us to devote larger areas of the Earth to energy production, seems fairly robust. (An arguable exception is breeder reactors, which in conjunction with extracting uranium from seawater might be considered a form of renewable energy. This is important.)

On the other hand, fans of solar energy argue that much smaller areas would be needed to supply the world’s power. There are two possible reasons, and I haven’t sorted them out yet:

1) They may be talking about electrical power, which is roughly one order of magnitude less than total power usage.

2) As Smil’s calculations show, solar power allows for significantly greater power density than wind or biofuels. Griffith’s area for ‘Renewistan’ may be large because it includes a significant amount of power from those other sources.

What do you folks think? I’ve got a lot of questions:

• what’s the power density for nuclear power?

• what’s the power density for sea-based wind farms?

and some harder ones, like:

• how useful is the concept of power density?

• how much land area would be devoted to power production in a well-crafted carbon-neutral economy?

and that perennial favorite:

• what am I ignoring that I should be thinking about?

If Saul Griffith’s calculations are wrong, and keeping the world from exceeding 450 ppm of CO₂ is easier than he thinks, we need to know!