Disclaimer: Written for an American site, so all standard US-centric qualifiers and apologies apply.
Energy Return on Investment versus Net Energy Yield
When looking at EROI, it is easy to get lost in the woods of arguing whether an EROI is 50 or 30, whether something is left out that should be included, whether something is included that should be left out. However, an important point is raised in an 2011 Oil Drum article, The Energy Return on Investment Threshold, which begins with the statement:
Hall and Day (2009) report that the EROI for coal might be as high as 80 and that for hydropower, EROI is 40. Does this mean that coal is twice as `good' as hydro? The answer is no ...
Don't worry about the next paragraph if you aren't comfortable with algebra, as you will see, the problem has already been worked out for you.
How could 80 fail to be twice as good as 40? Well, EROI looks at energy out for each one energy input used up
. That uses the amount of energy you lose along the way as the measuring stick, when for a lot of issues, that's the thing we want to measure. To give a useful measure of how much energy is used up in producing the energy, you need the concept of net energy. Net Energy is measured in absolute units (Watts, Joules, etc), but you can also take it as a percentage of the energy produced, to give the Net Energy Yield:
- EROI = Energy Out / Energy In
- Net Energy = Energy Out - Energy In
- Net Energy Yield = (Energy Out - Energy In) / Energy Out
- So: Net Energy Yield = (EROI-1)/EROI
And the figure on the right plots the EROI versus the Net Energy Yield (which it calls the "% Energy Out"). And that is why an EROI of 80 is not twice as good as an EROI of 40. The Net Energy Yield comparison for EROI of 80 vs 40 are yields of 98.75% vs 97.5% (79/80 vs 39/40), so the same energy flow results in 1.25% less energy available.
Its when we get down to the level marked on the figure as the EROI threshold that the differences start to buy. Instead of comparing EROI 80 vs EROI 40, what happens when we compare EROI 8 vs EROI 4? That is 87.5% vs 75%, a loss of 12.5% less energy available. Indeed, lets look at the Net Energy Yield for the hypothetical values in Charles Hall answer (EROI ~ Net Energy Yield %)
- 1.1 ~ 9%
- 1.2 ~ 17%
- 1.3 ~ 23%
- 3 ~ 67%
- 5 ~ 80%
- 7 ~ 86%
- 8 or 9 ~ 88%-89%
Indeed, he sets the threshold for a modern industrial society with the long childhood education required for a highly skilled workforce at just about the same point that David Murphy places the threshold for the "cliff" when drops in EROI start to really bite into our available Net Energy Yield.
What are the Sustainable, Renewable Net Energy Yields Available?
Just as the Big Oil funded Heritage, Cato and Reason foundations churn out reports on why providing Americans with any freedom of choice to pick an alternative to gasoline fueled cars and diesel fueled buses is a bad thing ~ since they are self-identified libertarians, and nothing says "liberty" like denying Americans choice ~ there seems to be a cottage industry in pushing the idea that a fully sustainable, renewable energy economy is an impossibility. Google for "Can the World Run on Renewable Energy" and "negative case" and you'll find several different revisions and country-specific versions of one of these arguments.
However, when we consider available resources and the EROI threshold of 8, the negative case is on shaky ground.
First and foremost comes wind power. In a meta-study of wind power EROI studies from 1997-2007, Kubiszewski, Cleveland and Endres (2009)
find an average over 60 operational studies of 19.8, for a net energy yield of 95%. And this will, of course, rise over the coming decade, since advances are still being made in the energy efficiency of wind turbines at relatively low wind-speeds. Clean Technica covered a press event by General Electric last month introducing its newest model wind turbine. According to GE Wind Products General Manager Keith Longtin
"We've made incredible gains since acquiring the property," Longtin stated, pointing out that today's GE turbines are operating at close to 98% availability (97.6%), the same as a thermal coal plant. Furthermore, he continued, "with the introduction of the 1.6-100, we've also improved the capacity factor (a measure of energy efficiency) from 35% ten years ago to over 50% today." Over 50% capacity factor is far above the capacity factor that most people think of when they think of wind turbines. Clearly, very significant strides have been made to get to such a high percentage.
Contributing to the boost in turbine availability and efficiency, GE's Brilliant 1.6-100 captures and converts more wind energy at lower (Class 3) wind speeds, which, by definition, blow at 7.5 meters per second (m/s).
The 1.6-100 integrates short term grid scale battery power storage. About 7% of wind power currently available to installed wind turbines is lost because of the fact that the grid cannot accept power increases from wind turbines as rapidly as the wind turbines can provide the power, and with short term grid scale battery power, that ramp up of power availability can be matched to the grid bottleneck. The batteries play a similar role when ramping down the wind turbines. It makes sense to integrate this scale of battery storage with the wind turbines, since they can use the power transformer that the wind turbines require in any event.
Kubiszewski, Cleveland and Endres (2009) also cite meta-studies of EROI estimates for other power sources. Notable among them are:
- Photo-Voltaic Solar with an average EROI of 6.7, though primarily from simulation studies rather than operational studies;
- nuclear with an average EROI of 15.8-9.1, depending on whether studies that omit the energy cost of one or more stage of fuel processing are included;
- and dammed hydropower with an EROI of 12, though as noted with limits to our ability to increase dammed hydropower
They do not include estimates of corn ethanol fuel, singled out by Charles Hall as an example of a fuel with an egregiously inadequate EROI. The basis for his remarks can be seen in David Murphy, Charles Hall and Bobby Powers (2011)
, which considers EROI studies of corn ethanol fuel, ranging from estimates of EROI of 0.8-1.5. We don't really need to wade into the details of their average estimate of 1.01 (a net energy yield of 1%), since an optimistic
EROI of 1.5 is a Net Energy Yield of only 33%.
It is important to distinguish corn ethanol from sugar cane ethanol. Corn ethanol requires substantial energy-intensive fertilizer inputs to get maximum yield for a plant creating a protein-intensive cereal grain, with so much of the plant discarded in the production of corn ethanol. Indeed, a major part of the difference between low EROI at the top end and extremely low estimates at the bottom end are whether the cogeneration of byproducts of corn ethanol biomass is counted, or whether the byproducts are treated as a product that must be returned to the field to sustain fertility.
The other extreme of a liquid biofuel that is often cited is sugar cane ethanol, with a plant that is specialized to produce a relatively large amount of simple carbohydrates, and which can grow and regrow two to three times per planting. An EROI figure of 8 for Brazilian sugarcane ethanol is often cited, but as Robert Rapier notes in the 2008 Oil Drum EROI round-up
, this is a specialized figure focusing just on oil independence:
The oft-cited Brazilian EROEI is really a cousin of EROEI. What is done to arrive at the 8 to 1 sugarcane EROEI is that they only count the fossil fuel inputs as energy. Boilers are powered by burning bagasse, but this energy input is not counted. (Also, electricity is sometimes exported, and credit is taken for this). For a true EROEI calculation, all energy inputs should be counted. So what we may see is that the EROEI for sugarcane is 2 to 1 (hypothetically) but since most inputs are not fossil-fuel based the EROEI based only on fossil-fuel inputs is 8 to 1.
An EROI of 2 is still substantially better than an EROI of (optimistically) 1.5 for corn ethanol, since that is an energy yield of 50% rather than an energy yield of 33%. However, it still relegate currently produced ethanol to the status of a secondary fuel source for specialized applications where the portability of a liquid fuel justifies reliance on such a low EROI fuel.
The most promising biomass energy source that is proven technology (though not currently widely deployed) is biocoal. As Engineer-Poet wrote in 2006
, in a long article worth setting aside time to read from beginning to end:
... [A] whale of a lot of energy is lost in conversions. The average refinery makes gasoline with 83% efficiency, but engines are so inefficient that more energy goes to refining losses than pushing the vehicle. An ethanol engine is potentially more efficient than the gasoline equivalent, but the conversion from biomass to ethanol loses so much that it takes more biomass energy than crude oil to do the same job! Biomass gasification may be more efficient than Iogen's hydrolization and fermentation, but even a 70%-efficient process yields barely 18% end-to-end efficiency at best. Still, the available energy from biomass looks to be several times the energy we actually use from crude oil. The conclusions are inescapable:
- There is sufficient biomass energy to replace motor fuel and then some... if the energy is not wasted.
- Using bio-ethanol in piston engines means taking between 4/5 and 9/10 of the captured energy and throwing it away.
- Even burning biomass as a replacement for e.g. coal in conventional powerplants means 60% losses or more.
- It looks impossible to grow enough biomass to take that path.
- The old paradigm won't work any more. A new systems approach is required.
- The essence of a successful system will be fewer conversions and minimizing losses.
Engineer-Poet's solution to reducing conversions and minimizing losses is biocoal: charcoal produced through partial burning in a sealed chamber that allows capturing the exhaust gases. The sealed chamber is the biggest difference between old-fashioned charcoal and biocoal production, since the exhaust gases can be used directly generate electricity. Assume that:
- 53.5% of the energy is yielded as charcoal (30% by weight).
- 88% of the remainder is yielded as chemical energy in hot gas (11.1 quads gas + 1.51 quads reaction heat + recycled heat).
- The gas can be converted to electricity at 50% efficiency.
Then the charcoal and the medium-BTU thermal gas has a crude energy loss of 12% of 47.5%, or 5,7%, so those products have an energy yield of 94.3%, and an EROI of 17.5. This is the figure that is directly equivalent to the EROI for creating various liquid biofuels.
Now, if all of this is converted to electricity, the EROI and energy yield is lower. The net energy yield for electricity is much lower if the biocoal is burned in conventional coal powered plants at an efficiency of about 33%, while the medium-BTU is converted into electricity at the production site using Solid Oxide fuel cells at an efficiency of 50%, then the electrical net energy yield is 38% (18%+20%), or an EROI of 1.6, However, if the biocoal is converted into electricity using Direct Carbon Fuel Cells with an efficiency of about 75%, then the electrical net energy yield is 60% (40%+20%), or an EROI for the electricity of 2.5.
Of course, the wind power and photovoltaic solar power is created as
electricity, so do not suffer the energy losses when converting solid and liquid fuels to electricity. At the other extreme, any thermal power plant that burned a coal or sugarcane ethanol fuel would operate at a negative net energy yield, acting as a net consumer of energy rather than as an energy source.
Current nuclear technologies are primarily Light Water reactors consuming uranium fuel without recycling. We have on the order of 70 years of usable uranium fuel sources at current rates of consumption, and as we work through our richest uranium fuel sources, the EROI and net energy yield of existing Light Water technology will continue to decline as the energy input for fuel enrichment continues to climb. (Note that a recent diary on EROI at daily kos
citing nuclear EROI of 75 from a paywalled study, appears to be a case of either rigging the study by ignoring fuel enrichment energy costs, or else based on simulations of hypothetical nuclear technologies under development.)
There are technologies that recycle fuel using side-effects of the nuclear chain reaction, to substantially reduce the energy cost of fuel enrichment, and these may offer EROI of 15 to 25 over a longer period of time, but most of these technologies carry a risk of nuclear proliferation. However, the thorium fuel cycle, which was not been pursued intensively in the United State precisely because it is less
useful than many rivals in the generation of weapons-grade nuclear materials, does offer a prospective recycling fuel cycle that might well offer useful supplementary source of electrical power.
EROI of a Sustainable Energy Portfolio
The first main element that matters for the maintenance of an advanced industrial economy is not the EROI and Net Energy Yield of specific components of the energy portfolio, but the overall EROI and Net Energy Yield of the total portfolio of energy sources.
It has been noted that the US could move to 20% windpower without requiring substantial changes in the way that we manage and regulate electricity generation. All that would be required would be some investment in long distance electricity transport from the Great Plains to population centers east of the Mississippi. No substantial investment in energy storage would be required, since shuffling around the operation of existing hydropower and existing gas-fueled peak power plants would cope with the volatility of wind power.
This could be used to argue that "we can use at most 20% windpower for our electrical needs", but that would be assuming that we are incapable of adapting our electrical supply system to cope with new types of power sources. That is a shaky assumption, since after all we were not handed our current system full-formed from some extra-terrestrial civilization, but rather developed it
to cope with the capacities and limitations of fuel-powered electricity supplies.
More importantly, though, this ignores portfolio effects. Solar power and windpower generation tend to peak at different times of the day, and the peak of solar power generation is well correlated to peak power demand. So 30% wind with 10% solar is easier to integrate into the grid than 20% wind alone. At the same time, for both, high production periods at a specific wind farm or solar producing region tends to be correlated with lower production in the other technology, and with wind farms and solar producing regions in other areas.
So the 20% "wind power threshold" based on integration of a specific wind resource into a specific grid interconnect can be reasonably projected to a 40% wind plus 20% solar threshold for a broadly distributed set of wind and solar power resources ... plus 40% of "something else.
Assume that half of that "something else" is catering to specialized needs where EROI gives way to other factors, for which I'll give an EROI of 1 and a net energy yield of 0%, and 20% of biocoal and biocoal production electrical generation (and note that the generation of power as a by-product of charcoal production can be scheduled to coincide with peak demand periods), at an EROI of 2.
What would be the end result? 40% (wind) of energy consumed in energy production has an EROI of 19, 20% (solar PV) has an EROI of 5.7, 20% (biocoal biomass) has an EROI of 2, and 20% (specialized/portable power sources) have an EROI of 1. That is a weighted average of 13.42, or a net energy yield of 92%, comfortably above the net energy cliff.
The Transport Challenge
However, this presents the US transport system with a substantial challenge.
To the extent that we can efficiently transport things using electricity, we know that it will be feasible to transport them using sustainable renewable power. And if the electrically powered transport can be done with a sufficient efficiency gain, we can pursue electrical transport and the development of sustainable electrical power sources in parallel, since the electrically powered transport automatically inherits any improvements to the sustainability of the electric power grid as they occur. By contrast, most fueled transport require some form of conversion to shift from an unsustainable to a sustainable power source.
Local electrical passenger transport and Active Transport alternatives include walking, cycling, ebikes, neighborhood electric vehicles, electric freeway-capable cars, trolleybuses, light rail and heavy rail. Many of these have local freight delivery versions, including freight cycles, neighborhood electric delivery vehicles, and electric conventional vans and light trucks, while a trolleybus, light rail or heavy rail corridor could also be used for local freight. Given advances in battery technology, an electric truck operating along a trolleybus route and departing from it to complete its trip on the public right of way could be readily integrated into a trolleybus corridor system.
Further, whatever our supply of high energy density liquid fuel may be, we can prioritize local freight shipments and allocate a given budget of low-EROI liquid fuels to performing those tasks.
However, if we consider the alternatives for long distance freight transport, to provide the 1,000mile+ freight movements that our economy relies upon, the list of alternatives drops away. Air freight and long distance highway freight consume large amounts of high energy density liquid fuels, using energy-intensive means of transport, and production of those fuels from sustainable energy sources imposes substantial energy losses from the conversion.
Long distance electric freight rail does not discriminate between unsustainable and sustainable sources of electricity, and we have ample potential for sustainable electricity sources to support electric freight rail.
My version of the Steel Interstate proposal
extends this to also operate the Steel Interstate electric freight rail system as Electricity Superhighways. This provides an essential complement to wind and solar power since, as already discussed, an ability to draw on wind and solar power from multiple regions results in a more stable total supply of renewable energy, and substantially eases the task of integrating wind and solar power into the grid.
A Transport Revolution
Where the Steel Interstate is most revolutionary, however, is in its efficiency.
The "energy cliff" argument of Charles Hall revolves around how much net energy yield we require to operate a modern industrial economy. This is a substantially different thing from the "energy cliff" argument of David Murphy, that observes an EROI threshold of about 8, where it becomes critical to be aware of the EROI of our energy sources.
David Murphy's threshold is simply arithmetic, and as such it does not in fact tell us what Net Energy Yield we need to maintain our industrial economy.
And Charles Hall's argument is surely correct regarding some
level of Net Energy Yield, what
level we need to maintain our industrial society surely depends on whether we rely on efficient or inefficient ways to do what we need to do.
And it is here where the Steel Interstate promises efficiency gains compared to our present reckless waste that are substantial enough to be called revolutionary. Long distance rail freight is substantially more efficient than diesel truck freight, even with the same diesel fuel source. And long distance electric rail freight is substantially more efficient than diesel rail freight. When the two efficiencies are combined, electric freight rail, consumes less than 10% as much energy per ton-mile as long distance diesel truck freight.
If we were to compare electric rail freight powered by sustainable electricity with an average EROI of 13 to long distance diesel truck freight, that would be equivalent to a oil with an EROI of over 130 ...
... while the large oil fields discovered before the 1960's with EROI in the range of 80-100, sustainable electric power with an EROI of 13 is effectively more abundant
for the task at hand than the big oil fields of West Texas and Saudi Arabia.
And those big oil fields are increasingly exhausted, with petroleum EROI falling toward 20 and sure to continue to fall.
That's what I call the Steel Interstate Energy Revolution.
Conversations, Considerations and Contemplates
As always, rather looking for some overarching conclusion, I now open the floor to the comments of those reading.
If you have an issue on some other area of sustainable transport or sustainable energy production, please feel free to start a new main comment. To avoid confusion among those who might be tempted to yell "off topic!", feel free to use the shorthand "NT:" in the subject line when introducing this kind of new topic.
And if you have a topic in sustainable transport or energy that you want me to take a look at in the coming month, be sure to include that as well.