Archive for the ‘fuel efficiency’ Category

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Taking Measure of Biofuel Limits

September 24, 2009

The current edition of American Scientist has a very good article on the fundamental biological limits of governing the production of biofuels.  Taking the Measure of Biofuel limits, by Thomas Sinclair addresses the two obvious limiting factors, light and water, and the perhaps less obvious limiting factor of nitrogen availability in the soil.

Thomas R. Sinclair is a professor in crop science at North Carolina State University with a Ph.D. from Cornell.  He specializes in the relationships between plant physiology, the environment, and crop yields.  He has edited several books as a Ballard Fellow at Harvard University.

Sinclair sets the stage by pointing out..

The U.S. Energy Independence and Security Act calls for 144 billion liters of ethanol per year in the U.S. transportation  fuel pool by 2022.  That equals 25 percent of the U.S. gasoline consumption today.  No more than about 4o percent is to be produced with maize, an important food and export crop.  Non-grain feedstock is supposed to provide the rest.

Before nations pin big hopes on biofuels, they must face some stark realities, however.  Crop physiology research has documented multiple limits to plant production on Earth.  To ramp up biofuel crop production, growers must adapt to those limits or find ways around them.  Such advances may not be as simple a some predict.  Plants and their evolutionary ancestors had hundreds of millions of years to optimize their biological machinery.  If further improvements were easy, they would probably already exist….

Plants cannot be grown without three crucial resource inputs: light, water, and nitrogen.  Each of these inputs is needed in substantial quantities, yet their availability in the field is limited…[T]he close relationship between the available amounts of these resources and the amount of plant mass they can produce – not human demand – will determine how much biofuel the world can produce.

Light

Sinclair considers the conversion efficiency of sunlight  and CO2 to sugars, which ultimately fuel the building of starch, cellulose, protein and lipids, for C3 plants (95% of all of Earth’s plants, but not highly CO2 fixation efficient) and the more efficient sugar-making C4 plants (corn, sugar cane and sorghum, for example).  He points out that “After hundreds of millions of years of evolution, these systems [for converting solar energy into the chemical energy of sugars] are highly efficient within the physical and thermodynamic constraints of photosynthesis and plant growth.”  Not much room has been left for improvement.

Bio-engineering advances may increase yields a little, but they cannot overcome the limits of the sunlight to sugar conversion ratios.  After the numbers are crunched he reveals that if the U.S. is to reach its biofuel goal of 58 billion liters of ethanol grown from corn (40 percent of 144 billion liters), it would require an additional 15 million hectares planted.  Similarly, the remaining 86 billion liters made from non-corn C4 grasses, which are not nearly as efficient as corn for this purpose,  would require at least an additional 48 million hectares.

Water

It may be obvious that in areas of limited water supply, plant growth will be limited by the amount of water available.  As plants transpire water out through their leaves, the rate of transpiration is:

T = G x VPD/k

where T is the transpiration (g/m2)
G is the plant growth (g/m2)
VPD is the Vapor Pressure Deficit, or the difference in the saturated water vapor pressure of air inside the plant leaves and the water vapor pressure of the outside atmosphere
k is a plant specific constant

The difference in the vapor pressure inside and outside a leaf (VPD) is what controls the rate of water loss through the stomata.  The VPD is large in arid regions because the vapor pressure of the water in the atmosphere is low. 

For a given environment  the VPD cannot be controlled – it is what it is.  So the only way for a plant to affect the transpiration, and thus prevent itself from drying out and dying in a arid environment, is to close down its stomata to reduce water loss.  But this also reduces the flow of CO2 into the leaves and O2 out, and consequently reduces or stops the plant’s growth.  There is no magic to get around this.  Sinclair  says…

“Despite claims that crop yields will be substantially increased by the application of biotechnology, the physical linkage between growth and transpiration imposes a barrier that is not amenable to genetic alteration.”

Under these circumstances the plant mass growth is nearly linear with water transpired.  So as more arid regions are put into crop use either crop yields per hectare will be lower, or the amount of irrigation will be higher.  This leads to the production of biofuels at the expense of aquifer depletion.

Nitrogen

Sinclair repeatedly points out that to be economically viable, biofuel crops must yeild at least 9 tonnes of plant mass per hectare of crop.  For C3 and C4 type plants this 9 tonne minimum requires the removal of 166 kg and 118 kg or nitrogen per hectare, respectively.  But, “Expectations for cellulosic yields are sometimes double or triple the 9-tonne-per-hectare yield” required for economic viability.  So, nitrogen removal from the soil will sometimes be double or triple also.   Some of this nitrogen is replaced by plant debris that is left behind and some comes from thunderstorms and some from organisms that fix atmospheric nitrogen.  But these sources are not enough to replace all the nitrogen that is removed with every harvest, and the available nitrogen will be less every year.  Sinclair explains…

“Although this decrease rate is usually small when compared to all the original organic matter in the soil, a cropping practice dependent on a continuous withdrawal clearly is not sustainable…  Nitrogen fertilizer of annual biofuel crops will inevitably be needed once soil organic matter decreases to levels limiting plant growth.”

Sinclair’s conclusion

Taking the limits of light, water and nitrogen in to account, for corn he concludes…

“The equivalent of 40 percent of today’s U.S. maize crop will be required to ethanol production while other domestic and export demands for maize also must be met.”

And for cellulosic derived ethanol he concludes that up to…

“50 million hectares of new land must be brought into high and sustainable agricultural production to achieve the required yields… it would be the most extensive and rapid land transformation in U.S. history… [L]and used for cellulosic feedstock must be in regions with sufficient rainfall to achieve needed yields.  The amount of water transpired by those crops could be large enough to influence the hydrologic balance of farming regions.”

and for in general…

“I]ncreased nitrogen supplementation required for the new crops will result in more nitrogen leaching into natural waterways…”

My final words

Sinclair indicates that between corn and other plants for ethanol, the U.S. may have to put as much as an additional 65 million hectares into crop production (15 million hectares for corn and 50 million hectares for other biofuel crops) to  generate 144 billion liters of ethanol.  This would replace only 25% of our gasoline usage.

How big is 65 million hectares?  It is the same as 650,000 square kilometers, and about the same as 160 million acres.  To put this in perspective, this is more than 10 times the acreage of corn planted in Iowa in 2007.   It is more than 150% of the corn acrage planted in the entire United States in 2007.

Look at the figures below.  The first image is from the USDA Census of Agriculture for 2002, and it shows the acreage planted in corn for grain in the United States that year.  Each dot on the map represents 10,000 acres.  To achieve 144 billion liters of ethanol we could need an additional 160 million acres of of corn and other crops, or more than 10 times the amount of corn acreage planted in Iowa.  The second figure shows the corn acreage of Iowa multiplied ten fold and added to the map of the United States.  This should give you some idea of the unprecedented agricultural multiplication that would be needed to satisfy the U.S. Energy Independence and Security Act.

corn acres

new crop area 2
Cartoon of ten times the corn acreage of Iowa added to the US. This gives some idea of what may be required to satisfy the U.S. Energy Independence and Security Act requirement of 144 billion liters of ethanol to replace 25% of U.S. gasoline usage.

Let’s face it – this ambitious goal of 144 billion liters of ethanol per year from biofuels is a very bad idea.  Our most precious resources are the land, water and resources for making fertilizer (which is primarily natural gas for nitrogen fertilizers).  The dumbest thing we can do is deplete our soil and aquifers, pollute our water with extra nitrogen fertilizer, and waste our natural gas to make ethanol.  If you think living with a shortage of gasoline is rough, try living with a shortage of food.

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I was (partially) wrong

August 20, 2009

I recieved  comment form the GM spokesman, Rob Peterson, about my last two posts lambasting the supposed 230 mile per gallon Chevy Volt.  Here is Rob’s comment  in its entirety.

This is Rob Peterson from GM.

Although the Volt has a 16 kWh battery, only 8 kWh is used. This will significantly impact the rest of your calculations and your synopsis. Please post a correction based on this fact.

As for the Volt’s city fuel efficiency rating of 230mpg – this is based on the EPA’s draft methodology. The same methodology which will be used for all other vehicles of this type.

r

I responded to Rob with two comments, which you can read here.  One of those comments questions his sincerity about “blaming” the 230 mile per gallon claim on the EPA.  However, he is essentially right about the the charging cycle of the 16 kWh battery only using about half of that.  He has asked me to “Please post a correction based on this fact.”  I have done so, but the final numbers for the vaunted Volt are still underwhelming.

Here is a table comparing miles per gallon, kWh per mile, and pounds of CO2 per mile for the Chevy Volt, the Toyota Prius, and the a couple of ancient Honda Civics.  You can read the details of how I derived the numbers for the Volt, using Rob’s partial capacity charge cycle scheme in the text below.  Note that the prices for the Honda Civics have been adjusted for inflation to 2009 dollars for a fair comparison.

milage chart copy

Now that I have posted a correction, can I expect Rob Peterson to post a retraction of GM’s preposterous 230 mile per gallon claim?  Not Likely.

I  have not yet been able to find an official specification for the number of kilowatt-hours per mile for the Volt.  I am hoping Rob will provide one.  I have found Rob’s description of the charging scheme for lithium-ion batteries to be essentially correct.  That is, the battery is typically charged by the electrical grid to around 90% of total capacity.  Then the car is propelled entirely off of battery power until it reaches about 30% capacity.  This is known as the “charge depletion” mode.  When the battery gets to about 30% of capacity the gasoline powered generator kicks in and maintains the charge at about 30% capacity.  This is known as the “charge sustaining” mode.  

Then, when the battery is plugged into the electrical grid it is recharged with grid energy from about 30% capacity back up to about 90% capacity    That is a range of about 60% of the total capacity.  So, for a 16 kilowatt-hour battery, a complete charge off the electric grid puts about 9.6 kWh (0.6 x 16 kWh) into the battery.  But an extra 10% or so is lost due to transmission line and battery conversion losses.  So the amount of power taken from the electrical grid will be about 10.6 kilowatt-hours. If that charge will propel the car for 40 miles, then that works out to 3.8 miles per kWh (or about 0.27 kilowatts per mile) 

I cannot find the value of about 0.27 kWh per mile anywhere in the specifications for the Volt, but I did find this somewhat cryptic statement at Chevy.com:

“Under the new procedure, the EPA weights plug-in electric vehicles as traveling more city miles than highway miles on only electricity. The EPA procedure would also note 25 kilowatt-hours/100 miles electrical efficiency in the city cycle.

So, lets accept the value of 25 kilowatt-hours/100 miles (0.25 kWh per mile) for the moment.  What is the affect that this will have on the numbers I reported for CO2 emissions?

The number of pounds of CO2 emitted per mile while powering the car with gasoline (known as the “charge sustaining” mode) will remain unchanged.  There are 19.4 pounds of CO2 produced per gallon of gasoline burned, and GM claims 50 miles per gallon in “charge sustaining” mode.  So:

( 19.4 lbs of CO2 / gallon) / (50 miles / gallon) =
0.39 lbs of CO2 per mile

This 0.39 lbs of CO2 per mile for the Volt running on gasoline (charge sustaining mode) is the same as for the Toyota Prius, because it also gets 50 miles per gallon.

Here is the same calculation for my ancient 1988 Honda Civic hatchback that got 47 miles to the gallon:

( 19.4 lbs of CO2 / Gallon) / (47 miles / gallon) =
0.41 lbs of CO2 per mile

And for the 197887 Honda Civic Coupe HF, which got 57 miles per gallon:

( 19.4 lbs of CO2 / Gallon) / (57 miles / gallon) =
0.34 lbs of CO2 per mile

Lets assume now that the Volt uses 0.25 kilowatt-hours per mile (“25 kilowatt-hours/100 miles’) when running off of power provided to the battery by the electric grid (known as the “charge depleting” mode).  On the average the grid yields 1.34 pounds of CO2 per kilowatt-hour. The grid transmission losses and grid to battery conversion losses  add up to about 10%.  So the amount of CO2 yielded per mile will be:

(1.34 lbs of CO2 per grid kWh) x (0.25 kWh per mile)  x 1.1 =
0.37 lbs of CO2 per mile

Almost identical to the CO2 emitted when it is running off of gasoline (0.39 lbs of CO2 per mile).  And it is also nearly identical to the amount of CO2 per mile as the  much cheaperPrius generates while running off of gasoline.

But here it the rub.  If the Volt is driven in an area where the electricity is predominantly generated with coal (by far the most common source or electricity generation in the US), then the CO2 emissions go way up.  That is because Coal emits about 2.1 pounds of CO2 per kilowatt-hour generated for the electric grid.  So again we can asume 10% for the sum of the grid transmission losses and grid to battery conversion losses.  Then the amount of CO2 that the Volt yields per mile driven in a region where coal is the primary source of electricity will be:

(2.1 lbs of CO2 per grid kWh) x (0.25 kWh per mile) x 1.1 =
0.58 lbs of CO2 per mile

If we really concerned about reducing CO2 (I’m not), saving energy (I am), creating American jobs (I am), and saving money (I am), then we should support the production of an American car that is similar to a 1988 Honda Civic.  Why argue the merits of a $40,000 car that few people will ever be able to afford?  A $15000 dollar car that gets as good or better mileage and generates as little or less CO2 would be bought by millions and have a much greater impact.

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Hypocrites, or simply confused?

March 14, 2009

It should be no surprise that President Obama is pushing for higher fuel economy standards.  I assume if you support Obama then you support government mandated higher fuel standards.   

I believe in voluntary adoption of higher fuel economy standards.  That is why I have driven a Honda Civic for 20 years.  Rising oil prices will insure that others will also opt for better mileage vehicles at time goes on.   And if you are worried about CO2, higher fuel economy standards won’t make any difference.  Because every drop of oil that you don’t burn helps keep the price low enough for somebody in India or China to burn it. 

So, are the owners of these vehicles hypocrites or just confused? 

cadillac-dts-2
Cadillac Deville DTS. 15 mpg city

Ford Exposition
Ford Expedition.   11 MPG city.

Cadillac Escalade
Cadillac Escalade.  About 14 mpg city.

Ford Yukon FlexFuel
GMC Yukon 11 mpg city. (Don’t worry about the terrible mileage, the owners have a dispensation directly from Gaia because it is a “flexfuel” vehicle.)

Chevy Trailblazer.  12 mpg city.
Chevy Trailblazer 12 mpg city.

Chevy Silverado.  14 mpg city.
Chevy Silverado 14 mpg city.

Lexus SUV
Lexus SUV. About 17 mpg city.  This picture is my favorite.  It was taken by my son on a school field trip to the Colorado state capitol.  I have not blocked out the license plate because I do not think that senate district 28 State Senator Suzanne Williams should enjoy an expectation of privacy. I would think that a state senator who resides on the Transportation Legislative Regulatory Commission (TLRC), is the vice-chair of the state Transportation Committee, and who supports a president that wants to have government mandated higher fuel standards would find something other than a Lexus SUV to drive around town.

 

Hummer

Hummer H1. About 10 mpg. This picture is from the Hummer Guy website.