Carbon footprints

California awards Zero Emission Vehicles or ZEVs its coveted white sticker, entitling its solo driver to drive in the high-occupancy-vehicle (HOV) or carpool lane.  The state’s rationale is that CO2-emitting vehicles are contributing to a gradual rise in temperature over the past century or so that is expected by 2100 to have raised the planet’s temperature 3 degrees C above its relatively steady level prior to 1750.  The white sticker is one of the state’s incentives for car-owning individuals to reduce their so-called carbon footprint, the rate at which they are contributing to the planet’s rising CO2.

The state considers a ZEV to be a vehicle that does not emit CO2.  Battery electric vehicles or BEVs fit that description.  The first production BEV was built in 1884 by Thomas Parker of London.  They were popular early in the 20th century, albeit not for emission reasons, but soon lost out to the more economical, powerful, lighter, and longer-range internal combustion engine vehicles (ICEVs) based on fossil fuels such as gasoline and diesel,

The popularity of BEVs was revived a century later when their ZEV status was recognized as contributing less to global warming than ICEVs.  Over the past fifteen years BEVs have gradually gained an enthusiastic following, with price highly correlated with range: each additional $1000 in price gains about 3 more miles of range.  To within an accuracy of 10%, a $33,000 Nissan Leaf has a range of a hundred miles, the $67,000 low end Tesla Model S extends this to two hundred miles, and the $100,000 high end Tesla further extends it to three hundred miles.  Whereas luxury brands such as BMW and Mercedes are priced to offer only 2.5 miles per $1000, the price of the high end Tesla is primarily to pay for the larger battery needed to maintain the range-to-price ratio.

Deep cycling of a lithium-ion battery shortens its life.  To maximize battery life all BEV vendors therefore recommend keeping the state of charge in a range something like 30% to 80%.   This effectively halves the usable range between recharges.  For long trips one can ignore this recommendation but only at the cost of reduced battery life.  For commuters with a daily commute less than half the range and with little time off for long vacation trips this is not a concern.  However for fleet owners, taxi drivers, traveling salesmen, etc. limited range and especially the longer refueling time are serious restrictions.

A far more recent ZEV entry is the fuel cell vehicle or FCV.  Unlike BEVs,  FCVs only came into existence towards the end of the 20th century.  Residents of Northern California in particular  had no access to either FCVs or their hydrogen infrastructure until early in 2016.  Today Northern California has 8 operational stations, namely 6 in the SF Bay Area, one in West Sacramento, and one in Truckee 30 miles from Reno.  The Harris Ranch station at Coalinga links North and South California, an additional one at Santa Barbara is just reachable via US-101 from the SF South Bay, and there are ten more stations around the LA area.  In addition there are 20 more stations with committed funding in various stages of completion, 4 non-retail (bus) stations, 6 older stations soon to be acquired, and 15 stations targeted for the next round of California Energy Commission (CEC) funding.  The total at that point is 61 stations (not counting the 4 non-retail ones); and CEC has state authorization to fund an additional 39 stations beyond that for a total of 100 retail stations over the next five years.

An important benefit of FCVs is their refueling time of 5-10 minutes from completely empty to completely full.   Another benefit is that each additional $1000 in price gains about 6 more miles of range, double that of BEVs.  A third benefit is that, mile for mile of range, an FCV “engine” consisting of a full tank, a fuel cell, and a traction battery weighs about a third of a BEVs lithium-ion battery.  For example the Mirai’s drivetrain including a full tank weighs about 400 lbs while the Tesla Model S 85’s battery as packaged weighs about 1200 lb.

So what’s not to love about FCVs?

Well, if hydrogen could be made economically using electrolysis of water powered by solar electricity, FCVs would truly be ZEVs.

But hydrogen today is made far more economically by a process known as steam reforming of methane.  This process has two steps, the result of which is that a mole of methane (aka natural gas or NG) and two moles of steam (aka water vapor at high temperature) is converted to four moles of hydrogen and one mole of CO2.  In the sort of formula beloved of chemists this reaction amounts to CH4 + 2H2O → 4H2 + CO2.

(The only difference between a mole and a molecule is that a mole of any substance contains Avogadro’s number (about 6 followed by 23 zeros) of molecules of that substance.  Whereas a molecule of CO2 weighs only 44 times as much as a single proton, a mole of CO2 weighs 44 g (grams) or about an ounce and a half.  A mole of hydrogen atoms weighs 1 g while a mole of hydrogen molecules, H2, weighs 2 g.  And a mole of H2O weighs 18 g while a mole of CH4 weighs 16 g.  Hence in the reaction of the preceding paragraph, every 16 g of natural gas and 36 g of steam are converted to 8 g of hydrogen and 44 g of CO2.  Scaled up by a factor of a thousand (kilomoles in place of moles), and in the usual units for each chemical, about 800 standard cubic feet (0.8 mmBTU) of natural gas with steam from about ten gallons of water becomes 8 kg of hydrogen and about 100 lbs of CO2.

This analysis assumes 100% efficiency.  In practice steam reforming is 65-75% efficient.  If we optimistically take 75% as the efficiency then we only get 6 kg of hydrogen from each 800 scf of natural gas, but still yielding a hundred pounds of CO2.

That CO2 is the Achilles’ heel of the whole zero-emission premise of fuel cell vehicles!

Now the US Environmental Protection Agency (EPA) has measured the combined city/highway fuel consumption of the Toyota Mirai FCV at 67 MPGe (miles per gallon equivalent), which in the case of hydrogen is miles per kg of hydrogen. (A gallon-equivalent is a unit of energy equal to 121 megajoules (MJ).)  Production of 6 kg of hydrogen by steam reformation emits 100 lbs of CO2, whence driving the Mirai 100 miles entails an implied emission of (100/67)*(100/6) = 25 lbs of CO2 per hundred miles.

Each gallon of gasoline consumed by an ICEV is estimated to produce 18 lbs of CO2.  This is the implied emission from a Mirai driven 18/25 = 72 miles.  It follows that when using hydrogen 100% of which is produced by steam reforming, the Mirai’s implied emission is that of a car rated at about 72 miles per gallon.

Hydrogen vendors try to offset their carbon footprint by including some hydrogen derived from renewables produced without emitting CO2.  The vendor True Zero for example claims that 1/3 of its hydrogen comes from renewables.  With such a fuel the Mirai’s implied emission becomes that of an ICEV rated at about 110 miles per gallon.

For a 4100 lb car that can accelerate from 0 to 60 mph in 9 seconds and reach a top speed of 111 mph, 110 miles per gallon is remarkable.

The CO2 produced in this way is customarily emitted to the atmosphere.  If instead it is suitably sequestered by any of the methods described in the relevant Wikipedia article this method of producing hydrogen will further reduce the carbon footprint to that necessary for the power consumed by the method.

Another method of producing hydrogen is by electrolysis of water.  Two electrodes are immersed in slightly conducting water and a voltage over about 1.5 volts is applied across them.  This voltage is sufficient to break the electron bonds holding the hydrogen and oxygen atoms of H2O together.  The hydrogen and oxygen atoms thereby liberated then bubble up from respectively the cathode (the negative electrode) and the anode (positive).  The oxygen is discarded and the hydrogen produced in this way has an energy of about 60% of that used to create it and the oxygen.

When electrolysis is powered entirely by renewables such as wind or solar the resulting hydrogen is itself 100% renewable in the sense that no CO2 was emitted in its production.  If however it is powered by a typical electric utility, much of that electricity will have been produced by burning fossil fuels such as coal, oil, and natural gas.

The same holds of the electricity used to charge the battery of a BEV.  The crucial difference here is that whereas the Mirai gets only 67 MPGe, the typical BEV gets 50% more than that or around 100 MPGe.  Hence for electricity from fossil fuels an FCV like the Mirai has an implied emission 50% more than a BEV.

Increasing use of renewables in electricity narrows this gap.  In the limit when electricity production entails no emission of CO2, both FCVs and BEVs have the same implied emissions from this source, namely zero.

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2 Responses to Carbon footprints

  1. Nick Austin says:

    You also need to consider upstream CH4 leakage.

    My natural gas powered van was considered an Inherently Low Emission vehicle by California, giving it access to white car pool lane stickers.

    I was disheartened when I discovered that the CO2 emissions reduction from using CH4 in an internal combustion engine (vs C8H16) were negated by upstream CH4 leakage from production.

    The good news is that the local criterion pollutants were substantially lower on the CH4 vs a conventional car. A fuel cell is obviously emits zero local criterion pollutants.


    • Excellent point, Nick. CH4’s Global Warming Potential (GWP) is greater than CO2’s because of its greater complexity: 5 atoms instead of 3 result in more vibration and rotation modes. But by that criterion vapors from gasoline are even worse because of their yet greater complexity.

      Locality was a big factor for me, not for CO2 but for the many other components of ICE emissions, some of which are carcinogenic. Google for the brochure “The Harmful Effects of Vehicle Exhaust”.

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