Monthly Archives: March 2013

Statistics Joke

A classic statistics joke concerns a person who’s afraid to fly; he goes to a statistician who explains that planes are very, very safe, especially if you fly a respectable airline in good weather. In that case, virtually the only problem you’ll have is the possibility of a bomb on board. The fellow thinks it over and decides that flying is still too risky, so the statistician suggests he plant a bomb on the airplane, but rig it to not go off. The statistician explains: while it’s very rare to have a bomb onboard an airplane, it’s really unheard of to have two bombs on the same plane.

It’s funny because …. the statistician left out the fact that an independent variable (number of bombs) has to be truly independent. If it is independent, the likelihood is found using a poisson distribution, a non-normal distribution where the greatest likelihood is zero bombs, and there are no possibilities for a negative bomb. Poisson distributions are rarely taught in schools for some reason.

By Dr. Robert E. Buxbaum, Mar 25, 2013. If you’ve got a problem like this (particularly involving chemical engineering) you could come to my company, REB Research.

Hydrogen versus Battery Power

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There are two major green energy choices that people are considering to power small-to-medium size, mobile applications like cars and next generation, drone airplanes: rechargeable, lithium-ion batteries and hydrogen /fuel cells. Neither choice is an energy source as such, but rather a clean energy carrier. That is, batteries and fuel cells are ways to store and concentrate energy from other sources, like solar or nuclear plants for use on the mobile platform.

Of these two, rechargeable batteries are the more familiar: they are used in computers, cell phones, automobiles, and the ill-fated, Boeing Dreamliner. Fuel cells are less familiar but not totally new: they are used to power most submarines and spy-planes, and find public use in the occasional, ‘educational’ toy. Fuel cells provided electricity for the last 30 years of space missions, and continue to power the international space station when the station is in the dark of night (about half the time). Batteries have low energy density (energy per mass or volume) but charging them is cheap and easy. Home electricity costs about 12¢/kWhr and is available in every home and shop. A cheap transformer and rectifier is all you needed to turn the alternating current electricity into DC to recharge a battery virtually anywhere. If not for the cost and weight of the batteries, the time to charge the battery (usually and hour or two), batteries would be the obvious option.

Two obvious problems with batteries are the low speed of charge and the annoyance of having to change the battery every 500 charges or so. If one runs an EV battery 3/4 of the way down and charges it every week, the battery will last 8 years. Further, battery charging takes 1-2 hours. These numbers are acceptable if you use the car only occasionally, but they get more annoying the more you use the car. By contrast, the tanks used to hold gasoline or hydrogen fill in a matter of minutes and last for decades or many thousands of fill-cycles.

Another problem with batteries is range. The weight-energy density of batteries is about 1/20 that of gasoline and about 1/10 that of hydrogen, and this affects range. While gasoline stores about 2.5 kWhr/kg including the weight of the gas tank, current Li-Ion batteries store far less than this, about 0.15 kWhr/kg. The energy density of hydrogen gas is nearly that of gasoline when the efficiency effect is included. A 100 kg of hydrogen tank at 10,000 psi will hold 8 kg of hydrogen, or enough to travel about 350 miles in a fuel-cell car. This is about as far as a gasoline car goes carrying 60 kg of tank + gasoline. This seems acceptable for long range and short-range travel, while the travel range with eVs is more limited, and will likely remain that way, see below.

The volumetric energy density of compressed hydrogen/ fuel cell systems is higher than for any battery scenario. And hydrogen tanks are far cheaper than batteries. From Battery University. http://batteryuniversity.com/learn/article/will_the_fuel_cell_have_a_second_life

The volumetric energy density of compressed hydrogen/ fuel cell systems is higher than for any battery scenario. And hydrogen tanks are far cheaper than batteries. From Battery University. http://batteryuniversity.com/learn/article/will_the_fuel_cell_have_a_second_life

Cost is perhaps the least understood problem with batteries. While electricity is cheap (cheaper than gasoline) battery power is expensive because of the high cost and limited life of batteries. Lithium-Ion batteries cost about $2000/kWhr, and give an effective 500 charge/discharge cycles; their physical life can be extended by not fully charging them, but it’s the same 500 cycles. The effective cost of the battery is thus $4/kWhr (The battery university site calculates $24/kWhr, but that seems overly pessimistic). Combined with the cost of electricity, and the losses in charging, the net cost of Li-Ion battery power is about $4.18/kWhr, several times the price of gasoline, even including the low efficiency of gasoline engines.

Hydrogen prices are much lower than battery prices, and nearly as low as gasoline, when you add in the effect of the high efficiency fuel cell engine. Hydrogen can be made on-site and compressed to 10,000 psi for less cost than gasoline, and certainly less cost than battery power. If one makes hydrogen by electrolysis of water, the cost is approximately 24¢/kWhr including the cost of the electrolysis unit.While the hydrogen tank is more expensive than a gasoline tank, it is much cheaper than a battery because the technology is simpler. Fuel cells are expensive though, and only about 50% efficient. As a result, the as-used cost of electrolysis hydrogen in a fuel cell car is about 48¢/kWhr. That’s far cheaper than battery power, but still not cheap enough to encourage the sale of FC vehicles with the current technology.

My company, REB Research provides another option for hydrogen generation: The use of a membrane reactor to make it from cheap, easy to transport liquids like methanol. Our technology can be used to make hydrogen either at the station or on-board the car. The cost of hydrogen made this way is far cheaper than from electrolysis because most of the energy comes from the methanol, and this energy is cheaper than electricity.

In our membrane reactors methanol-water (65-75% Methanol), is compressed to 350 psi, heated to 350°C, and reacted to produce hydrogen that is purified as it is made. CH3OH + H2O –> 3H2 + CO2, with the hydrogen extracted through a membrane within the reactor.

The hydrogen can be compressed to 10,000 psi and stored in a tank on board an automobile or airplane, or one can choose to run this process on-board the vehicle and generate it from liquid fuel as-needed. On-board generation provides a saving of weight, cost, and safety since you can carry methanol-water easily in a cheap tank at low pressure. The energy density of methanol-water is about 1/2 that of gasoline, but the fuel cell is about twice as efficient as a gasoline engine making the overall volumetric energy density about the same. Not including the fuel cell, the cost of energy made this way is somewhat lower than the cost of gasoline, about 25¢/kWhr; since methanol is cheaper than gasoline on a per-energy basis. Methanol is made from natural gas, coal, or trees — non-imported, low cost sources. And, best yet, trees are renewable.

Why the Boeing Dreamliner’s batteries burst into flames

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Boeing’s Dreamliner is currently grounded due to two of their Li-Ion batteries having burst into flames, one in flight, and another on the ground. Two accidents of the same type in a small fleet is no little matter as an airplane fire can be deadly on the ground or at 50,000 feet.

The fires are particularly bad on the Dreamliner because these lithium batteries control virtually everything that goes on aboard the plane. Even without a fire, when they go out so does virtually every control and sensor. So why did they burn and what has Boeing done to take care of it? The simple reason for the fires is that management chose to use Li-Cobalt oxide batteries, the same Li-battery design that every laptop computer maker had already rejected ten years earlier when laptops using them started busting into flames. This is the battery design that caused Dell and HP to recall every computer with it. Boeing decided that they should use a massive version to control everything on their flagship airplane because it has the highest energy density see graphic below. They figured that operational management would insure safety even without the need to install any cooling or sufficient shielding.

All lithium batteries have a negative electrode (anode) that is mostly lithium. The usual chemistry is lithium metal in a graphite matrix. Lithium metal is light and readily gives off electrons; the graphite makes is somewhat less reactive. The positive electrode (cathode) is typically an oxide of some sort, and here there are options. Most current cell-phone and laptop batteries use some version of manganese nickel oxide as the anode. Lithium atoms in the anode give off electrons, become lithium ions and then travel across to the oxide making a mixed ion oxide that absorbs the electron. The process provides about 4 volts of energy differential per electron transferred. With cobalt oxide, the cathode reaction is more or less CoO2 + Li+ + e– —> LiCoO2. Sorry to say this chemistry is very unstable; the oxide itself is unstable, more unstable than MnNi or iron oxide, especially when it is fully charged, and especially when it is warm (40 degrees or warmer) 2CoO2 –> Co2O3 +1/2O2. Boeing’s safety idea was to control the charge rate in a way that overheating was not supposed to occur.

Despite the controls, it didn’t work for the two Boeing batteries that burst into flames. Perhaps it would have helped to add cooling to reduce the temperature — that’s what’s done in lap-tops and plug-in automobiles — but even with cooling the batteries might have self-destructed due to local heating effects. These batteries were massive, and there is plenty of room for one spot to get hotter than the rest; this seems to have happened in both fires, either as a cause or result. Once the cobalt oxide gets hot and oxygen is released a lithium-oxygen fire can spread to the whole battery, even if the majority is held at a low temperature. If local heating were the cause, no amount of external cooling would have helped.

battery-materials-energy-densities-battery-university

Something that would have helped was a polymer interlayer separator to keep the unstable cobalt oxide from fueling the fire; there was none. Another option is to use a more-stable cathode like iron phosphate or lithium manganese nickel. As shown in the graphic above, these stable oxides do not have the high power density of Li-cobalt oxide. When the unstable cobalt oxide decomposed there was oxygen, lithium, and heat in one space and none of the fire extinguishers on the planes could put out the fires.

The solution that Boeing has proposed and that Washington is reviewing is to leave the batteries unchanged, but to shield them in a massive titanium shield with the vapors formed on burning vented outside the airplane. The claim is that this shield will protect the passengers from the fire, if not from the loss of electricity. This does not appear to be the best solution. Airbus had planned to use the same batteries on their newest planes, but has now gone retro and plans to use Ni-Cad batteries. I don’t think that’s the best solution either. Better options, I think, are nickel metal hydride or the very stable Lithium Iron Phosphate batteries that Segway uses. Better yet would be to use fuel cells, an option that appears to be better than even the best batteries. Fuel cells are what the navy uses on submarines and what NASA uses in space. They are both more energy dense and safer than batteries. As a disclaimer, REB Research makes hydrogen generators and purifiers that are used with fuel-cell power.

More on the chemistry of Boeing’s batteries and their problems can be found on Wikipedia. You can also read an interview with the head of Tesla motors regarding his suggestions and offer of help.

 

Two things are infinite

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Einstein is supposed to have commented that there are only two things that are infinite: the size of the universe and human stupidity, and he wasn’t sure about the former.

While Einstein still appears to be correct about the latter infinite, there is now more disagreement about the size of the universe. In Einstein’s day, it was known that the universe appeared to have originated in a big bang with all mass radiating outward at a ferocious rate. If the mass of the universe were high enough, and the speed were slow enough the universe would be finite and closed in on itself. That is, it would be a large black hole. But in Einstein’s day, the universe didn’t look to have enough mass. It thus looked like the universe was endless, but non-uniform. It appeared to be mostly filled with empty space — something that kept us from frying from the heat of distant stars.

Since Einstein’s day we’ve discovered more mass in the universe, but not quite enough to make us a black hole given the universe’s size. We’ve discovered neutron stars and black holes, dark concentrated masses, but not enough of them. We’ve discovered neutrinos, tiny neutral particles that fill space, and we’ve shown that they have rest-mass enough that neutrinos are now thought to make up most of the mass of the universe. But even with these dark-ish matter, we still have not found enough for the universe to be non-infinite, a black hole. Worse yet, we’ve discovered dark energy, something that keeps the universe expanding at nearly the speed of light when you’d think it should have slowed by now; this fast expansion makes it ever harder to find enough mass to close the universe (why we’d want to close it is an aesthetic issue discussed below).

Still, there is evidence for another, smaller mass item floating in space, the axion. This particle, and it’s yet-smaller companion, the axiono, may be the source of both the missing dark matter and the dark energy, see figure below. Axions should have masses about 10-7 eV, and should interact enough with matter to explain why there is more matter than antimatter while leaving the properties of matter otherwise unchanged. From normal physics, you’d expect an equal amount of matter and antimatter as antimatter is just matter moving backwards in time. Further, the light mass and weak interactions could allow axions to provide a halo around galaxies (helpful for galactic stability).

Mass of the Universe with Axions, no axions. Here is a plot from a recent SUSY talk (2010) http://susy10.uni-bonn.de/data/KimJEpreSUSY.pdf

Mass of the Universe with Axions, no axions. Here is a plot from a recent SUSY talk (2010) http://susy10.uni-bonn.de/data/KimJEpreSUSY.pdf

The reason you’d want the universe to be closed is aesthetic. The universe is nearly closed, if you think in terms of scientific numbers, and it’s hard to see why the universe should not then be closed. We appear to have an awful lot of mass, in terms of grams or kg, but appear to have only 20% of the required mass for a black hole. In terms of orders of magnitudes we are so close that you’d think we’d have 100% of the required mass. If axions are found to exist, and the evidence now is about 50-50, they will interact with strong magnetic fields so that they change into photons and photons change into axions. It is possible that the mass this represents will be the missing dark matter allowing our universe to be closed, and will be the missing dark energy.

As a final thought I’ve always wondered why religious leaders have been so against mention of “the big bang.” You’d think that the biggest boost to religion would be knowledge that everything appeared from nothing one bright and sunny morning, but they don’t seem to like the idea at all. If anyone who can explain that to me, I’d appreciate it. Thanks, Robert E. B.