November 26, 2006
BMW 7 Series Hydrogen Vehicle Reviewed

BMW has developed a prototype 7 series car which runs on either hydrogen or gasoline. Read the linked review from MIT's Technology Review below for the details. The company plans to produce only 100 of these cars and lease them out to selected customers in the United States and Europe for several months at a time in order to get feedback and experience with hydrogen. The engine is a standard internal combustion design rather than a fuel cell because BMW couldn't find a fuel cell that could deliver the power and perform with the reliability needed in a production automobile. BMW's engineers had to compromise on both the gasoline and hydrogen performance of this engine in order to build an engine that'll run on both gasoline and hydrogen. But a couple of other characteristics of the vehicle stood out to me in the reivew. First off, if hydrogen leaks past the pistons into the crankcase it can blow up the engine.

Still, the company has gone further than any other in regulating the combustion of hydrogen. Just three years ago, the engine would run for several minutes and then break down with a big bang, says Melcher. "Boom. We love explosions!" he laughs. It turned out that a little bit of hydrogen was leaking past the pistons, mixing with oil, and exploding. That problem was solved by modifying the piston rings to prevent leakage. Engine control systems also had to be modified to deal with the far faster combustion of hydrogen--it burns 100 times faster than gasoline--and to regulate it in such a way as to keep emissions of combustion byproducts like nitrogen oxides to trace levels.

This isn't a problem on a fairly new engine. But after, say, 100,000 miles the rings and cylinders get worn. Hydrogen (H2) is a much smaller molecule than the hydrocarbons in gasoline. So the wearing on a cylinder and rings will start letting hydrogen through much sooner than they will start letting gasoline through. So I expect a failure mode of exploding engines. Am I wrong to expect this?

But the more fundamental flaw is due to the need to use liquified hydrogen in order to make it sufficiently energy dense. Hydrogen must be kept very cold to stay in a liquid state. This car's hydrogen storage container is extremely well insulated. But a half tank of hydrogen will still heat up fast enough to evaporate away in just 9 days. That just seems unacceptable to me.

As the hydrogen becomes gaseous, pressure rises inside the tank. At a certain point, a pressure-relief valve opens. A little bit of hydrogen gas vents out (about 10 to 12 grams per hour), goes through a catalytic converter to turn it into water, and exits the car through a special pipe in the rear bumper. If you aren't driving the car, it takes only 17 hours before this venting starts. A half-full tank will almost completely "boil off" in nine days.

Granted, once you've lost your hydrogen you can still operate the vehicle with gasoline from the gasoline tank. But the need for a back-up gasoline tank uses up more space, makes the vehicle heavier, and the lost hydrogen costs money.

Hydrogen has 3 big problems as an automotive power source, the first two of which are illustrated in this car:

  • Hydrogen fuel cells are not ready yet. Hydrogen internal combustion engines have downsides as substitutes.
  • Liquid hydrogen storage does not make the grade due to losses from evaporation while sitting and the need to use energy while operating to cool the remaining hydrogen back down again. Solid room temperature hydrogen storage is needed. But the physicists and materials scientists still haven't solved that problem.
  • Hydrogen is currently made using hydrocarbons and the net result is probably less energy efficient than just burning the hydrocarbons directly.

I do not know when the fuel cell and solid hydrogen storage problems will be solved. But my suspicion is that the battery problem for electric cars will be solved first.

Then there is the problem of how to produce hydrogen in ways that do not pollute. First off, if the environmental goal is the reduction of carbon dioxide emissions (a more expensive goal to reach than the reduction of conventional pollutant emissions) then production of hydrogen from hydrocarbon fossil fuels makes it a lot easier to capture the carbon. The hydrogen production is done in large centralized facilities where the weight and durability of carbon capture equipment does not pose the problems that carbon capture would in cars.

Also, hydrogen could be produced from nuclear reactors designed to optimize the production of hydrogen. That might turn out to be the cheapest and environmentally friendliest way to produce hydrogen.

Hydrogen is not the only way to reduce carbon dioxide emissions from vehicles. Better batteries to enable the electric car is another approach. Also, biomass for liquid fuels is still another and more immediately adoptable approach.

Currently biomass is the big growth area. The high costs of fossil fuels seem likely to continue the shift toward biomass. But the increasing popularity of hybrid cars has increased the incentive for companies to develop better batteries. So I'm expecting battery technology to make some big advances in the next several years. Both biomass and batteries can advance by smaller steps driven by demand for existing products. Hydrogen has to make big strides on a multitude of problems without a current market to help fund its advance. So I'm much less optimistic about hydrogen in the short to medium term.

Share |      Randall Parker, 2006 November 26 01:28 PM  Energy Transportation

Engineer-Poet said at November 26, 2006 3:16 PM:

Hydrogen is going to lose to the PHEV; a tank doesn't fill as fast as the best batteries can charge, and a tank of it doesn't give the range of a backup tank of e.g. ethanol.

Hydrogen is so pathetic, I didn't even bother looking at it in my latest treatise, Energy, sustainability and agricultural policy.  I did find that we could replace all motor fuel - AND all coal and gas used for electrical generation - with biomass.  The only technology not already being used at pilot scale or bigger is the direct-carbon fuel cell, and it's a relatively small change from the molten-carbonate fuel cell.

downthehydrogenhighway said at November 28, 2006 11:21 PM:

You noted that hydrogen molecules (H2) are very tiny. Their ability to pass through very small openings means that all containers, pipes, and fittings used with hydrogen must be built and maintained to much higher tolerances and standards than equipment used for gasoline or natural gas or any other fuel. AFAIK there has been no serious attempt to factor in the cost that this represents.

Sione said at December 1, 2006 9:20 AM:

Perhaps what is required to burn hydrogen in an internal combustion engine is a new engine archiecture. It may be that what is needed is separate compression, combustion and expansion sections. They could be either reciprocating or even rotary. Some while ago I saw a paper about a new engine that featured a rotary compressor that delivered compressed air to a combustion chamber. Hot burnt gas was delivered to a reciprocating piston expander. That may get around the trouble with the crankcase explosions. Another method would be to separate the cylinder block from the crankcase (old crosshead steam engines and the prototype Rolls Royce Crecy are examples).

As an aside, large marine diesels used to suffer form the risk of crankcase explosions. People were killed. The problem has been largely overcome now.

In the end I think E-P is right. If you are going down this path it's better to burn biofuels. Storing H2 is difficult.

BTW Does anyone remmember the name of the Professor who was using solar energy to synthesise hydrocarbon fuels directly?


RickBrant said at December 8, 2006 12:55 PM:

> Hydrogen is going to lose to the PHEV; a tank doesn't
> fill as fast as the best batteries can charge,

Sorry, but that is not correct.

Last night I put 12 gallons of gasoline in my tank in less than five minutes.

One gallon of gas is good for about 34 kilowatt-hours. That's 34 kilowatts for an hour. 12 gallons = over 400 kwh.

Adding this energy to the car in five minutes means we are pumping energy into that gas tank at the rate of 4800 kilowatts. Nearly 5 MILLION watts.

Think of it as 440 volts at over 10,000 amps.

Please design for me a battery, charging station, charging regulators, wiring, etc., that can handle this? At a consumer-operated refueling station, without significant maintenance costs?

It's true that an electric power train can be more efficient than an i.c. driven one, but it's also true that batteries are not 100% efficient whereas gasoline tanks come very close -- this is approximately an even trade, so won't change the above.

And let's not even talk about the appalling energy density of batteries compared to a tank of gasoline!

Randall Parker said at December 8, 2006 8:54 PM:


Your 34 kwh pe gallon of gasoline depends on the time of year and jurisdiction. But 34 kwh per gallon is, yes, a good working number.

At 10 cents per kwh that's $3.40 if you buy that energy as electricity. The cost of electricity also varies.

But the storage density of gasoline and electricity are not directly comparable for two reasons:

A) Electric engines weigh far less than internal combustion engines and transmissions.

B) The lower conversion efficiency when burning gasoline in an engine.

Granted, charging up batteries takes time. But for local trips where you go home between outings the ability to plug into a home outlet makes the charging time much less important.

Electric cars do not have to equal gasoline cars in range or ease of refueling. They just have to reduce cost per mile travelled for shorter trips and do so in comfort. Advances in batteriy technology are getting us closer to the day when that'll happen. Pluggable hybrids with moderate battery improvements are the next step in that direction.

Sione Vatu said at December 10, 2006 11:07 AM:

There was some talk recently about a new type of lead-acid battery. Apparantly the lead was coated onto a substrate of some sort, the substrate providing the structure and mechanical strength so that the layer of lead could be very thin and its surface area maximised. The report I heard claimed that a great deal of weight could be saved in this way. Does anyone know about this? What were the specifics of the battery and what were its power and energy densities?

On a related topic, the founder of Aerovironment, Paul MacGready, claims that battery powered cars are likely to gain acceptable range (for most tasks) soon by utilising lithium battery technology. Also I read that Bert Rutan used to drive his GM EV-1 from Mojave to LA. He had to plan his trip and layover for meals while the car was recharged but he felt that this was not really an inconvenience for him. Alas, I can see other people laying over at the pub rather than a movie theatre, shopping centre or restaurant. Perhaps getting more range is also a safety issue.

Anyway, I was thinking that if the old lead acid technology is able to be reworked to be much lower in mass you may well see hybrid battery electric cars with several types of battery aboard, each dedicated to doing what its properties suit it to best. Hence my interest in the new lead acids.

BMW said at November 18, 2010 1:00 AM:

Hydrogen 7 is unique. While other manufacturers have developed fuel cell cars use hydrogen to create energy that powers an electric motor car BMW liquid hydrogen is burned directly in an internal combustion engine. It uses the same V12 engine to burn gasoline and hydrogen in a fuel tank for each - which is contrary to both body and space, the cockpit, but gives drivers the option of using clean combustion of hydrogen when they can find and gasoline goes from a hydrogen filling station.

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