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Hydrogen for transport and the B&E report

Oil provides most of the energy for transportation and oil will soon be in short supply. That is the reason why advocates of the hydrogen economy want to replace oil with hydrogen. Practical hydrogen powered trucks and automobiles must be the initial objective. Hydrogen sources and a hydrogen distribution system will be needed.

Much of what follows is a summary of a report, The Future of the Hydrogen Economy: Bright or Bleak? by two Swiss engineers, Ulf Bossel and Baldur Eliasson (B&E). [2] This report discusses hydrogen sources,  hydrogen storage, and hydrogen distribution. Hydrogen powered vehicles are not discussed. This report was written by and for engineers. All assertions are supported by facts, figures and calculations. It is trustworthy in every way although it's a slog to read it.

How do you get hydrogen?

Hydrogen has been an important article of commerce for a century. In 1905, the Germans developed the Haber process to make ammonia from nitrogen in the air and hydrogen. Ammonia is the starting point for making fertilizer and explosives. [4] Hydrogen is also used to hydrogenate vegetable oils, a process that converts healthful vegetable oils into something less healthful. Look for the word the next time you go grocery shopping.

Hydrogen may be produced by several methods. You will need an energy source and a hydrogen source. Today, the cheapest source of hydrogen is natural gas, which is both an energy source and a hydrogen source. The problem is that the resulting hydrogen has only 50% of the chemical energy of the original natural gas. Hydrogen can also be produced from oil but the energy efficiency is a little worse. If hydrogen is derived from fossil fuel, carbon dioxide, a greenhouse gas, is released in all cases.

The United States has lots of coal and it might seem like a good idea to use it to make hydrogen. Coal is an energy source but not a hydrogen source. Water must be used as a hydrogen source since coal has little or none. Coal gas, a mixture of carbon monoxide and hydrogen, is made by passing steam through a bed of white hot coal. This method has been used for two centuries and better methods exist, but the maximum theoretical energy efficiency is only 35%. The traditional problems with coal remain. These include acid rain producing sulfur dioxide, smoke containing heavy metals, ground water polluting ashes, and greenhouse gas. Coal consumption would increase 8 fold if coal derived hydrogen were to replace the oil used for transportation. Coal consumption at this rate would result in nightmarish levels of pollution. There isn't enough minable coal to sustain this rate very long.

This little critter, an algae, can make hydrogen from water and sunlight. Imagine a pond with a transparent cover for collecting the hydrogen that bubbles from great colonies of algae. It is also possible to design a device that is somewhat like a photocell that produces hydrogen instead of electricity. Both methods are far from practical but they hold promise.

It is possible to make hydrogen by "reforming" alcohol. Reforming always results in significant energy loss. Alcohol can be made from corn or sugar cane. Fossil fuel is used to grow corn and sugar cane. The energy available in the alcohol does not balance the fossil fuel energy expenditure. The energy balance is even less favorable after reforming. 

Electricity can be used to make hydrogen from water through electrolysis, an efficient process. Electrolysis of water has been known for centuries and it is a common classroom demonstration. This picture illustrates how. It works better if you add a little salt to the water.

Fossil fuel is the source of most of the energy used to make electricity in the United States. Hydrogen produced by electrolysis may be viewed as a convoluted way to convert fossil fuel to hydrogen. Fossil fuel (coal, natural gas, and oil) can be converted to hydrogen directly, without making electricity first. The direct methods use less fossil fuel for the same amount of produced hydrogen.

Electricity from wind turbines, solar arrays, etc. can also make hydrogen through electrolysis. It makes no sense to do so because the same electricity is better used to replace fossil fuel. The saved fossil fuel has three times more energy content. (Recall that converting fossil fuel to electricity is very inefficient.) The saved fossil fuel can be converted to hydrogen. The net effect is more hydrogen for the same energy inputs. This point cannot be overemphasized. [3]

In the extreme case, all fossil fuel would be replaced by sustainable electrical energy. In that case, the surplus electricity could be used to make hydrogen. Even this is a bad idea for reasons that will be explained.

How do you store hydrogen?

Hydrogen is difficult to store because has very low volumetric energy density. It is the simplest and lightest element--it's lighter than helium. Hydrogen is 3.2 times less energy dense than natural gas and 2700 times less energy dense than gasoline. Hydrogen contains 3.4 times more energy than gasoline on a weight basis. Hydrogen must be made more energy dense to be useful for transportation. There are three ways to do this. Hydrogen can be compressed, liquefied, or chemically combined.

Compressed hydrogen

Hydrogen compressed to 800 atmospheres (also called bars) occupies 3 times more volume than gasoline for the same energy. It is necessary to reach this density if a vehicle is to carry enough hydrogen to be practical. A pressure of 800 bars works out to 6 tons, or 12,000 lbs, per square inch. It is very difficult to contain such pressures safely in a lightweight tank. Catastrophic tank failure releases as much energy as an equal weight of dynamite. A tank made of high strength steel weighs 100 times more than the hydrogen it contains. A truck or an automobile using a steel tank would be impractical as the tank would weigh nearly as much as the vehicle.

High pressure hydrogen tanks made from carbon fiber may be a solution. Carbon fiber is a material used in aircraft and sporting goods. At the present time, carbon fiber tanks are very expensive. The DOE has proposed a performance goal as part of the FreedomCar initiative. The goal for 2005 is 4.5% as the ratio of hydrogen to tank weight at 800 atmospheres. [5] (page 6)

A typical 18 wheeled semi-truck carries two 90 gallon tanks, providing a range of 750 miles. A typical 4 cylinder sedan has an 18 gallon tank, providing a range of 575 miles. (The practical range would be somewhat less.) The diesel engine achieves an efficiency of 35% at cruising speeds. The gasoline engine achieves an efficiency of 25% at cruising speed. Both vehicles could be converted to hydrogen operation. Internal combustion engines (ICE) could be used resulting in an efficiency of 35%. Or, fuel cells could be used resulting in an efficiency of 45%.

The above information is presented here in tabular form:

The space, weight and expense of steel tanks make them impractical. Any gains in energy efficiency would be offset by losses incurred in hauling the very heavy tanks. Carbon fiber tanks of this size and performance do not exist--they are only goals. Gasoline, by contrast, requires only a small, low-tech tank.

Some notes about compressed hydrogen

The laws of thermodynamics dictate the amount of energy it takes to compress a gas. The physical properties of hydrogen make it the most difficult of all gasses to compress. At 800 bars, a perfect, single stage compressor consumes energy equal to 16% of the chemical energy in the hydrogen. (This is the energy that gets instantly released in the event of a tank failure.) It is possible to use a multistage compressors with intercoolers to achieve 12%. This is an estimate extrapolated from an actual multistage compressor working at 200 bars. A multistage compressor working at 800 bars does not exist. [7]

It is technologically challenging to compress hydrogen to 800 bars. Higher pressure would not result in much volume reduction. At these pressures,  hydrogen acts less like a gas and more like a liquid.

The laws of thermodynamics also dictate that energy losses occur when hydrogen is transferred from a storage tank to a vehicle. The design of the transfer lines and the pressure fittings is critical in keeping energy losses low.

Liquid hydrogen

Liquid (cryogenic) hydrogen also occupies 3 times more volume than gasoline for the same energy. (Paradoxically, there is more hydrogen in a gallon of gasoline than there is in a gallon of liquid hydrogen.) The advantage of hydrogen liquefaction is that a cryogenic hydrogen tank is much lighter. Hydrogen's physical properties means hydrogen is harder to liquefy than any other gas except helium. There are significant and inevitable energy losses when hydrogen is liquefied. This graph illustrates how energy losses depend strongly on plant capacity. Losses are 30% in the best case.

Liquid hydrogen is colder than any other substance except liquid helium. The advantage of liquid hydrogen is that it can be stored in relatively lightweight tanks. A tank for cryogenic hydrogen is like a thermos bottle, but it must work much better. It consists of a tank within a tank with a vacuum between the two. The inner tank must be supported without conducting heat from it. This is very difficult to do in a tank designed for a vehicle. Gasoline, by contrast, requires only a small, low-tech tank.

B&E estimates that a liquid hydrogen tank designed for automobile use will loose about 5% of its capacity every day, which is to say that all of it will be gone in 20 days. Losses of this magnitude are acceptable for, say, a taxicab fleet, but unacceptable to most people.

Hydrogen cannot be vented to the atmosphere because it is an explosion hazard and because it is a greenhouse gas. The vented hydrogen must be burned. A continuously running gas stove with one burner set to "medium" would do it.

A note about liquid hydrogen

Hydrogen molecules exist in two forms, ortho (the electron spins are anti-parallel) and para (the electron spins are parallel.) Room temperature hydrogen is a mixture of the two. However, liquid hydrogen turns into pure para hydrogen over the course of a few days. The process releases enough heat to turn the liquid hydrogen to gas in a few days. Liquid hydrogen can be catalytically converted to all para during the liquefaction process. If this were not done, 30% of the hydrogen would escape in two days even in a perfect cryogenic tank. Conversion adds to the cost and complexity of liquefaction.

Chemically combined hydrogen

Certain alkali metal hydrides release hydrogen when exposed to water. These metal hydrides hold enough hydrogen to make them useful for transportation. However, 70% of the energy is lost in the creation of the hydrides, making them unacceptable for widespread use.

Certain metals (platinum, zirconium, lanthanum) can be formed into "sponges" for hydrogen. But the "sponges" can hold only 1% of their weight in hydrogen and they are very expensive.

In both cases, the energy storage tank is heavy and expensive not to mention energy inefficient. These are not the qualities needed for vehicles designed for mass transportation. But the technology might have a niche application. Long lasting, but expensive, batteries for laptop computers may use these hydrogen sources to power small fuel cells. 

It is possible to combine hydrogen with carbon to produce methane or gasoline, both superior carriers of energy. If the carbon is taken from the atmosphere there would be no net greenhouse gas contribution. The carbon would only be "borrowed" to carry the hydrogen. The gasoline made in this way would not contain carcinogens (like benzene) and it could be very high octane.

How do you transport hydrogen?

A 40 ton truck can deliver 26 tons of gasoline to a conventional gasoline filling station. One daily delivery is sufficient for busy station. A 40 ton truck carrying compressed hydrogen can deliver only 400 kilograms. That is because of the weight of the tank capable of holding 200 atmospheres of pressure. An empty truck will weigh almost as much as a full one. The compressed hydrogen tank must be robust. The energy used to compress the hydrogen to 200 atmospheres would be released instantly if a tank ruptured. The fireball would cover a football field. Hydrogen is more energy dense than gasoline (by weight) and hydrogen powered transportation is more energy efficient. Yet the hydrogen filling station will require 15 deliveries every day, everything else being equal. The energy cost of truck transport becomes unacceptable unless the source of hydrogen is very close to the point of use. A cryogenic truck could carry more hydrogen but recall that the energy cost to liquefy hydrogen makes this infeasible in most cases.

Hydrogen can be transported by pipeline. According to B&E, it take about 4 times more energy to move hydrogen through a pipeline compared to natural gas.

Summary of Bossel and Eliasson Report

According to B&E, the hydrogen economy idea does not work for multiple reasons. They point out that there is no practical source of hydrogen, no good way to store hydrogen, and no good way to distribute hydrogen. Many of the problems of hydrogen stem from the physical and chemical properties of hydrogen. Technology cannot change these facts.

A compact and convenient energy carrier will be needed in the future. B&E suggest methane, ethane, methanol, ethanol, butane, octane, ammonia, etc. as better energy carriers.

It is difficult to understand the enthusiasm for hydrogen in view of the above. Hydrogen does not solve the energy problem and it is a bad choice for carrying energy.

[1] http://www.bp.com/ British Petroleum (BP) is a major oil company that publishes oil reserves data from the Oil and Gas Journal in its Statistical Review of World Energy. It is released annually in late June. The Review is widely referenced because it is convenient, complete, colorful and very well done. Information about hydro electricity, coal, and nuclear power are also included. It is also big at 2.2 megabytes.
[2] The Future of the Hydrogen Economy: Bright or Bleak? This is the April 2003, version 15 of the Bossel and Eliasson report. A 240 kilobyte PDF file which may be viewed on this website by clicking here.

[3] The Oil Factor subtitled, Protect Yourself--and Profit--from the Coming Energy Crisis, by Stephen Leeb. A new (2004) book about investing and the energy crisis. Leeb points out that the best use of wind or solar electricity to free up oil, gas and coal to make transportation fuels (page 85). Leeb also has a website:
http://www.leeb.net/

[4] http://www.princeton.edu/~hos/mike/texts/readmach/zmaczynski.htm Find out how the Haber process changed history.

http://www.llnl.gov/str/June03/Aceves.html Lawrence--Livermore National Laboratory tackles the problem of hydrogen storage for automotive use.

[5] http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/h2_storage_think_tank.pdf

[7] http://www.tinaja.com/h2gas01.asp Don Lancaster's collection of hydrogen resources. Also called Don Lancaster's Guru's Lair. Don holds forth on a wide range of topics. Surprisingly, this is a good place to learn about and thermodynamics. The terms "adiabatic" and "isothermal" are explained. Don is a little bombastic but everything he says is backed up by facts.

http://mb-soft.com/public2/hydrogen.html A good website devoted to the idea of the Hydrogen Economy.

http://www.tipmagazine.com/tip/INPHFA/vol-10/iss-1/p20.html The January/February of 2004 issue of The Industrial Physicist has a feature article called Bottling the hydrogen genie.

http://www.hydrogenprodevel.com/ "Hydrogen Prodev Daily News" is hosted by a Phd Chemical Engineer. If you want detailed information about the hydrogen economy technology, this is a good place to look.