The basics
Since the first oil crisis of the mid-1970s, and even before, energy experts have proposed that hydrogen could be used as the main source (or vehicle) for mankind’s energy needs in place of fossil fuels. Fossil fuels such as coal and oil are a finite resource. They will eventually run out. In addition, the combustion process yields large quantities of carbon dioxide, which contributes to global warming through the greenhouse effect. Worse still, many fossil fuels are contaminated with sulphur, heavy metals, and other dangerous pollutants.
By contrast, combining hydrogen with oxygen either through a fuel cell to produce electricity, or in an internal combustion engine to produce mechanical energy, is a completely clean process, yielding only water and energy as the final products (with some nitrogen compounds, if the hydrogen is burned in air).
The argument is that if we can find a way to separate the water in the oceans into hydrogen and oxygen, and then transport the hydrogen to the point of use, we can liberate the chemical energy of the hydrogen-oxygen reaction. The resulting energy can be used to drive vehicles, or produce light and/or heat, while the waste water will eventually find its way back to the oceans through clouds and rain. If this cycle can be made to work, mankind can make use of a closed, self-contained cycle without the environmental damage caused by large-scale oil extraction and consumption.
The term hydrogen economy is used because it requires a major shift in perspective, from a world in which wealth, value and power is based around control and management of fossil fuels, into one in which wealth and power derive from the management of hydrogen resources and systems.
The main techno-economic challenge for hydrogen economy proponents is to find a method to store and transport large quantities of hydrogen. Hydrogen is a gas at normal pressures and temperatures, and this gives it a very low energy density compared with petroleum, for example. It can be liquefied, but only at cryogenic temperatures and/or high pressure.
Secondary challenges include methods of separating large volumes of water into hydrogen and oxygen at reasonable cost. Traditionally, this is done by electrolysis—using electrical energy to dissociate the two gases. Leading candidates for creating that electricity are nuclear power, solar power, geothermal energy and other energy sources which do not consume large amounts of fossil fuel. Such plants would have to be located on the edge of a large supply of water, such as the ocean, or a large lake. The resulting hydrogen could then be pumped to distribution points closer to the centres of use.
If I write as if this is a very likely future, then I am far from alone. Almost all fuel experts believe that hydrogen is the fuel most likely to... replace petroleum. Perhaps longer term, we will have nuclear fusion reactors, but these systems are 50 to 100 years away from commercial exploitation. Hydrogen systems are only 5 to 10 years away. Most of the basic technological issues have been solved, and all that remains is getting the economics right and then psychological and fashion issues among the mass consumers. It is one of those large-scale changes that many noders are likely to witness in their lifetimes.
Transportation applications
As a liquid, a litre of petroleum at atmospheric temperature and pressure weighs around 700g, can be stored in a simple metal container and will take a standard car about 15 km. A litre of liquid hydrogen (at cryogenic temperature or higher pressure) will only take the same car about 4 km. If we supply 4 litres of hydrogen, to take it the same 15km, then those 4 litres of hydrogen weigh around 300g. However, the weight of the refrigeration system and/or pressure vessel will add considerably to the weight of the fuel.
The upside is that a kilo of liquid hydrogen contains roughly 2.6 times the chemical energy of a kilo of petroleum. The downside is that hydrogen is much more flammable and explosive than petroleum. A further problem is the high evaporation and loss rate from practical fuel tanks. Typical figures of 1 to 2 percent per day are quoted in the literature. This means a vehicle could lose half its fuel load in a few weeks of simply sitting on the driveway without ever being driven.
BMW built a liquid hydrogen powered car (745 hdi) as early as 1996. It had a tank capacity of 75 kg (equivalent in energy terms to 40 litres of gasoline). It offered a range in traffic of 400km, which is comparable to the same petrol-powered vehicle, and delivered fuel economy of 10km per litre of hydrogen fuel. (10 litres per 100km).
Note that liquid hydrogen has a density of 0.07 grams per cubic centimetre, or 1/14th that of water. Multiplying up, that 75 kg tank has a volume of around 1000 litres. A typical fuel tank in a conventional car has a volume of 100 litres, and the cabin space is say 1500 litres, so the hydrogen fuel tank is taking up around half the cabin space.
Since then, many more companies have developed hydrogen-powered vehicles. At the recent (January 2002) Detroit motor show, GM showed a concept vehicle, AUTOnomy, designed to carry large containers of hydrogen alongside fuel cells. In September 2001, the tyre company Michelin sponsored an event, dubbed 'Challenge Bibendum' designed to publicise alternatively-powered vehicles. European and Japanese manufacturers used the event to showcase their hydrogen-powered vehicles.
A Canadian company called Xcellsis entered the Zebus, a large 'bus aimed at public service duty, and powered by fuel cells supplied with pressurised hydrogen. The bus quite happily managed 440km (275 miles) across the Nevada deserts at speeds of up to 100 kph (65 mph) and now holds the world record for the longest distance travelled by a bus powered by fuel cells. The manufacturer has built three such buses and hopes they will be in commercial service during 2002. Honda, the Japanese auto maker offers its FCX-V4 vehicle, which has a range of 300 km (180 miles) on a single tank of compressed hydrogen gas. Prestigious trade paper Automotive News described the vehicle as, 'a pleasure to drive,' in contrast to GM's fuel cell-powered Zafira, which it described as, 'feeling like a developmental vehicle.' Challenge Bibendum will be repeated on a much larger scale in October 2002, based around an automotive trade show in Las Vegas. Expect to see many more hydrogen-powered vehicles.
The hydrogen fuel cycle is not speculation, but is now a technological reality. The biggest question is still economics, but as fossil fuels become more expensive, and environmental pollution becomes less acceptable, such vehicles are likely to become much more common on our roads. Legislative pressure in California and elsewhere strongly encourages vehicle manufactrurers to minimise the polluting effect of their vehicles, and all serious automotive companies are actively working on these technologies
Distribution and transportation
This is probably the biggest challenge facing the development of a hydrogen economy. As noted above, hydrogen is normally a gas with very high specific volume. It is not easily liquefied, and is flammable and even explosive when mixed with air.
Nevertheless, the situation is not impossible. Hydrogen is similar in some ways to methane, though hydrogen is much more volatile and the molecules can permeate through many more membranes than methane molecules. But the evidence shows that where there is an infrastructure for natural gas, then the same pipes can be used to distribute hydrogen, Similarly, trucks used to ship methane can also be used for hydrogen.
However, the cost of building such an infrastructure from scratch is very high. Electricity is usually delivered across overground wires supported on pylons, because the cost of burying and maintaining underground cables is too high. Underground pipelines are more expensive than cables.
Another issue that needs discussion in this context is the question of overall energy efficiency. Modern power stations use fossil fuels to drive turbines to create electricity. If we need to drive a wheel, we can choose to use a small internal combustion engine powered by fossil fuel, or to use the centrally-generated electricity to drive a motor. There is only a small difference in the overall efficiency of these two systems.
In a hydrogen economy, electricity (probably from solar power) is used to dissociate the hydrogen and oxygen in water. If then the hydrogen is transported to a home and then used for heating, it makes for a very inefficient overall energy use. Far better to use solar power to heat the home directly, rather than convert the solar power to electricity, use that to make hydrogen, ship the hydrogen to the home and finally burn the hydrogen for heat. Therefore, a successful hydrogen economy should include large numbers of small-scale renewable energy devices.
Costs
Perhaps surprisingly, the costs of hydrogen are not so different from the price of petroleum sold to consumers, at least in Europe. Night-time electricity can be bought for around $0.07 per kWh in the USA. This translates to a cost of some $0.11 per kWh for liquid hydrogen. A litre of gasoline contains just over 7 kWh of energy, so hydrogen can be made at a cost equivalent to $0.80 per litre of gas ($4 per gallon). This is similar to the price at the gas pump in Europe. Of course, government taxes contribute over 80 percent of the pump price, but the argument shows that consumers should not object too strongly to the price of hydrogen fuel.
In terms of efficiency, by far the best way to extract the energy from the hydrogen-oxygen reaction is in a fuel cell. Efficiencies around 80 percent can be achieved, compared with only perhaps 40 percent in an internal combustion engine.
However, despite these calculations, there is no doubt that the cost of a hydrogen-powered vehicle is substantially greater than a conventional diesel-powered car. This is an issue of volume production, and technological maturity. As the numbers of units manufactured increases, then the cost of making each unit falls. Today, many millions of diesel engines are made each year, but by comparison, only a few thousands of hydrogen fuel cells.
Similarly, the internal combustion engine is a mature technology. Engineers have spent over 100 years refining the design details. A fuel cell, by comparison, is a very new technology, and will over time become much cheaper.
Ultimately, the question is not so much about the dollar price in today's terms, but about whether the fossil-fuel economy is sustainable for another 100 years or more. And if it is not, then how can we build a future which is less dependent on our Jurassic legacy.