By George Monbiot (China Daily)
Updated: 2008-05-08 07:17

Because environmentalists tend to have a feeling for material constraints, we recognize that solutions cannot be conjured out of thin air. In some cases they just don't appear to exist.

There are two reasons why we make such a fuss about flying. The first is that, even as governments promise to cut emissions, everywhere airports are expanding. In Britain, the government expects the number of airline passengers to rise from 228 million in 2005 to 480 million in 2030. Before long, there will scarcely be a patch of sky without a jet in it.

The other is that there are no alternative means of propelling people through the air which are not more destructive than burning ordinary aviation fuel. Or so we think.

The airline companies prescribe two cures that are even worse than the disease. Even before they are deployed commercially in jets, biofuels are spreading hunger and deforestation.

At first sight, hydrogen seems more promising. If it is produced by electrolysis using renewable electricity, it's almost carbon free. The prohibitive issue is storage. Hydrogen contains just a quarter of the energy as the same volume of jet fuel (kerosene), which means that planes could fly long distances only if they were filled with gas, rather than passengers or cargo.

This means that if hydrogen planes are to fly commercially, they need much wider bodies than ordinary jetliners.

According to the Royal Commission on Environmental Pollution, "the combination of larger drag and lower weight would require flight at higher altitudes" than planes fuelled by kerosene.

A technology that is green at ground level becomes an environmental disaster in the stratosphere. Hydrogen's great advantage - that it produces only water when it burns - turns into a major liability: in the stratosphere, water vapor is a powerful greenhouse gas.

The commission estimates that hydrogen planes would exert a climate-changing effect "some 13 times larger than for a standard kerosene-fuelled subsonic aircraft".

But there is another use for this gas, though I am aware that it will go down like a lead balloon with most of my readers.

Though the airship designs have changed, their disadvantages have not disappeared. While a large commercial airliner cruises at about 900 kph, the maximum speed of an airship is roughly 150kph. At an average speed of 130kph, the journey from London to New York would take 43 hours.

Airships are more sensitive to wind than aeroplanes, which means that flights are more likely to be delayed. But they have one major advantage: the environmental cost could be reduced almost to zero.

Even when burning fossil fuels, the total climate-changing impact of an airship, according to researchers at the Tyndall Centre for Climate Change Research in southeast England, is 80 percent to 90 percent smaller than that of ordinary aircraft.

But the airship is also the only form of transport that can easily store hydrogen: you could inflate a hydrogen bladder inside the helium balloon. There might be a neat synergy here: one of the problems with airships is that they become lighter, and therefore harder to control, as the fuel is consumed. In this case they become heavier. Michael Stewart of the World SkyCat company suggests burning both gaseous and liquid hydrogen to keep the weight of the craft constant.

Airships fly much lower than planes, typically at about 1,300m, which means their emissions of water vapor have very little effect on temperature. If they were powered by hydrogen fuel cells, they would be almost silent, greatly reducing the effects for people on the ground. Though they are much slower than jets, the cabin can be built much wider, which means that traveling by airship would be rather like traveling by cruise ship, but at twice the speed and using a fraction of the fuel.

There are four small companies trying to get airships off the ground. Most of the new designs make use of aerodynamic lift as well as buoyancy (they are shaped like fat planes with stubby wings or tails), which means they are heavier and more stable than the old dirigibles and can land without help on the ground.

They can alight on and take off from almost any flattish surface, including water. But all of them have a problem with flotation - of the financial rather than the physical kind. While the price of carbon stays low, companies have no financial incentive to switch to a different form of transport.

Like so many other promising green technologies, this proposal is losing height in a hostile market. All the companies promoting large commercial airships are concentrating on freight, especially in places that are poorly served by roads. The danger here is that, if they take off, they could displace not jet transport but freight shipping - in which case, if they burn diesel, they are likely to cause a net increase in carbon pollution.

Paradoxically, the other major constraint could be an environmental one. Airships are one of several green technologies that might be killed by a shortage of materials. A new generation of solar panels relies on gallium and indium, whose global supplies appear close to exhaustion.

Is this proposal just a flight of fancy? Because airships feature in no official document, because they have not been considered by either government or major industry, I have no way of knowing. But like most greens I'm prepared to try almost anything, as long as it works.

The Guardian

(China Daily 05/08/2008 page9)