THE BMW Hydrogen 7 can be powered by either petrol or liquid hydrogen because the car's V12 internal combustion engine has been adapted to run on both. The Munich-based carmaker's project, which has so far seen the production of 100 such cars, is aimed at achieving the goal of sustainable mobility.

Linde, the world's largest industrial gas and engineering company, is BMW's partner in this environmentally friendly effort.

But there have been some criticisms. Some say that using hydrogen as a fuel to power a combustion engine - as opposed to using it in a fuel cell to create electricity - is not ecologically sound because hydrogen is in limited supply and manufacturing it requires large amounts of energy.

Fred Zheng, Linde's China and Asia representative for hydrogen solutions, gives BT a better understanding of the technology.

What is Linde's involvement in the BMW Hydrogen 7 project?

Linde provided the liquid hydrogen refuelling solutions for the project. These cover the whole hydrogen supply chain from hydrogen production, liquefaction, storage and transportation, to refuelling technologies.

Who makes the liquid hydrogen and how is it transported around the world?

For more than 70 years, Linde has liquefied hydrogen on an industrial scale and used it for a variety of applications. Most of the time, liquid hydrogen is transported by liquid hydrogen tankers. For BMW's Hydrogen 7 event, for instance, the tank carrying liquid hydrogen was transported from Melbourne to Singapore by sea freight. The liquid hydrogen was sourced from a liquefaction plant in China.

How much does liquid hydrogen cost and what kind of mileage can it give the Hydrogen 7?

Since hydrogen is an energy carrier, the price is largely dependent on the feedstock/primary energy it carries. The cost of today's industrial hydrogen depends on: feedstock, production capacity, order size, pressure, purity, distance to application and reliability of supply.

Moreover, the cost of hydrogen depends on the production methods. Per energy content, hydrogen is cheapest when produced from natural gas, which is, ex-works, cheaper than petrol at today's refuelling stations (including tax).

The Hydrogen 7's tanks can contain eight kg of liquid hydrogen, which allow the car to be driven up to 200 km.

Is it true that hydrogen production is extremely energy-intensive?

Centralised production of liquid hydrogen offers high efficiency due to economies of scale. Liquid hydrogen also offers advantages over compressed gaseous hydrogen in terms of logistics and transport. Transporting liquid hydrogen tends to be more economical and flexible than that for gas trailers because it has a considerably higher energy density than gaseous compressed hydrogen. Thus, more energy can be conveyed per transport unit. The considerable savings puts the slightly higher energy consumption necessary for liquefaction into perspective.

Does cooling the hydrogen for storage in the BMW's fuel tank use up as much as one-third of the energy in the tank?

Not that I know of. The insulated LH2 tank protects liquid hydrogen from heat. These cryo-tanks consist of an outer and inner metal vessel. The inner vessel holds the deep-cold hydrogen, while the outer vessel is at room temperature. To protect the liquid hydrogen from heat radiation, the inner vessel is wrapped with multiple sheets of metal foil and intermediate layers of glass wool.

A vacuum is maintained in the cavity between the inner and outer vessels to prevent heat from penetrating by convection. The inner vessel is connected to the outer vessels via mechanically stable and thermally insulating devices. Despite the complex measures, a slight heating of the liquid hydrogen does occur over time, leading to the rising pressure in the tank. In order to keep the pressure constant, some hydrogen must be vented. This is referred to as vaporisation losses.

This article was first published in The Business Times on Mar 8, 2008.