Feature

Hydrogen Power

The use of hydrogen as a fuel for vehicles has the potential to free New Zealand from its dependence on imported and non- renewable fuel sources

By Roger Green & Neil Glasson

Hydrogen has long been considered by many scientists as the ultimate fuel of the future. Research overseas has suggested using hydrogen to fuel city bus fleets, domestic and industrial premises, aircraft and power generating stations. At Canterbury University, researchers in the Mechanical Engineering Department are studying suitable means for using hydrogen as a fuel source for vehicles.

Hydrogen as a fuel is very clean burning, as the main product of its combustion is water. When burnt in air, the only significant pollutants are nitrogen oxides. The combustion of hydrogen does not produce the other pollutants associated with hydrocarbon-based fossil fuels, such as unburnt hydrocarbons, carbon monoxide, carbon dioxide and particulate matter.

As well as being practically non-polluting, hydrogen can be produced using almost any primary energy source. The possibility of manufacturing hydrogen from a variety of renewable energy sources makes a compelling argument in favour of its use as a fuel. Potential sources include wind power, solar power, geothermal power or hydropower.

The current techniques for the production of hydrogen are many and varied, but the basic reactions are steam reforming, carbon monoxide conversion and electrolysis.

The first two of these reactions are used for the majority of current hydrogen production. The point to note, however, is that both produce pollutants, whereas the third alternative -- electrolysis -- is a clean process.

A Transport Fuel Strategy

New Zealand is currently in the fortunate situation of being over 50% self-sufficient in road transport fuels. Most of this self-sufficiency stems from the availability of condensate and refined gas from the natural gas fields. During the next two decades, as the gas fields are depleted, we could return once again to almost total dependence on imported crude oil for road transport energy requirements.

In New Zealand, the two major sources of primary energy suitable for large-scale production of hydrogen are hydropower and coal. Electrolysis of water using hydroelectric power would provide a clean means of producing hydrogen. With state-of-the-art technology, the process would be about 80% efficient.

The surveyed reserves of hydropower which may be utilised in the future are of the same order of magnitude as the current road transport energy requirements. An increasing demand for electricity or road transport energy could mean that the hydropower plants would fall short of being able to supply a significant proportion of New Zealand's future road transport demands.

The other primary energy source to be considered is coal. Besides the need to develop cleaner processes for the manufacture of hydrogen using coal, there are the added environmental concerns associated with mining. These problems aside, there remains ample energy in New Zealand's coal reserves to fuel the transport fleet for many years to come.

Technical Hitches

The use of hydrogen in an internal combustion engine has the potential to be more efficient than petrol and is similar to that of a diesel engine. There are a number of problems, however.

One of the key problems is the storage of hydrogen on board a vehicle. There are basically three alternative methods for the storage of hydrogen: under pressure as a gas, bound in metal hydrides or liquified.

Current high pressure storage techniques require a large volume and have a prohibitively high mass. More advanced construction using fibre-wound aluminium vessels could provide an answer to this.

Metal hydride storage systems use special alloys which absorb hydrogen when supplied under pressure. They, too, have high mass, because of the hydride itself and the pressure vessel and heat exchanger required for the charging and release of hydrogen.

Hydrogen can be stored as a liquid at 20 K (-253^oC) in special super-vacuum insulated flasks. This system has the lowest mass and occupies the least volume. Other factors to consider in the selection of the storage technology include convenience, safety, time required to refuel and the pressure at which the hydrogen is needed in the engine. If high-pressure hydrogen gas is required, then the most attractive option is liquid storage.

Engine Combustion

Once the hydrogen is on board, the next problems occur in its combustion in an internal combustion engine. Hydrogen's very low ignition energy and wide limits of flammability tend to cause preignition and flashback of hydrogen at the inlet manifold. The high flame speed of hydrogen combustion promotes a rapid rise in pressure which results in engine "knocking". Its high specific volume causes a reduction in the charge energy if the hydrogen is taken in through the inlet manifold.

These problems can be minimised or even eliminated with careful attention to mixture preparation. A low pressure, direct injection system avoids the problems of reduced power and backfiring, although it is a little more complicated technically. The most advanced system of injecting hydrogen under high pressure late in the compression stroke has the potential to eliminate all the problems associated with hydrogen fuelling.

Such a system is being developed by Canterbury University engineers, with support from a scholarship from the Chartered Institute of Transport. It is hoped that the knowledge and experience gained in developing this advanced hydrogen injection system will lead to the development of a totally self-sufficient, low polluting, transport fuel in New Zealand. W

Roger Green is a Senior Lecturer and Neil Glasson a post-graduate student in the Mechanical Engineering Department, University of Canterbury.

Neil Glasson is a post-graduate student in the Mechanical Engineering Department, University of Canterbury.
Roger Green is a Senior Lecturer in the Mechanical Engineering Department, University of Canterbury.