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The Manapouri power station. PHOTO: STEPHEN JAQUIERY

The boosters are at it again, attracting the interest of foreign investors always eager to find a new place to splash their cash and suck the profits offshore.

This time it is the chimera of a green hydrogen plant to use the ‘‘surplus’’ Manapouri power if the Tiwai aluminium smelter shuts down in three years’ time.

Hydrogen is being touted as a clean source of energy that would power road transport. But it would be the least efficient solution to reducing our carbon footprint and a wasteful use of Manapouri electricity.

First, hydrogen is a place to store energy, an energy vector, not a source of energy.

‘‘Green hydrogen’’ is obtained by passing an electric current from a ‘‘renewable’’ generator through water, breaking the molecule of the liquid (electrolysis) and separating hydrogen from oxygen, without other emissions.

The first issue is that hydrogen needs a high level of electricity consumption to produce. The best electrolysis plants achieve a performance of 70%, meaning 30% of the energy is lost and does not accumulate in the hydrogen molecules produced. But this best performance only occurs under ideal conditions and with very sophisticated and costly plants. More realistically, the yield is about 50%. The other 50% of the energy is simply lost.

Secondly, the efficiency of hydrogen engines is low. Hydrogen can be burnt directly in a petrol engine but then only 15% to 20% of the hydrogen energy would be used (that is, only between 7.5% and 10% of the initially generated electrical energy).

Even by using the most efficient fuel cells (and making the engine more complex, because a battery is also required) the performance is around 50% (that is, only 25% of the initial electrical energy used).

By comparison, an electric motor directly driven from the grid (such as in an electric train) has efficiencies that are consistently above 75% or 80%. Hydrogen is more useful in providing industrial heat, but not in replacing diesel engines in our fleet of trucks and heavy machinery.

Then there are the very real storage problems. To achieve an acceptable volume energy density, hydrogen must be contained at high pressure. This is generally 750 atmospheres (this is the pressure at a depth of 7500 metres under the sea) in order to have an energy density that is only half that of natural gas at normal pressure.

These high pressures require additional energy to compress it and containers with exceptionally dense walls (more costs). It also must be refrigerated before compression to avoid the temperature rising too much (more energy expenditure). Let alone the dangers posed by a crack or a moderately strong impact on the tank. Hydrogen escapes easily from any container, even one with dense walls and well sealed. Losses of between 2% and 3% per day are normal, which implies that hydrogen has to be consumed within a few days of production.

Finally, hydrogen corrodes steel. In carbon steel tanks and pipes, hydrogen forms hydrides which over time make them brittle until they break. The solution is to cover them with special films called liners, but these are not without their problems (they withstand thermal contrasts and mechanical stresses poorly). Also, they are manufactured with oil, adding to the carbon footprint.

Oh, and then building and maintaining the entire hydrogen plant would consume massive amounts of fossil fuel products and create significant CO2 emissions. And oh, more energy would be required to purify the vast amounts of water needed.

The reality is that the energy losses of converting electricity to hydrogen for any energy use are large, ranging from 50% for production of hydrogen to be burnt immediately, to losses of more than 95% if it has to be stored under pressure to be consumed a few days later in the engines of trucks.

Even thinking about using Manapouri power for a green hydrogen plant at Tiwai is a waste of time and planning resources.

It would be a shocking waste of renewable energy that would be better directed into the national grid to meet the growing demands of EVs, electrified bulk transport, and future climate-driven hydro-power shortfalls.

• Dr Philip Temple is a member of the Wise Response Society committee. For the compilation of this article he would like to acknowledge Dr Antonio Turiel of the Institut de Ciencies del Mar, Barcelona, and Wise Response colleagues.