Our energy industry man returns to hydrogen fuel in the latest in the series on new and emerging powertrain technologies, and also considers lithium-ion batteries’ true carbon footprints
There are a number of ways of isolating and storing hydrogen: Electrolysis, chemical reformation and chemical reaction. We’ll consider the first two, as they lend themselves better to bulk production.
Water is separated into hydrogen and oxygen by passing a DC current through electrodes emerged in it. Hydrogen appears at the negative electrode and oxygen at the positive; the gases are collected separately.
If renewable electricity is used, the zero-emission characteristic of the hydrogen can be maintained. This is also a way of storing surplus energy from renewable sources.
The alternative bulk production method – reforming methane – would introduce a fossil fuel and destroy the unique zero-emission characteristic of hydrogen.
The first challenge of hydrogen as a fuel is its density. At ambient conditions, 1kg of hydrogen occupies 11.2m3 and its energy density is 39.4kW/h per kg.
To be useful, it must be stored at a high pressure; 11,600psi is typical. That compares with methane at 15.4kW/hr per kg and diesel at 13.3kw/hr per kg.
But when methane is used as a fuel, it is stored at 2,900psi. Diesel is stored at ambient pressure. The implications of this will be considered in the next article.
Lithium-ion batteries
Battery-powered EVs have no tailpipe CO2 emissions, but there is a carbon footprint associated with the production of lithium-ion batteries. Added to the carbon footprint of electricity production, this generates a very interesting result.
It has been reported in the e-book Life cycle assessment of lithium-ion batteries for plug-in hybrid-electric vehicles that in the manufacture of a lithium-ion battery, there is around 200kg of CO2 produced per kW/h of energy storage capacity.
A typical battery bus has a storage capacity of 324kW/h, meaning that manufacture of its battery produced 64,800kg of CO2.
Battery life is quoted as 4,000 cycles, equivalent to 16.2kg of CO2 per cycle. Range per cycle is around 250km, leading to a CO2 footprint of 64.8g/km.
Adding this to the CO2 footprint of the energy generated for charging of 779.3g/km, as previously reported, gives a total CO2 footprint of 847g/km. Electric buses may be clean locally, but globally is another matter.
Our industry expert speaks with authority and can back up all of his facts, but what do you think? Email editorial@divcom.co.uk if you agree or disagree with him.
