Reducing the Energy Carbon Content - Energy and Climate: How to Achieve a Successful Energy Transition - page 84

storage in the subsoil is difficult with small-capacity decentralised
facilities.
Hydrogen produced with CO 2 capture and storage, being of
neutral carbon content, can also be used for petroleum refining and
chemistry applications, in particular to obtain petroleum fuels of
better environmental quality.
In addition, hydrogen (and therefore energy) produced with CO 2
capture and storage from biomass has negative carbon content, with
a net removal of CO 2 from the atmosphere, as indicated previously.
. Although hydrogen produced by electrolysis, from nuclear or renew-
able origin electricity, is currently more expensive than hydrogen
produced from natural gas, it should eventually become cost-effec-
tive. There seems little point in converting it directly into electricity,
since it is simpler and cheaper to transport electricity as it is. Provided
that the electricity is obtained from nuclear or renewable sources, it
can be used in industry to improve the carbon balance. It can also be
used as fuel on board vehicles.
Useofhydrogenasameansofstoragecanbeusedtooffsettheintermittent
nature of the renewable energy supply or to modulate the supply of
electricity from base-load production, i.e. at constant power, which corre-
sponds to the conditions required with nuclear energy. In this case, a step to
produce hydrogen by electrolysis is necessary, followed by a step to generate
electricity in a fuel cell (or in a combined cycle). These successive conversions
increase the costs and decrease the overall efficiency.
With on-board storage, use of hydrogen as a fuel in a vehicle offers the
advantage of potentially avoiding CO 2 emissions while, at the same time,
diversifying the energy sources. The progress made several years ago in the
field of fuel cells raised hopes that a propulsion system consisting of a fuel
cell, powered by hydrogen and producing the electricity necessary to drive
the electric propulsion motors could represent the ideal solution sought in
the field of transport. These hopes are from being realised, however.
Numerous difficulties remain, concerning both use of hydrogen and devel-
opment of the fuel cells (cost, reliability). The storage system is also a difficult
issue. Hydrogen can be stored in gaseous form at high pressure (700 bar).
Hydrogen can also be stored in liquid phase, but at low temperature,
resulting in high costs and making extended storage difficult. Studies are
being conducted on alternative forms of storage, in particular as hydrides,
but no competitive solution has yet been found. With the current systems
implementing compressed hydrogen, a storage density of 4% by weight
can be reached, expressing the weight of hydrogen stored against the total