Environmental Engineering Reference
In-Depth Information
Hydrogen
A great deal has been written about the 'hydrogen economy', the idea that
hydrogen as a fuel could stand in the place currently occupied by oil and gas.
Hydrogen is a gas with a very low boiling point (-253˚C). There are several
technical problems associated with hydrogen use, including the difficulty of
storage. Hydrogen storage could be by way of compression or liquefaction,
or by reversible sorption processes or chemical reactions. Storage for use in
cars using methods that could compete economically with petrol or com-
pressed natural gas is a challenging proposition, but is receiving considerable
attention.
By far the largest problem is the question of the source of the hydrogen.
Hydrogen is currently derived by reacting water with fossil fuels at high tem-
peratures. This produces carbon dioxide and is no more sustainable than
any other fossil fuel burning technology. Electricity can be used to split water
into hydrogen and oxygen (electrolysis). In principle, the electricity can be
from a sustainable solar energy source. Unfortunately, solar electricity is rel-
atively expensive. Electrolysis of water to produce hydrogen entails substan-
tial conversion losses, and conversion of the energy in hydrogen to any form
of energy other than heat entails further substantial losses. In most cases it is
cheaper and more efficient to use the renewable electricity directly. The
direct splitting of water under sunlight (e.g. by using titanium dioxide) has
formidable technical obstacles, relating to corrosion and very low conversion
efficiency, which are far from resolution.
Fuel cells
Fuel cells convert gaseous fuels (e.g. hydrogen or natural gas) to electricity
without combustion. Fuel cells are being explored as replacements for fossil
fuel generators of electricity as they have the potential to be considerably
more efficient than conventional combustion, particularly in small systems.
Substantial technical obstacles still remain. Fuel cells are sometimes claimed
to be a renewable energy enabling technology because they could convert
hydrogen energy (produced by electrolysis using solar energy) to electricity
at relatively high efficiencies. However, there are many conversion losses in
this sequence.
Fuel cells may have important applications in saving energy. For exam-
ple, the use of fuel cells in houses to produce electricity from natural gas,
