Biomedical Engineering Reference
In-Depth Information
about 3.2, meaning that for every unit of energy employed in the production
of biodiesel, 3.2 units are harvested at the end of the process. Performances
with bioethanol from maize are even less encouraging being its energetic yield
about 1 or even less [1, 2].
Second, considering the present oil price, biofuels are still not economically
competitive. In fact, biodiesel is estimated to be economically convenient over
the limit of 80$/barrel (and even higher for bioethanol). So, at least on a short
term, biofuels introduction in the commercial networks needs to be financially
supported by governments willing to stimulate their utilization. However, in
a medium timespan perspective, when oil reserves will be exhausting and the
oil price will consequently rise, biodiesel will be a major option for vehicle
traction and investments will be paid back.
2.2 Hydrogen Biological Production by Fermentative
Processes
Hydrogen has some very interesting properties, which makes it the potential
energy source of the future. It is easily converted into electricity, and it has
very high energy content per weight unit. Moreover, its utilization has the
lowest possible environmental impact. It produces electricity by reacting with
oxygen and yielding water as the only waste. However, these advantages are
effective only if hydrogen is produced by an e
cient and clean process. Un-
fortunately, this is currently not the case, and hydrogen is mainly derived
from fossil fuels and its production generates considerable amounts of CO
2
in
addition to other air pollutants such as sulphur dioxide and nitrogen oxides
(http://www.fao.org/docrep/w7241e/w7241e05.htm).
Thus, hydrogen's positive impact on the energy system depends on the
development of new methods for its e
cient production. One major option is
the water electrolysis, which, however, requires an e
cient, clean, and cheap
method for electricity generation, which is at present not available.
For this reason, other options are receiving an increasing attention and,
among them, the biological H
2
production. In fact, some living organisms
areabletoproduceH
2
in anaerobic conditions thanks to metabolic reactions
coupled with fermentation, nitrogen fixation, or photosynthesis. This is a very
promising approach, as it has the potential to provide cheap hydrogen without
using fossil fuels and emitting pollutants.
Living organisms can synthesize hydrogen from protons and electrons, us-
ing special enzymes named hydrogenases. Hydrogenases fall into two major
classes: Ni-Fe and Fe-Fe hydrogenase identified from the metal content in
their active site [3]. They differ in the composition and in the structure of
the metal center active site as well as in their polypeptide sequence, but also
show some conserved properties, namely the presence of CN and/or CO lig-
ands in the active site. In general, Fe-Fe hydrogenases were shown to be more
