Biomedical Engineering Reference
In-Depth Information
Among the possible renewable energy sources, an interesting emerging
option is the exploitation of higher plants or algae and their capacity to harvest
solar light and convert it into chemical energy. This option is particularly
interesting over other methods of solar radiation exploitation because these
organisms are able to convert CO
2
into biomass, through the photosynthesis,
and thus their utilization will also contribute to reduce carbon dioxide levels.
A further advantage is that, unlike fossil fuels, photosynthetic organisms
are rather uniformly distributed over earth surface. Thus, their exploitation
would not require large plants, and energy will be produced locally in small
bio-power plants. This different spatial distribution of power plants has alone
a positive effect on energy e
ciencies: in fact, as a relevant percentage of
energy is lost only during its transport from production to utilization, the
reduction in distance would reduce significantly energy production needs.
Possible applications of plants or algae for energy production are described,
with particular attention on their possible exploitation for biofuels and bio-
hydrogen production.
2.1 Biological Production of Vehicle Traction Fuels:
Bioethanol and Biodiesel
Any fuel derived recently from living organisms or from their metabolic by-
product is defined as a biofuel. Unlike other natural resources like oil, coal,
and nuclear fuels, biofuels are a renewable energy source. They present the
advantage of a cleaner combustion in comparison to conventional fuels, and
thus have a lower impact on atmospheric CO
2
levels.
Liquid biofuels are mainly developed for vehicle traction. At present the
most promising options are bioethanol and biodiesel.
2.1.1 Bioethanol
Bioethanol can be utilized as a fuel in combustion engines in different ways:
(1) as hydrous ethanol (95% by volume), containing small percentage of water,
it can be used as a gasoline substitute in cars with modified engines. (2) As
anhydrous (or dehydrated) ethanol, free from water or at least 99% pure, it
can be blended with conventional fuels in ratios between 5 and 85% (E85).
As a 5% component, it can be used in all recently produced engines without
modification. Higher blends require modified engines installed in the so-called
flexible fuel vehicles. (3) Finally, bioethanol is also used to manufacture ethyl-
tertiary-butyl-ether (ETBE), a fuel additive for conventional gasoline.
Bioethanol is produced from starch plants (like grain, corn, and tubers-
like cassava), sugar plants (sugar beet or sugar cane), and - although not yet
on a large scale - from cellulose. Carbohydrates present in these plants are
used as a substrate for the microbial fermentation, and ethanol is subsequently
