Environmental Engineering Reference
In-Depth Information
(lack of carbonaceous emissions associated with its utilization) has to be
maintained at a global level, the development of economically viable production
methods starting from carbon-free primary sources is mandatory (as discussed in
Sect. 2.1
the technologies for carbon capture and sequestration are far to be
practically exploited). At the present state of technology, this means starting from
water as primary resource, and using non-fossil electricity to dissociate water in H
2
and O
2
, according to electrolysis process (see
Sect. 2.1.2
).
This immediately raises a problem of comparison in terms of well-to-wheels
efficiency between electricity and hydrogen, i.e., between BEVs and HFCEVs,
which will be faced later (see
Sect. 1.3
). The crucial aspect to be stressed here is
that both electricity and hydrogen can be considered as clean and sustainable
energy carriers only if they are produced by non-fossil sources, in particular if
hydrogen is derived from water by electrolysis using electricity generated by
renewable energy, the entire system from fuel production to end use on vehicles
has the potential to be ''zero emission'', in terms of both greenhouse gases and
local pollution.
Figure
1.1
shows that 6.4% of the energy consumed worldwide is produced
from conventional renewable sources (hydroelectric), while only a marginal
fraction is generated from new renewable energy sources (about 0.4%, not
reported in Fig.
1.1
). According to the most commonly accepted definition, a
renewable resource is one that can be replenished over a relatively short timescale
or is essentially limitless in supply. The new renewable resources—essentially
solar, wind and biomass, being tidal and geothermal very far from a commercial
exploitation—are worthy being greatly promoted because they constitute an
important alternative to fossil fuels. At this regards recent decisions of the
European Union (Decision N. 1639/2006/EC of the European Parliament, [
19
]) go
in this direction. The target is to reach a 20% share of energy from renewable
sources in the overall mix, focusing efforts on electricity, heating, and biofuels.
The main renewable source is the direct solar radiation, which can be exploited
for both direct and indirect electricity production. The power of the energy flow
coming from the sun depends on a number of parameters, such as season, latitude,
longitude, altitude, orientation, and inclination of the receiving surface, and at sea
level can reach the maximum value of about 1000 W/m
2
. It can be converted in
heat energy, which is either directly used in producing hot water or house heating
(solar thermal energy), or concentrated by mirror systems and then used for steam
production and consequent electricity by expansion in turbines (solar thermody-
namic energy). Moreover, the solar energy can be directly converted in electric
energy by using photovoltaic cells. These devices are composed by two thin layers
of semiconductors materials (n- and p-types, negative and positive, respectively),
between them an electric field is present due to the different nature of the two
materials (solar photovoltaic energy) [
20
]. The absorption of incident photons
(sunlight) generates an electron-hole pair, and when an external circuit is con-
nected to the cell, the electron and hole are separated by the structure of the device
(electrons to the negative terminals and holes to the positive terminal) thus gen-
erating electric power. The most mature solutions regarding the material used to
