Page 200 - Materials Chemistry, Second Edition

188 B. Ruggeri et al.

Fig. 7 Analogical model of the H 2 and CH 4 technology process at T = 35 °C with heat recovery
and a diameter of the bioreactor equal to 4 m

From a global point of view, in terms of energy sustainability, it is also nec-
essary to take into account the energy expenditure necessary for the production of
materials and the energy flows in different part of the World. In this context, the
useful energy effectively available from society adopting the AD technology is
21 %: at this point, it is clear that the complete ignorance of indirect energy is not
justified.
One consideration should be made regarding the modality of supplying direct
energy to run the plant: we have considered that the energy is produced through
the cogeneration of a part of the produced methane. This energy can be furnished
from different sources, for example, from a renewable one, such as solar energy or
wind power. In this case, the degree of sustainability would not change because the
use of renewable sources to furnish direct energy is removed from a different
energy service in society. In other words, the quality of the source to produce
direct energy in such a technology has no influence in the energy sustainability. In
fact, with the present approach, the useful energy remains constant. However,
using the exergy approach to score the sustainability of the technology, the use of
renewable energy sources rather than no-renewable ones does make the difference,
but in our opinion, this only introduces a false-perception of sustainability;
obviously other impacts, for example, Global Warming, could be different.